IAU Symposium 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) 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)
Incidence of (H ) activity decreases past M7 Gizis et al. (2000)
Strength of (H ) activity decreases past M7 Burgasser et al. (2002)
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 )
VB8 (M7) C IV spectral region ( A) shown Sequence of 15 exp. (5 min each) spanning two orbits Red shows the 1-sigma uncertainty limits
VB10 (M8) C IV spectral region ( A) shown Sequence of 15 exp. (10 min each) spanning four orbits Red shows the 1-sigma uncertainty limits Note: large flare in exposure 7
LHS2065 (M9) C IV spectral region ( A) shown Sequence of 15 exp. (10 min each) spanning four orbits Red shows the 1-sigma uncertainty limits
Integrated spectra - typical M dwarf emission
C IV time series – quiescent emission … except for that one big flare on VB 10!! (exp. 7)
Observed C IV emission fluxes Observed and Predicted C IV fluxes: VB 8 (M7) VB 10 (M8) 2.8(1.4) 0.9 LHS 2065 (M9) (units of ergs/s/cm 2 ) All within a factor of 2 of predicted!
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