A search for L dwarf binary systems

I. Neill Reid, John E. Gizis, J. Davy Kirkpatrick, David W Koerner

Research output: Contribution to journalArticle

175 Citations (Scopus)

Abstract

We present analysis of Hubble Space Telescope (HST) planetary camera images of 20 L dwarfs identified in the course of the Two Micron All Sky Survey. Four of the targets, 2MASSW J0746425 + 200032, 2MASSs J0850359 +105716, 2MASSW J0920122 + 351742, and 2MASSW J1146345 + 223053, have faint red companions at separations between 0″.07 and 0″.29 (1.6-7.6 AU). Ground-based infrared imaging confirms the last as a common proper motion companion. The surface density of background sources with comparable colors is extremely low, and we identify all four as physical binaries. In three cases, the bolometric magnitudes of the components differ by less than 0.3 mag. Since the cooling rate for brown dwarfs is a strong function of mass, similarity in luminosities implies comparable masses. The faint component in the 2M0850 system, however, is over 1.3 mag fainter than the primary in the I band and ∼0.8 mag fainter in Mbol. Indeed, 2M0850B is ∼0.8 mag fainter in I than the lowest luminosity L dwarf currently known, while the absolute magnitude we deduce at J is almost identical with MJ for G1 229B. We discuss the implications of these results for the temperature scale in the L/T transition region. 2M0850 is known to exhibit 6708 Å Li I absorption, indicating that the primary has a mass less than 0.06 M. Theoretical models predict that the magnitude difference implies a mass ratio of ≈0.75. The apparent binary fraction of the current sample, 20%, is comparable to the results of previous surveys of late-type M dwarfs in the field and in the Hyades. However, the mean separation of the L dwarf binaries in the current sample is smaller than the M dwarf value by a factor of 2, and only one system would be detected at the distance of the Hyades. We discuss the likely binary frequency among L dwarfs in light of these new data.

Original languageEnglish (US)
Pages (from-to)489-502
Number of pages14
JournalAstronomical Journal
Volume121
Issue number1
StatePublished - 2001
Externally publishedYes

Fingerprint

luminosity
temperature scales
proper motion
Hubble Space Telescope
mass ratios
cameras
color
cooling
temperature
analysis
rate

Keywords

  • Binaries
  • Brown dwarfs
  • General - Stars
  • Low-mass

ASJC Scopus subject areas

  • Space and Planetary Science
  • Astronomy and Astrophysics

Cite this

Reid, I. N., Gizis, J. E., Kirkpatrick, J. D., & Koerner, D. W. (2001). A search for L dwarf binary systems. Astronomical Journal, 121(1), 489-502.

A search for L dwarf binary systems. / Reid, I. Neill; Gizis, John E.; Kirkpatrick, J. Davy; Koerner, David W.

In: Astronomical Journal, Vol. 121, No. 1, 2001, p. 489-502.

Research output: Contribution to journalArticle

Reid, IN, Gizis, JE, Kirkpatrick, JD & Koerner, DW 2001, 'A search for L dwarf binary systems', Astronomical Journal, vol. 121, no. 1, pp. 489-502.
Reid IN, Gizis JE, Kirkpatrick JD, Koerner DW. A search for L dwarf binary systems. Astronomical Journal. 2001;121(1):489-502.
Reid, I. Neill ; Gizis, John E. ; Kirkpatrick, J. Davy ; Koerner, David W. / A search for L dwarf binary systems. In: Astronomical Journal. 2001 ; Vol. 121, No. 1. pp. 489-502.
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AB - We present analysis of Hubble Space Telescope (HST) planetary camera images of 20 L dwarfs identified in the course of the Two Micron All Sky Survey. Four of the targets, 2MASSW J0746425 + 200032, 2MASSs J0850359 +105716, 2MASSW J0920122 + 351742, and 2MASSW J1146345 + 223053, have faint red companions at separations between 0″.07 and 0″.29 (1.6-7.6 AU). Ground-based infrared imaging confirms the last as a common proper motion companion. The surface density of background sources with comparable colors is extremely low, and we identify all four as physical binaries. In three cases, the bolometric magnitudes of the components differ by less than 0.3 mag. Since the cooling rate for brown dwarfs is a strong function of mass, similarity in luminosities implies comparable masses. The faint component in the 2M0850 system, however, is over 1.3 mag fainter than the primary in the I band and ∼0.8 mag fainter in Mbol. Indeed, 2M0850B is ∼0.8 mag fainter in I than the lowest luminosity L dwarf currently known, while the absolute magnitude we deduce at J is almost identical with MJ for G1 229B. We discuss the implications of these results for the temperature scale in the L/T transition region. 2M0850 is known to exhibit 6708 Å Li I absorption, indicating that the primary has a mass less than 0.06 M⊙. Theoretical models predict that the magnitude difference implies a mass ratio of ≈0.75. The apparent binary fraction of the current sample, 20%, is comparable to the results of previous surveys of late-type M dwarfs in the field and in the Hyades. However, the mean separation of the L dwarf binaries in the current sample is smaller than the M dwarf value by a factor of 2, and only one system would be detected at the distance of the Hyades. We discuss the likely binary frequency among L dwarfs in light of these new data.

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