Environmental factors of Type Ia supernovae are key in understanding their nature, lightcurve evolution, and utility as cosmological standard candles. The progenitor ages (and many other properties) are bimodal, differing by roughly an order of magnitude. Is this reflected as well in the differences in their immediate surroundings in terms of gas and dust? The most powerful and direct way to address this issue is by imaging the reflected light from the dust itself via a light echo. In order for this approach to work, however, one must start imaging the vicinity of the supernova frequently and soon after the explosion is seen. We propose to maintain the imaging sequences crucial for understanding the three-dimensional dust distribution of two recent and key Type Ia supernovae, in a timely manner that will prevent otherwise significant holes in our knowledge. These observations are likely to be important in determining if the interstellar versus the circumstellar environments are more important in determining the appearance of Type Ia explosions, and thereby offer a clue as to the poorly-understood mass-loss history of SN Ia progenitors. JUSTIFICATION FOR VISIT TIME CONSTRAINTS: We have requested "Before" conditions

Our proposed exposure times and the expected signals to be returned by HST are reliably anticipated from previous exposures of earlier incarnations of these sources using imagers other than WFC3/UVIS. For SN 2006X our main priority will be to best determine the new echo geometry, hence we will use much of the 1 orbit requested for F555W observations (about 970s) that will define the echo locus most efficiently and match previous observations. We will also devote about 970s to F475W and 270s for F775W to provide sensitivity to any color change and to provide a reasonable match to previous HST/WFPC2 observations. For SN 1998bu we will make similar observations, but go deeper in the "echo geometry band" (F606W) and also include a deep exposure in F658N to capture optimally any non-echo CSM signal and provide an H-alpha line strength. JUSTIFICATION FOR VISIT TIMING REQUIREMENTS: We have requested a "Before" condition in the Visit Timing Requirements due to the rapidly changing nature of these targets' evolution and the unanticipated extension of the interval since the previous observations, in light of Servicing Mission 4. This is particularly critical in the case of SN 2006X (Target 1); there was a rapid series of HST images taken in May 2006 - January 2008, followed by an 18+ month delay. This time gap must be closed as soon as possible. The same argument applies to SN 1998bu (Target 2) but is less critical. There were five relevant epochs of HST imaging prior to January 2006, followed by a 42+ month delay. Closing these gaps as soon as possible increases the likelihood of maintaining adequate light echo coverage to 3-dimensionally map the surrounding material commensurate with the angular resolution of HST, which cannot be retrieved from later observations. Please, if at all possible, try to implement these Before conditions. Of course, the observations will still be of great value even if this is impossible. ISM IN EXTERNAL GALAXIES 12 false 0.0 0.0 false false false false false SUPPORTED EXT-STAR SUPERNOVA TYPE IA J2000 12 22 53.8800 +15 48 31.90 J2000 EXT-STAR SUPERNOVA TYPE IA J2000 10 46 46.0900 +11 50 7.00 J2000 WFC3/UVIS Imaging No No F775W WFC3/UVIS Imaging No No F475W WFC3/UVIS Imaging No No F555W WFC3/UVIS Imaging No No F814W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F475W WFC3/UVIS Imaging No No F658N The scheduling data was generated by CASM 17.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 11646 :visit-id "01" :proposal-version -1 :trans-version "20090130" :date "30-Jun-2009 12:58:47.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 1732464 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11646 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11646 :visit-id "01" :orbit-number 1 :pcs-time 241 :si-vis-utilization 2913 :al-overhead-at-end 116 :internal-dur-at-end 670 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3257 :pcs-gap 0 ) true 1 0 The scheduling data was generated by CASM 17.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 11646 :visit-id "02" :proposal-version -1 :trans-version "20090130" :date "25-Jun-2009 17:44:46.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 2887456 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11646 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11646 :visit-id "02" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2909 :al-overhead-at-end 116 :internal-dur-at-end 354 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3251 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11646 :visit-id "02" :orbit-number 2 :pcs-time 195 :si-vis-utilization 3047 :al-overhead-at-end 116 :internal-dur-at-end 354 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3251 :pcs-gap 0 ) true 2 0