18401( 2) - 18-Aug-2026 17:08:53 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18401 Version: 2 Check-in Time: 18-Aug-2026 17:08:53 STScI Edit Number: 0 Title FUV Spectroscopy of Transiting Exoplanetary Debris Orbiting near the Roche Limit of a White Dwarf ------------------------------------------------------------------------------------ Type Cycle GO 34 ------------------------------------------------------------------------------------ Investigators Contact? PI: Joseph Alexander Guidry Boston University Y CoI: Dr. JJ Hermes Boston University N CoI: Lou Baya Ould Rouis Boston University N CoI: Dr. Zachary Vanderbosch University of Texas at Austin N CoI: Dr. Mark Hollands University of Warwick N CoI: Prof. Boris T. Gaensicke University of Warwick N ------------------------------------------------------------------------------------ Abstract The circumstellar environments of white dwarf stars are often observed to be sites of the ongoing destruction of exoplanetary material. These stars are accreting from tidally disrupted minor planets. The geochemical makeup of these disintegrating objects are encoded within the spectra of these stars, constituting a powerful diagnostic for extracting the bulk composition of exoplanetary bodies. Ultraviolet (UV) spectroscopy is usually necessary to derive the most accurate abundances, especially for hot white dwarfs who emit most of their radiation in the UV, and since the predominant constituents of rocky material, such as iron, oxygen, and silicon, are most densely concentrated in the UV. We propose obtaining far-UV (FUV) spectroscopy with COS over seven consecutive orbits of a white dwarf, ZTF J1944+4557, accreting planetary material that shows transits from the debris repeating every 4.97 hours. This is the second case of transiting debris observed just outside the Roche limit of a white dwarf. In addition to taking an accurate census of the debris chemistry, we will create a time-tagged FUV light curve to search for small dust grains embedded in the disk. We will compare the FUV transit depths against those concurrently observed in the optical to constrain the minimum particle size of the dusty debris. Our proposed observations will place ZTF J1944+4557 into the broader context of the emerging class of white dwarfs showing transiting planetary debris, helping to begin to illuminate possible relationships between the orbital period and eccentricities of debris and dust production mechanisms, namely collisions. ------------------------------------------------------------------------------------ Observations Description ------------------------ The goal of this HST program is to obtain time-resolved COS far-ultraviolet spectroscopy of the white dwarf ZTF1944+4557 to (1) detect the photospheric metal absorption lines in this wavelength range and (2) to probe for color-dependence of the transit depth. The orbital period of the debris is 4.9704h. The observations will be collected over two visits, one of four orbits and one of three orbits. We will use the G130M grating in TIME-TAG mode and the primary science aperture (PSA) for both visits. The first visit (four orbits long) will 18401( 2) - 18-Aug-2026 17:08:53 - [ 2] be centered on 1291 Angstrom, while the second (three orbit) visit will be centered on 1222 Angstrom to provide complete coverage of the star's FUV spectrum. This will enhance our first scientific outcome. Splitting our program into two visits will allow us to test for potential variations in the particle size distribution from epoch to epoch. The target acquisition will be performed via COS/NUV ACQ IMAGE esposures designed to achieve S/N=40, as estimated using the ETC and MIRRORA. For the ETC simulation of the COS TIME-TAG spectrophotometry, we used a white dwarf model spectrum computed with the parameters from Bhattarcharjee et al. 2025 (PASP 137, 4202), i.e. Teff=20790 K, log g=8.27, and log(Ca/He)=-8.00, and we scaled all other elements to Bulk Earth abundances. We use the Gaia DR3 G-band Vega magnitude G=19.36 mag. ------------------------------------------------------------------------------------ 18401( 2) - 18-Aug-2026 17:08:53 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 ZTF-J1944+45 STAR, CIRCUMSTELLAR RA=19H44M31.9200S +/- J2000 V = 19.3 Gaia G=19.36 57 MATTER, DA 0.0001805", DEC=+45D57'53.13" +/- 0.000218" 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 -2.1282288288155827E-4 -0.010962999999999999 0.0023201 ------------------------------------------------------------------------------------------------------------------------------------ 18401( 2) - 18-Aug-2026 17:08:53 - [ 4] Visit: 01 Visit Requirements: BETWEEN 01-MAY-2027:00:00:00 AND 01-NOV-2027:00:00:00 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 ZTF-J1944+4 COS/NUV ACQ/IMA PSA MIRRORA 1 11.4 S 557 GE (11.4 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 ZTF-J1944+4 COS/FUV TIME-TA PSA G130M 1291 LIFETIME-POS=LP7, 1 1900 S 557 G BUFFER-TIME=5075, FP-POS=3 see below (2177 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 ZTF-J1944+4 COS/FUV TIME-TA PSA G130M 1291 BUFFER-TIME=5075, 1 2200 S 557 G LIFETIME-POS=LP7, FP-POS=3 see below (2571 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 ZTF-J1944+4 COS/FUV TIME-TA PSA G130M 1291 BUFFER-TIME=5075, 1 2200 S 557 G LIFETIME-POS=LP7, FP-POS=4 see below (2571 S) ------------------------------------------------------------------------------------------------------------------------------------ 5 ZTF-J1944+4 COS/FUV TIME-TA PSA G130M 1291 BUFFER-TIME=5075, 1 2200 S 557 G LIFETIME-POS=LP7, FP-POS=4 see below (2571 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 | ------------------------------------------------------------------------------------------------------------------------------------ ZTF-J1944+4557 1 COS/NUV ACQ/IMA PSA MIRRORA none none none none 1 N/A GE ZTF-J1944+4557 2 COS/FUV TIME-TA PSA G130M 1291 none none none none 1 2177.0 S G ZTF-J1944+4557 3 COS/FUV TIME-TA PSA G130M 1291 none none none none 2 2571.0 S G ZTF-J1944+4557 4 COS/FUV TIME-TA PSA G130M 1291 none none none none 3 2571.0 S G ZTF-J1944+4557 5 COS/FUV TIME-TA PSA G130M 1291 none none none none 4 2571.0 S G 18401( 2) - 18-Aug-2026 17:08:53 - [ 5] Visit: 02 Visit Requirements: BETWEEN 01-MAY-2027:00:00:00 AND 01-NOV-2027:00:00:00 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 ZTF-J1944+4 COS/NUV ACQ/IMA PSA MIRRORA 1 11.4 S 557 GE (11.4 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 ZTF-J1944+4 COS/FUV TIME-TA PSA G130M 1222 LIFETIME-POS=LP7, FP-POS=1, 1 2200 S 557 G BUFFER-TIME=10633 see below (2091 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 ZTF-J1944+4 COS/FUV TIME-TA PSA G130M 1222 LIFETIME-POS=LP7, FP-POS=2, 1 2200 S 557 G BUFFER-TIME=10633 see below (2571 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 ZTF-J1944+4 COS/FUV TIME-TA PSA G130M 1222 LIFETIME-POS=LP7, FP-POS=3, 1 2200 S 557 G BUFFER-TIME=10633 see below (2571 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 | ------------------------------------------------------------------------------------------------------------------------------------ ZTF-J1944+4557 1 COS/NUV ACQ/IMA PSA MIRRORA none none none none 1 N/A GE ZTF-J1944+4557 2 COS/FUV TIME-TA PSA G130M 1222 none none none none 1 2091.0 S G ZTF-J1944+4557 3 COS/FUV TIME-TA PSA G130M 1222 none none none none 2 2571.0 S G ZTF-J1944+4557 4 COS/FUV TIME-TA PSA G130M 1222 none none none none 3 2571.0 S G 18401( 2) - 18-Aug-2026 17:08:53 - [ 6] Summary Form for Proposal 18401 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures PSA ------------------------------------------------------------------------------------------------------------------------------------ Configurations COS/FUV, COS/NUV ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ/IMAGE, TIME-TAG ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters BUFFER-TIME=10633, BUFFER-TIME=5075, FP-POS=1, FP-POS=2, FP-POS=3, FP-POS=4, LIFETIME-POS=LP7 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements BETWEEN 01-MAY-2027:00:00:00 AND 01-NOV-2027:00:00:00 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G130M, MIRRORA ------------------------------------------------------------------------------------------------------------------------------------ Target Names ZTF-J1944+4557 ------------------------------------------------------------------------------------------------------------------------------------