Stellar-mass black holes in the Hyades star cluster?

Astrophysical models of binary-black hole mergers in the Universe require a significant fraction of stellar-mass black holes (BHs) to receive negligible natal kicks to explain the gravitational wave detections. This implies that BHs should be retained even in open clusters with low escape velocities ($\lesssim1~\mathrm{km \, s^{-1}}$). We search for signatures of the presence of BHs in the nearest open cluster to the Sun - the Hyades - by comparing density profiles of direct $N$-body models to data from $Gaia$. The observations are best reproduced by models with $2-3$ BHs at present. Models that never possessed BHs have an half-mass radius $\sim30\%$ smaller than the observed value, while those where the last BHs were ejected recently ($\lesssim150~$Myr ago) can still reproduce the density profile. In 50% of the models hosting BHs, we find BHs with stellar companion(s). Their period distribution peaks at $\sim10^3$ yr, making them unlikely to be found through velocity variations. We look for potential BH companions through large $Gaia$ astrometric and spectroscopic errors, identifying 56 binary candidates - none of which consistent with a massive compact companion. Models with $2-3$ BHs have an elevated central velocity dispersion, but observations can not yet discriminate. We conclude that the present-day structure of the Hyades requires a significant fraction of BHs to receive natal kicks smaller than the escape velocity of $\sim 3\, \mathrm{km \, s^{-1}}$ at the time of BH formation and that the nearest BHs to the Sun are in, or near, Hyades.Comment: 20 pages, 15 figures, 5 tables. Accepted for publication in MNRAS. Comments welcom

Radial velocities from Gaia BP/RP spectra

The Gaia mission has provided us full astrometric solutions for over $1.5$B sources. However, only the brightest 34M of those have radial velocity measurements. As a proof of concept, this paper aims to close that gap, by obtaining radial velocity estimates from the low-resolution BP/RP spectra that Gaia now provides. These spectra are currently published for about 220M sources, with this number increasing to the full $\sim 2$B Gaia sources with Gaia Data Release 4. To obtain the radial velocity measurements, we fit Gaia BP/RP spectra with models based on a grid of synthetic spectra, with which we obtain the posterior probability on the radial velocity for each object. Our measured velocities show systematic biases that depend mainly on colours and magnitudes of stars. We correct for these effects by using external catalogues of radial velocity measurements. We present in this work a catalogue of about $6.4$M sources with our most reliable radial velocity measurements and uncertainties $<300$ km s$^{-1}$ obtained from the BP/RP spectra. About 23% of these have no previous radial velocity measurement in Gaia RVS. Furthermore, we provide an extended catalogue containing all 125M sources for which we were able to obtain radial velocity measurements. The latter catalogue, however, also contains a fraction of measurements for which the reported radial velocities and uncertainties are inaccurate. Although typical uncertainties in the catalogue are significantly higher compared to those obtained with precision spectroscopy instruments, the number of potential sources for which this method can be applied is orders of magnitude higher than any previous radial velocity catalogue. Further development of the analysis could therefore prove extremely valuable in our understanding of Galactic dynamics.Comment: 14 pages, 17 figures, submitted to A&A, comments welcom

Astrobites as a Community-led Model for Education, Science Communication, and Accessibility in Astrophysics

Support for early career astronomers who are just beginning to explore astronomy research is imperative to increase retention of diverse practitioners in the field. Since 2010, Astrobites has played an instrumental role in engaging members of the community -- particularly undergraduate and graduate students -- in research. In this white paper, the Astrobites collaboration outlines our multi-faceted online education platform that both eases the transition into astronomy research and promotes inclusive professional development opportunities. We additionally offer recommendations for how the astronomy community can reduce barriers to entry to astronomy research in the coming decade

The Astrometric Contribution of Unresolved Stellar Companions

No astronomical body is alone - some fraction of the mass of a system will always be found in orbiting bodies, ranging from fragmented debris to planets, stars and compact objects. In general we ‘see’ the brighter component: one star often dominates the detectable light, obscuring the companion - and we must turn to indirect methods to infer its presence. Astrometry, the mapping of the motion of stars across the sky, is one such method. A significant mass concentrated in one or more collapsed companions will perturb the path. In the simplest and most ubiquitous case, a binary system, this perturbation is regular and easily described - a metronome beat added to the celestial motion. However, the combination of a star’s proper and parallactic motion with a binary orbit is non-linear, biasing our measurements of the system’s distance and motion and introducing extra astrometric noise. In this thesis we will detail the astrometric impact of a binary companion, how it changes our inferences about systems assumed to be single bodies, and how we can detect companions through the deviations from single body fits. This is especially relevant to the current and ongoing Gaia survey, which is building astrometric tracks at unprecedented (sub milli-arcsecond) precision and for the first time allowing us to detect a large number of unresolved but astrometrically significant binary systems. We present analytic expressions for the binary contribution, as well as an exploration of the behavior of simulated systems and detections of real binary candidates in the local galaxy