443 research outputs found
Discovery of an Unbound Hyper-Velocity Star in the Milky Way Halo
We have discovered a star, SDSS J090745.0+024507, leaving the Galaxy with a
heliocentric radial velocity of +853+-12 km/s, the largest velocity ever
observed in the Milky Way halo. The star is either a hot blue horizontal branch
star or a B9 main sequence star with a heliocentric distance ~55 kpc. Corrected
for the solar reflex motion and to the local standard of rest, the Galactic
rest-frame velocity is +709 km/s.
Because its radial velocity vector points 173.8 deg from the Galactic center,
we suggest that this star is the first example of a hyper-velocity star ejected
from the Galactic center as predicted by Hills and later discussed by Yu &
Tremaine. The star has [Fe/H]~0, consistent with a Galactic center origin, and
a travel time of <80 Myr from the Galactic center, consistent with its stellar
lifetime. If the star is indeed traveling from the Galactic center, it should
have a proper motion of 0.3 mas/yr observable with GAIA. Identifying additional
hyper-velocity stars throughout the halo will constrain the production rate
history of hyper-velocity stars at the Galactic center.Comment: 4 pages, submitted to ApJ Letter
The Homogeneity of Interstellar Krypton in the Galactic Disk
We present an analysis of high resolution HST Space Telescope Imaging
Spectrograph (STIS) observations of Kr I 1236 Angstrom absorption in seven
sight lines that probe a variety of interstellar environments. In combination
with krypton and hydrogen column densities derived from current and archival
STIS and Far-Ultraviolet Spectroscopic Explorer data, the number of sight lines
with reliable Kr/H ISM abundance ratios has been increased by 50% to
26--including paths that sample a range of nearly 5 orders of magnitude in
f(H_2), over 2 orders of magnitude in , and extending up to 4.8 kpc in
length. For sight lines contained entirely within the local spiral arm (the
Orion Spur), the spread of Kr/H ratios about the mean of log_10[N(Kr)/N(H)]_ISM
= -9.02+/-0.02 is remarkably tight (0.06 dex), less than the typical datapoint
uncertainty. Intriguingly, the only two sight lines that extend through
neighboring structures, in particular gas associated with the
Carina/Sagittarius Arm, exhibit relatively large, near-solar krypton abundances
(log_10[N(Kr)/N(H)]_combined = -8.75+0.09_-0.11). Although these deviations are
only measured at the 2 sigma level, they suggest the possibility that krypton
abundances beyond the Orion Spur may differ from the local value.Comment: 15 page, including 3 figures and 2 tables, accepted for publication
in Ap
The 'Forbidden' Abundance of Oxygen in the Sun
We reexamine closely the solar photospheric line at 6300 A, which is
attributed to a forbidden line of neutral oxygen, and is widely used in
analyses of other late-type stars.
We use a three-dimensional time-dependent hydrodynamical model solar
atmosphere which has been tested successfully against observed granulation
patterns and an array of absorption lines. We show that the solar line is a
blend with a Ni I line, as previously suggested but oftentimes neglected.
Thanks to accurate atomic data on the [O I] and Ni I lines we are able to
derive an accurate oxygen abundance for the Sun: log epsilon (O) = 8.69 +/-
0.05 dex, a value at the lower end of the distribution of previously published
abundances, but in good agreement with estimates for the local interstellar
medium and hot stars in the solar neighborhood. We conclude by discussing the
implication of the Ni I blend on oxygen abundances derived from the [O I] 6300
A line in disk and halo stars.Comment: 16 pages, 3 eps figures included; a more compact PostScript version
created using emulateapj.sty is available from
http://hebe.as.utexas.edu/recent_publi.html; to appear in ApJ
Star Formation in the Extreme Outer Galaxy: Digel Cloud 2 Clusters
As a first step for studying star formation in the extreme outer Galaxy
(EOG), we obtained deep near-infrared images of two embedded clusters at the
northern and southern CO peaks of Cloud 2, which is one of the most distant
star forming regions in the outer Galaxy (galactic radius R_g ~ 19 kpc). With
high spatial resolution (FWHM ~ 0".35) and deep imaging (K ~ 21 mag) with the
IRCS imager at the Subaru telescope, we detected cluster members with a mass
detection limit of < 0.1 M_{sun}, which is well into the substellar regime.
These high quality data enables a comparison of EOG to those in the solar
neighborhood on the same basis for the first time. Before interpreting the
photometric result, we have first constructed the NIR color-color diagram
(dwarf star track, classical T Tauri star (CTTS) locus, reddening law) in the
Mauna Kea Observatory filter system and also for the low metallicity
environment since the metallicity in EOG is much lower than those in the solar
neighborhood. The estimated stellar density suggests that an ``isolated type''
star formation is ongoing in Cloud 2-N, while a ``cluster type'' star formation
is ongoing in Cloud 2-S. Despite the difference of the star formation mode,
other characteristics of the two clusters are found to be almost identical: (1)
K-band luminosity function (KLF) of the two clusters are quite similar, as is
the estimated IMF and ages (~ 0.5--1 Myr) from the KLF fitting, (2) the
estimated star formation efficiencies (SFEs) for both clusters are typical
compared to those of embedded clusters in the solar neighborhood (~ 10 %). The
similarity of two independent clusters with a large separation (~ 25 pc)
strongly suggest that their star formation activities were triggered by the
same mechanism, probably the supernova remnant (GSH 138-01-94).Comment: 14pages, 11 figures; Accepted for publication in Ap
Elemental Abundance Ratios in Stars of the Outer Galactic Disk. II. Field Red Giants
We summarize a selection process to identify red giants in the direction of
the southern warp of the Galactic disk, employing VI_C photometry and
multi-object spectroscopy. We also present results from follow-up
high-resolution, high-S/N echelle spectroscopy of three field red giants,
finding [Fe/H] values of about -0.5. The field stars, with Galactocentric
distances estimated at 10 to 15 kpc, support the conclusion of Yong, Carney, &
de Almeida (2005) that the Galactic metallicity gradient disappears beyond R_GC
values of 10 to 12 kpc for the older stars and clusters of the outer disk. The
field and cluster stars at such large distances show very similar abundance
patterns, and, in particular, all show enhancements of the "alpha" elements O,
Mg, Si, Ca, and Ti and the r-process element Eu. These results suggest that
Type II supernovae have been significant contributors to star formation in the
outer disk relative to Type Ia supernovae within the past few Gyrs. We also
compare our results with those available for much younger objects. The limited
results for the H II regions and B stars in the outer disk also suggest that
the radial metallicity gradient in the outer disk is shallow or absent. The
much more extensive results for Cepheids confirm these trends, and that the
change in slope of the metallicity gradient may occur at a larger
Galactocentric distance than for the older stars and clusters. However, the
younger stars also show rising alpha element enhancements with increasing R_GC,
at least beyond 12 kpc. These trends are consistent with the idea of a
progressive growth in the size of the Galactic disk with time, and episodic
enrichment by Type II supernovae as part of the disk's growth. [Abridged]Comment: Accepted for publication in A
Metal Abundances in the Magellanic Stream
We report on the first metallicity determination for gas in the Magellanic
Stream, using archival HST GHRS data for the background targets Fairall 9, III
Zw 2, and NGC 7469. For Fairall 9, using two subsequent HST revisits and new
Parkes Multibeam Narrowband observations, we have unequivocally detected the
MSI HI component of the Stream (near its head) in SII1250,1253 yielding a
metallicity of [SII/H]=-0.55+/-0.06(r)+/-0.2(s), consistent with either an SMC
or LMC origin and with the earlier upper limit set by Lu et al. (1994). We also
detect the saturated SiII1260 line, but set only a lower limit of
[SiII/H]>-1.5. We present serendipitous detections of the Stream, seen in
MgII2796,2803 absorption with column densities of (0.5-1)x10^13 cm^-2 toward
the Seyfert galaxies III Zw 2 and NGC 7469. These latter sightlines probe gas
near the tip of the Stream (80 deg down-Stream of Fairall 9). For III Zw 2, the
lack of an accurate HI column density and the uncertain MgIII ionization
correction limits the degree to which we can constrain [Mg/H]; a lower limit of
[MgII/HI]>-1.3 was found. For NGC 7469, an accurate HI column density
determination exists, but the extant FOS spectrum limits the quality of the
MgII column density determination, and we conclude that [MgII/HI]>-1.5.
Ionization corrections associated with MgIII and HII suggest that the
corresponding [Mg/H] may range lower by 0.3-1.0 dex. However, an upward
revision of 0.5-1.0 dex would be expected under the assumption that the Stream
exhibits a dust depletion pattern similar to that seen in the Magellanic
Clouds. Remaining uncertainties do not allow us to differentiate between an LMC
versus SMC origin to the Stream gas.Comment: 30 pages, 8 figures, LaTeX (aaspp4), also available at
http://casa.colorado.edu/~bgibson/publications.html, accepted for publication
in The Astronomical Journa
Determining the Physical Properties of the B Stars I. Methodology and First Results
We describe a new approach to fitting the UV-to-optical spectra of B stars to
model atmospheres and present initial results. Using a sample of lightly
reddened stars, we demonstrate that the Kurucz model atmospheres can produce
excellent fits to either combined low dispersion IUE and optical photometry or
HST FOS spectrophotometry, as long as the following conditions are fulfilled:
1) an extended grid of Kurucz models is employed,
2) the IUE NEWSIPS data are placed on the FOS absolute flux system using the
Massa & Fitzpatrick (1999) transformation, and
3) all of the model parameters and the effects of interstellar extinction are
solved for simultaneously.
When these steps are taken, the temperatures, gravities, abundances and
microturbulence velocities of lightly reddened B0-A0 V stars are determined to
high precision. We also demonstrate that the same procedure can be used to fit
the energy distributions of stars which are reddened by any UV extinction curve
which can be expressed by the Fitzpatrick & Massa (1990) parameterization
scheme.
We present an initial set of results and verify our approach through
comparisons with angular diameter measurements and the parameters derived for
an eclipsing B star binary. We demonstrate that the metallicity derived from
the ATLAS 9 fits to main sequence B stars is essentially the Fe abundance. We
find that a near zero microturbulence velocity provides the best-fit to all but
the hottest or most luminous stars (where it may become a surrogate for
atmospheric expansion), and that the use of white dwarfs to calibrate UV
spectrophotometry is valid.Comment: 17 pages, including 2 pages of Tables and 6 pages of Figures.
Astrophysical Jounral, in pres
A massive cluster of Red Supergiants at the base of the Scutum-Crux arm
We report on the unprecedented Red Supergiant (RSG) population of a massive
young cluster, located at the base of the Scutum-Crux Galactic arm. We identify
candidate cluster RSGs based on {\it 2MASS} photometry and medium resolution
spectroscopy. With follow-up high-resolution spectroscopy, we use CO-bandhead
equivalent width and high-precision radial velocity measurements to identify a
core grouping of 26 physically-associated RSGs -- the largest such cluster
known to-date. Using the stars' velocity dispersion, and their inferred
luminosities in conjuction with evolutionary models, we argue that the cluster
has an initial mass of 40,000\msun, and is therefore among the most
massive in the galaxy. Further, the cluster is only a few hundred parsecs away
from the cluster of 14 RSGs recently reported by Figer et al (2006). These two
RSG clusters represent 20% of all known RSGs in the Galaxy, and now offer the
unique opportunity to study the pre-supernova evolution of massive stars, and
the Blue- to Red-Supergiant ratio at uniform metallicity. We use GLIMPSE,
MIPSGAL and MAGPIS survey data to identify several objects in the field of the
larger cluster which seem to be indicative of recent region-wide starburst
activity at the point where the Scutum-Crux arm intercepts the Galactic bulge.
Future abundance studies of these clusters will therefore permit the study of
the chemical evolution and metallicity gradient of the Galaxy in the region
where the disk meets the bulge.Comment: 49 pages, 22 figures. Accepted for publication in ApJ. Version with
hi-res figures can be found at http://www.cis.rit.edu/~bxdpci/RSGC2.pd
First Stellar Abundances in the Dwarf Irregular Galaxy Sextans A
We present the abundance analyses of three isolated A-type supergiant stars
in the dwarf irregular galaxy Sextans A from high-resolution spectra the UVES
spectrograph at the VLT. Detailed model atmosphere analyses have been used to
determine the stellar atmospheric parameters and the elemental abundances of
the stars. The mean iron group abundance was determined from these three stars
to be [(FeII,CrII)/H]=-0.99+/-0.04+/-0.06. This is the first determination of
the present-day iron group abundances in Sextans A. These three stars now
represent the most metal-poor massive stars for which detailed abundance
analyses have been carried out. The mean stellar alpha element abundance was
determined from the alpha element magnesium as
[alpha(MgI)/H]=-1.09+/-0.02+/-0.19. This is in excellent agreement with the
nebular alpha element abundances as determined from oxygen in the H II regions.
These results are consistent from star-to-star with no significant spatial
variations over a length of 0.8 kpc in Sextans A. This supports the nebular
abundance studies of dwarf irregular galaxies, where homogeneous oxygen
abundances are found throughout, and argues against in situ enrichment. The
alpha/Fe abundance ratio is [alpha(MgI)/FeII,CrII]=-0.11+/-0.02+/-0.10, which
is consistent with the solar ratio. This is consistent with the results from
A-supergiant analyses in other Local Group dwarf irregular galaxies but in
stark contrast with the high [alpha/Fe] results from metal-poor stars in the
Galaxy, and is most clearly seen from these three stars in Sextans A because of
their lower metallicities. The low [alpha/Fe] ratios are consistent with the
slow chemical evolution expected for dwarf galaxies from analyses of their
stellar populations.Comment: 40 pages, 8 figures, accepted for publication in A
Metallicity in the Galactic Center: The Arches cluster
We present a quantitative spectral analysis of five very massive stars in the
Arches cluster, located near the Galactic center, to determine stellar
parameters, stellar wind properties and, most importantly, metallicity content.
The analysis uses a new technique, presented here for the first time, and uses
line-blanketed NLTE wind/atmosphere models fit to high-resolution near-infrared
spectra of late-type nitrogen-rich Wolf-Rayet stars and OfI+ stars in the
cluster. It relies on the fact that massive stars reach a maximum nitrogen
abundance that is related to initial metallicity when they are in the WNL
phase. We determine the present-day nitrogen abundance of the WNL stars in the
Arches cluster to be 1.6% (mass fraction) and constrain the stellar metallicity
in the cluster to be solar. This result is invariant to assumptions about the
mass-luminosity relationship, the mass-loss rates, and rotation speeds. In
addition, from this analysis, we find the age of the Arches cluster to be
2-2.5Myr, assuming coeval formation
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