White Dwarfs

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”

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]

White Dwarfs in Globular Clusters

Cluster White Dwarf Spectroscopy

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

The Globular Cluster M4 Fainter white dwarfs are seen in this nearby cluster -> age = /- 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

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: M WD about 8 M Sun This result is in agreement with stellar models

Open Cluster M67

M67

Age of M67

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)

Field White Dwarfs

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)

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

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)

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

Determination of Mass (Field Objects) Spectroscopic Method: Line (Hydrogen) width is sensitive to pressure which is proportional to gravity g = GM/R 2 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

Masses of White Dwarfs

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

Magnetism in White Dwarfs

Zeeman (Landau) Splitting