H1504+65 is the hottest known white dwarf (Teff=200,000 K). It has an extraordinary surface composition. The surface is devoid of hydrogen and helium. It is mainly composed of carbon and oxygen (by equal amounts) and neon (2%). We obviously see the exposed core of a former red giant. The evolutionary history of this unique object is unknown. We have identified magnesium absorption lines in the soft X-ray photospheric Chandra spectrum, which suggests that H1504+65 may be an O-Ne-Mg white dwarf. We will test this hypothesis by abundance determinations of Mg and Na. If confirmed, then H1504+65 would be the most compelling case for the existence of single O-Ne-Mg white dwarfs.

We want to use COS gratings G130M and G160M, covering the range 1132-1775A. This provides the spectral resolution needed for a quantitative analysis of the narrow metal lines. Besides prominent C and O lines this range contains many Mg VII and Na VI suitable for spectroscopic analyses. We used the COS Exposure Time Calculator to determine the exposure times necessary to achieve S/N of about 30. From our experience with previous HST data analyses this S/N is necessary to determine abundances as accurate as 0.3 dex, which is required to answer our scientific questions. HOT STARS 12 false 0.0 0.0 false false false false false false SUPPORTED STAR PG1159 STAR J2000 15 02 9.6700 +66 12 18.70 J2000 COS/FUV Spectroscopic No No G130M 1291 COS/FUV Spectroscopic No No G160M 1600 The scheduling data was generated by CASM 17.2. CASM was run in Phase II mode. 1256381955000 16.601272372549886 (Tic-Visit :proposal-id 11601 :visit-id "01" :proposal-version -1 :trans-version "20080609" :date "27-Jun-2008 16:11:16.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "31" :si-bright-earth-limb-angle 20.0 :data-volume 1930832 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11601 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11601 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 3279 :al-overhead-at-end 0 :internal-dur-at-end 126 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3614 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11601 :visit-id "01" :orbit-number 2 :pcs-time 195 :si-vis-utilization 3419 :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 3614 :pcs-gap 0 ) true 2 0 (Tic-Visit :proposal-id 11601 :visit-id "51" :proposal-version -1 :trans-version "20100218" :date "13-Apr-2010 15:59:35.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "31" :si-bright-earth-limb-angle 20.0 :data-volume 1861264 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11601 :visit-id "51" :obset-id "51" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11601 :visit-id "51" :orbit-number 1 :pcs-time 333 :si-vis-utilization 3233 :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 3599 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11601 :visit-id "51" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3364 :al-overhead-at-end 0 :internal-dur-at-end 419 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3599 :pcs-gap 0 ) true 2 0