Summary of Cool Stars 13
Hamburg Germany
July 5-9, 2004
Jeffrey L. Linsky
JILA/University of Colorado
Boulder Colorado
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.”
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.”
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.
How are the coronae of solar-type stars
heated?
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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?]
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?
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.]
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.]
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.
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?
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.
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.]
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.]
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 fielddisk 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.
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.
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 midIR 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.
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.
How are stellar winds accelerated?
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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, timedependent 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.
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.
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.