  Excess infrared flux from white dwarf stars is likely to arise from a dusty
debris disk or a cool companion. In this work, we present near-infrared
spectroscopic observations with Keck/MOSFIRE, Gemini/GNIRS, and
Gemini/Flamingos-2 of seven white dwarfs with infrared excesses identified in
previous studies. We confirmed the presence of dust disks around four white
dwarfs (Gaia J0611-6931, Gaia J0006+2858, Gaia J2100+2122, and WD 0145+234) as
well as two new white dwarf brown dwarf pairs (Gaia J0052+4505 and Gaia
J0603+4518). In three of the dust disk systems, we detected for the first time
near-infrared metal emissions (Mg I, Fe I, and Si I) from a gaseous component
of the disk. We developed a new Markov Chain Monte Carlo framework to constrain
the geometric properties of each dust disk. In three systems, the dust disk and
the gas disk appear to coincide spatially. For the two brown dwarf white dwarf
pairs, we identified broad molecular absorption features typically seen in L
dwarfs. The origin of the infrared excess around Gaia J0723+6301 remains a
mystery. Our study underlines how near-infrared spectroscopy can be used to
determine sources of infrared excess around white dwarfs, which has now been
detected in hundreds of systems photometrically.

Solar and Stellar Astrophysics;Earth and Planetary Astrophysics