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Gaspard Duch^ene UC Berkeley
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Stellar multiplicity is an ubiquitous outcome of the star formation process. Characterizing the frequency and main characteristics of multiple systems and their dependencies on primary mass and environment is therefore a powerful tool to probe this process. While early attempts were fraught with selection biases and limited completeness, instrumentation breakthroughs in the last two decades now enable robust analyses. In this review, we summarize our current empirical knowledge of stellar multiplicity for Main Sequence stars and brown dwarfs, as well as among populations of Pre-Main Sequence stars and embedded protostars. Clear trends as a function of both primary mass and stellar evolutionary stage are identified that will serve as a comparison basis for numerical and analytical models of star formation.
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Our uncertainties about binary star systems (and triples and so on) limit our capabilities in literally every single one of the Thematic Areas identified for Astro2020. We need to understand the population statistics of stellar multiplicity and their variations with stellar type, chemistry, and dynamical environment: Correct interpretation of any exoplanet experiment depends on proper treatment of resolved and unresolved binaries; stellar multiplicity is a direct outcome of star and companion formation; the most precise constraints on stellar structure come from well-characterized binary systems; stellar populations heavily rely on stellar and binary evolution modeling; high-redshift galaxy radiation and reionization is controlled by binary-dependent stellar physics; compact objects are the outcomes of binary evolution; the interpretation of multi-messenger astronomy from gravitational waves, light, and neutrinos relies on understanding the products of binary star evolution; near-Universe constraints on the Hubble constant with Type Ia supernovae and gravitational-wave mergers are subject to systematics related to their binary star progenitors; local measures of dark-matter substructure masses are distorted by binary populations. In order to realize the scientific goals in each of these themes over the next decade, we therefore need to understand how binary stars and stellar multiplets are formed and distributed in the space of masses, composition, age, and orbital properties, and how the distribution evolves with time. This white paper emphasizes the interdisciplinary importance of binary-star science and advocates that coordinated investment from all astrophysical communities will benefit almost all branches of astrophysics.
We use the multi-epoch radial velocities acquired by the APOGEE survey to perform a large scale statistical study of stellar multiplicity for field stars in the Milky Way, spanning the evolutionary phases between the main sequence and the red clump. We show that the distribution of maximum radial velocity shifts (drvm) for APOGEE targets is a strong function of logg, with main sequence stars showing drvm as high as $sim$300 kms, and steadily dropping down to $sim$30 kms for logg$sim$0, as stars climb up the Red Giant Branch (RGB). Red clump stars show a distribution of drvm values comparable to that of stars at the tip of the RGB, implying they have similar multiplicity characteristics. The observed attrition of high drvm systems in the RGB is consistent with a lognormal period distribution in the main sequence and a multiplicity fraction of 0.35, which is truncated at an increasing period as stars become physically larger and undergo mass transfer after Roche Lobe Overflow during H shell burning. The drvm distributions also show that the multiplicity characteristics of field stars are metallicity dependent, with metal-poor ([Fe/H]$lesssim-0.5$) stars having a multiplicity fraction a factor 2-3 higher than metal-rich ([Fe/H]$gtrsim0.0$) stars. This has profound implications for the formation rates of interacting binaries observed by astronomical transient surveys and gravitational wave detectors, as well as the habitability of circumbinary planets.
According to theoretical considerations, multiplicity of hierarchical stellar systems can reach, depending on masses and orbital parameters, several hundred, while observational data confirm existence of at most septuple (seven-component) systems. In this study, we cross-match very high multiplicity (six and more components) stellar systems in modern catalogues of visual double and multiple stars, to find candidates to hierarchical systems among them. After cross-matching with catalogues of closer binaries (eclipsing, spectroscopic, etc.), some of their components were found to be binary/multiple themselves, which increases the systems degree of multiplicity. Optical pairs, known from literature or filtered by the authors, are flagged and excluded from the statistics. We have compiled a list of potentially very high multiplicity hierarchical systems that contains 10~objects. Their multiplicity does not exceed 12, and we discuss a number of ways to explain the lack of extremely high multiplicity systems.
We present a survey for the tightest visual binaries among 0.3-2 Msun members the Orion Nebula Cluster (ONC). Among 42 targets, we discovered 13 new 0.025-0.15 companions. Accounting for the Branch bias, we find a companion star fraction (CSF) in the 10-60 au range of 21+8/-5%, consistent with that observed in other star-forming regions (SFRs) and twice as high as among field stars; this excess is found with a high level of confidence. Since our sample is dominated by disk-bearing targets, this indicates that disk disruption by close binaries is inefficient, or has not yet taken place, in the ONC. The resulting separation distribution in the ONC drops sharply outside 60,au. These findings are consistent with a scenario in which the initial multiplicity properties, set by the star formation process itself, are identical in the ONC and in other SFRs and subsequently altered by the clusters dynamical evolution. This implies that the fragmentation process does not depend on the global properties of a molecular cloud, but on the local properties of prestellar cores, and that the latter are self-regulated to be nearly identical in a wide range of environments. These results, however, raise anew the question of the origin of field stars as the tight binaries we have discovered will not be destroyed as the ONC dissolves into the galactic field. It thus appears that most field stars formed in regions differ from well-studied SFRs in the Solar neighborhood, possibly due to changes in core fragmentation on Gyr timescales.
Context. Young loose nearby associations are unique samples of close (<150 pc), young (approx 5-100 Myr) pre-main sequence (PMS) stars. A significant number of members of these associations have been identified in the SACY collaboration. We can use the proximity and youth of these members to investigate key ingredients in star formation processes, such as multiplicity. Aims. We present the statistics of identified multiple systems from 113 confirmed SACY members. We derive multiplicity frequencies, mass-ratio, and physical separation distributions in a consistent parameter space, and compare our results to other PMS populations and the field. Methods. We have obtained adaptive-optics assisted near-infrared observations with NACO (ESO/VLT) and IRCAL (Lick Observatory) for at least one epoch of all 113 SACY members. We have identified multiple systems using co-moving proper-motion analysis and using contamination estimates. We have explored ranges in projected separation and mass-ratio of a [3-1000 au], and q [0.1-1], respectively. Results. We have identified 31 multiple systems (28 binaries and 3 triples). We derive a multiplicity frequency (MF) of MF_(3-1000au)=28.4 +4.7, -3.9% and a triple frequency (TF) of TF_(3-1000au)=2.8 +2.5, -0.8% in the separation range of 3-1000 au. We do not find any evidence for an increase in the MF with primary mass. The estimated mass-ratio of our statistical sample (with power-law index gamma=-0.04 +/- 0.14) is consistent with a flat distribution (gamma = 0). Conclusions. We show further similarities (but also hints of discrepancies) between SACY and the Taurus region: flat mass-ratio distributions and statistically similar MF and TF values. We also compared the SACY sample to the field (in the separation range of 19-100 au), finding that the two distributions are indistinguishable, suggesting a similar formation mechanism.
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