241 research outputs found
Reactions forming C(0,+)n=2,10, Cn=2,4H(0,+) and C3H(0,+) in the gas phase: semi empirical branching ratios
The aim of this paper is to provide a new set of branching ratios for
interstellar and planetary chemical networks based on a semi empirical model.
We applied, instead of zero order theory (i.e. only the most exoergic decaying
channel is considered), a statistical microcanonical model based on the
construction of breakdown curves and using experimental high velocity collision
branching ratios for their parametriza- tion. We applied the model to
ion-molecule, neutral-neutral, and ion-pair reactions implemented in the few
popular databases for astrochemistry such as KIDA, OSU and UMIST. We studied
the reactions of carbon and hydrocarbon species with electrons, He+, H+, CH+,
CH, C, and C+ leading to intermediate complexes of the type Cn=2,10, Cn=2,4 H,
C3 H2, C+n=2,10, Cn=2,4 H+, or C3 H+2 . Comparison of predictions with
measurements supports the validity of the model. Huge deviations with respect
to database values are often obtained. Effects of the new branching ratios in
time dependant chemistry for dark clouds and for photodissociation region
chemistry with conditions similar to those found in the Horsehead Nebula are
discussed
The IRAM-30m line survey of the Horsehead PDR: III. High abundance of complex (iso-)nitrile molecules in UV-illuminated gas
Complex (iso-)nitrile molecules, such as CH3CN and HC3N, are relatively
easily detected in our Galaxy and in other galaxies. We constrain their
chemistry through observations of two positions in the Horsehead edge: the
photo-dissociation region (PDR) and the dense, cold, and UV-shielded core just
behind it. We systematically searched for lines of CH3CN, HC3N, C3N, and some
of their isomers in our sensitive unbiased line survey at 3, 2, and 1mm. We
derived column densities and abundances through Bayesian analysis using a large
velocity gradient radiative transfer model. We report the first clear detection
of CH3NC at millimeter wavelength. We detected 17 lines of CH3CN at the PDR and
6 at the dense core position, and we resolved its hyperfine structure for 3
lines. We detected 4 lines of HC3N, and C3N is clearly detected at the PDR
position. We computed new electron collisional rate coefficients for CH3CN, and
we found that including electron excitation reduces the derived column density
by 40% at the PDR position. While CH3CN is 30 times more abundant in the PDR
than in the dense core, HC3N has similar abundance at both positions. The
isomeric ratio CH3NC/CH3CN is 0.15+-0.02. In the case of CH3CN, pure gas phase
chemistry cannot reproduce the amount of CH3CN observed in the UV-illuminated
gas. We propose that CH3CN gas phase abundance is enhanced when ice mantles of
grains are destroyed through photo-desorption or thermal-evaporation in PDRs,
and through sputtering in shocks. (abridged)Comment: Accepted for publication in Astronomy & Astrophysic
Dense molecular globulettes and the dust arc towards the runaway O star AE Aur (HD 34078)
Some runaway stars are known to display IR arc-like structures around them,
resulting from their interaction with surrounding interstellar material. The
properties of these features as well as the processes involved in their
formation are still poorly understood. We aim at understanding the physical
mechanisms that shapes the dust arc observed near the runaway O star AEAur
(HD34078). We obtained and analyzed a high spatial resolution map of the
CO(1-0) emission that is centered on HD34078, and that combines data from both
the IRAM interferometer and 30m single-dish antenna. The line of sight towards
HD34078 intersects the outer part of one of the detected globulettes, which
accounts for both the properties of diffuse UV light observed in the field and
the numerous molecular absorption lines detected in HD34078's spectra,
including those from highly excited H2 . Their modeled distance from the star
is compatible with the fact that they lie on the 3D paraboloid which fits the
arc detected in the 24 {\mu}m Spitzer image. Four other compact CO globulettes
are detected in the mapped area. These globulettes have a high density and
linewidth, and are strongly pressure-confined or transient. The good spatial
correlation between the CO globulettes and the IR arc suggests that they result
from the interaction of the radiation and wind emitted by HD 34078 with the
ambient gas. However, the details of this interaction remain unclear. A wind
mass loss rate significantly larger than the value inferred from UV lines is
favored by the large IR arc size, but does not easily explain the low velocity
of the CO globulettes. The effect of radiation pressure on dust grains also
meets several issues in explaining the observations. Further observational and
theoretical work is needed to fully elucidate the processes shaping the gas and
dust in bow shocks around runaway O stars. (Abridged)Comment: Accepted for publication in Astronomy & Astrophysic
The hyperfine structure in the rotational spectrum of CF+
Context. CF+ has recently been detected in the Horsehead and Orion Bar
photo-dissociation regions. The J=1-0 line in the Horsehead is double-peaked in
contrast to other millimeter lines. The origin of this double-peak profile may
be kinematic or spectroscopic. Aims. We investigate the effect of hyperfine
interactions due to the fluorine nucleus in CF+ on the rotational transitions.
Methods. We compute the fluorine spin rotation constant of CF+ using high-level
quantum chemical methods and determine the relative positions and intensities
of each hyperfine component. This information is used to fit the theoretical
hyperfine components to the observed CF+ line profiles, thereby employing the
hyperfine fitting method in GILDAS. Results. The fluorine spin rotation
constant of CF+ is 229.2 kHz. This way, the double-peaked CF+ line profiles are
well fitted by the hyperfine components predicted by the calculations. The
unusually large hyperfine splitting of the CF+ line therefore explains the
shape of the lines detected in the Horsehead nebula, without invoking intricate
kinematics in the UV-illuminated gas.Comment: 2 pages, 1 figure, Accepted for publication in A&
The IRAM-30m line survey of the Horsehead PDR: IV. Comparative chemistry of H2CO and CH3OH
Aims. We investigate the dominant formation mechanism of H2CO and CH3OH in
the Horsehead PDR and its associated dense core. Methods. We performed deep
integrations of several H2CO and CH3OH lines at two positions in the Horsehead,
namely the PDR and dense core, with the IRAM-30m telescope. In addition, we
observed one H2CO higher frequency line with the CSO telescope at both
positions. We determine the H2CO and CH3OH column densities and abundances from
the single-dish observations complemented with IRAM-PdBI high-angular
resolution maps (6") of both species. We compare the observed abundances with
PDR models including either pure gas-phase chemistry or both gas-phase and
grain surface chemistry. Results. We derive CH3OH abundances relative to total
number of hydrogen atoms of ~1.2e-10 and ~2.3e-10 in the PDR and dense core
positions, respectively. These abundances are similar to the inferred H2CO
abundance in both positions (~2e-10). We find an abundance ratio H2CO/CH3OH of
~2 in the PDR and ~1 in the dense core. Pure gas-phase models cannot reproduce
the observed abundances of either H2CO or CH3OH at the PDR position. Both
species are therefore formed on the surface of dust grains and are subsequently
photodesorbed into the gas-phase at this position. At the dense core, on the
other hand, photodesorption of ices is needed to explain the observed abundance
of CH3OH, while a pure gas-phase model can reproduce the observed H2CO
abundance. The high-resolution observations show that CH3OH is depleted onto
grains at the dense core. CH3OH is thus present in an envelope around this
position, while H2CO is present in both the envelope and the dense core itself.
Conclusions. Photodesorption is an efficient mechanism to release complex
molecules in low FUV-illuminated PDRs, where thermal desorption of ice mantles
is ineffective.Comment: 12 pages, 5 tables, 7 figures; Accepted for publication in A&
The IRAM-30m line survey of the Horsehead PDR: II. First detection of the l-C3H+ hydrocarbon cation
We present the first detection of the l-C3H+ hydrocarbon in the interstellar
medium. The Horsehead WHISPER project, a millimeter unbiased line survey at two
positions, namely the photo-dissociation region (PDR) and the nearby shielded
core, revealed a consistent set of eight unidentified lines toward the PDR
position. Six of them are detected with a signal-to-noise ratio from 6 to 19,
while the two last ones are tentatively detected. Mostly noise appears at the
same frequency toward the dense core, located less than 40" away. We
simultaneously fit 1) the rotational and centrifugal distortion constants of a
linear rotor, and 2) the Gaussian line shapes located at the eight predicted
frequencies. The observed lines can be accurately fitted with a linear rotor
model, implying a 1Sigma ground electronic state. The deduced rotational
constant value is Be= 11244.9512 +/- 0.0015 MHz, close to that of l-C3H. We
thus associate the lines to the l-C3H+ hydrocarbon cation, which enables us to
constrain the chemistry of small hydrocarbons. A rotational diagram is then
used to infer the excitation temperature and the column density. We finally
compare the abundance to the results of the Meudon PDR photochemical model.Comment: 9 pages, 7 PostScript figures. Accepted for publication in Astronomy
\& Astrophysics. Uses aa LaTeX macro
Cool and warm dust emission from M33 (HerM33es)
We study the far-infrared emission from the nearby spiral galaxy M33 in order
to investigate the dust physical properties such as the temperature and the
luminosity density across the galaxy. Taking advantage of the unique wavelength
coverage (100, 160, 250, 350 and 500 micron) of the Herschel Space Observatory
and complementing our dataset with Spitzer-IRAC 5.8 and 8 micron and
Spitzer-MIPS 24 and 70 micron data, we construct temperature and luminosity
density maps by fitting two modified blackbodies of a fixed emissivity index of
1.5. We find that the 'cool' dust grains are heated at temperatures between 11
and 28 K with the lowest temperatures found in the outskirts of the galaxy and
the highest ones in the center and in the bright HII regions. The
infrared/submillimeter total luminosity (5 - 1000 micron) is estimated to be
1.9x10^9 Lsun. 59% of the total luminosity of the galaxy is produced by the
'cool' dust grains (~15 K) while the rest 41% is produced by 'warm' dust grains
(~55 K). The ratio of the cool-to-warm dust luminosity is close to unity
(within the computed uncertainties), throughout the galaxy, with the luminosity
of the cool dust being slightly enhanced in the center of the galaxy.
Decomposing the emission of the dust into two components (one emitted by the
diffuse disk of the galaxy and one emitted by the spiral arms) we find that the
fraction of the emission in the disk in the mid-infrared (24 micron) is 21%,
while it gradually rises up to 57% in the submillimeter (500 micron). We find
that the bulk of the luminosity comes from the spiral arm network that produces
70% of the total luminosity of the galaxy with the rest coming from the diffuse
dust disk. The 'cool' dust inside the disk is heated at a narrow range of
temperatures between 18 and 15 K (going from the center to the outer parts of
the galaxy).Comment: 12 pages, 14 figures, accepted for publication in A&
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Cool gas and dust in M 33: Results from the HERschel M 33 Extended Survey (HERM33ES)
We present an analysis of the first space-based far-IR-submm observations of M 33, which measure the emission from the cool dust and resolve the giant molecular cloud complexes. With roughly half-solar abundances, M 33 is a first step towards young low-metallicity galaxies where the submm may be able to provide an alternative to CO mapping to measure their H2 content. In this Letter, we measure the dust emission cross-section σ using SPIRE and recent CO and H i observations; a variation in σ is present from a near-solar neighborhood cross-section to about half-solar with the maximum being south of the nucleus. Calculating the total H column density from the measured dust temperature and cross-section, and then subtracting the H i column, yields a morphology similar to that observed in CO. The H2/H i mass ratio decreases from about unity to well below 10% and is about 15% averaged over the optical disk. The single most important observation to reduce the potentially large systematic errors is to complete the CO mapping of M 33
The IRAM-30m line survey of the Horsehead PDR: I. CF+ as a tracer of C+ and a measure of the Fluorine abundance
C+ is a key species in the interstellar medium but its 158 {\mu}m fine
structure line cannot be observed from ground-based telescopes. Current models
of fluorine chemistry predict that CF+ is the second most important fluorine
reservoir, in regions where C+ is abundant. We detected the J = 1-0 and J = 2-1
rotational lines of CF+ with high signal-to-noise ratio towards the PDR and
dense core positions in the Horsehead. Using a rotational diagram analysis, we
derive a column density of N(CF+) = (1.5 - 2.0) \times 10^12 cm^-2. Because of
the simple fluorine chemistry, the CF+ column density is proportional to the
fluorine abundance. We thus infer the fluorine gas-phase abundance to be F/H =
(0.6 - 1.5) \times 10^-8. Photochemical models indicate that CF+ is found in
the layers where C+ is abundant. The emission arises in the UV illuminated skin
of the nebula, tracing the outermost cloud layers. Indeed, CF+ and C+ are the
only species observed to date in the Horsehead with a double peaked line
profile caused by kinematics. We therefore propose that CF+, which is
detectable from the ground, can be used as a proxy of the C+ layers.Comment: Accepted to A&A, 4 pages, 4 figures, 2 table
High strain rate deformation of porous sandstone and the asymmetry of earthquake damage in shallow fault zones
In contrast to coseismic pulverization of crystalline rocks, observations of coseismic pulverization in porous sedimentary rocks in fault damage zones are scarce. Also, juxtaposition of stiff crystalline rocks and compliant porous rocks across a fault often yields an asymmetric damage zone geometry, with less damage in the more compliant side. In this study, we argue that such asymmetry near the sub-surface may occur because of a different response of lithology to similar transient loading conditions. Uniaxial unconfined high strain rate loadings with a split Hopkinson pressure bar were performed on dry and water saturated Rothbach sandstone core samples. Bedding anisotropy was taken into account by coring the samples parallel and perpendicular to the bedding. The results show that pervasive pulverization below the grain scale, such as observed in crystalline rock, does not occur in the sandstone samples for the explored strain rate range (60–150 s−1). Damage is mainly restricted to the scale of the grains, with intragranular deformation occurring only in weaker regions where compaction bands are formed. The presence of water and the bedding anisotropy mitigates the formation of compaction bands and motivates intergranular dilatation. The competition between inter- and intragranular damage during dynamic loading is explained with the geometric parameters of the rock in combination with two classic micromechanical models: the Hertzian contact model and the pore-emanated crack model. In conclusion, the observed microstructures can form in both quasi-static and dynamic loading regimes. Therefore caution is advised when interpreting the mechanism responsible for near-fault damage in sedimentary rock near the surface. Moreover, the results suggest that different responses of lithology to transient loading are responsible for sub-surface damage zone asymmetry
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