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:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
list.dat 94 12 List of spectra
sp/* . 12 Individual spectra
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See also:
J/A+A/617/A7 : Nitrogen isotopic ratio in prestellar cores (Redaelli+, 2018)
Byte-by-byte Description of file: list.dat
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Bytes Format Units Label Explanations
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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
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Byte-by-byte Description of file (#): sp/*
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Bytes Format Units Label Explanations
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1- 12 E12.4 km/s Vlsr Velocity
14- 25 E12.4 K Tmb Main beam temperature
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Acknowledgements:
Elena Redaelli, eredaelli(at)mpe.mpg.de
(End) Elena Redaelli [MPE, Germany], Patricia Vannier [CDS] 20-Oct-2020