The Hourglass Nebulae of Sher 25 and SN 1987 A: Two of a Kind?

Abstract

We have performed a detailed study of the morphology and kinematics of the hourglass-shaped nebula around the blue supergiant Sher 25 in the galactic giant HII region NGC 3603. Near-infrared high resolution adaptive optics images in the Br gamma line and HST/NICMOS images in the HeI 1.08mu line were compared with iso-velocity maps in the H alpha and [NII] lines. The adaptive optics observations clearly resolved the width of the ring (0.9", i.e., 0.027 pc), yielding delta R / R = 1:8. We show that the H alpha and [NII] lines trace the entire silhouette of the hourglass. The bipolar lobes of the hourglass expand at 70 km/s, whereas the ring around the waist of the hourglass expands at 30 km/s. Both the ring and the bipolar lobes have about the same dynamical age, indicating a common origin and a major outburst and mass-loss event 6630 yr ago. The ionized mass within the hourglass is between 0.3 Mo and 0.6 Mo - quite comparable to the total mass suggested for the expanding (pre-supernova) shell around SN 1987 A. The hourglass structure around Sher 25 is similar to that of SN 1987 A in spatial extent, mass, and velocities. The major differences between these two nebulae might arise from environmental effects. Both internal and external ionization sources are available for Sher 25's nebula. Furthermore, Sher 25 and its hourglass-shaped nebula appear to be moving to the south-west with respect to the ambient interstellar medium, and ram pressure has apparently deformed the hourglass. We conclude that the circumstellar nebulae around SN 1987 A and Sher 25 are very similar and define a new class of nebulae around blue supergiants in their final evolutionary stage.