203 research outputs found
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
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
Fe XI emission lines in a high resolution extreme ultraviolet spectrum obtained by SERTS
New calculations of radiative rates and electron impact excitation cross
sections for Fe XI are used to derive emission line intensity ratios involving
3s^23p^4 - 3s^23p^33d transitions in the 180-223 A wavelength range. These
ratios are subsequently compared with observations of a solar active region,
obtained during the 1995 flight Solar EUV Research Telescope and Spectrograph
(SERTS). The version of SERTS flown in 1995 incorporated a multilayer grating
that enhanced the instrumental sensitivity for features in the 170 - 225 A
wavelength range, observed in second-order between 340 and 450 A. This
enhancement led to the detection of many emission lines not seen on previous
SERTS flights, which were measured with the highest spectral resolution (0.03
A) ever achieved for spatially resolved active region spectra in this
wavelength range. However, even at this high spectral resolution, several of
the Fe XI lines are found to be blended, although the sources of the blends are
identified in the majority of cases. The most useful Fe XI electron density
diagnostic line intensity ratio is I(184.80 A)/I(188.21 A). This ratio involves
lines close in wavelength and free from blends, and which varies by a factor of
11.7 between N_e = 10^9 and 10^11 cm^-3, yet shows little temperature
sensitivity. An unknown line in the SERTS spectrum at 189.00 A is found to be
due to Fe XI, the first time (to our knowledge) this feature has been
identified in the solar spectrum. Similarly, there are new identifications of
the Fe XI 192.88, 198.56 and 202.42 A features, although the latter two are
blended with S VIII/Fe XII and Fe XIII, respectively.Comment: 21 pages, 9 gigures, accepted for publication in the Astrophysical
Journa
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
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
Interstellar and Circumstellar Optical & Ultraviolet Lines Towards SN1998S
We have observed SN1998S which exploded in NGC3877, with the UES at the WHT
and with the E230M echelle of STIS aboard HST. Both data sets were obtained at
two seperate epochs. From our own Galaxy we detect interstellar absorption
lines of CaII, FeII, MgI, and probably MnII from the edge of the HVC Complex M.
We derive gas-phase abundances which are very similar to warm disk clouds in
the local ISM, which we believe argues against the HVC material having an
extragalactic origin. At the velocity of NGC3877 we detect interstellar MgI,
MgII, MnII, CaII, & NaI. Surprisingly, one component is seen to increase by a
factor of ~1 dex in N(NaI) and N(MgI) between the two epochs over which the
data were taken. Unusually, our data also show narrow Balmer, HeI, and
metastable FeII P-Cygni profiles, with a narrow absorption component
superimposed on the bottom of the profile's absorption trough. Both the broad
and narrow components of the optical lines are seen to increase substantially
in strength between the two epochs. Most of the low-ionization absorption can
be understood in terms of gas co-rotating with the disk of NGC 3877, providing
the SN is at the back of an HI disk with a similar thickness to that of our own
Galaxy. However, the variable absorption components, and the classic P-Cygni
emission profiles, most likely arise in slow-moving circumstellar outflows
originating from the red supergiant progenitor of SN1998S. [Abridged.]Comment: Accepted by ApJ, 26 pages including 9 figure
H_alpha Emission from High-Velocity Clouds and their Distances
We present deep Halpha spectroscopy towards several high-velocity clouds
(HVCs) which vary in structure from compact (CHVCs) to the Magellanic Stream.
The clouds range from being bright (~640 mR) to having upper limits on the
order of 30 to 70 mR. The Halpha measurements are discussed in relation to
their HI properties and distance constraints are given to each of the complexes
based on f_esc = 6% of the ionizing photons escaping normal to the Galactic
disk (f_escs = 1 - 2% when averaged over solid angle). The results suggest that
many HVCs and CHVCs are within a ~40 kpc radius from the Galaxy and are not
members of the Local Group at megaparsec distances. However, the Magellanic
Stream is inconsistent with this model and needs to be explained. It has bright
Halpha emission and little [NII] emission and appears to fall into a different
category than the currently detected HVCs. This may reflect the lower
metallicities of the Magellanic Clouds compared to the Galaxy, but the strength
of the Halpha emission cannot be explained solely by photoionization from the
Galaxy. The interaction of the Stream with halo gas or the presence of yet
unassociated young stars may assist in ionizing the Stream.Comment: ApJ accepted, see http://casa.colorado.edu/~mputman/pubs.html for the
full resolution versio
Distribution and Kinematics of O VI in the Galactic Halo
FUSE spectra of 100 extragalactic objects are analyzed to obtain measures of
O VI absorption along paths through the Milky Way thick disk/halo. Strong O VI
absorption over the approximate velocity range from -100 to 100 km/s reveals a
widespread but highly irregular distribution of thick disk O VI, implying the
existence of substantial amounts of hot gas with T ~ 3x10^5 K in the Milky Way
halo. Large irregularities in the distribution of the absorbing gas are found
to be similar over angular scales extending from less than one to 180 degrees,
indicating a considerable amount of small and large scale structure in the gas.
The overall distribution of Galactic O VI is not well described by a
symmetrical plane-parallel layer of patchy O VI absorption. The simplest
departure from such a model that provides a reasonable fit to the observations
is a plane-parallel patchy absorbing layer with a scale height of 2.3 kpc, and
a 0.25 dex excess of O VI in the northern Galactic polar region. The O VI
absorption has a Doppler parameter b = 30 to 99 km/s, with an average value of
60 km/s . Thermal broadening alone cannot explain the large observed profile
widths. The average O VI absorption velocities toward high latitude objects
range from -46 to 82 km/s, with a sample average of 0 km/s and a standard
deviation of 21 km/s. O VI associated with the thick disk moves both toward and
away from the plane with roughly equal frequency. A combination of models
involving the radiative cooling of hot fountain gas, the cooling of supernova
bubbles in the halo, and the turbulent mixing of warm and hot halo gases is
required to explain the presence of O VI and other highly ionized atoms found
in the halo. (abbreviated)Comment: 70 pages, single-spaced, PDF format. Bound copies of this manuscript
and two accompanying articles are available upon request. Submitted to ApJ
Distances to Galactic high-velocity clouds. I. Cohen Stream, complex GCP, cloud g1
The high- and intermediate-velocity interstellar clouds (HVCs/IVCs) are
tracers of energetic processes in and around the Milky Way. Clouds with
near-solar metallicity about one kpc above the disk trace the circulation of
material between disk and halo (the Galactic Fountain). The Magellanic Stream
consists of gas tidally extracted from the SMC, tracing the dark matter
potential of the Milky Way. Several other HVCs have low-metallicity and appear
to trace the continuing accretion of infalling intergalactic gas. These
assertions are supported by the metallicities (0.1 to 1 solar) measured for
about ten clouds in the past decade. Direct measurements of distances to HVCs
have remained elusive, however. In this paper we present four new distance
brackets, using VLT observations of interstellar \CaII H and K absorption
toward distant Galactic halo stars. We derive distance brackets of 5.0 to 11.7
kpc for the Cohen Stream (likely to be an infalling low-metallicity cloud), 9.8
to 15.1 kpc for complex GCP (also known as the Smith Cloud or HVC40-15+100 and
with still unknown origin), 1.0 to 2.7 kpc for an IVC that appears associated
with the return flow of the Fountain in the Perseus Arm, and 1.8 to 3.8 kpc for
cloud g1, which appears to be in the outflow phase of the Fountain. Our
measurements further demonstrate that the Milky Way is accreting substantial
amounts of gaseous material, which influences the Galaxy's current and future
dynamical and chemical evolution.Comment: Accepted by Ap
Highly-Ionized High-Velocity Gas in the Vicinity of the Galaxy
We report the results of an extensive FUSE study of high velocity OVI
absorption along 102 complete sight lines through the Galactic halo. The high
velocity OVI traces a variety of phenomena, including tidal interactions with
the Magellanic Clouds, accretion of gas, outflow from the Galactic disk,
warm/hot gas interactions in a highly extended Galactic corona, and
intergalactic gas in the Local Group. We identify 85 high velocity OVI features
at velocities of -500 < v(LSR) < +500 km/s along 59 of the 102 sight lines.
Approximately 60% of the sky (and perhaps as much as 85%) is covered by high
velocity H+ associated with the high velocity OVI. Some of the OVI is
associated with known high velocity HI structures (e.g., the Magellanic Stream,
Complexes A and C), while some OVI features have no counterpart in HI 21cm
emission. The smaller dispersion in the OVI velocities in the GSR and LGSR
reference frames compared to the LSR is necessary (but not conclusive) evidence
that some of the clouds are extragalactic. Most of the OVI cannot be produced
by photoionization, even if the gas is irradiated by extragalactic background
radiation. Collisions in hot gas are the primary OVI ionization mechanism. We
favor production of some of the OVI at the boundaries between warm clouds and a
highly extended [R > 70 kpc], hot [T > 10^6 K], low-density [n < 10^-4 cm^-3]
Galactic corona or Local Group medium. A hot Galactic corona or Local Group
medium and the prevalence of high velocity OVI are consistent with predictions
of galaxy formation scenarios. Distinguishing between the various phenomena
producing high velocity OVI will require continuing studies of the distances,
kinematics, elemental abundances, and physical states of the different types of
high velocity OVI features found in this study. (abbreviated)Comment: 78 pages of text/tables + 31 figures, AASTeX preprint format. All
figures are in PNG format due to astro-ph space restrictions. Bound copies of
manuscript and two accompanying articles are available upon request.
Submitted to ApJ
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