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Register a new userDiscovery of an equal-mass twin binary population reaching 1000+ AU separations
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We use a homogeneous catalog of 42,000 main-sequence wide binaries identified by Gaia to measure the mass ratio distribution, p(q), of binaries with primary masses $0.1<M_1/M_{odot}<2.5$, mass ratios $0.1 lesssim q<1$, and separations $50<s/{rm AU}<50,000$. A well-understood selection function allows us to constrain p(q) in 35 independent bins of primary mass and separation, with hundreds to thousands of binaries in each bin. Our investigation reveals a sharp excess of equal-mass twin binaries that is statistically significant out to separations of 1,000 to 10,000 AU, depending on primary mass. The excess is narrow: a steep increase in p(q) at $0.95 lesssim q<1$, with no significant excess at $qlesssim 0.95$. A range of tests confirm the signal is real, not a data artifact or selection effect. Combining the Gaia constraints with those from close binaries, we show that the twin excess decreases with increasing separation, but its width ($qgtrsim 0.95$) is constant over $0.01<a/{rm AU}<10,000$. The wide twin population would be difficult to explain if the components of all wide binaries formed via core fragmentation, which is not expected to produce strongly correlated component masses. We conjecture that wide twins formed at closer separations ($a lesssim 100$ AU), likely via accretion from circumbinary disks, and were subsequently widened by dynamical interactions in their birth environments. The separation-dependence of the twin excess then constrains the efficiency of dynamical widening and disruption of binaries in young clusters. We also constrain p(q) across $0.1 lesssim q<1$. Besides changes in the twin fraction, p(q) is independent of separation at fixed primary mass over $100 lesssim s/{rm AU} < 50,000$. It is flatter than expected for random pairings from the IMF but more bottom-heavy for wide binaries than for binaries with $alesssim$100 AU.
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Binary stars play a vital role in astrophysical research, as a good fraction of stars are in binaries. Binary fraction (BF) is known to change with stellar mass in the Galactic field, but such studies in clusters require binary identification and membership information. Here, we estimate the total and spectral-type-wise high mass-ratio (HMR) BF ($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.
Although many models have been proposed, the physical mechanisms responsible for the formation of low-mass brown dwarfs are poorly understood. The multiplicity properties and minimum mass of the brown-dwarf mass function provide critical empirical diagnostics of these mechanisms. We present the discovery via gravitational microlensing of two very low-mass, very tight binary systems. These binaries have directly and precisely measured total system masses of 0.025 Msun and 0.034 Msun, and projected separations of 0.31 AU and 0.19 AU, making them the lowest-mass and tightest field brown-dwarf binaries known. The discovery of a population of such binaries indicates that brown dwarf binaries can robustly form at least down to masses of ~0.02 Msun. Future microlensing surveys will measure a mass-selected sample of brown-dwarf binary systems, which can then be directly compared to similar samples of stellar binaries.
We present a near-IR survey for the visual multiples in the Orion molecular clouds region at separations between 100 and 1000 AU. These data were acquired at 1.6~$mu$m with the NICMOS and WFC3 cameras on the Hubble Space Telescope. Additional photometry was obtained for some of the sources at 2.05~$mu$m with NICMOS and in the $L$-band with NSFCAM2 on the IRTF. Towards 129 protostars and 197 pre-main sequence stars with disks observed with WFC3, we detect 21 and 28 candidate companions between the projected separations of 100---1000 AU, of which less than 5 and 8, respectively, are chance line of sight coincidences. The resulting companion fraction ($CF$) after the correction for the line of sight contamination is 14.4$^{+1.1}_{-1.3}$% for protostars and 12.5$^{+1.2}_{-0.8}$% for the pre-main sequence stars. These values are similar to those found for main sequence stars, suggesting that there is little variation in the $CF$ with evolution, although several observational biases may mask a decrease in the $CF$ from protostars to the main sequence stars. After segregating the sample into two populations based on the surrounding surface density of YSOs, we find that the $CF$ in the high stellar density regions ($Sigma_{YSO} > 45$~pc$^{-2}$) is approximately 50% higher than that found in the low stellar density regions ($Sigma_{YSO} < 45$~pc$^{-2}$). We interpret this as evidence for the elevated formation of companions at 100 to 1000 AU in the denser environments of Orion. We discuss possible reasons for this elevated formation.
Stars like our Sun form in self-gravitating dense and cold structures within interstellar clouds, called pre-stellar cores. Although much is known about the physical structure of dense clouds just before and soon after the switch-on of a protostar, the central few thousand astronomical units (au) of pre-stellar cores are unexplored. It is within these central regions that stellar systems assemble and fragmentation may take place, with the consequent formation of binaries and multiple systems. We present ALMA Band 6 observations (ACA and 12m array) of the dust continuum emission of the 8 Msun pre-stellar core L1544, with angular resolution of 2 x 1.6 (linear resolution 270 au x 216 au). Within the primary beam, a compact region of 0.1 Msun, which we call a kernel, has been unveiled. The kernel is elongated, with a central flat zone with radius Rker ~ 10 (~ 1400 au). The average number density within Rker is ~1 x 10^6 cm^{-3}, with possible local density enhancements. The region within Rker appears to have fragmented, but detailed analysis shows that similar substructure can be reproduced by synthetic interferometric observations of a smooth centrally concentrated dense core with a similar central flat zone. The presence of a smooth kernel within a dense core is in agreement with non-ideal magneto-hydro-dynamical simulations of a contracting cloud core with a peak number density of 1 x 10^7 cm^{-3}. Dense cores with lower central densities are completely filtered out when simulated 12m-array observations are carried out. These observations demonstrate that the kernel of dynamically evolved dense cores can be investigated at high angular resolution with ALMA.
The carbon-enhanced metal-poor (CEMP) stars constitute approximately one fifth of the metal-poor ([Fe/H] ~< -2) population but their origin is not well understood. The most widely accepted formation scenario, invokes mass-transfer of carbon-rich material from a thermally-pulsing asymptotic giant branch (TPAGB) primary star to a less massive main-sequence companion which is seen today. Recent studies explore the possibility that an initial mass function biased toward intermediate-mass stars is required to reproduce the observed CEMP fraction in stars with metallicity [Fe/H] < -2.5. These models also implicitly predict a large number of nitrogen-enhanced metal-poor (NEMP) stars which is not seen. We investigate whether the observed CEMP and NEMP to extremely metal-poor (EMP) ratios can be explained without invoking a change in the initial mass function. We confirm earlier findings that with current detailed TPAGB models the large observed CEMP fraction cannot be accounted for. We find that efficient third dredge up in low-mass (less than 1.25Msun), low-metallicity stars may offer at least a partial explanation to the large observed CEMP fraction while remaining consistent with the small observed NEMP fraction.
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