Investigation of the Progenitors and Outbursts of Classical and Recurrent Novae
Classical novae (CNe) are interacting binary systems whose outbursts are powered by a thermonuclear runaway in accreted material on the surface of a white dwarf (WD). The secondary stars in such systems fill their Roche lobe and material is transferred onto the WD primary star via an accretion disk.Recurrent novae (RNe) show many similarities to CNe, but have had more than one recorded outburst. They play an important role as one of the suspected progenitor systems of Type Ia supernovae (SNe) which are used as primary distance indicators in cosmology. Thus, it is important to investigate the nature of their central binary systems to determine the relation between the parameters of the central system and outburst type, and finally ascertain the population of novae that might be available to give rise to the progenitors of Type Ia SNe. The investigation looking for characteristics that may distinguish RNe from CNe systems, the selection of initial targets for detailed study, and results of the investigation are presented in this thesis.The proposal that RNe occupy a region separated from CNe in an outburst amplitude versus speed class diagram was adopted. Since the low amplitude results from the existence of an evolved secondary and/or high mass transfer rate in the quiescent system, RNe candidates should accordingly have low amplitude. The 93 novae with observed V amplitudes given in the literature and 43 novae with photographic amplitudes have been combined and plotted on an outburst amplitude versus rate of decline diagram from which 16 target novae suspected to be RNe candidates were selected for photometric and spectroscopic follow-up.Quiescent photometric magnitudes and spectra were obtained using RATCam on LT, FRODOSpec on LT, and RSS on SALT. Spectral type and luminosity class determined from the near-IR colour-magnitude diagrams were compared to those derived from the spectra. Determination of spectral types was accomplished by identifying specific lines and calculating indices from TiO bands, VO bands, and the Na atomic line for giants (finding 4 stars) and sub-giants/giants (3 stars). A spectral library template was used instead of the indices in cases of main-sequence stars (2 stars).Our investigation also confirmed the positions of AR Cir, V794 Oph and EU Sct where there had been some ambiguity previously. Ultimately, we suggest four prime RNe candidates (2 novae with giant secondaries - V3964 Sgr and EU Sct, and 2 novae with sub-giant secondaries - V794 Oph and V368 Aql) which are currently classified as CNe, to look for more than one outburst in archival plates or large sample sky surveys such as SMEI (see below).By introducing the high cadence full-sky space-based observational archive of the Solar Mass Ejection Imager (SMEI) which operated on the Coriolis satellite from 2003-2011, we derived light curves of one Mira (O Cet) as a general example and two novae with known outbursts during 2003-2011 (V2467 Cyg and V1187 Sco). The SMEI light curves potentially reveal more details than those given by ground-based observations. The pre-maximum halt was found in V2467 Cyg as well as oscillations in light curves found earlier than those found in previous studies. The precise date of maximum of each nova was provided.Four bright novae that are potentially RNe candidates (V4074 Sgr, V3964 Sgr, DK Lac and V368 Aql) were searched for second outbursts in the SMEI data, but none were found. Among the nova outbursts detected by SMEI, we found however unprecedented detail in first class data of the Recurrent Nova T Pyx in its 2011 outburst.We investigated the optical light curve of T Pyx during its 2011 outburst through compiling a database of SMEI and American Association of Variable Star Observers (AAVSO) observations. The SMEI light curve, covering t=1.5-49 days post-discovery, was divided into four phases based on the idealised nova optical light curve; the initial rise (1.5-3.3 days), the pre-maximum halt (3.3-13.3 days), the final rise (14.7-27.9 days), and the early decline (27.9 days, until the end of SMEI observations on day 49). The SMEI light curve contains a strongly detected period of 1.44+-0.05 days during the pre-maximum halt phase. These oscillations resemble those found in recent TNR models arising from instabilities in the expanding envelope. No spectral variations that mirror the light curve periodicity were found however. The marked dip at t~22-24 days just before light curve maximum at t=27.9 days may represent the same (shorter duration) phenomenon seen in other novae observed by SMEI and present in some TNR model light curves.The spectra of T Pyx from the 2m the Liverpool Telescope and the Small and Moderate Aperture Research Telescope System (SMARTS) 1.5m telescope were obtained from t=0.8-80.7 and 155.1-249.9 days, covering the major phases of development. The nova was observed very early in its rise where a distinct high velocity ejection phase was evident with derived Vej~4000 km/s initially. A marked drop at t=5.7 days, and then a gradual increase occurred in derived Vej to stabilise at ~1500 km/s at the pre-maximum halt. Here we propose two different stages of mass loss, a short-lived phase occurring immediately after outburst and lasting ~6 days followed by a more steadily evolving and higher mass loss phase. The overall spectral development follows that typical of a Classical Nova and comparison with the photometric behaviour reveals consistencies with the simple evolving pseudo-photosphere model of the nova outburst. Comparing optical spectra to X-ray and radio light curves, weak [Fe X] 6375A emission was marginally detected before the X-ray rise and was clearly present during the brightest phase of X-ray emission. If the onset of the X-ray phase and the start of the final decline in the optical are related to the cessation of significant mass loss, then this occurred at t~90-110 days.