Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userKinematics with Gaia DR2: The Force of a Dwarf
62
0
0.0
(
0
)
Added by
Ismael Enrique Carrillo Rivas
Publication date
2019
fields
Physics
and research's language is
English
Ask ChatGPT about the research
No Arabic abstract
We use Gaia DR2 astrometric and line-of-sight velocity information combined with two sets of distances obtained with a Bayesian inference method to study the 3D velocity distribution in the Milky Way disc. We search for variations in all Galactocentric cylindrical velocity components ($V_{phi}$, $V_R$ and $V_z$) with Galactic radius, azimuth, and distance from the disc mid-plane. We confirm recent work showing that bulk vertical motions in the $Rtext{-}z$ plane are consistent with a combination of breathing and bending modes. In the $xtext{-}y$ plane, we show that, although the amplitudes change, the structure produced by these modes is mostly invariant as a function of distance from the plane. Comparing to two different Galactic disc models, we demonstrate that the observed patterns can drastically change in short time intervals, showing the complexity of understanding the origin of vertical perturbations. A strong radial $V_R$ gradient was identified in the inner disc, transitioning smoothly from $16$ km s$^{-1}$ kpc$^{-1}$ at an azimuth of $30^circ<phi<45^circ$ ahead of the Sun-Galactic centre line, to $-16$ km s$^{-1}$ kpc$^{-1}$ at an azimuth of $-45^circ<phi<-30^circ$ lagging the solar azimuth. We use a simulation with no significant recent mergers to show that exactly the opposite trend is expected from a barred potential, but overestimated distances can flip this trend to match the data. Alternatively, using an $N$-body simulation of the Sagittarius dwarf-Milky Way interaction, we demonstrate that a major recent perturbation is necessary to reproduce the observations. Such an impact may have strongly perturbed the existing bar or even triggered its formation in the last $1text{-}2$ Gyr.
rate research
Read More
Context. Open clusters are very good tracers of the evolution of the Galactic disc. Thanks to Gaia, their kinematics can be investigated with an unprecedented precision and accuracy. Aims. The distribution of open clusters in the 6D phase space is revisited with Gaia DR2. Methods. The weighted mean radial velocity of open clusters was determined, using the most probable members available from a previous astrometric investigation that also provided mean parallaxes and proper motions. Those parameters, all derived from Gaia DR2 only, were combined to provide the 6D phase space information of 861 clusters. The velocity distribution of nearby clusters was investigated, as well as the spatial and velocity distributions of the whole sample as a function of age. A high quality subsample was used to investigate some possible pairs and groups of clusters sharing the same Galactic position and velocity. Results. For the high quality sample that has 406 clusters, the median uncertainty of the weighted mean radial velocity is 0.5 km/s. The accuracy, assessed by comparison to ground-based high resolution spectroscopy, is better than 1 km/s. Open clusters nicely follow the velocity distribution of field stars in the close Solar neighbourhood previously revealed by Gaia DR2. As expected, the vertical distribution of young clusters is very flat but the novelty is the high precision to which this can be seen. The dispersion of vertical velocities of young clusters is at the level of 5 km/s. Clusters older than 1 Gyr span distances to the Galactic plane up to 1 kpc with a vertical velocity dispersion of 14 km/s, typical of the thin disc. Five pairs of clusters and one group with five members are possibly physically related. Other binary candidates previously identified turn out to be chance alignment.
Using Gaia DR2 astrometry, we map the kinematic signature of the Galactic stellar warp out to a distance of 7 kpc from the Sun. Combining Gaia DR2 and 2MASS photometry, we identify, via a probabilistic approach, 599 494 upper main sequence stars and 12 616 068 giants without the need for individual extinction estimates. The spatial distribution of the upper main sequence stars clearly shows segments of the nearest spiral arms. The large-scale kinematics of both the upper main sequence and giant populations show a clear signature of the warp of the Milky Way, apparent as a gradient of 5-6 km/s in the vertical velocities from 8 to 14 kpc in Galactic radius. The presence of the signal in both samples, which have different typical ages, suggests that the warp is a gravitationally induced phenomenon.
The Gaia mission has opened a new window into the internal kinematics of young star clusters at the sub-km/s level, with implications for our understanding of how star clusters form and evolve. We use a sample of 28 clusters and associations with ages from 1-5 Myr, where lists of members are available from previous X-ray, optical, and infrared studies. Proper motions from Gaia DR2 reveals that at least 75% of these systems are expanding; however, rotation is only detected in one system. Typical expansion velocities are on the order of ~0.5 km/s, and, in several systems, there is a positive radial gradient in expansion velocity. Systems that are still embedded in molecular clouds are less likely to be expanding than those that are partially or fully revealed. One-dimensional velocity dispersions, which range from 1 to 3 km/s, imply that most of the stellar systems in our sample are supervirial and that some are unbound. In star-forming regions that contain multiple clusters or subclusters, we find no evidence that these groups are coalescing, implying that hierarchical cluster assembly, if it occurs, must happen rapidly during the embedded stage.
The kinematic properties of young stars that have not yet reached the stage of the main sequence are studied. The selection of these stars was recently carried out by Marton et al. (2019) and Vioque et al. (2020) according to the Gaia DR2 catalog using a number of photometric infrared surveys. We have determined the rotation parameters of the Galaxy and the parameters of the ellipsoids of the residual velocities. The linear velocity of the circular rotation of a solar region around the center of the Galaxy, found using 4431 stars, is equal to V_0=229.1+-4.4 km/s. The following ellipsoid parameters of their residual velocities are found from low-mass stars (of type T Tau): $sigma_{1,2,3}=(9.45,6.99,6.61)pm(0.94,0.43,0.32)$ km/s. For stars of intermediate masses (Herbig Ae/Be stars), their values turned out to be somewhat larger $sigma_{1,2,3}=(13.67,9.25,7.26)pm(2.40,2.44,0.88)$ km/s. Distant stars from both Catalogs trace the local spiral arm well. For 1212 stars, a new estimate of the pitch angle of the Local spiral arm is equal to i=-8.9+-0.1 deg.
To construct the rotation curve of the Galaxy, classical Cepheids with proper motions, parallaxes and line-of-sight velocities from the Gaia DR2 Catalog are used in large part. The working sample formed from literature data contains about 800 Cepheids with estimates of their age. We determined that the linear rotation velocity of the Galaxy at a solar distance is $V_0=240pm3$~km s$^{-1}$. In this case, the distance from the Sun to the axis of rotation of the Galaxy is found to be $R_0=8.27pm0.10$~kpc. A spectral analysis of radial and residual tangential velocities of Cepheids younger than 120 Myr showed close estimates of the parameters of the spiral density wave obtained from data both at present time and in the past. So, the value of the wavelength $lambda_{R,theta}$ is in the range of [2.4--3.0] kpc, the pitch angle $i_{R,theta}$ is in the range of [$-13^circ$,$-10^circ$] for a four-arm pattern model, the amplitudes of the radial and tangential perturbations are $f_Rsim12$~km s$^{-1}$ and $f_thetasim9$~km s$^{-1}$, respectively. Velocities of Cepheids older than 120 Myr are currently giving a wavelength $lambda_{R,theta}sim5$~kpc. This value differs significantly from one that we obtained from the samples of young Cepheids. An analysis of positions and velocities of old Cepheids, calculated by integrating their orbits backward in time, made it possible to determine significantly more reliable values of the parameters of the spiral density wave: wavelength $lambda_{R,theta}=2.7$~kpc, amplitudes of radial and tangential perturbations are $f_R=7.9$~km s$^{-1}$ and $f_theta=5$~km s$^{-1}$, respectively.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
