Image of the Week

The SB2 stars as seen by the RVS on Gaia

   
  The top plot shows the SB2 spectrum of HIP 70674 taken by Gaia's RVS in two different transits, corresponding to two different phases of the orbit of this binary system. The bottom plot shows the radial velocities of the two components measured from the RVS spectra (filled symbols: component A, open symbols: component B) compared with the predicted radial velocities (solid lines, green: component A, red: component B) as a function of the orbital phase. An outlier measurement has been removed from the diagram.  
 

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]

 

Image of the Week Archive

2017
16/08: Preliminary view of the Gaia sky in colour
07/07: Chariklo stellar occultation follow-up
24/04: Gaia reveals the composition of asteroids
20/04: Extra-galactic observations with Gaia
10/04: How faint are the faintest Gaia stars?
24/03: Pulsating stars to study Galactic structures
09/02: Known exoplanetary transits in Gaia data
31/01: Successful second DPAC Consortium Meeting
2016
23/12: Interactive and statistical visualisation of Gaia DR1 with vaex
16/12: Standard uncertainties for the photometric data (in GDR1)
25/11: Signature of the rotation of the galactic bar uncovered
15/11: Successful first DR1 Workshop
27/10: Microlensing Follow-Up
21/10: Asteroid Occultation
16/09: First DR1 results
14/09: Pluto Stellar Occultation
15/06: Happy Birthday, DPAC!
10/06: 1000th run of the Initial Data Treatment system
04/05: Complementing Gaia observations of the densest sky regions
22/04: A window to Gaia - the focal plane
05/04: Hipparcos interactive data access tool
24/03: Gaia spots a sunspot
29/02: Gaia sees exploding stars next door
11/02: A new heart for the Gaia Object Generator
04/02: Searching for solar siblings with Gaia
28/01: Globular cluster colour-magnitude diagrams
21/01: Gaia resolving power estimated with Pluto and Charon
12/01: 100th First-Look Weekly Report
06/01: Gaia intersects a Perseid meteoroid
2015
18/12: Tales of two clusters retold by Gaia
11/11: Lunar transit temperature plots
06/11: Gaia's sensors scan a lunar transit
03/11: Celebrity comet spotted among Gaia's stars
09/10: The SB2 stars as seen by Gaia's RVS
02/10: The colour of Gaia's eyes
24/09: Estimating distances from parallaxes
18/09: Gaia orbit reconstruction
31/07: Asteroids all around
17/07: Gaia satellite and amateur astronomers spot one in a billion star
03/07: Counting stars with Gaia
01/07: Avionics Model test bench arrives at ESOC
28/05: Short period/faint magnitude Cepheids in the Large Magellanic Cloud
19/05: Visualising Gaia Photometric Science Alerts
09/04: Gaia honours Einstein by observing his cross
02/04: 1 April - First Look Scientists play practical joke
05/03: RR Lyrae stars in the Large Magellanic Cloud as seen by Gaia
26/02: First Gaia BP/RP deblended spectra
19/02: 13 months of GBOT Gaia observations
12/02: Added Value Interface Portal for Gaia
04/02: Gaia's potential for the discovery of circumbinary planets
26/01: DIBs in three hot stars as seen by Gaia's RVS
15/01: The Tycho-Gaia Astrometric Solution
06/01: Close encounters of the stellar kind
2014
12/12: Gaia detects microlensing event
05/12: Cat's Eye Nebula as seen by Gaia
01/12: BFOSC observation of Gaia at L2
24/11: Gaia spectra of six stars
13/11: Omega Centauri as seen by Gaia
02/10: RVS Data Processing
12/09: Gaia discovers first supernova
04/08: Gaia flag arrives at ESAC
29/07: Gaia handover
15/07: Eclipsing binaries
03/07: Asteroids at the "photo finish"
19/06: Calibration image III - Messier 51
05/06: First Gaia BP/RP and RVS spectra
02/06: Sky coverage of Gaia during commissioning
03/04: Gaia source detection
21/02: Sky-background false detections in the sky mapper
14/02: Gaia calibration images II
06/02: Gaia calibration image I
28/01: Gaia telescope light path
17/01: First star shines for Gaia
14/01: Radiation Campaign #4
06/01: Asteroid detection by Gaia
2013
17/12: Gaia in the gantry
12/12: The sky in G magnitude
05/12: Pre-launch release of spectrophotometric standard stars
28/11: From one to one billion pixels
21/11: The Hipparcos all-sky map
15/10: Gaia Sunshield Deployment Test
08/10: Initial Gaia Source List
17/09: CU1 Operations Workshop
11/09: Apsis
26/08: Gaia arrival in French Guiana
20/08: Gaia cartoons
11/07: Model Soyuz Fregat video
01/07: Acoustic Testing
21/06: SOVT
03/06: CU4 meeting #15
04/04: DPCC (CNES) 
26/03: Gaia artist impression 
11/02: Gaia payload testing  
04/01: Space flyby with Gaia-like data
2012
10/12: DPAC OR#2. Testing with Planck
05/11: Galaxy detection with Gaia
09/10: Plot of part of the GUMS-10 catalogue
23/07: "Gaia" meets at Gaia
29/06: The Sky as seen by Gaia
31/05: Panorama of BAM clean room
29/03: GREAT school results
12/03: Scanning-law movie
21/02: Astrometric microlensing and Gaia
03/02: BAM with PMTS
12/01: FPA with all the CCDs and WFSs
2011
14/12: Deployable sunshield
10/11: Earth Trojan search
21/10: First Soyuz liftoff from the French Guiana
20/09: Fast 2D image reconstruction algorithm
05/09: RVS OMA
10/08: 3D distribution of the Gaia catalogue
13/07: Dynamical Attitude Model
22/06: Gaia's view of open clusters
27/05: Accuracy of the stellar transverse velocity
13/05: Vibration test of BAM mirrors
18/04: L. Lindegren, Dr. Honoris Causa of the Observatory of Paris
19/01: Detectability of stars close to Jupiter
05/01: Delivery of the WFS flight models
2010
21/12: The 100th member of CU3
17/11: Nano-JASMINE and AGIS
27/10: Eclipsing binary light curves fitted with DPAC code
13/10: Gaia broad band photometry
28/09: Measuring stellar parameters and interstellar extinction
14/09: M1 mirror
 
Please note: Entries from the period 2003-2010 are available in this PDF document.