Lecture I: Introduction to White Dwarfs

S. R. Kulkarni, E. Ofek

http://astro.caltech.edu/~srk/ay125/

Lecture 1: White Dwarfs (Introduction)

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”

Lecture 1: White Dwarfs (Introduction)

3

Sirius B

L= 4 r2T4

Lecture 1: White Dwarfs (Introduction)

4

White Dwarfs – Physical understanding

1926: Fermi & Dirac - statistical physics of electron gas

1926: Fowler – electron degeneracy pressure can support

stellar mass object against its gravity

1939: Chandrasekhar – derived the equations for the

structure, mass-radius-core composition

relations.

Lecture 1: White Dwarfs (Introduction)

5

Degenerate gas

Lecture 1: White Dwarfs (Introduction)

6

Interesting implications of degenerate electron gas

“inverted” mass radius relation

Lecture 1: White Dwarfs (Introduction)

7

Interesting implications of degenerate electron gas

Nuclear reactions are explosive

Novae

Type Ia supernovae

He flash in evolved stars

WD are condactive

Lecture 1: White Dwarfs (Introduction)

8

Observed properties

T=4,500 – 150,000 K

log g = 7 – 9 (cgs)

mass = 0.4 – 1.4 solar mass

mean radius = 0.012 solar radius

log L = -4.3 – 3 (in solar luminosity)

Upper limit on rotation : Vsini < 65 km/s

observed mass density 0.002 solar mass per cubic pc

formation rate: 4.5-7.5x10-13 pc-3 yr-1

Galactic scale height: 250-300 pc

Lecture 1: White Dwarfs (Introduction)

9

Spectroscopic classification

DA - only Balmer lines; no HeI or metals (75% of WD)

DB – HeI lines, no H or metals

DC – Continuous spectrum, no lines deeper than 5%

DO – strong HeII, HeI or H present

DZ – Metal lines only, no H or He

DQ – Carbon features

Temperature index in the range 0-9 in units of 5040K

Lecture 1: White Dwarfs (Introduction)

10

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)

Lecture 1: White Dwarfs (Introduction)

11

Lecture 1: White Dwarfs (Introduction)

12

Lecture 1: White Dwarfs (Introduction)

13

Lecture 1: White Dwarfs (Introduction)

14

Lecture 1: White Dwarfs (Introduction)

15

Lecture 1: White Dwarfs (Introduction)

16

DQZT=7740Klog(g)=8.0Mass from Orbit

Lecture 1: White Dwarfs (Introduction)

17

Lecture 1: White Dwarfs (Introduction)

18

Determination of Mass (Field Objects)

• Spectroscopic Method:Line (Hydrogen) width is sensitive to pressure

which is proportional to gravityg = GM/R2

* gravitational redshift (M/R)• Photometric Method:

Broad-band photometry fitted to black body yields Teff and angular size

Combine with parallax to get radius RUse Mass-Radius relation to derive Mass

Lecture 1: White Dwarfs (Introduction)

19

Lecture 1: White Dwarfs (Introduction)

20

Lecture 1: White Dwarfs (Introduction)

21

Lecture 1: White Dwarfs (Introduction)

22

Masses of White Dwarfs

Lecture 1: White Dwarfs (Introduction)

23

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 – Supported by cluster observations

Lecture 1: White Dwarfs (Introduction)

24

WD Cooling& evolution=M5/7L−5/7

Lecture 1: White Dwarfs (Introduction)

25

White Dwarfs in Globular Clusters

Lecture 1: White Dwarfs (Introduction)

26

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

Lecture 1: White Dwarfs (Introduction)

27

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

Lecture 1: White Dwarfs (Introduction)

28

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

Lecture 1: White Dwarfs (Introduction)

29

Lecture 1: White Dwarfs (Introduction)

30

Lecture 1: White Dwarfs (Introduction)

31

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)

Lecture 1: White Dwarfs (Introduction)

32

Lecture 1: White Dwarfs (Introduction)

33

Field White Dwarfs

Lecture 1: White Dwarfs (Introduction)

34

Lecture 1: White Dwarfs (Introduction)

35

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)

Lecture 1: White Dwarfs (Introduction)

36

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

Lecture 1: White Dwarfs (Introduction)

37

Magnetism in White Dwarfs

Lecture 1: White Dwarfs (Introduction)

38

Lecture 1: White Dwarfs (Introduction)

39

Lecture 1: White Dwarfs (Introduction)

40

Zeeman (Landau)Splitting

Lecture 1: White Dwarfs (Introduction)

41

Lecture 1: White Dwarfs (Introduction)

42

Pulsating WD

ZZ Cet: DA Teff~12,000+/-1000K P~100-1200s

GD358: DB Teff~28,000+/-1000K P~100-1200s

GW Vir: DO Teff~130,000+/-20,000 P~200-2000s

Lecture 1: White Dwarfs (Introduction)

43

End

Lecture 1: White Dwarfs (Introduction)

44

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

Lecture 1: White Dwarfs (Introduction)

45

Cluster White Dwarf Spectroscopy

Lecture 1: White Dwarfs (Introduction)

46

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

Lecture 1: White Dwarfs (Introduction)

47