Sensitive observations of radio recombination lines in Orion and W51: the data and detection of systematic recombination line blueshifts proportional to impact broadening

Sensitive spectral observations made in two frequency bands near 6.0 and 17.6 GHz are described for Orion and W51. Using frequency switching we were able to achieve a dynamic range in excess of 10,000 without fitting sinusoidal or polynomial baselines. This enabled us to detect lines as weak as T _A ∼1 mK in these strong continuum sources. Hydrogen recombination lines with Δn as high as 25 have been detected in Orion. In the Orion data, where the lines are stronger, we have also detected a systematic shift in the line center frequencies proportional to linewidth that cannot be explained by normal optical depth effects.     

The determination of the velocity amplitude of the total photospheric motion field

By the analysis of the profiles of 20 weak lines observed at five centre-to-limb positions on the solar disk, radial and tangential components of the velocity amplitude of the photospheric motion field are derived in the range of optical depth - 3.0 ? lgτ5 ? +0.5.

     

On the occurrence of convective motions in the upper photosphere

We have examined whether the motion field in the photosphere in the range of optical depths 0.25< ₀< 0.6 is dominated by thermal convection or by vibrations. The observed asymmetries of infrared Fraunhofer lines indicate the presence of motions, and the fact that the asymmetry is zero for lines of low excitation and increases with the excitation potential shows that these motions are chiefly convective in this part of the photosphere: upward moving elements appear to be hotter than downward moving ones.Assuming furthermore that the photosphere can be described by a three-column model, with temperature differences as given by Edmonds (1967), we find that in the range of optical depths given above, where T seems to vary between 80 and 160 °K, average convective velocities of 2.3 to 3.2 km/sec should occur. This result is in numerical agreement with (a) a previous one by the present authors (1967) derived from the variation of line asymmetry with depth in lines of one multiplet, (b) a finding by Lambert and Mallia (1968) deduced from absolute wavelength measurements of Fraunhofer lines, and (c) a recent result of Beckers (1968) found from a comparison of two granulation pictures obtained simultaneously with a narrow-band filter centred on the two wings of a faint line.     

Effects of high velocities on photoionization lines

We have treated formation of spectral lines in a commoving frame where photoionization is predominant over collisional processes. We have assumed that the radiation field for causing photoionization is a function of Planck function. We have also considered the situation in which the continuum contributes to the radiation in the line. In all the models the quantityB/A (ratio of outer to inner radii) is kept equal to 10 and the total optical depth is taken to be 10³. The velocity is assumed to be varying according to the lawdV/dτ ∼ < 1/τ whereτ is the optical depth (τ > 0) in the given shell. The velocities at the innermost radius (r =A) are set equal to 0 and at the outermost radius (r =B), the maximum velocities are taken to be 0, 1, 3 and 10 Doppler units. The calculated line profiles are those seen by an observer at infinity.P Cygni-type profiles are observed in the case of a medium with no continuum absorption. For a medium with continuum absorption double peaked asymmetric profiles are noticed when the velocities are small; the two emission peaks merge into a single asymmetric peak for larger velocities.     

A study of the green TiO band in the sunspot spectrum

The equivalent widths of the TiO lines in the α system have been measured on a high dispersion (11 mm/Å) spectrogram of large sunspot. The lines were so weak that the measurement was made by methods giving maximum and minimum equivalent widths, depending on the adopted continuum. The rotational temperature obtained in this way was about 3000 °K. The result is unaffected by stray light because there are no TiO lines in the undisturbed spectrum. The calculation of equivalent widths using several sunspot models (all of which can satisfy the observed data) shows that the logarithmic optical depth at the effective layer of molecular line formation is about -1.6.     

Absorption in the 21-cm Line in Primordial Matter Density Fluctuations at the Stage of Their Nonlinear Compression

The mechanisms of absorption formation in the cosmic microwave background (CMB) spectrum at the frequency of the 21-cm line of the transition between the ground-state hyperfine sublevels of the hydrogen atom are analyzed. We show that a strong nonlinearity at the compression stage of primordial matter density fluctuations can give rise to a significant (in depth) absorption even before the explosions of the first stars. In this case, the main effect is due to the heating of matter in a certain narrow range of temperatures under cloud compression. We consider a steady-state radiative transfer in the 21-cm line in a medium that represents a contracting primordial matter density fluctuation at a given redshift z modeled by a homogeneous spherically symmetric cloud in the state of collapse with an adiabatic change in the gas temperature. For a sequence of cloud states with different degrees of compression we have calculated the frequency profiles of the line in the flux of radiation emerging from the cloud. In the initial state we specify the cloud radius r₀, while the gas density is assumed to be equal to the mean cosmological density for a given redshift. We show that for a separate cloud at z = 20, r₀ = 1 kpc, and a degree of radius compression of 1.9 the absorption depth in the line center can reach 0.9 K. When averaged over an ensemble of clouds, the central frequency of the line and its width are determined by the details of the fluctuation evolution dynamics.

     

A diagnostic method for probing the possible twist of magnetic field lines in sunspots

In order to study the three-dimensional structure of sunspot magnetic fields it is necessary to determine whether the field lines are twisted, i.e., if the azimuthal angle of transverse field changes with depth. For this purpose we propose the following method. At a fixed point in a spot, and in a certain wavelength interval of a magnetic-sensitive spectral line, one may measure the two Stokes parameters Q and U and then calculate the azimuthal angle of the polarization plane. If the wavelength interval of observation is moved successively from the line center to a wing, one may draw the azimuth diagram by the method proposed by Makita (1986) and refined by us (Ye Shi-hui and Jin Jie-hai, 1987). According to our theoretical calculations, described in this paper, if sunspot field lines are sufficiently strongly twisted, the curve on this diagram contains loop structures. If the twist is rather weak, the curve is approximately semi-circular. From the direction in which the curve winds (clockwise or counterclockwise) one may infer whether the magnetic field is twisting in one direction or in the opposite. In the case of no twist at all, the curve is comparatively simple and similar to a parabola.When the sensitivity of observational data is high enough, our method can also be applied to regions of weak magnetic fields outside sunspots.This work has been supported by the National Natural Science Foundation of China under grant No. 9187006-01.     

On the origin of the wide HI absorption line towards Sgr A*

We have imaged a region of ∼ 5′ extent surrounding Sgr A* in the HI 21 cm-line absorption using the Very Large Array. A Gaussian decomposition of the optical depth spectra at positions within ∼ 2′ (∼ 5 pc at 8.5 kpc) of Sgr A* detects a wide line underlying the many narrow absorption lines. The wide line has a mean peak optical depth of 0.32 ± 0.12 centered at a mean velocity of V_1sr = −4 ± 15 km s{−1}. The mean full width at half maximum is 119 ± 42 km s⁻¹. Such a wide line is absent in the spectra at positions beyond ∼ 2′ from Sgr A*. The position-velocity diagrams in optical depth reveal that the wide line originates in various components of the circumnuclear disk (radius ∼ 1.3′ ) surrounding Sgr A*. These components contribute to the optical depth of the wide line in different velocity ranges. The position-velocity diagrams do not reveal any diffuse feature which could be attributed to a large number of HI clouds along the line of sight to Sgr A*. Consequently, the wide line has no implications either to a global population of shocked HI clouds in the Galaxy or to the energetics of the interstellar medium as was earlier thought.     

Lines formed in rotating and expanding atmospheres

Spectral lines formed in a rotating and expanding atmosphere have been computed in the frame of the observer at infinity. Two kinds of velocity laws are employed: (i) a uniform radial velocity of the gas and (ii) velocity increasing with radius (i.e. velocity gradients). The atmosphere has been assumed to be rotating with constant velocity. We have considered maximum radial and rotational velocities to be 10 and 3 mean thermal units respectively in an atmosphere whose geometrical thickness is 10 times the stellar radius. The total radial optical depth at line centre is taken to be about 100. In all cases, Doppler profile and a source function which is varying as 1/r ² have been used. Generally, the lines are broadened when rotation is introduced. However, when radial motion is also present, broadening becomes asymmetric and the red emission and blue absorption are enhanced.     

The Radio Recombination Line Intensities Of Atoms And Ions: Helium,Carbon And Oxygen

The radio recombination line intensities of heavy elements of helium, carbon and oxygen are calculated with accounting for dielectronic recombination. Dielectronic recombination rates are determined accurate to the second order of a perturbation theory and the rates are described as function of principal quantum number for helium-like atom or ion. Balance equations are solved for the departure coefficients from LTE b_n. The collision and spontaneous transition rates are accounted for the balance equations, in which non-equilibrium distribution source is dielectronic recombination. Non-equilibrium amplification coefficients are found as functions of a medium temperature, density and ion charge z = 1–3 for radio recombination lines. Optical depths are calculated for the heavy element low-frequency lines with the numbers 300 > n > 1200. For the chosen electronic temperatures and densities T_e = 0.8× 10⁴–10× 10⁴ K, N_e = 0.05–0.1 cm⁻³ the line optical depth is determined by the values τ_L∼ 0.1× 10⁻⁴–100× 10⁻⁴. Calculated for free-free transition rates, the optical depth is given by using the value τ_ff∼ 10⁻²τ_L.     

The center-to-limb variations of four Ca i lines in the photospheric spectrum at λ6500

We study the center-limb (CL) variation of the average profiles of four Ca i lines near 6500 and compare these observations with synthetic data obtained from several line formation models having different thermal structures, line parameters, LTE and non-LTE conditions, and micro and macroturbulence values, to assess the formation characteristics of our Ca i lines in the solar photosphere.Comparison of numerical results with observations indicates that non-LTE is indispensable to fit the CL variation of the central residual intensity for the line 6493, and anisotropic microturbulence is indispensable to improve the CL behavior of the equivalent widths for all lines. The Ca i line analysis favors a cool photospheric model, but this cannot be disentangled clearly from the effects of non-LTE and small-scale velocity fields on the grounds of the present line formation models.     

Water vapour in sunspots

The line intensities are calculated at temperatures of 263 K and 3500 K for the H₂O band 201 at 0.94 m. The possibility of detecting these lines in sunspots is discussed. The amount of H₂O is estimated to be 4.2 × 10²⁰ molecules or 0.013 g cm^–2 above optical depth equal to unity. However, other bands at longer wavelengths are more favourable for detection of H₂O.     

Circumstellar Na I and Ca II absorption lines of type Ia supernovae in the symbiotic scenario

The formation of circumstellar Na I and Ca II resonance absorption lines in a type Ia supernova is studied in the case where the supernova explodes in a binary system with a red giant. The model suggests a spherically symmetric wind and takes into account the nonstationary ionization and heating of the wind by X rays from the shock wave and by gamma rays from radioactive ⁵⁶Ni decay. For wind densities typical of a red giant, the expected optical depth of the wind in Na I lines is shown to be too small (τ < 10^?3) for their detection. Under the same conditions, the optical depth of the predicted Ca II 3934 Å absorption line is sufficient for its detection (τ > 0.1). It is concluded that the Na I and Ca II absorption lines detected in SN 2006X could not be formed in the red giant wind and are most likely related to clouds at distances exceeding the dust evaporation radius (r > 10¹⁷ cm). An upper limit for the rate of mass loss through a stationary wind with velocity u has been obtained from the absence of Ca II absorption lines in SN 2006X unrelated to the similar Na I components: ? < 10^?8 (u/10 km s^?1) M _⊙ yr^?1.     

The application of the quickest descent method to the analysis of the Hα line in loops

The quickest descent method and the multi-layer model are proposed to analyse symmetrical and asymmetrical line profiles of loops in this paper. By fitting observed lines, the physical parameters such as Doppler width, Δλ _D , the optical thickness at line centre,τ _λ0, and the line displacement, Δλ ₀, can be directly obtained, Using this method, some symmetrical and asymmetrical line profiles of both the loop prominence on February 18, 1984 and the loop on August 17, 1989 are fitted. It is found that the method mentioned in this paper is characterized by rapid convergence, high precision, and less scatter.     

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.     

Local burst model of CMB temperature fluctuations: scattering in primordial hydrogen lines

The propagation of an instantaneous burst of isotropic radiation from the time of its onset at some redshift z ₀ to the time of its detection at the present epoch (at z = 0) is considered within the framework of a flat Universe. Thomson scattering by free electrons and scattering in primordial hydrogen lines (Hα, Hβ, Pα, and Pβ) are believed to be the sources of opacity, with the single-scattering albedo in the lines being calculated by taking into account the deexcitation of the upper levels of the transitions being considered under the action of background blackbody radiation. The profiles of these lines in the burst spectrum at the present epoch have been constructed for various z0 at various distances from the burst center. To a first approximation, these profiles do not depend on the burst radiation spectrum and intensity. It is shown that the lines are purely absorption ones at a sufficiently large distance, but an emission component can appear with decreasing distance, which strengthens as the distance decreases, while the absorption component weakens. The absorption depth in the combined profile can reach 2 ×10^?4 for the Hα and Hβ lines and 7 × 10^?6 for the Pα and Pβ lines. In this case, the relative amplitude of the temperature fluctuations lies within the range 10^?7?10^?9. The calculations have been performed for bursts with different characteristic initial sizes. At the same z ₀, the hydrogen line profiles essentially coincide for sizes smaller than some value, and the contrast of the lines decreases with increasing burst size for greater ones.

     

The effective optical depth for the formation of absorption lines

The contribution function method used so far to define the effective depth for the formation of absorption lines is discussed and a new definition of the effective depth is proposed. The effective depth is the level where a thin slab having the equivalent optical thickness to the total line absorption is placed so as to give the observed line intensity.     

Spectral analysis of sunspot flares

We have qualitatively analyzed, in the H and K lines spectral region, 31 flares covering part of umbrae or penumbrae of sunspots. A strong narrowing of the emission lines has been observed over the umbrae, and the lines are, in general, much weaker than in common flares suggesting that the optical thickness is quite low in these parts. We have calculated the Stark broadening of the H line from the general theory, and it has been applied to obtain the electron density in 9 flare spectra. In all cases it has been found that n _e > 10¹³ cm^–3. Goldberg's method has been applied to find the kinetic temperature from the H and K lines of Ca ii, and from the ratio between the central intensities of the lines we have calculated the optical thickness in the K line. Much evidence supports the assumption that the flare emission is highly diluted in the cases considered, and we propose a two-component model for the calcium emission lines.

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Non-LTE profiles of the Ali autoionization lines

A non-LTE formulation is given for the transfer of radiation in the autoionizing lines of neutral aluminum at λ1932 and λ1936 through both the Bilderberg and Harvard-Smithsonian model atmospheres. Numerical solutions for the common source function of these lines and their theoretical line profiles are calculated and compared with the corresponding LTE profiles. Our results show that the non-LTE profiles provide a better match with the observations. They also indicate that the continuous opacity of the standard solar models should be increased in this wavelength region if the center-limb variations of observed and theoretical profiles of these lines are to be in reasonable agreement.

     

Detection of small scale structure in metal lines at the extreme solar limb

We describe a tangential limb spectrum at 5870 Å which geometrically probes the high photosphere through the low chromosphere. Velocity and brightness structures with sizes ranging from 500 to 1500 km are present in the stronger emission lines. Such structure is consistent between the Fe i and Ba ii lines, and emission knots in these lines coincide with continuum bright streaks. But no correlation is evident between structure in the He i D₃ line, emission in the Na i D₂ line, and emission in the Fe i and Ba ii lines as a group. Two classes of near-horizontal velocity structure are seen in the height range from 0 to 500 km above the limb: υ ? 1 km s^?1 for the weaker metals and υ ～ 7–10 km s^?1 for the Na i line. Differences in line opacity are suggested as the cause of the low correlation between the fine structure in the various lines.