Out of ∼ 500 Lithium-rich ROSAT counterparts, which were presumed to be low-mass pre-main sequence stars, 21 stars have been observed by HIPPARCOS. We study their parallaxes, proper motions, and photometric data. For 7 out of 10 Taurus and Lupus stars in our sample, proper motions and parallaxes are not inconsistent with membership to these associations, while most of the stars in Chamaeleon an...

We present a comprehensive analysis of X-ray variability of the late-type (dF7-dM) Pleiades stars, detected in all ROSAT-PSPC observations; X-ray variations on short (hours) and medium (months) time scales have been explored. We have grouped the stars in two samples: 89 observations of 42 distinct dF7-dK2 stars and 108 observations of 61 dK3-dM stars. The Kolmogorov-Smirnov test applied on all ...

We present our diagnosis of the role that massive stars play in the formation of lowand intermediate-mass stars in OB associations (the λ Ori region, the OriOB1, and LacOB1 associations). We find that the classical T Tauri stars and Herbig Ae/Be stars tend to line up between luminous O stars and brightrimmed or comet-shaped clouds; the closer to a cloud the progressively younger they are. Our p...

We present a classification of 267 new variable Btype stars discovered by Hipparcos. We have used two different classification schemes and they both result in only a few new β Cephei stars, a huge number of new slowly pulsating B stars, quite some supergiants with αCyg-type variations and variable CP stars, and further some new periodic Be stars and eclipsing binaries. Our results clearly point...

The J-type carbon (J)-stars constitute 10–15% of the observed carbon stars in both our Galaxy and the Large Magellanic Cloud (LMC). They are characterized by strong 13C absorption bands with low 12C/13C ratios along with other chemical signatures peculiar for typical carbon stars, e.g. a lack of s-process enhancement. Most of the J-stars are dimmer than the N-type carbon stars some of which, by...

We present detailed abundance results concerning CEMP (Carbon Enhanced Metal-Poor) stars and the various subclasses. Statistical arguments from radial velocity monitoring show that these stars are all binaries and have mass transferred from a former AGB. Therefore these stars are the direct witnesses of the nucleosynthesis production of the first low and intermediate mass stars. To put CEMP sta...

We present spectroscopic analyses of 17 faint blue stars in the globular cluster NGC 6752, using optical and UV spectrophotometric data and intermediate resolution optical spectra. Effective temperatures, surface gravities, and helium abundances of the stars are determined and compared to theoretical predictions. All stars are helium deficient by factors ranging from 3 to more than 100, indicat...

We present the result of a quantitative analysis of 12C/13C ratios in 26 J-type carbon stars. The 12C/13C ratios are determined from lines of the CN red system around 8000 Å, using the iso-intensity method and line-blanketed model atmospheres. The average of 12C/13C ratios in the 26 stars is 4.7 ± 2.8 (standard deviation). All the stars studied, except for two stars, have 12C/13C ratios smaller...

cancer is the second important reason of mortality in the world. in this regard melanoma was accounted as the most aggressive type of coetaneous cancer. among drug extracted from natural products from marine organisms have been focused investigations related to chemotherapeutic agents derived from echinoderms such as sea cucumbers and starfish.  in the present study, cytotoxic and apoptosis ind...

Most stars previously identified as Maunder minimum stars are old stars evolved off of the main sequence. Analysis of activity measurements from the California and Carnegie Planet Search program stars and Hipparcos parallaxes implies that the canonical age-chromospheric activity relation breaks down for stars older than ∼ 6 Gyr when activity is calculated from Mount Wilson S values. Stars only ...