307 research outputs found
The Hubble Constant
Considerable progress has been made in determining the Hubble constant over
the past two decades. We discuss the cosmological context and importance of an
accurate measurement of the Hubble constant, and focus on six high-precision
distance-determination methods: Cepheids, tip of the red giant branch, maser
galaxies, surface brightness fluctuations, the Tully-Fisher relation and Type
Ia supernovae. We discuss in detail known systematic errors in the measurement
of galaxy distances and how to minimize them. Our best current estimate of the
Hubble constant is 73 +/-2 (random) +/-4 (systematic) km/s/Mpc. The importance
of improved accuracy in the Hubble constant will increase over the next decade
with new missions and experiments designed to increase the precision in other
cosmological parameters. We outline the steps that will be required to deliver
a value of the Hubble constant to 2% systematic uncertainty and discuss the
constraints on other cosmological parameters that will then be possible with
such accuracy.Comment: To be published in Annual Reviews of Astronomy and Astrophysics, Vol.
48, 2010, consisting of 79 pages, 13 figures, 2 table
Making FORS2 fit for exoplanet observations (again)
For about three years, it was known that precision spectrophotometry with
FORS2 suffered from systematic errors that made quantitative observations of
planetary transits impossible. We identified the Longitudinal Atmospheric
Dispersion Compensator (LADC) as the most likely culprit, and therefore engaged
in a project to exchange the LADC prisms with the uncoated ones from FORS1.
This led to a significant improvement in the depth of FORS2 zero points, a
reduction in the systematic noise, and should make FORS2 again competitive for
transmission spectroscopy of exoplanets.Comment: To appear in the March issue of the ESO Messenge
Hot Horizontal-Branch Stars: The Ubiquitous Nature of the "Jump" in Stromgren u, Low Gravities, and the Role of Radiative Levitation of Metals
A "jump" in the BHB distribution in the V, u-y CMD was recently detected in
the GC M13. It is morphologically best characterized as a discontinuity in u,
u-y, with stars in the range 11,500<Teff(K)<20,000 deviating systematically
from (in the sense of appearing brighter and/or hotter than) canonical ZAHBs.
We present u, y photometry of 14 GCs obtained with 3 different telescopes
(Danish, NOT, HST) and demonstrate that the u-jump is present in every GC whose
HB extends beyond 11,500K, irrespective of [Fe/H], mixing history on the RGB,
and other GC parameters. We suggest that the u-jump is a ubiquitous feature,
intrinsic to all HB stars hotter than 11,500K. We draw a parallel between the
ubiquitous nature of the u-jump and the problem of low measured gravities among
BHB stars. We note that the "logg-jump" occurs over the same temperature range
as the u-jump, and that it occurs in every metal-poor GC for which gravities
have been determined--irrespective of [Fe/H], mixing history on the RGB, or any
other GC parameters. Furthermore, the u-jump and the logg-jump are connected on
a star-by-star basis. The two are likely different manifestations of the same
physical phenomenon. We present a framework which may simultaneously account
for the u-jump and the logg-jump. Reviewing spectroscopic data for several
field BHB stars, as well as two BHB stars in the GC NGC 6752, we find evidence
that radiative levitation of heavy elements takes place at Teff>11,500 K,
dramatically enhancing their abundances in the atmospheres of BHB stars in the
"critical" temperature region. Model atmospheres taking diffusion effects into
account are badly needed, and will likely lead to better overall agreement
between canonical evolutionary theory and observations for BHB stars.Comment: ApJ, Main Journal, accepted. Contains several changes and update
The enigmatic B[e]-star Henize 2-90: The non-spherical mass loss history from an analysis of forbidden lines
(abridged) We study the optical spectrum of the exciting B[e] star Hen 2-90
based on new high-resolution observations that cover the innermost 2". Our
investigation is splitted in two parts, a qualitative study of the presence of
the numerous emission lines and the classification of their line profiles which
indicate a circumstellar environment of high complexity, and a quantitative
analysis of numerous forbidden lines, e.g. [OI], [OII], [OIII], [SII], [SIII],
[ArIII], [ClII], [ClIII] and [NII]. We find a correlation between the different
ionization states of the elements and the velocities derived from the line
profiles: the highly ionized atoms have the highest outflow velocity while the
neutral lines have the lowest outflow velocity. The recent HST image of Hen
2-90 reveals a bipolar, highly ionized region, a neutral disk-like structure
and an intermediate region of moderate ionization. It seems that a
non-spherical stellar wind model is a good option to explain the ionization and
spatial distribution of the circumstellar material. We modelled the forbidden
lines under the assumption of a non-spherically symmetric wind based on the HST
image. We find that in order to fit the observed line luminosities, the mass
flux, surface temperature, and terminal wind velocities need to be latitude
dependent, which might be explained in terms of a rapidly rotating central
star. A rotation speed of 75-80 % of the critical velocity has been derived.
The total mass loss rate of the star was determined to be of order 3 10^{-5}
M_sun/yr. Such a wind scenario and the fact that compared to solar abundances
C, O, and N seem to be underabundant while S, Ar and Cl have solar abundances,
might be explained in terms of a rapidly rotating post-AGB star.Comment: 16 pages, 13 figures, accepted for publication in A&A. Table 4 is
included at the end of the paper. This table will only be available in the
online version of the paper and will not appear in the printed versio
A Catalog of Spectroscopically Confirmed White Dwarfs from the Sloan Digital Sky Survey Data Release 4
We present a catalog of 9316 spectroscopically confirmed white dwarfs from
the Sloan Digital Sky Survey Data Release 4. We have selected the stars through
photometric cuts and spectroscopic modeling, backed up by a set of visual
inspections. Roughly 6000 of the stars are new discoveries, roughly doubling
the number of spectroscopically confirmed white dwarfs. We analyze the stars by
performing temperature and surface gravity fits to grids of pure hydrogen and
helium atmospheres. Among the rare outliers are a set of presumed helium-core
DA white dwarfs with estimated masses below 0.3 Msun, including two candidates
that may be the lowest masses yet found. We also present a list of 928 hot
subdwarfs.Comment: Accepted by the Astrophysical Journal Supplements, 25 pages, 24
figures, LaTeX. The electronic catalog, as well as diagnostic figures and
links to the spectra, is available at http://das.sdss.org/wdcat/dr4
The mass of the sdB primary of the binary HS 2333+3927
Short period sdB binaries with cool companions are crucial to understand
pre-CV evolution, because they will evolve into cataclysmic variables, when the
sdB will have left the extended horizontal branch. Recently we discovered the
sixth such system, HS 2333+3927, consisting of an sdB star and an M dwarf
(period: 0.172 d) with a very strong reflection effect, but no eclipses. The
reflection is stronger than in any of the other similar systems which renders a
quantitative spectral analysis very difficult because the Balmer line profiles
may be disturbed by the reflected light. A spectroscopic analysis results in
Teff = 36500 K, log g = 5.70, and log (n_He/n_H) = -2.15. Mass-radius relations
were derived from the results of the analysis of light and radial-velocity
curves. Comparison with the mass-radius relation derived from the surface
gravity of the sdB star favours a rather low mass of 0.38 Msun for the primary.
The mass of the companion is 0.29 M_sun. HS 2333+3927 is the only known sdB+dM
system with a period above the CV period gap.Comment: 6 pages, 3 Fig., to appear in 14th European Workshop on White Dwarfs,
ASP Conference Series, eds. D. Koester, S. Moehle
Iron abundances of B-type post-Asymptotic Giant Branch stars in globular clusters: Barnard 29 in M 13 and ROA 5701 in omega Cen
High resolution optical and ultraviolet spectra of two B-type post-Asymptotic
Giant Branch (post-AGB) stars in globular clusters, Barnard 29 in M 13 and ROA
5701 in omega Cen, have been analysed using model atmosphere techniques. The
optical spectra have been obtained with FEROS on the ESO 2.2-m telescope and
the 2d-Coud\'e spectrograph on the 2.7-m McDonald telescope, while the
ultraviolet observations are from the GHRS on the HST. Abundances of light
elements (C, N, O, Mg, Al and S) plus Fe have been determined from the optical
spectra, while the ultraviolet data provide additional Fe abundance estimates
from Fe III absorption lines in the 1875-1900 {\AA} wavelength region. A
general metal underabundance relative to young B-type stars is found for both
Barnard 29 and ROA 5701. These results are consistent with the metallicities of
the respective clusters, as well as with previous studies of the objects. The
derived abundance patterns suggest that the stars have not undergone a gas-dust
separation, contrary to previous suggestions, although they may have evolved
from the AGB before the onset of the third dredge-up. However, the Fe
abundances derived from the HST spectra are lower than those expected from the
metallicities of the respective clusters, by 0.5 dex for Barnard 29 and 0.8 dex
for ROA 5701. A similar systematic underabundance is also found for other
B-type stars in environments of known metallicity, such as the Magellanic
Clouds. These results indicate that the Fe III ultraviolet lines may yield
abundance values which are systematically too low by typically 0.6 dex and
hence such estimates should be treated with caution.Comment: 15 pages, 3 figures. Accepted for publication in MNRA
Search for double degenerate progenitors of supernovae type Ia with SPY
We report on a large survey for double degenerate (DD) binaries as potential
progenitors of type Ia supernovae with the UVES spectrograph at the ESO VLT
(ESO SN Ia Progenitor surveY - SPY).Comment: in: "White Dwarfs", Proc. XIII Workshop on White Dwarfs, eds. D. de
Martino, R. Kalytis, R. Silvotti, J.E. Solheim, Kluwer, in press, 4 pages, 1
figur
The Look-back Time Evolution of Far-Ultraviolet Flux from the Brightest Cluster Elliptical Galaxies at z < 0.2
We present the GALEX UV photometry of the elliptical galaxies in Abell
clusters at moderate redshifts (z < 0.2) for the study of the look-back time
evolution of the UV upturn phenomenon. The brightest elliptical galaxies (M_r <
-22) in 12 remote clusters are compared with the nearby giant elliptical
galaxies of comparable optical luminosity in the Fornax and Virgo clusters. The
sample galaxies presented here appear to be quiescent without signs of massive
star formation or strong nuclear activity, and show smooth, extended profiles
in their UV images indicating that the far-UV (FUV) light is mostly produced by
hot stars in the underlying old stellar population. Compared to their
counterparts in nearby clusters, the FUV flux of cluster giant elliptical
galaxies at moderate redshifts fades rapidly with ~ 2 Gyrs of look-back time,
and the observed pace in FUV - V color evolution agrees reasonably well with
the prediction from the population synthesis models where the dominant FUV
source is hot horizontal-branch stars and their progeny. A similar amount of
color spread (~ 1 mag) in FUV - V exists among the brightest cluster elliptical
galaxies at z ~ 0.1, as observed among the nearby giant elliptical galaxies of
comparable optical luminosity.Comment: Accepted for publication in the Special GALEX ApJ Supplement,
December 200
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