Transition Region Heating and Structure in M Dwarfs: from Low Mass to Very Low Mass Stars
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Transition Region Heating and Structure in M Dwarfs:from Low Mass to Very Low
Mass StarsRachel OstenHubble 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)
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Magnetic Activity manifestations
in Solar-like Stars
H emission (104K)
Coronal emission(106K)
Radio radiation(nonthermal radiation)
Scaling laws constrain heating processes
Persistent & transient mag. activity
sunspotsWhite 2002
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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
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Quiescent Structures on Active M dwarfsBy combining spectroscopy with HST/STIS, FUSE,
EUVE, and Chandra, we can determine the characteristics of the quiescent emission
Osten et al. 2006
EV Lac: dM3.5eclassic flare staractive radio: X-ray
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Quiescent Structures on Active M dwarfs
Osten et al. 2006
Constant pressure
EV Lac
f ob
s/f p
red
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Quiescent Structures on Active M dwarfs
Osten et al. 2006
Energy Balance·Fc+·Fr = ·Fh
Consequence of large densities, presssures
Fr(Te)=nenH(Te) dsFc(Te)=-Te
5/2 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
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Take same approach & apply to very low mass
stars• Signatures of magnetic activity observed at
spectral types > M7: H, 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?
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Complexities in interpreting magnetic activity
signatures• Marked decrease in numbers of objects showing H in emission
• Breakdown in rotation-activity connection for ultracool stars & brown dwarfs: magnetic activity is dying
West et al. (2004)
But. . .
Although the absolute numbers of objects showing H in emission is dropping precipitously past M8, the average H properties are not: chromospheric heating efficiency is roughly the same
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X-ray emission from field dwarfs
Stelzer (2004)
flares
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
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Are we seeing a continuation of activity?
• X-ray spectra detected with persistent emission are qualitatively similar to quiet solar corona;
• Lx/LH 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)
Companionship to Gl 569A constrains age of brown dwarf pair 300-800 Myr; Stelzer (2004)
M2V
BD pair:Ba 55-87 Mjup
Bb 34-70 Mjup
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Study TR emission from 3 VLM stars
Hawley & Johns-Krull (2003)
M8
M7
M9
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Scaling lawsByrne & Doyle (1989) compared UV fluxes from dMe stars with two dMStars; scaling relations between C IV, He II, and X-ray fluxes
Power-law fits to dMe stars
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VB 8 VB 10 LHS 2065
Volume differential emission measures
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Comparison with dMe stars, Quiet Sun
Colu
mn
diff
ere
nti
al em
issi
on
measu
re
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Transition region heating
rates similar to the dMe
flare star EV LacCaveat: 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
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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
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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