137 research outputs found
Rare White dwarf stars with carbon atmospheres
White dwarfs represent the endpoint of stellar evolution for stars with
initial masses between approximately 0.07 msun and 8-10 msun, where msun is the
mass of the Sun (more massive stars end their life as either black holes or
neutron stars). The theory of stellar evolution predicts that the majority of
white dwarfs have a core made of carbon and oxygen, which itself is surrounded
by a helium layer and, for ~80 per cent of known white dwarfs, by an additional
hydrogen layer. All white dwarfs therefore have been traditionally found to
belong to one of two categories: those with a hydrogen-rich atmosphere (the DA
spectral type) and those with a helium-rich atmosphere (the non-DAs). Here we
report the discovery of several white dwarfs with atmospheres primarily
composed of carbon, with little or no trace of hydrogen or helium. Our analysis
shows that the atmospheric parameters found for these stars do not fit
satisfactorily in any of the currently known theories of post-asymptotic giant
branch evolution, although these objects might be the cooler counterpart of the
unique and extensively studied PG1159 star H1504+65. These stars, together with
H1504+65, might accordingly form a new evolutionary sequence that follow the
asymptotic giant branch.Comment: 7 pages, 1 figure, to appear in Nov 22nd 2007 edition of Natur
Abundance analyses of helium-rich subluminous B stars
The connection between helium-rich hot subdwarfs of spectral types O and B
(He-sdB) has been relatively unexplored since the latter were found in
significant numbers in the 1980's. In order to explore this connection further,
we have analysed the surface composition of six He-sdB stars, including LB
1766, LB 3229, SB 21 (= Ton-S 137 = BPS 29503-0009), BPS 22940-0009, BPS
29496-0010, and BPS 22956-0094. Opacity-sampled line-blanketed model
atmospheres have been used to derive atmospheric properties and elemental
abundances. All the stars are moderately metal-poor compared with the Sun
([Fe/H] ~ -0.5). Four stars are nitrogen-rich, two of these are carbon-rich,
and at least four appear to be neon-rich. The data are insufficient to rule out
binarity in any of the sample. The surface composition and locus of the N-rich
He-sdBs are currently best explained by the merger of two helium white dwarfs,
or possibly by the merger of a helium white dwarf with a post-sdB white dwarf.
C-rich He-sdBs require further investigation.Comment: Accepted 2010 July
Quantitative spectroscopy of extreme helium stars - Model atmospheres and a non-LTE abundance analysis of BD+102179?
Extreme helium stars (EHe stars) are hydrogen-deficient supergiants of
spectral type A and B. They are believed to result from mergers in double
degenerate systems. In this paper we present a detailed quantitative non-LTE
spectral analysis for BD+102179, a prototype of this rare class of
stars, using UVES and FEROS spectra covering the range from 3100 to 10
000 {\AA}. Atmosphere model computations were improved in two ways. First,
since the UV metal line blanketing has a strong impact on the
temperature-density stratification, we used the Atlas12 code. Additionally, We
tested Atlas12 against the benchmark code Sterne3, and found only small
differences in the temperature and density stratifications, and good agreement
with the spectral energy distributions. Second, 12 chemical species were
treated in non-LTE. Pronounced non-LTE effects occur in individual spectral
lines but, for the majority, the effects are moderate to small. The
spectroscopic parameters give = 17 300300 K and
= 2.800.10, and an evolutionary mass of 0.550.05 . The star
is thus slightly hotter, more compact and less massive than found in previous
studies. The kinematic properties imply a thick-disk membership, which is
consistent with the metallicity Fe/H and -enhancement.
The refined light-element abundances are consistent with the white dwarf merger
scenario. We further discuss the observed helium spectrum in an appendix,
detecting dipole-allowed transitions from about 150 multiplets plus the most
comprehensive set of known/predicted isolated forbidden components to date.
Moreover, a so far unreported series of pronounced forbidden He I components is
detected in the optical-UV.Comment: Accepted for publication in MNRAS, 26 pages, 19 Figure
Three carbon-enhanced metal-poor dwarf stars from the SDSS - Chemical abundances from CO^5BOLD 3D hydrodynamical model atmospheres
The origin of carbon-enhanced metal-poor stars enriched with both s and r
elements is highly debated. Detailed abundances of these types of stars are
crucial to understand the nature of their progenitors. The aim of this
investigation is to study in detail the abundances of SDSS J1349-0229, SDSS
J0912+0216 and SDSS J1036+1212, three dwarf CEMP stars, selected from the Sloan
Digital Sky Survey. Using high resolution VLT/UVES spectra (R ~ 30 000) we
determine abundances for Li, C, N, O, Na, Mg, Al, Ca, Sc, Ti, Cr, Mn, Fe, Co,
Ni and 21 neutron-capture elements. We made use of CO^5BOLD 3D hydrodynamical
model atmospheres in the analysis of the carbon, nitrogen and oxygen
abundances. NLTE corrections for C I and O I lines were computed using the Kiel
code. We classify SDSS J1349-0229 and SDSS J0912+0216 as CEMP-r+s stars. SDSS
J1036+1212 belongs to the class CEMP-no/s, with enhanced Ba, but deficient Sr,
of which it is the third member discovered to date. Radial-velocity variations
have been observed in SDSS J1349-0229, providing evidence that it is a member
of a binary system. The chemical composition of the three stars is generally
compatible with mass transfer from an AGB companion. However, many details
remain difficult to explain. Most notably of those are the abundance of Li at
the level of the Spite plateau in SDSS J1036+1212 and the large over-abundance
of the pure r-process element Eu in all three stars.Comment: 12 pages, 15 figures. Accepted for publication in A&
3D molecular line formation in dwarf carbon-enhanced metal-poor stars
We present a detailed analysis of the carbon and nitrogen abundances of two
dwarf carbon-enhanced metal-poor (CEMP) stars: SDSS J1349-0229 and SDSS
J0912+0216. We also report the oxygen abundance of SDSS J1349-0229. These stars
are metal-poor, with [Fe/H] < -2.5, and were selected from our ongoing survey
of extremely metal-poor dwarf candidates from the Sloan Digital SkySurvey
(SDSS). The carbon, nitrogen and oxygen abundances rely on molecular lines
which form in the outer layers of the stellar atmosphere. It is known that
convection in metal-poor stars induces very low temperatures which are not
predicted by `classical' 1D stellar atmospheres. To obtain the correct
temperature structure, one needs full 3D hydrodynamical models. Using CO5BOLD
3D hydrodynamical model atmospheres and the Linfor3D line formation code,
molecular lines of CH, NH, OH and C2 were computed, and 3D carbon, nitrogen and
oxygen abundances were determined. The resulting carbon abundances were
compared to abundances derived using atomic CI lines in 1D LTE and NLTE. There
is not a good agreement between the carbon abundances determined from C2 bands
and from the CH band, and molecular lines do not agree with the atomic CI
lines. Although this may be partly due to uncertainties in the transition
probabilities of the molecular bands it certainly has to do with the
temperature structure of the outer layers of the adopted model atmosphere. We
explore the influence of the 3D model properties on the molecular abundance
determination. In particular, the choice of the number of opacity bins used in
the model calculations and its subsequent effects on the temperature structure
and molecular line formation is discussed. (Abridged)Comment: Poster presented at IAU JD 10, Rio de Janeiro, 10-11 August 2009,
published in Memorie della Societa' Astronomica Italiana, Vol. 80 n.3 P.735.
One reference corrected, matches the published versio
Main-Sequence and sub-giant stars in the Globular Cluster NGC6397: The complex evolution of the lithium abundance
Thanks to the high multiplex and efficiency of Giraffe at the VLT we have
been able for the first time to observe the Li I doublet in the Main Sequence
(MS) stars of a Globular Cluster. At the same time we observed Li in a sample
of Sub-Giant (SG) stars of the same B-V colour. Our final sample is composed of
84 SG stars and 79 MS stars. In spite of the fact that SG and MS span the same
temperature range we find that the equivalent widths of the Li I doublet in SG
stars are systematically larger than those in MS stars, suggesting a higher Li
content among SG stars. This is confirmed by our quantitative analysis. We
derived the effective temperatures, from H fitting, and NLTE Li
abundances of the stars in our the sample, using 3D and 1D models. We find that
SG stars have a mean Li abundance higher by 0.1dex than MS stars, using both 1D
and 3D models. We also detect a positive slope of Li abundance with effective
temperature. These results provide an unambiguous evidence that the Li
abundance changes with evolutionary status. The physical mechanisms responsible
for this behaviour are not yet clear, and none of the existing models seems to
describe accurately these observations. Based on these conclusions, we believe
that the cosmological lithium problem still remains an open question.Comment: Proceedings of the contributed talk presented at the IAU Symposium
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Evaluation of various culture and staining techniques for the detection of extra pulmonary tuberculosis
Background: Though pulmonary tuberculosis form is the commonest presentation, the extra pulmonary tuberculosis (EPTB) is also an important emerging clinical problem. The objective of the current study was to compare two staining techniques, Ziehl-Neelsen (ZN) stain, fluorescent stain and two-culture medium, solid Löwenstein-Jensen (LJ) medium and liquid 7H9 Middle brook medium in MGIT (Mycobacterium Growth Indicator Tube) 320 system, for detection of Mycobacterium in clinically suspected patients of EPTB.Methods: A total of 100 clinically suspected cases of EPTB samples from various extrapulmonary sites had been collected. All the specimens were stained with ZN stain and fluorescent stain. The culture were processed after decontamination of specimens with NaOH-NALC method and thereafter inoculated on solid and liquid culture medium.Results: Out of the 100 EPTB specimens, 30 were found positive by any of the above techniques used. Out of 30 positive cases 18 showed positivity by ZN staining while 20 showed positivity by fluorescent staining technique. In two culture methods, 27 isolates were grown by any of the culture system. Out of 27, 22 and 26 specimens showed growth of MTB complex on LJ media and MGIT culture system respectively. In AFB smear positive specimens, the average turnaround time was found to be 8.45 days and 22.5 days in MGIT and LJ medium culture assay respectively. While the turnaround times in AFB smear negative cases, it was 16.5 days and 32.3 days in MGIT and LJ medium culture assay respectively.Conclusions: MGIT was a dependable, highly efficient system for recovery of MTB complex for EPTB specimens in combination with LJ media
Extremely metal-poor stars from the SDSS
We give a progress report about the activities within the CIFIST Team related
to the search for extremely metal-poor stars in the Sloan Digital Sky Survey's
spectroscopic catalog. So far the search has provided 25 candidates with
metallicities around or smaller -3. For 15 candidates high resolution
spectroscopy with UVES at the VLT has confirmed their extremely metal-poor
status. Work is under way to extend the search to the SDSS's photometric
catalog by augmenting the SDSS photometry, and by gauging the capabilities of
X-shooter when going to significantly fainter targets.Comment: 6 pages, 6 figures, Proceedings paper of the conference "A stellar
journey: A symposium in celebration of Bengt Gustafsson's 65th birthday
An abundance analysis of a chemically peculiar B star – JL 87*,**
Aims. The aim of this study is to understand the nature and origin of a chemically peculiar star JL 87 by measuring its physical parameters and chemical abundances.
Methods. Physical parameters – effective temperature, surface gravity and helium abundance were measured from a moderate resolution optical spectrum using fully line-blanketed LTE model atmospheres. The effective temperature and extinction were verified by comparing FUSE, IUE spectrophotometry and optical/IR broadband photometry with theoretical flux distributions from LTE model atmospheres. The photospheric chemical abundances were measured from a high-resolution optical spectrum using LTE model atmospheres and spectral synthesis.
Results. On the basis of its physical parameters and chemical abundances, we confirm that JL 87 is a chemically peculiar subluminous B star. It is significantly cooler, has a lower surface gravity and is more helium-rich than previously believed. It is moderately enriched in carbon and nitrogen, but its overall metallicity is slightly subsolar.
Conclusions. The shallow-mixing model of a late core-flash on a white-dwarf cooling track currently provides the most consistent agreement with the observable properties of JL 87
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