3-D Study of ISM in front of SN1987A using Light Echoes

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Session 49 -- Interstellar Dust and Polarization
Display presentation, Tuesday, 10, 1995, 9:20am - 6:30pm

[49.06] 3-D Study of ISM in front of SN1987A using Light Echoes

Jun Xu, Arlin P.S. Crotts (Columbia U.), William E. Kunkel (Las Campanas Observatory)

A thorough study of the ``interstellar'' light echo images from SN1987A over more than five years shows 12 dust structures in front of the supernova, ranging from 310 to 3240 lt-yrs. We measured the depths and inclination angles of these dust sheets by least $\chi^{2}$ fitting. Two bifurcated dust sheets, corresponding to the two major light echo rings, are clearly present. Some discrete dust bodies sit in between, and beyond them, a complicated structure extends above one of the sheet to the northwest. A superbubble other than N157C of 200 lt-yrs diameter is embedded in the southwestern part of this sheet. Good correlation was found between this dust complex and bright H$\alpha$ region of the western shell of the N157C-complex. Thus, we think the N157C-complex is located at about 1000 lt-yrs in front of SN1987A and causes the outer major echo ring. This shell is tilted about $40^{0}$ from the northeast to the southwest. Two dense dust clouds appear at about 3000 lt-yrs in the SN foreground. The southeastern one correlates with a prominent H$\alpha$ filament reaching out to the southwest from over N157C with an offset of about 20 lt-yrs. It is tilted at about $60^{0}$ from the northwest to the southeast. Based on this identification, we propose a 3D picture of large scale ISM structure: two dense gas shells embrace a 2000 lt-yr diameter superbubble filled with hot X-ray emitting gas. This region was earlier identified as a significant X-ray source. Because the upper limit of the size of superbubble in LMC is at most 1800 lt-yrs and is likely to be much lower, we think this giant superbubble is the merger of two or three smaller superbubbles. It was also found that this X-ray source is not as strong as those in R136, the center of 30 Doradus. Therefore, we advocate the picture that this region is a remnant of a historically active star formation region. The small offset of dust sheets from bright H$\alpha$ regions indicates a slowly expanding giant superbubble. We also suggest massive star formation in the past split and separated one of these clouds into two sheets separated by 700 lt-yrs, and created a complicated structure in between.

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