18420( 2) - 29-Jul-2026 16:39:17 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18420 Version: 2 Check-in Time: 29-Jul-2026 16:39:17 STScI Edit Number: 0 Title Exploring the composition of the most aluminium-rich object accreting onto a white dwarf ------------------------------------------------------------------------------------ Type Cycle GO 34 ------------------------------------------------------------------------------------ Investigators Contact? PI: Mr. Jamie Williams University of Warwick Y CoI: Prof. Boris T. Gaensicke University of Warwick N CoI: Snehalata Sahu University of Warwick N CoI: Dr. David John Wilson University of Colorado at Boulder N CoI: Dr. Thomas G. Wilson University of Warwick N CoI: Dr. Amy Bonsor University of Cambridge N ------------------------------------------------------------------------------------ Abstract The interior of low density rocky planets currently remains a mystery. Although there are theories that their interiors are dominated by carbon species or they are devoid of iron cores, there is no method to accurately probe their interior chemical compositions. However, white dwarfs accreting planetary material can be used to measure the chemical abundances of the accreted material. Their atmospheres are purely hydrogen or helium, hence measurements of any metal abundances can be used to determine the nature of planets in other systems. We have identified a white dwarf which is accreting a planet(esimal) with an enormous aluminium enhancement, much greater than that of mantle or crustal material. This could indicate it is a fragment or a building block of a low density rocky planet dominated by aluminium oxide which formed under an extremely high temperature, where iron was lost resulting in a planet with no core. With our exploratory spectrum, we are unable to detect or place strict upper limits on oxygen and iron. Oxygen is vital to confirm it can form oxides which dominate the interior. A stricter upper limit on iron is necessary to confirm it contains little or no iron core. Additional species such as chromium or nickel will further constrain the core mass fraction. At the high temperature of this white dwarf, oxygen and iron only have strong transitions in the far-ultraviolet, so we require one orbit of COS spectroscopy to search for these absorption features. ------------------------------------------------------------------------------------ Observations Description ------------------------ We aim to detect oxygen and iron in the atmosphere of a white dwarf with a very large aluminium abundance. We will use the COS G130M grating with a central wavelength of 1222A which is sensitive to both oxygen and iron lines. We aim for a S/N ~ 30 at the lines of interest. All four FP-POS are used to minimise fixed pattern noise. ------------------------------------------------------------------------------------ 18420( 2) - 29-Jul-2026 16:39:17 - [ 2] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 WDJ063541.34 STAR, DA RA=06H35M41.3371S +/- 0.02", J2000 V = 14.3 -052430.64 DEC=-05D24'30.63" +/- 0.02" 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 -0.0012503666590120532 0.155548 0.0181656 ------------------------------------------------------------------------------------------------------------------------------------ 18420( 2) - 29-Jul-2026 16:39:17 - [ 3] Visit: 02 (WDJ063541.34-052430.64) Visit Requirements: 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 WDJ063541.3 COS/FUV ACQ/PEA PSA G130M 1291 LIFETIME-POS=LP7 1 2 S (2 S) 4-052430.64 KXD ------------------------------------------------------------------------------------------------------------------------------------ 2 WDJ063541.3 COS/FUV ACQ/PEA PSA G130M 1291 LIFETIME-POS=LP7, 1 2 S (2 S) 4-052430.64 KD STEP-SIZE=0.9, CENTER=DEF, NUM-POS=5 ------------------------------------------------------------------------------------------------------------------------------------ 3 WDJ063541.3 COS/FUV TIME-TA PSA G130M 1222 FP-POS=ALL, LIFETIME-POS=LP7, 1 250 S 4-052430.64 G BUFFER-TIME=218 see below (805 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 | ------------------------------------------------------------------------------------------------------------------------------------ WDJ063541.34-052430 1 COS/FUV ACQ/PEA PSA G130M 1291 none none none none 1 N/A .64 KXD WDJ063541.34-052430 2 COS/FUV ACQ/PEA PSA G130M 1291 none none none none 1 N/A .64 KD WDJ063541.34-052430 3 COS/FUV TIME-TA PSA G130M 1222 none none none 1 1 200.0 S .64 G WDJ063541.34-052430 3 COS/FUV TIME-TA PSA G130M 1222 none none none 2 1 200.0 S .64 G WDJ063541.34-052430 3 COS/FUV TIME-TA PSA G130M 1222 none none none 3 1 200.0 S .64 G WDJ063541.34-052430 3 COS/FUV TIME-TA PSA G130M 1222 none none none 4 1 205.0 S .64 G 18420( 2) - 29-Jul-2026 16:39:17 - [ 4] Summary Form for Proposal 18420 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures PSA ------------------------------------------------------------------------------------------------------------------------------------ Configurations COS/FUV ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ/PEAKD, ACQ/PEAKXD, TIME-TAG ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters BUFFER-TIME=218, CENTER=DEF, FP-POS=ALL, LIFETIME-POS=LP7, NUM-POS=5, STEP-SIZE=0.9 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G130M ------------------------------------------------------------------------------------------------------------------------------------ Target Names WDJ063541.34-052430.64 ------------------------------------------------------------------------------------------------------------------------------------