Kepler and TESS weigh in on the purity of the DAV instability strip

Conference presentation, 20 min. July 2024: TASC8/KASC15 Workshop, Porto, Portugal.

Transcript

1. Kepler and TESS weigh in on the purity of the DAV instability strip J.J. Hermes
2. DA = Pure Hydrogen Atmosphere WD Aerts, Christensen-Dalsgaard, Kurtz 2010 Kepler and TESS weigh in on the purity of the DAV instability strip 4000 4500 5000 5500 6500 DA DB DZ DQ DC adapted from Wesemael et al. 1993
3. Kepler and TESS weigh in on the purity of the DAV instability strip Bergeron et al. 2004 “… ZZ Ceti stars occupy a pure region in the log(g)-Teff plane, a region where no nonvariable stars are found … ZZ Ceti pulsators represent a phase through which all DA stars must evolve.”
4. “…the vast majority, and probably all, of the hydrogen-rich white dwarfs evolve to become ZZ Ceti variables…” This is a >40-year-old debate in asteroseismology Fontaine et al. 1982 “We question the purity of the DAV instability strip, as we find several nonvariables within.” Mukadam et al. 2004
5. Two Ways to Explain a DA That Doesn’t Pulsate (NOV) 1. The temperature determination was incorrect 2. The DAV candidate was not observed long enough Gianninas et al. 2005 "Our new results indicate, but do not prove, a pure instability strip, because there are still other NOV stars that need to be observed again." Castanheira et al. 2007 "we revisit the analysis by Mukadam et al. ... Their erroneous conclusion of an instability strip containing several nonvariable stars is traced back to the low signal-to-noise ratio spectroscopic observations used in that survey."
6. Theory: All DA Should Pulsate When H is Partially Ionized Steen, Hermes, Guidry et al. 2024 (shown as stars are all Gaia variable WDs – we can measure < 1000 s signals from the Gaia light curves alone, even though average revisit time is >45 days!) most variable WDs cluster at DAV instability strip
7. a) short-period modes at blue edge have low amplitudes b) amplitudes start to increase as periods increase c) middle of the instability strip has very high amplitudes d) mode coupling leads to dramatic outbursts e) amplitudes die off strongly at <10,500 K Hermes et al. 2017, ApJS What Kepler/K2 said about the DAV instability strip:
8. Gaia1 Uniform Atmospheric Parameters of Known DAV known DAV from Bognar+ 2016, Hermes+ 2017, Vincent+ 2020, Guidry+ 2021, Romero+ 2022, 2024 empirical blue and red edges from Gaia SED fits 1given some quality cuts on ruwe, bp_rp_excess, & astrometric errors
9. TESS Has Observed >120 DA Near The Strip Romero et al. 2022 (first 74 TESS DAV) Romero et al. 2024 (32 new TESS DAV), arXiv: 2407.07260 Quality cuts applied for Gaia-derived Teff /log(g) parameters (Gentile Fusillo et al. 2021) empirical blue and red edges from Gaia SED fits
10. TESS Has Discovered ~100 New Bright DAVs Romero et al. 2022 (first 74 TESS DAV) Romero et al. 2024 (32 new TESS DAV), arXiv: 2407.07260 Quality cuts applied for Gaia-derived Teff /log(g) parameters (Gentile Fusillo et al. 2021) empirical blue and red edges from Gaia SED fits
11. ~20 DA Inside Instability Strip Are NOV to <0.5% Quality cuts applied for Gaia-derived Teff /log(g) parameters (Gentile Fusillo et al. 2021) empirical blue and red edges from Gaia SED fits NOV2 = Not variable to 2 ppt (0.2%)
12. However, Crowding and Sensitivity Limit TESS e.g., TIC 382303117 (G=16.9 mag) is NOV1.3 (not variable to <0.13%) in TESS but is a known DAV: HE 0532-5605 NOV2 = Not variable to 2 ppt (0.2%)
13. Kepler/K2 Observed ~2300 WDs (Though Many Faint) Hermes et al. 2017 (27 DAV) Hermes et al., in prep. (75 total DAV) Quality cuts applied for Gaia-derived Teff /log(g) parameters (Gentile Fusillo et al. 2021)
14. 22/26 Bright DA Inside the Instability Strip Pulsate in K2 Hermes et al. 2017 (27 DAV) Hermes et al., in prep. (75 total DAV) Quality cuts applied for Gaia-derived Teff /log(g) parameters (Gentile Fusillo et al. 2021)
15. At Least 3 DA are NOV to <0.1% amplitude in K2 Hermes et al. 2017 (27 DAV) Hermes et al., in prep. (75 DAV) EPIC 220488720 NOV0.5 11480(440) K 7.934(0.085) EPIC 220670150 NOV1.0 11800(390) K 7.947(0.070) EPIC 248474603 NOV0.6 10830(170) K 8.083(0.036) We have spectroscopic follow-up from SOAR for all DA close to the instability strip
16. One NOV Is Actually a Strongly Magnetic DA! Hermes et al. 2017 (27 DAV) Hermes et al., in prep. (75 DAV) EPIC 220488720 NOV0.5 11480(440) K 7.934(0.085) EPIC 220670150 NOV1.0 11800(390) K 7.947(0.070) EPIC 248474603 NOV0.6 10830(170) K 8.083(0.036) EPIC 211426122 NOV0.4 ~200 kG DAH We have spectroscopic follow-up from SOAR for all DA close to the instability strip
17. One NOV Is Actually a Strongly Magnetic DA! Hermes et al. 2017 (27 DAV) Hermes et al., in prep. (75 DAV) EPIC 211426122 NOV0.4 ~200 kG DAH We have spectroscopic follow-up from SOAR for all DA close to the instability strip EPIC 220670150 NOV1.0 11800(390) K 7.947(0.070) Hβ Hβ
18. The DAV Instability Strip is Still Remarkably Pure • Despite the varied histories before becoming a WD, ~all DA pulsate • Those that don’t pulsate are likely magnetic • Need a good Teff and strong limits (<0.05%) • Caveat: Not all WD are DA (~30% are non-DA) • Still, even metal-polluted and thin-H-layer WDs pulsate • Roughly >50 kG fields can fully suppress convection/driving
19. Bagnulo & Landstreet 2022 τ cool < 100 Myr τ cool < 1 Myr 350 Myr < τ cool < 800 Myr We see evidence that the incidence of strong magnetism in white dwarfs increases significantly the longer they cool (perhaps connections w/ core crystallization)
20. The DAV Instability Strip is Still Remarkably Pure • Despite the varied histories before becoming a WD, ~all DA pulsate • Those that don’t pulsate are likely magnetic • Hypothesis: the purity of the DAV instability strip tells us that the incidence of strong magnetism in single-star-evolved white dwarfs before crystallization is very low (likely <10%) • Need a good Teff and strong limits (<0.05%) • Caveat: Not all WD are DA (~30% are non-DA) • Still, even metal-polluted and thin-H-layer WDs pulsate • Roughly >50 kG fields can fully suppress convection/driving • We see evidence for a late emergence of magnetism in WDs
21. Extra Slides
22. 1Quality cuts: Pwd > 0.9 [1] ruwe < 1.2 [2] phot_bp_rp_excess_factor_corrected < 0.06 [2] parallax_over_error > 10 [2] mass_H > 0.5 [1] meanAV < 0.08 [1] emass_H < 0.1 [1] eteff_H < 750 [1] PDA > 0.97 [3] mean CROWDSAP > 0.5 [1] Gentile Fusillo et al. 2021 [2] Gaia Collaboration+ 2021 [3] Vincent et al. 2024
23. We also see a surface spot with the same period Surface: 10.17404 hr Towards core: 10.18± 0.27 hr Using l=1 and l=2 modes we measure a rotation period of Prot = 10.18 ± 0.27 hr in PG0112+104 l=1 l=2 Hermes et al. 2017, ApJ
24. He P.I. zone (30-20 kK) H P.I. zone (13-10 kK) N2 L l 2 DAV Propagation Diagram Core Surface p-modes σ2 > Ll 2, N2 convection zone log σ2 (s-2) g-modes σ2 < Ll 2, N2 DBV: aka V477 Her DAV: aka ZZ Ceti Fontaine & Brassard 2008