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

1. ChromoAstrology: What the stars can tell us about chromospheresT. R. Ayres (CASA)

2. Warning: this talk deals with lower rungs of Drake’s Ladder, where, sadly, sexiness is low, but on positive side, knowledge content is high Even so, a few surprises still crop up from time to time Chromospheres XXXXXXXXX

3. Sun Star .

4. one Sun many Stars

5. <- 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

6. Cartoon of solar chromosphere has complexified over past two decades (R.J. Rutten), but stellar view still is very much 1D

7. 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?

8. 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

9. Wilson-Bappu Effect:Barometer or Tachometer? Mg I + Mg IIresonance lines in early-G supergiantb Camelopardalis (deep core absorptions are ISM) (from STIS‘StarCAT’)

10. Average Mg IIk-line profiles from active & quiet G-type dwarfs. FWHMsare same, despite very different core fluxes

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

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

13. Left : k lines of G-type giantsupergiantsolar twin (a 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

14. 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)

15. Mg I in G giantsupergiantsolar twinNow, Mg Icores (and wings)do notbroaden with luminosity (although some photospheric absorptions do)

16. 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/nHincreases 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

17. 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 [a2] dynamo’, waves & shocks)

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

19. More examples (from SSS: Hall et al. 2008)

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

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

22. 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 Case Study: Cycles of Alpha Cen

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

24. 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

25. 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?

26. Fe XII l195 (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 dynamoresponsible for sunspots and their decadal cycling

27. 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

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

29. High-energy Yohkoh imaging, 1996-2006: 2-3 MK emission almost exclusively from active regions

30. Coronal histories: A=blueB=redSun=gray (Cyc 23 shifted in time); ROSAT, top panel; Chandra (dots) & XMM (shaded, scaled), lower. B has ~8 yr cycle, rising to new max. No clear period for A, modest cycle depth (‘flat activity’ star?)

31. C IV(upper) and Mg II (lower) of Bfrom IUE peak in ~1988, matching 8-yr X-ray cycle

32. 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

33. 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 forall types

34. 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

35. Chromospheric Dynamics Recent X-ray & FUV spectroscopic study of yellow giants (a Cen A reference solar twin). ‘CNOSi’ is combined flux accounting for ~1/3of TZ radiative losses. Orange curve is for age diverse G dwarfs TZ is not minor phenomenon

36. Left :montage of FUV line profiles (STIS). Right :average of Si IV+C IV+N V, fitted by double Gaussianline shapes (cf., the ‘narrow’ and ‘broad’ components of solar TZ emissions)

37. ChromoDynamics • TZ line shapes of yellow giants self-similar • Basic profile consists of redshiftednarrow component, not greatly different from star to star (except for rotational broadening); and ~blueshiftedbroad 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 [!] )

38. 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

39. FUV‘hot lines’ also detected in several graveyard giants by HST, but Si IV looked odd, and N Vdoublet was weak or missing. Distorted Si IV explained by blends with fluoresced H2lines.Curiously, de-blended profiles similar to legitimate coronal giants

40. Finally, recognized that Si IVemitting 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!

41. 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 *)

42. Final (provocative) Thoughts Many curiosities displayed by seemingly oddball X-ray stars (XRD syndrome, N-C giants, cool winds) might be explained if one adopts view that chromosphere is root of all evil, or at least of the hot gas we call coronal (as M. Aschwanden and others have suggested). This‘magnetic complexity zone’ is the natural home for wayward energetic phenomena Coronal heating inside the chromosphere would enhance smothering effects for stars with compact magnetic structures (H-gap?) and/or extended atmospheres (NCG), at quieter end of coronal scale (large active regions are another story).If solar paradigm must be revised thusly, we should carefully consider instrumental tactics for future, to avoid missing where high-energy action truly is Thankfully (for theory) Sun is so L-A, we see full range of heating mechanisms, none overwhelmingly dominant