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Emerging thoughts on the new class of DAHe white dwarfs

jjhermes
October 19, 2022

Emerging thoughts on the new class of DAHe white dwarfs

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

jjhermes

October 19, 2022
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  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)
  19. None
  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