The ELM Survey: A Targeted Search for the Most Extreme

White Dwarf Binary Systems

Alex GianninasAstronomy ColloquiumWesleyan UniversityFebruary 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)𝒈=

𝑮𝑴𝑹𝟐

Gravitational settling quickly creates a stratified internal structure

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C/O

HeH

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

7

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

HeH

He

H

ELM

𝑹𝟏𝑴𝟑

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

13

More Balmer lines are visible in ELM WDs due to

the lower log g

DA ELM

HH HH

ELM VI: 12 new ELM WDs

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Gianninas et al. (2015, in prep)𝑷 𝑲𝟑

𝟐𝝅 𝑮=𝑴 𝒇=¿¿¿

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

INDIVIDUALHIGHLIGHTS

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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 !

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

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

24Hermes 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 dwarfis indeed a background object

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

GMOS-North time resolved spectroscopy

Na I doublet H

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 ForegroundNelemans et al. (2001)

eLISA after 2 yearsAmaro-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

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

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PSR J1738+0333

• First pulsating ELM WD companion to a PSR!

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