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 Earths climate are they
correlated?

• Ulrich Cubasch Recent warming of the Earths
  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 lt10 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 and consequences 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
  lets 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 Fxgt6 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.