Sulphur and carbon isotopes towards Galactic centre clouds^★

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: phumire@mpifr-bonn.mpg.de
² Department of Astronomy, King Abdulaziz University, PO Box 80203, Jeddah 21589, Saudi Arabia
³ Xinjiang Astronomical Observatory, Chinese Academy of Sciences, 830011 Urumqi, PR China
⁴ Institut des Sciences Moléculaires (ISM), CNRS, Univ. Bordeaux, 351 cours de la Libération, 33400 Talence, France
⁵ Institute for Astrophysical Research, 725 Commonwealth Avenue, Boston University Boston, MA 02215, USA
⁶ Laboratoire d’astrophysique de Bordeaux, CNRS, Univ. Bordeaux, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
⁷ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany

Received: 20 April 2020
Accepted: 30 August 2020

Abstract

Context. Measuring isotopic ratios is a sensitive technique used to obtain information on stellar nucleosynthesis and chemical evolution.

Aims. We present measurements of the carbon and sulphur abundances in the interstellar medium of the central region of our Galaxy. The selected targets are the +50 km s⁻¹ Cloud and several line-of-sight clouds towards Sgr B2(N).

Methods. Towards the +50 km s⁻¹ Cloud, we observed the J = 2–1 rotational transitions of ¹²C³²S, ¹²C³⁴S, ¹³C³²S, ¹²C³³S, and ¹³C³⁴S, and the J = 3–2 transitions of ¹²C³²S and ¹²C³⁴S with the IRAM-30 m telescope, as well as the J = 6–5 transitions of ¹²C³⁴S and ¹³C³²S with the APEX 12 m telescope, all in emission. The J = 2–1 rotational transitions of ¹²C³²S, ¹²C³⁴S, ¹³C³²S, and ¹³C³⁴S were observed with ALMA in the envelope of Sgr B2(N), with those of ¹²C³²S and ¹²C³⁴S also observed in the line-of-sight clouds towards Sgr B2(N), all in absorption.

Results. In the +50 km s⁻¹ Cloud we derive a ¹²C/¹³C isotopic ratio of 22.1_−2.4^+3.3, that leads, with the measured ¹³C³²S/¹²C³⁴S line intensity ratio, to a ³²S/³⁴S ratio of 16.3_−2.4^+3.0. We also derive the ³²S/³⁴S isotopic ratio more directly from the two isotopologues ¹³C³²S and ¹³C³⁴S, which leads to an independent ³²S/³⁴S estimation of 16.3_−1.7^+2.1 and 17.9 ± 5.0 for the +50 km s⁻¹ Cloud and Sgr B2(N), respectively. We also obtain a ³⁴S/³³S ratio of 4.3 ± 0.2 in the +50 km s⁻¹ Cloud.

Conclusions. Previous studies observed a decreasing trend in the ³²S/³⁴S isotopic ratios when approaching the Galactic centre. Our result indicates a termination of this tendency at least at a galactocentric distance of 130₋₃₀⁺⁶⁰ pc. This is at variance with findings based on ¹²C/¹³C, ¹⁴N/¹⁵N, and ¹⁸O/¹⁷O isotope ratios, where the above-mentioned trend is observed to continue right to the central molecular zone. This can indicate a drop in the production of massive stars at the Galactic centre, in the same line as recent metallicity gradient ([Fe/H]) studies, and opens the work towards a comparison with Galactic and stellar evolution models.