Effective depth of spectral line formation in planetary atmospheres

The effective level of line formation for spectroscopic absorption lines has long been regarded as a useful parameter for determining average atmospheric values of the quantities involved in line formation. The identity of this parameter has recently been disputed. Here we reestablish the dependence of this parameter on the average depth at which photons are absorbed in a semi-infinite atmosphere and show that the mean depths derived by others are similar in nature and behavior.     

The profiles of fraunhofer lines in the presence of zeeman splitting

For the case of pure absorption lines (LTE) a method is described which enables the general computation of Zeeman-split line profiles. The magnetic field vector, the Doppler shift and the line absorption coefficient is permitted to vary arbitrarily with optical depth. Elliptical birefringence (e.g., Faraday rotation) of the solar atmosphere is taken into account. Some numerical examples are given and some interesting behaviors of the line profiles are discussed.     

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.     

A high galactic latitude HI 21 cm-line absorption survey using the GMRT: I. Observations and spectra

We have used the Giant Meterwave Radio Telescope (GMRT) to measure the Galactic HI 21-cm line absorption towards 102 extragalactic radio continuum sources, located at high (|b| > 15°) Galactic latitudes. The Declination coverage of the present survey is δ}> - 45°. With a mean rms optical depth of ∼ 0.003, this is the most sensitive Galactic HI 21-cm line absorption survey to date. To supplement the absorption data, we have extracted the HI 21-cm line emission profiles towards these 102 lines of sight from the Leiden Dwingeloo Survey of Galactic neutral hydrogen. We have carried out a Gaussian fitting analysis to identify the discrete absorption and emission components in these profiles. In this paper, we present the spectra and the components. A subsequent paper will discuss the interpretation of these results.     

Voyager 1 imaging and IRIS observations of Jovian methane absorption and thermal emission: Implications for cloud structure

Images from three filters of the Voyager 1 wide-angle camera were used to measure the continuum reflectivity and spectral gradient near 6000 Å and the 6190-Å band methane/continuum ratio for a variety of cloud features in Jupiter's atmosphere. The dark “barge” features in the North Equatorial Belt have anomalously strong positive continuum spectral gradients suggesting unique composition, probably not elemental sulfur. Methane absorption was shown at unprecedented spatial scales for the Great Red Spot and its immediate environment, for a dark barge feature in the North Equatorial Belt, and for two hot spot and plume regions in the North Equatorial Belt. Some small-scale features, unresolvable at ground-based resolution, show significant enhancement in methane absorption. Any enhancement in methane absorption is conspicuously absent in both hot spot regions with 5-μm brightness temperature 255°K. Methane absorption and 5-μm emission are correlated in the vicinity of the Great Red Spot but are anticorrelated in one of the plume hot spot regions. Methane absorption and simultaneously maps of 5-μm brightness temperature were quantitatively compared to realistic cloud structure models which include multiple scattering at 5 μm as well as in the visible. A curve in parameter space defines the solution to any observed quantity, ranging from a shallow atmosphere and thin NH₃ cloud to a deep atmosphere with a thick ammonia cloud. Without additional constraints, such as center-to-limb information, it is impossible to specify the NH₃ cloud optical depth and pressure of a deeper cloud top independently. Variability in H₂ quadrupole lines was also investigated and it was found that the constancy of the 4-0 S(1)-line equivalent width is consistent with the constancy of the methane 6190-Å band equivalent width at ground-based resolution, but the much greater variability of the 3-0 S(1) line is inconsistent with either the methane band or 4-0 S(1) line. In hot spot regions the 255°K brightness temperature requires a cloud optical depth of about 2 or less at 5 μm in the NH₃ cloud layer. To be consistent with the observed 6190-Å methane absorption in hot spot regions, the NH₃ cloud optical depth in the visible is about 7.5, implying that aerosols in hot spot regions have effective radii near 1 μm or less.     

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.

     

GMRT detection of HI 21 cm-line absorption from the peculiar galaxy in Abell 2125

Using the recently completed Giant Meterwave Radio Telescope, we have detected the HI 21 cm-line absorption from the peculiar galaxy C153 in the galaxy cluster Abell 2125. The HI absorption is at a redshift of 0.2533, with a peak optical depth of 0.36. The full width at half minimum of the absorption line is 100 km s⁻¹. The estimated column density of atomic Hydrogen is 0.7×10²²(T _s /100) cm⁻². The HI absorption is redshifted by ∼400km s⁻¹ compared to the [OIII] emission line from this system. We attribute this to an infalling cold gas or to an out-flowing ionised gas, or to a combination of both as a consequence of tidal interactions of C153 with either a cluster galaxy or the cluster potential.     

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.     

Depth-dependent line blanketing by neutral and ionized metals in a homogeneous model solar photosphere

Line blanketing due to both the neutral and ionized metals is treated here by the modified picket-fence method and applied to the case of a homogeneous model solar photosphere in strict radiative equilibrium. Improvement in fitting the observed photospheric radiation field is noted over a model in which the line absorption coefficient is constant with depth.This work was carried out while the author was at Georgetown College Observatory, Washington, D.C.     

Multiple velocities observed in HeI 1083 nm

We present a time sequence of slit spectra of Hei 1083.03 nm (and H) taken with the German Vacuum Tower Telescope at Tenerife. We find the signature of an onset of a downflow accelerating up to 42 km/s . The Hei 1083.03 nm line splits into two well separated components during the event, one of which shows the usual absorption profile of material which is almost at rest. We calculate the velocity, line depth and area of both line profiles. Possible explanations for these observed multiple velocities are discussed.     

The spatial variation of the 3-μm dust features in Circinus

We report spatially resolved variations in the 3.4-μm hydrocarbon absorption feature and the 3.3-μm polycyclic aromatic hydrocarbon (PAH) emission band in the Circinus galaxy over the central few arcsec. The absorption is measured towards warm emitting dust associated with Coronal line regions to the east and west of the nucleus. There is an absorption optical depth  τ_3.4 μm∼ 0.1  in the core which decreases to the west and increases to the east. This is consistent with increased extinction out to ∼40 pc east of the core, supported by the Coronal emission line intensities which are significantly lower to the east than the west. PAH emission is measured to be symmetrically distributed out to ±4 arcsec, outside the differential extinction region. The asymmetry in the 3.4-μm absorption band reflects that seen in the 9.7-μm silicate absorption band reported by Roche et al., and the ratio of the two absorption depths remains approximately constant across the central regions, with  τ_3.4 μm/τ_9.7 μm∼ 0.06 ± 0.01  . This indicates well-mixed hydrocarbon and silicate dust populations, with no evidence for significant changes near the nucleus.     

Mars: VLA observations of the Northern Hemisphere and the north polar region at wavelengths of 2 and 6 cm

Observations of Mars at wavelengths of 2 and 6 cm were made using the VLA in its A configuration. The season on Mars was late spring in the Northern Hemisphere (L_s = 60°). The sub-Earth latitude was 25°N, so the geometry for viewing the north polar region was optimal. Whole-disk brightness temperatures were estimated to be 193.2 ± 1.0°K at 2 cm and 191.2 ± 0.6°K at 6 cm (formal errors only). Since measurements of the polarized flux were taken at the same time, whole-disk effective dielectric constants could be estimated and from these estimates of subsurface densities could be made. The results of these calculations yielded a whole-disk effective dielectric constant of 2.34 ± 0.05, which implied a subsurface density of 1.24 ± 0.11 g cm⁻³ at 2 cm. The same calculations at 6 cm yielded an effective density of 1.45 ± 0.10 g cm⁻³ and dielectric constant of 2.70 ± 0.10. From the mapped data these parameters were also estimated as a function of latitude between latitudes of 15°S and 60°N. In addition to the effective dielectric constant and subsurface density, the radio absorption length of the subsurface was estimated. The radio absorption length for most of these latitudes was about 15 wavelengths with formal errors on the order of 5 or 10 wavelengths. The estimation of the effective dielectric constant at most latitudes was between 2 and 3.5 with only slight differences between the two different wavelengths. These estimates of the dielectric constant lead to estimation of the subsurface densities as a function of latitude. Most calculations of the subsurface density yielded results between 1 and 2 g cm⁻³ with errors on the order of 0.5 g cm⁻³. These results seem to imply that the subsurface is not much different than the surface as observed by the Viking and Mariner missions. In line with this, a comparison of the correlation of the dielectric constant at each wavelength with the thermal inertia determined from infrared measurements of the surface temperature shows that the correlation at 2 cm is slightly stronger than the correlation at 6 cm. Since the 2-cm radiation comes from a region closer to the surface than the 6-cm radiation, this decrease in correlation with depth is consistent with the idea that the physical makeup of the subsurface is varying slowly in the near subsurface region.     

A first order analysis of variations of the limb darkening and the shapes for solar Fraunhofer lines

New center-to-limb measurements in FeI lines show changes in both the line profiles and the limb darkening curves that appear to be characteristic of many other solar lines. Here we seek the constraints placed on the atmospheric model by these effects. We find that in addition to a depth varying source function we must also allow the ratio of the continuous absorption coefficient to the total absorption coefficient to pass through a minimum in the mid-photosphere. Such an effect is consistent with inward increases of the Doppler width and damping constant in the upper photosphere and an inward increase of the ionization for both iron and hydrogen in the low photosphere.The National Center for Atmospheric Research and Kitt Peak National Observatory are sponsored by the National Science Foundation.     

SCUBA observations of Hawaii 167

We present the first submillimetre observations of the z =2.36 broad absorption line system Hawaii 167. Our observations confirm the hypothesis that Hawaii 167 contains a massive quantity of dust, the optical depth of which is sufficient to extinguish completely our ultraviolet view of a central, buried quasar. The submillimetre luminosity and associated dust mass of Hawaii 167 are similar to those of the ultraluminous class of infrared galaxies, supporting the existence of an evolutionary link between these and the active galaxy population. Hawaii 167 appears to be a young quasar that is emerging from its dusty cocoon.     

GMRT observations of interstellar clouds in the 21cm line of atomic hydrogen

Nearby interstellar clouds with high (|ν|≥10km s⁻¹) random velocities although easily detected in NaI and CaII lines have hitherto not been detected (in emission or absorption) in the HI 21cm line. We describe here deep Giant Metrewave Radio Telescope (GMRT) HI absorption observations toward radio sources with small angular separation from bright O and B stars whose spectra reveal the presence of intervening high random velocity CaII absorbing clouds. In 5 out of the 14 directions searched we detect HI 21cm absorption features from these clouds. The mean optical depth of these detections is ∼0.09 and FWHM is ∼10km s⁻¹, consistent with absorption arising from CNM clouds.     

Exact solution of a problem in a stellar atmosphere using the Laplace transform and the Wiener-Hopf technique

The general equation for radiative transfer of line scattering intensity — including the effects of scattering, absorption and thermal emission — in the Milne-Eddington model is considered here. The scattering function is assumed to be quadratically anisotropic in the cosine of the scattering angle, and Planck's intensity function is assumed for thermal emission. The exact solutions for emergent intensity from the bounding face and the intensity at any optical depth are obtained by the method of the Laplace transform in combination with the Wiener-Hopf technique.     

A reconsideration of the effective depth of line formation in planetary atmospheres

A new definition of the effective depth of line formation is given which incorporates its dependence on the angle of emergence as well as on the position within the line. Solution obtained for isotropic scattering in the third approximation of discrete ordinates explains: (i) Variation of equivalent widths over the disc; (ii) Inverse dependence of this variation on line strength; and (iii) The phase effect for relative equivalent widths.     

类星体Lα森林吸收线密度与发射红移相关性的再考察

继文[1]之后,本文利用15个中—高红移类星体的Lα森林构成统计样本,再度考察了Lα森林吸收线密度N(Z_(abs))与发射红移Z_(em)的相关性。本文的统计样本包含的红移范围比文[1]更大:类星体的发射红移值从1.715到3.750,Lα森林吸收线红移范围从1.501到3.780;统计方法亦与文[1]略有差异。统计结果再度表明,Lα森林吸收线密度N(Z_(abs))明显依赖于类星体自身的发射红移Z_(em),这不仅在于Z_(em)越大的类星体其全部吸收线的平均数密度N(Z_(abs))也越大,更重要的是,对于相同的吸收红移值Z_(abs)而言,N(Z_(abs))在统计意义上明显地随Z_(em)的增大而增大。文末对所得的结果作了讨论。     

Temperature Dependence of Molecular Line Strengths and Fe i 1565 nm Zeeman Splitting in a Sunspot

Spectroscopic observations at 1565 nm were made in the eastern half of the main umbra of NOAA 9885 on 1 April 2002 using the National Solar Observatory McMath-Pierce Telescope at Kitt Peak with a tip-tilt image stabilization system and the California State University Northridge–National Solar Observatory infrared camera. The line depth of the OH blend at 1565.1 nm varies with the observed continuum temperature; the variation fits previous observations except that the continuum temperature is lower by 600 K. The equivalent width of the OH absorption line at 1565.2 nm shows a temperature dependence similar to previously published umbral molecular observations at 640 nm. A simple model of expected OH abundance based upon an ionization analogy to molecular dissociation is produced and agrees well with the temperature variation of the line equivalent width. A CN absorption line at 1564.6 nm shows a very different temperature dependence, likely due to complicated formation and destruction processes. Nonetheless a numerical fit of the temperature variation of the CN equivalent width is presented. Finally a comparison of the Zeeman splitting of the Fei 1564.8 nm line with the sunspot temperature derived from the continuum intensity shows an umbra somewhat cooler for a given magnetic field strength than previous comparisons using this infrared 1564.8 nm line, but consistent with these previous infrared measurements the umbra is hotter for a given magnetic field strength than magnetic and temperature measurements at 630.2 nm would suggest. Differences between the 630.2 nm and 1564.8 nm umbral temperature and magnetic field relations are explained with the different heights of formation of the lines and continua at these wavelengths.     

The spectrum of BAL QSO Q1303+308: intrinsic variability and line-locking stability

A recently taken WHT optical spectrum of the BAL QSO Q1303+308 shows interesting changes in the spectral characteristics of the absorption systems. In particular, the equivalent widths of many of the broad absorption lines have markedly increased with respect to observations taken 15 yr earlier by Foltz et al., with the more blueshifted components showing the greatest increase by a factor of ≈5 in optical depth. Perhaps the most remarkable feature of the data is that, in the rest frame of the quasar, the line-of-sight outflow velocity of the clouds has only increased by ≈55 km s⁻¹ in a rest-frame time interval of 5–6 yr. Various examples of doublet line locking are still visible in the system, and we present a provisional model analysis of the spectrum showing that theories based on small cloud velocity interactions via line locking can successfully explain the general appearance of the spectrum and its variability. Finally we highlight the need for further ongoing monitoring of this fascinating object.