WD+WD mergers in the Milky Way. 2. Most (>80%) simply create a weird WD. ~25% of all WDs are merger byproducts. 3. We have found stars ejected from the Galaxy that were involved in a WD supernova. Searching for signposts of failed white dwarf supernovae
dwarf staris • a stellar remnant that is no longer fusing in its core • the endpoint 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)
Can Observe WDs in the Act of Merging! Average distance between Earth and Moon: Brown et al. 2011; Hermes et al. 2012 M2 = 0.51 M ¤ M1 = 0.26 M ¤ SDSSJ0651+2844 is a 12.75-min WD+WD binary
| UHIfA | 7 Expected dP orb /dt = (-0.263 ± 0.020)ms/yr from GR Observed dP orb /dt = (-0.28688 ± 0.00072)ms/yr! M tot = 0.770 ±0.039 M ¤ L GW = 2.85 L ¤ LEM = 0.05 L ¤ SDSSJ0651+2844 is a 12.75-min WD+WD binary
merge into a single WD 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 M tot = 0.770 ±0.039 M ¤
havefound tens of WD+WD binaries with P orb < 1 hr They will all merge within 60 Myr, but none have total mass >1.1 M ¤ ZTF: Burdge et al. 2020 There should be many merger byproducts masquerading as ‘normal’ white dwarfs in the field
of Evolved Binaries Predicts Many Mergers Temmink et al. 2020 Population synthesis modeling expects >25% of all field white dwarfs to arise from stellar mergers
Collaboration, Babusiaux et al. 2018 JJ Hermes, Boston University | UHIfA | 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 by accident from SDSS) • Gentile Fusillo et al. 2021 catalog of ~360,000 high- confidence WDs from Gaia eDR3
JJ Hermes, Boston University | UHIfA | 12 El-Badry, Rix & Heintz 2021 Gaia has revealed >1500 wide (>100 au) WD+WD binaries that should be coeval We can estimate the total age of both WDs and make sure they agree (they usually do OK, with ~25% age uncertainties) Heintz, Hermes, El-Badry et al. 2022
JJ Hermes, Boston University | UHIfA | 13 However, roughly 30% of wide WD+WD have a more massive component that is hotter It should be cooler since it was a more-massive main-sequence star Thus ~30%of wide WD+WD binaries have had an age reset (once triples that merged). How do post-merger WDs look? Heintz, Hermes, El-Badry et al. 2022 (M1 is more massive component)
Composition | Mass | Rotation adapted from Wesemael et al. 1993 The majority (>2/3) of white dwarfs are Hydrogen dominated Mostly H-dominated 4000 4500 5000 5500 6500 DA: H JJ Hermes, Boston University | UHIfA | 14
H-dominated ~0.6 M ¤ The vast majority of white dwarfs have a mass near 0.55-0.65 M ¤ What Does a Merged WD Look Like? What is a Normal WD? JJ Hermes, Boston University | UHIfA | 15
(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 What Does a Merged WD Look Like? What is a Normal WD? JJ Hermes, Boston University | UHIfA | 16
a Normal ‘White Dwarf’? A ‘typical’ 0.6 solar-mass white dwarf H-dominated atmosphere ~0.6 M ¤ P rot = 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 Some strongly magnetic white dwarfs show large Zeeman splitting of the Balmer lines Gaensicke et al. 2002
Example of a Clear Merger Remnant 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" Composition | Mass | Rotation | Kinematics Mostly H-dominated ~0.6 M ¤ Prot = 0.5-2 days
reveals objects with older kinematics than cooling age suggests: evidence for age reset by merger Kinematics Can Reveal White Dwarf Mergers "Double White Dwarf Merger Products among High-mass White Dwarfs" JJ Hermes, Boston University | UHIfA | 22
reveals objects with older kinematics than cooling age suggests: evidence for age reset by merger Kinematics Can Reveal White Dwarf Mergers "Double White Dwarf Merger Products among High-mass White Dwarfs" JJ Hermes, Boston University | UHIfA | 23
2018b • LP 40-365 has v rad = +499 km/s • It is unbound to the Milky Way (v rf = 852 +/-10 km/s), a hyper-runaway not from Galactic center GD 492 (aka LP 40-365): A Star Unbound to the Milky Way JJ Hermes, Boston University | UHIfA | 31
Milky Way • LP 40-365 is one of the most metal-rich stars known • No H or He detected (<10-5 solar) • Heavy elements indicate near- Chandrasekhar-mass burning: [Mn/Fe] = 0.82 ± 0.18 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 (surface): The Sun: JJ Hermes, Boston University | UHIfA | 32
Raddi et al. 2019 JJ Hermes, Boston University | UHIfA | 33 • These are the most metal-rich stars ever found: No H, He detected • All have similar abundances • All have wild fast space motion • 0.20-0.28 M ¤ & 0.16-0.60 R ¤ • Hypothesis: These are the slung-shot remnants of an incomplete Type Iasupernova, ejected from <30-min binary!
H, He detected • All have similar abundances • All have wild fast space motion • 0.20-0.28 M ¤ & 0.16-0.60 R ¤ • Hypothesis: These are the slung-shot remnants of an incomplete Type Iasupernova, ejected from <30-min binary! GD 492: First of a Class of Partly Burnt Runaways JJ Hermes, Boston University | UHIfA | 34 Mark Garlick • Are they remnants of a He-star donor, or are they a fraction of the WD that mostly exploded?
Dwarf JJ Hermes, Boston University | UHIfA | 35 • TESS data of GD 492 showed 8.9-hr photometric variability; a surface spot Hermes, Putterman, et al. 2021
Dwarf JJ Hermes, Boston University | UHIfA | 36 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
Dwarf JJ Hermes, Boston University | UHIfA | 37 • If angular momentum is mostly conserved, 8.9-hr rotation today is too slow for it to have been the He-star donor: the donor would inflate in radius >20x less than a near-M Ch WD • Instead, is more likely to be a bound remnant from an underluminous supernova roughly 5 Myr ago ( P rot,i = P orb < 20 min P rot,f = 8.9 hr Hermes, Putterman, et al. 2021
merged in their past (a few every century in Milky Way) • We can pick some merger byproducts individually via: strong magnetism (>1 MG) & very fast (<1 hr) rotation • High space motion (fast kinematics) can reveal mergers as apparently young but actually old stars • We are now also finding more partly burnt, runaway supernova shards, likely failed Type Ia (Type Iax) events D. Berry, GSFC JJ Hermes, Boston University | UHIfA | 38
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