A High Angular Resolution Multiplicity Survey of the Open Clusters α Persei and Praesepe

Abstract

Two hundred and forty-two members of the Praesepe and α Persei clusters have been surveyed with high angular resolution 2.2 µm speckle imaging on the IRTF 3-m, the Hale 5-m, and the Keck 10-m, along with direct imaging using the near-infrared camera (NICMOS) aboard the Hubble Space Telescope (HST). The observed stars range in spectral type from B (~5 M SUN ) to early-M (~0.5 M SUN ), with the majority of the targets more massive than ~0.8 MSUN. The 39 binary and 1 quadruple systems detected encompass separations from 0".053 to 7".28; 28 of the systems are new detections and there are 9 candidate substellar companions. The results of the survey are used to test binary star formation and evolution scenarios and to investigate the effects of companion stars on X-ray emission and stellar rotation. The main results are:• Over the projected separation range of 26-581 AU and magnitude differences of ∆K<4.0 mag (comparable to mass ratios, q=m sec /m prim > 0.25), the companion star fraction (CSF) for α Persei is 0.09 ± 0.03 and for Praesepe is 0.10 ± 0.03. This fraction is consistent with the field G-dwarf value, implying that there is not a systematic decline in multiplicity with age at these separations on timescales of a few x 10 7 yrs. The combination of previous spectroscopic work and the current cluster survey results in a cluster binary separation distribution that peaks at 4 +1 -1.5 AU, a significantly smaller value than the peaks of both the field G-dwarf and the nearby T Tauri distributions. If the field G-dwarf distribution represents a superposition of distributions from the populations that contributed to the field, then the data implies that ~30% of field binaries formed in dark clouds like the nearby T Tauri stars and the remaining ~70% formed in denser regions.• An exploration of the binary star properties reveals a cluster CSF that increases with decreasing target mass and a cluster mass ratio distribution that rises more sharply for higher mass stars, but is independent of binary separation. These observational trends are consistent with several models of capture in small clusters and simulations of accretion following fragmentation in a cluster environment. Other types of capture and fragmentation are either inconsistent with these data or currently lack testable predictions.• Among the cluster A stars, there is a higher fraction of binaries in the subset with X-ray detections, consistent with the hypothesis that lower mass companions are the true source of X-ray emission.

Finding proto-spectroscopic binaries

“…It is therefore essential to probe the stellar multiplicity as early as possible, to gain a full understanding of the formation and the dynamical evolution of multiple systems. Ghez et al (1993); Leinert et al (1993); Simon et al (1995); Patience et al (2002); Beck et al (2003) and Duchêne et al (2004Duchêne et al ( , 2007 all noted differences in the BF of pre-MS (PMS) stars in different SFRs. In particular, the multiplicity of T Tauri stars with ages 10 6 yrs, appears to be higher by a factor of 2 in less dense regions such as Taurus compared to denser SFR such as Ophiuchus (e.g.…”

“…In particular, the multiplicity of T Tauri stars with ages 10 6 yrs, appears to be higher by a factor of 2 in less dense regions such as Taurus compared to denser SFR such as Ophiuchus (e.g. Simon et al 1995;Beck et al 2003;Ratzka et al 2005) or the field (Patience et al 2002;Duchêne et al 2004). Toward even younger ages, radio continuum (e.g.…”

Binary frequency of planet-host stars at wide separations

“…These searches also led to the discoveries of the first wide brown dwarf companion to HD 3651 (Mugrauer et al 2006b), a K7 dwarf with a sub-Saturn mass planet (Fischer et al 2004;Santos et al 2005), and the most evolved multiple system with an exoplanet (Butler et al 2001), composed of a K subgiant and a white dwarf (HD 27442; Chauvin et al 2006;Raghavan et al 2006;Mugrauer et al 2007a). Moreover, several close companions have been announced around planet-host stars, thanks to highresolution adaptive optics imaging (Els et al 2001;Patience et al 2002) and lucky imaging (Ginski et al 2012). All these surveys point towards a global multiplicity of at least 20% among solartype stars with exoplanets (Mugrauer et al 2007a).…”

The VLT/NaCo large program to probe the occurrence of exoplanets and brown dwarfs at wide orbits

“…The distribution of q values for a sample of binaries is the companion mass-ratio distribution (CMRD). Several surveys in the past decades focused on the detection of stellar binaries with the purpose of characterizing the occurrence of companions and their mass distribution both in the field (e.g., Raghavan et al 2010;Janson et al 2012) and in star-forming regions (e.g., Patience et al 2002). Reggiani & Meyer (2013), in an update of Reggiani & Meyer (2011), have shown that in the field the CMRD is consistent with being universal, independent of primary mass and separation in the range covered by the observations, and can be fit by a single power-law slope dN/dq ∝ q β , with β = 0.25 ± 0.29.…”