Other
Scientific paper
Feb 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006phdt........31p&link_type=abstract
PhD Thesis, Aryabhatta Research Institute of Observational Sciences, Nainital, India
Other
2
Stars, Active, Stellar Activity, Fr Cnc, Hd 81032, Lo Peg, Hd95559, Hd160934, Coronae, Chromosphere, Polarimeter, Sed, Kinematics, Rosat, 2Mass, Correlation, Radio Luminosity, X-Ray Luminosity, Ir Excess, Polarimetry, Polarization
Scientific paper
A multiwavelength study of the late-type active stars, selected on the basis of their X-ray and radio luminosities is presented in this thesis. For FR Cnc, a photometric period 0.8267 +/- 0.0004 d has been established. The strong variation in the phase and amplitude of the FR Cnc light curves when folded on this period implies the presence of evolving and migrating spots or spot groups on its surface. A photometric period of 18.802 +/- 0.074 has been discovered in the star HD 81032. The shape and amplitude of the photometric light curves of FR Cnc, HD 81032, HD 95559 and LO Peg are observed to be changing from one epoch to another. The change in the amplitude is mainly due to a change in the minimum of the light curve, and this May be due to a change in the spot coverage. This indicates that photometric variability is due to the presence of dark spots on the surface of active star. Two groups of spots are identified for FR Cnc and LO Peg. The spots are found to migrate, and migration periods of 0.97 year and 0.93 year are determined from the 4 years of data. A migration period of 1.12 years for one group of spots in LO Peg is also determined. Formation of a new group of spots in the star HD 95559 was also seen during our observations. A single large group of spots is found to migrate, and a migration period of 7.32 +/- 0.04 years is determined for HD 81032. The stars FR Cnc, HD 81032, HD 160934 and LO Peg are seen to be redder at the light minimum and we interpret this is due to the relatively cooler temperature of the darker regions present in the visible hemisphere. We find the lack of color-brightness correlation in the star HD 95559 and this May be due to the presence of bright faculae and plages like regions accompanied by dark spots in any one component of the this binary system. The optical spectroscopy of FR Cnc and HD 81032 carried out during 2002-2003, reveals the presence of strong and variable Ca II H and K, Halpha and Hbeta emission features indicative of high level of chromospheric activity. The chromospheric line emission for both stars seems to correlate with the photometric light curve, i.e. maximum at the light curve minimum, or minimum at the light curve maximum. The value of 5.3 for the ratio of the excess emission in Halpha to Hbeta, EHalpha/EHbeta for the star FR Cnc, suggests that the chromospheric emission May arise from an extended off-limb region. We have searched for the presence of color excesses in the near-IR JHK bands of these stars using 2MASS data, but none of them appear to have any significant color excess. The kinematics of the stars FR Cnc, HD 95559, HD 160934 and LO Peg suggest that these are younger than 0.6 Gyrs. The archival X-ray observations of HD 81032, HD 95559, HD 160934 and LO Peg carried out by with the ROSAT observatory were also analyzed. We did not find any significant variability in the X-ray light curve of the stars HD 95559. However, it appears from the X-ray light curve of HD 81032 that a moderate flare occurred during the RASS observations, with a peak of about 0.6 ct s^{-1} at approximately JD=244806.95 and half decay time of 2.6 x 10^4 s. A similar flare was also observed in HD 160934 with a peak about 0.2 ct s^{-1} (above its mean level) at JD = 2448123.12. A significant variability was found in the X-ray light curve of LO Peg. Rotational modulation appears to be present in the X-ray light curve of LO Peg. The best fit models to their X-ray spectra imply the presence of two coronal plasma components of differing temperatures and with sub-solar metal abundances. The inferred emission measures and temperatures of HD 95559 and LO Peg are similar to those found for other active dwarf stars. The observed X-ray spectrum and the inferred coronal plasma parameters for HD 81032 are typical of those seen in active stars such as RS CVn binaries. All of the optical and X-ray properties found for FR Cnc, HD 95559, HD 160934 and LO Peg are most consistent to the BY Dra type. However, for HD 81032 these properties suggest that it being an evolved RS! CVn binary of the long-period type Correlations between various physical quantities (Lx, Lrad, P and B-V) of active stars have been re-examined using a sample containing 248 active stars (101 dwarfs, 65 subgiants and 82 giants). It is a largest sample investigated so far. We did not find any appreciable changes in the correlations reported in previous studies.
In addition to above, an Imaging Polarimeter has been fabricated for use with liquid-N2 cooled CCD camera and is designed to suit 104-cm Sampurnanand telescope with an f/13 focus at ARIES, Naini Tal. The instrument measures the linear polarization in broad B, V and R band, and has a field of view 2' x 2'.
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