Puzzling Frequencies in First Overtone Cepheids

The OGLE project led to discovery of earlier unknown forms of multiperiodic pulsation in Cepheids. Often, the observed periods may be explained in terms of simultaneous excitation of two or rarely three radial modes. However, a secondary variability at about 0.6 of the dominant period, detected in a number of the first overtone (1O) pulsators inhabiting the Magellanic Clouds, seems to require a different explanation. After reviewing a possibility of explaining this signal in terms of radial and nonradial modes, I find that only unstable modes that may reproduce the observed period ratio are f-modes of high angular degrees (l=42-50). I discuss in detail the driving effect behind the instability and show that it is not the familiar opacity mechanism. Finally, I emphasize the main difficulty of this explanation, which requires high intrinsic amplitudes implying large broadening of spectral line.

Dipolar modes in luminous red giants

Lots of information on solar-like oscillations in red giants has been obtained thanks to observations with CoRoT and Kepler space telescopes. Data on dipolar modes appear most interesting. We study properties of dipolar oscillations in luminous red giants to explain mechanism of mode trapping in the convective envelope and to assess what may be learned from the new data. Equations for adiabatic oscillations are solved by numerical integration down to the bottom of convective envelope, where the boundary condition is applied. The condition is based on asymptotic decomposition of the fourth order system into components describing a running wave and a uniform shift of radiative core. If the luminosity of a red giant is sufficiently high, for instance at M = 2 Msun greater than about 100 Lsun, the dipolar modes become effectively trapped in the acoustic cavity, which covers the outer part of convective envelope. Energy loss caused by gravity wave emission at the envelope base is a secondary or negligible source of damping. Frequencies are insensitive to structure of the deep interior.

On the Efficiency of the Tidal Stirring Mechanism for the Origin of Dwarf Spheroidals: Dependence on the Orbital and Structural Parameters of the Progenitor Disky Dwarfs

(Abridged) The tidal stirring model posits the formation of dSph galaxies via the tidal interactions between rotationally-supported dwarfs and MW-sized host galaxies. Using a set of collisionless N-body simulations, we investigate the efficiency of the tidal stirring mechanism. We explore a wide variety of dwarf orbital configurations and initial structures and demonstrate that in most cases the disky dwarfs experience significant mass loss and their stellar components undergo a dramatic morphological and dynamical transformation: from disks to bars and finally to pressure-supported spheroidal systems with kinematic and structural properties akin to those of the classic dSphs in the Local Group (LG). Our results suggest that such tidal transformations should be common occurrences within the currently favored cosmological paradigm and highlight the key factor responsible for an effective metamorphosis to be the strength of the tidal shocks at the pericenters of the orbit. We demonstrate that the combination of short orbital times and small pericenters, characteristic of dwarfs being accreted at high redshift, induces the strongest transformations. Our models also indicate that the transformation efficiency is affected significantly by the structure of the progenitor disky dwarfs. Lastly, we find that the dwarf remnants satisfy the relation Vmax = sqrt{3} * sigma, where sigma is the 1D, central stellar velocity dispersion and Vmax is the maximum halo circular velocity, with intriguing implications for the missing satellites problem. Overall, we conclude that the action of tidal forces from the hosts constitutes a crucial evolutionary mechanism for shaping the nature of dwarf galaxies in environments such as that of the LG. Environmental processes of this type should thus be included as ingredients in models of dwarf galaxy formation and evolution.