HST studies of the white dwarfs in the globular cluster 47 Tucanae reveal the signatures of freezing in the core of the white dwarfs and the onset of convection in their surface

1. Alysa Obertas
White Dwarfs in
47 Tuc
Freezing or Not?
NASA/ESA

2. luminosity
temperature

3. luminosity
temperature

5. degenerate
core
envelope

6. Γ =
coulomb potential energy
thermal kinetic energy

7. Γ =
coulomb potential energy
thermal kinetic energy
Γ ≪ 1 ideal gas

8. Γ =
coulomb potential energy
thermal kinetic energy
Γ ≪ 1 ideal gas
Γ ∼ 1 liquid

9. Γ =
coulomb potential energy
thermal kinetic energy
Γ ≪ 1 ideal gas
Γ ∼ 1 liquid
Γ = Γc crystallization

10. Γ =
coulomb potential energy
thermal kinetic energy
Γ ≪ 1 ideal gas
Γ ∼ 1 liquid
Γ = Γc freezing ❄

11. latent heat
reduced cooling rate
‘pile up’ of WDs

12. Obertas+ (2018)

13. Obertas+ (2018)

14. Obertas+ (2018)

15. Obertas+ (2018)
neutrinos

16. Obertas+ (2018)
photons

17. Obertas+ (2018)
freezing?

18. degenerate
core
envelope

19. degenerate
core
envelope

20. Obertas+ (2018)

21. Obertas+ (2018)
evolution
track with
freezing &
convection

22. Obertas+ (2018)
core
begins
freezing

23. Obertas+ (2018)
0.1M☉
Frozen

24. Obertas+ (2018)
0.2M☉
Frozen

25. Obertas+ (2018)

26. Obertas+ (2018)

27. Obertas+ (2018)
evolution
track
without
freezing

28. Obertas+ (2018)
assume
core has
entire heat
capacity

29. Obertas+ (2018)

30. Obertas+ (2018)
evolution
track
without
freezing &
convection

31. Obertas+ (2018)
assume
radiative
transport

32. Obertas+ (2018)

33. Obertas+ (2018)
WDs in
47 Tuc

34. Obertas+ (2018)

35. Thank you!
• Low mass WDs not ideal to look for freezing
• Convective coupling does occur for old WDs
• It’s okay to be wrong