AKARI/FIS Mapping of the ISM-Wind Bow Shock around $\alpha $ Orionis

Abstract

Abstract We present $10$$\times$50scan maps around an M supergiant $\alpha $ Ori at 65, 90, 140, and 160$\mu$m 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 a stand-off distance of $4\farcm8$, position angle of 55$^{\circ}$ and inclination angle of 56$^{\circ}$. The shape of the bow shock suggests that the velocity of $\alpha $ Ori with respect to the local medium is $v_* = 40n_{\rm H}^{-1/2}$, where $n_{\rm H}$ is the hydrogen nucleus density at $\alpha $ Ori. We found that the local medium is of $n_{\rm H} =$ 1.5 to 1.9cm$^{-3}\rule[3mm]{0mm}{0mm}$ and the velocity of the local flow is at 11kms$^{-1}$ by using the most recent astrometric solutions for $\alpha $ Ori under the assumption that the medium local to $\alpha $ Ori 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 the interaction between the stellar wind and the interstellar medium.