J/A+A/644/A29         14N/15N ratio in YSOs                    (Redaelli+, 2020)

First sample of N2H+ nitrogen isotopic ratio measurements in low-mass protostars. Redaelli E., Bizzocchi L., Caselli P. <Astron. Astrophys. 644, A29 (2020)> =2020A&A...644A..29R 2020A&A...644A..29R (SIMBAD/NED BibCode)
ADC_Keywords: YSOs ; Molecular clouds ; Radio lines ; Spectroscopy Keywords: ISM: clouds - ISM: molecules - ISM: abundances - radio lines: ISM - stars: formation Abstract: The nitrogen isotopic ratio is considered an important diagnostic tool of the star formation process, and N2H+ is particularly important because it is directly linked to molecular nitrogen N2. However, theoretical models still lack to provide an exhaustive explanation for the observed14N/15N values. Recent theoretical works suggest that the14N/15N behaviour is dominated by two competing reactions that destroy N2H+: dissociative recombination and reaction with CO. When CO is depleted from the gas phase, if N2H+ recombination rate is lower with respect to the N15NH+ one, the rarer isotopologue is destroyed faster. In prestellar cores, due to a combination of low temperatures and high densities, most CO is frozen in ices onto the dust grains, leading to high levels of depletion. On the contrary, in protostellar cores, where temperature are higher, CO ices evaporate back to the gas phase. This implies that the N2H+ isotopic ratio in protostellar cores should be lower than the one in prestellar cores, and consistent with the elemental value of ∼440. We aim to test this hypothesis, producing the first sample of N2H+ /N15NH+ measurements in low mass protostars. We observe the N2H+ and N15NH+ lowest rotational transition towards six young stellar objects in Perseus and Taurusmolecular clouds. We model the spectra with a custom python code using a constant Tex approach to fit the observations. We discuss in appendix the validity of this hypothesis. The derived column densities are used to compute the nitrogen isotopic ratios. Our analysis yields an average of14N/15N|pro=420±15 in the protostellar sample. This is consistent with the protosolar value of 440, and significantly lower than the average value previously obtained in a sample of prestellar objects. Our results are in agreement with the hypothesis that, when CO is depleted from the gas-phase, dissociative recombinations with free electrons destroy N15NH+ faster than N2H+ , leading to high isotopic ratios in prestellar cores, where carbon monoxide is frozen onto dust grains. Description: We report the detection of N15NH+ (1-0) line with the IRAM 30m telescope in six young protostellar cores (Barnard 5, IRAS 03282, L448 C, L1448 IRS2, L1455, and L1527). We observe at the same time the N2H+ (1-0) line, allowing us to derive the 14N/15N ratio. The derived nitrogen isotopic ratios are consistent with the elemental value of 440, and significantly lower than the values found in a prestellar sample by Redaelli et al. (2018A&A...617A...7R 2018A&A...617A...7R, Cat. J/A+A/617/A7). File Summary: -------------------------------------------------------------------------------- FileName Lrecl Records Explanations -------------------------------------------------------------------------------- ReadMe 80 . This file list.dat 94 12 List of spectra sp/* . 12 Individual spectra -------------------------------------------------------------------------------- See also: J/A+A/617/A7 : Nitrogen isotopic ratio in prestellar cores (Redaelli+, 2018) Byte-by-byte Description of file: list.dat -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 1- 14 A14 --- Name Name 16- 17 I2 h RAh Right ascension (J2000) 19- 20 I2 min RAm Right ascension (J2000) 22- 26 F5.2 s RAs Right ascension (J2000) 28 A1 --- DE- Declination sign (J2000) 29- 30 I2 deg DEd Declination (J2000) 32- 33 I2 arcmin DEm Declination (J2000) 35- 38 F4.1 arcsec DEs Declination (J2000) 40- 49 A10 --- Mol Molecule 51- 70 A20 --- FileName Name of the spectrum file in subdirectory sp 73- 94 A22 --- SName Simbad name -------------------------------------------------------------------------------- Byte-by-byte Description of file (#): sp/* -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 1- 12 E12.4 km/s Vlsr Velocity 14- 25 E12.4 K Tmb Main beam temperature -------------------------------------------------------------------------------- Acknowledgements: Elena Redaelli, eredaelli(at)mpe.mpg.de
(End) Elena Redaelli [MPE, Germany], Patricia Vannier [CDS] 20-Oct-2020
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