The main data need for stellar atmosphere and spectrum modeling remains atomic and molecular transition data, particularly energy levels and transition cross-sections. We emphasize that data is needed for bound-free (b - f) as well as bound-bound (b - b), and collisional as well as radiative transitions. Data is now needed for polyatomic molecules as well as atoms, ions, and diatomic molecules. In addition, data for the formation of, and extinction due to, liquid and solid phase dust grains is needed. A prioritization of species and data types is presented, and gives emphasis to Fe group elements, and elements important for the investigation of nucleosynthesis and Galactic chemical evolution, such as the -elements and n-capture elements. Special data needs for topical problems in the modeling of cool stars and brown dwarfs are described

Short, C. I.

NASA Technical Reports Server

INVITED TALK NASA LAW, February 14-16, 2006, UNLV, Las Vegas
Data needs for stellar atmosphere and spectrum modeling
C. I. Short
Department of Astronomy & Physics and Institute for Computational Astrophysics, Saint
Mary’s University, Halifax, NS, Canada, B3H 3C3
ishort@ap.smu.ca
ABSTRACT
The main data need for stellar atmosphere and spectrum modeling remains
atomic and molecular transition data, particularly energy levels and transition
cross-sections. We emphasize that data is needed for bound-free (b − f) as well
as bound-bound (b − b), and collisional as well as radiative transitions. Data
is now needed for polyatomic molecules as well as atoms, ions, and diatomic
molecules. In addition, data for the formation of, and extinction due to, liquid
and solid phase dust grains is needed. A prioritization of species and data types
is presented, and gives emphasis to Fe group elements, and elements important
for the investigation of nucleosynthesis and Galactic chemical evolution, such as
the α-elements and n-capture elements. Special data needs for topical problems
in the modeling of cool stars and brown dwarfs are described.
1. Introduction
It has long been known that the radiative extinction of gas plays a major role in determin-
ing both the structure of a stellar atmosphere and the emergent spectral energy distribution.
Accurate knowledge of the extinction requires accurate knowledge of transition wavelengths,
λ, and of transition cross sections, σν (or, equivalently, for bound-bound (b− b) transitions,
oscillator strengths (gf values)), which in turn requires accurate knowledge of the energy
level (E-level) structure of atoms, ions, and molecules. With the discovery and classification
of an increasing number of brown dwarfs, for which atmospheric temperatures are lower than
that of the coolest objects that are traditionally regarded as stars, there is now a need for
molecular data for polyatomic molecules, in addition to that for diatomic molecules (Allard
et al. 2003).
https://ntrs.nasa.gov/search.jsp?R=20060052448 2019-08-29T18:46:29+00:00Z
– 91 –
The importance of the b − b transitions, which give rise to spectral lines, has been well
documented. However, bound-free (b− f) transitions also significantly determine the atmo-
spheric structure and spectral energy distribution of stars, and accurate knowledge of the
corresponding cross-sections, σν , is crucial. The importance of radiative transitions has been
long recognized because they contribute directly to extinction. However, for realistic treat-
ments, collisional transitions also play a central role in determining the equilibrium state of
the gas, and thus, through the physics of radiative transfer, the emergent spectrum.
As a point of general motivation, we note that there is a long tradition, still being practiced,
in quantitative stellar spectroscopy of “tuning” atomic parameters so that solar atmospheric
models match solar spectral features of interest. This approach is based on the assumption
that we have a correct model of the solar atmospheric structure and correct solar abundances.
Therefore, it is particularly disconcerting to note that the solar abundances of C, N, and
O have recently been revised downward by factors of two to three on the basis of including
turbulent hydrodynamic structure in the atmospheric model (Asplund 2000). Furthermore,
the solar abundance of Fe is still in dispute by a factor of 1.5 (Kostik et al. 1996). Clearly,
it would be helpful to remove the atomic data as a “variable” from this process.
In addition to atomic and molecular data, extinction cross sections for liquid and crystalline
aerosols of all types are needed. Again, it is in the cool brown dwarf objects that aerosols
play a significant role in the atmospheric structure and emergent spectrum (Allard et al.
2003).
2. Atomic data
2.1. Prioritization
One way to rank the chemical elements in urgency is to weight them by the product of
their relative abundance in stellar atmospheres and the richness of the line spectrum that
they contribute. With this ranking, iron (Fe) and the Fe group elements have the highest
priority. Another prioritization is to weight the elements by their importance to particular
problems in nucleosynthesis and Galactic chemical evolution. By this criterion, the α-process
and neutron capture (n-capture) elements deserve urgent consideration.
2.2. Fe group
Fig. 1 shows the E-level diagrams for atomic models of Fe I and II that are based on lab-
oratory and semi-empirical atomic data of Kurucz (1994) and Kurucz & Bell (1995). From
– 92 –
the energy gap between the highest included bound E-levels and the ionization limit, we
conclude that the atomic models are not complete. In addition to the E-levels and tran-
sitions displayed in Fig. 1, Kurucz (1992) has also made available data for hundreds of
levels and thousands of lines on the basis of theoretical quantum mechanical calculations.
Kurucz (1992) has demonstrated that inclusion of these additional theoretical lines in so-
lar atmospheric modeling reduces the predicted solar flux in the UV band, thus allowing
theoretical solar atmospheric models to fit the observed UV band flux much more closely
than was previously possible. However, when high resolution synthetic spectra computed
with the theoretical lines are compared to the observed solar spectrum, the match is worse
than when the theoretical lines are omitted from the calculation (Bell et al. (1996), Short
& Lester (1996)). This is due to the pervasiveness of significant errors in the E-level values
(and, hence, transition λ values) and in the gf values. It should be noted that Kurucz (1992)
himself acknowledged at the outset that the theoretical lines should only be used for stellar
atmospheric structure and broad-band flux level calculations. This points to the need for
more reliable and complete measurements of E-levels and gf values for Fe.
Need for completeness: It is worth noting that although many of the Fe transitions
seen in Fig. 1 give rise to very weak lines, it has been long understood that thousands of
weak lines of a particular element concentrated in a particular band, such as the near UV, act
collectively as a broad-band pseudo-continuous veiling opacity that can significantly affect
the atmospheric structure and the emergent flux level (the “iron curtain” effect) (Rutten
1986). Furthermore, in non-local thermodynamic equilibrium calculations (non-LTE), high
lying E-levels can act as an important channel of ionization to the ground state of the next
higher stage, and their neglect in the atomic model can lead to erroneous results for the
ionization equilibrium (see, for example, Short & Hauschildt (2005)).
There is a particular dearth of atomic data for Fe ionization stages greater than II, es-
pecially for b − b transitions of λ < 2000 A˚ (ie. vacuum UV and far UV (FUV) band
transitions). Stages III and IV, and UV transitions thereof, are important for the study of
hot stars (spectral class O and B), and the chromospheres and transition regions between
the chromospheres and coronae, of solar type and relatively cool stars. Stages XII through
XIV are useful for modeling the X-ray band spectra of stellar coronae.
LS coupling: Another area of concern is the lack of atomic data from quantum mechan-
ical calculations for transitions that are not predicted under the assumption of LS coupling.
For example, Fe III has been detected outside the H II regions around hot stars, yet of the 64
terms of Fe II, as predicted by LS coupling, none have transitions to Fe III of λ < 911 where
photo-ionizing flux may escape from the H II region. This indicates that non-LS coupling
transitions may be important, and the data for them should be measured.
– 93 –
Fig. 1.— Grotrian diagrams of the models of Fe I (left) and II (right) based on the laboratory
and semi-empirical data compiled by Kurucz (1994) and Kurucz & Bell (1995).
2.3. Nucleosynthesis and Galactic chemical evolution
α-process elements: There is a particularly urgent need for improved data, or any data
at all, for the following elements associated with He fusion and the α-process: 1) gf values
for b − b transitions of Mg I and Si II; 2) σν values for b − f transitions of C I and II, Mg I
and II, and Ca I and II; 3) low E (0-5 eV) e− and H collisional σν values for O I, Mg I, Al I,
and Ca I.
n-capture elements: The advent of 8-m class telescopes has allowed the acquisition
of spectra of faint Galactic halo red giant stars of sufficient quality to allow quantitative
analysis. This includes the extremely metal poor (XMP) stars, in which [Fe
H
] is less than
-3, and can be as low as -5. In these stars, the “iron curtain” referred to in Section 2 is
relatively transparent. As a result, investigators have detected spectral lines of relatively
heavy elements that are formed by the n-capture process (both the rapid (r-process) and
slow (s-process) ). The spectroscopic determination of the abundances of these elements is
important for the investigation of the astrophysical sites of nucleosynthesis, the formation
of the Galaxy, and the nature of the first stars. However, quantitative analysis will require
accurate and complete atomic data of the sort that has been long called for in traditional
light element analysis.
2.4. General atomic considerations
Bound-bound transitions: Given the constraints on measuring (or calculating) atomic
parameters, there has been a tendency for stellar spectroscopists to prioritize by emphasiz-
– 94 –
ing those transitions that give rise to spectral lines that are useful abundance diagnostics.
However, we note that extinction due to all transitions contributes to determining the radia-
tive equilibrium (RE) structure of a stellar atmosphere. Furthermore, in non-LTE modeling
determination of the excitation and ionization equilibria is a non-local problem such that the
rate of any transition may affect the rate of any other (multi-level non-LTE effects). Worse,
to the extent that different chemical species may have transitions that overlap in wavelength,
the computed strength of a spectral line of one chemical species may depend on the equilib-
rium state of other species (multi-species non-LTE effects) (see Short & Hauschildt (2006)
for examples of the non-LTE effect of Fe group elements on Sr and Ba lines). Therefore,
accurate modeling of any spectral feature can depend on accurate atomic data for many
transitions, not just the transition that directly corresponds to that spectral feature.
Bound-free transitions: Bound-bound transitions receive much attention because of the
obvious importance of spectral lines. However, photo-ionization and recombination rates are
also of central importance to calculating the equilibrium state of a gas and radiation field
in non-LTE, and the emergent spectrum in any case. Therefore, measured values of b − f
σν values are very useful. We note that auto-ionizing resonances can greatly complicate
the dependence of σν on λ, and cause values computed with simple analytic formulae to be
significantly inaccurate (see, for example, Bautista et al. (1998)).
Collisional transitions: Cross-sections for excitations and ionizations due to inelastic
collisions, mainly with e−’s and H I, have become increasingly important as a result of the
increase in non-LTE modeling. In non-LTE models, the excitation and ionization equilibria
of chemical species are calculated by solving a statistical equilibrium among all the processes
that populate and de-populate E-levels and ionization states, including collisional processes.
Therefore, collisional rates have a much more central importance than they do in LTE mod-
eling, and are particularly crucial when collisional transition rates are similar in magnitude
to radiative rates. In turn, the calculated opacity, and hence spectral line strength, depends
on the computed equilibria. As a result of current uncertainty in collisional σν values, the
results of non-LTE modeling must be carefully qualified by laborious perturbation analy-
ses. We note that collisional σν values are needed for dipole-forbidden as well as permitted
transitions.
Excited states: We note that for realistic modeling, σν values are needed for b− f and
collisional transitions arising from excited states as well as from the ground state.
– 95 –
3. Special problems requiring improved physical data
3.1. Brown dwarf and Jovian planetary atmospheres
Polyatomic molecular bands: As a result of their low temperatures (Teff < 3000
K), brown dwarf atmosphere chemistry admits significant polyatomic molecule formation.
Molecular bands that form in the visible and near UV spectral regions (ie. “hot” bands)
have a significant effect on both the atmospheric structure and the emergent spectral energy
distribution. However, there is a dearth of molecular data for especially important bands
of methane (CH4), water (H2O), ammonia (NH3), and iron hydride (FeH). For completely
accurate extinction modeling, there is also a need for data for metal hydrides in addition
to FeH. See Leggett et al. (2001) for examples of observed spectra with molecular bands
identified.
With the discovery of an increasing number of extra-solar “hot Jupiter” planets, brown
dwarf modeling has been extended downward in temperature to the regime of hot gas giant
planets. For these models there is a need for σν values for excitation of H2 by inelastic
collisions.
Atomic resonance line damping: Because the gas pressure in brown dwarf atmospheres
is relatively large, the resonance lines of some atomic species, most notably Na I and K I,
have Lorentzian damping wings that span over 1000 A˚ (see Allard et al. (2003)). As a
result, the wing of a single spectral line acts as a broad-band pseudo-continuous opacity
source that significantly affects both the atmospheric structure and the emergent spectral
energy distribution. Therefore, it is especially important to accurately model the line profile
and to have accurate knowledge of the atomic line broadening parameters.
Aerosols: The low temperatures of brown dwarf atmospheres also allow for the condensa-
tion of a large variety of liquid droplet and crystalline grain aerosols. Data is needed for the
extinction versus λ for these. Again, their extinction affects both the atmospheric structure
and the emergent spectrum (see Allard et al. (2003) for example). Important examples for
which data is needed are NH3 and NH4SH.
3.2. Stars: solar type and cool
CO collisional excitation: The fundamental vibrational (∆ν = 1) band of CO at 5µm
has been found generally to be stronger than expected in the spectra of solar type and
relatively cool stars. The most compelling explanation that has been put forward is that the
outer atmospheres of cool stars are thermally bifurcated, with the CO band forming in clouds
– 96 –
of cool gas that are embedded in warmer gas at altitudes that are considered to be purely
chromospheric in traditional 1D models (Ayres & Wiedemann 1989). If valid, then this
interpretation represents a major advance in moving beyond one of the main restrictions of
classical stellar spectrum interpretation, namely horizontal homogeneity. However, as noted
in Section 2.4, the true excitation equilibrium of a chemical species may depend on collisional
transition rates. Therefore, the correct interpretation of the strength of the ∆ν = 1 band
depends on the adopted value of σν for the excitation of CO by inelastic collisions with H I.
Currently, these σν values are estimated by scaling σν values for collisional excitation by H2,
and are uncertain by a factor of approximately 100. The large uncertainty limits the ability
to model the atmospheric structure on the basis of the ∆ν = 1 band with certainty.
SiO collisional excitation: Similarly, more accurate knowledge of the σν value for
collisional excitation by H I of SiO would be helpful for modeling maser emission in the
vicinity of very large cool stars. Masing occurs in an extreme non-LTE equilibrium, thus
collisional transition rates are important.
Silicate dust in winds: In cool stars with dusty winds the measured density of silicate
dust grains is greater than expected on the basis of the gas density in the wind. One
explanation is that the physical data relevant to silicate grain nucleation and condensation
is inadequate. Silicate grain formation has not been as well studied as that of carbon grains.
REFERENCES
Allard, N.F., Allard, F., Hauschildt, P.H., Kielkopf, J.F., Machin, L., 2003, A&A, 411, 473
Asplund, M., 2000, A&A, 359, 755
Ayres, T.R & Wiedemann, G.R, 1989, ApJ, 338, 1033
Bautista, M.A., Romano, P., & Pradhan, A.K., 1998, ApJS, 118, 259
Bell R.A., Paltoglou G. & Tripicco M.J., 1994, MNRAS, 268, 771
Kostik, R.I., Shchukina, N.G. & Rutten, R.J., 1996, A&A, 305, 325
Kurucz R.L., 1994, CD-ROM No 22, Atomic Data for Fe and Ni (Cambridge: SAO)
Kurucz, R.L., 1992, Rev. Mex. Astron. Astrofis., 23, 181
Kurucz, R.L., & Bell, B. 1995, CD-ROM 23, Atomic Line List (Cambridge: SAO)
Leggett, S.K., Allard, F., Geballe, T.R, Hauschildt, P.H., Schweitzer, A., 2001, ApJ, 548,
908
Rutten, R.J., 1986, in IAU Colloquium 94, Physics of Formation of Fe II lines outside LTE,
ed. R Viotti (Dordrecht: Reidel)
Short, C.I. & Hauschildt, P.H., 2006, ApJ, too appear in April
Short, C.I. & Hauschildt, P.H., 2005, ApJ, 618, 926
Short, C.I. & Lester, J.B., 1996, ApJ, 469, 898


Data Needs for Stellar Atmosphere and Spectrum Modeling

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server