What is a double or binary star ?

It's a pair of stars in orbit around a common centre of gravity under their mutual gravitational attraction. The individual stars in the system are called components: component A is the primary or the brightest and component B is the secondary or the faintest (in general). There are several types of binaries according to the techique of detection used. Here we mention four of them.
 

Visual Double Stars

The two components can be resolved visually through a telescope. Such systems have mostly been investigated for astrometric purposes: highly accurate measurements of the position of the star on the celestial sphere are obtained in order to gain knowledge of the orbit characteristics as well as visual estimates of the difference of magnitude (measure of the brightness) between the two components. In more recent times, observers used photometric techniques to get high precision measurements of the global magnitude and colours of these systems.
 

Astrometric Double Star

These are apparently single stars (not (yet) resolved). In this case the binarity is revealed by perturbations of the position with respect to the "background" or "reference stars". These systems are studied by photographic means: measurements show the motion of the photocentre (this is the weighted centre of the light intensity of the two components). From such a study we can obtain e.g. the luminosity, the semi-major axis of the astrometric orbit, the position of the center of mass of the system.
 

Spectroscopic Double Stars

Systems where the components are too close to each other to be resolved as separate stars, are studied by spectroscopy (study of the stellar atmospheres, radial velocity measurements via the Doppler effect).The study of their spectra shows periodic shifts due to the orbital motion around a mean velocity which is the motion in the radial direction of the system as a whole. We thus can obtain the spectroscopic orbit. It gives us the orbital period, information on the orbit itself such as the eccentricity and very useful, the mass ratio of the system. For example, the left figure illustrates the radial velocity curves for alpha Aurigæ A and B.
 

Eclipsing Double stars

When the orbit is aligned with respect to Earth, one component may occult the other one. These "eclipses" are seen as periodic perturbations (drops) on the light curve (luminosity as a function of time, e.g. HIC 31173 see figure). These double stars are called eclipsing binaries. From the study of their light curves, we have access to the period, the inclination, the luminosity ratio and the radii of the stars.
 

 

Double stars

Present estimates of the percentage of double versus single stars point toward a ratio of 60% or higher. This is also confirmed by the high number of new detections coming from ground-based and satellite surveys. But double stars are seriously complicating both the observations and the elaboration of theorical models. It frequently happened and still happens that studies of double stars are being neglected in favour of studies of single stars, galactic and extra-galactic research. However double stars have a serious advantage: the orbits
These data are extremely useful to:

  • refine the theories of Formation and Evolution of Single Stars
  • provide constraints on the possible scenarios of Binary Star Formation

 

Stellar structure

Scientists try to understand what processes are going on in the interior of stars. Models of internal structure are therefore elaborated in which stellar masses play a very important role. The only possible direct observation is by observing double stars: indeed, from the knowlegde of the

  • orbital motion (Keplerian orbit)
  • distance

we are able to determine the total mass of the system. This information is an essential parameter on the way to modelling also the stellar evolution.

 

Stellar formation

By studying the distributions of the orbital elements such as the orbital period, the eccentricity, the mass ratio,... of double stars as a function of population or age, one obtains:

  • constraints on scenarios of the formation of single stars
  • indications for possible scenarios of binary star formation

 

Diversity

The distributions of the orbital parameters show a very wide range in orbital period, (real) separation, mass ratio... There exists a panoply of widely differing objects with a variety of possible processes that may complicate the study of such systems. For example, in very close binaries interactions between the binary components may occur, such as:

  • mass transfer
  • apsidal motion (the orientation of the nodal line is changing with time)
  • common envelopes
  • peculiar chemical compositions

Such binaries can no longer be considered as two non-interfering stars at a common position in space: additional physical processes have to be taken into account. Additional observational constraints are obtained from their spectroscopic or light curve analyses.

 

Search for planetary systems ("exo-planets")

One can also look for systems with extremely low or high mass values. In the lower limit case, the studied systems may have a "brown dwarf" or a giant planet for companion. In the astrometric mode, similar to the detection of astrometric binaries, the reflex motion of the star gravitationally pulled by its planetary system is observed. This technique then provides a direct estimation of the mass of the planets. This search for "exo-planets" has been boosted a lot since the discovery of a solar-like star with a planetary companion (Mayor & Queloz, 1995 Nature 378, 355).

 


 

THE CCD TECHNIQUE AND DOUBLE STARS

The CCD (Charge Coupled Device) is a detector based on the technique of photon counting. The CCD chip consists of light sensitive micro-cells called "pixel" (picture element). Each pixel transforms the received photons into charges (e-). The read out of the CCD is done by a process of charge transfer. The electric signal is then processed by an ADC to produce a digitized image (frame). The following picture shows you the image CCD of a cluster: it is M16 in the constellation Serpens.
 
 
With a CCD, we can measure the angular separation between the two components of the double star, their positions and their magnitudes. We simultaneously obtain astrometric and photometric information. We thus study "intermediate" double systems, in other words double systems with angular separation between 1 and 10 arcsec (see table below).