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Searching for dying solar systems: Planets around White Dwarfs

Searching for dying solar systems: Planets around White Dwarfs. Matt Burleigh, Leicester Fraser Clarke, Oxford Emma Hogan, Leicester Simon Hodgkin, Cambridge. Improving Contrast. To understand extrasolar planets, we need their light!

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Searching for dying solar systems: Planets around White Dwarfs

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  1. Searching for dying solar systems:Planets around White Dwarfs Matt Burleigh, Leicester Fraser Clarke, Oxford Emma Hogan, Leicester Simon Hodgkin, Cambridge

  2. Improving Contrast • To understand extrasolar planets, we need their light! • None of the radial velocity planets can be imaged with current technology • Planet is too faint and too close to the star • Two solutions; • Remove the starlight (technology: AO, coronography, interferometry) • Remove the star (stellar evolution) Above: Gl229B – brown dwarf companion to nearby M dwarf Observe White Dwarfs!

  3. Surviving the RGB • Red Giant expands to maximum ~5AU • Planets inside 5AU are destroyed by Red Giant • Planets outside ~5AU will migrate outward due to mass loss and survive • Dynamical time ~10-30 yr • mass loss time ~1000-10000yr • planets stay bound

  4. The Benefits of White Dwarfs • WD's are ~10,000 times fainter than their progenitors • => Huge contrast gain • Planets orbit increases • => Big resolution gain • And >120 WDs within 20pc

  5. The Benefits of White Dwarfs • PLUS the WD progenitors are more massive than solar-type stars • Mainly A and B stars • Not generally being targeted by radial velocity programmes • So by targeting WDs we are probing frequency of planets around massive stars

  6. Planet brightness v age Solid lines Burrows 1997 models, dashed lines Burrows 2002 models Models assume evolution in isolation: no addition heating source or reflection component

  7. Imaging planets around white dwarfs • Large format NIR cameras on 8m-class telescopes • Typically reach J~24 in 1 hour (e.g. VLT+ISAAC, Gemini+NIRI) • Comparable with predicted magnitudes of planets around nearby white dwarfs • No immediate need for AO • Searching outside star’s PSF

  8. Cool companions to WDs • Becklin & Zuckerman, Probst in the 1980s • IR excess in WDs may indicate cool companion • First L dwarf discovered is a companion to a WD, GD165 • But brown dwarfs are not common companions to WDs • Only one more found so far (GD1400B, L6/7, Farihi et al. 2004) • Confirms brown dwarf desert at wide separations • No companions >10MJup found among Hyades WDs • Zinnecker & Friedrich, in prep. (HST/NICMOS)

  9. Strategy • Select young (<3Gyr), nearby (<20pc) white dwarfs • (0.2”yr < PM < few “/yr) • Sample ~40 stars • Progenitors mainly A/B stars (short main sequence lifetimes) • Obtain deep wide IR (J) images. • Total exp time ~1hr • Depth J~23.5 (GN) to ~24 (VLT) • Image quality typically 0.4”-0.6” • Wait 1—2 years… • Obtain 2nd epoch images of all systems to check for common proper motion companions

  10. Parameter space • For comparison with other planet search techniques; • Planet mass; >5 Mjup • Orbit; 5—1000 AU • Age; 0.5—3 Gyr • Star mass; 2—7 Msun (A and B stars) • Complimentary to other search techniques

  11. Not a discovery

  12. White dwarfs • Image depth J~24

  13. Two epochs for Proper Motion • One epoch in One colour tells us nothing. • Faint objects could be faint companions, or they could simply be far away… • Any object in the field could be a companion!! (orbital expansion) • 2nd epoch observations are needed to confirm companions via proper motion.

  14. ^ ~90” V • Two epochs • June 2002 GS+Flamingos-I • October 2003 VLT+ISAAC • WD motion ~1” between images • Image depth J~23.5 < ~120” >

  15. Motions in field • Arrows show direction and degree of motion x factor 20 • WD moved ~9 pixels in 15 months (~1”)

  16. A non-detection • Circles: 1s scatter on distribution of proper motions of background objects

  17. A detection? • Circles: 1s error on PMs • If associated, candidates are 7-10MJup • Would have originally orbited at ~65AU & ~75AU • First epoch June 2002, second June 2003

  18. Add third epoch (June 2004) • Candidates have gone away!

  19. Summary • White dwarfs open up more parameter space for planet surveys • Direct imaging of planets >5MJup • Probing frequency of planets around massive stars (>2MSun) • Sensitivity required is achievable with 8m telescopes in near-IR • Our survey is beginning to reach maturity • 2 epochs for 12 systems • 40 systems by 2006

  20. Further work…. • Is a sample of 40 enough? • Coronographic searches of nearby young stars indicate frequency of companions >5MJup beyond 75AU is <3% (McCarthy & Zuckerman 2004) • Plus want to probe to lower masses (<5MJup) • Spitzer mid-IR observations for photometric excesses (several programmes in progress)

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