Transcript Transition Region Heating and Structure in M Dwarfs

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)
Magnetic Activity manifestations
in Solar-like Stars
Persistent & transient mag. activity
Scaling laws constrain heating processes
Radio radiation
(nonthermal radiation)
Ha emission
(104K)
Coronal emission
(106K)
White 2002
sunspots
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 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
Quiescent Structures on Active M dwarfs
Osten et al. 2006
EV Lac
fobs/fpred
Constant pressure
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
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?
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
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
West et al. (2004)
But. . .
X-ray emission from field dwarfs
flares
Stelzer (2004)
quiescence
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
Are we seeing a continuation of
activity?
• 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 & JohnsKrull 2003)
BD pair:
Ba 55-87 Mjup
Bb 34-70 Mjup
M2V
Companionship to Gl 569A
constrains age of brown
dwarf pair 300-800 Myr;
Stelzer (2004)
Study TR emission from 3 VLM stars
M8
Hawley & Johns-Krull (2003)
M7
M9
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
Volume differential emission measures
VB 8
VB 10
LHS 2065
Column differential emission measure
Comparison with dMe stars, Quiet Sun
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
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 wellexposed STIS spectrum & high-res Chandra
spectrum, for comparison with EV Lac and VLM
stars