|Title (1st Abstract)||
Expansion Measurements of Young Type Ia Supernova Remnants and the Physics of Nonradiative Shocks
P. Frank Winkler
Joseph Putko / Middlebury College, Universidad de La Laguna
5. Collisionless shock waves in SNRs
We will report on optical proper motion measurements of the Tycho (SN1572) and SN1006 remnants, based on CCD images at multiple epochs with total baselines of over twenty years. Optical emission from both Tycho and SN1006 is almost entirely in the Balmer lines of hydrogen, and arises from pre-shock neutral atoms that pass unimpeded through the collisionless shock into the hot post-shock environment. The lifetime for these neutrals is short, so the Balmer radiation occurs only immediately behind the shock, making the Balmer filaments the best measure of the shock position over time.
Measurement of proper motions in Tycho is complicated by the fact that some of the brightest filaments do not evolve coherently over time. From images at seven different epochs we have identified filaments that maintain their morphology coherently, and we measure proper motions from 0.19 to 0.26 arcsec/yr, equivalent to an expansion index from 0.35 to 0.52. (The expansion index is the ratio of the current proper motion to the historical average—with 0.4 indicating Sedov expansion.)
For SN1006, we will report for the first time optical proper motion measurements carried out around much of the shell rim, where the Balmer emission is mostly extremely faint. Expansion rates vary by more than a factor of 2, from 0.27 to > 0.6 arcsec/yr, equivalent to expansion index values ranging from 0.34 to well above Sedov.
For both Tycho and SN1006, we compare the optical measurements with those in radio and X-ray bands, where emission arises from somewhat farther behind the shock. Past proper expansion measurements for young SNRs have sometimes found quite different values, though usually this has been for measurements carried out in different regions for different bands. We find generally good agreement between measurements in different bands for filaments located at similar positions around the shell rims.
Spectra of the Balmer filaments show both narrow and broad components: the former from cold neutrals that are collisionally excited after penetrating the shock, and the latter from hot neutrals that have undergone charge exchange with hot post-shock protons. From spectral profiles it is possible to extract information about the post-shock temperature, shock velocity, electron-proton temperature equilibration, and cosmic-ray acceleration. We will discuss progress we are making in these areas.
This work has been supported by the NSF through grant AST-0908566, and by NASA through grants Chandra GO2-13066 and HST GO-13432.003.