Transcript Stellar Activity

Stellar Activity
• Chromospheric activity is defined as:
– The variability of a chromosphere
and/or corona
– Spots (plage and dark spots)
– Flares
• Associated with convection, magnetic
fields, rotation
The Solar Magnetic Field
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Dynamo mechanism first proposed by Parker (1955) (the aW
dynamo); later dynamo models by Babcock, Durney, Rosner
Effectively “shell dynamo” models in that the field is generated at
the interface between the convective and radiative zones in the
interior
Radial differential rotation shears an initial poloidal field
Generates an internal toroidal field located at the base of the
convective zone
Small scale cyclonic motions within the toroidal field generate a
new poloidal field also in the vicinity of the base of the convective
zone
The regeneration of the poloidal field with opposite polarity marks
the beginning of a new 11-year cycle
Bundles of the toroidal field are broken off by turbulence in the
convective zone, rise to the surface, and appear as loop-like
structures, producing active regions with field strengths of 1-2
kGauss
Surface magnetic fields just a “shadow” of the stronger interior
fields (104-105 Gauss)
Solar magnetic field is structured, cyclic
Babcock Model of the Dynamo
Turbulent Dynamo Model
• Solar “intranetwork” magnetic fields
• Vary little during the solar cycle
• Magnetic fields produced by random convective
motions
– No rotation or differential rotation needed
– No radiative-convective boundary needed
• Field forms flux tubes, rise to surface, merge with
regions of opposite polarity, and are destroyed
• No cycles
• Coverage uniform over the stellar surface
• May work for fully convective M dwarfs
• But are the large field strengths possible?
Magnetic Fields in M Stars
• Measuring magnetic fields in M dwarfs is tricky
– Select IR lines with large Lande g factors
– Compare to lines with small Lande g factors
– Determine both field strength and filling factor (the
rest of the star assumed to have no field)
– Model line profiles with thermal, turbulent, collisional,
and rotational broadening
• Field strengths typically 2-4 kG with 50-80%
filling factors in dMe’s
• No evidence of globally organized fields (many
small active regions?)
• From limited data, fields do not seem to vary, even
when Ha varies a lot
The Outer Atmosphere
• Remember the Sun:
– From t = 1, temperature decreases to the TMR (temperature
minimum region)
– Energy balance still reflects radiative equilibrium
– Magnetic heating (non-radiative) causes the temperature to rise to
a plateau near 7000K (chromosphere); density falls by orders of
magnitude
– Plateau results from a balance between magnetic heating and
radiative cooling from collisionally excited Ha, Ca II K, Mg II k –
the principal diagnostic lines formed in the chromosphere
– Collisional excitation from electrons from ionizing H
– Then temperature rises abruptly through the transition region
(density too low, collisional excitation less, less cooling)
– Temperature stabilizes at ~106 K in the corona
– This picture is a global average in the Sun – we know it matches
neither quiescent nor active regions of the solar atmosphere
• In M dwarfs, a global average is the best we can do
Chromospheres of M dwarfs
• The chromosphere extends through the region of
partial hydrogen ionization
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About 1000 km in the Sun
Much broader in giants
Very compressed in M dwarfs
Explains the Wilson-Bappu effect
• With higher densities, cooling is much stronger
• Balmer lines are the primary source of cooling in M
dwarf chromospheres (and Ha is the principal
diagnostic line)
• Inconsistencies in fitting Ca II K, Mg II k, Balmer
and Lyman lines – attributed to inhomogeneous
surface structures (spots and plage)
• What provides the heating?
– In the Sun, acoustic heating may play some role
– In M dwarfs, probably not
The Transition Region
• Once H is ionized, collisional cooling is reduced and
temperature rises again
• Temperature rise is counter-intuitive because
cooling is provided by numerous resonance
transitions, and stops when cooling lessens
• The Energy balance is between conductive heating
and radiative cooling
• Principal emission lines from the TR are upper
ionization states of C, N, O, S, Si in the UV
• Behavior of the TR in M dwarfs differs from the
Sun, but only limited data are available because
dM’s are faint
• Less correlation between chromospheric and TR
lines in dM’s than in the Sun
The Corona
• M dwarfs are relatively bright x-ray sources
• Corona extends as much as a stellar radius above
the photosphere
• Temperatures up to a few million degrees
• Coronal emission primarily in soft x-rays (0.1-1
KeV), collisionally excited emission lines of high
ionization states of Fe and other heavy elements
• Structures defined by magnetic active regions
(loops with feet in photospheric active regions)
• Mass loss flows out along open field lines
• Diagnostic lines include C II, C IV, Si III, N V,
which are all highly temperature sensitive – used
to define temperature structure
• “Two component models” used to fit data
Spots and Spot Cycles
• The Sun provides a template
for understanding spots in
other stars
– Multi-year cycles
– Rotational modulation
– Age-rotation-activity
correlation
• Young stars don’t show
cyclic behavior, but older
stars do
• Some stars have very low
level of activity and no
cycles (Maunder minumum?)
• The Sun is brighter when it
is more active (more plage)
• In M dwarfs, very limited
evidence for spot modulation or
spot cycles
• Sometimes spots are present,
sometimes not
• Variable light levels – long
period, low amplitude
modulation? (mostly in dM’s with
M>0.5MSun)
• Spots may come and go on short
time scales or be distributed
evenly around the star
• Large isolated spots are NOT
common
• Evidence for turbulent dynamo?
Activity Cycles in Other Stars
• Chromospheric and coronal activity are characteristic of
most lower main sequence stars
• Rotational modulation is observed
– 50-100 Myr-old stars: 0.1-0.15 mag, P=days
– 500 Myr-old stars: 0.02-0.05 mag, P=days to weeks
– 5 Gyr-old stars: nearly constant on short timescales
• Stars often show longer term activity cycles like the Sun’s
– Young stars show changes in mean brightness of several % from
changes in surface markings, both bright and dark, but
brightness varies inversely with chromospheric activity
– Hyades show year-to-year brightness changes of order 0.04
mag over times of several years
– For older stars, long term brightness changes ~0.01 mag,
changes correlate with chromospheric activity
• Mt. Wilson Sample:
– 60% have periodic (or nearly) magnetic activity cycles
– 15% variable, with no obvious periodicity
– 10-15% non-variable (Maunder minimum stars?)
Activity Cycles
Young Stars
Old Stars
The Sun
Main Sequence
Age
1 Gyr
Few Gyr
4.6 Gyr
Mean
chromospheric
flux ratio
0.31
0.17
0.17
Mean rotation
period
9.1d
27 d
25 d
Cycle behavior
Periodic or
erratic; none
are flat
Periodic, ¼ are
flat
Periodic, 1/3
are flat
Correlation of
magnetic
activity and
flux?
inverse
correlated
correlated