Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998aas...193.7404b&link_type=abstract
American Astronomical Society, 193rd AAS Meeting, #74.04; Bulletin of the American Astronomical Society, Vol. 30, p.1365
Astronomy and Astrophysics
Astronomy
6
Scientific paper
NGC 4449 is a Magellanic irregular galaxy which lies at a distance of about 5 Mpc. It contains one exceedingly luminous young supernova remnant (SNR) identified by radio, optical, and X-ray means in the late-1970's that is readily observable even at this distance. This SNR shows a number of similarities to the galactic SNR Cas A, including high expansion velocities and enriched ejecta indicative of the remnant of a massive core-collapse supernova. However, the luminosity of this SNR is orders of magnitude higher than Cas A, presumably due both to its youth and expansion into a dense circumstellar environment. Because of its distance, no detailed spatial structure is resolvable for this SNR. We report new UV/optical HST data and ground-based optical data for this unique object. An HST/Faint Object Camera image shows the SNR to be only slightly broadened above a stellar source, allowing us to place an upper limit of 0.028'' (or 0.6 pc) on the diameter of the SNR. Improved S/N spectral line profiles from both the HST/Faint Object Spectrograph and the MDM Observatory in Arizona show broad wings on many lines, indicative of expansion velocities in excess of 6000 km s{-1}. Taken together, these data indicate an upper limit of 100 years on the age of this object (possibly less if some deceleration has occurred), or roughly 1/3--1/4 the age of Cas A. In addition to O-rich ejecta, strong lines of Ne, S, A, Mg, Fe, and possibly C and Si are seen but with somewhat different line widths suggestive of asymmetry in chemically distinct ejecta or possibly different levels of collisional de-excitation. This work is supported by STScI grant GO-06118.01-94A to the Johns Hopkins University.
Blair William Patrick
Fesen Robert A.
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