manual

About bacán

bacán calculates the spectrum of a photoionised nebula, assuming that it is optically thin, that recombination case B applies, and that the temperature and density of a given ion are homogenous.

Calculating a spectrum

When you load the web page, you should find a spectrum already calculated and plotted. This is a pure hydrogen spectrum at 10000K and 1000cm^-3. Abundances, temperatures and densities can be changed in the configuration settings below the plot. You can load a few other example calculations from the menu. The calculations are made as follows:

H⁺ and He²⁺

Recombination lines of hydrogenic ions are calculated using a subset of data from Storey and Hummer 1995. Tabulated logarithms of Case B emissivities are bilinearly interpolated in temperature and density to find the emission coefficients for the relevant values. Lines are calculated for transitions from levels up to n=25 to levels n=2-6. The valid density range is 10²-10⁸cm^-3, and the valid temperature range is 500-30,000K.

He⁺

The brighter singly-ionised helium lines (quantum number of upper level=3,4,5) are calculated using data from Porter et al. 2012, 2013. The data cover temperatures of 5,000-25,000K and densities of 10¹-10¹⁴cm^-3. Fainter helium lines are calculated by interpolating the ratios tabulated in Brocklehurst 1972. These data cover temperatures from 5,000-20,000K and 10⁴-10⁷cm^-3 for singlets, extending down to 2,500K and 10²cm^-3 for triplet lines.

Heavy element recombination lines

(to be documented)

Collisionally excited lines

For collisionally excited lines, level populations are solved using a javascript implementation of equib (Howarth 1983). Atomic data is taken from the CHIANTI database version 7, which covers temperatures from log(T_e)=2-6.4, and any density.

Continua

Continuous emission is calculated using coefficients tabulated in Ercolano and Storey 2006. These give the bound-free continua of H⁺, He⁺ and He²⁺, at temperatures between log(T_e)=2-5.

Data limits

Whenever the physical conditions specified are outside the range of the atomic data, the calculations are carried out at the relevant limit. This means that the range in which results should be maximally accurate is 5000K≤T_e≤20000K, and 10⁴≤n_e≤10⁷. At densities between 10² and 10⁴, only weak helium singlet lines are affected by a data limit.

Convolution

Once line fluxes are calculated, the synthesised spectrum is generated by convolving line and continuum fluxes with a Gaussian profile. If an observed spectrum is uploaded, the synthesised spectrum is sampled at the same wavelength points as the observed spectrum. In the absence of an observed spectrum, 5000 points uniformly distributed across the plot view are sampled.

The plot

• Zoom in and out of the plot by pressing "+" and "-".
• Highlight a selection by clicking and dragging, and that region will be shown.
• Right-click to clear that selection and return to the previous view.
• Press "a" to clear all selections and return to the default view.
• Ctrl-click and drag to pan

Loading a spectrum

If you want to compare a calculated spectrum to observations, you can upload a spectrum. Plain text files consisting of two columns of numbers are understood; anything else will be ignored. The columns should be wavelength in Angstroms and flux per unit wavelength. You can then adjust parameters to match the calculated spectrum to the observations. Setting the appropriate values of F(Hβ), c(Hβ) and radial velocity will scale, redden and shift the observed spectrum. Changing the spectral resolution will reconvolve the calculations to that resolution. Changing abundances, temperatures and densities will recalculate the predicted spectrum. Five parametrisations of extinction are available, three galactic and one each for the LMC and SMC.

Abundances can be entered logarithmically or linearly - whichever one you change, the other will be updated accordingly. You can also adjust abundances on the plot - when "show nearest line ID" is selected, scrolling the mousewheel will adjust the abundance of the ion which produces the nearest line to the cursor.

Examples

Example calculations are provided using published data for NGC 6543 (Wesson and Liu 2004) and NGC 6326 (Wesson et al. 2018), as well as the two simple cases of pure hydrogen, and hydrogen plus helium.

Name

bacán (also spelled bakán) is a Chilean word meaning "cool" or "awesome". It is pronounced with two short "a"s and the stress on the second syllable (International Phonetic Alphabet: /baˈkan/).