summary slide white dwarfs are empirically excellent flux standards caveats: binarity, pulsations, and magnetism when variable, white dwarfs reveal dynamic physics
CALSPEC white dwarfs jwst-docs.stsci.edu JJ Hermes, Boston University | STScI/JHU Colloquium | 5 … many frontiers are pushing towards <1% absolute flux calibration see also Calamida et al. 2019; Narayan et al. 2019
Original Kepler Mission: 46 white dwarfs observed, 20 every 1-min K2, through Campaign 18: 2166 white dwarfs observed, 552 every 1-min Kepler K2 provided a large, empirical test of white dwarf flux stability (e.g., Maoz et al. 2015) JJ Hermes, Boston University | STScI/JHU Colloquium | 11
Kp = 16.7 mag, 52,000 K DA JJ Hermes, Boston University | STScI/JHU Colloquium | 12 1 hr 24 hr 6 hr 2 hr Hermes et al. 2017, MNRAS not variable to 0.05%
• Before Gaia: ~35,000 white dwarfs (mostly from SDSS) • Nearly half a million candidates from Gaia DR2 JJ Hermes, Boston University | STScI/JHU Colloquium | 13 Gaia Collaboration, Babusiaux et al. 2018 Gentile Fusillo et al. 2019
JJ Hermes, Boston University | STScI/JHU Colloquium | 14 7.5 log(g) = 9 8.5 8.0 Grey: Only photometry Black: 72% of K2 WDs w/ spectra White dwarf cooling tracks Gaia CMD of all 2166 white dwarfs observed by Kepler
JJ Hermes, Boston University | STScI/JHU Colloquium | 15 7.5 8.5 8.0 Red: WD+MS from SDSS log(g) = 9 Gaia CMD of all 2166 white dwarfs observed by Kepler
Parsons et al. 2017 JJ Hermes, Boston University | STScI/JHU Colloquium | 17 Casewell et al. 2018 71.2 min 100% >11% 68.2 min Some overluminous white dwarfs have nearby brown dwarfs ~58 Jupiter-mass companion surviving common- envelope (shortest- period WD+BD system known) Companion: 51 ± 6 Jupiter masses 0.081-0.087 solar radii
JJ Hermes, Boston University | STScI/JHU Colloquium | 18 7.5 8.5 8.0 Red: WD+MS from SDSS log(g) = 9 We can generally omit suspected binaries as overluminous, based on their Gaia CMD position
JJ Hermes, Boston University | STScI/JHU Colloquium | 19 7.5 8.5 8.0 log(g) = 9 We can generally omit suspected binaries as overluminous, based on their Gaia CMD position HZ 43 DA1 (50,000 K) Nearby (3”) dM
JJ Hermes, Boston University | STScI/JHU Colloquium | 20 Sub-stellar transiting systems are rare, but present Gänsicke et al. 2016; Rappaport et al. 2016 Vanderburg et al. 2015 WD 1145+017: a disintegrating asteroid transiting a white dwarf >50%
JJ Hermes, Boston University | STScI/JHU Colloquium | 21 Vanderbosch et al. arXiv: 1908.09839 ~107 days >45% Sub-stellar transiting systems are rare, but present ZTF 0139+5245: transiting debris far outside the white dwarf tidal disruption radius 2019
JJ Hermes, Boston University | STScI/JHU Colloquium | 24 7.5 8.5 8.0 Blue: WD Pulsations log(g) = 9 Pulsations in white dwarfs are confined to narrow instability strips in temperature 30,000 K 20,000 K 10,000 K
170,000 K have the highest number of detected modes. The first class of pulsating s 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 H He CO Peering 6 Gyr into our Sun’s future… white dwarfs have non-radial g-mode pulsations driven by partial ionization of He or H See reviews by: Winget & Kepler 2008 Fontaine & Brassard 2008 Althaus, Córsico, Isern & García-Berro 2010 JJ Hermes, Boston University | STScI/JHU Colloquium | 25
JJ Hermes, Boston University | STScI/JHU Colloquium | 26 7.5 8.5 8.0 Blue: Pulsations log(g) = 9 >95% of isolated white dwarfs are <1% constant on 1-hr to 10- d timescales (omitting known/likely binaries & pulsators)
white dwarfs are empirically excellent flux standards caveats: binarity, pulsations >95% of isolated WDs are <1% constant on 1-hr to 10-d timescales eclipses, reflection effect from close companions, (debris) transits pulsations
1000 s 200 s 500 s 125 s White Dwarfs: g-modes (buoyancy restoring force) BiSON; Thompson et al. 2003 5 min 4 min 6 min Solar p-modes JJ Hermes, Boston University | STScI/JHU Colloquium | 29
Giammichele et al. 2015 5 nights on 3.6-m CFHT on Mauna Kea: V = 14.2 mag Actual signal The view from one telescope on the ground JJ Hermes, Boston University | STScI/JHU Colloquium | 30
JJ Hermes, Boston University | STScI/JHU Colloquium | 33 1 d 2 d 4 d None of the stars are currently in binaries: representative of single-star evolution of mostly 1-3 M¤ stars Isolated pulsating WDs rotate between 0.5-2.2 days k2wd.org Hermes et al. 2017, ApJS
JJ Hermes, Boston University | STScI/JHU Colloquium | 34 7.5 8.5 8.0 log(g) = 9 >95% of isolated white dwarfs are <1% constant on 1-hr to 10- d timescales (omitting known/likely binaries & pulsators) Blue: Pulsations
JJ Hermes, Boston University | STScI/JHU Colloquium | 35 7.5 8.5 8.0 log(g) = 9 >95% of isolated white dwarfs are <1% constant on 1-hr to 10- d timescales (omitting known/likely binaries & pulsators) Orange: Spots Blue: Pulsations
Kp = 17.7 mag 2.0409 days >4% Kp = 18.5 mag 2.2229 days >6% K2 uncovered a modest population of spotted white dwarfs! JJ Hermes, Boston University | STScI/JHU Colloquium | 36 Hermes et al. 2017, MNRAS
Long-cadence (30-min) exposures showed a significant peak in FT at 3 ppt = 0.3% 5 hr 2 hr 1 hr SDSS νNyq JJ Hermes, Boston University | STScI/JHU Colloquium | 38 SDSSJ082547.52+174818.4 g=18.9 mag K2 Campaign 5 A strongly magnetic WD observed in K2
JJ Hermes, Boston University | STScI/JHU Colloquium | 39 0.0 0.5 1.0 1.5 2.0 Rotational Phase °6 °4 °2 0 2 4 6 Relative Flux (%) νNyq 2νNyq 3νNyq 2 hr 30 min 20 min Folded SOAR High-speed photometry from SOAR shows this was a super- Nyquist signal at 18 min! K2 Campaign 5 SOAR, 5-pt smoothed A strongly magnetic WD observed in K2 rotating at 18 min! 18.0441 min >7%
JJ Hermes, Boston University | STScI/JHU Colloquium | 40 In K2 Campaign 10, CCD Module 4 failed one week into the campaign νNyq 1% FAP K2 discovered the fastest-rotating isolated WD Reding et al., in prep.
JJ Hermes, Boston University | STScI/JHU Colloquium | 41 A 317.287 s (5.29 min) signal! νNyq 3νNyq 7νNyq 11νNyq Reding et al., in prep. K2 discovered the fastest-rotating isolated WD 317.287 s >10% The signal seen in 30-min K2 data was reflected off the Nyquist 11 times!
JJ Hermes, Boston University | STScI/JHU Colloquium | 42 Spectrum changes occur along the 315.96 s (5.3 min) spin period! 5.1 MG Joins GD 356 as the only DAe SOAR The fastest-rotating isolated WD has magnetic emission! Reding et al., in prep.
white dwarfs are empirically excellent flux standards caveats: binarity, pulsations, and magnetism >95% of isolated WDs are <1% constant on 1-hr to 10-d timescales eclipses, (debris) transits, pulsations, spots It is possible that these spotted white dwarfs (strongly magnetic, rapidly rotating) are connected to WD+WD mergers à failed SNe Ia
Gaia provides an empirical look at white dwarf variability JJ Hermes, Boston University | STScI/JHU Colloquium | 44 Gaia Collaboration, Babusiaux et al. 2018 Gaia Collaboration, Evans et al. 2018 N obs
Hermes et al. 2018; 2020, in prep. see also Eyer et al. 2019, arXiv: 1912.07659 >46,000 WDs within 200pc WDs with the top 1% most sca7er for their magnitude… …cluster near WD instability strips! Cooling track, 0.6 M⊙ WD Using Gaia’s empirical photometric uncertainties … … is a good way to select (against) highly variable white dwarfs! JJ Hermes, Boston University | STScI/JHU Colloquium | 45
>46,000 WDs within 200pc Cooling track, 0.6 M⊙ WD Most of these variable WDs in Gaia are pulsating or spotted JJ Hermes, Boston University | STScI/JHU Colloquium | 46 Hermes et al. 2018; 2020, in prep. 27.9 min >4% >8% 16.6 min
WDs Evolve (Cool) à We Have Only Scratched the Surface of Analyzing the ~100 Pulsating White Dwarfs Observed by Kepler JJ Hermes, Boston University | STScI/JHU Colloquium | 48 Blue: Observed by Kepler Open: Ground-based SOAR
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 et al. 2015 3 months: 1 month: 1 week: Brightenings every ~2.7 d, lasting for 4.0-25.0 hr An unexpected, dynamic discovery in Kepler: outbursts JJ Hermes, Boston University | STScI/JHU Colloquium | 49
Quiescent pulsations (Dominant >800 s) PG 1149+057: Hermes et al. 2015 see also Bell et al. 2016 recurrence time: chaotic; days to weeks duration: 2-20 hr excess energy: 1033-34 erg 15% flux increase: 700 K T eff increase An unexpected, dynamic discovery confirmed in K2: outbursts JJ Hermes, Boston University | STScI/JHU Colloquium | 50
All white dwarfs pulsate at the appropriate temperature, and it appears all outburst at some point, too. This is likely a new phase of stellar evolution! Outbursting DAVs JJ Hermes, Boston University | STScI/JHU Colloquium | 51 Blue: Observed by Kepler Red: Outbursting DAV Open: Ground-based
GD 1212: Hermes et al. 2014 GD 1212: Data from the 9-day K2 engineering test run V=13.3 mag JJ Hermes, Boston University | STScI/JHU Colloquium | 52 2014
These outbursts may be responsible for shutting down pulsations in white dwarfs Outbursting DAVs JJ Hermes, Boston University | STScI/JHU Colloquium | 55 (see Wu & Goldreich 2001) Blue: Observed by Kepler Red: Outbursting DAV Open: Ground-based
n l=1 l=2 Solar p-modes, evenly spaced in frequency JJ Hermes, Boston University | STScI/JHU Colloquium | 56 How might outbursts work? Parametric resonance.
n l=1 l=2 JJ Hermes, Boston University | STScI/JHU Colloquium | 57 White dwarf g-modes, evenly spaced in period How might outbursts work? Parametric resonance.
l=1 l=2 Adiabatic Model: 11,245 K, 0.632 M ¤ , 10-4.12 M H /M WD Observed: 11,060(170) K, 0.64(0.03) M ¤ (Romero et al. 2012) (Gianninas et al. 2011) à driven damped ß outbursts are likely “limit cycles arising from sufficiently resonant 3-mode couplings between overstable parent modes and pairs of radiatively damped daughter modes” Luan & Goldreich 2018 JJ Hermes, Boston University | STScI/JHU Colloquium | 59
l=1 l=2 Adiabatic Model: 11,245 K, 0.632 M ¤ , 10-4.12 M H /M WD Observed: 11,060(170) K, 0.64(0.03) M ¤ (Romero et al. 2012) (Gianninas et al. 2011) à driven damped ß ω p = 897.7 µHz (l=1, n=24) JJ Hermes, Boston University | STScI/JHU Colloquium | 60
These outbursts may be responsible for shutting down pulsations in white dwarfs Outbursting DAVs JJ Hermes, Boston University | STScI/JHU Colloquium | 62 (see Wu & Goldreich 2001) Blue: Observed by Kepler Red: Outbursting DAV Open: Ground-based
white dwarfs are empirically excellent flux standards caveats: binarity, pulsations, and magnetism when variable, white dwarfs reveal dynamic physics summary slide >95% of isolated WDs are <1% constant; Gaia can assess variability eclipses, (debris) transits, pulsations, outbursts, spots some spots may reveal failed Type Ia supernovae outbursts (nonlinear mode coupling) could quench observable pulsations
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