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 first proposed by Parker (1955) (the  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 ( 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 H  varies a lot

The Outer Atmosphere Remember the Sun: –From  = 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 ~10 6 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 –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 H  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>0.5M Sun ) 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 – Myr-old stars: mag, P=days –500 Myr-old stars: 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 StarsOld StarsThe Sun Main Sequence Age 1 GyrFew Gyr4.6 Gyr Mean chromospheric flux ratio Mean rotation period 9.1d27 d25 d Cycle behavior Periodic or erratic; none are flat Periodic, ¼ are flat Periodic, 1/3 are flat Correlation of magnetic activity and flux? inversecorrelated