333 research outputs found
Gaia astrometric science performance - post-launch predictions
The standard errors of the end-of-mission Gaia astrometry have been
re-assessed after conclusion of the in-orbit commissioning phase of the
mission. An analytical relation is provided for the parallax standard error as
function of Gaia G magnitude (and V-I colour) which supersedes the pre-launch
relation provided in de Bruijne (2012).Comment: To be published in the proceedings of the GREAT-ITN conference "The
Milky Way Unravelled by Gaia: GREAT Science from the Gaia Data Releases", 1-5
December 2014, University of Barcelona, Spain, EAS Publications Series, eds
Nicholas Walton, Francesca Figueras, and Caroline Soubira
The intricate Galaxy disk: velocity asymmetries in Gaia-TGAS
We use the Gaia-TGAS data to compare the transverse velocities in Galactic
longitude (coming from proper motions and parallaxes) in the Milky Way disk for
negative and positive longitudes as a function of distance. The transverse
velocities are strongly asymmetric and deviate significantly from the
expectations for an axisymmetric Galaxy. The value and sign of the asymmetry
changes at spatial scales of several tens of degrees in Galactic longitude and
about 0.5 kpc in distance. The asymmetry is statistically significant at 95%
confidence level for 57% of the region probed, which extends up to ~1.2 kpc. A
percentage of 24% of the region studied shows absolute differences at this
confidence level larger than 5 km/s and 7% larger than 10 km/s. The asymmetry
pattern shows mild variations in the vertical direction and with stellar type.
A first qualitative comparison with spiral arm models indicates that the arms
are unlikely to be the main source of the asymmetry. We briefly discuss
alternative origins. This is the first time that global all-sky asymmetries are
detected in the Milky Way kinematics, beyond the local neighbourhood, and with
a purely astrometric sample.Comment: Accepted for publication in A&A Letter
Understanding the spiral structure of the Milky Way using the local kinematic groups
We study the spiral arm influence on the solar neighbourhood stellar
kinematics. As the nature of the Milky Way (MW) spiral arms is not completely
determined, we study two models: the Tight-Winding Approximation (TWA) model,
which represents a local approximation, and a model with self-consistent
material arms named PERLAS. This is a mass distribution with more abrupt
gravitational forces. We perform test particle simulations after tuning the two
models to the observational range for the MW spiral arm properties. We explore
the effects of the arm properties and find that a significant region of the
allowed parameter space favours the appearance of kinematic groups. The
velocity distribution is mostly sensitive to the relative spiral arm phase and
pattern speed. In all cases the arms induce strong kinematic imprints for
pattern speeds around 17 km/s/kpc (close to the 4:1 inner resonance) but no
substructure is induced close to corotation. The groups change significantly if
one moves only ~0.6 kpc in galactocentric radius, but ~2 kpc in azimuth. The
appearance time of each group is different, ranging from 0 to more than 1 Gyr.
Recent spiral arms can produce strong kinematic structures. The stellar
response to the two potential models is significantly different near the Sun,
both in density and kinematics. The PERLAS model triggers more substructure for
a larger range of pattern speed values. The kinematic groups can be used to
reduce the current uncertainty about the MW spiral structure and to test
whether this follows the TWA. However, groups such as the observed ones in the
solar vicinity can be reproduced by different parameter combinations. Data from
velocity distributions at larger distances are needed for a definitive
constraint.Comment: 18 pages, 21 figures, 4 tables; acccepted for publication in MNRA
Riding the kinematic waves in the Milky Way disk with Gaia
Gaia DR2 has delivered full-sky 6-D measurements for millions of stars, and
the quest to understand the dynamics of our Galaxy has entered a new phase. Our
aim is to reveal and characterize the kinematic sub-structure of the different
Galactic neighbourhoods, to form a picture of their spatial evolution that can
be used to infer the Galactic potential, its evolution and its components. We
take ~5 million stars in the Galactic disk from the Gaia DR2 catalogue and
build the velocity distribution of many different Galactic Neighbourhoods
distributed along 5 kpc in Galactic radius and azimuth. We decompose their
distribution of stars in the V_R-V_phi plane with the wavelet transformation
and asses the statistical significance of the structures found. We detect many
kinematic sub-structures (arches and more rounded groups) that diminish their
azimuthal velocity as a function of Galactic radius in a continuous way,
connecting volumes up to 3 kpc apart in some cases. The decrease rate is, on
average, of ~23 km/s/kpc. In azimuth, the kinematic sub-structures present much
smaller variations. We also observe a duality in their behaviour: some conserve
their vertical angular momentum with radius (e.g., Hercules), while some seem
to have nearly constant kinetic energy (e.g., Sirius). These two trends are
consistent with the approximate predictions of resonances and of phase mixing,
respectively. Besides, the overall spatial evolution of Hercules is consistent
with being related to the Outer Lindblad Resonance of the Bar. We also detect
structures without apparent counterpart in the vicinity of the Sun. The various
trends observed and their continuity with radius and azimuth allows for future
work to deeply explore the parameter space of the models. Also, the
characterization of extrasolar moving groups opens the opportunity to expand
our understanding of the Galaxy beyond the Solar Neighbourhood.Comment: 16 pages. Submitted to Astronomy and Astrophysics on 24th of May,
2018. Related on-line material available (see Appendix B
The evolution of the Sun's birth cluster and the search for the solar siblings with Gaia
We use self-consistent numerical simulations of the evolution and disruption
of the Sun's birth cluster in the Milky Way potential to investigate the
present-day phase space distribution of the Sun's siblings. The simulations
include the gravitational N-body forces within the cluster and the effects of
stellar evolution on the cluster population. In addition the gravitational
forces due to the Milky Way potential are accounted for in a self-consistent
manner. Our aim is to understand how the astrometric and radial velocity data
from the Gaia mission can be used to pre-select solar sibling candidates. We
vary the initial conditions of the Sun's birth cluster, as well as the
parameters of the Galactic potential. We show that the disruption time-scales
of the cluster are insensitive to the details of the non-axisymmetric
components of the Milky Way model and we make predictions, averaged over the
different simulated possibilities, about the number of solar siblings that
should appear in surveys such as Gaia or GALAH. We find a large variety of
present-day phase space distributions of solar siblings, which depend on the
cluster initial conditions and the Milky Way model parameters. We show that
nevertheless robust predictions can be made about the location of the solar
siblings in the space of parallaxes (), proper motions () and
radial velocities (). By calculating the ratio of the number of
simulated solar siblings to that of the number of stars in a model Galactic
disk, we find that this ratio is above 0.5 in the region given by: mas, masyr, and kms. Selecting stars from this region should increase the probability
of success in identifying solar siblings through follow up observations
[Abridged].Comment: 13 pages, 7 figures. Accepted for publication in MNRA
Kinematic groups across the MW disc: insights from models and from the RAVE catalogue
With the advent of the Gaia data, the unprecedented kinematic census of great
part of the Milky Way disc will allow us to characterise the local kinematic
groups and new groups in different disc neighbourhoods. First, we show here
that the models predict a stellar kinematic response to the spiral arms and bar
strongly dependent on disc position. For example, we find that the kinematic
groups induced by the spiral arm models change significantly if one moves only
~ 0.6 kpc in galactocentric radius, but ~ 2 kpc in azimuth. There are more and
stronger groups as one approaches the spiral arms. Depending on the spiral
pattern speed, the kinematic imprints are more intense in nearby vicinities or
far from the Sun. Secondly, we present a preliminary study of the kinematic
groups observed by RAVE. This sample will allow us, for the first time, to
study the dependence on Galactic position of the (thin and thick) disc moving
groups. In the solar neighbourhood, we find the same kinematics groups as
detected in previous surveys, but now with better statistics and over a larger
spatial volume around the Sun. This indicates that these structures are indeed
large scale kinematic features.Comment: 4 pages, 3 figures, to appear in the proceedings of "Assembling the
Puzzle of the Milky Way", Le Grand Bornand (April 17-22, 2011), C. Reyle, A.
Robin, M. Schultheis (eds.
BGM FASt: Besan\c{c}on Galaxy Model for Big Data. Simultaneous inference of the IMF, SFH and density in the Solar Neighbourhood
We develop a new theoretical framework to generate Besan\c{c}on Galaxy Model
fast approximate simulations (BGM FASt) to address fundamental questions of the
Galactic structure and evolution performing multi-parameter inference. As a
first application of our strategy we simultaneously infer the IMF, the star
formation history and the stellar mass density in the Solar Neighbourhood. The
BGM FASt strategy is based on a reweighing scheme, that uses a specific
pre-sampled simulation, and on the assumption that the distribution function of
the generated stars in the Galaxy can be described by an analytical expression.
To validate BGM FASt we execute a set of tests. Finally, we use BGM FASt with
an approximate Bayesian computation algorithm to obtain the posterior PDF of
the inferred parameters, by comparing synthetic versus Tycho-2 colour-magnitude
diagrams. Results: The validation shows a very good agreement between BGM FASt
and the standard BGM, with BGM FASt being times faster. By
analysing Tycho-2 data we obtain a thin disc star formation history decreasing
in time and a present rate of . The resulting total
stellar mass density in the Solar Neighbourhood is and the local dark matter density is . For the composite IMF we obtain a slope of
in the mass range between and
. The results of the slope at the high mass range are trustable up
to and highly depend on the choice of the extinction map (obtaining
and respectively,
for two different extinction maps). Systematic uncertainties are not included.
Conclusions: The good performance of BGM FASt demonstrates that it is a very
valuable tool to perform multi-parameter inference using Gaia data releases.Comment: Accepted for publication by A&A. 30 pages (23 pages of main body and
7 pages of Appendixes) , 15 figures and 4 table
The imprints of the Galactic Bar on the Thick Disk with RAVE
We study the kinematics of a local sample of stars, located within a cylinder of 500 pc radius centered on the Sun, in the RAVE data set. We find clear asymmetries in the v R vâ velocity distributions of thin and thick disk stars: there are more stars moving radially outward for low azimuthal velocities and more radially inward for high azimuthal velocities. Such asymmetries have been previously reported for the thin disk as being due to the Galactic bar, but this is the first time that the same type of structures are seen in the thick disk. Our findings imply that the velocities of thick-disk stars should no longer be described by Schwarzschilds, multivariate Gaussian or purely axisymmetric distributions. Furthermore, the nature of previously reported substructures in the thick disk needs to be revisited as these could be associated with dynamical resonances rather than to accretion events. It is clear that dynamical models of the Galaxy must fit the 3D velocity distributions of the disks, rather than the projected 1D, if we are to understand the Galaxy fully
Constraints on the Galactic bar from the Hercules stream as traced with RAVE across the Galaxy
Non-axisymmetries in the Galactic potential (spiral arms and bar) induce kinematic groups such as the Hercules stream. Assuming that Hercules is caused by the effects of the outer Lindblad resonance of the Galactic bar, we model analytically its properties as a function of position in the Galaxy and its dependence on the bar's pattern speed and orientation. Using data from the RAVE survey we find that the azimuthal velocity of the Hercules structure decreases as a function of Galactocentric radius, in a manner consistent with our analytical model. This allows us to obtain new estimates of the parameters of the Milky Way's bar. The combined likelihood function of the bar's pattern speed and angle has its maximum for a pattern speed of Omega(b) = (1.89 +/- 0.08) x Omega(0), where Omega(0) is the local circular frequency. Assuming a solar radius of 8.05 kpc and a local circular velocity of 238 km s(-1), this corresponds to Omega(b) = 56 +/- 2km s(-1) kpc(-1). On the other hand, the bar's orientation phi(b) cannot be constrained with the available data. In fact, the likelihood function shows that a tight correlation exists between the pattern speed and the orientation, implying that a better description of our best fit results is given by the linear relation Omega(b)/Omega(0) = 1.91+0.0044 (phi(b)(deg) - 48), with standard deviation of 0.02. For example, for an angle of phi(b) = 30 deg the pattern speed is 54.0 +/- 0.5 km s(-1) kpc(-1). These results are not very sensitive to the other Galactic parameters such as the circular velocity curve or the peculiar motion of the Sun, and are robust to biases in distance
Gaia DR2 view of the Lupus V-VI clouds: the candidate diskless young stellar objects are mainly background contaminants
Extensive surveys of star-forming regions with Spitzer have revealed
populations of disk-bearing young stellar objects. These have provided crucial
constraints, such as the timescale of dispersal of protoplanetary disks,
obtained by carefully combining infrared data with spectroscopic or X-ray data.
While observations in various regions agree with the general trend of
decreasing disk fraction with age, the Lupus V and VI regions appeared to have
been at odds, having an extremely low disk fraction. Here we show, using the
recent Gaia data release 2 (DR2), that these extremely low disk fractions are
actually due to a very high contamination by background giants. Out of the 83
candidate young stellar objects (YSOs) in these clouds observed by Gaia, only
five have distances of 150 pc, similar to YSOs in the other Lupus clouds, and
have similar proper motions to other members in this star-forming complex. Of
these five targets, four have optically thick (Class II) disks. On the one
hand, this result resolves the conundrum of the puzzling low disk fraction in
these clouds, while, on the other hand, it further clarifies the need to
confirm the Spitzer selected diskless population with other tracers, especially
in regions at low galactic latitude like Lupus V and VI. The use of Gaia
astrometry is now an independent and reliable way to further assess the
membership of candidate YSOs in these, and potentially other, star-forming
regions.Comment: Accepted for publication on Astronomy&Astrophysics Letter
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