White Dwarfs in Wide Binaries and the Age of the Galaxy

Abstract

We present a photometric study of wide binaries with suspected white dwarf (WD) components. These systems were initially identified on the basis of their reduced proper motions and colors by W. J. Luyten (Proper Motion Survey with the Forty-Eight Inch Schmidt Telescope [Minneapolis: Univ. Minnesota Press, 1963–]) and H. L. Giclas et al. (Lowell Proper Motion Survey, Northern Hemisphere [Flagstaff: Lowell Obs., 1971], Lowell Proper Motion Survey, Southern Hemisphere [Flagstaff: Lowell Obs., 1978]). These systems usually contain a WD and a main-sequence star; however, a few contain more than two components. We obtained BVRI CCD images for approximately 475 of the 512 systems in our sample. These systems were determined by a concurrent low-resolution spectroscopy program to contain about 325 relatively cool WDs. We use this sample to determine the WD luminosity function (WDLF) and, by inference, the minimum age of the Galaxy. The WDLF was constructed using the 1/Vmax technique (M. Schmidt, ApJ, 151, 393 [1968]), as modified by T. Oswalt & J. A. Smith (Proc. Ninth European Workshop on White Dwarfs, ed. D. Koester & K. Werner [Berlin: Springer, 1995], p. 24) to correct for sample incompleteness. Error estimates were determined using the conservative approach used by J. Liebert, C. C. Dahn, & D. G. Monet (ApJ, 332, 891 [1988]). Of the spectroscopically identified WDs, 152 met the selection and completeness criteria for our study and were included in the final sample used to derive the WDLF. Most stars will end their lives as white dwarfs, slowly radiating their stored thermal energy and cooling to invisibility. The cooling timescale, calibrated using pulsating WDs, is on the order of 10 Gyr to cool to . This cooling log (L/L ) ≈ 5 , rate is slow enough that the remnants of the first epoch of star formation in the solar neighborhood are still visible. Comparison of the observed WDLF with age-dependent WDLFs integrated numerically by M. A. Wood (Proc. Ninth European Workshop on White Dwarfs [Berlin: Springer, 1995], p. 41) suggests a lower limit age of the local Galactic disk of Gyr. Assuming a ∼1 Gyr delay between the big bang 0.9 9.7 0.8 and galaxy formation, and allowing for main-sequence evolutionary times, this minimum Galactic disk age then implies a lower limit to the age of the universe of about 11 Gyr. Newly revised ages derived from globular cluster studies (B. Chaboyer, P. Demarque, P. J. Kernan, & L. M. Krauss, ApJ, 494, 96 [1998]) and recently derived cosmological ages, corrected for Hipparcos data (M. W. Feast & R. M. Catchpole, MNRAS, 286, L1 [1997]), now agree with our age determination to within 1 j. Furthermore, our age estimate is in accord with those derived from nuclear-cosmochronology studies (e.g., C. Sneden, J. J. Cowen, D. L. Burris, & J. W. Truran, ApJ, 496, 235 [1998]) and meteoritic sample analyses (e.g., L. R. Nittler & R. Cowsik, Phys. Rev. Lett., 78, 175 [1997]). Our data show no clear observational evidence for an increase in the WDLF at crystallization, , so there is log (L/L ) ≈ 3.8 ,