Isolated massive star candidates in NGC 4242 with the Galaxy UV Legacy Project

¹ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg Mönchhofstr. 12-14 D-69120 Heidelberg, Germany
² Gemini Observatory/NSFs NOIRLab 950 N. Cherry Ave. Tucson AZ 85719, USA
³ Space Telescope Science Institute 3700 San Martin Dr Baltimore MD 21218, USA
⁴ Department of Astronomy, University of Wisconsin-Madison 475 North Charter St. Madison WI 53706, USA
⁵ Katonah NY 10536, USA
⁶ Department of Physics & Astronomy, The Johns Hopkins University, 3400 N. Charles St. Baltimore MD 21218, USA
⁷ Department of Astronomy, Oskar Klein Centre, Stockholm University, AlbaNova University Center SE-106 91 Stockholm, Sweden
⁸ Department of Astronomy, University of Massachusetts Amherst 710 North Pleasant Street Amherst MA 01003, USA
⁹ Dipartimento di Fisica, Università di Pisa Largo Bruno Pontecorvo 3 56127 Pisa, Italy
¹⁰ INFN Largo B. Pontecorvo 3 56127 Pisa, Italy
¹¹ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna Via Piero Gobetti 93/3 40129 Bologna, Italy
¹² Department of Physics & Astronomy, University of Sheffield Hounsfield Road Sheffield S3 7RH, United Kingdom
¹³ Department of Physics, The University of Auckland Private Bag 92019 Auckland, New Zealand
¹⁴ Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische Astrophysik Albert-Ueberle-Str. 2 69120 Heidelberg, Germany
¹⁵ Universität Heidelberg, Interdisziplinäres Zentrum für Wissenschaftliches Rechnen Im Neuenheimer Feld 225 69120 Heidelberg, Germany
¹⁶ Harvard-Smithsonian Center for Astrophysics 60 Garden Street Cambridge MA 02138, USA
¹⁷ Elizabeth S. and Richard M. Cashin Fellow at the Radcliffe Institute for Advanced Studies at Harvard University 10 Garden Street Cambridge MA 02138, USA
¹⁸ Instituto de Astronomía, Universidad Nacional Autónoma de México, Unidad Académica en Ensenada Km 103 Carr. Tijuana-Ensenada Ensenada 22860, Mexico
¹⁹ AURA for the European Space Agency (ESA), ESA Office, Space Telescope Science Institute 3700 San Martin Drive Baltimore MD 21218, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 2 June 2025
Accepted: 3 November 2025

Abstract

Context. There is considerable debate about the formation of massive stars, including whether a high-mass star must always form with a population of low-mass stars, or if it can also form in isolation. Massive stars found in the field are often considered to be runaways from star clusters or OB associations. However, there is evidence in the Milky Way and the Small Magellanic Cloud of high-mass stars that appear to be isolated in the field and they cannot be related to any known star cluster or OB association. Studies of more distant galaxies have been lacking so far.

Aims. We identified massive star candidates that appear isolated in the field of the nearby spiral galaxy NGC 4242 (at a distance of 5.3 Mpc) to explore how many candidates for isolated star formation we find in a galaxy outside the Local Group.

Methods. We identified 234 massive (M_ini ≥ 15 M_⊙) and young (≤10 Myr) field stars in NGC 4242 using the Hubble Space Telescope Solar Blind Channel of the Advanced Camera for Surveys, the UVIS channel of the Wide Field Camera 3 from the Galaxy UV Legacy Project (GULP), and optical data from the Legacy ExtraGalactic UV Survey (LEGUS). We investigated the surroundings of our targets within the range of projected distances expected for runaway stars, 74 pc and 204 pc.

Results. Within the threshold radii, 9.8% and 34.6% of our targets have no young star clusters, OB associations, or massive stars. This causes them to appear isolated. This fraction reduces to 3.2%−11.5% for the total number of massive stars expected from the observed UV star formation rate.

Conclusions. Our results show that there is a small population of young and massive potentially isolated field stars in NGC 4242.