1,701 research outputs found
Evolution of O Abundance Relative to Fe
We present a three-component mixing model for the evolution of O abundance
relative to Fe, taking into account the contributions of the first very massive
(> 100 solar masses) stars formed from Big Bang debris. We show that the
observations of O and Fe abundances in metal-poor stars in the Galaxy by
Israelian et al. and Boesgaard et al. can be well represented both
qualitatively and quantitatively by this model. Under the assumption of an
initial Fe ([Fe/H] = -3) and O inventory due to the prompt production by the
first very massive stars, the data at -3 < [Fe/H] < -1 are interpreted to
result from the addition of O and Fe only from type II supernovae (SNII) to the
prompt inventory. At [Fe/H] = -1, SNII still contribute O while both SNII and
type Ia supernovae contribute Fe. During this later stage, (O/Fe) sharply drops
off to an asymptotic value of 0.8(O/Fe)_sun. The value of (O/Fe) for the prompt
inventory at [Fe/H] = -3 is found to be (O/Fe) = 20(O/Fe)_sun. This result
suggests that protogalaxies with low ``metallicities'' should exhibit high
values of (O/Fe). The C/O ratio produced by the first very massive stars is
expected to be much less than 1 so that all the C should be tied up as CO and
that C dust and hydrocarbon compounds should be quite rare at epochs
corresponding to [Fe/H] < -3.Comment: 25 pages, 8 postscript figures, to appear in Ap
A Model for Abundances in Metal-Poor Stars
It is argued that the abundances of r-process related elements in stars with
-3<[Fe/H]<-1 can be explained by the contributions of three sources. The
sources are: the first generations of very massive (>100 solar masses) stars
that are formed from Big Bang debris and are distinct from SNII, and two types
of SNII, the H and L events, which can occur only at [Fe/H]>-3. The H events
are of high frequency and produce dominantly heavy (A>130) r-elements but no Fe
(presumably leaving behind black holes). The L events are of low frequency and
produce Fe and dominantly light (A<130) r-elements (essentially none above Ba).
By using the observed abundances in two ultra-metal-poor stars and the solar
r-abundances, the initial or prompt inventory of elements produced by the first
generations of very massive stars and the yields of H and L events can be
determined. The abundances of a large number of elements in a star can then be
calculated from the model by using only the observed Eu and Fe abundances. To
match the model results and the observational data for stars with -3<[Fe/H]<-1
requires that the solar r-abundances for Sr, Y, Zr, and Ba must be
significantly increased from the standard values. Whether the solar
r-components of these elements used here to obtain a fit to the stellar data
can be reconciled with those obtained from solar abundances by subtracting the
s-components calculated from models is not clear.Comment: 47 pages, 19 figures, to appear in Ap
Prompt Iron Enrichment, Two r-Process Components, and Abundances in Very Metal-Poor Stars
We present a model to explain the wide range of abundances for heavy
r-process elements (mass number A > 130) at low [Fe/H]. This model requires
rapid star formation and/or an initial population of supermassive stars in the
earliest condensed clots of matter to provide a prompt or initial Fe inventory.
Subsequent Fe and r-process enrichment was provided by two types of supernovae:
one producing heavy r-elements with no Fe on a rather short timescale and the
other producing light r-elements (A < or = 130) with Fe on a much longer
timescale.Comment: 5 pages, 2 postscript figures, to appear in ApJ
Langevin Analysis of Eternal Inflation
It has been widely claimed that inflation is generically eternal to the
future, even in models where the inflaton potential monotonically increases
away from its minimum. The idea is that quantum fluctuations allow the field to
jump uphill, thereby continually revitalizing the inflationary process in some
regions. In this paper we investigate a simple model of this process,
pertaining to inflation with a quartic potential, in which analytic progress
may be made. We calculate several quantities of interest, such as the expected
number of inflationary efolds, first without and then with various selection
effects. With no additional weighting, the stochastic noise has little impact
on the total number of inflationary efoldings even if the inflaton starts with
a Planckian energy density. A "rolling" volume factor, i.e. weighting in
proportion to the volume at that time, also leads to a monotonically decreasing
Hubble constant and hence no eternal inflation. We show how stronger selection
effects including a constraint on the initial and final states and weighting
with the final volume factor can lead to a picture similar to that usually
associated with eternal inflation.Comment: 22 pages, 2 figure
Detailed abundances of a large sample of giant stars in M 54 and in the Sagittarius nucleus
Homogeneous abundances of light elements, alpha and Fe-group elements from
high-resolution FLAMES spectra are presented for 76 red giant stars in M54, a
massive globular cluster (GC) lying in the nucleus of the Sagittarius dwarf
galaxy. We also derived detailed abundances for 27 red giants belonging to the
Sgr nucleus. Our abundances assess the intrinsic metallicity dispersion (~0.19
dex, rms scatter) of M54, with the bulk of stars peaking at [Fe/H]~-1.6 and a
long tail extending to higher metallicities, similar to omega Cen. The spread
in these probable nuclear star clusters exceeds those of most GCs: these
massive clusters are located in a region intermediate between normal GCs and
dwarf galaxies. M54 shows the Na-O anticorrelation, typical signature of GCs,
which is instead absent in the Sgr nucleus. The light elements (Mg, Al, Si)
participating to the high temperature Mg-Al cycle show that the pattern of
(anti)correlations produced by proton-capture reactions in H-burning is clearly
different between the most metal-rich and most metal-poor components in the two
most massive GCs in the Galaxy, confirming early result based on the Na-O
anticorrelation. As in omega Cen, stars affected by most extreme processing,
i.e. showing the signature of more massive polluters, are those of the
metal-rich component. This can be understood if the burst of star formation
giving birth to the metal-rich component was delayed by as much as 10-30 Myr
with respect to the metal-poor one. The evolution of these massive GCs can be
reconciled in the general scenario for the formation of GCs sketched in
Carretta et al.(2010a) taking into account that omega Cen could have already
incorporated the surrounding nucleus of its progenitor and lost the rest of the
hosting galaxy while the two are still observable as distinct components in M54
and the surrounding field.Comment: 22 pages (3 pages of appendix), 25 figures. Tables 2, 3, 5, 6, and 7
are only available in electronic form at the CDS Accepted for publication on
Astronomy and Astrophysic
Supernovae versus Neutron Star Mergers as the Major r-Process Sources
I show that recent observations of r-process abundances in metal-poor stars
are difficult to explain if neutron star mergers (NSMs) are the major r-process
sources. In contrast, such observations and meteoritic data on Hf182 and I129
in the early solar system support a self-consistent picture of r-process
enrichment by supernovae (SNe). While further theoretical studies of r-process
production and enrichment are needed for both SNe and NSMs, I emphasize two
possible direct observational tests of the SN r-process model: gamma rays from
decay of r-process nuclei in SN remnants and surface contamination of the
companion by SN r-process ejecta in binaries.Comment: 5 pages, to appear in ApJ
The normal chemistry of multiple stellar populations in the dense globular cluster NGC 6093 (M 80)
We present the abundance analysis of 82 red giant branch stars in the dense,
metal-poor globular cluster NGC 6093 (M 80), the largest sample of stars
analyzed in this way for this cluster. From high resolution UVES spectra of 14
stars and intermediate resolution GIRAFFE spectra for the other stars we
derived abundances of O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu,
Zn, Y, Zr, Ba, La, Ce, Pr, Nd, Sm, Eu. On our UVES metallicity scale the mean
metal abundance of M 80 is [Fe/H]=-1.791+/-0.006+/-0.076 (+/-statistical
+/-systematic error) with rms=0.023 (14 stars). M 80 shows star to star
variations in proton-capture elements, and the extension of the Na-O
anticorrelation perfectly fit the relations with (i) total cluster mass, (ii)
horizontal branch morphology, and (iii) cluster concentration previously found
by our group. The chemistry of multiple stellar populations in M 80 does not
look extreme. The cluster is also a typical representative of halo globular
clusters for what concerns the pattern of alpha-capture and Fe-group elements.
However we found that a significant contribution from the s-process is required
to account for the distribution of neutron-capture elements. A minority of
stars in M 80 seem to exhibit slightly enhanced abundances of s-process
species, compatible with those observed in M 22 and NGC 1851, although further
confirmation from larger samples is required.Comment: 18 pages, 21 figures, 10 tables; accepted for publication on
Astronomy and Astrophysic
Na-O Anticorrelation and HB. II. The Na-O anticorrelation in the globular cluster NGC 6752
We are studying the Na-O anticorrelation in several globular clusters of
different Horizontal Branch (HB) morphology in order to derive a possible
relation between (primordial) chemical inhomogeneities and morphological
parameters of the cluster population. We used the multifiber spectrograph
FLAMES on the ESO Very Large Telescope UT2 and derived atmospheric parameters
and elemental abundances of Fe, O and Na for about 150 red giant stars in the
Galactic globular cluster NGC 6752. The average metallicity we derive is
[Fe/H]=-1.56, in agreement with other results from red giants, but lower than
obtained for dwarfs or early subgiants. In NGC 6752 there is not much space for
an intrinsic spread in metallicity: on average, the rms scatter in [Fe/H] is
0.037+/-0.003 dex, while the scatter expected on the basis of the major error
sources is 0.039+/-0.003 dex. The distribution of stars along the Na-O
anticorrelation is different to what was found in the first paper of this
series for the globular cluster NGC 2808: in NGC 6752 it is skewed toward more
Na-poor stars, and it resembles more the one in M 13. Detailed modeling is
required to clarify whether this difference may explain the very different
distributions of stars along the HB.Comment: 13 pages, 8 figures; tables 2,3,5 available at CDS. Accepted for
publication on A&
The Na-O anticorrelation in horizontal branch stars. III. 47 Tuc and M 5
To check the impact of the multiple population scenario for globular clusters
on their HB, we present an analysis of the composition of 110 red HB (RHB)
stars in 47 Tucanae and of 61 blue HB (BHB) and 30 RHB stars in M5. In 47 Tuc
we found tight relations between the colours of the stars and their abundances
of p-capture elements. This strongly supports the idea that the He content -
which is expected to be closely correlated with the abundances of p-capture
elements - is the third parameter (after overall metallicity and age) that
determines the colour of HB stars. However, the range in He abundance must be
small (Delta Y<0.03) in 47 Tuc to reproduce our observations; this agrees with
previous analyses. There is possibly a correlation between the abundances of p-
and n-capture elements in 47 Tuc. If confirmed, this might suggest that AGB
stars of moderate mass contributed to the gas from which second-generation
stars formed. Considering the selection effects in our sample (which does not
include stars warmer than 11000 K and RR Lyrae variables) is important to
understand our results for M5. In this case, we find that, as expected, RHB
stars are Na-poor and O-rich, and likely belong to the primordial population.
There is a clear correlation of the [Na/O] ratio and N abundance with colour
along the BHB. A derivation of the He abundance for these stars yields a low
value of Y=0.22\pm 0.03. This is expected because HB stars of a putative
He-rich population in this cluster should be warmer than 11000 K, and would
accordingly not have been sampled by our analysis. However, we need some
additional source of scatter in the total mass loss of stars climbing up the
RGB to reproduce our results for M5. Finally, we found a C-star on the HB of 47
Tuc and a Ba-rich, fast-rotating, likely binary star on the HB of M5. These
stars are among the brightest and coolest HB stars.Comment: Accepted on Astronomy and Astrophysics. 16 pages, 19 figure
Determination of Nucleosynthetic Yields of Supernovae and Very Massive Stars from Abundances in Metal-Poor Stars
(Abridged) We determine the yields of Na to Ni for Type II supernovae (SNe
II) and the yield patterns of the same elements for Type Ia supernovae (SNe Ia)
and very massive (>100 M_sun) stars (VMS) using a phenomenological model of
stellar nucleosynthesis and the data on a number of stars with -4<[Fe/H]<-3, a
single star with [Fe/H]=-2.04, and the sun. We consider that there are two
distinct kinds of SNe II: the high-frequency SNe II(H) and the low-frequency
SNe II(L). We also consider that VMS were the dominant first-generation stars
formed from big bang debris. The yield patterns of Na to Ni for SNe II(H),
II(L), and Ia and VMS appear to be well defined. It is found that SNe II(H)
produce almost none of these elements, that SNe II(L) can account for the
entire solar inventory of Na, Mg, Si, Ca, Ti, and V, and that compared with SNe
II(L), VMS underproduce Na, Al, V, Cr, and Mn, overproduce Co, but otherwise
have an almost identical yield pattern. A comparison is made between the yield
patterns determined here from the observational data and those from ab initio
models of nucleosynthesis in SNe II and VMS. The evolution of the other
elements relative to Fe is shown to involve three distinct stages, the earliest
of which is directly related to the problems of early aggregation and
dispersion of baryonic matter. It is argued that the VMS contributions should
represent the typical composition of dispersed baryonic matter in the universe.Comment: 33 pages, 14 postscript figures, to appear in Ap
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