Spectral Types for Four OGLE-III Transit Candidates: Could These Be Planets?

Abstract

We present spectral types for OGLE (Optical Gravitational Lensing Experiment) transiting planet candidates OGLE-TR-134 through 137 based on low-resolution spectra taken at Kitt Peak. Our main objective is to aid those planning radial velocity monitoring of transit candidates. We obtain spectral types with an accuracy of 2 spectral subtypes, along with tentative luminosity classifications. Combining the spectral types with light-curve fits to the OGLE transit photometry, and with Two Micron All Sky Survey counterparts in two cases, we conclude that OGLE-TR-135 and 137 are not planetary transits, while OGLE-TR-134 and 136 are good candidates and should be observed with precision radial velocity monitoring to determine whether the companions are of planetary mass. OGLE-TR-135 is ruled out chiefly because a discrepancy between the stellar parameters obtained from the transit fit and those inferred from the spectra indicates that the system is a blend. OGLE-TR-137 is ruled out because the depth of the transit combined with the spectral type of the star indicates that the transiting object is stellar. OGLE-TR-134 and 136, if unblended main-sequence stars, are each orbited by a transiting object with radius below 1.4 RJ. The caveats are that our luminosity classification suggests that OGLE-TR-134 could be a giant (and therefore a blend), while OGLE-TR-136 shows a (much smaller) discrepancy of the same form as OGLE-TR-135, which may indicate that the system is a blend. However, since our luminosity classifications are uncertain at best, and the OGLE-TR-136 discrepancy can be explained if the primary is a slightly anomalous main-sequence star, the stars remain good candidates.