159 research outputs found
The ILIUM forward modelling algorithm for multivariate parameter estimation and its application to derive stellar parameters from Gaia spectrophotometry
I introduce an algorithm for estimating parameters from multidimensional data
based on forward modelling. In contrast to many machine learning approaches it
avoids fitting an inverse model and the problems associated with this. The
algorithm makes explicit use of the sensitivities of the data to the
parameters, with the goal of better treating parameters which only have a weak
impact on the data. The forward modelling approach provides uncertainty (full
covariance) estimates in the predicted parameters as well as a goodness-of-fit
for observations. I demonstrate the algorithm, ILIUM, with the estimation of
stellar astrophysical parameters (APs) from simulations of the low resolution
spectrophotometry to be obtained by Gaia. The AP accuracy is competitive with
that obtained by a support vector machine. For example, for zero extinction
stars covering a wide range of metallicity, surface gravity and temperature,
ILIUM can estimate Teff to an accuracy of 0.3% at G=15 and to 4% for (lower
signal-to-noise ratio) spectra at G=20. [Fe/H] and logg can be estimated to
accuracies of 0.1-0.4dex for stars with G<=18.5. If extinction varies a priori
over a wide range (Av=0-10mag), then Teff and Av can be estimated quite
accurately (3-4% and 0.1-0.2mag respectively at G=15), but there is a strong
and ubiquitous degeneracy in these parameters which limits our ability to
estimate either accurately at faint magnitudes. Using the forward model we can
map these degeneracies (in advance), and thus provide a complete probability
distribution over solutions. (Abridged)Comment: MNRAS, in press. This revision corrects a few minor errors and typos.
A better formatted version for A4 paper is available at
http://www.mpia.de/home/calj/ilium.pd
The expected performance of stellar parametrization with Gaia spectrophotometry
Gaia will obtain astrometry and spectrophotometry for essentially all sources
in the sky down to a broad band magnitude limit of G=20, an expected yield of
10^9 stars. Its main scientific objective is to reveal the formation and
evolution of our Galaxy through chemo-dynamical analysis. In addition to
inferring positions, parallaxes and proper motions from the astrometry, we must
also infer the astrophysical parameters of the stars from the
spectrophotometry, the BP/RP spectrum. Here we investigate the performance of
three different algorithms (SVM, ILIUM, Aeneas) for estimating the effective
temperature, line-of-sight interstellar extinction, metallicity and surface
gravity of A-M stars over a wide range of these parameters and over the full
magnitude range Gaia will observe (G=6-20mag). One of the algorithms, Aeneas,
infers the posterior probability density function over all parameters, and can
optionally take into account the parallax and the Hertzsprung-Russell diagram
to improve the estimates. For all algorithms the accuracy of estimation depends
on G and on the value of the parameters themselves, so a broad summary of
performance is only approximate. For stars at G=15 with less than two
magnitudes extinction, we expect to be able to estimate Teff to within 1%, logg
to 0.1-0.2dex, and [Fe/H] (for FGKM stars) to 0.1-0.2dex, just using the BP/RP
spectrum (mean absolute error statistics are quoted). Performance degrades at
larger extinctions, but not always by a large amount. Extinction can be
estimated to an accuracy of 0.05-0.2mag for stars across the full parameter
range with a priori unknown extinction between 0 and 10mag. Performance
degrades at fainter magnitudes, but even at G=19 we can estimate logg to better
than 0.2dex for all spectral types, and [Fe/H] to within 0.35dex for FGKM
stars, for extinctions below 1mag.Comment: MNRAS, in press. Minor corrections made in v
Estimation of stellar atmospheric parameters from SDSS/SEGUE spectra
We present techniques for the estimation of stellar atmospheric parameters
(Teff,logg,[Fe/H]) for stars from the SDSS/SEGUE survey. The atmospheric
parameters are derived from the observed medium-resolution (R=2000) stellar
spectra using non-linear regression models trained either on (1) pre-classified
observed data or (2) synthetic stellar spectra. In the first case we use our
models to automate and generalize parametrization produced by a preliminary
version of the SDSS/SEGUE Spectroscopic Parameter Pipeline (SSPP). In the
second case we directly model the mapping between synthetic spectra (derived
from Kurucz model atmospheres) and the atmospheric parameters, independently of
any intermediate estimates. After training, we apply our models to various
samples of SDSS spectra to derive atmospheric parameters, and compare our
results with those obtained previously by the SSPP for the same samples. We
obtain consistency between the two approaches, with RMS deviations of 150K in
Teff, 0.35dex in logg, and 0.22dex in [Fe/H]. The models are applied to
pre-processed spectra, either via Principal Components Analysis or a Wavelength
Range Selection method, which employs a subset of the full 3850-9000A spectral
range. This is both for computational reasons, and because it delivers higher
accuracy. From an analysis of cluster candidates with available SDSS
spectroscopy (M15, M13, M2, and NGC2420), we find evidence for small systematic
offsets in Teff and/or logg for the atmospheric parameter estimates from the
model trained on real data with the SSPP. Thus, this model turns out to derive
more precise, but less accurate, atmospheric parameters than the model trained
on synthetic data.Comment: 17 pages, 13 figures, accepted for publication in A&
The SEGUE Stellar Parameter Pipeline. II. Validation with Galactic Globular and Open Clusters
We validate the performance and accuracy of the current SEGUE (Sloan
Extension for Galactic Understanding and Exploration) Stellar Parameter
Pipeline (SSPP), which determines stellar atmospheric parameters (effective
temperature, surface gravity, and metallicity) by comparing derived overall
metallicities and radial velocities from selected likely members of three
globular clusters (M 13, M 15, and M 2) and two open clusters (NGC 2420 and M
67) to the literature values. Spectroscopic and photometric data obtained
during the course of the original Sloan Digital Sky Survey (SDSS-I) and its
first extension (SDSS-II/SEGUE) are used to determine stellar radial velocities
and atmospheric parameter estimates for stars in these clusters. Based on the
scatter in the metallicities derived for the members of each cluster, we
quantify the typical uncertainty of the SSPP values, sigma([Fe/H]) = 0.13 dex
for stars in the range of 4500 K < Teff < 7500 K and 2.0 < log g < 5.0, at
least over the metallicity interval spanned by the clusters studied (-2.3 <
[Fe/H] < 0). The surface gravities and effective temperatures derived by the
SSPP are also compared with those estimated from the comparison of the
color-magnitude diagrams with stellar evolution models; we find satisfactory
agreement. At present, the SSPP underestimates [Fe/H] for
near-solar-metallicity stars, represented by members of M 67 in this study, by
about 0.3 dex.Comment: 56 pages, 8 Tables, 15 figures, submitted to the Astronomical Journa
PCA Tomography: how to extract information from datacubes
Astronomy has evolved almost exclusively by the use of spectroscopic and
imaging techniques, operated separately. With the development of modern
technologies it is possible to obtain datacubes in which one combines both
techniques simultaneously, producing images with spectral resolution. To
extract information from them can be quite complex, and hence the development
of new methods of data analysis is desirable. We present a method of analysis
of datacube (data from single field observations, containing two spatial and
one spectral dimension) that uses PCA (Principal Component Analysis) to express
the data in the form of reduced dimensionality, facilitating efficient
information extraction from very large data sets. PCA transforms the system of
correlated coordinates into a system of uncorrelated coordinates ordered by
principal components of decreasing variance. The new coordinates are referred
to as eigenvectors, and the projections of the data onto these coordinates
produce images we will call tomograms. The association of the tomograms
(images) to eigenvectors (spectra) is important for the interpretation of both.
The eigenvectors are mutually orthogonal and this information is fundamental
for their handling and interpretation. When the datacube shows objects that
present uncorrelated physical phenomena, the eigenvector's orthogonality may be
instrumental in separating and identifying them. By handling eigenvectors and
tomograms one can enhance features, extract noise, compress data, extract
spectra, etc. We applied the method, for illustration purpose only, to the
central region of the LINER galaxy NGC 4736, and demonstrate that it has a type
1 active nucleus, not known before. Furthermore we show that it is displaced
from the centre of its stellar bulge.Comment: 13 pages, 16 figures, accepted for publication on MNRA
The SEGUE Stellar Parameter Pipeline. I. Description and Initial Validation Tests
We describe the development and implementation of the SEGUE (Sloan Extension
for Galactic Exploration and Understanding) Stellar Parameter Pipeline (SSPP).
The SSPP derives, using multiple techniques, radial velocities and the
fundamental stellar atmospheric parameters (effective temperature, surface
gravity, and metallicity) for AFGK-type stars, based on medium-resolution
spectroscopy and photometry obtained during the course of the original
Sloan Digital Sky Survey (SDSS-I) and its Galactic extension (SDSS-II/SEGUE).
The SSPP also provides spectral classification for a much wider range of stars,
including stars with temperatures outside of the window where atmospheric
parameters can be estimated with the current approaches. This is Paper I in a
series of papers on the SSPP; it provides an overview of the SSPP, and initial
tests of its performance using multiple data sets. Random and systematic errors
are critically examined for the current version of the SSPP, which has been
used for the sixth public data release of the SDSS (DR-6).Comment: 64 pages, 8 tables, 12 figures, submitted to the Astronomical Journa
The SEGUE Stellar Parameter Pipeline. III. Comparison with High-Resolution Spectroscopy of SDSS/SEGUE Field Stars
We report high-resolution spectroscopy of 125 field stars previously observed
as part of the Sloan Digital Sky Survey and its program for Galactic studies,
the Sloan Extension for Galactic Understanding and Exploration (SEGUE). These
spectra are used to measure radial velocities and to derive atmospheric
parameters, which we compare with those reported by the SEGUE Stellar Parameter
Pipeline (SSPP). The SSPP obtains estimates of these quantities based on SDSS
ugriz photometry and low-resolution (R = 2000) spectroscopy. For F- and G-type
stars observed with high signal-to-noise ratios (S/N), we empirically determine
the typical random uncertainties in the radial velocities, effective
temperatures, surface gravities, and metallicities delivered by the SSPP to be
2.4 km/s, 130 K (2.2%), 0.21 dex, and 0.11 dex, respectively, with systematic
uncertainties of a similar magnitude in the effective temperatures and
metallicities. We estimate random errors for lower S/N spectra based on
numerical simulations.Comment: 37 pages, 6 tables, 6 figures, submitted to the Astronomical Journa
Two Stellar Components in the Halo of the Milky Way
The halo of the Milky Way provides unique elemental abundance and kinematic
information on the first objects to form in the Universe, which can be used to
tightly constrain models of galaxy formation and evolution. Although the halo
was once considered a single component, evidence for its dichotomy has slowly
emerged in recent years from inspection of small samples of halo objects. Here
we show that the halo is indeed clearly divisible into two broadly overlapping
structural components -- an inner and an outer halo -- that exhibit different
spatial density profiles, stellar orbits and stellar metallicities (abundances
of elements heavier than helium). The inner halo has a modest net prograde
rotation, whereas the outer halo exhibits a net retrograde rotation and a peak
metallicity one-third that of the inner halo. These properties indicate that
the individual halo components probably formed in fundamentally different ways,
through successive dissipational (inner) and dissipationless (outer) mergers
and tidal disruption of proto-Galactic clumps.Comment: Two stand-alone files in manuscript, concatenated together. The first
is for the main paper, the second for supplementary information. The version
is consistent with the version published in Natur
The Milky Way Tomography With SDSS. III. Stellar Kinematics
We study Milky Way kinematics using a sample of 18.8 million main-sequence stars with r 20 degrees). We find that in the region defined by 1 kpc < Z < 5 kpc and 3 kpc < R < 13 kpc, the rotational velocity for disk stars smoothly decreases, and all three components of the velocity dispersion increase, with distance from the Galactic plane. In contrast, the velocity ellipsoid for halo stars is aligned with a spherical coordinate system and appears to be spatially invariant within the probed volume. The velocity distribution of nearby (Z < 1 kpc) K/M stars is complex, and cannot be described by a standard Schwarzschild ellipsoid. For stars in a distance-limited subsample of stars (< 100 pc), we detect a multi-modal velocity distribution consistent with that seen by HIPPARCOS. This strong non-Gaussianity significantly affects the measurements of the velocity-ellipsoid tilt and vertex deviation when using the Schwarzschild approximation. We develop and test a simple descriptive model for the overall kinematic behavior that captures these features over most of the probed volume, and can be used to search for substructure in kinematic and metallicity space. We use this model to predict further improvements in kinematic mapping of the Galaxy expected from Gaia and the Large Synoptic Survey Telescope.NSF AST-615991, AST-0707901, AST-0551161, AST-02-38683, AST-06-07634, AST-0807444, PHY05-51164NASA NAG5-13057, NAG5-13147, NNXO-8AH83GPhysics Frontier Center/Joint Institute for Nuclear Astrophysics (JINA) PHY 08-22648U.S. National Science FoundationMarie Curie Research Training Network ELSA (European Leadership in Space Astrometry) MRTN-CT-2006-033481Fermi Research Alliance, LLC, United States Department of Energy DE-AC02-07CH11359Alfred P. Sloan FoundationParticipating InstitutionsJapanese MonbukagakushoMax Planck SocietyHigher Education Funding Council for EnglandMcDonald Observator
Coude-feed stellar spectral library - atmospheric parameters
Context: Empirical libraries of stellar spectra play an important role in
different fields. For example, they are used as reference for the automatic
determination of atmospheric parameters, or for building synthetic stellar
populations to study galaxies. The CFLIB (Coude-feed library, Indo-US) database
is at present one of the most complete libraries, in terms of its coverage of
the atmospheric parameters space (Teff, log g and [Fe/H]) and wavelength
coverage 3460 - 9464 A at a resolution of 1 A FWHM. Although the atmospheric
parameters of most of the stars were determined from detailed analyses of
high-resolution spectra, for nearly 300 of the 1273 stars of the library at
least one of the three parameters is missing. For the others, the measurements,
compiled from the literature, are inhomogeneous.
Aims: In this paper, we re-determine the atmospheric parameters, directly
using the CFLIB spectra, and compare them to the previous studies.
Methods: We use the ULySS program to derive the atmospheric parameters, using
the ELODIE library as a reference.
Results: Based on comparisons with several previous studies we conclude that
our determinations are unbiased. For the 958 F,G, and K type stars the
precision on Teff, log g, and [Fe/H] is respectively 43 K, 0.13 dex and 0.05
dex. For the 53 M stars they are 82 K, 0.22 dex and 0.28 dex. And, for the 260
OBA type stars the relative precision on Teff is 5.1%, and on log g, and [Fe/H]
the precision is respectively 0.19 dex and 0.16 dex. These parameters will be
used to re-calibrate the CFLIB fluxes and to produce synthetic spectra of
stellar populations.Comment: 51 pages, accepted for publication in Astronomy and Astrophysic
- …