1,294 research outputs found
The chemical evolution of Manganese in different stellar systems
Aims. To model the chemical evolution of manganese relative to iron in three
different stellar systems: the solar neighbourhood, the Galactic bulge and the
Sagittarius dwarf spheroidal galaxy, and compare our results with the recent
and homogeneous observational data. Methods. We adopt three chemical evolution
models well able to reproduce the main properties of the solar vicinity, the
galactic Bulge and the Sagittarius dwarf spheroidal. Then, we compare different
stellar yields in order to identify the best set to match the observational
data in these systems. Results. We compute the evolution of manganese in the
three systems and we find that in order to reproduce simultaneously the [Mn/Fe]
versus [Fe/H] in the Galactic bulge, the solar neighbourhood and Sagittarius,
the type Ia SN Mn yield must be metallicity-dependent. Conclusions. We conclude
that the different histories of star formation in the three systems are not
enough to reproduce the different behaviour of the [Mn/Fe] ratio, unlike the
situation for [alpha/Fe]; rather, it is necessary to invoke
metallicity-dependent type Ia SN Mn yields, as originally suggested by
McWilliam, Rich & Smecker-Hane in 2003.Comment: 9 pages, 3 figures, submitted to A&
The Evolution of Oxygen and Magnesium in the Bulge and Disk of the Milky Way
We show that the Galactic bulge and disk share a similar, strong, decline in
[O/Mg] ratio with [Mg/H]. The similarity of the [O/Mg] trend in these two,
markedly different, populations suggests a metallicity-dependent modulation of
the stellar yields from massive stars, by mass loss from winds, and related to
the Wolf-Rayet phenomenon, as proposed by McWilliam & Rich (2004). We have
modified existing models for the chemical evolution of the Galactic bulge and
the solar neighborhood with the inclusion of metallicity-dependent oxygen
yields from theoretical predictions for massive stars that include mass loss by
stellar winds. Our results significantly improve the agreement between
predicted and observed [O/Mg] ratios in the bulge and disk above solar
metallicity; however, a small zero-point normalization problem remains to be
resolved. The zero-point shift indicates that either the semi-empirical yields
of Francois et al. (2004) need adjustment, or that the bulge IMF is not quite
as flat as found by Ballero et al. (2007); the former explanation is preferred.
Our result removes a previous inconsistency between the interpretation of
[O/Fe] and [Mg/Fe] ratios in the bulge, and confirms the conclusion that the
bulge formed more rapidly than the disk, based on the over-abundances of
elements produced by massive stars. We also provide an explanation for the
long-standing difference between [Mg/Fe] and [O/Fe] trends among disk stars
more metal-rich than the sun.Comment: 22 pages including 5 figures. Submitted to the Astronomical Journa
An Update on the 0Z Project
We give an update on our 0Z Survey to find more extremely metal poor (EMP)
stars with [Fe/H] < -3 dex through mining the database of the Hamburg/ESO
Survey. We present the most extreme such stars we have found from ~1550
moderate resolution follow up spectra. One of these, HE1424-0241, has highly
anomalous abundance ratios not seen in any previously known halo giant, with
very deficient Si, moderately deficient Ca and Ti, highly enhanced Mn and Co,
and low C, all with respect to Fe. We suggest a SNII where the nucleosynthetic
yield for explosive alpha-burning nuclei was very low compared to that for the
hydrostatic alpha-burning element Mg, which is normal in this star relative to
Fe. A second, less extreme, outlier star with high [Sc/Fe] has also been found.
We examine the extremely metal-poor tail of the HES metallicity distribution
function (MDF). We suggest on the basis of comparison of our high resolution
detailed abundance analyses with [Fe/H](HES) for stars in our sample that the
MDF inferred from follow up spectra of the HES sample of candidate EMP stars is
heavily contaminated for [Fe/H](HES) < -3 dex; many of the supposed EMP stars
below that metallicity are of substantially higher Fe-metallicity, including
most of the very C-rich stars, or are spurious objects.Comment: to appear in conference proceedings "First Stars III", ed. B. O'Shea,
A. Heger & T.Abel, 4 pages, 2 figure
Land and Life in Timor-Leste
Following the historic 1999 popular referendum, East Timor emerged as the first independent sovereign nation of the 21st Century. The years since these momentous events have seen an efflorescence of social research across the country drawn by shared interests in the aftermath of the resistance struggle, the processes of social recovery and the historic opportunity to pursue field-based ethnography following the hiatus of research during 24 years of Indonesian rule (1975-99). This volume brings together a collection of papers from a diverse field of international scholars exploring the multiple ways that East Timorese communities are making and remaking their connections to land and places of ancestral significance. The work is explicitly comparative and highlights the different ways Timorese language communities negotiate access and transactions in land, disputes and inheritance especially in areas subject to historical displacement and resettlement. Consideration is extended to the role of ritual performance and social alliance for inscribing connection and entitlement. Emerging through analysis is an appreciation of how relations to land, articulated in origin discourses, are implicated in the construction of national culture and differential contributions to the struggle for independence. The volume is informed by a range of Austronesian cultural themes and highlights the continuing vitality of customary governance and landed attachment in Timor-Leste
Optically-pumped saturable absorber for fast switching between continuous-wave and passively mode-locked regimes of a Nd:YVO4 laser
We report on the fast (~50 ÎĽs) remote-controlled switching between continuous-wave (cw), cw mode-locked (ML) and Q-switched ML modes of operation of a Nd:YVO4 laser using an optically-pumped saturable absorber (SA). Pulses as short as 40 ps with an average output power of 0.5 W are obtained in cw ML regime
The Evolution of Carbon and Oxygen in the Bulge and Disk of the Milky Way
The evolution of C and O abundances in the Milky Way can impose strong
constraints on stellar nucleosynthesis and help understanding the formation and
evolution of our Galaxy. The aim is to review the measured C and O abundances
in the disk and bulge of the Galaxy and compare them with model predictions. We
adopt two successful chemical evolution models for the bulge and the disk,
which assume the same nucleosynthesis prescriptions but different histories of
star formation. The data show a clear distinction between the trend of [C/O] in
the thick and thin Galactic disks, while the thick disk and bulge trends are
indistinguishable with a large (>0.5 dex) increase in the C/O ratio in the
range from -0.1 to +0.4 dex for [O/H]. In our models we consider yields from
massive stars with and without the inclusion of metallicity-dependent stellar
winds. The observed increase in the [C/O] ratio with metallicity in the bulge
and thick disk lies between the predictions utilizing the mass-loss rates of
Maeder (1992) and those of Meynet & Maeder (2002). A model without
metallicity-dependent yields completely fails to match the observations. Thus,
the relative increase in carbon abundance at high metallicity appears to be due
to metallicity-dependent stellar winds in massive stars. These results also
explain the steep decline of the [O/Fe] ratio with [Fe/H] in the Galactic
bulge, while the [Mg/Fe] ratio is enhanced at all [Fe/H]. (abridged)Comment: 18 pages, 6 figures, submitted to Astronomy & Astrophysic
The Frequency of Carbon Stars Among Extremely Metal-Poor Stars
We demonstrate that there are systematic scale errors in the [Fe/H] values
determined by the Hamburg/ESO Survey (and by inference by the HK Survey in the
past) for certain extremely metal poor highly C-enhanced giants. The
consequences of these scale errors are that a) the fraction of carbon stars at
extremely low metallicities has been overestimated in several papers in the
recent literature b) the number of extremely metal poor stars known is somewhat
lower than has been quoted in the recent literature c) the yield for extremely
metal poor stars by the HES Survey is somewhat lower than is stated in the
recent literature. A preliminary estimate for the frequency of Carbon stars
among the giants in the HES sample with -4 < [Fe/H] < -2.0 dex is 7.4 +-2.9%;
adding an estimate for the C-enhanced giants with [C/Fe] > 1.0 dex without
detectable C2 bands raises the fraction to 14 +-4$%.
We rely on the results of an extensive set of homogeneous detailed abundance
analyses of stars expected to have [Fe/H] < -3.0 dex selected from the HES to
establish these claims. We have found that the Fe-metallicity of the cooler
(Teff < 5200K) C-stars as derived from spectra taken with HIRES at Keck are a
factor of ~10 higher than those obtained via the algorithm used by the HES
project to analyze the moderate resolution follow-up spectra, which is
identical to that used until very recently by the HK Survey. This error in
Fe-abundance estimate for C-stars arises from a lowering of the emitted flux in
the continuum bandpasses of the KP (3933 A line of CaII) and particularly the
HP2 (Hdelta) indices used to estimate [Fe/H] due to absorption from strong
molecular bands.Comment: Accepted to the ApJL after a very lengthly duel with the 3
simultaneous referee
Outliers in the 0Z Survey
We have now completed detailed abundance analyses of more than 100 stars
selected as candidate extremely metal-poor stars with [Fe/H] < -3.0 dex. Of
these 18 are below -3.3 dex on the scale of the First Stars VLT project led by
Cayrel, and 57 are below -3.0 dex on that scale. Ignoring enhancement of carbon
which ranges up to very large values, and two C-rich stars with very high N as
well, there are 0 to 3 high or low strong outliers for each abundance ratio
tested from Mg to Ni. The outliers have been checked and they are real.
Ignoring the outliers, the dispersions are in most cases approximately
consistent with the uncertainties, except those for [Sr/Fe] and [Ba/Fe], which
are much larger. Approximately 6% of the sample are strong outliers in one or
more elements between Mg and Ni. This rises to ~15% if minor outliers for these
elements and strong outliers for Sr and Ba are included. There are 6 stars with
extremely low [Sr/Fe and [Ba/Fe], including one which has lower [Ba/H] than
Draco 119, the star found by Fulbright, Rich and Castro to have the lowest such
ratio known previously. There is one extreme r-process star.Comment: to be published in the Proceedings of 3rd Subaru International
Conference on Galactic Archeology, at Laforet Shuzenji, Japan, Nov 2011 The
proceedings will be a volume in the Astronomical Society of the Pacific
Conference Serie
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