Stellar Activity

1. 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

2. 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 loop-like structures, producing active regions with field strengths of 1-2 kGauss • Surface magnetic fields weak compared to stronger interior fields (104-105 Gauss)

3. Turbulent Dynamo Model What causes the solar small-scale magnetic field? • 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?

4. The Chromosphere • Remember the Sun: • 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

5. 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

6. 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

7. 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”

8. 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?)

9. 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.

10. 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.

11. 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

12. 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? Spots and Spot Cycles

13. 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

14. 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/

15. Spectrum of EV Lac & 5 Standards • 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. Johns-Krull & Valenti (1996)

16. 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)