Flare model chromospheres and photospheres

Homogeneous plane-parallel model atmospheres for solar flares have been constructed to approximately simulate observations of flares. The wings of the Ca II lines have been used to derive flare upper photosphere models, which indicate temperature increases of ～100 K over the temperature distribution in the pre-existing facula at a height of 300 km above τ₅₀₀₀ = 1. In the case of flares covering sunspots the temperature rise seems to occur much higher in the atmosphere. We solve the transfer and statistical equilibrium equations for a three-level hydrogen atom and a five-level calcium atom in order to obtain the chromospheric flare models. The general properties of flares, including n _e, N ₂, linear thickness, and Lyman continuum intensity are approximately reproduced. We find that with increasing flare importance the height of the upper chromosphere and transition region occur lower in the solar atmosphere, accounting for the factor of 60–600 increase in pressure in these regions relative to the quiet Sun. The Ca II line profiles agree with observations only by assuming a macro-velocity distribution that increases with height. Also the chromospheric parts of flares appear to be highly inhomogeneous. We show that shock and particle heated flare models do not agree with the observations and propose a thermal response model for flares. In particular, it appears that heating in the photosphere is an essential aspect of flares.     

Models of late-type stellar photospheres

Summary In this review we discuss recent work and progress in the modelling of photospheres of stars of spectral types F and later. Special emphasis is laid on advances as regards the consideration of atomic and molecular blanketing, non-LTE and convection and other dynamic processes. In a special chapter we discuss the possibilities of semi-empirical modelling of late-type photospheres. In the conclusions we find that much important work remains in this field, but that a considerable part of this work may in fact be carried out in a near future.     

On nonadiabatic waves in the photospheres of cool stars

The coupled set of equations of hydrodynamics and radiative transfer is derived for small disturbances in a plane, grey atmosphere. Only radiative transfer is taken into account in the energy equation; dynamical effects of radiation are ignored. A mean stationary radiative flux through the photosphere is taken into account. The radiative transfer equation is used by assuming the Eddington approximation, moreover, an exponential height profile of the temperature and an analytical opacity formula are supposed. For this model we obtained an asymptotic solution for plane nonadiabatic acoustic waves and radiation waves. The approach provides a detailed discussion of the interaction of nonadiabatic p‐modes and radiation waves in a realistic model of the photosphere of a solar‐like star.     

The structure of blue supergiant winds and the accretion in supergiant high-mass X-ray binaries

We have developed a stellar wind model for OB supergiants to investigate the effects of accretion from a clumpy wind on the luminosity and variability properties of high-mass X-ray binaries. Assuming that the clumps are confined by ram pressure of the ambient gas and exploring different distributions for their mass and radii, we computed the expected X-ray light curves in the framework of the Bondi–Hoyle accretion theory, modified to take into account the presence of clumps. The resulting variability properties are found to depend not only on the assumed orbital parameters but also on the wind characteristics. We have then applied this model to reproduce the X-ray light curves of three representative high-mass X-ray binaries: two persistent supergiant systems (Vela X−1 and 4U 1700−377) and the supergiant fast X-ray transient IGR J11215−5952. The model can reproduce the observed light curves well, but requiring in all cases an overall mass loss from the supergiant about a factor of 3–10 smaller than the values inferred from ultraviolet lines studies that assume a homogeneous wind.     

The supergiant HD 33579

The brightest star in either Magellanic cloud has been analyzed by the method of model atmospheres using observational data secured at Cerro Tololo. The results from abundance studies are in general accord with the conclusions by Przybylski. We find solar abundances for Fe, Sc, Mg, Si and, probably, Ca. The V, Cr, Si, Ti data suggest lower abundances but the evidence is not conclusive.     

Near-ir spectroscopy and the photospheres of embedded ysos

We present a study of weak near-IR absorption lines in 44 low luminosity YSOs. Using a spectral resolution of 1000 most Class II sources show CO overtone absorption bands of varying strength in the K window, whether they have optical counterparts or not. Class I sources tend to show featureless 2µm continua even though a S/N > 100 was achieved. High resolution (R=17000) echelle spectra were also obtained for a sub-sample of YSOs. Most show an unresolved¹²CO(2-0) bandhead, which when combined with inferred CO excitation temperatures and optical depths clearly points to a photospheric rather than a disk origin for the bands. They also show that embedded Class IIs are not rapidly rotating.We find an excellent correlation between increasing near-IR colour excess and decreasing band strength and interpret this in a straightforward way as due to veiling of the stellar photosphere by circumstellar dust emission at 1000-1200 K, probably from a disk. A veiling correction was applied and intrinsic indices obtained for many YSOs. The results provide confirmation that Class II sources are equivalent to T Tauri stars.     

On the size distribution of turbulent elements in the earth's atmosphere

Solar scintillation measurements made by Wessely and Mitchell are used to derive the size distribution of turbulent elements in the Earth's atmosphere. Assuming a stationary state of the Earth's atmosphere, the production rate of the turbulent elements is calculated.     

Study of the IR-excess supergiant HD 331319

We report the discovery of brightness variability in the IR-excess early F supergiant HD 331319, a candidate post-AGB star. Over three years of systematic U BV observations, the star showed low-amplitude (up to in V) quasi-periodic brightness variations on a time scale of ∼45 days. A preliminary analysis of our photometry indicates that HD 331319 and other typical post-AGBF supergiants have a similar pattern of variability. A study of the extinction toward HD 331319 leads us to conclude that the fraction of the circumstellar extinction is small for this star. We present low-resolution spectroscopy for HD 331319 and discuss the spectral classification of post-AGB F supergiants using HD 331319, HD 161796, HD 187885, and HD 56126 as examples.     

Spectroscopic study of the supergiant star 6 Cassiopeiae

Twenty-five coudé spectrograms (22 with dispersion 12 Å mm^–1 and three 7 Å mm^–1) of 6 Cassiopeiae (A3 Ia) have been studied. The observations were made at the Haute Provence Observatory. The results of the analysis suggest a correlation between the variations of the equivalent widths, the microturbulence and the radial velocity. The radial velocity and turbulent velocity present a rapid variation with time, even in intervals as short as about an hour. The hydrogen lines are slightly asymmetric but the strongest Feii lines are clearly asymmetric. We found that the amount of asymmetry of the strongest Feii lines (I>6) correlates with the loggf value, with the estimated laboratory intensityI, and with the equivalent widthW .The observations have been made at the Astronomical Observatory of Haute Provence (CNRS). This work has been supported by TUBITAK (Scientific and Technical Research Council of Turkey), and partially by CNR (Consiglio Nazionale delle Ricerche) of Italy.     

Radiative energy transfer in extended photospheres of the components of close binary systems

The aim of the present paper will be to set up, and solve, the equations governing transfer of radiation in semi-transparent envelopes of the stars; and, in order to do so, to employ a system of curvilinear (non-orthogonal) three-dimensional coordinates in which the radial coordinate has been identified with equipotential surfaces. Such coordinates are particularly suitable to a treatment of the problems arising in close binary systems, which render the outcome more than any other amenable to observable tests, but which has so far received but very scant attention.The introductory section of this paper will contain a statement of the problem; and its mathematical formulation in terms of Clairaut coordinates (cf. Kopal, 1980, 1989, Chapter V) will be outlined in Section 2; their methods in Section 3. Section 4 will then contain an application to the problem of distribution of surface brightness (limb-darkening) over the apparent discs of distorted components of close binary systems; while in Section 5 we shall do the same for radiative flux of distorted stars as a function of the phase (gravity darkening).The concluding Section 6 will then contain an outline of additional problems arising in this connection, to which we shall turn in successive parts of this series.     

The Lithium‐rich supergiant HD172365

The F‐type supergiant HD172365 was discovered by Luck (1982) to be a high Li abundance star. We find that the lithium abundance in this star is log A(Li) = 2.9 from a careful spectroscopic study. This value is larger than the abundance derived by Luck. We discuss the properties of this supergiant and tentatively conclude that HD172365 may, in fact, be a post‐blue straggler star with a mass ∼2 times greater than the “turn‐off point” mass for the open cluster IC 4756, of which HD172365 is a member. The star is formed by a merger of a close binary system. This conclusion is supported by, for example, its large projected rotational velocity, solar carbon abundance and high lithium content.     

Peculiarities of the abundance of chemical elements in the atmosphere of PMMR23-red supergiant in the Small Magellanic Cloud due to interstellar gas accretion

The chemical composition of the PMMR23 red supergiant located in the Small Magellanic Cloud (SMC) is analyzed. The abundance of 35 chemical elements and the upper limits of abundance for Tl and U are found. The relative abundance of heavy elements is higher by 0.6–1.0 dex with respect to iron peak elements. The spectra of several SMC red supergiants PMMR27, PMMR28, and PMMR144—located in the region where the velocities of stars and interstellar gas are quite high— show the emission components in the wings of the hydrogen line. This emission is not detected for PMMR23. A possibility of interstellar gas accretion on the atmospheres of PMMR23 and other supergiants in Magellanic Clouds is discussed. The analysis is carried out using spectra measured at ESO 3.6 m telescope with the spectral resolving power R = 30000.     

Spectral variability of the peculiar A-type supergiant 3Pup

Optical spectra taken in 1997–2008 are used to analyze the spectral peculiarities and velocity field in the atmosphere of the peculiar supergiant 3 Pup. The profiles of strong Fe II lines and of the lines of other iron-group ions have a specific shape: the wings are raised by emissions, whereas the core is sharpened by a depression. The latter feature becomes more pronounced with the increasing line strength, and the increasing wavelength. Line profiles are variable: the magnitude and sign of the absorption asymmetry, and the blue-to-red emission intensity ratios vary from one spectrum to another. The temporal V_r variations are minimal for the forbidden emissions and sharp shell cores of the absorption features of FeII(42), and other strong lines of iron-group ions. The average velocity for the above lines can be adopted as the systemic velocity: V_sys = 28.5 ± 0.5 km/s. The weakest photospheric absorptions and photospheric MgII, Si II absorptions exhibit well-defined day-to-day velocity variations of up to 7 km/s. Quantitative spectral classification yields the spectral type of A2.7±0.3 Ib. The equivalent widths and profiles of Hδ and Hγ, and the equivalent width of the OI 7774 Å triplet yield an absolute magnitude estimate of Mv=?5.5^m ± 0.3^m, implying the heliocentric distance of 0.7 kpc.     

On the possibility of disk-fed formation in supergiant high-mass X-ray binaries

We consider the existence of a neutron star magnetic field by the detected cyclotron lines. We collected data on nine sources of high-mass X-ray binaries with supergiant companions as a test case for our model, to demonstrate their distribution and evolution. The wind velocity, spin period and magnetic field strength are studied under different mass loss rates. In our model, correlations between mass-loss rate and wind velocity are found and can be tested in further observations. We examine the parameter space where wind accretion is allowed, avoiding the barrier of rotating magnetic fields, with robust data on the magnetic field of neutron stars. Our model shows that most sources(six of nine systems) can be fed by the wind with relatively slow velocity, and this result is consistent with previous predictions. In a few sources,our model cannot fit the standard wind accretion scenario. In these peculiar cases, other scenarios(disk formation, partial Roche lobe overflow) should be considered. This would provide information about the evolutionary tracks of various types of binaries, and thus exhibit a clear dichotomy behavior in wind-fed X-ray binary systems.