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Measuring the Evolutionary Rate of Cooling of ZZ Ceti Publications 7-1-2013 Measuring the Evolutionary Rate of Cooling of ZZ Ceti Anjum S. Mukadam University of Washington - Seattle Campus Ted von Hippel Embry-Riddle Aeronautical University, [email protected] et al. Follow this and additional works at: https://commons.erau.edu/publication Part of the Stars, Interstellar Medium and the Galaxy Commons Scholarly Commons Citation Mukadam, A. S., von Hippel, T., & al., e. (2013). Measuring the Evolutionary Rate of Cooling of ZZ Ceti. The Astrophysical Journal, 771(1). https://doi.org/10.1088/0004-637X/771/1/17 This Article is brought to you for free and open access by Scholarly Commons. It has been accepted for inclusion in Publications by an authorized administrator of Scholarly Commons. For more information, please contact [email protected]. The Astrophysical Journal, 771:17 (11pp), 2013 July 1 doi:10.1088/0004-637X/771/1/17 C 2013. The American Astronomical Society. All rights reserved. Printed in the U.S.A. MEASURING THE EVOLUTIONARY RATE OF COOLING OF ZZ Ceti Anjum S. Mukadam1,2, Agnes Bischoff-Kim3, Oliver Fraser1,2,A.H.Corsico´ 4,5, M. H. Montgomery6,7,8, S. O. Kepler9, A. D. Romero9,D.E.Winget6,8, J. J. Hermes6,8, T. S. Riecken1,2, M. E. Kronberg1,2,K.I.Winget6,8, Ross E. Falcon6,8, D. W. Chandler10, J. W. Kuehne8, D. J. Sullivan11, D. Reaves6,8, T. von Hippel12, F. Mullally13, H. Shipman7,14, S. E. Thompson13, N. M. Silvestri1,2, and R. I. Hynes15 1 Department of Astronomy, University of Washington, Seattle, WA 98195, USA 2 Apache Point Observatory, 2001 Apache Point Road, Sunspot, NM 88349, USA 3 Georgia College & State University, Milledgeville, GA 31061, USA 4 Facultad de Ciencias Astronomicas´ y Geof´ısicas, Universidad Nacional de La Plata, Argentina 5 Consejo Nacional de Investigaciones Cient´ıficas y Tecnicas´ (CONICET), Argentina 6 Department of Astronomy, University of Texas at Austin, Austin, TX 78759, USA 7 Delaware Asteroseismic Research Center, Mt. Cuba Observatory, Greenville, DE 19807, USA 8 McDonald Observatory, Fort Davis, TX 79734, USA 9 Universidade Federal do Rio Grande do Sul, Porto Alegre 91501-970, RS, Brazil 10 Meyer Observatory, Central Texas Astronomical Society, 3409 Whispering Oaks, Temple, TX 76504, USA 11 Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand 12 Embry-Riddle Aeronautical University, 600 South Clyde Morris Boulevard, Daytona Beach, FL 32114, USA 13 SETI Institute, NASA Ames Research Center, MS 244-30, Moffet Field, CA 94035, USA 14 Department of Physics and Astronomy, University of Delaware, Newark, DE 19716, USA 15 Department of Physics and Astronomy, Louisiana State University, Baton Rouge, LA 70803, USA Received 2012 December 28; accepted 2013 April 1; published 2013 June 11 ABSTRACT We have finally measured the evolutionary rate of cooling of the pulsating hydrogen atmosphere (DA) white dwarf ZZ Ceti (Ross 548), as reflected by the drift rate of the 213.13260694 s period. Using 41 yr of time-series photometry from 1970 November to 2012 January, we determine the rate of change of this period with time to be dP/dt = (5.2 ± 1.4) × 10−15 ss−1 employing the O − C method and (5.45 ± 0.79) × 10−15 ss−1 using a direct nonlinear least squares fit to the entire lightcurve. We adopt the dP/dt obtained from the nonlinear least squares program as our final determination, but augment the corresponding uncertainty to a more realistic value, ultimately arriving at the measurement of dP/dt = (5.5 ± 1.0) × 10−15 ss−1. After correcting for proper motion, the evolutionary rate of cooling of ZZ Ceti is computed to be (3.3 ± 1.1) × 10−15 ss−1. This value is consistent within uncertainties with the measurement of (4.19 ± 0.73) × 10−15 ss−1 for another similar pulsating DA white dwarf, G 117-B15A. Measuring the cooling rate of ZZ Ceti helps us refine our stellar structure and evolutionary models, as cooling depends mainly on the core composition and stellar mass. Calibrating white dwarf cooling curves with this measurement will reduce the theoretical uncertainties involved in white dwarf cosmochronometry. Should the 213.13 s period be trapped in the hydrogen envelope, then our determination of its drift rate compared to the expected evolutionary rate suggests an additional source of stellar cooling. Attributing the excess cooling to the emission of axions imposes a constraint on the mass of the hypothetical axion particle. Key words: stars: evolution – stars: individual (ZZ Ceti, R548) – stars: oscillations (including pulsations) – stars: variables: general – white dwarfs Online-only material: color figure 1. INTRODUCTION around 650–1000 s, larger amplitudes (up to 30%), nonlinear pulse shapes, and greater amplitude modulation (e.g., Kleinman ZZ Ceti (Ross 548) is the prototype of the class of hydrogen et al. 1998). The hot ZZ Ceti stars show relatively few pulsation atmosphere variable (DAV) white dwarfs that are pulsationally modes, shorter periods around 100–350 s with low amplitudes ∼ unstable in the temperature range 10,800–12,300 K for log g ≈ ( 0.1%–3%), and only a small degree of amplitude modulation 8 (Koester & Holberg 2001; Bergeron et al. 2004; Gianninas (Clemens 1993; Mukadam et al. 2006, 2007). Typical of the class et al. 2005, 2006). The ZZ Ceti stars constitute the coolest class of hot DAV pulsators, ZZ Ceti exhibits short periods at 187 s, 213 s, 274 s, 318 s, and 334 s with amplitudes in the range of of pulsating white dwarfs, with more than 150 members known 16 to date (e.g., Mukadam et al. 2004; Kepler & Castanheira 2009; 0.5–6.6 mma. The pulsation spectrum of ZZ Ceti exhibits a Castanheira et al. 2010;Greiss2012). very low degree of amplitude modulation over decades, making The pulsation characteristics of the hot ZZ Ceti stars closer it possible to measure the miniscule change of its pulsation to the blue edge of the instability strip are different from their period over that timespan to arrive at the rate of change of compatriots near the red edge. The distinct behavior of pulsation period with time, dP/dt. periods, amplitudes, and degree of amplitude modulation as Pulsation periods of model white dwarf stars increase as a function of temperature was demonstrated for a significant the star cools due to the increasing radius of the degeneracy sample of DAVs by Clemens (1993) and more recently by Kanaan et al. (2002) and Mukadam et al. (2006, 2007). Cool 16 One milli modulation amplitude (mma) equals 0.1% amplitude in intensity, ZZ Ceti stars typically show relatively longer pulsation periods corresponding to a 0.2% peak-to-peak change; 1 mma is equal to 1.085 mmag. 1 The Astrophysical Journal, 771:17 (11pp), 2013 July 1 Mukadam et al. 5.5 213s triplet Window 5 4.5 274s 4 triplet 3.5 3 2.5 2 1.5 de (mma) u 1 318s σ 334s 0.5 3 = 0.45 mma 187s Amplit 0 1 274.52s 212.95s 0.5 0 0.5 0 0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 Frequency (Hz) Figure 1. Discrete Fourier transform (DFT) of the 2007 August data on ZZ Ceti is shown in the top panel. The middle panel shows the prewhitened DFT obtained after subtracting two components of the 213 s and 274 s multiplets, as well as the 187 s, 318 s, and 334 s modes. The bottom panel shows the effect of additionally subtracting out the third low-amplitude component of the 213 s and 274 s triplets. boundary (Winget et al. 1983). Gravitational contraction de- Table 1 creases the periods, but is found to be very small in the tempera- High Signal-to-noise 2007 August ZZ Ceti Data Yield the Best-fit Periodicities Shown with the Listed Least Squares and Italicized Monte Carlo Uncertainties ture range of the ZZ Ceti instability strip compared to the hotter Computed using Period04 classes of pulsating white dwarfs (Kepler et al. 2000). Although this implies that the rate of change of period with time dP/dt Frequency Period Amplitude is governed by the rate of stellar cooling for the ZZ Ceti stars (μHz) (s) (mma) (Kepler et al. 2000), eigenmodes trapped in the outer hydrogen 4691.8904 ± 0.0055 213.13371 ± 0.00025 6.587 ± 0.070 envelope (e.g., 213 s) are still expected to show reduced values ± 0.0057 ± 0.00026 ± 0.082 of dP/dt due to first order effects from residual gravitational 4699.8685 ± 0.0090 212.77191 ± 0.00041 4.293 ± 0.069 contraction. ± 0.011 ± 0.00050 ± 0.079 4696.006 ± 0.031 212.9469 ± 0.0014 1.191 ± 0.069 ± 0.034 ± 0.0015 ± 0.075 ± ± ± 2. NEW OBSERVATIONS OF ZZ Ceti FROM 2002 TO 2012 3646.2442 0.0086 274.25481 0.00065 4.129 0.065 ± 0.010 ± 0.00077 ± 0.078 3639.321 ± 0.011 274.77652 ± 0.00087 3.146 ± 0.069 Since the last publication of this work in a refereed journal ± 0.013 ± 0.00097 ± 0.076 (Mukadam et al. 2003), we have obtained ten more years of 3642.694 ± 0.032 274.5221 ± 0.0024 1.206 ± 0.066 time-series photometry on ZZ Ceti. Most of these data were ± 0.035 ± 0.0027 ± 0.078 obtained at the 2.1 m Otto–Struve telescope at McDonald 3143.835 ± 0.038 318.0829 ± 0.0039 0.822 ± 0.061 Observatory with the time-series charge-coupled device (CCD) ± 0.043 ± 0.0043 ± 0.067 photometer named Argos (Nather & Mukadam 2004). The 2997.250 ± 0.038 333.6392 ± 0.0042 0.830 ± 0.061 journal of observations of these data is given in Table 5.We ± 0.050 ± 0.0056 ± 0.073 have not included data obtained in marginal weather or that 5351.703 ± 0.063 186.8564 ± 0.0022 0.502 ± 0.061 which was discarded due to timing problems.
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