Lucky Imaging of M subdwarfs

Abstract

Context. The knowledge of the binary properties of metal-poor and solar-metallicity stars can shed light on the potential differences between the formation processes responsible for both types of objects. Aims. The aim of the project is to determine the binary properties (separation, mass ratio, frequency of companions) for M subdwarfs, the low-metallicity counterparts of field M dwarfs, and investigate any potential differences between both populations. Methods. We have obtained high-resolution imaging in the optical for a sample of 24 early-M subdwarfs and nine extreme subdwarfs with the "Lucky Imaging" technique using the AstraLux instrument on the Calar Alto 2.2-m telescope. Results. We are sensitive to companions at separations larger than 0.1 arcsec and differences of ∼2 mag at 0.1 arcsec and ∼5 mag at 1 arcsec. We have found no companion around the 24 subdwarfs under study and one close binary out of nine extreme subdwarfs. A second image of LHS 182 taken three months later with the same instrument confirms the common proper motion of the binary separated by about 0.7 arcsec. Moreover, we do not confirm the common proper motion of the faint source reported by Riaz and collaborators at ∼2 arcsec from LHS 1074. Conclusions. We derive a binary frequency of 3 ± 3% for M subdwarfs from our sample of 33 objects for separations larger than about five astronomical units. Adding to our sample the additional 28 metal-poor early-M dwarfs observed with the Hubble Space Telescope by Riaz and collaborators, we infer a binary fraction of 3.7 ± 2.6% (with a 1σ confidence limit), significantly lower than the fraction of resolved binary M dwarfs (∼20%) over the same mass and separation ranges. This result suggests a sharp cut-off in the multiplicity fraction from G to M subdwarfs, indicating that the metallicity plays a role at lower masses and/or an environmental effect governing the formation of metal-poor M dwarfs compared to their metallicity counterparts.

Stellar Multiplicity

“…On the other hand, the frequency of VBs (a ≥ 10 AU) is markedly lower than for solar-metallicity stars (Allen, Poveda & Herrera 2000;Lodieu et al 2009;Zapatero Osorio & Martín 2004), though perhaps not for K-M dwarfs (Jao et al 2009). As a result, the total multiplicity frequency of Population II stars is CF The distribution of orbital period among solar-mass Population II multiple systems is characterized by a narrow peak in orbital period around log P ≈ 2-3, accompanied by a longperiod tail out to ∼ 10 4 AU that follows roughlyÖpik's law (Allen, Poveda & Herrera 2000) and a marked dearth of systems with orbital periods shorter than log P ≈ 1 , Rastegaev 2010.…”

CARMENES input catalogue of M dwarfs

“…In addition, J12025+084 (M1.5 V; ζ = 0.898 -LHS 320) was classified by Gizis (1997) as an sdM2.0 star and was investigated extensively afterwards with high-resolution imagers (Gizis & Reid 2000;Riaz et al 2008;Jao et al 2009;Lodieu et al 2009). However, we failed to detect any subdwarf signpost in our high signal-to-noise spectrum, which is partly consistent with the metallicity [Fe/H] = −0.6±0.3 measured by Rajpurohit et al (2014).…”

Multiplicity of cool dwarfs

“…Deacon et al (2012a) also identified a T5.5 companion to the M4.5 dwarf LHS 2803, which was independently discovered by Mužić et al (2012). Subdwarfs are a rare subset of low-mass objects that have also received more attention with modern surveys (e.g., Riaz et al 2008;Lodieu et al 2009;Jao et al 2009). SDSS has spectroscopically identified a large sample of M type subdwarfs (e.g., Lépine 2009).…”

“…Subdwarfs are a rare subset of low-mass objects that have also received more attention with modern surveys (e.g., Riaz et al 2008;Lodieu et al 2009;Jao et al 2009). SDSS has spectroscopically identified a large sample of M type subdwarfs (e.g., Lépine 2009).…”