Emerging thoughts on the new class of DAHe white dwarfs

Conference presentation, 30 min. October 2022: KITP Program, Santa Barbara, CA, USA.

Transcript

1. http://sites.bu.edu/buwd J.J. Hermes Emerging thoughts on the new class of DAHe white dwarfs + Josh Reding (UNC) Boris Gänsicke (Warwick) Nik Walters (UCL) Jay Farihi (UCL) Tyler Heintz (BU) Lou Baya Ould Rouis (BU)
2. DAHe: Maximum Emission at Photometric Minimum 5.0 MG Photometric max. Photometric min. Reding et al. 2020 SDSSJ1252-0234 (Prot = 5.2 min) (these are cool surface spots)
3. All DAHe Seem to Occupy a Narrow Part of the Gaia CMD Walters et al. 2021 in 2021, all four known DAHe occupied a narrow region in the Gaia CMD
4. All DAHe Seem to Occupy a Narrow Part of the Gaia CMD Walters et al. 2021 in 2021, all four known DAHe occupied a narrow region in the Gaia CMD with Josh Reding at UNC and an NOAO program we took SOAR & LDT spectra of any variable* WD at all masses within the range 10,500 < Teff < 6500 K *varindex > 0.0 (Guidry et al. 2021) ∩ excess_flux_error > 4 (Gentile Fusillo et al. 2021)
5. We Found 2 New DAHe Among 76 Variable WDs G = 17.4 mag (Prot = 84.6 min) B ~ 9 MG G = 16.9 mag (Prot = 72.6 min) B = 9.3 MG (within the range 10,500 < Teff < 6500 K) Hα Hβ φ = phot. max φ = phot. min φ = phot. max φ = phot. min Hα Hβ φ = phot. max φ = phot. min φ = phot. max φ = phot. min
6. We Found 2 Additional DAHe in SDSS-V Spectroscopy G = 18.1 mag (Prot = 5.6 hr) B ~ 55.5 MG G = 17.6 mag (Prot = 22.6 hr from ZTF) B ~ 25 MG Hα Wavelength [Å] Normalized Flux Gänsicke +
7. Physical Parameters of the Currently Known DAHe all 8 known DAHe range from 8600 < Teff < 7500 K and from 0.56 to 0.83 solar masses fast rotation periods, and all but one (J0412 Tremblay+ 2020) have ≥ 5 MG magnetic fields * derived from Gaia SED fits
8. All DAHe Still Occupy a Narrow Part of the Gaia CMD 0.6 M⊙ 1.2 M⊙ 200 pc sample in background
9. All DAHe Still Occupy a Narrow Part of the Gaia CMD why emission lines in DAHe? chromosphere? inversion layer? 0.6 M⊙ 1.2 M⊙ DAV instability strip 200 pc sample in background
10. Not All Spotted WDs in This Part of the CMD are DAHe
11. Not All Spotted WDs in This Part of the CMD are DAHe WDJ053714.90+675950.51 (G=16.3) Prot = 0.47 hr, B ~ 0.7 MG spectra from: Tremblay et al. 2020 (top right) Kilic et al. 2020 (bottom left) SPY (bottom right)
12. Not All Spotted WDs in This Part of the CMD are DAHe WDJ053714.90+675950.51 (G=16.3) Prot = 0.47 hr, B ~ 0.7 MG WDJ185218.63+183355.13 (G=16.9) Prot = 6.66 hr, B ~ 2.3 MG spectra from: Tremblay et al. 2020 (top right) Kilic et al. 2020 (bottom left) SPY (bottom right)
13. Not All Spotted WDs in This Part of the CMD are DAHe WDJ053714.90+675950.51 (G=16.3) Prot = 0.47 hr, B ~ 0.7 MG NLTT 5493 (G=15.5) Prot = 3.94 hr, B < 200 kG WDJ185218.63+183355.13 (G=16.9) Prot = 6.66 hr, B ~ 2.3 MG spectra from: Tremblay et al. 2020 (top right) Kilic et al. 2020 (bottom left) SPY (bottom right)
14. Not All Spotted WDs in This Part of the CMD are DAHe 241 monoperiodic WDs from K2 & TESS & Gaia; 68% with spectra 0.6 M⊙ 1.2 M⊙ magnetic spots occur throughout CMD, but DAHe are localized
15. Could DAHe Arise from Emergence of Strong Fields? Bagnulo & Landstreet 2022 40 pc sample: strong-field WDs emerge, especially after crystallization
16. Could DAHe Arise from Crystallization-Induced Dynamo? Ginzburg, Fuller, Kawka, Caiazzo 2022 (see also in Schreiber et al. 2021) Ohmic diffusion timescale all known DAHe range from 8600 < Teff < 7500 K and 0.56 to 0.83 solar masses * derived from Gaia SED fits sharp drop when crystallization- driven core convection zone reaches edge of He envelope
17. Emerging Thoughts on the New Class of DAHe 1. DAHe are WDs with cool spots and active chromospheres • Observed: 8600 < Teff < 7500 K and 0.56 < MWD < 0.83 Msun • 7/8 have strong (>5 MG) magnetic fields and rotate ~hours 2. Not all magnetic and/or spotted WD are DAHe in this range
18. Emerging Thoughts on the New Class of DAHe 1. DAHe are WDs with cool spots and active chromospheres • Observed: 8600 < Teff < 7500 K and 0.56 < MWD < 0.83 Msun • 7/8 have strong (>5 MG) magnetic fields and rotate ~hours • are we witnessing the emergence of a fossil field? • crystallization-induced global dynamo? (Ohmic diffusion time shortens for field to reach surface?) • unlikely to be a local surface dynamo given diversity in Prot (Rossby no. = Prot / [convective turnover] is o.o.m. different) 2. Not all magnetic and/or spotted WD are DAHe in this range • does envelope matter? (need some selection mechanism) • 0.3 + 0.4 Msun mergers? (BPS predicts few mergers to 0.7 Msun)
20. Cheng et al. 2020
21. The Outlier Case of WDJ0412+7549 TESS shows a clear 2.29 hr photometric signal 3 sectors TESS data Tremblay et al. 2020 emission only in H-alpha no RV changes detected in 3 consecutive nights no strong magnetism (< 0.5 MG)
22. Spots Really Do Reflect Surface Rotation Rate PG 0112+104: Hermes et al. 2017 l=1 modes Both l=1 and l=2 pulsations suggest a rotation period of 10.1±0.9 hr Surface spot with period of 10.17404 hr
23. 41 Spotted WDs from Kepler/K2 Spots occur across most of the CMD
24. We Found 2 New DAHe Among 76 Variable* WDs (within the range 10,500 < Teff < 6500 K) DAHe are fairly rare among variable WD in this range *varindex > 0.0 (Guidry et al. 2021) ∩ excess_flux_error > 4 (Gentile Fusillo et al. 2021)
25. All DAHe Still Occupy a Narrow Part of the Gaia CMD 0.6 M⊙ 1.2 M⊙ 200 pc sample in background stronger B fields