Creating PAH spectra

The (theoretical) database XML-files provide fundamental vibrational transitions at 0-Kelvin. To compare these with observations, they need to be transformed into a spectral density, i.e., spectra. When not dealing with absorption at 0-Kelvin, an emission model is required as well.

Emission models

The AmesPAHdbIDLSuite offers three PAH emission models. With increasing complexity they are the 'FixedTemperature', 'CalculatedTemperature', and 'Cascade' model. The first simply multiplies a blackbody at fixed given temperature with the integrated cross-section of each vibrational transition. The second first calculates the maximum attained temperature from the provided input and subsequently multiplies a blackbody at that fixed temperature with the integrated cross-section of each vibrational transition. The third averages the total emission over the entire cooling cascade (time).

Emission models are handled by the 'AmesPAHdbIDLSuite_Transitions'-object. The 'FixedTemperature'-model simply takes a temperature, in Kelvin, and, in their simplest form, both the 'CalculatedTemperature' and 'Cascade' models take an energy, in erg. The output of both 'Temperature' models and the 'Cascade' model are in erg/s and erg, respectively.

transitions->FixedTemperature,600D ; Kelvin

transitions->CalculatedTemperature,6D*1.603D-12; 6 eV

transitions->Cascade,6D*1.603D-12; 6 eV

Both the 'CalculatedTemperature' and 'Cascade'-methods accept the 'Approximate', 'Star', 'StellarModel', and 'ISRF'-keywords. With the 'Approximate'-keyword specified, calculations are performed using the PAH emission model from Bakes et al. (2001a, b). When the 'Star'-keyword is set, a stellar blackbody at the provided temperature is used to calculate the average energy absorbed by each PAH utilizing the PAH absorption cross-sections from Draine & Li (2007). In case the 'StellarModel'-keyword is provided as well, the input is considered to be a full-blown, for example, Kurucz stellar atmosphere model. The 'AmesPAHdbIDLSuite_CREATE_KURUCZ_STELLARMODEL_S' helper routine is provided to assist with molding the model data into the proper input format. Lastly, with the 'ISRF'-keyword set, the interstellar radiation field from Mathis et al. (1983) is used to calculate the average energy absorbed by each PAH.

transitions->CalculatedTemperature,17D3,/Star ; Kelvin

transitions->Cascade,/Approximate,/ISRF

FTAB_EXT,'ckp00_17000.fits',[1,10],angstroms,flam,EXT=1

transitions->Cascade, $
              AmesPAHdbIDLSuite_CREATE_KURUCZ_STELLARMODEL_S(angstroms, $
                                                             flam), $
              /Star, $
              /StellarModel

The 'Cascade'-method also accepts the 'Convolve'-keyword. When set and combined with either the 'Star', optionally with the 'StellarModel'-keyword, or 'ISRF'-keyword, will instead of calculating the average absorbed photon energy for each PAH, convolve the PAH emission with the entire radiation field.

transitions->Cascade,17D3,/Star,/Convolve ; Kelvin

NB This is computationally expensive.

Given the computational expense of the 'Cascade' model, the 'Cascade'-method transparently caches its results for faster subsequent access. However, this behavior can be disabled by setting the 'Cache'-keyword to '0'.

transitions->Cascade,6D*1.603D-12,Cache=0 ; 6 eV

The 'AmesPAHdbIDLSuite_Transitions'-object's 'Shift'-method can be used to redshift the fundamental transitions to simulate anharmonic effects.

transitions->Shift,-15D ; /cm

NB Red-shifting the fundamental vibrational transitions should be done after applying one of the three emission models described above.

Line profiles

Line profiles are handled by the 'AmesPAHdbIDLSuite_Transitions'-object and it provides three profiles; Lorentzian, Gaussian and Drude. Convolution with the keyword chosen line profile is achieved through the 'AmesPAHdbIDLSuite_Transitions'-object's 'Convolve'-method, which will return the convolved spectrum in the form of an 'AmesPAHdbIDLSuite_Spectrum'-object. The output will be that of the applied model with /cm^-1 added.

spectrum = transitions->Convolve(/Drude)

Optionally, the 'Convolve'-method accepts the 'FWHM', 'Grid', 'NPoints', and 'XRange'-keywords, which control the full-width-at-half-maximum of the selected line profile (in cm^-1), convolution onto a specified grid, the number of resolution elements in the generated spectrum, and the frequency range (in cm^-1) of the spectrum.

spectrum = transitions->Convolve(/Drude, FWHM=20D, Grid=myGrid)

The 'AmesPAHdbIDLSuite_Spectrum'-object exposes the convolved spectra and provides the 'Plot', and 'Write'-methods. The 'Plot'-method will display the spectrum of each PAH species in a different color. The 'Write'-method will write all spectra to a single text (.txt) file. Optionally, a prefix can be given that will be prepended to the filename.

spectrum->Plot

spectrum->Write,'myPrefix''

Optionally, the 'Wavelength', 'Stick', 'Oplot', 'Legend', and 'Color'-keywords can be given to the 'Plot'-method to control abscissa, stick representation, overplotting, legend and color, respectively. Through IDL's keyword inheritance mechanism additional keywords accepted by IDL's 'PLOT'-procedure can be passed.

spectrum->Plot,/Wavelength,XRANGE=[2.5,15],/XSTYLE