The M-dwarfs in Multiples (MinMs) survey - I. Stellar multiplicity among low-mass stars within 15 pc (1503.00724v1)

Abstract: We present a large-scale, volume-limited companion survey of 245 late-K to mid-M (K7-M6) dwarfs within 15 pc. Infrared adaptive optics (AO) data were analysed from the Very Large Telescope, Subaru Telescope, Canada-France-Hawaii Telescope, and MMT Observatory to detect close companions to the sample from $\sim$1 au to 100 au, while digitised wide-field archival plates were searched for wide companions from $\sim$100 au to 10,000 au. With sensitivity to the bottom of the main sequence over a separation range of 3 au to 10,000 au, multiple AO and wide-field epochs allow us to confirm candidates with common proper motions, minimize background contamination, and enable a measurement of comprehensive binary statistics. We detected 65 co-moving stellar companions and find a companion star fraction of $23.5 \pm 3.2$ per cent over the 3 au to 10,000 au separation range. The companion separation distribution is observed to rise to a higher frequency at smaller separations, peaking at closer separations than measured for more massive primaries. The mass ratio distribution across the $q = 0.2 - 1.0$ range is flat, similar to that of multiple systems with solar-type primaries. The characterisation of binary and multiple star frequency for low-mass field stars can provide crucial comparisons with star forming environments and hold implications for the frequency and evolutionary histories of their associated disks and planets.

• The paper reports that nearby M-dwarfs show a multiplicity rate of about 23.5%, potentially rising to 34.7% with undiscovered close companions.
• The study uses adaptive optics and archival plate data to cover separations from ~1 to 10,000 au, unveiling a denser population of close binary systems.
• Findings indicate that M-dwarfs form fewer and more tightly-bound multiples compared to higher-mass stars, illuminating key aspects of low-mass star formation.

Overview of the MinMs Survey on Stellar Multiplicity of M-Dwarfs

The paper titled "The M-dwarfs in Multiples (MinMs) survey - I. Stellar multiplicity among low-mass stars within 15 pc" presents a comprehensive investigation into the multiplicity characteristics of nearby low-mass stars, specifically targeting late-K to mid-M dwarfs within a 15 parsec range. With a focus on providing an empirically robust dataset, the research bridges gaps in our understanding of companion star frequency and distribution alongside primordial star-forming processes that govern low-mass stellar systems.

Methodology

This survey capitalized on a combination of archival data and new observations leveraging adaptive optics (AO) from several prominent facilities, including the Very Large Telescope and the Canada-France-Hawaii Telescope, to capture companions at separations of about 1 au to 100 au. Additionally, wide-field plate data were utilized to extend the companion-search range to separations of up to 10,000 au. This dual approach allowed for an expansive search spanning over 3.5 orders of magnitude in projected separation.

Key Findings

1. Companion Detection and Frequencies:
   • The paper unveiled 65 co-moving stellar companions distributed across 53 binary systems, and a small fraction in higher multiplicity configurations such as triples and quadruples.
   • The companion star fraction over the 3-10,000 au separation range was reported as $23.5 \pm 3.2\%$. Accounting for potentially undiscovered closer companions would increase this figure to an estimated $34.7\%$.
2. Separation and Mass Ratio Distributions:
   • The separation distribution reveals a trend towards an increasing number of close companions, highlighting a higher density of binaries at smaller separations.
   • The mass ratio distribution across the $q = 0.2 - 1.0$ range appears flat, markedly different from the trends observed in more massive stellar primaries.
3. Multiplicity Comparisons with More Massive Stars:
   • When compared with solar-type and A-type stars, the separation distributions stand in contrast — specifically, M-dwarfs exhibit a trend towards tighter orbital configurations relative to more massive counterparts.
   • The paper corroborates the declining trend in multiplicity fractions correlating inversely with primary mass, outlining M-dwarfs as less likely to form in multiple systems compared to higher mass stars.

Implications and Future Directions

The MinMs survey augments the empirical foundation necessary to extrapolate star formation characteristics from low-mass stellar companions in field settings. Insights from the separation and mass ratio distributions have potential ramifications on understanding binary formation dynamics, especially regarding environmental influences on low-mass star systems.

Future developments could focus on expanding radial velocity surveys and applying complementary methods to ascertain binary characteristics such as individual orbits, mass functions, and system architecture for binaries disturbed by additional stellar components. Improved spectroscopic surveys, for instance, could be pivotal in resolving more closely-bound spectroscopic binaries, hence refining the multiplicity statistics further.

In conclusion, through its extensive use of observational data and meticulous statistical analysis, the MinMs survey outlines essential comparative cosmological insights pertinent to understanding low-mass stellar evolution and the universality of star formation processes. This paper is thus a significant contribution, offering a pivotal lens on the less explored sector of the celestial population, the M-dwarfs and their multifaceted companion dynamics.