Friday, June 14, 2013

1306.3157 (Claude E. Mack III et al.)

A Cautionary Tale: MARVELS Brown Dwarf Candidate Reveals Itself To Be A Very Long Period, Highly Eccentric Spectroscopic Stellar Binary    [PDF]

Claude E. Mack III, Jian Ge, Rohit Deshpande, John P. Wisniewski, Keivan G. Stassun, B. Scott Gaudi, Scott W. Fleming, Suvrath Mahadevan, Nathan De Lee, Jason Eastman, Luan Ghezzi, Jonay I. Gonzalez Hernandez, Bruno Femenia, Leticia Ferreira, Gustavo Porto de Mello, Justin R. Crepp, Daniel Mata Sanchez, Eric Agol, Thomas G. Beatty, Dmitry Bizyaev, Howard Brewington, Phillip A. Cargile, Luiz N. da Costa, Massimiliano Esposito, Garret Ebelke, Leslie Hebb, Peng Jiang, Stephen R. Kane, Brian Lee, Marcio A. G. Maia, Elena Malanushenko, Victor Malanushenko, Daniel Oravetz, Martin Paegert, Kaike Pan, Carlos Allende Prieto, Joshua Peper, Rafael Rebolo, Arpita Roy, Basilio X. Santiago, Donald P. Schneider, Audrey Simmons, Robert J. Siverd, Stephanie Snedden, Benjamin M. Tofflemire
We report the discovery of a highly eccentric, double-lined spectroscopic binary star system (TYC 3010-1494-1), comprising two solar-type stars that we had initially identified as a single star with a brown dwarf companion. At the moderate resolving power of the MARVELS spectrograph and the spectrographs used for subsequent radial-velocity (RV) measurements (R ~ <30,000), this particular stellar binary mimics a single-lined binary with an RV signal that would be induced by a brown dwarf companion (Msin(i)~50 M_Jup) to a solar-type primary. At least three properties of this system allow it to masquerade as a single star with a very low-mass companion: its large eccentricity (e~0.8), its relatively long period (P~238 days), and the approximately perpendicular orientation of the semi-major axis with respect to the line of sight (omega~189 degrees). As a result of these properties, for ~95% of the orbit the two sets of stellar spectral lines are completely blended, and the RV measurements based on centroiding on the apparently single-lined spectrum is very well fit by an orbit solution indicative of a brown dwarf companion on a more circular orbit (e~0.3). Only during the ~5% of the orbit near periastron passage does the true, double-lined nature and large RV amplitude of ~15 km/s reveal itself. The discovery of this binary system is an important lesson for RV surveys searching for substellar companions; at a given resolution and observing cadence, a survey will be susceptible to these kinds of astrophysical false positives for a range of orbital parameters. Finally, for surveys like MARVELS that lack the resolution for a useful line bisector analysis, it is imperative to monitor the peak of the cross-correlation function for suspicious changes in width or shape, so that such false positives can be flagged during the candidate vetting process.
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