Radio emission from stellar active regions

Several types of stars present a variable radio emission. In this paper we present a time-dependent model aimed to reproduce the RS CVn emission, which however can be easily extended to other types of stars. The time evolution of a population of electrons following an initial power-law distribution is studied. It is shown that the spectrum of the emitted radiation evolves, in a time scale of a few days, from the typical spectrum of the flaring emission to the flat one observed in quiescent periods. It is also shown that the intensity distribution reproduces the core-halo structure observed using VLBI for a period of about one day after the flare.Department of Astronomy and Space Science, University of Florence, Italy. Published in Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 37, No. 5, pp. 634–643, May, 1994.     

Resonance detection of dark matter axions using a DC SQUID

A method for detecting dark matter axions in which a dc SQUID serves as a detector is proposed. The SQUID is shown to be able to detect the magnetic field perturbations induced by its interaction with axions. The resonance signal appears as a current step in the SQUID current–voltage characteristic. The voltage of the step corresponds to the axion mass, while its height depends on the axion energy density in near-Earth space. The proposed method is aimed at detecting axions with masses m_a ? 10^–4 eV, which are of interest for both cosmology and particle physics.     

Neutrino-antineutrino pair emission from thermally excited nuclei in stellar collapse

We examine the rate of neutrino-antineutrino pair emission by hot nuclei in collapsing stellar cores. The rates are calculated assuming that only allowed charge-neutral Gamow-Teller (GT₀) transitions contribute to the decay of thermally excited nuclear states. To obtain the GT₀ transition matrix elements, we employ the quasiparticle random phase approximation extended to finite temperatures within the thermo field dynamics formalism. The decay rates and the energy emission rates are calculated for the sample nuclei ⁵⁶Fe and ⁸²Ge at temperatures relevant to core collapse supernovae.     

Quen's gambit: Decuplet quarks and composite axions

Exotic quarks transforming as decuplets under SU(3)_c (quens) qith zero bare (current) mass plausibly condense at a scale exponentially removed from the ordinary triplet quark chiral symmetry breaking scale. The pseudoscalar pseudo-Goldstone boson remaining after the breaking of the quentic global chiral U(1)_A in the presence of QCD instantons is a likely composite invisible axion (CIA) candidate.     

Harmless axions in superstring theories

We explore the possibility of obtaining an axion in superstring theories whose decay constant lies in a phenomenologically acceptable range. We find in agreement with Choi and Kim that this is not possible in SO(32) theories. However, we find that it appears to be possible in the E₈ × E′₈ heterotic string theory.     

CP-Violation and axions in an effective technicolor model

CP violation is studied in an SO(10)⁵ effective technicolor model. One can generate a KM mixing matrix with a non-zero value for δ. The model has 5 chiral symmetries and hence 5 axions, of which 3 are invisible and 2 are heavy. Breaking these symmetries and minimizing the vacuum energy density fixes $\text{θ}$ to bezero. This solves the strong CP problem.     

Neutrinos,axions and conformal symmetry

We demonstrate that radiative breaking of conformal symmetry (and simultaneously electroweak symmetry) in the standard model with right-chiral neutrinos and a minimally enlarged scalar sector induces spontaneous breaking of lepton number symmetry, which naturally gives rise to an axion-like particle with some unusual features. The couplings of this ‘axion’ to standard model particles, in particular photons and gluons, are entirely determined (and computable) via the conformal anomaly, and their smallness turns out to be directly related to the smallness of the masses of the light neutrinos.     

Cosmological imprints of string axions in plateau

We initiate a study on various cosmological imprints of string axions whose scalar potentials have plateau regions. In such cases, we show that a delayed onset of oscillation rather generically leads to a parametric resonance instability. In particular, for ultralight axions, the parametric resonance can enhance the power spectrum slightly below the Jeans scale, alleviating the tension with the Lyman \(\alpha \) forest observations. We also argue that a sustainable resonance can lead to an emission of gravitational waves at the frequency bands which are detectable by gravitational wave interferometers and pulsar timing arrays and also to a succeeding oscillon formation.     

Propagation of axions in a strongly magnetized medium

The polarization operator of an axion in a degenerate gas of electrons occupying the ground-state Landau level in superstrong magnetic fields H≫H ₀=m _e ² c ³/eℏ=4.41×10¹³ G is investigated in a model with a tree-level axion-electron coupling. It is shown that a dynamic axion mass, which can fall within the allowed range of values 10⁻⁵ eV≲m _a≲10² eV, is generated under the conditions of strongly magnetized neutron stars. As a result, the dispersion relation for axions is appreciably different from that in a vacuum. Zh. éksp. Teor. Fiz. 115, 3–11 (January 1999)     

Search for axions emitted in nuclear magnetic transitions

Searches for invisible axions emitted in the electromagnetic transitions of isomeric nuclei are continued. It is found experimentally that the probability of axion emission in an M1 transition in ^125m Te is less than 8.5×10^?6 (90% C.L.).     

Starspots,stellar cycles and stellar flares: Lessons from solar dynamo models

We theoretically investigate the multistable behavior of a hybrid optomechanical system,in which a charged mechanical resonator is coupled via Coulomb interaction to an optomechanical cavity containing an optical parametric amplifier(OPA).It is shown that the multistable behavior of the mean intracavity photon number can be controlled flexibly by adjusting the nonlinear gain parameter of the OPA,the phase of the field pumping the OPA,the power and frequency of the field driving the cavity,and the Coulomb coupling strength between the two charged mechanical resonators.In particular,the increase of the nonlinear gain parameter can result in a transition from testability to Instability.Moreover,the effect of the Coulomb coupling strength on the bistable behavior of the steady-state positions of the two mechanical resonators is discussed.     

Starspots,stellar cycles and stellar flares: Lessons from solar dynamo models

In this review, we discuss whether the present solar dynamo models can be extrapolated to explain various aspects of stellar activity. We begin with a summary of the following kinds of data for solar-like stars:(i) data pertaining to stellar cycles from Ca H/K emission over many years;(ii) X-ray data indicating hot coronal activity;(iii) starspot data(especially about giant polar spots); and(iv) data pertaining to stellar superflares. Then we describe the current status of solar dynamo modelling—giving an introduction to the flux transport dynamo model, the currently favoured model for the solar cycle. While an extrapolation of this model to solar-like stars can explain some aspects of observational data, some other aspects of the data still remain to be theoretically explained. It is not clear right now whether we need a different kind of dynamo mechanism for stars having giant starspots or producing very strong superflares.     

Search for axions in thermal neutron capture by protons

A search for axions at a 500 MW light water power reactor was performed. From the measured upper limit on the n + p → d + a cross section the “standard” axion is ruled out.