Image of the Week

Spectroscopic variability of emission lines stars with GAia

 

 

 

Figure 1: RVS time series of X Per (HIP 18350). X Per is a member of the Be/X-ray class of binaries. The Be star (spectral type O9.5 III) has a neutron star companion that accretes the Be star circumstellar matter. The spectra show the typical Paschen lines in emission and a small nitrogen line around 863.5 nm. The double peaked shape of emission lines is due to a Keplerian rotating disc. In the first 22 months of the nominal mission, it was observed 28 times. Because of the orbital motion and the nature of the stellar components, the intensity and line-emission shape vary in time. Image credit: ESA/Gaia/DPAC/CU6-O.Marchal

 

During its mission, Gaia will scan all the sky in a repetitive way. The satellite spectrometer is expected to collect 6 billion spectra until the end of the mission and will provide thus access to all kind of objects until magnitude G=17. One of the key points of the survey is that every source will be observed about 8 times a year on average, which will allow the community to follow the evolution and the variation of the brightest sources through time. For the nominal mission, this will add up to about 40 observations on average, and with each mission extension this number will increase further.

The Radial Velocity Spectrometer (RVS) aims to supply radial velocities of stars by measuring the Doppler shift of their spectral lines in absorption. A small fraction of stars may however exhibit line emission in their spectrum. The intensity and shape of the emission depend on the physical conditions, the radiation field and the geometry of its formation region. Emission can be due to chrosmospheric activity (like in RS CVn stars), caused by matter transfer and accretion processes in binary interactions, or it may originate either in extended stellar atmospheres or in stellar envelopes such as equatorial discs.

Variable stars of type RS CVn are a good illustration of the first case, while Be (read “B”-“e” for B-type stars with emission lines) are representatives of the last one. In what follows we aim to provide a few time series examples of RVS spectra of these two categories of stars obtained during the validation of the spectroscopic processing. Because more upstream validation and treatment is needed still, the release of epoch RVS data is not foreseen for Gaia DR3, but for Gaia DR4. The figures below and above give a foretaste of the information contained in the emission-line star epoch spectra. Times in the graphics are given in units of Gaia's 6 hour revolution.

RS CVn stars are detached binary systems showing evidence for intense magnetic activity which is observed by the impact of spot coverage and chromospheric activity on their light and spectrum behavior. They have at least one cool component because of the presence of Ca II, H and K emission in their spectra, whose strength can be used to measure the chromospheric activity. In the RVS spectra, weak emission is also expected to be detected in the Ca II infrared triplet (Ca IRT) at 850.04, 854.44 and 866.45 nm.

 

 

Figure 2: RVS time series of the RS CVn system SZ Psc (HIP 114639). SZ Psc is a double-lined binary (orbital period 3.97d), with partial eclipses made of a F8 V-IV and a K1 IV component. The contribution of the two stars are seen, and a weak reversal of the Ca IRT line-cores due to emission is observed at most epochs. Remarkably, at cycle/time 1541.97rev. the emission cores popped up above the continuum during only one transit. The time sampling of the RVS spectra is not regular, but the event appeared and disappeared in less than 15 days. Image credit: ESA/Gaia/DPAC/CU6-O.Marchal

 

Be stars form a class of main sequence stars which have emission (mainly in the Hα line) formed in a Keplerian equatorial disc or a ring-like envelope. According to Milky-Way surveys, about 20% of the main sequence B-type stars display the Be phenomenon. Usually their measured rotational speed is subcritical, but larger than 75 percent of the breakup value, which makes them the fastest rotators among the main sequence stars (with projected rotational velocities from a few 10 km/s up to more than 400 km/s). Due to the centrifugal force, rapid rotation strongly affects stellar evolution, flattens the star (e.g. Be star Achernar) and generates non-uniform surface temperature and density distributions that have sensitive impact on its spectrum. Together with non-radial pulsation and/or small-scale weak magnetic fields, rotation is also widely thought to play a key role in the formation of the circumstellar disc.

 

 

Figure 3: Schematic view of Be stars’ emission line-profiles seen at different inclination angles, from pole-on (A) to equator-on (C). The shape and nature of the equatorial rotating disc lead to double peaked aspect angle-dependent line emission profiles. The separation of the two peaks, V and R, depends on the inclination angle, the rotation rate and disc size. Image credit: ESA/Gaia/DPAC/CU6-O.Marchal,Y.Frémat

 

Although emission is the strongest in the Hα line, emission can be detected at all wavelengths, depending on the temperature and stellar inclination angle. In the RVS wavelength domain, early type Be stars often show the higher members of the hydrogen Paschen series in emission, from P17 to P13. Be stars are also photometric and spectroscopic variable stars, with rapid to long-term variations (from few days to several years). Emission lines are therefore variable too. Their strength, as well as the relative intensity of their Violet and Red (known as V/R variability) emission peaks can change during outburst periods, either due to their orbital motion if they are part of multiple systems, because of a one-armed oscillation pattern that propagates across the disc, or due to changes of the disc physical structure.

Figure 4a and 4b: RVS time series of two single-line Be stars. FW CMa (HIP 35951 - Figure 4a - top) is of spectral type B3 V, while HIP 50044 (HD 88825 - Figure 4b - bottom) is a somewhat cooler B4 V star. Both sources have variable emission line-profiles superimposed to the photospheric absorption lines. The spectra of HIP 50044 show a clear contribution from the calcium triplet, stronger than the Paschen emission. The V/R variability is also stronger in this star. Image credit: ESA/Gaia/DPAC/CU6-O.Marchal

 

 

For further reading :

 

Credits: ESA/Gaia/DPAC/CU6, O. Marchal (CNRS/Observatoire de Paris), Y. Frémat (Royal Observatory of Belgium)

[Published: 29/05/2019]

 

Image of the Week Archive

2019
29/05: Spectroscopic variability of emission lines stars with Gaia
24/05: Evidence of new magnetic transitions in late-type stars
03/05: Atmospheric dynamics of AGB stars revealed by Gaia
25/04: Geographic contributions to DPAC
22/04: omega Centauri's lost stars
18/04: 53rd ESLAB symposium "the Gaia universe"
18/02: A river of stars
2018
21/12: Sonification of Gaia data
18/12: Gaia captures a rare FU Ori outburst
12/12: Changes in the DPAC Executive
26/11:New Very Low Mass dwarfs in Gaia data
19/11: Hypervelocity White Dwarfs in Gaia data
15/11: Hunting evolved carbon stars with Gaia RP spectra
13/11: Gaia catches the movement of the tiny galaxies surrounding the Milky Way
06/11: Secrets of the "wild duck" cluster revealed
12/10: 25 years since the initial GAIA proposal
09/10: 3rd Gaia DPAC Consortium Meeting
30/09: A new panoramic sky map of the Milky Way's Stellar Streams
25/09: Plausible home stars for interstellar object 'Oumuamua
11/09: Impressions from the IAU General Assembly
30/06: Asteroids in Gaia Data
14/06: Mapping and visualising Gaia DR2

25/04: In-depth stories on Gaia DR2

14/04: Gaia tops one trillion observations
16/03: Gaia DR2 Passbands
27/02: Triton observation campaign
11/02: Gaia Women In Science
29/01: Following-up on Gaia
2017
19/12: 4th launch anniversary
24/11: Gaia-GOSA service
27/10: German Gaia stamp in the making
19/10: Hertzsprung-russell diagram using Gaia DR1
05/10: Updated prediction to the Triton occultation campaign
04/10: 1:1 Gaia model arrives at ESAC
31/08: Close stellar encounters from the first Gaia data release
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
27/08: Quest for the Sun's siblings
 
Please note: Entries from the period 2003-2010 are available in this PDF document.