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Andrew H. Dublin

Publications and source records attributed to Andrew H. Dublin.

2 recordsLinked to original sources

Improvements to Asteroseismic Fitting of White Dwarfs in the Gaia Era

White dwarf asteroseismology aims to constrain the interior structures of pulsating white dwarf stars by fitting observed pulsation periods to those computed for stellar models. However, there remain several obstacles for achieving reliable results with the model grid-fitting approach. We simulate data for a parameter recovery experiment in one and two dimensions (mass and effective temperature) to demonstrate improved methodologies that address challenges for characterizing degenerate asteroseismic solutions. We show that interpolating model periods onto a finer grid can adequately resolve asteroseismic solutions that would be missed with sparse model grids. Incorporating absolute magnitude from Gaia astrometry into the statistical fitting is shown to reduce solution degeneracy. We define a seismic solution as a solution to the mode identification problem, and we show that fitting to consistent model pulsation modes can isolate each candidate solution in degenerate solution space. A criterion based on $χ^2$ for what should be considered a reasonable fit is adopted, and we identify all combinations of model periods that meet this criterion, not only those nearest to the measured periods. Finally, we demonstrate how fitting Gaussians to probability distributions allows for the robust characterization of each candidate solution, including uncertainties. Our accurate characterization of the degenerate solution landscape is supported by the comparison of our solutions to a direct marginalization of the likelihood function. These fitting approaches can be generalized to higher dimensions, where there are more than two free parameters.

astro-ph.SR

Mode Instability and a Massive, Isolated Outburst in the Pulsating White Dwarf GD 1212

We analyze a large brightening event that lasted for roughly half a day in the pulsating hydrogen-atmosphere white dwarf GD 1212 during K2 Campaign 12 of the extended Kepler mission. For the other 80 days of K2 observations, GD 1212 exhibited a rich spectrum of long-period (~1100 s) pulsations that underwent rapid variations in frequency and amplitude but did not exhibit any additional outbursts. We refine previous attempts at mode identification and find a likely sequence of dipole and quadrupole splittings that reveal an overall rotation rate of roughly 17.0 hr. The outburst at Day 61 is fully resolved by the 60-second-cadence K2 data, with the entire white dwarf becoming up to 17.5% brighter overall, from an approximately 850 K increase in effective temperature, with pulsational variability during the outburst showing shorter periods and higher amplitudes. Outbursts are believed to be the result of nonlinear mode coupling via parametric instability, whereby energy stored in linearly excited parent modes is rapidly transferred to damped child modes that dissipate near the surface. Additionally, we characterize a "failed" outburst that caused correlated pulsation frequency changes, an approximately 5 microHz increase, with a small approximately 0.35% corresponding brightness increase. GD 1212 is now the eighth pulsating hydrogen-atmosphere DAV white dwarf to show outburst behavior, although it exhibited the largest outburst yet and has the longest inferred recurrence timescale. This high-signal-to-noise record tracing pulsations through both large and small temperature excursions in GD 1212 provides unique insights into parametric resonance and nonlinear mode coupling in white dwarf pulsations.

astro-ph.SR