2MASS J05162881+2607387 is a Double-Lined Eclipsing Low-Mass Binary

ATel #680; Amanda J. Bayless (San Diego State University), Jerome A. Orosz (San Diego State University)
on 21 Dec 2005; 01:31 UT
Credential Certification: Jerome A. Orosz (orosz@sciences.sdsu.edu)

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The star known as 2MASS J05162881+2607387 (J0516) was discovered to be an eclipsing binary by Schuh et al. (2003, A&A, 410, 649). Assuming that the orbital period is the same as the eclipse period (1.29395 days), Schuh et al. suggested that J0516 consists of a K7V star and a brown dwarf. Schuh et al. also considered the possibility that the orbital period of J0156 was twice the eclipse period (i.e. 2.5879 days). In that case, their phased light curve had identical primary and secondary eclipses. Our spectroscopic results indicate that the orbital period of J0516 is in fact 2.5879 days.

We have analyzed 9 spectra (R=15,000) using the High Resolution Spectrograph (HRS) on the Hobby-Eberly Telescope (HET), obtained by the HET staff between 2005 October 10 and November 27. We also have the spectrum of a bright K7V star.

Radial velocities were derived using the "Broadening Function" technique (Rucinski 1999, IAU Coll. 170, eds. J. B. Hearnshaw and C. D. Scarfe, ASP Conf. Vol. 185, p. 82). The broadening functions have two clear peaks, indicating that the spectrum is double-lined. We find the following spectroscopic parameters:

K_1 = 89.2 +/- 0.8 km/sec; K_2 = 89.8 +/- 0.8 km/sec

Q = K_1/K_2 = M_2/M_1 = 0.99 +/- 0.01

The two stars have nearly identical line features, many of which roughly resemble the lines in the K7V comparison. However, there is one obvious difference: the stars in J0516 have no feature at Halpha, either in emission or absorption. In this regard, J0516 resembles some RS CVn BY Dra binaries with "filled in" Halpha line profiles (Fernandez-Figueroa, et al., 1994, ApJS, 90, 433). Our photometric light curves, discussed next, also point to relatively high levels of stellar activity.

Our differential light curves of J0516 were obtained over several nights between 2004 January-March (Epoch I) and 2004 October-2005 January (Epoch II), using the 1m at Mount Laguna Observatory. During Epoch I, the source was roughly 0.1 magnitudes brighter in V, R, and I compared to Epoch II, and also showed considerable out-of-eclipse variability. The phased light curves from Epoch II show little out-of-eclipse variability.

The light curves from Epoch II and the radial velocity curves were analyzed using our ELC code (Orosz & Hauschildt, 2000, A&A, 364, 265). We find the following preliminary results:

M_1 = 0.785 +/- 0.017 M_sun; M_2 = 0.780 +/- 0.015 M_sun;

R_1 = 0.887 +/- 0.023 R_sun; R_2 = 0.825 +/- 0.008 R_sun;

The ephemeris determined from the combined light and velocity curves is

P = 2.58791 +/- 0.00001 days; T_0 = HJD 2,452,251.512 +/- 0.005

Note that the above errors are statistical only, and do not include systematic errors (e.g. due to source variability). We also note that the phase coverage in the primary eclipse is less than ideal.

Double-lined eclipsing binaries containing low-mass stars are relatively rare, although the list of well-studied systems is growing (see Ribas, astro-ph/0511433, astro-ph/0511431). In the few low-mass binaries where precise measurements of the component masses and radii are available, current evolutionary models predict radii for stars that are significantly smaller than observed. Our preliminary results when taken at face value indicate that the stars in J0516 also show a similar trend where the radii are larger than model predictions.

J0516 is clearly a very important system in that it allows for further observational constraints for the lower main sequence. Owing to the enhanced variability, we encourage observers to obtain additional photometry so that the eclipse profiles can be better established.