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Register a new userNitrogen isotope fractionation in protoplanetary disks
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Authors
Ruud Visser
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Aims: The two stable isotopes of nitrogen, 14N and 15N, exhibit a range of abundance ratios both inside and outside the solar system. The elemental ratio in the solar neighborhood is 440. Recent ALMA observations showed HCN/HC15N ratios from 83 to 156 in six T Tauri and Herbig disks and a CN/C15 N ratio of 323 +/- 30 in one T Tauri star. We aim to determine the dominant mechanism responsible for these enhancements of 15N: low-temperature exchange reactions or isotope-selective photodissociation of N2. Methods: Using the thermochemical code DALI, we model the nitrogen isotope chemistry in circumstellar disks with a 2D axisymmetric geometry. Our chemical network is the first to include both fractionation mechanisms for nitrogen. The model produces abundance profiles and isotope ratios for several key N-bearing species. We study how these isotope ratios depend on various disk parameters. Results: The formation of CN and HCN is closely coupled to the vibrational excitation of H2 in the UV-irradiated surface layers of the disk. Isotope fractionation is completely dominated by isotope-selective photodissociation of N2. The column density ratio of HCN over HC15N in the disks inner 100 au does not depend strongly on the disk mass, the flaring angle or the stellar spectrum, but it is sensitive to the grain size distribution. For larger grains, self-shielding of N2 becomes more important relative to dust extinction, leading to stronger isotope fractionation. Between disk radii of ~50 and 200 au, the models predict HCN/HC15N and CN/C15N abundance ratios consistent with observations of disks and comets. The HCN/HC15N and CN/C15N column density ratios in the models are a factor of 2-3 higher than those inferred from the ALMA observations.
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We investigate the gas-phase and grain-surface chemistry in the inner 30 AU of a typical protoplanetary disk using a new model which calculates the gas temperature by solving the gas heating and cooling balance and which has an improved treatment of the UV radiation field. We discuss inner-disk chemistry in general, obtaining excellent agreement with recent observations which have probed the material in the inner regions of protoplanetary disks. We also apply our model to study the isotopic fractionation of carbon. Results show that the fractionation ratio, 12C/13C, of the system varies with radius and height in the disk. Different behaviour is seen in the fractionation of different species. We compare our results with 12C/13C ratios in the Solar System comets, and find a stark contrast, indicative of reprocessing.
The dominant reservoirs of elemental nitrogen in protoplanetary disks have not yet been observationally identified. Likely candidates are HCN, NH$_3$ and N$_2$. The relative abundances of these carriers determine the composition of planetesimals as a function of disk radius due to strong differences in their volatility. A significant sequestration of nitrogen in carriers less volatile than N$_2$ is likely required to deliver even small amounts of nitrogen to the Earth and potentially habitable exo-planets. While HCN has been detected in small amounts in inner disks ($<10$ au), so far only relatively insensitive upper limits on inner disk NH$_3$ have been obtained. We present new Gemini-TEXES high resolution spectroscopy of the 10.75 $mu$m band of warm NH$_3$, and use 2-dimensional radiative transfer modeling to improve previous upper limits by an order of magnitude to $rm [NH_3/H_{nuc}]<10^{-7}$ at 1 au. These NH$_3$ abundances are significantly lower than those typical for ices in circumstellar envelopes ($[{rm NH_3/H_{nuc}}]sim 3times 10^{-6}$). We also consistently retrieve the inner disk HCN gas abundances using archival Spitzer spectra, and derive upper limits on the HCN ice abundance in protostellar envelopes using archival ground-based 4.7 $mu$m spectroscopy ([HCN$_{rm ice}$]/[H$_2$O$_{rm ice}$]$<1.5-9$%). We identify the NH$_3$/HCN ratio as an indicator of chemical evolution in the disk, and use this ratio to suggest that inner disk nitrogen is efficiently converted from NH$_3$ to N$_2$, significantly increasing the volatility of nitrogen in planet-forming regions.
(Abridged) The terrestrial planets, comets, and meteorites are significantly enriched in 15N compared to the Sun and Jupiter. While the solar and jovian nitrogen isotope ratio is believed to represent the composition of the protosolar nebula, a still unidentified process has caused 15N-enrichment in the solids. Several mechanisms have been proposed to explain the variations, including chemical fractionation. However, observational results that constrain the fractionation models are scarce. While there is evidence of 15N-enrichment in prestellar cores, it is unclear how the signature evolves into the protostellar phases. Our aim is to measure the 14N/15N ratio around three nearby, embedded low-to-intermediate-mass protostars. Isotopologues of HCN and HNC were used to probe the 14N/15N ratio. A selection of H13CN, HC15N, HN13C, and H15NC transitions was observed with the APEX telescope. The 14N/15N ratios were derived from the integrated intensities assuming a standard 12C/13C ratio. The assumption of optically thin emission was verified using radiative transfer modeling and hyperfine structure fitting. Two sources, IRAS 16293A and R CrA IRS7B, show 15N-enrichment by a factor of around 1.5-2.5 in both HCN and HNC with respect to the solar composition. Solar composition cannot be excluded for the third source, OMC-3 MMS6. Furthermore, there are indications of a trend toward increasing 14N/15N ratios with increasing outer envelope temperature. The enhanced 15N abundances in HCN and HNC found in two Class~0 sources (14N/15N of 160-290) and the tentative trend toward a temperature-dependent 14N/15N ratio are consistent with the chemical fractionation scenario, but 14N/15N ratios from additional tracers are indispensable for testing the models. Spatially resolved observations are needed to distinguish between chemical fractionation and isotope-selective photochemistry.
The dominant form of nitrogen provided to most solar system bodies is currently unknown, though available measurements show that the detected nitrogen in solar system rocks and ices is depleted with respect to solar abundances and the interstellar medium. We use a detailed chemical/physical model of the chemical evolution of a protoplanetary disk to explore the evolution and abundance of nitrogen-bearing molecules. Based on this model we analyze how initial chemical abundances, provided as either gas or ice during the early stages of disk formation, influence which species become the dominant nitrogen bearers at later stages. We find that a disk with the majority of its initial nitrogen in either atomic or molecular nitrogen is later dominated by atomic and molecular nitrogen as well as NH$_{3}$ and HCN ices, where the dominant species varies with disk radius. When nitrogen is initially in gaseous ammonia, it later becomes trapped in ammonia ice except in the outer disk where atomic nitrogen dominates. For a disk with the initial nitrogen in the form of ammonia ice the nitrogen remains trapped in the ice as NH$_{3}$ at later stages. The model in which most of the initial nitrogen is placed in atomic N best matches the ammonia abundances observed in comets. Furthermore the initial state of nitrogen influences the abundance of N$_{2}$H$^{+}$, which has been detected in protoplanetary disks. Strong N$_{2}$H$^{+}$ emission is found to be indicative of an N$_{2}$ abundance greater than $n_{mathrm{N_{2}}}/n_{mathrm{H_{2}}}>10^{-6}$, in addition to tracing the CO snow line. Our models also indicate that NO is potentially detectable, with lower N gas abundances leading to higher NO abundances.
Molecular line emission from protoplanetary disks is a powerful tool to constrain their physical and chemical structure. Nevertheless, only a few molecules have been detected in disks so far. We take advantage of the enhanced capabilities of the IRAM 30m telescope by using the new broad band correlator (FTS) to search for so far undetected molecules in the protoplanetary disks surrounding the TTauri stars DM Tau, GO Tau, LkCa 15 and the Herbig Ae star MWC 480. We report the first detection of HC3N at 5 sigma in the GO Tau and MWC 480 disks with the IRAM 30-m, and in the LkCa 15 disk (5 sigma), using the IRAM array, with derived column densities of the order of 10^{12}cm^{-2}. We also obtain stringent upper limits on CCS (N < 1.5 x 10^{12} cm^{-3}). We discuss the observational results by comparing them to column densities derived from existing chemical disk models (computed using the chemical code Nautilus) and based on previous nitrogen and sulfur-bearing molecule observations. The observed column densities of HC3N are typically two orders of magnitude lower than the existing predictions and appear to be lower in the presence of strong UV flux, suggesting that the molecular chemistry is sensitive to the UV penetration through the disk. The CCS upper limits reinforce our model with low elemental abundance of sulfur derived from other sulfur-bearing molecules (CS, H2S and SO).
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