Asteroseismology of ultra-massive white dwarfs: New results from a multi-telescope survey

Asteroseismology of ultra-massive white dwarfs: New results from a multi-telescope survey Murat Uzundag FWO Postdoctoral Fellow Francisco C. De Gerónimo, Mukremin Kilic, Alberto Rebassa-Mansergas, Alex Brown, Alejandro H. Corsico 9th TESS/16th Kepler Asteroseismic Science Consortium Workshop DAV pulsating white dwarfs General Properties ●Atmosphere: Hydrogen-rich (DA type) ● 10,400 ≲ Teff ≲ 13,000 K & log g∼7 to ∼9.5 ●Population: Most common class of pulsating white dwarfs → Over 500 confirmed members (Bognar & Sódor 2016; Córsico et al. 2019a; Vincent et al. 2020; Guidry et al. 2021; Romero et al. 2022, 2024) Ultra-massive pulsating white dwarfs Pulsation Characteristics ●Mode Type: Non-radial g-mode pulsations ●κ–γ mechanism (Dolez & Vauclair 1981; Winget et al. 1982) ●Convective-driving mechanism (Brickhill 1991; Goldreich & Wu 1999) ●Harmonic Degree: ℓ ≤ 2 ●Radial Order: 1 ≲ k ≲ 15 ●Period Range: 100 s ≲ Π ≲1500 s ●Amplitude Range: 0.01 to 0.3 mma Ultra-massive pulsating white dwarfs Star Name Mass(M☉) N. Modes Reference BPM37093 1.13 8 modes Metcalfe et al. 2004 GD518 1.24 ≤3 modes Hermes et al. 2013 SDSSJ0840 1.16 ≤3 modes Curd et al. 2017 WDJ2124 1.16 ≤3 modes Rowan et al. 2019 WDJ0049 1.31 ≤2 modes Kilic et al. 2023 WDJ0551 1.13 ≤2 modes Hollands et al. 2020 Ultra-massive white dwarfs Formation Channel Degeneracy ● Two main paths: ➤ Single-star evolution from intermediate-mass stars (Althaus et al. 2010) ➤ Double WD mergers (~40%, Jewett et al. 2024) ● Observables (mass, temperature, log g) often can’t distinguish between them. ● Understanding formation is critical to link UMWDs to broader phenomena (e.g. SNe Ia, magnetism, Q-branch pileup). Our Goal: Combine asteroseismology, high-quality observations, and improved evolutionary models to resolve these fundamental questions. Image: NASA/CXC/M Weiss Core composition of DAVs Core Composition of White Dwarfs by Mass ●Low-Mass WDs: MWD <0.5 M⊙→ Expected to harbor helium (He) cores (Norris 2004; Althaus et al. 2017) Core composition of DAVs Core Composition of White Dwarfs by Mass ●Typical Mass Range: Most white dwarfs have masses in the range: 0.5 ≲ M⊙ ≲ 1.05 → Expected to have carbon–oxygen (CO) cores (Althaus et al. 2010) Core composition of UMWDs Core Composition of White Dwarfs by Mass ●Ultra-Massive WDs: MWD ≳ 1.05 M⊙ → Core composition remains uncertain! ● Possible options: ○Carbon–oxygen (CO) (Althaus et al. 2022) ○Oxygen–neon (ONe) (Siess 2010; Althaus et al. 2021) ○Hybrid CO–Ne (Denissenkov 2013, De Gerónimo et al. 2024) Searching for pulsating ultra-massive white dwarfs Selection Criteria define based on Jiménez-Esteban et al. 2023 and Jewett et al. 2024 focusing on the ZZ Ceti instability strip. Teff ≥ 11 000 K 0.9 M⊙ 1.1 M⊙ 1.3 M⊙ ●Unique pulsating DAQ white dwarf (C+H atmosphere) ● Atmospheric composition & kinematics suggest a merger origin ●19 nights of time-series photometry. ● Displays large amplitude and frequency modulations ●12 independent modes detected → Periods: ~595–1000 sec In prep. Conclusions What is the internal composition of UMWDs? ● UMWDs may have ONe, CO, or hybrid CO-Ne cores ● Current models cannot decisively constrain the core composition ● Disentangling core makeup is key to understanding progenitors and fate (e.g., SNe Ia) Conclusions What is the internal composition of UMWDs? ● UMWDs may have ONe, CO, or hybrid CO-Ne cores ● Current models cannot decisively constrain the core composition ● Disentangling core makeup is key to understanding progenitors and fate (e.g., SNe Ia) Conclusions What is the internal composition of UMWDs? ● UMWDs may have ONe, CO, or hybrid CO-Ne cores ● Current models cannot decisively constrain the core composition ● Disentangling core makeup is key to understanding progenitors and fate (e.g., SNe Ia) Crystallization & mode trapping in pulsating ultramassive WDs ● Core crystallization (>85–99%) alters mode propagation ● Complex pulsation spectra (e.g. WD J0135+5722: 19 modes) ● Requires advanced seismic modeling with tailored CO & ONe grids Conclusions What is the internal composition of UMWDs? ● UMWDs may have ONe, CO, or hybrid CO-Ne cores ● Current models cannot decisively constrain the core composition ● Disentangling core makeup is key to understanding progenitors and fate (e.g., SNe Ia) Crystallization & mode trapping in pulsating ultramassive WDs ● Core crystallization (>85–99%) alters mode propagation - key feature in the distribution on period spacing. ● Complex pulsation spectra (e.g. WD J0135+5722: 19 modes) ● Requires advanced seismic modeling with tailored CO & ONe grids How do mergers influence pulsational properties? ● Some UMWDs likely originate from WD mergers ● Pulsators like WD J0551+4135 exhibit unstable, modulated modes ● We lack constraints on how mergers affect internal rotation & pulsation modes Conclusions What is the internal composition of UMWDs? ● UMWDs may have ONe, CO, or hybrid CO-Ne cores ● Current models cannot decisively constrain the core composition ● Disentangling core makeup is key to understanding progenitors and fate (e.g., SNe Ia) Crystallization & mode trapping in pulsating ultramassive WDs ● Core crystallization (>85–99%) alters mode propagation - key feature in the distribution on period spacing. ● Complex pulsation spectra (e.g. WD J0135+5722: 19 modes) ● Requires advanced seismic modeling with tailored CO & ONe grids How do mergers influence pulsational properties? ● Some UMWDs likely originate from WD mergers ● Pulsators like WD J0551+4135 exhibit unstable, modulated modes ● We lack constraints on how mergers affect internal rotation & pulsation modes Thank you!