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:  H  frequency and strength decrease (Gizis, Burgasser)  X-ray results inconclusive (too faint) (Fleming)  Very strong flares observed in H  and X-rays (Fleming; Liebert; Rutledge; …)  Theory (atmospheres neutral) (Fleming; Mohanty & Basri)

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

4. Strength of (H  ) 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(H  )

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/cm 2 ) 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