Latest peer-reviewed papers :



A new outburst of the yellow hypergiant star ρ Cas

Kraus, M.Kolka, I.Aret, A.Nickeler, D. H.Maravelias, G.Eenmäe, T.Lobel, A.Klochkova, V. G.

Abstract :
Yellow hypergiants are evolved massive stars that were suggested to be in post-red supergiant stage. Post-red supergiants that evolve back to the blue, hot side of the Hertzsprung-Russell diagram can intersect a temperature domain in which their atmospheres become unstable against pulsations (the Yellow Void or Yellow Wall), and the stars can experience outbursts with short, but violent mass eruptions. The yellow hypergiant ρ Cas is famous for its historical and recent outbursts, during which the star develops a cool, optically thick wind with a very brief but high mass-loss rate, causing a sudden drop in the light curve. Here, we report on a new outburst of ρ Cas that occurred in 2013, accompanied by a temperature decrease of ̃3000 K and a brightness drop of 0.6 mag. During the outburst, TiO bands appear together with many low excitation metallic atmospheric lines characteristic for a later spectral type. With this new outburst, it appears that the time interval between individual events decreases, which might indicate that ρ Cas is preparing for a major eruption that could help the star to pass through the Yellow Void. We also analysed the emission features that appear during phases of maximum brightness and find that they vary synchronous with the emission in the prominent [Ca II] lines. We conclude that the occasionally detected emission in the spectra of ρ Cas, as well as certain asymmetries seen in the absorption lines of low to medium-excitation potential, are circumstellar in nature, and we discuss the possible origin of this material.


Keywords : stars: atmospheres; stars: individual: ρ Cas; stars: massive; supergiants; Astrophysics - Solar and Stellar Astrophysics 
Publication : Monthly Notices of the Royal Astronomical Society, Volume 483, Issue 3, p.3792-3809
Pub Date : March 2019

Circumstellar CO in metal-poor stellar winds: the highly irradiated globular cluster star 47 Tucanae V3

 McDonald, I.Boyer, M. L.Groenewegen, M.A.T.Lagadec, E.Richards, A. M. S.Sloan, G. C.Zijlstra, A. A.

Abstract :
We report the first detection of circumstellar CO in a globular cluster. Observations with ALMA have detected the CO J = 3-2 and SiO v = 1 J = 8 - 7 transitions at 345 and 344 GHz, respectively, around V3 in 47 Tucanae (NGC 104; [Fe/H] = -0.72 dex), a star on the asymptotic giant branch. The CO line is detected at 7σ at a rest velocity vLSR = -40.6 km s-1 and expansion velocity of 3.2 ± ̃0.4 km s-1. The brighter, asymmetric SiO line may indicate a circumstellar maser. The stellar wind is slow compared to similar Galactic stars, but the dust opacity remains similar to Galactic comparisons. We suggest that the mass-loss rate is set by the levitation of material into the circumstellar environment by pulsations, but that the terminal wind-expansion velocity is determined by radiation pressure on the dust: a pulsation-enhanced dust-driven wind. We suggest the metal-poor nature of the star decreases the grain size, slowing the wind and increasing its density and opacity. Metallic alloys at high altitudes above the photosphere could also provide an opacity increase. The CO line is weaker than expected from Galactic AGB stars, but its strength confirms a model that includes CO dissociation by the strong interstellar radiation field present inside globular clusters.


Keywords : stars: AGB and post-AGB; circumstellar matter; stars: mass-loss; stars: winds; outflows; globular clusters: individual: NGC 104; infrared: stars; Astrophysics - Solar and Stellar Astrophysics; Astrophysics - Astrophysics of Galaxies
Publication : Monthly Notices of the Royal Astronomical Society: Letters, Volume 484, Issue 1, p.L85-L89
Pub Date : March 2019

The VMC Survey. XXXIII. The tip of the red giant branch in the Magellanic Clouds

Groenewegen, M.A.T.Cioni, M. -R. L.Girardi, Grijs, R.Ivanov, V. D.Marconi, M.Muraveva, T.Ripepi, V.van Loon, J. Th.

Abstract :
In this paper JKs-band data from the VISTA Magellanic Cloud (VMC) survey are used to investigate the tip of the red giant branch (TRGB) as a distance indicator. A linear fit to recent theoretical models is used as the basis for the absolute calibration which reads MKs = -4.196 - 2.013 (J - Ks), valid in the colour range 0.75 < (J - Ks)< 1.3 mag and in the 2MASS system. The observed TRGB is found based on a classical first-order derivative filter and a second-order derivative filter applied to the binned luminosity function using the "sharpened" magnitude that takes the colour term into account. Extensive simulations are carried out to investigate any biases and errors in the derived distance modulus (DM). Based on these simulations criteria are established related to the number of stars per bin in the 0.5 mag range below the TRGB and related to the significance with which the peak in the filter response curve is determined such that the derived distances are unbiased. The DMs based on the second-order derivative filter are found to be more stable and are therefore adopted, although this requires twice as many stars per bin. Given the surface density of TRGB stars in the Magellanic Clouds (MCs), areas of ̃0.5 deg2in the densest parts to ̃10 deg2 in the outskirts of the MCs need to be considered to obtain accurate and reliable values for the DMs. The TRGB method is applied to specific lines-of-sight where independent distance estimates exist, based on detached eclipsing binaries in the Large and Small Magellanic Clouds (LMC, SMC), classical Cepheids in the LMC, RR Lyrae stars in the SMC, and fields in the SMC where the star formation history (together with reddening and distance) has been derived from deep VMC data. The analysis shows that the theoretical calibration is consistent with the data, that the systematic error on the DM is approximately 0.045 mag (about evenly split between the theoretical calibration and the method), and that random errors of 0.015 mag are achievable. Reddening is an important element in deriving the distance: we derive mean DMs ranging from 18.92 mag (for a typical E(B - V) of 0.15 mag) to 19.07 mag (E(B - V)̃0.04 mag) for the SMC, and ranging from 18.48 mag (E(B - V)̃0.12 mag) to 18.57 mag (E(B - V)̃0.05 mag) for the LMC. Based on observations made with VISTA at ESO under programme ID 179.B-2003.


Keywords : Magellanic Clouds; stars: distances
Publication : Astronomy & Astrophysics, Volume 622, id.A63, 26 pp.
Pub Date : February 2019

Diverse Variability of O and B Stars Revealed from 2-minute Cadence Light Curves in Sectors 1 and 2 of the TESS Mission: Selection of an Asteroseismic Sample

Pedersen, M.G.Chowdhury, S.Johnston, C.Bowman, D.M.Aerts, C.Handler, G.De Cat, P.Neiner, C.David-Uraz, A.Buzasi, D.Tkachenko, A.Simón-Díaz, S.Moravveji, E.Sikora, J.; et al


Abstract :
Uncertainties in stellar structure and evolution theory are largest for stars undergoing core convection on the main sequence. A powerful way to calibrate the free parameters used in the theory of stellar interiors is asteroseismology, which provides direct measurements of angular momentum and element transport. We report the detection and classification of new variable O and B stars using high-precision short-cadence (2 minutes) photometric observations assembled by the Transiting Exoplanet Survey Satellite (TESS). In our sample of 154 O and B stars, we detect a high percentage (90%) of variability. Among these we find 23 multiperiodic pulsators, 6 eclipsing binaries, 21 rotational variables, and 25 stars with stochastic low-frequency variability. Several additional variables overlap between these categories. Our study of O and B stars not only demonstrates the high data quality achieved by TESS for optimal studies of the variability of the most massive stars in the universe, but also represents the first step toward the selection and composition of a large sample of O and B pulsators with high potential for joint asteroseismic and spectroscopic modeling of their interior structure with unprecedented precision.


Keywords : asteroseismology; binaries: general; stars: evolution; stars: massive; stars: oscillations: including pulsations;  stars: rotation; Astrophysics - Solar and Stellar Astrophysics
Publication : The Astrophysical Journal Letters, Volume 872, Issue 1, article id. L9, 11 pp. (2019).
Pub Date : February 2019

A study of pulsation & rotation in a sample of A-K type stars in the Kepler field

Chowdhury, S.; Joshi, S.Engelbrecht, C.A.; De Cat, P.Joshi, Y.C.Paul, K. T.

We present the results of time-series photometric analysis of 15106 A-K type stars observed by the Kepler space mission. We identified 513 new rotational variables and measured their starspot rotation periods as a function of spectral type and discuss the distribution of their amplitudes. We examined the well-established period-color relationship that applies to stars of spectral types F5-K for all of these rotational variables and, interestingly, found that a similar period-color relationship appears to extend to stars of spectral types A7 to early-F too. This result is not consistent with the very foundation of the period-color relationship. We have characterized 350 new non-radial pulsating variables such as A- and F-type candidate δ Scuti, γ Doradus and hybrid stars, which increases the known candidate non-radial pulsators in the Kepler field significantly, by ̃20%. The relationship between two recently constructed observables, Energy and Efficiency, was also studied for the large sample of non-radial pulsators, which shows that the distribution in the logarithm of Energy (log (En)) can be used as a potential tool to distinguish between the non-radial pulsators, to some extent. Through visual inspection of the light curves and their corresponding frequency spectra, we found 23 new candidate red giant solar-like oscillators not previously reported in the literature. The basic physical parameters such as masses, radii and luminosities of these solar-like oscillators were also derived using asteroseismic relations.

Keywords : Astrophysics - Solar and Stellar Astrophysics
Pub Date : December 2018

Optimizing asteroid orbit computation for Gaia with normal points

Fedorets, G.Muinonen, K.Pauwels, T.Granvik, M.;

Abstract :
Context. In addition to the systematic observations of known solar-system objects (SSOs), a continuous processing of new discoveries requiring fast responses is implemented as the short-term processing of Gaia SSO observations, providing alerts for ground-based follow-up observers. The common independent observation approach for the purposes of orbit computation has led to unrealistically large ephemeris prediction uncertainties when processing real Gaia data. 
Aims: We aim to provide ground-based observers with a cloud of sky positions that is shrunk to a fraction of the previously expected search area by making use of the characteristic features of Gaia astrometry. This enhances the efficiency of Gaia SSO follow-up network and leads to an increased rate of asteroid discoveries with reasonably constrained orbits with the help of ground-based follow-up observations of Gaia asteroids. 
Methods: We took advantage of the separation of positional errors of Gaia SSO observations into a random and systematic component. We treated the Gaia observations in an alternative way by collapsing up to ten observations that correspond to a single transit into a single so-called normal point. We implemented this input procedure in the Gaia SSO short-term processing pipeline and the OpenOrb software. 
Results: We validate our approach by performing extensive comparisons between the independent observation and normal point input methods and compare them to the observed positions of previously known asteroids. The new approach reduces the ephemeris uncertainty by a factor of between three and ten compared to the situation where each point is treated as a separate observation. 
Conclusions: Our new data treatment improves the sky prediction for the Gaia SSO observations by removing low-weight orbital solutions. These solutions originate from excessive curvature of observations, introduced by short-term variations of Gaia attitude on the one hand, and, as a main effect, shrinking of systematic error bars in the independent observation case on the other hand. We anticipate that a similar approach may also be utilized in a situation where observations from a single observatory dominate.


Keywords : astrometry; celestial mechanics; minor planets; asteroids: general
Publication : Astronomy & Astrophysics, Volume 620, id.A101, 15 pp.
Pub Date : December 2018
DOI : 10.1051/0004-6361/201833197 
Bibcode : 2018A&A...620A.101F 

Hybrid Pulsations and Tidal Splitting detected in the Kepler Eclipsing and Spotted Binary System KIC 6048106

Samadi Ghadim, A.Lampens, P.Jassur, D. M.

Abstract :
We present a new asteroseismic analysis of KIC 6048106, a Kepler Algol-type eclipsing binary star in a circularized orbit with Porb=1.559361±0.000036 d. Based on a physical model for the binary and its corresponding set of fundamental parameters (Teff=7033±187 K, M1=1.55±0.11 M☉, R1=1.58±0.12 R☉ and Teff=4522±103 K, M2=0.33±0.07 M☉, R2=1.78±0.16 R☉, respectively for the primary and the secondary component), we obtained the residual light curve after removal of the full binary model, including a 290-d activity cycle for the secondary component. In this work, we used the method of Fourier analysis of the residual light curve in combination with least squares optimization for the frequency analysis. We detected seven dominant, independent gravity (g) modes as well as 34 low-amplitude acoustic (p) modes. The g modes in the range of 1.96-2.85 d-1 have a mean spacing of ∆Πmean=1517.92±131.54 s. Though of much lower amplitude, additional significant frequencies were detected in the intervals 7.49-15.2 d-1 and 19-22.5 d-1 (i.e., in the p mode region), with corresponding dominant modes νmax1=11.745±0.001 d-1 and νmax2=20.960±0.002 d-1. From its position in the HR diagram, we conclude that the primary component is the source of the detected hybrid pulsations. Consequently, the pulsation constants, Q, of the high frequencies cover the range of 0.028-0.064 d. Furthermore, ν43 (19.037±0.002 d-1) might correspond to the fundamental radial mode (Q=0.033±0.007 d). The other frequencies in the range of 19-22.5 d-1 could be radial or non-radial overtone modes. Moreover, the low-amplitude p modes show an equidistant splitting by forb, which we interpret as tidal splitting following theoretical predictions.


Keywords : Techniques: photometric; binaries: eclipsing; Stars: fundamental parameters; Stars: variables: delta Scuti; Stars: oscillations; starspots
Publication : Acta Astronomica, vol 68, no 4, p. 425-447
Pub Date : December 2018

Variability in Proto-planetary Nebulae. V. Velocity and Light Curve Analysis of IRAS 17436+5003, 18095+2704, and 19475+3119

Hrivnak, B.J.;  Van de Steene, G.Van Winckel, H.Lu, W.Sperauskas, J.

Abstract :
We have obtained contemporaneous light, color, and radial velocity data for three proto-planetary nebulae (PPNe) over the years 2007 to 2015. The light and velocity curves of each show similar periods of pulsation, with photometric periods of 42 and 50 days for IRAS 17436+5003, 102 days for IRAS 18095+2704, and 35 days for IRAS 19475+3119. The light and velocity curves are complex with multiple periods and small, variable amplitudes. Nevertheless, at least over limited time intervals, we were able to identify dominant periods in the light, color, and velocity curves and compare the phasing of each. The color curves appear to peak with or slightly after the light curves while the radial velocity curves peak about a quarter of a cycle before the light curves. Similar results were found previously for two other PPNe, although for them the light and color appeared to be in phase. Thus, it appears that PPNe are brightest when smallest and hottest. These phase results differ from those found for classical Cepheid variables, where the light and velocity differ by half a cycle, and are hottest at about average size and expanding. However, they do appear to have similar phasing to the larger-amplitude pulsations seen in RV Tauri variables. Presently, few pulsation models exist for PPNe, and these do not fit the observations well, especially the longer periods observed. Model fits to these new light and velocity curves would allow masses to be determined for these post-AGB objects, and thereby provide important constraints to post-AGB stellar evolution models of low- and intermediate-mass stars.


Keywords : stars: AGB and post-AGB; stars: individual: IRAS 17436+5003; IRAS 18095+2704; IRAS 19475+3119; stars: oscillations; stars: variables: general
Publication : The Astronomical Journal, Volume 156, Issue 6, article id. 300, 21 pp. (2018).
Pub Date : December 2018

Radio Continuum Spectra of Planetary Nebulae

Hajduk, M.van Hoof, P.A.M.Sniadkowska, K.Krankowski, A.Błaszkiewicz, L.Dabrowski, B.Zijlstra, A.

Abstract :
Radio continuum emission of planetary nebulae is a rich source of information about their structure and physical parameters. Although radio emission is well studied, planetary nebulae show higher spectral indices than expected for homogeneous sphere. A few competing models exist in the literature to explain this discrepancy. We propose that it is related to non-spherical morphology of most of planetary nebulae.

Keywords :  planetary nebulae; AGB and post-AGB; interstellar medium; radio continuum; winds; outflows
Pub Date : December 2018

The Cepheid period-luminosity-metallicity relation based on Gaia DR2 data

Groenewegen, M.A.T.

Abstract :
Aims: We use parallax data from the Gaia second data release (GDR2), combined with parallax data based on HIPPARCOS and HST data, to derive the period-luminosity-metallicity (PLZ) relation for Galactic classical cepheids (CCs) in the V, K, and Wesenheit WVK bands. 
Methods: An initial sample of 452 CCs are extracted from the literature with spectroscopically derived iron abundances. Reddening values, classifications, pulsation periods, and mean V- and K-band magnitudes are taken from the literature. Based on nine CCs with a goodness-of-fit (GOF) statistic smaller than 8 and with an accurate non-Gaia parallax (σπ comparable to that in GDR2), a parallax zero-point offset of -0.049 ± 0.018 mas is derived. Selecting a GOF statistic smaller than 8 removes about 40% of the sample most likely related due to binarity. Excluding first overtone and multi-mode cepheids and applying some other criteria reduces the sample to about 200 stars. 
Results: The derived PL(Z) relations depend strongly on the parallax zero-point offset. The slope of the PL relation is found to be different from the relations in the LMC at the 3σ level. Fixing the slope to the value found in the LMC leads to a distance modulus (DM) to the LMC of order 18.7 mag, larger than the canonical distance. The canonical DM of around 18.5 mag would require a parallax zero-point offset of order -0.1 mas. Given the strong correlation between zero point, period and metallicity dependence of the PL relation, and the parallax zero-point offset there is no evidence for a metallicity term in the PLZ relation. 
Conclusions: The GDR2 release does not allow us to improve on the current distance scale based on CCs. The value of and the uncertainty on the parallax zero-point offset leads to uncertainties of order 0.15 mag on the distance scale. The parallax zero-point offset will need to be known at a level of 3 μas or better to have a 0.01 mag or smaller effect on the zero point of the PL relation and the DM to the LMC. Table 1 is only available at the CDS via anonymous ftp to ( or via


Keywords : stars: distances; stars: variables: Cepheids; distance scale; parallaxes; Astrophysics - Solar and Stellar Astrophysics
Pub Date : November 2018

Radial velocity monitoring of (candidate) hybrid A- and F-type stars from the Kepler mission

Lampens, P.;  Vermeylen, L.De Cat, P.Sódor, Á.Bognár, Z.Frémat, Y.Skarka, M.Lehmann, H.


Abstract :
An ensemble of 50 candidate hybrid A/F-type stars from the Kepler mission was monitored during four years with the HERMES spectrograph attached to the Mercator telescope. From this survey, we obtained new radial velocities, new or improved atmospheric properties (Teff, log g, v sin i), and classified all our targets in terms of evidence for multiplicity, pulsation and/or fast rotation. An extension of 40 new candidate hybrid A- and F-type stars from the Kepler mission has been recently defined for a second survey to be performed under similar conditions. The new high-resolution spectra will be obtained with various small and medium-sized telescopes. For a number of newly detected stellar systems with good radial velocity coverage, we also computed much improved orbits by combining the radial velocities with the time delays obtained via the monitoring of the pulsation frequencies during the four years of Kepler photometry.


Keyword(s): technique: radial velocity ; stars: oscillations ; stars: binary systems ; stars: multiple systems ; fundamental stellar parameters
Note: Proceedings paper of poster presented at the international conference “Physics of Oscillating Stars” (PHOST), Banyuls-sur-mer (France), 03-07/09/2018
DOI: 10.5281/zenodo.1493166

Photometric study and search for variable stars in the intermediate-age open cluster NGC 2126

Chehlaeh, N.; Mkrtichian, D.Lampens, P.Komonjinda, S.Kim, S. -L.Van Cauteren, P.Kusakin, A. V.Glazunova, L.

Abstract : 
We present the results of an analysis of photometric time-series observations for NGC 2126 acquired during the years 2004, 2013, and 2015. NGC 2126 is an intermediate-age open cluster which has a population of stars inside the δ Scuti instability strip. The main purpose is to search for new variable stars, to study their light curves and pulsation spectra. Eleven variables are reported, two of which are new discoveries, eight are pulsating and three eclipsing binary stars, one of which has a pulsating component (V551 Aur). Our 10 times of light minimum along with others compiled from the literature were used to revise the orbital period of V551 Aur: Porb = 1.1731752(8) d. The Wilson-Devinney technique was used to analyse its light curves and to determine a new set of the system's parameters. We found a single pulsation period Ppul = 0.12964977(3) d in the residuals which lies near the 9th harmonic of the orbital period. We determined the accurate value of high proper motion star LHS 1809 as μαcos δ = 115.7± 0.1 mas yr-1 and μδ = -823.7 ± 0.1 mas yr-1. From the construction of the colour-magnitude diagram, we estimated the physical parameters of the cluster. We found that the shape of the colour-magnitude diagram at the turn-off point is well reproduced with an isochrone with log(t) of 9.1 ± 0.1 yr, metallicity of 0.019, E(B - V) = 0.27± 0.01 mag, and (m - M)0 = 10.80 ± 0.05 mag.

Keywords : techniques: photometric; binaries: eclipsing; stars: variables: δ Scuti; open clusters and associations: individual: NGC 2126
Publication : Monthly Notices of the Royal Astronomical Society, Volume 480, Issue 2, p.1850-1863
Pub Date : October 2018
Bibcode : 2018MNRAS.480.1850C

PACS and SPIRE range spectroscopy of cool, evolved stars

Nicolaes, D.Groenewegen, M.A.T.Royer, P.Lombaert, R.Danilovich, T.Decin, L.

Abstract :
Context. At the end of their lives AGB stars are prolific producers of dust and gas. The details of this mass-loss process are still not understood very well. Herschel PACS and SPIRE spectra which cover the wavelength range from ̃55 to 670 μm almost continuously, offer a unique way of investigating properties of AGB stars in general and the mass-loss process in particular as this is the wavelength region where dust emission is prominent and molecules have many emission lines. 
Aims: We present the community with a catalogue of AGB stars and red supergiants (RSGs) with PACS and/or SPIRE spectra reduced according to the current state of the art. 
Methods: The Herschel interactive processing environment (HIPE) software with the latest calibration is used to process the available PACS and SPIRE spectra of 40 evolved stars. The SPIRE spectra of some objects close to the Galactic plane require special treatment because of the weaker fluxes in combination with the strong and complex background emission at those wavelengths. The spectra are convolved with the response curves of the PACS and SPIRE bolometers and compared to the fluxes measured in imaging data of these sources. Custom software is used to identify lines in the spectra, and to determine the central wavelengths and line intensities. Standard molecular line databases are used to associate the observed lines. Because of the limited spectral resolution of the PACS and SPIRE spectrometers (̃1500), several known lines are typically potential counterparts to any observed line. To help identifications in follow-up studies the relative contributions in line intensity of the potential counterpart lines are listed for three characteristic temperatures based on local thermodynamic equilibrium (LTE) calculations and assuming optically thin emission. 
Results: The following data products are released: the reduced spectra, the lines that are measured in the spectra with wavelength, intensity, potential identifications, and the continuum spectra, i.e. the full spectra with all identified lines removed. As simple examples of how this data can be used in future studies we have fitted the continuum spectra with three power laws (two wavelength regimes covering PACS, and one covering SPIRE) and find that the few OH/IR stars seem to have significantly steeper slopes than the other oxygen- and carbon-rich objects in the sample, possibly related to a recent increase in mass-loss rate. As another example we constructed rotational diagrams for CO (and HCN for the carbon stars) and fitted a two-component model to derive rotational temperatures. The reduced spectra and the line subtracted spectra as well as Table E.1 are only available at the CDS via anonymous ftp to ( or via


Keywords : stars: AGB and post-AGB; stars: mass-loss; infrared: stars; Astrophysics - Solar and Stellar Astrophysics
Pub Date : October 2018

LAMOST Observations in the Kepler Field. II. Database of the Low-resolution Spectra from the Five-year Regular Survey

Zong, W.;  Fu, Jian-Ning;  De Cat, P.Shi, J.; et al

Abstract :
The LAMOST-Kepler (LK-) project was initiated to use the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) to make spectroscopic follow-up observations for the targets in the field of the Kepler mission. The Kepler field is divided into 14 subfields that are adapted to the LAMOST circular field with a diameter of 5°. During the regular survey phase of LAMOST, the LK-project took data from 2012 June to 2017 June and covered all 14 subfields at least twice. In particular, we describe in this paper the second Data Release of the LK-project, including all spectra acquired through 2015 May─2017 June together with the first round observations of the LK-project from 2012 June to 2014 September. The LK-project now counts 227,870 spectra of 156,390 stars, among which we have derived atmospheric parameters ({log}g, T eff, and [Fe/H]) and heliocentric radial velocity for 173,971 spectra of 126,172 stars. These parameters were obtained with the most recent version of the LAMOST Stellar Parameter Pipeline v 2.9.7. Nearly one half, namely 76,283 targets, are observed both by the LAMOST and Kepler telescopes. These spectra, establishing a large spectroscopy library, will be useful for the entire astronomical community, particularly for planetary science and stellar variability on Kepler targets. Based on observations collected with the Large Sky Area Multi-Object Fiber spectroscopic Telescope (LAMOST), which is located at the Xinglong Observatory, China.


Keywords : astronomical databases: miscellaneous; stars: fundamental parameters; stars: general; stars: statistics;  techniques: spectroscopic; Astrophysics - Solar and Stellar Astrophysics
Pub Date : October 2018
DOI 10.3847/1538-4365/aadf81 
Bibcode 2018ApJS..238...30Z 

Radio observations of planetary nebulae: no evidence for strong radial density gradients

Hajduk, M.van Hoof, P.A.M.Śniadkowska, K.Krankowski, A.Błaszkiewicz, L.Dąbrowski, B.Zijlstra, A. A. 

Abstract :
Radio-continuum observations trace the thermal emission of ionized plasma in planetary nebulae and bring useful information regarding nebular geometries. A model of a homogeneous sphere or shell cannot fit the nebular spectra and brightness temperatures. Two alternative models have been proposed in the literature: the first consists of two homogeneous components, while the other is a model of a shell with a significant radial density gradient. On the other hand, a prolate ellipsoidal shell model can fit the surface-brightness distribution of selected objects successfully. We verify the existing models using data collected in radio surveys covering a wide range of frequencies. In about 50 per cent of cases a density gradient can be excluded and none of the remaining objects could be confirmed. None of the observed planetary nebulae shows a spectral index of 0.6 in the optically thick part of the spectrum, which is the value predicted for a shell containing a strong radial density gradient. Radio spectra can be fitted with a model of a prolate ellipsoidal shell, but also by a shell containing temperature variations in planetary nebulae. At least eight planetary nebulae show two-component spectra, with one compact component showing much higher optical thickness than the other. Unexpectedly, the group of planetary nebulae with the lowest surface brightness shows non-negligible optical thickness. Their emission comes from compact and dense structures, comprising only a small part of the total nebular mass.


Keywords : planetary nebulae: general; radio continuum: general; stars: AGB and post-AGB; stars: winds; outflows; Astrophysics - Astrophysics of Galaxies; Astrophysics - Solar and Stellar Astrophysics
Pub Date : October 2018 

On the detection of CO and mass-loss of bulge OH/IR stars

Blommaert, J. A. D. L.Groenewegen, M.A.T.Justtanont, K.Decin, L.

Abstract :
We report on the successful search for CO (2-1) and (3-2) emission associated with OH/IR stars in the Galactic bulge. We observed a sample of eight extremely red asymptotic giant branch stars with the Atacama Pathfinder EXperiment telescope and detected seven. The sources were selected at a sufficiently high galactic latitude to avoid interference by interstellar CO, which hampered previous studies of inner galaxy stars. To study the nature of our sample and the mass-loss, we constructed the spectral energy distribution (SEDs) from photometric data and Spitzer IRS spectroscopy. In a first step, we apply radiative transfer modelling to fit the SEDs and obtain luminosities and dust mass-loss rates (MLRs). Through dynamical modelling, we then retrieve the total MLR and the gas-to-dust ratios. We derived variability periods of our stars. The luminosities range between approximately 4000 and 5500 L☉ and periods are shorter than 700 d. The total MLR ranges between 10-5 and 10-4 M☉ yr-1. Comparison with evolutionary models shows that the progenitor mass ≈1.5 M☉, similar to the bulge Miras of intermediate age (3 Gyr). The gas-to-dust ratios are between 100 and 400 and are similar to what is found for OH/IR stars in the galactic disc. One star, IRAS 17347-2319, has a very short period of approximately 300 d that may be decreasing further. It may belong to a class of Mira variables with a sudden change in period as observed in some Galactic objects. It would be the first example of an OH/IR star in this class and deserves further follow-up observations.

Keywords : stars: AGB and post-AGB; circumstellar matter; stars: mass-loss; galaxy: bulge; radio lines: stars; Astrophysics - Solar and Stellar Astrophysics; Astrophysics - Astrophysics of Galaxies
Pub Date : September 2018

Suppression of Dielectronic Recombination Due to Finite Density Effects. II. Analytical Refinement and Application to Density-dependent Ionization Balances and AGN Broad-line Emission

Nikolić, D.Gorczyca, T. W.Korista, K. T.Chatzikos, M.Ferland, G. J.Guzmán, F.van Hoof, P. A. M.Williams, R. J. R.Badnell, N. R.

Abstract :
We present improved fits to our treatment of suppression of dielectronic recombination at intermediate densities. At low densities, most recombined excited states eventually decay to the ground state, and therefore the total dielectronic recombination rate to all levels is preserved. At intermediate densities, on the other hand, collisions can lead to ionization of higher-lying excited states, thereby suppressing the dielectronic recombination rate. The improved suppression factors presented here, although highly approximate, allow summed recombination rate coefficients to be used to intermediate densities. There have been several technical improvements to our previously presented fits. For H- through B-like ions the activation log densities have been adjusted to better reproduce existing data. For B-, C-, Al-, and Si-like ions secondary autoionization is now included. The treatment of density discontinuity in electron excitations out of ground state H-, He-, and Ne-like ions has been improved. These refined dielectronic recombination suppression factors are used in the most recent version of the plasma simulation code Cloudy. We show how the ionization and emission spectrum change when this physics is included. Although these suppression factors improve the treatment of intermediate densities, they are highly approximate and are not a substitution for a complete collisional-radiative model of the ionization balance.


Keywords : atomic data; atomic processes; ISM: abundances; ISM: atoms; line: formation; plasmas; Physics - Atomic Physics
Pub Date : August 2018

Gaia Data Release 2. Processing the spectroscopic data

Sartoretti, P.Katz, D.Cropper, M.Panuzzo, P.Seabroke, G. M.Viala, Y.Benson, K.Blomme, R.; et al

Abstract :
Context. The Gaia Data Release 2 (DR2) contains the first release of radial velocities complementing the kinematic data of a sample of about 7 million relatively bright, late-type stars. 
Aims: This paper provides a detailed description of the Gaia spectroscopic data processing pipeline, and of the approach adopted to derive the radial velocities presented in DR2. 
Methods: The pipeline must perform four main tasks: (i) clean and reduce the spectra observed with the Radial Velocity Spectrometer (RVS); (ii) calibrate the RVS instrument, including wavelength, straylight, line-spread function, bias non-uniformity, and photometric zeropoint; (iii) extract the radial velocities; and (iv) verify the accuracy and precision of the results. The radial velocity of a star is obtained through a fit of the RVS spectrum relative to an appropriate synthetic template spectrum. An additional task of the spectroscopic pipeline was to provide first-order estimates of the stellar atmospheric parameters required to select such template spectra. We describe the pipeline features and present the detailed calibration algorithms and software solutions we used to produce the radial velocities published in DR2. 
Results: The spectroscopic processing pipeline produced median radial velocities for Gaia stars with narrow-band near-IR magnitude GRVS ≤ 12 (i.e. brighter than V 13). Stars identified as double-lined spectroscopic binaries were removed from the pipeline, while variable stars, single-lined, and non-detected double-lined spectroscopic binaries were treated as single stars. The scatter in radial velocity among different observations of a same star, also published in Gaia DR2, provides information about radial velocity variability. For the hottest (Teff≥ 7000 K) and coolest (Teff ≤ 3500 K) stars, the accuracy and precision of the stellar parameter estimates are not sufficient to allow selection of appropriate templates. The radial velocities obtained for these stars were removed from DR2. The pipeline also provides a first-order estimate of the performance obtained. The overall accuracy of radial velocity measurements is around 200-300 m s-1, and the overall precision is 1 km s-1; it reaches 200 m s-1for the brightest stars.


Keywords : techniques: spectroscopic; catalogs; techniques: radial velocities; surveys; methods: data analysis; Astrophysics - Astrophysics of Galaxies; Astrophysics - Instrumentation and Methods for Astrophysics; Astrophysics - Solar and Stellar Astrophysics
Pub Date : August 2018 

The VMC survey - XXXI: The spatially resolved star formation history of the main body of the Small Magellanic Cloud

Rubele, S.Pastorelli, G.Girardi, L.Cioni, Maria-Rosa L.Zaggia, S.Marigo, P.Bekki, K.Bressan, A.;  Clementini, Grijs, R.Emerson, J.Groenewegen, M.A.T.Ivanov, V. D.; et al.

Abstract :
We recover the spatially resolved star formation history across the entire main body and Wing of the Small Magellanic Cloud (SMC), using 14 deep tile images from the VISTA survey of the Magellanic Clouds (VMC) in the YJKs filters. The analysis is performed on 168 subregions of size 0.143 deg2 covering a total contiguous area of 23.57 deg2. We apply a colour-magnitude diagram (CMD) reconstruction method that returns the best-fitting star formation rate SFR(t), age-metallicity relation, distance and mean reddening, together with their confidence intervals, for each subregion. With respect to previous analyses, we use a far larger set of the VMC data, updated stellar models, and fit the two available CMDs (Y - Ks versus Ks and J - Ks versus Ks) independently. The results allow us to derive a more complete and more reliable picture of how the mean distances, extinction values, star formation rate, and metallicities vary across the SMC, and provide a better description of the populations that form its Bar and Wing. We conclude that the SMC has formed a total mass of (5.31 ± 0.05) × 108 M☉ in stars over its lifetime. About two-thirds of this mass is expected to be still locked in stars and stellar remnants. 50 per cent of the mass was formed prior to an age of 6.3 Gyr, and 80 per cent was formed between 8 and 3.5 Gyr ago. We also illustrate the likely distribution of stellar ages and metallicities in different parts of the CMD, to aid the interpretation of data from future astrometric and spectroscopic surveys of the SMC.


Keywords : Hertzsprung-Russell and colour-magnitude diagrams; galaxies: evolution; Magellanic Clouds; galaxies: stellar content; Astrophysics - Astrophysics of Galaxies; Astrophysics - Solar and Stellar Astrophysics
Publication : Monthly Notices of the Royal Astronomical Society, Volume 478, Issue 4, p.5017-5036
Pub Date : August 2018

The Belgian Repository of Fundamental Atomic Data and Stellar Spectra (BRASS) Identifying Fruitful Methods for Producing Atomic Data

Laverick, M.Lobel, A.Royer, P.Martayan, C.Merle, T.van Hoof, P.A.M.Van de Swaelmen, M.David, M.; Hensberge, H.Thienpont, E.

Abstract :
The Belgian repository of fundamental atomic data and stellar spectra (BRASS) aims to provide the largest systematic and homogeneous quality assessment to-date of input atomic data required for stellar spectral synthesis. In addition to quality-assessed atomic data, BRASS shall also provide of a range of extremely high-quality benchmark stellar spectra spanning late B-type stars to early K-type stars. In this paper, we summarise the project’s progress and available results to-date. We provide a brief comparison between our results and the BRASS project’s compiled and cross-matched atomic literature, with the goal of providing useful feedback to the atomic community on which methods may produce more reliable and accurate atomic data. We hope that the examples presented here stimulate further investigation by the atomic physics community.

Keywords :  atomic data; stellar spectra; oscillator strengths; atomic and molecular databases
Pub Date : July 2018

The Real-Time Evolution of V4334 Sgr

van Hoof, P.A.M.Kimeswenger, S.Van de Steene, G.Avison, A.Zijlstra, A.Guzman-Ramirez, L.Herwig, F.Hajduk, M.

Abstract :
V4334 Sgr (Sakurai’s object) is an enigmatic evolved star that underwent a very late thermal pulse a few years before its discovery in 1996. It ejected a new hydrogen-deficient nebula in the process. The source has been observed continuously since, at many wavelengths ranging from the optical to the radio regime. In this paper we evaluate these data and discuss the evolution of this object. We reach the conclusion that we have seen no evidence for photoionization of the nebula yet and that the spectral features we see are caused either by shocks or by dust. These shocks are an integral part of the hydrodynamic shaping that is now producing a new bipolar nebula inside the old planetary nebula (PN), implying that we have a detailed observational record of the very early stages of the shaping of a bipolar nebula.

Keywords :  stellar mass loss; stellar evolution; planetary nebulae; circumstellar dust
Pub Date : July 2018

Recent Development of the Atomic Line List

van Hoof, P.A.M.

Abstract :
The Atomic Line List is an online database of wavelengths and transition probabilities of atomic lines. It is primarily set up as a tool to help identify unknown spectral features. This paper briefly describes the web interface, how the line list is constructed, and what development is currently being undertaken for the next release.

Keywords : atomic data; spectroscopy; chemical abundances; oscillator strengths; atomic and molecular databases
Pub Date : June 2018

Proceedings of the 1st BINA workshop: Instrumentation and Science with the 3.6-m DOT and 4-m ILMT telescopes

De Cat, P.Surdej, J.Omar, A.De Becker, M.Joshi, S.

Abstract :
The first official BINA activity was the 1st BINA workshop (∼bina/). It was held from 15-18 November 2016 at the Uttarakhand Academy of Administration (UAoA, ATI; in Mallital Nainital (India). The organization was lead by Santosh Joshi (ARIES, India) who received financial support of DST (Govt. of India), BELSPO (Govt. of Belgium) and ARIES (host institute). About 150 people from India, Belgium and other countries expressed their interest for this workshop. However, due to financial limitations, only 107 astronomers could participate. They originated from 8 different countries (#88 from India, #10 from Belgium, #3 from Thailand, #2 from Japan, #1 from China, #1 from Russia, #1 from South Africa, and #1 from Taiwan), making it a true international workshop.  The focus of this workshop was on the “Instrumentation and Science with the 3.6-m DOT and 4-m ILMT telescopes”. Given that this was the first event where all the Indian and Belgian partners could meet in person, the aim was to give an overview of the current and future possibilities to base scientific projects on data obtained with the Indo-Belgian telescopes and/or other astronomical facilities in India. Furthermore, members of each partner institute have been given the opportunity to highlight their main scientific topics to stimulate collaborations with each other and international collaborators. The program of this 4-day meeting consisted of three types of activities.

Keywords : Astronomy; Astrophysics; Observatories
Pub Date : April 2018
Article : PDF-file

The Belgian repository of fundamental atomic data and stellar spectra (BRASS). I. Cross-matching atomic databases of astrophysical interest

Laverick, M.Lobel, A.Merle, T.Royer, P.Martayan, C.David, M.Hensberge, H.Thienpont, E.

Abstract :
Context. Fundamental atomic parameters, such as oscillator strengths, play a key role in modelling and understanding the chemical composition of stars in the Universe. Despite the significant work underway to produce these parameters for many astrophysically important ions, uncertainties in these parameters remain large and can propagate throughout the entire field of astronomy. 
Aims: The Belgian repository of fundamental atomic data and stellar spectra (BRASS) aims to provide the largest systematic and homogeneous quality assessment of atomic data to date in terms of wavelength, atomic and stellar parameter coverage. To prepare for it, we first compiled multiple literature occurrences of many individual atomic transitions, from several atomic databases of astrophysical interest, and assessed their agreement. In a second step synthetic spectra will be compared against extremely high-quality observed spectra, for a large number of BAFGK spectral type stars, in order to critically evaluate the atomic data of a large number of important stellar lines. 
Methods: Several atomic repositories were searched and their data retrieved and formatted in a consistent manner. Data entries from all repositories were cross-matched against our initial BRASS atomic line list to find multiple occurrences of the same transition. Where possible we used a new non-parametric cross-match depending only on electronic configurations and total angular momentum values. We also checked for duplicate entries of the same physical transition, within each retrieved repository, using the non-parametric cross-match. 
Results: We report on the number of cross-matched transitions for each repository and compare their fundamental atomic parameters. We find differences in log(gf) values of up to 2 dex or more. We also find and report that 2% of our line list and Vienna atomic line database retrievals are composed of duplicate transitions. Finally we provide a number of examples of atomic spectral lines with different retrieved literature log(gf) values, and discuss the impact of these uncertain log(gf) values on quantitative spectroscopy. All cross-matched atomic data and duplicate transition pairs are available to download at


Keywords : atomic data; methods: data analysis; astronomical databases: miscellaneous; Astrophysics - Solar and Stellar Astrophysics; Physics - Atomic Physics
Pub Date : April 2018

Variable stars in the Gaia era: Mira, RR Lyrae, δ and Type-II Cepheids

Groenewegen, M.A.T.

Abstract :
Classical variables like RR Lyrae, classical and Type-II Cepheids and Mira variables all follow period-luminosity relations that make them interesting as distance indicators. Especially the RR Lyrae and δ Cepheids are crucial in establishing the distance scale in the Universe, and all classes of variables can be used as tracers of galactic structure. I will present an overview of recent period-luminosity relations and review the work that has been done using the Gaia DR1 data so far, and discuss possibilities for the future.

Keywords : Cepheids; stars: variables: other; stars: distances; distance scale; stars: AGB and post-AGB; Magellanic Clouds; Astrophysics - Solar and Stellar Astrophysics
Pub Date : April 2018

Spectroscopic monitoring of bright A-F type candidate hybrid stars discovered by the Kepler mission

Lampens, P.;​ Frémat, Y.Vermeylen, L.; De Cat, P.; Dumortier, L.; Sódor, Á.; Sharka, M.; Bognár, Z.

Abstract :
We report on a study of 250 optical spectra for 50 bright A/F-type candidate hybrid pulsating stars from the Kepler field. Most of the spectra have been collected with the high-resolution spectrograph HERMES attached to the Mercator telescope, La Palma. We determined the radial velocities (RVs), projected rotational velocities, fundamental atmospheric parameters and provide a classification based on the appearance of the cross-correlation profiles and the behaviour of the RVs with time in order to find true hybrid pulsators. Additionally, we also detected new spectroscopic binary and multiple systems in our sample and determined the fraction of spectroscopic systems. In order to be able to extend this investigation to the fainter A-F type candidate hybrid stars, various high-quality spectra collected with 3-4 m sized telescopes suitably equipped with a high-resolution spectrograph and furthermore located in the Northern hemisphere would be ideal. This programme could be done using the new instruments installed at the Devasthal Observatory.

Keywords : Astronomy; Astrophysics; Variability; Spectroscopy; Stars
Pub Date : April 2018
Article : PDF-file

The BINA collaboration: science at the Royal Observatory of Belgium

De Cat, P.Cuypers, J.Blomme, R.Frémat, Y.Groenewegen, M.A.T.Lampens, P.Lobel, A.Pauwels, T.; Van de Steene, G.van Hoof, P.A.M.

Abstract : 
The Belgo-Indian Network for Astronomy and Astrophysics (BINA) is a collaboration between Indian and Belgian astronomical institutes with the main aim to optimize the scientific output of the Indo-Belgian telescopes, being the 4.0-m International Liquid Mirror Telescope and the 3.6-m Devasthal Optical Telescope. These new facilities are both located at the Devasthal Observatory near Nainital, India. In this contribution, we introduce projects that are of scientific interest for colleagues of the department "Astronomy and Astrophysics" of the Royal Observatory of Belgium (ROB). It serves as an invitation for Indian astronomers to participate. We highlight how these projects could benefit from observations with the Indo-Belgian telescopes by using instruments from the first-generation (currently offered) and/or the next-generation (development or design phase). We show that, from an ROB point-of-view, the BINA would be the most successful if the 3.6-m DOT would be equipped with an efficient optical high-resolution spectrograph.

Keywords : Astronomy ; Astrophysics ; Observatories ; Instrumentation ; Photometry ; Spectroscopy
Note : in Proceedings of the First Belgo-Indian Network for Astronomy & Astrophysics (BINA) workshop, November 2016, held in Nainital, India
Publication : Bulletin de la Société Royale des Sciences de Liège, 87, pp. 92-101 (2018)
Pub Date : April 2018
DOI : 10.25518/0037-9565.7528

Frequency and mode identification of γ Doradus from photometric and spectroscopic observations

Brunsden, E.Pollard, K. R.Wright, D. J.De Cat, P.Cottrell, P. L.

Abstract :
The prototype star for the γ Doradus class of pulsating variables was studied employing photometric and spectroscopic observations to determine the frequencies and modes of pulsation. The four frequencies found are self-consistent between the observation types and almost identical to those found in previous studies (1.3641 d-1, 1.8783 d-1, 1.4742 d-1, and 1.3209 d-1). Three of the frequencies are classified as l, m = (1, 1) pulsations and the other is ambiguous between l, m = (2, 0) and (2, -2) modes. Two frequencies are shown to be stable over 20 yr since their first identification. The agreement in ground-based work makes this star an excellent calibrator between high-precision photometry and spectroscopy with the upcoming TESS observations and a potential standard for continued asteroseismic modelling.

Keywords : line: profiles; techniques: spectroscopic; stars: oscillations; stars: variables: general; Astrophysics - Solar and Stellar Astrophysics
Publication : Monthly Notices of the Royal Astronomical Society, Volume 475, Issue 3, p.3813-3822
Pub Date : April 2018
DOI 10.1093/mnras/sty034
bibcode 2018MNRAS.475.3813B

KIC 6048106: an Algol-type eclipsing system with long-term magnetic activity and hybrid pulsations - I. Binary modelling

Samadi Ghadim, A.Lampens, P.Jassur, M.

Abstract :
The A-F-type stars and pulsators (δ Scuti-γ Dor) are in a critical regime where they experience a transition from radiative to convective transport of energy in their envelopes. Such stars can pulsate in both gravity and acoustic modes. Hence, the knowledge of their fundamental parameters along with their observed pulsation characteristics can help in improving the stellar models. When residing in a binary system, these pulsators provide more accurate and less model-dependent stellar parameters than in the case of their single counterparts. We present a light-curve model for the eclipsing system KIC 6048106 based on the Kepler photometry and the code PHOEBE. We aim to obtain accurate physical parameters and tough constraints for the stellar modelling of this intermediate-mass hybrid pulsator. We performed a separate modelling of three light-curve segments which show a distinct behaviour due to a difference in activity. We also analysed the Kepler Eclipse Time Variations (ETVs). KIC 6048106 is an Algol-type binary with F5-K5 components, a near-circular orbit and a 1.56-d period undergoing variations of the order of ∆ P/P̃eq 3.60× 10^{-7} in 287 ± 7 d. The primary component is a main-sequence star with M1 = 1.55 ± 0.11 M☉, R1 = 1.57 ± 0.12 R☉. The secondary is a much cooler subgiant with M2= 0.33 ± 0.07 M☉, R2 = 1.77 ± 0.16 R☉. Many small near-polar spots are active on its surface. The second quadrature phase shows a brightness modulation on a time-scale 290 ± 7 d, in good agreement with the ETV modulation. This study reveals a stable binary configuration along with clear evidence of a long-term activity of the secondary star.


Keywords : techniques: photometric; binaries: eclipsing; stars: fundamental parameters; stars: oscillations; starspots; stars: variables: δ Scuti
Pub Date : March 2018

Multi-technique investigation of the binary fraction of A-F type candidate hybrid variable stars discovered by Kepler

Lampens, P.Frémat, Y.Vermeylen, L.Sódor, Á.Skarka, M.De Cat, P.Bognár, Zs.De Nutte, R.Dumortier, L.Escorza, A.Oomen, G. M.;Van de Steene, G.Kamath, D.Laverick, M.Samadi, A.; , et al

Abstract :
Context. Hundreds of candidate hybrid pulsators of intermediate type A-F were revealed by recent space missions. Hybrid pulsators allow us to study the full stellar interiors, where both low-order p- and high-order g-modes are simultaneously excited. The true hybrid stars must be identified since other processes, related to stellar multiplicity or rotation, might explain the presence of (some) low frequencies observed in their periodograms. 
Aims: We measured the radial velocities of 50 candidate δ Scuti -γ Doradus hybrid stars from the Kepler mission with the Hermes and ACE spectrographs over a time span of months to years. We aim to derive the fraction of binary and multiple systems and to provide an independent and homogeneous determination of the atmospheric properties and v sin i for all targets. The long(er)-term objective is to identify the (probable) physical cause of the low frequencies. 
Methods: We computed one-dimensional cross-correlation functions (CCFs) in order to find the best set of parameters in terms of the number of components, spectral type(s), and v sin i for each target. Radial velocities were measured using spectrum synthesis and a two-dimensional cross-correlation technique in the case of double- and triple-lined systems. Fundamental parameters were determined by fitting (composite) synthetic spectra to the normalised median spectra corrected for the appropriate Doppler shifts. 
Results: We report on the analysis of 478 high-resolution Hermes and 41 ACE spectra of A/F-type candidate hybrid pulsators from the Kepler field. We determined their radial velocities, projected rotational velocities, and atmospheric properties and classified our targets based on the shape of the CCFs and the temporal behaviour of the radial velocities. We derived orbital solutions for seven new systems. Three preliminary long-period orbital solutions are confirmed by a photometric time-delay analysis. Finally, we determined a global multiplicity fraction of 27% in our sample of candidate hybrid stars. Based on observations obtained with the Hermes spectrograph, which is supported by the Research Foundation - Flanders (FWO), Belgium, the Research Council of KU Leuven, Belgium, the Fonds National de la Recherche Scientifique (F.R.S.-FNRS), Belgium, the Royal Observatory of Belgium, the Observatoire de Genève, Switzerland, and the Thüringer Landessternwarte Tautenburg, Germany. Tables 2 and 3 are only, and Tables C.1-C.3 are also, available at the CDS via anonymous ftp to ( or via

Keywords : techniques: spectroscopic; techniques: photometric; binaries: spectroscopic; stars: oscillations; stars: variables: δScuti; stars: rotation; Astrophysics - Solar and Stellar Astrophysics
Pub Date : February 2018

HR 8844: A New Transition Object between the Am Stars and the HgMn Stars?

Monier, R.Gebran, M.Royer, F.Kilicoglu, T.Frémat, Y.

Abstract :
While monitoring a sample of apparently slowly rotating superficially normal early-A stars, we have discovered that HR 8844 (A0 V) is actually a new chemically peculiar star. We first compared the high-resolution spectrum of HR 8844 with that of four slow rotators near A0V (ν Cap, ν Cnc, Sirius A, and HD 72660) to highlight similarities and differences. The lines of Ti II, Cr II, Sr II, and Ba II are conspicuous features in the high-resolution high signal-to-noise SOPHIE spectra of HR 8844 and much stronger than in the spectra of the normal star ν Cap. The Hg II line at 3983.93 Å is also present in a 3.5% blend. Selected unblended lines of 31 chemical elements from He up to Hg have been synthesized using model atmospheres computed with ATLAS9 and the spectrum synthesis code SYNSPEC48 including hyperfine structure of various isotopes when relevant. These synthetic spectra have been adjusted to the mean SOPHIE spectrum of HR 8844, and high-resolution spectra of the comparison stars. Chi-squares were minimized to derive abundances or upper limits to the abundances of these elements for HR 8844 and the comparison stars. HR 8844 is found to have underabundances of He, C, O, Mg, Ca, and Sc, mild enhancements of Ti, V, Cr, Mn, and distinct enhancements of the heavy elements Sr, Y, Zr, Ba, La, Pr, Sm, Eu, and Hg, the overabundances increasing steadily with atomic number. This chemical pattern suggests that HR 8844 may actually be a new transition object between the coolest HgMn stars and the Am stars.


Keywords : stars: abundances; stars: chemically peculiar; stars: early-type; stars: individual: HR 8844; Astrophysics - Solar and Stellar Astrophysics 
Pub Date : February 2018

Luminosities and mass-loss rates of Local Group AGB stars and red supergiants

Groenewegen, M.A.T.; Sloan, G. C.

Abstract :
Context. Mass loss is one of the fundamental properties of asymptotic giant branch (AGB) stars, and through the enrichment of the interstellar medium, AGB stars are key players in the life cycle of dust and gas in the universe. However, a quantitative understanding of the mass-loss process is still largely lacking.
Aims: We aim to investigate mass loss and luminosity in a large sample of evolved stars in several Local Group galaxies with a variety of metalliticies and star-formation histories: the Small and Large Magellanic Cloud, and the Fornax, Carina, and Sculptor dwarf spheroidal galaxies (dSphs).
Methods: Dust radiative transfer models are presented for 225 carbon stars and 171 oxygen-rich evolved stars in several Local Group galaxies for which spectra from the Infrared Spectrograph on Spitzer are available. The spectra are complemented with available optical and infrared photometry to construct spectral energy distributions. A minimization procedure was used to determine luminosity and mass-loss rate (MLR). Pulsation periods were derived for a large fraction of the sample based on a re-analysis of existing data.
Results: New deep K-band photometry from the VMC survey and multi-epoch data from IRAC (at 4.5 μm) and AllWISE and NEOWISE have allowed us to derive pulsation periods longer than 1000 days for some of the most heavily obscured and reddened objects. We derive (dust) MLRs and luminosities for the entire sample. The estimated MLRs can differ significantly from estimates for the same objects in the literature due to differences in adopted optical constants (up to factors of several) and details in the radiative transfer modelling. Updated parameters for the super-AGB candidate MSX SMC 055 (IRAS 00483-7347) are presented. Its current mass is estimated to be 8.5 ± 1.6 M, suggesting an initial mass well above 8 M in agreement with estimates based on its large Rubidium abundance. Using synthetic photometry, we present and discuss colour-colour and colour-magnitude diagrams which can be expected from the James Webb Space Telescope.

Tables A.1, A.2, B.1, B.2, and C.1 are available at the CDS via anonymous ftp to ( or via

Keywords : circumstellar matter; stars: mass-loss; Magellanic Clouds; stars: AGB and post-AGB; Astrophysics - Solar and Stellar Astrophysics; Astrophysics - Astrophysics of Galaxies
Pub Date : January 2018

Variable millimetre radiation from the colliding-wind binary Cygnus OB2 #8A

Blomme, R.Fenech, D. M.Prinja, R. K.Pittard, J. M.Morford, J. C.

Abstract : 
Context. Massive binaries have stellar winds that collide. In the colliding-wind region, various physically interesting processes occur, leading to enhanced X-ray emission, non-thermal radio emission, as well as non-thermal X-rays and gamma-rays. Non-thermal radio emission (due to synchrotron radiation) has so far been observed at centimetre wavelengths. At millimetre wavelengths, the stellar winds and the colliding-wind region emit more thermal free-free radiation, and it is expected that any non-thermal contribution will be difficult or impossible to detect.
Aims: We aim to determine if the material in the colliding-wind region contributes substantially to the observed millimetre fluxes of a colliding-wind binary. We also try to distinguish the synchrotron emission from the free-free emission.
Methods: We monitored the massive binary Cyg OB2 #8A at 3 mm with the NOrthern Extended Millimeter Array (NOEMA) interferometer of the Institut de Radioastronomie Millimétrique (IRAM). The data were collected in 14 separate observing runs (in 2014 and 2016), and provide good coverage of the orbital period.
Results: The observed millimetre fluxes range between 1.1 and 2.3 mJy, and show phase-locked variability, clearly indicating that a large part of the emission is due to the colliding-wind region. A simple synchrotron model gives fluxes with the correct order of magnitude, but with a maximum that is phase-shifted with respect to the observations. Qualitatively this phase shift can be explained by our neglect of orbital motion on the shape of the colliding-wind region. A model using only free-free emission results in only a slightly worse explanation of the observations. Additionally, on the map of our observations we also detect the O6.5 III star Cyg OB2 #8B, for which we determine a 3 mm flux of 0.21 ± 0.033 mJy.
Conclusions: The question of whether synchrotron radiation or free-free emission dominates the millimetre fluxes of Cyg OB2 #8A remains open. More detailed modelling of this system, based on solving the hydrodynamical equations, is required to give a definite answer. This work is based on observations carried out under project numbers S14AW and S16AU with the IRAM NOEMA Interferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain).

Keywords : binaries: spectroscopic; stars: winds; outflows; stars: individual: Cyg OB2 #8A; stars: individual: Cyg OB2 #8B; stars: massive; radio continuum: stars; Astrophysics - Solar and Stellar Astrophysics
Pub Date : December 2017
DOI 10.1051/0004-6361/201731403
Bibcode 2017A&A...608A..69B

Times of Minima of 116 Eclipsing Binary Systems (2010-2015)

Lampens, P.Van Cauteren, P.Ayiomamitis, A.Kleidis, S.Panagiotopoulos, K.Vanleenhove, M.Hambsch, J.Hautecler, H.Van Wassenhove, J.Vermeylen, L.

Abstract :
We present 201 times of CCD photometric minima collected from observing sites in Belgium and Greece during the years 2010 till 2015 for 116 eclipsing binary systems.

Keywords : CCD photometry
Pub Date : December 2017

Gravity darkening in stars with surface differential rotation

Zorec, J.Rieutord, M.Espinosa Lara, F.Frémat, Y.Domiciano de Souza, A.Royer, F.

Abstract :
Context. The interpretation of stellar apparent fundamental parameters (viewing-angle dependent) requires that they be treated consistently with the characteristics of their surface rotation law. 
Aims: We aim to develop a model to determine the distribution of the effective temperature and gravity, which explicitly depend on the surface differential rotation law and on the concomitant stellar external geometry. 
Methods: The basic assumptions in this model are: a) the external stellar layers are in radiative equilibrium; b) the emergent bolometric flux is anti-parallel with the effective gravity; c) the angular velocity in the surface obeys relations like Ω(θ) = Ωo [ 1 + αΥ(θ,k) ] where Υ(θ,k) = coskθ or sinkθ, and where (α,k) are free parameters. 
Results: The effective temperature varies with co-latitude θ, with amplitudes that depend on the differential-rotation law through the surface effective gravity and the gravity-darkening function (GDF). Although the derived expressions can be treated numerically, for some low integer values of k, analytical forms of the integral of characteristic curves, on which the determination of the GDF relies, are obtained. The effects of the quantities (η,α,k) (η = ratio between centrifugal and gravitational accelerations at the equator) on the determination of the Vsini parameter and on the gravity-darkening exponent are studied. Depending on the values of (η,α,k) the velocity V in the derived Vsini may strongly deviate from the equatorial rotational velocity. It is shown that the von Zeipel's-like gravity-darkening exponent β1 depends on all parameters (η,α,k) and that its value also depends on the viewing-angle I. Hence, there no unique interpretation of this exponent determined empirically in terms of (I,α). 
Conclusions: We stress that the data on rotating stars should be analyzed by taking into account the rotational effects through the GDF, by assuming k = 2 as a first approximation. Instead of the classic pair (η,β1), it would be more useful to determine the quantities (η,α,I) to characterize stellar rotation.


Keywords : stars: rotation; Astrophysics - Solar and Stellar Astrophysics
Pub Date : October 2017

The super-orbital modulation of supergiant high-mass X-ray binaries

Bozzo, E.Oskinova, L.Lobel, A.Hamann, W. -R.

Abstract :
The long-term X-ray light curves of classical supergiant X-ray binaries and supergiant fast X-ray transients show relatively similar super-orbital modulations, which are still lacking a sound interpretation. We propose that these modulations are related to the presence of corotating interaction regions (CIRs) known to thread the winds of OB supergiants. To test this hypothesis, we couple the outcomes of three-dimensional (3D) hydrodynamic models for the formation of CIRs in stellar winds with a simplified recipe for the accretion onto a neutron star. The results show that the synthetic X-ray light curves are indeed modulated by the presence of the CIRs. The exact period and amplitude of these modulations depend on a number of parameters governing the hydrodynamic wind models and on the binary orbital configuration. To compare our model predictions with the observations, we apply the 3D wind structure previously shown to well explain the appearance of discrete absorption components in the UV time series of a prototypical B0.5I-type supergiant. Using the orbital parameters of IGRJ 16493-4348, which has the same B0.5I donor spectral type, the period and modulations in the simulated X-ray light curve are similar to the observed ones, thus providing support to our scenario. We propose that the presence of CIRs in donor star winds should be considered in future theoretical and simulation efforts of wind-fed X-ray binaries.


Keywords : X-rays: stars; X-rays: binaries; gamma rays: stars; stars: massive; stars: neutron; Astrophysics - High Energy Astrophysical Phenomena
Pub Date : October 2017
DOI : 10.1051/0004-6361/201731930 
Bibcode : 2017A&A...606L..10B 

The very fast evolution of Sakurai's object

van de Steene, G. C.van Hoof, P.A.M.Kimeswenger, S.Zijlstra, A. A.Avison, A.Guzman-Ramirez, L.Hajduk, M.Herwig, F.

Abstract :
V4334 Sgr (a.k.a. Sakurai's object) is the central star of an old planetary nebula that underwent a very late thermal pulse a few years before its discovery in 1996. We have been monitoring the evolution of the optical emission line spectrum since 2001. The goal is to improve the evolutionary models by constraining them with the temporal evolution of the central star temperature. In addition the high resolution spectral observations obtained by X-shooter and ALMA show the temporal evolution of the different morphological components.

Keywords : planetary nebulae: individual (V4334 Sgr); evolution; Stars: AGB and post-AGB; circumstellar matter; Astrophysics - Solar and Stellar Astrophysics
Publication : Planetary Nebulae: Multi-Wavelength Probes of Stellar and Galactic Evolution, Proceedings of the International Astronomical Union, IAU Symposium, Volume 323, pp. 380-381
Pub Date : October 2017