248 research outputs found
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
Deriving Iodine-free spectra for high-resolution echelle spectrographs
We describe a new method to derive clean, iodine-free spectra directly from
observations acquired using high-resolution echelle spectrographs equipped with
iodine cells. The main motivation to obtain iodine-free spectra is to use
portions of the spectrum that are superimposed with the dense forest of iodine
absorption lines, in order to retrieve lines that can be used to monitor the
magnetic activity of the star, helping to validate candidate planets. In short,
we provide a straight-forward methodology to clean the spectra by using the
forward model used to derive radial velocities, the Line Spread Function
information plus the stellar spectrum without iodine to reconstruct and
subtract the iodine spectrum from the observations. We show our results using
observations of the star Ceti acquired with the PFS, HIRES and UCLES
spectrographs, reaching an iodine-free spectrum correction at the 1% RMS
level. We additionally discuss the limitations and further applications of the
method.Comment: 15 pages, 7 figures. Accepted for publication in A
A Detailed Study of Giants and Horizontal Branch Stars in M68: Atmospheric Parameters and Chemical Abundances
In this paper, we present a detailed high-resolution spectroscopic study of
post main sequence stars in the Globular Cluster M68. Our sample, which covers
a range of 4000 K in , and 3.5 dex in , is comprised of
members from the red giant, red horizontal, and blue horizontal branch, making
this the first high-resolution globular cluster study covering such a large
evolutionary and parameter space. Initially, atmospheric parameters were
determined using photometric as well as spectroscopic methods, both of which
resulted in unphysical and unexpected , , , and
[Fe/H] combinations. We therefore developed a hybrid approach that addresses
most of these problems, and yields atmospheric parameters that agree well with
other measurements in the literature. Furthermore, our derived stellar
metallicities are consistent across all evolutionary stages, with
[Fe/H] = 2.42 ( = 0.14) from 25 stars. Chemical
abundances obtained using our methodology also agree with previous studies and
bear all the hallmarks of globular clusters, such as a Na-O anti-correlation,
constant Ca abundances, and mild -process enrichment.Comment: Accepted to the Astronomical Journa
Clustering of the Diffuse Infrared Light from the COBE DIRBE maps. III. Power spectrum analysis and excess isotropic component of fluctuations
The cosmic infrared background (CIB) radiation is the cosmic repository for
energy release throughout the history of the universe. Using the all-sky data
from the COBE DIRBE instrument at wavelengths 1.25 - 100 mic we attempt to
measure the CIB fluctuations. In the near-IR, foreground emission is dominated
by small scale structure due to stars in the Galaxy. There we find a strong
correlation between the amplitude of the fluctuations and Galactic latitude
after removing bright foreground stars. Using data outside the Galactic plane
() and away from the center () we extrapolate
the amplitude of the fluctuations to cosec. We find a positive intercept
of nW/m2/sr at 1.25, 2.2,3.5 and 4.9 mic
respectively, where the errors are the range of 92% confidence limits. For
color subtracted maps between band 1 and 2 we find the isotropic part of the
fluctuations at nW/m2/sr. Based on detailed numerical and
analytic models, this residual is not likely to originate from the Galaxy, our
clipping algorithm, or instrumental noise. We demonstrate that the residuals
from the fit used in the extrapolation are distributed isotropically and
suggest that this extra variance may result from structure in the CIB. For
2\deg< \theta < 15^\deg, a power-spectrum analysis yields firm upper limits
of (\theta/5^\deg) \times\delta F_{\rm rms} (\theta) < 6, 2.5, 0.8, 0.5
nW/m2/sr at 1.25, 2.2, 3.5 and 4.9 mic respectively. From 10-100 mic, the upper
limits <1 nW/m2/sr.Comment: Ap.J., in press. 69 pages including 24 fig
PANIC: A Near-infrared Camera for the Magellan Telescopes
PANIC (Persson's Auxiliary Nasmyth Infrared Camera) is a near-infrared camera
designed to operate at any one of the f/11 folded ports of the 6.5m Magellan
telescopes at Las Campanas Observatory, Chile. The instrument is built around a
simple, all-refractive design that reimages the Magellan focal plane to a plate
scale of 0.125'' pixel^{-1} onto a Rockwell 1024x1024 HgCdTe detector. The
design goals for PANIC included excellent image quality to sample the superb
seeing measured with the Magellan telescopes, high throughput, a relatively
short construction time, and low cost. PANIC has now been in regular operation
for over one year and has proved to be highly reliable and produce excellent
images. The best recorded image quality has been ~0.2'' FWHM.Comment: 8 pages, 5 figures. To appear in "Astronomical Telescopes and
Instrumentation," Proc SPIE (Glasgow), June 2004. Version with higher
resolution figures is available at
http://cfa-www.harvard.edu/~pmartini/professional/publications/panic.pd
The Environment of ``E+A'' Galaxies
The violent star formation history of ``E+A'' galaxies and their detection
almost exclusively in distant clusters is frequently used to link them to the
``Butcher-Oemler effect'' and to argue that cluster environment influences
galaxy evolution. From 11113 spectra in the Las Campanas Redshift Survey, we
have obtained a unique sample of 21 nearby ``E+A" galaxies. Surprisingly, a
large fraction (about 75%) of these ``E+A''s lie in the field. Therefore,
interactions with the cluster environment, in the form of the ICM or cluster
potential, are not essential for ``E+A'' formation. If one mechanism is
responsible for ``E+A''s, their existence in the field and the tidal features
in at least 5 of the 21 argue that galaxy-galaxy interactions and mergers are
that mechanism. The most likely environments for such interactions are poor
groups, which have lower velocity dispersions than clusters and higher galaxy
densities than the field. In hierarchical models, groups fall into clusters in
greater numbers at intermediate redshifts than they do today. Thus, the
Butcher-Oemler effect may reflect the typical evolution of galaxies in groups
and in the field rather than the influence of clusters on star formation in
galaxies. This abstract is abridged.Comment: 39 uuencoded, compressed pages (except Fig 1), complete preprint at
ftp://ociw.edu/pub/aiz/eplusa.ps, ApJ, submitte
A Search for Stars of Very Low Metal Abundance. VI. Detailed Abundances of 313 Metal-Poor Stars
We present radial velocities, equivalent widths, model atmosphere parameters,
and abundances or upper limits for 53 species of 48 elements derived from high
resolution optical spectroscopy of 313 metal-poor stars. A majority of these
stars were selected from the metal-poor candidates of the HK Survey of Beers,
Preston, and Shectman. We derive detailed abundances for 61% of these stars for
the first time. Spectra were obtained during a 10-year observing campaign using
the Magellan Inamori Kyocera Echelle spectrograph on the Magellan Telescopes at
Las Campanas Observatory, the Robert G. Tull Coude Spectrograph on the Harlan
J. Smith Telescope at McDonald Observatory, and the High Resolution
Spectrograph on the Hobby-Eberly Telescope at McDonald Observatory. We perform
a standard LTE abundance analysis using MARCS model atmospheres, and we apply
line-by-line statistical corrections to minimize systematic abundance
differences arising when different sets of lines are available for analysis. We
identify several abundance correlations with effective temperature. A
comparison with previous abundance analyses reveals significant differences in
stellar parameters, which we investigate in detail. Our metallicities are, on
average, lower by approx. 0.25 dex for red giants and approx. 0.04 dex for
subgiants. Our sample contains 19 stars with [Fe/H] < -3.5, 84 stars with
[Fe/H] < -3.0, and 210 stars with [Fe/H] < -2.5. Detailed abundances are
presented here or elsewhere for 91% of the 209 stars with [Fe/H] < -2.5 as
estimated from medium resolution spectroscopy by Beers, Preston, and Shectman.
We will discuss the interpretation of these abundances in subsequent papers.Comment: Accepted for publication in the Astronomical Journal. 60 pages, 59
figures, 18 tables. Machine-readable versions of the long tables can be found
in the ancillary data file
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