3D spectroscopic analysis of helium-line white dwarfs

Abstract

In this paper, we present corrections to the spectroscopic parameters of DB and DBA white dwarfs with −10.0 ≤ log (H/He) ≤−2.0, 7.5 ≤ log g ≤9.0 and 12 000 K ≲ Teff ≲ 34 000 K, based on 282 3D atmospheric models calculated with the CO5BOLD radiation-hydrodynamics code. These corrections arise due to a better physical treatment of convective energy transport in 3D models when compared to the previously available 1D model atmospheres. By applying the corrections to an existing SDSS sample of DB and DBA white dwarfs, we find significant corrections both for effective temperature and surface gravity. The 3D log g corrections are most significant for Teff ≲ 18, 000 K, reaching up to −0.20 dex at log g = 8.0. However, in this low effective temperature range, the surface gravity determined from the spectroscopic technique, can also be significantly affected by the treatment of the neutral van der Waals line broadening of helium and by non-ideal effects due to the perturbation of helium by neutral atoms. Thus, by removing uncertainties due to 1D convection, our work showcases the need for improved description of microphysics for DB and DBA model atmospheres. Overall, we find that our 3D spectroscopic parameters for the SDSS sample are generally in agreement with Gaia DR2 absolute fluxes within 1-3σ for individual white dwarfs. By comparing our results to DA white dwarfs, we determine that the precision and accuracy of DB/DBA atmospheric models are similar. For ease of user application of the correction functions, we provide an example Python code.

A catalogue of white dwarfs in Gaia EDR3

“…Following the same methodology described for DR2 in Gentile Fusillo et al ( 2019) we then made use of a sample SDSS spectroscopically classified objects to map the distribution of white dwarfs and contaminant objects in G BP − G RP colour−G abs space and calculate probabilities of being a white dwarf (P WD ) for all objects in our sample. Coupled with Beaulieu & Bergeron (2019b), the sample has also been corrected for van der Waals effect (see Cukanovaite et al 2021). The dashed lines indicate the medians for the various samples.…”

“…In recent years Gaia DR2 photometry has been shown to be reliable for deriving white dwarf fundamental parameters, e.g. when compared to the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS), SDSS, or Javalambre Photometric Local Universe Survey (J-PLUS; Bergeron et al 2019;Gentile Fusillo et al 2019;López-Sanjuan et al 2019;Tremblay et al 2019a;McCleery et al 2020), albeit with moderate systematic offsets compared to spectroscopic results (Maíz Apellániz & Weiler 2018;Genest-Beaulieu & Bergeron 2019a;Narayan et al 2019;Tremblay et al 2019aTremblay et al , 2020Cukanovaite et al 2021). Gaia EDR3 photometric calibration has already been studied extensively (Fabricius et al 2021;Riello et al 2021) and is equally reliable as that of DR2 (see Extinction value (mag) derived from 3D reddening maps using +1σ EDR3 distance (Section 3.1)…”

“…The latter sample has been corrected for 3D effects by Tremblay et al (2019a). The DB samples are from Genest-Beaulieu & Bergeron (2019b) and Rolland, Bergeron & Fontaine (2018), with additional correction for 3D effects from Cukanovaite et al (2021). The Genest-Beaulieu & Bergeron (2019b) sample has also been corrected to put van der Waals broadening on the same scale as the other samples.…”

“…The Genest-Beaulieu & Bergeron (2019b) sample has also been corrected to put van der Waals broadening on the same scale as the other samples. For a more detailed discussion on these corrections, see Cukanovaite et al (2021).…”

Collisional effects in the blue wing of the Balmer-α line

“…Through the comparison of spectroscopic observations with models based on our best physics we gain an understanding of the composition, temperature, density, magnetic fields, turbulence, and motions of the objects of interest. An example, which is the key point of this paper, is the decades-old problem of the determination of the hydrogen abundance in helium-dominated white dwarf stars (Cukanovaite et al 2021). There is a discrepancy in the hydrogen abundance determined from optical spectra of Balmer-α and the abundance determined from ultraviolet spectra of Lyman-α that is possibly due to calculated opacities for these lines that have been incorporated into stellar atmosphere code.…”