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Transition Region Heating and Structure in M Dwarfs: from Low Mass to Very Low Mass Stars

Transition Region Heating and Structure in M Dwarfs: from Low Mass to Very Low Mass Stars. Rachel Osten Hubble Fellow University of Maryland/NASA GSFC. In collaboration with: Suzanne Hawley (U. Washington) Chris Johns-Krull (Rice U.)

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Transition Region Heating and Structure in M Dwarfs: from Low Mass to Very Low Mass Stars

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  1. Transition Region Heating and Structure in M Dwarfs:from Low Mass to Very Low Mass Stars Rachel Osten Hubble Fellow University of Maryland/NASA GSFC In collaboration with: Suzanne Hawley (U. Washington) Chris Johns-Krull (Rice U.) also J. Allred (U. Washington), A. Brown, G. M. Harper (Colorado)

  2. Magnetic Activity manifestationsin Solar-like Stars Persistent & transient mag. activity Scaling laws constrain heating processes Ha emission (104K) Coronal emission (106K) Radio radiation (nonthermal radiation) sunspots White 2002

  3. The Transition Region Couples the Chromosphere to the Corona • At lower regions of atmosphere, gas pressure, fluid motions dominate dynamics & structure (emission optically thick) • At higher regions of atmosphere, magnetic forces dominate (emission generally optically thin, opacity in some lines) • Multiple temperature diagnostics, can “invert” emission line fluxes to constrain the amount of material 1-D model of the solar atmosphere

  4. Quiescent Structures on Active M dwarfs By combining spectroscopy with HST/STIS, FUSE, EUVE, and Chandra, we can determine the characteristics of the quiescent emission EV Lac: dM3.5e classic flare star active radio: X-ray Osten et al. 2006

  5. Quiescent Structures on Active M dwarfs Osten et al. 2006 EV Lac Constant pressure fobs/fpred

  6. Quiescent Structures on Active M dwarfs Energy Balance ·Fc+·Fr = ·Fh Consequence of large densities, presssures Fr(Te)=nenH(Te) ds Fc(Te)=-Te5/2 dTe/ds Large energy inputs at coronal temperatures hard to envision under static energy balance  Steep temperature gradients, large conductive loss rates: dynamic situation leading to mass flows is inevitable  Flare heating arguments may instead be valid Osten et al. 2006

  7. Take same approach & apply to very low mass stars • Signatures of magnetic activity observed at spectral types > M7: Ha, UV, X-ray emission • Magnetic heating is able to occur, despite low degrees of ionization in atmospheres, large resistivities decouple matter & field • “Activity” appears to be decoupled from rotation, interiors are fully convective • Recent discovery of large magnetic field strengths (Reiners & Basri 2007) implies that large-scale fields can exist: what is their role in atmospheric heating?

  8. West et al. (2004) Complexities in interpreting magnetic activity signatures • Marked decrease in numbers of objects showing Ha in emission • Breakdown in rotation-activity connection for ultracool stars & brown dwarfs: magnetic activity is dying But. . . Although the absolute numbers of objects showing Ha in emission is dropping precipitously past M8, the average Ha properties are not: chromospheric heating efficiency is roughly the same

  9. X-ray emission from field dwarfs flares Stelzer (2004) Large scatter in coronal heating efficiency at early spectral types; range is similar to that in later spectral types, where span is due to quiescence/flares quiescence

  10. Are we seeing a continuation of activity? BD pair: Ba 55-87 Mjup Bb 34-70 Mjup • X-ray spectra detected with persistent emission are qualitatively similar to quiet solar corona; • Lx/LHa scaling same as for earlier M spectral type dwarfs (Fleming et al. 2003) • Detection of emission lines in HST/STIS spectra indicate transition region emission can be both persistent & transient in nature (Hawley & Johns-Krull 2003) M2V Companionship to Gl 569A constrains age of brown dwarf pair 300-800 Myr; Stelzer (2004)

  11. Study TR emission from 3 VLM stars M8 Hawley & Johns-Krull (2003) M7 M9

  12. Scaling laws Byrne & Doyle (1989) compared UV fluxes from dMe stars with two dM Stars; scaling relations between C IV, He II, and X-ray fluxes Power-law fits to dMe stars

  13. Volume differential emission measures VB 8 VB 10 LHS 2065

  14. Comparison with dMe stars, Quiet Sun Column differential emission measure

  15. Transition region heating rates similar to the dMe flare star EV Lac Caveat: don’t have a constraint on electron density, assume constant pressure at same value as for EV Lac transition region Power input (erg/s) is the same, to within factors of a few In EV Lac, the corona was where all hell was breaking loose

  16. Conclusions • More work is needed to understand discrepancies of Li, Na-like isoelectronic sequences • TR densities: constant pressure (into lower coronae?) Coronal densities imply large pressures, which necessitate large conductive fluxes • Disparity in emitting volumes at different coronal temperatures • Transition region fluxes for VLM stars consistent with those of dM, dMe stars, TR structures also apparently consistent

  17. Future Work • Add coronal information to VLM stars: T, EM can constrain losses & corresponding heat inputs • Add in AD Leo, another flare star with well-exposed STIS spectrum & high-res Chandra spectrum, for comparison with EV Lac and VLM stars

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