Watching stellar evolution all the way to the closing credits
Conference presentation, 25 min. September 2016: Understanding the roles of rotation, pulsation and chemical peculiarities in the upper main sequence, Windermere, Cumbria, UK.
U. North Carolina: Chris Clemens, Bart Dunlap, Erik Dennihy, Josh Fuchs, Stephen Fanale U. Warwick: Boris Gaensicke, Paul Chote, Roberto Raddi, Nicola Gentile Fusillo, Dave Armstrong, Pier-Emmanuel Tremblay U. Texas: Keaton J. Bell, Mike Montgomery, Don Winget U. Oklahoma: Mukremin Kilic, Alex Gianninas Harvard/Smithsonian: Warren R. Brown + S.O. Kepler, Alejandra Romero, Agnes Bischoff-Kim, Steve Kawaler, Alex Gianninas Watching Stellar Evolution All the Way to the Closing Credits
• White dwarfs are a remarkably homogenous byproduct of stellar evolution – Clustered mean mass, compositional stratification, simple evolution (just cooling) • Exploiting deviations from that simplicity yields rich insights Don Winget: “White Dwarfs Shed Their Complexity”
• Consensus: Metals are from accreted, tidally disrupted debris – 25-50% of all WDs are metal polluted (Koester et al. 2014) – WD debris is comparable to bulk Earth (dominated by Fe, O, S, Mg) – Some of this debris is water-rich! (Farihi et al. 2013) • Planetary systems around A stars are very common Metals in Typical WDs: Planetary Debris
Tremblay et al. 2016 Kleinman et al. 2013 • 80%+ of WDs have hydrogen-dominated atmospheres (DA) • Estimate masses from observed Balmer line profiles: Teff /log(g) Most White Dwarfs: 0.6 Solar Masses He-Core WDs G+ progenitors CO-Core WDs A/F/G progenitors ONe-Core WDs B+ progenitors < 0.45 M¤ excluded here
• The Galaxy is not old enough for a single star to evolve into a < 0.30 M¤ white dwarf • “Low-mass white dwarfs need friends” (Marsh et al. 1995) • Friends à binary companions – Effectively strip mass on RGB, leaving behind an ELM WD David A. Aguilar, CfA Extremely Low-Mass (ELM) White Dwarf Stars
phase = 0 • This is the most compact detached binary system currently known! In <1 Myr they’ll be really good friends. J0651+2844: A 12.75-min WD+WD Binary Brown et al. 2011
We expect dPorb /dt = (-0.26 ± 0.05)ms/yr; observe (-0.2891 ± 0.0028)ms/yr! – a measurement to <1% ! An Optical Detection of Gravitational Waves! Hermes et al. 2012, 2016 (in prep.)
Mean Earth--Moon separation Minimum WD+WD (J0651+2844, 12.75-min) Median ELM Survey (~5.4 hr) Maximum ELM Survey (J0815+2309, 25.8-hr) 1 R¤ The ELM Survey At a Glance Brown et al. 2016 + Gianninas, Kilic • 80+ ELM WD binaries solved • M1 range: 0.16-0.32 M¤ • Median M2 : 0.76 M¤
• Zeeman surface field limits exclude >200 kG for all 80 ELM WDs • These are stripped descendants of suppressed dipole mode red giants (Stello, Fuller, Cantiello, et al.) • No history of core He burning • Do all ELMs have <1.5 M¤ progenitors? Slow Ohmic diffusion? No strong internal B-fields? Aside: No ELM White Dwarf Is Strongly Magnetic Fit courtesy of Alex Gianninas Teff = 12240(180) K, log(g) = 5.75(04) à 0.17 M¤
Discovery of Pulsations in Low-Mass White Dwarfs Hermes et al. 2012, 2013 Kilic et al. 2015 • In October 2011 we discovered the first pulsating low-mass, He-core WD from McDonald Observatory: Now six known • The first pulsating ELM WD around a millisecond pulsar, PSR J1738+0333 • ELM white dwarfs have much longer pulsation periods (1100-6200 s) than C/O-core WDs (100-1400 s): less dense! • g-mode period spacing of ~100 s rather than ~40 s
Aside: White Dwarfs Are Good Flux Standards >95% of all spectroscopically confirmed white dwarfs in Kepler/K2 are flux constant to <1% on 30-min to 10-day timescales. Hermes et al. 2016 (in prep.)
K2 Already Doubled WD Rotation Measurements 10 1 100 101 102 White Dwarf Rotation Period (hr) 0 2 4 6 8 N K2 Asteroseismic Asteroseismic K2 Magnetic Magnetic 0.5 d 1.0 d 5.0 d 3 hr Hermes et al. 2016 (in prep.)
DA (hydrogen atmosphere) WDs pulsate when H partially ionized (DAVs, aka ZZ Cetis) in Figure 3. The pulsating pre-white dwarf PG 1159 stars, the DOVs, around 75, 170,000 K have the highest number of detected modes. The first class of pulsating st 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 obser locations of the instability strips, following the nonadiabatic calculations of C´ orsico, Althaus & Mi Bertolami (2006) for the DOVs, the pure He fits to the observations of Beauchamp et al. (1999) fo DBVs, and the observations of Gianninas, Bergeron & Fontaine (2006) and Castanheira et al. (200 Annu. Rev. Astro. Astrophys. 2008.46:157-199. Downloa by University of Texas - Austin on 01/28/0 Winget & Kepler 2008, ARA&A, 46, 157
Mike Montgomery • Pulsations: periodic brightness changes, caused by surface temperature variations • White dwarfs only show nonradial pulsations (strong surface gravity)
m = +1 m = -1 m = 0 1000 s 200 s 500 s 125 s 316.8 s 345.3 s n = Number of radial nodes l = Number of vertical nodes m = Number of horizontal + vertical nodes n l = 1 n = 5 l = 1 n = 6 Prot = 0.9 ± 0.2 day
A K2 View on Close, Evolved Binaries 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 • Post-RGB rotation influenced by common envelope ejection Hermes et al. 2015 l = 1 modes m = +1 m = 0 m = -1
Longest-period modes well-described by Lorentzian function Two modes in the DAV PG1149+057 Two modes in the DAV ATLASJ1342-0735 HWHM: 0.05 µHz HWHM: 1.54 µHz HWHM: 0.06 µHz HWHM: 2.23 µHz (No possible way to make this observation before Kepler.) Hermes et al. 2016 (in prep.)
• Damping rather than driving important for broadening; phase incoherence • ML2/α sets the base of convection zone and must be a free parameter Surface Core Broadened modes: bounded by the base of the convection zone! Montgomery et al. 2016 (in prep.)
The First Kepler Pulsating White Dwarf was Weird 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 pane shows the full Q11 light curve. The one-month shaded region in the top panel is expanded in the middle panel. The one-week shade region in the middle panel is expanded in the bottom panel. The solid line is the light curve smoothed with a 30-minute window. Th point-to-point scatter dominates the pulsation amplitudes in the light curve, so pulsations are not apparent to the eye. The dramati 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- ues of Te↵ = 11, 129 ± 115 K, log g = 8.34 ± 0.06, and tion rate. We summarize our findings and conclude i Section 5. KIC 4552982: Bell et al. 2015 3 months: 1 month: 1 week: Brightenings every ~2.7 d, lasting for 4.0-25.0 hr
In K2, Things Got Weirder • In the first K2 campaign we saw another case of outbursts • These outbursts are essentially rogue waves (or freak waves) on a pulsating star! • Never seen before in 40+ years of pulsating white dwarfs PG 1149+057: Hermes et al. 2015 Quiescence (1151.9 s, 1160.8 s, …) In Outburst (999.9 s, 896.6 s, …) g = 14.9 mag
oDAV1 10860 K 0.70 M¤ oDAV2 11060 K 0.64 M¤ oDAV3 10570 K 0.56 M¤ oDAV4 11190 K 0.62 M¤ oDAV5 10850 K 0.53 M¤ K2 keeps finding outbursting white dwarfs: Now 7 known! All aperiodic!
52 WDs within 2000 K of 10900 K do not outburst from K2 30-min-cadence data First 5 outbursting DAVs: Coolest DAVs, deepest convection zones Bell et al. 2016, arXiv: 1607.01392
Enough Energy in One Mode to Power Outbursts (3-day sliding window) PG 1149+057: Hermes et al. 2015 • Of order 1033-1034 erg per outburst • At least 1033 erg kinetic energy in a single mode (e.g., l=1,n=24 ωp )
• White dwarfs are a remarkably homogenous byproduct of stellar evolution – Clustered mean mass, compositional stratification, simpleevolution (just cooling) • Set boundary conditions on stellarand binaryevolution • K2 is changing the game: – Will more than triple measured rotation rates – Common-envelope evolution insights – Evidence of transfer of energy from nonlinear mode coupling – Ensemble asteroseismology,even constraints on convective efficiency White Dwarfs: Beyond The Closing Credits
Pulsations Persist in Outburst • White dwarf Teff = 11,060 K • é 14% mean flux = é 750 K • é >25% flux = é >1500 K Black line is 30-min running mean Event 1 Event 7 Quiescence
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