ChromoAstrology: What stars can tell us about chromospheres, or whatever T. R. Ayres (CASA)

Chromospheres XXXXXXXXX

Sun Star.

one Sun many Stars

<- p-a heating mech??? Cartoon of solar chromosphere has complexified over past two decades (K. Schrijver + B. DePontieu), but stellar view still is very much 1D

Coronal heating by accreting pineapples??!!

Outline H-R Diagram Wilson- Bappu Effect Rotation-Age-Activity Connections Activity Cycles Flux-Flux Correlations Atmospheric Dynamics (super new!) Buried Coronae Guiding questions: What can unresolved stellar chromospheres tell us about the solar counterpart? Is Sun ‘normal’ in cosmic scheme of things?

Chromospheric H-R Diagram Chromospheres appear to be confined to ‘cool stars’, in convective half of H-R diagram Coronae are seen at earlier types, but ‘ionization thermostat’ that inspires chromospheres dies out at same place convection fails Not a coincidence! Originally thought to signal lack of acoustic energy, but dynamo needs convection too

Wilson-Bappu Effect: Barometer or Tachometer? Mg I + Mg II resonance lines in early-G supergiant  Camelopardalis (deep core absorptions are ISM) (from STIS ‘StarCAT’)

Average Mg II k-line profiles from active & quiet G-type dwarfs. FWHMs are same, despite very different core fluxes

Average Mg I profiles: active dwarfs have higher wing intensities; lineshapes are similar to Ca II H & K in L-A G stars

Mg II h & k line wings also higher in active dwarfs. Similar behavior seen in Ca II H & K of plage vs. quiet-Sun

Left : k lines of G-type giant supergiant solar twin (  Cen A) Right : scaled profiles k line widens dramatically with increasing luminosity (W- B Effect) For dwarfs, FWHM is ~100 km/s, already beyond any plausible Doppler broadening

Like h & k cores, Mg II damping wings broaden with increasing luminosity: important clue to physical origin of W-B Effect (Same behavior is seen in Ca II H & K)

Mg I in G giant supergiant solar twin Now, Mg I cores (and wings) do not broaden with luminosity (although some photospheric absorptions do)

WBE = Barometer !!! W-B Effect owes its existence to decreasing mean density but increasing thickness of chromospheres with decreasing gravity, partly a consequence of H - opacity, a P 2 species (whereas Ca + and Mg + are P 1 and Mg 0 is P 2 ), but equally important is radiative cooling by metals and H, which depends on electron density through collisions (also P 2 ). Electrons provide ‘thermostat’ via partial ionization of hydrogen: n e /n H increases 10 4 x over K, accounting for great thickness of chromosphere, at nearly const T. Wings and outer emission edges of Mg II lines form outside Doppler core and thus can directly reflect changes in chromospheric column mass with gravity

Rotation-Age-Activity Connection ’Skumanich laws’ confirm importance of dynamo, creating high levels of activity in fast rotating stars, but also root of magnetic braking, which ultimately quenches activity. Recent issues: ‘saturation’ at high spin rates; ‘basal’ emissions at low end (‘little [  2 ] dynamo’, waves & shocks)

Stellar Activity Cycles Long term Ca II emissions of nearby field star closely mimic Sun’s cycle. Visible brightness changes of Sun only few milli- mags, yet 10x larger than entire chromospheric energy budget (Radick, Lockwood, Skiff, & Baliunas 1998)

Most late-type stars of near-solar color show long term variations in Ca II emission, many cyclic. Others, typically low R HK and often subgiants, are ‘flat activity’ (Radick et al. 1998)

Solar variations on long (and short) timescales fall close to stars of similar activity (Radick et al. ’98; Lockwood et al. 2007)

Case Study: Cycles of Alpha Cen Alpha Centauri triple system. Two solar-like stars about 20 au apart (Sun-Uranus); dim red dwarf 10,000 au away Slightly metal rich compared with Sun, slightly older by ~1 Gyr. G2V primary ( “A” ) is near twin of our own star

Alpha Cen X-rays first detected by HEAO-I ; binary later resolved by Einstein. Surprising result: little Alpha Cen B twice as X-ray luminous as big A ROSAT carried out long term coronal campaign in 1990’s

XMM (0.2-2 keV): a Cen A visible in first few frames; disappears by mid-2004 (Robrade+ 2005) Note: Secondary also fading

The `Fainting’ of Alpha Cen A Solar physicist frets over stunning 50x drop of Sun’s twin in soft X-rays Is Sun’s cycle depth (only ~5x in keV band) somehow abnormal in coronal scheme of things?

Fe XII 195 (1 MK) coronal emission persists at spot minimum (left ; max at right). ‘Fuzzy ball’ devolves from magnetic carpet: small clumps of flux built by local dynamo, independent of deep seated el jefe dynamo responsible for sunspots and their decadal cycling

Since ‘00 Alpha Cen orbital separation closing rapidly: no longer easily resolvable by XMM, still trivial for Chandra. HRC campaign (since Oct ‘05) *surprisingly* captures both stars

2007 Chandra LETGS spectrum shows strikingly different A than 7 yrs earlier: hard emissions gone, but key Fe IX & X (dominating energy losses) unchanged (actually, stronger)

High-energy Yohkoh imaging, : 2-3 MK emission almost exclusively from active regions

Cycles Summary Stellar HK activity cycles solar- like in amplitude & duration; flat activity stars common; long term cycles at low activity give way to stochastic behavior at high, dominated by rotational modulations. At low end, long term photometric changes positively correlated with Ca II; opposite is true at high activity Lesson of a Cen A: Appearance of X-ray cycles very dependent on energy bands & instrumental responses, especially for soft sources like Sun where bulk of coronal emission is >5 nm

Flux-Flux Correlations Coronal X-rays show good correlation with TZ C IV (except for ‘X-ray deficient stars’); Mg II & C IV well correlated for all types

Chromosphere and ‘Transition Zone’ show better correlations with each other than either does with the corona Oddballs (X-ray deficient Hertzsprung gap stars, ‘noncoronal’ red giants) where Mg II–C IV appears normal, but X- rays are anomalous Correlation power laws nonlinear, steeper than unity: increasing activity not just filling factor effect -- new heating sources must come into play

Chromospheric Dynamics Recent FUV HST/COS study of 50 Myr solar analog EK Draconis (  Cen A [shaded] reference solar twin). Note bright Fe XXI emission, and very broad chromospheric (C II) and transition zone lines (Si IV), the latter significantly redshifted.

EK Draconis displays two FUV flares during mere 20 min observation; Si IV affected greatly, Fe XXI not so much, and C II hardly at all.

Upper: EK Dra (Si IV 1393 left, 1402 center) Lower: Alpha Cen A; double Gaussian fits indicate multiple dynamical components; EK lines are strongly redshifted (warm coronal rain?).

ChromoDynamics TZ line shapes of EK Dra are remarkable Basic profile consists of redshifted narrow component; and even more redshifted broad component, with about equal flux ratio (like Alpha Cen A, curiously) Emphasizes prevalence of ‘relentless’ kinematic processes shaping upper chromospheres: perhaps analogous to TZ explosive events but not clear…

Buried Coronae ‘Noncoronal’ red giants thought to completely lack X-rays (post-MS expansion = ultra-slow spin = no dynamo), until archetype (Arcturus) finally dug out of ‘coronal graveyard’ by Chandra, albeit at pathetically low L X

FUV ‘hot lines’ also detected in several graveyard giants by HST, but Si IV looked odd, and N V doublet was weak or missing. Distorted Si IV explained by blends with fluoresced H 2 lines. Curiously, de-blended profiles similar to legitimate coronal giants

Finally, recognized that Si IV emitting gas selectively absorbed by overlying cooler material. N V clobbered by C I absorptions near b-f edge. X-rays would be attenuated by chromospheric atomic H and He Coronae buried alive!

Conclusions Chromospheres are fundamental property of cool stars, doubtless because waves, shocks & magnetism are ubiquitous features of convective atmospheres Chromosphere adjusts electron density and thickness to balance mech heating Energy deposition can be highly dynamic Corona tightly coupled to chromosphere Sun appears perfectly ‘normal’ (for L-A * )

Final (provocative) Thoughts