Publications by Ralph Schoenrich


Warp, waves, and wrinkles in the Milky Way

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 478 (2018) 3809-3824

R Schonrich, W Dehnen


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

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 475 (2018) 5487-5500

VS Aguirre, M Bojsen-Hansen, D Slumstrup, L Casagrande, D Kawata, I Ciuca, R Handberg, MN Lund, JR Mosumgaard, D Huber, JA Johnson, MH Pinsonneault, AM Serenelli, D Stello, J Tayar, JC Bird, S Cassisi, M Hon, M Martig, PE Nissen, HW Rix, R Schonrich, C Sahlholdt, WH Trick, J Yu


The origin of the Gaia phase-plane spiral

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 481 (2018) 1501-1506

J Binney, R Schonrich


Two chemically similar stellar overdensities on opposite sides of the plane of the Galactic disk.

Nature 555 (2018) 334-337

M Bergemann, B Sesar, JG Cohen, AM Serenelli, A Sheffield, TS Li, L Casagrande, KV Johnston, CFP Laporte, AM Price-Whelan, R Schönrich, A Gould

Our Galaxy is thought to have an active evolutionary history, dominated over the past ten billion years or so by star formation, the accretion of cold gas and, in particular, the merging of clumps of baryonic and dark matter. The stellar halo-the faint, roughly spherical component of the Galaxy-reveals rich 'fossil' evidence of these interactions, in the form of stellar streams, substructures and chemically distinct stellar components. The effects of interactions with dwarf galaxies on the content and morphology of the Galactic disk are still being explored. Recent studies have identified kinematically distinct stellar substructures and moving groups of stars in our Galaxy, which may have extragalactic origins. There is also mounting evidence that stellar overdensities (regions with greater-than-average stellar density) at the interface between the outer disk and the halo could have been caused by the interaction of a dwarf galaxy with the disk. Here we report a spectroscopic analysis of 14 stars from two stellar overdensities, each lying about five kiloparsecs above or below the Galactic plane-locations suggestive of an association with the stellar halo. We find that the chemical compositions of these two groups of stars are almost identical, both within and between these overdensities, and closely match the abundance patterns of stars in the Galactic disk. We conclude that these stars came from the disk, and that the overdensities that they are part of were created by tidal interactions of the disk with passing or merging dwarf galaxies.


On the Kinematic Signature of the Galactic Warp As Revealed By the LAMOST-TGAS Data

ASTROPHYSICAL JOURNAL 864 (2018) ARTN 129

Y Huang, R Schonrich, X-W Liu, B-Q Chen, H-W Zhang, H-B Yuan, M-S Xiang, C Wang, Z-J Tian


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

ASTROPHYSICAL JOURNAL 835 (2017) ARTN 224

BH Andrews, DH Weinberg, R Schonrich, JA Johnson


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

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 467 (2017) 1154-1174

R Schonrich, PJ McMillan


Non-local Thermodynamic Equilibrium Stellar Spectroscopy with 1D and 3D Models. II. Chemical Properties of the Galactic Metal-poor Disk and the Halo

Astrophysical Journal 847 (2017)

M Bergemann, R Collet, R Schönrich, R Andrae, M Kovalev, G Ruchti, CJ Hansen, Z Magic

© 2017. The American Astronomical Society. All rights reserved. From exploratory studies and theoretical expectations it is known that simplifying approximations in spectroscopic analysis (local thermodynamic equilibrium (LTE), 1D) lead to systematic biases of stellar parameters and abundances. These biases depend strongly on surface gravity, temperature and, in particular, for LTE versus non-LTE (NLTE), on metallicity of the stars. Here we analyze the [Mg/Fe] and [Fe/H] plane of a sample of 326 stars, comparing LTE and NLTE results obtained using 1D hydrostatic models and averaged models. We show that compared to the NLTE benchmark, the other three methods display increasing biases toward lower metallicities, resulting in false trends of [Mg/Fe] against [Fe/H] , which have profound implications for interpretations by chemical evolution models. In our best NLTE model, the halo and disk stars show a clearer behavior in the [Mg/Fe]-[Fe/H] plane, from the knee in abundance space down to the lowest metallicities. Our sample has a large fraction of thick disk stars and this population extends down to at least [Fe/H] ∼ -1.6 dex, further than previously proven. The thick disk stars display a constant [Mg/Fe] ≈ 0.3 dex, with a small intrinsic dispersion in [Mg/Fe] that suggests that a fast SN Ia channel is not relevant for the disk formation. The halo stars reach higher [Mg/Fe] ratios and display a net trend of [Mg/Fe] at low metallicities, paired with a large dispersion in [Mg/Fe] . These indicate the diverse origin of halo stars from accreted low-mass systems to stochastic/inhomogeneous chemical evolution in the Galactic halo.


Assessing distances and consistency of kinematics in Gaia/TGAS

Monthly Notices of the Royal Astronomical Society Oxford University Press 472 (2017) 3979–3998-

RA Schönrich, M Aumer

We apply the statistical methods by Schönrich, Binney & Asplund to assess the quality of distances and kinematics in the Radial Velocity Experiment (RAVE)–Tycho-Gaia Astrometric Solution (TGAS) and Large Sky Area Multiobject Fiber Spectroscopic Telescope (LAMOST)–TGAS samples of Solar neighbourhood stars. These methods yield a nominal distance accuracy of 1–2 per cent. 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 (vlos) by binning the samples in the nominal vlos uncertainties. We find: (i) the LAMOST vlos have a ∼−5 km s−1 offset; (ii) the average LAMOST measurement error for vlos is ∼7 km s−1, 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 vlos dispersion (e∥) 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∥. For LAMOST, this separation yields consistency with pure vlos measurement errors. We find an anomaly near longitude l ∼ (300 ± 60)° and distance s ∼ (0.32 ± 0.03) kpc on both sides of the galactic plane, which could be explained by either a localized distance error or a breathing mode.


The 13th data release of the Sloan Digital Sky Survey: First spectroscopic data from the SDSS-IV Survey mapping nearby galaxies at Apache Point Observatory

Astrophysicsl Journal Supplement Series Institute of Physics 233 (2017) 25-

FD Albareti, CA Allende Prieto, A Almeida, M Cappellari, R Schoenrich

The fourth generation of the Sloan Digital Sky Survey (SDSS-IV) began observations in 2014 July. It pursues three core programs: the Apache Point Observatory Galactic Evolution Experiment 2 (APOGEE-2), Mapping Nearby Galaxies at APO (MaNGA), and the Extended Baryon Oscillation Spectroscopic Survey (eBOSS). As well as its core program, eBOSS contains two major subprograms: the Time Domain Spectroscopic Survey (TDSS) and the SPectroscopic IDentification of ERosita Sources (SPIDERS). This paper describes the first data release from SDSS-IV, Data Release 13 (DR13). DR13 makes publicly available the first 1390 spatially resolved integral field unit observations of nearby galaxies from MaNGA. It includes new observations from eBOSS, completing the Sloan Extended QUasar, Emission-line galaxy, Luminous red galaxy Survey (SEQUELS), which also targeted variability-selected objects and X-ray-selected objects. DR13 includes new reductions of the SDSS-III BOSS data, improving the spectrophotometric calibration and redshift classification, and new reductions of the SDSS-III APOGEE-1 data, improving stellar parameters for dwarf stars and cooler stars. DR13 provides more robust and precise photometric calibrations. Value-added target catalogs relevant for eBOSS, TDSS, and SPIDERS and an updated red-clump catalog for APOGEE are also available. This paper describes the location and format of the data and provides references to important technical papers. The SDSS web site, http://www.sdss.org, provides links to the data, tutorials, examples of data access, and extensive documentation of the reduction and analysis procedures. DR13 is the first of a scheduled set that will contain new data and analyses from the planned ∼6 yr operations of SDSS-IV.


Chemical tagging with APOGEE: discovery of a large population of N-rich stars in the inner Galaxy

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 465 (2017) 501-524

RP Schiavon, O Zamora, R Carrera, S Lucatello, AC Robin, M Ness, SL Martell, VV Smith, DA Garcia-Hernandez, A Manchado, R Schonrich, N Bastian, C Chiappini, M Shetrone, JT Mackereth, RA Williams, S Meszaros, C Allende Prieto, F Anders, D Bizyaev, TC Beers, SD Chojnowski, K Cunha, C Epstein, PM Frinchaboy, AE Garcia Perez, FR Hearty, JA Holtzman, JA Johnson, K Kinemuchi, SR Majewski, D Muna, DL Nidever, DC Nguyen, RW O'Connell, D Oravetz, K Pan, M Pinsonneault, DP Schneider, M Schultheis, A Simmons, MF Skrutskie, J Sobeck, JC Wilson, G Zasowski


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

Monthly Notices of the Royal Astronomical Society Oxford University Press 470 (2017) 3685-3706

M Aumer, J Binney, R Schoenrich

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 R0 are favoured, in agreement with data for the Milky Way. For inside-out growing discs, today’s thick-disc stars at R0 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 [α/Fe] sequence outside R0. 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 ∼ R0 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.


Measuring the vertical age structure of the Galactic disc using asteroseismology and SAGA

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 455 (2016) 987-1007

L Casagrande, VS Aguirre, KJ Schlesinger, D Stello, D Huber, AM Serenelli, R Schoenrich, S Cassisi, A Pietrinferni, S Hodgkin, AP Milone, S Feltzing, M Asplund


The Gaia-ESO Survey: Hydrogen lines in red giants directly trace stellar mass

ASTRONOMY & ASTROPHYSICS 594 (2016) ARTN A120

M Bergemann, A Serenelli, R Schonrich, G Ruchti, A Korn, S Hekker, M Kovalev, L Mashonkina, G Gilmore, S Randich, M Asplund, H-W Rix, AR Casey, P Jofre, E Pancino, A Recio-Blanco, P de Laverny, R Smiljanic, G Tautvaisiene, A Bayo, J Lewis, S Koposov, A Hourihane, C Worley, L Morbidelli, E Franciosini, G Sacco, L Magrini, F Damiani, JM Bestenlehner


Age velocity dispersion relations and heating histories in disc galaxies

Monthly Notices of the Royal Astronomical Society Oxford University Press (2016)

M Aumer, JJ Binney, RA Schönrich

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 ∝ tβ and determine the exponents β bar R and β bar z derived from in-plane and vertical AVRs and βR and βz from heating histories. Values of βz are in almost all simulations larger than values of βbarz , wheras values of βR are similar to or mildly larger than values of β bar R. Moreover, values of βz (β bar z) are generally larger than values of βR (β bar 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 in uence the measured AVR: they tend to decrease β values by smearing out ages and thus measured dispersions. We compare AVRs and velocity ellipsoid shapes σz/σR 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.


The quiescent phase of galactic disc growth

Monthly Notices of the Royal Astronomical Society Oxford University Press (2015)

M Aumer, J Binney, R Schönrich

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.


GALACTIC DYNAMICS AND RADIAL MIGRATION

MILKY WAY UNRAVELLED BY GAIA: GREAT SCIENCE FROM THE GAIA DATA RELEASES 67-68 (2015) 339-345

R Schoenrich


RINGS AND RADIAL WAVES IN THE DISK OF THE MILKY WAY

ASTROPHYSICAL JOURNAL 801 (2015) ARTN 105

Y Xu, HJ Newberg, JL Carlin, C Liu, L Deng, J Li, R Schoenrich, B Yanny


Origin of the high v_los feature in the Galactic bar

Monthly Notices of the Royal Astronomical Society Oxford University Press 454 (2015) 3166-3184

M Aumer, R Schönrich

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.


Kinematic detection of the galactic nuclear disc

Astrophysical Journal Letters American Astronomical Society 812 (2015) L21-L21

R Schönrich, M Aumer, SE Sale

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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