284 research outputs found
Molecular column densities in selected model atmospheres
From an examination of predicted column densities, the following conclusions were drawn: (1) The SiO ought to be visible in carbon stars which were generated from triple alpha burning, but absent from carbon stars generated from the CNO bi-cycle. (2) Variation in the observed relative strengths of TiO and ZrO is indicative of real differences in the ratio Ti/Zr. (3) The TiO/ZrO ratio shows a small variation as C/O and effective temperature is changed. (4) Column density of silicon dicarbide (SiC2) is sensitive to abundance, temperature, and gravity; hence all relationships between the strength of SiC2 and other stellar parameters will show appreciable scatter. There is however, a substantial luminosity effect present in the SiC2 column densities. (5) Unexpectedly, SiC2 is anti-correlated with C2. (6) The presence of SiC2 in a carbon star eliminates the possibility of these stars having temperatures greater than or equal to 3000 K, or being produced through the CNO bi-cycle
Nucleosynthesis of Zinc and Iron-Peak Elements in Pop III Type II Supernovae : Comparison with abundances of Very Metal-Poor Halo Stars
We calculate nucleosynthesis in core-collapse explosions of massive Pop III
stars, and compare the results with abundances of metal-poor halo stars to
constrain the parameters of Pop III supernovae. We focus on iron-peak elements
and, in particular, we try to reproduce the large [Zn/Fe] observed in extremely
metal-poor stars. The interesting trends of the observed ratios [Zn, Co, Mn,
Cr, V/Fe] can be related to the variation of the relative mass of the complete
and incomplete Si-burning regions in supernova ejecta. We find that [Zn/Fe] is
larger for deeper mass-cuts, smaller neutron excess, and larger explosion
energies. The large [Zn/Fe] and [O/Fe] observed in the very metal-poor halo
stars suggest deep mixing of complete Si-burning material and a significant
amount of fall-back in Type II supernovae. Furthermore, large explosion
energies (E_51 >~ 2 for M ~ 13 Msun and E_51 >~ 20 for M >~ 20 Msun) are
required to reproduce [Zn/Fe] ~ 0.5. The observed trends of the abundance
ratios among the iron-peak elements are better explained with this high energy
(``Hypernova'') models rather than the simple ``deep'' mass-cut effect, because
the overabundance of Ni can be avoided in the hypernova models. We also present
the yields of pair-instability supernova explosions of M = 130 - 300 Msun
stars, and discuss that the abundance features of very metal-poor stars cannot
be explained by pair-instability supernovae.Comment: 32 pages, 19 figures, 18 tables. To appear in the Astrophysical
Journal 2002, 565. Table 18 of yields of Pop III Pair-Instability Supernovae
is replaced with a new on
An Extremely Lithium-Rich Bright Red Giant in the Globular Cluster M3
We have serendipitously discovered an extremely lithium-rich star on the red
giant branch of the globular cluster M3 (NGC 5272). An echelle spectrum
obtained with the Keck I HIRES reveals a Li I 6707 Angstrom resonance doublet
of 520 milli-Angstrom equivalent width, and our analysis places the star among
the most Li-rich giants known: log[epsilon(Li)] ~= +3.0. We determine the
elemental abundances of this star, IV-101, and three other cluster members of
similar luminosity and color, and conclude that IV-101 has abundance ratios
typical of giants in M3 and M13 that have undergone significant mixing. We
discuss mechanisms by which a low-mass star may be so enriched in Li, focusing
on the mixing of material processed by the hydrogen-burning shell just below
the convective envelope. While such enrichment could conceivably only happen
rarely, it may in fact regularly occur during giant-branch evolution but be
rarely detected because of rapid subsequent Li depletion.Comment: 7-page LaTeX file, including 2 encapsulated ps figures + 1 table;
accepted for publication in the Astrophysical Journal Letter
Anomalous RR Lyrae stars(?). III. CM Leonis
Time series of B,V,I CCD photometry and radial velocity measurements from
high resolution spectroscopy (R=30,000) covering the full pulsation cycle are
presented for the field RR Lyrae star CM Leonis. The photometric data span a 6
year interval from 1994 to 1999, and allow us to firmly establish the pulsation
mode and periodicity of the variable. The derived period P=0.361699 days (+/-
0.000001) is very close to the value published in the Fourth Edition of the
General Catalogue of Variable Stars (P=0.361732 days). However, contrary to
what was previously found, the amplitude and shape of the light curve qualify
CM Leo as a very regular first overtone pulsator with a prominent hump on the
rising branch of its multicolour light curves. According to an abundace
analysis performed on three spectra taken near minimum light (0.42 < phase <
0.61), CM Leo is a metal-poor star with metal abundance [Fe/H]=-1.93 +/- 0.20.
The photometric and radial velocity curves of CM Leo have been compared with
the predictions of suitable pulsational models to infer tight constraints on
the stellar mass, effective temperature, and distance modulus of the star. We
derive a true distance modulus of CM Leo of (m-M)0=13.11 +/- 0.02 mag and a
corresponding absolute magnitude of Mv=0.47 +/- 0.04. This absolute magnitude,
once corrected for evolutionary and metallicity effects, leads to a true
distance modulus of the Large Magellanic Cloud of (m-M)0=18.43 +/- 0.06 mag, in
better agreement with the long astronomical distance scale.Comment: 14 pages, 10 figures, accepted for publication in MNRA
The Low-z Intergalactic Medium. II. LyB, OVI, and CIII Forest
We present the results of a large survey of HI, OVI, and CIII absorption
lines in the low-redshift (z<0.3) intergalactic medium (IGM). We begin with 171
strong Lyalpha absorption lines (W>80 mA) in 31 AGN sight lines studied with
the Hubble Space Telescope and measure corresponding absorption from
higher-order Lyman lines with FUSE. Higher-order Lyman lines are used to
determine N_HI and b_HI accurately through a curve-of-growth (COG) analysis. We
find that the number of HI absorbers per column density bin is a power-law
distribution, dN/dN_HI=N^-beta, with beta_HI=1.68+-0.11. We made 40 detections
of OVI 1032,1038 and 30 detections of CIII 977 out of 129 and 148 potential
absorbers, respectively. The column density distribution of CIII absorbers has
beta_CIII=1.68+-0.04, similar to beta_HI but not as steep as beta_OVI=2.1+-0.1.
From the absorption-line frequency, dN_CIII/dz=12^+3_-2 for W(CIII)>30 mA, we
calculate a typical IGM absorber size r_0~400 kpc. The COG-derived b-values
show that HI samples material with T<10^5 K, incompatible with a hot IGM phase.
By calculating a grid of CLOUDY models of IGM absorbers with a range of
collisional and photoionization parameters, we find it difficult to
simultaneously account for the OVI and CIII observations with a single phase.
The observations require a multiphase IGM in which HI and CIII arise in
photoionized regions, while OVI is produced primarily through shocks. From the
multiphase ratio N_HI/N_CIII, we infer the IGM metallicity Z_C=0.12 Z_sun,
similar to our previous estimate of Z_O=0.09 Z_sun from OVI.Comment: 26 pages including five tables. One color figure. Accepted by Ap
Chemical Abundances Of Open Clusters From High-Resolution Infrared Spectra. I. NGC 6940
We present near-infrared spectroscopic analysis of 12 red giant members of
the Galactic open cluster NGC 6940. High-resolution (R45000) and high
signal-to-noise ratio (S/N > 100) near-infrared H and K band spectra were
gathered with the Immersion Grating Infrared Spectrograph (IGRINS) on the 2.7m
Smith Telescope at McDonald Observatory. We obtained abundances of H-burning
(C, N, O), (Mg, Si, S, Ca), light odd-Z (Na, Al, P, K), Fe-group
(Sc, Ti, Cr, Fe, Co, Ni) and neutron-capture (Ce, Nd, Yb) elements. We report
the abundances of S, P, K, Ce, and Yb in NGC 6940 for the first time. Many OH
and CN features in the H band were used to obtain O and N abundances. C
abundances were measured from four different features: CO molecular lines in
the K band, high excitation C I lines present in both near-infrared and
optical, CH and bands in the optical region. We have also determined
ratios from the R-branch band heads of first overtone (2-0) and
(3-1) (2-0) lines near 23440
\overset{\lower.5em\circ}{\mathrm{A}} and (3-1) lines at about
23730 \overset{\lower.5em\circ}{\mathrm{A}}. We have also investigated the HF
feature at 23358.3 \overset{\lower.5em\circ}{\mathrm{A}}, finding solar
fluorine abundances without ruling out a slight enhancement. For some elements
(such as the group), IGRINS data yield more internally
self-consistent abundances. We also revisited the CMD of NGC 6940 by
determining the most probable cluster members using Gaia DR2. Finally, we
applied Victoria isochrones and MESA models in order to refine our estimates of
the evolutionary stages of our targets.Comment: 16 pages, 10 figure
The Chemical Composition and Age of the Metal-Poor Halo Star BD +17^\circ 3248
We have combined new high-resolution spectra obtained with the Hubble Space
Telescope (HST) and ground-based facilities to make a comprehensive new
abundance analysis of the metal-poor, halo star BD +17^\circ 3248. We have
detected the third r-process peak elements osmium, platinum, and (for the first
time in a metal-poor star) gold, elements whose abundances can only be reliably
determined using HST. Our observations illustrate a pattern seen in other
similar halo stars with the abundances of the heavier neutron-capture elements,
including the third r-process peak elements, consistent with a scaled solar
system r-process distribution. The abundances of the lighter neutron-capture
elements, including germanium and silver, fall below that same scaled solar
r-process curve, a result similar to that seen in the ultra-metal-poor star CS
22892--052. A single site with two regimes or sets of conditions, or perhaps
two different sites for the lighter and heavier neutron-capture elements, might
explain the abundance pattern seen in this star. In addition we have derived a
reliable abundance for the radioactive element thorium. We tentatively identify
U II at 3859 A in the spectrum of BD +17^\circ 3248, which makes this the
second detection of uranium in a very metal-poor halo star. Our combined
observations cover the widest range in proton number (from germanium to
uranium) thus far of neutron-capture elements in metal-poor Galactic halo
stars. Employing the thorium and uranium abundances in comparison with each
other and with several stable elements, we determine an average
cosmochronological age for BD +17^\circ 3248 of 13.8 +/- 4 Gyr, consistent with
that found for other similar metal-poor halo stars.Comment: 58 pages, 4 tables, 11 figures; To appear in ApJ Typo correcte
Correlation between molecular lines and diffuse interstellar bands
Observations are presented of the Diffuse Interstellar Bands (DIB's) at 4726, 4763, and 4789 A and at 5780 and 5797 A together with the ultraviolet lines of CH and CN molecules for stars with different shapes of UV extinction curve. The new results concerning the relationship between different characteristics of the interstellar clouds; molecular lines, blue and yellow DIB's, and UV extinction curves are discussed
Nitrogen Abundances in Damped Ly alpha Galaxies
Nitrogen abundances have been derived in Damped Ly alpha (DLA) galaxies at
Zabs = 2.309, 2.827 and 3.025 toward the QSOs 0100+1300, 1425+6039 and
0347-3819 respectively. The behaviour of nitrogen relative to iron-peak and
alpha-elements has been investigated by considering all the extant NI
determinations for a total of 9 DLA galaxies. We have estimated the fraction of
iron locked into dust grains to convert the observed [N/Fe] ratios into overall
(dust plus gas) relative abundances, [N/Fe]corr. The ratios [N/alpha] have been
mostly determined by using sulphur as a tracer of alpha-elements which is
unaffected by dust. The [N/Fe] and [N/alpha] ratios show high dispersions, of
one order of magnitude or more, which have no equivalent in other
element-to-element ratios in DLAs. The lowest values of the [N/Fe]corr and
[N/alpha] ratios are at variance with the values measured in Galactic halo
stars of similar metallicity suggesting that part of the DLA galaxies do not
follow the chemical evolution of the Milky Way. The DLA nitrogen abundances and
their dispersion show some similarities with those observed in dwarf galaxies.
The behaviour of nitrogen abundance ratios can be ascribed, in general to the
delayed release of nitrogen in the course of evolution. However it is difficult
to conciliate this interpretation with the lowest [N/alpha] values measured,
since an expected enhancement of alpha-elements respect to the iron-peak
elements is not observed simultaneously in these DLA galaxies. In two cases,
relatively high [N/alpha] values are observed which require also a more complex
chemical evolution to be explained.Comment: 29 pages including 5 tables and figure captions,LaTeX, 8 figures, ApJ
accepte
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