479 research outputs found
The Production of HI in Photodissociation Regions and A Comparison with CO(1-0) Emission
The gas at the surfaces of molecular clouds in galaxies is heated and
dissociated by photons from young stars both near and far. HI resulting from
the dissociation of molecular hydrogen H2 emits hyperfine line emission at 21
cm, and warmed CO emits dipole rotational lines such as the 2.6 mm line of
CO(1-0). We use previously developed models for photodissociation regions
(PDRs) to compute the intensities of these HI and CO(1-0) lines as a function
of the total volume density n in the cloud and the far ultraviolet flux G0
incident upon it and present the results in units familiar to observers. The
intensities of these two lines behave differently with changing physical
conditions in the PDR, and, taken together, the two lines can provide a
ground-based radio astronomy diagnostic for determining n and G0 separately in
distant molecular clouds. This diagnostic is particularly useful in the range
Gzero <~ 100, 10 cm^{-3} <~ n <~ 10^5 cm^{-3}, which applies to a large
fraction of the volume of the interstellar medium in galaxies. If the molecular
cloud is located near discrete sources of far-UV (FUV) emission, the
PDR-generated HI and CO(1-0) emission on the cloud surface can be more easily
identified, appearing as layered ``blankets'' or ``blisters'' on the side of
the cloud nearest to the FUV source. As an illustration, we consider the
Galactic object G216 -2.5, i.e. ``Maddalena's Cloud'', which has been
previously identified as a large PDR in the Galaxy. We determine that this
cloud has n ~ 200 cm^{-3}, G0 ~ 0.8, consistent with other data.Comment: 13 Pages, 3 Figures. Accepted for publication in the Astrophysical
Journa
Formation of Narrow Dust Rings in Circumstellar Debris Disks
Narrow dust rings observed around some young stars (e.g., HR 4796A) need to
be confined. We present a possible explanation for the formation and
confinement of such rings in optically thin circumstellar disks, without
invoking shepherding planets. If an enhancement of dust grains (e.g., due to a
catastrophic collision) occurs somewhere in the disk, photoelectric emission
from the grains can heat the gas to temperatures well above that of the dust.
The gas orbits with super(sub)-Keplerian speeds inward (outward) of the
associated pressure maximum. This tends to concentrate the grains into a narrow
region. The rise in dust density leads to further heating and a stronger
concentration of grains. A narrow dust ring forms as a result of this
instability. We show that this mechanism not only operates around early-type
stars that have high UV fluxes, but also around stars with spectral types as
late as K. This implies that this process is generic and may have occurred
during the lifetime of each circumstellar disk. We examine the stringent
upper-limit on the H2 column density in the HR 4796A disk and find it to be
compatible with the presence of a significant amount of hydrogen gas in the
disk. We also compute the OI and CII infrared line fluxes expected from various
debris disks and show that these will be easily detectable by the upcoming
Herschel mission. Herschel will be instrumental in detecting and characterizing
gas in these disks.Comment: Accepted for publication in ApJ; 14 pages, 7 figure
Morphological Properties of PPNs: Mid-IR and HST Imaging Surveys
We will review our mid-infrared and HST imaging surveys of the circumstellar
dust shells of proto-planetary nebulae. While optical imaging indirectly probes
the dust distribution via dust-scattered starlight, mid-IR imaging directly
maps the distribution of warm dust grains. Both imaging surveys revealed
preferencially axisymmetric nature of PPN dust shells, suggesting that
axisymmetry in planetary nebulae sets in by the end of the asymptotic giant
branch phase, most likely by axisymmetric superwind mass loss. Moreover, both
surveys yielded two morphological classes which have one-to-one correspondence
between the two surveys, indicating that the optical depth of circumstellar
dust shells plays an equally important role as the inclination angle in
determining the morphology of the PPN shells.Comment: 6 pages + 8 figures, to appear in the proceedings of the conference,
"Post-AGB Objects (proto-planetary nebulae) as a Phase of Stellar Evolution",
Torun, Poland, July 5-7, 2000, eds. R. Szczerba, R. Tylenda, and S.K. Gorny.
Figures have been degraded to minimize the total file siz
86 GHz SiO maser survey of late-type stars in the Inner Galaxy. IV. SiO emission and infrared data for sources in the Scutum and Sagittarius-Carina arms, 20 deg < l < 50 deg
We present an 86 GHz SiO (v = 1, J = 2 ---> 1) maser search toward late-type
stars located within |b|<0.5 deg and 20 deg < l < 50 deg. This search is an
extension at longer longitudes of a previously published work. We selected 135
stars from the MSX catalog using color and flux criteria and detected 92 (86
new detections). The detection rate is 68%, the same as in our previous study.
The last few decades have seen the publication of several catalogs of point
sources detected in infrared surveys (MSX, 2MASS, DENIS, ISOGAL, WISE, GLIMPSE,
AKARI, and MIPSGAL). We searched each catalog for data on the 444 targets of
our earlier survey and for the 135 in the survey reported here. We confirm
that, as anticipated, most of our targets have colors typical of oxygen-rich
asymptotic giant branch (AGB) stars. Only one target star may have already left
the AGB. Ten stars have colors typical of carbon-rich stars, meaning a
contamination of our sample with carbon stars <=1.7%.Comment: 13 pages, 6 Figures, A&A accepte
An Attempt to Detect the Galactic Bulge at 12 microns with IRAS
Surface brightness maps at 12 microns, derived from observations with the
Infrared Astronomical Satellite (IRAS), are used to estimate the integrated
flux at this wavelength from the Galactic bulge as a function of galactic
latitude along the minor axis. A simple model was used to remove Galactic disk
emission (e.g. unresolved stars and dust) from the IRAS measurements. The
resulting estimates are compared with predictions for the 12 micron bulge
surface brightness based on observations of complete samples of optically
identified M giants in several minor axis bulge fields. No evidence is found
for any significant component of 12m emission in the bulge other than that
expected from the optically identified M star sample plus normal, lower
luminosity stars. Known large amplitude variables and point sources from the
IRAS catalogue contribute only a small fraction to the total 12 micron flux.Comment: Accepted for publication in ApJ; 13 pages of text including tables in
MS WORD97 generated postscript; 3 figures in postscript by Sigma Plo
CO in OH/IR stars close to the Galactic centre
Aims: A pilot project has been carried out to measure circumstellar CO
emission from three OH/IR stars close to the Galactic centre. The intention was
to find out whether it would be possible to conduct a large-scale survey for
mass-loss rates using, for example, the Atacama Large Millimeter Array (ALMA).
Such a survey would increase our understanding of the evolution of the Galactic
bulge.
Methods: Two millimetre-wave instruments were used: the Nobeyama Millimeter
Array at 115 GHz and the Submillimeter Array at 230 GHz. An interferometer is
necessary as a `spatial filter' in this region of space because of the
confusion with interstellar CO emission.
Results: Towards two of the stars, CO emission was detected with positions
and radial velocities coinciding within the statistical errors with the
corresponding data of the associated OH sources. However, for one of the stars
the line profile is not what one expects for an unresolved expanding
circumstellar envelope. We believe that this CO envelope is partially resolved
and that this star therefore is a foreground star not belonging to the bulge.
Conclusions: The results of the observations have shown that it is possible
to detect line profiles of circumstellar CO from late-type stars both within
and in the direction of the Galactic bulge. ALMA will be able to detect CO
emission in short integrations with sensitivity sufficient to estimate
mass-loss rates from a large number of such stars.Comment: 5 pages, 3 figure
Formation of Massive Primordial Stars in a Reionized Gas
We use cosmological hydrodynamic simulations with unprecedented resolution to
study the formation of primordial stars in an ionized gas at high redshifts.
Our approach includes all the relevant atomic and molecular physics to follow
the thermal evolution of a prestellar gas cloud to very high densities of
~10^{18} cm^{-3}. We locate a star-forming gas cloud within a reionized region
in our cosmological simulation. The first run-away collapse is triggered when
the gas cloud's mass is ~40 Msun. We show that the cloud core remains stable
against chemo-thermal instability and also against gravitational deformation
throughout its evolution. Consequently, a single proto-stellar seed is formed,
which accretes the surrounding hot gas at the rate ~10^{-3} Msun/year. We carry
out proto-stellar evolution calculations using the inferred accretion rate. The
resulting mass of the star when it reaches the zero-age main sequence is M_ZAMS
~40 Msun. We argue that, since the obtained M_ZAMS is as large as the mass of
the collapsing parent cloud, the final stellar mass should be close to this
value. Such massive, rather than exceptionally massive, primordial stars are
expected to cause early chemical enrichment of the Universe by exploding as
black hole-forming super/hypernovae, and may also be progenitors of high
redshift gamma-ray bursts. The elemental abundance patterns of recently
discovered hyper metal-poor stars suggest that they might have been born from
the interstellar medium that was metal-enriched by supernovae of these massive
primordial stars.Comment: Revised version. To appear in ApJ
H_2 Absorption and Fluorescence for Gamma Ray Bursts in Molecular Clouds
If a gamma ray burst with strong UV emission occurs in a molecular cloud,
there will be observable consequences resulting from excitation of the
surrounding H2. The UV pulse from the GRB will pump H2 into
vibrationally-excited levels which produce strong absorption at wavelengths <
1650 A. As a result, both the prompt flash and later afterglow will exhibit
strong absorption shortward of 1650 A, with specific spectroscopic features.
Such a cutoff in the emission from GRB 980329 may already have been observed by
Fruchter et al.; if so, GRB 980329 was at redshift 3.0 < z < 4.4 . BVRI
photometry of GRB 990510 could also be explained by H2 absorption if GRB 990510
is at redshift 1.6 < z < 2.3. The fluorescence accompanying the UV pumping of
the H2 will result in UV emission from the GRB which can extend over days or
months, depending on parameters of the ambient medium and beaming of the GRB
flash. The 7.5-13.6 eV fluorescent luminosity is \sim 10^{41.7} erg/s for
standard estimates of the parameters of the GRB and the ambient medium.
Spectroscopy can distinguish this fluorescent emission from other possible
sources of transient optical emission, such as a supernova.Comment: 13 pages, including 4 figures. submitted to Ap.J.(Letters
Why are massive O-rich AGB stars in our Galaxy not S-stars?
We present the main results derived from a chemical analysis carried out on a
large sample of galactic O-rich AGB stars using high resolution optical
spectroscopy (R~40,000-50,000) with the intention of studying their lithium
abundances and/or possible s-process element enrichment. Our chemical analysis
shows that some stars are lithium overabundant while others are not. The
observed lithium overabundances are interpreted as a clear signature of the
activation of the so-called ``Hot Bottom Burning'' (HBB) process in massive
galactic O-rich AGB stars, as predicted by the models. However, these stars do
not show the zirconium enhancement (taken as a representative for the s-process
element enrichment) associated to the third dredge-up phase following thermal
pulses. Our results suggest that the more massive O-rich AGB stars in our
Galaxy behave differently from those in the Magellanic Clouds, which are both
Li- and s-process-rich (S-type stars). Reasons for this unexpected result are
discussed. We conclude that metallicity is probably the main responsible for
the differences observed and suggest that it may play a more important role
than generally assumed in the chemical evolution of AGB stars.Comment: 4 pages, 2 figures, to appear in the proceedings of the conference
"Planetary Nebulae as astronomical tools" held in Gdansk, Poland, jun 28/jul
02, 200
Infrared astronomical satellite (IRAS) catalogs and atlases. Volume 1: Explanatory supplement
The Infrared Astronomical Satellite (IRAS) was launched on January 26, 1983. During its 300-day mission, IRAS surveyed over 96 pct of the celestial sphere at four infrared wavelengths, centered approximately at 12, 25, 60, and 100 microns. Volume 1 describes the instrument, the mission, and data reduction
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