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

Journal Article · Sun Aug 10 04:00:00 EDT 2014 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/791/1/30· OSTI ID:22365398

Medina, Amber A.; Raymond, John C.; Edgar, Richard J.; Caldwell, Nelson; Milisavljevic, Dan ^[1]

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1. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States)

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.

• OSTI ID: 22365398
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 1 Vol. 791; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ACCELERATION
COSMIC RADIATION
EMISSION
EQUILIBRIUM
ION TEMPERATURE
PHOTOIONIZATION
PROPER MOTION
PROTONS
RESOLUTION
SHOCK HEATING
SHOCK WAVES
SPECTROSCOPY
SUPERNOVA REMNANTS
VELOCITY