1,147 research outputs found
Arctic Oil and the S.S. Manhattan
The discovery of oil on Alaska’s North Slope in 1968 not only focused attention on that remote part of our world, it set the stage for one of the most exciting and significant scientific expeditions of our time—the voyage of the ice breaking tanker SS Manhattan through the Northwest Passage.
In the space available I would like to discuss with you that historic expedition, how it came to be, and comment on the results, as we see them
Searching for Dust around Hyper Metal-Poor Stars
We examine the mid-infrared fluxes and spectral energy distributions for
metal-poor stars with iron abundances [Fe/H] , as well as two
CEMP-no stars, to eliminate the possibility that their low metallicities are
related to the depletion of elements onto dust grains in the formation of a
debris disk. Six out of seven stars examined here show no mid-IR excess. These
non-detections rule out many types of circumstellar disks, e.g. a warm debris
disk ( K), or debris disks with inner radii AU, such as
those associated with the chemically peculiar post-AGB spectroscopic binaries
and RV Tau variables. However, we cannot rule out cooler debris disks, nor
those with lower flux ratios to their host stars due to, e.g. a smaller disk
mass, a larger inner disk radius, an absence of small grains, or even a
multicomponent structure, as often found with the chemically peculiar Lambda
Bootis stars. The only exception is HE0107-5240, for which a small mid-IR
excess near 10 microns is detected at the 2- level; if the excess is
real and associated with this star, it may indicate the presence of (recent)
dust-gas winnowing or a binary system.Comment: Accepted for publication in Ap
Could the Ultra Metal-poor Stars be Chemically Peculiar and Not Related to the First Stars?
Chemically peculiar stars define a class of stars that show unusual elemental
abundances due to stellar photospheric effects and not due to natal variations.
In this paper, we compare the elemental abundance patterns of the ultra
metal-poor stars with metallicities [Fe/H] to those of a subclass of
chemically peculiar stars. These include post-AGB stars, RV Tauri variable
stars, and the Lambda Bootis stars, which range in mass, age, binarity, and
evolutionary status, yet can have iron abundance determinations as low as
[Fe/H] . These chemical peculiarities are interpreted as due to the
separation of gas and dust beyond the stellar surface, followed by the
accretion of dust depleted-gas. Contrary to this, the elemental abundances in
the ultra metal-poor stars are thought to represent yields of the most
metal-poor supernova and, therefore, observationally constrain the earliest
stages of chemical evolution in the Universe. The abundance of the elements in
the photospheres of the ultra metal-poor stars appear to be related to the
condensation temperature of that element; if so, then their CNO abundances
suggest true metallicities of [X/H]~ -2 to -4, rather than their present
metallicities of [Fe/H] < -5.Comment: Accepted for ApJ. 17 pages, 10 figure
Testing Rotational Mixing Predictions with New Boron Abundances in Main Sequence B-type Stars
(Abridged) New boron abundances for seven main-sequence B-type stars are
determined from HST STIS spectroscopy around the BIII 2066A line. Boron
abundances provide a unique and critical test of stellar evolution models that
include rotational mixing since boron is destroyed in the surface layers of
stars through shallow mixing long before other elements are mixed from the
stellar interior through deep mixing. Boron abundances range from 12+log(B/H) =
1.0 to 2.2. The boron abundances are compared to the published values of their
stellar nitrogen abundances (all have 12+log(N/H) < 7.8, i.e., they do not show
significant CNO-mixing) and to their host cluster ages (4 to 16 Myr) to
investigate the predictions from models of massive star evolution with
rotational mixing effects (Heger & Langer 2000). Only three stars (out of 34)
deviate from the model predictions, including HD36591, HD205021, and HD30836.
These three stars suggest that rotational mixing could be more efficient than
currently modelled at the highest rotation rates.Comment: 10 figures, 7 tables; accepted for publication in the Astrophysical
Journa
Boron Abundances in Main Sequence B-type Stars: A Test of Rotational Depletion during Main Sequence Evolution
Boron abundances have been derived for seven main sequence B-type stars from
HST STIS spectra around the B III 2066 A line. In two stars, boron appears to
be undepleted with respect to the presumed initial abundance. In one star,
boron is detectable but it is clearly depleted. In the other four stars, boron
is undetectable implying depletions of 1 to 2 dex. Three of these four stars
are nitrogen enriched, but the fourth shows no enrichment of nitrogen. Only
rotationally induced mixing predicts that boron depletions are unaccompanied by
nitrogen enrichments. The inferred rate of boron depletion from our
observations is in good agreement with these predictions. Other boron-depleted
nitrogen-normal stars are identified from the literature. Also, several
boron-depleted nitrogen-rich stars are identified, and while all fall on the
boron-nitrogen trend predicted by rotationally-induced mixing, a majority have
nitrogen enrichments that are not uniquely explained by rotation.
The spectra have also been used to determine iron-group (Cr, Mn, Fe, and Ni)
abundances. The seven B-type stars have near solar iron-group abundances, as
expected for young stars in the solar neighborhood. We have also analysed the
halo B-type star, PG0832+676. We find [Fe/H] = -0.88 +/- 0.10, and the absence
of the B III line gives the upper limit [B/H]<2.5. These and other published
abundances are used to infer the star's evolutionary status as a post-AGB star.Comment: 31 pages, 14 figures. accepted to Ap
Evolutionary stage of the spectral variable BD+48 1220=IRAS 05040+4820
Based on high-resolution observations (R=60000 and 75000), we have studied
the optical spectral variability of the star BD+48 1220 (IRAS05040+4820). We
have measured the equivalent widths of numerous absorption lines of neutral
atoms and ions at wavelengths from 4500 to 6760 AA, as well as the
corresponding radial velocities. We use model atmospheres to determine
Teff=7900K, log g=0.0, microturbulence velocity xi_t=6.0, and the abundances
for 16 elements. The star's metallicity differs little from the solar value:
[Fe/H]=-0.10 dex. The main peculiarity of the chemical composition of the star
is a large He-excess, derived from the HeI 5876 A absorption, [He/H]=+1.04, and
the equally large O-excess, [O/Fe]=+0.72 dex. The C-excess is small,
[C/Fe]=+0.09 dex, and the ratio [C/O] 1. We obtained a revised relation
for the light-metal abundances: [Na/Fe]=+0.87 with [Mg/Fe]=-0.31. The barium
abundance is low, [Ba/Fe]=-0.84. The radial velocity of the star measured from
photospheric absorption lines over three years of observations varies in the
interval V_sun = -(7 - 15) km/s. Time variable differential line shifts have
been revealed. The entire set of available data (the luminosity Mv~-5m,
velocity V_lsr~-20 km/s, metallicity [Fe/H]=-0.10, and peculiarities of the
optical spectrum and chemical composition) confirms the status of BD+48 1220 as
a post-AGB star with He- and O-excesses belonging to the Galactic disk.Comment: 42 pages, 7 figures, 6 tables, accepted by Astronomy Report
Chemical Abundances of the Leo II Dwarf Galaxy
We use previously-published moderate-resolution spectra in combination with
stellar atmosphere models to derive the first measured chemical abundance
ratios in the Leo II dSph galaxy. We find that for spectra with SNR > 24, we
are able to measure abundances from weak Ti, Fe and Mg lines located near the
calcium infrared triplet (CaT). We also quantify and discuss discrepancies
between the metallicities measured from Fe I lines and those estimated from the
CaT features. We find that while the most metal-poor ([Fe/H] <-2.0]) Leo II
stars have Ca and Ti abundance ratios similar to those of Galactic globular
clusters, the more metal-rich stars show a gradual decline of Ti, Mg and Ca
abundance ratio with increasing metallicity. Finding these trends in this
distant and apparently dynamically stable dSph galaxy supports the hypothesis
that the slow chemical enrichment histories of the dSph galaxies is universal,
independent of any interaction with the Milky Way. Combining our spectroscopic
abundances with published broadband photometry and updated isochrones, we are
able to approximate stellar ages for our bright RGB stars to a relative
precision of 2-3 Gyr. While the derived age-metallicity relationship of Leo II
hints at some amount of slow enrichment, the data are still statistically
consistent with no enrichment over the history of Leo II.Comment: Accepted to A
- …