AAS Meeting #193 - Austin, Texas, January 1999
Session 74. Supernova Remnants
Display, Friday, January 8, 1999, 9:20am-6:30pm, Exhibit Hall 1

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[74.04] New Constraints on the Age of the Ultraluminous Supernova Remnant in NGC~4449

W.P. Blair (JHU/CAS), R.A. Fesen (Dartmouth)

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\arcsec\ (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 \rm 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.

The author(s) of this abstract have provided an email address for comments about the abstract: wpb@pha.jhu.edu

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