Nova Scenario Research Articles

Possible Formation Scenarios of ZTF J153932.16+502738.8—A Gravitational Source Close to the Peak of LISA’s Sensitivity

Abstract ZTF J153932.16+502738.8 (ZTFJ1539) is an eclipsing double-white-dwarf system with an orbital period of 6.91 minutes, and a significant source for LISA detection of gravitational waves. However, the massive white dwarf (WD), with a mass of about 0.61 M ⊙, has a high effective temperature (48,900 K), and the lower-mass WD, with a mass of about 0.21 M ⊙, has a low effective temperature (<10,000 K). This discrepancy challenges the popular theory of binary evolution. We investigate the formation of ZTFJ1539 via nova and Algol scenarios. Assuming that the massive WD in ZTFJ1539 just experiences a thermonuclear runaway, the nova scenario can explain the effective temperatures of the two WDs in ZTFJ1539. However, in order to enlarge a semi-detached orbit of about 4–5 minutes to a detached orbit of about 7 minutes, the nova scenario needs a much higher kick velocity of about 200 km s−1 during nova eruption. The high kick velocity can result in a high eccentricity of about 0.2–0.6. The Algol scenario can also produce ZTFJ1539 if we take a high efficient parameter for ejecting the common envelope and enhance the mass-loss rate via stellar wind trigger by tidal effect.

The diffusion-induced nova scenario: CK Vul and PB8 as possible observational counterparts

We propose a scenario for the formation of DA white dwarfs with very thin helium buffers. For these stars we explore the possible occurrence of diffusion-induced CNO- flashes, during their early cooling stage. In order to obtain very thin helium buffers, we simulate the formation of low mass remnants through an AGB final/late thermal pulse (AFTP/LTP scenario). Then we calculate the consequent white dwarf cooling evolution by means of a consistent treatment of element diffusion and nuclear burning. Based on physically sounding white dwarf models, we find that the range of helium buffer masses for these diffusion-induced novas to occur is significantly smaller than that predicted by the only previous study of this scenario. As a matter of fact, we find that these flashes do occur only in some low-mass (M < 0.6M) and low metallicity (Z_ZAMS <0.001) remnants about 10^6 - 10^7 yr after departing from the AGB. For these objects, we expect the luminosity to increase by about 4 orders of magnitude in less than a decade. We also show that diffusion-induced novas should display a very typical eruption lightcurve, with an increase of about a few magnitudes per year before reaching a maximum of M_V ~ -5 to -6. Our simulations show that surface abundances after the outburst are characterized by logNH/NHe ~ -0.15...0.6 and N>C>O by mass fractions. Contrary to previous speculations we show that these events are not recurrent and do not change substantially the final H-content of the cool (DA) white dwarf. (Abridged)

On the progenitor system of Nova V2491 Cygni

Nova V2491 Cyg is one of just two detected pre-outburst in X-rays. The light curve of this nova exhibited a rare "re-brightening" which has been attributed by some as the system being a polar, whilst others claim that a magnetic WD is unlikely. By virtue of the nature of X-ray and spectroscopic observations the system has been proposed as a recurrent nova, however the adoption of a 0.1 day orbital period is generally seen as incompatible with such a system. In this research note we address the nature of the progenitor system and the source of the 0.1 day periodicity. Through the combination of Liverpool Telescope observations with published data and archival 2MASS data we show that V2491 Cyg, at a distance of 10.5 - 14 kpc, is likely to be a recurrent nova of the U Sco-class; containing a sub-giant secondary and an accretion disk, rather than accretion directly onto the poles. We show that there is little evidence, at quiescence, supporting a ~ 0.1 day periodicity, the variation seen at this stage is likely caused by flickering of a re-established accretion disk. We propose that the periodicity seen shortly after outburst is more likely related to the outburst rather than the - then obscured - binary system. Finally we address the distance to the system, and show that a significantly lower distance (~ 2 kpc) would result in a severely under-luminous outburst, and as such favour the larger distance and the recurrent nova scenario.

Gamma-ray burst engine activity within the quark nova scenario: prompt emission, X-ray plateau and sharp drop-off

Extending the three-stage model for long GRB inner engines suggested by Staff, Ouyed & Bagchi, we interpret recent Swift satellite observations of early X-ray afterglow plateaus followed by a sharp drop-off or a shallow power-law decay. The three stages involve a neutron star phase, a quark star (QS) and a black hole phase. We find that the QS stage allows for more energy to be extracted from neutron star to QS conversion as well as from ensuing accretion on to the QS. The QS accretion phase naturally extends the engine activity and can account for both the prompt emission and irregular early X-ray afterglow activity. Following the accretion phase, the QS can spin down by emission of a baryon-free outflow. The magnetar-like magnetic field strengths resulting from the NS to QS transition provide enough spin-down energy, for the correct amount of time, to account for the plateau in the X-ray afterglow. In our model, a sharp drop-off following the plateau occurs when the QS collapses to a BH during the spin-down, thus shutting off the secondary outflow. We applied our model to GRB 070110 and GRB 060607A and found that we can consistently account for the energetics and duration during the prompt and plateau phases.

Quark-nova remnants

We explore the formation and evolution of debris ejected around quark stars in the Quark Nova scenario, and the application to Soft Gamma-ray Repeaters (SGRs) and Anomolous X-ray Pulsars (AXPs). If an isolated neutron star explodes as a Quark Nova, an Iron-rich shell of degenerate matter forms out of the fall-back (crust) material. Our model can account for many of the observed features of SGRs and AXPs such as: (i) the two types of bursts (giant and regular); (ii) the spin-up and spin-down episodes during and following the bursts with associated persistant increases in $\dot{P}$; (iii) the energetics of the boxing day burst, SGR1806$+$20; (iv) the presence of an Iron line as observed in SGR1900$+$14; (v) the correlation between the far-Infrared and the X-ray fluxes during the bursting episode and the quiescent phase; (vi) the hard X-ray component observed in SGRs during the giant bursts, and (vii) the discrepancy between the ages of SGRs/AXPs and their supernova remnants. We also find a natural evolutionary relationship between SGRs and AXPs in our model which predicts that only the youngest SGRs/AXPs are most likely to exhibit strong bursting. Many features of X-ray Dim Isolated Neutron stars (XDINs) are also accounted for in our model such as, (i) the two-component blackbody spectra; (ii) the absorption lines around 300 eV; and (iii) the excess optical emission.

Planets orbiting Quark Nova compact remnants

We explore planet formation in the Quark Nova scenario. If a millisecond pulsar explodes as a Quark Nova, a protoplanetary disk can be formed out of the metal rich fall-back material. The propeller mechanism transfers angular momentum from the born quark star to the disk that will go through viscous evolution with later plausible grain condensation and planet formation. As a result, earth-size planets on circular orbits may form within short radii from the central quark star. The planets in the PSR1257+12 system can be explained by our model if the Quark Nova compact remnant is born with a period of $\sim 0.5$ ms following the explosion. We suggest that a good portion of the Quark Nova remnants may harbour planetary systems.

The mysterious eruption of V838 Mon

V838 Mon is marking one of the most mysterious stellar outbursts on record. The spectral energy distribution of the progenitor resembles an under-luminous F main sequence star (at mag), that erupted into a cool supergiant following a complex and multi-maxima lightcurve (peaking at mag). The outburst spectrum show BaII, LiI and lines of several elements, with wide P-Cyg profiles and a moderate and retracing emission in the Balmer lines. A light-echo discovered expanding around the object helped to constrain the distance ( pc), providing MV=+4.45 in quiescence and MV=-4.35 at optical maximum (somewhat dependent on the still uncertain reddening). The general outburst trend is toward lower temperatures and larger luminosities, and continuing so at the time of writing. The object properties conflict with a classification within already existing categories: the progenitor was not on a post-AGB track and thus the similarities with the born-again AGB stars FG Sge, V605 Aql and Sakurai's object are limited to the cool giant spectrum at maximum; the cool spectrum, the moderate wind velocity (500 km s-1 and progressively reducing) and the monotonic decreasing of the low ionization condition argues against a classical nova scenario. The closest similarity is with a star that erupted into an M-type supergiant discovered in M 31 by Rich et al. ([CITE]), that became however much brighter by peaking at MV=-9.95, and with V4332 Sgr that too erupted into an M-type giant (Martini et al. [CITE]) and that attained a lower luminosity, closer to that of V838 Mon. M 31-RedVar, V4332 Sgr and V838 Mon could be all manifestations of a new class of astronomical objects.

Evolution of Cataclysmic Binaries

AbstractThe minimum period of hydrogen rich cataclysmic binaries at about 80 minutes is due to evolution driven by gravitational radiation. The nature of the gap between 2 and 3 hour periods is less clear. Magnetic braking may be responsible, but this would be in contradiction with the hibernating nova scenario. It is interesting that contact binaries of W UMa type are subject to little or no magnetic braking.