Signatures of UV radiation in low-mass protostars

I. Origin of HCN and CN emission in the Serpens Main region^★

¹ Astronomical Observatory of the Jagiellonian University, ul. Orla 171, 30-244 Kraków, Poland
² Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, ul. Grudziądzka 5, 87-100 Toruń, Poland
e-mail: agata.karska@umk.pl
³ Institute of Physical Chemistry Polish Academy of Sciences, ul. Kasprzaka 44/52, 01-224 Warszawa, Poland
⁴ Faculty of Physics, University of Warsaw, Pasteura 5, 02-093 Warsaw, Poland
⁵ Niels Bohr Institute & Centre for Star and Planet Formation, Copenhagen University, Øster Voldgade 5–7, 1350 Copenhagen K, Denmark
⁶ Leiden Observatory, Leiden University, PO Box 9513, 2300RA Leiden, The Netherlands
⁷ European Southern Observatory, Karl-Schwarzschild-Str 2, 85748 Garching near Munich, Germany
⁸ Institute of Astronomy and Astrophysics, Academia Sinica, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan, ROC
⁹ National Centre for Nuclear Research, ul. Pasteura 7, 02-093 Warszawa, Poland
¹⁰ Nicolaus Copernicus Astronomical Center, ul. Rabiańska 8, 87-100 Toruń, Poland
¹¹ University of Le Havre, Laboratoire Ondes et Milieux Complexes, UMR CNRS 6294, 75 Rue Bellot, 76600 Le Havre, France
¹² Univ Rennes, CNRS, IPR (Institut de Physique de Rennes) – UMR 6251, 35000 Rennes, France

Received: 19 March 2021
Accepted: 13 September 2021

Abstract

Context. Ultraviolet radiation (UV) influences the physics and chemistry of star-forming regions, but its properties and significance in the immediate surroundings of low-mass protostars are still poorly understood.

Aims. Our aim is to extend the use of the CN/HCN ratio, already established for high-mass protostars, to the low-mass regime to trace and characterize the UV field around low-mass protostars on ~0.6 × 0.6 pc scales.

Methods. We present 5′ × 5′ maps of the Serpens Main Cloud encompassing ten protostars observed with the EMIR receiver at the IRAM 30 m telescope in CN 1–0, HCN 1–0, CS 3–2, and some of their isotopologs. The radiative-transfer code RADEX and the chemical model Nahoon were used to determine column densities of molecules, gas temperature and density, and the UV field strength, G₀.

Results. The spatial distribution of HCN and CS are closely correlated with CO 6–5 emission, that traces outflows. The CN emission is extended from the central protostars to their immediate surroundings also tracing outflows, likely as a product of HCN photodissociation. The ratio of CN to HCN total column densities ranges from ~1 to 12 corresponding to G₀ ≈ 10¹–10³ for gas densities and temperatures typical for outflows of low-mass protostars.

Conclusions. UV radiation associated with protostars and their outflows is indirectly identified in a significant part of the Serpens Main low-mass star-forming region. Its strength is consistent with the values obtained from the OH and H₂O ratios observed with Herschel and compared with models of UV-illuminated shocks. From a chemical viewpoint, the CN to HCN ratio is an excellent tracer of UV fields around low- and intermediate-mass star-forming regions.