255 research outputs found
New stellar encounters discovered in the second Gaia data release
Passing stars may play an important role in the evolution of our solar
system. We search for close stellar encounters to the Sun among all 7.2 million
stars in Gaia-DR2 that have six-dimensional phase space data. We characterize
encounters by integrating their orbits through a Galactic potential and
propagating the correlated uncertainties via a Monte Carlo resampling. After
filtering to remove spurious data, we find 694 stars that have median (over
uncertainties) closest encounter distances within 5 pc, all occurring within 15
Myr from now. 26 of these have at least a 50% chance of coming closer than 1 pc
(and 7 within 0.5 pc), all but one of which are newly discovered here. We
further confirm some and refute several other previously-identified encounters,
confirming suspicions about their data. The closest encounter in the sample is
Gl 710, which has a 95% probability of coming closer than 0.08 pc (17 000 AU).
Taking mass estimates from Gaia astrometry and multiband photometry for
essentially all encounters, we find that Gl 710 also has the largest impulse on
the Oort cloud. Using a Galaxy model, we compute the completeness of the
Gaia-DR2 encountering sample as a function of perihelion time and distance.
Only 15% of encounters within 5 pc occurring within +/- 5 Myr of now have been
identified, mostly due to the lack of radial velocities for faint and/or cool
stars. Accounting for the incompleteness, we infer the present rate of
encounters within 1 pc to be 19.7 +/- 2.2 per Myr, a quantity expected to scale
quadratically with the encounter distance out to at least several pc.
Spuriously large parallaxes in our sample from imperfect filtering would tend
to inflate both the number of encounters found and this inferred rate. The
magnitude of this effect is hard to quantify.Comment: 12 pages. Accepted to A&A. Added to this version: section 3.2 and
Fig. 8 (CMD) with discussion of astrometric quality metrics; full versions of
tables 2 and 3 as ancillary dat
Precise Ages of Field Stars from White Dwarf Companions
Observational tests of stellar and Galactic chemical evolution call for the
joint knowledge of a star's physical parameters, detailed element abundances,
and precise age. For cool main-sequence (MS) stars the abundances of many
elements can be measured from spectroscopy, but ages are very hard to
determine. The situation is different if the MS star has a white dwarf (WD)
companion and a known distance, as the age of such a binary system can then be
determined precisely from the photometric properties of the cooling WD. As a
pilot study for obtaining precise age determinations of field MS stars, we
identify nearly one hundred candidates for such wide binary systems: a faint WD
whose GPS1 proper motion matches that of a brighter MS star in Gaia/TGAS with a
good parallax (). We model the WD's multi-band
photometry with the BASE-9 code using this precise distance (assumed to be
common for the pair) and infer ages for each binary system. The resulting age
estimates are precise to () for () MS-WD systems.
Our analysis more than doubles the number of MS-WD systems with precise
distances known to date, and it boosts the number of such systems with precise
age determination by an order of magnitude. With the advent of the Gaia DR2
data, this approach will be applicable to a far larger sample, providing ages
for many MS stars (that can yield detailed abundances for over 20 elements),
especially in the age range 2 to 8\,\Gyr, where there are only few known star
clusters.Comment: 9 pages, 5 figures, 1 catalog; Submitted to Ap
Inferring the three-dimensional distribution of dust in the Galaxy with a non-parametric method: Preparing for Gaia
We present a non-parametric model for inferring the three-dimensional (3D)
distribution of dust density in the Milky Way. Our approach uses the extinction
measured towards stars at different locations in the Galaxy at approximately
known distances. Each extinction measurement is proportional to the integrated
dust density along its line-of-sight. Making simple assumptions about the
spatial correlation of the dust density, we can infer the most probable 3D
distribution of dust across the entire observed region, including along sight
lines which were not observed. This is possible because our model employs a
Gaussian Process to connect all lines-of-sight. We demonstrate the capability
of our model to capture detailed dust density variations using mock data as
well as simulated data from the Gaia Universe Model Snapshot. We then apply our
method to a sample of giant stars observed by APOGEE and Kepler to construct a
3D dust map over a small region of the Galaxy. Due to our smoothness constraint
and its isotropy, we provide one of the first maps which does not show the
"fingers of god" effect.Comment: Minor changes applied. Final version accepted for publication in A&A.
15 pages, 17 figure
Precise Ages of Field Stars from White Dwarf Companions
Observational tests of stellar and Galactic chemical evolution call for the joint knowledge of a star’s physical parameters, detailed element abundances, and precise age. For cool main-sequence (MS) stars the abundances of many elements can be measured from spectroscopy, but ages are very hard to determine. The situation is different if the MS star has a white dwarf (WD) companion and a known distance, as the age of such a binary system can then be determined precisely from the photometric properties of the cooling WD. As a pilot study for obtaining precise age determinations of field MS stars, we identify nearly one hundred candidate for such wide binary systems: a faint WD whose GPS1 proper motion matches that of a brighter MS star in Gaia/TGAS with a good parallax (σ ≤ 0.05). We model the WD’s multi-band photometry with the BASE-9 code using this precise distance (assumed to be common for the pair) and infer ages for each binary system. The resulting age estimates are precise to ≤ 10% (≤ 20%) for 42 (67) MS-WD systems. Our analysis more than doubles the number of MS-WD systems with precise distances known to date, and it boosts the number of such systems with precise age determination by an order of magnitude. With the advent of the Gaia DR2 data, this approach will be applicable to a far larger sample, providing ages for many MS stars (that can yield detailed abundances for over 20 elements), especially in the age range 2 to 8 Gyr, where there are only few known star clusters
Evidence of a dynamically evolving Galactic warp
In a cosmological setting, the disc of a galaxy is expected to continuously
experience gravitational torques and perturbations from a variety of sources,
which can cause the disc to wobble, flare and warp. Specifically, the study of
galactic warps and their dynamical nature can potentially reveal key
information on the formation history of galaxies and the mass distribution of
their halos. Our Milky Way presents a unique case study for galactic warps,
thanks to the detailed knowledge of its stellar distribution and kinematics.
Using a simple model of how the warp's orientation is changing with time, we
here measure the precession rate of the Milky Way's warp using 12 million giant
stars from Gaia Data Release 2, finding that it is precessing at km/s/kpc in the direction of Galactic rotation,
about one third the angular velocity at the Sun's position in the Galaxy. The
direction and magnitude of the warp's precession rate favour the scenario that
the warp is the results of a recent or ongoing encounter with a satellite
galaxy, rather than the relic of the ancient assembly history of the Galaxy.Comment: Published in Nature Astronomy. Final accepted version here:
https://www.nature.com/articles/s41550-020-1017-3 and full-text access via
SharedIt link: https://rdcu.be/b2ph
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