Showing papers by "Ralph Schönrich published in 2019"

Distances and parallax bias in Gaia DR2

Abstract: We derive Bayesian distances for all stars in the radial velocity sample of Gaia DR2, and use the statistical method of Schonrich, Binney & Asplund to validate the distances and test the Gaia parallaxes. In contrast to other methods, which rely on special sources, our method directly tests the distances to all stars in our sample. We find clear evidence for a near-linear trend of distance bias f with distance s, proving a parallax offset δp. On average, we find δp=-0.054 mas (parallaxes in Gaia DR2 need to be increased) when accounting for the parallax uncertainty underestimate in the Gaia set (compared to δp=-0.048 mas on the raw parallax errors), with negligible formal error and a systematic uncertainty of about 0.006 mas. The value is in concordance with results from asteroseismic measurements, but differs from the much lower bias found on quasar samples. We further use our method to compile a comprehensive set of quality cuts in colour, apparent magnitude, and astrometric parameters. Lastly, we find that for this sample δp appears to strongly depend on σp (when including the additional 0.043 mas) with a statistical confidence far in excess of 10σ and a proportionality factor close to 1, though the dependence varies somewhat with σp. Correcting for the σp dependence also resolves otherwise unexplained correlations of the offset with the number of observation periods nvis and ecliptic latitude. Every study using Gaia DR2 parallaxes/distances should investigate the sensitivity of its results on the parallax biases described here and-for fainter samples-in the DR2 astrometry paper. (Less)

The chemical evolution of r-process elements from neutron star mergers: the role of a 2-phase interstellar medium

Abstract: Neutron star mergers (NM) are a plausible source of heavy r-process elements such as Europium, but previous chemical evolution models have either failed to reproduce the observed Europium trends for Milky Way thick disc stars (with [Fe/H] ~ -1) or have done so only by adopting unrealistically short merger timescales. Using analytic arguments and numerical simulations, we demonstrate that models with a single-phase interstellar medium (ISM) and metallicity-independent yields cannot reproduce observations showing [Eu/alpha] > 0 or [Eu/Fe] > [alpha/Fe] for alpha-elements such as Mg and Si. However, this problem is easily resolved if we allow for a 2-phase ISM, with hot-phase cooling times \tau_{cool} of order 1 Gyr and a larger fraction of NM yields injected directly into the cold star-forming phase relative to alpha-element yields from core collapse supernovae (ccSNe). We find good agreement with observations in models with a cold phase injection ratio f_{c,NM}/f_{c,ccSN} of order 2, and a characteristic merger timescale \tau_NM=150 Myr. We show that the observed super-solar [Eu/alpha] at intermediate metallicities implies that a significant fraction of Eu originates from NM or another source besides ccSNe, and that these non-ccSN yields are preferentially deposited in the star-forming phase of the ISM at early times.

More than just a wrinkle: a wave-like pattern in Ug versus Lz from Gaia data

Abstract: We present a newly found wave-like pattern in mean Galactocentric radial velocity U¯¯¯¯g versus guiding centre radius Rg or angular momentum Lz of stars in the RV subsample of Gaia DR2. The short-wave pattern has a wavelength of order 1.2kpc in Rg or 285kpckms−1 in Lz. The pattern shows only weak changes with Galactocentric radius R and little change in strength in particular with the vertical energy Ez of the stars or the distance to the Galactic plane |z|. The pattern is to first order symmetric around the plane, i.e. has no significant odd terms in z. There is a weak phase shift with the pattern moving towards slightly lower Lz (i.e. trailing) with |z| and Ez. However, we observe a highly significant phase shift in Galactic azimuth ϕ, which is different for different peaks. The peak around Lz∼2100kpckms−1 only shows a weak change with ϕ, while the rest of the pattern shows a clearly detectable shift of dLz/dϕ=(200±22)kpckms−1rad−1⁠. If we consider all peaks to belong to the same pattern, this would suggest a wavenumber m = 4. We further find that the wave-like pattern in Ug appears to be related to the W¯¯¯¯¯ versus Lz pattern detected in Gaia DR1. A comparison of the U¯¯¯¯g−Lz wave pattern with changes of U¯¯¯¯g versus R, which have been previously discussed, suggests that the latter can be understood as just the U¯¯¯¯g−Lz pattern washed out by blurring (i.e. orbital excursions around their guiding centre) of disc stars.

The tilt of the local velocity ellipsoid as seen by Gaia

Abstract: 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 Schonrich 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 $\alpha = (0.952 \pm 0.007)\arctan (|z|/R)$, 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 $\alpha = (0.909-1.038)\arctan (|z|/R)$, and of halo stars straddles $\alpha = (0.927-1.063)\arctan (|z|/R)$. We caution against the use of reciprocal parallax for distances in studies of the tilt, as this can lead to serious artefacts.

Resonance sweeping by a decelerating bar

Abstract: We provide the first quantitative evidence for the deceleration/growth of the Galactic bar from local stellar kinematics thus confirming dynamical friction within expectations for a typical dark matter halo. The kinematic response of the stellar disk to a decelerating bar is studied using secular perturbation theory and test particle simulations. We show that the velocity distribution at any point in the disk affected by a naturally slowing bar is qualitatively different from that perturbed by a steadily rotating bar with the same current pattern speed $\Omega_p$ and amplitude. When the bar slows with rate $d\Omega_p/dt$, its resonances sweep through phase space. Depending on $d{\Omega}_p/dt$, they trap and drag along a portion of previously free orbits. This enhances occupation on resonances, but also changes the distribution of stars within the resonant region. Helped by this accumulation of orbits near the boundary of the resonant region, the decelerating bar model reproduces with its corotation resonance the offset and strength of the Hercules stream in the local $v_R-v_\varphi$ plane and the double-peaked structure of $ $ in the $L_z-\varphi$ plane. On the outer/inner Lindblad resonances and other higher order resonances, resonant dragging by a slowing bar is associated with a continuing increase in radial action. We compare the model to data in the action plane, identifying multiple resonance ridges. This work shows models using a constant bar pattern speed ($d\Omega_p/dt=0$) likely lead to qualitatively wrong conclusions. Most importantly we provide the first quantitative estimate of the slowing rate of the bar: $d\Omega_p/dt = (-5.0 \pm 2.5)$ km/s/kpc/Gyr.