833 research outputs found
A survey of interstellar HI from L alpha absorption measurements 2
The Copernicus satellite surveyed the spectral region near L alpha to obtain column densities of interstellar HI toward 100 stars. The distance to 10 stars exceeds 2 kpc and 34 stars lie beyond 1 kpc. Stars with color excess E(B-V) up to 0.5 mag are observed. The value of the mean ratio of total neutral hydrogen to color excess was found to equal 5.8 x 10 to the 21st power atoms per (sq cm x mag). For stars with accurate E(B-V), the deviations from this mean are generally less than a factor of 1.5. A notable exception is the dark cloud star, rho Oph. A reduction in visual reddening efficiency for the grains that are larger than normal in the rho Oph dark cloud probably explains this result. The conversion of atomic hydrogen into molecular form in dense clouds was observed in the gas to E(B-V) correlation plots. The best estimate for the mean total gas density for clouds and the intercloud medium, as a whole, in the solar neighborhood and in the plane of the galaxy is 1.15 atoms per cu. cm; those for the atomic gas and molecular gas alone are 0.86 atoms per cu cm and 0.143 molecules per cu cm respectively. For the intercloud medium, where molecular hydrogen is a negligible fraction of the total gas, atomic gas density was found to equal 0.16 atoms per cu cm with a Gaussian scale height perpendicular to the plane of about 350 pc, as derived from high latitude stars
Effective area calibration of the reflection grating spectrometers of XMM-Newton. II. X-ray spectroscopy of DA white dwarfs
White dwarf spectra have been widely used as a calibration source for X-ray
and EUV instruments. The in-flight effective area calibration of the reflection
grating spectrometers (RGS) of XMM-Newton depend upon the availability of
reliable calibration sources. We investigate how well these white dwarf spectra
can be used as standard candles at the lowest X-ray energies in order to gauge
the absolute effective area scale of X-ray instruments. We calculate a grid of
model atmospheres for Sirius B and HZ 43A, and adjust the parameters using
several constraints until the ratio of the spectra of both stars agrees with
the ratio as observed by the low energy transmission grating spectrometer
(LETGS) of Chandra. This ratio is independent of any errors in the effective
area of the LETGS. We find that we can constrain the absolute X-ray spectrum of
both stars with better than 5 % accuracy. The best-fit model for both stars is
close to a pure hydrogen atmosphere, and we put tight limits to the amount of
helium or the thickness of a hydrogen layer in both stars. Our upper limit to
the helium abundance in Sirius B is 4 times below the previous detection based
on EUVE data. We also find that our results are sensitive to the adopted
cut-off in the Lyman pseudo-continuum opacity in Sirius B. We get best
agreement with a long wavelength cut-off. White dwarf model atmospheres can be
used to derive the effective area of X-ray spectrometers in the lowest energy
band. An accuracy of 3-4 % in the absolute effective area can be achieved.Comment: 15 pages, 7 figures, accepted for publication in Astronomy &
Astrophysics, main journa
Searching for dark clouds in the outer galactic plane I -- A statistical approach for identifying extended red(dened) regions in 2MASS
[Abridged] Though the exact role of infrared dark clouds in the formation
process is still somewhat unclear, they seem to provide useful laboratories to
study the very early stages of clustered star formation. Infrared dark clouds
have been identified predominantly toward the bright inner parts of the
galactic plane. The low background emission makes it more difficult to identify
similar objects in mid-infrared absorption in the outer parts. This is
unfortunate, because the outer Galaxy represents the only nearby region where
we can study effects of different (external) conditions on the star formation
process. The aim of this paper is to identify extended red regions in the outer
galactic plane based on reddening of stars in the near-infrared. We argue that
these regions appear reddened mainly due to extinction caused by molecular
clouds and young stellar objects. The work presented here is used as a basis
for identifying star forming regions and in particular the very early stages.
We use the Mann-Whitney U-test, in combination with a friends-of-friends
algorithm, to identify extended reddened regions in the 2MASS all-sky JHK
survey. We process the data on a regular grid using two different resolutions,
60" and 90". The two resolutions have been chosen because the stellar surface
density varies between the crowded spiral arm regions and the sparsely
populated galactic anti-center region. We identify 1320 extended red regions at
the higher resolution and 1589 at the lower resolution run. The majority of
regions are associated with major molecular cloud complexes, supporting our
hypothesis that the reddening is mostly due to foreground clouds and embedded
objects.Comment: Accepted for publication in A&A -- 9 pages, 5 figures (+ on-line only
tables
A New Probe of the Molecular Gas in Galaxies: Application to M101
Recent studies of nearby spiral galaxies suggest that photodissociation
regions (PDRs) are capable of producing much of the observed HI in galaxy
disks. In that case, measurements of the HI column density and the
far-ultraviolet (FUV) photon flux provide a new probe of the volume density of
the local underlying H_2. We develop the method and apply it to the giant Scd
spiral M101 (NGC 5457). We find that, after correction for the best-estimate
gradient of metallicity in the ISM of M101 and for the extinction of the
ultraviolet emission, molecular gas with a narrow range of density from 30-1000
cm^-3 is found near star- forming regions at all radii in the disk of M101 out
to a distance of 12' (approximately 26 kpc), close to the photometric limit of
R_25 = 13.5'.
In this picture, the ISM is virtually all molecular in the inner parts of
M101. The strong decrease of the HI column density in the inner disk of the
galaxy at R_G < 10 kpc is a consequence of a strong increase in the dust-to-gas
ratio there, resulting in an increase of the H_2 formation rate on grains and a
corresponding disappearance of hydrogen in its atomic form.Comment: accepted for publication in The Astrophysical Journal (1 August
2000); 29 pages including 20 figures (7 gif); AAS LaTex; contact authors for
full resolution versions of gif figure
On the fidelity of the core mass functions derived from dust column density data
Aims: We examine the recoverability and completeness limits of the dense core
mass functions (CMFs) derived for a molecular cloud using extinction data and a
core identification scheme based on two-dimensional thresholding.
Methods: We performed simulations where a population of artificial cores was
embedded into the variable background extinction field of the Pipe nebula. We
extracted the cores from the simulated extinction maps, constructed the CMFs,
and compared them to the input CMFs. The simulations were repeated using a
variety of extraction parameters and several core populations with differing
input mass functions and differing degrees of crowding.
Results: The fidelity of the observed CMF depends on the parameters selected
for the core extraction algorithm for our background. More importantly, it
depends on how crowded the core population is. We find that the observed CMF
recovers the true CMF reliably when the mean separation of cores is larger than
their mean diameter (f>1). If this condition holds, the derived CMF is accurate
and complete above M > 0.8-1.5 Msun, depending on the parameters used for the
core extraction. In the simulations, the best fidelity was achieved with the
detection threshold of 1 or 2 times the rms-noise of the extinction data, and
with the contour level spacings of 3 times the rms-noise. Choosing larger
threshold and wider level spacings increases the limiting mass. The simulations
show that when f>1.5, the masses of individual cores are recovered with a
typical uncertainty of 25-30 %. When f=1 the uncertainty is ~60 %. In very
crowded cases where f<1 the core identification algorithm is unable to recover
the masses of the cores adequately. For the cores of the Pipe nebula f~2.0 and
therefore the use of the method in that region is justified.Comment: 9 pages, 6 figures, accepted for publication in A&
The White Dwarf Distance to the Globular Cluster 47 Tucanae and its Age
We present a new determination of the distance (and age) of the Galactic
globular cluster 47 Tucanae (NGC 104) based on the fit of its white dwarf (WD)
cooling sequence with the empirical fiducial sequence of local WD with known
trigonometric parallax, following the method described in Renzini et al.
(1996). Both the cluster and the local WDs were imaged with HST+WFPC2 using the
same instrument setup. We obtained an apparent distance modulus of
consistent with previous ground-based determinations and
shorter than that found using HIPPARCOS subdwarfs. Coupling our distance
determination with a new measure of the apparent magnitude of the main sequence
turnoff, based on our HST data, we derive an age of Gyr.Comment: Accepted for publication on the Astrophysical Journa
Molecular clouds under the influence of massive stars in the Galactic HII region G353.2+0.9
The Galactic HII region G353.2+0.9 is excited by the massive open cluster
Pismis-24. By analyzing (sub-)mm molecular-line and -continuum we study the
detailed morphology of the gas and dust, as well as their physical parameters
and their variation across the PDR. We observed various molecules and
transitions to derive the physical properties of the molecular gas through line
ratios, and both LTE and non-LTE analyses. The physical properties of the gas
were derived with a Bayesian approach for the non-LTE analysis. Based on the
continuum data at 870 micron, we derived the dust mass and the column density
of H2, and thus the molecular abundances. The total mass of the gas in the
region is ca. 2000 Mo, while that of the dust is ca. 21 Mo. A velocity gradient
in the region suggests that the expansion of the ionized gas is pushing the
molecular gas away from the observer. We unambiguously identify the ionization
front, at the location of which we detect an increase in gas density and
temperature. We find at least 14 clumps at different positions and LSR
velocities. We derive kinetic temperatures in the ranges 11-45 K (CS) and 20-45
K (CN). The H2 number density is typically around 1e5 cm^-3 from CS and few 1e5
cm^-3 from CN, with maxima above 1e6 cm^-3. The abundances of the molecules
observed vary across the region, and appear to be higher in regions further
away from the ionization front.Comment: 14 pages, 11 figures. Accepted for publication in Astronomy and
Astrophysic
Far-Ultraviolet Color Gradients in Early-Type Galaxies
We discuss far-UV (1500 A) surface photometry and FUV-B color profiles for 8
E/S0 galaxies from images taken with the Ultraviolet Imaging Telescope,
primarily during the Astro-2 mission. In three cases, the FUV radial profiles
are more consistent with an exponential than a de Vaucouleurs function, but
there is no other evidence for the presence of a disk or of young, massive
stars. In all cases except M32 the FUV-B color becomes redder at larger radii.
There is a wide range of internal radial FUV-B color gradients. However, we
find no correlation between the FUV-B color gradients and internal metallicity
gradients based on Mg absorption features. We conclude that metallicity is not
the sole parameter controlling the "UV upturn component" in old populations.Comment: 11 pages; tar.gz file includes LaTeX text file, 3 PostScript figures.
Paper to be published in ApJ Letter
The Hubble Legacy Archive NICMOS Grism Data
The Hubble Legacy Archive (HLA) aims to create calibrated science data from
the Hubble Space Telescope archive and make them accessible via user-friendly
and Virtual Observatory (VO) compatible interfaces. It is a collaboration
between the Space Telescope Science Institute (STScI), the Canadian Astronomy
Data Centre (CADC) and the Space Telescope - European Coordinating Facility
(ST-ECF). Data produced by the Hubble Space Telescope (HST) instruments with
slitless spectroscopy modes are among the most difficult to extract and
exploit. As part of the HLA project, the ST-ECF aims to provide calibrated
spectra for objects observed with these HST slitless modes. In this paper, we
present the HLA NICMOS G141 grism spectra. We describe in detail the
calibration, data reduction and spectrum extraction methods used to produce the
extracted spectra. The quality of the extracted spectra and associated direct
images is demonstrated through comparison with near-IR imaging catalogues and
existing near-IR spectroscopy. The output data products and their associated
metadata are publicly available through a web form at http://hla.stecf.org and
via VO interfaces. In total, 2470 spectra of 1923 unique targets are included
in the current release.Comment: 18 pages, 21 figures, accepted for publication in Astronomy &
Astrophysic
Evidence for the Large-Scale Dissociation of Molecular Gas in the Inner Spiral Arms of M81
We compare the detailed distributions of HI, H alpha, and 150 nm far-UV
continuum emission in the spiral arms of M81 at a resolution of 9" (linear
resolution 150 pc at 3.7 Mpc distance). The bright H alpha emission peaks are
always associated with peaks in the far-UV emission. The converse is not always
true; there are many regions of far-UV emission with little corresponding H
alpha. The HI and the far-UV are always closely associated, in the sense that
the HI is often brightest around the edges of the far-UV emission. The effects
of extinction on the morphology are small, even in the far-UV. Extensive far-UV
emission, often with little corresponding H alpha, indicates the presence of
many ``B-stars'', which produce mostly non-ionizing UV photons. These far-UV
photons dissociate a small fraction of an extensive layer of H_2 into HI. The
observed morphology can be understood if ``chimneys'' are common in the spiral
arms of M81, where holes are blown out of the galactic disk, exposing the
bright HII regions and the corresponding far-UV associated with vigorous star
formation. These ``naked'' star-forming regions show little obscuration. H_2 is
turned into HI by UV photons impinging on the interior surfaces of these
chimneys. The intensity of the far-UV radiation measured by UIT can dissociate
the underlying H_2 with a typical density of ~10 H nucleii cm**-3 to produce
the observed amount of HI in the spiral arms of M81. Except for thin surface
layers locally heated in these photo-dissociation regions close to the far-UV
sources, the bulk of the molecular gas in the inner disk of M81 is apparently
too cold to produce much 12CO(1-0) emission.Comment: 12 pages, Latex. 8 postscript files. Better quality versions of the
figures available from ftp://star.herts.ac.uk/pub/Knapen/m81uv . Accepted,
Ap
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