A New ZZ Ceti Star: SDSSJ

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1 A New ZZ Ceti Star: SDSSJ Michael A. Wolfe 1, Anjum S. Mukadam 2, Scott F. Anderson 3, Nicole M. Silvestri 4 and Daryl Haggard 5 ABSTRACT A new ZZ Ceti star, SDSSJ , has been discovered from a variability survey conducted in Equatorial Stripe 82 from the Sloan Digital Sky Survey (SDSS). Among an initial catalog of 10 4 candidate photometric variables in Stripe 82 that also have archival SDSS spectra, more than two dozen were spectrally classifiable as white dwarfs. Follow up of these candidate variable white dwarfs is just beginning, but early observations of two of these have been accomplished via high speed time series photometry with the prime focus Argos CCD camera on the 2.1 m telescope at McDonald Observatory. Although one of the two Argos observed candidates does not conclusively reveal variability on such short timescales, the other object SDSSJ is strongly confirmed in our high speed photometry as a new ZZ Ceti star. This new variability- and spectrally-selected ZZ Ceti star has a pulsational period of P = s ± 2.1 s and temperature T = 11,400 K ± 170 K. SDSSJ may be well suited for studying convection in ZZ Ceti stars because of its single high amplitude mode and subsequent non-sinusoidal light curve. Subject headings: Stars 1. Introduction White dwarfs are the stellar remnants of most stars and as these stellar remnants evolve pulsations are observed. Pulsations arise as the white dwarf cools and passes through three temperature regions which manifest as three instability strips (Robinson 1979; Fontaine et al. 1985, 2003). The pulsations associated with the instability strips are observed in the non-radial g-mode as the white dwarfs decrease in temperature (Winget 1998), with pulsation periods in the range from 100 to 1200 s. 1 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218; mwolfe@stsci.edu 2 Department of Astronomy, University of Washington, Seattle, WA 98195; anjum@astro.washington.edu 3 Department of Astronomy, University of Washington, Seattle, WA 98195; anderson@astro.washington.edu 4 Department of Astronomy, University of Washington, Seattle, WA 98195; nms@astro.washington.edu 5 Department of Astronomy, University of Washington, Seattle, WA 98195; dhaggard@astro.washington.edu

2 2 The atmospheres of 80% of white dwarfs are dominated by hydrogen (Fleming et al. 1986) and are given the spectral classification of DA. These DA variables (DAVs), also known as ZZ Ceti stars, have a log g 8 and a temperature range of 11,000 to 12,500 K (Bergeron et al. 1995; Koester & Allard 2000). The pulsation periods and amplitudes of DAVs show a distinct trend with temperature (Clemens 1993). The cooler DAVs (cdavs) show longer periods, 600 to 1000 s, larger amplitudes (up to 30%), and greater variability in amplitude as these cooler DAVs have many modes that are transient. The hot DAVs (hdavs) reveal relatively few pulsation modes, periods in the range of 100 to 300 s, and low amplitudes 0.1% - 3% (Kleinman et al. 1998). This well understood period-temperature and amplitude-temperature correlation allows categorization of DAVs into hdavs and cdavs (for further discussion see section 6 of Mukadam et al. 2004). ZZ Ceti stars have applications to a variety of studies. New identifications of such stars enable individual or ensemble follow-up studies in diverse fields such as: stellar structure, planet discovery, calibration of white dwarf cooling curves, crystallization in white dwarfs, asteroseismological distances, convection studies in ZZ Ceti stars, and establishing the ZZ Ceti instability strip boundaries (for further details see Mukadam et al. 2004). Here, we report on a new ZZ Ceti star, identified from a combination of variability and spectral selection in the Sloan Digital Sky Survey (hereafter, SDSS). In section 2, we discuss the initial selection of candidate variable white dwarfs from SDSS. In section 3 we emphasize the acquisition, reduction and analysis of follow-up high-speed photometric observations for two initial candidate variable white dwarfs. In 4 we present our results, confirming SDSSJ as a new ZZ Ceti star. A brief summary and conclusions are provided in Selection of Variable White Dwarf Candidates from SDSS 2.1. Sloan Digital Sky Survey The Sloan Digital Sky Survey (SDSS) is a large-area optical survey, providing accurate CCD imaging photometry for 10 8 objects and spectra of 10 6 objects over its nearly π steradians of sky coverage (Pier et al. 2003; York et al. 2000). The photometry comes in five filters (u, g, r, i, z) with a limiting magnitude at about r < 22.5 (Fukugita et al. 1996; Gunn et al. 1998; Hogg et al. 2001; Smith et al. 2002; Gunn et al. 2006; Tucker et al. 2006) and is accurate to 0.02 mag rms for (brighter) objects that are constrained by systematic errors (Scranton et al. 2002; Ivezić et al. 2003), and also with a zero-point error of about 0.02 mag (Ivezić et al. 2004). The main portions of the survey encompasses nearly 10,000 deg 2 especially centered near the northern Galactic cap, but with an additional few hundred square degree region near the southern Galactic cap. The large area surveyed and high caliber of photometry provided by SDSS make the survey stand out among contemporary surveys (Sesar et al. 2006). Substantial multi-epoch imaging observations were obtained in the southern Galactic cap portion. The Equatorial Stripe 82 region in particular (22 h 24 m < α J2000 < 04 h 08 m; < δ J2000 < 1.27 ) includes 300 deg 2, and was re-observed

3 3 in multi-epoch observations; this Stripe 82 database is the largest in SDSS for variability studies. This variability database grew over time as additional epochs were added, e.g., now including up to 65 epochs for many objects, though the current study is based on a much earlier (many fewer epochs) version of this database. SDSS is also the most comprehensive large-area sky survey in providing quality multi-object spectroscopy. Using dual fiber-fed spectrographs, SDSS obtains 640 spectra at once in a 7 deg 2 FOV, typically with co-added exposure times of greater than 45 minutes. SDSS acquired multiobject spectroscopy to a typical surface density of order 10 2 spectra per square degree SDSS Variability- and Spectral-Selection The variable white dwarfs studied in this paper were selected from a database created by Ẑeljko Ivezić (private communication) by considering multi-epoch imaging photometry in the well-studied 300 deg 2 Stripe 82 region. The particular early version of the Ivezić et al. catalog discussed herein included 48,471 photometric variable candidates among Stripe 82 objects that are: morphologically stellar (mainly stars and quasars); observed in at least 4 SDSS epochs; and that were candidate photometric variables, as defined in the subsequent paragraph. From the full Stripe 82 imaging catalog (i.e., including the non-variables) it has been shown by Ivezić et al. (2003) and Ivezić et al. (2007) that a Gaussian distribution closely matches the distribution of photometric errors derived from the multi-epoch SDSS observations. The 48,471 Stripe 82 objects initially considered here as candidate variables are those whose measured magnitudes in the multi-epoch photometric data have a scatter exceeding that expected due to random (Gaussian) statistics for a non-variable object. Comparison of such ensemble Gaussian expectations with the observed photometric variations (or lack thereof) for each object allows assignment of a χ 2 measure of their likely variability. Non-variable objects would have a χ 2 value of about unity by this measure (for additional details see Sesar et al. 2007), while variable objects would be found as outliers in this distribution. Specifically, the 48,471 objects initially considered here as candidate variables all have χ 2 values in g and r multi-epoch photometric scatter that exceed a value of 3. In other words, the scatter in multi-epoch g and r photometric magnitudes for each of these 48,471 objects is significantly larger than expected for a non-variable object. This technique for finding variables is valid not only because the errors follow an approximate Gaussian distribution but also because of the high accuracy of the photometry, i.e., the rms is about 0.02 mag (Scranton et al. 2002). Among the 48,471 such potentially photometric variables thereby cataloged in Stripe 82 (at the time of our study), there were 10,782 in the SDSS that have associated SDSS spectra. Of these 10,782 spectra 10,088 remained after some spectra were excluded due to no photometric matches, duplication, superpositions, unacceptable photometric error (σ p > 0.1), and mismatches at 10. These remaining spectra were individually classified by visual and/or algorithmic inspection, and

4 4 27 potentially variable white dwarfs (i.e., with confirming SDSS spectra) were thereby identified. 3. Argos High Speed Camera and SDSS Spectral Data and Analysis 3.1. Argos Instrumentation and Observing Strategy In order to confirm and characterize in detail the variability of the candidate variable white dwarfs, we have begun to obtain good time-resolution detailed follow-on lightcurves. Our first such follow-up lightcurve observing run, reported herein, utilized Argos, a CCD Camera system at the prime focus of the 2.1m McDonald Observatory telescope that was designed to optimize high-speed time series measurements of oscillating white dwarf stars (Nather & Mukadam 2004). The earlier practical limit using the photomultiplier tube (PMT) photometers on the 2.1 m telescope was about 17.0 magnitudes. Light curves are now obtained, of comparable quality at 19.4 magnitudes, with a gain of about a factor of 9 in overall sensitivity; this is essential to observe many of the fainter ZZ Ceti candidates. In the observational stratagem to confirm ZZ Ceti stars with Argos, it is typical to observe bright candidates (14.5 g 17.5) for hours each and faint candidates (18.0 g 19.5) for 2 hours each. Data extraction routines allow real time analysis of the light curves and a Fourier transform (FT) of the star. If any interesting features are observed in the light curve data then the star is observed for a longer duration of time. When a pulsator is discovered the star is observed for a few hours and observed a second time to confirm the variability (Mukadam et al. 2004) Argos Data Acquisition Early high speed time series photometry was obtained for two of our candidate variable white dwarfs (both DAs), SDSSJ and SDSSJ , using Argos on the 2.1 m telescope at McDonald Observatory; the two SDSS white dwarfs were observed on two nights in 2007, August 27, 2007 and September 04, 2007 (UT). Information pertaining to the observations can be found in Table 1 and the columns are observation run, designation, date, start time, end time, exposure time, filter, and number of frames, respectively. The exposure times ranged from s per frame. A 1 mm thick Schott glass BG40 filter 6 was used for all of the observations in order to suppress the red part of the spectrum and to measure amplitudes comparable to earlier blue sensitive detectors like photo-multiplier tubes with bi-alkali photocathodes (Kanaan et al. 2000; Mukadam et al. 2004). 6 The transmission of this filter can be found at

5 Argos Data Reduction and Analysis of SDSS Spectra The light curves of the two Argos observed white dwarfs have been sky subtracted and were extracted from the CCD frames using weighted circular aperture photometry (O Donoghue et al. 2000). Extinction variations were corrected for and the light curve of the target star was divided by the sum of one or more comparison stars on the CCD chip. Brighter comparison stars were employed as divisors because their light curves have a higher signal to noise ratio (S/N). The data were brought to the same fractional amplitude scale and the arrival times of the photons were converted to the Barycentric Coordinate System (TCB; Standish 1998) after completion of the preliminary reduction steps previously stated. A discrete Fourier transform (DFT) was then computed for all light curves (Mukadam et al. 2004). The SDSS spectra have also been compared to standard white dwarf spectral models to obtain basic gravity and temperature diagnostics. The designation, temperature, temperature error, logarithmic surface gravity, logarithmic surface gravity error, χ 2 value, spectral type, magnitude in g, r, and i, magnitude error in g, r, and i, proper motion (PM), and proper motion error can be found in Table 2. The proper motions came from the catalogues of Bramich et al. (2008); specifically the Higher Level Catalogue (HLC). The two stars, SDSSJ and SDSSJ , have log g values which are consistent with white dwarfs. Furthermore, the temperature of white dwarf, SDSSJ , was found to lie within the temperature range of the instability strip which is 10,800 K to 12,300 K (Mukadam et al. 2007), although the temperature of SDSSJ lies outside the instability strip. The temperatures, log g, associated errors, and χ 2 values were calculated using autofit (for details see Kleinman et al. 2004; Finley et al. 1997). A static, plane-parallel stellar atmosphere in radiative, hydrostatic and local thermodynamic equilibrium model is used in autofit. Figures 1 and 2 show the output of autofit. The top panels show likelihood contours of 1, 2, 3, 5, and 10σ, where the subtype represents the T eff (effective temperature) of the white dwarf in units of 50,400/T eff (Sion et al. 1983). The middle panels present the SDSS spectra of SDSSJ and SDSSJ , while the bottom panels (red line) highlight the model fit to the Balmer features (Kleinman et al. 2004). From the Argos lightcurves we assess the variability or non-variability of an object, by measur- Table 1: Argos Observation Log Run SDSSJ Date Start Time End Time Exp Time Filter # of Frames Number (UTC) (UTC) (UTC) per Frame (s) A :13:07 04:19:52 15 BG40 28 A :22:13 06:26:13 20 BG A :11:02 05:25:32 30 BG40 150

6 6 Fig. 1. SDSSJ is from the sample of potentially variable white dwarfs. The 3 numbers at the top of this figure designate the SDSS spectroscopic plate, fiber, and MJD, respectively. The top panel shows likelihood contours of 1, 2, 3, 5, and 10σ, where the subtype represents the T eff (effective temperature) of the white dwarf in units of 50,400/T eff. The middle panel show the SDSS spectrum of SDSSJ while the bottom panel highlights (red line) the model fit to the Balmer features. ing the pulse amplitude against a threshold level set at approximately twice the average amplitude (mma 7 ). If this threshold is not exceeded anywhere in the DFT of the DAV candidate, then all peaks are thought to be most likely consistent with noise. Accordingly, a limit on variability is then defined as the highest white noise peak. Of course, if the DFTs of the reference stars reflect the same highest white noise peak, then this peak is not used to determine the variability detection threshold. Instead, the second highest white noise peak is then used in the testing process, and this repeated until a detection threshold can be established. If the detection threshold is significantly exceeded (by 3σ of the white background noise) then the object is re-observed to determine whether the peak is pure noise or real (Mukadam et al. 2004). A thorough discussion of the reliability of 7 One millimodulation amplitude (mma) corresponds to a 0.1% change in intensity. Table 2: White Dwarf Parameters SDSSJ Temp. σ T log(g) σ log(g) χ 2 Spectral g σg r σr i σ i PM PM error Type arcsec/yr arcsec/yr DA DA

7 7 Fig. 2. SDSSJ is from the sample of potentially variable white dwarfs. The 3 numbers at the top of this figure designate the SDSS plate, fiber, and MJD, respectively. The top panel shows likelihood contours of 1, 2, 3, 5, and 10σ, where the subtype represents the T eff (effective temperature) of the white dwarf in units of 50,400/T eff. The middle panel shows the SDSS spectrum of SDSSJ , while the bottom panel (red line) highlights the model fit to the Balmer features. detecting a periodic signal in noisy data is given by Scargle (1982). 4. Detection Technique and the Pulsation Properties of the New ZZ Ceti Star There are several detection techniques that are employed in the hunt for ZZ Ceti stars (for details on these strategies see Mukadam et al. 2004) and in the particular method of using color-color diagrams in conjunction with spectroscopic fits, while effective, is biased to finding new ZZ Ceti stars that lie within the instability strip. The discovery technique mentioned in 2 does not suffer from this bias as there are no assumptions made about the location of the ZZ Ceti star in color-color or color-magnitude space. Therefore, there exists the possibility of discovering a DAV outside of the instability strip, while by definition, the color-color diagram and spectroscopic fit method would not recognize the white dwarf as being variable. Additionally, the purity of the instability strip is in question (Mukadam et al. 2005); thus it is of potential interest to identify both non-variables which lie within the instability strip boundaries, as well as to use search techniques such as that described here that are capable of finding ZZ Ceti stars outside the strip. Furthermore, as a consequence of this detection technique, there is also the potential of discovering new hot pulsators for which the instability strip is not defined at all.

8 8 Of the two stars observed with Argos only one was convincingly confirmed to be a DAV or ZZ Ceti star. The DAV is SDSSJ and the associated pulsation characteristics, frequency, period, period error, amplitude, amplitude error, respectively, can be found in Table 3. The DFT can found in Figure 3 along with the light curve. It is quite evident from the DFT of Figure 3 that the pulsations are distinct from the white background noise, greatly exceeding the 3σ limit. It is also quite clear that there exist harmonics of the fundamental frequency (see Table 3). In addition, the light curve itself shows unmistakable periodic deviations in intensity. The period of SDSSJ is s; this falls within the domain of the periods that are indicative of cdavs (Mukadam et al. 2004). Furthermore, SDSSJ is an ideal candidate for studying convection in ZZ Ceti stars because of its single high amplitude mode and subsequent non-sinusoidal light curve (Montgomery 2005). Argos data for the other candidate DA, SDSSJ , do not (thus far) confirm it as a DAV. In Figure 4 there is a peak at 2000 s that is just barely above the 3σ detection threshold. But because this value is just minimally above threshold, classification of SDSSJ as a DAV at this time would be premature; additional high speed time series photometry is needed for a conclusive determination. If this object were confirmed as a ZZ Ceti star, it would be interesting, as it falls outside the typical temperature range boundaries for the instability strip. 5. Conclusions Among candidate photometric variables in SDSS Stripe 82 are 10 4 objects also having archival SDSS spectra, permitting classification; somewhat over two dozen cases are spectrally classifiable as white dwarfs, mainly DAs. These candidate variable white dwarfs may comprise a less biased set than found in many other surveys dedicated to the discovery of DAVs, which have tended to explore primarily objects that fall within standard (though not uniformly agreed upon) multicolor realizations of white dwarf instability strips. Two of these new candidate DAVs were observed at McDonald Observatory using high speed time series photometry. SDSSJ shows no marked, high-amplitude periodic variations (though further high speed photometry is needed to determine definitively whether or not it is a ZZ Ceti star). On the other hand, SDSSJ is strongly confirmed as a new ZZ Ceti star (cdav) with a period of s and a temperature of 11,400 K, and well within the temperature Table 3: Pulsation Characteristics for SDSSJ Harmonic Period (s) σ P (2) Amplitude (mma) σ A (mma) Fundamental First Second

9 9 Fractional Intensity Amplitude Amplitude (mma) SDSSJ (2007 Sep 4 UT) Run A1575, McD 2.1m, 30s exp, BG40 filter Time (s) P=740.8s P/2=368.9s P/3=245.7s DFT 3σ = 10.2 mma Window Frequency (Hz) Fig. 3. Argos light curve data for SDSSJ In the DFT, it is quite evident that there are distinct pulsations at period s that are markedly above the detection threshold and therefore not consistent with noise, confirming this as a new ZZ Ceti star. The DFT also shows two harmonics. The light curve directly shows the pulsations, as well as their non-sinusoidal character. zone of the instability strip. The large amplitude and non-sinusoidal variations of the SDSSJ pulsations may render it a useful case for studying convection in ZZ Ceti stars. 6. Acknowledgments Thanks goes to McDonald Observatory for the opportunity to acquire observations without which this work would not have been possible. Many thanks to Daniel J. Lennon for his numerous comments that have been incorporated resulting in a much enhanced paper. The instrument grant from the NSF (AST ) was helpful in providing trips for observations to McDonald Observatory. We thank Željko Ivezić for kindly providing us early access to his variability database for SDSS Stripe 82. Funding for the SDSS and SDSS-II has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, the U.S. Department of Energy, the National Aeronautics and Space Administration, the Japanese Monbukagakusho, the Max Planck Society, and the Higher Education Funding Council for England. The SDSS Web Site is The SDSS is managed by the Astrophysical Research Consortium for the Participating Institu-

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