chromoastrology: what the stars can tell us about chromospheres t. r. ayres (casa)
TRANSCRIPT
ChromoAstrology: What the stars can tell us
about chromospheresT. 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 (R.J. Rutten), but stellar view still is very much
1D
Cartoon of solar chromosphere has complexified over past two decades (R.J. Rutten), but stellar view still is very much
1D
Outline• H-R Diagram• Wilson- Bappu Effect• Rotation-Age-Activity Connections• Activity Cycles• Flux-Flux Correlations• Atmospheric Dynamics• 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)
More examples (from SSS: Hall et al. 2008)
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
New 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
C IV (upper) and Mg II (lower) of B from IUE peak in ~1988, matching 8-yr X-ray cycle
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 X-ray & FUV spectroscopic study of yellow giants ( Cen A reference solar twin). ‘CNOSi’ is combined flux accounting for ~1/3 of TZ radiative losses. Orange curve is for age diverse G dwarfs
TZ is not minor phenomenon
Left : montage of FUV line profiles (STIS). Right : average of Si IV+C IV+N V, fitted by double Gaussian line shapes (cf., the ‘narrow’ and ‘broad’ components of solar TZ emissions)
ChromoDynamics• TZ line shapes of yellow giants self-similar• Basic profile consists of redshifted narrow
component, not greatly different from star to star (except for rotational broadening); and ~blueshifted broad component, which differs in strength and width, mainly with activity
• Emphasizes prevalence of ‘relentless’ kinematic processes shaping upper chromospheres: undoubtedly analogous to TZ explosive events (Solar twin has 4x brighter BC than would be inferred from FUV studies of Sun itself [!] )
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