Identifying QCD Phase Transitions via the Gravitational Wave Frequency from a Supernova Explosion

We investigate the nonradial oscillations of newly born neutron stars (NSs) and strange quark stars (SQSs). This is done with the relativistic nuclear field theory with hyperon degrees of freedom employed to describe the equation of state (EoS) for the stellar matter in NSs, and with both the MIT bag model and the Nambu–Jona-Lasinio model adopted to construct the configurations of the SQSs. We find that the gravitational-mode (g-mode) eigenfrequencies of newly born SQSs are significantly lower than those of NSs, which is independent of models implemented to describe the EoS for the strange quark matter. Meanwhile, the eigenfrequencies of the other modes of nonradial oscillations, e.g., fundamental (f)- and pressure (p)-modes, are much larger than those of the g-mode, and are related to the stiffness of the EoSs. In light of the first direct observation of gravitational waves (GWs), it is promising to employ GWs to identify the QCD phase transition in high-density strong-interaction matter.

Acoustic modes of pulsating axion stars: Nonradial oscillations

We compute the lowest frequency nonradial oscillation modes of dilute axion stars. The effective potential that enters into the Schrödinger-like equation, several associated eigenfunctions and the large as well as the small frequency separations are shown as well.

Nonradial oscillations of neutron stars and emitted gravitational waves: Computing strongly damped normal modes

In this paper, we compute eigenfrequencies of strongly damped normal modes arising from the coupling of the nonradial oscillations of a neutron star to the oscillations of the space-time metric, so-called “w-modes”, by integrating all involved differential equations in the complex plane. Regarding the interior of the star, we use the so-called “complex-plane strategy”. Specifically, we integrate the differential equations of the nonradial fluid oscillations of a general-relativistic polytropic model, simulating the star, along a straight-line contour placed parallel to the real axis and at small imaginary distance from it, thus avoiding a singularity at the stellar center. Regarding the exterior of the star, we use a method proposed by Andersson, Kokkotas and Schutz, following a slightly different terminating procedure. Specifically, (i) we integrate the equations along a straight-line contour lying parallel to the so-called “anti-Stokes lines”, on which the exponential divergence of the solution is drastically suppressed, so that the outgoing and ingoing waves become comparable; and (ii) we carry out one final integration up to a “common reference point”, thus comparing all results at this point. We verify the reliability and accuracy of the method by comparing our numerical results to corresponding ones appearing in the bibliography.

A working hypothesis about the cause of Be stars: Episodic outward leakage of low-frequency waves excited by the iron-peak κ-mechanism

Observations indicate that a circumstellar disk is formed around a Be star while the stellar rotation is below the break-up velocity. I propose a working hypothesis to explain this mystery by taking account of the effect of leaky waves upon angular momentum transfer.In B-type stars near the main sequence, low-frequency nonradial oscillations are excited by the κ-mechanism in the iron bump. They transport angular momentum from the driving zone to the surface. As a consequence, the angular momentum is gradually deposited near the stellar surface. This results in a gradual increase in the “critical frequency for g-modes”, and g-modes eventually start to leak outward, long before the surface rotation reaches the break-up velocity. This leads to a substantial amount of angular momentum loss from the star, and a circumstellar disk is formed. The oscillations themselves will be soon damped owing to kinetic energy loss. Then the envelope of the star spins down and angular momentum loss stops soon. The star returns to being quiet and remains calm until nonradial oscillations are newly built up by the κ-mechanism to sufficient amplitude and a new episode begins.According to this view, the interval of episodic Be-star activity corresponds to the growth time of the oscillation, and it seems in good agreement with observations.

Multi-mode oscillations in classical Cepheids and RR Lyrae-type stars

I review different types of multi-mode pulsations observed in classical Cepheids and in RR Lyrae-type stars. The presentation concentrates on the newest results, with special emphasis on recently detected nonradial oscillations.

Non-radial pulsations in the CoRoT Be Star 102761769

We investigate non-radial pulsations of the CoRoT IR1 Be Star 102761769, with a projected stellar rotation estimated to be 120±15 km/s. If the star is a typical galactic Be star it rotates near the critical velocity. We propose an alternative scenario, where the star could be seen nearly equator-on rotating at a relatively moderate velocity say, ≈ 120 km/s and therefore the nonradial oscillations could be modeled. In order to identify the pulsation modes of the observed frequencies, we computed a set of models representative of CoRoT 102761769 by means of the adiabatic pulsation package FILOU. Results indicate that the two frequencies are compatible with a high-g mode as predicted by pulsation models of Be stars.

Radial and nonradial oscillations of massive supergiants

Stability of radial and nonradial oscillations of massive supergiants is discussed. The kappa-mechanism and strange-mode instability excite oscillations having various periods in wide ranges of the upper part of the HR diagram. In addition, in very luminous (log L/L⊙ ≳ 5.9) models, monotonously unstable modes exist, which probably indicates the occurrence of optically thick winds. The instability boundary is not far from the Humphreys-Davidson limit. Furthermore, it is found that there exist low-degree (ℓ = 1, 2) oscillatory convection modes associated with the Fe-opacity peak convection zone, and they can emerge to the stellar surface so that they are very likely observable in a considerable range in the HR diagram. The convection modes have periods similar to g-modes, and their growth-times are comparable to the periods. Theoretical predictions are compared with some of the supergiant variables.