ABSTRACT
We construct from Gaia eDR3 an extensive catalogue of spatially resolved binary stars within ≈1 kpc of the Sun, with projected separations ranging from a few au to 1 pc. We estimate the ...probability that each pair is a chance alignment empirically, using the Gaia catalogue itself to calculate the rate of chance alignments as a function of observables. The catalogue contains 1.3 (1.1) million binaries with >90 per cent (>99 per cent) probability of being bound, including 16 000 white dwarf – main-sequence (WD + MS) binaries and 1400 WD + WD binaries. We make the full catalogue publicly available, as well as the queries and code to produce it. We then use this sample to calibrate the published Gaia DR3 parallax uncertainties, making use of the binary components’ near-identical parallaxes. We show that these uncertainties are generally reliable for faint stars (G ≳ 18), but are underestimated significantly for brighter stars. The underestimates are generally $\leq30{{\ \rm per\ cent}}$ for isolated sources with well-behaved astrometry, but are larger (up to ∼80 per cent) for apparently well-behaved sources with a companion within ≲4 arcsec, and much larger for sources with poor astrometric fits. We provide an empirical fitting function to inflate published σϖ values for isolated sources. The public catalogue offers wide ranging follow-up opportunities: from calibrating spectroscopic surveys, to precisely constraining ages of field stars, to the masses and the initial–final mass relation of WDs, to dynamically probing the Galactic tidal field.
ABSTRACT We combine a catalogue of wide binaries constructed from Gaia DR2 with Fe/H abundances from wide-field spectroscopic surveys to quantify how the binary fraction varies with metallicity over ...separations 50 ≲ s/au ≲ 50 000. At a given distance, the completeness of the catalogue is independent of metallicity, making it straightforward to constrain intrinsic variation with Fe/H. The wide binary fraction is basically constant with Fe/H at large separations (s ≳ 250 au) but becomes quite rapidly anticorrelated with Fe/H at smaller separations: for 50 < s/au < 100, the binary fraction at $\rm Fe/H = -1$ exceeds that at $\rm Fe/H = 0.5$ by a factor of 3, an anticorrelation almost as strong as that found for close binaries with a < 10 au. Interpreted in terms of models where disc fragmentation is more efficient at low Fe/H, our results suggest that 100 < a/au < 200 is the separation below which a significant fraction of binaries formed via fragmentation of individual gravitationally unstable discs rather than through turbulent core fragmentation. We provide a public catalogue of 8407 binaries within 200 pc with spectroscopically determined Fe/H for at least one component.
ABSTRACT
We show that the radial velocity variable star in the black hole (BH) candidate NGC 1850 BH1 cannot be a normal ${\approx}5\hbox{-}{\rm M}_{\odot }$ subgiant, as was proposed, but is an ...overluminous stripped-envelope star with mass ≈1 M⊙. The result follows directly from the star’s observed radius and the orbital period–density relation for Roche lobe-filling stars: The star’s density, as constrained by the observed ellipsoidal variability, is too low for its mass to exceed ${\approx}1.5\, {\rm M}_{\odot }$. This lower mass significantly reduces the implied mass of the unseen companion and qualitative interpretation of the system, such that a normal main-sequence companion with mass $2.5\!-\!5\, {\rm M}_{\odot }$ is fully consistent with the data. We explore evolutionary scenarios that could produce the binary using mesa and find that its properties can be matched by models in which an ${\approx}5\hbox{-}{\rm M}_{\odot }$ primary loses most of its envelope to a companion and is observed in a bloated state before contracting to become a core helium burning sdOB star. This is similar to the scenario proposed to explain the binaries LB-1 and HR 6819. Though it likely does not contain a BH, NGC 1850 BH1 provides an interesting test case for binary evolution models, particularly given its membership in a cluster of known age.
ABSTRACT
We use a homogeneous catalogue 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 < M1/M⊙ < 2.5, mass ...ratios 0.1 ≲ q < 1, and separations $50 \lt s/{\rm au} \lt 50\, 000$. 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 1000–10 000 au, depending on primary mass. The excess is narrow: a steep increase in p(q) at 0.95 ≲ q < 1, with no significant excess at q ≲ 0.95. A range of tests confirm the signal is real, not a data artefact 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 ≳ 0.95) is constant over $0.01 \lt a/{\rm au} \lt 10\, 000$. 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 ≲ 100 au), likely via accretion from circumbinary discs, 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 ≲ q < 1. Besides changes in the twin fraction, p(q) is independent of separation at fixed primary mass over $100 \lesssim s/{\rm au} \lt 50\, 000$. It is flatter than expected for random pairings from the initial mass function but more bottom-heavy for wide binaries than for binaries with a ≲100 au.
We investigate the lowest-mass quiescent galaxies known to exist in isolated environments ( ; 1.5 Mpc from a more massive galaxy). This population may represent the lowest stellar mass galaxies in ...which internal feedback quenches galaxy-wide star formation. We present a Keck/Echelle Spectrograph and Imager long-slit spectroscopy for 27 isolated galaxies in this regime (20 quiescent galaxies and 7 star-forming galaxies). We measure emission line strengths as a function of radius and place galaxies on the Baldwin-Phillips-Terlevich (BPT) diagram. Remarkably, 16 of 20 quiescent galaxies in our sample host central active galactic nucleus (AGN)-like line ratios. Only five of these quiescent galaxies were identified as AGN-like in the Sloan Digital Sky Survey due to a lower spatial resolution and signal-to-noise ratio. We find that many of the quiescent galaxies in our sample have spatially extended emission across the non-star-forming regions of BPT-space. While quenched galaxies in denser environments in this mass range often show no evidence for AGN activity, a significant fraction of quiescent galaxies in isolation host AGNs despite their overall passive appearances.
We present an extensive and pure sample of ultrawide binary stars with separations of 0.01 s/pc 1 in the solar neighborhood. Using data from Gaia DR2, we define kinematic subpopulations via the ...systems' tangential velocities, i.e., disk-like (v ,tot ≤ 40 km s−1), intermediate (v ,tot = 40-85 km s−1), and halo-like (v ,tot ≥ 85 km s−1) binaries, presuming that these velocity cuts represent a rough ordering in the binaries' age and metallicity. Through stringent cuts on astrometric precision, we can obtain pure binary samples at such wide separations with thousands of binaries in each sample. Fitting a smoothly broken power law for the separation distribution, we find that its slope at s = 102.5-4 au is the same for all subpopulations, p(s) ∝ sγ with γ −1.54. However, the logarithmic slope of p(s) steepens at s 104 au. We find some evidences that the degree of steepening increases with the binaries' age, with a slope change of only Δγ 0.5 for disk-like stars, but Δγ 1 for halo-like stars. This trend is contrary to what might be expected if steepening at wide separations were due to gravitational perturbations by molecular clouds or stars, which would preferentially disrupt disk binaries. If we were to interpret steepening at s 104 au as a consequence of disruption by MAssive Compact Halo Objects (MACHOs), we would have to invoke a MACHO population inconsistent with other constraints. As a more plausible alternative, we propose a simple model to predict the separation distribution of wide binaries formed in dissolving star clusters. This model generically predicts γ −1.5 as observed, with steepening at larger separations due to the finite size of binaries' birth clusters.
Abstract
We present a new set of high-resolution hydrodynamic cosmological zoom-in simulations that apply the Feedback In Realistic Environments physics to both Local Group (LG)-like and isolated ...Milky Way (MW)-like volumes (10 host systems in total with a baryonic particle mass ${\simeq } 3500\!-\!7000\,{\rm M}_\odot$). We study the stellar mass functions, circular velocity or mass profiles, and velocity dispersions of the dwarf galaxy populations. The simulations reproduce the stellar mass function and central densities of MW satellite dwarfs for $M_\ast \ge 10^{5.5}\,{\rm M}_\odot$ and predict the existence of ∼3 unidentified galaxies with $M_\ast \sim 10^5\,{\rm M}_\odot$ within 300 kpc of the MW. Overall, we find no evidence for the classical missing satellites or too-big-to-fail (TBTF) problems for satellite galaxies in our sample. Among the satellites, TBTF is resolved primarily by subhalo disruption and overall mass-loss; central density profiles of subhaloes are of secondary importance. For non-satellite galaxies, our LG-like simulations predict as many as ∼10 as-of-yet unseen galaxies at distances $0.3\!-\!1\, \mathrm{Mpc}$ from both hosts, with $M_\ast \simeq 10^{5-6}\,{\rm M}_\odot$ (in haloes with Vmax ∼ 20 km s−1), albeit with large halo-to-halo variance. None of our simulations produces a compact, baryon-dominated, high-density dwarf elliptical-type galaxy (with Vcirc ≳ 35 km s−1 at $r\lt 1\,$ kpc), of which six may appear in the LG (but none in the MW). It may therefore remain a challenge to reproduce the full diversity of the dwarf population, including both the highest and lowest density systems.
ABSTRACT We examine the effects of stellar feedback and bursty star formation on low-mass galaxies (Mstar = 2 × 106 − 5 × 1010 M ) using the Feedback in Realistic Environments (FIRE) simulations. ...While previous studies emphasized the impact of feedback on dark matter profiles, we investigate the impact on the stellar component: kinematics, radial migration, size evolution, and population gradients. Feedback-driven outflows/inflows drive significant radial stellar migration over both short and long timescales via two processes: (1) outflowing/infalling gas can remain star-forming, producing young stars that migrate ∼1 kpc within their first 100 Myr, and (2) gas outflows/inflows drive strong fluctuations in the global potential, transferring energy to all stars. These processes produce several dramatic effects. First, galaxies' effective radii can fluctuate by factors of >2 over ∼200 Myr, and these rapid size fluctuations can account for much of the observed scatter in the radius at fixed Mstar. Second, the cumulative effects of many outflow/infall episodes steadily heat stellar orbits, causing old stars to migrate outward most strongly. This age-dependent radial migration mixes-and even inverts-intrinsic age and metallicity gradients. Thus, the galactic-archaeology approach of calculating radial star formation histories from stellar populations at z = 0 can be severely biased. These effects are strongest at Mstar 107-9.6 M , the same regime where feedback most efficiently cores galaxies. Thus, detailed measurements of stellar kinematics in low-mass galaxies can strongly constrain feedback models and test baryonic solutions to small-scale problems in ΛCDM.
A 33 M ⊙ black hole (BH) was recently discovered in an 11.6-year binary only 590 pc from the Sun. The system, Gaia BH3, contains a 0.8 M ⊙ low-metallicity giant () that is a member of the ED-2 ...stellar stream. This paper investigates whether the system could have formed via isolated binary evolution. I construct evolutionary models for metal-poor massive stars with initial masses ranging from 35 − 55 M ⊙ , which reach maximum radii of 1150 − 1800 R ⊙ as red supergiants. I then explore what combinations of initial orbit, mass loss, and natal kick can produce the period and eccentricity of Gaia BH3. Initial orbits wide enough to accommodate the BH progenitor as a red supergiant can match the observed period and eccentricity, but only if the BH formed with a significant natal kick (). These models are disfavored because such a kick would have ejected the binary from the ED-2 progenitor cluster. I conclude that Gaia BH3 likely formed through dynamical interactions, unless the BH progenitor did not expand to red supergiant dimensions. Only about 1 in 10,000 stars in the solar neighborhood have metallicities as low as Gaia BH3. This suggests that BH companions are dramatically over-represented at low-metallicity, though caveats related to small number statistics apply. The fact that the luminous star in Gaia BH3 has been a giant – greatly boosting its detectability – only for $$1% of the time since the system’s formation implies that additional massive BHs remain to be discovered with only moderately fainter companions. Both isolated and dynamically-formed BH binaries with orbits similar to Gaia BH3 are likely to be discovered in Gaia DR4.
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