We have no hope of understanding the structure and evolution of a class of astrophysical objects if we cannot identify the dominant energy source of those objects.The Disk Instability Model (DIM) postulates that Dwarf Nova (DN) outbursts are powered by runaway accretion from an accretion disk onto a White Dwarf (WD) in a red dwarf-WD mass transferring binary. Ominously, HST observations (e.g. Sion et al. 2001) of WD surface abundances hint at a significant shortcoming of the DIM. The data from the present proposal will be able to unequivocally demonstrate if the observed highly Carbon-depleted and Nitrogen-enhanced abundances on WD surfaces (NOT predicted by DIM) vary with binary orbital phase, or throughout a DN quiescence cycle, or from cycle to cycle. These same data will test if predicted (but never observed) Local Thermonuclear Runaways ("Nuclear-powered mini-novas") occur on the WDs of DN. Such events could trigger or even power DN, providing the long-sought physical mechanism of DN eruptions that DIM lacks. As a "free" bonus, the same data may also directly detect the diffusion of accreted metals in a WD atmosphere for the first time, or provide significant limits on the diffusion rate. The core goals of this proposal are 1) to detect and monitor abundance variations on the white dwarf of the dwarf nova U Gem during one quiescence cycle, and 2) to determine if the WD surface abundances vary from one quiescence cycle to the next. To achieve our goals we will need to acquire spectra taken at given intervals starting when the system recovers from outburst. This makes our proposal a Target of Opportunity (ToO), with 100% chance of occurring once every 4-6 months. We request observations at four epochs during one outburst cycle: roughly 2,7,20 and 60 days after outburst ends. The first observations should be taken as soon as possible after the system returns to quiescence, when the abundance pattern is most likely to have been altered due to the outburst activity. At the first epoch we request 3 HST orbits, which matches the orbital period of U Gem. This will enable us to definitively search for abundance variations ACROSS the surface of the WD, (as we see the WD from different angles), as hinted at by the work of Long et al (2006). At 7, 20 and 60 days after eruption we conservatively request only 1 HST orbit to determine if significant variations in abundances occur due to diffusion. Only if strongly phase-dependent variations are seen in the first 3 orbits will we request, in the future, 3-orbit visits at EVERY epoch. Finally, we request one more orbit at 7 days after the NEXT eruption, to compare the WD surface composition at similar times in different outburst cycles. The orbital period of U Gem is 4.25 hr with a maximum orbital radial velocity of 107.1|*|2.1 km/s. For an exposure time of 10 minutes, taken at orbital phase |*|0.3 or |*|0.7, when the velocity shift of the spectral lines changes the fastest, the smearing of the spectral lines will only be |*|30 km/s, i.e. 0.13 |*|A at 1300 |*|A. The rotational velocity of the WD will reduce the resolution to 100 km/s|*|1 (R |*|3000). We chose the G140L grating to obtain excellent spectral coverage (900 to 2200 |*|A) with COS, to observe a wide variety of CNO AND metal lines (see Godon et al 2006 for the detailed lists of ion lines we will use). The strongest-line producing ions we will monitor are: HeII, CI, CII, CIV,NI,NII,NIII,OIV,SiII, SiIII, SiIV,AlII,SI,SIV,SVI . The 2.5 arcsec aperture of COS is no problem as there are no stars brighter than 20th magnitude within 20 arcsec of U Gem. Detailed line lists in Godon et al 2008 show that multiple ionization stages of each of C, N, O, P, Si, and S are available in this wavelength range...more than sufficient for our abundance analyses. (The 1155-1260 |*|A gap, including Lyman alpha, makes no difference to our analyses.) The minimum SNR needed to unambiguously distinguish abundance changes of a few times solar (see simulation in Fig. 2) is 35 (all previous observations only achieved SNR<15 and could only demonstrate qualitatively that CNO abundances are non-solar). Given the orbital visibility time for U Gem of 53 min, 22 min overhead for the first orbit (6 min for guide star acquisition, 6 min for target acquisition, 8 min for the first exposure in orbit and 1x2 min for 2 additional exposures) we have 1860 s per orbit to expose. Using a mean of 4 calibrated FOS spectra (y11i0402t, y11i0403t, y11i0502t and y11i0503t) obtained during quiescence we determined the SNR achievable in the available observing time. For the G140L grating we used a Kurucz stellar atmosphere equivalent to an OV5 star, scaling it to a flux of 5e-14 at 1700 |*|A, determined from the FOS spectra. We find that SNR=35-40 is easily achieved in 1 orbit with COS. U Gem outbursts last approximately 10-20 days with a rise from minimum (|*|14.5 mag) to maximum brightness (|*|9 mag) lasting about 2 days, with a subsequent return to minimum |*|8-18 days later. This means that STScI will be alerted at least two weeks prior to when the observations have to begin. In fact the continuous AAVSO monitoring of U Gem is so complete that we will be able to give STScI a |*|heads up|*| about 30 days before each anticipated eruption. We will take the first observation as soon as the system returns to quiescence. We will be able to give to STScI a precise date of when the observations should be taken one week in advance, once the onset of decline has started. We will work closely with STScI planners to minimize disruption to the HST schedule. By observing only up to 9 weeks after the return to quiescence, we ensure that our last observations will not occur during the next outburst, since the shortest time between any two U Gem outbursts is 100 days. RESOLVED STELLAR POPULATIONS 12 false false 0.0 0.0 false false false AVAILABLE STAR DWARF NOVA INTERACTING BINARY J2000 J2000 UNIDENTIFIED BLANK FIELD This target is a blank piece of sky which is the bright object safe pointing and is 9.062 arcseconds away at a PA 1.6 degrees from Target #1. COS/FUV Spectroscopic No No G140L 1280 Timing link suitability includes constraints ([02 After 01 by 5.0 to 6.0 days] [03 After 01 by 18.0 to 19.0 days] [04 After 01 by 40.0 to 43.0 days]) The scheduling data was generated by CASM 20.2. CASM was run in Phase II mode. 1356002770000 278.68281347332066 (Tic-Visit :proposal-id 12926 :visit-id "01" :proposal-version -1 :trans-version "20120604" :date "11-Dec-2012 10:57:22.0" :trans-errors '(:WARNINGS 2 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 3 :saa-model "31" :si-bright-earth-limb-angle 20.0 :data-volume 1113488 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12926 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12926 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2724 :al-overhead-at-end 0 :internal-dur-at-end 105 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3244 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12926 :visit-id "01" :orbit-number 2 :pcs-time 222 :si-vis-utilization 2836 :al-overhead-at-end 0 :internal-dur-at-end 57 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3244 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12926 :visit-id "01" :orbit-number 3 :pcs-time 222 :si-vis-utilization 2836 :al-overhead-at-end 0 :internal-dur-at-end 385 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3244 :pcs-gap 0 ) false 3 0 Timing link suitability includes constraints ([02 After 01 by 5.0 to 6.0 days]) The scheduling data was generated by CASM 20.2. CASM was run in Phase II mode. 1356496289000 277.96066301722425 (Tic-Visit :proposal-id 12926 :visit-id "02" :proposal-version -1 :trans-version "20120604" :date "11-Dec-2012 10:57:24.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "31" :si-bright-earth-limb-angle 20.0 :data-volume 278416 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12926 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12926 :visit-id "02" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2724 :al-overhead-at-end 0 :internal-dur-at-end 433 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3244 :pcs-gap 0 ) false 1 0 Timing link suitability includes constraints ([03 After 01 by 18.0 to 19.0 days]) The scheduling data was generated by CASM 20.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 12926 :visit-id "03" :proposal-version -1 :trans-version "20120604" :date "11-Dec-2012 10:57:26.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "31" :si-bright-earth-limb-angle 20.0 :data-volume 278416 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12926 :visit-id "03" :obset-id "03" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12926 :visit-id "03" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2724 :al-overhead-at-end 0 :internal-dur-at-end 433 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3244 :pcs-gap 0 ) false 1 0 Timing link suitability includes constraints ([04 After 01 by 40.0 to 43.0 days]) The scheduling data was generated by CASM 20.2. CASM was run in Phase II mode. 1359581472000 80.0 (Tic-Visit :proposal-id 12926 :visit-id "04" :proposal-version -1 :trans-version "20120604" :date "21-Dec-2012 15:52:33.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "31" :si-bright-earth-limb-angle 20.0 :data-volume 278416 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12926 :visit-id "04" :obset-id "04" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12926 :visit-id "04" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2724 :al-overhead-at-end 0 :internal-dur-at-end 433 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3244 :pcs-gap 0 ) false 1 0 The scheduling data was generated by CASM 20.2. CASM was run in Phase II mode. 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(Tic-Visit :proposal-id 12926 :visit-id "04" :proposal-version -1 :trans-version "20120604" :date "21-Dec-2012 15:52:33.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "31" :si-bright-earth-limb-angle 20.0 :data-volume 278416 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12926 :visit-id "04" :obset-id "04" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12926 :visit-id "04" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2724 :al-overhead-at-end 0 :internal-dur-at-end 433 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3244 :pcs-gap 0 ) false 1 0