28 research outputs found
The main sequence from F to K stars of the solar neighbourhood in SDSS colours
For an understanding of Galactic stellar populations in the SDSS filter
system well defined stellar samples are needed. The nearby stars provide a
complete stellar sample representative for the thin disc population. We compare
the filter transformations of different authors applied to the main sequence
stars from F to K dwarfs to SDSS filter system and discuss the properties of
the main sequence. The location of the mean main sequence in colour-magnitude
diagrams is very sensitive to systematic differences in the filter
transformation. A comparison with fiducial sequences of star clusters observed
in g',r',i' show good agreement. Theoretical isochrones from Padua and from
Dartmouth have still some problems especially in (r-i)-colour.Comment: 11 pages, 15 figures, 1 online table available at
ftp://ftp.ari.uni-heidelberg.de/staff/just/stars.tab, accepted by A
Galactic Globular and Open Clusters in the Sloan Digital Sky Survey. II. Test of Theoretical Stellar Isochrones
We perform an extensive test of theoretical stellar models for main-sequence
stars in ugriz, using cluster fiducial sequences obtained in the previous paper
of this series. We generate a set of isochrones using the Yale Rotating
Evolutionary Code (YREC) with updated input physics, and derive magnitudes and
colors in ugriz from MARCS model atmospheres. These models match cluster main
sequences over a wide range of metallicity within the errors of the adopted
cluster parameters. However, we find a large discrepancy of model colors at the
lower main sequence (Teff < ~4500 K) for clusters at and above solar
metallicity. We also reach similar conclusions using the theoretical isochrones
of Girardi et al. and Dotter et al., but our new models are generally in better
agreement with the data. Using our theoretical isochrones, we also derive
main-sequence fitting distances and turn-off ages for five key globular
clusters, and demonstrate the ability to derive these quantities from
photometric data in the Sloan Digital Sky Survey. In particular, we exploit
multiple color indices (g - r, g - i, and g - z) in the parameter estimation,
which allows us to evaluate internal systematic errors. Our distance estimates,
with an error of sigma(m - M) = 0.03-0.11 mag for individual clusters, are
consistent with Hipparcos-based subdwarf fitting distances derived in the
Johnson-Cousins or Stromgren photometric systems.Comment: 26 pages, 28 figures. Accepted for publication in ApJ. Version with
high resolution figures available at
http://spider.ipac.caltech.edu/~deokkeun/sdss_iso.pd
The kinematics of late type stars in the solar cylinder studied with SDSS data
We study the velocity distribution of Milky Way disk stars in a
kiloparsec-sized region around the Sun, based on ~ 2 million M-type stars from
DR7 of SDSS, which have newly re-calibrated absolute proper motions from
combining SDSS positions with the USNO-B catalogue. We estimate photometric
distances to all stars, accurate to ~ 20 %, and combine them with the proper
motions to derive tangential velocities for this kinematically unbiased sample
of stars. Based on a statistical de-projection method we then derive the
vertical profiles (to heights of Z = 800 pc above the disk plane) for the first
and second moments of the three dimensional stellar velocity distribution. We
find that = -7 +/- 1 km/s and = -9 +/- 1 km/s, independent of height
above the mid-plane, reflecting the Sun's motion with respect to the local
standard of rest. In contrast, changes distinctly from -20 +/- 2 km/s in
the mid-plane to = -32 km/s at Z = 800 pc, reflecting an asymmetric drift
of the stellar mean velocity that increases with height. All three components
of the M-star velocity dispersion show a strong linear rise away from the
mid-plane, most notably \sigma_{ZZ}, which grows from 18 km/s (Z = 0) to 40
km/s (at Z = 800 pc). We determine the orientation of the velocity ellipsoid,
and find a significant vertex deviation of 20 to 25 degrees, which decreases
only slightly to heights of Z = 800 pc. Away from the mid-plane, our sample
exhibits a remarkably large tilt of the velocity ellipsoid towards the Galactic
plane, which reaches 20 deg. at Z = 800 pc and which is not easily explained.
Finally, we determine the ratio \sigma^2_{\phi\phi}/\sigma^2_{RR} near the
mid-plane, which in the epicyclic approximation implies an almost perfectly
flat rotation curve at the Solar radius.Comment: 18 pages, 9 figures, accepted to Astron.
Leo V: A Companion of a Companion of the Milky Way Galaxy
We report the discovery of a new Milky Way dwarf spheroidal galaxy in the
constellation of Leo identified in data from the Sloan Digital Sky Survey. Leo
V lies at a distance of about 180 kpc, and is separated by about 3 degrees from
another recent discovery, Leo IV. We present follow-up imaging from the Isaac
Newton Telescope and spectroscopy from the Hectochelle fiber spectrograph at
the Multiple Mirror Telescope. Leo V's heliocentric velocity is 173.4 km/s,
which is offset by about 40 km/s from that of Leo IV. A simple interpretation
of the kinematic data is that both objects may lie on the same stream, though
the implied orbit is only modestly eccentric (e = 0.2)Comment: Submitted to ApJ (Letters
The Least Luminous Galaxy: Spectroscopy of the Milky Way Satellite Segue 1
We present Keck/DEIMOS spectroscopy of Segue 1, an ultra-low luminosity (M_V
= -1.5) Milky Way satellite companion. While the combined size and luminosity
of Segue 1 are consistent with either a globular cluster or a dwarf galaxy, we
present spectroscopic evidence that this object is a dark matter-dominated
dwarf galaxy. We identify 24 stars as members of Segue 1 with a mean
heliocentric recession velocity of 206 +/- 1.3 kms. We measure an internal
velocity dispersion of 4.3+/-1.2 kms. Under the assumption that these stars are
in dynamical equilibrium, we infer a total mass of 4.5^{+4.7}_{-2.5} x 10^5
Msun in the case where mass-follow-light; using a two-component maximum
likelihood model, we determine a similar mass within the stellar radius of 50
pc. This implies a mass-to-light ratio of ln(M/L_V) = 7.2^{+1.1}_{-1.2} or
M/L_V = 1320^{+2680}_{-940}. The error distribution of the mass-to-light ratio
is nearly log-normal, thus Segue 1 is dark matter-dominated at a high
significance. Using spectral synthesis modeling, we derive a metallicity for
the single red giant branch star in our sample of [Fe/H]=-3.3 +/- 0.2 dex.
Finally, we discuss the prospects for detecting gamma-rays from annihilation of
dark matter particles and show that Segue 1 is the most promising satellite for
indirect dark matter detection. We conclude that Segue 1 is the least luminous
of the ultra-faint galaxies recently discovered around the Milky Way, and is
thus the least luminous known galaxy.Comment: 12 pages, 6 figures, ApJ accepte
Multi-Element Abundance Measurements from Medium-Resolution Spectra. II. Catalog of Stars in Milky Way Dwarf Satellite Galaxies
We present a catalog of Fe, Mg, Si, Ca, and Ti abundances for 2961 red giant
stars that are likely members of eight dwarf satellite galaxies of the Milky
Way (MW): Sculptor, Fornax, Leo I, Sextans, Leo II, Canes Venatici I, Ursa
Minor, and Draco. For the purposes of validating our measurements, we also
observed 445 red giants in MW globular clusters and 21 field red giants in the
MW halo. The measurements are based on Keck/DEIMOS medium-resolution
spectroscopy combined with spectral synthesis. We estimate uncertainties in
[Fe/H] by quantifying the dispersion of [Fe/H] measurements in a sample of
stars in monometallic globular clusters. We estimate uncertainties in Mg, Si,
Ca, and Ti abundances by comparing our medium-resolution spectroscopic
measurements to high-resolution spectroscopic abundances of the same stars. For
this purpose, our DEIMOS sample included 132 red giants with published
high-resolution spectroscopy in globular clusters, the MW halo field, and dwarf
galaxies. The standard deviations of the differences in [Fe/H] and [alpha/Fe]
(the average of [Mg/Fe], [Si/Fe], [Ca/Fe], and [Ti/Fe]) between the two samples
is 0.15 and 0.16, respectively. This catalog represents the largest sample of
multi-element abundances in dwarf galaxies to date. The next papers in this
series draw conclusions on the chemical evolution, gas dynamics, and star
formation histories from the catalog presented here. The wide range of dwarf
galaxy luminosity reveals the dependence of dwarf galaxy chemical evolution on
galaxy stellar mass.Comment: 26 pages, 22 figures, 4 machine-readable tables (available in the
source file; click "Other formats"); accepted for publication in ApJ
Supplements; updated acknowledgments in v
Recommended from our members
Photometric calibrations for 21st century science
The answers to fundamental science questions in astrophysics, ranging from the history of the expansion of the universe to the sizes of nearby stars, hinge on our ability to make precise measurements of diverse astronomical objects. As our knowledge of the underlying physics of objects improves along with advances in detectors and instrumentation, the limits on our capability to extract science from measurements is set, not by our lack of understanding of the nature of these objects, but rather by the most mundane of all issues: the precision with which we can calibrate observations in physical units. In principle, photometric calibration is a solved problem - laboratory reference standards such as blackbody furnaces achieve precisions well in excess of those needed for astrophysics. In practice, however, transferring the calibration from these laboratory standards to astronomical objects of interest is far from trivial - the transfer must reach outside the atmosphere, extend over 4{pi} steradians of sky, cover a wide range of wavelengths, and span an enormous dynamic range in intensity. Virtually all spectrophotometric observations today are calibrated against one or more stellar reference sources, such as Vega, which are themselves tied back to laboratory standards in a variety of ways. This system's accuracy is not uniform. Selected regions of the electromagnetic spectrum are calibrated extremely well, but discontinuities of a few percent still exist, e.g., between the optical and infrared. Independently, model stellar atmospheres are used to calibrate the spectra of selected white dwarf stars, e.g. the HST system, but the ultimate accuracy of this system should be verified against laboratory sources. Our traditional standard star systems, while sufficient until now, need to be improved and extended in order to serve future astrophysics experiments. This white paper calls for a program to improve upon and expand the current networks of spectrophotometrically calibrated stars to provide precise calibration with an accuracy of equal to and better than 1% in the ultraviolet, visible and near-infrared portions of the spectrum, with excellent sky coverage and large dynamic range
A Complete Spectroscopic Survey of the Milky Way Satellite Segue 1: The Darkest Galaxy
We present the results of a comprehensive Keck/DEIMOS spectroscopic survey of
the ultra-faint Milky Way satellite galaxy Segue 1. We have obtained velocity
measurements for 98.2% of the stars within 67 pc (10 arcmin, or 2.3 half-light
radii) of the center of Segue 1 that have colors and magnitudes consistent with
membership, down to a magnitude limit of r=21.7. Based on photometric,
kinematic, and metallicity information, we identify 71 stars as probable Segue
1 members, including some as far out as 87 pc. After correcting for the
influence of binary stars using repeated velocity measurements, we determine a
velocity dispersion of 3.7^{+1.4}_{-1.1} km/s, with a corresponding mass within
the half-light radius of 5.8^{+8.2}_{-3.1} x 10^5 Msun. The stellar kinematics
of Segue 1 require very high mass-to-light ratios unless the system is far from
dynamical equilibrium, even if the period distribution of unresolved binary
stars is skewed toward implausibly short periods. With a total luminosity less
than that of a single bright red giant and a V-band mass-to-light ratio of 3400
Msun/Lsun, Segue 1 is the darkest galaxy currently known. We critically
re-examine recent claims that Segue 1 is a tidally disrupting star cluster and
that kinematic samples are contaminated by the Sagittarius stream. The
extremely low metallicities ([Fe/H] < -3) of two Segue 1 stars and the large
metallicity spread among the members demonstrate conclusively that Segue 1 is a
dwarf galaxy, and we find no evidence in favor of tidal effects. We also show
that contamination by the Sagittarius stream has been overestimated. Segue 1
has the highest measured dark matter density of any known galaxy and will
therefore be a prime testing ground for dark matter physics and galaxy
formation on small scales.Comment: 24 pages, 4 tables, 11 figures (10 in color). Submitted for
publication in ApJ. V3 revised according to comments from the refere
The Star Formation History and Extended Structure of the Hercules Milky Way Satellite
We present imaging of the recently discovered Hercules Milky Way satellite
and its surrounding regions to study its structure, star formation history and
to thoroughly search for signs of disruption. We robustly determine the
distance, luminosity, size and morphology of Hercules utilizing a bootstrap
approach to characterize our uncertainties. We derive a distance to Hercules of
kpc via a comparison to empirical and theoretical isochrones. As
previous studies have found, Hercules is very elongated, with
and a half light radius of pc. Using
the color magnitude fitting package StarFISH, we determine that Hercules is old
( Gyr) and metal poor (), with a spread in metallicity, in
agreement with previous spectroscopic work. We infer a total absolute magnitude
of . Our innovative search for external Hercules structure both
in the plane of the sky and along the line of sight yields some evidence that
Hercules is embedded in a larger stream of stars. A clear stellar extension is
seen to the Northwest with several additional candidate stellar overdensities
along the position angle of Hercules out to 35' (1.3 kpc). While
the association of any of the individual stellar overdensities with Hercules is
difficult to determine, we do show that the summed color magnitude diagram of
all three is consistent with Hercules' stellar population. Finally, we estimate
that any change in the distance to Hercules across its face is at most 6
kpc; and the data are consistent with Hercules being at the same distance
throughout.Comment: 50 pages, 15 figures, submitted to the Astrophysical Journa