The TOPGöt high-mass star-forming sample

I. Methyl cyanide emission as tracer of early phases of star formation^★

¹ Dipartimento di Fisica e Astronomia, Universitá degli Studi di Firenze, Via Sansone 1, 50019 Sesto Fiorentino, Italy
e-mail: chiara.mininni@inaf.it
² INAF Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
³ Istituto di Astrofisica e Planetologia Spaziali, INAF, Via Fosso del Cavaliere 100, 00133 Roma, Italy
⁴ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
⁵ Centro de Astrobiología (CSIC, INTA), Ctra. de Ajalvir, km. 4, Torrejón de Ardoz, 28850 Madrid, Spain
⁶ Department of Physical Science, Graduate School of Science, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
⁷ National Astronomical Observatory of Japan, National Institutes of Natural Science, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁸ Joint Institute for VLBI ERIC, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
⁹ Leiden Observatory, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands
¹⁰ INAF, Istituto di Radioastronomia, Italian ARC, Via P. Gobetti 101, Bologna, Italy
¹¹ Max-Planck-Institut für Extraterrestrische Physik, Gießenbachstraße 1, 85748 Garching bei München, Germany
¹² European Southern Observatory (ESO), Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
¹³ Excellence Cluster Origins, Boltzmannstrasse 2, 85748 Garching bei München, Germany

Received: 30 December 2020
Accepted: 14 July 2021

Abstract

Aims. The TOPGöt project studies a sample of 86 high-mass star-forming regions in different evolutionary stages from starless cores to ultra compact HII regions. The aim of the survey is to analyze different molecular species in a statistically significant sample to study the chemical evolution in high-mass star-forming regions, and identify chemical tracers of the different phases.

Methods. The sources have been observed with the IRAM 30 m telescope in different spectral windows at 1, 2, and 3 mm. In this first paper, we present the sample and analyze the spectral energy distributions (SEDs) of the TOPGöt sources to derive physical parameters such as the dust temperature, T_dust, the total column density, N_H2, the mass, M, the luminosity, L, and the luminosity-to-mass ratio, L∕M, which is an indicator of the evolutionary stage of the sources. We use the MADCUBA software to analyze the emission of methyl cyanide (CH₃CN), a well-known tracer of high-mass star formation.

Results. We built the spectral energy distributions for ~80% of the sample and derived T_dust and N_H2 values which range between 9−36 K and ~3 × 10²¹−7 × 10²³ cm⁻², respectively. The luminosity of the sources spans over four orders of magnitude from 30 to 3 × 10⁵ L_⊙, masses vary between ~30 and 8 × 10³ M_⊙, and the luminosity-to-mass ratio L∕M covers three orders of magnitude from 6 × 10⁻² to 3 × 10² L_⊙∕M_⊙. The emission of the CH₃CN(5_K-4_K) K-transitions has been detected toward 73 sources (85% of the sample), with 12 nondetections and one source not observed in the frequency range of CH₃CN(5_K-4_K). The emission of CH₃CN has been detected toward all evolutionary stages, with the mean abundances showing a clear increase of an order of magnitude from high-mass starless cores to later evolutionary stages. We found a conservative abundance upper limit for high-mass starless cores of X_CH3CN < 4.0 × 10⁻¹¹, and a range in abundance of 4.0 × 10⁻¹¹ < X_CH3CN < 7.0 × 10⁻¹¹ for those sources that are likely high-mass starless cores or very early high-mass protostellar objects. In fact, in this range of abundance we have identified five sources previously not classified as being in a very early evolutionary stage. The abundance of CH₃CN can thus be used to identify high-mass star-forming regions in early phases of star-formation.