Using data from the literature, we made a list of individual estimates of the solar Galactocentric distance, which were performed after 2017 by different methods. These values have not yet been used to calculate the best value of mean $R_0$. For the sample containing 21 estimates, based on the standard approach, we found the weighted mean ${\overline R_0}=8.14$ kpc with the dispersion $\sigma=0.16$ kpc, and using the median statistics, we obtained the estimate $R_0=8.15\pm0.11$ kpc. For practical use, the value $R_0=8.1\pm0.1$ kpc can be recommended.

A New Estimate of the Best Value for the Solar Galactocentric Distance

Context. Mapping the Galactic spiral structure is a di ﬃ cult task since the Sun is located in the Galactic plane and because of dust extinction. For these reasons, molecular masers in radio wavelengths have been used with great success to trace the Milky Way spiral arms. Recently, Gaia parallaxes have helped in investigating the spiral structure in the Solar extended neighborhood. Aims. In this paper, we propose to determine the location of the spiral arms using Cepheids since they are bright, young supergiants with accurate distances (they are the ﬁrst ladder of the extragalactic distance scale). They can be observed at very large distances; therefore, we need to take the Galactic warp into account. Methods. Thanks to updated mid-infrared photometry and to the most complete catalog of Galactic Cepheids, we derived the pa- rameters of the warp using a robust regression method. Using a clustering algorithm, we identiﬁed groups of Cepheids after having corrected their Galactocentric distances from the (small) e ﬀ ects of the warp. Results. We derived new parameters for the Galactic warp, and we show that the warp cannot be responsible for the increased disper- sion of abundance gradients in the outer disk reported in previous studies. We show that Cepheids can be used to trace spiral arms, even at large distances from the Sun. The groups we identify are consistent with previous studies explicitly deriving the position of spiral arms using young tracers (masers, OB(A) stars) or mapping overdensities of upper main-sequence stars in the Solar neighborhood thanks to Gaia data.

/pdf/tracing-the-milky-way-warp-and-spiral-arms-with-classical-1t0mgmzm.pdf

Tracing the Milky Way warp and spiral arms with classical Cepheids

http://academic.oup.com/mnras/advance-article-pdf/doi/10.1093/mnras/stab2971/40820713/stab2971.pdf

Spiral density-wave structure parameters in the solar neighbourhood derived from longitudinal velocities of Gaia EDR3 OB stars: 3D approach

We have analyzed the kinematics of OB2 stars with proper motions and parallaxes selected by Xu et al. from the Gaia EDR3 catalog. The relative parallax errors for all the stars in this sample do not exceed 10%. Based on a sample of 9750 stars, the group velocity components $${{(U,V,W)}_{ \odot }} = $$ (7.21, 7.46, 8.52) ± (0.13, 0.20, 0.10) km/s were obtained and the parameters of the angular velocity of rotation of the Galaxy: $${{\Omega }_{0}} = 29.712 \pm 0.062$$ km/s/kpc, $$\Omega _{0}^{'} = - 4.014 \pm 0.018$$ km/s/kpc2, and $$\Omega _{0}^{{''}} = 0.674 \pm 0.009$$ km/s/kpc3. The circular velocity of rotation of the solar neighborhood around the center of the Galaxy is $${{V}_{0}} = 240.7 \pm 3.0$$ km/s for the assumed distance of the Sun to the galactic center $${{R}_{0}} = 8.1 \pm 0.1$$ kpc. It is shown that the influence of the systematic correction to the trigonometric parallaxes of the Gaia EDR3 catalog with the value $$\Delta \pi = - 0.040$$ mas does not exceed the $$\sim {\kern 1pt} 1\sigma $$ level of the errors of the sought-for kinematic parameters of the model. Based on the proper motions of OB stars, the following variances of the residual velocities were found: $$({{\sigma }_{1}},{{\sigma }_{2}},{{\sigma }_{3}}) = (11.79,9.66,7.21) \pm (0.06,0.05,0.04)$$ km/s. It is shown that the first axis of this ellipsoid slightly deviates from the direction to the center of the Galaxy $${{L}_{1}} = 12.4^\circ \pm 0.1^\circ $$ , and the third axis is oriented almost exactly to the north pole of the Galaxy, $${{B}_{3}} = 83.7^\circ \pm 0.1^\circ $$ . 

/pdf/galaxy-rotation-parameters-from-ob2-stars-with-proper-245c9rtk.pdf

Galaxy Rotation Parameters from OB2 Stars with Proper Motions and Parallaxes from the Gaia EDR3 Catalog

Using data from the literature, we made a list of individual estimates of the solar Galactocentric distance, which were performed after 2017 by different methods. These values have not yet been used to calculate the best value of mean $${{R}_{0}}$$
 . For the sample containing 21 estimates, based on the standard approach, we found the weighted mean $${{\bar {R}}_{0}} = 8.14$$
 kpc with the dispersion $$\sigma = 0.16$$
 kpc, and using the median statistics, we obtained the estimate $${{R}_{0}} = 8.15 \pm 0.11$$
 kpc. For practical use, the value $${{R}_{0}} = 8.1 \pm 0.1$$
 kpc can be recommended.

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 $V_0=240\pm3$~km s$^{-1}$. In this case, the distance from the Sun to the axis of rotation of the Galaxy is found to be $R_0=8.27\pm0.10$~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 $f_R\sim12$~km s$^{-1}$ and $f_\theta\sim9$~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.

V. V. Bobylev

Papers

Analysis of galaxy kinematics based on Cepheids from the Gaia DR2 Catalogue