The classical nova V2491 Cyg was once suggested to be a recurrent nova. We have broadly reproduced the light curve of V2491 Cyg by a nova outburst model on a cold $1.36~M_odot$ white dwarf (WD), which strongly suggests that V2491 Cyg is a classical n
ova outbursting on a cold very massive WD rather than a recurrent nova outbursting on a warmer WD like the recurrent nova RS Oph. In a long-term evolution of a cataclysmic binary, an accreting WD has been settled down to a thermal equilibrium state with the balance of gravitational energy release and neutrino loss. The central temperature of the WD is uniquely determined by the energy balance. The WD is hot (cold) for a high (low) mass-accretion rate. We present the central temperatures, ignition masses, ignition radii, and recurrence periods for various WD masses and mass-accretion rates. In a classical nova, which corresponds to a low mass-accretion rate, the WD is cool and strongly degenerated and the ignition mass is large, which result in a strong nova outburst. In a recurrent nova, the WD is relatively warmer because of a high mass accretion rate and the outburst is relatively weaker. The gravitational energy release substantially contributes to the luminosity during the recurrent nova outbursts. We compare physical properties between classical novae and recurrent novae and discuss the essential differences between them.
Massive star evolution is still poorly understood, and observational tests are required to discriminate between different implementations of physical phenomenon in stellar evolution codes. By confronting stellar evolution models with observed propert
ies of blue supergiants, such as pulsations, chemical composition and position in the Hertzsprung-Russell diagram, we aim at determining which of the criterion used for convection (Schwarzschild or Ledoux) is best able to explain the observations. We compute state-of-the-art stellar evolution models with either the Schwarzschild or the Ledoux criterion for convection. Models are for $14$ to $35,M_odot$ at solar or Large Magellanic Cloud metallicity. For each model, we compute the pulsation properties to know when radial modes are excited. We then compare our results with the position of blue supergiants in the Hertzsprung-Russell diagram, with their surface chemical composition, and with their variability. Our results at Large Magellanic Cloud metallicity shows only a slight preference for the Ledoux criterion over the Schwarzschild one in reproducing at the same time the observed properties of blue supergiants, even if the Schwarzschild criterion cannot be excluded at this metallicity. We check that changing the overshoot parameter at solar metallicity does not improve the situation. We also check that our models are able to reproduce the position of Galactic blue supergiant in the flux-weighted-gravity -- luminosity relation. We confirm that overall, models computed with the Ledoux criterion are slightly better in matching observations. Our results also support the idea that most Galactic $alpha$ Cyg variables are blue supergiants of the group 2, i.e. stars that have been through a previous red supergiant phase where they have lost large amount of mass.
The relation of period spacing ($Delta P$) versus period ($P$) of dipole prograde g modes is known to be useful to measure rotation rates in the g-mode cavity of rapidly rotating $gamma$ Dor and slowly pulsating B (SPB) stars. In a rapidly rotating s
tar, an inertial mode in the convective core can resonantly couple with g modes propagative in the surrounding radiative region. The resonant coupling causes a dip in the $P$-$Delta P$ relation, distinct from the modulations due to the chemical composition gradient. Such a resonance dip in $Delta P$ of prograde dipole g modes appears around a frequency corresponding to a spin parameter $2f_{rm rot}{rm(cc)}/ u_{rm co-rot} sim 8-11$ with $f_{rm rot}$(cc) being the rotation frequency of the convective core and $ u_{rm co-rot}$ the pulsation frequency in the co-rotating frame. The spin parameter at the resonance depends somewhat on the extent of core overshooting, central hydrogen abundance, and other stellar parameters. We can fit the period at the observed dip with the prediction from prograde dipole g modes of a main-sequence model, allowing the convective core to rotate differentially from the surrounding g-mode cavity. We have performed such fittings for 16 selected $gamma$ Dor stars having well defined dips, and found that the majority of $gamma$ Dor stars we studied rotate nearly uniformly, while convective cores tend to rotate slightly faster than the g-mode cavity in less evolved stars.
The Kepler spacecraft observed the hot subdwarf star PHL 417 during its extended K2 mission, and the high-precision photometric lightcurve reveals the presence of 17 pulsation modes with periods between 38 and 105 minutes. From follow-up ground-based
spectroscopy we find that the object has a relatively high temperature of 35 600 K, a surface gravity of $log g / {rm cm,s^{-2}},=,5.75$ and a super-solar helium abundance. Remarkably, it also shows strong zirconium lines corresponding to an apparent +3.9 dex overabundance compared with the Sun. These properties clearly identify this object as the third member of the rare group of pulsating heavy-metal stars, the V366 Aquarii pulsators. These stars are intriguing in that the pulsations are inconsistent with the standard models for pulsations in hot subdwarfs, which predicts that they should display short-period pulsations rather than the observed longer periods. We perform a stability analysis of the pulsation modes based on data from two campaigns with K2. The highest amplitude mode is found to be stable with a period drift, $dot{P}$, of less than $1.1cdot10^{-9}$ s/s. This result rules out pulsations driven during the rapid stages of helium flash ignition.
We propose a theory for the MMRD relation of novae, using free-free emission model light curves built on the optically thick wind theory. We calculated $(t_3,M_{V,rm max})$ for various sets of $(dot M_{rm acc}, M_{rm WD})$, where $M_{V,rm max}$ is th
e peak absolute $V$ magnitude, $t_3$ is the 3-mag decay time from the peak, and $dot M_{rm acc}$ is the mass accretion rate on to the white dwarf (WD) of mass $M_{rm WD}$. The model light curves are uniquely characterized by $xequiv M_{rm env}/M_{rm sc}$, where $M_{rm env}$ is the hydrogen-rich envelope mass and $M_{rm sc}$ is the scaling mass at which the wind has a certain wind mass-loss rate. For a given ignition mass $M_{rm ig}$, we can specify the first point $x_0= M_{rm ig}/M_{rm sc}$ on the model light curve, and calculate the corresponding peak brightness and $t_3$ time from this first point. Our $(t_3, M_{V,rm max})$ points cover well the distribution of existing novae. The lower the mass accretion rate, the brighter the peak. The maximum brightness is limited to $M_{V,rm max} gtrsim -10.4$ by the lowest mass-accretion rate of $dot M_{rm acc} gtrsim1 times 10^{-11}~M_odot$ yr$^{-1}$. A significant part of the observational MMRD trend corresponds to the $dot M_{rm acc}sim5times10^{-9}~M_odot$ yr$^{-1}$ line with different WD masses. A scatter from the trend line indicates a variation in their mass-accretion rates. Thus, the global trend of an MMRD relation does exist, but its scatter is too large for it to be a precision distance indicator of individual novae. We tabulate $(t_3, M_{V,rm max})$ for many sets of $(dot M_{rm acc},M_{rm WD})$.
Strong magnetic fields in chemically peculiar A-type (Ap) stars typically suppress low-overtone pressure modes (p modes) but allow high-overtone p modes to be driven. KIC 11296437 is the first star to show both. We obtained and analysed a Subaru spec
trum, from which we show that KIC 11296437 has abundances similar to other magnetic Ap stars, and we estimate a mean magnetic field modulus of $2.8pm0.5$ kG. The same spectrum rules out a double-lined spectroscopic binary, and we use other techniques to rule out binarity over a wide parameter space, so the two pulsation types originate in one $delta$ Sct--roAp hybrid pulsator. We construct stellar models depleted in helium and demonstrate that helium settling is second to magnetic damping in suppressing low-overtone p modes in Ap stars. We compute the magnetic damping effect for selected p and g modes, and find that modes with frequencies similar to the fundamental mode are driven for polar field strengths $lesssim4$ kG, while other low-overtone p modes are driven for polar field strengths up to $sim$1.5 kG. We find that the high-order g modes commonly observed in $gamma$ Dor stars are heavily damped by polar fields stronger than 1--4 kG, with the damping being stronger for higher radial orders. We therefore explain the observation that no magnetic Ap stars have been observed as $gamma$ Dor stars. We use our helium-depleted models to calculate the $delta$ Sct instability strip for metallic-lined A (Am) stars, and find that driving from a Rosseland mean opacity bump at $sim$$5times10^4$ K caused by the discontinuous H-ionization edge in bound-free opacity explains the observation of $delta$ Sct pulsations in Am stars.
KIC 10685175 (TIC 264509538) was discovered to be a rapidly oscillating Ap star from {it Kepler} long cadence data using super-Nyquist frequency analysis. It was re-observed by TESS with 2-min cadence data in Sectors 14 and 15. We analyzed the TESS l
ight curves, finding that the previously determined frequency is a Nyquist alias. The revised pulsation frequency is $191.5151 pm 0.0005$d$^{-1}$ ($P = 7.52$min) and the rotation frequency is $0.32229 pm 0.00005$d$^{-1}$ ($P_{rm rot} = 3.1028$d). The star is an oblique pulsator with pulsation amplitude modulated by the rotation, reaching pulsation amplitude maximum at the time of the rotational light minimum. The oblique pulsation generates a frequency quintuplet split by exactly the rotation frequency. The phases of sidelobes, the pulsation phase modulation, and a spherical harmonic decomposition all show this star to be pulsating in a distorted quadrupole mode. Following the oblique pulsator model, we calculated the rotation inclination $i$ and magnetic oblique $beta$ of this star, which provide detailed information of pulsation geometry. The $i$ and $beta$ derived by the best fit of pulsation amplitude and phase modulation through a theoretical model differ from those calculated for a pure quadrupole, indicating the existence of strong magnetic distortion. The model also predicts the polar magnetic field strength is as high as about 6kG which is predicted to be observed in a high resolution spectrum of this star.
ASASSN-16oh is a peculiar transient supersoft X-ray source without a mass-ejection signature in the field of the Small Magellanic Cloud. Maccarone et al. (2019) concluded that ASASSN-16oh is the first dwarf nova with supersoft X-ray that originated f
rom an equatorial accretion belt on a white dwarf (WD). Hillman et al. (2019) proposed a thermonuclear runaway model that both the X-rays and $V$/$I$ photons are emitted from the hot WD. We calculated the same parameter models as Hillman et al.s and found that they manipulated on/off the mass-accretion, and their best fit $V$ light curves are 6 mag fainter, and decay about 10 times slower, than that of ASASSN-16oh. We propose a nova model induced by a high rate of mass accretion during a dwarf nova outburst, i.e., the X-rays originate from the surface of the hydrogen-burning WD whereas the $V/I$ photons are from the irradiated disk. Our model explains the main observational properties of ASASSN-16oh. We also obtained thermonuclear runaway models with no mass ejection for a wide range of parameters of the WD mass and mass accretion rates including both natural and forced novae in low-metal environments of $Z=0.001$ and $Z=0.0001$. They are a new type of periodic supersoft X-ray sources with no mass ejection, and also a bright transient in $V$/$I$ bands if they have a large disk. We suggest that such objects are candidates of Type Ia supernova progenitors because its mass is increasing at a very high efficiency $(sim 100 %)$.
Type Ia supernovae (SNe Ia) often show high-velocity absorption features (HVFs) in their early phase spectra; however the origin of the HVFs is unknown. We show that a near-Chandrasekhar-mass white dwarf (WD) develops a silicon-rich layer on a carbon
-oxygen (CO) core before it explodes as an SN Ia. We calculated the nuclear yields in successive helium shell flashes for 1.0 $M_odot$, 1.2 $M_odot$, and 1.35 $M_odot$ CO WDs accreting helium-rich matter with several mass-accretion rates ranging from $1 times 10^{-7}~M_odot$ yr$^{-1}$ to $7.5 times 10^{-7}~M_odot$ yr$^{-1}$. For the $1.35~M_odot$ WD with the accretion rate of $1.6 times 10^{-7}~M_odot$ yr$^{-1}$, the surface layer developed as helium burning ash and consisted of 40% $^{24}$Mg, 33% $^{12}$C, 23% $^{28}$Si, and a few percent of $^{20}$Ne by weight. For a higher mass accretion rate of $7.5 times 10^{-7}~M_odot$ yr$^{-1}$, the surface layer consisted of 58% $^{12}$C, 31% $^{24}$Mg, and 0.43% $^{28}$Si. For the $1.2~M_odot$ WDs, silicon is produced only for lower mass accretion rates (2% for $1.6 times 10^{-7}~M_odot$ yr$^{-1}$). No substantial silicon ($< 0.07%$) is produced on the $1.0~M_odot$ WD independently of the mass-accretion rate. If the silicon-rich surface layer is the origin of Si II HVFs, its characteristics are consistent with that of mass increasing WDs. We also discuss possible Ca production on very massive WDs ($ gtrsim 1.38~M_odot$).
We present the results of a multisite photometric observing campaign on the rapidly oscillating Ap (roAp) star 2MASS 16400299-0737293 (J1640; $V=12.7$). We analyse photometric $B$ data to show the star pulsates at a frequency of $151.93$ d$^{-1}$ ($1
758.45 mu$Hz; $P=9.5$ min) with a peak-to-peak amplitude of 20.68 mmag, making it one of the highest amplitude roAp stars. No further pulsation modes are detected. The stellar rotation period is measured at $3.6747pm0.0005$ d, and we show that rotational modulation due to spots is in anti-phase between broadband and $B$ observations. Analysis and modelling of the pulsation reveals this star to be pulsating in a distorted quadrupole mode, but with a strong spherically symmetric component. The pulsational phase variation in this star is suppressed, leading to the conclusion that the contribution of $ell>2$ components dictate the shape of phase variations in roAp stars that pulsate in quadrupole modes. This is only the fourth time such a strong pulsation phase suppression has been observed, leading us to question the mechanisms at work in these stars. We classify J1640 as an A7 Vp SrEu(Cr) star through analysis of classification resolution spectra.
