408 research outputs found
Presupernova evolution and explosive nucleosynthesis of zero metal massive stars
We present a new set of zero metallicity models in the range 13-80 together to the associated explosive nucleosynthesis. These models are
fully homogeneous with the solar metallicity set we published in Limongi &
Chieffi (2006) and will be freely available at the web site
http://www.iasf-roma.inaf.it./orfeo/public{\_}html. A comparison between these
yields and an average star that represents the average behavior of most of the
very metal poor stars in the range confirms previous
findings that only a fraction of the elemental [X/Fe] may be fitted by the
ejecta of core collapse supernovae.Comment: 39 pages, 8 figures, 2 tables, accepted for publication in ApJ
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&
Carbon-Enhanced Metal-Poor Stars in the Early Galaxy
Very metal-deficient stars that exhibit enhancements of their carbon
abundances are of crucial importance for understanding a number of issues --
the nature of stellar evolution among the first generations of stars, the shape
of the Initial Mass Function, and the relationship between carbon enhancement
and neutron-capture processes, in particular the astrophysical s-process. One
recent discovery from objective-prism surveys dedicated to the discovery of
metal-deficient stars is that the frequency of Carbon-Enhanced Metal-Poor
(CEMP) stars increases with declining metallicity, reaching roughly 25% for
[Fe/H] < -2.5. In order to explore this phenomenon in greater detail we have
obtained medium-resolution (2 A) spectroscopy for about 350 of the 413 objects
in the Christlieb et al. catalog of carbon-rich stars, selected from the
Hamburg/ESO objective prism survey on the basis of their carbon-enhancement,
rather than metal deficiency. Based on these spectra, and near-IR JHK
photometry from the 2MASS Point Source Catalog, we obtain estimates of [Fe/H]
and [C/Fe] for most of the stars in this sample, along with reasonably accurate
determinations of their radial velocities. Of particular importance, we find
that the upper envelope of carbon enhancement observed for these stars is
nearly constant, at [C/H] ~ -1.0, over the metallicity range -4.0 < [Fe/H] <
-2.0; this same level of [C/H] applies to the most iron-deficent star yet
discovered, HE 0107-5240, at [Fe/H] = -5.3.Comment: Contributed paper to The Eigth Nuclei in the Cosmos conference, to
appear (in refereed form) in Nuclear Physics
On the oxygen abundance of HE0107-5240
We have determined the oxygen abundance of HE0107-5240 from UV-OH lines
detected in VLT/UVES spectra. Using a plane-parallel LTE model atmosphere, we
derive [O/Fe] = +2.4, and a similar analysis of CD -38 245 yields [O/Fe] =
+1.0. We estimate systematic errors due to 3D effects to be in the order of 0.3
to 0.4 dex. That is, our derived O abundances are likely overestimates: effects
from thermal inhomogeneities due to convection may require that the abundances
should be reduced by 0.3-0.4 dex or even more. Radial velocity data for
HE0107-5240 based on high-resolution spectra show that over a time span of 373
days the radial velocity was constant at 44.5 km/s, with a 1 sigma scatter of
the measurements of 0.5 km/s. However, it can not yet be ruled out that
HE0107-5240 is a very long period and/or low amplitude binary. These results
provide new constraints on scenarios for the origin of the abundance pattern of
HE0107-5240. In particular, it seems unlikely that the large overabundances of
CNO have been produced in a medium-mass AGB star which later evolved to a white
dwarf. The oxygen abundance of HE0107-5240 is significantly smaller than the
prediction of Umeda & Nomoto (2003) from calculated yields of a ~25 solar mass
Population III star exploding as a supernova of low explosion energy (E_exp = 3
x 10^50 erg) with mixing and fallback. The scenario of Limongi et al. (2003),
involving two Population III supernovae, predicts an oxygen abundance of [O/Fe]
= +4.1 for HE0107-5240, in strong contradiction with the observed value. In
conclusion, none of the above mentioned scenarios, in their present
realizations, can satisfactorly explain the abundance pattern of HE0107-5240.Comment: Submitted to ApJ Letter
A Catalogue of Field Horizontal Branch Stars Aligned with High Velocity Clouds
We present a catalogue of 430 Field Horizontal Branch (FHB) stars, selected
from the Hamburg/ESO Survey (HES), which fortuitously align with high column
density neutral hydrogen (HI) High-Velocity Cloud (HVC) gas. These stars are
ideal candidates for absorption-line studies of HVCs, attempts at which have
been made for almost 40 years with little success. A parent sample of 8321 HES
FHB stars was used to extract HI spectra along each line-of-sight, using the HI
Parkes All-Sky Survey. All lines-of-sight aligned with high velocity HI
emission with peak brightness temperatures greater than 120mK were examined.
The HI spectra of these 430 probes were visually screened and cross-referenced
with several HVC catalogues. In a forthcoming paper, we report on the results
of high-resolution spectroscopic observations of a sample of stars drawn from
this catalogue.Comment: 7 pages, 4 figures. ApJS accepted. Full catalogue and all online-only
images available at
http://astronomy.swin.edu.au/staff/cthom/catalogue/index.htm
Hierarchical Structure Formation and Chemical Evolution of Galaxies
We present an analytical and phenomenological model for metal enrichment in
halos based on hierarchical structure formation. This model assumes that
astration of normal stellar populations along with SNe II already occurs at
very high redshift. For halos that are not disrupted by SN II explosions, the
chemical evolution of the gas and stars is explicitly determined by the rate of
gas infall as compared with the astration rate and the corresponding rate of
metal production by SNe II per H atom in the gas. This model provides a good
description of the data on [Fe/H] for damped Ly alpha systems over a wide range
of redshift 0.5 < z < 5. For all halos not disrupted by SN II explosions, if
there is a cessation of gas infall, the metallicities of stars follow a bimodal
distribution. This distribution is characterized by a sharp peak at the value
of [Fe/H] corresponding to the time of infall cessation and by a broad peak at
a higher value of [Fe/H] corresponding to the subsequent period of astration
during which the bulk of the remaining gas forms stars. Such a distribution may
be compared to that observed for the Galactic halo stars. If the gas in a halo
is rapidly lost upon cessation of infall, then an assemblage of stars with a
very sharply-defined [Fe/H] value will be left behind. This assemblage of stars
may be accreted by a larger system and become a globular cluster of the larger
system. We also discuss the masses and metallicities of the globular clusters
in this model. (Abridged)Comment: 24 pages, 10 figures, to appear in Ap
Galactic Archeology with 4MOST
4MOST is a new wide-field, high-multiplex spectroscopic survey facility for
the VISTA telescope of ESO. Starting in 2022, 4MOST will deploy more than 2400
fibres in a 4.1 square degree field-of-view using a positioner based on the
tilting spine principle. In this ontribution we give an outline of the major
science goals we wish to achieve with 4MOST in the area of Galactic Archeology.
The 4MOST Galactic Archeology surveys have been designed to address
long-standing and far-reaching problems in Galactic science. They are focused
on our major themes: 1) Near-field cosmology tests, 2) Chemo-dynamical
characterisation of the major Milky Way stellar components, 3) The Galactic
Halo and beyond, and 4) Discovery and characterisation of extremely metal-poor
stars. In addition to a top-level description of the Galactic surveys we
provide information about how the community will be able to join 4MOST via a
call for Public Spectroscopic Surveys that ESO will launch.Comment: To be published in "Rediscovering our Galaxy", IAU Symposium 334,
Eds. C. Chiappini, I. Minchev, E. Starkenburg, M. Valentin
The Era of Massive Population III Stars: Cosmological Implications and Self-Termination
The birth and death of the first generation of stars have important
implications for the thermal state and chemical properties of the intergalactic
medium (IGM) in the early universe. Sometime after recombination, the neutral,
chemically pristine gas was reionized by ultraviolet photons emitted from the
first stars, but also enriched with heavy elements when these stars ended their
lives as energetic supernovae. Using the results from previous high-resolution
cosmological simulations of early structure formation that include radiative
transfer, we show that a significant volume fraction of the IGM can be
metal-polluted, as well as ionized, by massive Population III stars formed in
small-mass (10^6-10^7 Msun) halos early on. If most of the early generation
stars die as pair-instability supernovae with energies up to 10^{53} ergs, the
volume-averaged mean metallicity will quickly reach Z ~ 10^{-4}Zsun by a
redshift of 15-20, possibly causing a prompt transition to the formation of a
stellar population that is dominated by low-mass stars. In this scenario, the
early chemical enrichment history should closely trace the reionization history
of the IGM, and the end of the Population III era is marked by the completion
of reionization and pre-enrichment by z=15. We conclude that, while the
pre-enrichment may partially account for the ``metallicity-floor'' in
high-redshift Lyman-alpha clouds, it does not significantly affect the
elemental abundance in the intracluster medium.Comment: Version accepted by ApJ. Minor revisions and a few citations adde
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