118 research outputs found
Evidence for quasi-chemically homogeneous evolution of massive stars up to solar metallicity
Long soft gamma ray bursts (LGRBs) are usually associated with the death of
the most massive stars. A large amount of core angular momentum in the phases
preceding the explosion is required to form LGRBs. A very high initial
rotational velocity can provide this angular momentum. Such a velocity strongly
influences the way the star evolves: it is chemically homogeneously mixed and
evolves directly towards the blue part of the HR diagram from the main
sequence. We have shown that chemically homogeneous evolution (CHE) takes place
in the SMC, at low metallicity. We want to see if there is a metallicity
threshold above which such an evolution does not exist. We perform a
spectroscopic analysis of H-rich early-type WN stars in the LMC and the Galaxy.
We use the code CMFGEN to determine the fundamental properties and the surface
composition of the target stars. We then place the stars in the HR diagram and
determine their evolution. We show that both the LMC and Galactic WNh stars we
selected cannot be explained by standard stellar evolution. They are located on
the left of the main sequence but show surface abundances typical of CN
equilibrium. In addition, they still contain a large amount of hydrogen. They
are thus core-H burning objects. Their properties are consistent with CHE. We
determine the metallicity of the Galactic stars from their position and
Galactic metallicity gradients, and conclude that they have 0.6<Z<1.0. A
moderate coupling between the core and the envelope is required to explain that
stellar winds do not extract to much angular momentum to prevent a blueward
evolution. In view of the findings that some long gamma ray bursts appear in
solar environments, CHE may be a viable way to form them over a wide range of
metallicities.Comment: 10 pages, 10 figures. Accepted in Astronomy and Astrophysic
On the abundances of GRO J1655-40
Context: The detection of overabundances of -elements and lithium in the secondary star of a black-hole binary provides important insights about the formation of a stellar-mass black-hole. -enhancement might theoretically also be the result of pollution by the nucleosynthesis occurring during an outburst, or through spallation by the jet. Aims: We study the abundances, and their possible variations with time, in the secondary star of the runaway black-hole binary GRO J1655--40, in order to understand their origin. Methods: We present a detailed comparison between a Keck spectrum obtained in 1998 found in the literature, archival VLT-UVES data taken in 2004 and new VLT-UVES spectra obtained early 2006. We carefully determine the equivalent widths of different -elements (Mg, O, Ti, S and Si) with their associated uncertainty. We use the well-studied comparison star HD 156098 as well as synthetic spectra to match the spectrum of GRO J1655--40 in order to determine the abundances of these elements. Results: We see no significant variations of equivalent widths with time. Our fit using HD 156098 reveals that there is significant overabundance of oxygen in all our spectra, but no overabundances of any of the other -elements. Finally, we do not detect the lithium line at 6707 \AA. Conclusions: We show that there is no detected pollution in GRO J1655--40 after the burst in 2005. Moreover, we argue that uncertainties in the equivalent widths were previously underestimated by a factor of 3. Consequently, our results challenge the existence of general overabundances of -elements observed in this galactic black-hole binary, and thus the accepted interpretation that they are of supernova origin. The physical cause of the overabundance of oxygen remains unclear
Looking for imprints of the first stellar generations in metal-poor bulge field stars
© 2016 ESO. Context. Efforts to look for signatures of the first stars have concentrated on metal-poor halo objects. However, the low end of the bulge metallicity distribution has been shown to host some of the oldest objects in the Milky Way and hence this Galactic component potentially offers interesting targets to look at imprints of the first stellar generations. As a pilot project, we selected bulge field stars already identified in the ARGOS survey as having [Fe/H] 1 and oversolar [α/Fe] ratios, and we used FLAMES-UVES to obtain detailed abundances of key elements that are believed to reveal imprints of the first stellar generations. Aims. The main purpose of this study is to analyse selected ARGOS stars using new high-resolution (R ∼ 45 000) and high-signal-tonoise (S=N > 100) spectra. We aim to derive their stellar parameters and elemental ratios, in particular the abundances of C, N, the α-elements O, Mg, Si, Ca, and Ti, the odd-Z elements Na and Al, the neutron-capture s-process dominated elements Y, Zr, La, and Ba, and the r-element Eu. Methods. High-resolution spectra of five field giant stars were obtained at the 8 m VLT UT2-Kueyen telescope with the UVES spectrograph in FLAMES-UVES configuration. Spectroscopic parameters were derived based on the excitation and ionization equilibrium of Fe i and Fe ii. The abundance analysis was performed with a MARCS LTE spherical model atmosphere grid and the Turbospectrum spectrum synthesis code. Results.We confirm that the analysed stars are moderately metal-poor (-1:04≤[Fe/H]≤-0:43), non-carbon-enhanced (non-CEMP) with [C/Fe] ≤+0:2, and α-enhanced.We find that our three most metal-poor stars are nitrogen enhanced. The α-enhancement suggests that these stars were formed from a gas enriched by core-collapse supernovae, and that the values are in agreement with results in the literature for bulge stars in the same metallicity range. No abundance anomalies (Na-O, Al-O, Al-Mg anti-correlations) were detected in our sample. The heavy elements Y, Zr, Ba, La, and Eu also exhibit oversolar abundances. Three out of the five stars analysed here show slightly enhanced [Y/Ba] ratios similar to those found in other metal-poor bulge globular clusters (NGC 6522 and M 62). Conclusions. This sample shows enhancement in the first-to-second peak abundance ratios of heavy elements, as well as dominantly s-process element excesses. This can be explained by different nucleosynthesis scenarios: (a) the main r-process plus extra mechanisms, such as the weak r-process; (b) mass transfer from asymptotic giant branch stars in binary systems; (c) an early generation of fast-rotating massive stars. Larger samples of moderately metal-poor bulge stars, with detailed chemical abundances, are needed to better constrain the source of dominantly s-process elements in the early Universe
Properties of WNh stars in the Small Magellanic Cloud: evidence for homogeneous evolution
We derive the physical properties of three WNh stars in the SMC to constrain
stellar evolution beyond the main sequence at low metallicity and to
investigate the metallicity dependence of the clumping properties of massive
stars. We compute atmosphere models to derive the stellar and wind properties
of the three WNh targets. A FUV/UV/optical/near-infrared analysis gives access
to temperatures, luminosities, mass loss rates, terminal velocities and stellar
abundances. All stars still have a large hydrogen mass fraction in their
atmosphere, and show clear signs of CNO processing in their surface abundances.
One of the targets can be accounted for by normal stellar evolution. It is a
star with initial mass around 40-50 Msun in, or close to, the core He burning
phase. The other two objects must follow a peculiar evolution, governed by fast
rotation. In particular, one object is likely evolving homogeneously due to its
position blue-ward of the main sequence and its high H mass fraction. The
clumping factor of one star is found to be 0.15+/-0.05. This is comparable to
values found for Galactic Wolf-Rayet stars, indicating that within the
uncertainties, the clumping factor does not seem to depend on metallicity.Comment: 16 pages. A&A accepte
High-resolution abundance analysis of red giants in the globular cluster NGC 6522
The [Sr/Ba] and [Y/Ba] scatter observed in some galactic halo stars that are
very metal-poor stars and in a few individual stars of the oldest known Milky
Way globular cluster NGC 6522,have been interpreted as evidence of early
enrichment by massive fast-rotating stars (spinstars). Because NGC 6522 is a
bulge globular cluster, the suggestion was that not only the very-metal poor
halo stars, but also bulge stars at [Fe/H]~-1 could be used as probes of the
stellar nucleosynthesis signatures from the earlier generations of massive
stars, but at much higher metallicity. For the bulge the suggestions were based
on early spectra available for stars in NGC 6522, with a medium resolution of
R~22,000 and a moderate signal-to-noise ratio. The main purpose of this study
is to re-analyse the NGC 6522 stars previously reported using new
high-resolution (R~45,000) and high signal-to-noise spectra (S/N>100). We aim
at re-deriving their stellar parameters and elemental ratios, in particular the
abundances of the neutron-capture s-process-dominated elements such as Sr, Y,
Zr, La, and Ba, and of the r-element Eu. High-resolution spectra of four giants
belonging to the bulge globular cluster NGC 6522 were obtained at the 8m VLT
UT2-Kueyen telescope with the UVES spectrograph in FLAMES-UVESconfiguration.
The spectroscopic parameters were derived based on the excitation and
ionization equilibrium of \ion{Fe}{I} and \ion{Fe}{II}. Our analysis confirms a
metallicity [Fe/H] = -0.95+-0.15 for NGC 6522, and the overabundance of the
studied stars in Eu (with +~0.2 < [Eu/Fe] < +~0.4) and alpha-elements O and Mg.
The neutron-capture s-element-dominated Sr, Y, Zr, Ba, La now show less
pronounced variations from star to star. Enhancements are in the range 0.0 <
[Sr/Fe] < +0.4, +0.23 < [Y/Fe] < +0.43, 0.0 < [Zr/Fe] < +0.4, 0.0 < [La/Fe] <
+0.35,and 0.05 < [Ba/Fe] < +0.55.Comment: date of acceptation: 31/07/2014, in press, 24 pages, 19
figures,Astronomy & Astrophysics, 201
Dust during the Reionization
The possibility that population III stars have reionized the Universe at
redshifts greater than 6 has recently gained momentum with WMAP polarization
results. Here we analyse the role of early dust produced by these stars and
ejected into the intergalactic medium. We show that this dust, heated by the
radiation from the same population III stars, produces a submillimetre excess.
The electromagnetic spectrum of this excess could account for a significant
fraction of the FIRAS (Far Infrared Absolute Spectrophotometer) cosmic far
infrared background above 700 micron. This spectrum, a primary anisotropy
() spectrum times the dust emissivity law, peaking in the
submillimetre domain around 750 micron, is generic and does not depend on other
detailed dust properties. Arcminute--scale anisotropies, coming from
inhomogeneities in this early dust, could be detected by future submillimetre
experiments such as Planck HFI.Comment: 6 pages, 8 figures, accepted by A&A; clarifications made, typos
fixed, results more exactly calculate
Spectroscopy of SMC Wolf-Rayet Stars Suggests that Wind-Clumping does not Depend on Ambient Metallicity
The mass-loss rates of hot, massive, luminous stars are considered a decisive
parameter in shaping the evolutionary tracks of such stars and influencing the
interstellar medium on galactic scales. The small-scale structures (clumps)
omnipresent in such winds may reduce empirical estimates of mass-loss rates by
an evolutionarily significant factor of >=3. So far, there has been no direct
observational evidence that wind-clumping may persist at the same level in
environments with a low ambient metallicity, where the wind-driving opacity is
reduced. Here we report the results of time-resolved spectroscopy of three
presumably single Population I Wolf-Rayet stars in the Small Magellanic Cloud,
where the ambient metallicity is ~1/5 Z_Sun.We detect numerous small-scale
emission peaks moving outwards in the accelerating parts of the stellar
winds.The general properties of the moving features, such as their velocity
dispersions,emissivities and average accelerations, closely match the
corresponding characteristics of small-scale inhomogeneities in the winds of
Galactic Wolf-Rayet stars.Comment: 9 pages, 3 figures; accepted by ApJ Letter
How did the metals in a giant star originate?
The chemical composition of stars with extremely low metal contents (taking
``metals'' to mean all elements other than hydrogen and helium) provides us
with information on the masses of the stars that produced the first metals.
Such a direct connection is not possible, however, if the surface of the star
has been polluted by enriched material, either dredged from the star's interior
or transferred from a companion star. Here we argue that, in the case of
HE0107-5240 (ref. 1), the most iron poor star known, the oxygen abundance could
be a discriminant: a ratio of [O/Fe] exceeding +3.5 would favour a pristine
origin of metals, whereas an [O/Fe] ratio of less than +3 would favour the
pollution hypothesis. Using this criterion, we suggest how the required
information on oxygen abundance might be obtained.Comment: to appear in Nature Brief Communications issue 24 April 200
All-Sky spectrally matched UBVRI-ZY and u'g'r'i'z' magnitudes for stars in the Tycho2 catalog
We present fitted UBVRI-ZY and u'g'r'i'z' magnitudes, spectral types and
distances for 2.4M stars, derived from synthetic photometry of a library
spectrum that best matches the Tycho2 BtVt, NOMAD Rn and 2MASS JHK_{2/S}
catalog magnitudes. We present similarly synthesized multi-filter magnitudes,
types and distances for 4.8M stars with 2MASS and SDSS photometry to g<16
within the Sloan survey region, for Landolt and Sloan primary standards, and
for Sloan Northern (PT) and Southern secondary standards.
The synthetic magnitude zeropoints for BtVt, UBVRI, ZvYv, JHK_{2/S},
JHK_{MKO}, Stromgren uvby, Sloan u'g'r'i'z' and ugriz are calibrated on 20
calspec spectrophotometric standards. The UBVRI and ugriz zeropoints have
dispersions of 1--3%, for standards covering a range of color from -0.3 < V-I <
4.6; those for other filters are in the range 2--5%.
The spectrally matched fits to Tycho2 stars provide estimated 1-sigma errors
per star of ~0.2, 0.15, 0.12, 0.10 and 0.08 mags respectively in either UBVRI
or u'g'r'i'z'; those for at least 70% of the SDSS survey region to g<16 have
estimated 1-sigma errors per star of ~0.2, 0.06, 0.04, 0.04, 0.05 in u'g'r'i'z'
or UBVRI.
The density of Tycho2 stars, averaging about 60 stars per square degree,
provides sufficient stars to enable automatic flux calibrations for most
digital images with fields of view of 0.5 degree or more. Using several such
standards per field, automatic flux calibration can be achieved to a few
percent in any filter, at any airmass, in most workable observing conditions,
to facilitate inter-comparison of data from different sites, telescopes and
instruments.Comment: 36 pages, 30 figures, 3 printed tables, several electronic tables,
accepted PASP Dec 201
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