244 research outputs found
Tracing the atomic nitrogen abundance in star-forming regions with ammonia deuteration
Partitioning of elemental nitrogen in star-forming regions is not well
constrained. Most nitrogen is expected to be partitioned among atomic nitrogen,
molecular nitrogen (N2), and icy N-bearing molecules, such as ammonia (NH3) and
N2. Atomic nitrogen is not directly observable in the cold gas. In this paper,
we propose an indirect way to constrain the amount of atomic nitrogen in the
cold gas of star-forming clouds, via deuteration in ammonia ice, the
[ND2H/NH2D]/[NH2D/NH3] ratio. Using gas-ice astrochemical simulations, we show
that if atomic nitrogen remains as the primary reservoir of nitrogen during
cold ice formation stages, the [ND2H/NH2D]/[NH2D/NH3] ratio is close to the
statistical value of 1/3 and lower than unity, whereas if atomic nitrogen is
largely converted into N-bearing molecules, the ratio should be larger than
unity. Observability of ammonia isotopologues in the inner hot regions around
low-mass protostars, where ammonia ice has sublimated, is also discussed. We
conclude that the [ND2H/NH2D]/[NH2D/NH3] ratio can be quantified using a
combination of VLA and ALMA observations with reasonable integration times, at
least toward IRAS 16293-2422 where high molecular column densities are
expected.Comment: Accepted for publication in MNRAS, 12 pages, 9 figures, 1 Tabl
Analytical Formulas of Molecular Ion Abundances and N2H+ Ring in Protoplanetary Disks
We investigate the chemistry of ion molecules in protoplanetary disks,
motivated by the detection of NH ring around TW Hya. While the ring
inner radius coincides with the CO snow line, it is not apparent why NH
is abundant outside the CO snow line in spite of the similar sublimation
temperatures of CO and N. Using the full gas-grain network model, we
reproduced the NH ring in a disk model with millimeter grains. The
chemical conversion of CO and N to less volatile species (sink effect
hereinafter) is found to affect the NH distribution. Since the
efficiency of the sink depends on various parameters such as activation
barriers of grain surface reactions, which are not well constrained, we also
constructed the no-sink model; the total (gas and ice) CO and N abundances
are set constant, and their gaseous abundances are given by the balance between
adsorption and desorption. Abundances of molecular ions in the no-sink model
are calculated by analytical formulas, which are derived by analyzing the
full-network model. The NH ring is reproduced by the no-sink model, as
well. The 2D (R-Z) distribution of NH, however, is different among the
full-network model and no-sink model. The column density of NH in the
no-sink model depends sensitively on the desorption rate of CO and N, and
the flux of cosmic ray. We also found that NH abundance can peak at the
temperature slightly below the CO sublimation, even if the desorption energies
of CO and N are the same.Comment: accepted to Ap
Changes in Peripheral Anterior Chamber Depth of a Case of Relapsing Polychondritis with Recurrent Secondary Angle Closure Glaucoma§
A case of relapsing polychondritis showed IOP elevations three times during the follow-up due to the angle-closure mechanism. The peripheral anterior chamber depth (ACD) showed a good correlation with IOP elevation, but central ACD did not. The peripheral ACD could be more related to IOP elevation than central ACD
Tracing the atomic nitrogen abundance in star-forming regions with ammonia deuteration
Partitioning of elemental nitrogen in star-forming regions is not well constrained. Most nitrogen is expected to be partitioned among atomic nitrogen (N i), molecular nitrogen (N2), and icy N-bearing molecules, such as NH3 and N2. N i is not directly observable in the cold gas. In this paper, we propose an indirect way to constrain the amount of N i in the cold gas of star-forming clouds, via deuteration in ammonia ice, the [ND2H/NH2D]/[NH2D/NH3] ratio. Using gas–ice astrochemical simulations, we show that if atomic nitrogen remains as the primary reservoir of nitrogen during cold ice formation stages, the [ND2H/NH2D]/[NH2D/NH3] ratio is close to the statistical value of 1/3 and lower than unity, whereas if atomic nitrogen is largely converted into N-bearing molecules, the ratio should be larger than unity. Observability of ammonia isotopologues in the inner hot regions around low-mass protostars, where ammonia ice has sublimated, is also discussed. We conclude that the [ND2H/NH2D]/[NH2D/NH3] ratio can be quantified using a combination of Very Large Array and Atacama Large Millimeter/submillimeter Array observations with reasonable integration times, at least towards IRAS 16293−2422, where high molecular column densities are expected
The Detection of Hot Molecular Cores in the Small Magellanic Cloud
We report the first detection of hot molecular cores in the Small Magellanic
Cloud, a nearby dwarf galaxy with 0.2 solar metallicity. We observed two
high-mass young stellar objects in the SMC with ALMA, and detected emission
lines of CO, HCO+, H13CO+, SiO, H2CO, CH3OH, SO, and SO2. Compact hot-core
regions are traced by SO2, whose spatial extent is about 0.1 pc, and the gas
temperature is higher than 100 K based on the rotation diagram analysis. In
contrast, CH3OH, a classical hot-core tracer, is dominated by extended (0.2-0.3
pc) components in both sources, and the gas temperature is estimated to be
39+-8 K for one source. Protostellar outflows are also detected from both
sources as high-velocity components of CO. The metallicity-scaled abundances of
SO2 in hot cores are comparable among the SMC, LMC, and Galactic sources,
suggesting that the chemical reactions leading to SO2 formation would be
regulated by elemental abundances. On the other hand, CH3OH shows a large
abundance variation within SMC and LMC hot cores. The diversity in the initial
condition of star formation (e.g., degree of shielding, local radiation field
strength) may lead to the large abundance variation of organic molecules in hot
cores. This work, in conjunction with previous hot-core studies in the LMC and
outer/inner Galaxy, suggests that the formation of a hot core would be a common
phenomenon during high-mass star formation across the metallicity range of
0.2-1 solar metallicity. High-excitation SO2 lines will be a useful hot-core
tracer in the low-metallicity environments of the SMC and LMC.Comment: Accepted for publication in ApJL, 17 pages, 8 figures, 4 tables.
arXiv admin note: text overlap with arXiv:2109.1112
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