Tidally disrupted globular clusters are transformed into thin, dynamically cold streams of stars that are extremely valuable tracers of the large- and small-scale distribution of mass in the Galaxy. ...Using data from the Gaia second data release combined with Pan-STARRS photometry, we present a sample of highly probable members of the longest cold stream in the Milky Way, GD-1. The resulting map of GD-1: (1) extends the apparent length of the stream by 20°, (2) reveals plausible locations for the progenitor, (3) detects high-contrast gaps along the stream, and (4) indicates the existence of stream members perturbed off the main stream track. These discoveries are only possible because of the exquisite astrometry from Gaia, which permits a clean separation of the stream from Milky Way stars. The additional length and a proper treatment of the progenitor will aid in dynamical modeling of GD-1 for mapping the large-scale dark matter distribution. The complex morphology of the stream points to a turbulent history; detailed phase-space properties of the perturbed stream members could potentially constrain dark matter substructure in the Milky Way.
Aims. Several kinematic and chemical substructures have been recently found amongst Milky Way halo stars with retrograde motions. It is currently unclear how these various structures are related to ...each other. This Letter aims to shed light on this issue. Methods. We explore the retrograde halo with an augmented version of the Gaia DR2 RVS sample, extended with data from three large spectroscopic surveys, namely RAVE, APOGEE, and LAMOST. In this dataset, we identify several structures using the HDBSCAN clustering algorithm. We discuss their properties and possible links using all the available chemical and dynamical information. Results. In concordance with previous work, we find that stars with Fe/H < −1 have more retrograde motions than those with Fe/H > −1. The retrograde halo contains a mixture of debris from objects like Gaia-Enceladus, Sequoia, and even the chemically defined thick disc. We find that the Sequoia has a smaller range in orbital energies than previously suggested and is confined to high energy. Sequoia could be a small galaxy in itself, but since it overlaps both in integrals-of-motion space and chemical abundance space with the less bound debris of Gaia-Enceladus, its nature cannot yet be fully settled. In the low-energy part of the halo, we find evidence for at least one more distinct structure: Thamnos. Stars in Thamnos are on low-inclination, mildly eccentric retrograde orbits, moving at vϕ ≈ −150 km s−1, and are chemically distinct from the other structures. Conclusions. Even with the excellent Gaia DR2 data, piecing together all the fragments found in the retrograde halo remains challenging. At this point, we are very much in need of large datasets with high-quality high-resolution spectra and tailored high-resolution hydrodynamical simulations of galaxy mergers.
We present a model for the interaction of the GD-1 stellar stream with a massive perturber that naturally explains many of the observed stream features, including a gap and an off-stream spur of ...stars. The model involves an impulse by a fast encounter, after which the stream grows a loop of stars at different orbital energies. At specific viewing angles, this loop appears offset from the stream track. A quantitative comparison of the spur-and-gap features prefers models where the perturber is in the mass range of 106-108 M . Orbit integrations back in time show that the stream encounter could not have been caused by any known globular cluster or dwarf galaxy with a determined orbit, and mass, size, and impact parameter arguments show that it could not have been caused by a molecular cloud in the Milky Way disk. The most plausible explanation for the gap-and-spur structure is an encounter with a dark matter substructure, like those predicted to populate galactic halos in ΛCDM cosmology. However, the expected densities of ΛCDM subhalos in this mass range and in this part of the Milky Way are 2 -3 lower than the inferred density of the GD-1 perturber. This observation opens up the possibility that detailed observations of streams could measure the mass spectrum of dark matter substructures and even identify individual substructures and their orbits in the Galactic halo.
Short-period binary star systems dissipate orbital energy through tidal interactions that lead to tighter, more circular orbits. Using a sample of binaries with subgiant, giant, and red clump star ...members that is nearly an order of magnitude larger than that of Verbunt & Phinney, we reexamine predictions for tidal circularization of binary stars with evolved members. We confirm that binary star systems in our sample predicted to have circular orbits (using equilibrium tide theory) generally have negligible measured eccentricities. At a fixed stellar mass, the transition period is correlated with the surface gravity (i.e., size) of the evolved member, indicating that the circularization timescale must be shorter than the evolutionary timescale along the giant branch. A few exceptions to the conclusions above are mentioned in the discussion. Some of these exceptions are likely systems in which the spectrum of the secondary biases the radial velocity measurements, but four appear to be genuine, short-period, moderate-eccentricity systems.
Multi-epoch radial velocity measurements of stars can be used to identify stellar, substellar, and planetary-mass companions. Even a small number of observation epochs can be informative about ...companions, though there can be multiple qualitatively different orbital solutions that fit the data. We have custom-built a Monte Carlo sampler (The Joker) that delivers reliable (and often highly multimodal) posterior samplings for companion orbital parameters given sparse radial velocity data. Here we use The Joker to perform a search for companions to 96,231 red giant stars observed in the APOGEE survey (DR14) with ≥3 spectroscopic epochs. We select stars with probable companions by making a cut on our posterior belief about the amplitude of the variation in stellar radial velocity induced by the orbit. We provide (1) a catalog of 320 companions for which the stellar companion's properties can be confidently determined, (2) a catalog of 4898 stars that likely have companions, but would require more observations to uniquely determine the orbital properties, and (3) posterior samplings for the full orbital parameters for all stars in the parent sample. We show the characteristics of systems with confidently determined companion properties and highlight interesting systems with candidate compact object companions.
Abstract
Collisions and interactions between gas-rich galaxies are thought to be pivotal stages in their formation and evolution, causing the rapid production of new stars, and possibly serving as a ...mechanism for fueling supermassive black holes (BHs). Harnessing the exquisite spatial resolution (∼0${^{\prime\prime}_{.}}$5) afforded by the first ∼170 deg2 of the Hyper Suprime-Cam (HSC) survey, we present our new constraints on the importance of galaxy–galaxy major mergers (1 : 4) in growing BHs throughout the last ∼8 Gyr. Utilizing mid-infrared observations in the WISE all-sky survey, we robustly select active galactic nuclei (AGN) and mass-matched control galaxy samples, totaling ∼140000 spectroscopically confirmed systems at i < 22 mag. We identify galaxy interaction signatures using a novel machine-learning random forest decision tree technique allowing us to select statistically significant samples of major mergers, minor mergers / irregular systems, and non-interacting galaxies. We use these samples to show that galaxies undergoing mergers are a factor of ∼2–7 more likely to contain luminous obscured AGN than non-interacting galaxies, and this is independent of both stellar mass and redshift to z < 0.9. Furthermore, based on our comparison of AGN fractions in mass-matched samples, we determine that the most luminous AGN population (LAGN ≳ 1045 erg s−1) systematically reside in merging systems over non-interacting galaxies. Our findings show that galaxy–galaxy interactions do, on average, trigger luminous AGN activity substantially more often than in secularly evolving non-interacting galaxies, and we further suggest that the BH growth rate may be closely tied to the dynamical time of the merger system.
We report and discuss the discovery of a significant difference in the chemical abundances of a comoving pair of bright solar-type stars, HD 240430 and HD 240429. The two stars have an estimated 3D ...separation of 0.6 pc ( 0.01 pc projected) at a distance of r 100 pc with nearly identical 3D velocities, as inferred from Gaia TGAS parallaxes and proper motions, and high-precision radial velocity measurements. Stellar parameters determined from high-resolution spectra obtained with the High Resolution Echelle Spectrometer (HIRES) at the Keck Observatory indicate that the two stars are ∼4 Gyr old. The more metal-rich of the two, HD 240430, shows an enhancement of refractory ( K) elements by 0.2 dex and a marginal enhancement of (moderately) volatile elements ( K; , , , , and ). This is the largest metallicity difference found in a wide binary pair to date. Additionally, HD 240430 shows an anomalously high surface lithium abundance ( ), higher than its cooler companion by 0.5 dex. The proximity in phase-space and ages between the two stars suggests that they formed together with the same composition, which is at odds with the observed differences in metallicity and abundance patterns. We therefore suggest that the star HD 240430, "Kronos," accreted of rocky material after birth, selectively enhancing the refractory elements as well as lithium in its surface and convective envelope.
Abstract
The gas content of the complete compilation of Local Group dwarf galaxies (119 within 2 Mpc) is presented using H
i
survey data. Within the virial radius of the Milky Way (224 kpc here), 53 ...of 55 dwarf galaxies are devoid of gas to limits of
M
H
i
< 10
4
M
⊙
. Within the virial radius of M31 (266 kpc), 27 of 30 dwarf galaxies are devoid of gas (with limits typically <10
5
M
⊙
). Beyond the virial radii of the Milky Way and M31, the majority of the dwarf galaxies have detected H
i
gas and H
i
masses higher than the limits. When the relationship between gas content and distance is investigated using a Local Group virial radius, more of the nondetected dwarf galaxies are within this radius (85 ± 1 of the 93 nondetected dwarf galaxies) than within the virial radii of the Milky Way and M31. Using the Gaia proper-motion measurements available for 38 dwarf galaxies, the minimum gas density required to completely strip them of gas is calculated. Halo densities between 10
−5
and 5 × 10
−4
cm
−3
are typically required for instantaneous stripping at perigalacticon. When compared to halo density with radius expectations from simulations and observations, 80% of the dwarf galaxies with proper motions are consistent with being stripped by ram pressure at Milky Way pericenter. The results suggest that a diffuse gaseous galactic halo medium is important in quenching dwarf galaxies, and that a Local Group medium also potentially plays a role.
Theoretical models of stellar evolution predict that most of the lithium inside a star is destroyed as the star becomes a red giant. However, observations reveal that about 1% of red giants are ...peculiarly rich in lithium, often exceeding the amount in the interstellar medium or predicted from the big bang. With only about 150 lithium-rich giants discovered in the past four decades, and no distinguishing properties other than lithium enhancement, the origin of lithium-rich giant stars is one of the oldest problems in stellar astrophysics. Here we report the discovery of 2330 low-mass (1-3 M ) lithium-rich giant stars, which we argue are consistent with internal lithium production that is driven by tidal spin-up by a binary companion. Our sample reveals that most lithium-rich giants have helium-burning cores ( ), and that the frequency of lithium-rich giants rises with increasing stellar metallicity. We find that while planet accretion may explain some lithium-rich giants, it cannot account for the majority that have helium-burning cores. We rule out most other proposed explanations for the origin of lithium-rich giants. Our analysis shows that giants remain lithium-rich for only about two million years. A prediction from this lithium depletion timescale is that most lithium-rich giants with a helium-burning core have a binary companion.
The primary sample of the Gaia Data Release 1 is the Tycho-Gaia Astrometric Solution (TGAS): 2 million Tycho-2 sources with improved parallaxes and proper motions relative to the initial catalog. ...This increased astrometric precision presents an opportunity to find new binary stars and moving groups. We search for high-confidence comoving pairs of stars in TGAS by identifying pairs of stars consistent with having the same 3D velocity using a marginalized likelihood ratio test to discriminate candidate comoving pairs from the field population. Although we perform some visualizations using (bias-corrected) inverse parallax as a point estimate of distance, the likelihood ratio is computed with a probabilistic model that includes the covariances of parallax and proper motions and marginalizes the (unknown) true distances and 3D velocities of the stars. We find 13,085 comoving star pairs among 10,606 unique stars with separations as large as 10 pc (our search limit). Some of these pairs form larger groups through mutual comoving neighbors: many of these pair networks correspond to known open clusters and OB associations, but we also report the discovery of several new comoving groups. Most surprisingly, we find a large number of very wide ( pc) separation comoving star pairs, the number of which increases with increasing separation and cannot be explained purely by false-positive contamination. Our key result is a catalog of high-confidence comoving pairs of stars in TGAS. We discuss the utility of this catalog for making dynamical inferences about the Galaxy, testing stellar atmosphere models, and validating chemical abundance measurements.
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