The Solar Neighborhood. Xxviii. The Multiplicity Fraction of Nearby Stars From 5 to 70 Au and the Brown Dwarf Desert Around M Dwarfs

Abstract

We report on our analysis of Hubble Space Telescope/NICMOS snapshot high-resolution images of 255 stars in 201 systems within ∼10 pc of the Sun. Photometry was obtained through filters F110W, F180M, F207M, and F222M using NICMOS Camera 2. These filters were selected to permit clear identification of cool brown dwarfs through methane contrast imaging. With a plate scale of 76 mas pixel −1 , NICMOS can easily resolve binaries with subarcsecond separations in the 19. 5×19. 5 field of view. We previously reported five companions to nearby M and L dwarfs from this search. No new companions were discovered during the second phase of data analysis presented here, confirming that stellar/substellar binaries are rare. We establish magnitude and separation limits for which companions can be ruled out for each star in the sample, and then perform a comprehensive sensitivity and completeness analysis for the subsample of 138 M dwarfs in 126 systems. We calculate a multiplicity fraction of 0.0 +3.5 −0.0 % for L companions to M dwarfs in the separation range of 5-70 AU, and 2.3 +5.0 −0.7 % for L and T companions to M dwarfs in the separation range of 10-70 AU. We also discuss trends in the color-magnitude diagrams using various color combinations and present astrometry for 19 multiple systems in our sample. Considering these results and results from several other studies, we argue that the so-called brown dwarf desert extends to binary systems with low-mass primaries and is largely independent of primary mass, mass ratio, and separations. While focusing on companion properties, we discuss how the qualitative agreement between observed companion mass functions and initial mass functions suggests that the paucity of brown dwarfs in either population may be due to a common cause and not due to binary formation mechanisms.

Stellar Multiplicity

“…Like solar-type binaries, short period M-dwarf binaries are significantly biased towards high-q systems (Reid & Gizis 1997a). In addition, low-q systems are limited to the earliest M-type dwarfs (Dieterich et al 2012). Fitting the RECONS sample, we find that γ = 1.9 ± 1.7.…”

Using binary statistics in Taurus-Auriga to distinguish between brown dwarf formation processes

“…the shape of the IMF around the HBML (Thies & Kroupa 2007, 2008Thies et al 2015), the binary statistics of BDs which are different from higher mass stars (Thies & Kroupa 2008;Dieterich et al 2012, Sec. 5.3.4), the generation of free-floating planetary mass objects and BDs (Stamatellos & Whitworth 2009;Li et al 2015;Forgan et al 2015;Vorobyov 2016) and the lack of BD companions to solar-type stars at low separations known as the BD desert (Marcy & Butler 2000;Grether & Lineweaver 2006;Dieterich et al 2012;Evans et al 2012;Wilson et al 2016). All these observables are expected from peripheral fragmentation models and are thus no unambigious tracers of a single star-formation mode (see Sec.…”

The big problems in star formation: The star formation rate, stellar clustering, and the initial mass function

“…Brown dwarfs represent a possible exception to this statement, since they are both intrinsically rarer than stars and easily concealed by a stellar companion. Fortunately there appear to be few brown dwarf-stellar binaries (e.g., Dieterich et al 2012), but the exact form of the IMF at low masses is quite sensitive to exactly how rare they are, since K and M stars are so numerous that even a small number of brown dwarf companions to them might represent a non-negligible contribution to the total number of brown dwarfs.…”