Defining the DS Spectral Type for Oxygen-atmosphere White Dwarfs

The Sloan Digital Sky Survey (SDSS) has revolutionized the study of white dwarfs (WDs), with the number of spectroscopically-confirmed WDs exploding to ∼30,000 (e.g., Kepler et al. 2019). As a result, new WD classes have been discovered that do not fit into existing spectral types, particularly WDs with spectra dominated by features of oxygen (Liebert et al. 2003). In this research note, we propose an extension to the accepted WD spectral type system to accommodate these WDs. We also identify the best remaining designations for any future discoveries of new spectral types.

WD spectral types are described by McCook & Sion (1999) and references therein. The McCook & Sion system is driven by a pragmatic philosophy. Based on observable features in optical spectra of WDs, it strives to avoid confusion when reading historical papers, and it seeks to use designations that parallel familiar spectral types used for non-degenerate stars. It also seeks to limit the number of spectral classes; for example, WD spectra dominated by metals are identified as type DZ, regardless of which metallic element dominates optical spectral features.

Unlike spectral types for non-degenerate stars, the WD spectral types are not unambiguously related to effective temperature of the WD. For this reason, Sion et al. (1983) introduced the temperature index, defined as 10 × θ_eff = 50, 400/T_eff. Further refinement of the temperature index is described in McCook & Sion (1999).

Driven to retain this philosophy and greatly inspired by the methodology of Kirkpatrick et al. (1999) in defining brown dwarf spectral types, we examine the McCook & Sion system for all potential spectral types that would provide unambiguous and as-yet unused identifiers for WD spectra. The results are presented in Table 1. We find the following spectral types remain unclaimed and unproblematic for potential use: DJ, DT, DR, DS, DT, DW, and DY.

In 2010, Gänsicke et al. (2010) announced the discovery of two WDs with oxygen features in the optical spectra: SDSS J092208.19+292810.9, a cool DQ with atomic oxygen features, and SDSS J110239.69+205439.4, with a spectrum dominated by atomic oxygen. In 2016, the discovery of the hot WD SDSS J124043.01+671034.68 with an oxygen-dominated spectrum was announced (Kepler et al. 2016). To date, the moniker "Dox" has been used to refer to these WDs, but this could be confused with spectral type DOX, indicating a peculiar DO WD.

We propose the use of DS for WDs exhibiting significant oxygen features. DS could be considered as shorthand for the German Sauerstoff (oxygen), and/or for Carl Wilhelm Scheele, the Swedish scientist who independently discovered oxygen; Joseph Priestly published his discovery first. Additionally, evolved stars exhibiting ZrO molecular bands have been classified as S-type stars, so there is precedence for using the letter S for oxygen-related features.

While one might be tempted to propose multiple new spectral types depending on the ionization state of oxygen observed in the optical spectrum, we refrain from doing so. We wish to avoid an alphabet soup of WD spectral types beyond the fairly complex system that already exists. Further, differing ionization species in photospheric spectra are primarily indicative of temperature, and the temperature index already in use fills that purpose.

In summary, we encourage the community to adopt the spectral type of DS for WDs exhibiting spectra dominated by oxygen features. We also present a complete list of unadopted and unproblematic WD spectral types in the event that future work uncovers yet new classes of WD spectra.