http://jjherm.es J.J. Hermes Hubble Fellow University of North Carolina at Chapel Hill Rumblings in the Stellar Graveyard: White Dwarf Pulsations with K2 and TESS
U. North Carolina: Chris Clemens, Bart Dunlap, Erik Dennihy, Josh Fuchs, Stephen Fanale U. Warwick: Boris Gaensicke, Roberto Raddi, N. P. Gentile Fusillo, P.-E. Tremblay, Paul Chote U. Texas: Keaton J. Bell, Mike Montgomery, Don Winget, Zach Vanderbosch + Steve Kawaler, Agnes Bischoff-Kim, Judi Provencal, S.O. Kepler, Alejandra Romero Rumblings in the Stellar Graveyard: White Dwarf Pulsations with K2 and TESS
Kepler/K2 pulsating white dwarfs have provided opportunity to: • Establish the range of WD envelope masses • Empirically constrain the efficiency of convection in WDs • Witness nonlinear mode coupling • Measure the endpoints of angular momentum evolution • Test for radial differential rotation All without extremely detailed asteroseismic fits
Sandra Greiss et al. 2016 http://www2.warwick.ac.uk/fac/sci/physics/ research/astro/research/catalogues/kis stars WDs Just two known WDs at Kepler launch In 2012 we started Kepler INT Survey (U,g,r,i,Hα ) down to 20th mag: Found 10 new pulsating WDs
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
White Dwarfs are Finally Getting the Space Treatment PG1159-035, V=14.9 mag -- poster child for WET (March 1989, 9 sites, 90.8% duty cycle over 12.0 days) SDSSJ0106+0145, g=16.2 mag -- typical K2 light curve (K2 Campaign 8, 96.0% duty cycle over 78.7 days) Winget et al. 1991 Hermes et al. 2017, in prep. l = 1 l = 2 l = 2 l = 2 l = 1 l = 1
White Dwarfs Seismologist’s Dilemma: Often Few Modes For any one hot DAV: 1. Small number of independent modes observed 2. Best model hinges on mode identification 3. Hidden free parameters (core profile, layer masses), with 8+ degrees of freedom e.g., WD0111+0018, 6 hr ground-based data e.g., WD0111+0018, 78.7-d K2 data Hermes et al. 2013 Hermes et al. 2017, in prep.
White Dwarfs Seismologist’s Dilemma: Often Few Modes For any one hot DAV: 1. Small number of independent modes observed 2. Best model hinges on mode identification 3. Hidden free parameters (core profile, layer masses), with 8+ degrees of freedom e.g., WD0111+0018, 6 hr ground-based data e.g., WD0111+0018, 78.7-d K2 data Hermes et al. 2013 Hermes et al. 2017, in prep.
Spectroscopy Yields Effective Temperatures and Masses SDSS SOAR spectroscopy yields WD mass We have obtained SOAR spectra of all DAVs observed by K2 so far: k2wd.org
The Kepler/K2 DAV Instability Strip Our DAVs span all edge to edge of DAV instability strip (We will address purity by the end of K2) Hermes et al. 2017, k2wd.org Josh Fuchs et al. 2017, in prep. Stay tuned: Recent UNC Ph.D. Josh Fuchs is exploring strip with minimal systematics (same instrument, methods, models) Empirical edges by Tremblay+ 2015
239 eigenperiods from 75 hot DAVs (mostly ground-based, no combination frequencies) Histogram of eigenperiods (again, no combination frequencies) 0 5 10 15 20 25 30 50 100 150 200 250 300 350 400 450 500 0 5 10 15 20 25 30 50 100 150 200 250 300 350 400 450 Clemens et al. 2017, in prep. Insights from the Aggregated Periods of DAVs Mode Amplitude (ppt) N Mode Period (s) Mode Period (s)
38 eigenperiods from 16 hot DAVs (l=1, m=0, no combination frequencies, mostly Kepler/K2) 0 1 2 3 4 5 6 7 8 50 100 150 200 250 300 350 400 450 Clemens et al. 2017, in prep. Insights from the Aggregated Periods of DAVs n = 1 l = 1 n = 2 l = 1 n = 3 l = 1 Kepler makes mode identification relatively trivial Mode Period (s) N n=1 n=2 n=3 n=4
Kepler Has Revealed a Dichotomy of Mode Linewidths Two modes within the same DAV show very different linewidths Many of the broadened modes appear relatively Lorentzian in shape (Absolutely no way to have made these measurements before Kepler) Hermes et al. 2017, in prep.
Kepler Has Revealed a Dichotomy of Mode Linewidths Results from fitting Lorentzians to the 27 DAVs through K2 Campaign 8: Clear dichotomy at ~800 s Hermes et al. 2017, in prep. Stephen Fanale
Kepler Has Revealed a Dichotomy of Mode Linewidths • Damping rather than driving important for broadening; phase incoherence Broadened modes: bounded by the base of the convection zone! Mike Montgomery et al. 2017, in prep.
Kepler Has Revealed a Dichotomy of Mode Linewidths Mike Montgomery et al. 2017, in prep. 0.4 0.6 0.8 1.0 1.2 1.4 ↵ 0.675 0.700 0.725 0.750 0.775 0.800 0.825 0.850 Fraction Correct 0 200 400 600 800 1000 1200 1400 Period (s) 0 1 2 3 4 5 6 HWHM (µHz) ML2/↵ = 0.6 0 200 400 600 800 1000 1200 1400 Period (s) 0 1 2 3 4 5 6 HWHM (µHz) ML2/↵ = 0.9 Given spectroscopic Teff /log(g), we can calculate the critical period for a mode reflecting off the base of the convection zone This empirically constrains ML2/α
Not the Whole Linewidth Story – Lots of Resonances First 5 days: 885.243(0.057) µHz Last 5 days: 888.285(0.067) µHz Some longer-period modes appear to change very quickly in frequency: Nonlinear mode coupling appears to be the only way the star can transfer that much energy so quickly!
Not the Whole Linewidth Story – Lots of Resonances Weikai Zong et al. 2016 Zong et al. have explored three-mode coupling to explain amplitude/phase changes in compact objects
This outburst phenomenon never seen before in 40+ years of pulsating white dwarf studies A surprising discovery with Kepler: Aperiodic Outbursts Quiescent pulsations (1151.9 s, 1160.8 s, …) In Outburst (999.9 s, 896.6 s, …) PG 1149+057: Hermes et al. 2015
A surprising discovery with Kepler: Aperiodic Outbursts Keaton Bell et al. 2017 We see outbursts in 6 of the 27 DAVs observed through Campaign 8 These are aperiodic brightenings causing up to 15% mean flux increases (>750 K Teff increases) Pulsations persist in outburst, and are consistent with the star having a thinner convection zone
A surprising discovery with Kepler: Aperiodic Outbursts Keaton Bell et al. 2017 We see outbursts in 6 of the 27 DAVs observed through Campaign 8 These are aperiodic brightenings causing up to 15% mean flux increases (>750 K Teff increases) Pulsations persist in outburst, and are consistent with the star having a thinner convection zone
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 Rotation Rates Usually Fall Readily from K2 Data 0.5 d 1 d 2 d 4 d
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 The fastest-rotating pulsating white dwarf (1.13 hr) is also the most massive (0.87 M¤ ) – descended from a single 4.0 M¤ ZAMS progenitor Hermes et al. 2017c, ApJL, 841, L2; arXiv: 1704.08690
>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 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
PG 0112+104: Hermes et al. 2017a l=1 modes l=2 modes The Most Evolved Test of Radial Differential Rotation PG 0112+104 is a ~31,000 K pulsating He-atmosphere WD (DBV)
The Most Evolved Test of Radial Differential Rotation PG 0112+104: Hermes et al. 2017a l=1 modes n (n) n=2 n=3 n=4 n=5 n=6 Frequency splittings and overtone spacings behave in concert: Modes trapped to different depths Early hints: rigid rotation Period spacing difference (s)
We also see a surface spot Surface: 10.17404 hr Towards core: 10.1±0.9hr PG 0112+104: Hermes et al. 2017a 10.17404 hr surface spot rotation period The Most Evolved Test of Radial Differential Rotation Using l=1 and l=2 modes we measure a rotation period of 10.1±0.9 hr in PG 0112+104 (better asteroseismic modeling will improve this uncertainty)
The Most Evolved Test of Radial Differential Rotation “PG0112+104 rotates rigidlyover its outer 70% in radius with a period of Prot = 10.18 ± 0.27 hr” Based on full seismic model See Poster 2.7 by Noemi Giammichele et al.
WG8: TESS White Dwarf Candidate Follow-Up Ongoing! Zsófia Bognár et al., in prep. Time (d) Amplitude (mag) KonkolyObservatory: New I=14.3 mag DAV! S. Charpinet
Conclusion: More White Dwarfs Yield Firmer Conclusions! Kepler/K2 pulsating white dwarfs provide opportunity to: • Establish the range of WD envelope masses § Most have canonically thick (10-4 MH /M ★ ) hydrogen envelopes • Empirically constrain the efficiency of convection in WDs § ML2/α > 0.8 from mode linewidths bounded by base of convection zone • Witness nonlinear mode coupling § Outbursts (and frequency changes) on day-week timescales • Measure the endpoints of angular momentum evolution § Endpoints of 1.7-3.0 M¤ stars rotate at 35 ± 28 hr, but >3.0 M¤ faster • Test for radial differential rotation § White dwarfs appear to rotate rigidly, but more tests on the way! All of these constraints significantly improve by observing more pulsating WDs with K2 and TESS!
>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 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
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