AAS Meeting #193 - Austin, Texas, January 1999
Session 51. Supernova Remnants
Oral, Thursday, January 7, 1999, 10:00-11:00am, Room 8 (A,B,C)

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[51.04D] Optical Spectroscopy and Numerical Modeling of Nonradiative Shocks in Young Supernova Remnants

P. Ghavamian, P. M. Hartigan (Rice University), J. C. Raymond (CfA), R. C. Smith (CTIO)

When a shock propagates through a partially ionized, low density medium (n~1 cm-3), the collisional excitation of cold neutral atoms and their charge exchange with hot postshock ions produces both narrow and broad lines of Balmer emission. The width of the broad component and its flux ratio relative to the narrow component indicates the ion temperature and shock velocity, therefore Balmer line profiles offer a valuable diagnostic of physical conditions immediately behind a nonradiative shock (so named because radiative cooling in these objects is dynamically unimportant). Several major uncertainties, however, complicate the interpretation of nonradiative shock spectra. First, the lack of two-body collisions between electrons and ions means that they are often heated to different temperatures. An observed Balmer line profile can therefore correspond to a range of different shock velocities. Second, the intensity of the H-alpha narrow component is enhanced via Lyman-beta trapping; this leads to inaccurate prediction of the shock velocity from the broad-to- narrow flux ratio. Thus far, scant attention has been paid to emission lines blueward of H-alpha. The goal of our project has been to acquire high S/N, complete optical spectra of the brightest nonradiative shocks in a selected sample of young SNRs (4000 < \lambda < 7000 Å). Our targets include the supernova remnants SN 1572 (Tycho, V~ 2000 km/s), RCW 86 (V~ 600 km/s) and nonradiative portions of the Cygnus Loop (V~ 400 km/s). For the first time, we have been able to measure the broad-to-narrow ratio in both H-beta and H-alpha, as well as separate Balmer decrements for the broad and narrow components. We present here the results of these observations and use numerical models to predict the ionization structure and electron-ion equilibration for the observed objects, as well as the contribution of Lyman line trapping to their optical spectra.

The author(s) of this abstract have provided an email address for comments about the abstract: parviz@sparky.rice.edu

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