The vibrational excitation of PAHs

We have to evaluate the temperature that describes the vibrational excitation of a PAH as a function of time (e.g., internal energy) during the IR emission process. From a statistical physics point of view, the excitation temperature is connected to the average energy in a mode. Normally, we would average the energy over all species to arrive at the temperature or equivalently average the energy of one particle over a long time period. For PAHs, we have to be careful in this averaging. For a...

The shock spectrum and shock diagnostics

We will first focus on radiative J-shocks. Gas cooling has been extensively discussed in Chapter 2. Because of the high electron velocities and cross sections for electronic excitation, the excitation of species behind a strong (ionizing) shock is generally dominated by inelastic electron collisions. For the excitation of neutral atomic fine-structure lines, proton collisions are also important. In neutral postshock gas, hydrogen atoms dominate the collisional excitation process (Sections 2.5...

Grainsurface chemistry

The composition of the accreting gas sets the stage for grain-surface chemistry. Here, we will focus on dense cloud cores that are shielded from ambient FUV photons except for H2 formation, grain-surface chemistry and ice-mantle formation are limited in the more diffuse phases of the ISM because of the effects of photodesorption (Section 10.6.2). While hydrogen is mainly in the form of H2, cosmic-ray ionization keeps a low level of atomic hydrogen around. Assuming each ionization eventually...

Stardust

Using a variety of extraction techniques, genuine individual stardust grains have been recovered from carbonaceous meteorites with an isotopic composition that betrays the nucleosynthetic heritage of their stellar birthsite. These isotopic anomalies often exceed orders of magnitude and, therefore, probably reflect on the isotopic composition of the material from which the grain condensed. For the smaller grains, these isotopic anomalies refer to an average over many grains. However,...

Arp arp o at

Figure 8.1 A schematic representation of the cooling curve, plotted as A kT. (In thermal equilibrium L 0, where L is the net cooling , A kT r P.) Above this curve, heating exceeds cooling and vice versa below. At constant pressure and constant heating rate (per H atom), the equilibrium points are given by the intersection of a horizontal line with this equilibrium curve. There is then a region in which more than one solution exists the multiphase zone indicated by dashed lines. Within this...

Other large molecules

The concepts developed for interstellar PAH molecules can also be applied directly to other large molecules such as C60 and other fullerenes as well as various types of carbon chains. While there is no evidence for these species in the IR spectra of the ISM, electronic transitions of small carbon chains (C2, C3) have been detected in absorption towards many diffuse clouds. The abundances of these species are, however, small. Current thinking is that the diffuse interstellar bands are caused by...

Molecular clouds

Molecular clouds differ in a number of important aspects from the atomic clouds discussed in Chapter 8. They tend to be denser and have a higher column density. As a result, the intensity of the dissociating FUV radiation field is lower and gas-phase chemistry in these clouds is primarily driven by cosmic-ray ionization. Accretion on grains is another process that becomes more important at these higher densities and ice mantles - absent in the diffuse ISM - become prevalent inside molecular...

Ihm i m m i i r

SIII OIII I SilI I NIII ArIII I NeIII I OI SIII OIII I SilI I NIII ArIII I NeIII I OI Figure 7.11 The extremely rich infrared spectrum of the HII region, K3-50, measured by the Infrared Space Observatory. This spectrum shows a multitude of components. The continuum emission is due to warm dust (see Chapter 5). Superimposed, we recognize the IR emission bands due to large polycyclic aromatic hydrocarbon (PAH) molecules (see Chapter 6). Absorption bands due to silicates and ices located in a...

JJ I I I I I

Time-dependent effects of H2 in photodissociation regions have been considered in 4 , 5 , and 6 . The effects on the emitted spectrum are discussed in 7 . The chemistry of dense photodissociation regions is discussed in great detail in 8 . Various aspects of chemistry in photodissociation regions are also discussed in 9 and 10 . The earliest observations on photodissociation regions were made using the Kuiper Airborne Observatory, 11 , 12 , and 13 more recent studies are 14 , 15 , 16 , and 17 ....

Physical processes 521 Absorption and scattering by small particles

The dust optical depth, t , is given by where Cext (A) is the extinction cross section, nd is the dust density, and L the pathlength. The relation between extinction in optical depth units, t (A), and in magnitudes, Aa , is given by The extinction cross section of a grain is often expressed in terms of the extinction efficiency, Qext, and the geometric cross section, ad, Qext (A) . (5.3) Extinction is the sum of absorption and scattering processes viz., Qext (A) Qabs(A) + Qsca (A). (5.4) The...

Spectroscopy

Table 2.1 summarizes typical properties of transitions. These are, of course, directly related to the binding energies of the species involved. Electronic binding energies for atoms increase from left to right in the periodic system from about 5 eV to some 20 eV. For hydrogen and helium the lowest electronic transitions are fairly high (10.2 and 21.3 eV, respectively) a substantial fraction of the ionization energy. Multi-electron systems have electronic orbitals that are low in energy compared...

Photophysics of PAHs

The photophysics of large molecules has been discussed in Section 2.1.1. A small, neutral PAH molecule in the singlet electronic ground state (S0) can absorb UV photons of fairly specific energies, corresponding to discrete electronic transitions, taking the molecule up to the S1, S2, or higher state (Fig. 6.2). This electronic excitation can be followed by a variety of de-excitation processes, including ionization and photodissociation. In terrestrial environments, collisional de-excitation is...

Components of the interstellar medium

The gas in the ISM is organized in a variety of phases. The physical properties of these phases are summarized in Table 1.1. The 21cm line of atomic hydrogen traces the neutral gas of the ISM. This neutral gas can also be observed in optical and UV absorption lines of various elements towards bright background stars. The neutral medium is organized in cold 100 K) diffuse HI clouds (cold neutral medium, CNM) and warm ( 8000K) intercloud gas (warm neutral medium, WNM). A standard HI cloud (often...

The expansion of HII regions

The ionization of the interstellar medium by stellar photons has been discussed in detail in Chapter 7. Here we will focus on the dynamical effects of the ionized gas on its surroundings. First, we will examine the general characteristics of ionization fronts (Section 12.2.1). We will then apply this to the formation and evolution of an HII region in a homogeneous medium. In particular, when a massive star is suddenly turned on in a homogeneous environment, an ionization front will expand so...

Photodissociation regions

Photodissociation regions (PDRs) are regions where FUV photons dominate the energy balance or chemistry of the gas. In this chapter, we will examine the physical characteristics of dense and luminous PDRs near bright O and B stars, starting with the ionization balance (Section 9.2) and the energy balance (Section 9.3) of the gas in PDRs. We follow that with a discussion of the dust temperature in PDRs. The chemistry of PDRs (Section 9.5) is very similar to that of diffuse clouds (cf. Section...

Interstellar extinction

The presence of dust in the interstellar medium was first recognized by its reddening effect on the light from distant stars. The apparent magnitude, m, of a star is given by m(A) M(A) + 5log d + Aa, (5.85) with M the absolute magnitude, d the distance, and Aa the extinction due to dust. Now, the extinction can be derived by comparing a reddened star with a nearby star with the same spectral type, resulting in a magnitude difference given by The color excess between two wavelengths, A1 and A2,...

List of symbols

Nomenclature in astronomy is often very confusing if not intentionally obfuscating. Every subfield arbitrarily introduces its own symbols and a book encompassing many different subfields has to dance a fine line between consistency with common usage in the literature and avoiding internal confusion. I have tried to avoid using the same symbols for more than one quantity as much as possible but did not quite succeed. This glossary provides a list of symbols that are used throughout the book,...

Sr f10Mo

And we find the somewhat counterintuitive result that, for a given ionizing star, the mass of ionized gas increases as the density decreases. Basically, there are fewer recombinations per unit volume at a lower density and hence the neutral fraction is lower. As a result, the same number of ionizing photons can be used to keep a larger number of atoms ionized and the mass of ionized gas increases. The total recombination rate integrated over the nebula is, of course, independent of density and...