Studies of activity in Late type stars Mathew Varghese Mekkaden [Ph.D Thesis]

Doctor of Philosophy Indian Institute of Astrophysics, Bangalore 1996

Photometric and spectroscopic variability of late type stars are usually interpreted
as evidence for magnetic activity. The present picture of stellar
activity, evolved largely based on solar analogy, includes cool dark spots
with enhanced photospheric magnetic fields, which is manifested ill the light
variation due to rotational modulation, and the associated chromospheric
emissions. The domain of stellar activity encompasses the T Tauri stars,
young late type stars, and the evolved binary systems like the RS eVn, W
UMa, Algol and FK Com type objects. While the weak emission T Tauri
stars, young late type stars and the RS ev n systems show evidences for the
presence of cool spots, the active classical T Tauri stars indicate thf! presence
of hot spots that are supposed to be produced by an entirely different
mechanism.
The present investigation was started with the goal of studying the
nature of activity in a selected sample of RS eVn systems, young activechromosphere
objects and T Tauri stars, and also to examine the observed
differences in the levels of activity exhibited by them. For this purpose we
have carried out extensive multi-band photometry of all the objects under
investigation. We have also carried out near-simultaneous photometry, spectroscopy
and polarimetry of T Tauri stars that are included in the present
study.
The present study includes two RS eVn systems, two young activechromosphere
stars, two classical T Tauri stars and two weak emission T
Tauri stars.
The thesis has been divided into eight chapters. The first chapter contains
a general introduction to RS eVn systems, young active-chromosphere stars and T Tauri stars listing out their main observational characteristics.
The objectives of the present study are also given. The second chapter describes
the method of observations and data reduction techniques.
Chapter 3 deals with the spot model developed by us to solve the observed
light curves for the spot parameters. This model employs the method
of least squares using differential correction to the parameters. An investigation
of the synthetic light curves for various combination of spot parameters
is also made. The computer program developed by us is fairly general; there
is no restriction either on the nature of spots or on their location.
The analysis of U BV RI and uvby photometry of the RS CVn systems
HD 81410 and HD 127535 is presented in chapter 4. The most significant
finding from the analysis of the photometry of HD 81410 is that the star has
two preferred longitudes about which the spots are generally formed. The
modeling of the light curves indicates a large range in spot temperature (r..J
1000 K). In the case of HD 127535 the photometric period is found to be
almost the same as the orbital period. There is also an indication of an
activity cycle of about 10 years for HD 127535.
Chapter 5 deals with the analysis of the photometry of the young activechromosphere
stars HD 139084 and HD 155555. It is noticed that the amplitudes
of light variation in both these stars are comparatively smaller than
that usually observed in RS eVn systems. The brightness at light maximum
observed any two different epochs were not the same. We attribute
this phenomenon to a large longitudinal spread of spot groups. The analysis
of light curves of HD 155555 gives evidences for the presence of short-lived
photospheric plages. It is found that the observed light variability is mainly
due to the brighter and hotter component.
The observation and analysis of the weak emission T Tauri stars FK
Ser and V410 Tau are the subject of chapter 6. We have detected periodic
light variation in FK Ser and also the presence of hot plages co-existing
with cool spots on its surface. The H Ct. observations also give evidence for
the presence of cool spots. In V 410 Tau the large amplitude light variation
and the corresponding H Ct. emission strength variation have given conclusive
evidence for the cool spot hypothesis. The linear polarization measurements exhibit a periodic behaviour which is in anticorrelation with light variation
and hence in agreement with the cool spot phenomenon.
The observations and analysis of the classical T Tauri stars TW Hya
and V4046 Sgr are discussed in chapter 7. We have derived the photometric
periods of these stars. The wavelength dependence of amplitude observed in
the light curves of TW Hya shows that the modulation of the light is caused
by hot spots. Apart from periodic light variations, TW Hya also show sudden
brightening that last for one or two days. The light variation in V4046 Sgr
has very low amplitudes which is interpreted as due to the low inclination
and magnetically controlled flow from the accretion disc. The H a emission
strength in TW Hya does not show any indication of periodicity, while in
V4046 Sgr there is an indication for the same. Both TW Hya and V4046 Sgr
show variable linear polarization.
In chapter 8 the results of the present study are summarized and the
further investigations needed to answer some of the related problems are
discussed. The tables and figures are numbered sequentially in each chapter
and the corresponding chapter number is prefixed whenever a reference is
made to them. The references, which are listed in the alphabetical order, are
given at the end.