Coronae of cool stars

With the hypothesis generally adopted as responsible for the heating of the solar corona an attempt was made to predict the coronal temperature for a few cool stars.Stars with effective temperatures between 2500K and 3700K, with logg-values of 3 or 4, appear to have coronal temperatures only 600 to 25000K higher thanT _eff.In order to determine the possibility of coronal heating caused by sound waves we made a comparison between the frequency of the sound waves and the cut-off frequencies.     

Analysis of the hyades giant stars ε and γ Tau

Two hyades giant stars, and Tau, have been studied from an analysis of strong line profiles. We get for Tau,T _e =4750K and logg=2.7, and for Tau,T _e =4700K and logg=2.8. Hydrogen-to-metal ratio for the two stars is nearly the same as that of solar value.     

The winds of WC10 central stars of planetary nebulae

We present the results of an analysis of the winds of two WC10 central stars of planetary nebulae, CPD-56°8032 and He 2-113. These two stars have remarkably similar spectra, although the former exhibits somewhat broader emission line widths. High resolution spectra (up to R=50 000) were obtained in May 1993 for both objects at the 3.9 m AAT, using the UCL Echelle Spectrograph. The fluxes in individual Cii auto-ionising multiplet components, many of which were blended, were derived. Lines originating from auto-ionising resonance states situated in the C²⁺ continuum are very sensitive to the electron temperature, since the population of the these levels is close to LTE. The measured widths and profile shapes of these transitions are presented and are in excellent agreement with those predicted on the basis of their calculated auto-ionising lifetimes. The wind electron temperature is derived for both stars from the ratio of the fluxes in four such transitions (T _e =18 500 K±1 500 K for CPD-56° 8032 andT _e =13 600 K±800 K for He 2-113). Optical depth effects are investigated using normal recombination lines to obtain an independent wind electron temperature estimate in excellent agreement with the dielectronic line analysis.     

Mass loss from coronae and its effect upon stellar rotation

The acoustic energy-generation rate from the convective zone was calculated for various models. Results show that chromosphere and corona can be expected around stars with temperature lower than 8000K at the main sequence, and lower than 6500K at logg=2.When a star is rotating rapidly, mass loss from its corona is large, and can be an effective mechanism of braking the stellar rotation. If this mechanism is effective, we can explain the slow rotation of stars later than F2 to be the result of the loss of the angular momentum through a stellar wind that is effective in their main sequence phase. Stars with massM>1.5M lose mass through a stellar wind during their contraction phase. The mass-loss rate is larger than the solar value because of the larger energy input into the chromosphere-corona system and because of the smaller gravitational potential at the surface. T Tauri stars may be the observational counterparts for such stars. As the duration of contraction phase is very short (less than 10⁷ years), the braking mechanism works only in the presence of a strong magnetic field (Ap) or in the presence of a companion (Am).Presented at the Trieste Colloquium on Mass Loss from Stars, September 12–16, 1968.     

The radiation field in photospheric models for extreme supergiants

On the basis of assumed photospheric temperature models for 36 extreme supergiants (logg _e -values of 1, 0.5 and 0;T _e ranging from approx. 3700–33 000 K) photospheric fluxesS(τ_λ) were computed for 36 wavelengths ranging from 100 Å to 60 000 Å. The hot models are in perfect radiative equilibrium; the cooler show deviations up to 10%, sometimes even larger. Only in the relatively deep parts of the photospheres (τ₅?1) the radiation field at each geometrical level can be characterized by one unique radiation temperature; for smaller τ₅-values there are large deviations from local thermal equilibrium. The influence of deviations from local thermodynamical equilibrium on the fluxes is briefly examined, and appears small but for the shortest wavelengths. In tables and graphs we give for these models πF(γ)-values, integrated fluxes, effective temperatures, coloursU, B andV, and the Balmer discontinuityD.     

A model atmosphere analysis of the binary 112 Herculis

Double-lined spectra have been obtained at 2 Å/mm for the Mn star 112 Herculis AB. The components are shown to be a B6.5V star and A2V star. Hydrogen-line-blanketed models are used to calculate atmospheric abundances. The best models appear to beT _e=13500 K, logg=4.0, _T =2.5 km/sec for the primary; andT _e=10000K, logg=4.2, _T =6 km/sec for the secondary. Both stars appear to have the same abundances. Helium, carbon, magnesium, aluminum, silicon, calcium, vanadium, and nickel appear to be considerably underabundant; phosphorus, scandium, titanium, manganese, iron, gallium, and mercury considerably overabundant; and chromium, strontium, and zirconium normal in abundance.     

Large-Scale Structure of the Atmosphere of the Quiet Sun,Coronal Holes,and Plages as Deduced by Tomography Study

We present here the results of emission tomography studies, based on a new differential deconvolution method (DDM) of Laplace transform inversion, which we use for reconstruction of the coronal emission measure distributions in the quiet Sun, coronal holes and plage areas. Two methods are explored. The first method is based on the deconvolution of radioemission brightness spectra in a wide wavelength range (1 mm–100 cm) for temperature profile reconstructions from the corona to the deeper chromosphere. The second method uses radio brightness measurements in the cm–dm range to give a coronal column emission measure (EM).Our results are based on RATAN-600 observations in the range 2.0–32 cm supplemented by the data of other observatories during the period near minimum solar activity. This study gives results that agree with known estimates of the coronal EM values, but reveals the absence of any measurable quantities of EM in the transition temperature region 3 × 10⁴ –10⁵ K for all studied large-scale structures. The chromospheric temperature structure (T _e = 20,000–5800 K) is quite similar for all objects with extremely low-temperature gradients at deep layers.Some refraction effects were detected in the decimeter range for all Types of large-scale structures, which suggests the presence of dense and compact loops (up to N _e =(1–3)× 10⁹ cm^-3 number density) for the quiet-Sun coronal regions with temperature T _e > 5× 10^-5 K.     

The paschen discontinuity in B stars

Measurements of the Paschen discontinuity in stars withT _eff10⁴K leads to the conclusion that theD _P/D_B ratio increases with temperature faster than expected. The increase ofD _P/D_B with (logg)^–1 is also steep.

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Résumé Les mesures de la discontinuité de Paschen dans les étoiles deT _eff10⁴K nous montrent que le rapportD _P/D_B augmente avec la température plus rapidement que prevu. L'augment deD _P/D_B avec (logg)^–1 est aussi remarquable.

     

Trapping of test particles in static and spherical space-time geometries

Estimates of the pulsation massesM _Q and of massesM _g=gR²/G (g is the surface gravity;R, the mean stellar radius; andG, the gravitational constant) of 89 Delta Scuti-variables have been obtained. An intercomparison of three kinds of masses —M _e, M_g, andM _Q — is performed; the evolutionary massesM _e of the same variables were estimated in a previous paper (Tsvetkov, 1986). It is pointed out that within the limits of the accuracy of determination, the three kinds of masses agree for the majority of the stars under study. For several varaibles, however, there is a considerable discrepancy between the estimates of the pulsation massesM _Q and the estimates of the massesM _e andM _g. Arguments are represented, which may alleviate or even remove this mass discrepancy for some of these stars. It is shown that the results from the comparison between the three kinds of masses of the considered Delta Scuti-stars are not very sensitive to the choice of the system of evolutionary tracks and the adopted chemical composition as well as to the evolutionary phases of the variables (core or shell hydrogen burning). The variations of the chemical composition in most of the stars under study are probably not very large.     

Non-LTE effects in Al I lines

We present the theoretical analysis of the Al I line formation in the spectra of late-type stars ignoring the assumption of local thermodynamic equilibrium (LTE). The calculations were based on the 39-level aluminum atom model for one-dimensional hydrostatic stellar atmosphere models with the parameters: T _eff from 4000 to 9000 K, log g = 0.0–4.5, and metallicity [A] = 0.0;–1.0;–2.0;–3.0;–4.0. The aluminum atom model and the method of calculations were tested by the study of line profiles in the solar spectrum. We refined the oscillator strengths and Van-der-Vaals broadening constants C ₆ of the investigated transitions. We conclude that the Al I atom is in the overionization state: the 3p level is underpopulated in the line formation region. This leads to the line weakening, as compared with the LTE results. The overionization effect becomes more pronounced with increasing temperature and decreasing metallicity. We show that the use of various atomic data (ionization cross-sections) for the low levels of Al I does not change the behavior of non-LTE deviations, whereas the value of these deviations varies essentially. For nine selected Al I lines we calculated the grids of theoretical non-LTE corrections (ΔX _NLTE = logɛ _NLTE − log ɛ _LTE) to the Al abundances determinedwith the LTE assumption. The non-LTE corrections are positive and significant for the stars with temperatures T _eff > 6000 K. These corrections weakly depend on log g, and increase with declining stellar metallicity.     

Particle acceleration in impulsive solar flares

Second-step acceleration of nonrelativistic protons and ions in impulsive solar flares is discussed extending our earlier calculations for relativistic electrons. We derive the relevant particle transport equation, discussing in detail the influence of the particle's effective charge and mass number on the various momentum gain (stochastic acceleration, diffusive shock wave acceleration) and loss (Coulomb interactions, particle escape) processes. Analytical solutions for the ion-momentum spectra in the hard-sphere approximation are given. The inclusion of Coulomb losses modify the particle spectra significantly at kinetic energies smaller than E _B = 0.64( _e /5.0) MeV nucl.^–1 from the well-known Bessel function variation in long-duration flares. For equal injection conditions this modification explains the observed much smaller ion fluxes from impulsive flares at high energies as compared to long-duration flares. We also calculate the ³He/⁴He-isotope variation as a function of momentum in impulsive flares in the hard-sphere approximation and find significant variations near E _m = 0.38(T _e /2 × 10⁶ K) MeV nucl.^–1, where T _e is the electron temperature of the coronal medium.     

Determination of Distances to Galaxies of the NGC 1023 Group. The Hubble Constant

On the basis of photographs from the 6 m Large Azimuthal Telescope and the Hubble Space Telescope, VRI photometry of stars in 11 galaxies in the NGC 1023 group has been carried out. The distances to these galaxies were determined by the method of brightest stars. The distances to NGC 925 and NGC 1023 were determined from the position of the top of the red giant branch (the TRGB method). From the calculated average and root-mean-square distances to the NGC 1023 group (10.3 ± 2.2 Mpc and 9.7 ± 0.5 Mpc) the Hubble constant in this direction was determined: H _0R = 75 ± 8 km·sec^-1·Mpc^-1 and H _0M = 81 ± 5 km ·sec^-1·Mpc^-1.     

Study of the June 30, 1973 trans-polar coronal hole

Radially and tangentially polarized pictures of the solar corona obtained near 4500 Å during the 30 June, 1973 solar total eclipse have been used to derive a model of a trans-polar coronal hole. The hole is identified by using OSO-7 EUV spectroheliograms. The line of sight coincides with the privileged plan of the hole over the N-polar region. A new method of absolute calibration is used. The Saito (1970) method is applied to determine the electron densities. Extrapolated values of densities down to the surface are lower than have ever been observed although derived hydrostatic temperatures are certainly not: N _e × 10⁷ cm^–3 and T = 2 × 10⁶ K. The morphological peculiarities of polar regions are considered.On leave from Institut d'Astrophysique, CNRS, Paris as NRC Research Associate.     

Coronae around helium stars and X-ray sources

Calculations of the acoustic energy generation for helium-rich composition show that the maximum acoustic energy generation is located around 12000 K at logg=4 and 15000 K at logg=6. The author's suggestion in his last paper that a helium star Sgr may have a corona seems to be justified. X-ray from a corona around a helium star is strongest when the physical parameters of the star are logg6 andT _e15000 K. But the total energy flux is too small to account for the observed X-ray sources.Presented at the Trieste Colloquium on Mass Loss from Stars, September 12–16, 1968.     

Radii of some Ap,Am and A stars

Based on the observed energy curves of nine Ap stars, three Am stars, four normal A stars and one F0 V magnetic star, their radii have been estimated.Thence, the bolometric magnitudesM _bo1 have been obtained and a plot between logT _e andM _bo1 of these stars shows that a majority of Ap and Am stars are a little above the zero-age Main-Sequence, suggesting that they are slightly more evolved as compared to the normal A stars.The bolometric corrections derived from the aboveM _bo1 are much closer to those computed by Mihalas than to the ones given by Davis and Webb, the latter being about O ^m 1 more negative than the former.     

On the coronal lines in the chromosphere at the 1970 eclipse

Spectrographic observations of the flash spectrum were made by the Kwasan Observatory at the total solar eclipse on 7 March, 1970. The integrated intensities of Fexiv 5303, Fex 6374, and the continuum were measured on the spectrograms as a function of height above the Sun's limb. It was found that a large amount of emission in the coronal lines originates in the interspicular regions of the chromosphere. Analysis of the data yielded that the interspicular regions consist of coronal material of T _e = 1.6 × 10⁶–1.2 × 10⁶ and log N _e = 8.5–9.5, and that a decrease in T _e and an increase in N _e occur with decreasing height.     

The relationship between UV and X-ray active region structures

Yohkoh and the Coronal Diagnostic Spectrometer (CDS) on the Solar and Heliospheric Observatory (SOHO) jointly observed two brightenings in active region NOAA 7981 on 6 August 1996. Combining the UV data from CDS with information from the high time resolution coronal images obtained with the Soft X-ray Telescope (SXT) on Yohkoh, provides us with important information on the relationship between the transition region and corona. Our observations show that cool plasma (T_e = 2.2 x 10^-5 K) can lie at the same altitude as the hot coronal plasma (T_e = 1–4 x 10⁶ K). The lower temperature structure is not formed from the cooling of the hotter coronal loop. We are also able to observe a low temperature cut-off of T_e = 1–4 x 10⁶ K for a loop which repeatedly brightened over the period of approximately one day.     

Simulation of energy distributions in optical and IR spectra of late-type M4–M6 dwarfs

Spectral types (M4–M6), effective temperatures T _ef (2700–2900 K), and free fall accelerations logg (4.0–4.5) are determined for five M dwarfs using their energy distributions in the spectral range λλ = 680…840 nm. Stellar spectra with resolutions R = 4000 were obtained using the IMACS spectrograph mounted on the ESO Walter Baade 6.5-m telescope. The spectral types are derived from spectral indices and the effective temperatures of the stars are estimated based on their spectral types. Values of T _ef and logg are also derived from the comparison between the observed and theoretical energy distributions, calculated both for dust-free, standard NextGen model atmospheres of red dwarfs, and for semiempirical models considering the presence of dust in stellar atmospheres according to the technique developed by Pavlenko et al. We determine dust parameters for stellar atmospheres of these stars, and establish that it is necessary to account for the decrease in concentration of TiO molecules due to their condensation on dust grains, when T _ef < 3000 K. We conclude that the radiation scattering by dust grains does not have an appreciable effect on energy distributions in the spectra of the considered stars.     

Mgix and Sixi line ratios in the sun

New theoretical emission line ratios for the Be-sequence ions Mgix and Sixi are presented. A comparison with observational data for two solar flares and an active region loop obtained with the Harvard EUV spectrometer and NRL XUV spectroheliograph aboard Skylab reveals that these plasmas are in ionization equilibrium at coronal temperatures. Unfortunately most of the density diagnostics are not particularly useful under solar plasma conditions, as they vary only slightly over the electron density range 10⁸–10¹³cm^–3. However the Sixi ratioI(³ P ^e ₂ -³ P ^o ₂)/I(³ P ^o ₁ –¹ S ^e ₀) is density sensitive in the range 10⁸ to 10¹⁰cm^–3, which is representative of electron densities found in solar active regions or small flares.     

Amplitude ratios of several groups of variable stars

The observed positions of classical cepheids, RR Lyrae stars, Scuti stars and dwarf cepheids in the logg-logT _e plane form a continuous sequence, thereby defining the location of maximum instability. The amplitude ratio (the ratio of radial velocity amplitude to light amplitude) is small for variables at the upper end of the instability strip and increases almost linearly towards the lower end of the strip. The theory of radial pulsation predicts the trend of this correlation.