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IAU Symposium 219 22 July 2003

IAU Symposium 219 22 July 2003. Persistent Transition Region Emission in Very Low Mass Stars Suzanne L. Hawley University of Washington and Christopher M. Johns-Krull Rice University. Magnetic activity disappears at late types?. Conjecture: (Rutledge et al. 2000)

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IAU Symposium 219 22 July 2003

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  1. IAU Symposium 219 22 July 2003 Persistent Transition Region Emission in Very Low Mass Stars Suzanne L. Hawley University of Washington and Christopher M. Johns-Krull Rice University

  2. Magnetic activity disappears at late types? • Conjecture: (Rutledge et al. 2000) • High temperature plasma only exists during flares (no quiescent TR/corona) • Evidence: • Ha frequency and strength decrease (Gizis, Burgasser) • X-ray results inconclusive (too faint) (Fleming) • Very strong flares observed in Ha and X-rays (Fleming; Liebert; Rutledge; …) • Theory (atmospheres neutral) (Fleming; Mohanty & Basri)

  3. Incidence of (Ha) activity decreases past M7 Gizis et al. (2000)

  4. Strength of (Ha) activity decreases past M7 Burgasser et al. (2002)

  5. Searching for TR emission in M7-M9 dwarfs • Test: • Use HST to search for TR emission (C IV) to very faint limits • Choose most active, brightest late-type stars as targets (M7-VB8; M8-VB10; M9-LHS2065) to obtain best limits • Exposure times chosen so that limits would be x10 less than predicted based on scaling from earlier type M dwarfs: F(C IV) ~ 0.1 F(Ha)

  6. VB8 (M7) C IV spectral region (1500-1600A) shown Sequence of 15 exp. (5 min each) spanning two orbits Red shows the 1-sigma uncertainty limits

  7. VB10 (M8) C IV spectral region (1500-1600A) shown Sequence of 15 exp. (10 min each) spanning four orbits Red shows the 1-sigma uncertainty limits Note: large flare in exposure 7

  8. LHS2065 (M9) C IV spectral region (1500-1600A) shown Sequence of 15 exp. (10 min each) spanning four orbits Red shows the 1-sigma uncertainty limits

  9. Integrated spectra - typical M dwarf emission

  10. C IV time series – quiescent emission … except for that one big flare on VB 10!! (exp. 7)

  11. Observed C IV emission fluxes • Observed and Predicted C IV fluxes: • VB 8 (M7) 1.4 2.0 • VB 10 (M8) 2.8(1.4) 0.9 • LHS 2065 (M9) 0.7 1.0 (units of 10-15 ergs/s/cm2) All within a factor of 2 of predicted!

  12. Conclusion: late M dwarfs maintainTR/corona • Persistent, quiescent, high temperature plasma does exist on active M7-M9 stars as observed in C IV and other transition region emission • Recent Chandra observations also show persistent, quiescent X-ray emission on VB 10 (Fleming et al) • Need observations at lower masses (later types, brown dwarfs) • See: Hawley & Johns-Krull 2003, ApJL, 588, L109 for more details

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