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)

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

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

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

Quiescent Structures on Active M dwarfs

Osten et al. 2006

Constant pressure

EV Lac

f ob

s/f p

red

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

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?

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

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

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

Study TR emission from 3 VLM stars

Hawley & Johns-Krull (2003)

M8

M7

M9

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

VB 8 VB 10 LHS 2065

Volume differential emission measures

Comparison with dMe stars, Quiet Sun

Colu

mn

diff

ere

nti

al em

issi

on

measu

re

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

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

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