Two-Body Relaxation in NGC 362 as measured by white dwarfs

1. Relaxation in NGC 362
Javiera Parada
May 8th 2019
A Richer Universe
In collaboration with:
I. Caiazzo, J. Heyl, H. Richer (UBC), J. Kalirai, P.Tremblay, M. Correnti, J. Cummings, B. Hansen, L.
Watkins,A. Bellini, J.Anderson

2. • Introduction
• Proper motion cleaning
• Modelling
• Results
• Conclusions
Introduction
Determine the relaxation time in the core of
the globular cluster NGC 362 by tracking the
positions of the youngest white dwarfs.

3. • Introduction
• Proper motion cleaning
• Modelling
• Results
• Conclusions
Observations in the UV
HST/WFC3 UV filters F225W and F275W.

4. • Introduction
• Proper motion cleaning
• Modelling
• Results
• Conclusions
Observations in the UV
White dwarfs

5. • Introduction
• Proper motion cleaning
• Modelling
• Results
• Conclusions
Colour-magnitude diagram

6. • Introduction
• Proper motion cleaning
• Modelling
• Results
• Conclusions
Colour-magnitude diagram
White dwarfs
are bright in
the UV

7. • Introduction
• Proper motion cleaning
• Modelling
• Results
• Conclusions
Colour-magnitude diagram
White dwarfs
Small Magellanic
Cloud stars

8. • Introduction
• Proper motion cleaning
• Modelling
• Results
• Conclusions
Colour-magnitude diagram
White dwarfs
Small Magellanic
Cloud stars
Other stars we
don’t like

9. • Introduction
• Proper motion cleaning
• Modelling
• Results
• Conclusions
Proper Motion Cleaning

10. • Introduction
• Proper motion cleaning
• Modelling
• Results
• Conclusions
Proper Motion Cleaning

11. • Introduction
• Proper motion cleaning
• Modelling
• Results
• Conclusions
Completeness corrections – SMC background stars

12. • Introduction
• Proper motion cleaning
• Modelling
• Results
• Conclusions
Colour-magnitude diagram

13. • Introduction
• Proper motion cleaning
• Modelling
• Results
• Conclusions
Selecting stars
• Select stars based
on their position
in the CMD.
• Clear white dwarf
sequence.

14. • Introduction
• Proper motion cleaning
• Modelling
• Results
• Conclusions
Tracking positions - Distributions of stars

15. • Introduction
• Proper motion cleaning
• Modelling
• Results
• Conclusions
Tracking positions - Distributions of stars
• Young white dwarfs
have recently lost close
to half their mass and
are just beginning to
migrate outwards due
to relaxation.
• Older white dwarfs are
already further out.

16. • Introduction
• Proper motion cleaning
• Modelling
• Results
• Conclusions
The relaxation time
• As defined in Heyl et at. 2015 (ApJ, 804:43)
𝑡"#$%& =
𝑎*
+
𝜅

17. • Introduction
• Proper motion cleaning
• Modelling
• Results
• Conclusions
Time dependent core radius
• As defined in Heyl et at. 2015 (ApJ, 804:43)
• Time dependent core radius
𝑡"#$%& =
𝑎*
+
𝜅

18. • Introduction
• Proper motion cleaning
• Modelling
• Results
• Conclusions
Modelling the distributions
• As defined in Heyl et at. 2015 (ApJ, 804:43)
• Time dependent core radius
• King model
𝑡"#$%& =
𝑎*
+
𝜅

19. • Introduction
• Proper motion cleaning
• Modelling
• Results
• Conclusions
Modelling the distributions
• As defined in Heyl et at. 2015 (ApJ, 804:43)
• Time dependent core radius
• King model
𝑡"#$%& =
𝑎*
+
𝜅

20. • Introduction
• Proper motion cleaning
• Modelling
• Results
• Conclusions
Modelling the distributions

21. • Introduction
• Proper motion cleaning
• Modelling
• Results
• Conclusions
Modelling the distributions

22. • Introduction
• Proper motion cleaning
• Modelling
• Results
• Conclusions
Estimating 𝑎* and 𝜅
Binning
Unbinned
likelihood
estimation
White dwarfs
ages
Dividing WDs
into faint and
bright WDs

23. • Introduction
• Proper motion cleaning
• Modelling
• Results
• Conclusions
White dwarfs ages
White dwarfs
ages
WD birth rate
(Counting stars)
WD cooling
models
𝑡- =
1
𝑁̇
1
1
𝐶3
35-
Ages from
F225W
magnitude

24. • Introduction
• Proper motion cleaning
• Modelling
• Results
• Conclusions
Isochrone & cooling models
• 11 Gyr isochrone
built using MESA
models.
• Initial mass of main
sequence stars
currently becoming
WDs is 0.81 M⦿
• Cooling curve for
the 0.54 M⦿ white
dwarf generated
from a 0.81 M⦿ main-
sequence star.