White dwarfs serve as empirical final boundary conditions to stellar evolution. So how fast do white dwarfs rotate? Expectations if conserving angular momentum from main sequence: 2.5 M¤ ZAMS Prot,initial : 10 hr à 0.6 M¤ WD Prot,WD : 2 min
• Despite 220+ known pulsating WDs, just 16 ground-based measurements: Typical Prot : 0.5-2.0 days (not 2 min!) Most White Dwarfs Rotate at Roughly 0.5-2 Days Stars apparently shed most of their angular momentum before they become white dwarfs. But how?! • Pulsations are the best way to measure white dwarf rotation rates (few known with spots, v sin i not helpful)
Giant Kepler legacy: RGB cores are rotating much slower than any models can predict on their first ascent up AGB Cantiello et al. 2014 observed (Mosser+ 2012) plus Tayler-Spruit (magnetic torques from dynamo-driven fields in radiative regions) with hydrodynamic rotational instabilities (e.g. Eddington- Sweet; Heger+ 2000) Internal gravity waves also insufficient (Fuller+ 2014) pure angular momentum conservation
Cantiello et al. 2014 observed (Mosser+ 2012) plus Tayler-Spruit (magnetic torques from dynamo-driven fields in radiative regions) with hydrodynamic rotational instabilities (e.g. Eddington- Sweet; Heger+ 2000) Internal gravity waves also insufficient (Fuller+ 2014) pure angular momentum conservation Giant Kepler legacy: RGB cores are rotating much slower than any models can predict on their first ascent up AGB Essentially all constraints on angular momentum evolution in stars observed by Kepler are for <3 M¤ stars
Original Kepler Mission (4 years): Just 20 white dwarfs observed, 6 pulsating WDs (just two >3 months) K2 through Campaign 10: >1000 white dwarf candidates observed 35 more pulsating WDs K2 has given us hundreds of candidate pulsating white dwarfs to observe
This outburst phenomenon never seen before in 40+ years of pulsating white dwarf studies See poster #6 by Keaton Bell A surprising discovery with Kepler: Aperiodic Outbursts Quiescent pulsations Outburst
m = +1 m = -1 m = 0 125 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 ± 0.1 day Fourier transform, 75 days data from K2 Campaign 1 200 s 500 s 1000 s We Can Decompose Pulsations into Spherical Harmonics
1 10 100 White Dwarf Rotation Period (hr) 0 2 4 6 8 10 N Kepler & K2 Kawaler (2015) Kepler & K2 have doubled the number of white dwarfs with measured internal rotation periods using asteroseismology Hermes et al. 2017, in prep. None of the stars here are currently in binaries: Representative of single-star evolution Pulsation SplittingsReveal Rotation Rates 0.5 d 1 d 2 d 4 d
1 10 100 White Dwarf Rotation Period (hr) 0 2 4 6 8 10 N Kepler & K2 Kawaler (2015) Kepler & K2 have doubled the number of white dwarfs with measured internal rotation periods using asteroseismology Hermes et al. 2017, in prep. None of the stars here are currently in binaries: Representative of single-star evolution Pulsation SplittingsReveal Rotation Rates 0.5 d 1 d 2 d 4 d
• EPIC 211914185, aka SDSSJ0837+1856 • Kp = 18.9 mag • Observed with short-cadence in C5 as part of a search for WD transits (PI: S. Redfield) The most rapidly rotating pulsating WD is massive SDSS Hermes et al. 2017, ApJL, 841, L2; arXiv: 1704.08690 500 s 200 s 118 s Nyquist ambiguity EPIC 211914185
The most rapidly rotating pulsating WD is massive 500 s 200 s 118 s Hermes et al. 2017, ApJL, 841, L2; arXiv: 1704.08690 Ground-based time-series photometry breaks Nyquist ambiguity EPIC 211914185
The most rapidly rotating pulsating WD is massive SDSS Hermes et al. 2017, ApJL, 841, L2; arXiv: 1704.08690 SOAR spectroscopy yields WD mass Teff : 13,590± 340 K log(g) = 8.434 ± 0.052 MWD : 0.87 ± 0.03 M¤ Using cluster-calibrated initial-to-final mass relation: MProg. : 4.0 ± 0.5 M¤ The fastest-rotating pulsating white dwarf is also the most massive
Could This Massive White Dwarf Be a Merger Byproduct? Hermes et al. 2017, ApJL, 841, L2; arXiv: 1704.08690 Evolution models suggest that <110 s l=1 modes can only exist in white dwarfs with maximally thick hydrogen envelopes à Prefer single-star evolution, (but main-sequence merger not completely ruled out) HST far UV ?
1 10 100 WD Rotation Period (hr) 0.4 0.5 0.6 0.7 0.8 0.9 WD Mass (M⊙ ) 1.0 1.5 2.0 2.5 3.0 3.5 4.0 ZAMS Progenitor Mass (M⊙ ) 1 10 100 White Dwarf Rotation Period (hr) 0 2 4 6 8 10 N Kepler & K2 Kawaler (2015) 1 d 2 d 4 d Hermes et al. 2017, in prep. We Can Finally Probe WD Rotation as a Function of Mass We have obtained WHT/SOAR spectra of all pulsating white dwarfs in Kepler/K2: All will be publicly available soon at k2wd.org
>70% of Field WDs between 0.51-0.73 M¤ (evolved 1.7-3.0 M¤ ZAMS) These WDs rotate at 0.5-2.2 d (WD Prot : 35 ± 28 hr) Link emerging between higher WD mass and faster rotation Expect dozens to 100more pulsating WDs before K2 runs out of fuel! 1 10 100 0 1 2 3 4 N 1.7 2.0 M ZAMS WD Prot = 1.48 ± 0.94 d 1 10 100 0 1 2 3 4 N 2.0 2.5 M ZAMS WD Prot = 1.35 ± 0.74 d 1 10 100 0 1 2 3 4 N 2.5 3.0 M ZAMS WD Prot = 1.32 ± 1.04 d 1 10 100 White Dwarf Rotation Period (hr) 0 1 2 3 4 N 3.5 4.0 M ZAMS WD Prot = 0.17 ± 0.15 d We Can Finally Probe WD Rotation as a Function of Mass
100 101 102 White Dwarf Rotation Period (hr) 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 WD Mass (M ) 100 101 102 White Dwarf Rotation Period (hr) 0 2 4 6 8 10 12 14 N K2 Asteroseismic Asteroseismic J1136+0409 K2 Magnetic Magnetic 1 hr 0.5 d 5 d 2.5 M¤ 6.5 M¤ 4.5 M¤ ZAMS Progenitor: 10 min Burleigh et al. 1999 2 d REJ0317-853, the fastest rotating isolated white dwarf (727.5 s), is both massive (>1.28 M¤ ) and strongly magnetic (>200 MG) REJ0317-853is very likely a merger byproduct
Cantiello et al. 2014 Evolution models including Tayler-Spruit 0.5-2.2 d 1.1 hr tHe = core He ignition Stellar evolution models including Tayler-Spruit still predict too-fast WD rotation, but only by factors of a few
Ground-based rotation rates of pulsating white dwarfs are hard: 1-day (11.57 µHz) aliasing (18.9 hr over 5 nights on a 3.6-m telescope) V = 14.2 mag Actual signal Still, 16 WDs well-constrained with ground-based data: Prot = 2 hr to 2.4 d Giammichele et al. 2015 1 10 100 White Dwarf Rotation Period (hr) 0 1 2 3 4 5 N Kawaler (2015) 1 d 2 d
1 d 2 d 4 d 1.7-3.0 M¤ ZAMS stars evolve to rotate at 0.5-2.2 d as WDs > = 0.621 σ = 0.059 within 1σ of field WDs (0.51-0.73) > = 35 ± 28 hr Tremblay et al. 2016 Hermes et al. 2017, in prep.
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