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High-resolution speckle masking interferometry and radiative transfer modeling of the oxygen-rich AGB star AFGL 2290
We present the first diffraction-limited speckle masking observations ofthe oxygen-rich AGB star AFGL 2290. The speckleinterferograms were recorded with the Russian 6 m SAO telescope. At thewavelength 2.11 microns a resolution of 75 milli-arcsec (mas) wasobtained. The reconstructed diffraction-limited image reveals that thecircumstellar dust shell (CDS) of AFGL 2290 is atleast slightly non-spherical. The visibility function shows that thestellar contribution to the total 2.11 microns flux is less than ~ 40%,indicating a rather large optical depth of the circumstellar dust shell.The 2-dimensional Gaussian visibility fit yields a diameter ofAFGL 2290 at 2.11 microns of 43 masx51 mas, whichcorresponds to a diameter of 42 AUx50 AU for an adopted distance of 0.98kpc. Our new observational results provide additional constraints on theCDS of AFGL 2290, which supplement the informationfrom the spectral energy distribution (SED). To determine the structureand the properties of the CDS we have performed radiative transfercalculations for spherically symmetric dust shell models. The observedSED approximately at phase 0.2 can be well reproduced at all wavelengthsby a model with T_eff=2000 K, a dust temperature of 800 K at the innerboundary r1, an optical depth tau_ {V}=100 and a radius forthe single-sized grains of a_gr=0.1 microns . However, the 2.11 micronsvisibility of the model does not match the observation. Exploring theparameter space, we found that grain size is the key parameter inachieving a fit of the observed visibility while retaining the match ofthe SED, at least partially. Both the slope and the curvature of thevisibility strongly constrain the possible grain radii. On the otherhand, the SED at longer wavelengths, the silicate feature in particular,determines the dust mass loss rate and, thereby, restricts the possibleoptical depths of the model. With a larger grain size of 0.16 micronsand a higher tau_ {V}=150, the observed visibility can be reproducedpreserving the match of the SED at longer wavelengths. Nevertheless, themodel shows a deficiency of flux at short wavelengths, which isattributed to the model assumption of a spherically symmetric dustdistribution, whereas the actual structure of the CDS aroundAFGL 2290 is in fact non-spherical. Our studydemonstrates the possible limitations of dust shell models which areconstrained solely by the spectral energy distribution, and emphasizesthe importance of high spatial resolution observations for thedetermination of the structure and the properties of circumstellar dustshells around evolved stars. Based on data collected at the 6~mtelescope of the Special Astrophysical Observatory in Russia
| The Photometric Variability of Sun-like Stars: Observations and Results, 1984--1995
Using differential Stromgren b, y photometry, we monitored thebrightness variations of 41 program stars and their 73 comparison starsfrom 1984 through 1995. The predominantly main-sequence program starsspanned ranges of temperature and mean chromospheric activity centeredon solar values. About 40% of all the stars showed measurablevariability, typically at levels below 0.01 mag (~1%), on bothnight-to-night and year-to-year timescales. The variability correlatedwith mean chromospheric activity and advancing spectral type. We presentdifferential light curves and statistical descriptions of ourobservations.
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