Published on November 13, 2007
Stellar Surface Structures: Stellar Surface Structures Surface features associated with activity Chromospheres Spots Plage Coronal holes Flares Types of stellar activity Solar-type stars M dwarfs Active stars Star spots Doppler imaging Spots on A stars Stellar oscillations Star Spot Properties: Star Spot Properties Size – large, medium, small Inactive, weak-dynamo stars (small spots only) Active, strong-dynamo stars (big spots) Light curve amplitudes may be 0.5 mag Nature of spots indeterminate Temperatures Do all of a star’s spots have the same temperature? Do spots have umbra/penumbra structure? How does a spot temperature evolve as it forms and vanishes? Magnetic fields How do fields correlate with other spot properties? Spot locations – polar, equatorial, both? Most have dark polar spots with strong B Spot lifetimes Plage: Plage What is plage? - Regions of strong (~340 G), vertical magnetic field Seen in white light as bright regions around sunspots Much higher contrast in Ca II K Feature of upper photosphere and lower chromosphere Sun varies by ~15% in Ca II K line emission over a solar cycle Fields replenished from sunspot fields, drift poleward to merge with intranetwork fields Ca II K emission is rotationally modulated What are Coronal Holes?: What are Coronal Holes? Regions of the solar corona where the magnetic field lines diverge outward from the Sun Develop in regions adjacent to areas of similar polarity Low density, material flows outward – source of much of the solar wind Solar Flares: Solar Flares Magnetic flux tubes “reconnect” in the corona (coronal loops) Electrons accelerate down the magnetic field lines toward the lower atmosphere, producing microwave emission Electrons collide with ions, producing hard x-rays, white light emission from chromosphere Chromospheric plasma heated to coronal temperatures, hot plasma flows up into the corona Shock front moves downward to heat the photospheric base As the density of the corona increases, it is further heated by the energetic electrons. Soft x-rays from the corona then heat the chromosphere M Dwarf Flares: M Dwarf Flares The mechanism for M dwarf flares is different than in the Sun Blue and UV continuum increase by several magnitudes in seconds (unlike the Sun, where the contrast to the photospheric background is less) Typically a black body of 8,000 –10,000K The source of the white light is still unknown Strong,broad emission lines in the UV and optical – Balmer, Ca II K + He I, He II, Ca II IR triplet, numerous singly and doubly ionized metals UV emission lines also stronger Broadening mechanism unknown Soft x-ray emission rises more slowly M dwarf flux dims just before the outburst Coronal mass ejections? Mass loss rates from flares estimated at 10-13 MSun per year Activity Cycles: Activity Cycles Activity Cycles: Activity Cycles Long term chromospheric activity indices for several stars showing different patterns of activity cycles Age-Activity Relation: Age-Activity Relation In solar-type stars, age-activity relation is well defined Young stars have stronger Ca II K line emission (flux proportional to t-1/2) M dwarfs don’t fit the solar-type relation Activity is more prolonged; Activity is a function of both age AND mass dMe stars are kinematically younger than dM stars In older clusters, activity “turns on” at later spectral type Activity Distributions: Activity Distributions Rotation: Rotation In survey for rotation, 25% of stars have rotation rates above 2 km/sec Later type dwarfs MORE LIKELY to have measurable rotation Earlier type M dwarfs that rotate are usually young Rotation takes longer to decay in the later M dwarfs No strong correlation between activity level and the rate of rotation A low threshold for rotation to maintain activity in M dwarfs Among very late M dwarfs, some rapid rotators DO NOT show activity Effects of Activity on M Dwarfs: Effects of Activity on M Dwarfs Activity affects color, luminosity, TiO band strength Active stars are redder/brighter TiO stronger or weaker depending on the particular band Mapping Starspots: Mapping Starspots Direct imaging – limited application Photometric light curves Intensity vs. time Equatorial spots Eclipsing binaries Doppler imaging Intensity vs. radial velocity vs. time See Vogt and Penrod 1983 (PASP, 95, 565) Doppler Imaging from Vogt & Penrod 1983: Doppler Imaging from Vogt & Penrod 1983 As a spot moves across the star the line profile changes. From an observed line profile, one can construct an image of the surface of the star. This technique has been applied to many different types of stars. Surfaces of T Tauri Stars: Surfaces of T Tauri Stars Cool circumstellar disk around a late-type, magnetically active star Light variations at all wavelengths, timescales Mass accretion Magnetic fields Periodic light variations – rotational modulation Amplitudes 0.05 – 0.5 mag Periods stable over several years Cool spots cover a large fraction of the surface, typically polar, similar to RS CVn’s Some evidence for warm spots as well Magnetic fields ~1kG Spots on the ZAMS: Spots on the ZAMS Two Pleiades dwarfs – K5V, M0V Vsini=60-70 km/sec Periods ~10 hours Inclinations ~ 50-60 degrees Again, dark polar spots Starspots on Active Stars?: Starspots on Active Stars? A few dozen stars with Doppler images RS CVn T Tauri FK Comae W Uma Young single dwarfs BY Dra Rotation periods from 0.31 to 19 days (165 to 25 km/sec) Radii from 0.77 to 16 RSun Temperatures from 4000 K to 6000 K PMS to class III giants Generally show dark polar spots – unlike the Sun. Why do they differ from the solar paradigm? Faster rotation Mostly deeper convective zones Presence of polar spots remains controversial Strassmeier’s Spot: Strassmeier’s Spot Spots in HR 1099 (Vogt & Hatzes): Spots in HR 1099 (Vogt & Hatzes) Doppler images of HR 1099 (RS CVn star) from 1981-1989 Star dominated by a large polar spot Smaller spots form in equatorial regions and migrate toward pole Spots merge together and may merge into polar spot Polar rotation fixed with orbital period Equatorial rotation slightly faster Some spots persist over years Spot patterns reminiscent of solar coronal holes Activity on Active Stars – NOT “Solar-like”: Activity on Active Stars – NOT “Solar-like” Starspot latitude Solar-type – mostly equatorial Active stars – mostly polar Chromospheres Solar type - ~ 0.01 solar radii Active stars - ~ one stellar radius Rotation activity relations Solar type – strong correlation Active stars – activity saturates at 15 km/sec Activity cycles Solar type – long term periodicities in Ca II K Active stars – most show no evidence for cycles Types of Stellar Activity: Types of Stellar Activity Solar type stars Young stars/Active stars M dwarfs T Tauri stars The Sun is not representative Different spot locations and sizes Different migration patterns Filling factors Cycles or no cycles Different dependences on rotation rates Spots on Ap Stars: Spots on Ap Stars 10-15% of late B-early F stars have magnetic fields (Sr-Cr-Eu stars, Si stars) Oblique rotator model – dipole field inclined to rotation axis (and also decentered) High Teff and stable atmospheres Radiative and gravitational forces push atoms up or down Length scales ~104 km Time scales 102-104 years Magnetic fields suppress motion of ions across field lines Element may accumulate where field horizontal, deplete where field is vertical (or vice versa) Expect polar spots, equatorial rings Si depleted in polar spots, enhanced in rings Cr enhanced in polar spots, depleted in rings Element diffusion along horizontal field lines may cause surface abundance distributions to evolve with time (time scale ~108 years) Si II Spots on Cu Vir (Si Ap Star): Si II Spots on Cu Vir (Si Ap Star) Stellar Oscillations: Stellar Oscillations Solar acoustic (p-mode) oscillations ~5 min, 107 modes Stellar obs. limited to lowest order modes in integrated light About 15 min, amplitudes a few parts per million Radial velocity vs. photometric techniques p-modes vs. g-modes p-modes: Pressure is restoring force g-modes: Buoyancy is restoring force White dwarfs, delta Scuti’s, roAp stars, etc.