The shapes of light curves of Mira-type variables

Using a sample of 454 mira light curves from the ASAS survey we study the shape of the light variations in this kind of variable stars. Opposite to earlier studies, we choose a general approach to identify any deviation from a sinusoidal light change. We find that about 30% of the studied light curves show a significant deviation from the sinusoidal reference shape. Among these stars two characteristic light curve shapes of comparable frequency could be identified. Some hint for a connection between atmospheric chemistry and light curve shape was found, but beside that no or only very weak relations between light curve shape and other stellar parameters seem to exist.Comment: 7 pages, 7 figures, accepted for publication in A

Astrophysics : Contributions of Indian Scientists .

A glimpse of astronomy and astrophysics from the earliest times to the end of the nineteenth century is given. This is followed by important contributions to astrophysics in the twentieth century with detailed accounts of the work of MN Saha, DS Kothari and a few others. This review ends with a brief account of the astrophysical activities in which a large number of scientists are presently involved

Light-induced disassembly of dusty bodies in inner protoplanetary discs: implications for the formation of planets

Laboratory experiments show that a solid-state greenhouse effect in combination with thermophoresis can efficiently erode a dust bed in a low-pressure gaseous environment. The surface of an illuminated, light absorbing dusty body is cooler than the dust below the surface (solidstate greenhouse effect). This temperature gradient leads to a directed momentum transfer between gas and dust particles and the dust particles are subject to a force towards the surface(thermophoresis). If the thermophoretic force is stronger than gravity and cohesion, dust particles are ejected. Applied to protoplanetary discs, dusty bodies smaller than several kilometres in size which are closer to a star than about 0.4 au are subject to a rapid and complete disassembly to submillimetre size dust aggregates by this process. While an inward-drifting dusty body is destroyed, the generated dust is not lost for the disc by sublimation or subsequent accretion on to the star but can be reprocessed by photophoresis or radiation pressure. Planetesimals cannot originate through aggregation of dust inside the erosion zone. If objects larger than several kilometres already exist, they prevail and further grow by collecting dust from disassembled smaller bodies. The pile-up of solids in a confined inner region of the disc, in general, boosts the formation of planets. Erosion is possible in even strongly gas-depleted inner regions as observed for TW Hya. Reprocessing of dust through light-induced erosion offers one possible explanation for growth of large cores of gas-poor giant planets in a gas-starved region as recently found around HD 149026b

Rotation and mass loss in early type stars

The effects of rotation on the rate of mass loss for O and B stars has been reviewed, and the causes for conflicting results discussed

Near-IR variability properties of a selected sample of AGB stars

We present the results of a near-infrared monitoring programme of a selected sample of stars, initially suspected to be Mira variables and OH/IR stars, covering more than a decade of observations. The objects monitored cover the typical range of IRAS colours shown by O-rich stars on the Asymptotic Giant Branch and show a surprisingly large diversity of variability properties. 16 objects are confirmed as large-amplitude variables. Periods between 360 and 1800 days and typical amplitudes from 1 to 2 magnitudes could be determined for nine of them. In three light curves we find a systematic decrease of the mean brightness, two light curves show pronounced asymmetry. One source, IRAS 07222-2005, shows infrared colours typical of Mira variables but pulsates with a much longer period (approx. 1200 days) than a normal Mira. Two objects are ither close to (IRAS 03293+6010) or probably in (IRAS 18299-1705) the post-AGB phase. In IRAS 16029-3041 we found a systematic increase of the H-K colour of approximately 1 magnitude, which we interpret as evidence of a recent episode of enhanced mass loss. IRAS 18576+0341, a heavily obscured Luminous Blue Variable was also monitored. The star showed a continued decrease of brightness over a period of 7 years (1995 - 2002).Comment: 9 pages + 3 appendix, 36 figures, photometry table, accepted in Astronomy & Astrophysic

On non-LTE H<SUB>2</SUB><SUP>+</SUP> as missing solar opacity

A careful examination has revealed that use of incomplete reaction processes and incorrect rates have led Krishna Swamy and Stecher to overestimate the H2+ opacity by a factor of 104 at &#955; = 2000 &#197;. H2+ is not a significant source of opacity in the solar atmosphere

Low Temperature Opacities

Previous computations of low temperature Rosseland and Planck mean opacities from Alexander & Ferguson (1994) are updated and expanded. The new computations include a more complete equation of state with more grain species and updated optical constants. Grains are now explicitly included in thermal equilibrium in the equation of state calculation, which allows for a much wider range of grain compositions to be accurately included than was previously the case. The inclusion of high temperature condensates such as Al$_2$O$_3$ and CaTiO$_3$ significantly affects the total opacity over a narrow range of temperatures before the appearance of the first silicate grains. The new opacity tables are tabulated for temperatures ranging from 30000 K to 500 K with gas densities from 10$^{-4}$ g cm$^{-3}$ to 10$^{-19}$ g cm$^{-3}$. Comparisons with previous Rosseland mean opacity calculations are discussed. At high temperatures, the agreement with OPAL and Opacity Project is quite good. Comparisons at lower temperatures are more divergent as a result of differences in molecular and grain physics included in different calculations. The computation of Planck mean opacities performed with the opacity sampling method are shown to require a very large number of opacity sampling wavelength points; previously published results obtained with fewer wavelength points are shown to be significantly in error. Methods for requesting or obtaining the new tables are provided.Comment: 39 pages with 12 figures. To be published in ApJ, April 200

The Spitzer Spectroscopic Survey of S-type Stars

S-type AGB stars are thought to be in the transitional phase between M-type and C-type AGB stars. Because of their peculiar chemical composition, one may expect a strong influence of the stellar C/O ratio on the molecular chemistry and the mineralogy of the circumstellar dust. In this paper, we present a large sample of 87 intrinsic galactic S-type AGB stars, observed at infrared wavelengths with the Spitzer Space Telescope, and supplemented with ground-based optical data. On the one hand, we derive the stellar parameters from the optical spectroscopy and photometry, using a grid of model atmospheres. On the other, we decompose the infrared spectra to quantify the flux-contributions from the different dust species. Finally, we compare the independently determined stellar parameters and dust properties. For the stars without significant dust emission, we detect a strict relation between the presence of SiS absorption in the Spitzer spectra and the C/O ratio of the stellar atmosphere. These absorption bands can thus be used as an additional diagnostic for the C/O ratio. For stars with significant dust emission, we define three groups, based on the relative contribution of certain dust species to the infrared flux. We find a strong link between group-membership and C/O ratio. We show that these groups can be explained by assuming that the dust-condensation can be cut short before silicates are produced, while the remaining free atoms and molecules can then form the observed magnesium sulfides or the carriers of the unidentified 13 and 20 micron features. Finally, we present the detection of emission features attributed to molecules and dust characteristic to C-type stars, such as molecular SiS, hydrocarbons and magnesium sulfide grains. We show that we often detect magnesium sulfides together with molecular SiS and we propose that it is formed by a reaction of SiS molecules with Mg.Comment: Accepted for publication in A&

Physical atmospheric parameters for late-type stars

For a gas mixture, the relation between total gas pressure, partial pressure of atomic hydrogen, mean molecular weight, and several other auxiliary quantities has been determined as a function of electron piessure and θ = 5040/T, for three hydrogen-to-helium abundance ratios. The effect of molecular hydrogen has been incorporated

Some comments on empirical FITS to stellar mass loss rates

Two empirical fits to stellar mass loss rates have been examined and found lacking in physical content