Synoptic Imaging Polarimetry Observations of CRL 2688: What's Whipping the Egg?

0 Assigned ID: 144 ----- Attempting Submission 1 (Wed Apr 06 17:44:34 GMT 2016) ----- HST Phase I Proposal 144 (dhines_CRL2688.apt) successfully submitted. Receipt: # 144-1 ----- Attempting Submission 2 (Fri Apr 08 14:29:02 GMT 2016) ----- HST Phase I Proposal 144 (dhines_CRL2688.apt) successfully submitted. Receipt: # 144-2 ----- Attempting Submission 3 (Fri Apr 08 14:58:52 GMT 2016) ----- I'm submitting w/o post flash because the ETC estimates > 50 electrons per frame on the background for the specific dates of observations and target coords. 175065403 ----- Attempting Submission 1 (Wed Jul 20 22:39:07 GMT 2016) ----- Synoptic Imaging Polarimetry Observations of CRL 2688: What's Whipping the Egg? We propose to obtain new imaging polarimetry of the archetypical pre-planetary nebula, CRL 2688 (The Egg Nebula) with the ACS/WFC in the F606W and F775W filters. Pre-planetary nebulae are objects caught in the short-lived, rapidly changing transition phase between the AGB and planetary nebula (PN) stages of stellar evolution. Their morphology bridges the gap between the spherical mass-loss of AGB stars and the surprisingly non-spherical morphologies observed in PN. The mechanism(s) that drives this departure from sphericity is highly debated, and has many implications for stellar and galactic evolution. CRL 2688 is the nearest (to Earth) pre-planetary nebula. Its complex morphology exhibiting lobes, rings, "search light beams," and a dust lane -- all of which shine in highly polarized light -- make it an ideal specimen for detailed investigation with the high spatial resolution and exquisite polarimetric precision and accuracy provided by HST/ACS. Our proposed observations will be compared with previous ACS imaging polarimetry obtained in 2002 (providing a ~14 year baseline) to: 1) look for secular changes in properties of the scattering particles and scattering geometries within the lobes, rings, SLBs and dust lane; 2) form perpendiculars to the polarization position angles to pinpoint the location of the central illuminating source -- comparison between epochs will place constraints on changes in that source; 3) precisely measure proper motions in the lobes, SLBs, and rings. Our results will provide new insights into the processes that transform the spherical envelopes of a dying low mass stars into the often highly aspherical structures witnessed in planetary nebulae. We will observe CRL 2688 with the WFC on ACS using two broad-band filters (F606W & F775W) combined with the POLV polarizers to obtain imaging polarimetry from 0.48-0.85|um. We use POLV polarizers, as they are most polarimetrically efficient at the desired wavelengths. We chose the F606W to match the observations obtained in 2002; therefore this is a duplicate, but the source structures have changed since then, which is the reason for obtaining new observations. We also include F775W observations to determine the wavelength dependence of the polarization, which in turn places strong constraints on the scattering particles and their scattering geometry. To minimize any systematics caused by residual field distortions in our images relative to the 2002 observations, we constrain the new observations to have a spacecraft orientation (roll) that is within a few degrees of the previous 2002 observations. We use a 4 point box dither to mitigate cosmic rays. We also include a postfash to mitigate CTE effects. Stellar Physics 12 false false false false false false SUPPORTED EXT-STAR POST-AGB STAR YES Highly polarized (30%-70%) preplanetary nebula. The magnitue is an average surface brightness in V-mag/sq. arcsec J2000 J2000 The scheduling data was generated by CASM 24.0. CASM was run in Phase I mode. ACS/WFC Imaging No Yes F606W The scheduling data was generated by CASM 24.0. CASM was run in Phase I mode. ACS/WFC Imaging No Yes F775W POS-TARG DITHER 0.262 LINE 0.192 false true 68.14 4 18.39 ACS-WFC-DITHER-BOX 1

false

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