Discovery of an oxygen white dwarf The vast majority of stars will eventually evolve into a white dwarf, a small, hot, and extremely dense object made of leftover material from the star's core. Stellar evolution theory suggests that white dwarfs should be mostly made of helium, carbon, or oxygen, but even a tiny amount of hydrogen or helium floats to the surface and hides the underlying composition. Kepler et al. searched through thousands of white dwarf spectra and discovered one that has an atmosphere dominated by oxygen, with no contamination by hydrogen or helium (see the Perspective by Gänsicke). This pristine object confirms the long-postulated theory and will be an important test case for stellar evolution. Science, this issue p. 67; see also p. 37

Abstract Stars born with masses below around 10 solar masses end their lives as white dwarf stars. Their atmospheres are dominated by the lightest elements because gravitational diffusion brings the lightest element to the surface. We report the discovery of a white dwarf with an atmosphere completely dominated by oxygen, SDSS J124043.01+671034.68. After oxygen, the next most abundant elements in its atmosphere are neon and magnesium, but these are lower by a factor of ≥25 by number. The fact that no hydrogen or helium are observed is surprising. Oxygen, neon, and magnesium are the products of carbon burning, which occurs in stars at the high-mass end of pre–white dwarf formation. This star, a possible oxygen-neon white dwarf, will provide a rare observational test of the evolutionary paths toward white dwarfs.

White dwarf stars are the end product of stellar evolution for all stars born with masses below 8 to 11 solar masses (M ☉ ). The limit depends on the initial composition on the main sequence, in particular the abundances of the heavy elements (the metallicity), but also on uncertainties of the models and input physics. Among these are the nuclear reaction rates of C+He and C+C and the treatment of convection in the asymptotic giant branch (1, 2). About 80% of white dwarfs have atmospheres dominated by H, and the remainder by He. All other elements are only small traces, much less abundant than in the Sun. The reason for this unusual pattern is separation in the strong gravitational field (3). The lightest elements present very rapidly float to the surface once the white dwarf cools below about 100,000 K effective temperature (T eff ). Except for the basic division of the two groups, which suggests different evolutionary channels, the atmosphere of the white dwarfs in their later cooling evolution has thus lost all memory of the previous evolutionary phases. There are only a few, very rare, exceptions to this rule. At very high effective temperature, T eff > 200,000 K, two stars (H1504+65 and RX J0439.8-6809) (4) show no visible He or H but a C/O mixture. The limits on the He abundance are rather high, and it is quite possible that these stars will develop H or He atmospheres as they cool to lower effective temperatures, when gravitational separation becomes efficient.

Between 22,000 K ≥ T eff ≥ 18,000 K, there isa small group of stars, called Hot DQ white dwarfs (5, 6), which have C-dominated atmospheres. Their origin is not yet clear, but a likely scenario is that the carbon is dredged up from below the atmosphere once the convection zone reaches deep enough (7). If this scenario is correct, the DQ stars demonstrate that underneath the He layer there is a C layer resulting from the previous He-burning stage on the asymptotic giant branch. Another scenario is their formation by a merger of two white dwarf stars (8). At lower effective temperature, around 12,000 K, there is another small group of stars with strong O lines in their spectra; they have He-dominated atmospheres, but the next most abundant element is O, followed by C (9–11). It is plausible that their composition is related to the pre–white dwarf evolution, specifically C burning, but the reason that they appear at this temperature and this O/C ratio is not understood. To aid in our understanding of the late phases of low and intermediate mass star evolution, we searched for new white dwarf stars through the 4.5 million spectra in Data Release (DR) 12 (12) of the Sloan Digital Sky Survey (SDSS) (13).

One of the results of our search was SDSS J124043.01+671034.68 (spectrum with Plate-Modified Julian Date-Fiber 7120-56720-0894), which covers 3600 to 10,400 Å with resolving power R = λ/δλ ~ 2000. The spectrum (Fig. 1) exhibits many O I spectral lines, appearing similar to the group of cool stars with strong oxygen lines in their spectra (10, 11). The absence of any He lines could be understood if the stellar effective temperature were near 11,000 K. However, closer inspection shows several lines of ionized Mg II and even O II, which require T eff > 20,000 K. Temperatures ~20,000 K are also obtained from the SDSS photometry and the ultraviolet Galaxy Evolution Explorer (GALEX) measurements (14). At this temperature, the H and He lines, if these elements were present in the atmosphere, should be very strong. The absence of any He and H lines is only possible if O is the most abundant element. A detailed analysis (see the supplementary materials) confirmed this, with T eff = 21,600 K and surface gravity log g = 7.93 ± 0.17, where g = GM/R2 is the surface gravity in centimeter-gram-second units, with G the gravitational constant, M the stellar mass, and R the radius. Table 1 shows the atmospheric composition ratios determined from our modeling (see the supplementary materials).

Fig. 1 Spectrum of SDSS J124043.01+671034.68. The four upper panels show important regions for O, Mg, Si, and Ne. The two lower panels show the regions of the strongest He line (5877 Å) and Hα with abundances at the upper limit (red) and without these elements (blue).

Table 1 Elemental abundance ratios by number. Abundance ratios were determined from the atmospheric modeling (see the supplementary text). Uncertainties are 69% for O/Ne, 28% for O/Mg, and 46% for O/Si; for the other elements, we only estimated upper limits. View this table:

The surface gravity is typical for white dwarfs (13) and corresponds to a mass of 0.56 ± 0.09 M ☉ , using the white dwarf mass-radius relation for stars without outer H layer (15), but it is theoretically not expected for a star with an oxygen atmosphere. From the estimated log g solution and the SDSS photometry in the ugriz filters, we estimated the distance of the star as 360 ± 50 pc (see the supplementary materials). This is close enough that a more accurate distance, and therefore mass, should be determined once the Global Astrometric Interferometer for Astrophysics (GAIA) parallaxes become available in the next few years.

Carbon burning occurs at a temperature around T = (8 to 12) × 108 K, corresponding to center-of-mass energies from 1 to 3 MeV (16). Classical single-star evolution theory predicts that an O/Ne/Mg core composition is only produced in massive stars originally between 6 and 10.6 M ☉ , which reach sufficiently high core temperatures to proceed to C burning (17–23). Such high-mass main-sequence stars would result in massive (M > 1 M ☉ ) white dwarf stars (24).

Single-star simulations (25) show that there is another possible evolutionary path. If there is a particularly violent very late thermal pulse during the post-asymptotic-giant-branch phase, it can destroy the remaining stellar envelope containing He and H. Recent simulations show that the lowest mass for C ignition occurs for a hybrid model, a C/O core surrounded by an O/Ne/Mg layer, in turn surrounded by a C/O/He/H envelope (22, 23). In their model of an initial 5.5 M ☉ , the off-center C ignition does not reach the center, because convective boundary mixing (overshooting) causes the stalling of the C flame. In that model, the C burning ignited near the 0.3 M ☉ mass coordinate and propagated upward until the first convective burning was initiated. If the upper layers are lost, the star would be left with an O/Ne/Mg envelope (an oxygen-neon white dwarf), as we observe. For the complete removal of the H/He/C envelope, a late C-shell flash, strong mass loss, and/or close binary evolution, including mergers of white dwarfs, are possible scenarios. That several different scenarios for the late evolution might be needed is also suggested by the discussion of progenitor types and evolutionary paths observed in type Ia supernovae (26–29).

Distinct from hot pre–white dwarfs with oxygen lines (4) and cool oxygen line spectra white dwarfs (10, 11), which have C and He rich atmospheres, we have found a star with oxygen 25 times more abundant than any other element, by number, the only one known among the roughly 32,000 SDSS white dwarf stars (13). Time-series spectroscopy of the object will be required to search for a close binary companion, which could point toward the evolutionary path of such a rare object.

Supplementary Materials www.sciencemag.org/content/352/6281/67/suppl/DC1 Supplementary Text Tables S1 to S11 Model Spectrum References (30–45)