1. The ELM Survey: A Targeted Search for the Most Extreme White Dwarf Binary Systems
Alex Gianninas
Astronomy Colloquium
Wesleyan University
February 25th, 2015

2. Outline Intro: Stellar Evolution & White Dwarfs 101
Extremely Low Mass White Dwarfs
The ELM Survey
Highlights from 2014
Takeaways

3. Stellar Evolution and White Dwarfs 101
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)

5. Stellar Evolution and White Dwarfs 101
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= 𝑮𝑴 𝑹 𝟐

6. Gravitational settling quickly creates a stratified internal structureHe
Gravitational settling quickly creates a stratified internal structure
C/O H He
Non-DA (He-rich)
C/O
C/O
DA (H-rich)
Hot DQ

7. Extremely Low Mass (ELM) WDs have M < 0
Extremely Low Mass (ELM) WDs have M < 0.3 M and form in compact binaries
Gianninas et al. (2011, ApJ, 743, 138)
Binary Evolution

8. Extremely Low-Mass (ELM) WDs have He cores and are formed in compact binaries
Iben & Tutukov (1984, ApJ, 54, 335)

9. ELM WDs have He cores and considerably larger radii
𝑹~ 𝟏 𝑴 𝟑
ELM WDs have He cores and considerably larger radii H H He He
C/O
ELM

10. 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 : new ELM WDs

11. SDSS colors work well for choosing ELM WD candidates
Brown et al. (2012, ApJ, 744, 142)
Gianninas et al. (2015, in prep)

12. Radial Velocity Follow-Up
FLWO 1.5m
MMT 6.5m
KPNO 4m

13. More Balmer lines are visible in ELM WDs due to the lower log g
The optical spectra of ELM WDs are dominated by the Hydrogen Balmer lines DA
ELM H H H H
J : systemic velocity ~ -300 km/s, possible Halo WD
J : first ELM WD from LAMOST candidates
More Balmer lines are visible in ELM WDs due to the lower log g

14. ELM VI: 12 new ELM WDs 𝑷 𝑲 𝟑 𝟐𝝅𝑮 = 𝑴 𝒇 = (𝒎 𝟐 𝐬𝐢𝐧 𝒊 ) 𝟑 (𝒎 𝟏 +𝒎 𝟐 ) 𝟐
J : systemic velocity ~ -300 km/s, possible Halo WD
J : first ELM WD from LAMOST candidates
𝑷 𝑲 𝟑 𝟐𝝅𝑮 = 𝑴 𝒇 = (𝒎 𝟐 𝐬𝐢𝐧 𝒊 ) 𝟑 (𝒎 𝟏 +𝒎 𝟐 ) 𝟐
Gianninas et al. (2015, in prep)

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

16. Standard Spectroscopic Technique
Gianninas et al. (2015, in prep)

17. 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)
Discuss lower limit for (ELM) WD masses
Gianninas et al. (2015 ApJ, in prep)

18. Some ELM WDs may be Type Ia progenitors
Brown et al. (2013, ApJ, 769, 66)
Super-Chandrasekhar mass systems not necessarily Type Ia progenitors
WD+NS binaries? No radio or X-ray detections

19. Some ELM WDs are likely progenitors of AM CVn systems
Gianninas et al. (2015, in prep)

20. Cooler ELM WD binaries have longer periods or else they would have already merged
K-S statistic =
Gianninas et al. (2015, in prep)

21. INDIVIDUAL HIGHLIGHTS

22. J0651: the poster child for ELM WDs
Shortest period ELM WD binary  P = min!
Eclipsing!
Hermes et al. (2012, ApJ, 757, 21)

23. The shift in the midpoint of the eclipses reveals that the orbit is decaying !
Hermes et al. (2012, ApJ, 757, 21)

24. The rate of decay agrees with the prediction of General Relativity!
Hermes et al. (2015, in prep)

25. PSR B1913+16: The Hulse-Taylor pulsar
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!
Weisberg & Taylor (2005, ASPC, 328, 25)

26. WD 0931+444: a new 20-min ELM WD! Preliminary SDSS DR10 selection
Kilic et al. (2014, MNRAS, 441, L1 )

27. WD 0931+444 looks like a DA+dM… DA+dM1 : Silvestri et al. (2006)
Rebassa-Mansergas et al. (2007,2010)
Heller et al. (2009)
MMT 6.5m
SDSS
Kilic et al. (2014, MNRAS, 441, L1 )

28. Gemini data shows that M dwarf is indeed a background object
GMOS-North time resolved spectroscopy
Na I doublet H
Kilic et al. (2014, MNRAS, 441, L1 )

29. ELM WDs are among the best gravitational wave verification sources for eLISA
eLISA after 2 years
Amaro-Seoane et al. (2013)
Average Galactic Foreground
Nelemans et al. (2001)
Kilic et al. (2015, ASSP, 40, 167)

30. Several PSR have ELM WD companions
Recent example: triple system PSR J (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

31. ELM WD instability strip is an extension of ZZ Ceti Instability Strip
Gianninas et al. (2014, ApJ, 794,35)

32. Several PSR have ELM WD companions
Recent example: triple system PSR J (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 J and PSR J (Steinfadt et al. 2010)

33. PSR J1909-3744 No strong evidence for pulsations
Kilic et al. (2015, MNRAS, 446,L26)

34. PSR J1738+0333 First pulsating ELM WD companion to a PSR!
Kilic et al. (2015, MNRAS, 446, L26)

35. Updated ELM WD instability strip
Kilic et al. (2015, MNRAS, 446, L26)

36. 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

37. Collaborators 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