Image of the Week

Hunting evolved carbon stars with Gaia RP spectra


Figure 1: Epoch RP spectra of the O-rich Mira star T Aqr (top panel, pulsation period of 203 days) and the C-rich Mira star RU Vir (bottom panel, pulsation period of 425 days) at various phases of their pulsation cycle, colour-coded according to the colour scale on the right of the figures. The horizontal axis represents an arbitrary, dimensionless pseudo-wavelength. The flux on the Y-axis is given in units of photo-electrons per second per sample. The displayed spectra approximately cover the wavelength range from 640 to 1100 nm. Image credit: ESA/Gaia/DPAC, Mowlavi et al.

Long-period variables (LPVs) are red giant stars whose luminosities vary by up to several magnitudes with periods from tens to a thousand days. They are important in estimating distances (they obey period-luminosity relations) and ages (they are intermediate-age stars). LPVs are also used in tracing the structure of the Galaxy and they play an important role in the chemical evolution of the Galaxy. LPVs form two broad distinct groups according to their surface chemical signature: O-rich when the C/O ratio is less than unity, and C-rich when this ratio is above unity (the solar ratio is about 0.5).

The spectra of the two classes of red giant stars are fundamentally different. Spectra of O-rich red giants show wide absorption features mainly due to TiO (Titanium Oxide) molecular bands, while the spectra of C-rich giants show wide absorption features mainly due to  CH (Methylidyne) and C2 (Diatomic Carbon) molecules. These features are clearly visible in the low-resolution spectra taken by the RP spectro-photometer on board of Gaia. This is shown in Figure 1 for an example of each class. In particular, the width of the absorption features is larger for C-rich than for O-rich stars, while O-rich stars show more absorption features.

These distinct spectral features enable us in principle to disentangle O-rich from C-rich stars based on Gaia RP spectra. The procedure to perform this, however, should take into account the sensitivity of the spectral features to stellar surface temperature. Long-period variables are indeed characterized by surface temperature variations up to several hundred Kelvin over a pulsation cycle, in concordance with their large brightness and colour variations. The effect of these variations on the spectral shape is shown with the case of the O-rich star, T Aqr, shown in Figure 1. In particular, the absorption feature present at pseudo-wavelengths around 40 when the star is coolest (faintest) is not present when the star is hottest (brightest). It is therefore necessary to perform spectral analysis on a subset of the epoch spectra depending on the spectral features that need to be considered.

Several analysis techniques are being considered within the Gaia DPAC consortium to disentangle O-rich and C-rich LPVs taking into account the variability of their RP spectra. We illustrate in this Image of the Week one of these techniques, based on the width of the spectral absorption features, that is expected to be larger for C-rich than for O-rich giants. The pseudo-wavelength separation between the two highest peaks detected in each spectrum, Δλ*, is taken as a proxy for the spectral absorption width (see Figure 1). We compute this proxy for each epoch spectrum of a given star, and take the median value median(Δλ*). Tests performed on a sample of twelve well-known LPVs, listed in Table 2 of Mowlavi et al. (2018), confirm the larger value of the wavelength separation for C-rich stars compared to O-rich stars, with median(Δλ*)>7 for the former stars and median(Δλ*)<=7 for the latter stars.



Figure 2: Left panel: Gaia-2MASS diagram recopied from Figure 2 of Lebzelter et al. (2018). The horizontal axis gives the difference between the two Wesenheit functions WRP,BP-RP = GRP – 1.3 (GBP GRP) and WKs,J-Ks = Ks – 0.686 (J – Ks), with GBP and GRP the Gaia integrated photometry in the blue and in the red spectro-photometers, respectively, and KS and J the 2MASS near-infrared magnitudes. See Lebzelter et al. (2018) for more details. Right panel: Same as left panel, but with the points colour-coded according to the wavelength separation between the two maxima in the RP spectra of each star, in blue if  median(Δλ*)>7 and in red if median(Δλ*)<=7. Image credit for left image: Lebzelter, A&A, 616, L13, 2018, reproduced with permission © ESO].  Image credit for right image: ESA/Gaia/DPAC, Mowlavi et al.



To validate the procedure on a large sample of red giants, we consider the LPVs from the Large Magellanic Cloud that have been published in Gaia DR2. The combination of optical Gaia and infrared 2MASS photometry was shown by Lebzelter et al. (2018) to allow a clear identification of C-rich relative to O-rich stars. Their Figure 2 is reproduced in the left panel of Figure 2. In the right panel, we plot the same diagram, but colour-coded according to the value of median(Δλ*) derived from their Gaia RP spectra (unpublished in Gaia DR2). The distinction between O-rich and C-rich stars, which is here purely based on Gaia data, is unambiguous, with only very few misclassified cases.

Additional procedures and validations are underway. In particular, the above technique is being adapted to be more robust against the absence of the λ*=40 absorption feature in O-rich giants, by considering only RP spectra that correspond to times when the stars are reddest in their pulsation cycles. The results are planned to be published in Gaia DR3.

Credits: ESA/Gaia/DPAC, Nami Mowlavi, Isabelle Lecoeur-Taïbi, Maria Süveges, Thomas Lebzelter, Francesca De Angeli, Laurent Eyer, Dafydd Evans, Michal Pawlak, and the Gaia CU7 Geneva and CU5 Cambridge teams.

[Published: 15/11/2018]

Image of the Week Archive


25/07: 10 years of Gaia science operations

23/07: How binary stars change their stellar dance with age

25/06: Dynamical masses across the Hertzsprung-Russell diagram

28/05: Did Gaia find its first neutron star?

26/04: A textbook solar eruption

22/04: Gaia's contribution to discovering distant worlds

16/04: Gaia spots Milky Way's most massive black hole of stellar origin

02/04: The Gaia Cataclysmic Variable hook


19/12: 10 Science topics to celebrate Gaia's 10 years in space

31/10: Gaia observes cosmic clock inside a heavenly jewel

10/10: Gaia Focused Product Release stories

27/09: Does the Milky Way contain less dark matter than previously thought?

22/09: Mass-luminosity relation from Gaia's binary stars

13/09: Gaia DPAC CU8 seminars

13/06: Gaia's multi-dimensional Milky Way

18/05: Mapping the Milky Way

15/05: Goonhilly station steps in to save Gaia science data

25/04: The Gaia ESA Archive

05/04: Dual quasar found to be hosted by an ongoing galaxy merger at redshift 2.17

21/03: GaiaVari: a citizen science project to help Gaia variability classificaton

09/02: Missing mass in Albireo Ac: massive star or black hole?

31/01: Gaia reaches to the clouds – 3D kinematics of the LMC

25/01: Meet your neighbours: CNS5 - the fifth catalogue of nearby stars

18/01: A single-object visualisation tool for Gaia objects


25/11: 100 months of Gaia data

23/11: The astonishment

09/11: Gamma-Ray Burst detection from Lagrange 2 point by Gaia

04/11: Gaia's first black hole discovery: Gaia BH1

26/10: Are Newton and Einstein in error after all?

21/10: Gaia ESA Archive goes live with third data release

06/10: Mapping the interstellar medium using the Gaia RVS spectra

26/09: Gaia on the hunt for dual quasars and gravitational lenses

23/09: Gaia's observation of relativistic deflection of light close to Jupiter

13/06: Gaia Data Release 3

10/06: MK classification of stars from BP/RP spectrophotometry across the Hertzsprung-Russell diagram

09/06: BP/RP low-resolution spectroscopy across the Hertzsprung-Russell diagram

27/05: Cepheids and their radial velocity curves

23/05: The Galaxy in your preferred colours

19/05: GaiaXPy 1.0.0 released, a tool for Gaia's BP/RP spectra users

11/05: Systemic proper motions of 73 galaxies in the Local group

28/03: Gaia query statistics

16/03: Gaia's first photo shooting of the James Webb Space Telescope

08/03: Gaia's women in science - coordination unit 8

25/02: Not only distances: what Gaia DR3 RR Lyrae stars will tell us about our Galaxy and beyond

11/02: Gaia's women in science

31/01: Astrometric orbit of the exoplanet-host star HD81040

12/01: The Local Bubble - source of our nearby stars

05/01: A Milky-Way relic of the formation of the Universe


23/12: Signal-to-Noise ratio for Gaia DR3 BP/RP mean spectra

22/12: The 7 October 2021 stellar occultation by the Neptunian system

01/12: Observation of a long-predicted new type of binary star

24/09: Astrometric microlensing effect in the Gaia16aye event

22/09: the power of the third dimension - the discovery of a gigantic cavity in space

16/09: An alternative Gaia sky chart

25/08: Gaia Photometric Science Alerts and Gravitational Wave Triggers

09/07: How Gaia unveils what stars are made of

23/06: Interviews with CU3

27/04: HIP 70674 Orbital solution resulting from Gaia DR3 processing

30/03: First transiting exoplanet by Gaia

26/03: Apophis' Yarkovsky acceleration improved through stellar occultation

26/02: Matching observations to sources for Gaia DR4


22/12: QSO emission lines in low-resolution BP/RP spectra

03/12: Gaia Early Data Release 3

29/10: Gaia EDR3 passbands

15/10: Star clusters are only the tip of the iceberg

04/09: Discovery of a year long superoutburst in a white dwarf binary

12/08: First calibrated XP spectra

22/07: Gaia and the size of the Solar System

16/07: Testing CDM and geometry-driven Milky Way rotation Curve Models

30/06: Gaia's impact on Solar system science

14/05: Machine-learning techniques reveal hundreds of open clusters in Gaia data

20/03: The chemical trace of Galactic stellar populations as seen by Gaia

09/01: Discovery of a new star cluster: Price-Whelan1

08/01: Largest ever seen gaseous structure in our Galaxy


20/12: The lost stars of the Hyades

06/12: Do we see a dark-matter like effect in globular clusters?

12/11: Hypervelocity star ejected from a supermassive black hole

17/09: Instrument Development Award

08/08: 30th anniversary of Hipparcos

17/07: Whitehead Eclipse Avoidance Manoeuvre

28/06: Following up on Gaia Solar System Objects

19/06: News from the Gaia Archive

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

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
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
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
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
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
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
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
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
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.