Recent and impending satellite launches are bringing enormous improvements in spectral resolution, spectral range, sensitivity and intensity calibration to UV and X-ray astronomy. Data from STIS on HST and the CDS, SUMER and UVCS instruments aboard the SOHO satellite provide a glimpse of the diagnostic power of observations with the new generation of satellites. FUSE will extend this level of spectral resolution to the 912-1200 range. The grating spectrometers on the AXAF and XMM satellites will resolve X-ray spectral features that have hitherto been perceived as blends whose components could not be unambiguously separated.
The overall goals of astrophysical spectral analysis at these wavelengths have not changed greatly over the past two decades. The difference is that the new instruments and the accumulating high quality atomic data make it possible to imagine getting reliable, accurate, unambiguous results. From an emission line spectrum, one ought to be able to derive the temperature distribution, the pressure and the elemental abundances. From these physical parameters of the emitting gas, one hopes to understand the physical processes responsible for generating and exciting the emitting gas. If both the data and the atomic rates are good enough, it is possible to go beyond the standard simplifying assumptions and search for indications of non-Maxwellian electron distributions (indicative of heating processes or steep temperature gradients), time-dependent ionization states (as a way to infer the history of the emitting plasma), optical depths in the emission lines (as a way to derive the geometrical structure of unresolved sources), and even electro-magnetic fields (through their effects on processes such as dielectronic recombination).
The current state of the atomic data leaves a great deal to be desired. The uncertainties in excitation ionization and recombination rates will dominate over the measurement errors for many observations of emission lines from astrophysical plasmas-an inefficient use of very expensive satellite observatories. Analysis of stellar continuum spectra will be limited by uncertainties in wavelengths and oscillator strengths.
The following sections list some of the important processes that should be
measured in the laboratory or computed in order to make full use of X-ray
and UV observations. It should be kept in mind that, with some important
exceptions, that the importance of a given rate is more or less proportional
to the astrophysical abundance of the element involved. H, He, C, N, O, Ne,
Na, Mg, Al, Si, S, Ar, Ca, Fe and Ni account for most of the observable
emission lines. Less abundant elements are detected, however, and their
abundances can be crucial to understanding things as diverse as supernova
explosions or radiative levitation in stellar atmospheres.
Line Identification
Wavelengths of the strong lines of the more abundant elements are known quite
accurately, and for many purposes the strong lines provide adequate, easily
measured diagnostics. Wavelengths are less reliable for less abundant elements,
for complex atoms and ions, and for weak lines of the abundant elements.
While oscillator strengths for enormous numbers of transitions are available
from Opacity Project or HULLAC calculations, the wavelengths predicted in these
calculations are generally inadequate for line identification without considerable
additional effort. About 30% of the emission lines detected in the low solar
corona by the SUMER instrument have not been identified (Feldman et al 1997).
The situation is likely to be worse for the 40-150 band observable
with the AXAF LETG.
Identification of these features is important for understanding line blends
and for the additional diagnostic possibilities the line present.
Absorption lines can be detected for a greater range of elemental abundance and
lines strength, particularly for high signal-to-noise ultraviolet
spectroscopy of narrow-lined, chemically peculiar and normal B- and A-type
stars. Understanding these spectra requires accurate wavelengths and
transition probabilities for an enormous number of lines, spanning the
wavelength range 1150-3200A. There is an especially acute need for atomic
data at the shorter VUV wavelengths (< 1700 A), where the line crowding
increases and the accuracy and completeness of existing laboratory work is
much lower than at longer wavelengths. These investigations would
benefit greatly from expanded large scale
studies that could provide wavelengths with sub milli-Angstrom accuracy and
improved estimates of transition probabilities (to 10-20% accuracy) for
large numbers of lines of the second and third spectra of iron-group and
other elements at these VUV wavelengths. There are numerous examples
of observable isotope shifts (IS) and hyperfine
structure (hfs) in the UV spectrum of chi Lupi. A comprehensive data base of IS
and hfs parameters is very important. It is not easy to
predict in which ions these nuclear effects will have a discernible
influence on stellar line strengths and abundances derived therefrom.
Oscillator Strengths and Photoionization Cross Sections
The OPAL and Opacity Project calculations represent such a large advance
in the quantity and quality of the data that it is tempting to regard
oscillator strengths and photoionization cross sections as a solved problem,
and the general agreement between the two sets of calculations provides confidence
that the predictions are basically correct. Howver, it would be
extremely useful to have better data for the more complex ions (especially
low ionization states of the iron group elements), better estimates of the
uncertainties in the numbers, and laboratory benchmark measurements of the computed
resonance structure in photoionization cross sections. Resonances may be
especially important for questions involving ionization by particular strong
emission lines such as Ly or He II 304.
Ionization Rate Coefficients
The ionization state of a plasma is often used as an indicator of the
electron temperature, but this is only as accurate as the ionization and
recombination rates used in computing the ionization balance. The level
of agreement among current ionization balance calculations suggests that
the temperature inferred from the ionization state is only reliable to
about 0.1 dex in log T. Moreover, some problems that have persisted for decades,
such as the inconsistency among the emission measures derived from Li-like,
Be-like and B-like ions, are likely to be caused by errors in the computed
ionization states. It is necessary to reach about 10% accuracy in the
ionization rates to match the uncertainties in instrumental calibration.
This should be achievable with storage ring experiments and crossed beams
experiments. Measurements for complex ions are important, as the theoretical
rates are especially uncertain. Inner shell excitation followed by autoionization
is important for several isoelectronic sequences. The Oak Ridge measurements
of ionization cross sections of iron ions through are exemplary,
and it is important to extend them to higher ions. It is important to measure
ionization rates from metastable levels for interpretation of plasmas such as
the solar transition region. The metastable levels of Be-like and Mg-like
ions have statistical weights much larger than those of the ground states, so
they have an especially large effect.
Recombination Rate Coefficients
Radiative recombination is the inverse of photoionization, and is therefore
covered by the photoionization cross section discussion above. The difference
is that determining the recombination rates requires photoionization cross sections from
many excited levels, particularly if one is interested in the contribution to
emission lines. In general, the radiative recombination rate dominates for
ions that have low dielectronic recombination rates due to the lack of low-lying
excited levels; H-like, He-like, Ne-like and Ar-like ions.
Dielectronic recombination (DR) is the dominant recombination path for most ions
in collisionally ionized plasmas. The standard uncertainty estimate for
calculated DR rates is
30%, but there are larger discrepancies than that even among sophisticated
calculations for simple ions. The problems become worse for complex ions
(e.g. Fe II-Fe VIII) and for temperatures where kT is small compared with the
excitation energies of the resonance lines (as found in photoionized plasmas).
Dielectronic recombination by way of forbidden lines has been recently shown
to be important for some highly charged ions at low temperatures.
The dependence of the DR rate on density and electric or magnetic field
is seldom included in astrophysical model calculations, and DR from
metastable levels is not usually considered.
The DR rates under the density and field conditions of astrophysical plasmas
appear to be the largest contributor to the uncertainty in ionization
balance, and hence to the plasma parameters inferred from the ionization
state. They are therefore among the most important quantities to measure.
Charge transfer, especially with neutral hydrogen, is an important contribution
to the recombination rates of ions in photoionized plasmas, especially if
the ionizing radiation has a hard spectrum. Laboratory measurements for
collision energies of the order a few eV are valuable.
Collisional Excitation Rates
If we are to avoid having uncertainties in the collisional excitation rates
dominate the uncertainties in derived physical parameters, we need
collision strengths accurate to about 10%. This level is at the limits
of the capabilities of crossed beams and storage ring experiments.
Typical uncertainties quoted are 10% for Distorted Wave calculations
and perhaps 10% for the Close Coupling method. Particular attention
must be paid to resonance structure in the collision cross sections,
especially for fairly complex ions. For most astrophysical applications,
the cross section will be integrated over a Maxwellian distribution, so
the individual resonances are not as important as the overall contribution
of the resonances. In some cases cascades from a large number of higher
lying levels, each of which has a fairly small cross section, can be very
important. Many weak lines add together to form a quasi continuum which
can be very important for abundance determinations. EBIT measurements
are especially promising for investigating large numbers of transitions.
Excitations from metastable levels are challenging from the laboratory
perspective, but they are important in ions where the metastable population
exceeds the ground state population, such as Be-like and Mg-like ions
at high densities. Collisions with protons have been studied in the
laboratory, but mostly at energies well above those important for
astrophysical spectroscopy. Proton (and alpha particle) collisions are
determine the fine structure populations in many ions under coronal
conditions, and in fast shock waves they can excite resonance lines in
the UV.
Other Laboratory Astrophysics
The NASA laboratory astrophysics program has traditionally provided atomic
data needed for the interpretation of data collected from satellites.
Recent laboratory efforts to investigate the physics of shock waves, particularly
the growth of instabilities and the interaction of shock waves with
density inhomogeneities, show considerable promise. Strong shocks of a
variety of types (purely hydrodynamic, high Mach number, low Mach number,
radiative, MHD, in fully ionized plasmas) can be generated and well
diagnosed on large inertial confinement laser facilities. Such work
does not find a natural niche in the current programs (as exemplified
by the ROSS NRA structure). Consideration
should be given to the question of whether to broaden the NASA laboratory
effort to include investigations of plasma physics, hydrodynamics, or other
disciplines outside the usual scope of the NASA laboratory astrophysics
program.