Summary of Cool Stars 13

1. Summary of Cool Stars 13 Hamburg Germany July 5-9, 2004 Jeffrey L. Linsky JILA/University of Colorado Boulder Colorado

2. Who said this? • (1) “Who is not attending anything?” • (2) “You get what scientists call a mess.” • (3) “The purpose of a diagnostic tool is not to verify anything.” • (4) “Recently the agreement of theory and observations has gone downhill.” • (5) “A unique harmonization of data.” • (6) “I feel like I should be selling something.”

3. Who said this ? (continued) • (7) “Of course we need more candidates and more data.” • (8) “The synthetic models are wrong. They are always wrong.” • (9) “I think that this thing is running out of battery.” • (10) “In order to give you the impression that we have done our job…” • (11) “The next talk will be on the weather in Hamburg – rain and clouds” • (12) “My sophisticated model – a horizontal line at zero.”

4. Solar activity and the Earth’s climate: are they correlated? • Ulrich Cubasch: Recent warming of the Earth’s climate (larger than seen in the last 1000 years) cannot be explained only by solar forcing. [Politically important.] • Sami Solanki: The Sun has been more active during the last 60 years than in the previous 1100 years. • Phil Judge: If Tau Ceti is a reliable indicator of solar activity during the Maunder minimum, then the Sun during MM had some Ca II emission and thus a magnetic network and probably a magnetic cycle. Tau Ceti TR lines show no redshifts (a magnetic effect). • Jason Wright (P): Most or all [?] of the solar-type stars with very low Ca II emission are evolved subgiants about 1 magnitude above the MS rather than solar analogs in a Maunder minimum state.

5. How are the coronae of solar-type stars heated? • Hardi Peter: Self-consistent 3D MHD model of a solar active region with heating due to braiding of magnetic flux (Parker model) can explain DEM(T) and Doppler shifts as f(T). These important results were not explained by previous 1D models. • Karel Schrijver: Potential field extrapolations of solar global magnetic field with different functional heating laws. Best fit to Yohkoh images is with a heating law consistent with DC heating by braided coronal magnetic fields with reconnection at the Alfven speed (like Peter). Predicts flux-flux relations for active stars. [Simulations include the essential physics approximately.] • Sam Krucker: RHESSI low energy nonthermal spectra of flares may answer the question of whether microflares can explain coronal heating. • Massimo Landi (P): None of the commonly used heating mechanisms reproduce solar X-ray observations. Loop temperatures and TR emission lines are best reproduced by loops with small cross-sectional areas at the base and expand upwards. [Importance of geometry.] • Alessandra Telleschi(P): Time scale for change from hot coronae/IFIP to cool corona/FIP in young stars. • Giovanni Peres(P): Scaling laws relating T, P, volumetric heating, and loop lengths. [What is the path from scaling laws to understanding physical processes in coronae?]

6. Are the coronae of PMS stars and brown dwarfs heated in qualitatively different ways than the Sun? • Eric Feigelson: X-ray saturation observed in the Orion stars depends on age like other samples of stars, but the dependence on rotation is different. Why? Accreting PMS stars show lower X-ray emission than nonaccreting stars? Why?

7. What are the basic properties of stellar coronal structures? Why are the hottest plasmas dense and compact? • Jan-Uwe Ness: XMM and Chandra spectra of Fe XXI and Fe XXII imply different electron densities than EUVE spectra. [We need high S/N and spectral resolution and better understanding of atomic physics to make progress.] • Paola Testa (P): Ratios of He-like Mg XI lines indicate high density plasma covering 0.0001 to 0.1 of active stars (flares?), whereas O VII line ratios indicate cool low density plasma covering up to 1.0 of active stars. [Is this right?] • Manuel Guedel (P): XMM-Newton observations of the eclipsing M dwarf primary CM Dra including primary and secondary eclipses. Reconstruction of the coronal structure is crude [and not unique but a powerful technique for the future.]

8. What are we learning about stellar magnetic fields? • Moira Jardine: Models that produce mixed magnetic polarity at the poles of rapid rotator stars predict enhanced meridional flows. Mixed polarity at the poles may explain the absence of X-ray cycles. • Jeff Valenti: Measurements of 2-3 kG magnetic fields in active K dwarfs and PMS stars from the analysis of near-IR spectra. First measurement of a uniform magnetic field in the accretion shock of a PMS star using spectropolarimetry. • Soren Dorch (P): MHD simulations show that M giants/supergiants like Betelgeuse could have 500 G surface magnetic fields, which could influence dust and wind formation. • Nils Ryde (P): The 12 micron Mg I line is very Zeeman sensitive. A good tool for measuring photospheric magnetic fields with new IR spectrographs (e.g., TEXES). • Michaelo Weber (P): STELLA will obtain Doppler images. [Important to study diverse stars and monitor interesting stars.]

9. What is the physics behind crazy coronal abundances? • Marc Audard: Important to compare coronal abundances with measured stellar photospheric abundances. Abundance changes must occur in the chromosphere where FIP <10 eV elements are ionized, but the physical process not well understood. • David Garcia-Alvarez: Existance of very hot coronal plasma plays a role in FIP/IFIP perhaps by chromospheric evaporation or ionization. • Manfred Cuntz(P): The effects of time-dependent ionization are most pronounced in simulations of magnetic flux tubes with narrow spreading with height (high magnetic filling factors). • Jorge Sanz-Forcada(P): For some stars IFIP goes away when one compares coronal with stellar photospheric abundances.

10. Evidence for andconsequences of high energy particle acceleration • Sam Krucker: RHESSI images show locations of the thermal and nonthermal components of solar flares. Detect a plasmoid rising from a reconnecting loop. Evidence for nonthermal electrons and protons in similar nearby loops. • Rachel Osten (P): Detected variable 3.6 cm and 6 cm emission from the M8.5 V star TVLM513-46546 at 10.5 pc. Why is there gyrosynchrotron emission from relativistic electrons from a fully convective star? Is radio emission from very cool stars common or not?

11. New insights concerning stellar flares • Marc Audard: The Neupert effect is observed during flares on several stars supports the chromospheric evaporation scenario. • Jan-Uwe Ness: Coronal electron densities decrease with time during a flare on Proxima Centauri.

12. Do A-type stars have chromospheres and coronae? • Beate Stelzer: Adaptive optics, Chandra X-ray images, and IR spectroscopy still do not rule out X-ray emission from faint close companions to B stars. [So look for X-ray variability and hard X-ray spectra from cool companions.] • Eric Feigelson: Young A and B stars in Orion are either very weak or dark X-ray sources. [Suggestive of no low mass companions.] • Seth Redfield (P): Horned shape of the C III 977A and O VI 1032A lines of Altair (A7V) provide the first evidence for limb brightening on stars. [Doppler imaging feasible with FUSE and Con-X.] • Christian Schroder (P): There are 73 apparently single A-type stars in the RASS and pointing error boxes. Some have Lx values that look to be too high for late-type companions. • Jurgen Schmitt (P): Some MCP (magnetic chemically peculiar) stars with spectral types B2p-A0p are strong X-ray sources. [Probably wind-driven magnetosphere mechanism rather than coronal sources.]

13. What are we learning about stellar interior structure and dynamos? • Jorgen Christensen-Dalsgaard: To get a good match of observed with predicted frequencies, solar models must include settling of heavy elements and relativistic motions of electrons. But the new lower [O/H] value by Asplund et al (2004) is a serious challenge. • John Barnes: Differential rotation decreases with mass until stars rotate as solid bodies at M1-2V. So, the alpha effect must dominate magnetic field generation in M dwarfs. • Michael Weber (P): Study of differential rotation of 5 RS CVn-type giants using time series Doppler images shows that some stars are solar-like (equator faster than pole) and some are reversed. [Why?] • Wolfgang Dobler: Theoretical models for fully convective stars can and do generate large scale magnetic fields. [So, M dwarfs should have large scale magnetic loops and very energetic flares.]

14. Interaction of stars with disks • Eric Feigelson: Deep penetration of hard X-rays and MeV protons from PMS stellar flares can change the chemistry, ionization, and turbulence in disks that can determine whether there are hot Jupiters or habitable Earths. [Important connection to the rest of astronomy.] • Scott Gregory: Computed potential field extrapolations to determine accretion channels for PMS stars. Compared accretion footpoints to Zeeman Doppler images of LQ Hya and AB Dor. [Now let’s get more realistic about field-disk interactions.] • Ray Jayarardhana: Accreting brown dwarfs are slow rotators, so disk locking scenario applies to young BDs. • Jochen Eisloffel: For VLM stars and BDs disk locking is probably not a major issue for stellar rotation.

15. New insights concerning the use of coronal spectral diagnostics • M. Matranga (P): Evidence for opacity in the Fe XVII 16.78A and 15.01A lines. [If so,] then path length ~0.3 Rstar.

16. What is Spitzer telling us about PMS stars ? • John Stauffer: Class I objects near the tips of “elephant trunks” [photodissociation regions], so that is where star formation occurs. Time scale for A star debris disk dissipation about 100 Myr. • Adam Burgasser: Spitzer is providing the first good mid-IR spectra of metal-poor L and T subdwarfs. No good model atmospheres to fit the data. • Michael Cushing: First detection of 7.8 μ CH4 and 10.5 μ NH3 bands in BDs. • Kevin Luhman: Spitzer excellent for discovery of Class I BDs. First widely-separated BD binary system provides best evidence yet that BDs formed by cloud fragmentation rather than by ejection from a multiple system.

17. What is new about brown dwarfs? • Kelle Cruz: The stellar luminosity function turns up at MJ = 14-15 (the stellar-BD boundary) but Spitzer data are needed to determine the LF fainter than MJ =16 (L7). • Subhanjoy Mohanty: At low masses (0.01 MSun) stellar radii appear to be too large. So present evolutionary tracks may be in error due to early turn on of deuterium burning. • Herve Bouy: First dynamical mass for a brown dwarf. Important test of theoretical models.

18. How are stellar winds accelerated? • Stephen Cranmer: Speed, density, and mass flux of the solar wind depends on the magnetic field topology (large expansion factor produces slow wind). Strong departures from Maxwellian implies wave dissipation important (ion-cyclotron waves not yet observed). [Magnetic field geometry critically important.] • Brian Wood: A whole new field of research – dwarf star winds. Apparent decrease in mass loss rates at log Fx>6 could be due to a topological change in coronal magnetic fields to nearly dipolar (polar spots). [Diverse subfields are now connecting.] • Susanne Hoefner: Importance of including the essential microphysics when modeling AGB atmospheres and winds (frequency-dependent radiative transfer, time-dependent dust formation, pulsations, etc.). Much information in time series. • Cian Crowley: Empirical wind velocity laws for giants in symbiotic binaries from N(HI) vs phase data are inconsistent with generally used beta scaling laws. • Klaus-Peter Schroeder (P): New semiempirical mass loss relation different from the “classical” Reimers law. Important for AGB stars. • Alex Lobel: Spatially-resolved spectroscopy of Betelgeuse provides evidence for wind acceleration in the upper chromosphere. Coexistence of warm gas and cold dust in the upper chromosphere may require time-dependent wind acceleration models.

19. Some minor concerns • .ppt presentors should recognize that Light green cannot be seen against a bright background on the screen. • Dark red, blue, and violet are not visible against a dark background.

20. For the future • Science by simulations is a powerful tool for identifying the importance of different physical processes. • Future spectroscopic missions for UV and X-rays are in the distant future and the reliability of present analysis tools is uncertain. So, we need to create a rich data archive (legacy) for future analysis and reanalysis during the data drought. • New data, especially from new spectral regions, will rejuvenate the field (Spitzer, ALMA, ground-based spectroscopy and interferometry, etc.) • Always emphasize uniqueness. • Cool stars are “kühl” because (in various ways) they provide insights concerning broader issues in astrophysics.