What's Shaking with Extremely Low-Mass White Dwarfs

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http://jjherm.es J.J. Hermes Hubble Fellow University of North Carolina at Chapel Hill What's shaking with extremely low-mass white dwarfs

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All stars like the Sun eventually run out of fuel. When they do, they lose all their envelope and only the core remains: a white dwarf star.

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Tremblay et al. 2016, MNRAS, 461, 2100 Hydrogen Balmer-line fits to low-resolution spectra allow us to estimate Teff and log(g), and thus mass CO-Core WDs ONe-Core WDs Only isolated WDs Most isolated white dwarfs ~0.62 M¤

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Bloemen et al. 2012, MNRAS, 422, 2600 2.2 ± 0.2 M¤ , 9500 K, 2.14 ± 0.08 R¤ A star primary + 0.228 ± 0.014 M¤ , 14500 K, 0.044 ± 0.002 R¤ WD Porb = 5.188675 days, i = 87.0 ± 0.4 deg KOI-74

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The Galaxy is not old enough for a single star to evolve into a <0.35 M¤ WD “Low-mass white dwarfs need friends” (Marsh et al. 1995, MNRAS, 275, 828) David A. Aguilar, CfA 0.228 ± 0.014 M¤ , 14500 K, 0.044 ± 0.002 R¤ WD KOI-74

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van Kerkwijk et al. 2015, ASPC, 328, 357 Low-mass WDs first inferred in timing residuals around pulsars assuming i=60, MMSP =1.4 M¤

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van Kerkwijk et al. 2015, ASPC, 328, 357 Low-mass WDs first inferred in timing residuals around pulsars only those with optical constraints

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van Kerkwijk et al. 2011, ApJ, 728, 95 Not all low-mass white dwarfs have “friends” – many are being actively ablated from a nearby MSP: Black Widow Pulsars PSR B1957+20 (Bow shock, artist’s rendition)

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Motivation and Outline • How well can we determine mass of a low-mass white dwarf? – Used to constrain pulsar masses when in MSP+WD binaries • What is the cooling age of a low-mass white dwarf? – WD cooling ages often compared to MSP spin-down ages – Observational constraints on H-envelope thickness and thus cooling rates • How does the galactic gravitational wave foreground sound? – The mHz regime it is awash in persistent sources from close WD+WD binaries D. Berry, GSFC

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Bassa et al. 2006, A&A, 456, 295 PSR J1911–5958B: V = 22.1 mag Pulsar opticalcounterparts are usually very faint! 8.2m VLT

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log(g)= log(g)= log(g)= log(g)=8 “The ELM Survey” An SDSS color-selected search for new extremely low-mass (ELM, <0.3 M¤ ) white dwarfs Orbits solved from 6.5m MMT, 4m KPNO, 4.1m SOAR Warren Brown, Mukremin Kilic, Alex Gianninas, et al.

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Brown et al. 2013, ApJ, 769, 66 “The ELM Survey” An SDSS color-selected search for new extremely low-mass (ELM, <0.3 M¤ ) white dwarfs Orbits solved from 6.5m MMT, 4m KPNO, 4.1m SOAR: • 82 WD+WD Porb <1.1 d • Most 15.5 < V < 18.5 log(g)= log(g)= log(g)= log(g)=8

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Kilic et al. 2016, MNRAS, 460, 4176 No clear pulsar companions to WDs so far in ELM Survey: all likely WD+WD binaries Chandra x-ray upper limits a factor of 2 more sensitive than MSPs detected in x-rays in 47 Tuc Null x-ray Null radio

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Mean Earth--Moon separation Minimum ELM Survey (J0651+2844, 12.75-min) Median ELM Survey (≈7 hr) Maximum ELM Survey (J0815+2309, 25.8-hr) 1 R¤ µWD2 = 0.74 ± 0.24 M¤ (Andrews et al. 2014, ApJ, 797, L32)

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Discovery spectroscopy determines the binary and atmospheric parameters blue-shifted red-shifted Brown et al. 2012, ApJ, 744, 142 Teff = 10,540 ± 170 K log(g) = 6.01 ± 0.06 à M1 = 0.17 M¤ Porb = 87.996 ± 0.006 min K1 = 508 ± 4 km s-1 M2 > 1.10 M¤ if M1 = 0.17 M¤ tmerge < 170 Myr SDSS J1471+6526

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Discovery spectroscopy determines the binary and atmospheric parameters Teff = 10,540 ± 170 K log(g) = 6.01 ± 0.06 à M1 = 0.17 M¤

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a b c d d c b a Usually, isochrones do not radically overlap on an H-R diagram

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≤ 0.18 M¤ WDs: residual pp-burning; simple but very slow evolution >0.18 M¤ WDs: undergo unstable CNO burning flashes: Their evolution is not simple cooling Panei et al. 2007, MNRAS, 382, 779 Stars evolve from A-J

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adapted from Althaus et al. 2013, A&A, 557, A19 • Two ELM white dwarfs of different masses often cross the same points in a Teff –log(g) diagram • There is a non-uniqueness to using Teff ,log(g) for ELM WD mass

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Adapted from Althaus et al. 2013, A&A, 557, A19 • There is a non-uniqueness to using Teff ,log(g) for ELM WD mass • For example, take an object at 10,000 K with log(g) = 6.60:

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Bassa et al. 2006, A&A, 456, 295 PSR J1911–5958A: MPSR = 1.34 ± 0.08 M¤ 8.2m VLT

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Pulsar Masses Are Often Tied to Getting ELM White Dwarf Masses Correct

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Alina Istrate et al. 2016, A&A, 595, 35 Metallicity certainly affects severity of CNO flashes These are diffusion-induced, unstable H burning episodes 0.28 M¤ 0.28 M¤ 0.28 M¤ Lower metallicity

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Alina Istrate et al. 2016, A&A, 595, 35 We are finding that ELM WD models must include rotation CNO flashes likely cause shear affecting internal angular momentum: Prot ~ hours (sub-Porb ) 0.23 M¤ 0.23 M¤ 0.23 M¤

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Lowest-Gravity White Dwarfs All Show Metals Hermes et al. 2014, MNRAS, 444, 1674 Diffusion should cause metals to sink out in <1 Myr (>100 Myr cooling ages)

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Istrate et al. 2016, A&A, 595, 35 Rotational support (Eddington-Sweet circulation) can support metals at the gravities observed!

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M1 ~ 0.19 M¤ (M2 ~ 0.90 M¤ ) R1 = 0.093 ± 0.013 R¤ i = 86.9 ± 0.4 deg Porb = 5.90724895(41) hr -20 vrot = 50+30 km s-1 Prot = 2.3+2.0 hr Hermes et al. 2014, MNRAS, 444, 1674 -1.0 GALEX J1717+6757 Most low-mass white dwarfs rotate faster than Porb !

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Original Kepler Mission (4 years): Just 20 white dwarfs observed, K2 through Campaign 13: >1250 white dwarfs observed K2 has given us thousands of candidate white dwarfs to observe k2wd.org

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Marsh et al. 2017, in prep. We observed the first eclipsing ELM WD every minute for 78+ days w/ K2 NLTT 11748 spectra show: M1 ~ 0.162 M¤ M2 ~ 0.74 M¤ (i = 89.7 deg) secondary primary Römer delay: -4.44 ± 0.39 s offset: q = M2 /M1 = 4.6 ± 0.13 (if e < 10-4)

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Marsh et al. 2017, in prep. We observed the first eclipsing ELM WD every minute for 78+ days w/ K2 NLTT 11748 primary (massive WD passing in front of ELM WD) w/out gravitational lensing w/ lensing Light curve fits (see also Kaplan et al. 2014): R1 ~ 0.043 R¤ R2 ~ 0.011 R¤ T1 ~ 8700 K T2 ~ 7590 K spectra show: M1 ~ 0.162 M¤ M2 ~ 0.74 M¤ (i = 89.7 deg)

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Yellow: Tidally distorted ELM WDs White: Eclipsing WD+WD binaries Pink: Eclipsing WD+A stars from Kepler Hermes et al. 2014, ApJ, 792, 39 The Empirical He-WD Mass-Radius Relation

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Istrate et al. 2016, A&A, 595, 35 0.23 M¤

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Istrate et al. 2016, A&A, 595, 35 A ‘typical’ white dwarf 0.6 M¤ electron degenerate C/O core non-degenerate He layer non-degenerate H layer [thermal reservoir] [insulating blanket] A low-mass white dwarf 0.2 M¤ electron degenerate He core non-degenerate H layer

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Hermes et al. 2012, ApJ, 750, L28 Hermes et al. 2013, ApJ, 765, 102 Hermes et al. 2013, MNRAS, 436, 3573 Kilic et al. 2015, MNRAS, 446, 26 We can use pulsations to check H-layer thicknesses and thus calibrate cooling ages: Asteroseismology!

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Kilic et al. 2015, MNRAS, 446, 26 We have also found a pulsating ELM WD around a MSP! Antoniadis et al. 2012, MNRAS, 423, 3316 "Looking at individual acquisition frames, the scatter of the magnitude difference was ∼0.05 mag, somewhat larger than expected based on measurement noise, though with no obvious correlation with orbital phase." V = 21.3 mag Pulsation periods of 1790-3060 s (0.50-0.85 hr)

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McDonald 2.1m KPNO 2.1m SOAR 4m, PROMPT 0.41m SAAO 1m SSO 2m Mt. John 1m NARIT 2.4m Lulin 1m BOAO 1.8m Xinglong 2.16m Tian-Shan 1m Peak Terskol 2m CrAO 1.25m Tuebingen & Mt. Suhora 0.6m Haleakala 2m CTAS 0.7m • Whole Earth Telescope run from 2013 April/May on J1518+0658 – 17 sites in coordinated global campaign – 54 days, ~40% duty cycle

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• Whole Earth Telescope run from 2013 April/May on J1518+0658 – 17 sites in coordinated global campaign – 54 days, ~40% duty cycle relative amplitude

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ELM White Dwarfs Strong Sources of Gravity Waves (Pulsations) and Gravitational Waves!

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Gravitational Waves Gravity Waves ≠

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Mean Earth--Moon separation Minimum ELM Survey (J0651+2844, 12.75-min) Median ELM Survey (≈7 hr) Maximum ELM Survey (J0815+2309, 25.8-hr) 1 R¤

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phase = 0 • We detected eclipses in April 2011 • This is the most compact detached binary system currently known! J0651+2844: A 12.75-min WD+WD Binary Brown et al. 2011, ApJ, 737, L23

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Photo by Taylor Chonis Eclipses were detected on 1 April 2011 from the 2.1m telescope at McDonald Observatory on the first night of a 14-night run

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The Rock House fire started on 9 April 2011 near Marfa, TX It eventually burned >310,000 acres (>1270 km2) Photos I took on 9 April 2011

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The Rock House fire eventually burned >310,000 acres Photo by Don Winget on 11 April 2011

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We re-started monitoring the 12.75-min binary on 13 April 2011, the first photons collected at McDonald after the Rock House fire started. Photo by Frank Cianciolo on 17 April 2011

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J0651+2844: A 12.75-min WD+WD Binary Average distance between the Earth and the Moon: 384,400 km M2 = 0.50 ± 0.04 M¤ M1 = 0.25 ± 0.04 M¤

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Orbital Decay in J0651+2844 After just 13 months we confirmed orbital decay from gravitational radiation. Hermes et al. 2012, ApJ, 757, L21

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We expect dPorb /dt = (-0.26 ± 0.05) ms/yr from GR (point masses) and observe (-0.2875 ± 0.0011) ms/yr! – a 0.4% measurement! Hermes et al. 2018, in prep.

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• This 12.75-min WD+WD binary is decaying > 3.8x faster than the 7.75-hr Hulse-Taylor binary pulsar (first indirect detection of gravitational waves) Weisberg et al. 2010 J0651+2844 PSR B1913+16 dP/dt = -0.288 ms/yr dP/dt = -0.076 ms/yr Orbital Decay in a 12.75-min WD+WD binary

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• Tidal torques should increase the rate of orbital decay in J0651+2844 – Additional angular momentum is lost from the orbit to spin-up the WDs to remain synchronized, leading to >5% faster rate of orbital decay (e.g., Piro 2011, ApJ, 740, L53; Fuller & Lai 2012, MNRAS, 421, 426) The Fate of the WDs in J0651+2844

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• J0651+2844 is an excellent verification source for direct detection of gravitational waves with eLISA: forb = 2.6136738(32) mHz • Expect Roche lobe contact in less than 1 million years • J0651+2844 should be detectable by eLISA with S/N > 7 within six months Kilic, Brown & Hermes 2013 J0651 is an Excellent eLISA Verification Source eLISAplanned sensitivity known detached binaries known interacting binaries (AM CVns)

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population synthesis by Valeriya Korol et al. 2017, MNRAS, 470, 1894 • In next decade, we expect to find 1000s of eclipsing WD+WD binaries (just 7 known to date) hundreds (GAIA) thousands (LSST)

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Conclusions • Determining the mass of a low-mass white dwarf has caveats – Complicated evolution due to CNO-flash episodes • Low-mass white dwarfs are likely fast rotators • Cooling ages strongly affected by outer H layer thickness – Asteroseismologyis finally getting off the ground for He-core WDs • 100s of close, eclipsing WD+WD binaries from Gaia/LSST – The mHz regime it is awash in persistent sources from close WD+WD binaries D. Berry, GSFC