Astrophysics

Gravito-inertial waves are excited at the interface of convective and radiative regions and by the Reynolds stresses in the bulk of the convection zones of rotating stars and planets. Such waves have notable asteroseismic signatures in the frequency spectra of rotating stars, particularly among rapidly rotating early-type stars, which provides a means of probing their internal structure and dynamics. They can also transport angular momentum, chemical species, and energy from the excitation region to where they dissipate in radiative regions. To estimate the excitation and convective parameter dependence of the amplitude of those waves, a monomodal model for stellar and planetary convection as described in Paper I is employed, which provides the magnitude of the rms convective velocity as a function of rotation rate. With this convection model, two channels for wave driving are considered: excitation at a boundary between convectively stable and unstable regions and excitation due to Reynolds-stresses. Parameter regimes are found where the sub-inertial waves may carry a significant energy flux, depending upon the convective Rossby number, the interface stiffness, and the wave frequency. The super-inertial waves can also be enhanced, but only for convective Rossby numbers near unity. Interfacially excited waves have a peak energy flux near the lower cutoff frequency when the convective Rossby number of the flows that excite them are below a critical Rossby number that depends upon the stiffness of the interface, whereas that flux decreases when the convective Rossby number is larger than this critical Rossby number.

We identify IRAS 16115-5044, which was previously classified as a protoplanetary nebula (PPN), as a candidate luminous blue variable (LBV). The star has high luminosity (>10 5.75 L_Sun), ensuring supergiant status, has a temperature similar to LBVs, is photometrically and spectroscopically variable, and is surrounded by warm dust. Its near-infrared spectrum shows the presence of several lines of HI, He I, Fe II, Fe [II], MgII, and Na I with shapes ranging from pure absorption and P Cygni profiles to full emission. These characteristics are often observed together in the relatively rare LBV class of stars, of which only ≈ 20 are known in the Galaxy. The key to the new classification is the fact that we compute a new distance and extinction that yields a luminosity significantly in excess of those for post-AGB PPNe, for which the initial masses are <8 M_Sun. Assuming single star evolution, we estimate an initial mass of ≈ 40 M_Sun.

Before reaching their quiescent terminal white-dwarf cooling branch, some low-mass helium-core white dwarf stellar models experience a number of nuclear flashes which greatly reduce their hydrogen envelopes. Just before the occurrence of each flash, stable hydrogen burning may be able to drive global pulsations that could be relevant to shed some light on the internal structure of these stars through asteroseismology. We present a pulsational stability analysis applied to low-mass helium-core stars on their early white-dwarf cooling branches going through CNO flashes in order to study the possibility that the ε mechanism is able to excite gravity-mode pulsations. We carried out a nonadiabatic pulsation analysis for low-mass helium-core white-dwarf models going through CNO flashes during their early cooling phases. We found that the ε mechanism due to stable hydrogen burning can excite low-order ( ??1,2 ) gravity modes with periods between ??0 and 500 s, for stars with 0.2025??M ??/ M ????.3630 located in an extended region of the logg??T eff diagram with effective temperature and surface gravity in the ranges 15000??T eff ??8000 K and 5.8?�logg??.1 , respectively. Since the timescales required for these modes to reach amplitudes large enough to be observable are shorter than their corresponding evolutionary timescales, the detection of pulsations in these stars is feasible. If a low-mass white dwarf star were found to pulsate with low-order gravity modes in this region of instability, it would confirm our result that such pulsations can be driven by the ε mechanism. In addition, confirming a rapid rate of period change in these pulsations would support that these stars actually experience CNO flashes, as predicted by evolutionary calculations.

Stellar models often use relations between the temperature T and optical depth ? to evaluate the structure of their optically-thin outer layers. We fit a novel analytic function to the Hopf function q(?) of a radiation-coupled hydrodynamics simulation of near-surface convection with solar parameters by Trampedach et al. (2014). The fit is accurate to within 0.82 per cent for the solar simulation and to within 13 per cent for all the simulations that are not on either the low-temperature or low-gravity edges of the grid of simulations.

We present the first BVR photometry, period variation, and photometric light-curve analysis of two poorly studied eclipsing binaries V1321 Cyg and CR Tau. Observations were carried out from November 2017 to January 2020 at the observatory of Uzhhorod National University. Period variations were studied using all available early published as well as our minima times. We have used newly developed ELISa code for the light curve analysis and determination of photometric parameters of both systems. We found that V1321 Cyg is a close detached eclipsing system with a low photometric mass ratio of q=0.28 which suggests that the binary is a post mass transfer system. No significant period changes in this system are detected. CR Tau is, on the other hand, a semi-detached system where the secondary component almost fills its Roche lobe. We detected a long-term period increase at a rate of 1.49? 10 ?? d/y , which support mass transfer from lower mass secondary component to the more massive primary.

People cannot witness the stellar evolution process of a single star obviously in most cases because of its extremely secular time-scale, except for some special time nodes in it (such as the supernova explosion). But in some specific evolutionary phases, we have the chances to witness such process gradually on human times-scales. When a star evolved leaving from the main sequence, the hydrogen nuclei fusion in its core is gradually transferring into the shell. In the Hertzsprung-Russell diagram, its evolutionary phase falls into the Hertzsprung gap, which is one of the most rapidly evolving phases in the life of a star. Here we report a discovery of a rapidly evolving high-amplitude δ Scuti star KIC6382916 (J19480292+4146558) which is crossing the Hertzsprung gap. According to the analysis of the archival data, we find three independent pulsation modes of it, whose amplitudes and frequencies are variating distinctly in 4 years. The period variation rates of the three pulsation modes are one or two orders larger than the best seismic model constructed by the standard evolution theory, which indicates the current theory cannot precisely describe the evolution process in this rapidly evolving phase and needs further upgrades. Moreover, the newly introduced Interaction Diagram can help us to find the interactions between the three independent pulsation modes and their harmonics/combinations, which opens a new window to the future asteroseismology.

The majority of massive stars ( >8 M ??) in OB associations are found in close binary systems. Nonetheless, the formation mechanism of these close massive binaries is not understood yet. Using literature data, we measured the radial-velocity dispersion ( ? RV ) as a proxy for the close binary fraction in ten OB associations in the Galaxy and the Large Magellanic Cloud, spanning an age range from 1 to 6 Myrs. We find a positive trend of this dispersion with the cluster's age, which is consistent with binary hardening. Assuming a universal binary fraction of f bin = 0.7, we converted the ? RV behavior to an evolution of the minimum orbital period P cutoff from ??9.5 years at 1 Myr to ??1.4 days for the oldest clusters in our sample at ??6 Myr. Our results suggest that binaries are formed at larger separations, and they harden in around 1 to 2 Myrs to produce the period distribution observed in few million year-old OB binaries. Such an inward migration may either be driven by an interaction with a remnant accretion disk or with other young stellar objects present in the system. Our findings constitute the first empirical evidence in favor of migration as a scenario for the formation of massive close binaries.

Runaway stars can result from core-collapse supernovae in multiple stellar systems. If the supernova disrupts the system, the companion gets ejected with its former orbital velocity. A clear identification of a runaway star can yield the time and place of the explosion as well as orbital parameters of the pre-supernova binary system. Previous searches have mostly considered O- and B-type stars as runaway stars because they are always young in absolute terms (not much older than the lifetime of the progenitor) and can be detected up to larger distances. We present here a search for runaway stars of all spectral types. For late-type stars, a young age can be inferred from the lithium test. We used Gaia data to identify and characterise runaway star candidates in nearby supernova remnants, obtained spectra of 39 stars with UVES at the VLT and HDS at the Subaru telescope and found a significant amount of lithium in the spectra of six dwarf stars. We present the spectral analysis, including measurements of radial velocities, atmospheric parameters and lithium abundances. Then we estimate the ages of our targets from the Hertzsprung-Russell diagram and with the lithium test, present a selection of promising runaway star candidates and draw constraints on the number of ejected runaway stars compared to model expectations.

High-precision space-based photometry obtained by the \emph{Kepler} and \emph{TESS} missions has revealed evidence of rotational modulation associated with main sequence (MS) A and late-B type stars. Generally, such variability in these objects is attributed to inhomogeneous surface structures (e.g. chemical spots), which are typically linked to strong magnetic fields ( B≳100G ) visible at the surface. It has been reported that ≈44 ~per~cent of all A-type stars observed during the \emph{Kepler} mission exhibit rotationally modulated light curves. This is surprising considering that ≲10 ~per~cent of all MS A-type stars are known to be strongly magnetic (i.e. they are Ap/Bp stars). We present a spectroscopic monitoring survey of 44 A and late-B type stars reported to exhibit rotational modulation in their \emph{Kepler} light curves. The primary goal of this survey is to test the hypothesis that the variability is rotational modulation by comparing each star's rotational broadening ( vsini ) with the equatorial velocities ( v eq ) inferred from the photometric periods. We searched for chemical peculiarities and binary companions in order to provide insight into the origin of the apparent rotational modulation. We find that 14 stars in our sample have vsini> v eq and/or have low-mass companions that may contribute to or be responsible for the observed variability. Our results suggest that more than 10 ~per~cent of all MS A and late-B type stars may exhibit inhomogeneous surface structures; however, the incidence rate is likely ≲30 ~per~cent.

(shortened) Planet forming discs are believed to be very weakly turbulent in the regions outside of 1 AU. For this reason, it is now believed that magnetized winds could be the dominant mechanism driving accretion in these systems. However, there is today no self-consistent way to describe discs subject to a magnetized wind, in a way similar to the α disc model. In this article, I explore in a systematic way the parameter space of wind-driven protoplanetary discs and present scaling laws which can be used in reduced models ? la alpha-disc. Methods: I compute a series of self-similar wind solutions, assuming the disc is dominated by ambipolar and Ohmic diffusions. These solution are obtained by looking for stationary solutions in the finite-volume code PLUTO using a relaxation method and continuation. Results: Self-similar solutions are obtained for values of plasma beta ranging from 10^2 to 10^8, for several Ohmic and ambipolar diffusion strengths. Mass accretions rates of the order of 10^{-8} Msun/yr are obtained for poloidal field strength beta=O(10^4) or equivalently 1 mG at 10 AU. The resulting magnetic lever arms are typically lower than 2, possibly reaching 1.5 in weakest field cases. Finally, I provide a complete set of scaling laws and semi-analytical wind solutions, which can be used to fit and interpret observations. Conclusions: Magnetized winds are unavoidable in protoplanetary discs as soon as they are embedded in an ambient poloidal magnetic field. Very detailed disc microphysics are not always needed to describe them, and simplified models such as self-similar solutions manage to capture most of the physics seen in full 3D simulations. The remaining difficulty to get a complete theory of wind-driven accretion lies in the transport of the large scale field, which remains poorly constrained and not well understood.