Search results for "-type"

A comparison between the X-ray variable Sun and solar-like main sequence stars

2003

We analyze the time variations of the solar X-ray luminosity observed with Yohkoh/SXT with the aim to compare the X-ray variability of the Sun with that of the other solar-like main sequence stars as function of the relevant time scales. Since the observational set-up and strategies used to observe the Sun dier from those used for the other stars, we have explored the solar X-ray variability properties starting from the available solar data, trying to reproduce the observational procedures adopted for the stars. We have quantified how the solar variability amplitude increases with the explored time scales and found that solar-cycle variability can contribute at most up to 60% to the spread …

First results of an Hα based search of classical Be stars in the Perseus Arm and beyond

2013

We investigate a region of the Galactic plane, between 120 <= l <= 140 and -1 <= b <= +4, and uncover a population of moderately reddened (E(B-V) \sim 1) classical Be stars within and beyond the Perseus and Outer Arms. 370 candidate emission line stars (13 <= r <= 16) selected from the INT Photometric H-alpha Survey of the Northern Galactic plane (IPHAS) have been followed up spectroscopically. A subset of these, 67 stars with properties consistent with those of classical Be stars, have been observed at sufficient spectral resolution (Delta_lambda \sim 2 - 4 Angstrom) at blue wavelengths to narrow down their spectral types. We determine these to a precision estimated to be…

On X-ray variability in ROSAT-PSPC observations of F7-K2 stars

2002

We have analyzed the X-ray variability of dF7-dK2 stars in the solar neighborhood detected with the pointed ROSAT-PSPC observations. Our data base is the sample of all stars listed in the CNS3 catalog (Gliese & Jahrei β 1991) having a $B-V$ color between 0.5 and 0.9; it includes 70 pointed observations of 40 distinct stars or multiple systems. We have applied the unbinned Kolmogorov-Smirnov test on all X-ray photon time series of our sample: only 10 observations relative to 8 distinct stars are variable at a confidence level greater than 99% and 4 of them belong to multiple systems. For the subsample of 9 stars observed both at the beginning and at the end of the mission, we can study the v…

The age evolution of Ca II emission in late type stars

2008

It has been widely demonstrated that chromospheric emission is dependent on rotation rate (Hartmann and Noyes, 1987, Catalano, 1990, for recent reviews). Since the rotation decays with time, the chromospheric activity of single main sequence stars is also dependent on age. Catalano and Marilli (1983) suggested that the chromospheric emission of solar mass stars decays exponentially with the square root of the time, as recently confirmed by Barry (1988). Magnetic activity depends also on the convection parameters, i.e. on the mass. Since the evolution of surface rotation is faster for lower mass stars (Catalano et al., 1988), the internal rotation distribution, and therefore the dynamo actio…

Short-term variations in Be stars observed by the CoRoT and Kepler space missions

2010

AbstractThe corot and kepler space missions are collecting very high-precision long-duration photometric data of many Be stars, allowing us to better understand the origin of their short-term variability and the link between these variations and the Be phenomenon. In this paper, we present a brief summary of the results obtained in the analysis of several Be stars observed with corot in terms of pulsations. In addition, we show that variations of the Be star HD 175869 can be explained as two active regions separated by 150 degrees or as unstable pulsating modes in a star with an extensive mixing in radiative layers corresponding to a core overshooting of 0.35Hp. A preliminary study of the p…

The X-ray Luminosity Distributions of the high-metallicity open cluster Blanco 1

2003

We present X-ray Luminosity Distributions (XLDs) of late-type members (dF, dG, dK, dM) of the Blanco 1 cluster, based on ROSAT-HRI data and new astrometric-photometric membership obtained from the GSC-II project. For the first time we present the XLD of dM stars of this cluster. The high metallicity of Blanco 1 allows us to investigate the role of chemical composition on the coronal emission of late-type stars. Comparison between X-ray Luminosity Distributions of Blanco 1 and Pleiades, NGC 2516 and α  Per suggests a possible metallicity effect in dM stars.

On the Age, Spectral Type, Orbit, and Comparison to Evolutionary Models of AB Dor C

2005

High-resolution spectroscopy of two carbon stars with long-term obscuration events

2012

In this paper, we provide an analysis of the high-resolution optical spectra of two carbon variables, V1983 Cyg and V2074 Cyg, which are presumed to be cool R Corona Borealis (RCB) stars, according to their photometric behaviour. The stars show irregular long-term visual light variations of relatively large amplitude, similar to those exhibited by RCB variables. Based on the comparison of our high-resolution spectra with those of various carbon stars, we conclude that both variables have spectral properties typical for cool N-type carbon stars. However, two major differences from the spectra of normal carbon stars have been noted: the presence of high-velocity (∼−60 km s−1) components in th…

X-ray variability of Pleiades late-type stars as observed with the ROSAT-PSPC

2003

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 X-ray photon time series show that the percentage of cases of significant vari- ability is quite similar on both samples, suggesting that the presence of variability does not depend on mass for the time scales and mass range explored. The comparison between the Time X-ray Amplitude Distribution functi…

Signatures of pulsations and mass loss in the spectra of post-AGB stars

2011

AbstractThe results of high-resolution spectroscopy and radial velocity monitoringa are presented for selected post-AGB candidate stars. Time series of high-resolution spectra for HD235858 shows spliting of low-excitation atomic lines and significant changes in the intensity of C2 and CN lines originated in the extended atmosphere significantly affected by pulsations and outflow. Mass ejection was confirmed for cool R CrB candidate stars DY Per, V1983 Cyg, and V2074 Cyg.