Low-mass binaries from CoRoT: stringent tests for stellar models Abstract: Analyses of double-lined eclipsing binary systems provide masses and radii of stars with enough accuracy to use these measures as constraints for stellar structure models. Tests over models have unveiled discrepancies between their predictions and observations of low-mass stars, however the number of well-known low-mass eclipsing binaries is still scarce. It is expected that many of the transiting objects that CoRoT is providing will be low-mass eclipsing binaries, therefore we have been conducting a selection of candidates for spectroscopic follow-up. Preliminary estimates using the CoRoT light curves have already resulted in some good candidate systems. The combination of the precise photometry from CoRoT with radial velocity measurements will lead to accurate stellar properties for these systems and to thorough tests of models with a larger sample of low-mass stars than now. J. C. Morales 1, I. Ribas 1,2, C. Maceroni 3, C. Jordi 1,4, A. P. Hatzes 5 & E. W. Guenther 5 1 Institut d’Estudis Espacials de Catalunya (Barcelona, Spain), 2 Institut de Ciències de l’Espai (CSIC, Bellaterra, Spain), 3 Istituto Nazionale Di Astrofisica – Osservatorio Astronomico di Roma (Roma, Italy), 4 Universitat de Barcelona – Institut de Ciències del Cosmos (Barcelona, Spain), 5 Thuringer Landessternwarte (Tautenburg, Germany)‏ Bibliography Baraffe, I., Chabrier, G., Allard, F. & Hauschildt, P. H. 1998, A&A, 337, 403 Chabrier, G., Gallardo, J. & Baraffe, I. 2007, A&A, 462, L17 Ribas, I., Morales, J.C., Jordi, C., Baraffe, I., Chabrier, G. & Gallardo, J. 2008, Mem. Soc. Astron. Italiana, 79, 562 Wilson, R. E. & Devinney, E. J. 1971, ApJ, 166, 605 A small mass ratio low-mass eclipsing binary Some CoRoT targets classified as EB’s were selected for spectroscopic follow- up. CoRoT target E turned to be a single-lined spectroscopic binary, i.e. only radial velocity of the primary component was detected. This was an indication that the secondary component could be a very low-mass star. Radial velocity analysis resulted in an orbital period of 5.2 days and a mass function for the system of 2.57  M . Taking into account the spectral type of the primary (G5 star), a mass ratio of ~0.16 could be inferred. The raw R band CoRoT light curve of this system only showed primary eclipses convolved with a modulation that could be produced by spot activity effects as observed in other low-mass stars. For the preliminary analysis, this modulation was subtracted from data removing a running average and the resulting light curve was phase-folded and subsequently binned with larger bins in the primary eclipse phases and where secondary eclipses were expected. The resulting light curve show a shallow secondary eclipse (see figure below). First analysis of this light curve using the Wilson-Devinney code (Wilson & Devinney, 1971) provide two possible solutions indicating an eccentric orbit for the system with complementary angles of periastron passage and with radii of about 1 R  and 0.2 R  for primary and secondary components, respectively. High resolution spectrophotometric observations would be very valuable to refine the analysis of this interesting system. Accurate measures of the masses and radii of this system could provide stringent test to the evolutionary models at both ends of the low-mass domain. Besides continuous minima timing follow- up could serve as a check for the presence of apsidal motion or other bodies in the system. Fit to the radial velocity curve.Fits to the R band CoRoT light curve. Selection of eclipsing binaries For each CoRoT target that is classified as an eclipsing binary, we have drawn its light curves to select the best EB candidates. Since our interest is centered on low- mass binaries, a preliminary analysis of the target is performed by different methods:  Cross-correlation with 2MASS catalog: this provides infrared colors which are used to estimate effective temperatures through V-K color calibrations.  Duration of eclipses: it is used to estimate the total mass of the system. For circular and edge-on orbits, the duration of eclipses is proportional to the sum of stellar radii which in turn is a direct measure of the total mass when M≈R (in solar units) is assumed for low-mass stars. After this analysis, some good candidates of low-mass stars, as well as other interesting systems, were selected for spectroscopic follow-up observations to get radial velocity curves. The analysis of both light curves and radial velocity curves will provide fundamental properties with accuracies suitable to test stellar models. P~22 days T eff ~3700 K P~28 days T eff ~4000 K P~21 days T eff ~4700 KP~5.2 days T eff ~3700 K Light curves of a sample of the selected stars for spectroscopic follow-up. A total of 13 objects from LRc01 data were selected for observation with FLAMES+UVES on the VLT. Fundamental properties of low-mass stars Till now, double-lined eclipsing binary stars (EB's) are the best way to obtain accurate fundamental properties (such as masses and radii) of stars. In the last few years, several analyses have unveiled that low-mass stars in EB's have 10% larger radii and 5% colder effective temperatures than theoretical evolutionary models predict while luminosities are correctly predicted (Ribas et al and references therein). These discrepancies have been attributed to the effects of high magnetic activity on these stars, since they are fast rotators due to the synchronization of spin with orbital motion. The magnetic activity could both disturb the interior structure through the inhibition of convection, or to appear as spot activity on the surface of stars (Chabrier et al. 2007). However, the number of known systems is still scarce, therefore, discovery of new low-mass EB's would through more light to this issue, especially if they are long period binaries, since they are expected to be less active stars. Mass-Radius (left) and Mass-Effective temperature relationships for low-mass stars (DLEB, double.lined EB’s; SLEB, single-lined EB’s; Interferometry, interferometric measures). Solid line is a 1 Gyr model from Baraffe et al. (1998).   