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Title:
Evidence for Shock Precursors in Tycho's Supernova Remnant
Authors:
Ghavamian, Parviz; Raymond, John; Hartigan, Patrick; Blair, William P.
Affiliation:
AA(Postal address: Department of Space Physics and Astronomy, Rice University, Houston, TX 77251-1892 , ), AB(Postal address: Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138; ), AC(Postal address: Department of Space Physics and Astronomy, Rice University, Houston, TX 77251-1892 , ), AD(Postal address: Center for Astrophysical Sciences, Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218-2686 .; Visiting Astronomer, Kitt Peak National Observatory, National Optical Astronomy Observatories. KPNO is operated by AURA, Inc., under contract to the National Science Foundation)
Publication:
The Astrophysical Journal, Volume 535, Issue 1, pp. 266-274. (ApJ Homepage)
Publication Date:
05/2000
Origin:
UCP
ApJ Keywords:
ISM: individual (Tycho's Supernova), ISM: Structure, Shock Waves, ISM: Supernova Remnants
DOI:
10.1086/308811
Bibliographic Code:
2000ApJ...535..266G

Abstract

We report optical spectroscopy and imaging of Tycho's supernova remnant (SNR) which suggest that two different precursors are associated with the blast wave. In long-slit spectra of the eastern rim, we find diffuse Hα emission extending >~1' ahead of Knot g, a bright nonradiative shock propagating at velocity ~2000 km s-1. In addition to the hydrogen lines, the diffuse spectrum also exhibits [S II] λλ6716, 6731; [N II] λ6583 and weak [O III] λ5007 emission. Narrowband imagery of Tycho's SNR shows that the diffuse emission extends ahead of most of the Balmer-dominated optical filaments and drops sharply in the remnant interior, suggesting that the diffuse gas is overrun by the supernova shock. The line ratios of the diffuse gas are compared with predictions from a simple He II λ304 photoionization precursor model. The models predict that the radiative transfer of He II λ304 photons in the precursor produces spatially distinct ionization layers of H0, He0-He+, N0-N++, O0-O++, and S+-S+3 ahead of the shock. The models suggest that the diffuse emission arises from collisional excitation in mostly neutral gas which has been heated to a temperature ~12,000 K. In high-resolution echelle spectra of Knot g, we find that the narrow Hα component is broadened to 44+/-4 km s-1, suggesting that the upstream gas may be further heated before it crosses the shock, perhaps by a fast neutral or cosmic-ray precursor. High-resolution spectroscopy of the Hα line in both the diffuse region and Knot g will be required to definitively establish the existence of a second precursor.
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