AT 2018lqh: Black Hole Born from a Rotating Star?

Recently an intriguing transient, AT 2018lqh, with only a day-scale duration and a high luminosity of 7 × 1042 erg s−1, was discovered. While several possibilities are raised on its origin, the nature of this transient is yet to be unveiled. We propose that a black hole (BH) with ∼30 M ⊙ forming from a rotating blue supergiant can generate a transient like AT 2018lqh. We find that this scenario can consistently explain the optical/UV emission and the tentative late-time X-ray detection, as well as the radio upper limits. If super-Eddington accretion onto the nascent BH powers the X-ray emission, continued X-ray observations may be able to test the presence of an accretion disk around the BH.

Generalization of Vaidya’s Metric for A Radiating Star to Rotating Star

Schwarzschild's external solution of Einstein’s gravitational field equations in the general theory of relativity for a static star has been generalized by Vaidya [1], taking into account the radiation of the star. Here, we generalize Vaidya’s metric to a star that is rotating and radiating. Although, there is a famous Kerr solution [2] for a rotating star, but here is a simple solution for a rotating star which may be termed as a zero approximate version of the Kerr solution. Results are discussed.

Hot and counter-rotating star-forming disc galaxies in IllustrisTNG and their real-world counterparts

ABSTRACT A key feature of a large population of low-mass, late-type disc galaxies are star-forming discs with exponential light distributions. They are typically also associated with thin and flat morphologies, blue colours, and dynamically cold stars moving along circular orbits within co-planar thin gas discs. However, the latter features do not necessarily always imply the former, in fact, a variety of different kinematic configurations do exist. In this work, we use the cosmological hydrodynamical IllustrisTNG simulation to study the nature and origin of dynamically hot, sometimes even counter-rotating, star-forming disc galaxies in the lower stellar mass range (between $5\times 10^9\, \mathrm{M_{\odot }}$ and $2\times 10^{10}\, \mathrm{M_{\odot }}$). We find that being dynamically hot arises in most cases as an induced transient state, for example due to galaxy interactions and merger activities, rather than as an age-dependent evolutionary phase of star-forming disc galaxies. The dynamically hot but still actively star-forming discs show a common feature of hosting kinematically misaligned gas and stellar discs, and centrally concentrated on-going star formation. The former is often accompanied by disturbed gas morphologies, while the latter is reflected in low gas and stellar spins in comparison to their dynamically cold, normal disc counterparts. Interestingly, observed galaxies from MaNGA with kinematic misalignment between gas and stars show remarkably similar general properties as the IllustrisTNG galaxies, and therefore are plausible real-world counterparts. In turn, this allows us to make predictions for the stellar orbits and gas properties of these misaligned galaxies.

Rotation of the convective core in γ Dor stars measured by dips in period spacings of g modes coupled with inertial modes

The relation of period spacing (ΔP) 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 γ Dor and slowly pulsating B (SPB) stars. In a rapidly rotating star, 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 - ΔP relation, distinct from the modulations due to the chemical composition gradient. Such a resonance dip in ΔP of prograde dipole g modes appears around a frequency corresponding to a spin parameter 2frot(cc)/νco-rot ∼ 8 − 11 with frot(cc) being the rotation frequency of the convective core and νco-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 γ Dor stars having well defined dips, and found that the majority of γ Dor stars we studied rotate nearly uniformly, while convective cores tend to rotate slightly faster than the g-mode cavity in less evolved stars.

A backward-spinning star with two coplanar planets

It is widely assumed that a star and its protoplanetary disk are initially aligned, with the stellar equator parallel to the disk plane. When observations reveal a misalignment between stellar rotation and the orbital motion of a planet, the usual interpretation is that the initial alignment was upset by gravitational perturbations that took place after planet formation. Most of the previously known misalignments involve isolated hot Jupiters, for which planet–planet scattering or secular effects from a wider-orbiting planet are the leading explanations. In theory, star/disk misalignments can result from turbulence during star formation or the gravitational torque of a wide-orbiting companion star, but no definite examples of this scenario are known. An ideal example would combine a coplanar system of multiple planets—ruling out planet–planet scattering or other disruptive postformation events—with a backward-rotating star, a condition that is easier to obtain from a primordial misalignment than from postformation perturbations. There are two previously known examples of a misaligned star in a coplanar multiplanet system, but in neither case has a suitable companion star been identified, nor is the stellar rotation known to be retrograde. Here, we show that the star K2-290 A is tilted by 124○±6○ compared with the orbits of both of its known planets and has a wide-orbiting stellar companion that is capable of having tilted the protoplanetary disk. The system provides the clearest demonstration that stars and protoplanetary disks can become grossly misaligned due to the gravitational torque from a neighboring star.

Magnetorotational core collapse of possible GRB progenitors. III. Three-dimensional models

We explore the influence of non-axisymmetric modes on the dynamics of the collapsed core of rotating, magnetized high-mass stars in three-dimensional simulations of a rapidly rotating star with an initial mass of $M_{\rm \small {ZAMS}} = 35 \, M_{\odot }$ endowed with four different pre-collapse configurations of the magnetic field, ranging from moderate to very strong field strength and including the field predicted by the stellar evolution model. The model with the weakest magnetic field achieves shock revival due to neutrino heating in a gain layer characterized by a large-scale, hydrodynamic m = 1 spiral mode. Later on, the growing magnetic field of the proto neutron star launches weak outflows into the early ejecta. Their orientation follows the evolution of the rotational axis of the proto neutron star, which starts to tilt from the original orientation due to the asymmetric accretion flows impinging on its surface. The models with stronger magnetization generate mildly relativistic, magnetically driven polar outflows propagating over a distance of 104 km within a few 100 ms. These jets are stabilized against disruptive non-axisymmetric instabilities by their fast propagation and by the shear of their toroidal magnetic field. Within the simulation times of around 1 s, the explosions reach moderate energies and the growth of the proto neutron star masses ceases at values substantially below the threshold for black hole formation, which, in combination with the high rotational energies, might suggest a possible later proto-magnetar activity.

Internal circulation in tidally locked massive binary stars: Consequences for double black hole formation

Context. Steady-state currents, so-called Eddington–Sweet circulation, result in the mixing of chemical elements in rotating stars, and in extreme cases lead to a homogeneous composition. Such circulation currents are also predicted in tidally deformed binary stars, which are thought to be progenitors of double black-hole merger events. Aims. This work aims to quantitatively characterise the steady-state circulation currents in components of a tidally locked binary system and to explore the effects of such currents on numerical models. Methods. Previous results describing the circulation velocity in a single rotating star and a tidally and rotationally distorted binary star are used to deduce a new prescription for the internal circulation in tidally locked binaries. We explore the effect of this prescription numerically with a detailed stellar evolution code for binary systems with initial orbital periods between 0.5 and 2.0 days, primary masses between 25 and 100 M⊙ and initial mass-ratios qi = 0.5, 0.7, 0.9, 1.0 at metallicity Z = Z⊙/50. Results. When comparing circulation velocities in the radial direction for the cases of a single rotating star and a binary star, it is found that the average circulation velocity in the binary star may be described as an enhancement to the circulation velocity in a single rotating star. This velocity enhancement is a simple function depending on the masses of the binary components and amounts to a factor of approximately two when the components have equal masses. After applying this enhancement to stellar models, it is found that the formation of double helium stars through efficient mixing occurs for systems with higher initial orbital periods, lower primary masses and lower mass ratios, compared to the standard circulation scenario. Taking into account appropriate distributions for primary mass, initial period and mass ratio, models with enhanced mixing predict 2.4 times more double helium stars being produced in the parameter space than models without. Conclusions. We conclude that the effects of companion-induced circulation have strong implications for the formation of close binary black holes through the chemically homogeneous evolution channel. Not only do the predicted detection rates increase but double black-hole systems with mass ratios as low as 0.8 may be formed when companion-induced circulation is taken into account.

Rotational modulation and single g-mode pulsation in the B9pSi star HD 174356?

ABSTRACT Chemically peculiar (CP) stars of the upper main sequence are characterized by specific anomalies in the photospheric abundances of some chemical elements. The group of CP2 stars, which encompasses classical Ap and Bp stars, exhibits strictly periodic light, spectral, and spectropolarimetric variations that can be adequately explained by the model of a rigidly rotating star with persistent surface structures and a stable global magnetic field. Using observations from the Kepler K2 mission, we find that the B9pSi star HD 174356 displays a light curve variable in both amplitude and shape, which is not expected in a CP2 star. Employing archival and new photometric and spectroscopic observations, we carry out a detailed abundance analysis of HD 174356 and discuss its photometric and astrophysical properties in detail. We employ phenomenological modelling to decompose the light curve and the observed radial velocity variability. Our abundance analysis confirms that HD 174356 is a silicon-type CP2 star. No magnetic field stronger than 110 G was found. The star’s light curve can be interpreted as the sum of two independent strictly periodic signals with $P_1=4{_{.}^{\rm d}}043\, 55(5)$ and $P_2=2{_{.}^{\rm d}}111\, 69(3)$. The periods have remained stable over 17 yr of observations. In all spectra, HD 174356 appears to be single-lined. From the simulation of the variability characteristics and investigation of stars in the close angular vicinity, we put forth the hypothesis that the peculiar light variability of HD 174356 arises in a single star and is caused by rotational modulation due to surface abundance patches (P1) and g-mode pulsation (P2).

The impact of unresolved magnetic spots on high-precision radial velocity measurements

ABSTRACT The Doppler method of exoplanet detection has been extremely successful, but suffers from contaminating noise from stellar activity. In this work, a model of a rotating star with a magnetic field based on the geometry of the K2 star ϵ Eridani is presented and used to estimate its effect on simulated radial velocity (RV) measurements. A number of different distributions of unresolved magnetic spots were simulated on top of the observed large-scale magnetic maps obtained from 8 yr of spectropolarimetric observations. The RV signals due to the magnetic spots have amplitudes of up to 10 m s−1, high enough to prevent the detection of planets under 20 Earth masses in temperate zones of solar-type stars. We show that the RV depends heavily on spot distribution. Our results emphasize that understanding stellar magnetic activity and spot distribution is crucial for the detection of Earth analogues.