Context. Ongoing and future massive spectroscopic surveys will collect large numbers (106–107) of stellar spectra that need to be analyzed. Highly automated software is needed to derive stellar ...parameters and chemical abundances from these spectra. Aims. We developed a new method of estimating the stellar parameters Teff, log g, M/H, and elemental abundances. This method was implemented in a new code, SP_Ace (Stellar Parameters And Chemical abundances Estimator). This is a highly automated code suitable for analyzing the spectra of large spectroscopic surveys with low or medium spectral resolution (R = 2000–20 000). Methods. After the astrophysical calibration of the oscillator strengths of 4643 absorption lines covering the wavelength ranges 5212–6860 Å and 8400–8924 Å, we constructed a library that contains the equivalent widths (EW) of these lines for a grid of stellar parameters. The EWs of each line are fit by a polynomial function that describes the EW of the line as a function of the stellar parameters. The coefficients of these polynomial functions are stored in a library called the “GCOG library”. SP_Ace, a code written in FORTRAN95, uses the GCOG library to compute the EWs of the lines, constructs models of spectra as a function of the stellar parameters and abundances, and searches for the model that minimizes the χ2 deviation when compared to the observed spectrum. The code has been tested on synthetic and real spectra for a wide range of signal-to-noise and spectral resolutions. Results. SP_Ace derives stellar parameters such as Teff, log g, M/H, and chemical abundances of up to ten elements for low to medium resolution spectra of FGK-type stars with precision comparable to the one usually obtained with spectra of higher resolution. Systematic errors in stellar parameters and chemical abundances are presented and identified with tests on synthetic and real spectra. Stochastic errors are automatically estimated by the code for all the parameters. A simple Web front end of SP_Ace can be found at http://dc.g-vo.org/SP_ACE, while the source code will be published soon.
Context.
Ongoing and future massive spectroscopic surveys will collect very large numbers (10
6
–10
7
) of stellar spectra that need to be analyzed. Highly automated software is needed to derive ...stellar parameters and chemical abundances from these spectra.
Aims.
We present the new version of SP_Ace (Stellar Parameters And Chemical abundances Estimator) a code that derives stellar parameters and elemental abundance from stellar spectra. The new version covers a larger spectral resolution interval (
R
= 2000−40 000) and its new library covers bluer wavelengths (4800–6860 Å).
Methods.
SP_Ace relies on the General-Curve-Of-Growth (GCOG) library based on 6700 absorption lines whose oscillator strengths were calibrated astrophysically. We developed the calibration method and applied it to all the lines. From the new line list obtained we build the GCOG library, adopting an improved method to correct for the opacity of the neighboring lines. We implemented a new line profile for the code SP_Ace that better reproduces that of synthetic spectra. This new version of SP_Ace and the GCOG library has been tested on synthetic and real spectra to establish the accuracy and precision of the derived stellar parameters.
Results.
SP_Ace can derive the stellar parameters
T
eff
, log
g
, M/H, and chemical abundances with satisfactory results; the accuracy depends on the spectral features that determine the quality, such as spectral resolution, signal-to-noise ratio, and wavelength coverage. Systematic errors were identified and quantified where possible. The source code is publicly available.
We made new estimates of the Galactic escape speed at various Galactocentric radii using the latest data release of the RAdial Velocity Experiment (RAVE DR4). Compared to previous studies we have a ...database that is larger by a factor of 10, as well as reliable distance estimates for almost all stars. Our analysis is based on statistical analysis of a rigorously selected sample of 90 high-velocity halo stars from RAVE and a previously published data set. We calibrated and extensively tested our method using a suite of cosmological simulations of the formation of Milky Way-sized galaxies. Our best estimate of the local Galactic escape speed, which we define as the minimum speed required to reach three virial radii R340, is 533+54-41 km s-1 (90% confidence), with an additional 4% systematic uncertainty, where R340 is the Galactocentric radius encompassing a mean overdensity of 340 times the critical density for closure in the Universe. From the escape speed we further derived estimates of the mass of the Galaxy using a simple mass model with two options for the mass profile of the dark matter halo: an unaltered and an adiabatically contracted Navarro, Frenk & White (NFW) sphere. If we fix the local circular velocity, the latter profile yields a significantly higher mass than the uncontracted halo, but if we instead use the statistics for halo concentration parameters in large cosmological simulations as a constraint, we find very similar masses for both models. Our best estimate for M340, the mass interiorto R340 (dark matter and baryons), is 1.3+0.4-0.3 × 1012 M⊙ (corresponds to M200 = 1.6+0.5-0.4 × 1012 M⊙). This estimate is in good agreement with recently published, independent mass estimates based on the kinematics of more distant halo stars and the satellite galaxy Leo I.
We provide AAVSO Photometric All-Sky Survey (APASS) photometry in the Landolt BV and Sloan g r i bands for all 425, 743 stars included in the fourth RAVE Data Release. The internal accuracy of the ...APASS photometry of RAVE stars, expressed as the error of the mean of data obtained and separately calibrated over a median of four distinct observing epochs and distributed between 2009 and 2013, is 0.013, 0.012, 0.012, 0.014, and 0.021 mag for the B, V, g, r, and i bands, respectively. In the process, we find that the reddening caused by a homogeneous slab of dust, extending for 140 pc on either side of the Galactic plane and responsible for E poles B-V = 0.036 + or - 0.002 at the Galactic poles, is a suitable approximation of the actual reddening encountered at Galactic latitudes b 25degrees.
We investigate the kinematic parameters of the Milky Way disk using the Radial Velocity Experiment (RAVE) and Geneva-Copenhagen Survey (GCS) stellar surveys. We do this by fitting a kinematic model ...to the data and taking the selection function of the data into account. The large size of the RAVE survey allows us to get precise values for most parameters. Using a simple model for vertical dependence of kinematics, we find that it is possible to match the Sgr A* proper motion without any need for V being larger than that estimated locally by surveys like GCS.
Abstract
Using the Radial Velocity Experiment (RAVE) survey, we recently brought to light a gradient in the mean galactocentric radial velocity of stars in the extended solar neighbourhood. This ...gradient likely originates from non-axisymmetric perturbations of the potential, among which a perturbation by spiral arms is a possible explanation. Here, we apply the traditional density wave theory and analytically model the radial component of the two-dimensional velocity field. Provided that the radial velocity gradient is caused by relatively long-lived spiral arms that can affect stars substantially above the plane, this analytic model provides new independent estimates for the parameters of the Milky Way spiral structure. Our analysis favours a two-armed perturbation with the Sun close to the inner ultra-harmonic 4:1 resonance, with a pattern speed and a small amplitude per cent of the background potential (14 per cent of the background density). This model can serve as a basis for numerical simulations in three dimensions, additionally including a possible influence of the Galactic bar and/or other non-axisymmetric modes.
We present a new analysis of the LAMOST DR1 survey spectral database performed with the code SP_Ace, which provides the derived stellar parameters , , Fe/H, and /H for 1,097,231 stellar objects. We ...tested the reliability of our results by comparing them to reference results from high spectral resolution surveys. The expected errors can be summarized as ∼120 K in , ∼0.2 in , ∼0.15 dex in Fe/H, and ∼0.1 dex in /Fe for spectra with S/N > 40, with some differences between dwarf and giant stars. SP_Ace provides error estimations consistent with the discrepancies observed between derived and reference parameters. Some systematic errors are identified and discussed. The resulting catalog is publicly available at the LAMOST and CDS websites.
The RAdial Velocity Experiment survey, combined with proper motions and distance estimates, can be used to study in detail stellar kinematics in the extended solar neighbourhood (solar suburb). Using ...72 365 red-clump stars, we examine the mean velocity components in 3D between 6 < R < 10 kpc and −2 < Z < 2 kpc, concentrating on north-south differences. Simple parametric fits to the (R, Z) trends for V
φ and the velocity dispersions are presented. We confirm the recently discovered gradient in mean Galactocentric radial velocity, V
R, finding that the gradient is marked below the plane (δ〈V
R〉/δR = −8 km s−1 kpc−1 for Z < 0, vanishing to zero above the plane), with a Z gradient thus also present. The vertical velocity, V
Z
, also shows clear, large-amplitude (|V
Z
| = 17 km s−1) structure, with indications of a rarefaction-compression pattern, suggestive of wave-like behaviour. We perform a rigorous error analysis, tracing sources of both systematic and random errors. We confirm the north-south differences in V
R and V
Z
along the line of sight, with the V
R estimated independent of the proper motions. The complex three-dimensional structure of velocity space presents challenges for future modelling of the Galactic disc, with the Galactic bar, spiral arms and excitation of wave-like structures all probably playing a role.
The velocity dispersions of stars near the Sun are known to increase with stellar age, but age can be difficult to determine, so a proxy like the abundance of alpha elements (e.g., Mg) with respect ...to iron, alpha /Fe, is used. Here we report an unexpected behavior found in the velocity dispersion of a sample of giant stars from the Radial Velocity Experiment survey with high-quality chemical and kinematic information, in that it decreases strongly for stars with Mg/Fe > 0.4 dex (i.e., those that formed in the first gigayear of the Galaxy's life). These findings can be explained by perturbations from massive mergers in the early universe, which have affected the outer parts of the disk more strongly, and the subsequent radial migration of stars with cooler kinematics from the inner disk. Similar reversed trends in velocity dispersion are also found for different metallicity subpopulations. Our results suggest that the Milky Way disk merger history can be recovered by relating the observed chemo-kinematic relations to the properties of past merger events.
Context. Radial velocity surveys such as the RAdial Velocity Experiment (RAVE) provide us with measurements of hundreds of thousands of nearby stars most of which belong to the Galactic thin, thick ...disk or halo. Ideally, to study the Galactic disks (both thin and thick) one should make use of the multi-dimensional phase-space and the whole pattern of chemical abundances of their stellar populations. Aims. In this paper, with the aid of the RAVE survey, we study the thin and thick disks of the Milky Way, focusing on the latter. We present a technique to disentangle the stellar content of the two disks based on the kinematics and other stellar parameters such as the surface gravity of the stars. Using the Padova Galaxy model, we checked the ability of our method to correctly isolate the thick disk component from the Galaxy mixture of stellar populations. Methods. We introduce selection criteria in order to clean the observed radial velocities from the Galactic differential rotation and to take into account the partial sky coverage of RAVE. We developed a numerical technique to statistically disentangle thin and thick disks from their mixture. Results. We deduce the components of the solar motion relative to the local standard of rest (LSR) in the radial and vertical direction, the rotational lag of the thick disk component relative to the LSR, and the square root of the absolute value of the velocity dispersion tensor for the thick disk alone. The analysis of the thin disk is presented in another paper. We find good agreement with previous independent parameter determinations. In our analysis we used photometrically determined distances. In the Appendix we show that similar values can be found for the thick disk alone as derived in the main sections of our paper even without the knowledge of photometric distances.
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