Discovery of an equal-mass "twin" binary population reaching 1000+ AU separations (1906.10128v2)

Abstract: 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 $q\lesssim 0.95$. 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 ($q\gtrsim 0.95$) 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 $a\lesssim$100 AU.

Overview of "Discovery of an Equal-Mass 'Twin' Binary Population Reaching 1000+ AU Separations"

The paper conducted by El-Badry et al. unveils the presence of a statistically robust population of equal-mass "twin" binary stars spanning unusually wide orbital separations, exceeding 1000 astronomical units (AU). Utilizing the extensive, high-precision dataset from the Gaia mission, the researchers have systematically cataloged approximately 42,000 wide binaries nearing the solar neighborhood. This meticulous analysis sheds light on the mass ratio distribution across a variety of separations and primary masses, primarily focusing on main-sequence stars within the mass range of 0.1 to 2.5 solar masses.

Detailed Insights and Numerical Strengths

The paper remarkably highlights a pronounced peak in the mass ratio distribution (referred to as $p(q)$) of wide binaries at nearly equal masses ($q \approx 0.95 - 1$), a phenomenon traditionally associated with much closer binaries. The researchers categorized the binaries into 35 statistically independent bins based on primary mass and separation, allowing them to accurately capture variation within these parameters.

The authors report a twin excess evident in all primary mass bins but with differing separation ranges. For instance, the excess in the $0.4 < M_1/M_{\odot} < 0.6$ mass bin is significant up to 15,000 AU with a twin fraction ($F_{\rm twin}$) peaking around 5%. Conversely, for solar-type stars, the twin excess is present but diminishes beyond 1,000 AU. This nuanced discovery of the wide-separation twin excess challenges the prevalent notion that strong mass correlation only occurs over small separations, prompting a reevaluation of traditional binary star formation theories.

Theoretical and Practical Implications

The findings indicate that the formation of these wide twins cannot be solely ascribed to traditional core fragmentation models, which inherently predict less constrained, more random mass correlations. Instead, the authors propose that these wide twin binaries initially formed at much closer separations - possibly within a circumbinary disk - and subsequently migrated outward due to dynamical interactions in their birth environments. Such a scenario postulates that the dynamics within young clusters play a significant role in altering the separation of these twin binaries, offering an intriguing window into the complexities of early stellar evolution and binary dynamics.

Future Prospects in Astrophysical Research

The implications of this paper extend beyond enhancing our understanding of binary star formation processes; it also sets a precedent for considering dynamical interactions in shaping binary populations. Future work could explore the environmental factors and specific conditions under which such dynamic widening occurs, integrating results from high-fidelity simulations of binary-star disk interactions.

In addition, as the precision of astrometric missions like Gaia improves, similar methodologies could be employed to investigate other stellar populations or environments, potentially revealing further anomalous features significant to the understanding of stellar systems' origins.

In conclusion, El-Badry et al.'s research offers a meticulous and data-rich examination of a new aspect of binary star configurations, proposing a compelling, albeit complex, mechanism that challenges conventional formation models. This work invites further theoretical exploration and simulation-based verification to validate the proposed mechanism of twin formation and examine its broader applicability across different stellar environments.