THE AGES OF THE DISK AND THE HALO FROM NEARBY WHITE DWARFSMUKREMIN KILIC, JEFFREY A. MUNN, H. HARRIS, TED VON HIPPEL, J. LIEBERT, K. WILLIAMS, E. JEFFERY, S. DEGENNARO

ABSTRACTWe present a detailed analysis of the white

dwarf luminosity functions derived from thelocal 40 pc sample and the deep proper mo-tion catalog of Munn et al. Many previousstudies have ignored the contribution of thickdisk white dwarfs to the Galactic disk luminos-ity function, which results in an erroneous agemeasurement. We demonstrate that the ratioof thick/thin disk white dwarfs is roughly 20%in the local sample. Simultaneously fitting forboth disk components, we derive ages of 6.8-7.0 Gyr for the thin disk and 8.7 ± 0.1 Gyr forthe thick disk from the local 40 pc sample. Sim-ilarly, we derive ages of 7.4-8.2 Gyr for the thindisk and 9.5-9.9 Gyr for the thick disk from thedeep proper motion catalog, which shows noevidence of a deviation from a constant star for-mation rate in the past 2.5 Gyr. We constrain thetime difference between the onset of star for-mation in the thin disk and the thick disk tobe 1.6+0.3

−0.4 Gyr. The faint end of the luminosityfunction for the halo white dwarfs is less con-strained, resulting in an age estimate of 12.5+1.4

−3.4

Gyr for the Galactic inner halo.

THE 40 PC SAMPLE

Figure 1: The observed luminosity function for the local40 pc sample of white dwarfs (points with error bars, Limo-ges et al. 2015) compared to the best-fit synthetic whitedwarf luminosity function (solid lines). The top panelshows the model fits assuming a population of 100% thindisk stars, whereas the bottom panel shows the fits using acomposite population where the ratio of thick disk to thindisk white dwarfs is 22%. Dashed and dotted lines showthe contribution from the thin disk and thick disk whitedwarfs, respectively.

THE DEEP PROPER MOTION SURVEY SAMPLE

Figure 2: Left: The white dwarf luminosity function from the deep proper motion survey. The top panel showsthe model fits assuming a population of 100% thin disk stars, whereas the bottom panel shows the fits using acomposite population where the ratio of thick disk to thin disk white dwarfs is 35%. Dashed and dotted linesshow the contribution from the thin disk and thick disk white dwarfs, respectively.Right: Thin disk and thick disk age constraints as a function of the ratio of thick disk to thin disk white dwarfs.The black, blue, magenta, and red lines show the constraints from luminosity functions using the scale heightsof 200-900 pc, 200-700 pc, 200-500 pc, and 300 pc, respectively.

THE HALO SAMPLE

Figure 3: Munn et al. (2017) luminosity function for thevtan = 200–500 km s−1 halo white dwarf sample. Solid,dashed, and dotted lines show model luminosity functionsfor 12.5, 13.9, and 15.0 Gyr old halo samples, respectively.This luminosity function implies a halo age of 12.5+1.4

−3.4 Gyr.

RESULTS

Figure 4: Age-metallicity relation based on the whitedwarf luminosity functions for the open cluster NGC 6791,globular clusters 47 Tuc, M4, and NGC 6397 (Hansen et al.2013), and field thin disk, thick disk, and halo stars fromthis study. The error bars cover the age ranges estimatedfrom both the 40 pc local sample and the deep proper mo-tion survey sample.

CONCLUSIONSThis is the first time ages for all three major components of the Galaxy are obtained from a sam-

ple of field white dwarfs that is large enough to contain significant numbers of disk and halo objects.The resultant ages agree reasonably well with the age estimates for the oldest open and globularclusters. Perhaps the most important result of this analysis is not the absolute age measurements,but instead the demonstration of age differences between the different kinematic populations ofwhite dwarfs.

The enhanced star formation rate observed in the 40 pc local sample is only relevant for theimmediate Solar neighborhood, as we do not find any evidence of a deviation from a constantstar formation rate in the past 2.5 Gyr in the Deep Proper Motion Survey Sample. Gaia parallaxeswill enable us to create disk luminosity functions as a function of distance and study their starformation histories. In addition, the Large Synoptic Survey Telescope will identify ∼ 105 halo whitedwarfs, which will enable precise age estimates for the inner halo.

Fore more information, please see Kilic et al. (2017), ApJ, 837, 162.

REFERENCES• [1] Limoges et al. ApJS. (2015). 219. 19 • [2] Munn et al. AJ. (2017). 153. 10 • [3] Hansen et al. Nature. (2013). 500. 51

CONTACT INFORMATION[Email] kilic@ou.edu [Web] www.nhn.ou.edu/∼kilic