Optical depth effects on the formation of spectral lines in rotating and expanding spherical atmospheres

We have investigated the effects of increasing optical depths on spectral lines formed in a rotating and expanding spherical shell. We have assumed a shell whose outer radius is 3 times the inner radius, with the radial optical depths equal to 10, 50, 100, 500. We have employed a constant velocity with no velocity gradients in the shell. The shell is assumed to be rotating with velocities varying as 1/_ρ, where_ρ is the perpendicular distance from the axis of rotation, implying the conservation of angular momentum. Two expansion (radial) velocities are treated: (1)V = 0 (static case) and (2)V = 10 mean thermal units. The maximum rotational velocities areV _rot = 0, 5, 10 and 20. In the shell where there are no radial motions, we obtain symmetric lines with emission in the wings forV _rot = 0 and 5 while forV _rot ≥ 10 we obtain symmetric absorption lines. In the case of an expanding shell, we obtain lines with central emission.     

Resonant scattering in galaxy clusters for anisotropic gas motions on various spatial scales

The determination of the characteristic amplitudes and directions of hot gas motions in galaxy clusters from observations of the brightest resonance lines is discussed. Gas motions affect (i) the spectral line shape through the Doppler effect and (ii) the distortions of the radial surface brightness profiles in lines during resonant scattering. Radiative transfer calculations have been performed by the Monte Carlo method in the FeXXV resonance line at 6.7 keV for the Perseus cluster A426. As a result, (a) radial motions have been shown to reduce the scattering efficiency much more dramatically than purely tangential motions; (b) large-scale gas motions have been shown to affect weakly the scattering efficiency; and (c) the uncertainty in measuring the characteristics of gas motions using resonant scattering has been estimated from existing and future observations of clusters.     

The Problem of the Height Dependence of Magnetic Fields in Sunspots

To understand the physics of sunspots, it is important to know the properties of their magnetic field, and especially its height stratification plays a substantial role. There are mainly two methods to assess this stratification, but they yield different magnetic gradients in the photospheric layers. Determinations based on the several spectral lines of different formation heights and the slope of their profiles result in gradients of ?2 to ?3 G?km^?1, or even steeper. This is similar for the total magnetic field strength and for the vertical component of the magnetic field. The other option is to determine the horizontal partial derivatives of the magnetic field, and with the condition \(\operatorname{div} {{\boldsymbol {B}}} = 0\) also the vertical derivative is known. With this method, gradients of ?0.5 G?km^?1 and even shallower are obtained. Obviously, these results do not agree. If chromospheric spectral lines are included, only shallow gradients around ?0.5 G?km^?1 are obtained. Shallow gradients are also found from gyro-resonance measurements in the radio wave range 300?–?2000 GHz.Some indirect methods are also considered, but they cannot clarify the total picture. An analysis of a numerical simulation of a sunspot indicates a shallow gradient over a wide height range, but with slightly steeper gradients in deep layers.Several ideas to explain the discrepancy are also discussed. With no doubts cast on Maxwell’s equations, the first one is to look at the uncertainties of the formation heights of spectral lines, but a wider range of these heights would require an extension of the solar photosphere that is incompatible with observations and the theory of stellar atmospheres. Submerging and rising magnetic flux might play a role in the outer penumbra, if the resolution is too low to separate them, but it is not likely that this effect acts also in the umbra. A quick investigation assuming a spatial small scale structure of sunspots together with twist and writhe of individual flux tubes shows a reduction of the measured magnetic field strength for spectral lines sensitive to a larger height range. However, sophisticated investigations are required to prove that the explanation for the discrepancy lies here, and the problem of the height gradient of the magnetic field in sunspots is still not solved.     

The AXAF low-energy transmission grating spectrometer LETGS: Diagnostic capabilities for the study of stellar coronae

We study the diagnostic capabilities of the high-resolution, Low-Energy Transmission Grating Spectrometer, LETGS, of NASA's planned Advanced X-ray Astrophysics Facility, AXAF, for optically thin stellar coronae. Spectra are simulated on the basis of isothermal and source loop models and are analyzed with particular emphasis on the extraction of the differential emission measure distribution. The AXAF-LETGS is shown to be particularly sensitive for plasma at temperatures between 0.5 and 15 MK. Emission from temperatures in excess of 20 MK can be observed, but the lack of strong spectral lines hampers accurate temperature determinations. We simulate spectra of close binaries to demonstrate the observability of the Doppler effects associated with orbital motions. We present lists of spectral lines that can be used for density diagnostics, and we simulate and compare various spectra at different electron densities.     

On determining the microturbulent velocities of solar prominences

The classical method for determining the velocities of microturbulent motions in solar prominences is generalized to account for the possible opacity of the spectral lines. A new characteristic of a line is introduced which, for a given line formation mechanism, can be used to determine the optical thickness of the emitting region. The method is applied to lines in the EUV region observed with the SUMER spectrograph as part of the SOHO space program. Comparison with observational data not only confirms the validity of this mechanism for line formation, but also shows that the optical thickness of the medium is small for these lines. Difficulties involved in determining the kinetic temperature and, therefore, the microturbulent velocities, are discussed. Based on lines of various ions, this velocity is estimated to be on the order of 30–40 km/s.     

Spectral classification of the cool components of symbiotic stars

We have made near infrared spectroscopic observations of 10 symbiotic and 27 K-M comparison stars. For stars later than M3, we found the band depth of the triple-headed TiO absorption band to be sensitive to temperature and insensitive to gravity. We fitted the spectral type to the band depth with a standard error of 0.22 of a subtype and derived the spectral types for 6 symbiotic stars. We measured the EW of a large number of lines including the CaII lines (very sensitive to luminosity), the FeI and TiO lines (moderately sensitive) and the NaI lines (not sensitive). The EW of these lines vary with the spectral type, particularly for stars later than M3, both spectral type and luminosity effects must be considered. We give the luminosity classes for 10 of the brighter symbiotic stars, they are all giant stars, showing no features of bright giants or supergiants.     

二维太阳光谱观测方法和技术

经典的太阳光谱观测是一维的，它有很大的局限性。从５０年代起，天文工作者采用多种方法开展二维太阳光谱观测，已经研制出一系列仪器，建立完整的资料归算程序，取得优良成果。在二维观测资料的基础上，用理论方法推出深度分布，可以得出三维的立体图像，这会成为太阳研究的主要方法之一。     

Turbulence in clusters of galaxies and X-ray line profiles

Large-scale bulk motions and hydrodynamic turbulence in the intergalactic gas that fills clusters of galaxies significantly broaden X-ray emission lines. For lines of heavy ions (primarily helium-like and hydrogen-like iron ions), the hydrodynamic broadening is appreciably larger than the thermal broadening. Since clusters of galaxies have a negligible optical depth for resonant scattering in the forbidden and intercombination lines of these ions, these lines are not additionally broadened. At the same time, they are very intense, which allows deviations of the spectrum from the Gaussian spectrum in the line wings to be investigated. The line shape proves to be an important indicator of bulk hydrodynamic processes. Doppler probing of turbulence becomes possible, because the cryogenic detectors of the X-ray observatories now ready for launch and being planned will have a high energy resolution (from 5 eV for ASTRO-E2 to 1–2 eV for Constellation-X and XEUS). We use the spectral representation of a Kolmogorov cascade in the inertial range to calculate the characteristic shapes of radiation lines. Significant deviations in the line profiles from the Gaussian profile (shape asymmetry, additional peaks, sharp breaks in the exponential tails) are expected for large-scale turbulence. The kinematic SZ effect and the X-ray line profiles carry different information about the hydrodynamic velocity distribution in clusters of galaxies and complement each other, allowing the redshift, the peculiar velocity of the cluster, and the bulk velocity dispersion to be separated.     

On the propagation of chromospheric condensations in solar flares (I)

We simulate dynamically the downward propagation of the chromospheric condensation, which originates following the chromospheric evaporation during solar flares. Our attention is concentrated on the lower part of the atmosphere. The top of the chromosphere (base of the transition region) is regarded as the top boundary. The condensation is mimicked by assuming an impulsive pressure increase at the top boundary. Using such a method, we compute in detail the evolution process of a condensation. The results show that the condensation can penetrate into the deeper atmosphere, though it becomes very weak at the later phase. Moreover, we also discuss the possibility that the mass motions in the condensation may cause the asymmetries of some spectral lines as observations have indicated.     

The solar temperature reversal and convective motions

Given an equation of State, the three equations of mass, momentum and energy conservation determine the vortex free flow fields in a star. The solar data processed by Athay (1966) is a measure of the dissipation of the fluid, while the use of an empirical temperature profile (BCA, 1968) permits values for the gradients in the equations of fluid flow rendering them algebraic, with solutions independent of boundary conditions. This approach serves as a consistency check on the methods with which data have been processed by inquiring to what degree the basic conservation laws have been satified, and by directly implying a velocity field which can be scrutinized for its observational consequences. We find that the most consistent interpretation to this data is that the temperature minimum should not be coincident with flux maximum as the BCD model photosphere is, and that the temperature profile must rise higher at the 1500 km. region above optical depth one. Furthermore, more emissions must occur in the U-V metals at this region to support such a temperature increase, but at least an order of magnitude decrease in H^? emission should also be expected, as well as some suppression of the Balmer. As a theoretical consequence horizontal stratification of the velocity field occurs, suggesting either cellular motions or differential rotation with surface velocities as high as 60 km s^?1, but on the average near 30 to 40 km s^?1, across the solar surface.     

On the contribution of horizontal granular motions to observed limb-effect curves

From the comparison of 59 iron lines at the center of the solar disk with laboratory wavelenghts, the mean vertical velocity of solar granulation and its depth dependence is determined. These values are used to calculate limb-effect curves. The differences to observed curves are interpreted as mean horizontal motions. These motions yield again a depth dependence showing Doppler shifts toward the observer in deep layers and away from the observer in high layers for regions away from the disk center. Values from - 400 m s^–1 through + 500 m s^–1 are obtained.     

The derivation of the parameters of coronal mass motions from noise storm observations

Based on a total of 152 events, we studied the spectral parameters of the continuum component of isolated noise storms (Lantos, 1982) subdividing the data set according to flare-related and non-flare-related events. There were typical differences between both classes with regard to starting frequency and frequency extent. The flare-related events can mostly be attributed to loops rising into the corona with velocities as being known for transients of moderate velocity and erupting prominences. The non-flare-related events must evidently be ascribed to a broad variety of mass motions but on the average these mass motions have a lower outward drift velocity than those associated with the flare-related events.Deceased 22 March, 1992     

Stokes profiles inversion techniques

Inversion techniques of the radiative transfer equation for polarized light are presented as one of the best current procedures to infer the vector magnetic field, as well as other quantities governing the physical state of the atmospheric layers that photons are coming from. Several characteristics of the various available inversion procedures are pointed out. They are mostly based on the diagnostic contents of the spectral lines as well as on the main hypotheses assumed in these procedures. In particular, the role of gradients in the atmospheric quantities is emphasized as of paramount importance in any diagnostic analysis and, hence, in any interpretation of inversion results.     

The 6.4-keV fluorescent iron line from cluster cooling flows

The fate of the cooling gas in the central regions of rich clusters of galaxies is not well understood. In one plausible scenario clouds of atomic or molecular gas are formed. However the mass of the cold gas, inferred from measurements of low-energy X-ray absorption, is hardly consistent with the absence of powerful CO or 21-cm emission lines from the cooling flow region. Among the factors which may affect the detectability of the cold clouds are their optical depth, shape and covering fraction. Thus, alternative methods to determine the mass in cold clouds, which are less sensitive to these parameters, are important.   For the inner region of the cooling flow (e.g. within a radius of ∼50–100 kpc) the Thomson optical depth of the hot gas in a massive cooling flow can be as large as ∼ 0.01. Assuming that the cooling time in the inner region is few times shorter than the lifetime of the cluster, the Thomson depth of the accumulated cold gas can be accordingly higher (if most of the gas remains in the form of clouds). The illumination of the cold clouds by the X-ray emission of the hot gas should lead to the appearance of a 6.4-keV iron fluorescent line, with an equivalent width proportional to τ_T. The equivalent width only weakly depends on the detailed properties of the clouds, e.g. on the column density of individual clouds, as long as the column density is less than a few 10²³ cm⁻². Another effect also associated exclusively with the cold gas is a flux in the Compton shoulder of bright X-ray emission lines. It also scales linearly with the Thomson optical depth of the cold gas. With the new generation of X-ray telescopes, combining large effective area and high spectral resolution, the mass of the cold gas in cooling flows (and its distribution) can be measured.     

Optical spectroscopic classification and membership of young M dwarfs in star-forming regions

The spectral type is a key parameter in calibrating the temperature which is required to estimate the mass of young stars and brown dwarfs. We describe an approach developed to classify low-mass stars and brown dwarfs in the Trapezium Cluster using red optical spectra, which can be applied to other star-forming regions. The classification uses two methods for greater accuracy: the use of narrow-band spectral indices which rely on the variation of the strength of molecular lines with spectral type and a comparison with other previously classified young, low-mass objects in the Chamaeleon I star-forming region. We have investigated and compared many different molecular indices and have identified a small number of indices which work well for classifying M-type objects in nebular regions. The indices are calibrated for young, pre-main-sequence objects whose spectra are affected by their lower surface gravities compared with those on the main sequence. Spectral types obtained are essentially independent of both reddening and nebular emission lines.
Confirmation of candidate young stars and brown dwarfs as bona fide cluster members may be accomplished with moderate resolution spectra in the optical region by an analysis of the strength of the gravity-sensitive Na doublet. It has been established that this feature is much weaker in these very young objects than in field dwarfs. A sodium spectral index is used to estimate the surface gravity and to demonstrate quantitatively the difference between young (1–2 Myr) objects, and dwarf and giant field stars.     

Contamination of the 5394 Å spectral region by telluric lines

The spectral region in the vicinity of 5394 Å contains three prominent photospheric spectral lines, which can be used as a solar plasma diagnostic tool. The occurrence of telluric lines in this region is a potential source of systematic and random errors in these solar spectral lines. The goal of our investigation was to determine the telluric line contamination of this interesting spectral region. Several series of high-resolution solar spectra within an interval of about 4 Å around the 5394 Å wavelength were observed at different zenith distances of the Sun. Comparison of these spectra has permitted identification of telluric lines in this spectral interval. The observations were carried out with the horizontal solar spectrograph of the Heliophysical Observatory in Debrecen. Telluric feature blending was identified in the blue and red wings of the Fe I 5393.2 Å line, and in the local continuum of the Mn I 5394.7 Å line. The blue wing of the Fe I 5395.2 Å line is contaminated by a weak telluric feature too. The red continuum of this line has a more prominent telluric contamination. A dozen of water vapor telluric lines that determined the observed telluric features were identified in this spectral interval. The profiles of three telluric lines that have a significant influence on both the profiles of solar spectral lines and the level of local continuum were derived, and the variation of their parameters (equivalent width and central depth) with air mass were analyzed.     

The Transverse Velocity Field Of An Euv Solar Prominence

Previous studies have shown that the measured velocity field in solar prominences exhibits a slightly different behaviour depending on the observational conditions, on the investigation method, and possibly on the type of prominence. Observations of prominences seen at the limb reveal strong downward motions, whereas upflows are detected as Doppler shifts in filaments on the disk. In order to shed new light on this point, we have investigated the mass motions in a solar prominence by using a new method for calculating the geometric distortion between subsequent images. Flows perpendicular to the line of sight have been determined in several layers of the prominence-corona atmosphere, using extreme ultraviolet (EUV) lines formed at different temperature levels (T=104–106 K). We show that the motions mainly have a vertical direction, oriented both upwards and downwards. The velocity pattern can change rapidly during time intervals exceeding 10–15 min. We also find that the measured velocity field shows a similar pattern in all the studied lines.     

The distribution of microlensed light-curve derivatives: the relationship between stellar proper motions and transverse velocity

We present a method for computing the probability distribution of microlensed light-curve derivatives both in the case of a static lens with a transverse velocity, and in the case of microlensing that is produced through stellar proper motions. The distributions are closely related in form, and can be considered equivalent after appropriate scaling of the input transverse velocity. The comparison of the distributions in this manner provides a consistent way to consider the relative contribution to microlensing (both large and small fluctuations) of the two classes of motion, a problem that is otherwise an extremely expensive computational exercise. We find that the relative contribution of stellar proper motions to the microlensing rate is independent of the mass function assumed for the microlenses, but is a function of optical depth and shear. We find that stellar proper motions produce a higher overall microlensing rate than a transverse velocity of the same magnitude. This effect becomes more pronounced at higher optical depth. With the introduction of shear, the relative rates of microlensing become dependent on the direction of the transverse velocity. This may have important consequences in the case of quadruply lensed quasars such as Q2237+0305, where the alignment of the shear vector with the source trajectory varies between images.     

The dawn polar cap boundary at high altitude

Comparison of hot plasma data from ATS-6 and GEOS-1 when the satellites were near dawn L.T. conjunction reveals the presence of strong gradients separating plasmas differing by more than two orders of magnitude in keV particle fluxes. These gradients are observed at off-equatorial geomagnetic latitudes of 25–30° on field lines outside the synchronous orbit. They are associated with magnetic storms and are distinct from magnetopause crossings. Interpretation of these events in terms of a boundary between magnetospheric and polar-cap plasma leads to the following conclusions: (1) the polar cap/lobe region is essentially devoid of keV plasma at these times; (2) the field lines defining this boundary are significantly distorted from a dipolar to a more stretched form consistent with the presence of a storm-ring current, (3) smaller substorm-scale motions are superposed on the gross motion of the boundary with some evidence present for structure in the plasma spatial profile, and (4) magnetosheath-like plasma finds access to the inner magnetosphere at dawn L.T., much as it does near noon, along polar-cap boundary-layer field lines which close through the low latitude magnetospheric boundary layer.     

Spectral observations of spicules at two heights in the solar chromosphere

An observational program at the Sacramento Peak Observatory in 1965 provided high-dispersion spectra of the solar chromosphere in several spectral regions simultaneously. These regions included various combinations of the spectral lines Hα, Hβ and H?, the D₃-line of Hei, the infrared triplet of Oi, and the H- and K-lines and the infrared triplet of Caii. With the use of an image slicer the observations were made simultaneously at two heights in the solar chromosphere separated by several thousand kilometers. From these data we draw the following conclusions:

1. Emission of different lines arises in the same chromospheric features. The intensity ratio of lines of different elements varies significantly from spicule to spicule. For the H- and K-lines of ionized calcium, this ratio remains constant, independent of wavelength throughout the line, overall intensity, and height in the chromosphere. Two rare-earth lines in the wing of the H-line show no spicular structure at all.
2. The line-of-sight velocities of many features reverse as a function of time, although most spicules show velocities in only one direction. The simultaneous spectra at two heights show most spicules to have the same line-of-sight velocity at both. There may be an additional class of features, mostly rapidly moving, whose members have line-of-sight velocities that increase with height. These features comprise perhaps 10% of the total. Velocity changes occur simultaneously, to within 20 sec, at two heights separated by 1800 km, indicating velocities of propagation of hundreds of km/sec. The velocity field of individual features is often quite complicated; many spectral features are inclined to the direction of dispersion, implying that differential mass motions are present.
3. The existence of anomalously broad H and K profiles is real. Even with high dispersion and the best seeing, such profiles are not resolved into smaller features. The central reversal in K, H and Hα appears to remain unshifted when the wings are displaced in wavelength, indicating that the reversal is non-spicular.