SDSS1102+2054 is a cool white dwarf with a helium-dominated atmosphere that exhibits strong oxygen and carbon lines in its optical spectrum. The oxygen-to-carbon (O/C) abundance ratio implied by the line strengths is much larger than unity, which rules out out a carbon-oxygen core composition and provides strong evidence that SDSS1102+2054 is a naked oxygen-neon (ONe) core. Stellar evolution models predict that such extreme O/C abundance ratios are achieved only in the most massive stars that just avoid core-collapse, and we hence believe that SDSS1102+2054 descends from an intermediate mass star close to the 8-10Msun boundary between stars leaving behind either white dwarfs, or neutron stars. As such, this white dwarf has the potential to provide important constraints on our understanding of the evolution of intermediate mass stars, in particular testing predictions of mass loss, the efficiency of convective core mixing, and the relevant nuclear reaction rates. We propose to obtain intermediate-resolution HST/COS spectroscopy of SDSS1102+2054 to overcome the limitations of the available ground-based data and accurately measure its effective temperature, mass, and Mg abundance, and stringently test the current models of stellar evolution.

SDSS1102+2054 is the first cool white dwarf with a very oxygen (and carbon)-rich atmosphere. We need to obtain a low-res FUV/NUV spectrum to establish the spectral energy distribution, and from there determine the effective temperature of the star. This will be done by a combination of COS/G140L and COS/G230L observations. The G140L observations are spread over three orbits, and use two different FP-POS (1,4) settings to deal with fixed pattern noise. The NUV range is sampled in two orbits with three G230L observations, centred on 2635A, 2950A, 3360A. The target is sufficiently faint to be acquired with NUV imaging, MIRRORA + PSA. HOT STARS 12 false 0.0 0.0 false false false false SUPPORTED STAR DZ No GALEX data at the position, this is a cool white dwarf with a predicted flux of 1.5e-14/erg/cm2/s/AA across the NUV,and rapidly dropping at lambda < 2000A. UPDATE - we have obtained a Swift UVOT UVW1 observation (Target ID=31907), which gvies a flux density [erg/s/cm^2/A] of 4.82 +/- 0.18 (stat) +/- 0.15 (sys) x 10^-16 at 2591A, i.e. about a factor ~3 lower than the predicted flux. I have updated the target acq exposure times from 7sec to 16sec. J2000 11 02 39.6900 +20 54 39.40 J2000 COS/FUV Spectroscopic No No G140L 1280 COS/NUV Spectroscopic No No G230L 2635 COS/NUV Spectroscopic No No G230L 2950 COS/NUV Spectroscopic No No G230L 3360 The scheduling data was generated by CASM 18.3.1. CASM was run in Phase II mode. (Tic-Visit :proposal-id 12168 :visit-id "01" :proposal-version -1 :trans-version "20100505" :date "25-Jan-2011 10:43:20.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 6 :saa-model "31" :si-bright-earth-limb-angle 20.0 :data-volume 913344 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12168 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12168 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2916 :al-overhead-at-end 0 :internal-dur-at-end 69 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3249 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12168 :visit-id "01" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3027 :al-overhead-at-end 0 :internal-dur-at-end 97 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3249 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12168 :visit-id "01" :orbit-number 3 :pcs-time 222 :si-vis-utilization 3027 :al-overhead-at-end 0 :internal-dur-at-end 116 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3249 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12168 :visit-id "01" :orbit-number 4 :pcs-time 222 :si-vis-utilization 3027 :al-overhead-at-end 0 :internal-dur-at-end 116 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3249 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12168 :visit-id "01" :orbit-number 5 :pcs-time 222 :si-vis-utilization 3027 :al-overhead-at-end 0 :internal-dur-at-end 116 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3249 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12168 :visit-id "01" :orbit-number 6 :pcs-time 222 :si-vis-utilization 3027 :al-overhead-at-end 0 :internal-dur-at-end 428 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3249 :pcs-gap 0 ) false 6 0 COS.ta.004574 COS.ta.004574 COS.A331104 COS.A331104 COS.A331104 COS.A331104 COS.A331100 COS.A331095 COS.A331099