Second submission 1931607640 Assigned ID: 7077 ----- Attempting Submission 1 (Tue Apr 14 16:16:26 GMT 2026) ----- HST Phase I Proposal 7077 successfully submitted. Receipt: # 7077-1 ----- Attempting Submission 2 (Wed Apr 15 23:14:57 GMT 2026) ----- HST Phase I Proposal 7077 successfully submitted. Receipt: # 7077-2 First submission -540248356 ----- Attempting Submission 1 (Tue Aug 18 15:57:41 GMT 2026) ----- 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. 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 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. P.I. Guidry is an expert in using large time-domain surveys to discover and characterize white dwarf variability, both in the optical and mid-infrared. Guidry has been involved in the discovery of every transiting debris system following WD1145+017 and ZTF J0139+5245, the first two known systems. He will lead the analysis of the proposed science. Hermes is an expert on white dwarf variability, and has been involved in the discovery of all new transiting debris systems since WD1145+017. Hermes is also an expert in the analysis of time-tagged HST data. Hermes and Guidry have guaranteed access to the 1.8-meter Perkins Telescope in Flagstaff, AZ, which is 100% owned and operated by Boston University. This will allow maximal scheduling flexibility to guarantee ground-based, optical, time-series photometry of our target regardless of when it is scheduled to be observed by HST. Vanderbosch is an expert on white dwarf variability, and has been involved in the discovery of all new transiting debris systems since WD1145+017, excluding WD1054-226. Hollands is an expert in the modeling of white dwarf spectra, especially those accreting planetary material. He will assist in the analysis of extracting abundances from the spectra we obtain. Gaensicke has been PI of 20 HST programs, carrying out ultraviolet spectroscopy of white dwarfs, including large, medium, small GO programs and several snapshot programs. He has in-depth experience in the analysis and interpretation of the observations of accreting white dwarfs, both in binary systems, and evolved planetarysystems. He will assist in implementing the observations and their analysis. Ould Rouis is an expert in the handling and interpretation of far-ultraviolet COS spectra, especially those of massive, accreting white dwarfs. She will assist in the identification of metal lines that indicate ongoing accretion and help constrain observational biases that may effect the interpretation of our results. Stellar Physics and Stellar Types Exoplanets and Exoplanet Formation 6 false true false false false false false SUPPORTED STAR CIRCUMSTELLAR MATTER DA NO 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. GAIA-DR3-2080201713998110336 J2000 J2000 Gaia G=19.36 COS/FUV Spectroscopic No No G130M 1291 /Users/astrojoe/Research/Observing_Proposals/HST/Cycle34_ZTFJ1944_COS/PhaseII/OP-cache/ZTFJ1944+4557_Cycle34_COS_PhaseII-v01.xml 1787068164874 The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. 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CASM was run in Phase II mode. 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