Sapaki: Galactic O3If* star possibly born in isolation

¹ Departamento de Astronomía, Universidad de La Serena, Avenida Juan Cisternas 1200, La Serena, Chile
e-mail: matiassebastian.zarricueta@userena.cl
² Carnegie Observatories, Las Campanas Observatory, Casilla 601, La Serena, Chile
³ Rubin Observatory Project Office, 950 N. Cherry Ave., Tucson, AZ 85719, USA

Received: 5 January 2023
Accepted: 4 May 2023

Abstract

Context. The study of high-mass stars found to be isolated in the field of the Milky Way may help to probe the feasibility of the core-accretion mechanism in the case of massive star formation. The existence of truly isolated stars may efficiently probe the possibility that individual massive stars can be born in isolation.

Aims. We observed WR67a (hereafter Sapaki), an O3If* star that appears to be isolated close to the center of a well-developed giant cavity that is aptly traced by 8.0 μm hot dust emission.

Methods. We acquired medium-resolution (R  =  4100) and moderate signal-to-noise (S/N  =  95 at 4500 Å) spectra for Sapaki in the range of 3800 − 10 500 Å with the Magellan Echellette (MagE) at Las Campanas Observatory. We computed the line-of-sight total extinctions. Additionally, we restricted its heliocentric distance by using a range of different estimators. Moreover, we measured its radial velocity from several lines in its spectrum. Finally, we analyzed its proper motions from Gaia to examine its possible runaway status.

Results. The star has been classified as having the spectral type O3If* given its resemblance to standard examples of the class. In addition, we found that Sapaki is highly obscured, reaching a line-of-sight extinction value of A_V = 7.87. We estimated the heliocentric distance to be in the range of d = 4 − 7 kpc. We also estimated its radial velocity to be V_r = −34.2 ± 15.6 km s⁻¹. We may also discard its runaway status solely based on its 2D kinematics. Furthermore, by analyzing proper motions and parallaxes provided by Gaia, we found only one other star with compatible measurements.

Conclusions. Given its apparent non-runaway status and the absence of clustering, Sapaki appears to be a solid candidate for isolated high-mass star formation in the Milky Way.