Galactic archaeology and asteroseismology of white dwarf stars

Galactic archaeology
and asteroseismology of
white dwarf stars
http://jjherm.es
@jotajotahermes
J.J. Hermes

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JJ Hermes, Boston University | SAC Seminar | 4
1. White Dwarfs
are Stellar Fossils
that We Can
Directly Age-Date
2. We Are Using TESS
to Better Model These
Stellar Fossils

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• a stellar remnant that is
no longer fusing in its
core
• the endpoints of
everything < 8 M¤
• electron degeneracy
limits WD mass to <
1.4 M¤
A ‘typical’ 0.6 solar-mass
white dwarf
electron degenerate
C/O core
(r = 8500 km)
non-degenerate He
layer
(260 km) non-degenerate H
layer
(30 km)
[thermal reservoir]
[insulating blanket]
What Do We Mean by ‘White Dwarf’?

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A ‘typical’ 0.6 solar-mass
white dwarf
electron degenerate
C/O core
(r = 8500 km)
non-degenerate He
layer
(260 km) non-degenerate H
layer
(30 km)
[thermal reservoir]
[insulating blanket]
White Dwarfs are Excellent for Ages: They Simply Radiate
Their Stored Heat, Which is Not Replenished
65,000 K (0.001 Gyr)
25,000 K (0.02 Gyr)
13,000 K (0.31 Gyr)
10,500 K (0.56 Gyr)
7100 K (1.5 Gyr)
5100 K (5 Gyr)
3300 K
(11 Gyr)

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Gaia has revolutionized our ability to nd white dwarfs
Gaia Collaboration,
Babusiaux et al. 2018
• Before Gaia we knew of
~35,000 white dwarfs
(mostly from SDSS)
• Gentile Fusillo et al. 2019
catalog nearly half
a million candidates
from Gaia DR2

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Gaia eDR3: We Are Directly Testing Ages in Wide WD+WD
Tyler Heintz (BU), also
Kareem El-Badry (Berkeley)
El-Badry, Rix & Heintz 2021:
arXiv: 2101.05282
1565 high-conﬁdence, wide
(>100 au) binaries with 2 WDs!

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Soon We Will Have a Better-Calibrated Sample of Wide WD
Co-PI: Jennifer Van Saders JJ Hermes, Boston University | SAC Seminar | 9
200 pc sample from
El-Badry et al. 2018, 2021
>16,000 high-confidence, wide (>100 au) binaries with 1 WD!

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0.6 M¤
0.9 M¤
1.1 M¤
Gaia Reveals Interesting Physics in White Dwarf Cooling
Tremblay et al. 2019

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0.6 M¤
0.9 M¤
1.1 M¤
White dwarfs within 100 pc in Gaia
• Gaia CMD showed an
overdensity aside from
expected cooling tracks

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DA (H-atmosphere) white dwarfs within 100 pc in Gaia
• The overdensity
appears better focusing only
on DA white dwarfs

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20% of core is
crystallized
80% of core is
crystallized
DA (H-atmosphere) white dwarfs within 100 pc in Gaia
• The overdensity
appears better focusing only
on DA white dwarfs

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107.5 yr,
0.4 M¤
107.5 yr,
1.3 M¤
+100.02 yr
Onset of crystallization
80% of mass is
crystallized
Crystallization causes a pile-up in white dwarf cooling
Predicted theoretically by
Van Horn 1968

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No crystallization
Latent heat from crystallization
Gaia Observed
Latent heat from
crystallization, plus
gravitational energy
from 16O sedimentation
This overdensity is exactly where we expect crystallization!
Gaia WD Luminosity Function

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(What is latent heat? Why does it stall cooling?)
• Latent heat: extra energy
required to carry out a
first-order phase transition
• (When smoking a brisket
the moisture in the meat
undergoes a 1st-order
phase transition at 160F
[70C] from evaporative
cooling)

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“Long after our roiling, boiling life-giving star runs out of fuel,
it will slowly form a cold, dead, super-dense crystal sphere
about the size of the Earth that will linger like a translucent
tombstone.” – Deborah Netburn, The Los Angeles Times

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Adding tangential velocities to the Gaia CMD…
100 pc sample from
Gentile Fusillo et al. 2019 JJ Hermes, Boston University | SAC Seminar | 18
0.6 M¤
1.2 M¤

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1. Fundamental physics from White Dwarfs in Gaia CMD
• Most (>90%) of WDs Should Cool
Predictably & Eventually Crystallize,
Releasing Latent Heat
• We are Testing Expectations with Wide,
Coeval WD+WD Binaries
65,000 K (0.001 Gyr)
25,000 K (0.02 Gyr)
13,000 K (0.31 Gyr)
10,500 K (0.56 Gyr)
7100 K (1.5 Gyr)
5100 K (5 Gyr)
3300 K
(11 Gyr)

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He
P.I. zone
(30-20 kK)
H
P.I. zone
(13-10 kK)
N2
Ll
2
DAV Propagation Diagram
Core Surface
p-modes
σ2 > L
l
2, N2
convection
zone
log σ2 (s-2)
g-modes
σ2 < L
l
2, N2
DBV:
aka V477 Her
DAV:
aka ZZ Ceti
Fontaine & Brassard 2008
Aerts, JCD, Kurtz 2010 JJ Hermes, Boston University | SAC Seminar | 20

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White Dwarfs: g-modes, not all
modes are observed excited
(much like delta Scuti p-modes)
5 min 4 min
6 min
Solar p-modes
BiSON; Thompson et al. 2003
2 min
3 min
4 min
5 min
6 min
10 min
n

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White Dwarfs: g-modes, not all
modes are observed excited
(much like delta Scuti p-modes)
Adiabatic Model: 11,245 K, 0.632 M¤
,
10-4.12 MH
/MWD
(Romero et al. 2012)
2 min
3 min
4 min
5 min
6 min
10 min
l=1
l=2

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KIC 08626021: Østensen et al.
2011, 2013
The Most Studied Kepler Pulsating WD: A Hot DBV
Red: Location of
m=0 models of
Giammichele et
al. 2018
Blue: Location of
m=0 models of
Bischoff-Kim et
al. 2014

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Core Surface
ß 99% of mass
X(O) = 78.03% ± 4.2%
X(C) = 21.96% ± 4.2%
X(He) = 0.0113% ± 0.006%
KIC 08626021: Giammichele et
al. 2018
Toulouse Models Predict A Large, O-Dominated Core

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Toulouse Models Neglected to Include Neutrino Cooling
Timmes et al. 2018
KIC 08626021: Giammichele et
al. 2018

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Toulouse Models Now Updated to Include Neutrino Cooling
KIC 08626021: Charpinet et al.
2019
8 observed modes &
many free parameters
There is great potential
in getting this right!
Gaia CMD position
infers: 0.73 ± 0.12 M¤
Black: Models of Giammichele et al. 2018
Red: Models of Charpinet et al. 2019 including
neutrino cooling

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7.5
8.5 8.0
Blue: K2 Pulsating WDs log(g) = 9
20,000 K
10,000 K
Kepler/K2 Observed 90 Pulsating White Dwarfs
Hermes et al. 2021, in prep.
81 DAV (H-atm.) WDs
First 27 published in Hermes et al. 2017
Grey: All 2166 WDs
7 DBV (He-atm.) WDs
Analysis in Vanderbosch et al. 2021
2 DOV (pre-WDs, C/O-atm.)

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1000 s 200 s
500 s 125 s
l=1
l=2
White Dwarf Seismologist’s Dilemma: Often Few Modes

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1000 s 200 s
500 s 125 s
l=1
l=2
(consecutive g-modes evenly
spaced in period, not frequency)

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l = 1
n = 1
l = 1
n = 2
l = 1
n = 3
SDSSJ0051+0339, g=17.6, K2 Campaign 8
Chris Clemens et al. in prep.
Kepler and TESS makes some short-period
mode identification relatively trivial

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0
1
2
3
4
5
6
7
8
50 100 150 200 250 300 350 400 450
Mode Period (s)
N
l = 1
n = 1
l = 1
n = 2
l = 1
n = 3
n = 1 n = 2 n = 3 n = 4
If we only plot identified l=1 (m=0) modes:

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0
1
2
3
4
5
6
7
8
50 100 150 200 250 300 350 400 450
Mode Period (s)
N
n = 1 n = 2 n = 3 n = 4
l = 1
n = 1
l = 1
n = 2
l = 1
n = 3
If we only plot identified l=1 (m=0) modes:

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0
1
2
3
4
5
6
7
8
50 100 150 200 250 300 350 400 450
l=1 DAV periods, observed
Full evolutionary models computed
by Romero et al. 2012

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Drawing from a random distribution of all
hydrogen layer masses
0
1
2
3
4
5
6
7
8
50 100 150 200 250 300 350 400 450
0
1
2
3
4
5
6
7
8
50 100 150 200 250 300 350 400 450
l=1 random MH
simulation

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0
1
2
3
4
5
6
7
8
50 100 150 200 250 300 350 400 450
0
1
2
3
4
5
6
7
8
50 100 150 200 250 300 350 400 450
0
1
2
3
4
5
6
7
8
50 100 150 200 250 300 350 400 450
l=1 random MH
simulation
l=1 canonical MH
simulation
Only drawing from the models with
canonically thick (10-4 MH
/M★
)
hydrogen layers
Most (>80%) of DAs Have Thick H Layers

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0
1
2
3
4
5
6
7
8
50 100 150 200 250 300 350 400 450
0
1
2
3
4
5
6
7
8
50 100 150 200 250 300 350 400 450
0
1
2
3
4
5
6
7
8
50 100 150 200 250 300 350 400 450
l=1 random MH
simulation
l=1 canonical MH
simulation
Only drawing from the models with
canonically thick (10-4 MH
/M★
)
hydrogen layers
Most (>80%) of DAs Have Thick H Layers
10-15 s offset: Could be that He-layer
masses too thick in canonical models
à Would lead to systematically
younger WD cooling ages (~10%)

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We Have Strong Motivation to Keep Going with TESS

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We Have Strong Motivation to Keep Going with TESS
Blue: TESS Pulsating WDs
Grey: All 3383 WDs with T < 17 mag 7.5
8.5 8.0
log(g) = 9
20,000 K
10,000 K
So Far TESS Has Detected Oscillations in 62 Pulsating WDs

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White Dwarfs Are Faint, But TESS Data is Still Valuable

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WD 0158-160 (TIC 257459955): T=14.1 mag
Bell et al. 2019
TESS DAVs: Bognar et al. 2020
9 observed l=1 modes
Gaia parallax implies a
distance of 68.14 ± 0.28 pc
Seismic distance, Model 2: 66.5 ± 2.5 pc
Seismic distance, Model 1/3: 87.7 ± 7.9 pc

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The 20-Second Cadence Is Often Necessary for Our Science
Pulsation amplitudes have been, on
average, 35% underestimated by the
2-min cadence so far in Cycle 3

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2. TESS Will Keep Refining Models of White Dwarf Interiors
• Boutique Asteroseismology of WDs Still
Hard, But Improving, Especially With
Ensemble Approaches at Short Periods
• In Many Cases, 20-second TESS
Data Is Crucial
• We Are Working Onwards Towards
Constraining WD Core Compositions
and Envelopes!
65,000 K (0.001 Gyr)
25,000 K (0.02 Gyr)
13,000 K (0.31 Gyr)
10,500 K (0.56 Gyr)
7100 K (1.5 Gyr)
5100 K (5 Gyr)
3300 K
(11 Gyr)

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Adding tangential velocities to the Gaia CMD…
100 pc sample from
Gentile Fusillo et al. 2019 JJ Hermes, Boston University | SAC Seminar | 44
0.6 M¤
1.2 M¤

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Massive WDs descended from 4-8 solar-mass ZAMS stars
Cheng et al. 2019 JJ Hermes, Boston University | SAC Seminar | 45
• Massive WDs
should come from
stars that spend
<0.5 Gyr on the
main sequence

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Something is delaying cooling in some massive WDs
• Models include
crystallization:
something else is
slowing them down!
• Fast kinematics
suggests these have
had their ages
“reset”!
• But why are they
piling up?
Cheng et al. 2019

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0.0 0.2 0.4 0.6 0.8 1.0
0.0
0.2
0.4
0.6
0.8
1.0
Sedimentation Cooling Delay [Gyr]
°3.5
°3.0
°2.5
°2.0
°1.5
0
2
4
Q Branch
0.9 MØ C/O WD
Z = 0.035
Clustering, hNi = 3,000
Clustering, hNi = 1,000
Clustering, hNi = 300
Single-particle Diffusion
°3.5
°3.0
°2.5
°2.0
°1.5
0
2
4
Q Branch
1.0 MØ C/O WD
Z = 0.035
°3.5
°3.0
°2.5
°2.0
°1.5
0
2
4
Q Branch
1.1 MØ C/O WD
Z = 0.035
°3.5
°3.0
°2.5
°2.0
°1.5
log(L/LØ)
0
2
4
Q Branch
1.18 MØ C/O WD
Z = 0.035
0.0 0.2 0.4 0.6 0.8 1.0
m [MØ]
0.0
0.2
0.4
0.6
Mass Fraction
22Ne
12C
16O
initial
ﬁnal
Something is delaying cooling in some massive WDs
• That something could be 22Ne
sedimentation (perhaps in
clusters) in C/O-rich cores
Bauer et al. 2020
Bildsten & Hall 2001

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1. Fundamental physics from White Dwarfs in Gaia CMD
• Most (>90%) of WDs Should Cool
Predictably & Crystallize, Releasing Latent
Heat
• We are Testing Expectations with Wide,
Coeval WD+WD Binaries
• Pile-ups among ~7% of massive WDs
reveal crystallization PLUS poorly
modeled physics (perhaps related to
mergers & sedimentation of 22Ne
clusters?)
65,000 K (0.001 Gyr)
25,000 K (0.02 Gyr)
13,000 K (0.31 Gyr)
10,500 K (0.56 Gyr)
7100 K (1.5 Gyr)
5100 K (5 Gyr)
3300 K
(11 Gyr)

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Galactic archaeology and asteroseismology of white dwarf stars

Galactic archaeology and asteroseismology of white dwarf stars

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