The enigmatic young brown dwarf binary FU Tau: accretion and activity

B. Stelzer*, A. Scholz, C. Argiroffi, G. Micela

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

10 Citations (Scopus)

Abstract

FU Tau belongs to a rare class of young, wide brown dwarf binaries. We have resolved the system in a Chandra X-ray observation and detected only the primary, FU TauA. Hard X-ray emission, presumably from a corona, is present but, unexpectedly, we also detect a strong and unusually soft component from FU TauA. Its X-ray properties, so far unique among brown dwarfs, are very similar to those of the T Tauri star TW Hya. The analogy with TW Hya suggests that the dominating soft X-ray component can be explained by emission from accretion shocks. However, the typical free-fall velocities of a brown dwarf are too low for an interpretation of the observed X-ray temperature as a post-shock region. On the other hand, velocities in excess of the free-fall speed are derived from archival optical spectroscopy, and independent pieces of evidence for strong accretion in FU Tau A are found in optical photometry. The high X-ray luminosity of FU TauA coincides with a high bolometric luminosity confirming an unexplained trend among young brown dwarfs. In fact, FU Tau A is overluminous with respect to evolutionary models while FU Tau B is on the 1-Myr isochrone suggesting non-contemporaneous formation of the two components in the binary. The extreme youth of FU TauA could be responsible for its peculiar X-ray properties, in terms of atypical magnetic activity or accretion. Alternatively, rotation and magnetic field effects may reduce the efficiency of convection which in turn affects the effective temperature and radius of FU Tau A, shifting its position in the Hertzsprung-Russell diagram. Although there is no direct proof of this latter scenario so far, we present arguments for its plausibility.

Original languageEnglish
Pages (from-to)1095-1102
Number of pages8
JournalMonthly Notices of the Royal Astronomical Society
Volume408
Issue number2
DOIs
Publication statusPublished - 21 Oct 2010

Keywords

  • MAGNETOCENTRIFUGALLY DRIVEN FLOWS
  • TW-HYDRAE
  • X-RAY-EMISSION
  • MAGNETIC-FIELDS
  • accretion, accretion discs
  • FORMING REGION
  • SIGMA ORI CLUSTER
  • stars: pre-main-sequence
  • brown dwarfs
  • LOW-MASS STARS
  • MOLECULAR CLOUD
  • 2MASS J05352184-0546085
  • stars: activity
  • X-rays: stars
  • EVOLUTIONARY MODELS

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