JJ Hermes, Boston University | MIT | 2 What is a normal ‘White Dwarf’? • a stellar remnant that is no longer fusing in its core • the endpoints of everything < 8 M¤ • electron degeneracy limits WD mass to < 1.4 M¤ A ‘typical’ 0.6 solar-mass white dwarf electron degenerate C/O core (r = 8500 km) non-degenerate He layer (260 km) non-degenerate H layer (30 km)
• a stellar remnant that is no longer fusing in its core • the endpoints of everything < 8 M¤ • we need a way to grow the WD mass above the Chandrasekhar limit • a stellar remnant that is no longer fusing in its core • the endpoints of everything < 8 M¤ • electron degeneracy limits WD mass to < 1.4 M¤ JJ Hermes, Boston University | MIT | 3 Type Ia Supernova: Fully Disrupted White Dwarfs HST: SN1994D A ‘typical’ 0.6 solar-mass white dwarf electron degenerate C/O core (r = 8500 km)
JJ Hermes, Boston University | MIT | 4 We Now Often Observe WDs in the Act of Merging! Average distance between Earth and Moon: Brown et al. 2011; Hermes et al. 2012 M 2 = 0.51 M ¤ M 1 = 0.26 M ¤ SDSSJ0651+2844 is a 12.75-min WD+WD binary
JJ Hermes, Boston University | MIT | 5 We Now Often Observe WDs in the Act of Merging! SDSSJ0651+2844 is a 12.75-min WD+WD binary Hermes et al. 2022, in prep.
Hermes et al. 2022, in prep. JJ Hermes, Boston University | MIT | 6 Expected dPorb /dt = (-0.263 ± 0.020) ms/yr from GR Observed dPorb /dt = (-0.28688 ± 0.00072) ms/yr! M tot = 0.770 ± 0.039 M ¤ L GW = 2.85 L ¤ L EM = 0.05 L ¤ SDSSJ0651+2844 is a 12.75-min WD+WD binary
JJ Hermes, Boston University | MIT | 7 There should be many merger byproducts masquerading as ‘normal’ white dwarfs in the field M tot = 0.770 ± 0.039 M ¤ that will merge in less than 1 million years D. Berry, GSFC SDSSJ0651+2844 is a 12.75-min WD+WD binary
JJ Hermes, Boston University | MIT | 8 ZTF has found >15 WD+WD binaries with Porb < 1 hr that will all merge within 60 Myr, but none have total mass >1.1 M¤ Burdge et al. 2020 There should be many merger byproducts masquerading as ‘normal’ white dwarfs in the field
Gaia revolutionized our ability to find white dwarfs Gaia Collaboration, Babusiaux et al. 2018 JJ Hermes, Boston University | MIT | 9 Sun-like stars white dwarfs Bluer (hotter) Redder (cooler) Absolute G Magnitude (Distance Normalized) • Before Gaia we knew of ~35,000 white dwarfs (mostly from SDSS) • Gentile Fusillo et al. 2021 catalog of ~359,000 high- confidence WDs from Gaia eDR3
JJ Hermes, Boston University | MIT | 10 What is a normal ‘White Dwarf’? Composition | Mass | Rotation 4000 4500 5000 5500 6500 DA DA: H DB: He DQ: C2 (‘Swan bands’) DC: [continuum] DZ: [metals] DB DZ DQ DC adapted from Wesemael et al. 1993 The majority (>2/3) of white dwarfs are H dominated Mostly H-dominated
JJ Hermes, Boston University | MIT | 11 What is a normal ‘White Dwarf’? Composition | Mass | Rotation Tremblay et al. 2016 Mostly H-dominated ~0.6 M ¤ The vast majority of white dwarfs have a mass near 0.55-0.65 M¤
JJ Hermes, Boston University | MIT | 12 What is a normal ‘White Dwarf’? Composition | Mass | Rotation Winget & Kepler 2008 Mostly H-dominated ~0.6 M ¤ maximum associated with the onset of significant partial ionization. Observations soon caught up. A systematic survey of the DB white dwarf stars de that the brightest DB with the broadest He I lines, GD 358, did indeed pulsate i g-modes—remarkably similar to the large-amplitude DAV pulsators (Winget et al. 19 The observed pulsating white dwarf stars lie in three strips in the H-R diagram, in Figure 3. The pulsating pre-white dwarf PG 1159 stars, the DOVs, around 7 170,000 K have the highest number of detected modes. The first class of pulsating 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 A 13-Gyr isochrone with z = 0.019 from Marigo et al. (2007), on which we have drawn the ob Annu. Rev. Astro. Astrophys. 2008.46:157-199. Downloaded from arjour by University of Texas - Austin on 01/28/09. For personal Asteroseismology can probe core composition, mass, and WD rotation
JJ Hermes, Boston University | MIT | 13 What is a normal ‘White Dwarf’? Hermes et al. 2017 1000 s 200 s 500 s 125 s Typical K2 data from a pulsating white dwarf 345.3 s l = 1 n = 6 Prot = 0.9 ± 0.2 day Composition | Mass | Rotation Mostly H-dominated ~0.6 M ¤ Asteroseismology can probe core composition, mass, and WD rotation
JJ Hermes, Boston University | MIT | 14 What is a normal ‘White Dwarf’? Prot = 0.5-2 days 1 10 100 WD Rotation Period (hr) 0.4 0.5 0.6 0.7 0.8 0.9 WD Mass (MØ) 1.0 1.5 2.0 2.5 3.0 3.5 4.0 ZAMS Progenitor Mass (MØ) 1 10 100 White Dwarf Rotation Period (hr) 0 2 4 6 8 10 12 14 16 N TESS Kepler/K2 pre-Kepler 1 d 2 d 4 d Hermes et al. 2017 The vast majority of pulsating white dwarfs rotate between 0.5-2 days Composition | Mass | Rotation Mostly H-dominated ~0.6 M ¤ TESS update from Romero et al. 2021, in prep.
JJ Hermes, Boston University | MIT | 15 What is a normal ‘White Dwarf’? A ‘typical’ 0.6 solar-mass white dwarf H-dominated atmosphere ~0.6 M ¤ Prot = 0.5-2 days The majority of white dwarfs are pretty boring – that’s why they’re great flux standards!
JJ Hermes, Boston University | MIT | 16 What are possible signposts of merged WDs? Composition | Mass | Rotation | Kinematics Mostly H-dominated ~0.6 M ¤ Prot = 0.5-2 days 4000 4500 5000 5500 6500 DA: H Some strongly magnetic white dwarfs show large Zeeman splitting of the Balmer lines Gaensicke et al. 2002
JJ Hermes, Boston University | MIT | 17 An extreme example of a clear merger remnant Composition | Mass | Rotation | Kinematics Mostly H-dominated ~0.6 M ¤ Prot = 0.5-2 days Caiazzo et al. 2021 P rot = 416.2 s B = 600-900 MG R ~ 2100 km M > 1.3 M ¤ ZTF J1901+1458 "A highly magnetized and rapidly rotating white dwarf as small as the Moon"
JJ Hermes, Boston University | MIT | 18 Photometric variability can select for WD mergers Gaia’s empirical photometric uncertainties can select variables n obs Gaia Collaboration, Evans et al. 2018 Guidry et al. 2021
JJ Hermes, Boston University | MIT | 19 Photometric variability can select for WD mergers Gaia’s empirical photometric uncertainties can select variables Bluer (hotter) Redder (cooler) 0.6 M¤ 0.9 M ¤ 1.2 M¤ Guidry et al. 2021
JJ Hermes, Boston University | MIT | 24 How can we start to put estimates on merger rates? Temmink et al. 2020 Population synthesis expects >25% of all white dwarfs to arise from stellar mergers
JJ Hermes, Boston University | MIT | 25 Dunlap & Clemens 2015 see also Wegg & Phinney 2012 Most hot, massive WDs were formed recently (<1 Gyr) à low velocity dispersions M < 0.75 M¤ Teff > 15,000 K M > 0.90 M¤ Teff > 15,000 K within 100 pc How can we start to put estimates on merger rates?
JJ Hermes, Boston University | MIT | 26 Cheng et al. 2020 Modeling WDs with anomalous kinematics reveals objects with older kinematics than cooling age suggests: mergers How can we start to put estimates on merger rates? "Double White Dwarf Merger Products among High-mass White Dwarfs"
discovery: Vennes et al. 2017 follow-up: Raddi et al. 2018a, 2018b • LP 40-365 has vrad = +499 km/s; vrf = 852 +/- 10 km/s • It is unbound, a hyper-runaway not from Galactic center GD 492 = LP 40-365: A Hyper-Runaway WD JJ Hermes, Boston University | MIT | 34
GD 492 = LP 40-365: A Hyper-Runaway WD • LP 40-365 is one of the most metal-rich stars known • No H or He detected (< 10-5) • Heavy elements indicate core C, Si burning discovery: Vennes et al. 2017 follow-up: Raddi et al. 2018a, 2018b He H Ne O Mg all else all else <0.005% H GD 492: The Sun: JJ Hermes, Boston University | MIT | 35
GD 492: first of a class of partly burnt runaways • These are the most metal- rich stars ever found: No H, He detected • All have similar abundances • All have extremely fast space motion • Hypothesis: These are the slung-shot remnants of an incomplete Type Ia supernova, ejected from <30-min binary! Raddi et al. 2019 JJ Hermes, Boston University | MIT | 36
GD 492: rst of a class of partly burnt runaways • These are the most metal- rich stars ever found: No H, He detected • All have similar abundances • All have extremely fast space motion • Hypothesis: These are the slung-shot remnants of an incomplete Type Ia supernova, ejected from <30-min binary! JJ Hermes, Boston University | MIT | 37 Mark Garlick
GD 492: TESS suggests it is a surviving white dwarf JJ Hermes, Boston University | MIT | 38 Hermes, Putterman, et al. 2021 • TESS FFI data of GD 492 showed 8.9-hr variability
GD 492: TESS suggests it is a surviving white dwarf JJ Hermes, Boston University | MIT | 39 Hermes, Putterman, et al. 2021 "8.9 hr Rotation in the Partly Burnt Runaway Stellar Remnant LP 40-365 (GD 492)" • TESS FFI data of GD 492 showed 8.9-hr variability • Variability confirmed from five orbits of archival time- tagged HST ultraviolet data
GD 492: TESS suggests it is a surviving white dwarf JJ Hermes, Boston University | MIT | 40 Hermes, Putterman, et al. 2021 • If angular momentum is mostly conserved rotation is likely too slow for it to have been the donor • More evidence its radius has increased substantially and it is actually a bound remnant from an underluminous supernova roughly 5 Myr ago Prot,i = Porb = minutes Prot,f = 8.9 hr
• Roughly 25% of all field white dwarfs have interacted or merged in their past • We can pick some merger byproducts individually via: strong magnetism, fast rotation, and/or high mass • High space motion (fast kinematics) can reveal apparently young but actually old systems • We are now also finding more partly burnt runaway supernova shards D. Berry, GSFC
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