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A study of the 13C and 15N fractionation in low-mass starless cores

Sigurd, Jensen

Publisher
Zenodo
DOI
10.5281/zenodo.17108797
Repository
zenodo.org
Language
en

Abstract

The nitrogen isotopic ratio 14N/15N can provide crucial insights into the chemical evolution during star and planet formation (e.g., Ceccarelli+2014). The 14N/15N ratio is often determined assuming a constant 12C/13C ratio (double isotope method). However, this assumption has been challenged by recent astrochemical models of carbon fractionation which show a significant variation in carbon fractionation in starless cores at different chemical ages and physical conditions (Colzi+2020, Sipilä+2023). I will present IRAM 30m observations of HCN, H13CN, and HC15N toward 6 low-mass starless cores (Jensen+2024). The 12C/13C and 14N/15N ratios for HCN are derived using the double isotope method and directly through 1D line-radiative transfer modeling of the emission lines. The results imply that a fixed 12C/13C ratio is a poor assumption for starless cores and reveal a substantial difference between the nitrogen fractionation ratio in HCN derived using direct and indirect methods. This can strongly affect the interpretation of the chemical processing in fractionation studies and questions the reliability of the double isotope method.

Full text

FRONTIERS: The astrochemical journey from young stellar nurseries to exoplanets A study of the 13C and 15N fractionation in low-mass starless cores Sigurd S. Jensen, Silvia Spezzano, Paola Caselli, Olli Sipilä, Elena Redaelli, Katharina Giers, Judit Ferrer Asensio CAS-MPE, Garching Credit: Bill Saxton (NRAO) Chemical evolution during starand planet formation Dense core Planetary system Protostellar phase Protoplanetary disk Credit: Bill Saxton (NRAO) Chemical evolution during starand planet formation Nitrogen and carbon fractionation! of HCN Inheritance or reset? Impact of the environment in fractionation Spezzano et al. (2022) Local environment / Core structure Cloud environment Jensen et al. (2019) •Nitrogen isotopic fractionation (14N/15N) is a powerful tracer of chemical evolution. ! •Fractionation processes depend on the physical conditions during the formation of the nitrogen bearing molecule.! •So far, nitrogen fractionation is puzzling, with both increased and decreased 14N/15N ratios observed (with respect to Solar Nebula values), depending on the molecular carrier.! •Several different processes at play:! •Isotope-selective photodissociation! •Nucleosynthetic effects! •Chemical imbalances (endothermic reactions) Tracing chemical evolution during starand planet formation Nitrogen fractionation Tracing chemical evolution during starand planet formation Nitrogen fractionation of HCN References in Jensen et al. (2024)! Tracing chemical evolution during starand planet formation Nitrogen fractionation of HCN Main goal: Increasing sample size References in Jensen et al. (2024)! •Two methods:! •Indirect double isotope method: observations of H13CN and HC15N and assumed fixed 12C/13C ratio (68, Milam+2005).! •Direct method: Observations of (optically thick) HCN, along with H13CN and HC15N. Spectral line modeling to determine both 14N/15N and 12C/13C ratios. Determining the 14N/15N ratio of HCN Observations of starless and pre-stellar cores Credit: Bill Saxton (NRAO) IRAM 30m telescopeIRAM 30m Credit:IRAM,K.Zacher FTS50 for a 50 kHz resolution to resolve HFS Direct method Pre-stellar core TMC2 Jensen et al. (2024)! Direct method Jensen et al. (2024)! Direct method Direct Indirect Jensen et al. (2024)! A lower 12C/13C ratio in young cores? •Direct method suggest a lower values:! L1495 (45, Magalhães et al. 2018), ! B1-b (30, Daniel et al. 2013)! L483 (34, Agúndez et al. 2019)! This work.! •Modeling suggest ratio evolves over time:! Loison et al. 2020, Colzi et al. 2020, Sipilä et al. 2023 Sipilä et al. (2023)! Loison et al. (2020)! Summary •Using the indirect method we estimated the HCN 14N/15N ratio for 6 cores. The results for the indirect method are close to the PSN values of around 440.! •Using the direct method, we are able to estimate both 14N/15N and 12C/13C ratios for five out of six cores. The results show lower 12C/13C ratios between 20-40 for 4/5 cores.! •We need to be careful using the indirect method. However, the direct method requires a well-defined source structure.! •The results show no clear evolution between the pre-stellar and protostellar phases. Protoplanetary disks are generally lower, but most are derived using the indirect method. We need more data! FRONTIERS: The astrochemical journey from young stellar nurseries to exoplanets