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تسجيل مستخدم جديدGalactic Rotation from Cepheids with Gaia DR2 and Effects of Non-Axisymmetry
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Daisuke Kawata
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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$^{-1}$.
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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 Cepheid
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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.
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ide, spatially resolved companions of the targets of our reference samples of Galactic CCs and RRLs. Methods. Angularly resolved common proper motion pairs were detected using a simple progressive selection algorithm to separate the most probable candidate companions from the unrelated field stars. Results. We found 27 resolved, high probability gravitationally bound systems with CCs out of 456 examined stars, and one unbound star embedded in the circumstellar dusty nebula of the long-period Cepheid RS Pup. We found seven spatially resolved, probably bound systems with RRL primaries out of 789 investigated stars, and 22 additional candidate pairs. We report in particular new companions of three bright RRLs: OV And (companion of F4V spectral type), RR Leo (M0V), and SS Oct (K2V). In addition, we discovered resolved companions of 14 stars that were likely misclassified as RRLs. Conclusions. The detection of resolved non-variable companions around CCs and RRLs facilitates the validation of their Gaia DR2 parallaxes. The possibility to conduct a detailed analysis of the resolved coeval companions of CCs and old population RRLs will also be valuable to progress on our understanding of their evolutionary path.
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