Rogue Waves on Pulsating White Dwarf Stars U. Texas: Keaton J. Bell, Mike Montgomery, Don Winget U. Warwick: Boris Gaensicke, Paul Chote, Roberto Raddi, Nicola Gentile Fusillo, Dave Armstrong U. North Carolina: Chris Clemens, Bart Dunlap, Erik Dennihy, Josh Fuchs + Steve Kawaler, Alex Gianninas, Pier-Emmanuel Tremblay, Agnes Bischoff-Kim
White dwarfs are compact: This 0.25 M¤ + 0.50 M¤ WD+WD is the most compact detached binary system currently known Brown et al. 2011; Hermes et al. 2012 Mean Earth-Moon separation (J0651+2844, 12.75-min) 1 R¤
Some WD+WD binaries are so close that gravitational waves rob energy from the orbit, causing them to rapidly shrink J0651+2844 PSR B1913+16 (aka Hulse-Taylor binary pulsar) dPorb /dt = -0.289 ms/yr dPorb /dt = -0.076 ms/yr Hermes et al. 2012 Weisberg et al. 2010
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
Van Grootel et al. 2012 The blue edge where pulsations turn on is well predicted (Brickhill 1991, Goldreich& Wu 1999) (1999). We have reproduced some of their results in Figure 9. These calculations were obtained through full nonadiabatic cal- culations of complete stellar models under the assumption that the outer convection zone (due to H recombination) reacts in- stantaneously to the perturbations caused by the oscillatory mo- tions in the star. This is appropriate because, as first pointed out by Brickhill (1983), the convective turnover timescale is about 2 to 3 orders of magnitude smaller than the measured pulsation periods of ZZ Ceti stars near the blue edge. This is exactly the opposite of the frozen convection hypothesis used in all of the early nonadiabatic investigations of pulsating DA and DB white dwarfs. As indicated in Figure 9, the results do de- pend on the assumed version of the mixing-length theory used in the construction of the equilibrium stellar models. These should not be confused with the model atmospheres and the synthetic spectra computed by Bergeron et al. (1995) whereby a calibration of the mixing-length theory, the ML2=α ¼ 0:6 parametrization, was achieved as mentioned above. That cali- bration, we recall, only applies in the atmospheric layers, in regions where the observable flux comes from. In comparison, the theoretical blue edge is most sensitive to the physical conditions found at the base of the convection zone (see the next section), well below the atmospheric layers. In fact, the location of the theoretical blue edge is a measure of the FIG. 9.—Instability domain in the log g À Teff diagram for the ZZ Ceti stars. The positions of the pulsators are indicated by the filled circles, while those of the nonvariable stars are given by the open circles. The error cross in the lower 1054 FONTAINE & BRASSARD 4000 5000 6000 7000 8000 9000 10000 11000 12000 0 500 1000 1500 2000 T eff (K) Period (s) TDC, β=(0,0), ML2/ α=1.0, 0.60 M s 5000 6000 7000 8000 9000 10000 11000 12000 0 500 1000 1500 2000 2500 T eff (K) Period (s) TDC, β=(1,−1), ML2/ α=1.0, 0.60 M s Fig. 7. Periods (in seconds) of the excited l = 1 g-modes as functions of the effective temperature along the 0.6 M⊙ evolutionary sequence computed with the detailed atmosphere modeling. The size of a dot is a measure of the excitation of that particular mode. Top panel: FC. Middle panel: TDC with β = (0, 0). Bottom panel: TDC with β = (1, −1). a 2 f T a b o i a p c o f o p d m t a d w m t o g T p m e s i e The red edge where pulsations turn off is observed several thousand K hotter than non-adiabatic calculations predict
Mike Montgomery • Pulsations: periodic brightness changes, caused by surface temperature variations • White dwarfs only show nonradial pulsations (strong surface gravity) • g-modes with periods of 100-1400 s (1.5-23 min)
THE MAIN LOG Observations at Maidanak observatory in Uzbekistan. Aug 1994 Observers: E. Meistas, and local assistant Alexey V. Chernyshev Jul 27th Uzbeks introduced new rules for the visas ... spent long 8 night hours in the old stinking Russian bus, which, using longest possible route and stopping more than ten times for the repairs, after which passengers were supposed to push the bus to start the engine, brought us to Shakhrisabz. Jul 28th Old military jeep, which exhaust went more inside than via its pipes, after 5 hours brought us to Maidanak [Observatory]. ... Some windows of our living house were broken, no clean sheets, they had nobody to prepare meals, and they were poor even with the food supply: no butter, meat, sugar. Running water system was not working anymore, not to mention hot water, which we had here always before.
THE MAIN LOG Observations at Maidanak observatory in Uzbekistan. Aug 1994 Observers: E. Meistas, and local assistant Alexey V. Chernyshev Jul 29th I checked telescope; tracking and positioning were working, but telescope mirrors needed cleaning... Jul 30th Managed to repair distiller and to get 3 l of water late in the evening only. Decided to wash mirrors next day. Still lots of yellow Afghanistan dust in the sky. Jul 31st Washed mirrors, cleaned telescope inner surfaces from thick dust layer. Started the full scale system test.
THE MAIN LOG Observations at Maidanak observatory in Uzbekistan. Aug 1994 Observers: E. Meistas, and local assistant Alexey V. Chernyshev Aug 1st All day no clouds, but wind increasing to the evening. Worked all night. Aug 3rd All day clear sky with some clouds. Quite strong wind in day time but diminished before the night.
THE MAIN LOG Observations at Maidanak observatory in Uzbekistan. Aug 1994 Observers: E. Meistas, and local assistant Alexey V. Chernyshev Aug 5th It was first night there on the mountain without me. I was at that time in Kitab Hospital severely injured by the Tashkent Astrophysical Institute Director son Iskander Yuldashbaev, apparently mentally ill young man of about 21.
THE MAIN LOG Observations at Maidanak observatory in Uzbekistan. Aug 1994 Observers: E. Meistas, and local assistant Alexey V. Chernyshev Aug 5th It was first night there on the mountain without me. I was at that time in Kitab Hospital severely injured by the Tashkent Astrophysical Institute Director son Iskander Yuldashbaev, apparently mentally ill young man of about 21. He did some cleaning, then came to my room ... suddenly saying no words grabbed my hair with his left hand and hit my throat with a broken knife from our kitchen. I jumped from my chair, ran in horror out of the house, but he managed to hit me twice into my back until I was out. I ran to the Russian house for the help all in the blood. It was no phone connection with outside world and two of them had to run all the way to Maidanak to soldiers, and in three hours at last I was delivered to Kitab hospital in rather weak condition.
THE MAIN LOG Observations at Maidanak observatory in Uzbekistan. Aug 1994 Observers: E. Meistas, and local assistant Alexey V. Chernyshev Aug 5th It was first night there on the mountain without me. I was at that time in Kitab Hospital severely injured by the Tashkent Astrophysical Institute Director son Iskander Yuldashbaev, apparently mentally ill young man of about 21. He did some cleaning, then came to my room ... suddenly saying no words grabbed my hair with his left hand and hit my throat with a broken knife from our kitchen. I jumped from my chair, ran in horror out of the house, but he managed to hit me twice into my back until I was out. I ran to the Russian house for the help all in the blood. It was no phone connection with outside world and two of them had to run all the way to Maidanak to soldiers, and in three hours at last I was delivered to Kitab hospital in rather weak condition. ... He is in a custody now and cannot say the reason either, says he did not like the way I looked at him. But he was smart enough to steal before that event good sum of my money ... Until helicopter arrived I explained the basics of the work with the quilt program to Alexey - my assistant. Luckily I trained him in previous nights on almost everything...
THE MAIN LOG Observations at Maidanak observatory in Uzbekistan. Aug 1994 Observers: E. Meistas, and local assistant Alexey V. Chernyshev Aug 8th All day clear sky, in the evening no clouds too, but bad seeing. I was in Kitab at russian astronomer place all throat bandaged, practically defected from Kitab hospital, where black bugs were running on the walls at night even in the patient's beds, over the face too. Throat is badly swollen and hurts. Aug 10th Alexey arrived from the Maidanak in the afternoon. Everything seems OK. Aug 11th Aug 12th I lived in the Russian hotel in Kitab ... working with data: writing logs, marking bad points, making .op files. Tomorrow night Uzbeks promised to bring me to the Samarkand airport. My throat is swollen, still hurts and ugly. END OF CAMPAIGN HERE IN THE UZBEKISTAN ------------------------------------------------------------------------
g=17.1 mag DAV in K2 Campaign 5 Just 12 hr of a 78.8-day K2 light curve My throat is swollen, still hurts and ugly. END OF CAMPAIGN HERE IN THE UZBEKISTAN ------------------------------------------------------------------------
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 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, 4.0-25.0 hr durations
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 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. 3 months: 1 month: 1 week: KIC 4552982: Bell et al. 2015 1000 s 400 s Prot ≈ 0.73 d
A Second Case of Outbursts in a Cool DAV • In K2 Campaign 1 we saw another case • These outbursts are essentially rogue waves (or freak waves) on a pulsating star! PG 1149+057: Hermes et al. 2015, ApJ, 810, L5
A Second Case of Outbursts in a Cool DAV • No companion earlier than L3: This is happening on the white dwarf SDSS image K2 pixels 11,060 K, log(g)=8.06 model (3σ uncertainties smaller than each point) PG 1149+057: Hermes et al. 2015, ApJ, 810, L5
A Second Case of Outbursts in a Cool DAV K2 Campaign 1 full light curve Prot ≈ 1.2 d PG 1149+057: Hermes et al. 2015, ApJ, 810, L5 Outbursts every ~8 d, 9.3-36.4 hr durations
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
Potential Outburst Mechanisms in Cool DAVs • Magnetic flares 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 related to pulsations Base of convection zone Surface Deeper
Potential Outburst Mechanisms in Cool DAVs • Possible mechanism: Nonlinear mode coupling, via parametric instability (Wu & Goldreich2001) • Mode density increases for cooler DAVs: more modes with which to couple The first two outbursting DAVs
A Second Case of Outbursts in a Cool DAV (3-day sliding window) PG 1149+057: Hermes et al. 2015, ApJ, 810, L5 • ~1034 erg per outburst • At least 1038 erg kinetic energy in ωp
0 500 1000 1500 2000 2500 Frequency (µHz) 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 Amplitude (%) 1% FAP for a single peak 1% FAP for three peaks 500 s 1000 s A Campaign 5 white dwarf showed at least 13 outbursts recurring every ~4.7 d, 19.4-52.1 hr durations Bell, Hermes et al., in prep. A Third Case of Outbursts in a Cool DAV
Hot off the space telescope: A Campaign 6 white dwarf showed at least 33 outbursts, recurring every ~2.4 d, 6.8-17.0 hr durations Bell, Hermes et al., in prep. 1000 s 500 s A Fourth Case of Outbursts in a Cool DAV!
Why Have We Never Seen This Before? All ground-based photometry, including WET, traditionally divides out second-order polynomial to compensate for differential extinction GD66, taken 2007-11-07 from McDonald Observatory
Three Commonalities Among 4 Outbursting WDs 1. Repeated, aperiodic outbursts; recurrence of days, duration of hours 2. Temperatures at the cool, red edge of the DAV instability strip 3. Long-period (800-1400 s) pulsations with a shorter-period (350-500 s) l=1 mode visible
• Confirmed that planets are disrupted onto white dwarfs • Outbursts occur in the coolest pulsating white dwarfs with the deepest convection zones. Is this how WD pulsations shut down? K2 is Changing the Way we Look at WDs
§ Ensemble asteroseismology of white dwarfs § Diversity in envelope thicknesses?Affect on cooling ages. § Up to 100 white dwarf rotation rates § Incidence of magnetism in white dwarfs § More remnant planetary systems § More close, evolved binaries § Refining WDs as “Flux Standards” K2 begins observing Field 9 (towards Earth) in a week. It has fuel to make it beyond Field 18 (barring safe mode events). What More Can We Expect from K2?
• Spherical symmetry = spherical harmonics • Eases mode identification • 1 month Kepler data • Frequency splittingsyield Prot = 0.9 ± 0.2 day l = 1 modes m = +1 m = -1 m = 0 1000 s 200 s 500 s 125 s
l = 0, m = 0 l = 2, m = 2 l = 2, m = 0 Radial Modes Nonradial Modes White Dwarf Pulsations in Observation • Pulsations: periodic brightness changes, caused by surface temperature variations • White dwarfs only show nonradial pulsations (strong surface gravity) Róbert Szabó
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