Stellar Activity Chromospheric activity is defined as The

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 a. W 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 k. Gauss • 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 7000 K (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.

M 67 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 d. M’s with M>0. 5 MSun) • 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 k. G with 50 -80% filling factors in d. Me’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 M 5 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 Ti. O (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 k. G magnetic fields • 50% of Gliese 729 is covered by 2. 6 ± 0. 3 k. G fields Johns-Krull & Valenti (1996)