AKARI/FIS Mapping of the ISM-Wind Bow Shock around Alpha Ori

TL;DR

This study uses AKARI infrared data to map and analyze the shape and properties of the bow shock around Alpha Ori, revealing details about the star's motion, local medium density, and potential instabilities.

Contribution

First detailed infrared mapping of Alpha Ori's bow shock with shape fitting and analysis of stellar motion and local medium properties.

Findings

Bow shock stand-off distance of 4.8 arcminutes.

Local medium density estimated at 1.5 to 1.9 cm^(-3).

Possible vortex ring indicating instabilities on the shock surface.

Abstract

We present 10' x 50' scan maps around an M supergiant Alpha Ori at 65, 90, 140 and 160 microns obtained with the AKARI Infrared Astronomy Satellite. Higher spatial resolution data with the exact analytic solution permit us to fit the de-projected shape of the stellar wind bow shock around Alpha Ori to have the stand-off distance of 4.8', position angle of 55 degrees and inclination angle of 56 degrees. The shape of the bow shock suggests that the peculiar velocity of Alpha Ori with respect to the local medium is v_* = 40 (n_H)^(-1/2), where n_H is the hydrogen nucleus density at Alpha Ori. We find that the local medium is of n_H = 1.5 to 1.9 cm^(-3) and the velocity of the local flow is at 11 km s^(-1) by using the most recent astrometric solutions for Alpha Ori under the assumption that the local medium is moving away from the Orion OB 1 association. AKARI images may also reveal a vortex ring due to instabilities on the surface of the bow shock as demonstrated by numerical models. This research exemplifies the potential of AKARI All-Sky data as well as follow-up observations with Herschel Space Telescope and Stratospheric Observatory for Infrared Astronomy for this avenue of research in revealing the nature of interaction between the stellar wind and interstellar medium.