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Sakurai's object (V4334 Sgr) in 1998: The R CrB phase has set in
Photometric BVRI observations of V4334 Sgr during 43 nights in 1998 arepresented. They show that the star has entered the R CrB phase. Threedeep minima with amplitudes between 1 mag and 7 mag in V were observedin 1998. The change in B-V, V-R, and V-I reflected mainly the reddeningin the circumstellar envelope and, possibly, further cooling of thestar. The previously observed light variations are found to be describedby a systematic increase of the only pulsation period from 9 to 40 daysin late 1997 and, possibly, to 60 days in late 1998. By the C_2 and CNabsorption bands and the NaI lines, in 1998 the star is classified asC-J4 C25 j5 in Keenan's revised system. The continuum energydistribution in the wavelength range 4500-7500 A in late April 1998 isfitted by the spectral energy distribution of a G5 Ib supergiant. Theextinction law in the circumstellar dust envelope of V4334 Sgr is shownto essentially match the interstellar extinction law at opticalwavelengths. The luminosity of the star in 1996-1997 was constant if itseffective temperature corresponded to the scale of normal supergiantsand if the interstellar reddening is assumed to be E(B-V) = 0.8.
| The distance of Sakurai's Object
Sakurai's Object seems to be undergoing a very late Helium-flash, makingit a unique observational event with very important implications for ourunderstanding of late stellar evolution. We present the first individualdistance determination: using the extinction method we have derived adistance of 1.1kpc for the planetary nebula surrounding the star. Thisaccurate distance will be a crucial step forward in our efforts tounderstand the nature and the evolution of Sakurai's Object.
| Supergiants and the Galactic metallicity gradient. II - Spectroscopic abundances for 64 distant F- to M-type supergiants
The metallicity gradient in the Galactic disk from in situ stars withvisual magnitude ranging from 6 to 10 is analyzed. Atmosphericparameters and detailed chemical abundances for 64 Population Isupergiants of spectral types F through M and luminosity classes Iathrough II have been determined. The derived Fe/H ratios ranging from-0.5 to + 0.7 show a mean value of +0.13 with an estimated uncertaintyof + or - 0.2. A subset of 25 supergiants fainter than 7th magnitudelying in the direction of the Galactic center shows a Fe/H mean of +0.18+ or - 0.04, while a similar sample of 15 faint supergiants lying in thedirection of the Galactic anticenter shows a lower Fe/H mean of +0.07 +or - 0.06. For a sample of bright supergiants analyzed by Luck andLambert (1985), the mean abundance pattern for all 64 stars showed thefollowing: deficient C and O along with enhancement of N, indicatingmixing of CNO-cycled material to the stellar surfaces; an apparent Srenhancement attributed to departures from LTE; and an essentially solarpattern of other chemical elements.
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Osservazione e dati astrometrici
| Costellazione: | Sagittario |
| Ascensione retta: | 17h53m17.12s |
| Declinazione: | -17°29'01.1" |
| Magnitudine apparente: | 8.267 |
| Distanza: | 10000000 parsec |
| Moto proprio RA: | -0.1 |
| Moto proprio Dec: | -1 |
| B-T magnitude: | 9.859 |
| V-T magnitude: | 8.399 |
Cataloghi e designazioni:
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