Abstract
We report the detection of eclipses in GJ 3236, a bright (I = 11.6) very low mass binary system with an orbital period of 0.77 days. Analysis of light- and radial velocity curves of the system yielded component masses of 0.38 +/- 0.02 and 0.28 +/- 0.02 Msol. The central values for the stellar radii are larger than the theoretical models predict for these masses, in agreement with the results for existing eclipsing binaries, although the present 5% observational uncertainties limit the significance of the larger radii to approximately 1 sigma. Degeneracies in the light curve models resulting from the unknown configuration of surface spots on the components of GJ 3236 currently dominate the uncertainties in the radii, and could be reduced by obtaining precise, multi-band photometry covering the full orbital period. The system appears to be tidally synchronized and shows signs of high activity levels as expected for such a short orbital period, evidenced by strong Halpha emission lines in the spectra of both components. These observations probe an important region of mass-radius parameter space around the predicted transition to fully-convective stellar interiors, where there are a limited number of precise measurements available in the literature.
Highlights
Detached, double-lined eclipsing binaries provide a largely model-independent means to precisely and accurately measure fundamental stellar properties, masses and radii
We carried out a preliminary analysis of the available light curves before starting to obtain spectroscopic observations
This is clear from comparing the results for the three solutions reported in Table 9, where we find significant (>1 standard deviation) differences in the orbital inclination i, ratio of effective temperatures T2/T1, and in (R1 + R2)/a, which depend on the assumptions made regarding the locations of the spots on the components of the binary
Summary
Double-lined eclipsing binaries provide a largely model-independent means to precisely and accurately measure fundamental stellar properties, masses and radii. The only two with parameters determined to better than 2% are CM Dra and CU Cnc. For SDSS J031824−010018, the knowledge of the parameters is limited largely by radial velocity errors, since this system is extremely faint (SDSS r = 19.3) and has an extremely short orbital period (0.41 days), meaning long integrations cannot be used to obtain better signal to noise. In order to better constrain the stellar mass–radius relation on the main sequence, more bright, low-mass eclipsing binaries are needed to yield extremely precise masses and radii
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