[Paper Review] Detection of N15NH+ in L1544

L. Bizzocchi, P. Caselli+1 more|arXiv (Cornell University)|Jan 27, 2010

Astro and Planetary Science3 citations

TL;DR

This study reports the first detection of N¹⁵NH⁺ in the cold prestellar core L1544 using the IRAM 30 m telescope. The measured ¹⁴N/¹⁵N abundance ratio of 446 ± 71 in N¹⁵NH⁺ matches the protosolar value (~450), providing direct observational support for nitrogen isotope fractionation in cold interstellar environments via gas-phase ion-molecule reactions and selective depletion of CO, as predicted by chemical models.

ABSTRACT

Excess levels of 15N isotopes which have been detected in primitive solar system materials are explained as a remnant of interstellar chemistry which took place in regions of the protosolar nebula. Chemical models of nitrogen fractionation in cold clouds predict an enhancement in the gas-phase abundance of 15N-bearing molecules, thus we have searched for 15N variants of the N2H+ ion in L1544, which is one of the best candidate sources for detection owing to its low central core temperature and high CO depletion. With the IRAM 30m telescope we have obtained deep integrations of the N2H+(1-0) line at 91.2 GHz. The N2H+(1-0) line has been detected toward the dust emission peak of L1544. The 14N/15N abundance ratio in N2H+ resulted 446+/-71, very close to the protosolar value of ~450, higher than the terrestrial ratio of ~270, and significantly lower than the lower limit in L1544 found by Gerin et al. (2009, ApJ, 570, L101) in the same object using ammonia isotopologues.

Motivation & Objective

• To test theoretical models of nitrogen isotope fractionation in cold, dense interstellar environments by searching for ¹⁵N-bearing molecular ions.
• To investigate whether the enhanced ¹⁵N abundance observed in primitive solar system materials originated in cold prestellar cores via gas-phase ion-molecule reactions.
• To determine the ¹⁴N/¹⁵N ratio in a key nitrogen-bearing ion, N¹⁵NH⁺, in L1544, a prototypical quiescent, CO-depleted core.
• To compare the observed ¹⁴N/¹⁵N ratio in N¹⁵NH⁺ with those derived from ammonia isotopologues and terrestrial/protosolar values to assess the validity of current chemical models.

Proposed method

• Deep integration of the N¹⁵NH⁺ (1-0) rotational transition line at 91.2 GHz using the IRAM 30 m telescope toward the dust emission peak of L1544.
• Spectral analysis using the radiative transfer equation to derive the excitation temperature and column density of N¹⁵NH⁺, accounting for hyperfine structure and Einstein A coefficients.
• Calculation of the Einstein A coefficient for the F=2–1 hyperfine transition using a weighted average of dipole moments (μ = 3.31 ± 0.20 D), yielding A = 3.23 ± 0.40 × 10⁻⁵ s⁻¹.
• Use of the spin-rotational partition function Q_sr(T_ex) and the main beam brightness temperature T_mb to compute the column density via the radiative transfer equation.
• Comparison of the derived N¹⁵NH⁺ column density with that of N₂H⁺ (1.83 × 10¹³ cm⁻²) to determine the ¹⁴N/¹⁵N abundance ratio.
• Propagation of uncertainties from A, T_ex, and integrated intensity to estimate the final error budget on the abundance ratio.

Experimental results

Research questions

• RQ1 Is the ¹⁴N/¹⁵N ratio in N¹⁵NH⁺ in L1544 consistent with the protosolar value of ~450?
• RQ2 Does the observed ¹⁴N/¹⁵N ratio in N¹⁵NH⁺ support the theoretical prediction that nitrogen isotope fractionation occurs in cold, CO-depleted prestellar cores?
• RQ3 How does the ¹⁴N/¹⁵N ratio in N¹⁵NH⁺ compare with the lower limit derived from non-detection of ¹⁵NH₂D in the same source by Gerin et al. (2009)?
• RQ4 Is the observed abundance ratio in N¹⁵NH⁺ compatible with the predictions of time-dependent gas/solid chemical models of nitrogen fractionation?
• RQ5 What does the N¹⁵NH⁺ abundance imply about the relative fractionation of different nitrogen-bearing species in cold cores?

Key findings

• The N¹⁵NH⁺ (1-0) line was successfully detected toward the dust peak of L1544 using the IRAM 30 m telescope.
• The derived ¹⁴N/¹⁵N abundance ratio in N¹⁵NH⁺ is 446 ± 71, which is in excellent agreement with the protosolar value of ~450.
• This ratio is significantly higher than the terrestrial ¹⁴N/¹⁵N ratio (~270) and lower than the lower limit of 700 inferred from the non-detection of ¹⁵NH₂D in L1544 by Gerin et al. (2009).
• The measured ¹⁴N/¹⁵N ratio in N¹⁵NH⁺ is consistent with the elemental isotopic ratio in the local interstellar medium, suggesting that N¹⁵NH⁺ may not be strongly fractionated beyond the elemental abundance.
• The result challenges current models predicting strong enhancement of ¹⁵N in N₂H⁺ due to selective depletion and ion-molecule reactions, as the observed ratio does not show the expected overabundance.
• The detection of N¹⁵NH⁺ supports the role of cold, CO-depleted environments in nitrogen isotope fractionation, but highlights the need for further observations of other isotopologues, such as ¹⁵NNH⁺, to test model predictions.