16 research outputs found
ChemInform Abstract: Four 9-Alkylthiophenanthrenes at 193 K (Alkyl: Methyl, Ethyl, Propyl and Butyl)
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ChemInform Abstract: Two 9-Alkylthiophenanthrenes at 193 K (Alkyl = Dodecyl and Tetradecyl).
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The Effect of Weak Cosmic-ray Heating Events on the Desorption of H2
No full textThe typical amount of molecular hydrogen (H-2) in interstellar ices is not known, but significant freeze-out of H-2 on dust grains is not expected. However, chemical models ubiquitously predict large amounts of H-2 freeze-out in dense cloud conditions, and specialized treatments are needed to control the H-2 population on grains. Here we present a numerical desorption model where the effect of weak heating events induced by cosmic rays (CRs) that heat grains to temperatures of a few tens of kelvin at high frequencies is included, improving upon earlier desorption models that only consider strong heating events (maximum grain temperature close to 100 K) that occur at a low frequency. A temperature of a few tens of kelvin is high enough to induce efficient desorption of H-2, but we find that even the weak heating events do not occur often enough to lead to significant H-2 desorption. Taking the weak heating events into account does affect the predicted abundances of other lightly bound species, but the effect is restricted to low column densities. We make here the canonical assumption that the grains are spherical with a radius of 0.1 mu m. It is conceivable that in the case of a grain size distribution, weak heating events could provide a boost to H-2 desorption coming off small grains, which are the most numerous. Further studies are still required to better quantify the role of CRs in the desorption of H-2 and other weakly bound species
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 textDeuteration of c-C
No full textContext. In the centre of pre-stellar cores, the deuterium fractionation is enhanced due to the cold temperatures and high densities. Therefore, the chemistry of deuterated molecules can be used to probe the evolution and the kinematics in the earliest stages of star formation.
Aims. We analyse emission maps of cyclopropenylidene, c-C3H2, to study the distribution of the deuteration throughout the prototypical pre-stellar core L1544.
Methods. We used single-dish observations of c-C3H2, c-H13CC2H, c-C3HD, and c-C3D2 towards the pre-stellar core L1544, performed at the IRAM 30 m telescope. We derived the column density and deuterium fraction maps, and compared these observations with non-local thermodynamic equilibrium radiative transfer simulations.
Results. The highest deuterium fractions are found close to the dust peak at the centre of L1544, where the increased abundance of H2D+ ions drives the deuteration process. The peak values are N(c-C3HD)/N(c-C3H2) = 0.17 ± 0.01, N(c-C3D2)/N(c-C3H2) = 0.025 ± 0.003, and N(c-C3D2)/N(c-C3HD) = 0.16 ± 0.03, which is consistent with previous single-pointing observations. The distributions of c-C3HD and c-C3D2 indicate that the deuterated forms of c-C3H2 in fact trace the dust peak and not the c-C3H2 peak.
Conclusions. The N(c-C3D2)/N(c-C3HD) map confirms that the process of deuteration is more efficient towards the centre of the core and demonstrates that carbon-chain molecules are still present at high densities. This is likely caused by an increased abundance of He+ ions destroying CO, which increases the number of carbon atoms in the gas phase
Experimental febrile seizures are precipitated by a hyperthermia-induced respiratory alkalosis
No full textFebrile seizures are frequent during early childhood, and prolonged (complex) febrile seizures are associated with an increased susceptibility to temporal lobe epilepsy. The pathophysiological consequences of febrile seizures have been extensively studied in rat pups exposed to hyperthermia. The mechanisms that trigger these seizures are unknown, however. A rise in brain pH is known to enhance neuronal excitability. Here we show that hyperthermia causes respiratory alkalosis in the immature brain, with a threshold of 0.2-0.3 pH units for seizure induction. Suppressing alkalosis with 5% ambient CO2 abolished seizures within 20 s. CO2 also prevented two long-term effects of hyperthermic seizures in the hippocampus: the upregulation of the I(h) current and the upregulation of CB1 receptor expression. The effects of hyperthermia were closely mimicked by intraperitoneal injection of bicarbonate. Our work indicates a mechanism for triggering hyperthermic seizures and suggests new strategies in the research and therapy of fever-related epileptic syndromes
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&
PRODIGE – envelope to disk with NOEMA
No full textContext. The 12C/13C isotope ratio has been derived towards numerous cold clouds (~20-50 K) and a couple of protoplanetary disks and exoplanet atmospheres. However, direct measurements of this ratio in the warm gas (>100 K) around young low-mass protostars remain scarce but are required to study its evolution during star and planet formation.
Aims. We aim to derive 12C/13C ratios from the isotopologues of the complex organic molecules (COMs) CH3OH and CH3CN in the warm gas towards seven Class 0/I protostellar systems to improve our understanding of the evolution of the 12C/13C ratios during star and planet formation.
Methods. We used the data that were taken as part of the PROtostars & DIsks: Global Evolution (PRODIGE) large program with the Northern Extended Millimetre Array (NOEMA) at 1 mm. The 13C isotopologue of CH3OH was detected towards seven sources of the sample, those of CH3CN were detected towards six sources. The emission spectra were analysed by deriving synthetic spectra and population diagrams assuming conditions of local thermodynamic equilibrium.
Results. The emission of CH3OH and CH3CN is spatially unresolved in the PRODIGE data with a resolution of ~1″(~300 au) for the seven targeted systems. Rotational temperatures derived from both COMs exceed 100 K, telling us that they trace the gas of the hot corino, where CH3CN probes hotter regions than CH3OH on average (290 K versus 180 K). The column density ratios between the 12C and 13C isotopologues range from 4 to 30, which is lower by factors of a few up to an order of magnitude than the expected isotope ratio of the local interstellar medium of ~68. We conducted astrochemical models to understand the origins of the observed low ratios. We studied potential precursor molecules of CH3 OH and CH3 CN since the model does not include COMs, assuming that the ratio is transferred in reactions from the precursors to the COMs. The model predicts 12C/13C ratios close to the ISM value for CO and H2CO, precursors of CH3OH, in contrast to our observational results. For the potential precursors of CH3CN (CN, HCN, and HNC), the model predicts low 12C/13C ratios close to the protostar (<300 au). Hence, they may also be expected for CH3CN.
Conclusions. Our results show that an enrichment in 13C in COMs at the earliest protostellar stages is likely inherited from the precursor species of the COMs, whose 12C/13C ratios are set during the prestellar stage via isotopic exchange reactions. This also implies that low 12C/13C ratios observed at later evolutionary stages such as protoplanetary disks and exoplanetary atmospheres could at least partially be inherited. A definitive conclusion on 12C/13C ratios in protostellar environments requires observations at higher angular and spectral resolution that simultaneously cover a broad bandwidth, to tackle current observational limitations, and additional modelling efforts
