As noted in Chapter 5, previous surveys have been done to determine M dwarf multiplicity, but most have studied samples on the order of a hundred stars. We reproduce in Table 8.1 the listing of those efforts outlined in Chapter 5, Table 5.2, with the addition of the multiplicity rate reported by each survey. In the notes column, we provide information related to each particular study that indicates whether a direct comparison is possible or even relevant.
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Table 8.1: Previous M Dwarf Multiplicity Studies - Mul- tiplicity Rates
Reference # Mult. Notes
Skrutskie et al. (1989) 55 ... multiplicity not reported Henry & McCarthy (1990) 27 34 ±9
Henry (1991) 74 20 ±5
Fischer & Marcy (1992) 28-62 42 ±9 varied sample Simons et al. (1996) 63 40
Delfosse et al. (1999a) 127 ... multiplicity not reported Law et al. (2006) 32 7+7−3 M5 - M8
Endl et al. (2006) 90 ... Jovian search Law et al. (2008) 77 13.6 +6−4.5 late-type M’s Bergfors et al. (2010) 124 32 ±6 young M0 - M6 Dhital et al. (2010) 1342 ... wide binary search Law et al. (2010) 36 ... wide binary search Dieterich et al. (2012) 126 ... brown dwarf search Janson et al. (2012) 701 27 ±3 young M0 - M5 Janson et al. (2014b) 286 21-27 >M5
Ward-Duong et al. (2015) 245 23.5 ±3.2 K7 - M6
this work 1122 27.8 ±1.3 all trig. distances
At first glance, it is evident that the multiplicity fractions reported by others varies over a large range, from 7% to 42%. However, it is necessary to probe deeper to understand the limitations of each survey.
Some of the surveys (Skrutskie et al. 1989; Delfosse et al. 1999a) did not report a multi- plicity fraction in their results, so they will not be addressed. Others explored the regions around M dwarfs in search of different types of objects (brown dwarfs or exoplanets) or at dif- ferent separation regimes and are thus not relevant to the present comparison. Dhital et al. (2010) and Law et al. (2010) explored only the wide binary fraction, while other searches
were for substellar objects, brown dwarfs in the case of Dieterich et al. (2012) and Jovian mass planets in the case of Endl et al. (2006). While these searches for substellar objects can be considered lower limits for the types of companions found around M dwarfs, we note that stellar companions are not always reported. Law et al. (2006, 2008) probed different sample sizes of late-type M dwarfs using the same technique and report multiplicity rates that are different by a factor of two, but still within their large errors. We note that the mul- tiplicity rate calculated here for the lowest mass bin of M dwarfs agrees with that reported in Law et al. (2008).
A number of the other samples for M dwarf multiplicity determination were volume- limited. Henry & McCarthy (1990) searched the 5 pc sample of M dwarfs, while Henry (1991) and Simons et al. (1996) extended the volume searched to 8 pc. Fischer & Marcy (1992) searched a varied sample of M dwarfs within 20 pc. The samples of Bergfors et al. (2010); Janson et al. (2012), and Janson et al. (2014b) were all within 52 pc, but had mostly photometric parallaxes. The only other sizeable survey that is volume-limited and has trigonometric parallaxes available is that of Ward-Duong et al. (2015); however, their sample does not include any late M dwarfs.
We find that our multiplicity fraction result agrees with most of the more recent surveys (Bergfors et al. 2010; Janson et al. 2012, 2014b; Ward-Duong et al. 2015) (32%, 27%, 21 — 27%, 24%, respectively). Our results also agree with the earlier studied of Henry & McCarthy (1990) and Henry (1991) (34% and 20%) within the errors, but are smaller than the studies of Fischer & Marcy (1992) and Simons et al. (1996) (42% and 40%). It is likely that some
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of the earlier works simply did not have enough targets from which to calculate accurate results with low statistical errors.
We find a somewhat larger multiplicity fraction than that of Ward-Duong et al. (2015), who had a volume-limited sample of late-K to mid-M dwarfs, all with trigonometric par- allaxes, although results agree within the error bars. We note that the fraction that they report should be larger, as (a) it includes the more massive M dwarfs, which have a higher multiplicity rate, (b) it samples well the smaller separations, which dominate the multiplicity rate, and (c) their stars were all closer (distances <15pc) and therefore their sample should be more complete.
The answer lies in the fact that they used parallaxes only fromHIPPARCOS(van Leeuwen 2007). Examination of the sample studied here reveals an additional 308 M dwarfs with par- allaxes from sources other than van Leeuwen (2007) that place them within 15 pc, 247 of which are within the color-limits of their sample (3.65 < (V −K) . 6.8)a. Of course, un-
earthing every parallax to every nearby M dwarf is no trivial task. It is much simpler to use one source for trigonometric distances. But the benefit of access to the RECONS 25 pc Database for creation of the sample presented here is emphasized.
Due to all of the targets in our multiplicity sample having accurate trigonometric paral- laxes, the study presented here has a number of advantages over ones conducted by others. All of the targets considered were reliably known to be within 25 pc. Because we measured
V RI photometry for all targets lacking it, we were able to use data all on the same photo- metric system, combined with the existing parallaxes, to calculate MV and thus, estimate
a
masses. Most other surveys were forced to use less accurate types of distances to draw conclusions from their data. We were also able to calculate projected separations that were more accurate than those of others. Our survey was comprehensive in two search regimes, while also able to infer the presence of candidate companions using other methods.