1,776 research outputs found
Fluorine in a Carbon-Enhanced Metal-Poor Star
The fluorine abundance of the Carbon-Enhanced Metal-Poor (CEMP) star HE
1305+0132 has been derived by analysis of the molecular HF (1-0) R9 line at
2.3357 microns in a high-resolution (R = 50,000) spectrum obtained with the
Phoenix spectrometer and Gemini-South telescope. Our abundance analysis makes
use of a CNO-enhanced ATLAS12 model atmosphere characterized by a metallicity
and CNO enhancements determined utilizing medium-resolution (R = 3,000) optical
and near-IR spectra. The effective iron abundance is found to be [Fe/H] = -2.5,
making HE 1305+0132 the most Fe-deficient star, by more than an order of
magnitude, for which the abundance of fluorine has been measured. Using
spectral synthesis, we derive a super-solar fluorine abundance of A(19F) = 4.96
+/- 0.21, corresponding to a relative abundance of [F/Fe] = 2.90. A single line
of the Phillips C_2 system is identified in our Phoenix spectrum, and along
with multiple lines of the first-overtone vibration-rotation CO (3-1) band
head, C and O abundances of A(12C) = 8.57 +/- 0.11 and A(16O) = 7.04 +/- 0.14
are derived. We consider the striking fluorine overabundance in the framework
of the nucleosynthetic processes thought to be responsible for the
C-enhancement of CEMP stars and conclude that the atmosphere of HE 1305+0132
was polluted via mass transfer by a primary companion during its asymptotic
giant branch phase. This is the first study of fluorine in a CEMP star, and it
demonstrates that this rare nuclide can be a key diagnostic of nucleosynthetic
processes in the early Galaxy.Comment: 13 pages, 3 figures; Accepted for publication in ApJ Letter
Telling the tale of the first stars
HE 0107-5240 is a star in more than once sense of the word. Chemically, it is
the most primitive object yet discovered, and it is at the centre of debate
about the origins of the first elements in the Universe.Comment: 3 pages, 0 figures, published in Nature "News and Views," Apr. 24,
200
The stellar content of the Hamburg/ESO survey VI. The metallicity distribution of main-sequence turnoff stars in the Galactic halo
We determine the metallicity distribution function (MDF) of the Galactic halo
based on metal-poor main-sequence turnoff-stars (MSTO) which were selected from
the Hamburg/ESO objective-prism survey (HES) database. Corresponding follow-up
moderateresolution observations (R ~ 2000) of some 682 stars (among which 617
were accepted program stars) were carried out with the 2.3m telescope at the
Siding Spring Observatory (SSO). Corrections for the survey volume covered by
the sample stars were quantitatively estimated and applied to the observed MDF.
The corrections are quite small, when compared with those for a previously
studied sample of metal-poor giants. The corrected observational MDF of the
turnoff sample was then compared with that of the giants, as well as with a
number of theoretical predictions of Galactic chemical evolution, including the
mass-loss modified Simple Model. Although the survey-volume corrected MDFs of
the metal-poor turnoff and the halo giants notably differ in the region of
[Fe/H] > -2.0, below [Fe/H] ~ -2.0, (the region we scientifically focus on
most) both MDFs show a sharp drop at [Fe/H] ~ -3.6 and present rather similar
distributions in the low-metallicity tail. Theoretical models can fit some
parts of the observed MDF, but none is found to simultaneously reproduce the
peak as well as the features in the metal-poor region with [Fe/H] between -2.0
to -3.6. Among the tested models only the GAMETE model, when normalized to the
tail of the observed MDF below [Fe/H] ~ -3.0, and with Z_{cr} =
10^{-3.4}Z_{\odot}, is able to predict the sharp drop at [Fe/H] ~ -3.6.Comment: 10 pages, 11 figures, accepted for publication in A&
Modelling the observed properties of carbon-enhanced metal-poor stars using binary population synthesis
The stellar population in the Galactic halo is characterised by a large
fraction of CEMP stars. Most CEMP stars are enriched in -elements (CEMP-
stars), and some of these are also enriched in -elements (CEMP- stars).
One formation scenario proposed for CEMP stars invokes wind mass transfer in
the past from a TP-AGB primary star to a less massive companion star which is
presently observed. We generate low-metallicity populations of binary stars to
reproduce the observed CEMP-star fraction. In addition, we aim to constrain our
wind mass-transfer model and investigate under which conditions our synthetic
populations reproduce observed abundance distributions. We compare the CEMP
fractions and the abundance distributions determined from our synthetic
populations with observations. Several physical parameters of the binary
stellar population of the halo are uncertain, e.g. the initial mass function,
the mass-ratio and orbital-period distributions, and the binary fraction. We
vary the assumptions in our model about these parameters, as well as the wind
mass-transfer process, and study the consequent variations of our synthetic
CEMP population. The CEMP fractions calculated in our synthetic populations
vary between 7% and 17%, a range consistent with the CEMP fractions among very
metal-poor stars recently derived from the SDSS/SEGUE data sample. The results
of our comparison between the modelled and observed abundance distributions are
different for CEMP- stars and for CEMP- stars. For the latter, our
simulations qualitatively reproduce the observed distributions of C, Na, Sr,
Ba, Eu, and Pb. Contrarily, for CEMP- stars our model cannot reproduce the
large abundances of neutron-rich elements such as Ba, Eu, and Pb. This result
is consistent with previous studies, and suggests that CEMP- stars
experienced a different nucleosynthesis history to CEMP- stars.Comment: 17 pages, 11 figures, accepted for publication on Astronomy and
Astrophysic
The Dichotomy of the Halo of the Milky Way
We summarize evidence that the halo of the Milky Way comprises two different,
and broadly overlapping, stellar components. The two structures exhibit
different chemical compositions, spatial distributions, and kinematics. These
results were obtained through an analysis of more than 20,000 calibration stars
from the Sloan Digital Sky Survey (SDSS). The duality of the stellar halo
directly impacts galaxy formation models, for the Milky Way and other large
spirals.Comment: 6 pages, 4 figures, to appear in Proceedings of the OMEG07
Conference, held in December 200
A Collision of Subclusters in Abell 754
We present direct evidence of a collision of subclusters in the galaxy
cluster Abell 754. Our comparison of new optical data and archival ROSAT PSPC
X-ray data reveal three collision signatures predicted by n-body/hydrodynamical
simulations of hierarchical cluster evolution. First, there is strong evidence
of a non-hydrostatic process; neither of the two major clumps in the galaxy
distribution lies on the off-center peak of the X-ray emission from the
intracluster gas. Second, the peak of the X-ray emission is elongated
perpendicular to the collision axis defined by the centroids of the two galaxy
clumps. Third, there is evidence of compression-heated gas; one of A754's two
X-ray temperature components (Henry & Briel 1995) is among the hottest observed
in any cluster and hotter than that inferred from the velocity dispersion of
the associated galaxy clump. These signatures are consistent with the
qualitative features of simulations (Evrard 1990a,b) in which two subclusters
have collided in the plane of the sky during roughly the last Gyr. The
detection of such collisions is crucial for understanding both the dynamics of
individual clusters and the underlying cosmology. First, for systems like A754,
estimating the cluster X-ray mass from assumptions of hydrostatic equilibrium
and isothermality is incorrect and may produce the discrepancies sometimes
found between X-ray masses and those derived from gravitational lens models
(Babul & Miralda-Escude 1994). Second, the fraction of nearby clusters in which
subclusters have collided in the last Gyr is especially sensitive to the mean
mass density parameter Omega_0 (cf. Richstone et al. 1992; Evrard et al. 1993;
Lacey & Cole 1993). With a large, well-defined cluster sample, it will be
possible to place a new and powerful constraint on cosmological models.Comment: 4 pages + 1 color figure (Postscript). Accepted for Publication in
ApJ Letter
Chronography of the Milky Way's Halo System with Field Blue Horizontal-Branch Stars
In a pioneering effort, Preston et al. reported that the colors of blue
horizontal-branch (BHB) stars in the halo of the Galaxy shift with distance,
from regions near the Galactic center to about 12 kpc away, and interpreted
this as a correlated variation in the ages of halo stars, from older to
younger, spanning a range of a few Gyrs. We have applied this approach to a
sample of some 4700 spectroscopically confirmed BHB stars selected from the
Sloan Digital Sky Survey to produce the first "chronographic map" of the halo
of the Galaxy. We demonstrate that the mean de-reddened gr color,
, increases outward in the Galaxy from 0.22 to 0.08 (over a
color window spanning [0.3:0.0]) from regions close to the Galactic center
to ~40 kpc, independent of the metallicity of the stars. Models of the expected
shift in the color of the field BHB stars based on modern stellar evolutionary
codes confirm that this color gradient can be associated with an age difference
of roughly 2-2.5 Gyrs, with the oldest stars concentrated in the central ~15
kpc of the Galaxy. Within this central region, the age difference spans a mean
color range of about 0.05 mag (~0.8 Gyrs). Furthermore, we show that
chronographic maps can be used to identify individual substructures, such as
the Sagittarius Stream, and overdensities in the direction of Virgo and
Monoceros, based on the observed contrast in their mean BHB colors with respect
to the foreground/background field population.Comment: 6 pages, 4 figures, ApJ letter
Spatially valid proprioceptive cues improve the detection of a visual stimulus
Vision and proprioception are the main sensory modalities that convey hand location and direction of movement. Fusion of these sensory signals into a single robust percept is now well documented. However, it is not known whether these modalities also interact in the spatial allocation of attention, which has been demonstrated for other modality pairings. The aim of this study was to test whether proprioceptive signals can spatially cue a visual target to improve its detection. Participants were instructed to use a planar manipulandum in a forward reaching action and determine during this movement whether a near-threshold visual target appeared at either of two lateral positions. The target presentation was followed by a masking stimulus, which made its possible location unambiguous, but not its presence. Proprioceptive cues were given by applying a brief lateral force to the participantâs arm, either in the same direction (validly cued) or in the opposite direction (invalidly cued) to the on-screen location of the mask. The dâČ detection rate of the target increased when the direction of proprioceptive stimulus was compatible with the location of the visual target compared to when it was incompatible. These results suggest that proprioception influences the allocation of attention in visual spac
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