Search for H$_3^+$ isotopologues toward CRL 2136 IRS 1. (arXiv:1910.04165v1 [astro-ph.SR])

Search for H$_3^+$ isotopologues toward CRL 2136 IRS 1. (arXiv:1910.04165v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Goto_M/0/1/0/all/0/1">Miwa Goto</a> (1), <a href="http://arxiv.org/find/astro-ph/1/au:+Geballe_T/0/1/0/all/0/1">T. R. Geballe</a> (2), <a href="http://arxiv.org/find/astro-ph/1/au:+Harju_J/0/1/0/all/0/1">Jorma Harju</a> (3,4), <a href="http://arxiv.org/find/astro-ph/1/au:+Caselli_P/0/1/0/all/0/1">Paola Caselli</a> (4), <a href="http://arxiv.org/find/astro-ph/1/au:+Sipila_O/0/1/0/all/0/1">Olli Sipil&#xe4;</a> (4), <a href="http://arxiv.org/find/astro-ph/1/au:+Menten_K/0/1/0/all/0/1">Karl M. Menten</a> (5), <a href="http://arxiv.org/find/astro-ph/1/au:+Usuda_T/0/1/0/all/0/1">Tomonori Usuda</a> (6) ((1) Universit&#xe4;ts-Sternwarte M&#xfc;nchen, Ludwig-Maximilians-Universit&#xe4;t, (2) Gemini Observatory, (3) University of Helsinki, (4) Max-Planck-Institut f&#xfc;r extraterrestrische Physik, (5) Max-Planck-Institut f&#xfc;r Radioastronomie, (6) Thirty Meter Telescope Japan, National Astronomical Observatory of Japan)

Deuterated interstellar molecules frequently have abundances relative to
their main isotopologues much higher than the overall elemental D-to-H ratio in
the cold dense interstellar medium. The H$_3^+$ and its isotopologues play a
key role in the deuterium fractionation; however, the abundances of these
isotopologues have not been measured empirically with respect to H$_3^+$ to
date. Our aim was to constrain the relative abundances of H$_2$D$^+$ and
D$_3^+$ in the cold outer envelope of the hot core CRL 2136 IRS 1. We carried
out three observations targeting H$_3^+$ and its isotopologues using the
spectrographs CRIRES at the VLT, iSHELL at IRTF, and EXES on board SOFIA. In
addition, the CO overtone band at 2.3 $mu$m was observed by iSHELL to
characterize the gas on the line of sight. The H$_3^+$ ion was detected toward
CRL 2136 IRS 1 as in previous observations. Spectroscopy of lines of H$_2$D$^+$
and D$_3^+$ resulted in non-detections. The 3$sigma$ upper limits of $N({rm
H_2D^+})/N({rm H_3^+})$ and $N({rm D_3^+})/N({rm H_3^+})$ are 0.24 and 0.13,
respectively. The population diagram for CO is reproduced by two components of
warm gas with the temperatures 58 K and 530 K, assuming a local thermodynamic
equilibrium (LTE) distribution of the rotational levels. Cold gas ($<$20 K) makes only a minor contribution to the CO molecular column toward CRL 2136 IRS 1. The critical conditions for deuterium fractionation in a dense cloud are low temperature and CO depletion. Given the revised cloud properties, it is no surprise that H$_3^+$ isotopologues are not detected toward CRL 2136 IRS 1. The result is consistent with our current understanding of how deuterium fractionation proceeds.

Deuterated interstellar molecules frequently have abundances relative to
their main isotopologues much higher than the overall elemental D-to-H ratio in
the cold dense interstellar medium. The H$_3^+$ and its isotopologues play a
key role in the deuterium fractionation; however, the abundances of these
isotopologues have not been measured empirically with respect to H$_3^+$ to
date. Our aim was to constrain the relative abundances of H$_2$D$^+$ and
D$_3^+$ in the cold outer envelope of the hot core CRL 2136 IRS 1. We carried
out three observations targeting H$_3^+$ and its isotopologues using the
spectrographs CRIRES at the VLT, iSHELL at IRTF, and EXES on board SOFIA. In
addition, the CO overtone band at 2.3 $mu$m was observed by iSHELL to
characterize the gas on the line of sight. The H$_3^+$ ion was detected toward
CRL 2136 IRS 1 as in previous observations. Spectroscopy of lines of H$_2$D$^+$
and D$_3^+$ resulted in non-detections. The 3$sigma$ upper limits of $N({rm
H_2D^+})/N({rm H_3^+})$ and $N({rm D_3^+})/N({rm H_3^+})$ are 0.24 and 0.13,
respectively. The population diagram for CO is reproduced by two components of
warm gas with the temperatures 58 K and 530 K, assuming a local thermodynamic
equilibrium (LTE) distribution of the rotational levels. Cold gas ($<$20 K)
makes only a minor contribution to the CO molecular column toward CRL 2136 IRS
1. The critical conditions for deuterium fractionation in a dense cloud are low
temperature and CO depletion. Given the revised cloud properties, it is no
surprise that H$_3^+$ isotopologues are not detected toward CRL 2136 IRS 1. The
result is consistent with our current understanding of how deuterium
fractionation proceeds.

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