A summary of additional programs on eclipsing binaries C. Maceroni INAF- Osservatorio Astronomico di Roma 3rd COROT Week, Liège, December 4 -7, 2002

A Summary of Additional Programs on close binaries Purpose Program for: Field WW * Class GB/ Follow-up Comment Derivation of precise stellar parameters Short run Exo –planet R2E EGB Spectroscopy needed for radial velocity determination. Stellar activity studies. Short & long runs Exo (short/long)+ Seismo (short) R1E, R2E+S EGB/SGB Target characterization needed. Determination of limb darkening and gravity darkening Detached system selection necessary. Oscillations in binaries Long run Seismology R1S SGB As secondary targets. Selection & characterization By -product sample studies Archival data R3E Follow -up For individual systems (*) According to W. Weiss notation: R1 = long run, R2 = short run, R3 = archival data; S=seismology field, E= Exoplanet field

A summary of Additional Programs on eclipsing binaries Possible Additional programs based on binaries presented in previous COROT weeks: Derivation of precise basic stellar parameters (masses, radii, luminosities) from “well - behaved” systems, i.e. detached binaries, and of atmospheric properties. Limb darkening, gravity darkening, the effects of tidal distortion and reflection are second - order effect in the light curves, that will be possible to study with the precision of COROT. study of the manifestations of stellar activity (spots, flares, stellar activity cycles) and use them to derive information on rotational period and differential rotation (from spot migration) in late -type components (see Pagano et al., CSW2/3 for a more detailed description). The effects of stellar activity on the p - mode peaks has also to be taken into account (see Lanza and Rodono’ 2002 A&A 390, 167). Program a) shall be performed in the Exoplanetary field, (program type: R2:E ), program b) in both fields (R1:E + R2:E+S). Both will need target characterization from the ground.

Additional programs on eclipsing binaries (2) c) Oscillations in binaries. In principle, looking for (solar) oscillation in suitable eclipsing binaries would have the advantage of knowing the masses, radii and the orbit inclination angle, and therefore the rotational velocity. There are a few possible binary targets with MV < 9.5, in the COROT fields (circular fields of radius 12o centered at d =0, a = 6h 50m [SC4ac], and at d =0 a = 18h 50m [SC4c]), they are however very poorly known, so that characterization from GB programs is essential. (program type R1:S, as secondary targets) d) Archival programs, dedicated to the study of the by -product binary sample from the planetary search and of individual new interesting systems (program type R3).

Determination of precise physical parameters Detached double -lined eclipsing binaries are the primary source of information on stellar masses, radii and luminosities. Accurate data (at some percent level) exists for ~100 systems, with however few objects with masses below the solar value (in particular the lower MS is calibrated by only two binaries: YY Gem and CM Dra). A selection should be made, after characterization of the possible targets in the COROT exo –fields. to choose detached double lined systems. The situation is particularly severe for low - mass stars in the Pre – Main Sequence (PMS) stage, for which it is crucial to provide observational constraints to the current evolutionary models, still affected by strong uncertainties in the input physics. Follow up of COROT, observations providing precise velocity curves (uncertainty ~ 1 kms-1 on K for some percent on the mass determination) shall be planned. New suitable targets might as well be discovered during the mission (then R3 program possible).

Second order effects in the light curves Atmosphere structure: While in principle limb darkening can be derived from light curve fitting with synthetic models, the current precision of the light curves is not sufficient and theoretical values are assumed. For a close but detached binary the effect of varying limb darkening is some mmag (see figure). The limb darkening effect is observed only during eclipses, in detached binaries these may last just some hours, so that the length of the integration time may be critical for achieving the required precision (if 8 minutes, it might still be good for many systems, otherwise the faster windows would be needed). Internal structure: A similar situation applies for closer (tidally interacting) binaries and gravity darkening effect, whose variations with the stellar structure are too small to be detected from ground (again some mmag). It has to be noticed that gravity darkening affects the whole light curve, so that it should show up in long term continuous photometry amplitude spectrum in the frequency domain. Systems with just ellipsoidal variations (no eclipses) could be included in the observable sample.

Example: The effect of changing the limb darkening The effect of changing the (linear) limb - darkening coefficient by Dx= 0.1 on the synthetis V light curve of a detached binary with the following parameters: P=2.41 d; i=86 °, Tp=8164 K, Ts=7178 K rp=0.18 r s=0.15 (fractional radii) Ls/Lp=0.362 ( V805 Aql, detached binary with two MS components) The plot shows the difference between the light curves (computed with the WD synthetic light-curve program) of two models with x=0.57 and x=0.67. The steepest variation, during primary eclipse, lasts 0.03 P, i.e. ~ 1.8 h.

How many known eclipsing binary targets in the Exo-fields? A query of the Simbad database for the two COROT fields of view SC4ac and SC4c returns a total of – respectively - 173 and 241 systems for the following variable types (Simbad coding in brackets): eclipsing binaries (EB*), cataclysmic variables, (cataclyV*), RS CVn binaries (RS*), XR binaries (XR) and ellipsoidal variables (El*). When the exoplanetary field magnitude constraints are approximately taken into account as 11.5 ≤ mB ≤ 16.5 (most Simbad objects have only the B magnitude) the samples shrink to 123 and 156 objects subdivided as follows: EB* Al* βL* WU* No/NL DN Tot SC4c 38 84 7 18 7 2 156 SC4ac 6 87 15 10 1 4 123 EB: generic eclipsing binary, Al*: Algol type, βL*: β Lyr type, WU*: WUMa contact binary, No/NL: Nova, Nova-like, DN: dwarf nova. Detached binaries can be found in the first two classes. These (relatively faint) systems are in general poorly known: only for ~10% an approximate spectral classification is available. Target characterization is needed to select the systems suitable for the various programs.

New binaries: an example of a magnitude limited search for variability ( OGLE-I microlensing experiment ) OGLE-I 12 fields 15’x15’, photometry of a few million stars in (mainly) I band: 14<I<18 mag. Being a ground-based experiment with a ~1m telescope no comparison with COROT in accuracy (~0.01-0.02 mag in good cases). Periodicity search was between 0.1d ≤ P ≤ 100d. Typically (ex in the field of central BW: BWC) 40% of stars with photometry good enough for analysis were selected as suspected variables, but only 1.25% of those finally turned out to be recognized as periodic variables. Eclipsing binaries are 56% of the total with a prevalence (72.5%) of contact binaries. Many miscellaneous variables could actually be grazing eclipsing or ellipsoidal binaries. Therefore a percentage of ≈1% of stars is a reasonable estimate for the fraction of eclipsing binaries. Distribution of 2293 variables present in the OGLE-I catalog field puls. Misc. eclipsing EA EB EW BWC 31 71 116 30 4 77 BW1-4 71 264 465 94 10 347 BW5-8 64 228 352 76 7 251 BW9-11 59 224 348 62 10 248 total 225 787 1281 262 31 923 % of var 10% 34% 56% % of bin. subtypes 20% 2.5% 72.5% It has to be noticed that contact binaries turn out to be the great majority of all eclipsing systems!

Conclusions All the programs require target characterization by ground based observations or follow-up (for those based on archival data). Most programs will be performed in the Exo-planet field. At present only the programs related to the study of stellar activity benefit of a dedicated program of target characterization, (mainly thanks to the work in progress at Catania Observatory). Only part of the necessary observations could presumably be done with ESO or national telescopes (e.g. TNG @ Canary Islands). Therefore, a coordinated effort of the available facilities is the minimum necessary effort for the success of many APs. As the EGBWG will take care of target characterization only in the sky areas covered by the EXO-CCD during the long runs, the formation of an AP-GBWG shall as well be considered.