added proper motion for target 2 ----- Attempting Submission 2 (Wed Mar 11 14:22:59 GMT 2009) ----- On the evolutionary status of extremely hot helium stars - are the O(He) stars successors of the R CrB stars? We propose UV spectroscopy of the four unique post-AGB stars of spectral type O(He) in order to understand the origin of their peculiar surface abundances. These stars are the only known amongst the hottest post-AGB stars (effective temperatures > 100,000 K) whose atmospheres are composed of almost pure helium. This chemistry markedly differs from that of the hydrogen-deficient post-AGB evolutionary sequence with objects which have carbon dominated atmospheres (PG1159 stars and Wolf-Rayet central stars). While PG1159 and Wolf-Rayet stars are the result of a late helium-shell flash, this scenario cannot explain the O(He) stars. Instead, they are possibly double-degenerate mergers. We speculate that the four O(He) stars represent evolved RCrB stars, which also have helium-dominated atmospheres. We aim to determine the C, N, O, and Si abundances precisely, in order to proof this evolutionary link.
We propose to take medium-resolution high S/N spectra with COS/FUV of the four O(He) stars. With grating G140L we cover the wavelength range 1105 / 2203A including many diagnostic metal lines. A resolution of about 0.5A is sufficient for our purpose because the lines are relatively broad. We aim to achieve S/N > 50 at 1425A which is necessary from our experience with STIS/G140L observations. The respective exposure time have been determined with the COS Exposure Time Calculator using theoretical NLTE model atmospheres. For any of the four objects, the S/N can be obtained within one orbit. HOT STARS 12 false 0.0 0.0 false false false false false SUPPORTED STAR PLANETARY NEBULA CENTRAL STAR This object was generated by the targetselector and retrieved from the SIMBAD database. FUSE: log (Flambda / erg/cm**2/sec/A) = -12.5 J2000 05 57 2.1400 -75 40 22.50 J2000 STAR PLANETARY NEBULA CENTRAL STAR This object was generated by the targetselector and retrieved from the SIMBAD database. FUSE: log (Flambda / erg/cm**2/sec/A) = -13.2 J2000 11 52 29.2300 -42 17 38.70 J2000 STAR POST-AGB STAR This object was generated by the targetselector and retrieved from the SIMBAD database. FUSE: log (Flambda / erg/cm**2/sec/A) = -12.8 J2000 15 22 56.7000 +66 04 42.10 J2000 STAR POST-AGB STAR This object was generated by the targetselector and retrieved from the SIMBAD database. FUSE: log (Flambda / erg/cm**2/sec/A) = -13.1 J2000 22 08 25.8000 +82 44 52.00 J2000 COS/FUV Spectroscopic No No G140L 1105 COS/FUV Spectroscopic No No G140L 1105 COS/FUV Spectroscopic No No G140L 1105 COS/FUV Spectroscopic No No G140L 1105 The scheduling data was generated by CASM 17.2. CASM was run in Phase II mode. /atmosarray/rauch/HST/Cycle17/phase1/OP-cache/ohe_2009-v01.xml (Tic-Visit :proposal-id 11699 :visit-id "01" :proposal-version -1 :trans-version "20080609" :date "11-Mar-2009 09:54:34.0" :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 1322000 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11699 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11699 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 3324 :al-overhead-at-end 0 :internal-dur-at-end 447 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3658 :pcs-gap 0 ) true 1 0 The scheduling data was generated by CASM 17.2. CASM was run in Phase II mode. /atmosarray/rauch/HST/Cycle17/phase1/OP-cache/ohe_2009-v02.xml (Tic-Visit :proposal-id 11699 :visit-id "02" :proposal-version -1 :trans-version "20080609" :date "11-Mar-2009 09:54:39.0" :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 591504 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11699 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11699 :visit-id "02" :orbit-number 1 :pcs-time 333 :si-vis-utilization 3017 :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 3351 :pcs-gap 0 ) true 1 0 The scheduling data was generated by CASM 17.2. CASM was run in Phase II mode. /atmosarray/rauch/HST/Cycle17/phase1/OP-cache/ohe_2009-v03.xml (Tic-Visit :proposal-id 11699 :visit-id "03" :proposal-version -1 :trans-version "20080609" :date "11-Mar-2009 09:54:42.0" :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 661072 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11699 :visit-id "03" :obset-id "03" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11699 :visit-id "03" :orbit-number 1 :pcs-time 333 :si-vis-utilization 3280 :al-overhead-at-end 0 :internal-dur-at-end 438 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3614 :pcs-gap 0 ) true 1 0 The scheduling data was generated by CASM 17.2. CASM was run in Phase II mode. /atmosarray/rauch/HST/Cycle17/phase1/OP-cache/ohe_2009-v04.xml (Tic-Visit :proposal-id 11699 :visit-id "04" :proposal-version -1 :trans-version "20080609" :date "11-Mar-2009 09:54:46.0" :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 661072 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11699 :visit-id "04" :obset-id "04" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11699 :visit-id "04" :orbit-number 1 :pcs-time 333 :si-vis-utilization 3329 :al-overhead-at-end 0 :internal-dur-at-end 438 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3662 :pcs-gap 0 ) true 1 0