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Observations of the Type II-P (plateau) Supernova (SN) 1999em and Type IIn(narrow emission line) SN 1998S have enabled estimation of the profile of theSN ejecta, the structure of the circumstellar medium (CSM) established by thepre-SN stellar wind, and the nature of the shock interaction. SN 1999em is thefirst Type II-P detected at both X-ray and radio wavelengths. The Chandra X-raydata indicate non-radiative interaction of SN ejecta with a power-law densityprofile (rho \propto r^{-n} with n ~ 7) with a pre-SN wind with a low mass-lossrate of ~2 \times 10^{-6} M_sun/yr for a wind velocity of 10 km/sec, inagreement with radio mass-loss rate estimates. The Chandra data show anunexpected, temporary rise in the 0.4--2.0 keV X-ray flux at ~100 days afterexplosion. SN 1998S, at an age of >3 years, is still bright in X-rays and isincreasing in flux density at cm radio wavelengths. Spectral fits to theChandra data show that many heavy elements (Ne, Al, Si, S, Ar, and Fe) areoverabundant with respect to solar values. We compare the observed elementalabundances and abundance ratios to theoretical calculations and find that ourdata are consistent with a progenitor mass of approximately 15-20 M_sun if theheavy element ejecta are radially mixed out to a high velocity. If the X-rayemission is from the reverse shock wave region, the supernova density profilemust be moderately flat at a velocity ~10^4 km/sec, the shock front isnon-radiative at the time of the observations, and the mass-loss rate is 1-2\times 10^{-4} M_sun/yr for a pre-supernova wind velocity of 10 km/sec. Thisresult is also supported by modeling of the radio emission which implies thatSN 1998S is surrounded by a clumpy or filamentary CSM established by a highmass-loss rate, ~2 \times 10^{-4} M_sun/yr, from the pre-supernova star.Comment: 14 pages, 10 figures, accepted by ApJ, includes new data (one new  obs. each of SN 1999em and SN 1998S), expanded discussion of spectral fit

X‐Ray, Optical, and Radio Observations of the Type II Supernovae 1999em and 1998S