1,266 research outputs found
O2 signature in thin and thick O2-H2O ices
Aims. In this paper we investigate the detectability of the molecular oxygen
in icy dust grain mantles towards astronomical objects. Methods. We present a
systematic set of experiments with O2-H2O ice mixtures designed to disentangle
how the molecular ratio affects the O2 signature in the mid- and near-infrared
spectral regions. All the experiments were conducted in a closed-cycle helium
cryostat coupled to a Fourier transform infrared spectrometer. The ice mixtures
comprise varying thicknesses from 8 10 to 3 m. The
absorption spectra of the O2-H2O mixtures are also compared to the one of pure
water. In addition, the possibility to detect the O2 in icy bodies and in the
interstellar medium is discussed. Results. We are able to see the O2 feature at
1551 cm even for the most diluted mixture of H2O : O2 = 9 : 1,
comparable to a ratio of O2/H2O = 10 % which has already been detected in situ
in the coma of the comet 67P/Churyumov-Gerasimenko. We provide an estimate for
the detection of O2 with the future mission of the James Webb Space Telescope
(JWST).Comment: 11 pages, 10 figures, article in press, to appear in A&A 201
On the evolution of the molecular line profiles induced by the propagation of C-shock waves
We present the first results of the expected variations of the molecular line
emission arising from material recently affected by C-shocks (shock
precursors). Our parametric model of the structure of C-shocks has been coupled
with a radiative transfer code to calculate the molecular excitation and line
profiles of shock tracers such as SiO, and of ion and neutral molecules such as
H13CO+ and HN13C, as the shock propagates through the unperturbed medium. Our
results show that the SiO emission arising from the early stage of the magnetic
precursor typically has very narrow line profiles slightly shifted in velocity
with respect to the ambient cloud. This narrow emission is generated in the
region where the bulk of the ion fluid has already slipped to larger velocities
in the precursor as observed toward the young L1448-mm outflow. This strongly
suggests that the detection of narrow SiO emission and of an ion enhancement in
young shocks, is produced by the magnetic precursor of C-shocks. In addition,
our model shows that the different velocity components observed toward this
outflow can be explained by the coexistence of different shocks at different
evolutionary stages, within the same beam of the single-dish observations.Comment: 7 pages, 4 figures, accepted for publication in Ap
Modeling of Immunosenescence and Risk of Death from Respiratory Infections: Evaluation of the Role of Antigenic Load and Population Heterogeneity
It is well known that efficacy of immune functions declines with age. It results in an increase of severity and duration of respiratory infections and also in dramatic growth of risk of death due to these diseases after age 65. The goal of this work is to describe and investigate the mechanism underlying the age pattern of the mortality rate caused by infectious diseases and to determine the cause-specific hazard rate as a function of immune system characteristics. For these purposes we develop a three-compartment model explaining observed risk-of-death. The model incorporates up-to-date knowledge about cellular mechanisms of aging, disease dynamics, population heterogeneity in resistance to infections, and intrinsic aging rate. The results of modeling show that the age-trajectory of mortality caused by respiratory infections may be explained by the value of antigenic load, frequency of infections and the rate of aging of the stem cell population (i.e. the population of T-lymphocyte progenitor cells). The deceleration of infection-induced mortality at advanced age can be explained by selection of individuals with a slower rate of stem cell aging. Parameter estimates derived from fitting mortality data indicate that infection burden was monotonically decreasing during the twentieth century, and changes in total antigenic load were gender-specific: it experienced periodic fluctuations in males and increased approximately two-fold in females
Deuteration as an evolutionary tracer in massive-star formation
Theory predicts, and observations confirm, that the column density ratio of a
molecule containing D to its counterpart containing H can be used as an
evolutionary tracer in the low-mass star formation process. Since it remains
unclear if the high-mass star formation process is a scaled-up version of the
low-mass one, we investigated whether the relation between deuteration and
evolution can be applied to the high-mass regime. With the IRAM-30m telescope,
we observed rotational transitions of N2D+ and N2H+ and derived the deuterated
fraction in 27 cores within massive star-forming regions understood to
represent different evolutionary stages of the massive-star formation process.
Results. Our results clearly indicate that the abundance of N2D+ is higher at
the pre-stellar/cluster stage, then drops during the formation of the
protostellar object(s) as in the low-mass regime, remaining relatively constant
during the ultra-compact HII region phase. The objects with the highest
fractional abundance of N2D+ are starless cores with properties very similar to
typical pre-stellar cores of lower mass. The abundance of N2D+ is lower in
objects with higher gas temperatures as in the low-mass case but does not seem
to depend on gas turbulence. Our results indicate that the N2D+-to-N2H+ column
density ratio can be used as an evolutionary indicator in both low- and
high-mass star formation, and that the physical conditions influencing the
abundance of deuterated species likely evolve similarly during the processes
that lead to the formation of both low- and high-mass stars.Comment: Accepted by A&AL, 4 pages, 2 figures, 2 appendices (one for Tables,
one for additional figures
Deuterium fractionation on interstellar grains studied with modified rate equations and a Monte Carlo approach
The formation of singly and doubly deuterated isotopomers of formaldehyde and
of singly, doubly, and multiply deuterated isotopomers of methanol on
interstellar grain surfaces has been studied with a semi-empirical modified
rate approach and a Monte Carlo method in the temperature range 10-20 K.
Agreement between the results of the two methods is satisfactory for all major
and many minor species throughout this range. If gas-phase fractionation can
produce a high abundance of atomic deuterium, which then accretes onto grain
surfaces, diffusive surface chemistry can produce large abundances of
deuterated species, especially at low temperatures and high gas densities.
Warming temperatures will then permit these surface species to evaporate into
the gas, where they will remain abundant for a considerable period. We
calculate that the doubly deuterated molecules CHD2OH and CH2DOD are
particularly abundant and should be searched for in the gas phase of
protostellar sources. For example, at 10 K and high density, these species can
achieve up to 10-20% of the abundance of methanol.Comment: 27 pages, 3 figures, Planetary and Space Science, in pres
Chemical differentiation in regions of high mass star formation II. Molecular multiline and dust continuum studies of selected objects
The aim of this study is to investigate systematic chemical differentiation
of molecules in regions of high mass star formation. We observed five prominent
sites of high mass star formation in HCN, HNC, HCO+, their isotopes, C18O, C34S
and some other molecular lines, for some sources both at 3 and 1.3 mm and in
continuum at 1.3 mm. Taking into account earlier obtained data for N2H+ we
derive molecular abundances and physical parameters of the sources (mass,
density, ionization fraction, etc.). The kinetic temperature is estimated from
CH3C2H observations. Then we analyze correlations between molecular abundances
and physical parameters and discuss chemical models applicable to these
species. The typical physical parameters for the sources in our sample are the
following: kinetic temperature in the range ~ 30-50 K (it is systematically
higher than that obtained from ammonia observations and is rather close to dust
temperature), masses from tens to hundreds solar masses, gas densities ~ 10^5
cm^{-3}, ionization fraction ~ 10^{-7}. In most cases the ionization fraction
slightly (a few times) increases towards the embedded YSOs. The observed clumps
are close to gravitational equilibrium. There are systematic differences in
distributions of various molecules. The abundances of CO, CS and HCN are more
or less constant. There is no sign of CO and/or CS depletion as in cold cores.
At the same time the abundances of HCO+, HNC and especially N2H+ strongly vary
in these objects. They anti-correlate with the ionization fraction and as a
result decrease towards the embedded YSOs. For N2H+ this can be explained by
dissociative recombination to be the dominant destroying process. N2H+, HCO+,
and HNC are valuable indicators of massive protostars.Comment: 15 pages, 8 figure
N2H+(1-0) survey of massive molecular cloud cores
We present the results of N2H+(1-0) observations of 35 dense molecular cloud
cores from the northern and southern hemispheres where massive stars and star
clusters are formed. Line emission has been detected in 33 sources, for 28
sources detailed maps have been obtained. The optical depth of (23-12)
component toward peak intensity positions of 10 sources is ~ 0.2-1. In total,
47 clumps have been revealed in 26 sources. Integrated intensity maps with
aspect ratios < 2 have been fitted with a power-law radial distribution
convolved with the telescope beam. Mean power-law index is close to
unity corresponding to the density profile provided N2H+
excitation conditions do not vary inside these regions. Line widths of the
cores either decrease or stay constant with distance from the center. The ratio
of rotational to gravitational energy is too low for rotation to play a
significant role in the dynamics of the cores. A correlation between mean line
widths and sizes of clumps has been found.Comment: 17 pages, Late
Gas Kinematics and Excitation in the Filamentary IRDC G035.39-00.33
Some theories of dense molecular cloud formation involve dynamical
environments driven by converging atomic flows or collisions between
preexisting molecular clouds. The determination of the dynamics and physical
conditions of the gas in clouds at the early stages of their evolution is
essential to establish the dynamical imprints of such collisions, and to infer
the processes involved in their formation. We present multi-transition 13CO and
C18O maps toward the IRDC G035.39-00.33, believed to be at the earliest stages
of evolution. The 13CO and C18O gas is distributed in three filaments
(Filaments 1, 2 and 3), where the most massive cores are preferentially found
at the intersecting regions between them. The filaments have a similar
kinematic structure with smooth velocity gradients of ~0.4-0.8 km s-1 pc-1.
Several scenarios are proposed to explain these gradients, including cloud
rotation, gas accretion along the filaments, global gravitational collapse, and
unresolved sub-filament structures. These results are complemented by HCO+,
HNC, H13CO+ and HN13C single-pointing data to search for gas infall signatures.
The 13CO and C18O gas motions are supersonic across G035.39-00.33, with the
emission showing broader linewidths toward the edges of the IRDC. This could be
due to energy dissipation at the densest regions in the cloud. The average H2
densities are ~5000-7000 cm-3, with Filaments 2 and 3 being denser and more
massive than Filament 1. The C18O data unveils three regions with high CO
depletion factors (f_D~5-12), similar to those found in massive starless cores.Comment: 20 pages, 14 figures, 6 tables, accepted for publication in MNRA
Mid-J CO Shock Tracing Observations of Infrared Dark Clouds I
Infrared dark clouds (IRDCs) are dense, molecular structures in the
interstellar medium that can harbour sites of high-mass star formation. IRDCs
contain supersonic turbulence, which is expected to generate shocks that
locally heat pockets of gas within the clouds. We present observations of the
CO J = 8-7, 9-8, and 10-9 transitions, taken with the Herschel Space
Observatory, towards four dense, starless clumps within IRDCs (C1 in
G028.37+00.07, F1 and F2 in G034.43+0007, and G2 in G034.77-0.55). We detect
the CO J = 8-7 and 9-8 transitions towards three of the clumps (C1, F1, and F2)
at intensity levels greater than expected from photodissociation region (PDR)
models. The average ratio of the 8-7 to 9-8 lines is also found to be between
1.6 and 2.6 in the three clumps with detections, significantly smaller than
expected from PDR models. These low line ratios and large line intensities
strongly suggest that the C1, F1, and F2 clumps contain a hot gas component not
accounted for by standard PDR models. Such a hot gas component could be
generated by turbulence dissipating in low velocity shocks.Comment: 14 pages, 8 figures, 5 tables, accepted by A&A, minor updates to
match the final published versio
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