Shown in sequence as a movie, the systematic variation of emission with
velocity expected for an expanding shell becomes clear.
GSH 23.0-0.7+117 is a small HI shell in the inner Galaxy. It is found
near the terminal velocity of Galactic HI, so its distance is fairly
well determined. The radius of this shell is only 15 parsec. In the
paper we discuss the possibility that GSH 23.0-0.7+117 is a stellar
wind bubble. HI shells like this one are dominated by swept-up
interstellar gas and provide few clues to the nature of their source.
One clue is the expansion energy of the shell: 2 x 1048
ergs. This energy is readily provided in the form of stellar wind by a
single massive star over the course of its life. The age of the shell is
estimated at 1 Myr, with an upper limit of 2 Myr.
In this paper we show for the first time that the upper limit to the
continuum emission associated with the HI shell puts an additional
constraint on the source of the shell. We use the observed HI shell as
a screen of neutral material that captures any ionizing flux from the
source inside the shell. The upper limit to the (thermal) radio
continuum flux provides an upper limit to the number of Ly continuum
photons produces inside the shell. We take into account the fact that
the observed HI shell is incomplete, and that ionizing photons may
escape throught holes in the part of the shell that we do see.
The winds of massive stars are driven by the ultraviolet luminosity,
so the energy released in stellar wind and the ionizing luminosity of
these stars are closely related. We conclude that is is almost
impossible to reconcile the energy required to blow this bubble by a
stellar wind with the limit to the ionizing flux derived from the
radio continuum.
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HI channel maps of GSH23.0-0.7+117. The LSR velocity is indicated
in the lower right corner of each panel. Note the clear ring of
emission and the "cap" around 108 km/s. The red shifted side of
the shell is invisible. Click on the image for a larger
version.
Shown in sequence as a movie, the systematic variation of emission with
velocity expected for an expanding shell becomes clear.
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