Aims. We model the effects of the spiral arms of the Milky Way on the disk stellar kinematics in the Gaia observable space. We also estimate the Gaia capabilities of detecting the predicted ...signatures. Methods. We use both controlled orbital integrations in analytic potentials and self-consistent simulations. We introduce a new strategy to investigate the effects of spiral arms, which consists of comparing the stellar kinematics of symmetric Galactic longitudes (+l and −l), in particular the median transverse velocity as determined from parallaxes and proper motions. This approach does not require the assumption of an axisymmetric model because it involves an internal comparison of the data. Results. The typical differences between the transverse velocity in symmetric longitudes in the models are of the order of ~2 km s-1, but can be larger than 10 km s-1 for certain longitudes and distances. The longitudes close to the Galactic centre and to the anti-centre are those with larger and smaller differences, respectively. The differences between the kinematics for +l and −l show clear trends that depend strongly on the properties of spiral arms. Thus, this method can be used to quantify the importance of the effects of spiral arms on the orbits of stars in the different regions of the disk, and to constrain the location of the arms, main resonances and, thus, pattern speed. Moreover, the method allows us to test different origin scenarios of spiral arms and the dynamical nature of the spiral structure (e.g. grand design versus transient multiple arms). We estimate the number of stars of each spectral type that Gaia will observe in certain representative Galactic longitudes, their characteristic errors in distance and transverse velocity, and the error in computing the median velocity as a function of distance. We will be able to measure the median transverse velocity exclusively with Gaia data, with precision smaller than ~1 km s-1 up to distances of ~4–6 kpc for certain giant stars, and up to ~2–4 kpc and better kinematic precision (≲0.5 km s-1) for certain sub-giants and dwarfs. These are enough to measure the typical signatures seen in the models. Conclusions. The Gaia catalogue will allow us to use the presented approach successfully and improve significantly upon current studies of the dynamics of the spiral arms of our Galaxy. We also show that a similar strategy can be used with line-of-sight velocities, which could be applied to Gaia data and to upcoming spectroscopic surveys.
Aims. We develop a new theoretical framework to generate Besançon 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 initial-mass function (IMF), the star formation history and the stellar mass density in the solar neighbourhood. Methods. 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 evaluate the performance of our strategy we execute a set of validation tests. Finally, we use BGM FASt together with an approximate Bayesian computation algorithm to obtain the posterior probability distribution function of the inferred parameters, by automatically comparing synthetic versus Tycho-2 colour-magnitude diagrams. Results. The validation tests show a very good agreement between equivalent simulations performed with BGM FASt and the standard BGM code, with BGM FASt being ∼104 times faster. From the analysis of the Tycho-2 data we obtain a thin-disc star formation history decreasing in time and a present rate of 1.2 ± 0.2 M ⊙ yr−1. The resulting total stellar volume mass density in the solar neighbourhood is 0.051−0.005+0.002 M⊙ pc−3 0 . 051 − 0.005 + 0.002 M ⊙ pc −3 $ 0.051_{-0.005}^{+0.002}\,M_{\odot}\,{\rm{p}}{{\rm{c}}^{ - 3}}\ $ and the local dark matter density is 0.012 ± 0.001 M ⊙ pc−3. For the composite IMF, we obtain a slope of α2 = 2.1−0.3+0.1 α 2 = 2.1 − 0.3 + 0.1 $ \alpha_2={2.1}_{-0.3}^{+0.1} $ in the mass range between 0.5 M⊙ and 1.53 M⊙. The results of the slope at the high-mass range are trustable up to 4 M⊙ and highly dependent on the choice of extinction map (obtaining α3 = 2.9−0.2+0.2 α 3 = 2.9 − 0.2 + 0.2 $ \alpha_3={2.9}_{-0.2}^{+0.2} $ and α3 = 3.7−0.2+0.2 α 3 = 3.7 − 0.2 + 0.2 $ \alpha_3={3.7}_{-0.2}^{+0.2} $ , respectively, for two different extinction maps). Systematic uncertainties coming from model assumptions 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.
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 Ωb = (1.89 ± 0.08) × Ω0, where Ω0 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 Ωb = 56 ± 2 km s-1 kpc-1. On the other hand, the bar’s orientation φb 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 Ωb/Ω0 = 1.91 + 0.0044(φb(deg) − 48), with standard deviation of 0.02. For example, for an angle of φb = 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.
Context.
Thanks to ongoing efforts to compute accurate stellar ages, we are able to characterise stars in different regions of the Milky Way. The
Gaia
and
Kepler
space-missions, along with ...ground-based spectroscopic surveys such as APOGEE, provide a unique way to study the chemo-kinematics relations as a function of age through the Galactic stellar populations and provide new constraints to Galactic evolution models.
Aims.
We investigate the properties of the double sequences of the Milky Way discs visible in the
α
/Fe versus Fe/H diagram, which are usually associated to the chemical thin and thick discs at the solar circle. In the framework of Galactic formation and evolution, we discuss the complex relationships between age, metallicity,
α
/Fe, and the radial, azimuthal, and vertical components of the space velocities.
Methods.
We study stars with measured chemical and seismic properties from the APOGEE spectroscopic survey and the
Kepler
satellite, respectively. In addition, astrometry from the
Gaia
satellite is available for the majority of the sample. We separate the
α
/Fe−Fe/H diagram into three stellar populations: the thin disc, the high-
α
metal-poor thick disc, and the high-
α
metal-rich thick disc and characterise each of these in the age-chemo-kinematics parameter space. Because of the model-dependent nature of the ages inferred from asteroseismology, and because they depend on the quality of the input spectroscopic information, we compare results obtained from different APOGEE data releases (DR14 and DR16). We also use age determinations from two recent works in the literature. In addition, we use the Besançon stellar populations synthesis model to highlight selection biases and mechanisms (such as mergers and secular evolution) not included in the model.
Results.
The thin disc exhibits a flat age–metallicity relation while
α
/Fe increases with stellar age. We confirm no correlation between radial and vertical velocities with Fe/H,
α
/Fe, and age for each stellar population. Considering both samples,
V
φ
decreases with age for the thin disc, while
V
φ
increases with age for the high-
α
metal-poor thick disc. We show that this difference is not due to sample selection. Although the age distribution of the high-
α
metal-rich thick disc is very close to that of the high-
α
metal-poor thick disc between 7 and 14 Gyr, its kinematics seems to follow that of the thin disc. This feature, not predicted by the hypotheses included in the Besançon Galaxy Model, suggests a different origin and history for this population. Finally, we show that there is a maximum dispersion of the vertical velocity,
σ
Z
, with age for the high-
α
metal-poor thick disc around 8 Gyr. The comparisons with the Besançon Galaxy Model simulations suggest a more complex chemo-dynamical scheme to explain this feature, most likely including mergers and radial migration effects.
Context. The stellar α/Fe abundance is sometimes used as a proxy for stellar age, following standard chemical evolution models for the Galaxy, as seen by different observational results. Aim. In this ...work, we aim to show that the open cluster NGC 6705/M 11 has a significant α-enhancement α/Fe > 0.1 dex, despite its young age (~300 Myr), challenging the current paradigm. Methods. We used high resolution (R > 65 000) high signal-to-noise (~70) spectra of eight red clump stars, acquired within the OCCASO survey. We determined very accurate chemical abundances of several α elements, using an equivalent width methodology (Si, Ca and Ti), and spectral synthesis fits (Mg and O). Results. We obtain Si/Fe = 0.13 ± 0.05, Mg/Fe = 0.14 ± 0.07, O/Fe = 0.17 ± 0.07, Ca/Fe = 0.06 ± 0.05, and Ti/Fe = 0.03 ± 0.03. Our results place these clusters within the group of young α/Fe-enhanced field stars recently found by several authors in the literature. The ages of our stars have an uncertainty of around 50 Myr, much more precise than for field stars. By integrating the cluster’s orbit in several non-axisymmetric Galactic potentials, we establish the M 11’s most likely birth radius as lying between 6.8–7.5 kpc from the Galactic centre, not far from its current position. Conclusions. With the robust open cluster age scale, our results prove that a moderate α/Fe-enhancement is no guarantee for a star to be old, and that not all α-enhanced stars can be explained with an evolved blue straggler scenario. Based on our orbit calculations, we further argue against a Galactic bar origin of M 11.
We use 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 shows absolute differences at this confidence level larger than 5 km s-1 and 7% larger than 10 km s-1. 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 probably not 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.
ABSTRACT
We analyse the kinematics of disc stars observed by the RAdial Velocity Experiment (RAVE) survey in and beyond the solar neighbourhood. We detect significant overdensities in the velocity ...distributions using a technique based on the wavelet transform. We find that the main local kinematic groups are large‐scale features, surviving at least up to ∼1 kpc from the Sun in the direction of antirotation, and also at below the Galactic plane. We also find that for regions located at different radii than the Sun, the known groups appear shifted in the vR–vφ velocity plane. For example, the Hercules group has a larger azimuthal velocity for regions inside the solar circle and a lower value outside. We have also discovered a new group at in the solar neighbourhood and confirmed the significance of other previously found groups. Some of these trends detected for the first time are consistent with dynamical models of the effects of the bar and the spiral arms. More modelling is required to definitively characterize the non‐axisymmetric components of our Galaxy using these groups.
We present a method to identify ultrafaint dwarf galaxies (UFDGs) candidates in the halo of the Milky Way using the future Gaia catalogue and we explore its detection limits and completeness. The ...method is based on the Wavelet Transform and searches for overdensities in the combined space of sky coordinates and proper motions, using kinematics in the search for the first time. We test the method with a Gaia mock catalogue that has the Gaia Universe Model Snapshot as a background, and use a library of around 30 000 UFDGs simulated as Plummer spheres with a single stellar population. For the UFDGs, we use a wide range of structural and orbital parameters that go beyond the range spanned by real systems, where some UFDGs may remain undetected. We characterize the detection limits as function of the number of observable stars by Gaia in the UFDGs with respect to that of the background and their apparent sizes in the sky and proper motion planes. We find that the addition of proper motions in the search improves considerably the detections compared to a photometric survey at the same magnitude limit. Our experiments suggest that Gaia will be able to detect UFDGs that are similar to some of the known UFDGs even if the limit of Gaia is around 2 mag brighter than that of SDSS, with the advantage of having a full-sky catalogue. We also see that Gaia could even find some UFDGs that have lower surface brightness than the SDSS limit.
Context. The chemical abundances of stars encode information on their place and time of origin. Stars formed together in e.g. a cluster, should present chemical homogeneity. Also disk stars ...influenced by the effects of the bar and the spiral arms might have distinct chemical signatures depending on the type of orbit that they follow, e.g. from the inner versus outer regions of the Milky Way. Aims. We explore the correlations between velocity and metallicity and the possible distinct chemical signatures of the velocity over-densities of the local Galactic neighbourhood. Methods. We use the large spectroscopic survey RAVE and the Geneva Copenhagen Survey. We compare the metallicity distribution of regions in the velocity plane (vR,vφ) with that of their symmetric counterparts (−vR,vφ). We expect similar metallicity distributions if there are no tracers of a sub-population (e.g. a dispersed cluster, accreted stars), if the disk of the Galaxy is axisymmetric, and if the orbital effects of the bar and the spiral arms are weak. Results. We find that the metallicity-velocity space of the solar neighbourhood is highly patterned. A large fraction of the velocity plane shows differences in the metallicity distribution when comparing symmetric vR regions. The typical differences in the median metallicity are of 0.05 dex with statistical significant of at least 95% confidence, and with values up to 0.6 dex. For stars with low azimuthal velocity vφ, the ones moving outwards. These include stars in the Hercules and Hyades moving groups and other velocity branch-like structures. For higher vφ, the stars moving inwards have higher metallicity than those moving outwards. We have also discovered a positive gradient in vφ with respect to metallicity at high metallicities, apart from the two known positive and negative gradients for the thick and thin disks. Conclusions. The most likely interpretation of the metallicity asymmetry is that it is mainly due to the orbital effects of the Galactic bar and the radial metallicity gradient of the disk. We present a simulation that supports this idea.
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