New stellar bow shocks and bubbles found around runaway stars

¹ Departament de Física Quàntica i Astrofísica, Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona, IEEC-UB, Martí i Franquès 1, 08028 Barcelona, Spain
² Instituto Argentino de Radioastronomía (CONICET–CICPBA–UNLP), C.C No 5. 1894 Villa Elisa, Argentina

^★ Corresponding author; mcarretero@fqa.ub.edu

Received: 1 July 2024
Accepted: 5 December 2024

Abstract

Context. Runaway stars with peculiar high velocities can generate stellar bow shocks. Only a few bow shocks show clear radio emission.

Aims. Our goal is to identify and characterize new stellar bow shocks around O and Be runaway stars in the infrared (IR), as well as to study their possible radio emission and nature.

Methods. Our input data is a catalog of O and Be runaways compiled using Gaia DR3. We used WISE IR images to search for bow shock structures around these runaways, Gaia DR3 data to determine the actual motion of the runaway stars corrected for interstellar medium (ISM) motion caused by Galactic rotation, and archival radio data to search for emission signatures. We finally explored the radio detectability of these sources under thermal and nonthermal scenarios.

Results. We found nine new stellar bow shock candidates, three new bubble candidates, and one intermediate structure candidate. One of them is an in situ bow shock candidate. We also found 17 already known bow shocks in our sample, though we discarded one, and 62 miscellaneous sources showing some IR emission around the runaways. We geometrically characterized the sources in IR using the WISE-4 band and estimated the ISM density at the bow shock positions, obtaining median values of ∼6 and ∼4 cm⁻³ using 2D and 3D peculiar velocities, respectively. Most of the new discovered bow shocks come from new runaway discoveries. Within our samples we found that ∼24% of the O-type runaway stars show bow shocks, while this decreases to ∼3% for Be-type runaway stars. Two bow shocks present radio emission but not as clear counterparts, and two others show hints of radio emission. The physical scenarios indicate that two sources could still be compatible with nonthermal radio emission.

Conclusions. The new sample of O and Be runaway stars allowed us to discover both new stellar bow shocks and bubbles. Their geometrical characterization can be used to assess the physical scenario of the radio emission. Deeper radio observations are needed to unveil a population of radio-detected bow shocks, and the physical scenarios occurring in these sources.