IoW_20151009 - Gaia

Image of the Week

The SB2 stars as seen by the RVS on Gaia

More than half of the stars in our Galaxy belong to binary or multiple stellar systems. The majority of these stellar systems, when observed by Gaia, will be detected as single sources, and the spectra obtained by Gaia's Radial Velocity Spectrometer (RVS) will be composed of the sum of the light of the components. For a fraction of the binary systems, when the two components are bright and have a similar luminosity, the RVS spectra will show double peaked absorption lines. In these cases, it is possible to estimate the radial velocity of each component from the analysis of a single transit.

The Single Transit Analysis team, a subgroup of DPAC Coordination Unit 6, developed a pipeline module that takes care of identifying this type of spectroscopic binaries and of measuring the radial velocity of the two components. As a first step, the algorithm estimates the probability that the observed spectrum is from a binary system, using an ad-hoc implementation of the TODCOR algorithm (Zucker & Mazeh 1994). Afterwards, the algorithm performs a non-linear fit of the observed spectrum with a combination of two spectra with different radial velocities and luminosities. The two spectra are templates extracted from a library of synthetic stellar spectra (with different atmospheric parameters) convolved with the instrumental profile and with the effect of the rotation of the stars. The algorithm determines the radial velocities, the luminosity ratio, the atmospheric parameters (temperature, gravity and metallicity), and the rotational velocities by finding the set that produces the best fit. The measured radial velocities of the two components, produced by the CU6 pipeline, will be analysed by CU4 to estimate the orbital parameters of the binary systems.

In order to characterise the performances of the above algorithm, the CU6 team processed and analysed the RVS spectra of some known double-lined spectroscopic binary (SB2) stars. The resulting radial velocities were compared with the radial velocities predicted by the orbital parameters from the SB9 catalogue (Pourbaix et al. 2004).

The image above shows the example of HIP 70674, an SB2 spectroscopic binary with visual magnitude V=7.99, composed of two stars of almost the same luminosity. The stars are orbiting each other in a circular orbit in four days (Griffin & Suchkov 2003). HIP 70674 was observed by the RVS in August 2014, December 2014, and March 2015. The top plot shows the spectrum of HIP 70674 in two different transits, corresponding to two different phases of the orbit. The spectrum was obtained when the difference between the two radial velocities is maximum. The absorption lines in this spectrum show very clearly a double-peaked profile. The bottom spectrum was obtained when the radial velocity difference was much smaller. In this case, the absorption lines of the two components are nearly superposed. The bottom plot shows the radial velocities of the two components measured from the RVS spectra in the six transits, compared with the predicted radial velocity curves, as a function of the orbital phase.

A first glance at the figure shows that a fairly good agreement exists between the radial velocities measured with the module developed in the CU6 pipeline and the predicted ones. These encouraging results validate the algorithm and the capacity of detecting spectroscopic binary stars from a single-transit analysis, for bright stars with significant radial velocity difference between the two components.

Credits: ESA/Gaia/DPAC/CU6/Yassine Damerdji (Observatoire d'Alger/Institut  d'Astrophysique et de Géophysique de Liège) & Pasquale Panuzzo (CNRS/Observatoire de Paris)

[Published: 09/10/2015]