Infrared observations from planned space facilities (e.g., ISO (Infrared Space Observatory), SIRTF (Space Infrared Telescope Facility)) will yield a large and uniform sample of high-quality data from both photometric and spectroscopic measurements. To maximize the scientific returns of these space missions, complementary theoretical studies must be undertaken to interpret these observations. A crucial step in such studies is the construction of phenomenological models in which we parameterize the observed radiation characteristics in terms of the physical source properties. In the last decade, models with increasing degree of physical realism (in terms of grain properties, physical processes, and source geometry) have been constructed for infrared sources. Here we review current capabilities available in the phenomenological modeling of infrared sources and discuss briefly directions for future research in this area

Kwok, Sun

Leung, Chun Ming

English

NASA Technical Reports Server

As_m_cai i_Jared Sprc_coi_: Yu_rt @b_trva_on_ D_c_onJ
ASP Cov_ftrtnce Sides, Vol. 41. ig93
_m K,,_k (td.)
NASA-CR-203013
/
PHENOMENOLOGICAL MODELING OF INFRARED SOURCES:
RECENT ADVANCES
CHUN MING LEUNG
Department of Physics, Rcnsselaer Polytechnic Institute, Troy, New York
12180-3590, USA
ABSTRACT Infrared observations from planned space facilities (e.g., ISO,
SIRTF) will yield a large and uniform sample of high-quality data from both
photometric and spectroscopic measurements. To maximize the scientific
relurns of these space missions, complementary theoretical studies must be
undertaken lo interpret these observations. A crucial step in such studies is the
conslruction of phenomenological models in which we parameterize the observed
radiation characteristics in terms of the physical source properties. In the last
decade, models with increasing degree of physical realism (in lerrns of grain
properties, physical processes, and source geometry) have been constructed for
infrared sources. Here we review current capabilities available in the
phenomenological modeling of infrared sources and discuss briefly directions for
future research in this area.
1. INTRODUCTION & OVERVIEW
Infrared radialion is the primary tracer of the dust componenl in the universe. Dust
grains, although a minor constituent, play a very important role in the thermodynamics
and evolution of many astronomical objects. In the last decade, NASA has launched a
number of space-based observatories: the Infrared Astronomical Satellite (IRAS), the
Hubble Space Telescope (HS'r), and the Cosmic Background Explorer (COBE). The
IRAS, launched in 1983, carried out a comprehensive infrared survey of the sky and a
wide variety of astronomical objects were detecled in the infrared. The IRAS has
stimulated advances in most branches of astrophysics and the IRAS database remains
a vital tool for current research in infrared astronomy. Most recently the astronomical
community in the US has identified the 1990's as the decade of infrared aslronomy.
To maximize the scientific returns of past and future space missions (e.g., ISO
and SIRT_ complementary theoretical studies have been undertaken. A vital
component of these studies is the construction of phenomenological models in which
we seek to parameterize the observed radiation characteristics of infrared sources in
terms of their physical source properties (see Figure 1).
"lOft
https://ntrs.nasa.gov/search.jsp?R=19990034119 2020-06-15T22:28:32+00:00Z
Phenomenological Modelling 191
Scoville and Kwan 19"/6; Rowan-Robinson 1980; Wolfire and Cassinelli 1986, Egan
ctal. 1988). In the last decade, significant progress has been made in the development
of radiation transport models for infrared sources. ModeLs with increasing degree of
physical realism have been constructed, in Figure 2 we summarize schematically the
recent advances made. Clearly models of infrared sources have hecome increasingly
sophisticated Below we brictrly descdb¢ a few selected resuiLs to demonstrate the
recent progress made in the modeling of infrared sources
[ pA= I t J
I GRAIN PROPERTIES I
• grayu pow_-lawopac_
• singlegrain
• ,singlesizegrain
• spte_ st_
• temp.-indepen_t opacity
•nan_ayo_
• m_plegraintypes
• size 01stdl3ution
• _,_oes
• temp.-Oepementopac_
[ PHYSICALPROCESSES J
• isotrof_c radka_o_
• si_jJe o¢no sca.ed_
• equi_Un r_ating
• classicalnucleationtheory
• anisouop¢rackation
• aniso_o_mul_pt_scatmnng
• ec_u_ansient tmet,,g
. maslefequa@o,al_oac_
[ SOURCEGEOIIETRY 1
• 1-Dslaborsphere * 1-Dsphece,2-Ddi_
[ MODELINGAPPROACH J
• anay1_alorsemi-analybcal
• singleso_ce
• r_mencal
• singlesource,classofsources
Figure 2 - Schematic diagram comparing the past and present capabilities in the
phenomcnological modeling of infrared source_
190 C.M. Leung
1. Perkxm statistical analyses and correlation studies, and deve_ classification
schemes, ¢.9., histograms, two-colo¢ dagrams, _orrela_n Oots, ck_ss_rw.at_ of spec_
features.
2. Constmclphenomenological models _ each class o! o_ecls Io parammerize the
observed radiat_ d_am_dstics in leans of I_r _ source prcg_eaies.
3. From the _ical models, dewlo_ self.consistent physical models wtf_ con
exp_n oo_entty p_esen_ and fu_re _.
I
I I
• luminosityand spectral energy dislribution
of heat source
• source geometry &dimensions
• _operlies of grainspecies [e.g., opacity,
=bedo,scattenng_ _on)
• densily (fistribulJonof dust grains
• other localhealing mechanisms(e.g.,
v_cosily, collisionswill_gas pa_cle_)
• temperature dis_ibulJons of eac_ grain
spec_
• characteristicsof internal radiationfield
• intemity vadabon acrosssource s_rface at
different wave_,_s
• chaacteristics of grainspecial features
• eme_genlen_gy spectrumof heat so_ce
and dusl sllell
[ OBSERVATIONAL CONSTRAINTS & DIAGNOSTIC PROBES I
• spe_ energy _st_bu_on {fluxspeclrumI _ source lumh'_os_
• sudace I_htness & appa_enlemissions_zeas a functionof wa_length _ dust density
distribution& geometri:at sou_c_size
• strength and st_peof emission/abso_ion lealuces ¢:o grain oompo_lion & source opacity
• color temperatures incliflere_l pails of spec_um _ oust temperature distribution
• flux ratios o_colo,-colordiagrams ¢_ sourceevolution
Figure I - Schematic diagram to show thai a crucial step in the Iheorctical
studies of infrared sources is the construclion of phenomenological models.
To m¢_el the observed characteristics of an infrared source, typically one solves
the equation of radiation transport in a dusty medium, subject to the constraint of
radiative equilibrium. For spherically symmetric geometry, the problem has been
solved by many author's (e.g., Lcung 1975; Apruzcsc 1976; Jones and Merrill 1976;
192 C.M. Leung
2. SOME RECENT ADVANCES
2.1 Grain Properties
In astrophysical environments dust grains have irregular shapes mo6t likely formed by
fractal growth p_, e.g., paNicle-by-paNicle aggregation or cluster-by-cluster
aggregation (Witten and Cares 1986). On the other hand, spherical dust grains are
often assumed in models of infrared sources so that the dust opacity can be calculated
from the Mie theory. A computational technique now exists for calculating the dust
opacity for grains of irregular shapes (Draine 1988; Bazell and Dwek 1990). In this
method, called "discrete dipole approximation", a fractal grain is approximated by a
collection of dipoles. Using realistic dust opacities for fractal grains, recently Fogel
and Leung (1992) have constructed radiation transport models for infrared sources.
Compared to models with spherical grains of Ihe same composition and volume,
models with fraetal grains show a shift in the peak flux toward longer wavelengths,
implying that fractal dust grains are cooler than spherical grains. These differences
can be attributed to a lower ratio (p) of volume to geometric cross section (averaged
over orientation) for the less compact fractal grains. The ratio p varies with the fractal
dimension. Grains with a higher p attain a higher temperature. Furthermore, grains
having the same fractal dimension show almost no difference in their absorption
cross sections and energy spectrun_ implying that the overall grain shape plays only
a minor role in the thermal properties.
In modeling infrared sources, the opacity of dust grains is often assumed to be
temperature-independent. However, the optical constants of many grain types actually
change with grain temperature. In particular, for water ice, which has a feature at 3.1
p.m, the absorption coefficients change with temperature. The ice feature is seen in the
spectra of molecular clouds and circumstellar dust shells of both young and evolved
stars (Whittet 1992). As the temperature increases, water ice changes from an
amorphous state to a crystalline state. This irreversible phase change, which occurs at
around 100 K, narrows the ice feature and increases the peak strength, thus producing
different profiles for the 3.1 I.tm feature as the temperature increases. Since the grain
temperature changes with position in infrared sources, it is crucial that temperature-
dependent effects in grain opacity be incorporated in detailed models to interpret
observations of ice features. This will allow us to probe the evolution of ice mantle on
grains. Recently laboratory data on temperature-dependent optical constants for water
ice (Hudgins cl al. 1992) have become available, making it possiblc to include these
effects in radiation transport models. Work is underway to study this problem.
2.2 Physicai Processes
Among the unexpected results from IRAS was the discovery of excess mid-infrared
emission detected in many infrared sources, e.g., diffuse clouds, dark globules, visual
reflection nebulae, and high-latitude dust clouds or infrared cirrus, it is now bclievctt
tlmt the cmissi_m at short wavelengths (< 30 i.tm) comes from transicnt heating of very
small grains and large polycyclic aromatic hydrocarl_ms or PAtts (f_}r a review sec
194 C.M. Leung
::::::i:'::'::: _ .... .,_ ' ".... '_::' ........ _:i:: " :_:: ::_;::i'_: "i_:" :i::::":;:_!:.: _" ' " i_'>:" :%::
_ _" " .-_ .... ::::" o.':::i. ". ....... :,' "..'::::._ ..... "" .
I"" S F..EI I='" ruSKI
v_
radi_ie_ unhu_y
flux c_)m_wvutlon
c_mpuUngharchvm
IINI IK)4_
. ob_rv_l lumlno_ty
vt_ible/_nbm'_l Ilux
appar_n_ _murc_
J'THEORETICAL J
• r,O
• orc_r_c_fermlmlequa_
• sca_,s_m__o.nc_,_mnd
• _ F ; consWnt
• scalarmachines
• r,z,O,)
• _ cliffemntiaJequations
• tensor;complex, non-imuitive
• J r Fz clr + rJ Fr dz = constanl
• vectcx/parallelmachines
I OBSERVATIONAL I
• a_/s embed(_or_
• orimta_ indepemmt
• conr,_ecl
• increases.i_
• embed_cxpa_tyexpos_
• del_ongecxneVy/cxien_c_
• tx_rrelal_
• no ._mp_ereia_
Figure 3 - Schematic diagram comparing the theoretical and observational
considerations in modeling infrared sources with I-D and 2-D geometries.
2. 4 Modeling Approach
Unlike ground-based observations, space missions generally yield large quantities of
data. Interpreting such data requires a different strategy which utilizes extensive
modeling. As an example, the IRAS survey has yielded a large and unifonn sample of
high-quality data from observations of asymptotic giant branch stars. In particular,
the IRAS dala indicates that many carbon slars, especially those with optically thin
dust shells, have large fluxes at 60 and 100 p.m. It has been suggested that either an
extended single dust shell or a two-shell model, with a remnant shell from an earlier
mass-loss episode (Willems 1987), can explain the cxcc_ fluxes. To test these
hypotheses, models of dust around carbon stars have been constructed: models with
either a single C-rich dust shell or double shells (Egan and l..¢ung 1991). Figure 4
shows the color-color diagrams for selected carbon stars in the IRAS-LRS catalog.
The thick solid line is the blackbody line. The gray-shaded areas indicate the limits on
the colors for carbon stars of various ages imposed by the mcvJcls. CIcarly single-shell
m(xlcls cannot explain the observed color distribution, while two-shell m(xlcls with
Phenomenological Modelling 193
Pugel and Leger 1989). Temperature fluctuations in small grains occur whenever the
energy input from photons or energetic particles is considerably larger than the
average energy content of the grain. This non-equilibrium grain heating can change
significantly the energy distribution of the radiation field in infrared sources. By
formulating the radiation transport problem involving transient heating in a fashion
similar to a non-LTE line transfer problem involving many transitions, Lis & Leung
(1991a,b) recently constructed models which treat self-coasistently the thermal
coupling between the transient heating of small grains and the equilibrium heating of
conventional large grains. They used the models to interpret the IRAS observations of
the Barnard 5 cloud and a diffuse cloud in Chamaeleon. In both cases, Iongward of
100 _m, the emission is dominated by large grains. Between 30-100 ttm, the emission
is produced mainly by very small grains. Shortward of 30 v.m, both PAils and small
grains are responsible for the emission. Typically very small grains and PAHs
account for 10-20% of the total opacity in the visible, and 5-20% of the total dust
mass of the cloud. Furthermore, to produce the observed infrared limb brightening,
the spatial distribution of small grains and PAlls must be mote extended than that of
large grains. This has important significance in understanding the origin of small
grains and PAHs. Thus detailed radiation transport models now exist which
incorporate both the transient heating of very small grains and the equilibrium heating
ofconventional large grains.
Another important progress deals with the study of grain formation in stellar
outlfows, using a master-equation approach (Egan and Leung, this volume). This
method allows a self-consistent treatment of grain nucleation and growth, it is found
that, compared to the classical nucleation theory, the new method predicts fewer but
larger grains. This has important observational implications.
2.3 Source Geometry
Although there is growing observational and theoretical evidence for a large number of
disk-shaped or toroidal objects of astrophysical interest (e.g., circumstellar disks,
protoplanctary disks, protostellar accretion disks, bipolar molecular flows, and disk
galaxies), most models currently in use invoke the assumplion of spherical geometry.
A few attempts have been made to model infrared sources with nonspherical geometry.
Lefevre el al. (1983) performed Monte Carlo simulations of ellipsoidal dust shells
around cool stars, while Ghosh & Tandon (198.5) calculated dust temperature
distributions in cylindrical clouds with embedded stars. The latter work has been
extended by Dent (1988) to the case of circumstellar disks around young stars. Most
recently, Spagna et al. (1991) considered radiation transport in disk-shaped interstellar
clouds heated externally by the ambient interstellar radiation field. While theoretical
tools are available for modeling infrared sources with different geometries, the typical
thousand-fold increase in computing requirement for 2-D geometries makes it
impractical to do any extensive modeling. In Figure 3 we compare the two geometries
(I-D sphere vs. 2-D disk) from theoretical and observational considerations. Since
establishing source geometry places severe constraints on the origin, dynamics, and
properties of infrared sources, it is crucial to consider realistic geometries.
PhenomenologicalModelling 195
either a C-rich or an O-rich remnant shell can. Thus by modeling a class of sources
and comparing the results with observations, one can idemify trends and determine
more reliably the physical source paramelers.
od
"x
to
,.<
o
i i I i ! i ! " -
Carbon SLars _n IRAS-LRSC
S_n61¢ 5heft Models
Doll/bJO _heJJ ModeJs
ResJoJ_ of Overlap
300If
Figure 4 - Color-color diagrams for selected carbon stats in Ihe IRAS data
(filled circles). The shaded areas indicate regions occupied by models.
3. CONCLUDING REMARKS AND OUTLX)OK
To summarize, much progress has been made in the development of phenomenoiogical
models for infrared sources. Models with increasing degree of physical realism have
become available. Future research in this area should incorporate other important
physical processes so that self-consistent physical models can ultimately bc
constructed: a) (ransfer of [x)iarized radiation, b) radiation hydrodynamics, c) radiation
transport in 3-D geometries, d) grain nucleation and growth, and e) gas-phase and
grain-surface chemistry. In addition, improvements in computational techniques, e.g.,
be,er algorithms and iteration procedure, should be made.
Another critical research area deals with automation of modeling. Constructing
computer models for specific infrared sources (model fitting) is a labor-intensive task,
since one needs to adjust many model parameters and run many models. It is not
unusual to require several hundreds of models before one finds a few models which
can fit the observations. To expedile the analysis of spaced-based infrared
observations, a critical task is h) automate the m(x.lcling process so Ihal rc.,;carcher_
196 C.M.Leung
canmodelinfraredsourcesonworkstationsi realtime.Whencomputermodeling
becomes as easy as doing least-square fits, i! will become a rouline part of data
analysis. With automation researchers can perform computer experiments to test
various hypotheses and determine Ihe physical parameters for infrared sources. With
such a research tool, predictions on certain observational consequences can be made
readily. This will stimulate further observations and theoretical studies, efforts which
are essential to the future space-based missions such as ISO and SIR'IF.
ACKNOWLEDGMENTS
At Rensselaer, research in the computer modeling of infrared sources has been
supported by the US Air Force (granl AFOSR 89-0104) and NASA (grants NAGW-
577, NAGW-1103, NAGW-2817, and HAG 5-1182).
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