Spectral analysis of cool white dwarfs accreting from planetary systems: from the ultraviolet to the optical

Abstract

The accretion of planetary debris into the atmospheres of white dwarfs leads to the presence of metal lines in their spectra. Cool metal-rich white dwarfs, which left the main-sequence many Gyr ago, allow the study of the remnants of the oldest planetary systems. Despite their low effective temperatures (Teff), a non-neglible amount of their flux is emitted in the near ultraviolet (NUV), where many overlapping metal lines can potentially be detected. We have observed three metal-rich cool white dwarfs with the Space Telescope Imaging Spectrograph (STIS) onboard the Hubble Space Telescope (HST), and compare the results determined from the NUV data with those previously derived from the analysis of optical spectroscopy. For two of the white dwarfs, SDSS J1038−0036 and SDSS J1535+1247, we find reasonable agreement with our previous analysis and the new combined fit of optical and NUV data. For the third object, SDSS J0956+5912, including the STIS data leads to a ten percent lower Teff, though we do not identify a convincing explanation for this discrepancy. The unusual abundances found for SDSS J0956+5912 suggest that the accreted parent-body was composed largely of water ice and magnesium silicates, and with a mass of up to ≃ 2 × 1025 g. Furthermore SDSS J0956+5912 shows likely traces of atomic carbon in the NUV. While molecular carbon is not observed in the optical, we demonstrate that the large quantity of metals accreted by SDSS J0956+5912 can suppress the C2 molecular bands, indicating that planetary accretion can convert DQ stars into DZs (and not DQZs/DZQs).

Missing metals in DQ stars: A simple explanation

“…We qualitatively replicate their finding that a high metal pollution of log Ca/He ≳ −8.5 is required to strongly suppress the Swan bands at that temperature and carbon abundance. This good agreement between independent calculations indicates that differences with respect to the constitutive physics of our atmosphere code and that used in Hollands et al 2022(Koester 2010 are negligible in the context of this work.…”

“…Their Figure 3 shows how their models predict that the Swan bands disappear if a sufficient quantity of polluting metals is added to the atmosphere while keeping the other parameters constant (specif- ically, T eff = 8100 K, log g = 8.02, log H/He = −3.2 1 , and log C/He = −4.5). However, Hollands et al (2022) argue that this disappearance of the Swan bands cannot explain the extreme rarity of DQZ stars, since they find that the Swan bands are suppressed only at extreme levels of metal pollution. Therefore, they conclude that while the Swan bands can be inhibited to the point of becoming undetectable in some strongly polluted white dwarfs, they are generally too weakly suppressed to explain the dearth of DQZ white dwarfs.…”

“…We examine how the strength of the Swan bands changes for a normal-mass white dwarf with T eff = 8000 K and log C/He = −4.5 as a function of the external pollution level log Ca/He 2 (no hydrogen is included). These atmospheric parameters are similar to those used in Figure 3 of Hollands et al (2022), but there are small differences since in that work the hydrogen and individual metal abundances were adjusted to the specific case of SDSS J095645.12+591240.7. We qualitatively replicate their finding that a high metal pollution of log Ca/He ≳ −8.5 is required to strongly suppress the Swan bands at that temperature and carbon abundance.…”

“…For their analysis, Hollands et al (2022) chose a fixed carbon abundance of log C/He = −4.5. This is not the ideal value to study the Swan bands' suppression: DQ white dwarfs at 8000 K typically have 10 times less carbon than that.…”

“…Approximately 15% of all helium-atmosphere white dwarfs are DZs (McCleery et al 2020;Hollands et al 2022). Under the assumption that the DQ phenomenon is independent from the external accretion of rocky planetesimals, about 15% of DQ white dwarfs should also have atmospheres polluted by elements heav-ier than carbon.…”