Signatures of UV radiation in low-mass protostars

Agnieszka Mirocha,Łukasz Tychoniec,Agata Karska,Marcin Gronowski,Marcin Gładkowski,Lars E Kristensen,Michał Żółtowski,Miguel Figueira,Daniel Harsono

doi:10.1051/0004-6361/202140833

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.

Highlights

The formation of low-mass stars is associated with many physical phenomena
IRAM 30 m / Eight MIxer Receiver (EMIR) observations of CN and HCN emission pinpoint the location of the impact of Ultraviolet radiation (UV) radiation on the chemistry of low-mass star forming region in the Serpens Main
Column densities of HCN corrected for optical depth and determined from a simple scaling of H13CN agree within a factor of 2. – For typical physical conditions of the gas in low-mass star forming regions column density ratio of CN and HCN is in the range of 1–12

Summary

Introduction

The formation of low-mass stars is associated with many physical phenomena. The inside-out collapse of dense cores is accompanied by the ejection of bipolar outflows and the formation of embedded disks Ultraviolet (UV) radiation can be produced in mass accretion on the central object or bow shocks, and it irradiates the outflow cavities in the envelopes (Spaans et al 1995; van Kempen et al 2009a; Visser et al 2012; Drozdovskaya et al 2015). The importance of UV radiation for star formation was initially considered only in the context of massive stars, where the central stars are the main source of UV photons from early stages (Cesaroni 2005; Zinnecker & Yorke 2007).

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