White Dwarfs

1. White Dwarfs

2. References • D. Koester, A&A Review (2002) “White Dwarfs: Recent Developments” • Hansen & Liebert, Ann Rev A&A (2003) “Cool White Dwarfs” • Wesemael et al. PASP (1993) “An Atlas of Optical Spectra of White-Dwarf Stars” • Wickramsinghe & Ferrario PASP (2000) “Magnetism in Isolated & Binary White Dwarfs”

3. How stars die • Stars above 8 Msun form neutron stars and black holes • Below 8 Msun the stars condense to O-Ne-Mg white dwarfs (high mass stars) or usually C-O white dwarfs • Single stars do not form He white dwarfs but can form in binary stars [*] • We know of no channel to form H white dwarfs of some reasonable mass [other than Brown Dwarfs]

4. White Dwarfs in Globular Clusters

5. Cluster White Dwarf Spectroscopy

6. White Dwarfs in Clusters • Chronometers: Use cooling models to derive the ages of globular clusters • Yardsticks: Compare nearby and cluster white dwarfs. • Forensics: Diagnose the long dead population of massive stars

7. The Globular Cluster M4 • Fainter white dwarfs are seen in this nearby cluster -> age = 12.7 +/- 0.7 Gyr M4 formed at about z=6 Disk formed at about z=1.5 • dN/dM, differential mass spectrum dN/dM propto M-0.9

8. White Dwarfs in Open Clusters Open Clusters have a wide range of ages (100 Myr to 9 Gyr, the age of the disk) • Use white dwarfs as chronometers • Derive initial-mass to final-mass mapping Key Result: MWD about 8 MSun This result is in agreement with stellar models

9. Open Cluster M67

10. M67

11. Age of M67

13. Field White Dwarfs • Identified by large proper motion yet faint object • LHS (Leuyten Half Second) • NLTT (New Leuyten Two Tenths) • Blue Objects (found in quasar surveys) • Very Hot objects (found in X-ray surveys)

15. Field White Dwarfs

17. Old White Dwarfs • Microlensing observations indicate presence of 0.5 Msun objects in the halo • Old white white dwarfs expected in our disk, thick disk and halo • These old white dwarfs are paradoxically blue (cf cool brown dwarfs)

19. Spectroscopic Classification • DA, strong Hydrogen lines • DB, strong He I lines • DO, strong He II lines • DC, no strong lines (“continuous”) spectrum • DZ, strong metal lines (excluding carbon) • DQ, strong carbon lines Multiple families shown in decreasing order e.g. DAB, DQAB, DAZ

20. Spectroscopic Features: A few comments • Strong gravity of white dwarfs result in rapid settling of elements e.g. Hydrogen always rises to the top and can mask other elements • Given the above white dwarf atmosphere modeling is generally considered to be more tractable than for other stars • If trace elements are seen as in DZ white dwarfs then they must be of recent origin (e.g. accretion from the ISM, comets etc)

26. DQZ T=7740K log(g)=8.0 Mass from Orbit

28. Determination of Mass (Field Objects) • Spectroscopic Method: Line (Hydrogen) width is sensitive to pressure which is proportional to gravity g = GM/R2 • Photometric Method: Broad-band photometry fitted to black body yields Teff and angular size Combine with parallax to get radius R Use Mass-Radius relation to derive Mass

32. Masses of White Dwarfs

33. Magnetism in Isolated White Dwarfs • About 5% of field white dwarfs exhibit strong magnetism • On an averge these white dwarfs have larger mass • Some rotate rapidly and some not at all • Magnetism thus influences the initial-final mapping relation • Or speculatively some of these are the result of coalescence of white dwarfs

34. Magnetism in White Dwarfs

37. Zeeman (Landau) Splitting