|
FGLA |
Aim |
The following
text is an extract from the proposal which has been submitted to the DFG in
Dec. 1999
|
1. Astrophysical background |
|
The space between the stars is not
empty, but is filled with a tenuous interstellar medium (ISM) which in the
Galaxy is characterized by an average density of 1 hydrogen nucleus per cm3.
This medium consists of distinct phases with very different thermodynamical
properties (see Hollenbach and Thronson 1987). Most of the total mass of the
galactic interstellar gas, which amounts to 6x109 solar masses, is
contained in cool and "dense" clouds. There are two types of such
clouds: molecular clouds containing mostly molecular hydrogen, and clouds of
atomic hydrogen. Molecular clouds are characterized by kinetic temperatures
of 10-15 K and gas densities between a few hundred and 107
hydrogen molecules per cm3 in their dense cores. Hot molecular
cores associated with star formation can have temperatures up to 200 K
leading to the release of grain mantle components into the gas phase. Compared
with quiescent molecular clouds, diffuse atomic hydrogen clouds have higher
average temperatures (80 K) and lower densities (50 hydrogen atoms per
cm3). Outflows from evolved stars and protoplanetary disks are
characterized by even higher densities and temperatures up to the grain
formation / destruction temperatures. The interstellar medium is an
integral component of the various classes of normal and active galaxies. In
one of the most important evolutionary processes in the universe, the
molecular cloud phase of the ISM is transformed into stars which in their
later stages enrich the gas with heavy elements. A fraction of these heavy
elements has condensed into submicron- and micron-sized solid particles
(cosmic "dust"). Molecular line and continuum radiation observed at
infrared, millimeter and radio wavelengths has frequently be used to
determine the physical conditions (temperature, particle density, column
density, magnetic fields) in the interstellar medium (Genzel 1991). The
infrared bands caused by aromatic hydrocarbons have been recently recognized
to be an important indicator for star formation at higher red-shifts. Atoms, molecules, and grains are
more than just probes of the interstellar medium, but act as active players;
they provide heating and cooling through energetic photoelectrons, fine structure
line emission, and gas-grain collisions. The degree of ionization in
molecular clouds and protoplanetary accretion disks and the coupling of the
magnetic field with matter is determined by the shielding effect of dust
grains and the depletion of metals. Radiation pressure acting on grains and
gas causes molecular outflows from evolved stars and may even prevent the
formation of extremely massive stars. Opacity-limited fragmentation may be
decisive for the determination of the lower mass end of the initial mass
function of stars. The chemical evolution of protoplanetary disks resembles
the chemistry in the early solar nebula, which finally determines the
composition and structure of the planets. |
Table 1: Types of molecular processes
(after van Dishoeck 1999)
Chemical processes
|
Ion-molecule reactions |
X+ + YZ ® XY+ + Z |
|
Charge transfer |
X+ + YZ ® X + YZ+ |
|
Neutral-neutral reaction |
X + YZ ® XY + Z |
|
Radiative association |
X + Y ® (XY)* ® XY + h n |
|
Grain surface formation |
X + Y + g ® XY + g |
|
Associative detachment |
X– + Y ® XY + e– |
|
Photodissociation |
XY + hn ® X + Y |
|
Dissociative recombination |
XY+ + e– ® X + Y |
|
Collisional dissociation |
XY + M ® X + Y + M |
Heating and cooling processes
|
Photoelectric heating |
Grain/PAH + hn ® grain/PAH + + e* |
|
Cosmic ray heating |
H2 + cosmic ray ® H2+ + e* |
|
CO line cooling |
CO(J) + coll. ® CO(J*)
® CO(J’) + hn |
|
Oi line cooling |
O(3P2) + coll. ® O(3P1) ® O(3P2) + hn |
|
Cii line cooling |
C+(3P1/2)
+ coll. ® C+(3P3/2)
® C+(3P1/2)
+ hn |
|
Gas grain heating/cooling |
gas + grain ® gas’ + grain’ |
|
A comprehensive description of the ISM has to include a wide variety
of basic physical and chemical processes that determine the evolution of the
medium and the observable properties (van Dishoeck 1997, Hartquist and
Williams 1998, Ehrenfreund et al. 1999). The typical chemical
processes and the interactions of the matter with radiation are summarized in
Table 1. Their importance for diffusive and molecular clouds has been
thoroughly discussed by van Dishoeck (van Dishoeck 1998). The atoms, molecules, the dust particles and the radiation field
interact with each other in an extremely complex manner. Concerning the
chemical processes one usually distinguishes between neutral and ionic
reactions and reactions which occur on grain surfaces. Ion-molecule
reactions have dominated for long time the reaction models developed for
describing molecular clouds and protoplanetary accretion disks (Herbst and
Klemperer 1973, Black and Dalgarno 1973) since many of them have no barriers
(Gerlich 1993, Smith M.A. 1993). A very important reaction mechanism at the
low temperatures and densities of the ISM is radiative association, a
challenge for laboratory experiments (Gerlich and Horning 1992). Based on new experimental evidence (Smith 1997, Chasting et al.
1998, Rowe et al. 1999, Kaiser et al. 1998) it has been shown
that also neutral-neutral reactions can have large enough rate
coefficients at low temperatures that they may be important. Very relevant
for understanding the evolution of interstellar matter is the process of isotopic
fractionation occurring predominantly at the low temperatures of
interstellar clouds but also in protoplanetary accretion disks (Wootten 1987,
Millar et al. 1989, Aikawa and Herbst 1999). Not much quantitative knowledge exists on the role of grain
surfaces in the chemical balance of the ISM. Although there is no
experimental proof, it is commonly assumed that H2 is formed via
catalytic reactions on dust particles (Hollenbach and Salpeter 1971). Also
for the formation of hydrogenated species such as H2O, NH3 and
CH4 the importance of grain surface reactions has been postulated
(e.g. Tielens and Hagen 1982, d'Hendecourt et al. 1985, Tielens and
Allamandola 1987, Herbst 1993). So far, more than 100 interstellar molecules have been identified
including neutral and ionic radicals with unsaturated bonds and complex
polycyclic aromatic hydrocarbons. Many more will be found since the wide
temperature range, the low densities as well as the lack of containing
"walls" make molecular and diffuse interstellar clouds to very
special chemical factories. One also has to consider that all species and
processes involved are strongly influenced by the environmental radiation
fields, energetic cosmic particles, turbulent motions, and shock fronts
produced by stellar winds and supernova explosions. In the denser regions of
protoplanetary accretion disks also ternary reactions can become
important (e.g. Willacy et al. 1998). Finally, the freeze-out
of molecules on cold grains and desorption processes have to be
considered both in dense and cold molecular cloud cores and the outer regions
of protoplanetary disks. The lifecycle of dust grains is closely connected to that of
stars (e.g. Millar and Williams 1993, Dorschner and Henning 1995, Henning
1999). Interstellar grains form in the cooling outflows from evolved stars
and supernovae. The C/O ratio determines if carbonaceous grains and
carbides (C/O >
1) or oxides including silicates (C/O < 1)
prevail (Cherchneff 1998, Gail and Sedlmayr 1998). Cases exist where
silicates are stored in circumstellar disks and carbonaceous matter exists in
the envelope region. Sputtering and grain-grain collisions in interstellar
shocks lead to the disintegration of interstellar grains and the production
of very small grains (VSGs) (Jones et al. 1996). VSGs are stochastically
heated by the interstellar and circumstellar radiation field. Molecular ice
mantles composed mainly of H2O, CO, and CO2 are formed
in molecular clouds and circumstellar envelopes (Schutte 1999). Grain-grain
collisions with low relative velocities are the process leading from
micron-sized particles immersed in protoplanetary disks to kilometer-sized
planetesimals (Beckwith et al. 2000). |
Table 2: Carriers of infrared
emission components.
(after
Tielens et al. 1999)
|
Carrier |
IR emission component |
Number |
Size |
Abundance of C locked up (ppm) |
|
PAHs (uncertain) |
UIR bands |
20-100 |
4-10 |
14 |
|
PAH clusters |
Plateaus |
100-1000 |
10-20 |
8 |
|
VSG |
25 µm |
1.000-10.000 |
20-30 |
7 |
|
Small grains |
60µm |
100.000 |
50 |
16 |
|
Classic grains |
8 > 100 µm |
|
≥ 100 |
35 |
|
The ubiquitous unidentified infrared bands (UIRs) are widely
attributed to polycyclic aromatic hydrocarbon (PAHs) molecules (Leger and
Puget 1984, Allamandola et al. 1985) although it has been shown by
laboratory emission experiments that small PAHs do not match with the
astrophysical observations (Schlemmer et al. 1994, Cook et al.
1996). PAHs can either accompany grain formation or are produced by the
erosion of carbonaceous grains in the diffuse interstellar medium (Tielens
1997). In addition, carbon-chain molecules exist both in the diffuse ISM and
molecular clouds (Irvine et al.1987, Thaddeus 1994). Both carbon-chain
molecules and PAHs have been proposed as carriers of the mysterious diffuse
interstellar bands (DIBs) (Tielens and Snow 1995, Tulej et al.
1998, Romanini et al. 1999, Maier et al. 1998). The size of the
carriers of infrared emission components are summarized in Table 2. In
addition, Table 3 gives abundance estimates for large interstellar molecules.
Spectroscopic evidence exists for amorphous and crystalline silicates,
carbonaceous grains, carbides and to a lesser degree for sulfides and other
metal oxides (Henning 1998, 1999). |
Table 3: Abundances of large
molecules.
(after
Tielens et al. 1999).
|
Species |
Niche |
Abundance |
Abundance of C locked up (ppm) |
|
PAHs |
The Universe |
10-7 |
14 |
|
C-chains |
Diffuse ISM |
<6×10-9 |
<3×10-1 |
|
C60 |
Diffuse ISM |
3×10-8 |
2 |
1.2 Observations
|
We are now in the golden
age of observational astronomy. Observations over the whole range of the
electromagnetic spectrum both from the ground and space have provided a
wealth of ISM data of a class and quality that is unprecedented. Millimeter
line and infrared spectroscopy have been decisive for the determination of
the physical and chemical conditions in the ISM (Sandford 1996, van Dishoeck
and Hogerheijde 1999). The Infrared Space Observatory ISO recently led to a
truly revolutionary step in our understanding of the interstellar medium by
opening the spectral range from 2.4 µm to 200 µm for spectroscopy. Detection
includes (i) the ubiquitous nature of the UIR bands, (ii) the widespread
presence of water in warm dense gas, (iii) the detection of a whole set of H2
ro-vibrational and pure rotational lines, (iv) a complete inventory of
molecular ices, (v) the detection of the sharp signatures of crystalline
Mg-rich silicates at mid-infrared wavelengths in various environments, and
(vi) the detection of long carbon chain molecules in interstellar space. Early
results of the ISO mission have been summarized in 1996 in a special issue of
Astronomy and Astrophysics (Vol. 315) and are more completely covered by the
proceedings of the Paris ISO meeting (Cox and Kessler 1999). New infrared
missions such as SIRTF, FIRST and the NGST, which will allow an even better
and deeper infrared view into the universe, are already scheduled. The
American-German airborne observatory SOFIA will succeed the highly successful
Kuiper Airborne Observatory (Haas et al. 1995) in 2002/2003.
|
1.3 Challenges for Laboratory
Astrophysics
|
|
In order to exploit the truly unique observational data, a dedicated
research effort is required covering the physics and chemistry of molecules,
clusters, nanoparticles and grains under the extreme conditions of
interstellar space. Worldwide, there are several related activities in this
interdisciplinary field of research as mentioned in the preface. This field
which is briefly called laboratory
astrophysics (Ehrenfreund et al. 1999), encompasses
various domains as schematically indicated in the figure on the next page. Activities
include systematic empirical laboratory investigations as well as basic
research where fundamental questions are answered either by experiment or
basic theory. The results from these efforts can be either used directly for
understanding and predicting observational facts or they are utilized as
parameters in large scale models. There is general agreement that most of the
present models describing physical and chemical processes in the ISM, treat
only parts of the problem and that they use parameters and concepts often not
based on reliable data. Because of the wide area
of fundamental research needed for astronomy, a major aim is to address those
topics which seem likely to yield the greatest scientific return. There are
still many open questions in gas-phase
reactions at low temperatures and at low densities. Processes of specific
interest include exothermic ion-molecule reactions which are hindered by a
small barrier (Gerlich 1993, Smith D. 1993) or the formation of fast
products by exothermic reactions which significantly affect chemical
networks. Other open questions concern the formation of H2, the
balance between characteristic molecules (e.g. SO and CS), and the formation
of typical products (e.g. C7– or ammonia). Critical
quantities are rates for recombination of electrons with cold ions (H3+),
or the photostability of large molecules such as PAHs.
|
1.3 Challenges for Laboratory
Astrophysics
Laboratory Astrophysics
In astrochemistry, most poorly understood is probably the interaction
of gas with grain surfaces. The parameters used in models for sticking and
desorption of molecules on grains or for chemical reactions occurring on the
surfaces or in ice layers, are often taken from crude measurements or simple
models. Also the formation and growth of the carbonaceous and silicatic
solids from the gas phase is poorly understood. Grain processes which have
to be simulated include sputtering by energetic particles and radiation,
collisional destruction and growth, as well as crystal/amorphous
interconversion. Thermal annealing and crystallization assisted by the presence
of OH groups is a special field of interest after the ISO detection of
crystalline silicates in rather different cosmic environments.
The structural, spectral, and scattering properties of micron- and
submicron-sized solid grains and grain aggregates have to be known if the
information contained in observed spectral energy distributions, intensity and
polarization maps should be completely exploited. The influence of such factors
as temperature, finite size, shape, and morphology on optical properties of
cosmic dust analogues have to be investigated. Without the knowledge of the
spectral properties of gas phase molecules the mystery of the carriers of the
Diffuse Interstellar Bands and the Unidentified Infrared Bands will remain
unsolved.
Special challenges are related to the role of hydrogen, the most
abundant molecule in the Universe. Our knowledge on this molecule has
significantly augmented in recent years as became apparent on the international
conference H2 in Space (Combes and Pineau des Forets 1999),
however, it also became obvious that many more laboratory studies are needed
for astrophysical interpretations. This concerns the formation of H2
and the ortho / para ratio under the conditions prevailing in
space as well as the energy balance of the ISM (Flower et al.
1999). Other important quantities are the H/D ratio, a measure of
galactic evolutions or the amount of dark matter in the form of H2
(Valentijn and van der Werf 1999).
The field of laboratory astrophysics is at a point where a comprehensive
effort such as our proposed FGLA shall have a major impact by
concentrating in a coherent way experimental and theoretical expertises and by
providing an attractive discussion forum for modelers and astronomers.
2 Scientific concept of the research group
2.1 Introduction
The overview given in the last section demonstrates that laboratory
astrophysics is a wide interdisciplinary field. It is obvious that a
Forschergruppe, like our FGLA must concentrate on specific aims which
are selected according to the challenges of modern astronomy, the scientific
experience of the researchers and the potential of the involved laboratories. As
a fundamental concept for our proposal we have selected six major fields which
are depicted in the following graph. Some more topics will be integrated by
external cooperation (see Section 2.3.3).
Concept of FGLA
The included numbers refer to the projects TP1 to TP10 of this proposal.
Two of them deal with basic theory, one with modeling. All other contributions
are experimental efforts. The relations between the groups which are indicated
by thin lines, will be detailed in 3.5 of the individual projects. Basic theory
is not only needed for understanding experimental results on a profound level
but an important aspect for laboratory astrophysics is that theoretical
descriptions must help to generalize the results for efficiently integrating
them into models of the ISM. In modeling, very sophisticated and realistic
descriptions can be made today due to advances in computer power. We think that
this subfield is important to be integrated into our FGLA since it is an
important link to the astrophysical community. Experiments on gas phase
collisions are dealt with in three projects. The availability of radicals (H
and C atoms) will open up a new important area in ISM chemistry. The physics
and chemistry of grains, solids, and surfaces is another important subject of
the FGLA. Optical properties of interstellar matter in its various forms
and under different excitation schemes link together the three fields:
laboratory research, astrophysical modeling, and observations. Also
spectroscopic studies and light scattering are included. These six selected
fields are the basis of our joint activities and will be discussed in some
detail on the following pages.
2.2 Basic theory
For simple systems, such as electron-atom recombination or absorption
and emission of photons, the treatment from first principles is standard
nowadays. For treating collision dynamics highly sophisticated quantum
chemical methods have been developed if only a few atoms are involved. Larger
molecules of interstellar relevance can be calculated to spectroscopic precision
(Botschwina et al 1995, Botschwina and Hey 1999). For larger systems,
Hartree-Fock methods (Szabo and Ostlund 1982), density functional methods
(Parr and Yang 1989) or tight-binding approaches (Gorringe et al 1997)
are used. Also interest in surface related phenomena is growing and dynamical
processes on grains are being handled by more and more detailed approaches. Here,
density functional approaches are a good compromise because they still allow
for a realistic treatment of the electronic states.
Because for large systems not all degrees of freedom can be treated
explicitly, methods of dissipative quantum dynamics such as the path
integral method (Weiss 1999) or the density matrix method (Blum 1996) have to
be used. Presently, this field is developing towards dynamical simulations of
relevant subspaces, e.g. using efficient algorithms for propagating a
wavepacket in time by an integration of the time-dependent Schrödinger
equation. It has been demonstrated that the use of non-Hermitian Hamiltonians
in these schemes allows for an approximate modeling of dissipative
surroundings. Alternatively, density matrix approaches can be used, with the
advantage that coupling to a dissipative surrounding can be included more
easily. For any of these dynamic simulation schemes, it remains crucial that a
relevant low-dimensional subspace has to be identified beforehand, containing
e.g. a single reaction coordinate and a dissipative coupling to further degrees
of freedom treated as a heat bath in thermal equilibrium.
All above mentioned theoretical methods are of interest for our FGLA
but cannot be persuaded within the two theoretical projects; however, we will
concentrate on large systems which are of special interest for understanding
the role of interstellar dust. The more "simple" gas phase reaction
dynamics (e.g. state specific cross sections measured in TP 4) will be treated
with the help of external cooperations. TP1 shall contribute to our
understanding of the elementary steps which control the dynamical evolution
of complex interstellar matter. The method applied will be a
density-functional based tight-binding method, combining a precision close to
more sophisticated ab initio methods with an outstanding numerical
performance. Interaction potentials, geometries, formation energies and
vibrational properties will be calculated for a variety of molecular
structures. Besides these gas phase processes it is intended to treat the
interaction of molecules with cold surfaces.
In TP2, the interaction between a nanoparticle and a molecule on its
surface will be investigated within the reduced density matrix formalism. Dissipative
quantum dynamics will be used to calculate the competition between electron
transfer and fluorescence. The work is closely related to the single
molecule spectroscopy project TP6. Both projects shall investigate the
influence of the nanoparticle size and of the type of adsorbed aromatic
hydrocarbons on the electron transfer rate.
2.3 Modeling
Large-scale modeling of the interstellar and circumstellar medium is
extremely important if one wants to understand the different phase transitions
including the formation of stars in molecular cloud cores and of planets in
circumstellar disks which are driven by gravity, radiation forces, gas
pressure, and magnetic fields. Such calculations must be based on a detailed
knowledge of microscopic processes such as ionization, chemical reactions, and
grain evolution. The numerical modeling is critical for a better understanding
of the influence of the elementary reaction steps which control the
dynamical evolution and to filter out the dominant key processes.
It would go far beyond the possibilities of the FGLA to
contribute to this field in a systematic way. Therefore, we selected in TP3 the
numerical modeling of chemical reactions in protoplanetary disks as the numerical
modeling project of the research group. There are at least five different
reasons for this choice: (1) The chemistry in protoplanetary disks is largely
unexplored (see description in TP3) in strong contrast to a lot of work which
has been done for molecular clouds and outflows from AGB stars (e.g. Herbst
1995, Millar 1998, van Dishoeck 1998, and references therein); (2) The complex
interplay between the dynamics and the chemistry in the disks makes these
systems extremely interesting for a long-term program; (3) The physical
conditions in the disks require knowledge of surface reactions, neutral-neutral
reactions, molecule-ion chemistry, and ternary reactions at different
temperature regimes; (4) Millimeter and submillimeter interferometry with new
projects such as ALMA on the horizon will deliver high-quality astronomical
data which can test the predictions of the models; (5) We can build on our
experience of the large-scale modeling of disks (Klahr et al. 1999;
Steinacker and Henning 1999; Bell et al. 1998) and the treatment of
chemical processes in disks (see TP3). A close collaboration between this
astrophysical project and the theoretical projects (TP1 and TP2) as well as the
experimental projects (TP4 and TP5) will foster the integration of the
different subgroups. The numerical modeling project will provide information on
what is important to measure and what is the required accuracy of the
laboratory data.
Here, we should note that we will establish connections to the theory
group of the AIU in Jena, which developed the necessary continuum and line
transfer codes for an interpretation of astronomical data (e.g.
Ossenkopf 1997, Wolf et al. 1999). We will also continue with our
established contacts to other theory groups which perform large-scale modeling
of protoplanetary accretion disks and molecular cloud cores (E. Herbst, T.
Millar, E. van Dishoeck, W. Kley, H.W. Yorke). These collaborations will be
essential for establishing a strong position of the FGLA in the
astronomical community.
2.4 Gas phase collisions
Gas phase studies appear to be in a mature state compared with other
areas of astrophysical chemistry, and many laboratory results have been
included in model calculations (Aikawa & Herbst 1999, Terzieva & Herbst
1998). Nonetheless, there are still many uncertainties and unsolved problems,
some of which are related to the fact that the temperature range of
interstellar interest became only accessible to experiments in recent years by
using a new flow-reactor technique (for a review see: Smith 1994, and
references therein) or cooled ion traps (Gerlich 1992, Gerlich 1993). Since
rate coefficients often depend dramatically on temperature there is a great
need for more measurements carried out under astrophysically relevant
conditions
Some aspects of molecular dynamics at very low energies are very
specific to the interstellar environment. On the one hand, it is evident that
small barriers or endothermicities in bimolecular reactions play an important
role at low temperature collisions. On the other hand, excitation of internal
degrees of freedom in a reactant which is otherwise cold has a reverse effect
when released in such a collision. Usually these effects are simply neglected.
However, there are known cases where these effects play a significant role in
the formation or alteration of interstellar molecules. This topic will be one
major aspect of TP4.
One very important process in molecule formation at the very low
densities and temperatures of molecular clouds is the radiative association.
In this process, two species collide and stick together aided by emission of a
photon. The success of such an encounter is determined by the competition
between (1) the lifetime of the collision complex with respect to
redissociation as compared to (2) its lifetime with respect to the emission of
a photon. For small molecules, this competition is strongly in favor of the
redissociation (generally > 106 / 1). But although
only every millionth collision might lead to success, radiative association
between ions and neutrals became measurable due to the very high sensitivity of
ion trapping techniques combined with single ion detection. However, only very
few systems have been studied so far (Gerlich 1993, Gerlich 1994, Sorgenfrei
and Gerlich 1993). Future studies also in the FGLA will be concerned
with associative reactions with CO, the second most abundant molecule in the
ISM, and the question at which size of a molecular ion a target molecule
"sticks" to it, i.e. association at every collision occurs.
Isotope exchange reactions are an example for a reaction which is
slightly endothermic in one direction due to the difference in zero point
energy of the corresponding reactants and products. This phenomenon leads to
isotopic fractionation in the astrophysical environment. The
prediction of reliable barrier heights for such systems is important because
differences to observational abundances can be used as tracers to the temporal
and spatial evolution of the corresponding astrophysical objects (Walmsley et
al. 1999). Several laboratory experiments along these lines have been made
in one participating group (e.g. Gerlich 1994), a typical example is the system
C2H2+ + HD and other isotopic variants. Here,
isotopic fractionation leads to an enhancement of C2HD+.
Further experiments for other key reactions, e.g. H3+ + H2
and isotopic variants, will be performed in TP4.
In the cold environment of molecular clouds, it can not be assumed that
all internal degrees of freedom (spin-orbit levels, rotation, vibration) of
molecules are thermally equilibrated. First observations of transitions of
spin-orbit levels occurred in the 1980s and consisted of forbidden infrared
transitions of neutral atomic oxygen, Oi, and singly ionized carbon, Cii. The
basic processes of collisional and radiative excitation (and relaxation) of
fine structure states is still very poorly understood, i.e. rarely treated
experimentally or theoretically. First results are dealing with the fine
structure relaxation of Al by Ar between 30 and 300 K (Le Picard et al.
1998). Relaxation of ArII has been studied in one participating group using a
special trap arrangement (Haufler 1997, Schlemmer 1998) which allows the
overlap of the trap volume with a molecular beam. Radiative lifetimes in the
tens of seconds regime have been determined with this technique. The work in
TP4 will deal with the important question how these radiating levels are
populated via collisions and also try to determine lifetimes for spin orbit excited
C+.
Another system relevant to the ISM is the formation of ammonia
which is started by the initial reaction N+ + H2 →
NH+ + H, followed by a chain of hydrogen abstraction reactions. The
endothermicity is of the same order of magnitude as the smallest rotational
excitation and the fine structure energy of the three spin-orbit states of N+ (3P0,1,2).
In TP4, new technical approaches will be developed to find out which energy
form increases the formation probability of NH+ and thus also of
ammonia.
Collisions with atomic species, especially H atoms dominate in the ISM
environment. However, due to experimental difficulties only very few laboratory
studies deal with this important topic of gas phase chemistry (McEwan et al.
1999). TP5 and TP7 will specialize in collisions of ions with atomic targets
(H, C, N, O) at low temperatures. In general, rate coefficients for neutral-neutral
reactions are much smaller than for the ion-molecule case. However, more
recent investigations show that especially in low temperature collisions with radicals
rate coefficients can be of the same order of magnitude as compared to ionic
systems (Chasting et al. 1998). The latter class of reactions will not
be studied within the FGLA. This and several other very important topics
in gas phase chemistry can be included to the FGLA thanks to the well
established cooperation within the: TMR network, connections to the TSR in
Heidelberg) and other facilities such as the free electron laser in Amsterdam
or the infrared detector.
2.5 Carbon structures
Carbon belongs to the most abundant non-volatile elements in the
universe (see e.g. the review by Henning and Salama 1998). Due to its unique
ability to form three types of hybridized orbitals, carbon can form a large
number of molecules with a variety of isomers. It is not surprising, that many
of them (unsaturated chain-like hydrocarbons, polyynes, PAHs, etc.) have
already been identified in the interstellar medium (e.g. in Thaddeus 1999). Also
bare carbon molecules have been discovered in the atmosphere of carbon
rich stars (Hinkle et al. 1988, Bernath et al. 1989). Small carbon
particles are the most probable carriers of the interstellar extinction
hump at 217 nm (Fitzpatrick and Massa 1986) and also of the 3.4 µm
absorption band of the diffuse interstellar medium (Pendleton and Chiar 1997).
The relation between the carbonaceous species in space, their history
and evolution is, however, not understood yet. This is mainly due to a lack in
basic physical and chemical data on the formation dynamics as well as on the
spectroscopic properties of the carbon structures. Although complex chemical
networks have been developed to describe the formation of carbonaceous
molecules and ions in different regions of space, these depend critically on
rate coefficients which are poorly known for the relevant physical conditions. For
example, the laboratory data suggesting the anion (Tulej et
al. 1998) as a carrier of several diffuse interstellar bands (DIBs) opened
the questions about the formation mechanisms of such ionic species in the
interstellar medium. Ruffle and coworkers (1999) calculated the abundance of
using an extended reaction network, the results depend, however, on rate
coefficients of several key reactions (radiative electron attachment,
photodetachment, and reactions with hydrogen and carbon atoms) that have not
been experimentally investigated till now. Compared to the rich spectroscopic
experimental work on carbon molecules (especially in the last decade), the information
about reaction dynamics of carbon species is quite limited.
In the FGLA, the carbon-containing molecules will be studied in
several projects. The FGLA intends to contribute to a detailed knowledge
of the reaction dynamics of carbon species that is substantial for explanation
of their growth and destruction in the interstellar medium. Reactions
between carbon species will be studied in TP7. The growth of bare carbon
structures will be investigated in a wide temperature range between 10 and
1000 K. The observation of isomerization of the reaction products in
reactions of type Cm+ + Cn is
the main aim of TP7.
Solid carbonaceous
particles show a wide range of structural properties depending sensitively on
the condensation process and determining in turn their behavior (see also
2.2.6). The properties of carbon nanostructures will be investigated within the
FGLA in several projects. In TP7, single isolated carbon
nanoparticles will be studied that will be directly grown in a trapping
apparatus. The charge state of the carbon nanostructures and its influence on
the optical properties and reactivity of the surface of the grains will be one
of the topics of TP5 and TP7. TP8 will focus on production of carbonaceous
nanoparticles by using diverse gas aggregation and CVD sources. For
characterization of the nanoparticles, electron microscopy and several
spectroscopic methods will be applied. An important point of all these
experiments is not only to characterize the produced nanostructures, but also
to provide insight into formation mechanisms of the carbon nanoparticles under
various conditions. The experiments aim to obtain information necessary for
understanding the condensation processes of carbon in the ISM.
Studies on
carbon-containing molecules will be carried out also by several other projects
of the FGLA. TP4 and TP5 will study interactions of small hydrocarbons
with H atoms. The analysis of isomerization (e.g. the linear and the
ring structure of C3H2+) in radiative
association is one of the topics of TP5. The experimental work on
carbon-containing molecules within FGLA will be supported by the
theoretical project TP1. Using the density-functional based tight-binding
method, TP1 will provide precise calculations of geometries, formation energies
and vibrational properties for the molecules experimentally investigated in
TP4, 5 and 7.
The experiments of the FGLA
on carbon structures will surely profit from external collaborations.
Especially the spectroscopic work in the groups in Heidelberg
(W. Krätschmer), Cologne (T. Giesen), and Basel (J.P. Maier) is
of substantial relevance to our projects. For the structural investigations
planned on carbonaceous grains, the same holds for the activities of the groups
in Saclay (C. Reynaud, R. Papoular) and Naples (V. Mennella, L. Colangeli).
Further, our studies will certainly benefit from the tight connections to
combustion (R. Schlögl, Berlin and K. Siegmann, Zürich) and aerosol
science (M. Schnaiter, Karlsruhe).
2.6 Grains and surfaces
Grains determine the
thermal, dynamical, and ionization structure of the cool phases of the ISM.
Molecular hydrogen, other molecules like CO2, and molecular ices
form on grain surfaces. Examples for the importance of grains in astrophysical
processes (see 2.1) are the coupling of the magnetic field to the dense
molecular cloud cores and the radiation pressure on grains, which may limit the
formation of massive stars. One of the main unsolved problems is the role of
grains in the formation of planetesimals during the evolution of protoplanetary
disks. The properties of small solid particles and their interaction with the
gas phase as well as with electromagnetic radiation will be the subject of a
number of projects both in Chemnitz and Jena. It is now generally assumed that
grains play a significant role in interstellar chemistry in dense clouds.
However, even for formation of H2, almost no experiments have
been conducted under astrophysical conditions (e.g. Pirronello et al.
1999 and references therein). Based on the recent development of nanoparticle
mass spectrometry (NPMS) in Chemnitz (Schlemmer et al. 1999), TP5 will
deal with the formation of molecular hydrogen on a single isolated
nanoparticle.
The investigation of
structural and optical properties of grains as well as of growth processes has
already been a strong research activity of the Jena group (Henning and Mutschke
1999, Schnaiter et al. 1998). This will be continued in the projects
TP8, 9 and 10 of the FGLA. New aspects will be added, based on the
concerted experience of the FGLA. Also the collaboration with
F. Huisken (MPI für Strömungsforschung, Göttingen) including development
of the production methods of carbonaceous (Schnaiter et al. 1999)
and silicate nanoparticles will be of big importance.
A new and important research
goal pursued in TP7 and TP8 is to understand the formation processes of
particles from the gas phase. These condensation processes determine the
solid-state structure of the condensed particles (especially for carbonaceous
grains, see also 2.2.5) which in turn affect their optical properties (see
below). TP7 will approach the problem by studying basic growth reactions,
whereas TP8 will look at condensed structures and condensation conditions from
a "macroscopic" point of view. In our view, important properties of
grains which might effect their role in the ISM have been overlooked to date.
In TP5 and TP7, we will look for the influence of the particles charge and
quantum size effects on the optical properties and on the reactivity of a
particle. Surface reactions will be quite extensively studied in TP5. The main
goal of this projects is to experimentally analyze the interaction of an
isolated nanoparticle with hydrogen atoms and to study their recombination on
nanoparticle surfaces.
On the one hand the optical
properties of small grains and grain agglomerates are a diagnostic instrument
for the determination of the grain structure and on the other hand the
basic input to astrophysical modeling. Although optical measurements will be
used as diagnostic tools in all projects dealing with grains, the projects TP6,
TP7, TP9, and TP10 are directly dedicated to luminescence, absorption, and
scattering properties of solid particles. An important point is that all these
projects will not only perform measurements of optical data but will correlate
them to structural properties by using either optical measurement techniques of
high spatial resolution or accompanying (electron)microscopy. This will result
in new insights into the microphysical structures determining small-grain
optical behavior which is urgently needed for the interpretation of new
observational data (see 2.1.2).
2.7 Optical properties,
spectroscopy, light scattering
Observed spectral lines are
due to transitions between atomic and molecular states and usually well
understood. Broad features can be due to complex molecules in the gas phase,
clusters, or also molecules frozen out on solid-state material. Many of the observed
spectral features are still not identified. One example is the mystery of
the DIBs. Many, most probably molecular carriers are responsible for the
plenty of these absorption lines (see 2.1). To date, the only molecule that
shows compelling matches with the observed DIBs is the anion
studied in the group of J. Maier (Tulej et al. 1998). The carriers
of the unidentified infrared bands (UIR) are also still unknown.
However, the emission bands are attributed to polycyclic aromatic hydrocarbons
(PAHs). Also a number of emission bands of circumstellar dust discovered in the
last years have not been completely identified - the "crystalline silicate
bands" (Molster et al. 1999), the 11 µm (Speck et al.
1997), 13 µm (Sloan et al. 1996), 21 µm (Kwok et al.
1995), and 30 µm features (Szczerba et al. 1999) of AGB and
post-AGB stars.
The discrete absorption of
radiation by gas phase molecules - in ranges from microwave to UV is well
studied and close interactions with spectroscopic groups (R.J. Saykally,
J.P. Maier, T. Giesen) will contribute to the know-how of the FGLA.
The projects of the FGLA will also profit from the experiments on laser
induced reactions (SPP Sternentstehung). However, there are many
photon/gas-phase interactions, which are less well understood. Completely
unknown, for example, is the radiative association spectrum of ion-molecule
complexes. As molecules increase in size, they may become stable against
photodestruction by VUV radiation due to efficient internal relaxation
processes. In TP6, the spectroscopic properties of polycyclic aromatic
hydrocarbons (PAHs) adsorbed on silicon nanoparticles will be studied
using techniques of single-molecule confocal microscopy as well as
magnetic resonance. The photostability of PAHs will be investigated and
interactions between nanoparticles and PAHs will be described in detail.
The absorption/emission and
scattering cross sections of the dust are crucial for the thermal and dynamical
structure of star-forming regions, protoplanetary accretion disks, and
circumstellar envelopes around evolved stars (Dorschner and Henning 1995).
Therefore, related studies should comprise not only the efforts to identify the
species causing observed features, but should rather perform systematic
studies on the optical properties of small particles in dependence on their
structure. The interaction of small particles with electromagnetic radiation is
of interest in a wide wavelength region (from the VUV to millimeter
wavelengths) and at material temperatures ranging from 10 K to the
sublimation point. Questions such as the transition from dielectric to metallic
behavior by increasing the percolation strength, the presence of hot spots, the
question of quantum heating, and the luminescence of small particles deserve
special attention (see, e.g., the contributions in Greenberg and Li 1997 and
Ehrenfreund et al. 1999).
An important point is also
the development of spectroscopic techniques allowing to study the laboratory
analogs under conditions similar to those expected for their cosmic pendants.
There is nearly a complete lack of experiments on isolated small particles. The
matrix isolation technique here will provide new results on the optical
properties of laboratory dust analogues (Schnaiter et al. 1996). In TP8,
these will be related to the inner solid-state structure of gas-phase
condensed nanoparticles, whereas in TP9 morphological influences on the
infrared spectrum of submicron particles are studied. In the latter project,
the introduction of new techniques like levitation in aerodynamic flows is one
of the major goals. Several projects of the FGLA will apply the
established trapping technique to isolate small particles and to
investigate their optical properties. In TP7, carbon nanoparticles will be
grown directly in the trapping apparatus and their blackbody radiation at high
temperatures will be studied. The influence of the charge state of the
nanoparticles and, in cooperation with TP5, the influence of hydrogenation of
the particle surfaces are of interest.
In a variety of cosmic
environments dust particles are expected to be very complex in structure. Their
morphology might be described and understood by the concepts of fractal
physics. However, they differ from spheres and, therefore, rather poor
predictions for their optical properties can be given nowadays. Especially, the
angle dependent light scattering depends sensitively on the shape and structure
of the particles. The understanding of the scattering properties of complex
grain structures is essential for the physics of these environments, e.g. by
influencing radiation transport or for interpretation of observations, e.g. of
dichroic extinction, which might trace magnetic fields and even particle-gas
flows. Although scattering theory has considerably improved our understanding
of aggregate optical properties (e.g. Henning and Stognienko 1996), experiments
are restricted so far to analogue experiments in the microwave range (Gustafson
1995, Greenberg et al. 1961) or to very small aggregates (Bottiger et
al. 1980). New possibilities will be opened by particle growth experiments in
TP10. This project aims to study light scattering on individual aggregates
which are produced in a natural agglomeration process (Wurm and Blum 1998).
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