Painting a portrait of the Galactic disc with its stellar clusters

Context. The large astrometric and photometric survey performed by the Gaia mission allows for a panoptic view of the Galactic disc and its stellar cluster population. Hundreds of stellar clusters were only discovered after the latest Gaia data release (DR2) and have yet to be characterised. Aims. Here we make use of the deep and homogeneous Gaia photometry down to G = 18 to estimate the distance, age, and interstellar reddening for about 2000 stellar clusters identified with Gaia DR2 astrometry. We use these objects to study the structure and evolution of the Galactic disc. Methods. We relied on a set of objects with well-determined parameters in the literature to train an artificial neural network to estimate parameters from the Gaia photometry of cluster members and their mean parallax. Results. We obtain reliable parameters for 1867 clusters. Our catalogue confirms the relative lack of old stellar clusters in the inner disc (with a few notable exceptions). We also quantify and discuss the variation of scale height with cluster age, and we detect the Galactic warp in the distribution of old clusters. Conclusions. This work results in a large and homogeneous cluster catalogue, allowing one to trace the structure of the disc out to distances of ∼4 kpc. However, the present sample is still unable to trace the outer spiral arm of the Milky Way, which indicates that the outer disc cluster census might still be incomplete.

Unlocking Galactic Wolf–Rayet stars with Gaia DR2 – I. Distances and absolute magnitudes

ABSTRACT We obtain distances to 383 Galactic Wolf–Rayet (WR) stars from Gaia DR2 parallaxes and Bayesian methods, with a prior based on H ii regions and dust extinction. Distances agree with those from Bailer-Jones et al. for stars up to 2 kpc from the Sun, though deviate thereafter due to differing priors, leading to modest reductions in luminosities for recent WR spectroscopic results. We calculate visual and K-band absolute magnitudes, accounting for dust extinction contributions and binarity, and identify 187 stars with reliable absolute magnitudes. For WR and O stars within 2 kpc, we find a WR/O ratio of 0.09. The distances are used to generate absolute magnitude calibrations and obtain the Gaia colour–magnitude diagram for WR stars. Average vWR-band absolute magnitudes for WN stars range from –3.6 mag (WN3–4) to –7.0 mag (WN8–9ha), and –3.1 (WO2–4) to –4.6 mag (WC9), with standard deviations of ∼0.6 mag. Using H ii region scale heights, we identify 31 WR stars at large (3σ, |z| ≥ 156 pc) distances from the mid-plane as potential runaways accounting for the Galactic warp, of which only four involve WN8–9 stars, contrary to previous claims.

Galactic warp from the kinematics of OB stars

The warp in Milky Way seen through the young stellar disk shows the complex structures. Its spatial distribution reveals the asymmetry in the vertical distance to the galactic midplane, whereas the kinematics shows the os- cillating motion. We analyze the warp using about 25,000 OB stars from Gaia Data Release 2, with Galactocentric azimuth (φ) range from 135◦ to 225◦ . We detect the warp from two distinctive regions. First region (warp down) lies on 100◦ < l < 150◦ and second regions (warp up) lies on 150◦ < l < 220◦ . The vertical position towards the Galactic midplane (Z) from all samples shows the line-of-nodes of the warp is on φ = 174◦ ± 2◦ . The maximum deviation in warp down region reachs 0.1 kpc and 0.2 kpc in warp up region. The vertical velocity (VZ) from both regions are dominated by negative VZ . From the kinematics dis- tribution, it can be concluded that there is an oscillati1ng motion. Considering with the previous study, the oscillating motion was caused by the gravitational interaction with the Galaxy satellites. From this, we can constrain the origin of the warp is developing from the tidal interaction between Milky Way and its satellites.

Vertical structure and kinematics of the Galactic outer disk

We report on measurements of parallax and proper motion for four 22 GHz water maser sources as part of the VERA Outer Rotation Curve project. All the sources show Galactic latitudes of &gt;2° and Galactocentric distances of &gt;11 kpc at the Galactic longitude range of 95° &lt; l &lt; 126°. The sources trace the Galactic warp reaching to 200–400 pc, and also the signature of the warp to 600 pc toward the north Galactic pole. The new results, along with previous results in the literature, show that the maximum height of the Galactic warp increases with Galactocentric distance. Also, we examined velocities perpendicular to the disk for the sample, and found oscillatory behavior between the vertical velocities and Galactic heights. This behavior suggests the existence of bending (vertical density) waves, possibly induced by a perturbing satellite (e.g., the passage of the Sagittarius dwarf galaxy).

Gaia kinematics reveal a complex lopsided and twisted Galactic disc warp

Context. There are few warp kinematic models of the Galaxy able to characterise both structure and kinematics, since these require high accuracy at large distances. These models are necessary to shed light on the lopsidedness of the warp and the twisting of the line-of-nodes of the stellar warp already seen in gas and dust. Aims. We use the vertical information coming from the Gaia Data Release 2 astrometric data up to G = 20 mag to characterise the structure of the Galactic warp, the related vertical motions, and the dependency of Galactic warp on age. Methods. We analyse two populations up to Galactocentric distances of 16 kpc: a young bright sample mainly formed by OB stars and an older one of red giant branch (RGB) stars. We use two methods (the pole count maps of great circle bands and Galactic longitude – proper motion in latitude lines) based on the Gaia observables, together with 2D projections of the positions and proper motions in the Galactic plane. Results. This work confirms the age dependency of the Galactic warp, both in position and kinematics, the height of the Galactic warp being of the order of 0.2 kpc for the OB sample and 1.0 kpc for the RGB at a Galactocentric distance of 14 kpc. Both methods find that the onset radius of the warp is 12 ∼ 13 kpc for the OB sample and 10 ∼ 11 kpc for the RGB. From the RGB sample, we find from Galactocentric distances larger than 10 kpc that the line-of-nodes twists away from the Sun-anticentre line towards Galactic azimuths ≈180−200° increasing with radius, though possibly influenced by extinction. Also, the RGB sample reveals a slightly lopsided stellar warp with ≈250 pc difference between the up and down sides. The line of maximum of proper motions in latitude is systematically offset from the line-of-nodes estimated from the spatial data, which our warp models predict as a kinematic signature of lopsidedness. We also show a prominent wave-like pattern of a bending mode different in the OB and RGB samples. Both positions and kinematics also reveal substructures that might not be related to the large-scale Galactic warp or to the bending mode. Conclusions. Gaia Data Release 2 data reveals a high degree of complexity in terms of both position and velocity that triggers the need for complex kinematic models flexible enough to combine both wave-like patterns and an S-shaped lopsided warp.

Warps, Waves, and Phase Spirals in the Milky Way

The stellar disc of the Milky Way exhibits clear departures from planarity, the most conspicuous manifestation being the Galactic Warp but also includes an apparent corrugation pattern in number counts around 15kpc from the Galactic centre, a wave like pattern in the vertical velocities of stars as a function of guiding radius, asymmetries about the midplane in both number counts and bulk motions, and phase spirals in the z–vz projection of the local stellar distribution function. We discuss the physics of these phenomena and, in particular, suggest a possible avenue for inferring the vertical force in the Solar Neighbourhood from phase spirals. We apply Dynamic Mode Decomposition, a technique widely used in the realm of fluid mechanics, to simulations of disc galaxy simulations. This method appears to be particularly well-suited to the study of nonlinear processes such as the coupling of warps and spirals, first discussed by Masset and Tagger.