Stars are the Endpoints of Stellar Evolution • a stellar remnant that is no longer fusing in its core • the endpoints of all stars that begin their lives < 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)
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
| Pitt/CMU | 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
= 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 There should be many merger byproducts masquerading as ‘normal’ white dwarfs in the field
Facility (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
also search for WD+WD binaries from radial-velocity changes Maoz, Hallakoun & Badenes 2018 Badenes & Maoz 2012 Modeling RV changes shows WDs have a ~10% binary fraction <4 au The observed WD merger rate is 6-7 times higher than the SN Ia rate Chandra et al. 2021, using SDSS-V
of Evolved Binaries Predicts Many Mergers Temmink et al. 2020 Population synthesis modeling expects >25% of all white dwarfs to arise from stellar mergers
Collaboration, Babusiaux et al. 2018 JJ Hermes, Boston University | Pitt/CMU | 11 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 ~360,000 high- confidence WDs from Gaia eDR3
JJ Hermes, Boston University | Pitt/CMU | 12 El-Badry, Rix & Heintz 2021 Gaia has revealed >1500 wide (>100 au) WD+WD binaries Tyler Heintz (BU) • Both WD components should be coeval, so we should infer the same total age! • We have used this sample to empirically test WD age determination (good to at least 25%)
JJ Hermes, Boston University | Pitt/CMU | 14 Roughly 40% of wide WD+WD have a more massive component that is hotter – its age was “reset” by a merger Thus ~40% of wide WD+WD binaries were once triples Heintz, Hermes, El-Badry et al. 2022 Tyler Heintz (BU)
Distinguish Merged WDs? What is a Normal WD? Composition | Mass | Rotation adapted from Wesemael et al. 1993 The majority (>2/3) of white dwarfs are H dominated Mostly H-dominated 4000 4500 5000 5500 6500 DA: H
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¤ Can We Distinguish Merged WDs? What is a Normal WD?
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 Can We Distinguish Merged WDs? What is a Normal WD?
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 Can We Distinguish Merged WDs? What is a Normal WD?
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, Kepler, Hermes et al. 2022 Can We Distinguish Merged WDs? What is a Normal WD?
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!
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
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"
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 Kinematics are Another Way to Reveal Actually Old Stars
al. 2020 Modeling WDs with anomalous kinematics reveals objects with older kinematics than cooling age suggests: mergers Kinematics are Another Way to Reveal Actually Old Stars "Double White Dwarf Merger Products among High-mass White Dwarfs"
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 | Pitt/CMU | 32
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 | Pitt/CMU | 33
• 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 | Pitt/CMU | 34
• 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 | Pitt/CMU | 35 Mark Garlick
JJ Hermes, Boston University | Pitt/CMU | 36 Hermes, Putterman, et al. 2021 • TESS data of GD 492 showed 8.9-hr photometric variability; a surface spot
JJ Hermes, Boston University | Pitt/CMU | 37 Hermes, Putterman, et al. 2021 "8.9 hr Rotation in the Partly Burnt Runaway Stellar Remnant LP 40-365 (GD 492)" • TESS data of GD 492 showed 8.9-hr photometric variability; a surface spot • Variability confirmed from five orbits of archival time- tagged HST ultraviolet data
JJ Hermes, Boston University | Pitt/CMU | 38 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
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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