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 N$_2$H$^+$ ring around TW Hya. While the ring inner radius coincides with the CO snow line, it is not apparent why N$_2$H$^+$ is abundant outside the CO snow line in spite of the similar sublimation temperatures of CO and N$_2$. Using the full gas-grain network model, we reproduced the N$_2$H$^+$ ring in a disk model with millimeter grains. The chemical conversion of CO and N$_2$ to less volatile species (sink effect hereinafter) is found to affect the N$_2$H$^+$ 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$_2$ 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 N$_2$H$^+$ ring is reproduced by the no-sink model, as well. The 2D (R-Z) distribution of N$_2$H$^+$, however, is different among the full-network model and no-sink model. The column density of N$_2$H$^+$ in the no-sink model depends sensitively on the desorption rate of CO and N$_2$, and the flux of cosmic ray. We also found that N$_2$H$^+$ abundance can peak at the temperature slightly below the CO sublimation, even if the desorption energies of CO and N$_2$ 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