Nova Research Articles

Formation of CN and CO Molecules in the Envelope of Nova V1391 Cas During the Near-maximum Phase

Moderate-resolution spectra (R ≈ 2700) of the classical nova V1391 Cas were obtained in the near-infrared wavelength range of 0.95–2.45 μm on UT 2020 August 14.41 using the Keck II telescope and the NIRES spectrograph. The spectral analysis revealed simultaneous detection of CO and CN molecular features. The CN red-system band was observed solely in absorption, while the CO first overtone bands, encompassing both 12CO and 13CO, displayed a combination of emission and absorption features at different velocities, resembling a P-Cygni profile. This specific CO profile is unprecedented in classical novae observations. Additionally, the negative velocity shifts in the absorption bands of both CN and CO were found to be consistent with each other.

Search for CP -Violating Neutrino Nonstandard Interactions with the NOvA Experiment

This Letter reports a search for charge-parity (CP) symmetry violating nonstandard interactions (NSI) of neutrinos with matter using the NOvA Experiment, and examines their effects on the determination of the standard oscillation parameters. Data from νμ(ν¯μ)→νμ(ν¯μ) and νμ(ν¯μ)→νe(ν¯e) oscillation channels are used to measure the effect of the NSI parameters ϵeμ and ϵeτ. With 90% CL the magnitudes of the NSI couplings are constrained to be |ϵeμ|≲0.3 and |ϵeτ|≲0.4. A degeneracy at |ϵeτ|≈1.8 is reported, and we observe that the presence of NSI limits sensitivity to the standard CP phase δCP. Published by the American Physical Society 2024

Discovery of a Bow-shock Nebula Around the Z Cam-type Cataclysmic Variable SY Cancri

We report the serendipitous discovery of a bow-shock nebula around the cataclysmic variable (CV) SY Cancri. In addition, SY Cnc lies near the edge of a faint Hα-emitting nebula with a diameter of about 15′ . The orientation of the bow shock is consistent with the direction of SY Cnc’s proper motion. Nebulae are extremely rare around CVs, apart from those known to have undergone classical-nova (CN) outbursts; bow shocks and off-center nebulae are even more unusual. Nevertheless, the properties of SY Cnc and its nebulosity are strikingly similar to those of V341 Ara, another CV that is also associated with a bow shock and is likewise off-center with respect to its faint Hα nebula. Both stars are binaries with optically thick accretion disks, belonging to the classes of Z Cam CVs or nova-like variables. We discuss three scenarios to explain their properties. They may have resulted from chance encounters with interstellar gas clouds, with the stars leaving in their wakes material that is recombining after being photoionized by ultraviolet radiation from the CVs. Alternatively, the large nebulae could be ejecta from unobserved CN outbursts in the recent past, which have been decelerated through collisions with the interstellar medium (ISM), while the stars continue to snowplow through the gas. Or the faint Hα nebulae may be ambient ISM that was flash-ionized by a CN outburst in the past and is now recombining.

The Accelerating Decline of the Mass Transfer Rate in the Recurrent Nova T Pyxidis* * Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of University for Research in Astronomy, Inc. under NASA contract NAS 5-26555.

The recurrent nova T Pyxidis (T Pyx) has erupted six times since 1890, with its last outburst in 2011, and the relatively short recurrence time between classical nova explosions indicates that T Pyx must have a massive white dwarf (WD) accreting at a high rate. It is believed that, since its outburst in 1890, the mass transfer rate in T Pyx was very large due to a feedback loop where the secondary is heated by the hot WD. The feedback loop has been slowly shutting off, reducing the mass transfer rate, and thereby explaining the magnitude decline of T Pyx from ∼13.8 (before 1890) to 15.7 just before the 2011 eruption. We present an analysis of the latest Hubble Space Telescope far-ultraviolet and optical spectra, obtained 12 yr after the 2011 outburst, showing that the mass transfer rate has been steadily declining and is now below its preoutburst level by about 40%: Ṁ∼1−3×10−7M⊙ yr−1 for a WD mass of ∼1.0–1.4 M ⊙, an inclination of 50°–60°, reddening of E(B − V) = 0.30 ± 0.05, and a Gaia Data Release 3 distance of 2860−471+816 pc. This steady decrease in the mass transfer rate in the ∼decade after the 2011 outburst is in sharp contrast with the more constant preoutburst ultraviolet continuum flux level from archival International Ultraviolet Explorer spectra. The flux (i.e., Ṁ ) decline rate is 29 times faster now in the last ∼decade than observed since 1890 to ∼2010. The feedback loop shut off seems to be accelerating, at least in the decade following its 2011 outburst. In all eventualities, our analysis confirms that T Pyx is going through an unusually peculiar short-lived phase.

A 9 Month Hubble Space Telescope Near-UV Survey of M87. II. A Strongly Enhanced Nova Rate near the Jet of M87

The 135 classical novae that we have discovered in M87 with two Hubble Space Telescope imaging surveys appear to be strongly concentrated along that galaxy’s jet. Detailed simulations show that the likelihood that this distribution occurred by chance is of order 0.3%. The novae near the jet display outburst characteristics (peak luminosities, colors, and decline rates) that are indistinguishable from novae far from the jet. We explore whether the remarkable nova distribution could be caused by the jet’s irradiation of the hydrogen-rich donors in M87's cataclysmic binaries. This explanation, and others extant in the literature that rely on increased binary mass transfer rates, fail by orders of magnitude in explaining the enhanced nova rate near the jet. An alternate explanation is the presence of a genuine surplus of nova binary systems near the jet, perhaps due to jet-induced star formation. This explanation fails to explain the lack of nova enhancement along M87's counterjet. The enhanced rate of novae along M87's jet is now firmly established, and unexplained.

Soft X-ray emission from the classical nova AT 2018bej

Context. Classical novae are known to demonstrate a supersoft X-ray source (SSS) state following outbursts. This state is associated with residual thermonuclear burning on the white dwarf (WD) surface. During its all-sky survey (eRASS1), the eROSITA telescope on board the Spectrum-Roentgen-Gamma observatory discovered a bright new SSS, whose position is consistent with the known classical nova AT 2018bej in the Large Magellanic Cloud. There were two eROSITA spectra obtained during the eRASS1 and eRASS2 monitoring epochs and one XMM-Newton grating spectrum close to the eRASS1 epoch. Aims. We aim to describe the eROSITA and follow-up XMM-Newton spectra of AT 2018bej with our local thermodynamic equilibrium (LTE) atmosphere models. We focussed on the evolution of the hot WD properties between the eRASS1 and eRASS2 epochs, especially with respect to the change in carbon abundance. Methods. A grid of LTE model atmosphere spectra was calculated for different values of the effective temperature (from Teff = 525 to 700 kK in steps of 25 kK), surface gravity (six values), and chemical composition, assuming approximately equal hydrogen and helium number fractions, and five different values of carbon and nitrogen abundances. Results. Both eRASS1 and XMM 0.3–0.6 keV spectral analyses yield a temperature of the WD of Teff~ 600 kK and a WD radius of 8000–8700 km. A simultaneous fitting of the eROSITA spectra for two epochs (eRASS1 and eRASS2) with a common WD mass parameter demonstrates a decrease in Teff, accompanied by an increase in the WD radius and a decrease in the carbon abundance. However, these changes are marginal and remain within the errors. The derived WD mass is estimated to be 1.05–1.15 M⊙. Conclusions. We traced a minor evolution of the source on a half-year timescale accompanied by a decrease in the carbon abundance and concluded that LTE model atmospheres can be used to analyse the available X-ray spectra of classical novae during their SSS state.

Hydrodynamical shear mixing in subsonic boundary layers and its role in the thermonuclear explosion of classical novae

Hydrodynamical shear mixing in subsonic boundary layers and its role in the thermonuclear explosion of classical novae

Jet-induced Enhancement of the Nova Rate in M87

We argue that the Bondi accretion mechanism has the potential to enhance classical nova production near the M87 jet. According to our estimates, the jet's influence can increase the nova rate in its vicinity by up to a factor of two over the background rate by triggering explosions on single white dwarfs. This result may offer a straightforward explanation for the recently observed excess of novae near the M87 jet.

Constraining the mass-spectra in the presence of a light sterile neutrino from absolute mass-related observables

The framework of three-flavor neutrino oscillation is a well-established phenomenon, but results from the short-baseline experiments, such as the Liquid Scintillator Neutrino Detector (LSND) and MiniBooster Neutrino Experiment (MiniBooNE), hint at the potential existence of an additional light neutrino state characterized by a mass-squared difference of approximately 1 eV2. The new neutrino state is devoid of all Standard Model (SM) interactions, commonly referred to as a “sterile” state. In addition, a sterile neutrino with a mass-squared difference of 10−2 eV2 has been proposed to improve the tension between the results obtained from the Tokai to Kamioka (T2K) and the NuMI Off-axis νe Appearance (NOνA) experiments. Further, the nonobservation of the predicted upturn in the solar neutrino spectra below 8 MeV can be explained by postulating an extra light sterile neutrino state with a mass-squared difference around 10−5 eV2. The hypothesis of an additional light sterile neutrino state introduces four distinct mass spectra depending on the sign of the mass-squared difference. In this paper, we discuss the implications of the above scenarios on the observables that depend on the absolute mass of the neutrinos; namely, the sum of the light neutrino masses (Σ) from cosmology, the effective mass of the electron neutrino from beta decay (mβ), and the effective Majorana mass (mββ) from neutrinoless double beta decay. We show that some scenarios can be disfavored by the current constraints of the above variables. The implications for projected sensitivity of Karlsruhe Tritium Neutrino Experiment (KATRIN) and future experiments like Project-8, next Enriched Xenon Observatory (nEXO) are discussed. Published by the American Physical Society 2024

Mixing of Hot Shocked Plasma with Cold Gas in Nova YZ Ret 2020

The origin of bright X-ray emission lines that appear late in a nova eruption remains largely a puzzle. We present two high-resolution X-ray grating spectra of the classical nova YZ Ret, observed 77 and 115 days post-eruption, using XMM-Newton and Chandra, respectively. Both spectra feature resolved emission lines blueshifted by v = –1500 km s−1 and broadened by σ v = 500 km s−1. The two spectra are well described by a collisionally ionized plasma of kT ∼ 70 eV that dimmed by a factor of ∼40 between the two exposures. The spectra also show narrow radiative recombination continua (RRCs) of C+4, C+5, and N+5, indicating the interaction of the hot ionized plasma with cold electrons of kT ∼ 2 eV. The high-n Rydberg series of C+4 is anomalously bright, allowing us to measure the electron density through continuum lowering, which is in agreement with the He-like N+5 density diagnostic of n e = (1.7 ± 0.4) × 1011 cm−3. The high population of these high-n levels constitutes the best evidence to date of charge exchange (CX) with neutral H in an astrophysical ionized plasma. The remarkable fact that the velocity and plasma temperature are the same after 38 days, despite the high density and decreasing flux, is evidence for ongoing heating. We suggest the heating is due to a reverse shock in the nova ejecta, which forms a thin X-ray shell. The narrow RRCs and CX are attributed to direct mixing with cold gas, which overtakes the hot plasma either from the shock front or through the contact discontinuity.

Recurrent Nova V2487 Oph Had Superflares in 1941 and 1942 with Radiant Energies of 1042.5±1.6 erg

V2487 Ophiuchi (V2487 Oph) is a recurrent nova with classical nova eruptions in 1900 and 1998, and it is also the most extreme known superflare star. These superflares are roughly hour-long flares with amplitudes and optical energies reaching up to 1.10 mag and 1039.21 erg, respectively, with the superflares recurring once a day. The V2487 Oph superflares are certainly operating with the same mechanism as all the other types of superflare stars, where magnetic loops are twisted and stretched until reconnection occurs, whereupon ambient electrons are accelerated to relativistic energies and then emit bremsstrahlung radiation from X-ray to radio. V2487 Oph is unique among known superflare stars in that one of the loop footprints is in an accretion disk. This exact mechanism was theoretically predicted by M. R. Hayashi and colleagues in 1996. Now, I have found two superflares recorded on Harvard archival photographs from 1941 and 1942. These two superflares have B-magnitude amplitudes of >1.83 and >2.00 mag and total radiated energies of 1042.4 and 1042.5 erg, respectively, with bolometric corrections. Each has emitted energies of ∼30 billion Carringtons, in units of the most energetic solar flare. Further, I find superflares in Zwicky Transient Factory light curves, so V2487 Oph has been superflaring from 1941 to 2023. For the observed number distribution of dN/dE = 4E −2 superflares per year, for E in units of 1041 erg, the emitted energy in superflare light is 1042.1 erg in each year, or 1044.1 erg from 1941 to 2023.

Physics of nova outbursts: Theoretical models of classical nova outbursts with optically thick winds on 1.2 M⊙ and 1.3 M⊙ white dwarfs

Abstract We present time-dependent nova outburst models with optically thick winds for 1.2 and 1.35$\, M_{\odot }$ white dwarfs (WDs) with a mass-accretion rate of $5 \times 10^{-9}\, M_{\odot }$ yr−1 and for a 1.3$\, M_{\odot }$ WD with $2 \times 10^{-9}\, M_{\odot }$ yr−1. The X-ray flash occurs 11 d before the optical peak of the 1.2$\, M_{\odot }$ WD and 2.5 d before the peak of the 1.3$\, M_{\odot }$ WD. The wind mass-loss rate of the 1.2$\, M_{\odot }$ WD (1.3$\, M_{\odot }$ WD) reaches a peak of $6.4 \times 10^{-5}\, M_{\odot }$ yr−1 ($7.4 \times 10^{-5}\, M_{\odot }$ yr−1) at the epoch of the maximum photospheric expansion with the lowest photospheric temperature of log Tph (K) = 4.33 (4.35). The nuclear energy generated during the outburst is lost in the form of radiation (61% for the 1.2$\, M_{\odot }$ WD; 47% for the 1.3$\, M_{\odot }$ WD), gravitational energy of ejecta (39%; 52%), and kinetic energy of the wind (0.28%; 0.29%). We found an empirical relation for fast novae between the time to optical maximum from the outburst tpeak and the expansion timescale τexp. With this relation, we are able to predict the time to optical maximum tpeak from the ignition model (at t = 0) without following a time-consuming nova wind evolution.

Search for Lorentz violations through the sidereal effect at the NOνA experiment

Long-baseline neutrino oscillation experiments offer a unique laboratory to test the fundamental Lorentz symmetry, which is at the heart of both the standard model of particle and general relativity theory. The sidereal modulation in neutrino events will act as the smoking-gun experimental signature of Lorentz and CPT violation. In this study, we investigate the impact of the sidereal effect on standard neutrino oscillation measurements within the context of the NuMI Off-Axis νe Appearance Experiment (NOνA) experiment. Additionally, we assess the sensitivity of the NOνA experiment to detect Lorentz-violating interactions, taking into account the sidereal effect. Furthermore, we highlight the potential of the NOνA experiment to set new constraints on anisotropic Lorentz-violating parameters. Published by the American Physical Society 2024

A Multiwavelength Light-curve Model of the Classical Nova V339 Del: A Mechanism for the Coexistence of Dust Dip and Supersoft X-Rays

The classical nova V339 Del 2013 is characterized by a 1.5 mag dip of the V light curve owing to a dust shell formation, with which soft X-ray emissions coexist. We present a Strömgren y-band light curve, which represents continuum emission, not influenced by strong [O iii] emission lines. The y light curve monotonically decreases in marked contrast to the V light curve that shows a 1.5 mag dip. We propose a multiwavelength light-curve model that reproduces the y and V light curves as well as the gamma-ray and X-ray light curves. In our model, a strong shock arises far outside the photosphere after optical maximum, because later ejected matter collides with earlier ejected gas. Our shocked shell model explains optical emission lines, Hα, hard X-ray, and gamma-ray fluxes. A dust shell forms behind the shock that suppresses [O iii]. This low flux of [O iii] shapes a 1.5 mag drop in the V light curve. Then, the V flux recovers with an increasing contribution from [O iii] lines, while the y flux does not. However, the optical depth of the dust shell is too small to absorb the photospheric (X-ray) emission of the white dwarf. This is the reason that a dust shell and soft X-ray radiation coexist. We determined the white dwarf mass to be M WD = 1.25 ± 0.05 M ☉ and the distance modulus in the V band to be (m − M) V = 12.2 ± 0.2; the distance is d = 2.1 ± 0.2 kpc for the reddening of E(B − V) = 0.18.

The Recurrent Nova V745 Sco had a Classical Nova Eruption in 1897 July

Recurrent nova V745 Sco is discovered to have a long-lost nova eruption in 1897 July. Now with four known eruptions (in 1897, 1937, 1989, and 2014), plus two likely missed eruptions (around 1917 and 1963), the average recurrence time is 23 yr, with a fractional rms of 13%.

The Long-term Activity of the Postnovae Q Cygni and BK Lyncis

We show the postnova activity of Q Cyg (Nova Cyg 1876) and BK Lyn (probable Nova Lyn 101). We use both CCD and photographic photometric observations. We show that both systems lie close to the upper limit of the luminosity in which dwarf nova (DN) outbursts occur. Q Cyg shows a novalike high-state activity. Random fluctuations (typically 0.6 mag) from a well-defined curve of the moving averages of brightness often occur on the timescale of weeks. The random fluctuations were suppressed during one fainter interval lasting several months but increased during another. In the author’s interpretation, clumps in the disk wind are likely to play a role in these fluctuations, especially when the luminosity of Q Cyg is near the upper limit of the range in which DN outbursts occur. BK Lyn was observed to spend about 100 yr in a very long state of a high luminosity on the upper limit of the region of DN outbursts before undergoing a time segment in which DN outbursts were present. We find that the individual DN outbursts in BK Lyn all show similar decay rates and fade more gradually than those of DNe that do not also show classical nova eruptions. We attribute it, along with the low amplitude of DN outbursts and the high quiescent luminosity, to the role of extra light. These outburst peaks, higher than the surrounding segments of the flat light curve, speak in favor of the ER UMa-type with superoutburst cycles and standstills rather than the high state in a novalike variable.

Correction to: Study of the fastest classical nova, V1674 Her: photoionization and morpho-kinemetic model analysis

Correction to: Study of the fastest classical nova, V1674 Her: photoionization and morpho-kinemetic model analysis

The Peculiar Bursting Nature of CP Pup

Abstract The classical nova CP Puppis has been observed to have particularly puzzling and peculiar properties. In particular, this classical nova displays occasional bursts in its long-term ASAS-SN light curve. Here we report on 5 sectors of TESS data displaying 2 of these rapid bursts, lasting ∼ 1 day. Based on the estimated lower energy limits of the bursts we discuss whether the bursts may be examples of micronovae resulting from localised thermonuclear explosion. Furthermore, its orbital period remains uncertain, with several inconsistent periodic signals appearing in spectroscopic and photometric observations at various wavelengths. Although we cannot unambiguously unravel the physical origin of the signals, the previously suggested nature of CP Puppis as a long orbital period system may be a viable explanation. The recurrence time of the bursts in CP Puppis, together with the unexplained variable modulations make it a prime candidate for intense monitoring.

Hydrodynamic Simulations of Oxygen–Neon Classical Novae as Galactic 7Li Producers and Potential Accretion-induced Collapse Progenitors* *We dedicate this paper to the memory of R. Mark Wagner. Friend, collaborator, exemplary observational astronomer, and contributor to studies of classical novae and transient objects for more than 40 years. He will be missed.

We report on studies of classical nova (CN) explosions where we follow the evolution of thermonuclear runaways (TNRs) on oxygen–neon (ONe) white dwarfs (WDs). Using NOVA, a 1D hydrodynamic computer code, we accrete solar matter until the TNR is ongoing and then switch to a mixed composition. This approach is guided by the results of multidimensional studies of TNRs in WDs, which find that sufficient mixing with WD core material occurs after the TNR is well underway, and levels of enrichment of the CNONeMg elements are reached that agree with observations of CN ejecta abundances. Because the amount of accreted material is inversely proportional to the oxygen abundance, by first accreting solar matter, the amount of accreted material is larger than in those simulations with an initially enriched composition. We vary the mass of the WD (from 0.6 M ⊙ to 1.35 M ⊙) and the composition of the mixed materials. Our results show large enrichments of 7Be in the ejected gases, implying that ONe CNe and CO CNe may be responsible for a significant fraction (∼100 M ⊙) of the Galactic 7Li (∼1000 M ⊙). The production of 22Na and 26Al in CN explosions and the γ-ray emission predicted by our simulations are discussed. The WDs in all our simulations eject less material than they accrete and we predict that the WD is growing in mass as a consequence of the CN outburst. ONe CNe, therefore, may be an important channel for accretion-induced collapse events.

On an apparent dearth of recurrent nova super-remnants in the Local Group

ABSTRACT The Andromeda Galaxy is home to the annually erupting recurrent nova (RN) M 31N 2008-12a (12a); the first nova found to host a nova super-remnant (NSR). An NSR is an immense structure surrounding a RN, created from many millions of eruptions sweeping up material in the local environment to form a shell tens of parsecs across. Theory has demonstrated that NSRs should be found around all recurrent novae (RNe), even those systems with long periods between eruptions. Befittingly, the second NSR was found around the Galactic classical (and long suspected recurrent) nova, KT Eridani. In this Paper, we aim to find more of these phenomena through conducting the first ever survey for NSRs in M 31 and the Large Magellanic Cloud (LMC). We find that the surroundings of fourteen RNe in M 31 as well as the surroundings of the four RNe in the LMC do not show any evidence of vast parsec-scale structures in narrow-band (H α and $[{\rm S\, {\small II}}]$) images, unlike the one clearly seen around 12a, and therefore conclude that observable NSRs are either rare structures, or they are too faint (or small) to be detected in our existing data sets. Yet, the NSR surrounding 12a would also likely to have been overlooked in our study if it were approximately one magnitude fainter. Searches for NSRs around other RNe ‘masquerading’ as classical novae may prove to be fruitful as would whole surveys of other Local Group galaxies.