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Register a new userWide binaries are rare in open clusters
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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.
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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.
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.
Modelling dust formation in single stars evolving through the carbon-star stage of the asymptotic giant branch (AGB) reproduces well the mid-infrared colours and magnitudes of most of the C-rich sources in the Large Magellanic Cloud (LMC), apart from a small subset of extremely red objects (EROs). The analysis of EROs spectral energy distribution suggests the presence of large quantities of dust, which demand gas densities in the outflow significantly higher than expected from theoretical modelling. We propose that binary interaction mechanisms that involve common envelope (CE) evolution could be a possible explanation for these peculiar stars; the CE phase is favoured by the rapid growth of the stellar radius occurring after C$/$O overcomes unity. Our modelling of the dust provides results consistent with the observations for mass-loss rates $dot M sim 5times 10^{-4}~dot M/$yr, a lower limit to the rapid loss of the envelope experienced in the CE phase. We propose that EROs could possibly hide binaries of orbital periods $sim$days and are likely to be responsible for a large fraction of the dust production rate in galaxies.
Binary stars play a vital role in astrophysical research, as a good fraction of stars are in binaries. Binary fraction (BF) is known to change with stellar mass in the Galactic field, but such studies in clusters require binary identification and membership information. Here, we estimate the total and spectral-type-wise high mass-ratio (HMR) BF ($f^{0.6}$) in 23 open clusters using unresolved binaries in color-magnitude diagrams using textit{Gaia} DR2 data. We introduce the segregation index (SI) parameter to trace mass segregation of HMR (total and mass-wise) binaries and the reference population. This study finds that in open clusters, (1) HMR BF for the mass range 0.4--3.6 Msun (early M to late B type) has a range of 0.12 to 0.38 with a peak at 0.12--0.20, (2) older clusters have a relatively higher HMR BF, (3) the mass-ratio distribution is unlikely to be a flat distribution and BF(total) $sim$ (1.5 to 2.5) $times f^{0.6}$, (4) a decreasing BF(total) from late B-type to K-type, in agreement with the Galactic field stars, (5) older clusters show radial segregation of HMR binaries, (6) B and A/F type HMR binaries show radial segregation in some young clusters suggesting a primordial origin. This study will constrain the initial conditions and identify the major mechanisms that regulate binary formation in clusters. Primordial segregation of HMR binaries could result from massive clumps spatially segregated in the collapse phase of the molecular cloud.
Black hole-main sequence star (BH-MS) binaries are one of the targets of the future data releases of the astrometric satellite {it Gaia}. They are supposed to be formed in two main sites: a galactic field and star clusters. However, previous work has never predicted the number of BH-MS binaries originating in the latter site. In this paper, we estimate the number of BH-MS binaries formed in open clusters and detectable with {it Gaia} based on the results of {it N}-body simulations. By considering interstellar extinction in the Milky Way (MW) and observational constraints, we predict $sim 10$ BH-MS binaries are observable. We also find that chemical abundance patterns of companion MSs will help us to identify the origin of the binaries as star clusters. Such MSs are not polluted by outflows of the BH progenitors, such as stellar winds and supernova ejecta. Chemical anomalies might be a good test to confirm the origin of binaries with relatively less massive MSs ($lesssim 5M_{odot}$), orbital periods ($sim 1.5;$year) and higher eccentricities ($e gtrsim 0.1$).
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