Studies of classical and recurrent novae G. C. Anupama [Ph.D Thesis]

Doctor of Philosophy Indian Institute of Astrophysics, Bangalore 1990

Novae belong to the cataclysmic variable class of objects, which includes dwarf
novae, recurrent novae and classical novae. These systems undergo out bursts ranging
from Δm ∼ 2-5 mag for dwarf novae, Δm ∼ 7-9 mag for recurrent novae, to
Δm ∼ 9-> 14 mag for classical novae, with the inter-outburst periods being ∼
weeks to years for dwarf novae, ∼ decades for recurrent novae and ∼ 104 years for
classical novae. Cataclysmic variables are interacting binary star systems consisting
of a Roche-lobe filling secondary, on or near main sequence, losing hydrogen-rich
material through the inner Lagrangian point onto an accretion disc that surrounds
the primary, which is a white dwarf in most cases. A classical nova outburst is
caused by a thermonuclear runaway on the surface of the white dwarf primary,
whereas in dwarf novae the outburst is due to accretion disc instabilities, caused by
factors such as enhanced mass transfer.
Novae serve as valuable astrophysical laboratories. The physics of accretion
onto compact, evolved objects, thermonuclear runaways on semi-degenerate surfaces,
line formation and transfer processes in moving atmospheres, and formation
of dust in the ejected matter are some of the astrophysical problems that can be
understood better by detailed studies of novae. Such problems are also encountered
in other instances of ejection of matter such as supernovae and planetary nebulae.
In this study we present optical spectroscopic data of the classical novae LW
Serpentis 1978, Nova Scuti 1989 and of the recurrent nova RS Ophiuchi 1985,
obtained during outbursts. Spectroscopic data of the recurrent novae T Coronae
Borealis, RS Ophiuchi and T Pyxidis and the classical nova GK Persei 1901 obtained
during quiescence are also presented. Also, CCD images of the shells of GK Persei,
and T Pyxidis are presented. Most of the data used in the study were obtained with
the 102 cm reflector at the Vainu Bappu Observatory (VBO). In addition, some
data obtained at the European Southern Observatory (ESO), kindly made available
by H.W. Duerbeck and also some archival data from the International Ultraviolet
Explorer (IUE), kindly made available by A. Cassatella are made use of. All data
were reduced at VBO using the locally developed RESPECT software package, as
also the STARLINK package with locally developed application routines. These
data have been used to study the physical conditions in the nova envelopes, and
the components of the binary system.
Chapter I contains a general introduction to the field, and the outline of the dissertation. Chapter II describes the procedure of data reduction.
Results on three classical novae are presented in Chapter III. The outburst
spectrum of the moderately slow nova LW Serpent is 1978 compares well with that
of a typical nova. Based on moderate-resolution Ha-line profile, a kinematical
model for the shell of LW Serpentis is proposed. The spectrum of Nova Scuti 1989,
also a moderately slow nova, compares well with LW Serpent is at similar epochs.
The fluxes in emission lines have been used in determining the physical conditions
in the ejected material. Spectra observed during oscillations in the light curve
show that the variations are mostly in the continuum, and hence imply a change in
photospheric radius.
CCD images of the shell of nova GK Persei obtained in the lines of [N II] and
[0 III] are used to determine the expansion of the shell by comparison with data
available in literature. The shell is asymmetric with bulk of the emission arising in
the southwest quadrant in the [N II] image. There is a difference in the distribution
of [0 III] and [N II] emission indicating chemical. inhomogeneities. The spectrum
at quiescence is decomposed into those of KO-2 IV secondary and the hot accretion
disc. The mass transfer rate is estimated to be ∼ 10-10 С yr-1 . The He/H
abundance in the accretion disc is ≤ 0.24.
Results on three recurrent novae are presented in Chapter IV. Although the
outburst characteristics of recurrent novae are similar to those of classical novae, the
cause of the outburst in these systems is rather uncertain. Outburst in most cases
appears to be due to thermonuclear runaways on massive white dwarfs. However,
in the case of recurrent novae RS Ophiuchi and T Coronae Borealis, alternative
explanations also exist, according to which the primary in these systems is a main
sequence star, with outbursts attributed to instabilities in the disc.
The fluxes in the Ha, He I and He II emission lines during the out burst of RS
Ophiuchi have been used to determine the electron density and helium abundance
(He/H= 0.16) in the envelope. Based on an estimate of the number of hydrogen
and helium ionizing photons, the temperature and radius of the ionizing source
have been determined. The results obtained during the late stages of outburst
are consistent with the primary being a white dwarf rather than a main sequence
star. The coronal line :B.uxes have been used to determine the temperature in the
shocked ejecta. Spectra obtained during quiescence indicate that the secondary is
an Mʘ ± 1 giant. The presence of strong 0 18446 Å emission line implies presence
of Ly(3 fluorescence and a high temperature for the ionizing source. The presence
of 0 I 7774 Å. line in absorption indicates that the accretion disc is optically thick.
The quiescence spectrum is decomposed into the spectra of cool secondary and
hot accretion disc. The mass transfer rate is estimated to be '" 10-6 Mʘ yr-1 .
Spectra of the recurrent nova T Coronae Borealis obtained during its quiescence
phase (1985-1990) show that the secondary is an M4 ± 1 giant. The emission lines
in the spectra are variable in strength. The Ha flux shows a long term variation
with a period ∼ 2400 days. Superposed over this is an orbital phase dependent
variation with maxima at '" 0.5 and rv 0.9 phase. The estimated mass transfer rate
is '" 10-7 Mʘ yr-1 .
Images of T Pyxidis in [N II] and [0 III] reveal a bright shell ejected in the
1944 outburst and a faint extension due to the 1920 outburst. Comparison of
VBO and ESO images shows that the bright shell expanded by 0.2 ± 0.1 arcsec
in 3 years. The differences in [0 III] and [N II] images suggest the presence of
chemical inhomogeneities as in the case of GK Persei. The spectrum at quiescence is
dominated by the accretion disc, and has a high degree of excitation. The estimated
mass transfer rate is ('V 10-8 Mʘ yr-1 , and HelH abundance is ≤0.24.
The results of the study are summarized in Chapter V. A mention is also made of
the problems that require further investigation using medium and large telescopes,
and multiwavelength long~term monitoring.
The tables, figures and equations are numbered sequentially in each section,
with the chapter and section numbers indicated by suffixes.