169 research outputs found
A Na I Absorption Map of the Small-Scale Structure in the Interstellar Gas Toward M15
Using the DensePak fiber optic array on the KPNO WIYN telescope, we have
obtained high S/N echelle spectra of the Na I D wavelength region toward the
central 27" x 43" of the globular cluster M15 at a spatial resolution of 4".
The spectra exhibit significant interstellar Na I absorption at LSR velocities
of +3 km/s (LISM component) and +68 km/s (IVC component). Both components vary
appreciably in strength on these scales. The derived Na I column densities
differ by a factor of 4 across the LISM absorption map and by a factor of 16
across the IVC map. Assuming distances of 500 pc and 1500 pc for the LISM and
IVC clouds, these maps show evidence of significant ISM structure down to the
minimum scales of 2000 AU and 6000 AU probed in these absorbers. The
smallest-scale N(Na I) variations observed in the M15 LISM and IVC maps are
typically comparable to or higher than the values found at similar scales in
previous studies of interstellar Na I structure toward binary stars. The
physical implications of the small and larger-scale Na I features observed in
the M15 maps are discussed in terms of variations in the H I column density as
well as in the Na ionization equilibrium.Comment: 11 pages, 3 figures, accepted for publication in ApJ Letter
A non-LTE abundance analysis of the post-AGB star ROA 5701
An analysis of high-resolution Anglo-Australian Telescope (AAT)/ University
College London Echelle Spectrograph (UCLES) optical spectra for the ultraviolet
(UV)-bright star ROA 5701 in the globular cluster omega Cen (NGC 5139) is
performed, using non-local thermodynamic equilibrium (non-LTE) model
atmospheres to estimate stellar atmospheric parameters and chemical
composition. Abundances are derived for C, N, O, Mg, Si and S, and compared
with those found previously by Moehler et al. We find a general metal
underabundance relative to young B-type stars, consistent with the average
metallicity of the cluster. Our results indicate that ROA 5701 has not
undergone a gas-dust separation scenario as previously suggested. However, its
abundance pattern does imply that ROA 5701 has evolved off the AGB prior to the
onset of the third dredge-up.Comment: 9 pages, 2 figures. Accepted for publication in MNRAS (Online Early
Detection of Voigt Spectral Line Profiles of Hydrogen Radio Recombination Lines toward Sagittarius B2(N)
We report the detection of Voigt spectral line profiles of radio
recombination lines (RRLs) toward Sagittarius B2(N) with the 100-m Green Bank
Telescope (GBT). At radio wavelengths, astronomical spectra are highly
populated with RRLs, which serve as ideal probes of the physical conditions in
molecular cloud complexes. An analysis of the Hn(alpha) lines presented herein
shows that RRLs of higher principal quantum number (n>90) are generally
divergent from their expected Gaussian profiles and, moreover, are well
described by their respective Voigt profiles. This is in agreement with the
theory that spectral lines experience pressure broadening as a result of
electron collisions at lower radio frequencies. Given the inherent technical
difficulties regarding the detection and profiling of true RRL wing spans and
shapes, it is crucial that the observing instrumentation produce flat baselines
as well as high sensitivity, high resolution data. The GBT has demonstrated its
capabilities regarding all of these aspects, and we believe that future
observations of RRL emission via the GBT will be crucial towards advancing our
knowledge of the larger-scale extended structures of ionized gas in the
interstellar medium (ISM)
HI spectra and column densities toward HVC and IVC probes
We show 21-cm line profiles in the direction of stars and extragalactic
objects, lying projected on high- and intermediate-velocity clouds (HVCs and
IVCs). About half of these are from new data obtained with the Effelsberg 100-m
telescope, about a quarter are extracted from the Leiden-Dwingeloo Survey (LDS)
and the remaining quarter were observed with other single-dish telescopes. HI
column densities were determined for each HVC/IVC. Wakker (2001) (Paper I) uses
these in combination with optical and ultraviolet high-resolution measurements
to derive abundances. Here, an analysis is given of the difference and ratio of
N(HI) as observed with a 9 arcmin versus a 35 arcmin beam. For HVCs and IVCs
the ratio N(HI-9 arcmin)/N(HI-35 arcmin) lies in the range 0.2-2.5. For
low-velocity gas this ratio ranges from 0.75 to 1.3 (the observed ratio is
0.85-1.4, but it appears that the correction for stray radiation is slightly
off). The smaller range for the low-velocity gas may be caused by confusion in
the line of sight, so that a low ratio in one component can be compensated by a
high ratio in another -- for 11 low-velocity clouds fit by one component the
distribution of ratios has a larger dispersion. Comparison with higher angular
resolution data is possible for sixteen sightlines. Eight sightlines with HI
data at 1 arcmin-2 arcmin resolution show a range of 0.75-1.25 for N(HI-2
arcmin)/N(HI-9 arcmin), while in eight other sightlines N(HI-Ly-alpha)/N(HI-9
arcmin) ranges from 0.74 to 0.98.Comment: To appear in the "Astrophysical Journal Supplement"; 45 pages;
degraded figures (astro-ph restriction) - ask for good version
B-type supergiants in the SMC: Rotational velocities and implications for evolutionary models
High-resolution spectra for 24 SMC and Galactic B-type supergiants have been
analysed to estimate the contributions of both macroturbulence and rotation to
the broadening of their metal lines. Two different methodologies are
considered, viz. goodness-of-fit comparisons between observed and theoretical
line profiles and identifying zeros in the Fourier transforms of the observed
profiles. The advantages and limitations of the two methods are briefly
discussed with the latter techniques being adopted for estimated projected
rotational velocities (\vsini) but the former being used to estimate
macroturbulent velocities. Only one SMC supergiant, SK 191, shows a significant
degree of rotational broadening (\vsini 90 \kms). For the remaining
targets, the distribution of projected rotational velocities are similar in
both our Galactic and SMC samples with larger values being found at earlier
spectral types. There is marginal evidence for the projected rotational
velocities in the SMC being higher than those in the Galactic targets but any
differences are only of the order of 5-10 \kms, whilst evolutionary models
predict differences in this effective temperature range of typically 20 to 70
\kms. The combined sample is consistent with a linear variation of projected
rotational velocity with effective temperature, which would imply rotational
velocities for supergiants of 70 \kms at an effective temperature of 28 000 K
(approximately B0 spectral type) decreasing to 32 \kms at 12 000 K (B8 spectral
type). For all targets, the macroturbulent broadening would appear to be
consistent with a Gaussian distribution (although other distributions cannot be
discounted) with an half-width varying from approximately 20 \kms
at B8 to 60 \kms at B0 spectral types.Comment: 4 figures, 8 pages, submitted to Astronomy and Astrophysic
Chemical compositions of Four B-type Supergiants in the SMC Wing
High-resolution UCLES/AAT spectra of four B-type supergiants in the SMC South
East Wing have been analysed using non-LTE model atmosphere techniques to
determine their atmospheric parameters and chemical compositions. The principle
aim of this analysis was to determine whether the very low metal abundances
(1.1 dex compared with Galactic value) previously found in the Magellanic
Inter Cloud region (ICR) were also present in SMC Wing. The chemical
compositions of the four targets are similar to those found in other SMC
objects and appear to be incompatible with those deduced previously for the
ICR. Given the close proximity of the Wing to the ICR, this is difficult to
understand and some possible explanations are briefly discussed.Comment: 11 pages, 2 figues, A&A accepte
Current Status of the SuperWASP Project
We present the current status of the SuperWASP project, a Wide Angle Search
for Planets. SuperWASP consists of up to 8 individual cameras using ultra-wide
field lenses backed by high-quality passively cooled CCDs. Each camera covers
7.8 x 7.8 sq degrees of sky, for nearly 500 sq degrees of sky coverage.
SuperWASP I, located in LaPalma, is currently operational with 5 cameras and is
conducting a photometric survey of a large numbers of stars in the magnitude
range ~7 to 15. The collaboration has developed a custom-built reduction
pipeline and aims to achieve better than 1 percent photometric precision. The
pipeline will also produce well sampled light curves for all the stars in each
field which will be used to detect: planetary transits, optical transients, and
track Near-Earth Objects. Status of current observations, and expected rates of
extrasolar planetary detections will be presented. The consortium members,
institutions, and further details can be found on the web site at:
http://www.superwasp.org.Comment: 3 pages, 2 figures, submitted to the Proceedings of the 13th Cool
Stars Workshop, Ed. F. Favata, ESA-S
Soft X-ray emission lines of Fe XV in solar flare observations and the Chandra spectrum of Capella
Recent calculations of atomic data for Fe XV have been used to generate
theoretical line ratios involving n = 3-4 transitions in the soft X-ray
spectral region (52-83 A), for a wide range of electron temperatures and
densities applicable to solar and stellar coronal plasmas. A comparison of
these with solar flare observations from a rocket-borne spectrograph (XSST)
reveals generally good agreement between theory and experiment. In particular,
the 82.76 A emission line in the XSST spectrum is identified, for the first
time to our knowledge in an astrophysical source. Most of the Fe XV transitions
which are blended have had the species responsible clearly identified, although
there remain a few instances where this has not been possible. The line ratio
calculations are also compared with a co-added spectrum of Capella obtained
with the Chandra satellite, which is probably the highest signal-to-noise
observation achieved for a stellar source in the 25-175 A soft X-ray region.
Good agreement is found between theory and experiment, indicating that the Fe
XV lines are reliably detected in Chandra spectra, and hence may be employed as
diagnostics to determine the temperature and/or density of the emitting plasma.
However the line blending in the Chandra data is such that individual emission
lines are difficult to measure accurately, and fluxes may only be reliably
determined via detailed profile fitting of the observations. The co-added
Capella spectrum is made available to hopefully encourage further exploration
of the soft X-ray region in astronomical sources.Comment: 27 pages, 10 figures, Astrophysical Journal, in pres
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