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High Accuracy Atomic Physics
In Astronomy
COOL STARS
and ATOMIC PHYSICS
Andrea Dupree
Harvard-Smithsonian
CfA 7 Aug. 2006
OUTLINE
How does atomic physics
influence our understanding
of the atmospheres of cool stars ???
Three critical examples:
1. Identifications
2. Wavelengths
3. Coll. X-sections
temperatures
dynamics
densities
Will draw from highly ionized species characteristic of 10MK,
to singly ionized atoms observed in cool star spectra….
Supergiants
Cool , extended
Giant stars
Solar type
S
Identification of Ions allows EMD
Emission Measure distributions quite different from
the well-known solar case (Sanz-Forcada et al. 2004)
Highly Ionized Species
FUSE spectra of cool stars show Fe XVIII at 974.86A.
Identified in solar flare spectra.
Feldman and Doschek 1991
Young et al. 2001
Dupree et al. 2003
Redfield et al, 2003.
Radial Velocity of Fe XVIII
emission lines …
Reveals coincidence of Fe XVIII with
the stellar photospheric velocities ,
Suggests that high T plasma,
6.8 K (dex) is anchored close
to the stellar surface reminiscent
of low-lying coronal loops…
High Temperature Species Anchored in
Warm
Wind
Fe XIX
Fe XVIII
FUSE Cool Star Team;
Redfield et al. 2003
Symbiotic Star: AG Dra
This stellar system consists of a
red giant whose wind and surrounding
nebula is photoionized by a hot white
dwarf companion. Spectrum is complex
with narrow nebular emission, and the
surprising presencs of high ionization
forbidden lines. These conditions are
quite different from ‘coronal’ plasmas
(collisionally-dominated).
HST/STIS spectra reveal
forbidden lines: Ca VII, Fe VII, Mg V,
Mg VI, Si VII, and for the first time,
2 transitions of Mg VII between terms
of the 2s 2 2p 2 3P-1D configuration
(Young, P. et al. 2006).
Energy levels and density diagnostics
Separation of ground 3P levels
(from IR astronomical spectra)
plus UV wavelengths define
1D energy levels in Mg VII
Four density diagnostics using
Mg ion ratios do not give consistent results,
although the electron density appears to be high.
High ionization appears to require nearby source
of photoionization. Other problems remain that
might be resolved by detailed modeling.
(Young et al. 2006)
EUV spectra offer many coronal diagnostics
Spectra from the EUVE
satellite contain ions
Fe IX-XXIV (not FeXVII)
allow both T and Ne to
be defined in cool star
coronas.
(Sanz-Forcada et al. 2003)
Density diagnostics suggest
small coronal structures
Electron densities are high.
The observed line flux in
combination with the density
diagnostic suggest small
emitting volumes (<0.01 R)
and continuous heating.
FLUXobs ~ Ne 2 ΔV
Sanz-Forcada et al. 2004
EUV radiance spectrum of the Sun
CHIPS EUV spectrum
of the whole Sun reveals
differences from the
standard solar irradiance
models (red line).
Courtesy of M. Hurwitz
(2006)
Fluorescent processes in extended
atmospheres
Betelgeuse – a supergiant
Dupree and Brickhouse 1998
Imaged in the ultraviolet by HST
Narrow lines appeared in emission in far UV (ORPHEUS) spectra
of cool giants and supergiants near 1140Ǻ. Possibly fluorescent
lines from low ionization species.
Higher resolution led to confirmation
Fe II can be produced by H-Lyman-α pumping from
4s a4D and cascade to 4s a6D and 3d7 a4F.
May provide an indirect diagnostic of stellar Lyman-α profile .
Harper et al (2004) hypothesized that Fe II lines away from
H-Ly α (Δλ > 1.8Ǻ) should be weak (marked by *).
FUSE Spectra show puzzling differences
FUSE spectra of luminous stars do not show consistent patterns.
(Dupree et al. 2005)
Unresolved Problem: C III
profiles
Profiles of the C III 1176Ǻ line
3P-3P, in luminous cool stars differ
substantially from the solar profile.
Check center to limb behavior.
(Dupree et al 2005)