Recent observation of the tidally excited stellar oscillations in the main-sequence binary KOI-54 by the Kepler satellite provides a unique opportunity for studying dynamical tides in eccentric ...binary systems. We develop a general theory of tidal excitation of oscillation modes of rotating binary stars and apply our theory to tidally excited gravity modes (g-modes) in KOI-54. The strongest observed oscillations, which occur at 90 and 91 times the orbital frequency, are likely due to prograde m= 2 modes (relative to the stellar spin axis) locked in resonance with the orbit. The remaining flux oscillations with frequencies that are integer multiples of the orbital frequency are likely due to nearly resonant m= 0 g-modes; such axisymmetric modes generate larger flux variations compared to the m= 2 modes, assuming that the spin inclination angle of the star is comparable to the orbital inclination angle. We examine the process of resonance mode locking under the combined effects of dynamical tides on the stellar spin and orbit and the intrinsic stellar spindown. We show that KOI-54 can naturally evolve into a state in which at least one m= 2 mode is locked in resonance with the orbital frequency. Our analysis provides an explanation for the fact that only oscillations with frequencies less than 90-100 times the orbital frequency are observed. We have also found evidence from the published Kepler result that three-mode non-linear coupling occurs in the KOI-54 system. We suggest that such non-linear mode coupling may explain the observed oscillations that are not harmonics of the orbital frequency.
The inner moons of Jupiter and Saturn migrate outwards due to tidal energy dissipation within the planets, the details of which remain poorly understood. We demonstrate that resonance locking between ...moons and internal oscillation modes of the planet can produce rapid tidal migration. Resonance locking arises due to the internal structural evolution of the planet and typically produces an outward migration rate comparable to the age of the Solar system. Resonance locking predicts a similar migration time-scale but a different effective tidal quality factor Q governing the migration of each moon. The theory also predicts nearly constant migration time-scales a function of semimajor axis, such that effective Q values were larger in the past. Recent measurements of Jupiter and Saturn's moon systems find effective Q values that are smaller than expected (and are different between moons), and which correspond to migration time-scales of ∼10 Gyr. If confirmed, the measurements are broadly consistent with resonance locking as the dominant source of tidal dissipation in Jupiter and Saturn. Resonance locking also provides solutions to several problems posed by current measurements: it naturally explains the exceptionally small Q governing Rhea's migration, it allows the large heating rate of Enceladus to be achieved in an equilibrium eccentricity configuration, and it resolves evolutionary problems arising from present-day migration/heating rates.
Abstract We present SN 2023zaw—a subluminous ( M r = −16.7 mag) and rapidly evolving supernova ( t 1/2, r = 4.9 days), with the lowest nickel mass (≈0.002 M ⊙ ) measured among all stripped-envelope ...supernovae discovered to date. The photospheric spectra are dominated by broad He i and Ca near-infrared emission lines with velocities of ∼10,000−12,000 km s −1 . The late-time spectra show prominent narrow He i emission lines at ∼1000 km s −1 , indicative of interaction with He-rich circumstellar material. SN 2023zaw is located in the spiral arm of a star-forming galaxy. We perform radiation-hydrodynamical and analytical modeling of the lightcurve by fitting with a combination of shock-cooling emission and nickel decay. The progenitor has a best-fit envelope mass of ≈0.2 M ☉ and an envelope radius of ≈50 R ⊙ . The extremely low nickel mass and low ejecta mass (≈0.5 M ⊙ ) suggest an ultrastripped SN, which originates from a mass-losing low-mass He-star (zero-age main-sequence mass < 10 M ⊙ ) in a close binary system. This is a channel to form double neutron star systems, whose merger is detectable with LIGO. SN 2023zaw underscores the existence of a previously undiscovered population of extremely low nickel mass (<0.005 M ☉ ) stripped-envelope supernovae, which can be explored with deep and high-cadence transient surveys.
ABSTRACT
Massive stars exhibit a variety of instabilities, many of which are poorly understood. We explore instabilities induced by centrifugal forces and angular momentum transport in massive ...rotating stars. First, we derive and numerically solve linearized oscillation equations for adiabatic radial modes in polytropic stellar models. In the presence of differential rotation, we show that centrifugal and Coriolis forces combined with viscous angular momentum transport can excite stellar pulsation modes, under both low- or high-viscosity conditions. In the low-viscosity limit, which is common in real stars, we demonstrate how to compute mode growth/damping rates via a work integral. Finally, we build realistic rotating $30\, \mathrm{M}_\odot$ star models and show that overstable (growing) radial modes are predicted to exist for most of the star’s life, in the absence of non-adiabatic effects. Peak growth rates are predicted to occur while the star is crossing the Hertzsprung–Russell gap, though non-adiabatic damping may dominate over viscous driving, depending on the effective viscosity produced by convective and/or magnetic torques. Viscous instability could be a new mechanism to drive massive star pulsations and is possibly related to instabilities of luminous blue variable stars.
Super slowly spinning stars in close binaries Fuller, Jim; Felce, Catherine
Monthly notices of the Royal Astronomical Society. Letters,
10/2023, Letnik:
527, Številka:
1
Journal Article
Odprti dostop
ABSTRACT
Stars in short-period binaries typically have spins that are aligned and synchronized with the orbit of their companion. In triple systems, however, the combination of spin and orbital ...precession can cause the star’s rotation to evolve to a highly misaligned and sub-synchronous equilibrium known as a Cassini state. We identify a population of recently discovered stars that exhibit these characteristics and which are already known to have tertiary companions. These third bodies have a suitable orbital period to allow the inner binary to evolve into the sub-synchronous Cassini state, which we confirm with orbital evolution models. We also compute the expected stellar obliquity and spin period, showing that the observed rotation rates are often slower than expected from equilibrium tidal models. However, we show that tidal dissipation via inertial waves can alter the expected spin–orbit misalignment angle and rotation rate, potentially creating the very slow rotation rates in some systems. Finally, we show how additional discoveries of such systems can be used to constrain the tidal physics and orbital evolution histories of stellar systems.
Tidal dissipation in compact white dwarf (WD) binary systems significantly influences the physical conditions (such as surface temperature and rotation rate) of the WDs prior to mass transfer or ...merger. In these systems, the dominant tidal effects involve the excitation of gravity waves and their dissipation in the outer envelope of the star. We calculate the amplitude of tidally excited gravity waves in low-mass (0.3 M) helium (He) core WDs as a function of the tidal forcing frequency ω. Like carbon-oxygen (CO) WDs studied in our previous paper, we find that the dimensionless tidal torque F(ω) (inversely proportional to the effective tidal quality factor) depends on ω in an erratic way. On average, F(ω) scales approximately as ω6, and is several orders of magnitude smaller for He WDs than for CO WDs. We find that tidal torques can begin to synchronize the WD rotation when the orbital period is less than about an hour, although a nearly constant asynchronization is maintained even at small periods. We examine where the tidally excited gravity waves experience non-linear breaking or resonant absorption at a critical layer, allowing us to estimate the location and magnitude of tidal heating in the WD envelope. We then incorporate tidal heating in the mesa stellar evolution code, calculating the physical conditions of the WD as a function of orbital period for different WD models. We find that tidal heating makes a significant contribution to the WD luminosity for short-period (∼10 min) systems such as SDSS J0651+2844. We also find that for WDs containing a hydrogen envelope, tidal heating can trigger runaway hydrogen shell burning, leading to a nova-like event before the onset of mass transfer.
The fastest stars in the Galaxy Jahaveri, Kareem; Shen, Ken J.; Chandra, Vedant ...
The open journal of astrophysics,
07/2023, Letnik:
6
Journal Article
Planet engulfment signatures in twin stars Behmard, Aida; Sevilla, Jason; Fuller, Jim
Monthly notices of the Royal Astronomical Society,
02/2023, Letnik:
518, Številka:
4
Journal Article
Recenzirano
Odprti dostop
ABSTRACT
Planet engulfment can be inferred from enhancement of refractory elements in the photosphere of the engulfing star following accretion of rocky planetary material. Such refractory ...enrichments are subject to stellar interior mixing processes, namely thermohaline mixing induced by an inverse mean-molecular-weight gradient between the convective envelope and radiative core. Using mesa stellar models, we quantified the strength and duration of engulfment signatures following planet engulfment. We found that thermohaline mixing dominates during the first ∼5–45 Myr post-engulfment, weakening signatures by a factor of ∼2 before giving way to depletion via gravitational settling on longer time-scales. Solar metallicity stars in the 0.5–1.2 M⊙ mass range have observable signature time-scales of ∼1 Myr–8 Gyr, depending on the engulfing star mass and amount of material engulfed. Early type stars exhibit larger initial refractory enhancements but more rapid depletion. Solar-like stars (M = 0.9–1.1 M⊙) maintain observable signatures (>0.05 dex) over time-scales of ∼20 Myr–1.7 Gyr for nominal 10 M⊕ engulfment events, with longer-lived signatures occurring for low-metallicity and/or hotter stars (1 M⊙, ∼2–3 Gyr). Engulfment events occurring well after the zero-age main sequence produce larger signals due to suppression of thermohaline mixing by gravitational settling of helium (1 M⊙, ∼1.5 Gyr). These results indicate that it may be difficult to observe engulfment signatures in solar-like stars that are several Gyr old.
ABSTRACT
The Tayler instability is an important but poorly studied magnetohydrodynamic (MHD) instability that likely operates in stellar interiors. The non-linear saturation of the Tayler instability ...is poorly understood and has crucial consequences for dynamo action and angular momentum transport in radiative regions of stars. We perform three-dimensional MHD simulations of the Tayler instability in a cylindrical geometry, including strong buoyancy and Coriolis forces as appropriate for its operation in realistic rotating stars. The linear growth of the instability is characterized by a pre-dominantly m = 1 oscillation with growth rates roughly following analytical expectations. The non-linear saturation of the instability appears to be caused by secondary shear instabilities and is also accompanied by a morphological change in the flow. We argue, however, that non-linear saturation likely occurs via other mechanisms in real stars where the separation of scales is larger than those reached by our simulations. We also observe dynamo action via the amplification of the axisymmetric poloidal magnetic field, suggesting that Tayler instability could be important for magnetic field generation and angular momentum transport in the radiative regions of evolving stars.
ABSTRACT
Strong magnetic fields are observed in a substantial fraction of upper main sequence stars and white dwarfs. Many such stars are observed to exhibit photometric modulations as the magnetic ...poles rotate in and out of view, which could be a consequence of magnetic perturbations to the star’s thermal structure. The magnetic pressure is typically larger than the gas pressure at the star’s photosphere, but much smaller than the gas pressure in the star’s interior, so the expected surface flux perturbations are not clear. We compute magnetically perturbed stellar structures of young $3 \, \mathrm{M}_\odot$ stars that are in both hydrostatic and thermal equilibrium, and which contain both poloidal and toroidal components of a dipolar magnetic field as expected for stable fossil fields. This provides semi-analytical models of such fields in baroclinic stably stratified regions. The star’s internal pressure, temperature, and flux perturbations can have a range of magnitudes, though we argue the most likely configurations exhibit flux perturbations much smaller than the ratio of surface magnetic pressure to surface gas pressure, but much larger than the ratio of surface magnetic pressure to central gas pressure. The magnetic pole is hotter than the equator in our models, but a cooler magnetic pole is possible depending on the magnetic field configuration. The expected flux variations for observed field strengths are δL/L ≲ 10−6, much smaller than those observed in magnetic stars, suggesting that observed perturbations stem from changes to the emergent spectrum rather than changes to the bolometric flux.
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