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Session 45 - Interstellar Medium I.
Display session, Tuesday, January 16
North Banquet Hall, Convention Center
We are continuing to study the structure of the approximately 2\times10^8 cubic light years of the interstellar medium (ISM) in the Large Magellanic Cloud illuminated by the light echoes from SN 1987A. High resolution (10 km s^-1) [N II]-emission echelle spectra in a 6\arcmin \times 6\arcmin region around SN1987A were obtained on the CTIO 4m telescope. The map shows a complicated velocity structure consistent with that reported before for the ISM, but which now can be correlated with the three-dimesnional structure revealed by the light echoes. Three components, V_hel = 265, 277 and 285 km s^-1 were identified with the N157C superbubble (the R1170-complex seen in light echoes [Xu, Crotts amp; Kunkel 1995]) around the OB association LH 90. The radius of this 230-pc-diameter superbubble was found to expand at 10 km s^-1, with a lifetime of 7\times 10^6 years and a total energy of 2\times 10^51 ergs determined from its radius and velocity according to superbubble theory. The V_hel = 235 km s^-1 component corresponds to the near side of 600 pc giant superbubble of Xu, Crotts amp; Kunkel (1994). This bubble is 3 \times 10^7 years old, and has blown out of the LMC disk. Two other components, V_hel = 255 and 245 km s^-1, are identified as the inner major light echo ring (a double-shell structure) at about 130 pc in front of SN1987A. There are also two high velocity components, 300 and 313 km s^-1, which may be the far side of a superbubble in which SN1987A exploded. These are seen neither in absorption towards SN 1987A, nor yet in the light echoes, hence are probably behind the SN. We also noticed that there are two components at 259 and 301 km s^-1 within 20\arcsec of SN1987A. These structures are probably due to the emission from the red supergiant wind of the SN progenitor. The time it took the SN1987A progenitor to move to the current location is comparable to the lifetime of N157C and the SN, as well as perhaps the time since collapse of the asserted superbubble around the SN. This may suggest that the SN acquired its velocity while being formed in the collapse of the giant superbubble, which may have also created N157C.
