Optically thin sources

Figure 2 Spectra of optically thin and thick models with n a r and observed with various apertures. The aperture sizes are for a distance of 500 pc. Note that the optically thin source has a power law spectrum in the near IR and that the wavelength of peak emission is redshifted for larger apertures.

In this chapter we shall be concerned only with the shape of lines emitted by an optically thin source. The change in the observed intensity distribution which can occur as a result of self-absorption by ground-state or metastable atoms will be discussed in Chapter 10. 

Optically thin sources, t, (L) < 1. To investigate the intensity of radiation emitted by a uniformly excited column of gas of length L, we set the incident intensity,... 

Martin, P.J., Bendavid, A., Netterfield, R.P., Kinder, T.J., Jahan, F. and Smith, G. (1999), Plasma deposition of tribological and optical thin film materials with a filtered cathodic arc source , Surface and Coatings Technology, 112, 257-260. 

Recently, experiments to measure the isotopic fractionation associated with the CO photodissociation have been performed at the Advanced Light Source (ALS, Chakraborty et al. 2008). Photodissociation of CO by UV synchrotron photons (1016 photons per second) at four different wavelengths (A = 107.61, 105.17, 97.62, 94.12nm) leads to C and O atoms, which subsequently recombine to form CO2. In the experiments, optical depths of relevant C160 lines were several orders of magnitude thicker compared to the optical depth of C180 and C170, which were optically thin. 

Because of the low spectral background intensities the limits of detection from the side of the source will be low and for metals will be down to the pg/g range in a classical Grimm lamp [479]. Also elements such as B, P, S, C, As,... can be determined. The plasma is optically thin, but for resonance lines of the matrix elements,... 

As viewed from the top, the plasma has a circular, doughnut shape. The sample is injected as an aerosol through the centre of the doughnut. This characteristic of the source confines the sample to a narrow region and provides an optically thin emission source and a chemically inert atmosphere. Normally, samples are introduced as a solution into the plasma and argon is used as a carrier gas for the sample introduction. The much higher temperatures of the plasma compared to flame make ICP-AES more effective in detecting lower concentrations of refractory elements such as Ta, W and Zr, and rare earth elements. 

The physical-mathematical description of the heat transfer in optically thin aerogels leads to coupled nonlinear integrodifferential equations which cannot be treated analytically. In the heat transfer equation, (23.1), the heat source term O considers the net radiative heat flux into the infinitesimal volume element where this equation is defined ... 

This expression assumes that the source is optically thin, that is, every photon emitted is able to escape from the source without being re-absorbed by atoms in the lower level i. Self-absorption is usually only important if i is the ground state or is a metastable level. 

Abstract. We have obtained hi d c Lition spectra of the CO band-head emission from five young stars. We find a variety of line shapes, and we compare the profiles with those predicted by models of accretion disks and neutral winds. Standard accretion disks are able to reproduce the fluxes, profiles, and optical d ths of our sources, with accretion rates of 10 Mq yr. Winds successfully account for the fluxes and profiles provided the wind does not cool adiabatically, but remains hot at least out to where the density is too low to excite CO emission. However, our wind modd predicts optically-thin emission, which is inconsistent with low-resdhition data. We therefore prefer disks as the origin of the CO band-head emission in young stars. 

For AFGL 2343, we used the simple model discussed by Sopka et d. (1985), where the circumstellar shell is assumed to be optically thin and heated by the central star. In accordance with Bujarrabal et al. (1992), we assume that AFGL 2343 is a typical pop I star on the asymptotic giant branch (AGB), with 10 L luminosity at distance 1.5 kpc. We derive a mass loss rate in the shell of 1.5 x 10 M yr (from the extended emission), and an upper limit to the current mass loss rate inside the shell of M < 2 X 10 M yr (from t he stellar point source flux at 8.5 /on). 

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