Atomic Line List: Instructions

When attempting to identify an observed line usually many possible candidates can be found in the line list. In order to facilitate narrowing down the number of possible identifications a selection tool is presented which allows imposing, apart from the wavelength, several additional criteria.

All fields in the request form have default values and are therefore optional

Each field will be discussed separately below. In the examples it is assumed that Angstrom has been chosen as the wavelength unit.

Wavelength range:
This field allows you to supply a wavelength range. This can be done in two ways: you can supply a lower and upper limit for the region or, if you are only trying to identify a single line, you can supply the central wavelength and the 1 sigma error (68 % confidence value) for that line. If the first number is smaller than the second number, this implies that the first option has been chosen, and otherwise the second option.

Lower/Upper Limit: When this option is used, all lines with wavelengths in the range
Llow <= Lc < Lhigh
will be included as possible identifications.

Center/Sigma: When this option is used, all lines for which the difference between the observed central wavelength and the predicted wavelength in the table is smaller than the combined uncertainty in both wavelengths, will satisfy the condition.
The computed wavelengths (Lc) in the line list all have an estimate for the 1 sigma uncertainty (DLc) assigned to them. When the Center/Sigma option is used these numbers are compared to the user supplied wavelength (Lo) and its 1 sigma uncertainty (DLo) in the following way:
|Lc-Lo| <= (DLc2 + DLo2)½.
This can be useful when you are trying to identify high ionization lines. The wavelengths for these lines can have low accuracy in the line list and hence can be "far away" from the observed wavelength. This option provides a simple way of including these lines in the query results. But also more generally, when an accurate measurement of the observed wavelength is available, this option can be useful.

Wavelength accuracy: All wavelengths in the line list have relative accuracies of 5 % or better. The default is to list all lines, irrespective of their accuracy. When a relative accuracy in percent is entered in this box, only those lines with accuracies better than or equal to the prescribed value are included in the search. Values larger than 5 % are meaningless and will be ignored.

Wavelength unit:
This option allows you to make a custom choice for the unit in which the wavelengths will be given (and printed in the output). The following choices are supported: Ångstrom (default), nanometer, micrometer, wavenumbers in cm-1, gigaherz and teraherz.

Wavelength type:
Vacuum:Give wavelengths in vacuum.
Air: Give wavelengths in air. They are calculated from the vacuum wavelengths using the five-parameter formula for the refractive index of air given in: Peck E.R., Reeder K., 1972, J. Opt. Soc. Am., 62, 958. This expression is valid for wavelengths between 1850 Å and 17000 Å. It assumes that the air has a temperature of 15 degrees Celsius and contains 0.033% of CO2.
Note 1: This option is ignored when the wavelength range starts below 2000 Å.
Note 2: The conversion formula is extrapolated for wavelengths longward of 17000 Å, but its validity is cannot be guaranteed in this case.

Default all ionization stages of all elements will be searched to find a possible identification. This field can be used to restrict the search to a range of elements and/or ionization stages. The elements should be entered by their usual symbolic names (e.g. Fe) and the ionization stages by the usual spectroscopic notation (e.g. I for neutral, II for singly ionized etc.).

Several lines of input can be combined, each containing entries like:

Fe I include lines of neutral Iron
Ni I-III include lines of ionization stages I thru III of Nickel
Na include lines of all ionization stages of Sodium
Mg-Sinclude lines of all elements Magnesium thru Sulphur
Sc-V I-III include lines of ionization stages I thru III of all elements Scandium thru Vanadium
NB - This entry is not case sensitive. At least one space should be typed between the element name and the ionization stage.

Minimum abundance, Depletion factor:
With this command it is possible to impose a lower limit on the abundances of elements to be considered for possible identifications. Default is to consider arbitrary low abundances.

The elements are assumed to have standard cosmic abundances. Most of the values were taken from:

Grevesse N., 1984, Phys. Scr. T8, 49
For nebular conditions this is not a realistic assumption since most metals will be depleted on grains. To simulate this it is possible to supply a depletion factor df. This factor will be used to calculate the actual abundance A from the cosmic abundance Ac using the formula
A(elm) = Ac(elm) - df*sd(elm).
where sd is the standard depletion for each element taken from:
Cowie L.L., Songaila A., 1986, Ann. Rev. Astron. Astrophys. 24, 499
If the transition type Nebular is chosen (see section Transitions) the default value for the depletion factor is 1.0 and 0.0 otherwise.

Lower/Upper level energy range:
Default is to consider all values for the lower/upper level energy to find a possible identification. To restrict the search a range of energies can be supplied as follows:
10000 selects levels between 0 cm-1 and 10000 cm-1.
10000-60000 selects levels between 10000 cm-1 and 60000 cm-1.
ground selects only levels belonging to the ground term (not case sensitive).

Energy unit:
This option allows you to make a custom choice for the unit in which the energy levels will be given (and printed in the output). The following choices are supported: cm-1 (default), eV and Rydberg.

Maximum for principal quantum number n:
Default is to consider all possible values for the principal quantum number n to find possible identifications. However, transitions involving electrons with a very high quantum number n tend to be weaker and can therefore be less likely identifications. These transitions can be suppressed using this option.

Select a specific multiplet:
This option can be used to find all lines in a specific multiplet within a certain wavelength range. The lower and upper level term should be entered here exactly as they appear in the output of the query. The spectrum to which this multiplet belongs should of course also be supplied in the Element/spectrum field. A typical example would look something like:
Wavelength range: 2000 - 10000
Element/spectrum: Fe I
Select multiplet: b3F4-z3Go
Wavelength type: Air
Which would give the following result:
6475.650   Fe I  E1  b3F4-z3Go   4-3   20641.11 - 36079.37
6575.043   Fe I  E1  b3F4-z3Go   3-3   20874.48 - 36079.37
6609.134   Fe I  E1  b3F4-z3Go   4-4   20641.11 - 35767.56
6646.956   Fe I  E1  b3F4-z3Go   2-3   21038.99 - 36079.37
6712.699   Fe I  E1  b3F4-z3Go   3-4   20874.48 - 35767.56
6783.288   Fe I  E1  b3F4-z3Go   4-5   20641.11 - 35379.21

Default is to consider all types of transitions to find a possible identification. To alter this you first have to choose one of the following three buttons:
All:The default, consider all transition types.
Nebular: Consider only allowed transitions of Hydrogen or Helium and only magnetic dipole or electric quadrupole transitions of other elements. A side effect of this choice is that the limits on the lower and upper levels will only be applied to the forbidden transitions. This allows the selection of ground state forbidden transitions only (by giving an upper level limit of e.g. 25000 cm-1) while still getting all the information on the Hydrogen and Helium lines. This is very useful for identifying lines in spectra of PN, H II regions etc.
Select: Make a custom choice from the following four buttons:
E1: allowed transitions.
IC: intercombination or semi-forbidden transitions.
M1: magnetic dipole forbidden transitions.
E2: electric quadrupole forbidden transitions.

Fine structure of transitions in hydrogen-like spectra:
In hydrogen-like ions, levels with the same principal quantum number have very small energy differences. As a result of this, all transitions between levels with the same set of principal quantum numbers have nearly identical wavelengths. Under normal astrophysical conditions all these fine structure components can not be resolved and will blend into one line. The wavelength of this blend is the weighted average of all the components and is calculated assuming LTE level populations. For completeness we have included the fine structure components of all transitions between the lowest levels of each hydrogen-like ion in the line list. The default is to suppress this information. If the option Show is selected, the fine structure components will be included in the output. Transitions between two levels with the same principal quantum number are not subject to this rule and will always be included in the output.

Transitions from auto-ionizing levels:
The default is to suppress transitions originating from auto-ionizing levels in the output. This option allows you to include these transitions. In this context, all levels with energies higher than the ionization potential going to the ground state of the next ion are considered auto-ionizing levels.

Output format:
This option allows you to check the various items you want included in your output. The wavelength or wavenumber of the transition is always included and need not be checked. The header will indicate if the wavelengths are valid for air or vacuum, as well as the unit for the wavelength. The following optional items can be checked:
Wavelength accuracy: Includes a rough estimate for the 1-sigma uncertainty (68% confidence interval) of the wavelength.
Spectrum: Gives Fe I for allowed transitions, Fe I] for intercombination transitions, [Fe I] for forbidden transitions.
Transition Type: Gives E1 for allowed transitions etc. (see the section on transitions).
Configuration: Gives the electronic configuration of the lower resp. upper level. Entries like 15* denote the principal quantum number n for hydrogen-like ions. For these the wavelength of the transition and the level energies have been averaged over all allowed values of l and j assuming LTE level populations.
Term: Gives the spectroscopic term for the lower resp. upper level. A lowercase "o" at the end of the term indicates odd parity in plain mode.
Angular momentum: Gives the angular momentum (when "as J" is checked) or statistical weight (when "as g" is checked) for the lower resp. upper level. An asterisk in the output indicates more than one allowed value of J for that specific term and transition. When "combine with term" is checked, the angular momentum will be given together with the term (this option is ignored in plain mode).
Transition probability: Gives the transition probability for that particular line (currently only available for a limited set of lines).
Transition probability flags: Gives information about the source and the reliability of the transition probability. The field consists of a number (possibly zero) of characters followed by one digit. Lower case characters pertain to the lower level and upper case characters to the upper level. The characters have the following meaning:
U -- The level identification was marked uncertain in the Opacity Project data.
R -- The order of the levels in the Opacity Project data is reversed compared to laboratory measurements.
C -- The level identification given in the Opacity Project data has been altered.
The single digit identifies the source of the transition probability. The references are listed in the documentation page.
Level energies: Gives the energy of the lower resp. upper level in the chosen wavelength unit. This unit will be indicated in the header
Literature references: The reference for the level information. If two numbers are present, the first is for the lower level and the second for the upper level. The references are listed in the documentation page.

Output mode:
PlainThe resulting output will be printed in plain ascii (default).
LaTeXThe resulting output will be printed in a form that can be included in a LaTeX file.

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