A Comprehensive Search for Stellar Bowshock Nebulae in the Milky Way: A Catalog of 709 Mid-Infrared Selected Candidates

Abstract

We identify 709 arc-shaped mid-infrared nebula in 24 µm Spitzer Space Telescope or 22 µm Wide Field Infrared Explorer surveys of the Galactic Plane as probable dusty interstellar bowshocks powered by early-type stars. About 20% are visible at 8 µm or shorter mid-infrared wavelengths as well. The vast majority (660) have no previous identification in the literature. These extended infrared sources are strongly concentrated near Galactic mid-Plane with an angular scale height of ∼0.6 • . All host a symmetrically placed star implicated as the source of a stellar wind sweeping up interstellar material. These are candidate "runaway" stars potentially having high velocities in the reference frame of the local medium. Among the 286 objects with measured proper motions, we find an unambiguous excess having velocity vectors aligned with the infrared morphology -kinematic evidence that many of these are "runaway" stars with large peculiar motions responsible for the bowshock signature. We discuss a population of "in-situ" bowshocks (∼103 objects) that face giant H II regions where the relative motions between the star and ISM may be caused by bulk outflows from an overpressured bubble. We also identify ∼58 objects that face 8 µm bright-rimmed clouds and apparently constitute a sub-class of in-situ bowshocks where the stellar wind interacts with a photo-evaporative flow from an eroding molecular cloud interface (i.e., "PEF bowshocks"). Orientations of the arcuate nebulae exhibit a correlation over small angular scales, indicating that external influences such as H II regions are responsible for producing some bowshock nebulae. However, the vast majority of this sample appear to be isolated (499 objects) from obvious external influences.

Bow shocks, bow waves, and dust waves – III. Diagnostics

“…Grids of galactic coordinates (light blue lines, parallel to the box sides) and equatorial coordinates (tilted magenta lines) are shown. The stellar source and the bow shock axis, as determined by Kobulnicky et al (2016) are indicated by an orange star and an orange line, respectively, where the line extends from −2R 0 to +2R 0 . The automatically traced arc shapes using the "mean" and "peak" methods (see text) are shown by blue and red dots, respectively.…”

“…Angular ranges of θ = ±45 • and ±90 • are shown by gray shaded boxes. The R 0 value tabulated by Kobulnicky et al (2016) is shown by a horizontal blue line. Panels (e, h, k) show equivalent plots for the surface brightness.…”

“…2. Using the central source coordinates, C 0 and nominal bowshock radius, R 0 from Kobulnicky et al (2016), construct a pixel mask that includes only those pixels with separations from the source that satisfy 1 2 R 0 |C − C 0 | 3R 0 . This mask will be used for all subsequent operations, which serves to help avoid confusion from the star itself and other bright sources in the field of view.…”

“…Stellar bow shocks are produced by the relative motion between a star and its surrounding medium, and are commonly detected as curved arcs of emission at optical (Gull & Sofia 1979;Brown & Bomans 2005), infrared (van Buren & McCray 1988;Kobulnicky et al 2016), or radio (van Buren et al 1990Benaglia et al 2010) wavelengths. The canonical theory for these objects is that they are formed by a two-shock interaction between the stellar wind and the interstellar medium (Pikel'ner 1968; Dyson & de Vries 1972), which is distorted due to the supersonic motion of the star (Baranov et al 1970;Wilkin 1996).…”

“…The largest number of bow shocks have been detected around high-mass OB stars, via their mid-infrared dust emission (van Buren et al 1995;Noriega-Crespo et al 1997;Povich et al 2008;Kobulnicky et al 2010;Peri et al 2012Peri et al , 2015Sexton et al 2015;Kobulnicky et al 2016;Bodensteiner et al 2018), and these have typical sizes ranging from 0.01 pc to 1 pc. In particularly dense environments, such as the inner Orion Nebula (Smith et al 2005) and the Galactic center region (Geballe et al 2004) they may be as small as 0.003 pc and emit at near-infrared wavelengths (Tanner et al 2005;Sanchez-Bermudez et al 2014).…”