Multiplicity Among M Dwarfs

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Session 46 -- Late Type Stars
Display presentation, Wednesday, 9:20-6:30, Heller Lounge Room

[46.03] Multiplicity Among M Dwarfs

D.A. Fischer (SFSU/UCSC), G.W. Marcy (SFSU/UC Berkeley)

We examine surveys of M dwarfs within 20 pc to determine the incidence of stellar companions. Observational data are drawn from high-quality surveys, including IR-array imaging, precise velocities, IR speckle interferometry, and visual imaging, and their respective incompleteness is determined. Each technique permits detection of companions down to the H-burning limit, and each is nearly complete (owing to the proximity of M dwarfs) within a specific range of separation, the farthest being $\approx 10^4$ AU. The number of companions per primary per AU of semimajor axis, %dN/da$, is computed and found to decline monotonically toward larger semimajor axes. The period distribution (in bins of $\Delta \log P$) exhibits a unimodal, broad maximum with a peak in the range $P = 9 - 220$ yr, corresponding to separations 3-30 AU, similar to that for G dwarf binaries. Integrating over all semimajor axes yields the average number of companions per primary, $0.55 \pm 0.09$, which includes those companions clumped in multiple systems. Thus, 58\% of nearby M dwarf primaries are single and $42\% \pm 9\%$ have companions, similar to Henry and McCarthy's binary frequency of 34\%. The M dwarf binary frequency is lower than that for G dwarfs ($\approx 57\%$), owing partly to the smaller range of companion masses included, i.e., companions less massive than the M- of G-type primary. The mass function of companions to M dwarfs is roughly flat in shape, similar to the field mass function. Companions to G dwarfs also exhibit the field mass function, which provides support for protostellar "capture" as the dominant mechanism by which binaries form.

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