18484( 3) - 05-Oct-2026 21:09:15 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18484 Version: 3 Check-in Time: 05-Oct-2026 21:09:15 STScI Edit Number: 0 Title Planet or EUV variation? Chandra and Hubble observations of a variable white dwarf ------------------------------------------------------------------------------------ Type Cycle GO 34 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. David John Wilson University of Colorado at Boulder Y CoI: Mr. Jamie Williams University of Warwick N CoI: Prof. Boris T. Gaensicke University of Warwick N CoI: Snehalata Sahu University of Warwick N CoI: Dr. Andrew Swan University of Warwick N ------------------------------------------------------------------------------------ Abstract We request Chandra and Hubble observations to characterise the X-ray and ultraviolet variability of the hot white dwarf HS 0209+0832. The white dwarf is accreting material from a remnant planetary system with chemical abundances that match no known object. TESS observations reveal a periodic 4.4 day signal, an extremely rare variability mode for hot white dwarfs. We suggest two hypotheses, both of which would be extraordinary discoveries: First, this may be only the second known example of an EUV variable white dwarf; Second, the signals could reveal an evaporating giant planet. We will obtain a soft X-ray light curve and spectrum of the system to search for EUV variation to test these hypotheses, along with phase-resolved ultraviolet spectra to search for metal line variability. ------------------------------------------------------------------------------------ Observations Description ------------------------ The aim is to aquire phase resolved E140M spectra of the white dwarf HS0209+0832 (Feige 20). Eight exposures should be taken over the 4.399 day period identified with TESS. We estimate (etc run 2370262) that a single E140M observation should return S/N of about 10 to 20 over the spectrum. Data should be obtained in time-tag mode. The ephemeris of the periodic behaviour is unknown so the submission date was used. ------------------------------------------------------------------------------------ 18484( 3) - 05-Oct-2026 21:09:15 - [ 2] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 FEIGE-20 STAR, DA, DZ RA=02H12M4.8544S, J2000 V = 13.84 DEC=+08D46'50.89" Reference Frame: ICRS Extended: NO Comments: This object was generated by the targetselector and retrieved from the SIMBAD database. SIMBAD listed proper motion for this target. When retrieving targets with PM from SIMBAD, APT requests the coordinates be calculated with an epoch of the year 2000. Do not modify this epoch. Always review coordinates using the Target Confirmation tool, which graphically displays the PM. Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2000 0.005401575373839003 -0.025774000073397474 0.0120996 ------------------------------------------------------------------------------------------------------------------------------------ 18484( 3) - 05-Oct-2026 21:09:15 - [ 3] Visit: 01 Visit Requirements: Period 4.399 D AND ZERO-PHASE HJD2461315 On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 FEIGE-20 STIS/CCD ACQ F28X50LP MIRROR 1 1 S (1 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 FEIGE-20 STIS/FUV TIME-TA 0.2X0.2 E140M 1425 BUFFER-TIME=272 1 1600 S PHASE 0 TO 0.125 -MAMA G see below (1761 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ FEIGE-20 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A FEIGE-20 2 STIS/FUV TIME-TA 0.2X0.2 E140M 1425 none none none none 1 1761.0 S -MAMA G 18484( 3) - 05-Oct-2026 21:09:15 - [ 4] Visit: 02 Visit Requirements: Period 4.399 D AND ZERO-PHASE HJD2461315 On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 FEIGE-20 STIS/CCD ACQ F28X50LP MIRROR 1 1 S (1 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 FEIGE-20 STIS/FUV TIME-TA 0.2X0.2 E140M 1425 BUFFER-TIME=272 1 1600 S PHASE 0.125 TO 0.25 -MAMA G see below (1761 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ FEIGE-20 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A FEIGE-20 2 STIS/FUV TIME-TA 0.2X0.2 E140M 1425 none none none none 1 1761.0 S -MAMA G 18484( 3) - 05-Oct-2026 21:09:15 - [ 5] Visit: 03 Visit Requirements: Period 4.399 D AND ZERO-PHASE HJD2461315 On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 FEIGE-20 STIS/CCD ACQ F28X50LP MIRROR 1 1 S (1 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 FEIGE-20 STIS/FUV TIME-TA 0.2X0.2 E140M 1425 BUFFER-TIME=272 1 1600 S PHASE 0.25 TO 0.375 -MAMA G see below (1761 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ FEIGE-20 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A FEIGE-20 2 STIS/FUV TIME-TA 0.2X0.2 E140M 1425 none none none none 1 1761.0 S -MAMA G 18484( 3) - 05-Oct-2026 21:09:15 - [ 6] Visit: 04 Visit Requirements: Period 4.399 D AND ZERO-PHASE HJD2461315 On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 FEIGE-20 STIS/CCD ACQ F28X50LP MIRROR 1 1 S (1 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 FEIGE-20 STIS/FUV TIME-TA 0.2X0.2 E140M 1425 BUFFER-TIME=272 1 1600 S PHASE 0.375 TO 0.5 -MAMA G see below (1761 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ FEIGE-20 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A FEIGE-20 2 STIS/FUV TIME-TA 0.2X0.2 E140M 1425 none none none none 1 1761.0 S -MAMA G 18484( 3) - 05-Oct-2026 21:09:15 - [ 7] Visit: 05 Visit Requirements: Period 4.399 D AND ZERO-PHASE HJD2461315 On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 FEIGE-20 STIS/CCD ACQ F28X50LP MIRROR 1 1 S (1 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 FEIGE-20 STIS/FUV TIME-TA 0.2X0.2 E140M 1425 BUFFER-TIME=272 1 1600 S PHASE 0.5 TO 0.625 -MAMA G see below (1761 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ FEIGE-20 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A FEIGE-20 2 STIS/FUV TIME-TA 0.2X0.2 E140M 1425 none none none none 1 1761.0 S -MAMA G 18484( 3) - 05-Oct-2026 21:09:15 - [ 8] Visit: 06 Visit Requirements: Period 4.399 D AND ZERO-PHASE HJD2461315 On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 FEIGE-20 STIS/CCD ACQ F28X50LP MIRROR 1 1 S (1 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 FEIGE-20 STIS/FUV TIME-TA 0.2X0.2 E140M 1425 BUFFER-TIME=272 1 1600 S PHASE 0.625 TO 0.75 -MAMA G see below (1761 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ FEIGE-20 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A FEIGE-20 2 STIS/FUV TIME-TA 0.2X0.2 E140M 1425 none none none none 1 1761.0 S -MAMA G 18484( 3) - 05-Oct-2026 21:09:15 - [ 9] Visit: 07 Visit Requirements: Period 4.399 D AND ZERO-PHASE HJD2461315 On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 FEIGE-20 STIS/CCD ACQ F28X50LP MIRROR 1 1 S (1 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 FEIGE-20 STIS/FUV TIME-TA 0.2X0.2 E140M 1425 BUFFER-TIME=272 1 1600 S PHASE 0.75 TO 0.875 -MAMA G see below (1761 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ FEIGE-20 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A FEIGE-20 2 STIS/FUV TIME-TA 0.2X0.2 E140M 1425 none none none none 1 1761.0 S -MAMA G 18484( 3) - 05-Oct-2026 21:09:15 - [ 10] Visit: 08 Visit Requirements: Period 4.399 D AND ZERO-PHASE HJD2461315 On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 FEIGE-20 STIS/CCD ACQ F28X50LP MIRROR 1 1 S (1 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 FEIGE-20 STIS/FUV TIME-TA 0.2X0.2 E140M 1425 BUFFER-TIME=272 1 1600 S PHASE 0.875 TO 0 -MAMA G see below (1761 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ FEIGE-20 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A FEIGE-20 2 STIS/FUV TIME-TA 0.2X0.2 E140M 1425 none none none none 1 1761.0 S -MAMA G 18484( 3) - 05-Oct-2026 21:09:15 - [ 11] Summary Form for Proposal 18484 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures 0.2X0.2, F28X50LP ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD, STIS/FUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ, TIME-TAG ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters BUFFER-TIME=272 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PHASE 0 TO 0.125, PHASE 0.125 TO 0.25, PHASE 0.25 TO 0.375, PHASE 0.375 TO 0.5, PHASE 0.5 TO 0.625, PHASE 0.625 TO 0.75, PHASE 0.75 TO 0.875, PHASE 0.875 TO 0, Period 4.399 D AND ZERO-PHASE HJD2461315 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements E140M, MIRROR ------------------------------------------------------------------------------------------------------------------------------------ Target Names FEIGE-20 ------------------------------------------------------------------------------------------------------------------------------------