158 research outputs found
GHRS and ORFEUS-II Observations of the Highly Ionized Interstellar Medium Toward ESO141-055
We present Goddard High Resolution Spectrograph and ORFEUS-II measurements of
Si IV, CIV, N V, and O VI absorption in the interstellar medium of the Galactic
disk and halo toward the nucleus of the Seyfert galaxy ESO141-055. The high
ionization absorption is strong, with line strengths consistent with the
spectral signature expected for hot (log T = 5-6) collisionally ionized gas in
either a ``Galactic fountain'' or an inhomogeneous medium containing a mixture
of conductive interfaces and turbulent mixing layers. The total O VI column
density of log N ~ 15 suggests that the scale height of O VI is large (>3 kpc)
in this direction. Comparison of the high ion column densities with
measurements for other sight lines indicates that the highly ionized gas
distribution is patchy. The amount of O VI perpendicular to the Galactic plane
varies by at least a factor of ~4 among the complete halo sight lines thus far
studied. In addition to the high ion absorption, lines of low ionization
species are also present in the spectra. With the possible exception of Ar I,
which may have a lower than expected abundance resulting from partial
photoionization of gas along the sight line, the absorption strengths are
typical of those expected for the warm, neutral interstellar medium. The sight
line intercepts a cold molecular cloud with log N(H2) ~ 19. The cloud has an
identifiable counterpart in IRAS 100-micron emission maps of this region of the
sky. We detect a Ly-alpha absorber associated with ESO141-055 at z = 0.03492.
This study presents an enticing glimpse into the interstellar and intergalactic
absorption patterns that will be observed at high spectral resolution by the
Far Ultraviolet Spectroscopic Explorer.Comment: 24 pages + 8 figures, uses aaspp4.sty. Accepted for publication in
Ap
New Observations of the Interstellar Medium in the Lyman Break Galaxy MS 1512-cB58
We present the results of a detailed study of the interstellar medium of MS
1512-cB58, an L* Lyman break galaxy at z = 2.7276, based on new spectral
observations obtained with the Echelle Spectrograph and Imager on the Keck II
telescope at 58 km/s resolution. We focus in particular on the chemical
abundances and kinematics of the interstellar gas and our main findings are as
follows. Even at this relatively early epoch, the ISM of this galaxy is already
highly enriched in elements released by Type II supernovae; the abundances of
O, Mg, Si, P, and S are all about 2/5 of their solar values. In contrast, N and
the Fe-peak elements Mn, Fe, and Ni are underabundant by a factor of about 3.
Based on current ideas of stellar nucleosynthesis, these results can be
understood if most of the metal enrichment in cB58 has taken place within the
last 300 million years, the timescale for the release of N from intermediate
mass stars. cB58 appears to be an example of a galaxy in the process of
converting its gas into stars on a few dynamical timescales; quite possibly we
are witnessing the formation of a galactic bulge or an elliptical galaxy. The
energetic star formation activity has stirred the interstellar medium to high
velocities of up to 1000 km/s. The net effect is a bulk outflow of the ISM at a
speed of 255 km/s and at a rate which exceeds the star formation rate. It is
unclear whether this gas will be lost or retained by the galaxy. We point out
that the chemical and kinematic properties of cB58 are markedly different from
those of most damped Lyman alpha systems at the same redshift.Comment: 38 pages, LaTeX, 9 Postscript Figures. Accepted for publication in
the Astrophysical Journal. Sections 3.3 and 5.3 expanded, and two additional
figures included, following referee's repor
Emission lines of Fe XI in the 257--407 A wavelength region observed in solar spectra from EIS/Hinode and SERTS
Theoretical emission-line ratios involving Fe XI transitions in the 257-407 A
wavelength range are derived using fully relativistic calculations of radiative
rates and electron impact excitation cross sections. These are subsequently
compared with both long wavelength channel Extreme-Ultraviolet Imaging
Spectrometer (EIS) spectra from the Hinode satellite (covering 245-291 A), and
first-order observations (235-449 A) obtained by the Solar Extreme-ultraviolet
Research Telescope and Spectrograph (SERTS). The 266.39, 266.60 and 276.36 A
lines of Fe XI are detected in two EIS spectra, confirming earlier
identifications of these features, and 276.36 A is found to provide an electron
density diagnostic when ratioed against the 257.55 A transition. Agreement
between theory and observation is found to be generally good for the SERTS data
sets, with discrepancies normally being due to known line blends, while the
257.55 A feature is detected for the first time in SERTS spectra. The most
useful Fe XI electron density diagnostic is found to be the 308.54/352.67
intensity ratio, which varies by a factor of 8.4 between N_e = 10^8 and 10^11
cm^-3, while showing little temperature sensitivity. However, the 349.04/352.67
ratio potentially provides a superior diagnostic, as it involves lines which
are closer in wavelength, and varies by a factor of 14.7 between N_e = 10^8 and
10^11 cm^-3. Unfortunately, the 349.04 A line is relatively weak, and also
blended with the second-order Fe X 174.52 A feature, unless the first-order
instrument response is enhanced.Comment: 9 pages, 5 figures, 13 tables; MNRAS in pres
How useful are crowdsourced air temperature observations? An assessment of Netatmo stations and quality control schemes over the United Kingdom
Observations of the real-time state of the atmosphere are required in order to initialize numerical weather prediction (NWP) models. As NWP resolution improves, more observations are needed, to better capture regional variations in atmospheric conditions. In particular, surface observations are necessary to reflect conditions experienced on the surface. One proposed opportunity to increase the number of surface observations available for assimilation into NWP is to crowdsource the data from home weather stations. This study investigates the outdoor air temperature measurements made by Netatmo home weather stations, through validation against a calibrated laboratory chamber and by evaluating quality control schemes that are applied to a UK-wide network of Netatmo stations. In a series of controlled lab experiments, it was found that the Netatmo temperature sensor was accurate to 0.3°C. The response to fluctuations in temperature is lagged, with τ (the time taken for 63% of the change to be measured) calculated as 12.7 min for a near-instantaneous decrease in temperature. Netatmo temperature observations were compared with Met Office MIDAS hourly weather observations. A warm bias in excess of 1°C was present in the Netatmo temperature observations, which was lessened by the three quality control schemes tested, but still in excess of 0.5°C. Hence, Netatmo temperature measurements have potential to be assimilated in NWP in the United Kingdom, but work is required to find a suitable agreed quality control scheme to filter out anomalous observations in the United Kingdom
Abundances and Physical Conditions in the Warm Neutral Medium Towards mu Columbae
We present ultraviolet interstellar absorption line measurements for the
sightline towards the O9.5 V star mu Columbae obtained with the Goddard High
Resolution Spectrograph (GHRS) on board the Hubble Space Telescope. These
archival data represent the most complete GHRS interstellar absorption line
measurements for any line of sight towards an early-type star. The 3.5 km/s
resolution of the instrument allow us to accurately derive the gas-phase column
densities of many important ionic species in the diffuse warm neutral medium
using a combination of apparent column density and component fitting
techniques, and we study in detail the contamination from ionized gas along
this sightline. The low-velocity material shows gas-phase abundance patterns
similar to the warm cloud (cloud A) towards the disk star zeta Oph, while the
component at v = +20.1 km/s shows gas-phase abundances similar to those found
in warm halo clouds. We find the velocity-integrated gas-phase abundances of
Zn, P, and S relative to H along this sightline are indistinguishable from
solar system abundances. We discuss the implications of our gas-phase abundance
measurements for the composition of interstellar dust. The relative ionic
column density ratios of the intermediate velocity components show the imprint
both of elemental incorporation into grains and (photo)ionization. The
components at v = -30 and -48 km/s along this sightline likely trace shocked
gas with very low hydrogen column densities. Appendices include a new
derivation of the GHRS instrumental line spread function, and a new very
accurate determination of the total H I column along this sightline. (Abridged)Comment: Accepted for publication in the Astrophysical Journal. 80 pages
including 19 embedded figures and 12 embedded tables. Version with higher
resolution figures can be downloaded from
http://fuse.pha.jhu.edu/~howk/Papers/papers.htm
Vertical abundance stratification in the blue horizontal branch star HD135485
It is commonly believed that the observed overabundances of many chemical
species relative to the expected cluster metallicity in blue horizontal branch
(BHB) stars appear as a result of atomic diffusion in the photosphere. The slow
rotation of BHB stars (with T_eff > 11,500K), typically v sin{i} < 10 km/s, is
consistent with this idea. In this work we search for observational evidence of
vertical chemical stratification in the atmosphere of HD135485. If this
evidence exists, it will demonstrate the importance of atomic diffusion
processes in the atmospheres of BHB stars. We undertake an extensive abundance
stratification analysis of the atmosphere of HD135485, based on recently
acquired high resolution and S/N CFHT ESPaDOnS spectra and a McDonald-CE
spectrum. Our numerical simulations show that nitrogen and sulfur reveal
signatures of vertical abundance stratification in the stellar atmosphere. It
appears that the abundances of these elements increase toward the upper
atmosphere. This fact cannot be explained by the influence of microturbulent
velocity, because oxygen, carbon, neon, argon, titanium and chromium do not
show similar behavior and their abundances remain constant throughout the
atmosphere. It seems that the iron abundance may increase marginally toward the
lower atmosphere. This is the first demonstration of vertical abundance
stratification of metals in a BHB star.Comment: 8 pages, 5 figures, accepted to A&
A Faraday Rotation Search for Magnetic Fields in Large Scale Structure
Faraday rotation of radio source polarization provides a measure of the
integrated magnetic field along the observational lines of sight. We compare a
new, large sample of Faraday rotation measures (RMs) of polarized extragalactic
sources with galaxy counts in Hercules and Perseus-Pisces, two nearby
superclusters. We find that the average of RMs in these two supercluster areas
are larger than in control areas in the same galactic latitude range. This is
the first RM detection of magnetic fields that pervade a supercluster volume,
in which case the fields are at least partially coherent over several
megaparsecs. Even the most conservative interpretation of our observations,
according to which Milky Way RM variations mimic the background supercluster
galaxy overdensities, puts constraints on the IGM magneto-ionic ``strength'' in
these two superclusters. We obtain an approximate typical upper limit on the
field strength of about 0.3 microGauss l/(500 kpc), when we combine our RM data
with fiducial estimates of electron density from the environments of giant
radio galaxies, and of the warm-hot intergalactic medium (WHIM).Comment: 8 pages, 3 figures, 1 table, to appear in the Astrophysical Journa
The VLT-FLAMES Tarantula Survey XVIII. Classifications and radial velocities of the B-type stars
We present spectral classifications for 438 B-type stars observed as part of the VLT-FLAMES Tarantula Survey (VFTS) in the 30 Doradus region of the Large Magellanic Cloud. Radial velocities are provided for 307 apparently single stars, and for 99 targets with radial-velocity variations which are consistent with them being spectroscopic binaries. We investigate the spatial distribution of the radial velocities across the 30 Dor region, and use the results to identify candidate runaway stars. Excluding potential runaways and members of two older clusters in the survey region (SL 639 and Hodge 301), we determine a systemic velocity for 30 Dor of 271.6 ± 12.2 kms-1 from 273 presumed single stars. Employing a 3σ criterion we identify nine candidate runaway stars (2.9% of the single stars with radial-velocity estimates). The projected rotational velocities of the candidate runaways appear to be significantly different to those of the full B-type sample, with a strong preference for either large (≥345 kms-1) or small (≤65 kms-1) rotational velocities. Of the candidate runaways, VFTS 358 (classified B0.5: V) has the largest differential radial velocity (−106.9 ± 16.2 kms-1), and a preliminary atmospheric analysis finds a significantly enriched nitrogen abundance of 12 + log (N/H) ≳ 8.5. Combined with a large rotational velocity (ve sin i = 345 ± 22 kms-1), this is suggestive of past binary interaction for this star
The NCAS mobile dual-polarisation Doppler X-band weather radar (NXPol)
In recent years, dual-polarisation Doppler X-band radars have become a widely used part of the atmospheric scientist's toolkit for examining cloud dynamics and microphysics and making quantitative precipitation estimates. This is especially true for research questions that require mobile radars. Here we describe the National Centre for Atmospheric Science (NCAS) mobile X-band dual-polarisation Doppler weather radar (NXPol) and the infrastructure used to deploy the radar and provide an overview of the technical specifications. It is the first radar of its kind in the UK. The NXPol is a Meteor 50DX manufactured by Selex- Gematronik (Selex ES GmbH), modified to operate with a larger 2.4m diameter antenna that produces a 0.98 halfpower beam width and without a radome. We provide an overview of the technical specifications of the NXPol with emphasis given to the description of the aspects of the infrastructure developed to deploy the radar as an autonomous observing facility in remote locations. To demonstrate the radar's capabilities, we also present examples of its use in three recent field campaigns and its ongoing observations at the NERC Facility for Atmospheric Radio Research (NFARR)
Simulating the formation of molecular clouds. I. Slow formation by gravitational collapse from static initial conditions
We study the formation of H2 in the ISM, using a modified version of the
astrophysical magnetohydrodynamical code ZEUS-MP that includes a
non-equilibrium treatment of the formation and destruction of H2. We examine
two different approximations to treat the shielding of H2 against
photodissociation: a local approximation, which gives us a solid lower bound on
the amount of shielding, and a method based on ray-tracing that is considerably
more accurate in some circumstances but that produces results that are harder
to clearly interpret. Either approximation allows one to perform
three-dimensional high-resolution simulations of cloud formation with only
modest computational resources. We also include a detailed treatment of the
thermal behaviour of the gas.
In this paper, we focus on the problem of molecular cloud formation in
gravitationally unstable, initially static gas. We show that in these
conditions, and for initial densities consistent with those observed in the
cold, neutral atomic phase of the interstellar medium, H2 formation occurs on a
timescale t > 10 Myr, comparable to or longer than the gravitational free-fall
timescale of the cloud. We also show that the collapsing gas very quickly
reaches thermal equilibrium and that the equation of state of the gas is
generally softer than isothermal.
Finally, we demonstrate that although these results show little sensitivity
to variations in most of our simulation parameters, they are highly sensitive
to the assumed initial density n_i. Reducing n_i significantly increases the
cloud formation timescale and decreases the amount of hydrogen ultimately
converted to H2. (Abridged).Comment: 89 pages, 40 figures, AASTex. Results section significantly revised
and extended. Includes results from a large number of new simulations
performed using a treatment of H2 photodissociation based on ray-tracing.
This version matches that accepted by ApJ
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