A NearIR Spectroscopic Search for VeryLowMass Companions to White Dwarfs

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A Near-IR Spectroscopic Search for Very-Low-Mass
Companions to White Dwarfs

• P. Steele, M. Burleigh P. Dobbie
• University of Leicester, UK
• Anglo-Australian Observatory, Australia

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Overview

• Background
• Analysis of Data
• Results
• PG1234482
• Ongoing Future Work
• Conclusions

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BackgroundWhy White Dwarfs?

• White Dwarfs are 10,000x less luminous than their
  MS progenitors.
• Ideal Candidates for Spectroscopic search for
  low-mass companions (Brown Dwarfs/Gas Giants).
• Look in Near/Mid-IR.

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BackgroundWhite Dwarfs in Binaries

• Many examples of white dwarfs with early to mid-M
  type companions.
• Unresolved systems identified through excess in
  near-IR photometry
• A further proportion revealed through excess
  emission in optical spectra.

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BackgroundExample Binary System
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BackgroundWhite Dwarfs in Binaries

• In contrast late-M and brown dwarf companions to
  white dwarfs are rare.
• Can be revealed as near-IR photometric excesses
  or by features characteristic of a cool companion
  in the IR spectrum.
• Only 3 confirmed brown dwarf companions - GD165,
  GD1400, WD0317

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BackgroundWhy?

• In some apparently isolated WDs there are
  unusual heavy metal abundances.
• Implies external accretion.
• Unless high-Z material from inter-stellar clouds
  then there must be some unseen source associated
  with the WD.
• E.g. EG102 - Mg, Al, Si Ca observed.

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BackgroundWhy?

• Recently gaseous/dust disks observed around some
  WDs (GD362, WD1150).
• Revealed through excess K emission.
• Remnants of planetary systems?
• Place constraints on fraction of normal stars
  with low mass (sub)-stellar companions.

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BackgroundExample Dust/Gas Disk
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Analysis of DataBuilding our Spectra

• Model WD - generate pure-H synthetic spectrum
  using T and log g.
• Normalize model to V mag. of WD.
• Extract Observed Near-IR spectrum of WD.
• Scale observed spectrum to 2MASS J, H and K
  photometric fluxes.

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Analysis of DataBuilding our Spectra
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Analysis of DataLooking for Cool Companions

• Look for difference between level/shape of
  observed synthetic fluxes.
• Search for specific features typical of M, L or T
  dwarfs.
• E.g. KI , NaI CO (1.25, 2.2 2.3 microns
  respectively).
• Convolve BD spectra to type/limit.

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Analysis of DataLooking for Cool Companions
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Analysis of DataLooking for Cool Companions
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ResultsA Near-IR Spectroscopic Search for
Very-Low-Mass Companions to DA White Dwarfs
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ResultsPG1234482

• Previous photometric evidence suggest low mass
  companion. (Green 1986).
• Debes, Sigurdsson Woodgate (2003) - M8,
  Mullally et al. (2006) - L0.
• Recently we used WHT service time to get an HK
  spectrum.

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ResultsWHT HK Spectrum of PG1234482
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ResultsPG1234482

• At a distance of 135pc (Farihi et al. 2005)
  spectrum looks like L1.
• Other distance estimates lead to spectral type of
  M9.
• Need high resolution, high S/N K spectroscopy to
  spectral type from CO.
• Conclusion Type L01 (M9-L1).

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Ongoing Future Work

• Mass Limits (Spitzer)
• DODO
• SuperWASP
• UKIDSS

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Conclusions

• Very-low-mass companions to white dwarfs are
  rare. (3 BDs confirmed).
• No T spectral types or cooler known.
• Dwarfs and dust/gas disks revealed through IR
  excess.
• Best bet for planet hunters are transits.