A multiwavelength study of the early evolution of the classical nova LMC 1988 1

We study star-formation-inducing mechanisms in galaxies through multiwavelength measurements of a sample of dwarf galaxies in the Virgo cluster described in Paper I. Our main goal is to test how star-formation-inducing mechanisms depend on several parameters of the galaxies, such as morphological type and hydrogen content. We derive the star formation rate and star formation histories of the galaxies, and check their dependence on other parameters.   Comparison of the sample galaxies with population synthesis models shows that these objects have significantly lower metallicity than the solar value. The colours can generally be explained as a combination of two different stellar populations: a young (3–20 Myr) metal-poor population which represents the stars currently forming presumably in a starburst, and an older (0.1–1 Gyr) population of previous stellar generations. There is evidence that the older stellar population was also formed in a starburst. This is consistent with the explanation that star formation in this type of objects takes place in short bursts followed by long quiescent periods.   No significant correlation is found between the star formation properties of the sample galaxies and their hydrogen content. Apparently, when star formation occurs in bursts, other parameters influence the star formation properties more significantly than the amount of atomic hydrogen. No correlation is found between the projected Virgocentric distance and the rate of star formation in the galaxies, suggesting that tidal interactions are not significant in triggering star formation in cluster dwarf galaxies.     

A review of the classical nova outburst

We review the recent observational and theoretical studies of the nova outburst. The observational studies have not only identified a new class of novae but theoretical simulations of this class have been found to be in excellent agreement with the observations. This new class consists of outbursts occurring on ONeMg white dwarfs in close binary systems in contrast to the other outbursts which are occurring on CO white dwarfs. We also review the effects of the ⁺ nuclei and show how their presence has a major effect on the evolution.Paper presented at the IAU Colloquium No. 93 on Cataclysmic Variables. Recent Multi-Frequency Observations and Theoretical Developments, held at Dr. Remeis-Sternwarte Bamberg, F. R. G., 16–19 June, 1986.     

Interacting winds in classical nova outbursts

Classical nova outbursts occur in binary systems containing a white dwarf accretor and a Roche-lobe-filling main-sequence star. The outburst is due to a thermonuclear runaway in the accreted material on the surface of the white dwarf, and results in the ejection of up to 10^–4 M of material at velocities of several hundred to a few thousand kilometres per second. There is now strong evidence that the mass ejection takes place via a wind with secularly increasing velocity. The fast ejecta catches up with slower moving material ejected earlier in the outburst, forming a layer of shock-heated gas which gives rise to a short burst of soft X-ray emission. This emission was observed in V838 Her (Nova Herculis 1991), and was succesfully accounted for by the interacting winds model. In this paper, we present 2.5-D numerical hydrodynamics calculations of interacting winds in novae which consider the effects of the binary system on shaping the mass-loss, and show that many of the features seen in the optical shells of novae many years after outburst can be accounted for.This author is supported by a PPARC research assistantship     

Spectral evolution of the classical symbiotic star V1413 Aql during a new activity cycle

We present the results of our spectroscopic observations for the classical symbiotic star V1413 Aql performed in 2008–2011. Various states of the hot component are considered: almost quiescence in 2008, very slow outburst onset in 2009, outburst maximum in 2010, and gradual brightness decline in the erupted hot component in 2011. We have established that, according to a spectroscopic criterion, in 2008 the system was in quiescence. We have shown that the parameters of the hot component during its outburst can be determined only by modeling the differences of the spectra taken at different eclipse phases. The active hot component of V1413 Aql at the outburst maximum is shown to have had the record late (for a symbiotic star) spectral type K2. At various stages of the new activity cycle, we have modeled the continuumenergy distribution based on a standard three-componentmodel, a model including a standard accretion disk and a red giant, and amodel including a supergiant (of various spectral types) and a red giant. The parameters of the system’s components have been determined.     

V4633 Sgr – a probable second asynchronous polar classical nova

Photometric observations of V4633 Sgr (Nova Sagittarii 1998) during 1998–2005 reveal the presence of a stable photometric periodicity at   P ₁= 180.8 min  which is probably the orbital period of the underlying binary system. A second period was present in the light curve of the object for 6 yr. Shortly after the nova eruption it was measured as   P ₂= 185.6 min  . It has decreased monotonically in the following few years reaching the value   P ₂= 183.9 min  in 2003. In 2004 it was no longer detectable. We suggest that the second periodicity is the spin of the magnetic white dwarf of this system that rotates nearly synchronously with the orbital revolution. According to our interpretation, the post-eruption evolution of Nova V4633 Sgr follows a track similar to the one taken by V1500 Cyg (Nova Cygni 1975) after that nova eruption, on a somewhat longer time-scale. The asynchronism is probably the result of the nova outburst that led to a considerable expansion of the white dwarf's photosphere. The increase in the moment of inertia of the star was associated with a corresponding decrease in its spin rate. Our observations have followed the spinning-up of the white dwarf resulting from the contraction of its outer envelope as the star is slowly returning to its pre-outburst state. It is thus the second known asynchronous polar classical nova.     

Variability and multiperiodic oscillations in the X-ray light curve of the classical nova V4743 Sgr

The classical nova V4743 Sgr was observed with XMM–Newton for about 10 h on 2003 April 4, 6.5 months after optical maximum. At this time, this nova had become the brightest supersoft X-ray source ever observed. In this paper, we present the results of a time-series analysis performed on the X-ray light curve (LC) obtained in this observation, and in a previous shorter observation done with Chandra 16 d earlier. Intense variability, with amplitude as large as 40 per cent of the total flux, was observed both times. Similarities can be found between the two observations in the structure of the variations. Most of the variability is well represented as a combination of oscillations at a set of discrete frequencies lower than 1.7 mHz. At least five frequencies are constant over the 16 d time interval between the two observations. We suggest that a period in the power spectrum of both LCs at the frequency of 0.75 mHz and its first harmonic are related to the spin period of the white dwarf (WD) in the system, and that other observed frequencies are signatures of non-radial WD pulsations. A possible signal with a 24 000 s period is also found in the XMM–Newton LC: a cycle and a half are clearly identified. This period is consistent with the 24 278 s periodicity discovered in the optical LC of the source and thought to be the orbital period of the nova binary stellar system.     

The evolution of nova V5558 Sgr during the decline stage

We report the spectroscopic follow-up of the nova V5558 Sgr carried out in 2008 and 2009 at the Loiano Observatory, Italy, during the decline stage, as a part of an ongoing monitoring campaign. The spectra suggest that V5558 Sgr has entered the nebular stage, as shown by the presence of high ionization lines, and confirm that it is very similar to the slow nova V723 Cas, as previously suggested. We have estimated the decline time by three magnitudes, 170 ± 2 days, typical of slow novae. We have revised the estimation of the absolute magnitude at maximum (?6.3 to ?5.9) and the distance (1.3–1.6 kpc) of V5558 Sgr, in agreement with previous results. The revised white dwarf mass (0.58–0.63 M_⊙) confirms that V5558 Sgr is a critical system whose mass is close to the lower limit to trigger the nova outburst.     

Beginning of the super‐soft phase of the classical nova V2491 Cygni

We present the results of soft X‐ray studies of the classical nova V2491 Cygni using the Suzaku observatory. On day 29 after outburst, a soft X‐ray component with a peak at ∼0.5 keV has appeared, which is tantalising evidence for the beginning of the super‐soft X‐ray emission phase. We show that an absorbed blackbody model can describe the observed spectra, yielding a temperature of 57 eV, neutral hydrogen column density of 2 × 10²¹ cm^–2, and a bolometric luminosity of ∼10³⁶ erg s^–1. However, at the same time, we also found a good fit with an absorbed thin‐thermal plasma model, yielding a temperature of 0.1 keV, neutral hydrogen column density of 4 × 10²¹ cm^–2, and a volume emission measure of ∼10⁵⁸ cm^–3. Owing to low spectral resolution and low signal‐to‐noise ratio below 0.6 keV, the statistical parameter uncertainties are large, but the ambiguity of the two very different models demonstrates that the systematic errors are the main point of concern. The thin‐thermal plasma model implies that the soft emission originates from optically thin ejecta, while the blackbody model suggests that we are seeing optically thick emission from the white dwarf (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A dwarf nova in Taurus

In searchs for flare stars in the vicinity of the Pleiades cluster, three flares were detected in 1970,1972, and 1977 in a star with the coordinates α ₁₉₅₀ = 3 ^h 48 ^m ·9, δ _l950 = 25‡15’.8. The star’s brightness at a minimum is >21 ^m ·5. The star was tentatively assigned to the U Gem type. To confirm this assumption, we examined photographic plates for the period of 1947–1987. As a result, we found 12 more flares. The average recurrence time based on the 1963–1977 observations is about 330 days, and the maximum flare amplitude is >6 ^m .Narrow superflares and steady-state flares lasting over 40 days have been observed in the star. The results show that this is a dwarf nova of the UGSU subtype. Translated from Astrofizika, Vol. 42, No. 1, pp. 47–52, January–March, 1999.     

Diagnostics of the early explosion phase of a classical nova using its X-ray emission: A model for the X-ray outburst of CI Camelopardalis in 1998

We have computed a spherically symmetric model for the interaction of matter ejected during the outburst of a classical nova with the stellar wind from its optical component. This model is used to describe the intense X-ray outburst (the peak 3–20 keV flux was ～2 Crab) of the binary system CI Camelopardalis in 1998. According to our model, the stellar wind from the optical component heated by a strong shock wave produced when matter is ejected from the white dwarf as the result of a thermonuclear explosion on its surface is the emission source in the standard X-ray band. Comparison of the calculated and observed time dependences of the mean radiation temperature and luminosity of the binary system during its outburst has yielded very important characteristics of the explosion. We have been able to measure the velocity of the ejected matter immediately after the onset of the explosion for the first time: it follows from our model that the ejected matter had a velocity of ～2700 km s^?1 even on 0.1–0.5 day after the outburst onset and it flew with such a velocity for the first 1–1.5 day under an external force, possibly, the radiation pressure from the white dwarf. Subsequently, the matter probably became transparent and began to decelerate. The time dependence of the mean radiation temperature at late expansion phases has allowed us to estimate the mass of the ejected matter, ～10^?7–10^?6 M _⊙. The mass loss rate in the stellar wind required to explain the observed peak luminosity of the binary system during its outburst has been estimated to be \(\dot M\) ～ (1 ? 2) × 10^?6 M _⊙ yr^?1.     

Simulations and interpretation of the 6-cm MERLIN images of the classical nova V723 Cas

We compare the predictions of simple models for the radio emission from classical novae with the MERLIN radio observations of nova V723 Cas (Nova Cas 1995). Spherically symmetric and ellipsoidal radiative transfer models are implemented in order to generate synthetic emission maps. These are then convolved with an accurate representation of the u v coverage of MERLIN. The parameters and geometry of the shell model are based on those returned by fitting models to the observed light curve. This allows direct comparison of the model images with the nine 6-cm MERLIN images of V723 Cas.
It is found that the seemingly complex structure (clumping, apparent rotation) evident in the observations can actually be reproduced with a simple spherical emission model. The simulations show that a 24-h track greatly reduces the instrumental effects and the synthetic radio map is a closer representation of the true (model) sky brightness distribution. It is clear that interferometric arrays with sparse u v coverage (e.g. MERLIN, VLBA) will be more prone to these instrumental effects especially when imaging ring-like objects with time-dependent structure variations. A modelling approach such as that adopted here is essential when interpreting observations.     

A survey of nova remnants

Exnovae and nova shells are generally faint and difficult to observe. Only a few have been studied, and even less of them in sufficient detail. We give a progress report on our survey of exnovae and nova shells, especially on the results of a spectral survey of novae in the southern Milky Way. The three-dimensional structure in the light of H derived for the nebula of GK Per is displayed.Based on observations obtained at the European Southern Observatory, La Silla, Chile, and at the Centro Astronomico Hispano-Aleman Calar Alto, Spain, operated by the Max-Planck-Institut für Astronomie, Heidelberg.Paper presented at the IAU Colloquium No. 93 on Cataclysmic Variables. Recent Multi-Frequency Observations and Theoretical Developments, held at Dr. Remeis-Sternwarte Bamberg, F.R.G., 16–19 June, 1986.     

Spectroscopic evolution of nova LW serpentis 1978 during its early decline

Optical spectroscopic data are presented on nova LW Serpentis 1978, obtained during its decline fromV 9.0 to ≃10.2 (compared to a value of ∼ 8.0 at recorded maximum). The spectrum and its evolution compare well with a typical nova, though the principal absorption (∼ −750 km s^−l) was very weak in comparison with the diffuse-enhanced absorption (∼ −1300 km s⁻¹). The principal absorption could be identified only in the lines of O I λλ7774, 8446, and in moderate-resolution observations of Hα. The salient features of spectral evolution follow: The near-infrared triplet of Ca n continuously weakened. O I λ8446 was always brighter than O I λ 7774, indicating continued importance of Lyman Β fluorescence. The lines due to [O I], [N II] and N n brightened considerably near the end of our observations (37 days from maximum). The Hα emission line was asymmetric all through with more emission towards the red. Its emission profile showed considerable structure. Based on the individual peaks in the Hα line profile, a kinematical model is proposed for the shell of LW Ser. The model consists of an equatorial ring, and a polar cone on the side away from the earth. The nearer polar cone did not show significant emission of Hα during our observations. The polar axis of the shell is inclined at a small angle (∼ 15‡) to the line of sight.     

The evolution of the binary population in globular clusters: a full analytical computation

I present a simplified analytical model that simulates the evolution of the binary population in a dynamically evolving globular cluster. A number of simulations have been run spanning a wide range in initial cluster and environmental conditions by taking into account the main mechanisms of formation and destruction of binary systems. Following this approach, I investigate the evolution of the fraction, the radial distribution, the distribution of mass ratios and periods of the binary population. According to these simulations, the fraction of surviving binaries appears to be dominated by the processes of binary ionization and evaporation. In particular, the frequency of binary systems changes by a factor of 1–5 depending on the initial conditions and on the assumed initial distribution of periods. The comparison with the existing estimates of binary fractions in Galactic globular clusters suggests that significant variations in the initial binary content could exist among the analysed globular cluster. This model has been also used to explain the observed discrepancy found between the most recent N -body and Monte Carlo simulations in the literature.     

Photometric and spectroscopic evolution of the symbiotic nova V1016 Cygni after its outburst

We have constructed the light curves of the symbiotic nova V1016 Cyg for the period 1971–2007 in a homogeneous photometric system close to UBV using our observations with the Zeiss-600 SAI telescope. Based on the observational data obtained with the 125-cm SAI telescope in 2000–2007, we have performed absolute spectrophotometry of the star in the range λ3700–9300 Å. The derived line intensities are compared with the data of other authors in the preceding years (1965–1988). The behavior of nebular lines showed the variations in electron density and, probably, electron temperature in the [OIII] emission region caused by a variable stellar wind from the hot component. All the available observations of the star confirm the theoretical conclusion that the nova-like outburst of V1016 Cyg was produced by a thermonuclear flash in the accreted envelope of a white dwarf.     

A TiO study of the dwarf nova IP Pegasi

We present red spectra in the region ∼ λ 7000–8300 Å of the eclipsing dwarf nova IP Peg, with simultaneous narrow-band photometry centred at 7322 Å. We show that by placing a second star on the slit we can correct for the telluric absorption bands which have hitherto made the TiO features from the secondary star unusable. We use these TiO features to carry out a radial velocity study of the secondary star, and find this gives an improvement in the signal-to-noise ratio of a factor of 2 compared with using the Na  i doublet. In contrast with previous results, we find no apparent ellipticity in the radial velocity curve. As a result we revise the semi-amplitude to K ₂=331.3±5.8 km s⁻¹, and thus the primary and secondary star masses to 1.05^-0.07_+0.14 M⊙ and 0.33^-0.05_+0.14 M⊙ respectively. Although this is the lowest mass yet derived for the secondary star, it is still overmassive for its observed spectral type. However, the revised mass and radius bring IP Peg into line with other cataclysmic variables in the mass–radius–period relationships.
By fitting the phase-resolved spectra around the TiO bands to a mean spectrum, we attempt to isolate the light curve of the secondary star. The resulting light curve has marked deviations from the expected ellipsoidal shape. The largest difference is at phase 0.5, and can be explained as an eclipse of the secondary star by the disc, indicating that the disc is optically thick when viewed at high inclination angles.     

Hints of a second explosion(a quark nova) in Cassiopeia A supernova

We show that the explosive transition of the neutron star(NS)to a quark star(QS)(a Quark Nova)in Cassiopeia A(Cas A)a few days following the supernova(SN)proper can account for several of the puzzling kinematic and nucleosynthetic features that are observed.The observed decoupling between Fe and44Ti and the lack of Fe emission within44Ti regions is expected in the QN model owing to the spallation of the inner SN ejecta by relativistic QN neutrons.Our model predicts the44Ti to be more prominent to the NW of the central compact object(CCO)than in the SE and little of it along the NE-SW jets,in agreement with Nu Star observations.Other intriguing features of Cas A are addressed,such as the lack of a pulsar wind nebula and the reported few percent drop in the CCO temperature over a period of 10 yr.     

A modern view of the dwarf nova Z Chamaeleontis

Some recent photometric and spectroscopic results for this ultrashort period system are summarized, and several straightforward consequences of these results for our understanding of the evolution of cataclysmic binaries are pointed out: The space motion of Z Cha is characteristic of the old disk population. The white dwarf cannot be composed primarily of He, unless it grew by accretion by at least 20%. The inferred masses of the component stars, combined with the usual gravitational quadrupole formula, probably do not suffice to explain the inferred rate of mass transfer, even in quiescence. The secondary star does not lie on the computed evolutionary tracks in the period-mass diagram of Paczynski and Sienkiewicz.Paper presented at the IAU Colloquium No. 93 on Cataclysmic Variables. Recent Multi-Frequency Observations and Theoretical Developments, held at Dr. Remeis-Sternwarte Bamberg, F.R.G., 16–19 June, 1986.