arXiv subjects

R. Schönrich

Publications and source records attributed to R. Schönrich.

4 recordsLinked to original sources

arXiv2019

The Tilt of the Local Velocity Ellipsoid as Seen by Gaia

The Gaia Radial Velocity Spectrometer (RVS) provides a sample of 7,224,631 stars with full six-dimensional phase space information. Bayesian distances of these stars are available from the catalogue of Schönrich et al. (2019). We exploit this to map out the behaviour of the velocity ellipsoid within 5 kpc of the Sun. We find that the tilt of the disc-dominated RVS sample is accurately described by the relation $α= (0.952 \pm 0.007)\arctan (|z|/R)$, where ($R,z$) are cylindrical polar coordinates. This corresponds to velocity ellipsoids close to spherical alignment (for which the normalising constant would be unity) and pointing towards the Galactic centre. Flattening of the tilt of the velocity ellipsoids is enhanced close to the plane and Galactic centre, whilst at high elevations far from the Galactic center the population is consistent with exact spherical alignment. Using the LAMOST catalogue cross-matched with Gaia DR2, we construct thin disc and halo samples of reasonable purity based on metallicity. We find that the tilt of thin disc stars straddles $α= (0.909-1.038)\arctan (|z|/R)$, and of halo stars straddles $α= (0.927-1.063)\arctan (|z|/R)$. We caution against the use of reciprocal parallax for distances in studies of the tilt, as this can lead to serious artefacts.

astro-ph.GA↗

arXiv2018

On the kinematic signature of the Galactic warp as revealed by the LAMOST-TGAS data

Using a sample of about 123,000 stars with accurate 3D velocity measurements from the LAMOST-TGAS data, we confirm the kinematic signature of the Galactic warp found by Schönrich & Dehnen recently. The data reveal a clear trend of increasing mean vertical velocity $\overline{V_{z}}$ as a function of absolute vertical angular momentum $L_{z}$ and azimuthal velocity $V_ϕ$ for guiding center radius $R_{g}$ between 6.0 and 10.5 kpc. The trend is consistent with a large-scale Galactic warp. Similar to Schönrich & Dehnen, we also find a wave-like pattern of $\overline{V_{z}}$ versus $L_{z}$ with an amplitude of $\sim 0.9$ km s$^{-1}$ on a scale of $\sim 2.0$ kpc, which could arise from bending waves or a winding warp. Finally, we confirm a prominent, localized peak in $\overline{V_z}$ near $L_z \sim 2150$ kpc km s$^{-1}$ (corresponding to $R_{g} \sim 9$ kpc and $V_ϕ \sim 255$ km s$^{-1}$). The additional line-of-sight velocity information from LAMOST reveals that stars in this feature have a large, inward radial velocity of $V_{R} \sim -13.33 \pm 0.59$ km s$^{-1}$ and a small radial velocity dispersion $σ_{R} \sim 25.27 \pm 0.89$ km s$^{-1}$, suggesting that a stellar stream gives rise to this feature.

astro-ph.GA↗

arXiv2018

Confirming chemical clocks: asteroseismic age dissection of the Milky Way disk(s)

Investigations of the origin and evolution of the Milky Way disk have long relied on chemical and kinematic identification of its components to reconstruct our Galactic past. Difficulties in determining precise stellar ages have restricted most studies to small samples, normally confined to the solar neighbourhood. Here we break this impasse with the help of asteroseismic inference and perform a chronology of the evolution of the disk throughout the age of the Galaxy. We chemically dissect the Milky Way disk population using a sample of red giant stars spanning out to 2~kpc in the solar annulus observed by the {\it Kepler} satellite, with the added dimension of asteroseismic ages. Our results reveal a clear difference in age between the low- and high-$α$ populations, which also show distinct velocity dispersions in the $V$ and $W$ components. We find no tight correlation between age and metallicity nor [$α$/Fe] for the high-$α$ disk stars. Our results indicate that this component formed over a period of more than 2~Gyr with a wide range of [M/H] and [$α$/Fe] independent of time. Our findings show that the kinematic properties of young $α$-rich stars are consistent with the rest of the high-$α$ population and different from the low-$α$ stars of similar age, rendering support to their origin being old stars that went through a mass transfer or stellar merger event, making them appear younger, instead of migration of truly young stars formed close to the Galactic bar.

astro-ph.GA↗

arXiv2009

Dust properties in M31.I.Basic properties and a discussion on age-dependent dust heating

Context. Spitzer Space Telescope observations and dust emission models are used to discuss the distribution of dust and its characteristics in M31. Together with GALEX FUV, NUV, and SDSS images we studied the age dependence of the dust heating process. Methods.Spitzer IRAC/MIPS maps of M31 were matched together and compared to dust emission models allowing to constrain the dust mass, the intensity of the mean radiation field, the abundance of Polycyclic Aromatic Hydrocarbons (PAH) particles. The total infrared emission (TIR) was analyzed in function of UV and Optical colors and compared to predictions of models which consider the age-dependent dust heating. Results. We demonstrate that cold-dust component emission dominates the infrared spectral energy distribution of M31. The mean intensity of the radiation field heating the dust is low (typically U<2, where U=1 is the value in the solar surrounding). Due to the lack of submillimetric measurements the dust mass (M_{dust}) is only weakly constrained by the infrared spectrum. We show that across the spiral-ring structure of M31 a fraction >3% of the total dust mass is in PAHs. UV and optical colors are correlated to (TIR/FUV) ratios in \sim 670 pc-sized regions overall the disk of M31, although deviating from the IRX-beta relationship for starburst galaxies. We derived that in 83% of the regions analyzed across the 10kpc ring more than 50% of the energy absorbed by the dust is rediated at λ> 4000 Åand that dust in M31 appears mainly heated by populations a few Gyr old even across the star-forming ring. The attenuation is varying radially peaking near 10kpc and decreasing faster in the inner regions of M31 than in the outer regions. We finally derived the attenuation map of M31 at 6"/px resolution (\sim 100 pc/px along the plane of M31).[abridged]

astro-ph.GA↗