White Dwarfs: Not Every One is So Simple DZ (atmospheric metals) 25-50% of white dwarfs are polluted with metals • Metals are coming from remnant planetary systems!
• log(g) = 8.0, so metals should quickly sink out of photosphere
• Abundances: debris is rocky and chemically diverse, like solar system meteorites (Gänsicke+12) • Some debris is rocky & water-‐‑rich (Farihi+13, Raddi+2015) • Infrared excesses often seen: directly detecting debris disks
• Exoplanet compositions!
The Scars of Tidally Disrupted Planetesimals
Vanderburg+ 2015 K2: First Transiting Planetesimals Around WDs • This WD has metal pollution and an IR excess (dust disk)
• See talk by Andrew Vanderburg tomorrow: A disintegrating minor planet transiting a white dwarf
K2: Dozens of Short-Period WD Binaries • By the end of K2 we will get orbital periods for dozens of pre-‐‑ Cataclysmic Variables (WD+dM binaries)
Rebassa-Mansergas, Hermes+ in prep. 99,860 K
0.59 Msun WD
Reflection off dM every 19.9 hr
Convection Drives White Dwarf Pulsations • DA (hydrogen atmosphere) WDs pulsate when H partially ionized (DAVs, aka ZZ Cetis)
g-modes—remarkably similar to the large-amplitude DAV pulsators (Winget et al. 1 The observed pulsating white dwarf stars lie in three strips in the H-R diagram, in Figure 3. The pulsating pre-white dwarf PG 1159 stars, the DOVs, around 7 170,000 K have the highest number of detected modes. The first class of pulsating 5.5 5.0 4.5 Planetary Nebula Main sequence DOV DBV DAV 4.0 3.5 3.0 log [T eff (K)] 4 2 0 –2 –4 log (L/L ) Figure 3 A 13-Gyr isochrone with z = 0.019 from Marigo et al. (2007), on which we have drawn the ob Annu. Rev. Astro. Astrophys. 2008.46:157-199. Downloaded f by University of Texas - Austin on 01/28/09. For Winget & Kepler 2008 Partial ionization zone can store & release energy
A New K2 View on Common-Envelope Evolution 10 1 100 101 102 White Dwarf Rotation Period (hr) 0 1 2 3 4 5 6 N Non-magnetic CVs Pulsating white dwarfs J1136+0409 J1136+0409 Prot : 2.49 ± 0.53 hr
~Days ~Minutes • No isolated WD rotates this fast
• No accretion history in J1136+0409
• RGB core evolution influenced by common envelope ejection
Hermes et al. 2015, MNRAS, 451, 1701
Convective Driving: WD Cools, Periods Increase Mukadam+ 2006 V. Van Grootel et al.: The instab Fig. 2. Structure of the envelope of our representative evolving 0.6 M DA white dwarf. The ordinate gives the fractional mass depth in loga rithmic units. The small dots define “isocontours” of opacity, and som Surface Core Base of convection zone deepens as WD cools
Van Grootel+ 2012
The First Kepler DAV Showed Something Funny 2 Bell et al. Fig. 1.— Representative sections of the Kepler light curve of KIC 4552982 in units of days since the start of observations. The top p shows the full Q11 light curve. The one-month shaded region in the top panel is expanded in the middle panel. The one-week sh region in the middle panel is expanded in the bottom panel. The solid line is the light curve smoothed with a 30-minute window. point-to-point scatter dominates the pulsation amplitudes in the light curve, so pulsations are not apparent to the eye. The dram increases in brightness are discussed in detail in Section 3. to medium-resolution spectra for the white dwarf and fit the Balmer line profiles to models to determine its val- tion rate. We summarize our findings and conclud Section 5. KIC 4552982: Bell+ 2015 3 months: 1 month: 1 week:
The First Kepler DAV Showed Something Funny 2 Bell et al. Fig. 1.— Representative sections of the Kepler light curve of KIC 4552982 in units of days since the start of observations. The top p shows the full Q11 light curve. The one-month shaded region in the top panel is expanded in the middle panel. The one-week sh region in the middle panel is expanded in the bottom panel. The solid line is the light curve smoothed with a 30-minute window. point-to-point scatter dominates the pulsation amplitudes in the light curve, so pulsations are not apparent to the eye. The dram increases in brightness are discussed in detail in Section 3. to medium-resolution spectra for the white dwarf and fit the Balmer line profiles to models to determine its val- tion rate. We summarize our findings and conclud Section 5. KIC 4552982: Bell+ 2015 3 months: 1 month: 1 week: e measured equivalent durations of the 186 outbursts at were recorded without interruption from gaps in the ta is displayed in Figure 4 and the continua used for e example outbursts are the dashed lines in Figure 3. e median outburst equivalent duration is 6.8 minutes he corresponding outburst is displayed in the second nel of Figure 3). Since the Kepler point-to-point scat- is 1.8% for this target, we are limited to detecting ly large outbursts by eye and so are undoubtedly in-
A Second Case of Outbursts in a Cool DAV PG 1149+057: Hermes et al. 2015, ApJ, 810, L5 • No companion earlier than L3: This is happening on the white dwarf
SDSS image K2 pixels 11,000 K, log(g)=8.0 model (3σ uncertainties smaller than each point)
(Aside: The Ecliptic is Full of Asteroids!) • Always check the target pixels if you see a blip in your K2 light curve
Data from J1136+0409 (WD+dM from earlier) h5p://barentsen.github.io/k2flix/
K2flix
A Second Case of Outbursts in a Cool DAV PG 1149+057: Hermes et al. 2015, ApJ, 810, L5 • These outbursts are essentially rogue waves in a pulsating star!
• The pulsations persist in outburst
Potential Outburst Mechanisms in Cool DAVs • Magnetism unlikely: τdynamical
only a few s for WDs
• Nuclear burning unlikely: T < 106 K at τthermal of recurrence timescale (~7.7 d)
• Rocky accretion unlikely: No spectroscopic metal lines
• Most likely connected to pulsations
Base of convection zone Surface Deeper
Most Modes Bounded by Base of Convection Zone • Growth time ~days:
γ-‐‑1 ≈ nτω
• For ~1000 s modes:
– Radial order n ~ 20
– τω = τthermal at top
of mode cavity τω ~ hrs
Goldreich & Wu I, 1999
Potential Outburst Mechanisms in Cool DAVs • Nonlinear mode coupling, via parametric instability? (Wu & Goldreich 1999) • Still an open question!
The two outbursting DAVs
K2’s Expanding View of Pulsating White Dwarfs • So far, 20 pulsating white dwarfs with >1 month short-‐‑cadence Kepler data
Not variable with K2 SC Not outbursting from LC First 3 DAVs that outburst Ground-based DAVs
§ Ensemble Asteroseismology of White Dwarfs
§ White Dwarf Rotation Rates
§ Incidence of Magnetism in White Dwarfs
§ More Remnant Planetary Systems
§ Close, Evolved Binaries
§ Refining WDs as “Flux Standards”
Likely a WD+BD binary
very close to Roche-‐‑lobe filling
Christmas in July (and October, and March) 2.5m INT spectroscopy:
24,430(310) K, 0.39(05) Msun WD
White Dwarfs Aren’t All Great Flux Standards • For nonvariable white dwarfs, stay away from convective surfaces: DAs
< 13,000 K DBs
< 25,000 K
g-modes—remarkably similar to the large-amplitude DAV pulsators (Winget et al. 1 The observed pulsating white dwarf stars lie in three strips in the H-R diagram, in Figure 3. The pulsating pre-white dwarf PG 1159 stars, the DOVs, around 7 170,000 K have the highest number of detected modes. The first class of pulsating 5.5 5.0 4.5 Planetary Nebula Main sequence DOV DBV DAV 4.0 3.5 3.0 log [T eff (K)] 4 2 0 –2 –4 log (L/L ) Figure 3 A 13-Gyr isochrone with z = 0.019 from Marigo et al. (2007), on which we have drawn the ob Annu. Rev. Astro. Astrophys. 2008.46:157-199. Downloaded f by University of Texas - Austin on 01/28/09. For Winget & Kepler 2008
• Confirmed that planets are disrupted onto white dwarfs
• From pulsations: A close binary WD with truncated RGB evolution rotates much faster than isolated WDs
• Outbursts in the coolest DAVs with very deep convection zones
What Have We Learned from K2 so Far?
§ Ensemble Asteroseismology of White Dwarfs
§ Can test for diversity in envelope thicknesses, which would affect cooling ages
§ White Dwarf Rotation Rates
§ Incidence of Magnetism in White Dwarfs
§ More Remnant Planetary Systems
§ Close, Evolved Binaries
§ Refining WDs as “Flux Standards”
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