ChromoAstrology: What stars can tell us about chromospheres, or whatever

T. R. Ayres (CASA)

Chromospheres

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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 P2 species (whereas Ca+and Mg+are P1 and Mg0 is P2 ), but equally important is radiative

cooling by metals and H, which depends on electron density

through collisions (also P2 ). Electrons provide ‘thermostat’ via partial ionization of hydrogen: ne/nH

increases 104x over 5000-8000 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 RHK 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 2006-07

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 0.2-2 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, 1996-2006: 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 LX

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 H2 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