281 research outputs found
Two distinct halo populations in the solar neighborhood. Evidence from stellar abundance ratios and kinematics
Precise abundance ratios are determined for 94 dwarf stars with 5200 < Teff <
6300 K, -1.6 < [Fe/H] < -0.4, and distances D < 335 pc. Most of them have halo
kinematics, but 16 thick-disk stars are included. Equivalent widths of atomic
lines are measured from VLT/UVES and NOT/FIES spectra with resolutions R =
55000 and R = 40000, respectively. An LTE abundance analysis based on MARCS
models is applied to derive precise differential abundance ratios of Na, Mg,
Si, Ca, Ti, Cr, and Ni with respect to Fe.
The halo stars fall into two populations, clearly separated in [alpha/Fe],
where alpha refers to the average abundance of Mg, Si, Ca, and Ti. Differences
in [Na/Fe] and [Ni/Fe] are also present with a remarkably clear correlation
between these two abundance ratios. The `high-alpha' stars may be ancient disk
or bulge stars `heated' to halo kinematics by merging satellite galaxies or
they could have formed as the first stars during the collapse of a
proto-Galactic gas cloud. The kinematics of the `low-alpha' stars suggest that
they have been accreted from dwarf galaxies, and that some of them may
originate from the omega Cen progenitor galaxy.Comment: Accepted for publication in A&A as a four-page Letter with five pages
of online materia
Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars. Detailed abundance analysis of OGLE-2008-BLG-209S
AIMS. Our aims are twofold. First we aim to evaluate the robustness and
accuracy of stellar parameters and detailed elemental abundances that can be
derived from high-resolution spectroscopic observations of microlensed dwarf
and subgiant stars. We then aim to use microlensed dwarf and subgiant stars to
investigate the abundance structure and chemical evolution of the Milky Way
Bulge. [ABRIDGED] METHODS. We present a detailed elemental abundance analysis
of OGLE-2008-BLG-209S, the source star of a new microlensing event towards the
Bulge, for which we obtained a high-resolution spectrum with the MIKE
spectrograph on the Magellan Clay telescope. We have performed four different
analyses of OGLE-2008-BLG-209S. [ABRIDGED] We have also re-analysed three
previous microlensed dwarf stars OGLE-2006-BLG-265S, MOA-2006-BLG-099S, and
OGLE-2007-BLG-349S with the same method. This homogeneous data set, although
small, enables a direct comparison between the different stellar populations.
RESULTS. We find that OGLE-2008-BLG-209S is a subgiant star that has a
metallicity of [Fe/H] ~-0.33. It possesses [alpha/Fe] enhancements similar to
what is found for Bulge giant stars at the same metallicity, and what also is
found for nearby thick disc stars at the same metallicity. In contrast, the
previous three microlensing dwarf stars have very high metallicities,
[Fe/H]>+0.4, and more solar-like abundance ratios, i.e. [alpha/Fe]~0. The
decrease in the [alpha/Fe] ratio with [Fe/H] is the typical signature of
enrichment from low and intermediate mass stars. We furthermore find that the
results for the four Bulge stars, in combination with results from studies of
giant stars in the Bulge, seem to favour a secular formation scenario for the
Bulge.Comment: Accepted for publication in A&A, 17 pages, online table will be
available in published version, or by contacting the first autho
The RAVE Survey: Constraining the Local Galactic Escape Speed
We report new constraints on the local escape speed of our Galaxy. Our
analysis is based on a sample of high velocity stars from the RAVE survey and
two previously published datasets. We use cosmological simulations of disk
galaxy formation to motivate our assumptions on the shape of the velocity
distribution, allowing for a significantly more precise measurement of the
escape velocity compared to previous studies. We find that the escape velocity
lies within the range 498\kms < \ve < 608 \kms (90 per cent confidence), with
a median likelihood of 544\kms. The fact that \ve^2 is significantly
greater than 2\vc^2 (where \vc=220\kms is the local circular velocity)
implies that there must be a significant amount of mass exterior to the Solar
circle, i.e. this convincingly demonstrates the presence of a dark halo in the
Galaxy. For a simple isothermal halo, one can calculate that the minimum radial
extent is kpc. We use our constraints on \ve to determine the mass
of the Milky Way halo for three halo profiles. For example, an adiabatically
contracted NFW halo model results in a virial mass of
and virial radius of
kpc (90 per cent confidence). For this model the circular
velocity at the virial radius is 142^{+31}_{-21}\kms. Although our halo
masses are model dependent, we find that they are in good agreement with each
other.Comment: 19 pages, 9 figures, MNRAS (accepted). v2 incorporates minor cosmetic
revisions which have no effect on the results or conclusion
Early Galactic Evolution of Carbon, Nitrogen and Oxygen
We present results on carbon, nitrogen, and oxygen abundances for a sample of
unevolved metal-poor stars with metallicities in the range -0.3< [Fe/H]< -3.
Oxygen abundances derived from different indicators are compared showing
consistently that in the range 0.3 >[Fe/H]>-3.0, the [O/Fe] ratio increases
from approximately 0 to 1. We find a good agreement between abundances based on
the forbidden line, the OH and IR triplet lines when gravities based on
Hipparcos} parallaxes are considered for the sample stars. Gravities derived
from LTE ionization balance in metal-poor stars with [Fe/H]< -1 are likely too
low, and could be responsible for an underestimation of the oxygen abundances
derived using the [OI] line. [C/Fe] and [N/Fe] ratios appear to be constant,
independently of metallicity, in the same range. However, they show larger
scatter than oxygen at a given metallicity, which could reflect the larger
variety of stellar production sites for these other elements.Comment: 10 pages, 3 figures, To appear in the proceedings of the conference
"The Chemical Evolution of The Milky Way: Stars versus Clusters", eds. F.
Matteucci and F. Giovannelli, Vulcano, Italy, September 20-24 199
Abundances and Kinematics of Field Halo and Disk Stars I: Observational Data and Abundance Analysis
We describe observations and abundance analysis of a high-resolution,
high-S/N survey of 168 stars, most of which are metal-poor dwarfs. We follow a
self-consistent LTE analysis technique to determine the stellar parameters and
abundances, and estimate the effects of random and systematic uncertainties on
the resulting abundances. Element-to-iron ratios are derived for key alpha,
odd, Fe-peak, r- and s-process elements. Effects of Non-LTE on the analysis of
Fe I lines are shown to be very small on the average. Spectroscopically
determined surface gravities are derived that are generally close to those
obtained from Hipparcos parallaxes.Comment: 41 pages, 7 Postscript figures. Accepted for publication in the A
A multi-wavelength study of the radio source G296.7-0.9: confirmation as a Galactic supernova remnant
We present a multi-wavelength study of the radio source G296.7-0.9. This
source has a bilateral radio morphology, a radio spectral index of -0.5 +/-
0.1, sparse patches of linear polarisation, and thermal X-rays with a bright
arc near the radio boundary. Considering these characteristics, we conclude
that G296.7-0.9 is a supernova remnant (SNR). The age and morphology of the SNR
in the context of its environment suggest that the source is co-located with an
HII region, and that portions of the shock front have broken out into a lower
density medium. We see no evidence for a neutron star or pulsar wind nebula
associated with SNR G296.7-0.9.Comment: 11 pages with 9 figures and 2 tables. Accepted for publication in
MNRA
A search for p-modes and other variability in the binary system 85 Pegasi using MOST photometry
Context: Asteroseismology has great potential for the study of metal-poor
stars due to its sensitivity to determine stellar ages. Aims: Our goal was to
detect p-mode oscillations in the metal-poor sub-dwarf 85 Peg A and to search
for other variability on longer timescales. Methods: We have obtained
continuous high-precision photometry of the binary system 85 Pegasi with the
MOST space telescope in two seasons (2005 & 2007). Furthermore, we redetermined
vsini for 85 Peg A using high resolution spectra obtained through the ESO
archive, and used photometric spot modeling to interpret long periodic
variations. Results: Our frequency analysis yields no convincing evidence for
p-modes significantly above a noise level of 4 ppm. Using simulated p-mode
patterns we provide upper RMS amplitude limits for 85 Peg A. The light curve
shows evidence for variability with a period of about 11 d and this periodicity
is also seen in the follow up run in 2007; however, as different methods to
remove instrumental trends in the 2005 run yield vastly different results, the
exact shape and periodicity of the 2005 variability remain uncertain. Our
re-determined vsini value for 85 Peg A is comparable to previous studies and we
provide realistic uncertainties for this parameter. Using these values in
combination with simple photometric spot models we are able to reconstruct the
observed variations. Conclusions: The null-detection of p-modes in 85 Peg A is
consistent with theoretical values for pulsation amplitudes in this star. The
detected long-periodic variation must await confirmation by further
observations with similar or better precision and long-term stability. If the
11 d periodicity is real, rotational modulation of surface features on one of
the components is the most likely explanation.Comment: 11 pages, 9 figures, accepted for publication in A&
Non-LTE line formation for heavy elements in four very metal-poor stars
Stellar parameters and abundances of Na, Mg, Al, K, Ca, Sr, Ba, and Eu are
determined for four very metal-poor stars (-2.66 < [Fe/H] < -2.15) based on
non-LTE line formation and analysis of high-resolution (R ~60000 and 90000)
high signal-to-noise (S/N > 200) observed spectra. A model atom for H I is
presented. An effective temperature was obtained from the Balmer Halpha and
Hbeta line wing fits, the surface gravity from the Hipparcos parallax if
available and the non-LTE ionization balance between Ca I and Ca II. Based on
the hyperfine structure affecting the Ba II resonance line, the fractional
abundance of the odd isotopes of Ba was derived for HD 84937 and HD 122563 from
a requirement that Ba abundances from the resonance line and subordinate lines
of Ba II must be equal. For each star, non-LTE leads to a consistency of Teff
from two Balmer lines and to a higher temperature compared to the LTE case, by
up to 60 K. Non-LTE effects are important in spectroscopic determination of
surface gravity from Ca I/Ca II. For each star with a known trigonometric
gravity, non-LTE abundances from the lines of two ionization stages agree
within the error bars, while a difference in the LTE abundances consists of
0.23 dex to 0.40 dex for different stars. Departures from LTE are found to be
significant for the investigated atoms, and they strongly depend on stellar
parameters. For HD 84937, the Eu/Ba ratio is consistent with the relative solar
system r-process abundances, and the fraction of the odd isotopes of Ba, f_odd,
equals 0.43+-0.14. The latter can serve as a constraint on r-process models.
The lower Eu/Ba ratio and f_odd = 0.22+-0.15 found for HD 122563 suggest that
the s-process or the unknown process has contributed significantly to the Ba
abundance in this star.Comment: accepted for publication in A&A, November 16, 200
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