The Hourglass Nebulae of Sher 25 and SN 1987A: 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 H II region NGC 3603. Near-infrared high-resolution adaptive optics images in the Brγ line and HST/NICMOS images in the He I 1.08 μm line were compared with isovelocity maps in the Hα and [N II] lines. The adaptive optics observations clearly resolved the width of the ring (09, i.e., 0.027 pc), yielding δR/R = 1/8. We show that the Hα and [N II] lines trace the entire silhouette of the hourglass. The bipolar lobes of the hourglass expand at 70 km s-1, whereas the ring around the waist of the hourglass expands at 30 km s-1. 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 and 0.6 M☉—quite comparable to the total mass suggested for the expanding (presupernova) shell around SN 1987A. The hourglass structure around Sher 25 is similar to that of SN 1987A in spatial extent, mass, and velocity. 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 southwest with respect to the ambient interstellar medium, and ram pressure has apparently deformed the hourglass. We conclude that the circumstellar nebulae around SN 1987A and Sher 25 are very similar and define a new class of nebulae around blue supergiants in their final evolutionary stage.