Infrared hydrogen emission lines in the spectra of Be stars

For the interpretation of observed hydrogen emission lines in the spectrum of Cas calculations of the relative intensities of spectral lines of the Balmer, Paschen, Brackett and Pfund series are carried out, by using the theory of moving stellar envelopes.It is shown that in the spectra of Be stars, which have opaque envelopes and therefore a slow Balmer decrement, infrared hydrogen lines must be relatively strong.It is also found that the observed relative line intensities of Cas are in qualitative agreement with those calculated for the case of a simple model-envelope which consists of low and high opacity regions.     

Highly excited hydrogen lines in stellar spectra. I

The solution of the hydrogenic Schrödinger equation are given with two boundary conditions imposed on the wave function, for distances of the order of magnitude of one hundred times the Bohr radius from the central nucleus. Thus the shifts and splitting of the H-H₂₆ lines are given which arise from the non-vacant environment. The Inglis-Teller limit is revised since from the shifts it follows that it gives a correct charged particle density only in the case of extremely high electron and ion densities. The gas density is derived from the number of the visible Balmer lines, and usually the widths and the contours but not the coalescence include information on the charged particle density.     

Highly excited hydrogen lines in stellar spectra. II

In the first part of the paper the shifts of the last visible balmer lines as a function of the gas density were predicted theoretically. Here spectroscopic research for these shifts is reported: 10A/mm^–1 spectra of six stars were examined and published material of other authors was also used.The shifts are of tenth Angström order of magnitude and they are used to measure the gas density in a well defined region of the stellar atmosphere. It is shown that usually there is no real coalescence of Balmer lines. The lack of Balmer lines with high main quantum number is explained adequately in most cases without Stark and Doppler effect. Evidence is given against using the Inglis-Teller formula because its physical foundation is inadequate. It gives false values in general.Stars with peculiar spectra—helium and carbon Wolf-Rayet stars—are briefly mentioned as well because the last visible Balmer-like line of Hei orCiv gives a density value in their atmospheres which is a useful first guess in understanding their spectra better.     

Relationship between intensities of strong emission lines in the spectra of H II regions and their chemical compositions

The method of determination of heavy element abundances in H II regions (the strong-line method) uses the assumption that some combinations of strong emission line intensities in spectra of H II regions can serve as indicators of metallicities and electron temperatures in nebulosities. Three sets of strong lines are considered, namely, A (R ₃, R ₂, N ₂, S ₂ lines), B (R ₃, R ₂, N ₂ lines), and C (R ₃, N ₂, S ₂ lines). Strong line intensities are normalized to the H _?? intensity). We searched for an unambiguous relationship between strong emission line intensities of these line sets in spectra of H II regions and their compositions. The extensive model grid for H II regions is computed. Chemical compositions of nebulosities and intensities of A and C lines are shown to be related unambiguously. For the B line set, 5% of model H II regions do not have any unambiguous relationship, namely, the models with appreciably different oxygen and nitrogen abundances in H II regions can have similar intensities of the B set lines. The versions of strong-line method (calibrations) using the A and C lines are more reliable than those based on the B lines.     

The behaviour of the infrared lines of neutral oxygen and the balmer lines of hydrogen in the spectra of early-type stars with emission lines

The equivalent widths of the oxygen lines at 7774 and 8446 and of H (and some H) have been measured for 22 early-type, emission-line stars. A strong correlation between H and 8446 intensities has been found, although there is no such correlation between H and 7774. This confirms the probability that Bowen's mechanism is operative (the neutral oxygen 3³ D state is overpopulated because the excitation energy of Ly- nearly coincides with that of theOi 1025 line). The possibility of using 8446 and H equivalent widths for a comparison of oxygen to hydrogen abundances in these stars is discussed.     

Influence of inelastic collisions with hydrogen atoms on the formation of AlI and SiI lines in stellar spectra

We have performed calculations by abandoning the assumption of local thermodynamic equilibrium (within the so-called non-LTE approach) for Al I and Si I with model atmospheres corresponding to stars of spectral types F–G–Kwith differentmetal abundances. To take into account inelastic collisions with hydrogen atoms, for the first time we have applied the cross sections calculated by Belyaev et al. using model approaches within the formalism of the Born–Oppenheimer quantum theory. We show that for Al I non-LTE leads to higher ionization (overionization) than in LTE in the spectral line formation region and to a weakening of spectral lines, which is consistent with earlier non-LTE studies. However, our results, especially for the subordinate lines, differ quantitatively from the results of predecessors. Owing to their large cross sections, the ion-pair production and mutual neutralization processes Al I(nl) + HI(1s) ? Al II(3s ²) + H^? provide a close coupling of highly excited Al I levels with the Al II ground state, which causes the deviations from the equilibrium level population to decrease compared to the calculations where the collisions only with electrons are taken into account. For three moderately metal-deficient dwarf stars, the aluminum abundance has been determined from seven Al I lines in different models of their formation. Under the assumption of LTE and in non-LTE calculations including the collisions only with electrons, the Al I 3961 ?A resonance line gives a systematically lower abundance than the mean abundance from the subordinate lines, by 0.25–0.45 dex. The difference for each star is removed by taking into account the collisions with hydrogen atoms, and the rms error of the abundance derived from all seven Al I lines decreases by a factor of 1.5–3 compared to the LTE analysis. We have calculated the non- LTE corrections to the abundance for six subordinate Al I lines as a function of the effective temperature (4500 K ≤ T _eff ≤ 6500 K), surface gravity (3.0 ≤ log g ≤ 4.5), and metal abundance ([M/H] = 0, ?1, ?2, and ?3). For Si I including the collisions with HI leads to the establishment of equilibrium populations in the spectral line formation region even in hot metal-deficient models and to vanishingly small departures from LTE in spectral lines.     

The relative intensities of CI lines in the solar EUV spectrum

The relative intensity of two Ci lines at 1993.6 Å and 1657.4 Å, observed in the limb spectrum of the sun, is a factor 2.6 × 10³ larger than that expected if both lines were optically thin. It is shown that the observed intensity ratio may be explained in terms of the transfer of photons from 1657.4 Å to 1993.6 Å due to a large optical depth in the line at 1657.4 Å. The observed upper limit on the relative intensity of two further lines at 1992.0 Å and 1657.0 Å has been used to show that the line at 1993.6 Å is optically thin. Hence it is shown that (1657.4 Å) = 1300, and (1993.6 Å) = 0.44. These values provide an independent evaluation of optical depths against which chromospheric models may be checked. Assuming a mean temperature of T _e = 8000 °K, and a mean scale height of 350 km, the optical depths lead to a mean hydrogen-particle density of N (H) = 1.4 × 10¹² cm^–3.     

The anomalous intensities of helium lines in the quiet solar transition region

Previous studies using observations made at low spatial and spectral resolution showed that the resonance lines of He  i and He  ii are anomalously strong in the quiet Sun when compared with other transition region lines formed at similar temperatures. Here, the higher spatial and spectral resolution provided by the Coronal Diagnostic Spectrometer ( cds ) instrument on board the Solar and Heliospheric Observatory ( SOHO ) is used to re-examine the behaviour of the He  i and He  ii lines and other transition region lines, in quiet regions near Sun centre. Supergranulation cell boundaries and cell interiors are examined separately. Near-simultaneous observations with the sumer instrument provide information on the lower transition region and the electron pressure. While the lines of He  i and He  ii have a common behaviour, as do the other transition region lines, the behaviour of the helium lines relative to the other transition region lines is significantly different. The emission measure distributions that account for all transition region lines, except those of helium, fail to produce sufficient emission in the He  i and He  ii resonance lines by around an order of magnitude, in both supergranulation cell boundary and cell interior regions. The electron pressure appears to be higher in the cell interiors than in the average cell boundaries, although the uncertainties are large. While the VAL-D model gives a closer match to the He  i 584.3-Å line, it does not successfully reproduce other transition region lines.     

Equator-pole effect in the central intensities of some strong solar Fraunhofer lines

Uncorrected central intensities of the Fraunhofer lines NaD₁ and D₂, Mgb₁ and b₂, and Hβ have been determined at a number of points along the S-W limb of the sun, from equator to pole, using photographic spectra taken at the Locarno station of the Göttingen Observatory. No significant pole-equator variation could be found in excess of the observational errors which are of the order of 2%.     

The anomalous intensities of helium lines in a coronal hole

Observations made at the quiet Sun-centre with the Coronal Diagnostic Spectrometer (CDS) and Solar Ultraviolet Measurements of Emitted Radiation (SUMER) instruments on the Solar and Heliospheric Observatory ( SOHO ) have shown that the intensities of the resonance lines of He  i and He  ii are significantly larger than predicted by emission measure distributions found from other transition region lines. The intensities of the helium lines are observed to be lower in coronal holes than in the quiet Sun. Any theory proposed to account for the behaviour of the helium lines must explain the observations of both the quiet Sun and coronal holes. We use observations made with SOHO to find the physical conditions in a polar coronal hole. The electron pressure is found using the C  iii 1175-Å and N  iii 991.5-Å lines, as the C  iii line at 977.0 Å becomes optically thick in some regions at high latitudes. The mean electron pressure is a factor of ≃2 lower than that at the quiet Sun-centre. The mean coronal electron temperature is     . The helium lines are enhanced with respect to other transition region lines but by factors which are ≃ 30 per cent smaller than at the quiet Sun-centre. The mean ratios of the intensities of the He  i 537.0- and 584.3-Å lines and of the He  i and He  ii 303.8-Å lines vary little with the type of region studied. These ratios are compared with those predicted by models of the transition region, taking into account the radiative transfer in the helium lines. No significant variation is found in the relative abundances of carbon and silicon.