Hot carbon-dominated, magnetic, and metal-polluted white dwarfs: Spectroscopic insights from a machine-learning-selected 500 pc sample Gaia

The latest data release provides astrometric measurements for 1.8 billion sources and low-resolution spectra for 220 million sources, including approximately 100,000 white dwarfs. Although useful for pre-classification, spectra lack the resolution required for accurate spectral typing and reliable parameter determination, motivating dedicated spectroscopic follow-up observations. We assessed the reliability of machine-learning spectral classifications derived from spectra by comparing them with medium-resolution spectroscopy. We determined the nature of objects classified as `massive helium-rich (DB)' white dwarfs by automated methods, and characterised the properties of warm and hot carbon-dominated (DQ) white dwarfs, as well as magnetic and metal-polluted objects. We observed 255 white dwarfs along the B- and Q-branches with the Gran Telescopio Canarias, equipped with the OSIRIS instrument (R We assigned spectral types through visual inspection and compared them with machine-learning classifications applied to spectra. For objects labelled as `massive DBs', we independently determined their atmospheric spectral compositions. We identified magnetic white dwarfs via Zeeman splitting and estimated first-order magnetic field strengths when possible. Machine-learning classifications are highly accurate (>90% for spectral types included in their training sets), despite the low resolution of spectra. We determine that objects classified as `massive DBs' are largely magnetic white dwarfs and warm DQs, with only five of 112 observed objects (4.46%) confirmed as genuine DBs. Warm DQs are found along the Q-branch and exhibit unusually high tangential velocities. We provide spectral classifications for 255 white dwarfs and demonstrate that random forest algorithms reliably classify low-resolution spectra for the main spectral types. We also determine the nature of objects classified as `massive DBs' and identify a large population of magnetic white dwarfs and carbon-rich objects. We identify several rare subtypes including two DAB, one DBAZ, one DZAB, two DZBA, 14 DAH, one DAQ, one DQZA, four hot, and 29 warm DQ stars, along with 63 magnetic white dwarfs. The location and kinematics of warm DQs are consistent with previous studies, supporting their proposed origin as merger remnants.