475 research outputs found
The Polytropic Equation of State of Interstellar Gas Clouds
Models are presented for the polytropic equation of state of
self-gravitating, quiescent interstellar gas clouds. A detailed analysis,
including chemistry, thermal balance, and radiative transfer, is performed for
the physical state of the gas as a function of density, metallicity, velocity
field, and background radiation field. It is found that the stiffness of the
equation of state strongly depends on all these physical parameters, and the
adiabatic index varies between 0.2-1.4. The implications for star formation, in
particular at high redshift and in starburst galaxies, and the initial stellar
mass function are discussed.Comment: Accepted by Ap
L-band ATS 5/Orion/S. S. Manhattan marine navigation and communication experiment Final report
L-band signals relayed by synchronous satellite for navigation and data communicatio
Far-Infrared and Sub-Millimeter Observations and Physical Models of the Reflection Nebula Ced 201
ISO [C II] 158 micron, [O I] 63 micron, and H_2 9 and 17 micron observations
are presented of the reflection nebula Ced 201, which is a photon-dominated
region illuminated by a B9.5 star with a color temperature of 10,000 K (a cool
PDR). In combination with ground based [C I] 609 micron, CO, 13CO, CS and HCO+
data, the carbon budget and physical structure of the reflection nebula are
constrained. The obtained data set is the first one to contain all important
cooling lines of a cool PDR, and allows a comparison to be made with classical
PDRs. To this effect one- and three-dimensional PDR models are presented which
incorporate the physical characteristics of the source, and are aimed at
understanding the dominant heating processes of the cloud. The contribution of
very small grains to the photo-electric heating rate is estimated from these
models and used to constrain the total abundance of PAHs and small grains.
Observations of the pure rotational H_2 lines with ISO, in particular the S(3)
line, indicate the presence of a small amount of very warm, approximately 330
K, molecular gas. This gas cannot be accommodated by the presented models.Comment: 32 pages, 7 figures, in LaTeX. To be published in Ap
Search for Interstellar Water in the Translucent Molecular Cloud toward HD 154368
We report an upper limit of 9 x 10^{12} cm-2 on the column density of water
in the translucent cloud along the line of sight toward HD 154368. This result
is based upon a search for the C-X band of water near 1240 \AA carried out
using the Goddard High Resolution Spectrograph of the Hubble Space Telescope.
Our observational limit on the water abundance together with detailed chemical
models of translucent clouds and previous measurements of OH along the line of
sight constrain the branching ratio in the dissociative recombination of H_3O+
to form water. We find at the level that no more than 30% of
dissociative recombinations of H_3O+ can lead to H_2O. The observed spectrum
also yielded high-resolution observations of the Mg II doublet at 1239.9 \AA
and 1240.4 \AA, allowing the velocity structure of the dominant ionization
state of magnesium to be studied along the line of sight. The Mg II spectrum is
consistent with GHRS observations at lower spectral resolution that were
obtained previously but allow an additional velocity component to be
identified.Comment: Accepted by ApJ, uses aasp
Models for Dusty Lyman alpha Emitters at High Redshift
Models are presented for the Lyman alpha emission of dusty high-redshift
galaxies by combining the Press-Schechter formalism with a treatment of the
inhomogeneous dust distribution inside galaxies. It is found that the amount of
Lyman alpha radiation escaping from the galaxies strongly depends on the time
over which the dust is produced through stellar activity, and on the ambient
inhomogeneity of the HII regions that surround the ionizing OB stars. Good
agreement is found with recent observations, as well as previous
non-detections. Our models indicate that the dust content builds up in no more
than approximately 5x10^8 yr, the galactic HII regions are inhomogeneous with a
cloud covering factor of order unity, and the overall star formation efficiency
is at least about 5%. It is predicted that future observations can detect these
Lyman alpha galaxies upto redshifts of about 8.Comment: 16 pages, 4 figures, submitted to Ap
Search for grain growth towards the center of L1544
In dense and cold molecular clouds dust grains are surrounded by thick icy
mantles. It is however not clear if dust growth and coagulation take place
before the switch-on of a protostar. This is an important issue, as the
presence of large grains may affect the chemical structure of dense cloud
cores, including the dynamically important ionization fraction, and the future
evolution of solids in protoplanetary disks. To study this further, we focus on
L1544, one of the most centrally concentrated pre-stellar cores on the verge of
star formation, and with a well-known physical structure. We observed L1544 at
1.2 and 2 mm using NIKA, a new receiver at the IRAM 30 m telescope, and we used
data from the Herschel Space Observatory archive. We find no evidence of grain
growth towards the center of L1544 at the available angular resolution.
Therefore, we conclude that single dish observations do not allow us to
investigate grain growth toward the pre-stellar core L1544 and high sensitivity
interferometer observations are needed. We predict that dust grains can grow to
200 m in size toward the central ~300 au of L1544. This will imply a dust
opacity change by a factor of ~2.5 at 1.2 mm, which can be detected using the
Atacama Large Millimeter and submillimeter Array (ALMA) at different
wavelengths and with an angular resolution of 2".Comment: 12 pages, 14 figures. Accepted for publication in A&
Fysische karakterisering van compactie als gevolg van vegetatieverandering in twee ontbossingssequenties in Costa Rica
The Abundance and Emission of H2O and O2 in Clumpy Molecular Clouds
Recent observations with the Submillimeter Wave Astronomy Satellite indicate
abundances of gaseous H2O and O2 in dense molecular clouds which are
significantly lower than found in standard homogeneous chemistry models. We
present here results for the thermal and chemical balance of inhomogeneous
molecular clouds exposed to ultraviolet radiation in which the abundances of
H2O and O2 are computed for various density distributions, radiation field
strengths and geometries. It is found that an inhomogeneous density
distribution lowers the column densities of H2O and O2 compared to the
homogeneous case by more than an order of magnitude at the same A_V. O2 is
particularly sensitive to the penetrating ultraviolet radiation, more so than
H2O. The S140 and rho Oph clouds are studied as relevant test cases of
star-forming and quiescent regions. The SWAS results of S140 can be
accommodated naturally in a clumpy model with mean density of 2x10^3 cm-3 and
enhancement I_UV=140 compared with the average interstellar radiation field, in
agreement with observations of [CI] and 13CO of this cloud. Additional
radiative transfer computations suggest that this diffuse H2O component is
warm, ~60-90 K, and can account for the bulk of the 1_10-1_01 line emission
observed by SWAS. The rho Oph model yields consistent O2 abundances but too
much H2O, even for [C]/[O]=0.94, if I_UV<10 respectively <40 for a mean density
of 10^3 respectively 10^4 cm-3. It is concluded that enhanced photodissociation
in clumpy regions can explain the low H2O and O2 abundances and emissivities
found in the large SWAS beam for extended molecular clouds, but that additional
freeze-out of oxygen onto grains is needed in dense cold cores.Comment: To be published in ApJ
Pure species within a continuum of genetic and morphological variation:Sympatric oaks at the edge of their range
Background and Aims Studies on oaks (Quercus spp.) have often been hampered by taxonomic confusion, a situation further compounded by the occurrence of extensive interspecific hybridization. In the present study, a combination of genetic and morphological analyses was used to examine sympatric populations of Q. petraea and Q. robur at the north-western edge of their ranges in Northern Ireland, since it had previously been suggested that hybridization could facilitate the apparent rapid, long-distance dispersal of oaks following the glaciations. Methods Samples were collected from 24 sites across Northern Ireland that had been previously designated as ancient or semi-natural woodland. Genotypes were obtained from a total of 950 trees using 12 nuclear microsatellite loci, and admixture coefficients were calculated based on a Bayesian clustering approach. Individuals were also classified as Q. petraea, Q. robur or hybrids based on two objective morphometric characters shown previously to delineate pure individuals effectively. Genetically ‘pure’ individuals of both species, as defined by the Bayesian clustering, were also genotyped for five chloroplast microsatellites. Key Results Genetic and morphological analyses both indicated the presence of pure individuals of both species, as well as a continuum of intermediates. There was a good agreement between the molecular and morphological classification, with a generally clear separation between pure individuals. Conclusions Despite millennia of hybridization and introgression, genetically and morphologically pure individuals of both Q. petraea and Q. robur can be found at the edge of their range, where both species occur sympatrically. The high proportion of individuals exhibiting introgression compared with previous studies may reflect the historical role of hybridization in facilitating dispersal following the glaciations. This is further supported by the significantly higher chloroplast diversity in Q. robur compared with Q. petraea
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