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
• Dynamo mechanism Parker (1955) (the aW dynamo); Babcock,
Durney, Rosner
• “shell dynamo” field is generated between the convective
and radiative zones
• Radial differential rotation shears an initial poloidal field
• Generates an internal toroidal field 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 looplike structures, producing active regions with field strengths
of 1-2 kGauss
• Surface magnetic fields weak compared to stronger interior
fields (104-105 Gauss)
What causes the solar
small-scale magnetic field?
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?
The
Chromosphere
•
Remember the Sun:
•
In M dwarfs, a global average is the best we can do
– 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
Chromospheres of M dwarfs
• The chromosphere extends through the region of
partial hydrogen ionization
–
–
–
–
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
Measuring
Chromospheric
Activity
• Trace the change in the emission of the
calcium K line along a slit placed across the
Sun.
• the amount of emission changes as the slit
passes over magnetically active and quiet
areas on the solar surface
The Ca II K
line index
• Narrow band filter centered on the Ca
II K line
• Measure the strength of the emission
compared to nearby “continuum”
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?)
H & K
Obs in
Solar
Type
Stars
• Representative Ca II H&K observations of Sun-like stars from
the Mount Wilson program (Baliunas)
• Chromospheric activity is expressed in terms of the Mt.
Wilson S index.
Chromospheric
Activity in
Solar-Type
Stars
• Chromospheric activity in 800 southern G dwarfs
(Soderblom)
• log R’HK is a common measure for expressing the
activity level
• the Sun lies at B – V = 0.65 and log R’HK ≈ -4.95, in
the middle of the “inactive” star classification band.
M67 Data from
Giampapa
• The Sun is in a relatively
moderate state of
activity.
• About 40% of the time
the Sun is likely to be
either significantly more,
or significantly less,
active.
• A change to either of
these states is likely to
cause significant changes
in the Earth's climate.
• Excursions in the
luminosity of the Sun
from about 0.2% - 0.5%
are possible, compared
with the observed 0.1%
variations
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?
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
M Dwarf Magnetic
Field Models
• Red dwarf stars of
type M5 or smaller are
fully convective
• Turbulent motion
generates and enhances
magnetic fields
• Fields appear the form
of solar (or stellar)
spots, or flares
• Simulated magnetic
fields in fully
convective stars
Wolfgang Dobler: http://www.kis.uni-freiburg.de/~dobler/
Spectrum of
EV Lac & 5
Standards
Johns-Krull & Valenti (1996)
• Most features are TiO (strengthen with increasing spectral type)
• Zeeman-sensitive Fe I line at 8468.40 Å
• Zeeman-split components are visibly shifted out of the line core
and into the wings, allowing a fairly direct determination of the
magnetic field strength.
EV Lac +
Gliese 729
• Zeeman components
are indicated
• 50% of the
photosphere of EV
Lac is covered by 3.8
± 0.5 kG magnetic
fields
• 50% of Gliese 729 is
covered by 2.6 ± 0.3
kG fields
Johns-Krull & Valenti (1996)