arXiv subjects

Ralph Schönrich

Publications and source records attributed to Ralph Schönrich.

At least 37 records · Page 2Linked to original sources

arXiv2019

More than just a wrinkle: A wave-like pattern in radial velocity vs. angular momentum from Gaia Data

We present a newly found wave--like pattern in mean Galactocentric radial velocity U_g vs. guiding centre radius R_g or angular momentum L_z of stars in the RV subsample of Gaia DR2. The short-wave pattern has a wavelength of order 1.2 kpc in R_g or 285 kpc km/s in L_z. The pattern shows only weak changes with Galactocentric radius R and little change in strength in particular with the vertical energy E_z of the stars or respectively 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 weak phase shift with the pattern moving towards slightly lower L_z (i.e. trailing) with |z| and E_z. However, we observe a highly significant phase shift in Galactic azimuth phi, which is different for different peaks. The peak around L_z ~ 2100 kpc km/s only shows a weak change with galactic azimuth, while the rest of the pattern shows a clearly detectable shift of d L_z /d phi = (200 +/- 22) kpc km/s/rad. 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 U_g appears to be related to the W vs. L_z pattern detected in Gaia DR1. A comparison of the U_g-L_z wave pattern with changes of U_g vs. R, which have been previously discussed, suggests that the latter can be understood as just the U_g-L_z pattern washed out by blurring (i.e. orbital excursions around their guiding-centre) of disc stars.

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arXiv2019

Distances and parallax bias in Gaia DR2

We derive Bayesian distances for all stars in the RV sample of Gaia DR2, and use the statistical method of Schoenrich, Binney & Asplund(2012) 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 delta p. On average, we find delta p = -0.054 mas (parallaxes in Gaia DR2 need to be increased) when accounting for the parallax uncertainty under-estimate in the Gaia set (delta p = -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. Last, we find that for this sample delta p appears to strongly depend on the parallax uncertainty sigma_p (when including the additional 0.043 mas) with a statistical confidence far in excess of 10σand a proportionality factor close to 1, though the dependence varies somewhat with sigma_p. Correcting for the sigma_p dependence also resolves otherwise unexplained correlations of the offset with the number of observation periods n_{vis} and ecliptic latitude. Every study using Gaia DR2 parallaxes/distances should investigate the sensitivity of their results on the parallax biases described here and - for fainter samples - in the DR2 astrometry paper.

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arXiv2019

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

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 τ_{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 τ_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.

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arXiv2018

Warp, Waves, and Wrinkles in the Milky Way

We derive unbiased distance estimates for the Gaia-TGAS dataset by correcting for the bias due to the distance dependence of the selection function, which we measure directly from the data. From these distances and proper motions, we estimate the vertical and azimuthal velocities, $W$ and $V_ϕ$, and angular momentum $L_z$ for stars in the Galactic centre and anti-centre directions. The resulting mean vertical motion $\overline{W}$ shows a linear increase with both $V_ϕ$ and $L_z$ at $10 σ$ significance. Such a trend is expected from and consistent with the known Galactic warp. This signal extends to stars with guiding centre radii $R_g<R_0$, placing the onset of the warp at $R\lesssim7{\rm kpc}$. At equally high significance, we detect a previously unknown wave-like pattern of $\overline{W}$ over guiding centre $R_g$ with amplitude $\sim1{\rm kms}^{-1}$ and wavelength $\sim2.5{\rm kpc}$. This pattern is present in both the centre and anti-centre directions, consistent with a winding (corrugated) warp or bending wave, likely related to known features in the outer disc (TriAnd and Monoceros over-densities), and may be caused by the interaction with the Sgr dwarf galaxy $\sim1{\rm Gyr}$ ago. The only significant deviation from this simple fit is a stream-like feature near $R_g\sim9{\rm kpc}$ ($|L_z|\sim2150{\rm kpckms}^{-1}$).

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arXiv2017

Assessing distances and consistency of kinematics in Gaia/TGAS

We apply the statistical methods by Schoenrich, Binney & Asplund to assess the quality of distances and kinematics in the RAVE-TGAS and LAMOST-TGAS samples of Solar neighbourhood stars. These methods yield a nominal distance accuracy of 1-2%. Other than common tests on parallax accuracy, they directly test distance estimations including the effects of distance priors. We show how to construct these priors including the survey selection functions (SSFs) directly from the data. We demonstrate that neglecting the SSFs causes severe distance biases. Due to the decline of the SSFs in distance, the simple 1/parallax estimate only mildly underestimates distances. We test the accuracy of measured line-of-sight velocities (v_los) by binning the samples in the nominal v_los uncertainties. We find: a) the LAMOST v_los have a ~ -5 km/s offset; b) the average LAMOST measurement error for v_los is ~7 km/s, significantly smaller than, and nearly uncorrelated with the nominal LAMOST estimates. The RAVE sample shows either a moderate distance underestimate, or an unaccounted source of v_los dispersion (e_v) from measurement errors and binary stars. For a subsample of suspected binary stars in RAVE, our methods indicate significant distance underestimates. Separating a sample in metallicity or kinematics to select thick-disc/halo stars, discriminates between distance bias and e_v. For LAMOST, this separation yields consistency with pure v_los measurement errors. We find an anomaly near longitude l~(300+/-60)deg and distance s~(0.32+/-0.03)kpc on both sides of the galactic plane, which could be explained by either a localised distance error or a breathing mode.

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arXiv2017

Migration and kinematics in growing disc galaxies with thin and thick discs

We analyse disc heating and radial migration in N-body models of growing disc galaxies with thick and thin discs. Similar to thin-disc-only models, galaxies with appropriate non-axisymmetric structures reproduce observational constraints on radial disc heating in and migration to the Solar Neighbourhood (Snhd). The presence of thick discs can suppress non-axisymmetries and thus higher baryonic-to-dark matter fractions are required than in models that only have a thin disc. Models that are baryon-dominated to roughly the Solar radius R_0 are favoured, in agreement with data for the Milky Way. For inside-out growing discs, today's thick-disc stars at R_0 are dominated by outwards migrators. Whether outwards migrators are vertically hotter than non-migrators depends on the radial gradient of the thick disc vertical velocity dispersion. There is an effective upper boundary in angular momentum that thick disc stars born in the centre of a galaxy can reach by migration, which explains the fading of the high-alpha sequence outside R_0. Our models compare well to Snhd kinematics from RAVE-TGAS. For such comparisons it is important to take into account the azimuthal variation of kinematics at R ~ R_0 and biases from survey selection functions. The vertical heating of thin disc stars by giant molecular clouds is only mildly affected by the presence of thick discs. Our models predict higher vertical velocity dispersions for the oldest stars than found in the Snhd age-velocity dispersion relation, possibly because of measurement uncertainties or an underestimation of the number of old cold stars in our models.

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arXiv2017

Understanding inverse metallicity gradients in galactic discs as a consequence of inside-out formation

The early stages of a galaxy's evolution leave an imprint on its metallicity distribution. We discuss the origins and evolution of radial metallicity gradients in discs of spiral galaxies using an analytical chemical evolution model. We explain how radial metallicity gradients in stellar populations are determined by three factors: the radial metallicity profile of the star-forming ISM, radial changes in the star-formation history (in particular inside-out formation), and radial mixing of stars. Under reasonable assumptions, inside-out formation steepens the negative ISM metallicity gradient, but contributes positively to the stellar metallicity gradient, up to inverting the metallicity profile to a positive d[Fe/H]/dR. This reconciles steep negative d[Fe/H]/dR in some high redshift galaxies to generally flatter gradients in local observations. We discuss the evidence for inverse radial metallicity gradients (positive d[X/H]/dR) at high redshifts and the inverse relationship between azimuthal velocity and the metallicity (positive dV_ϕ/d[Fe/H]) of stars for the Milky Way's thick disc. The former can be achieved by high central gas-loss rates and re-distribution processes, e.g. re-accretion of enriched material in conjunction with inside-out formation, and near-disc galactic fountaining. For the Milky Way thick disc, we show that the positive dV_ϕ/d[Fe/H] correlation points to comparable timescales for inside-out formation, initial metal enrichment and SNIa enrichment. We argue that the original ISM metallicity gradient could be inferred with better data from the high-metallicity tail of the alpha enhanced population. Including inside-out formation in our models changes the local vertical metallicity gradient by about -0.2dex/kpc, in line with local measurements.

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arXiv2016

Inflow, Outflow, Yields, and Stellar Population Mixing in Chemical Evolution Models

Chemical evolution models are powerful tools for interpreting stellar abundance surveys and understanding galaxy evolution. However, their predictions depend heavily on the treatment of inflow, outflow, star formation efficiency (SFE), the stellar initial mass function, the Type Ia supernova delay time distribution, stellar yields, and stellar population mixing. Using flexCE, a flexible one-zone chemical evolution code, we investigate the effects of and trade-offs between parameters. Two critical parameters are SFE and the outflow mass-loading parameter, which shift the knee in [O/Fe]-[Fe/H] and the equilibrium abundances that the simulations asymptotically approach, respectively. One-zone models with simple star formation histories follow narrow tracks in [O/Fe]-[Fe/H] unlike the observed bimodality (separate high-alpha and low-alpha sequences) in this plane. A mix of one-zone models with inflow timescale and outflow mass-loading parameter variations, motivated by the inside-out galaxy formation scenario with radial mixing, reproduces the two sequences better than a one-zone model with two infall epochs. We present [X/Fe]-[Fe/H] tracks for 20 elements assuming three different supernova yield models and find some significant discrepancies with solar neighborhood observations, especially for elements with strongly metallicity-dependent yields. We apply principal component abundance analysis (PCAA) to the simulations and existing data to reveal the main correlations amongst abundances and quantify their contributions to variation in abundance space. For the stellar population mixing scenario, the abundances of alpha-elements and elements with metallicity-dependent yields dominate the first and second principal components, respectively, and collectively explain 99% of the variance in the model. flexCE is a python package available at https://github.com/bretthandrews/flexCE.

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arXiv2016

Age velocity dispersion relations and heating histories in disc galaxies

We analyse the heating of stellar discs by non axisymmetric structures and giant molecular clouds (GMCs) in N-body simulations of growing disc galaxies. The analysis resolves long-standing discrepancies between models and data by demonstrating the importance of distinguishing between measured age-velocity dispersion relations (AVRs) and the heating histories of the stars that make up the AVR. We fit both AVRs and heating histories with formulae proportional to t^beta and determine the exponents beta_R and beta_z derived from in-plane and vertical AVRs and ~beta_R and ~beta_z from heating histories. Values of beta_z are in almost all simulations larger than values of ~beta_z, whereas values of beta_R are similar to or mildly larger than values of ~beta_R. Moreover, values of beta_z (~beta_z) are generally larger than values of beta_R (~beta_R). The dominant cause of these relations is the decline over the life of the disc in importance of GMCs as heating agents relative to spiral structure and the bar. We examine how age errors and biases in solar neighbourhood surveys influence the measured AVR: they tend to decrease beta values by smearing out ages and thus measured dispersions. We compare AVRs and velocity ellipsoid shapes sigma_z/sigma_R from simulations to Solar neighbourhood data. We conclude that for the expected disc mass and dark halo structure, combined GMC and spiral/bar heating can explain the AVR of the Galactic thin disc. Strong departures of the disc mass or the dark halo structure from expectation spoil fits to the data.

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arXiv2016

The quiescent phase of galactic disc growth

We perform a series of controlled N-body simulations of growing disc galaxies within non-growing, live dark matter haloes of varying mass and concentration. Our initial conditions include either a low-mass disc or a compact bulge. New stellar particles are continuously added on near-circular orbits to the existing disc, so spiral structure is continuously excited. To study the effect of combined spiral and giant molecular cloud (GMC) heating on the discs we introduce massive, short-lived particles that sample a GMC mass function. An isothermal gas component is introduced for a subset of the models. We perform a resolution study and vary parameters governing the GMC population, the histories of star formation and radial scale growth. Models with GMCs and standard values for the disc mass and halo density provide the right level of self-gravity to explain the age velocity dispersion relation of the Solar neighbourhood (Snhd). GMC heating generates remarkably exponential vertical profiles with scaleheights that are radially constant and agree with observations of galactic thin discs. GMCs are also capable of significantly delaying bar formation. The amount of spiral induced radial migration agrees with what is required for the metallicity distribution of the Snhd. However, in our standard models the outward migrating populations are not hot enough vertically to create thick discs. Thick discs can form in models with high baryon fractions, but the corresponding bars are too long, the young stellar populations too hot and the discs flare considerably.

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arXiv2015

Kinematic Detection of the Galactic Nuclear Disc

We report the detection of the Galactic nuclear disc in line-of-sight kinematics of stars, measured with infrared spectroscopy from APOGEE. This stellar component of the nuclear disc has an extent and rotation velocity V ~ 120kms comparable to the gas disc in the central molecular zone. The current data suggest that this disc is kinematically cool and has a small vertical extent of order 50pc. The stellar kinematics suggest a truncation radius/steep decline of the stellar disc at a galactocentric radius R ~ 150pc, and provide tentative evidence for an overdensity at the position of the ring found in the molecular gas disc.

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arXiv2015

Origin of the high v_los feature in the Galactic bar

We analyse a controlled N-body + smoothed particle hydrodynamics simulation of a growing disc galaxy within a non-growing, live dark halo. The disc is continuously fed with gas and star particles on near-circular orbits and develops a bar comparable in size to the one of the Milky Way (MW). We extract line of sight velocity v_los distributions from the model and compare it to data recently obtained from the APOGEE survey which show distinct high velocity features around v_los ~ 200 km/s. With an APOGEE like selection function, but without any scaling nor adjustment, we find v_los distributions very similar to those in APOGEE. The stars that make up the high v_los features at positive longitudes l are preferentially young bar stars (age <~ 2-3 Gyr) which move away from us along the rear side of the bar. At negative l, we find the corresponding low v_los feature from stars moving towards us. At l>10 degrees the highest v_los stars are a mixture of bar and background disc stars which complicates the interpretation of observations. The main peak in v_los is dominated by fore- and background stars. At a given time, ~40-50 per cent of high v_los stars occupy x_1 like orbits, but a significant fraction are on more complex orbits. The observed feature is likely due to a population of dynamically cool, young stars formed from gas just outside the bar and subsequently captured by the growing bar. The high v_los features disappear at high latitudes |b|>~2 degrees which explains the non-detection of such features in other surveys.

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arXiv2015

Rings and Radial Waves in the Disk of the Milky Way

We show that in the anticenter region, between Galactic longitudes of $110^\circ<l<229^\circ$, there is an oscillating asymmetry in the main sequence star counts on either side of the Galactic plane using data from the Sloan Digital Sky Survey. This asymmetry oscillates from more stars in the north at distances of about 2 kpc from the Sun to more stars in the south at 4-6 kpc from the Sun to more stars in the north at distances of 8-10 kpc from the Sun. We also see evidence that there are more stars in the south at distances of 12-16 kpc from the Sun. The three more distant asymmetries form roughly concentric rings around the Galactic center, opening in the direction of the Milky Way's spiral arms. The northern ring, 9 kpc from the Sun, is easily identified with the previously discovered Monoceros Ring. Parts of the southern ring at 14 kpc from the Sun (which we call the TriAnd Ring) have previously been identified as related to the Monoceros Ring and others have been called the Triangulum Andromeda Overdensity. The two nearer oscillations are approximated by a toy model in which the disk plane is offset by of the order 100 pc up and then down at different radii. We also show that the disk is not azimuthally symmetric around the Galactic anticenter and that there could be a correspondence between our observed oscillations and the spiral structure of the Galaxy. Our observations suggest that the TriAnd and Monoceros Rings (which extend to at least 25 kpc from the Galactic center) are primarily the result of disk oscillations.

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arXiv2014

Fundamental stellar parameters and metallicities from Bayesian spectroscopy. Application to low- and high-resolution spectra

We present a unified framework to derive fundamental stellar parameters by combining all available observational and theoretical information for a star. The algorithm relies on the method of Bayesian inference, which for the first time directly integrates the spectroscopic analysis pipeline based on the global spectrum synthesis and allows for comprehensive and objective error calculations given the priors. Arbitrary input datasets can be included into our analysis and other stellar quantities, in addition to stellar age, effective temperature, surface gravity, and metallicity, can be computed on demand. We lay out the mathematical framework of the method and apply it to several observational datasets, including high- and low-resolution spectra (UVES, NARVAL, HARPS, SDSS/SEGUE). We find that simpler approximations for the spectroscopic PDF, which are inherent to past Bayesian approaches, lead to deviations of several standard deviations and unreliable errors on the same data. By its flexibility and the simultaneous analysis of multiple independent measurements for a star, it will be ideal to analyse and cross-calibrate the large ongoing and forthcoming surveys, like Gaia-ESO, SDSS, Gaia and LSST.

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arXiv2014

Testing the Asteroseismic Mass Scale Using Metal-Poor Stars Characterized with APOGEE and Kepler

Fundamental stellar properties, such as mass, radius, and age, can be inferred using asteroseismology. Cool stars with convective envelopes have turbulent motions that can stochastically drive and damp pulsations. The properties of the oscillation frequency power spectrum can be tied to mass and radius through solar-scaled asteroseismic relations. Stellar properties derived using these scaling relations need verification over a range of metallicities. Because the age and mass of halo stars are well-constrained by astrophysical priors, they provide an independent, empirical check on asteroseismic mass estimates in the low-metallicity regime. We identify nine metal-poor red giants (including six stars that are kinematically associated with the halo) from a sample observed by both the Kepler space telescope and the Sloan Digital Sky Survey-III APOGEE spectroscopic survey. We compare masses inferred using asteroseismology to those expected for halo and thick-disk stars. Although our sample is small, standard scaling relations, combined with asteroseismic parameters from the APOKASC Catalog, produce masses that are systematically higher (<ΔM>=0.17+/-0.05 Msun) than astrophysical expectations. The magnitude of the mass discrepancy is reduced by known theoretical corrections to the measured large frequency separation scaling relationship. Using alternative methods for measuring asteroseismic parameters induces systematic shifts at the 0.04 Msun level. We also compare published asteroseismic analyses with scaling relationship masses to examine the impact of using the frequency of maximum power as a constraint. Upcoming APOKASC observations will provide a larger sample of ~100 metal-poor stars, important for detailed asteroseismic characterization of Galactic stellar populations.

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arXiv2013

Rotational signature of the Milky Way stellar halo

We measure the rotation of the Milky Way stellar halo on two samples of Blue Horizontal Branch (BHB) field halo stars from the Sloan Digital Sky Survey (SDSS) with four different methods. The two samples comprise 1582 and 2563 stars respectively and reach out to ~50 kpc in galactocentric distance. Two of the methods to measure rotation rely exclusively on line-of-sight velocities, namely the popular double power-law model and a direct estimate of the de-projected l.o.s. velocity. The other two techniques use the full 3D motions: the radial velocity based rotation estimator of Schönrich, Binney & Asplund (2012) and a simple 3D azimuthal velocity mean. In this context we a) critique the popular model and b) assess the reliability of the estimators. All four methods agree on a weakly prograde or non-rotating halo. Further, we observe no duality in the rotation of sub-samples with different metallicities or at different radii. We trace the rotation gradient across metallicity measured by Deason et al. (2011) on a similar sample of BHB stars back to the inclusion of regions in the apparent magnitude-surface gravity plane known to be contaminated. In the spectroscopically selected sample of Xue et al. (2011), we flag ~500 hot metal-poor stars for their peculiar kinematics w.r.t. to both their cooler metal-poor counter-parts and to the metal-rich stars in the same sample. They show a seemingly retrograde behaviour in line-of-sight velocities, which is not confirmed by the 3D estimators. Their anomalous vertical motion hints at either a pipeline problem or a stream-like component rather than a smooth retrograde population.

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arXiv2013

A new calibration for the Blue Horizontal Branch

We suggest a simple analytic approximation for magnitudes and hence distances of Blue Horizontal Branch (BHB) stars in Sloan colours. Precedent formulations do not offer a simple closed formula, nor do they cover the full dependences, e.g. on metallicity. Further, using BHB star samples from the Sloan Digital Sky Survey (SDSS) we validate our distance calibration directly on field stars instead of globular clusters and assess the performance of other available distance calibrations. Our method to statistically measure distances is sufficiently accurate to measure the colour and metallicity dependence on our sample and can be applied to other sets of stars or filters.

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arXiv2012

Radial Flows and Angular Momentum Conservation in Galactic Chemical Evolution

We study the effects of radial flows on Galactic chemical evolution. A simple analytic scheme is developed prescribing the coupling of infall from the intergalactic medium and radial flows within the disc based on angular momentum conservation. We show that model parameters are tightly constrained by the observed [Fe/H]-abundance gradient in the Galactic disc. By this comparison the average rotational velocity of the onfalling material can be constrained to 0.7 < v/V_c < 0.75, or respectively ~ 160 km/s when assuming a constant disc circular velocity of V_c = 220 km/s. We test the robustness of this value against the influence of other processes. For a very simple model of inside-out formation this value changes only by Δv/V_c ~ 0.1, i.e. ~ 20 km/s, and significantly less on more realistic scenarios, showing that inside-out formation does not alone explain the abundance gradient. Effects of other uncertain parameters, e.g. star formation history and star formation efficiency have very small impact. Other drivers of inflow beyond our explicit modelling are assessed by adding a fixed inflow across the whole disc. The churning amplitude only mildly affects the results mostly by slightly flattening the metallicity gradient in the inner disc. A new process causing radial gas flows due to the ejection of material by stars moving on non-circular orbits is studied and seems to contribute negligibly to the total flows. We further show that gaseous outer discs cannot be the main source feeding the persistent star formation in the inner regions by a direct inflow.

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