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Register a new userUnusually Wide Binaries: Are They Wide or Unusual?
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We describe an astrometric and spectroscopic campaign to confirm the youth and association of a complete sample of candidate wide companions in Taurus and Upper Sco. Our survey found fifteen new binary systems (3 in Taurus and 12 in Upper Sco) with separations of 3-30 (500-5000 AU) among all of the known members with masses of 2.5-0.012 Msun. The total sample of 49 wide systems in these two regions conforms to only some expectations from field multiplicity surveys. Higher-mass stars have a higher frequency of wide binary companions, and there is a marked paucity of wide binary systems near the substellar regime. However, the separation distribution appears to be log-flat, rather than declining as in the field, and the mass ratio distribution is more biased toward similar-mass companions than the IMF or the field G dwarf distribution. The maximum separation also shows no evidence of a limit at <5000 AU until the abrupt cessation of any wide binary formation at system masses of ~0.3 Msun. We attribute this result to the post-natal dynamical sculpting that occurs for most field systems; our binary systems will escape to the field intact, but most field stars are formed in denser clusters and do not. In summary, only wide binary systems with total masses <0.3 Msun appear to be unusually wide.
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The population statistics of binary stars are an important output of star formation models. However populations of wide binaries evolve over time due to interactions within a systems birth environment and the unfolding of wide, hierarchical triple systems. Hence the wide binary populations observed in star forming regions or OB associations may not accurately reflect the wide binary populations that will eventually reach the field. We use Gaia DR2 data to select members of three open clusters, Alpha~Per, the Pleiades and Praesepe and to flag cluster members that are likely unresolved binaries due to overluminosity or elevated astrometric noise. We then identify the resolved wide binary population in each cluster, separating it from coincident pairings of unrelated cluster members. We find that these clusters have an average wide binary fraction in the 300-3000,AU projected separation range of 2.1$pm^{0.4}_{0.2}$% increasing to 3.0$pm^{0.8}_{0.7}$% for primaries with masses in the 0.5-1.5,$M_{odot}$ range. This is significantly below the observed field wide binary fraction, but shows some wide binaries survive in these dynamically highly processed environments. We compare our results with another open cluster (the Hyades) and two populations of young stars that likely originated in looser associations (Young Moving Groups and the Pisces-Eridanus stream). We find that the Hyades also has a deficit of wide binaries while the products of looser associations have wide binary fractions at or above field level.
M-dwarfs in extremely wide binary systems are very rare, and may thus have different formation processes from those found as single stars or close binaries in the field. In this paper we search for close companions to a new sample of 36 extremely wide M-dwarf binaries, covering a spectral type range of M1 to M5 and a separation range of 600 - 6500 AU. We discover 10 new triple systems and one new quadruple system. We carefully account for selection effects including proper motion, magnitude limits, the detection of close binaries in the SDSS, and other sample biases. The bias-corrected total high-order-multiple fraction is 45% (+18%/-16%) and the bias-corrected incidence of quadruple systems is < 5%, both statistically compatible with that found for the more common close M-dwarf multiple systems. Almost all the detected companions have similar masses to their primaries, although two very low mass companions, including a candidate brown dwarf, are found at relatively large separations. We find that the close-binary separation distribution is strongly peaked towards < 30AU separations. There is marginally significant evidence for a change in high-order M-dwarf multiplicity with binding energy and total mass. We also find 2-sigma evidence of an unexpected increased high-order-multiple fraction for the widest targets in our survey, with a high-order-multiple fraction of 21% (+17%/-7%) for systems with separations up to 2000AU, compared to 77% (+9%/-22%) for systems with separations > 4000AU. These results suggest that the very widest M-dwarf binary systems need higher masses to form or to survive.
Understanding the formation of wide binary systems of very low mass stars (M $le$ 0.1 Msun) is challenging. The most obvious route is via widely separated low-mass collapsing fragments produced through turbulent fragmentation of a molecular core. However, close binaries/multiples from disk fragmentation can also evolve to wide binaries over a few initial crossing times of the stellar cluster through tidal evolution. Finding an isolated low mass wide binary system in the earliest stage of formation, before tidal evolution could occur, would prove that turbulent fragmentation is a viable mechanism for (very) low mass wide binaries. Here we report high resolution ALMA observations of a known wide-separation protostellar binary, showing that each component has a circumstellar disk. The system is too young to have evolved from a close binary and the disk axes are misaligned, providing strong support for the turbulent fragmentation model. Masses of both stars are derived from the Keplerian rotation of the disks; both are very low mass stars.
In spite of its importance for the study of star formation at all mass domains, the nearby young sigma Orionis cluster still lacks a comprehensive survey for multiplicity. We try to fill that observational gap by looking for wide resolved binaries with angular separations between 0.4 and 4.0 arcsec. We search for companions to 331 catalogued cluster stellar members and candidates in public K-band UKIDSS images outside the innermost 1 arcmin, which is affected by the glare of the bright, eponymous sigma Ori multiple system, and investigate their cluster membership with colour-magnitude diagrams and previous knowledge of youth features. Of the 18 identified pairs, ten have very low individual probabilities of chance alignment (< 1 %) and are considered here as physical pairs. Four of them are new, while the other six had been discovered previously, but never investigated homogeneously and in detail. Projected physical separations and magnitude differences of the ten probably bound pairs range from 180 to 1220 au, and from 0.0 to 3.4 mag in K, respectively. Besides, we identify two cluster stars with elongated point spread functions. We determine the minimum frequency of wide multiplicity in the interval of projected physical separations s = 160-1600 au in sigma Orionis at 3.0^{+1.2}_{-1.1} %. We discover a new Lindroos system, find that massive and X-ray stars tend to be in pairs or trios, conclude that multiplicity truncates circumstellar discs and enhances X-ray emission, and ascribe a reported lithium depletion in a young star to unresolved binarity in spectra of moderate resolution. When accounting for all know multiples, including spectroscopic binaries, the minimum frequency of multiplicity increases to about 10 %, which implies that of the order of 80-100 unknown multiple systems still await discovery in sigma Orionis.
We examine the distribution of on-sky relative velocities for wide binaries previously assembled from GAIA DR2 data and focus on the origin of the high velocity tail of apparently unbound systems which may be interpreted as evidence for non-Newtonian gravity in the weak field limit. We argue that this tail is instead explicable in terms of a population of hidden triples, i.e. cases where one of the components of the wide binary is itself a close binary unresolved in the GAIA data. In this case the motion of the photocentre of the inner pair relative to its barycentre affects the apparent relative proper motion of the wide pair and can make pairs that are in fact bound appear to be unbound. We show that the general shape of the observed distributions can be reproduced using simple observationally motivated assumptions about the population of hidden triples.
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