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

“If the disk accumulates material from the envelope more quickly than mass can accrete onto the star, then it could grow more massive and violate the Toomre stability criterion (Toomre 1964), fragmenting to form a bound companion.”

“The latter conclusion also matches the results of Kouwenhoven et al. (2007b) for intermediate-mass multiple systems in Upper Scorpius.”

“…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).”

“The infrared-selected Luhman et al. catalog of Tau-Aur Class II sources is used as the basis of our sample: it includes 179 young stellar “systems” composed of 227 (known) individual stars with spectral types from B8 to M9 (e.g., Luhman 2000, 2004; Kraus et al. 2011).”

“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.”

“LkCa 5 has a very low-mass companion (Kraus et al. 2011) that contributes a negligible amount of flux at optical wavelengths.”

“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.”

“2002), environment (e.g. Köhler et al. 2006; King et al. 201; Sana et al. 2013) and primary mass (e.g. Abt 1983; Lada 2006; Lafrenière et al. 2008; Kraus et al. 2011; Sana & Evans 2011)is therefore crucial to our understanding of star formation.”

“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.”

“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.”

“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.”

“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 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).”

“…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).”

“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.”

“…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).”

“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.”

“…2009b) and radial velocity monitoring (e.g., Mathieu et al. 1997) to more specialized methods like lunar occultations (Simon et al. 1992, 1995; Richichi et al. 1999), speckle interferometry (Ghez et al. 1993; Leinert et al. 1993), and most recently aperture-mask interferometry (Kraus et al. 2011).”

“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.”

“NRM data were analyzed following the method of Kraus et al. (2008) and Kraus et al. (2011).”

“Presuming a similar binary fraction as in the Taurus Class II population (e.g., Kraus et al. 2011), we could be missing up to ∼6–10 additional multiple systems in this sample.”

“T-Tauri stars and Class II objects have a higher multiplicity fraction than their main-sequence descendants (Kraus et al. 2011).”

“We assume that the physical processes of disk and core fragmentation are relatively independent (Kraus et al. 2011).”

“Given that the sub-mm dust cavity of UX Tau A and DM Tau are ∼ 25 AU (Andrews et al. 2011; Kraus et al. 2011; Ingleby et al. 2013; Alcalá et al. 2014) and ∼ 19 AU (Andrews et al. 2011; Kraus et al. 2011; Ingleby et al. 2013) respectively, our results also imply that the [O i] 6300 Å NC traces gas…”

“…with the Pipeline 19 The status for wide companions is less clear; see Stauffer et al. (1991); Itoh et al. (2008); Kraus & Hillenbrand (2009); Kraus et al. (2011); Herczeg & Hillenbrand (2014) 20 Some data, Python code, Starfish configuration files, and other documents related to this paper…”

“They have been resolved as close pairs by Gaia and Kraus et al. (2011), respectively, which would explain the poor fits.”

“…close brown dwarf companions within P 100 d, commonly known as the brown dwarf desert, but that the frequency of brown dwarf companions increases with increasing separation (Grether & Lineweaver 2006; Kraus et al. 2008, 2011; Csizmadia et al. 2015; Shahaf & Mazeh 2019; Wagner, Apai & Kratter 2019).”

“The multiplicity of class I young stellar objects (YSOs) in embedded clusters and pre-MS stars in young associations is high and ranges from 20 to 60% for the ∼101.5- 102.5 AU separation range and mass ratios of ∼0.1-1 (see the comprehensive review by Duchene et al. 2007 and also Kraus et al. 2011).”

“The five benchmark associations are ordered in luminosity as expected from previous studies (e.g., see discussions in Mamajek 2008; Hillenbrand 2009; Kraus et al. 2011 and Fang et al. 2013).”

“Indeed, very recently, Kraus et al. (2011, 2012) have presented results of a high-resolution imaging study of the Taurus Auriga star-forming region of mostly solar type stars (M ∼ 0.25 − 2.5M⊙) and compared the fraction of pre-MS stars hosting disks at different ages.”

“The study by Kraus et al. (2011) suggests that this assumption is in good agreement with observations (though this was only checked for a small sample of triples).”

“(1) R10a, and references therein; (2) Reipurth & Zinnecker 1993; (3) AW05, and references therein; (4) Kraus et al. 2011, and references therein; (5) Chauvin et al. 2005; (6) Andrews et al. 2010, and references therein; and (7) Looper et al. 2010a.”

“Indeed, such external perturbations are likely if the majority of stars of interest (i.e., solar-type stars) are born in binary or multiple systems (Ghez et al. 1993; Kraus et al. 2011; Marks & Kroupa 2012).”

“Stellar companions can also clear the inner disk (Artymowicz & Lubow 1994) but many stars harboring transitional disks are single stars (Kraus et al. 2011).”

“Two circumbinary disks, CoKu Tau/4 (7.8 AU; Ireland & Kraus 2008) and ROXs42c (23 AU; Kraus et al. 2011), also do not show Pfβ emission, although it should be noted that this does not appear to be universally true, as close binary DP Tau (projected separation 15.5 AU Kraus et al. 2011) has detectable Pfβ emission.”

“This is true especially at wider separations, where the separation distribution of young binaries is roughly log-uniform, but that of field stars declines more steeply (e.g. Connelley et al. 2008; Kraus et al. 2011).”

“We adapt the Bayesian statistical analysis method established in Allen (2007) and utilized as well in Kraus et al. (2011) to our survey for binarity of exoplanet host stars.”

“Because TWHya and HD163296 do not have a resolved inner gas gap, no values are provided here.”

“…systems formation to be in the range Observations of the Taurus-Auriga star-forming region show a wide binary frequency of ∼ 5− 10 % in the range 1000 − 5000 AU (Kraus et al. 2011), which is 2-3 times larger than found in the fractal models, and even higher compared with the homogeneous models.”

“Although the Taurus-Auriga class II sample is nearly complete for spectral types earlier than M8–M9, binary samples are relatively incomplete for stars with spectral types later than roughly M4 (Kraus et al. 2011).”

“In the TaurusAuriga star-forming region, Kraus et al. (2011) report a flat mass ratio distribution for primaries in the range 0.7 − 2.5 M , but for primaries in the mass range 0.25− 0.7 M they find a bias toward equal-mass systems.”

“…decreased likelihood of an infrared excess in multiple systems as opposed to isolated stars (reflecting absence of an inner disk, Cieza et al. 2009; Kraus et al. 2011) or decreased millimeter-wave continuum emission (reflecting loss of material in either the inner or outer disk, Jensen et al.…”

“…(Figure 10), new developments in the hydrodynamical simulations of star cluster formation (Bate 2009, 2011), and new observations of young stellar clusters (Kraus et al. 2008, 2011; Evans et al. 2012), we re-examine the nature of the IMF discontinuity at masses close to the hydrogen burning limit.”

“Highcontrast imaging has ruled out stellar and brown dwarf companions around LkCa 15, hinting that the gap may be opened by a young giant planet (Thalmann et al. 2010; Pott et al. 2010; Kraus et al. 2011).”

“The contrasts of these recovered planets are similar to limits achieved for diskless stars with SAM in Kraus et al. (2011) and Lacour et al. (2011).”

“…2012), and so the operating hypothesis is that the observed structures are produced by dynamical interactions with very faint (e.g., Pott et al. 2010; Kraus et al. 2011), presumably planetary-mass, companions (although more massive stellar companions must be ruled out; e.g., Ireland & Kraus 2008).”

“Our analysis of the HIRES data is very similar to the methods described in Kraus et al. (2011b) and Kraus et al. (2014).”

“Our analysis of the HIRES data is very similar to the methods described in Kraus et al. (2011b) and Kraus et al. (2014).”

“This estimate was based on the expected multiplicity statistics of G, K and M-type stars, taken from Kraus et al. (2011), which indicates ∼50% of solar type stars have a companion.”

“…with high mm-fluxes (by expelling material to large radius, reducing the mm-flux) are well above the current sensitivity levels of direct imaging studies of transition discs which currently are readily capable of detecting 10 MJ objects (Kraus et al. 2011; Kraus & Ireland 2012; Kraus et al. 2012).”

“In addition, we plot JHK photometry for a list of spectroscopically confirmed Taurus members that do not possess circumstellar disks (Luhman et al. 2010) or known binary companions (Kraus et al. 2011, 2012).”

“This is consistent with the results of Kraus et al. (2011), who obtained a lower bound of 3.9+2.6−1.2% for the frequency of substellar companions over the range 5– 5000AU by combining the results of their aperture masking survey of Taurus-Auriga members with previous direct imaging results.”

“This technique has been responsible for a series of recent high profile discoveries (Kraus et al. 2008, 2011), including the first two newborn substellar companions captured in the process of formation (Huelamo et al. 2011; Kraus & Ireland 2012).”

“…of binary properties in the field (e.g. Fischer & Marcy (1992), Reid & Gizis (1997) for M dwarfs, Metchev & Hillenbrand (2009) and Raghavan et al. (2010) for solar-type stars) and in clusters or associations (e.g. Leinert et al. 1993; Ghez et al. 1993; Patience et al. 2002; Kraus et al. 2011).”

“Indeed, observations of the occurrence of protoplanetary disks among single stars versus binaries has shown that disks exist but are much less common among <40 AU binaries (e.g., Ghez et al. 1997; White & Ghez 2001; Cieza et al. 2009; Kraus et al. 2011, 2012).”

“The binarity of the latter population is now fairly well known (e.g., Kraus et al. 2011).”

“These results confirm previous detection limits obtained by the same aperture masking technique on the Keck telescope (Kraus et al. 2011; Hinkley et al. 2011) In terms of scientific impact, this observational domain is important because it corresponds to a few astronomical units at a hundred…”

“To this end, we have compiled the multiplicity properties of each system in our sample (Kenyon, Gomez & Whitney 2008; Kraus et al. 2011; White et al. 2013, in preparation).”

“LkCa 4 is a single star, showing no spectroscopic or imaging evidence for a companion (e.g. White & Ghez 2001; Kraus et al. 2011).”

“Although a significantly elevated fraction of multi-stellar systems in young star clusters is observationally established (Ghez et al. 1993; Kraus et al. 2011), it seems natural to additionally expect a corresponding correlation between hot Jupiter spin-orbit misalignments and the existence of…”

“…would be required to restrict the disk sizes (in line and continuum) to the observed values, while no companion has been found at such distances with typical contrast limits of about∆m ≃ 5 from IR/optical studies of these stars (e.g. Ghez et al. 1993; Kraus et al. 2011, for even closer companions).”

“Our models predict that we should find only one background star with separation ρ 2′′ and brightness K 19 in Taurus, so the candidates inside this limit all seem very promising as potential analogs to 2M1207b.”

“Kraus et al. (2011, 2012) found 10 Class II/III binaries in Taurus-Auriga with projected separations less than 50AU and detected disks (either circumstellar or circumbinary).”

“A more exotic scenario could be envisioned, in which the inner planet forms in the standard manner, while the outer companion arises from a gravitational instability in the circumstellar disk at the time of the star formation (e.g. Kraus et al. 2011).”

“Both are well studied single wTTSs (from direct imaging and spectroscopic monitoring, e.g., Kraus et al. 2011; Nguyen et al. 2012) and bona fide members of the Taurus L1495 dark cloud (Xiao et al. 2012), showing clear photometric variations of large amplitudes (Grankin et al. 2008; Xiao et al.…”

“This has led to the coining of the phrase “brown dwarf desert” to represent the paucity of companion masses in between the planetary and stellar domains (Marcy & Butler 2000; Grether & Lineweaver 2006; Sahlmann et al. 2011; Kraus et al. 2011; Cheetham et al. 2015).”

“The mean system masses for the TaurusAuriga sample (derived using the component masses given in Table 5 of Kraus et al. 2011) for the binaries, triples, quadruples, quintuple, and sextuple, respectively, are 1.0 ± 0.1 M⊙, 1.3 ± 0.1 M⊙, 2.0 ± 0.3 M⊙, 2.9 M⊙, and 3.2 M⊙.”

“The data analysis follows the same prescription as in Kraus et al. (2008); Hinkley et al. (2011a); Kraus et al. (2011), so we refer the reader to these works.”

“Although the targets of Kraus et al. (2008, 2011) overlap with ours, this study is the first large-scale and systematic exploration of exoplanets around YSOs.”

“Almost half of all main-sequence solar-type stars are found in binary or higher-order multiple systems (e.g., Raghavan et al. 2010; Duchêne & Kraus 2013), and it is thought that the fraction of multiple systems is even higher among pre-main sequence stars (e.g., Duchêne 1999; Kraus et al. 2011).”

“′′05; Kraus et al. 2011).”

“Kraus et al. (2008, 2011) performed a high-resolution imaging studies of Taurus-Auriga and Upper Sco star-forming regions to identify companions down to 8 to 12 MJ .”