Observing Changes in the Primary Shock Front of the Cygnus Loop: An HST 20+ Year Perspective

Revised submission 1350446496 Assigned ID: 289 ----- Attempting Submission 1 (Thu Apr 04 22:39:56 GMT 2019) ----- HST Phase I Proposal 289 (apt_v2.apt) successfully submitted. Receipt: # 289-1 ----- Attempting Submission 2 (Fri Apr 05 17:32:29 GMT 2019) ----- FLASH level may be too high for this exposure or a long subexposure. See extended explanation in the diagnostic browser FLASH level may be too high for this exposure or a long subexposure. See extended explanation in the diagnostic browser Changed post-flash value from 12e- to 20e- on the recommendation of the CS 951244699 ----- Attempting Submission 1 (Wed Jul 17 21:47:19 GMT 2019) ----- ----- Attempting Submission 2 (Wed Jun 24 22:00:51 GMT 2020) ----- Post-flash changed based on advise from CS to the new Cycle 28 value Observing Changes in the Primary Shock Front of the Cygnus Loop: An HST 20+ Year Perspective We propose to re-observe the faint, primary blast wave of the Cygnus Loop supernova remnant with WFC3, a region that has been observed twice before with WFPC2, in 1997 and 2001. The region contains a delicate rippled sheet of plasma visible in H-alpha, that represents a section of the primary non-radiative supernova shock, and an adjacent [O III] 5007-emitting filament that is a shock in transition to becoming radiative. This would be the third epoch in H-alpha, and the second in [O III] 5007. With the recent accurate GAIA DR2-derived distance to the Cygnus Loop (735 +/- 25 pc), and the proper motion in H-alpha from epochs 1 and 2, the derived shock velocity is 245 km/s. The epoch 3 H-alpha data and long time baseline will permit a sensitive search for signs of deceleration of the filament, either globally or locally along portions of the filament structure. The two epochs of [O III] will measure or constrain any changes in the filament structure over ~20 years due to catastrophic cooling, thermal instabilities, or density variations that are predicted to occur as SN remnant shocks turn radiative. The Cygnus Loop provides a unique target to test for these changes since the densities and shock velocities involved provide for the possibility of observable changes on a reasonable timescale. Two WFC3/UVIS filters, F656N and F502N, will be used to image a single field in the Cygnus Loop supernova remnant, targeting a narrow, extended H-alpha emitting filament, with portions of it emitting in [OIII]. There are two visits, each with three orbits. The first visit uses F656N, and the second visit uses F502N. For each visit, a dither pattern to cover the chip gap, plus a subpattern to remove cosmic rays and bad pixels have been specified. This results in two sub-exposures per orbit. Orientation requirements have been specified in order to get the entire filament on a single UVIS detector, with the second detector covering a region towards the interior of the remnant (i.e. southwest of the filament). The range for the allowed orientation is such that the exact target co-ordinates (specified in the proposal for the UVIS CENTER aperture) will need to be tweaked after the visits have been scheduled in the long-range plan. Therefore we request that once we the target is scheduled we are given the opportunity to update the target co-ordinates in the Phase II proposal before execution. We have also specified that visit 02 be obtained at the same orientation as visit 01, to best match the relative locations of the H-alpha and [OIII] emission. Also, we have specified that the two visits be obtained within 60 days of each other, so they are included in the same visibility window, and there is no significant temporal evolution of the shock between them. Drs. Blair, Raymond, and Sankrit were the co-authors on the paper presenting the first two epochs of imaging this region in the Cygnus Loop (Blair, Sankrit and Raymond 2005). All three co-investigators are experienced in the use of HST for the study of supernova remnants. Dr. Raymond is a world-acknowledged expert on shock astrophysics. Stellar Physics 6 false false false false false false false SUPPORTED ISM EMISSION LINE NEBULA FILAMENT SHOCK FRONT SNR J2000 J2000 H-alpha surface brightness 6.0 +/- 0.5 E-16 erg/s/cm2/arcsec2 WFC3/UVIS Imaging No No F656N WFC3/UVIS Imaging No No F502N POS-TARG MOSAIC DITHER 3 2.414 WFC3-UVIS-DITHER-LINE-3PT false true 1.485 2 85.759 POS-TARG WFC3-UVIS-GAP-LINE 46.84 1

true

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