Magnetic absorption lines in stellar spectra

We analyze the process of absorption which is produced under conditions of strong magnetic fields in the magnetosphere of a compact stellar source. The magnitude of the magnetic field lies in the range 10¹²-10¹³ Gauss, which are common values in modelling pulsars.Analyzing the first absorption lines (N = 0 toN 3) we arrive to the conclusions that the orientation of electron's spin does not change if it absorbs a photon. It means it maintains its orientation opposite to the external magnetic field after the absorption. For the fundamental line (N = 0 toN = 1) the dominant polarization of the photon is. For the next two transition lines (N = 0 toN = 2 andN = 0 toN = 3), the polarization is. In the case that the absorption lines belong to one of the first three transition lines, then the mean photon energy can be approached with the relation =AN B and thus we get an error of 13.6% with respect to values obtained by the theoretical expression. Also we applied our absorption transition probabilities some known pulsar spectra and we determine which transition feature corresponds in their spectra.Presented at the 2nd UN/ESA Workshop, held in Bogotá, Colombia, 9-13 November, 1992.     

Comparison of unidentified absorption lines of quasar spectra with unidentified lines of stellar spectra

Conclusions Thus, we have considered the possibility of comparing unidentified lines of quasar spectra with unidentified lines of stellar spectra. Since the majority of absorption lines in the spectrum of Oph have wavelength shorter than 1000 Å, a comparison could be made in practice only with the absorption lines of Pup. Significant excesses in the number of coincidences over the mean level were not found, but in the case of the quasar 2351–154 the number of such coincidences within the reliable system is somewhat higher than for the other spectra. It is interesting that all the lines in the spectrum of this quasar that we distinguished belong to the unidentified lines.Of course, such comparison of the absorption lines of quasar spectra and the lines of stellar spectra can be regarded only as a weak attempt to win back in the L forest at least some lines for the absorption systems. However, for more distant absorption systems that could possibly be detected this method could play a more important part.Institute of Cosmic Research, USSR Academy of Sciences. Translated from Astrofizika, Vol. 38, No. 1, pp. 27–33, January–February, 1989.     

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.     

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.     

The absorption lines in the spectra of high redshift qsos

Based on the spectra of 4 high-redshift quasars (resolution 2A) obtained by us [1–5] using IPCS on the RGO Cassegrain spectrograph of the AAT, we point out the following. 1. Auto-correlation peaks at in PKS 0805+046 and PKS 1442+101 suggest that the large number of absorption lines shortward of L_α in high redshift quasars are due to absorption by hydrogen clouds. 2. The distribution of absorption lines and the correlation function of indicate that PKS 0528-250 may be an exception, requiring further observation. 3. Absorption redshift systems containing metallic lines may be produced by either matter ejected from the quasar, or an associated galaxy cluster or an intervening galaxy. 4. The randomness in the column density and the dispersion velocity deduced from the curve of growth of the pair supports the hypothesis that the pure L_α absorption comes from primitive hydrogen clouds in the early, exploding universe. 5. The number of hydrogen clouds per unit redshift interval is determined by the data of absorption lines of quasars with Z > 3.     

On the natural line width of absorption lines in strong magnetic stellar fields

In the last years, the interest in explaining the existence of a singularity at low frequencies ( = 0) in the differential cross section of photon scattering by electrons, in a homogeneous magnetic field, for the transitions between the Landau levelsN = 1 andN = 0, has greatly increased. It is the purpose of this paper to present a calculation of the cross section for the absorption of radiation by an electron in a strong magnetic stellar field, using the Wigner-Weisskopf approximation to better understand this kind of singularity in the Compton cross section.Presented at the 2nd UN/ESA Workshop, held in Bogotá, Colombia, 9-13 November, 1992.     

QSO absorption lines (invited review)

Some techniques and selection effects in establishing uniform samples of absorption line systems are presented. Summaries of the basic properties of various absorbers are given. Finally, attention is paid to some special, interesting problems which are now widely discussed.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984.     

The continuous absorption coefficient of a stellar atmosphere

It is pointed out that, when calculating the continuous absorption coefficient in a stellar atmosphere, it is advantageous to use the coefficient per particle of the most abundant element instead of the usual coefficient per gram of matter. The sources of continuous opacity considered are 1) absorption by H^-, HI, H₂^-, H ₂⁺, HeI, HeII, CI, CII, CIII, NI, NII, OI, OII, NaI, MgI, MgII, AlI, AlII, SiI, SiII, ClI, KI, CaII; and 2) Rayleigh scattering by HI, HeI, CI, NI, OI, H₂, and 3) Thomson scattering of free electrons. The calculations are illustrated by the results for a solar-type photosphere.     

The classification of stellar spectra using statistical image moments

A modeling of the response of the 1549 CIV emission line profile in the spectrum of Seyfert galaxy NGC 5548 to a sharp continuum drop which happened in 1989 is carried out on the basis of observational data taken from the literature. A more rapid response of the red wing of CIV profile suggesting the accretion of CIV-emitting clouds in the broad-line region (BLR) is reproduced. It is shown that the accretion velocity near the central object is consistent with a free fall law whereas at large distances from the center it varies approximately asr ^–2. The best agreement between observed CIV profile changes and modeled ones is obtained when the mass of the central objectM 1.2 × 10⁸ M . The first approximation of the BLR geometry is also found: the inflow takes place only outside two wide cones with half-opening angle 65° which are disposed like a sand-glass and filled by outflowing material. The angle between the axis of such biconical structure and our line of sight is found to be small ( 9°). It is shown that the outer radius of an accretion disk around the central object does not exceed ~ I light-day (i.e., < 100 Schwarzschild radii), and the outer radius of CIV-emitting BLR is 14 light-days. The nature of a broad blue excess in the CIV profile which did not respond to the continuum change is discussed.     

An investigation of Fe xvi emission lines in solar and stellar extreme-ultraviolet and soft X-ray spectra

New fully relativistic calculations of radiative rates and electron impact excitation cross-sections for Fe  xvi are used to determine theoretical emission-line ratios applicable to the 251–361 and 32–77 Å portions of the extreme-ultraviolet (EUV) and soft X-ray spectral regions, respectively. A comparison of the EUV results with observations from the Solar Extreme-Ultraviolet Research Telescope and Spectrograph (SERTS) reveals excellent agreement between theory and experiment. However, for emission lines in the 32–49 Å portion of the soft X-ray spectral region, there are large discrepancies between theory and measurement for both a solar flare spectrum obtained with the X-Ray Spectrometer/Spectrograph Telescope (XSST) and for observations of Capella from the Low-Energy Transmission Grating Spectrometer (LETGS) on the Chandra X-ray Observatory . These are probably due to blending in the solar flare and Capella data from both first-order lines and from shorter wavelength transitions detected in second and third order. By contrast, there is very good agreement between our theoretical results and the XSST and LETGS observations in the 50–77 Å wavelength range, contrary to previous results. In particular, there is no evidence that the Fe  xvi emission from the XSST flare arises from plasma at a much higher temperature than that expected for Fe  xvi in ionization equilibrium, as suggested by earlier work.     

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.     

Broad absorption lines in the spectra of planetary nebulae as indicators of fast flows

The possibility of using broad absorption lines (on the order of 100 km/sec or more) of the Mg II 2800 doublet, formed in regions of planetary nebulae with fast collimated flows and, probably, manifested against the background of the short-wavelength wing of the Balmer continuum of the spectrum as indicators of such flows, is discussed. For this purpose, using existing analytical (dynamical) models of mass-loaded isothermal flows and numerical (photoionization) models of planetary nebulae, theoretical line profiles are calculated in the Sobolev approximation and their dependence on certain parameters is shown. An advantage of the proposed method consists in the possibility of revealing fast flows in faint, star-like planetary nebulae. Translated from Astrofizika, Vol. 42, No. 4, pp. 519–530, October–December, 1999.     

The transfer of polarized radiation in spectral lines: Solar-type stellar atmospheres

The polarization structure in several spectral lines in solar type stars is computed using the method described by McKenna (1981, 1984a). The frequency redistribution function used for these calculations is a linear combination ofR _II andR _III. The line profiles and polarization structures have been computed for several weak solar resonance lines includingKi 7664 Å, Sri 4607 Å, Baii 4554 Å, for various polar angles along the stellar disk. Both the line profiles and polarization structures as well as the center to limb behavior of the line center polarization agree well with observations.The somewhat stronger resonance line Cai 4227 Å shows a different polarization structure when compared to the weaker solar resonance lines. It is found that for strong resonance lines the proper redistribution function to be used is a linear combination ofR _III andR _v (see McKenna, 1981, 1984b; Heinzel, 1981). The major reason for this is that for strong resonance lines both the upper and lower levels are broadened by collisions. This violates the assumptions upon which the redistribution functionsR _II andR _III are based.     

Automated classification of sloan digital sky survey (SDSS) stellar spectra using artificial neural networks

Automated techniques have been developed to automate the process of classification of objects or their analysis. The large datasets provided by upcoming spectroscopic surveys with dedicated telescopes urges scientists to use these automated techniques for analysis of such large datasets which are now available to the community. Sloan Digital Sky Survey (SDSS) is one of such surveys releasing massive datasets. We use Probabilistic Neural Network (PNN) for automatic classification of about 5000 SDSS spectra into 158 spectral type of a reference library ranging from O type to M type stars.     

Highn emission and absorption lines of the sun (II)

Considering the electronic pressure broadening of highn solar recombination lines, an upper wavelength limit of the observable region of possible lines is derived. The reason that no lines have been observed is discussed.     

The standard systems of stellar rotation measurements (II)

The limb darkening effect on the measurement of stellar rotation is discussed in this paper. It is shown that this effect plays an important role in the measurement of V_e sin i. In the extreme case with the limb darkening coefficient of 1.0, it may cause a difference up to 17%. As a sequel to paper [1], this work presents the following new explanation for the systematic differences between Slettebak's new and old systems. The main causes of the systematic differences are: (1) The old system made an inadequate twice repeated correction for the limb darkening. (2) Owing to the historical reason, the old system used too large limb darkening coefficients.