Interstellar deuterated ammonia: from NH3 to ND3 (original) (raw)
Related papers
Deuterated Ammonia in Galactic Protostellar Cores
The Astrophysical Journal, 2001
We report on a survey of NH 2 D towards protostellar cores in low-mass star formation and quiescent regions in the Galaxy. Twenty-three out of thirty-two observed sources have significant (> ∼5σ) NH 2 D emission. Ion-molecule chemistry, which preferentially enhances deuterium in molecules above its cosmological value of 1.6 × 10 −5 sufficiently explains these abundances. NH 2 D/NH 3 ratios towards Class 0 sources yields information about the "fossil remnants" from the era prior to the onset of core collapse and star formation. We compare our observations with predictions of gas-phase chemical networks.
Ammonia cores in high mass star formation regions
Astronomy and Astrophysics, 2006
We observed a sample of 35 water masers not coincident with known HII regions and/or low mass young stellar objects (YSOs) with the Effelsberg 100 m telescope in the NH 3 (J, K) = (1, 1), (2, 2), (3, 3) and (4, 4) transitions. Sixteen sources were detected in the NH 3 emission. The detection rate is 46%. All these sixteen sources have NH 3 (1, 1) and (2, 2) emission, among which four sources have NH 3 (3, 3) emission. Comparing with the IRAS and the 2MASS data, we analyzed the relationship between the detection rate and the infrared color, the dust temperature and the source distance. All the detected sources were mapped and 17 cores were obtained (one source IRAS 20215+3725 has two cores). From the detected sources five cores do not coincide with radio continuum or IRAS and MSX point sources. Excluding one core that has no MSX data available, the remaining eleven cores are coincident with IRAS or MSX point sources. The typical size and mass of the cores are 1.6 pc and 1.5 × 10 3 M ⊙ , respectively. The average line widths of the NH 3 (1, 1) and (2, 2) are 1.54 and 1.73 km s −1 . The average kinetic temperature of the gas is about 19 K. These values are much larger than those of low mass cores. The NH 3 cores that coincide with IRAS sources (referred to as Group I) have slightly larger line widths (1.65 and 1.75 km s −1 for the (1, 1) and (2, 2) lines, respectively) and larger masses (1.8 × 10 3 M ⊙ ) than the mean values of the sample. For this type of core the kinetic temperature correlates with the line width. The line width appears to correlate with the bolometric luminosity and the core size. Despite the average luminosity of 2.9 × 10 4 L ⊙ , there is no detectable 6 cm emission. These are candidates for high mass protostars or precursors of UC HII regions. The NH 3 cores with peaks offset from infrared sources (referred to as Group II) have an average size of 1.7 pc and an average line width of 1.50 km s −1 for the (1, 1) line. The line width of the (1, 1) emission is smaller than that of the group I. The average mass is 9.4 × 10 2 M ⊙ . One possible explanation for the deviation is that the NH 3 peak and the infrared source correspond to different clumps. These cores are potential high mass star formation sites and may be at an earlier evolutionary stage than those with IRAS point sources. This type of core is seen in mapping observations, and can be easily missed by single-spectrum observations toward the IRAS position.
Stratified NH and ND emission in the prestellar core 16293E in L1689N
Astronomy & Astrophysics, 2015
Context. High degrees of deuterium fractionation are commonly found in cold prestellar cores and in the envelopes around young protostars. As it brings strong constraints to chemical models, deuterium chemistry is often used to infer core history or molecule formation pathways. Whereas a large number of observations are available regarding interstellar deuterated stable molecules, relatively little is known about the deuteration of hydride radicals, as their fundamental rotational transitions are at high frequencies where the atmosphere is mostly opaque. Aims. Nitrogen hydride radicals are important species in nitrogen chemistry, as they are thought to be related to ammonia formation. Observations have shown that ammonia is strongly deuterated, with [NH 2 D]/[NH 3 ] ∼ 10%. Models predict similarly high [ND]/[NH] ratios, but so far only one observational determination of this ratio is available, towards the envelope of the protostar IRAS16293-2422. To test model predictions, we aim here to determine [ND]/[NH] in a dense, starless core. Methods. We observed NH and ND in 16293E with the HIFI spectrometer on board the Herschel Space Observatory as part of the CHESS guaranteed time key programme, and derived the abundances of these two species using a non local thermodynamic equilibrium radiative transfer model. Results. Both NH and ND are detected in the source, with ND in emission and NH in absorption against the continuum that arises from the cold dust emission. Our model shows, however, that the ND emission and the NH absorption originate from different layers in the cloud, as further evidenced by their different velocities. In the central region of the core, we can set a lower limit to the [ND]/[NH] ratio of > ∼ 2%. This estimate is consistent with recent pure gas-phase models of nitrogen chemistry.
Astronomy and Astrophysics, 2010
Context. The deuterium fractionation, D frac , has been proposed as an evolutionary indicator in pre-protostellar and protostellar cores of low-mass star-forming regions. Aims. We investigate D frac , with high angular resolution, in the cluster environment surrounding the UCH ii region IRAS 20293+3952. Methods. We performed high angular resolution observations with the IRAM Plateau de Bure Interferometer (PdBI) of the ortho-NH 2 D 1 11-1 01 line at 85.926 GHz and compared them with previously reported VLA NH 3 data. Results. We detected strong NH 2 D emission toward the pre-protostellar cores identified in NH 3 and dust emission, all located in the vicinity of the UCH ii region IRAS 20293+3952. We found high values of D frac 0.1-0.8 in all the pre-protostellar cores and low values, D frac < 0.1, associated with young stellar objects. Conclusions. The high values of D frac in pre-protostellar cores could be indicative of evolution, although outflow interactions and UV radiation could also play a role.
Deuterium chemistry of dense gas in the vicinity of low-mass and massive star-forming regions
Monthly Notices of the Royal Astronomical Society, 2014
The standard interstellar ratio of deuterium to hydrogen (D/H) atoms is ∼1.5 × 10 −5. However, the deuterium fractionation is in fact found to be enhanced, to different degrees, in cold, dark cores, hot cores around massive star-forming regions, lukewarm cores, and warm cores (hereafter hot corinos) around low-mass star-forming regions. In this paper, we investigate the overall differences in the deuterium chemistry between hot cores and hot corinos. We have modelled the chemistry of dense gas around low-mass and massive star-forming regions using a gas-grain chemical model. We investigate the influence of varying the core density, the depletion efficiency of gaseous species on to dust grains, the collapse mode and the final mass of the protostar on the chemical evolution of star-forming regions. We find that the deuterium chemistry is, in general, most sensitive to variations of the depletion efficiency on to grain surfaces, in agreement with observations. In addition, the results showed that the chemistry is more sensitive to changes in the final density of the collapsing core in hot cores than in hot corinos. Finally, we find that ratios of deuterated sulphur bearing species in dense gas around hot cores and corinos may be good evolutionary indicators in a similar way as their non-deuterated counterparts.
Chemical fractionation of deuterium in the protosolar nebula
Monthly Notices of the Royal Astronomical Society, 2017
Understanding the gas-grain chemistry of deuterium in star-forming objects may help to explain their history and present state. We aim to clarify how processes in ices affect the deuterium fractionation. In this regard, we investigate a Solar-mass protostellar envelope using an astrochemical rate-equation model that considers bulk-ice chemistry. The results show a general agreement with the molecular D/H abundance ratios observed in low-mass protostars. The simultaneous processes of ice accumulation and rapid synthesis of HD on grain surfaces in the pre-stellar core hamper the deuteration of icy species. The observed very high D/H ratios exceeding 10 per cent, i.e. super-deuteration, are reproduced for formaldehyde and dimethyl ether, but not for other species in the protostellar envelope phase. Chemical transformations in bulk ice lower D/H ratios of icy species and do not help explaining the super-deuteration. In the protostellar phase, the D 2 O/HDO abundance ratio was calculated to be higher than the HDO/H 2 O ratio owing to gas-phase chemistry. Species that undergo evaporation from ices have a high molecular D/H ratio and a high gas-phase abundance.
NH3 observations of the S235 star-forming region: Dense gas in inter-core bridges
Publications of the Astronomical Society of Japan, 2019
Star formation is thought to be driven by two groups of mechanisms; spontaneous collapse and triggered collapse. Triggered star formation mechanisms further diverge into cloud–cloud collision (CCC), “collect and collapse” (C&C) and shock-induced collapse of pre-existing, gravitationally stable cores, or “radiation driven implosion” (RDI). To evaluate the contributions of these mechanisms and establish whether these processes can occur together within the same star-forming region, we performed mapping observations of radio-frequency ammonia and water maser emission lines in the S235 massive star-forming region. Via spectral analyses of main, hyperfine, and multi-transitional ammonia lines we explored the distribution of temperature and column density in the dense gas in the S235 and S235AB star-forming region. The most remarkable result of the mapping observations is the discovery of high-density gas in inter-core bridges which physically link dense molecular cores that house young p...
Molecular D/H ratios in the dense gas surrounding low-mass protostars
Planetary and Space Science, 2002
Observations of [HDCO]/[H2CO] and [DCN]/[HCN] ratios have been made towards a selection of low-mass protostellar cores in three di erent star forming regions. Most [HDCO]/ [H2CO] ratios are between 0.05 and 0.07, similar to the values observed towards the dark clouds, TMC-1 and L134N. [DCN]/[HCN] ratios are ∼ 0:04, higher than those seen in TMC-1, around the low-mass protostar, IRAS 16293, and in the Orion Hot Core, but similar values to the Orion Compact Ridge and L134N. These results are compared with predictions from chemical models, and to other observations made in these sources. We ÿnd best agreement between models and observations by assuming that interaction between gas phase molecules and dust grains has impacted on the chemistry during the cold pre-collapse phase of the cloud's history. There are no marked di erence between molecular D/H ratios towards di erent regions, or between Class 0 and I protostars. However, the di erence between the [DCN]/[HCN] ratios we have measured and those previously observed towards Hot Molecular Cores leads us to suggest that there are signiÿcant evolutionary di erences between high and low mass star forming regions.
AMMONIA THERMOMETRY OF STAR-FORMING GALAXIES
The Astrophysical Journal, 2013
With a goal toward deriving the physical conditions in external galaxies, we present a study of the ammonia (NH 3 ) emission and absorption in a sample of star forming systems. Using the unique sensitivities to kinetic temperature afforded by the excitation characteristics of several inversion transitions of NH 3 , we have continued our characterization of the dense gas in star forming galaxies by measuring the kinetic temperature in a sample of 23 galaxies and one galaxy offset position selected for their high infrared luminosity. We derive kinetic temperatures toward 13 galaxies, 9 of which possess multiple kinetic temperature and/or velocity components. Eight of these galaxies exhibit kinetic temperatures > 100 K, which are in many cases at least a factor of two larger than kinetic temperatures derived previously. Furthermore, the derived kinetic temperatures in our galaxy sample, which are in many cases at least a factor of two larger than derived dust temperatures, point to a problem with the common assumption that dust and gas kinetic temperatures are equivalent. As previously suggested, the use of dust emission at wavelengths greater than 160 µm to derive dust temperatures, or dust heating from older stellar populations, may be skewing derived dust temperatures in these galaxies to lower values. We confirm the detection of high-excitation OH 2 Π 3/2 J=9/2 absorption toward Arp 220 . We also report the first detections of non-metastable NH 3 inversion transitions toward
NITROGEN ISOTOPIC FRACTIONATION IN INTERSTELLAR AMMONIA
The Astrophysical Journal, 2010
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 ± 50 (3σ), is particularly well constrained and falls in between the local interstellar medium/proto-solar value of ∼450 and the terrestrial atmospheric value of 272. The NGC 1333 measurement is consistent with the Barnard 1 result, but has a larger uncertainty. We do not see evidence for the very high 15 N enhancements seen in cometary CN. Sensitive observations of a larger, carefully selected sample of prestellar cores with varying temperatures and gas densities can significantly improve our understanding of the nitrogen fractionation in the local interstellar medium and its relation to the isotopic ratios measured in various solar system reservoirs.