20 research outputs found
ChemInform Abstract: Four 9-Alkylthiophenanthrenes at 193 K (Alkyl: Methyl, Ethyl, Propyl and Butyl)
No full textChemInform Abstract: The Formation of Dibenzodioxocin Structures by Oxidative Coupling. A Model Reaction for Lignin Biosynthesis.
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Experimental characterization and analysis of the static behavior of twin-ridge AlGaInAs laterally coupled lasers (LCDL)
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Tracing the contraction of the pre-stellar core L1544 with HC
No full textContext. Spectral line profiles of several molecules observed towards the pre-stellar core L1544 appear double-peaked. For abundant molecular species this line morphology has been linked to self-absorption. However, the physical process behind the double-peaked morphology for less abundant species is still under debate.
Aims. In order to understand the cause behind the double-peaked spectra of optically thin transitions and their link to the physical structure of pre-stellar cores, we present high-sensitivity and high spectral resolution HC17O+ J =1−0 observations towards the dust peak in L1544.
Methods. We observed the HC17O+(1−0) spectrum with the Institut de Radioastronomie Millimétrique (IRAM) 30 m telescope. By using state-of-the-art collisional rate coefficients, a physical model for the core and the fractional abundance profile of HC17O+, the hyperfine structure of this molecular ion is modelled for the first time with the radiative transfer code lo
ChemInform Abstract: Synthesis and Pharmacological Properties of 4(5)-(2-Ethyl-2,3-dihydro- 2-silainden-2-yl)imidazole, a Silicon Analogue of Atipamezole.
No full textA low cosmic-ray ionisation rate in the pre-stellar core Ophiuchus/H-MM1
No full textAims. We test the use of three common molecular ions, ortho-H2D+ (oH2D+), N2H+, and DCO+, as probes of the internal structure and kinematics of a dense, starless molecular cloud core.
Methods. The pre-stellar core H-MM1 in Ophiuchus was mapped in the oH2D+(110 − N2H+(4 − 3), and DCO+ (5 − 4) lines with the Large APEX sub-Millimeter Array (LAsMA) multi-beam receiver of the Atacama Pathfinder EXperiment (APEX) telescope. We also ran a series of chemistry models to predict the abundance distributions of the observed molecules, and to estimate the effect of the cosmic-ray ionisation rate on their abundances.
Results. The three line maps show different distributions. The oH2D+ map is extended and outlines the general structure of the core, N2H+ mainly shows the density maxima, and the DCO+ emission peaks are shifted towards one edge of the core where a region of enhanced desorption had previously been found. According to the chemical simulation, the fractional oH2D+ abundance remains relatively high in the centre of the core, and its column density correlates strongly with the cosmic-ray ionisation rate, ζH2. Simulated line maps constrain the cosmic-ray ionisation rate to be low, between 5 × 10−18 s−1 and 1 × 10−17 s−1 in the H-MM1 core. This estimate agrees with the gas temperature measured in the core.
Conclusions. The present observations show that very dense, cold gas in molecular clouds can be traced by mapping the ground-state line of oH2D+ and high-J transitions of DCO+ and N2H+, despite the severe depletion of the latter two molecules. Modelling line emission of oH2D+ provides a straightforward method of determining the cosmic-ray ionisation rate in dense clouds, where the primary ion, H3+, is not observable
Similar levels of deuteration in the pre-stellar core L1544 and the protostellar core HH211
No full textIn the centre of pre-stellar cores, deuterium fractionation is enhanced due
to the low temperatures and high densities. Therefore, the chemistry of
deuterated molecules can be used to study the earliest stages of star
formation. We analyse the deuterium fractionation of simple molecules,
comparing the level of deuteration in the envelopes of the pre-stellar core
L1544 in Taurus and the protostellar core HH211 in Perseus. We used single-dish
observations of CCH, HCN, HNC, HCO, and their C-, O- and
D-bearing isotopologues, detected with the Onsala 20m telescope. We derived the
column densities and the deuterium fractions of the molecules. Additionally, we
used radiative transfer simulations and results from chemical modelling to
reproduce the observed molecular lines. We used new collisional rate
coefficients for HNC, HNC, DNC, and DCN that consider the hyperfine
structure of these molecules. We find high levels of deuteration for CCH (10%)
in both sources, consistent with other carbon chains, and moderate levels for
HCN (5-7%) and HNC (8%). The deuterium fraction of HCO is enhanced towards
HH211, most likely caused by isotope-selective photodissociation of CO.
Similar levels of deuteration show that the process is likely equally efficient
towards both cores, suggesting that the protostellar envelope still retains the
chemical composition of the original pre-stellar core. The fact that the two
cores are embedded in different molecular clouds also suggests that
environmental conditions do not have a significant effect on the deuteration
within dense cores. Radiative transfer modelling shows that it is necessary to
include the outer layers of the cores to consider the effects of extended
structures. Besides HCO observations, HCN observations towards L1544 also
require the presence of an outer diffuse layer where the molecules are
relatively abundant.Comment: 27 pages, 17 figures, accepted for publication in A&
