The CoRoT Legacy Book c The authors, 2016 DOI: 10.1051/978-2-7598-1876-1.c035 III.5 Stellar classification in CoRoT faint stars fields C. Damiani1; 2, J.-C. Meunier1, C. Moutou1, M. Deleuil1, and F. Baudin2 1 Aix-Marseille Universite,´ CNRS, Laboratoire d’Astrophysique de Marseille, UMR 7326, 13388, Marseille, France e-mail: [email protected] 2 Universite´ Paris-Sud, CNRS, Institut d’Astrophysique Spatiale, UMR8617, 91405 Orsay Cedex, France A final refined treatment of the light curves has been re- 1. Introduction cently applied to the complete CoRoT data set and a new The extremely precise photometric time-series provided by and ultimate version of N2 products has been released. The the faint stars channel of the CoRoT mission serve a very spectral classification has also been revised for this last ver- large number of scientific objectives. In addition to ma- sion, taking into account the particularities of each CoRoT jor contributions in the faint stars field, let us recall also runs. While an overview of the FOSC method is given in that the CoRoT faint stars chanel has allowed to produce Deleuil et al.(2009), they do not provide an estimation of important results in stellar physics (see the numerous con- the reliability of the final results. In this paper, we give a tributions in this book). For either the core or additional thorough description of the method in Sect.2. In Sect.3 we programs, both the selection of the targets and the exploita- give the statistical error expected from the FOSC as esti- tion of the light curve require at least a basic knowledge of mated using a synthetic survey of the Milky Way. In Sect.4 the targets, such as their colours or spectral type and lu- we compare our results to the classification obtained by minosity class. spectroscopic campaigns. We give our conclusions in Sect.5. This is one of the reasons why the Exodat (Deleuil et al. 2009) database was built. Its prime objective was to pro- vide position, colours and stellar classification for the stars 2. Classification method in the observable zones of CoRoT, the so-called ``CoRoT eyes''. Exodat is an information system and a database that The FOSC method is adapted from the automatic classifi- gathers an extended body of data of various origin, and de- cation technique developed by Hatziminaoglou et al.(2002) livers it to the community through a user friendly web inter- for celestial objects classification and quasar identification. face1(Agneray et al. 2014). It contains data for more than It consists of fitting the apparent broad-band photometry 51 million stars providing information for both the potential magnitudes of the target to the spectral energy distribu- targets of the exoplanet program (with 11 ≤ r ≤ 16), and tion (SED) of template stars of different spectral types. also the fainter background sources. The later are needed in The templates are taken from the Pickles stellar spectral order to estimate the targets' level of light contamination. flux library UVKLIB (Pickles 1998). This library encom- To help the selection of the targets and the preparation of passes spectral types from O to M and luminosity classes the observations, a first order stellar classification (FOSC) from supergiants to dwarfs with good completeness and uni- was conducted using broad-band multi-colour photometry formity. To avoid introducing too many degeneracies, only available through Exodat. Any detailed analysis of targets solar abundance templates are used in the FOSC. Hence, of particular interest would require complementary observa- the classification is done over 106 different spectral types tions for a better characterisation. Indeed it has been done including dwarfs, subgiants, giants, bright giants and su- on a target to target basis, especially for planet hosting pergiants with effective temperatures (Teff ) comprised be- stars, but also on more consequent samples (see Sect.4). tween 3:398 ≤ log Teff ≤ 4:600. There are 36 templates Overall, no more than 15% of the targets observed over the for dwarf stars, which roughly correspond to a sampling mission lifetime were the object of complementary observa- of about one template every two subclasses in the main- tions. Notwithstanding, stellar parameters are essential to sequence. Templates of extragalactic sources, white dwarfs the scientific exploitation of the CoRoT light curves. For the and brown dwarfs are not used in the FOSC because it is vast majority of targets, to this day, and at least until the expected that the number of such objects in the considered third data release of the GAIA mission, scheduled in late range of magnitudes and galactic regions is small enough 2017/20182, the only source of information concerning the to be safely neglected. Templates of binary stars are not spectral classification of all stars observed in the exoplanet used neither despite the fact that the proportion of binary channel is Exodat. stars in the sample is expected to be significant. Again, this limitation is conceded to avoid too many degeneracies, 1 http://cesam.lam.fr/exodat/ because broad-band photometry does not allow to clearly 2 see http://www.cosmos.esa.int/web/gaia/release distinguish the binary nature of the target. If the spectral 137 The CoRoT Legacy Book type of the two components are similar, the resulting spec- For each target, the apparent magnitudes are converted tral classification would give the same result when using to fluxes which are normalised to the flux in the band where the template of a single star, since it is insensitive to the the photometric error is the smallest. The photometric er- absolute magnitude of the target. On the other hand, if ror i is also converted to an error in flux σi, taking into the spectral type of the two components are very different, account the error on the zero point 0i for the corresponding the colours of the binary would largely be undistinguishable filter i, using the relationship3 from the colours of the brightest component, due to the ex- q tremely different luminosities of the two components in the 2:3 2 2 σi = + : (1) considered wavelength range. In those cases, the spectral 2:5 i 0i type of a binary determined with the FOSC method will The observed flux is then confronted with the computed thus correspond to the one of the primary star. In interme- fluxes for all the reddened templates by computing diate cases, we expect that the spectral type determination will be degraded when the target is a binary. However to 2 n i i F − F (EB−V ) ensure the reliability of the result, the binary nature of X obs modj χ2(E ) = (2) the target should be known before attempting the classi- j B−V 2 σi fication. This was not the case prior to the mission, but i=1 the results of the planetary transit search in CoRoTlight i i where F is the observed flux in the i band, F (EB−V ) curves has identified a number of eclipsing binaries. In this obs modj is the flux in the same band for the template j reddened case a second order classification, suited for binary targets, with a colour excess E , σ is the estimated error of the would produce better results. This is out of the scope of this B−V i observed flux in this band and n is the number of bands paper, and will be tackled in a dedicated study of CoRoT that are used. binaries. Finding the minimum χ2 then leads to the attribution To take into account the effect of interstellar absorption, of a spectral type and a colour excess that best reproduce the SEDs of the UVKLIB library are used to produce a li- the observed magnitudes. The reduced χ2 is used as quality brary of reddened templates. They were created for a range index QI of colour excess from EB−V = 0 to EB−V = 4 in steps of 0.05. The extinction law is taken from Fitzpatrick(1999) QI = χ2=(n − 3): (3) using the average value of the ratio of total to selective ex- AV As already pointed out by Hatziminaoglou et al.(2002), a tinction RV = = 3:1. The actual value of RV does EB−V 2 not matter here given that in the wavelength range consid- classification scheme based on the χ technique may lead to ered in this study (350 nm ≤ λ ≤ 2:2 µm), the extinction degeneracies, which are class-dependent. Multiple minima curve is insensitive to R variations. For a given value of the may occur in the parameter space and such degeneracies V can only be solved by including additional information in colour excess EB−V , the reddened flux distribution is then multiplied with the appropriate relative spectral response the classification procedure. In the following we describe curves of the different bands. the strategy used to avoid the main degeneracies. The bands used are Harris B and V and Sloan-Gunn r0 and i0 for about two thirds of the targets, RGO U for 2.1. Dwarfs and giants degeneracy only 3% of them, and 2MASS J, H, and Ks bands for almost all of them (98%). We will refer to this set of magni- The effect of luminosity on stellar energy distributions is tude as the \Obscat" catalogue. When the B, r0 or i0 bands often seen in some metallic lines that are usually shallow are not available, the PPMXL catalogue (Roeser et al. and/or narrow. It is thus very difficult to distinguish dwarf 2010) was used instead. It combines the USNO-B1.0 blue, and giant stars when adjusting SEDs to broad-band pho- red and infrared magnitudes with the 2MASS magnitudes tometry only.