295 research outputs found
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&
A single low-energy, iron-poor supernova as the source of metals in the star SMSS J 031300.36-670839.3
The element abundance ratios of four low-mass stars with extremely low
metallicities indicate that the gas out of which the stars formed was enriched
in each case by at most a few, and potentially only one low-energy, supernova.
Such supernovae yield large quantities of light elements such as carbon but
very little iron. The dominance of low-energy supernovae is surprising, because
it has been expected that the first stars were extremely massive, and that they
disintegrated in pair-instability explosions that would rapidly enrich galaxies
in iron. What has remained unclear is the yield of iron from the first
supernovae, because hitherto no star is unambiguously interpreted as
encapsulating the yield of a single supernova. Here we report the optical
spectrum of SMSS J031300.36- 670839.3, which shows no evidence of iron (with an
upper limit of 10^-7.1 times solar abundance). Based on a comparison of its
abundance pattern with those of models, we conclude that the star was seeded
with material from a single supernova with an original mass of ~60 Mo (and that
the supernova left behind a black hole). Taken together with the previously
mentioned low-metallicity stars, we conclude that low-energy supernovae were
common in the early Universe, and that such supernovae yield light element
enrichment with insignificant iron. Reduced stellar feedback both chemically
and mechanically from low-energy supernovae would have enabled first-generation
stars to form over an extended period. We speculate that such stars may perhaps
have had an important role in the epoch of cosmic reionization and the chemical
evolution of early galaxies.Comment: 28 pages, 6 figures, Natur
The Dynamical Equilibrium of Galaxy Clusters
If a galaxy cluster is effectively in dynamical equilibrium then all galaxy
populations within the cluster must have distributions in velocity and position
that individually reflect the same underlying mass distribution, although the
derived virial masses can be quite different. Specifically, within the CNOC
cluster sample the virial radius of the red galaxy population is, on the
average, a factor of smaller than that of the blue population.
The red galaxies also have a smaller RMS velocity dispersion, a factor of within our sample. Consequently, the virial mass calculated from the
blue galaxies is times larger than from the red galaxies.
However, applying the Jeans equation of stellar-hydrodynamical equilibrium to
the red and blue subsamples separately give statistically identical cluster
mass profiles. This is strong evidence that these clusters are effectively
equilibrium systems, and therefore empirically demonstrates that the masses in
the virialized region are reliably estimated using dynamical techniques.Comment: Submitted for publication in ApJLetts. 12 pages as a uufile, also
available at http://manaslu.astro.utoronto.ca/~carlberg/cnoc/br/br.ps.g
Gamma-rays from millisecond pulsars in Globular Clusters
Globular clusters (GCs) with their ages of the order of several billion years
contain many final products of evolution of stars such as: neutron stars, white
dwarfs and probably also black holes. These compact objects can be at present
responsible for the acceleration of particles to relativistic energies.
Therefore, gamma-ray emission is expected from GCs as a result of radiation
processes occurring either in the inner magnetosperes of millisecond pulsars or
in the vicinity of accreting neutron stars and white dwarfs or as a result of
interaction of particles leaving the compact objects with the strong radiation
field within the GC. Recently, GeV gamma-ray emission has been detected from
several GCs by the new satellite observatory Fermi. Also Cherenkov telescopes
reported interesting upper limits at the TeV energies which start to constrain
the content of GCs. We review the results of these gamma-ray observations in
the context of recent scenarios for their origin.Comment: 20 pages, 9 figures, will be published in Astrophysics and Space
Science Series (Springer), eds. N. Rea and D.F. Torre
Membership of 23 stars towards the bulge globular clusters NGC 6528 and NGC 6553
Low resolution spectra of 23 stars towards the bulge globular clusters NGC
6528 and NGC 6553 are analysed. Radial velocities and atmospheric parameters
are derived in order to check their membership in the clusters. Effective
temperatures were obtained from photometric data for stars with Teff > 3800 K,
whereas for cooler stars, they were derived from equivalent widths of TiO
bands. Calibrations of W(TiO) as a function of stellar parameters based on a
grid of synthetic spectra are presented. Metallicities were derived from a
comparison of the observed spectra to a grid of synthetic spectra. The sample
comprises evolutionary stages from the Red Giant Branch to the Horizontal
Branch, with parameters in the range 3200 <= Teff <= 5000 K and -0.5 <= log g
<= 2.4. The mean metallicities obtained for NGC 6528 and NGC 6553 are [Fe/H] ~
-0.5 and -0.7, in both cases with [Mg/Fe] = +0.3; assuming the same
overabundance for the elements O, Mg, Si, S, Ca and Ti, this gives
[Z/Zsolar] = -0.25 and -0.45. Membership verification by means of low
resolution spectra is a crucial step in preparing targets for high resolution
spectroscopy with 8m class telescopes.Comment: Accepted for publication in A&
The VVDS type-1 AGN sample: The faint end of the luminosity function
In a previous paper (Gavignaud et al. 2006), we presented the type-1 Active
Galactic Nuclei (AGN) sample obtained from the first epoch data of the
VIMOS-VLT Deep Survey (VVDS). The sample consists of 130 faint, broad-line AGN
with redshift up to z=5 and 17.5< I <24.0, selected on the basis of their
spectra. In this paper we present the measurement of the Optical Luminosity
Function up to z=3.6 derived from this sample, we compare our results with
previous results from brighter samples both at low and at high redshift. Our
data, more than one magnitude fainter than previous optical surveys, allow us
to constrain the faint part of the luminosity function up to high redshift. By
combining our faint VVDS sample with the large sample of bright AGN extracted
from the SDSS DR3 (Richards et al., 2006b) and testing a number of different
evolutionary models, we find that the model which better represents the
combined luminosity functions, over a wide range of redshift and luminosity, is
a luminosity dependent density evolution (LDDE) model, similar to those derived
from the major X-surveys. Such a parameterization allows the redshift of the
AGN space density peak to change as a function of luminosity and explains the
excess of faint AGN that we find at 1.0< z <1.5. On the basis of this model we
find, for the first time from the analysis of optically selected samples, that
the peak of the AGN space density shifts significantly towards lower redshift
going to lower luminosity objects. This result, already found in a number of
X-ray selected samples of AGN, is consistent with a scenario of "AGN cosmic
downsizing", in which the density of more luminous AGN, possibly associated to
more massive black holes, peaks earlier in the history of the Universe, than
that of low luminosity ones.Comment: 13 pages, 10 figures, submitted to A&
Pre-FLAMES Survey: Observations of Selected Stellar Fields
This paper presents the first set of fully calibrated images and associated
stellar catalogs of the Pre-FLAMES survey being carried out by the ESO Imaging
Survey (EIS) project. The primary goal of this survey is to provide the ESO
community with data sets from which suitable target lists can be extracted for
follow-up observations with the new VLT facility FLAMES. For this purpose 160
stellar fields have been selected for observations in B, V and I using the
8kx8k Wide Field Imager (WFI) at the MPG/ESO 2.2 m telescope at La Silla. These
data have been used to assess the observing strategy adopted and to define
suitable reduction techniques and procedures for the preparation of input
catalogs for FLAMES. The images and catalogs presented here are publicly
available and can be requested from the URL address ``http://www.eso.org/eis''.Comment: 18 pages, 28 figures available in gif format. The postscript version
of the paper with all the figures encapsulated is available at
http://www.eso.org/science/eis/eis_pub/eis_pub.html. Astronomy &
Astrophysics, accepte
Melanopsin-expressing amphioxus photoreceptors transduce light via a phospholipase C signaling cascade
© The Author(s), 2012. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in PLoS One 7 (2012): e29813, doi:10.1371/journal.pone.0029813.Melanopsin, the receptor molecule that underlies light sensitivity in mammalian ‘circadian’ receptors, is homologous to invertebrate rhodopsins and has been proposed to operate via a similar signaling pathway. Its downstream effectors, however, remain elusive. Melanopsin also expresses in two distinct light-sensitive cell types in the neural tube of amphioxus. This organism is the most basal extant chordate and can help outline the evolutionary history of different photoreceptor lineages and their transduction mechanisms; moreover, isolated amphioxus photoreceptors offer unique advantages, because they are unambiguously identifiable and amenable to single-cell physiological assays. In the present study whole-cell patch clamp recording, pharmacological manipulations, and immunodetection were utilized to investigate light transduction in amphioxus photoreceptors. A Gq was identified and selectively localized to the photosensitive microvillar membrane, while the pivotal role of phospholipase C was established pharmacologically. The photocurrent was profoundly depressed by IP3 receptor antagonists, highlighting the importance of IP3 receptors in light signaling. By contrast, surrogates of diacylglycerol (DAG), as well as poly-unsaturated fatty acids failed to activate a membrane conductance or to alter the light response. The results strengthen the notion that calcium released from the ER via IP3-sensitive channels may fulfill a key role in conveying - directly or indirectly - the melanopsin-initiated light signal to the photoconductance; moreover, they challenge the dogma that microvillar photoreceptors and phoshoinositide-based light transduction are a prerogative of invertebrate eyes.This work was supported by the National Science Foundation of the USA (grant 0918930)
Derivation of Human Differential Photoreceptor-like Cells from the Iris by Defined Combinations of CRX, RX and NEUROD
Examples of direct differentiation by defined transcription factors have been provided for beta-cells, cardiomyocytes and neurons. In the human visual system, there are four kinds of photoreceptors in the retina. Neural retina and iris-pigmented epithelium (IPE) share a common developmental origin, leading us to test whether human iris cells could differentiate to retinal neurons. We here define the transcription factor combinations that can determine human photoreceptor cell fate. Expression of rhodopsin, blue opsin and green/red opsin in induced photoreceptor cells were dependent on combinations of transcription factors: A combination of CRX and NEUROD induced rhodopsin and blue opsin, but did not induce green opsin; a combination of CRX and RX induced blue opsin and green/red opsin, but did not induce rhodopsin. Phototransduction-related genes as well as opsin genes were up-regulated in those cells. Functional analysis; i.e. patch clamp recordings, clearly revealed that generated photoreceptor cells, induced by CRX, RX and NEUROD, responded to light. The response was an inward current instead of the typical outward current. These data suggest that photosensitive photoreceptor cells can be generated by combinations of transcription factors. The combination of CRX and RX generate immature photoreceptors: and additional NEUROD promotes maturation. These findings contribute substantially to a major advance toward eventual cell-based therapy for retinal degenerative diseases
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