Tracing Planetary Accretion in a 3 Gyr-old Hydrogen-Rich White Dwarf: The Extremely Polluted Atmosphere of LSPM J0207+3331

TL;DR

This work targets the cool, hydrogen-rich white dwarf LSPM J0207+3331 to overcome biases toward helium-rich systems and to directly measure exoplanetary material in an ancient system. Through multi-instrument spectroscopy and photometry, the authors determine $T_{ m eff}=5910\pm98$ K and $\log g=8.11\pm0.03$ while detecting 13 heavy elements, including Sr, and constraining the carbon abundance to $\log({\rm C/H})< -7.3$. The inferred abundance pattern is consistent with accretion from a carbon-depleted, bulk-Earth–like body, best described as a differentiated rocky object with a core mass fraction of about 55%, and the high total accretion rate $\log \dot{M}=9.81$ g s$^{-1}$ indicates ongoing delivery of material. The infrared excess can be explained by a single silicate disk, and the discovery of Ca II H&K core emission reveals additional atmospheric processes at play, collectively advancing our understanding of long-term planetary system evolution around white dwarfs and the importance of including metals in atmospheric models for cool polluted WDs.

Abstract

We report the detection of 13 heavy elements (Na, Mg, Al, Si, Ca, Ti, Cr, Mn, Fe, Co, Ni, Cu, and Sr) in the photosphere of LSPM J0207+3331, a ~3 Gyr old hydrogen-rich white dwarf with an effective temperature comparable to that of the Sun. Upper limits on carbon, obtained through the absence of molecular CH, suggest accretion from a carbon-volatile-depleted source. The accreted parent body exhibits slight deficits of Mg and Si relative to Fe but otherwise bulk Earth-like abundance patterns; a reasonable interpretation is that LSPM J0207+3331 is accreting a massive differentiated rocky body that had a core mass fraction higher than the Earth's. The high level of pollution indicates that substantial accretion events can still occur even after 3 Gyr of cooling. We also detect weak Ca II H & K line-core emission, making this only the second known isolated polluted white dwarf to exhibit this phenomenon and suggesting the presence of additional physical processes in or above the upper atmosphere. Our analysis also highlights the critical importance of including heavy elements in the model atmosphere structure calculations for highly polluted hydrogen-rich white dwarfs. Neglecting their contribution significantly impacts the inferred thermodynamic structure, leading to inaccuracies in derived stellar parameters. Finally, we show that the observed 11.3 microns infrared excess can be explained by a single silicate dust disk rather than a two-ring disk model.

Figures (13)

• Figure 1: Photometric fit for LSPM J0207+3331. All spectral bands listed in Table \ref{['tab:photometry']} are shown with error bars. The best-fit pure hydrogen model (filled circles) is compared to a pure helium model (open circles). Photometric data excluded from the fit are highlighted in red.
• Figure 2: Models covering H$\alpha$ with He abundances of $\log({\rm He/H})= -2, -1,$ and 0 overplotted on the MagE spectrum.
• Figure 3: Temperature (left) and pressure (right) structure as a function of the optical depth for a hydrogen-rich white dwarf model with $T_{\rm eff}$ = 5000 K and $\log g$ = 8.0. Cases explored include no metals and metal abundances between log(Ca/H) = -6 and -10.
• Figure 4: Deviations between fitted and the model's true $T_{\rm eff}$ (top) and $\log g$ (bottom) when models with heavy elements are fit using a pure-hydrogen grid, shown as a function of the white dwarf's true $T_{\rm eff}$ for metal abundances from $\log({\rm Ca/H}) = -9.0$ to $-6.0$ for $\log g$ = 8.0. The experiment is done using Pan-STARRS $grizy$ and 2MASS $JHK_s$ bandpasses.
• Figure 5: Spectral energy distribution for a model white dwarf with $T_{\rm eff}$ = 4000 K and $\log g$ = 8.0 for various metallicities. The 2MASS $JHK_s$ spectral bands are indicated by dashed lines.