JJ Hermes edited sectionIntroduction_.tex  about 10 years ago

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Multi-site campaigns coordinated across the globe via the Whole Earth Telescope (WET, \citealt{1990ApJ...361..309N}) have proved the richness of well-resolved WD pulsation spectra. For example, less than a week of nearly continuous observations of the helium-atmosphere (DBV) GD\,358 revealed more than 180 significant periodicities in the power spectrum, providing exquisite constraints on the helium-envelope mass, $(2.0\pm1.0) \times 10^{-6}$\,\mstar, the overall mass, $0.61\pm0.03$\,\msun, and the magnetic field strength, $1300\pm300$\,G \citep{1994ApJ...430..839W}. Similarly, roughly 11 days of nearly continuous photometry on the pre-WD PG\,1159$-$035 revealed 125 individual periodicities, accurately constraining the mass, rotation rate and magnetic field of this DOV \citep{1991ApJ...378..326W}.  % ++++++++++++++++++++++++++++ GD1212 LC +++++++++++++++++++++++++++++ %  \begin{figure*}  \centering{\includegraphics[width=0.995\textwidth]{f1.eps}}  \caption{A portion of the {\em K2} short-cadence photometry collected on the pulsating white dwarf GD\,1212 in 2014\,February. This 9.0-day light curve (shown without any smoothing) establishes the capabilities of the extended {\em Kepler} mission on this $K_p=13.3$ mag star. An additional 2.6\,days of data, not shown, were collected on GD\,1212 in 2014\,January and included in our analysis. \label{fig:GD1212lc}}  \end{figure*}  % ==================================================================== %  DAVs have also been extensively studied by some half-dozen WET campaigns, with varying results. In part, this is a result of how pulsation modes excited in DAVs are characteristically influenced by the WD effective temperature: hotter DAVs tend to have fewer modes, lower amplitudes and shorter-period pulsations, while cooler DAVs driven by substantially deeper convection zones tend to have more modes at higher amplitude and longer periods \citep{2006ApJ...640..956M}. WET campaigns have borne this out. More than 5 days of nearly continuous monitoring of the hot DAV G226$-$29 revealed just one significant triplet pulsation mode \citep{1995ApJ...447..874K}, whereas the cooler DAV G29$-$38 has more a dozen modes of relatively high amplitude \citep{1994ApJ...436..875K}.  In fact, G29$-$38 illustrates the challenges faced to performing asteroseismology of cooler DAVs: although the WD exhibits at least 19 independent oscillation frequencies, there is significant amplitude and phase modulation of these modes, which change dramatically from year-to-year \citep{1990ApJ...357..630W,1998ApJ...495..424K}. Another excellent example of this complex behavior is the cool DAV HL\,Tau\,76 \citep{2006A&A...446..237D}, which shows 34 independent periodicities along with many oscillation frequencies at linear combinations of the mode frequencies. The complex mode amplitude and frequency variations are likely the result of longer-period pulsations having much shorter linear growth times, increasing the prevalence of amplitude and phase changes in cooler DAVs with longer periods (e.g., \citealt{1999ApJ...511..904G}).  The {\em Kepler} mission has already uniquely contributed to long-term distinctions between the handful of hot and cool DAVs eventually found in the original pointing. The longest-studied by {\em Kepler}, the cool DAV ($11{,}130$\,K) KIC\,4552982 discovered from ground-based photometry \citep{2011ApJ...741L..16H}, shows considerable frequency modulation in the long-period modes present between $770-1330$\,s (Bell et al. 2014, in prep.). A much hotter DAV was also observed for six months, KIC\,11911480 ($12{,}160$\,K), which shows at least six independent pulsation modes from $172.9-324.5$\,s that are incredibly stable and evidence consistent splitting from a $3.5\pm0.5$\,day rotation rate \citep{2014MNRAS.tmp...98G}. \citep{2014MNRAS.438.3086G}.  After the failure of a second reaction wheel in 2013\,May, the {\em Kepler} spacecraft has been repurposed as {\em K2} to observe fields in the direction of the ecliptic. {\em K2} is poised to begin uninterrupted observations of fields in the ecliptic for approximately 75\,days. As part of an initial test to monitor the {\em K2} pointing behavior on long timescales, short-cadence photometry was collected every minute on the cool DAV GD\,1212 during a preliminary 27-day engineering run in 2014\,January and February.  GD\,1212 ($V=13.3$ mag) was discovered to pulsate by \citet{2006AJ....132..831G}, with roughly 0.5\% relative amplitude photometric variability dominant at 1160.7\,s. The most recent model atmosphere fits to spectroscopy of GD\,1212 find this WD has a \teff\ $= 11{,}270\pm170$ K and \logg\ $= 8.18\pm0.05$, which corresponds to a mass of $0.71\pm0.03$ \msun\ \citep{2011ApJ...743..138G}. This puts GD\,1212 at a distance of roughly 17 pc, although GD\,1212 has the lowest proper motion of any WD within 25 pc of the Sun, $33.6\pm1.0$ mas yr$^{-1}$ \citep{2009AJ....137.4547S}.  In this paper we provide a preliminary analysis of the unique extended {\em Kepler} observations of GD\,1212. In Section 2 we outline the observations and reductions, reductions. We analyze the independent pulsation modes  and provide a light curve analysis nonlinear combination frequencies  in Section 3. Sections 3 and 4, respectively.  We reserve Section 4 5  for a preliminary asteroseismic interpretation of the results, and conclude with a discussion of these results in the context of an extended {\em Kepler} mission to the ecliptic in Section 5. 6.