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Title: Electron-ion equilibrium and shock precursors in the northeast limb of the Cygnus Loop

Abstract

We present an observational study using high-resolution echelle spectroscopy of collisionless shocks in the Cygnus Loop supernova remnant. Measured Hα line profiles constrain pre-shock heating processes, shock speeds, and electron-ion equilibration (T{sub e} /T{sub i} ). The shocks produce faint Hα emission line profiles, which are characterized by narrow and broad components. The narrow component is representative of the pre-shock conditions, while the broad component is produced after charge transfer between neutrals entering the shock and protons in the post-shock gas, thus reflecting the properties of the post-shock gas. We observe a diffuse Hα region extending about 2.'5 ahead of the shock with line width ∼29 km s{sup –1}, while the Hα profile of the shock itself consists of broader than expected narrow (36 km s{sup –1}) and broad (250 km s{sup –1}) components. The observed diffuse emission arises in a photoionization precursor heated to about 18,000 K by He I and He II emission from the shock, with additional narrow component broadening originating from a thin cosmic-ray precursor. Broad to narrow component intensity ratios of ∼1.0 imply full electron-ion temperature equilibration T{sub e} ≅ T{sub i} in the post-shock region. Broad component line widths indicate shock velocities of aboutmore » 400 km s{sup –1}. Combining the shock velocities with proper motions suggests that the distance to the Cygnus Loop is ∼890 pc, significantly greater than the generally accepted upper limit of 637 pc.« less

Authors:
; ; ; ;  [1];  [2]
  1. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States)
  2. Department of Physics and Astronomy, Dartmouth College, 6127 Wilder Lab, Hanover, NH 03755 (United States)
Publication Date:
OSTI Identifier:
22365398
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 791; 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; ACCELERATION; COSMIC RADIATION; EMISSION; EQUILIBRIUM; ION TEMPERATURE; PHOTOIONIZATION; PROPER MOTION; PROTONS; RESOLUTION; SHOCK HEATING; SHOCK WAVES; SPECTROSCOPY; SUPERNOVA REMNANTS; VELOCITY

Citation Formats

Medina, Amber A., Raymond, John C., Edgar, Richard J., Caldwell, Nelson, Milisavljevic, Dan, and Fesen, Robert A., E-mail: amedina@cfa.harvard.edu, E-mail: jraymond@cfa.harvard.edu, E-mail: edgar@head.cfa.harvard.edu, E-mail: amedina7@nmsu.edu. Electron-ion equilibrium and shock precursors in the northeast limb of the Cygnus Loop. United States: N. p., 2014. Web. doi:10.1088/0004-637X/791/1/30.
Medina, Amber A., Raymond, John C., Edgar, Richard J., Caldwell, Nelson, Milisavljevic, Dan, & Fesen, Robert A., E-mail: amedina@cfa.harvard.edu, E-mail: jraymond@cfa.harvard.edu, E-mail: edgar@head.cfa.harvard.edu, E-mail: amedina7@nmsu.edu. Electron-ion equilibrium and shock precursors in the northeast limb of the Cygnus Loop. United States. doi:10.1088/0004-637X/791/1/30.
Medina, Amber A., Raymond, John C., Edgar, Richard J., Caldwell, Nelson, Milisavljevic, Dan, and Fesen, Robert A., E-mail: amedina@cfa.harvard.edu, E-mail: jraymond@cfa.harvard.edu, E-mail: edgar@head.cfa.harvard.edu, E-mail: amedina7@nmsu.edu. Sun . "Electron-ion equilibrium and shock precursors in the northeast limb of the Cygnus Loop". United States. doi:10.1088/0004-637X/791/1/30.
@article{osti_22365398,
title = {Electron-ion equilibrium and shock precursors in the northeast limb of the Cygnus Loop},
author = {Medina, Amber A. and Raymond, John C. and Edgar, Richard J. and Caldwell, Nelson and Milisavljevic, Dan and Fesen, Robert A., E-mail: amedina@cfa.harvard.edu, E-mail: jraymond@cfa.harvard.edu, E-mail: edgar@head.cfa.harvard.edu, E-mail: amedina7@nmsu.edu},
abstractNote = {We present an observational study using high-resolution echelle spectroscopy of collisionless shocks in the Cygnus Loop supernova remnant. Measured Hα line profiles constrain pre-shock heating processes, shock speeds, and electron-ion equilibration (T{sub e} /T{sub i} ). The shocks produce faint Hα emission line profiles, which are characterized by narrow and broad components. The narrow component is representative of the pre-shock conditions, while the broad component is produced after charge transfer between neutrals entering the shock and protons in the post-shock gas, thus reflecting the properties of the post-shock gas. We observe a diffuse Hα region extending about 2.'5 ahead of the shock with line width ∼29 km s{sup –1}, while the Hα profile of the shock itself consists of broader than expected narrow (36 km s{sup –1}) and broad (250 km s{sup –1}) components. The observed diffuse emission arises in a photoionization precursor heated to about 18,000 K by He I and He II emission from the shock, with additional narrow component broadening originating from a thin cosmic-ray precursor. Broad to narrow component intensity ratios of ∼1.0 imply full electron-ion temperature equilibration T{sub e} ≅ T{sub i} in the post-shock region. Broad component line widths indicate shock velocities of about 400 km s{sup –1}. Combining the shock velocities with proper motions suggests that the distance to the Cygnus Loop is ∼890 pc, significantly greater than the generally accepted upper limit of 637 pc.},
doi = {10.1088/0004-637X/791/1/30},
journal = {Astrophysical Journal},
number = 1,
volume = 791,
place = {United States},
year = {Sun Aug 10 00:00:00 EDT 2014},
month = {Sun Aug 10 00:00:00 EDT 2014}
}
  • Emission from vibrationally excited H2 has been discovered which is associated with the bright optical shock-excited filaments to the northeast of the Cygnus Loop supernova remnant. Infrared spectroscopy and infrared and optical narrow-band images of the shock-excited gas have been obtained in an effort to understand the mechanism of H2 excitation. A shock model with a magnetic precursor is proposed which explains quantitatively the observed H2 surface brightness, level population, and relation to optical emission. A shock with a magnetic precursor can also account for some of the anomalous properties of nonradiative shocks. 64 refs.
  • Radio observations of the northeastern part of the Cygnus Loop obtained with the VLA at 18 cm with a resolution of 4.7 arcsec are presented and discussed. In general, the brightest radio filaments match the brightest optical filaments very well, particularly those which are bright in H-alpha. The agreement in position, morphology, and relative brightness is particularly good where all four optical species are bright. However, a few bright radio features have no bright optical counterparts. The general radio and optical structure of the region can be explained by a model in which shocked interstellar clouds form filaments due tomore » unstable cooling and compression. The features appear to be ropelike, although it is not possible to rule out that they are irregular sheets seen edge-on. A large gradation in cooling on a scale of parsecs is evidenced by the sequence of the optical species in emission, with a corresponding increase in density generally evidenced by the continuum radio emission. 19 references.« less
  • Infrared Fabry-Perot spectra at 25 km/s resolution have been obtained for the H2 line emission associated with the fast shocks to the NE of the Cygnus Loop supernova remnant. Profiles of the 1-0 S(1) H2 emission associated with both the optically bright radiative shocks and the faint nonradiative shocks have been observed. The profiles are resolved and have deconvolved widths of 20 km/s. This result favors excitation of the H2 emission in a magnetic precursor over excitation by a radiative precursor. Shocks with precursors may play an important role in Herbig-Haro objects, and in star-forming cloud cores. 25 refs.
  • We present 24 {mu}m and 70 {mu}m images of a non-radiative shock in the Cygnus Loop supernova remnant, obtained with the Multiband Imaging Photometer on board the Spitzer Space Telescope. The observed emission is from dust grains heated in the post-shock region. The 70 {mu}m to 24 {mu}m flux ratio depends on the dust heating and the dust destruction rates, and thereby it is a sensitive tracer of the gas density and temperature in the shocked plasma. We model the dust emission and grain destruction in the post-shock flow, and find that the observed 70 {mu}m to 24 {mu}m fluxmore » ratios are produced for post-shock densities, n{sub H}{approx}2.0 cm{sup -3} and electron temperatures of about 0.20 keV. We find that about 35% of the dust has been destroyed in the shock, and that non-thermal sputtering (i.e. sputtering due to bulk motion of the grains relative to the gas) contributes significantly to the dust destruction.« less