Nova Models Research Articles

AN EXTENDED GRID OF NOVA MODELS. III. VERY LUMINOUS, RED NOVAE

Extremely luminous, red eruptive variables like RV in M31 are being suggested as exemplars of a new class of astrophysical object. Our greatly extended series of nova simulations shows that classical nova models can produce very red, luminous eruptions. In a poorly studied corner of 3-D nova parameter space (very cold, low-mass white dwarfs, accreting at very low rates) we find bona fide classical novae that are very luminous and red because they eject very slowly moving, massive envelopes. A crucial prediction of these nova models - in contrast to the predictions of merging star ("mergeburst") models - is that a hot remnant, the underlying white dwarf, will emerge after the massive ejected envelope has expanded enough to become optically thin. This blue remnant must fade on a timescale of decades - much faster than a "mergeburst", which must fade on timescales of millennia or longer. Furthermore, the cooling nova white dwarf and its expanding ejecta must become redder in the years after eruption, while a contracting mergeburst must become hotter and bluer. We predict that red novae will always brighten to L~1000 Lsun for about 1 year before rising to maximum luminosity at L~10^6 - 10^7 Lsun. The maximum luminosity attainable by a nova is likely to be L~10^7 Lsun, corresponding to M-12. In an accompanying paper we describe a fading, luminous blue candidate for the remnant of M31-RV; it is observed with HST to be compatible only with the nova model.

THE RED NOVA-LIKE VARIABLE IN M31—A BLUE CANDIDATE IN QUIESCENCE

M31-RV was an extraordinarily luminous (~10^6 Lsun) eruptive variable, displaying very cool temperatures (roughly 1000 Kelvins) as it faded. The photometric behavior of M31-RV (and several other very red novae, i.e. luminous eruptive red variables) has led to several models of this apparently new class of astrophysical object. One of the most detailed models is that of "mergebursts": hypothetical mergers of close binary stars. These are predicted to rival or exceed the brightest classical novae in luminosity, but to be much cooler and redder than classical novae, and to become slowly hotter and bluer as they age. This prediction suggests two stringent and definitive tests of the mergeburst hypothesis. First, there should always be a cool red remnant, and NOT a hot blue remnant at the site of such an outburst. Second, the inflated envelope of a mergeburst event should be slowly contracting, hence it must display a slowly rising effective temperature. We have located a luminous, UV-bright object within 0.4 arcsec (1.5 sigma of the astrometric position) of M31-RV in archival WFPC2 images taken 10 years after the outburst: it resembles an old nova. Twenty years after the outburst, the object remains much too hot to be a mergeburst. Its behavior remains consistent with that of theoretical nova models which erupt on a low mass white dwarf. Future Hubble UV and visible images could determine if the M31-RV analogs (in M85 and in M99) are also behaving like old novae.

The orbital period and system parameters of the recurrent nova T Pyx

T Pyx is a luminous recurrent nova that accretes at a much higher rate than is expected for its photometrically determined orbital period of about 1.8 hours. We here provide the first spectroscopic confirmation of the orbital period, P =1.829 hours (f=13.118368(11) c/d), based on time-resolved optical spectroscopy obtained at the VLT and the Magellan telescopes. We also derive an upper limit of the velocity semi-amplitude of the white dwarf, K1 = 17.9 +/- 1.6 km/s, and estimate a mass ratio of q = 0.20 +/- 0.03. If the mass of the donor star is estimated using the period-density relation and theoretical main-sequence mass-radius relation for a slightly inflated donor star, we find M2 = 0.14 +/- 0.03 Msun. This implies a mass of the primary white dwarf M1 = 0.7 +/- 0.2 Msun. If the white-dwarf mass is > 1 Msun, as classical nova models imply, the donor mass must be even higher. We therefore rule out the possibility that T Pyx has evolved beyond the period minimum for cataclysmic variables. We find that the system inclination is constrained to be approximately 10 degrees, confirming the expectation that T Pyx is a low-inclination system. We also discuss some of the evolutionary implications of the emerging physical picture of T Pyx. In particular, we show that epochs of enhanced mass transfer (like the present) may accelerate or even dominate the overall evolution of the system, even if they are relatively short-lived. We also point out that such phases may be relevant to the evolution of cataclysmic variables more generally.

NON-EQUIPARTITION OF ENERGY, MASSES OF NOVA EJECTA, AND TYPE Ia SUPERNOVAE

The total masses ejected during classical nova eruptions are needed to answer two questions with broad astrophysical implications: Can accreting white dwarfs be pushed towards the Chandrasekhar mass limit to yield type Ia supernovae? Are Ultra-luminous red variables a new kind of astrophysical phenomenon, or merely extreme classical novae? We review the methods used to determine nova ejecta masses. Except for the unique case of BT Mon (nova 1939), all nova ejecta mass determinations depend on untested assumptions and multi-parameter modeling. The remarkably simple assumption of equipartition between kinetic and radiated energy (E_kin and E_rad, respectively) in nova ejecta has been invoked as a way around this conundrum for the ultra-luminous red variable in M31. The deduced mass is far larger than that produced by any classical nova model. Our nova eruption simulations show that radiation and kinetic energy in nova ejecta are very far from being in energy equipartition, with variations of four orders of magnitude in the ratio E_kin/E_rad being commonplace. The assumption of equipartition must not be used to deduce nova ejecta masses; any such "determinations" can be overestimates by a factor of up to 10,000. We data-mined our extensive series of nova simulations to search for correlations that could yield nova ejecta masses. Remarkably, the mass ejected during a nova eruption is dependent only on (and is directly proportional to) E_rad. If we measure the distance to an erupting nova and its bolometric light curve then E_rad and hence the mass ejected can be directly measured.

Effect of flow rules and elastic strains on pressuremeter test results in dense sand

Simple numerical methods that allow to obtain stress and strain paths in sand during self-boring pressuremeter tests are presented in this paper. The material is considered to undergo deformation in drained and plane-strain conditions. The flow rules of Rowe, Cambridge-type (Cam clay and Nova) model, and the sawtooth model are used in the analysis. Elastic strains are also considered in relation with the flow rule of Rowe. The proposed approach has been evaluated using a reference pressuremeter test performed on Ticino sand in a calibration chamber. The results indicate that the stress–strain–volume change responses of the sand predicted by the various models are quite similar, with and without consideration of the elastic strain components.

NewP30(p,γ)S31resonances and oxygen-neon nova nucleosynthesis

Enriched isotopic abundance ratios of $^{30}\mathrm{Si}$/$^{28}\mathrm{Si}$ in several presolar SiC and graphite grains qualitatively indicate massive oxygen-neon (ONe) nova origins but fall short of hydrodynamic ONe nova model ejecta predictions by as much as an order of magnitude. The astrophysical $^{30}\mathrm{P}$($p,\ensuremath{\gamma}$)$^{31}\mathrm{S}$ reaction rate uncertainty at ONe nova temperatures ($0.10lTl0.35$ GK) spans several orders of magnitude through which the predicted amount of ejected $^{30}\mathrm{Si}$ can vary by a factor of 100. By measuring triton momenta from the $^{31}\mathrm{P}$($^{3}\mathrm{He}$,$t$)$^{31}\mathrm{S}$ reaction at 20 MeV, the energies of $^{30}\mathrm{P}$ $+p$ resonances in the Gamow window for ONe novae have been determined to better than \ifmmode\pm\else\textpm\fi{}3 keV, and two new resonances at ${E}_{\mathrm{c}.\mathrm{m}.}=194.0(25)$ and 266.4(27) keV that likely dominate the $^{30}\mathrm{P}$($p,\ensuremath{\gamma}$)$^{31}\mathrm{S}$ rate for $0.08lTl0.25$ GK have been resolved. A resulting increase in the experimentally determined $^{30}\mathrm{P}$($p,\ensuremath{\gamma}$)$^{31}\mathrm{S}$ reaction rate puts it in accord with Hauser-Feshbach statistical model estimates for $0.08lTl0.40$ GK, supporting conclusions drawn from ONe nova model studies that employed a Hauser-Feshbach rate.

Neutron stars as cosmic laboratories to explore hadronic matter at ultra-high densities

We examine the present status of the theoretical calculations for the internal structure of neutron stars, and the connection with the microscopic properties of ultradense hadronic matter. We discuss the possibility to have quark deconfinement phase transition in the core of neutron stars, and we explore some of its astrophysical implications as the quark-deconfinement nova model for gamma-ray bursts.

A comparison of measured and calculated toroidal Alfvén eigenmode damping rates in Alcator C-Mod

Measured damping rates of stable toroidal Alfvén eigenmodes (TAEs) have been compared with damping rates calculated with the NOVA-K code for a number of experimental conditions on Alcator C-Mod to test the NOVA-K damping model. Very low amplitude ( 10−7 at the wall) stable TAEs are excited in Ohmic plasmas with a pair of active MHD antennas inside C-Mod. By sweeping the excited frequency of the antennas through the expected TAE frequency, the frequency width of a mode resonance can be measured with poloidal field pick-up coils on outboard limiters to determine the damping rate of that mode. The calculated equilibrium and main plasma parameters at the time of the resonance are input into the NOVA-K code to then calculate the expected damping rate for the measured toroidal mode number and compare with the experimental value. The calculations include continuum damping, Landau damping on both the electrons and the background and impurity ions and collisional damping on the electrons. Comparisons have been made for inner wall limited and diverted cases over a range of ITER-relevant moderate toroidal mode numbers (4 ⩽ |n | ⩽ 9). Good agreement between the experiment and theoretical calculations can be obtained with reasonable q profiles for the plasma conditions in which perturbative NOVA models are applicable, but the results are found to depend very sensitively on the assumed q profile.

HR4049: signature of nova nucleosynthesis?

The post-Asymptotic Giant Branch (AGB) star HR4049 is in an eccentric binary system with a relatively short period probably surrounded by a dusty circumbinary disk. Extremely anomalous oxygen isotopic ratios, 16 O/ 17 O ≃ 16 O/ 18 O ≃ 7, have been measured from CO 2 molecules likely residing in the disk. Such a composition cannot be explained in the framework of AGB and post-AGB evolution while it can be qualitatively associated with the nucleosynthesis occurring during nova outbursts. We discuss nova models, the presence of a white dwarf companion to HR4049 and possible scenarios for the dynamical evolution of this binary system. Circumbinary disks in which mixing occurs between redgiant and nova material may also be invoked as the site of formation of some rare types of meteoritic presolar grains.

Studies of the 18F( p, α) 15O reaction rate with a 18F beam at the HRIBF

The rates of the 18 F( p , α ) 15 O and 18 F( p , γ) 19 Ne reactions at T = 0.1 – 0.4 GK are necessary to understand the emission of positron-annihilation gamma rays expected from novae. Observations of such gamma rays by space-based telescopes would provide direct tests of nova models. These rates, however, are uncertain because of the unknown level structure of 19 Ne above the 18 F+ p threshold. Missing levels in 19 Ne were searched for by stopping a 24-MeV 18 F beam in a thick polypropylene target, and scattered protons were detected in a silicon-strip detector. The 18 F( p , p ) 18 F excitation function has been measured from E c.m. = 0.3 - 1.3 MeV covering most energies of astrophysical interest, and a significant discrepancy was observed with a recent compilation of 19 Ne resonance parameters.

Is T Leonis a superoutbursting intermediate polar?

We present an XMM-Newton analysis of the cataclysmic variable T Leo. The X-ray light curve shows sinusoidal variation on a period P_x equal to 0.89^{+0.14}_{-0.10} times the previously spectroscopically determined orbital period. Furthermore, we find a signal in the power spectrum at 414 sec that could be attributed to the spin period of the white dwarf. If true, T Leo would be the first confirmed superoutbursting intermediate polar IP). The spin profile is double-peaked with a peak separation of about 1/3 spin phases. This appears to be a typical feature for IPs with a small magnetic field and fast white dwarf rotation. An alternative explanation is that the 414 sec signal is a Quasi-periodic Oscillation (QPO) that is caused by mass transfer variation from the secondary, a bright region (``blob'') rotating in the disc at a radius of approximately ~9 Rwd or - more likely - a travelling wave close to the inner disc edge of a dwarf nova with a low field white dwarf. The XMM-Newton RGS spectra reveal double peaked emission for the O VIII Ly alpha line. Scenarios in the IP and dwarf nova model are discussed (an emitting ring in the disc, bright X-ray spot on disc edge, or emitting accretion funnels), but the intermediate polar model is favoured. Supported is this idea by the finding that only the red peak appears to be shifted and the `blue' peak is compatible with the rest wavelength. The red peak thus is caused by emission from the northern accretion spot when it faces the observer. Instead, the peak at the rest wavelength is caused when the southern accretion funnel is visible just on the lower edge of the white dwarf - with the velocity of the accreting material being perpendicular to the line of sight.

Impact of nuclear reaction rate uncertainties on Nova models

We have, for the first time, determined the effect of nuclear reactions rate uncertainties on nova model predictions by simultaneously considering uncertainties in all relevant reaction rates. Our unique Monte Carlo approach enables a robust determination of uncertainties of nuclear origin in predictions of synthetized abundances, including radioisotopes which may be observable. This technique also enables us to identify which reactions most influence the production of each isotope, thereby guiding future measurements.

Insights from Pulsating Nova Envelopes

AbstractBased on a linear and non-linear study of radial pulsations in the envelopes of classical novae (Schenker, 1999), I discuss the results both from the point of view of pulsation theory as well as their consequences for current nova models. Starting from initially static envelope structures at various stages during the decline of a nova outburst, strong ‘running-wave’ instabilities have been found that rapidly grow into shocks. Improved analytical concepts give a new direction to the interpretation of such highly non-adiabatic radial pulsations. For direct observational confirmation, a search for short period variability in the UV and soft X-ray is suggested during the very late decline phase. Speculative consequences for mass-loss scenarios in novae due to these instabilities will need some more work in the future.

Presolar Grains from Novae

We report the discovery of five SiC grains and one graphite grain isolated from the Murchison carbonaceous meteorite whose major-element isotopic compositions indicate an origin in nova explosions. The grains are characterized by low 12C/13C (4-9) and 14N/15N (5-20) ratios, large excesses in 30Si (30Si/28Si ratios range to 2.1 times solar) and high 26Al/27Al ratios. These isotopic signatures are theoretically predicted for the ejecta from ONe novae and cannot be matched by any other stellar sources. Previous studies of presolar grains from primitive meteorites have shown that the vast majority formed in red giant outflows and supernova ejecta. Although a classical nova origin was suggested for a few presolar graphite grains on the basis of 22Ne enrichments, this identification is somewhat ambiguous since it is based only on one trace element. Our present study presents the first evidence for nova grains on the basis of major element isotopic compositions of single grains. We also present the results of nucleosynthetic calculations of classical nova models and compare the predicted isotopic ratios with those of the grains. The comparison points toward massive ONe novae if the ejecta are mixed with material of close-to-solar composition.

Non‐LTE Modeling of Nova Cygni 1992

We present a grid of nova models that have an extremely large number of species treated in non-LTE and apply it to the analysis of an extensive time series of ultraviolet spectroscopic data for Nova Cygni 1992. We use ultraviolet colors to derive the time development of the effective temperature of the expanding atmosphere during the fireball phase and the first 10 days of the optically thick wind phase. We find that the nova has a pure, optically thick wind spectrum until about 10 days after the explosion. During this interval, we find that synthetic spectra based on our derived temperature sequence agree very well with the observed spectra. We find that a sequence of hydrogen-deficient models provides an equally good fit, provided the model effective temperature is shifted upward by ~1000 K. We find that high-resolution UV spectra of the optically thick wind phase are fit moderately well by the models. We find that a high-resolution spectrum of the fireball phase is better fit by a model with a steep density gradient, similar to that of a supernova, than by a nova model.

Nucleosynthesis in ONeMg Novae: Models versus Observations to Constrain the Masses of ONeMg White Dwarfs and Their Envelopes

Nucleosynthesis in ONeMg novae has been investigated with the wide ranges of three parameters, i.e., the white dwarf mass, the envelope mass at ignition, and the initial composition. A quasi-analytic one- zone approach is used with an up-to-date nuclear reaction network. The nucleosynthesis results show correlation with the peak temperatures or the cooling timescales during outbursts. Among the com- binations of white dwarf and envelope masses that give the same peak temperature, the explosion is more violent for a lower white dwarf mass owing to its smaller gravitational potential. Comparison of the nucleosynthesis results with observations implies that at least two-thirds of the white dwarf masses for the observed ONeMg novae are ^1.1 which is signi—cantly lower than estimated by previous M _ , hydrodynamic studies but consistent with the observations of V1974 Cyg. Moreover, the envelope masses derived from the comparison are which is in good agreement with the ejecta masses Z10~4 M _ , estimated from observations but signi—cantly higher than in previous hydrodynamic studies. With such a low-mass white dwarf and a high-mass envelope, a nova can produce interesting amounts of the c-ray emitters 7Be, 22Na, and 26Al. We suggest that V1974 Cyg has produced 22Na as high as the upper limit derived from the COMPTEL survey. In addition, a nonnegligible part, if not the majority, of the Galac- tic 26Al may originate from ONeMg novae. Both the future International Gamma-Ray Astrophysical L aboratory (INT EGRAL ) survey for these c-ray emitters and abundance estimates derived from ultra- violet, optical, and near-infrared spectroscopy will impose severe constraints on the current nova models. Subject headings: novae, cataclysmic variablesnuclear reactions, nucleosynthesis, abundances ¨ white dwarfs

Black-Hole X-Ray Transients: The Effect of Irradiation on Time-Dependent Accretion Disk Structure

Some effects of irradiation on time-dependent accretion-disk models for black hole X-ray novae are presented. Two types of irradiation are considered: direct irradiation from the inner hot disk and indirect irradiation as might be reflected by a corona or chromosphere above the disk. The shadowing effect of the time-dependent evolution of the disk height and consequent blocking of the outer disk by the inner and middle portions of the disk from the direct irradiation is included. The direct irradiation of the disk by inner layers where the soft X-ray flux is generated is found to have only a small effect on the outer disk because of shadowing. Mild indirect irradiation that flattens, but otherwise does not affect the light curve substantially, still has interesting non-linear effects on the structure of the disk as heating and cooling waves propagate. The irradiated disks do not always make simple transitions between the hot and cold states, but can linger at intermediate temperatures or even return temporarily to the hot state, depending on the irradiation and the activity in adjacent portions of the disk.

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

LMC 1988 1 was a moderately fast, CO, dust-forming classical nova. It was observed extensively in many different wavelength regions during its early evolution. The UV, optical and infrared light curves of LMC 1988 1 are very similar to those of the Galactic Nova V1668 Cyg 1978. As in V1668 Cyg, LMC 1988 1 showed the presence of an optically thin dust shell ∼55 d after visual maximum which lasted for about two months. We successfully fit the UV and optical spectra of LMC 1988 1 taken within the first two months of its outburst (when the atmosphere was still optically thick) with synthetic spectra computed using PHOENIX nova model atmospheres. The synthetic spectra reproduce most of the features seen in the observations and provide V-band magnitudes consistent with the observed light curve. From the synthetic spectral fits to the observed data, we find that the bolometric luminosity was approximately constant at (3.0 ± 0.3) × 1038 erg s−1 (assuming a distance of 53 kpc to the LMC) during the first 45 d after visual maximum. The fits are improved by increasing the CNO abundances to ∼10 times their solar values, implying that the ejecta were enriched in CNO elements.

Nucleosynthesis in Classical Novae: CO versus ONe White Dwarfs

Detailed nucleosynthesis in the ejecta of classical novae has been determined for a grid of hydrodynamic nova models. The reported 14 evolutionary sequences, followed from the onset of accretion up to the explosion and ejection stages, span a range of CO and ONe white dwarf masses (0.8-1.35 M☉) and mixing levels between the accreted envelope and the underlying white dwarf core (25%-75%). The synthesis of each isotope from 1H to 40Ca is discussed, along with its sensitivity to model parameters. Special emphasis is placed on isotopes such as 13C,15N, and 17O, whose synthesis may account for a significant fraction of their Galactic content. Production of the radioactive isotopes 7Be,22Na, and 26Al is also analyzed, since they may provide a direct test of the thermonuclear runaway model through their γ-ray emission. The resulting elemental yields reproduce the spectroscopic abundance determinations of several well-studied classical novae fairly well.

Properties of discontinuous and nova-amplified mass transfer in cataclysmic variables

We investigate the effects of discontinuous mass loss in recurrent outburst events on the long-term evolution of cataclysmic variables (CVs). Similarly we consider the effects of frictional angular momentum loss (FAML), i.e. interaction of the expanding nova envelope with the secondary. The Bondi-Hoyle accretion model is used to parametrize FAML in terms of the expansion velocity vexp of the nova envelope at the location of the secondary; we find that small vexp causes strong FAML. Numerical calculations of CV evolution over a wide range of parameters demonstrate the equivalence of a discontinuous sequence of nova cycles and the corresponding mean evolution (replacing envelope ejection by a continuous wind), even close to the mass-transfer instability. A formal stability analysis of discontinuous mass transfer confirms this, independent of details of the FAML model. FAML is a consequential angular momentum loss that amplifies the mass-transfer rate driven by systemic angular momentum losses such as magnetic braking. We show that for a given vexp and white dwarf mass the amplification increases with secondary mass and is significant only close to the largest secondary mass consistent with mass-transfer stability. The amplification factor is independent of the envelope mass ejected during the outburst, whereas the mass-transfer amplitude induced by individual nova outbursts is proportional to it. In sequences calculated with nova model parameters taken from Prialnik & Kovetz, FAML amplification is negligible, but the outburst amplitude in systems below the period gap with a white dwarf mass ≃ 0.6 M⊙ is larger than a factor of 10. The mass-transfer rate in such systems is smaller than 10−11 M⊙ yr−1 for ≃ 0.5 Myr (≃ 10 per cent of the nova cycle) after the outburst. This offers an explanation for intrinsically unusually faint CVs below the period gap.