MAPPING THE SHORES OF THE BROWN DWARF DESERT. II. MULTIPLE STAR FORMATION IN TAURUS–AURIGA

By A. Kraus, Michael J. Ireland, F. Martinache, L. Hillenbrand. 2011

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We have conducted a high-resolution imaging study of the Taurus–Auriga star-forming region in order to characterize the primordial outcome of multiple star formation and the extent of the brown dwarf desert. Our survey identified 16 new binary companions to primary stars with masses of 0.25–2.5 M☉, raising the total number of binary pairs (including components of high-order multiples) with separations of 3–5000 AU to 90. We find that ∼2/3–3/4 of all Taurus members are multiple systems of two or more stars, while the other ∼1/4–1/3 appear to have formed as single stars; the distribution of high-order multiplicity suggests that fragmentation into a wide binary has no impact on the subsequent probability that either component will fragment again. The separation distribution for solar-type stars (0.7–2.5 M☉) is nearly log-flat over separations of 3–5000 AU, but lower-mass stars (0.25–0.7 M☉) show a paucity of binary companions with separations of ≳200 AU. Across this full mass range, companion masses are well described with a linear-flat function; all system mass ratios (q = MB/MA) are equally probable, apparently including substellar companions. Our results are broadly consistent with the two expected modes of binary formation (free-fall fragmentation on large scales and disk fragmentation on small scales), but the distributions provide some clues as to the epochs at which the companions are likely to form.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/d9092295d91fdf71df1f34481bdbff65a7fb02f1

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THE GEMINI/NICI PLANET-FINDING CAMPAIGN: THE FREQUENCY OF PLANETS AROUND YOUNG MOVING GROUP STARS (2013, methodology): “We adapt the method established in Allen (2007) and Kraus et al. (2011) for studying stellar binarity in the context of a direct imaging survey of exoplanets.”
Stellar Multiplicity (2013, methodology): “The latter conclusion also matches the results of Kouwenhoven et al. (2007b) for intermediate-mass multiple systems in Upper Scorpius.”
The Close Binary Fraction of Solar-type Stars Is Strongly Anticorrelated with Metallicity (2018, background): “…stars, i.e., slightly older Class II and III protostars, in various star-forming environments such as Taurus, Upper Scorpius, Ophiuchus, and Orion exhibit a factor of ≈2 binary excess across intermediate separations a≈10–100 au (Ghez et al. 1993; Kraus et al. 2008, 2011; Duchêne et al. 2018).”
AN OPTICAL SPECTROSCOPIC STUDY OF T TAURI STARS. I. PHOTOSPHERIC PROPERTIES (2014, background): “LkCa 5 has a very low-mass companion (Kraus et al. 2011) that contributes a negligible amount of flux at optical wavelengths.”
THE VLA NASCENT DISK AND MULTIPLICITY SURVEY OF PERSEUS PROTOSTARS (VANDAM). II. MULTIPLICITY OF PROTOSTARS IN THE PERSEUS MOLECULAR CLOUD (2016, background): “The best characterized group of young stellar multiples is in Taurus, where Kraus et al. (2011) combined new observations down to 3 au scales with previous multiplicity searches.”
THE IMPACT OF STELLAR MULTIPLICITY ON PLANETARY SYSTEMS. I. THE RUINOUS INFLUENCE OF CLOSE BINARY COMPANIONS (2016, methodology): “The majority of all solar-type stars form with at least one binary companion (Duquennoy & Mayor 1991; Kraus et al. 2008, 2011; Raghavan et al. 2010), so the impact of stellar binarity on planet occurrence could represent one of the largest remaining systematic uncertainties in the Kepler era.”
THE ROLE OF MULTIPLICITY IN DISK EVOLUTION AND PLANET FORMATION (2011, background): “Taurus-Auriga is one of the nearest and best studied star-forming regions, and our knowledge of its circumstellar disk population is generally considered the most comprehensive for any star-forming region.”
FROM BINARIES TO MULTIPLES. II. HIERARCHICAL MULTIPLICITY OF F AND G DWARFS (2014, methodology): “Kraus et al. (2011) already tried to model hierarchical multiplicity in a Monte-Carlo simulation by producing inner sub-systems with the generating period distribution and applying the stability cutoff.”
Mind Your Ps and Qs: The Interrelation between Period (P) and Mass-ratio (Q) Distributions of Binary Stars (2016, methodology): “…have also concluded that the statistics of solar-type binaries in open clusters are indistinguishable from those in the field for a broad range of cluster densities and ages τ ≈ 3 Myr - 7 Gyr (Bouvier et al. 1997; Köhler et al. 2006; Kraus et al. 2011; Geller & Mathieu 2012; King et al. 2012).”
ACCRETION RATES FOR T TAURI STARS USING NEARLY SIMULTANEOUS ULTRAVIOLET AND OPTICAL SPECTRA (2013, background): “DK Tau A, HN Tau A and RW Aur A all have companions at large separations of >1.4′′or ∼200 AU (Kraus et al. 2011; White & Ghez 2001), resolved by STIS.”
TRANSITIONAL DISKS AS SIGNPOSTS OF YOUNG, MULTIPLANET SYSTEMS (2011, background): “Kraus et al. (2011) also found no stellar companions in the inner holes of DM Tau, GM Aur, LkCa 15 and UX Tau A in their non-redundant aperture masking survey of the Taurus star-forming region.”
LkCa 15: A YOUNG EXOPLANET CAUGHT AT FORMATION? (2011, background): “All of these stars were chosen from our previous binary survey (Kraus et al. 2011) and are known to have no stellar companions at angular separations of >20 mas and no brown dwarf companions at >40 mas.”
Long-baseline interferometric multiplicity survey of the Sco-Cen OB association (2013, methodology): “The formalism for the application of the above Bayesian statistics to the analysis of multiplicity populations was first introduced by Allen (2007), though we present it in a similar way to Kraus et al. (2011).”
The Formation and Early Evolution of Low-Mass Stars and Brown Dwarfs (2012, background): “…of wide binaries and their average separation decrease steadily from solar-mass stars to brown dwarfs (Konopacky et al. 2007; Kraus & Hillenbrand 2007, 2009a, 2012; Kraus et al. 2011b; Kraus, White & Hillenbrand 2006; Lafrenière et al. 2008; Neuhäuser et al. 2002, K. Todorov, in preparation).”
They are small worlds after all: revised properties of Kepler M dwarf stars and their planets (2015, methodology): “NRM data were analyzed following the method of Kraus et al. (2008) and Kraus et al. (2011).”
Gravitational Instabilities in Circumstellar Disks (2016, background): “On the contrary, Kraus et al. (2011) find that for young stars in Taurus, the lower mass objects M∗ < 0.7M typically have companions with smaller separations consistent with disk fragmentation (< 100AU ,) whereas stars with 0.7M < M∗ < 2.5M have more companions at larger separations.”