5 research outputs found
The Age Distribution of Stellar Orbit Space Clumps
The orbit distribution of young stars in the Galactic disk is highly
structured, from well-defined clusters to streams of stars that may be widely
dispersed across the sky, but are compact in orbital action-angle space. The
age distribution of such groups can constrain the timescales over which
co-natal groups of stars disperse into the `field'. Gaia data have proven
powerful to identify such groups in action-angle space, but the resulting
member samples are often too small and have too narrow a CMD coverage to allow
robust age determinations. Here, we develop and illustrate a new approach that
can estimate robust stellar population ages for such groups of stars. This
first entails projecting the predetermined action-angle distribution into the
5D space of positions, parallaxes and proper motions, where much larger samples
of likely members can be identified over a much wider range of the CMD. It then
entails isochrone fitting that accounts for a) widely varying distances and
reddenings; b) outliers and binaries; c) sparsely populated main sequence
turn-offs, by incorporating the age information of the low-mass main sequence;
and d) the possible presence of an intrinsic age spread in the stellar
population. When we apply this approach to 92 nearby stellar groups identified
in 6D orbit space, we find that they are predominately young (
Gyr), mono-age populations. Many groups are established (known) localized
clusters with possible tidal tails, others tend to be widely dispersed and
manifestly unbound. This new age-dating tool offers a stringent approach to
understanding on which orbits stars form in the solar neighborhood and how
quickly they disperse into the field.Comment: Accepted for publication in Ap
A red giant orbiting a black hole
We report spectroscopic and photometric follow-up of a dormant black hole
(BH) candidate from Gaia DR3. We show that the system, which we call Gaia BH2,
contains a red giant and a dark companion with mass that is very likely a BH. The orbital period, days, is much longer than that of any previously studied BH
binary. Our radial velocity (RV) follow-up over a 6-month period spans most of
the orbit's dynamic range in RV and is in excellent agreement with predictions
of the Gaia solution. UV imaging and high-resolution optical spectra rule out
all plausible luminous companions that could explain the orbit. The star is a
bright (), slightly metal-poor () low-luminosity
giant (; ; ). The binary's orbit is moderately eccentric
(). The giant is strongly enhanced in elements, with , but the system's Galactocentric orbit is typical of the
thin disk. We obtained X-ray and radio nondetections of the source near
periastron, which support BH accretion models in which the net accretion rate
at the horizon is much lower than the Bondi-Hoyle-Lyttleton rate. At a distance
of 1.16 kpc, Gaia BH2 is the second-nearest known BH, after Gaia BH1. Its orbit
-- like that of Gaia BH1 -- seems too wide to have formed through common
envelope evolution. Gaia BH1 and BH2 have orbital periods at opposite edges of
the Gaia DR3 sensitivity curve, perhaps hinting at a bimodal intrinsic period
distribution for wide BH binaries. Dormant BH binaries like Gaia BH1 and Gaia
BH2 likely significantly outnumber their close, X-ray bright cousins, but their
formation pathways remain uncertain.Comment: 22 pages, 15 figures. Submitted to MNRA
The Age Distribution of Stellar Orbit Space Clumps
The orbit distribution of young stars in the Galactic disk is highly structured, from well-defined clusters to streams of stars that may be widely dispersed across the sky, but are compact in orbital action-angle space. The age distribution of such groups can constrain the timescales over which conatal groups of stars disperse into the âfield.â Gaia data have proven powerful in identifying such groups in action-angle space, but the resulting member samples are often too small and have too narrow a colorâmagnitude diagram (CMD) coverage to allow robust age determinations. Here, we develop and illustrate a new approach that can estimate robust stellar population ages for such groups of stars. This first entails projecting the predetermined action-angle distribution into the 5D space of positions, parallaxes, and proper motions, where much larger samples of likely members can be identified over a much wider range of the CMD. It then entails isochrone fitting that accounts for: (a) widely varying distances and reddenings; (b) outliers and binaries; (c) sparsely populated main-sequence turnoffs, by incorporating the age information of the low-mass main sequence; and (d) the possible presence of an intrinsic age spread in the stellar population. When we apply this approach to 92 nearby stellar groups identified in 6D orbit space, we find that they are predominantly young (âČ1 Gyr), mono-age populations. Many groups are established (known) localized clusters with possible tidal tails, while others tend to be widely dispersed and manifestly unbound. This new age-dating tool offers a stringent approach to understanding on which orbits stars form in the solar neighborhood and how quickly they disperse into the field
A Sun-like star orbiting a black hole
International audienceWe report discovery of a bright, nearby () Sun-like star orbiting a dark object. We identified the system as a black hole candidate via its astrometric orbital solution from the Gaia mission. Radial velocities validated and refined the Gaia solution, and spectroscopy ruled out significant light contributions from another star. Joint modeling of radial velocities and astrometry constrains the companion mass to M2 = 9.62 ± 0.18 Mâ. The spectroscopic orbit alone sets a minimum companion mass of M2 > 5 Mâ; if the companion were a 5 Mâ star, it would be 500 times more luminous than the entire system. These constraints are insensitive to the mass of the luminous star, which appears as a slowly-rotating G dwarf (, log g = 4.5, M = 0.93 Mâ), with near-solar metallicity () and an unremarkable abundance pattern. We find no plausible astrophysical scenario that can explain the orbit and does not involve a black hole. The orbital period, Porb = 185.6 days, is longer than that of any known stellar-mass black hole binary. The system's modest eccentricity (e = 0.45), high metallicity, and thin-disk Galactic orbit suggest that it was born in the Milky Way disk with at most a weak natal kick. How the system formed is uncertain. Common envelope evolution can only produce the system's wide orbit under extreme and likely unphysical assumptions. Formation models involving triples or dynamical assembly in an open cluster may be more promising. This is the nearest known black hole by a factor of 3, and its discovery suggests the existence of a sizable population of dormant black holes in binaries. Future Gaia releases will likely facilitate the discovery of dozens more