119 research outputs found
Evaluating GAIA performances on eclipsing binaries
No full textThe orbits and physical parameters of three close, double-lined G0 eclipsing binaries have been derived combining photometry
from the Hipparcos/Tycho mission with 8480–8740 Å ground-based spectroscopy.
The setup is mimicking the photometric and spectroscopic observations that
should be obtained by GAIA.
The binaries considered here are all of G0 spectral type, but each with its
own complications: V781 Tau is an overcontact system with components of
unequal temperature, UV Leo shows occasional surface spots and GK Dra
contains a δ Scuti variable. Such peculiarities will be common among
binaries to be discovered by GAIA. We find that the values of
masses, radii and temperatures for such stars can be
derived with a 1–2% accuracy using the adopted GAIA-like observing mode
SP_Ace: a new code to derive stellar parameters and elemental abundances
No full textContext. 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_AC
Investigating the Milky Way with the RAVE chemical catalog
No full textRAVE is a large spectroscopic survey of the Milky Way, aiming at observing up to one million stars by 2012 and at obtaining radial velocities and atmosphere parameters (see Steinmetz et al. [6], Zwitter et al. [7], Siebert et al. [5]). Despite to their medium resolution (R ∼ 7500), and the small wavelength window (8410–8795Å) RAVE spectra are suitable to perform a chemical abundance analysis. For 234,227 spectra (to date April 2011) we derived abundance estimates for up to 7 elements, which makes RAVE the largest chemical abundances database existing today. We present here the radial chemical gradients of the Milky Way by using RAVE data
SP_Ace v1.4 and the new GCOG library for deriving stellar parameters and elemental abundances
No full textContext. Ongoing and future massive spectroscopic surveys will collect very 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 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 Teff, 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
NGC 6738: Not a real open cluster
No full textA photometric, astrometric and spectroscopic investigation of the poorly
studied open cluster NGC 6738 has been performed in order to ascertain its
real nature. NGC 6738 is definitely not a physical stellar ensemble:
photometry does not show a defined mean sequence, proper motions and
radial velocities are randomly distributed, spectro-photometric parallaxes
range between 10 and 1600 pc, and the apparent luminosity function is identical to
that of the surrounding field. NGC 6738 therefore appears to be an apparent
concentration of a few bright stars projected on patchy background absorption.
Kinematics and binaries in young stellar aggregates
No full textBetween 1996 and 2003 we obtained 226 high resolution spectra of 16
stars in the field of the young open cluster NGC 6913, to
constrain its main properties and study its internal kinematics. Twelve
of the program stars turned out to be members, one of them probably unbound.
Nine are binaries (one eclipsing and another double lined) and for seven of
them the observations allowed us to derive the orbital elements. All but two of
the nine discovered binaries are cluster members. In spite of the young
age (a few Myr), the cluster already shows signs that could be interpreted
as evidence of dynamical relaxation and mass segregation. However, they may
be also the result of an unconventional formation scenario. The dynamical
(virial) mass as estimated from the radial velocity dispersion is larger
than the cluster luminous mass, which may be explained by a combination of
the optically thick interstellar cloud that occults part of the cluster, the
unbound state or undetected very wide binary orbit of some of the
members that inflate the velocity dispersion and a high inclination for the
axis of possible cluster angular momentum. All the discovered binaries are
hard enough to survive average close encounters within the cluster and do
not yet show signs of relaxation of the orbital elements to values typical of
field binaries
Spectroscopic signatures of extratidal stars around the globular clusters NGC 6656 (M 22), NGC 3201, and NGC 1851 from RAVE
No full textRAVE stars in K2: I. Improving RAVE red giants spectroscopy using asteroseismology from K2 Campaign 1
No full textWe present a set of 87 RAVE stars with detected solar like oscillations, observed during Campaign 1 of the K2 mission (RAVE K2-C1 sample). This data set provides a useful benchmark for testing the gravities provided in RAVE data release 4 (DR4), and is key for the calibration of the RAVE data release 5 (DR5). The RAVE survey collected medium-resolution spectra (R = 7500) centred in the Ca II triplet(8600 Å) wavelength interval, which although being very useful for determining radial velocity and metallicity, even at low S/N, is known be affected by a log (g)-Teff degeneracy. This degeneracy is the cause of the large spread in the RAVE DR4 gravities for giants. The understanding of the trends and offsets that affects RAVE atmospheric parameters, and in particular log (g), is a crucial step in obtaining not only improved abundance measurements, but also improved distances and ages. In the present work, we use two different pipelines, GAUFRE and Sp-Ace, to determine atmospheric parameters and abundances by fixing log (g) to the seismic one. Our strategy ensures highly consistent values among all stellar parameters, leading to more accurate chemical abundances. A comparison of the chemical abundances obtained here with and without the use of seismic log (g) information has shown that an underestimated (overestimated) gravity leads to an underestimated (overestimated) elemental abundance (e.g. [Mg/H] is underestimated by ∼0.25 dex when the gravity is underestimated by 0.5 dex). We then perform a comparison between the seismic gravities and the spectroscopic gravities presented in the RAVE DR4 catalogue, extracting a calibration for log (g) of RAVE giants in the colour interval 0.50 < (J-KS) < 0.85. Finally, we show a comparison of the distances, temperatures, extinctions (and ages) derived here for our RAVE K2-C1 sample with those derived in RAVE DR4 and DR5. DR5 performs better than DR4 thanks to the seismic calibration, although discrepancies can still be important for objects for which the difference between DR4/DR5 and seismic gravities differ by more than ∼0.5 dex. The method illustrated in this work will be used for analysing RAVE targets present in the other K2 campaigns, in the framework of Galactic Archaeology investigations
LAMOST DR1: Stellar Parameters and Chemical Abundances with SP_Ace
No full textAbstract
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 [α/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 [α/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
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