ﻻ يوجد ملخص باللغة العربية
Context. The increased sensitivity and high spectral resolution of millimeter telescopes allow the detection of an increasing number of isotopically substituted molecules in the interstellar medium. The 14N/ 15N ratio is difficult to measure directly for carbon containing molecules. Aims. We want to check the underlying hypothesis that the 13C/ 12C ratio of nitriles and isonitriles is equal to the elemental value via a chemical time dependent gas phase chemical model. Methods. We have built a chemical network containing D, 13C and 15N molecular species after a careful check of the possible fractionation reactions at work in the gas phase. Results. Model results obtained for 2 different physical conditions corresponding respectively to a moderately dense cloud in an early evolutionary stage and a dense depleted pre-stellar core tend to show that ammonia and its singly deuterated form are somewhat enriched in 15N, in agreement with observations. The 14N/ 15N ratio in N2H+ is found to be close to the elemental value, in contrast to previous models which obtain a significant enrichment, as we found that the fractionation reaction between 15N and N2H+ has a barrier in the entrance channel. The large values of the N2H+/15NNH+ and N2H+/ N15NH+ ratios derived in L1544 cannot be reproduced in our model. Finally we find that nitriles and isonitriles are in fact significantly depleted in 13C, questioning previous interpretations of observed C15N, HC15N and H15NC abundances from 13C containing isotopologues.
Using the Green Bank Telescope (GBT), we have obtained accurate measurements of the $^{14}$N/$^{15}$N isotopic ratio in ammonia in two nearby cold, dense molecular clouds, Barnard~1 and NGC 1333. The $^{14}$N/$^{15}$N ratio in Barnard~1, $334 pm 50$
C-fractionation has been studied from a theoretical point of view with different models of time-dependent chemistry, including both isotope-selective photodissociation and low-temperature isotopic exchange reactions. Recent chemical models predict th
Although ammonia is an abundant molecule commonly observed towards the dense interstellar medium, it has not yet been established whether its main formation route is from gas-phase ion-molecule reactions or grain-surface hydrogen additions on adsorbe
The formation of organic compounds is generally assumed to result from abiotic processes in the Solar System, with the exception of biogenic organics on Earth. Nitrogen-bearing organics are of particular interest, notably for prebiotic perspectives b
We upgraded the chemical network from the UMIST Database for Astrochemistry 2006 to include isotopes such as ^{13}C and ^{18}O. This includes all corresponding isotopologues, their chemical reactions and the properly scaled reaction rate coefficients