The ELM Survey: A Targeted Search for the Most

Extreme White Dwarf Binary Systems

Alex Gianninas Astronomy Colloquium

Wesleyan University February 25th, 2015

Outline

• Intro: Stellar Evolution & White Dwarfs 101

• Extremely Low Mass White Dwarfs

• The ELM Survey

• Highlights from 2014

• Takeaways

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Stellar Evolution and White Dwarfs 101

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• No H burning, WDs cool over several billion years (Gyrs)

• Teff ~ 150,000 K → 3000 K • M ~ 0.6 M • R ~ 0.01 R ~ 1 R⊕ • log g ~ 8.0 (log g = 4.4, log g ⊕ = 3.0)

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Stellar Evolution and White Dwarfs 101

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• No H burning, WDs cool over several billion years (Gyrs)

• Teff ~ 150,000 K → 3000 K • M ~ 0.6 M • R ~ 0.01 R ~ 1 R⊕ • log g ~ 8.0 (log g = 4.4, log g ⊕ = 3.0)

𝒈𝒈 = 𝑮𝑮𝑮𝑮𝑹𝑹𝟐𝟐

Gravitational settling quickly creates a stratified internal structure

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C/O

He H

DA (H-rich)

C/O

He

Non-DA (He-rich)

C/O

Hot DQ

Extremely Low Mass (ELM) WDs have M < 0.3 M and form in compact binaries

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Gianninas et al. (2011, ApJ, 743, 138)

Binary Evolution

Extremely Low-Mass (ELM) WDs have He cores and are formed in compact binaries

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Iben & Tutukov (1984, ApJ, 54, 335)

ELM WDs have He cores and considerably larger radii

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C/O

He H

He

H

ELM

𝑹𝑹~𝟏𝟏𝑮𝑮𝟑𝟑

The ELM Survey • The ELM Survey is an ongoing, targeted search

for ELM WDs – M < 0.30 M (5 < log g < 7) – Found in short-period (P < 1 day) binary systems

• Motivation – Progenitors of SN Ia, .Ia, AM CVn, R CrB – Neutron star companions – Gravitational wave verification sources – Laboratories to test GR, tidal effects

• Papers I – V + VI : 62 + 12 new ELM WDs

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SDSS colors work well for choosing ELM WD candidates

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Brown et al. (2012, ApJ, 744, 142) Gianninas et al. (2015, in prep)

Radial Velocity Follow-Up

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FLWO 1.5m MMT 6.5m KPNO 4m

The optical spectra of ELM WDs are dominated by the Hydrogen Balmer lines

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More Balmer lines are visible in ELM WDs due to the lower log g

DA ELM

Hβ Hγ Hγ Hδ

ELM VI: 12 new ELM WDs

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Gianninas et al. (2015, in prep) 𝑷𝑷𝑲𝑲𝟑𝟑

𝟐𝟐𝝅𝝅𝑮𝑮= 𝑮𝑮𝒇𝒇 =

(𝒎𝒎𝟐𝟐𝐬𝐬𝐬𝐬𝐬𝐬 𝒊𝒊)𝟑𝟑 (𝒎𝒎𝟏𝟏+𝒎𝒎𝟐𝟐)𝟐𝟐

Hydrogen Balmer lines are very sensitive to both Teff and log g

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Standard Spectroscopic Technique

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Gianninas et al. (2015, in prep)

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Gianninas et al. (2015 ApJ, in prep)

There is still a lot of uncertainty in the evolutionary models

• Althaus et al. (2013, A&A, 557, A19)

• Istrate et al. (2014, A&A, 571, L3)

Some ELM WDs may be Type Ia progenitors

• Super-Chandrasekhar mass systems not necessarily Type Ia progenitors

• WD+NS binaries? No radio or X-ray detections

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Brown et al. (2013, ApJ, 769, 66)

Some ELM WDs are likely progenitors of AM CVn systems

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Gianninas et al. (2015, in prep)

Cooler ELM WD binaries have longer periods or else they would have already merged

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Gianninas et al. (2015, in prep)

K-S statistic = 0.5622

INDIVIDUAL HIGHLIGHTS

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J0651: the poster child for ELM WDs

• Shortest period ELM WD binary → P = 12.75 min!

• Eclipsing!

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Hermes et al. (2012, ApJ, 757, 21)

The shift in the midpoint of the eclipses reveals that the orbit is decaying !

23 Hermes et al. (2012, ApJ, 757, 21)

The rate of decay agrees with the prediction of General Relativity!

24 Hermes et al. (2015, in prep)

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PSR B1913+16: The Hulse-Taylor pulsar

Weisberg & Taylor (2005, ASPC, 328, 25)

• Orbital decay of a binary pulsar system (PSR +NS)

• Test of GR • 1993 Nobel Prize in

Physics • It took 30 years to

display the same period shift as J0651!

WD 0931+444: a new 20-min ELM WD!

• Preliminary SDSS DR10 selection

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Kilic et al. (2014, MNRAS, 441, L1 )

WD 0931+444 looks like a DA+dM…

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Kilic et al. (2014, MNRAS, 441, L1 )

MMT 6.5m SDSS

DA+dM1 : Silvestri et al. (2006) Rebassa-Mansergas et al. (2007,2010) Heller et al. (2009)

Gemini data shows that M dwarf is indeed a background object

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Kilic et al. (2014, MNRAS, 441, L1 )

GMOS-North time resolved spectroscopy

Na I doublet Hα

ELM WDs are among the best gravitational wave verification sources for eLISA

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Kilic et al. (2015, ASSP, 40, 167)

Average Galactic Foreground Nelemans et al. (2001)

eLISA after 2 years Amaro-Seoane et al. (2013)

Several PSR have ELM WD companions

• Recent example: triple system PSR J0337+1715 (Ransom et al., 2014, Nature)

• Several of the ELM WD companions are in the correct range of Teff and log g (i.e. instability strip) to pulsate

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ELM WD instability strip is an extension of ZZ Ceti Instability Strip

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Gianninas et al. (2014, ApJ, 794,35)

Several PSR have ELM WD companions

• Recent example: triple system PSR J0337+1715 (Ransom et al., 2014, Nature)

• Several of the ELM WD companions are in the correct range of Teff and log g (i.e. instability strip) to pulsate

• No pulsations detected for ELM companions of PSR J1012+5307+ and PSR J1911-5958 (Steinfadt et al. 2010)

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PSR J1909-3744

• No strong evidence for pulsations

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Kilic et al. (2015, MNRAS, 446,L26)

PSR J1738+0333

• First pulsating ELM WD companion to a PSR!

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Kilic et al. (2015, MNRAS, 446, L26)

Updated ELM WD instability strip

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Kilic et al. (2015, MNRAS, 446, L26)

TAKEAWAYS

• ELM Survey has discovered 74 ELM WDs and many more are in the pipeline – Starting to be able to do some statistics

• 2014 Highlights: – J0651 : detection of orbital decay

– Discovery of a new 20-min system : 2nd gravitational wave verification source

– Discovery of first PSR + pulsating ELM WD

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Collaborators

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University of Oklahoma: Mukremin Kilic, Paul Canton Smithsonian Astrophysical Observatory: Warren Brown, Scott Kenyon University of Warwick: JJ Hermes Université de Montréal: Patrick Dufour, Pierre Bergeron University of Texas: Keaton Bell, Samuel Harrold Instituto de Astrofisica de Canarias: Carlos Allende Prieto