Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.astro.spbu.ru/staff/afk/AtDatCentre/AtDatCat/Introduction.pdf Дата изменения: Fri Nov 19 16:06:59 2010 Дата индексирования: Tue Oct 2 04:14:54 2012 Кодировка:

Introduction This catalogue contains mainly the atomic data needed in the astrophysical investigations of low-density plasma. It is a revised and extended version of the "Catalogue of Atomic Data for the Rarefied Astrophysical Plasma" (Golovatyj et al. (1991)). Low-density plasma. As different forms of the low-density astrophysical plasma we can consider the intergalactic and the interstellar media, the matter of the gaseous nebulae (planetary and diffuse), the regions in the vicinity of active galaxies and quasars, the solar and stellar coronae and several other astronomical sub jects. The ion concentration in the plasma varies in broad limits from ab out 10-7 cm
-3

to 109 cm

-3

and kinetic temp erature from 5·103 K to

107 K. In these plasma ob jects there proceeds generation of emission sp ectral lines. The lowdensity astrophysical plasma is practically transparent in all sp ectral lines excluded the series of resonance lines which arise due to electron transitions to the ground state of most abundant ions of abundant elements. The b ound state p opulations and corresp onding line intensities of such targets are predominantly determined by the cascade processes which p opulate the states. The preceding absorption processes in the radiative transfer are photoionization and excitation in optically thick resonance sp ectral lines. In the low-density astrophysical plasma the ma jority of atoms and ions are placed in the ground state. In calculations of sp ectra generated by such plasma structures b esides probabilities of sp ontaneous transitions only the induced transition rates to the low excited states and to continua are needed. This circumstance reduces essentially the atomic data set indisp ensable in computations of the sp ectra of the low-density plasma targets compared with the set needed in calculations of sp ectra of essentially denser stellar atmospheres. We shall use the following notations: Ai and Bi are the i-fold ions of elements A and B, the

3


notations n(Ai ) we use for numb er density of corresp onding atoms or ions and the symb ols n

k

denote the numb er densities of ion in the atomic state k (the occupation numb er). The excited states we denote by prime added to the ion symb ol, say Ai , but the autoionization states are doubly primed (Ai ). As usual we shall use a standard notation XI for neutral atom, XI I for its singly ionized ion et al. This means, for example, that symb ols N3+ and NIV for triple ionized nitrogen are identical. The cross-sections and rates of physical processes in a low-density astrophysical plasma. The full list of physical processes, proceeding in a rarefied astrophysical plasma includes the impact, fusion and decay processes which are accompanied by electron transitions from one state to another. In the present pap er we confine our analysis with the following most imp ortant processes: the radiation transitions b etween discrete b ound levels( Ai + Ai ), the photoionization and the photorecombination (Ai + Ai+1 +e), the excitation and deactivation (de-excitation) by electron impacts (Ai +e Ai +e), the dielectronic recombination and autoionization (Ai Ai +e), the charge transfer (Ai+1 +Bj Ai +Bj+1 ), the electron impact ionization (Ai +e Ai+1 +2e). The reversed process of the last process in the list ­ the triple impact recombination is negligible in conditions of a rarefied astrophysical plasma. More detailed description of physical processes studied by us is given in the corresp onding sections of the explanatory text to the atomic data tables. Here we only mention that for reactions of the typ e A + B A +B the transition rate p er unit time and unit volume is given by expression < v > n(A)n(B). The

4


averaged reaction rate in this expression is defined by

q =< v >=

(v )vf (v ) dv ,

where f (v ) is the velocity distribution of colliding particles taken to b e the Maxwellian one, characterized by temp erature T . If one of the colliding particles is photon we have to integrate over the photon frequency distribution of the external radiation field. The external radiation field distribution in the most cases we shall assume to b e the diluted Planckian one, sp ecified by effective stellar temp erature T . The units of measure. For convenience we shall give here the units of measure for quantities, describing the cross-sections and rates of elementary processes. a. The energy units. In the atomic sp ectroscopy the energy of particles and photons usually is expressed in electron-volts (eV), ergs, wave numb ers (cm
-1

) , Kelvin degrees (K) and Ryd-

b erg units (Ry ). The transformation coefficients b etween the quantities are illustrated by the following table: unit of measure 1 eV 1 erg 1 cm-1 1K 1 Ry eV 1 6.242+11 1.240-4 8.617-5 13.606 erg 1.602-12 1 1.986-16 1.381-16 2.180-11 cm-1 8065.48 5.034+15 1 6.950-1 1.097+5 K 11604.5 7.243+15 1.439 1 1.579+5 Ry 7.350-2 4.587+10 9.113-6 6.334-6 1

b. The cross-section units. The cross-sections in atomic physics are usually measured in cm2 , megabarns (Mb = 10-18 cm2 ), or the hydrogen ground-state Bohr orbit areas (a2 = 0 8.797 · 10-17 cm2 ). c. The process rates. In two-particle collisions its rate is measured in cm3 s
-1

units. The

higher-order impacts for conditions of rarefied astrophysical plasma media can b e ignored. In the present manual only the data concerning elementary processes, which proceed in rarefied astrophysical plasma media are given. Both the numerical values and the approximate 5


half-empirical formulae for cross-sections and process rates, compiled from different published pap ers, have b een presented. The formulae for computation of intensities of sp ectral lines and continua of rarefied plasma have b een given. The computation results are given for different mechanisms of line formation. In the present study we represent only the atomic data for computation of sp ectra of low-density astrophysical plasma (most known kinds of it are gaseous nebulae), their thermal and ionization structure.

6