Searching for Massive Remnant Planetary System Hosts with White Dwarf Gaia XP Spectra

Remnant planetary systems around massive white dwarfs (M WD > 0.8, M) are intrinsically rare and remain poorly characterized. As the evolutionary endpoints of intermediate-mass B- and A-type stars, these white dwarfs provide valuable constraints on the formation and survival of planetary systems around higher-mass progenitors. In the absence of large, dedicated spectroscopic surveys, extremely-low-resolution Gaia XP spectra provide a powerful means of identifying candidate metal-polluted white dwarfs. We obtained follow-up observations with low-resolution optical spectroscopy (R=500-4000) to search primarily for Ca II H & K absorption in massive, metal-polluted white dwarf candidates selected from Gaia XP spectra to increase the sample of massive remnant planetary system hosts among objects with effective temperatures below about15, 000 K. We find that Gaia XP spectra recover metal-polluted white dwarfs efficiently, confirming 82 plus or minus 5% recovery of photospheric metals (59 of 72 targets) in high-confidence candidates of our sample (with classifier probability P DZ > 0.65). However, Gaia photometric masses derived from G, G BP, and G RP broad-band photometry are not sufficient to identify the most massive systems, as reliable mass determinations require detailed metal-enriched atmospheric fits. We present a selection of cool white dwarfs confirmed to be both massive and metal polluted, demonstrating that Gaia XP spectra can substantially expand the sample of white dwarf exoplanet hosts. Our search led to the discovery of a new transiting debris system, SDSS J1624+5404, the 21st published system to date and eighth to show periodic transits.