The K supergiant runaway star HD 137071

Abstract

Context. Extensive work exists on runaway massive stars with peculiar motions that are much higher than those typical of the extreme Population I to which they belong. Work on runaways has focused almost exclusively on O and B stars, most of which undergo a red supergiant phase before ending their lives as supernovae. Very few examples are known of red supergiant runaways, all of which descend from the more massive O-type precursors, but none from the lower mass B-type precursors, although runaway statistics of B-type stars suggest that K-type runaways must be relatively numerous. Aims. We study HD 137071, a star that has so far been considered to be a normal K-type red giant. Its parallax measured by Gaia and the derived luminosity suggest that it is a supergiant, whereas its derived distance to the Galactic plane and its spatial velocity of 54.1 km s−1 with respect to the local standard of rest suggest that it is also a runaway star. However, intrinsic limitations in determining the trigonometric parallaxes of cool supergiants, even in the Gaia era, require accurate spectral classifications for confirmation. Methods. We present visible spectroscopy obtained with the 2.2m telescope at Calar Alto Observatory and compare it with the spectra of MK standard stars to produce an accurate spectral classification, including the determination of its luminosity class. We complement this information with astrometric data from the Gaia DR2 catalog. Results. We reliably classify HD 137071 as a K4II star and establish its membership to the extreme Population I. This agrees with the luminosity derived using the Gaia DR2 parallax measurement. Kinematical data from the Gaia DR2 catalog confirm its high spatial velocity and runaway nature. By combining the spectral classification with astrometric information, recent Galactic potential models, and evolutionary models for high-mass stars, we trace the motion of HD 137071 back to the proximities of the Galactic plane and speculate which of the two proposed mechanisms for the production of runaway stars may be responsible for the high velocity of HD 137071. The available data favor the formation of HD 137071 in a massive binary system where the more massive companion underwent a supernova explosion about 32 Myr ago.