Schematic diagram showing the effect of orientation on observed ionized gas velocities for a cometary H ii region shaped by a bow shock, champagne flow, and strong stellar wind. Due to the stellar wind, the ionized gas is confined to a swept-up shell, which gives rise to two velocity components. The two expected Gaussians components (from the near and far sides of the shell) are sketched for three different lines of sight. In the case of 90° inclination (w.r.t. the line of sight), the two velocity components are emitted at corresponding positions on the sides of the shell, so the velocities are symmetric with respect to the systemic velocity (left panel). However, in the case of 75° inclination (w.r.t the line of sight), the two components come from shifted positions along the shell and thus, the offset from the systemic velocity is not the same for the two components (right panel). The second case reflects the velocity structure of the southern H ii region, suggesting that the exciting star is moving towards us.
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