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Register a new userCharting the Galactic acceleration field I. A search for stellar streams with Gaia DR2 and EDR3 with follow-up from ESPaDOnS and UVES
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We present maps of the stellar streams detected in the Gaia Data Release 2 (DR2) and Early Data Release 3 (EDR3) catalogs using the STREAMFINDER algorithm. We also report the spectroscopic follow-up of the brighter DR2 stream members obtained with the high-resolution CFHT/ESPaDOnS and VLT/UVES spectrographs as well as with the medium-resolution NTT/EFOSC2 spectrograph. Two new stellar streams that do not have a clear progenitor are detected in DR2 (named Hrid and Gunnthra), and seven are detected in EDR3 (named Gaia-6 to Gaia-12). Several candidate streams are also identified. The software also finds very long tidal tails associated with the 15 globular clusters NGC 288, NGC 1261, NGC 1851, NGC 2298, NGC 2808, NGC 3201, M 68, $omega$Cen, NGC 5466, Palomar 5, M 5, NGC 6101, M 92, NGC 6397 and NGC 7089. These stellar streams will be used in subsequent contributions in this series to chart the properties of the Galactic acceleration field on $sim$ 100 pc to $sim$ 100 kpc scales.
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We measure the Suns velocity with respect to the Galactic halo using Gaia Early Data Release 3 (EDR3) observations of stellar streams. Our method relies on the fact that, in low-mass streams, the proper motion of stars should be directed along the stream structure in a non-rotating rest frame of the Galaxy, but the observed deviation arises due to the Suns own reflex motion. This principle allows us to implement a simple geometrical procedure, which we use to analyse 17 streams over a $sim 3-30$ kpc range. Our constraint on the Suns motion is independent of any Galactic potential model, and it is also uncorrelated with the Suns galactocentric distance. We infer the Suns velocity as $V_{R,odot}=8.88^{+1.20}_{-1.22},rm{kms^{-1}}$ (radially towards the Galactic centre), $V_{phi,odot}=241.91^{+1.61}_{-1.73},rm{kms^{-1}}$ (in the direction of Galactic rotation) and $V_{z,odot}=3.08^{+1.06}_{-1.10},rm{kms^{-1}}$ (vertically upwards), in global agreement with past measurements through other techniques; although we do note a small but significant difference in the $V_{z,odot}$ component. Some of these parameters show significant correlation and we provide our MCMC output so it can be used by the reader as an input to future works. The comparison between our Suns velocity inference and previous results, using other reference frames, indicates that the inner Galaxy is not moving with respect to the inertial frame defined by the halo streams.
We apply a simple axisymmetric disc model to 218 Galactic Cepheids whose accurate measurements of the distance and velocities are obtained by cross-matching an existing Cepheids catalogue with the Gaia DR2 data. Our model fit determines the local centrifugal speed, $V_mathrm{c}$ $-$ defined as the rotation speed required to balance the local radial gravitational force $-$ at the Suns location to be $V_{c}(R_0)=236pm 3$ km s$^{-1}$ and the Suns azimuthal and radial peculiar motions to be $V_{odot}=12.4pm0.7$ km s$^{-1}$ and $U_{odot}=7.7pm0.9$ km s$^{-1}$, respectively. These results are obtained with strong priors on the solar radius, $R_0=8.2pm0.1$ kpc, and Suns angular rotation velocity, $Omega_{odot}=30.24pm0.12$ km s$^{-1}$ kpc$^{-1}$. We also applied the axisymmetric model to mock data from an N-body/hydrodynamic simulation of a Milky Way-like galaxy with a bar and spiral arms. We find that our axisymmetric model fit to the young stars recovers the local centrifugal speed reasonably well, even in the face of significant non-axisymmetry. However, the local centrifugal speed determined from our Cepheid sample could suffer from systematic uncertainty as large as 6 km s$^{-1}$.
Context: Chamaeleon is the southernmost low-mass star-forming complex within 200 pc from the Sun. Its stellar population has been extensively studied in the past, but the current census of the stellar content is not complete yet and deserves further investigation. Aims: We take advantage of the second data release of the textit{Gaia} space mission to expand the census of stars in Chamaeleon and to revisit the properties of the stellar populations associated to the Chamaeleon I (Cha I) and Chamaeleon II (Cha II) dark clouds. Methods: We perform a membership analysis of the sources in the textit{Gaia} catalogue over a field of 100 deg$^{2}$ encompassing the Chamaeleon clouds, and use this new census of cluster members to investigate the 6D structure of the complex. Results: We identify 188 and 41 high-probability members of the stellar populations in Cha I and Cha II, respectively, including 19 and 7 new members. Our sample covers the magnitude range from $G=6$ to $G=20$ mag in Cha I, and from $G=12$ to $G=18$ mag in Cha II. We confirm that the northern and southern subgroups of Cha I are located at different distances ($191.4^{+0.8}_{-0.8}$ pc and $186.7^{+1.0}_{-1.0}$ pc), but they exhibit the same space motion within the reported uncertainties. Cha II is located at a distance of $197.5^{+1.0}_{-0.9}$ pc and exhibits a space motion that is consistent with Cha I within the admittedly large uncertainties on the spatial velocities of the stars that come from radial velocity data. The median age of the stars derived from the Hertzsprung-Russell diagram (HRD) and stellar models is about 1-2 Myr, suggesting that they are somewhat younger than previously thought. We do not detect significant age differences between the Chamaeleon subgroups, but we show that Cha II exhibits a higher fraction of disc-bearing stars compared to Cha I.
We characterize the kinematic and chemical properties of 589 Galactic Anticenter Substructure Stars (GASS) with K-/M- giants in Integrals-of-Motion space. These stars likely include members of previously identified substructures such as Monoceros, A13, and the Triangulum-Andromeda cloud (TriAnd). We show that these stars are on nearly circular orbits on both sides of the Galactic plane. We can see velocity($V_{Z}$) gradient along Y-axis especially for the south GASS members. Our GASS members have similar energy and angular momentum distributions to thin disk stars. Their location in [$alpha$/M] vs. [M/H] space is more metal poor than typical thin disk stars, with [$alpha$/M] textbf{lower} than the thick disk. We infer that our GASS members are part of the outer metal-poor disk stars, and the outer-disk extends to 30 kpc. Considering the distance range and $alpha$-abundance features, GASS could be formed after the thick disk was formed due to the molecular cloud density decreased in the outer disk where the SFR might be less efficient than the inner disk.
The second data release of the Gaia mission has revealed, in stellar velocity and action space, multiple ridges, the exact origin of which is still debated. Recently, we demonstrated that a large Galactic bar with pattern speed 39 km/s/kpc does create most of the observed ridges. Among those ridges, the Hercules moving group would then be associated to orbits trapped at the co-rotation resonance of the bar. Here we show that a distinctive prediction of such a model is that the angular momentum of Hercules at the Suns radius must significantly decrease with increasing Galactocentric azimuth, i.e. when getting closer to the major axis of the bar. We show that such a dependence of the angular momentum of trapped orbits on the azimuth would on the other hand not happen close to the outer Lindblad resonance of a faster bar, unless the orbital distribution is still far from phase-mixed, namely for a bar perturbation younger than ~ 2 Gyr. Using Gaia DR2 and Bayesian distances from the StarHorse code, and tracing the average Galactocentric radial velocity as a function of angular momentum and azimuth, we show that the Hercules angular momentum changes significantly with azimuth as expected for the co-rotation resonance of a dynamically old large bar.
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