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Register a new userKinematic Properties of Young Intermediate- and Low-Mass Stars from the Gaia DR2 Catalogue
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We have studied the kinematic properties of young pre-main-sequence stars. We have selected these stars based on data from the Gaia DR2 catalogue by invoking a number of photometric infrared surveys. Using 4564 stars with parallax errors less than 20%, we have found the following parameters of the angular velocity of Galactic rotation: $Omega_0 =28.84pm0.10$ km s$^{-1}$ kpc$^{-1}$, $Omega^{}_0=-4.063pm0.029$ km s$^{-1}$ kpc$^{-2}$ and $Omega^{}_0=0.766pm0.020$ km s$^{-1}$ kpc$^{-3}$, where the Oort constants are $A=16.25pm0.33$ km s$^{-1}$ kpc$^{-1}$ and $B=-12.58pm0.34$ km s$^{-1}$ kpc$^{-1}$. The circular rotation velocity of the solar neighborhood around the Galactic center is $V_0=230.7pm4.4$ km s$^{-1}$ for the adopted Galactocentric distance of the Sun $R_0=8.0pm0.15$ kpc. The residual velocity dispersion for the stars considered is shown to be low, suggesting that they are extremely young. The residual velocity dispersion averaged over three coordinates is $sim$11 km s$^{-1}$ for Herbig Ae/Be stars and $sim$7 km s$^{-1}$ for T Tauri stars.
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We determined the chemical and kinematic properties of the Galactic thin and thick disk using a sample of 307,246 A/F/G/K-type giant stars from the LAMOST spectroscopic survey and Gaia DR2 survey. Our study found that the thick disk globally exhibits no metallicity radial gradient, but the inner disk ($R le 8$ kpc) and the outer disk ($R>8$ kpc) have different gradients when they are studied separately. The thin disk also shows two different metallicity radial gradients for the inner disk and the outer disk, and has steep metallicity vertical gradient of d[Fe/H]/d$|z|$ $=-0.12pm0.0007$ dex kpc$^{-1}$, but it becomes flat when it is measured at increasing radial distance, while the metallicity radial gradient becomes weaker with increasing vertical distance. Adopting a galaxy potential model, we derived the orbital eccentricity of sample stars and found a downtrend of average eccentricity with increasing metallicity for the thick disk. The variation of the rotation velocity with the metallicity shows a positive gradient for the thick disk stars and a negative one for the thin disk stars. Comparisons of our observed results with models of disk formation suggest that radial migration could have influenced the chemical evolution of the thin disk. The formation of the thick disk could be affected by more than one processes: the accretion model could play an indispensable role, while other formation mechanisms, such as the radial migration or heating scenario model could also have a contribution.
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.
We have studied a sample of more than 25 000 young stars with proper motions and trigonometric parallaxes from the Gaia DR2 catalogue. The relative errors of their parallaxes do not exceed 10%. The selection of stars belonging to active star-forming regions was made by Marton et al. based on data from the Gaia DR2 catalogue by invoking infrared measurements from the WISE and Planck catalogues. Low-mass T Tauri stars constitute the majority of sample stars. The following parameters of the angular velocity of Galactic rotation have been found from them: $Omega_0 =28.40pm0.11$ km s$^{-1}$ kpc$^{-1}$, $Omega^{}_0=-3.933pm0.033$ km s$^{-1}$ kpc$^{-2}$ and $Omega^{}_0=0.804pm0.040$ km s$^{-1}$ kpc$^{-3}$. The Oort constants are $A=15.73pm0.32$ km s$^{-1}$ kpc$^{-1}$ and $B=-12.67pm0.34$ km s$^{-1}$ kpc$^{-1}$, while the circular rotation velocity of the solar neighborhood around the Galactic center is $V_0=227pm4$ km s$^{-1}$ for the adopted Galactocentric distance of the Sun $R_0=8.0pm0.15$ kpc.
We estimate the mass of the Milky Way (MW) within 21.1 kpc using the kinematics of halo globular clusters (GCs) determined by Gaia. The second Gaia data release (DR2) contained a catalogue of absolute proper motions (PMs) for a set of Galactic GCs and satellite galaxies measured using Gaia DR2 data. We select from the catalogue only halo GCs, identifying a total of 34 GCs spanning $2.0 < r < 21.1$ kpc, and use their 3D kinematics to estimate the anisotropy over this range to be $beta = 0.46^{+0.15}_{-0.19}$, in good agreement, though slightly lower than, a recent estimate for a sample of halo GCs using HST PM measurements further out in the halo. We then use the Gaia kinematics to estimate the mass of the MW inside the outermost GC to be $M(< 21.1 mathrm{kpc}) = 0.21^{+0.04}_{-0.03} 10^{12} mathrm{M_odot}$, which corresponds to a circular velocity of $v_mathrm{circ}(21.1 mathrm{kpc}) = 206^{+19}_{-16}$ km/s. The implied virial mass is $M_mathrm{virial} = 1.28^{+0.97}_{-0.48} 10^{12} mathrm{M_odot}$. The error bars encompass the uncertainties on the anisotropy and on the density profile of the MW dark halo, and the scatter inherent in the mass estimator we use. We get improved estimates when we combine the Gaia and HST samples to provide kinematics for 46 GCs out to 39.5 kpc: $beta = 0.52^{+0.11}_{-0.14}$, $M(< 39.5 mathrm{kpc}) = 0.42^{+0.07}_{-0.06} 10^{12} mathrm{M_odot}$, and $M_mathrm{virial} = 1.54^{+0.75}_{-0.44} 10^{12} mathrm{M_odot}$. We show that these results are robust to potential substructure in the halo GC distribution. While a wide range of MW virial masses have been advocated in the literature, from below $10^{12} mathrm{M_odot}$ to above $2 times 10^{12}mathrm{M_odot}$, these new data imply that an intermediate mass is most likely.
We present a catalogue of 73,221 white dwarf candidates extracted from the astrometric and photometric data of the recently published Gaia DR2 catalogue. White dwarfs were selected from the Gaia Hertzsprung-Russell diagram with the aid of the most updated population synthesis simulator. Our analysis shows that Gaia has virtually identified all white dwarfs within 100 pc from the Sun. Hence, our sub-population of 8,555 white dwarfs within this distance limit and the colour range considered, $-,0.52<(G_{rm BP}-G_{rm RP})<0.80$, is the largest and most complete volume-limited sample of such objects to date. From this sub-sample we identified 8,343 CO-core and 212 ONe-core white dwarf candidates and derived a white dwarf space density of $4.9pm0.4times10^{-3},{rm pc^{-3}}$. A bifurcation in the Hertzsprung-Russell diagram for these sources, which our models do not predict, is clearly visible. We used the Virtual Observatory tool VOSA to derive effective temperatures and luminosities for our sources by fitting their spectral energy distributions, that we built from the UV to the NIR using publicly available photometry through the Virtual Observatory. From these parameters, we derived the white dwarf radii. Interpolating the radii and effective temperatures in hydrogen-rich white dwarf cooling sequences, we derived the surface gravities and masses. The Gaia 100 pc white dwarf population is clearly dominated by cool ($sim$ 8,000 K) objects and reveals a significant population of massive ($M sim 0.8 M_{odot}$) white dwarfs, of which no more than $sim$ $30-40 %$ can be attributed to hydrogen-deficient atmospheres, and whose origin remains uncertain.
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