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Title: The density and mass of unshocked ejecta in Cassiopeia a through low frequency radio absorption

Characterizing the ejecta in young supernova remnants is a requisite step toward a better understanding of stellar evolution. In Cassiopeia A the density and total mass remaining in the unshocked ejecta are important parameters for modeling its explosion and subsequent evolution. Low frequency (<100 MHz) radio observations of sufficient angular resolution offer a unique probe of unshocked ejecta revealed via free-free absorption against the synchrotron emitting shell. We have used the Very Large Array plus Pie Town Link extension to probe this cool, ionized absorber at 9'' and 18.''5 resolution at 74 MHz. Together with higher frequency data we estimate an electron density of 4.2 cm{sup –3} and a total mass of 0.39 M {sub ☉} with uncertainties of a factor of ∼2. This is a significant improvement over the 100 cm{sup –3} upper limit offered by infrared [S III] line ratios from the Spitzer Space Telescope. Our estimates are sensitive to a number of factors including temperature and geometry. However using reasonable values for each, our unshocked mass estimate agrees with predictions from dynamical models. We also consider the presence, or absence, of cold iron- and carbon-rich ejecta and how these affect our calculations. Finally we reconcile the intrinsicmore » absorption from unshocked ejecta with the turnover in Cas A's integrated spectrum documented decades ago at much lower frequencies. These and other recent observations below 100 MHz confirm that spatially resolved thermal absorption, when extended to lower frequencies and higher resolution, will offer a powerful new tool for low frequency astrophysics.« less
Authors:
 [1] ;  [2] ;  [3] ;  [4]
  1. Physics and Engineering Department, West Virginia Wesleyan College, Buckhannon, WV 26201 (United States)
  2. U.S. Naval Research Laboratory, Washington, DC 20375 (United States)
  3. Minnesota Institute for Astrophysics, School of Physics and Astronomy, University of Minnesota, 116 Church Street SE, Minneapolis, MN 55455 (United States)
  4. National Radio Astronomy Observatory, P.O. Box O, Socorro, NM 87801 (United States)
Publication Date:
OSTI Identifier:
22357205
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 785; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ABSORPTION; ASTROPHYSICS; CARBON; DENSITY; ELECTRON DENSITY; EXPLOSIONS; FORECASTING; MASS; RESOLUTION; SIMULATION; SPACE; SPECTRA; STAR EVOLUTION; SUPERNOVA REMNANTS; TELESCOPES