Reliability of NH<sub>3</sub>as the temperature probe of cold cloud cores

Juvela, M.; Harju, J.; Ysard, N.; Lunttila, T.

doi:10.1051/0004-6361/201118257

Cited by 16 publications

(13 citation statements)

References 19 publications

(43 reference statements)

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“…The standard analysis described in Ho & Townes (1983), Walmsley & Ungerechts (1983), and Ungerechts et al (1986) was used. Ac- cording to the modelling results of Juvela et al (2012), the ammonia spectra trace faithfully the real mass averaged gas temperature. In Fig.…”

Section: Lte Analysis Of the Observed Spectrasupporting

confidence: 69%

Deuteration of ammonia in the starless core Ophiuchus/H-MM1

Harju

Daniel

Sipilä

et al. 2017

A&A

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Context. Ammonia and its deuterated isotopologues probe physical conditions in dense molecular cloud cores. The time-dependence of deuterium fractionation and the relative abundances of different nuclear spin modifications are supposed to provide means of determining the evolutionary stages of these objects. Aims. We aim to test the current understanding of spin-state chemistry of deuterated species by determining the abundances and spin ratios of NH 2 D, NHD 2 , and ND 3 in a quiescent, dense cloud. Methods. Spectral lines of NH 3 , NH 2 D, NHD 2 , ND 3 , and N 2 D + were observed towards a dense, starless core in Ophiuchus with the APEX, GBT, and IRAM 30-m telescopes. The observations were interpreted using a gas-grain chemistry model combined with radiative transfer calculations. The chemistry model distinguishes between the different nuclear spin states of light hydrogen molecules, ammonia, and their deuterated forms. Different desorption schemes can be considered. Results. High deuterium fractionation ratios with NH 2 D/NH 3 ∼ 0.4, NHD 2 /NH 2 D ∼ 0.2, and ND 3 /NHD 2 ∼ 0.06 are found in the core. The observed ortho/para ratios of NH 2 D and NHD 2 are close to the corresponding nuclear spin statistical weights. The chemistry model can approximately reproduce the observed abundances, but predicts uniformly too low ortho/para-NH 2 D, and too large ortho/para-NHD 2 ratios. The longevity of N 2 H + and NH 3 in dense gas, which is prerequisite to their strong deuteration, can be attributed to the chemical inertia of N 2 on grain surfaces. Conclusions. The discrepancies between the chemistry model and the observations are likely to be caused by the fact that the model assumes complete scrambling in principal gas-phase deuteration reactions of ammonia, which means that all the nuclei are mixed in reactive collisions. If, instead, these reactions occur through proton hop/hydrogen abstraction processes, statistical spin ratios are to be expected. The present results suggest that while the deuteration of ammonia changes with physical conditions and time, the nuclear spin ratios of ammonia isotopologues do not probe the evolutionary stage of a cloud.

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Section: Lte Analysis Of the Observed Spectrasupporting

confidence: 69%

Deuteration of ammonia in the starless core Ophiuchus/H-MM1

Harju

Daniel

Sipilä

et al. 2017

A&A

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“…The inversion doublets have indeed hyperfine components due to (mainly) the quadrupole moment of the 14 N nucleus. Asssuming the excitation temperature of each hyperfine transition (within a doublet) is the same, one can derive the opacities τ (11) and τ (22) from which the excitation temperature T1,2 can be obtained (Ho et al 1979;Juvela et al 2012). In order to derive the kinetic temperature from the measured T1,2 excitation temperature, one needs to calibrate the ammonia thermometer, Eq.…”

Section: The Ammonia Thermometermentioning

confidence: 99%

Collisional excitation of NH3 by atomic and molecular hydrogen

Bouhafs

Rist

Daniel

et al. 2017

Monthly Notices of the Royal Astronomical Society

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We report extensive theoretical calculations on the rotation-inversion excitation of interstellar ammonia (NH 3 ) due to collisions with atomic and molecular hydrogen (both para-and ortho-H 2 ). Close-coupling calculations are performed for total energies in the range 1-2000 cm −1 and rotational cross sections are obtained for all transitions among the lowest 17 and 34 rotation-inversion levels of ortho-and para-NH 3 , respectively. Rate coefficients are deduced for kinetic temperatures up to 200 K. Propensity rules for the three colliding partners are discussed and we also compare the new results to previous calculations for the spherically symmetrical He and para-H 2 projectiles. Significant differences are found between the different sets of calculations. Finally, we test the impact of the new rate coefficients on the calibration of the ammonia thermometer. We find that the calibration curve is only weakly sensitive to the colliding partner and we confirm that the ammonia thermometer is robust.

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“…Because of the smaller optical depth, more of the C 18 O line photons escape, leading to a smaller excitation temperature. This is also clearly significant when the kinetic temperature varies within the cloud (Juvela et al 2012a).…”

Section: Discussionmentioning

confidence: 89%

The physical state of selected cold clumps

Parikka

Juvela

Pelkonen

et al. 2015

A&A

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Context. The study of prestellar cores is essential to understanding the initial stages of star formation. With Herschel more cold clumps have been detected than ever before. For this study we have selected 21 cold clumps from 20 Herschel fields observed as a follow-up on original Planck detections. We have observed these clumps in 13 CO (1−0), C 18 O (1−0), and N 2 H + (1−0) lines. Aims. Our aim is to find out if these cold clumps are prestellar. We have examined to what extent independent analysis of the dust and the molecular lines lead to similar conclusions about the masses of these objects. Methods. We calculate the clump masses and densities from the dust continuum and molecular line observations and compare these to each other and to the virial and Bonnor-Ebert masses calculated for each clump. Finally we examine two of the fields with radiative transfer models to estimate CO abundances. Results. When excitation temperatures could be estimated, the column densities derived from molecular line observations were comparable to those from dust continuum data. The median column density estimates are 4.2 × 10 21 cm −2 and 5.5 × 10 21 cm −2 for the line and dust emission data, respectively. The calculated abundances, column densities, volume densities, and masses all have large uncertainties and one must be careful when drawing conclusions. Abundance of 13 CO was found in modeling the two clumps in the field G131.65+9.75 to be close to the usual value of 10 −6 . The abundance ratio of 13 CO and C 18 O was ∼10. Molecular abundances could only be estimated with modeling, relying on dust column density data. Conclusions. The results indicate that most cold clumps, even those with dust color temperatures close to 11 K, are not necessarily prestellar.

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Reliability of NH₃as the temperature probe of cold cloud cores

Cited by 16 publications

References 19 publications

Deuteration of ammonia in the starless core Ophiuchus/H-MM1

Deuteration of ammonia in the starless core Ophiuchus/H-MM1

Collisional excitation of NH3 by atomic and molecular hydrogen

The physical state of selected cold clumps

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Reliability of NH3as the temperature probe of cold cloud cores

Cited by 16 publications

References 19 publications

Deuteration of ammonia in the starless core Ophiuchus/H-MM1

Deuteration of ammonia in the starless core Ophiuchus/H-MM1

Collisional excitation of NH3 by atomic and molecular hydrogen

The physical state of selected cold clumps

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Reliability of NH₃as the temperature probe of cold cloud cores