Variable stars in the globular cluster NGC 3201. I. Multimode SX Phe-type variables

We report on the discovery of eleven multimode SX Phoenicis--type blue stragglers in the field of the southern globular cluster NGC 3201. In these variables both radial and non-radial modes are excited. For three variables the derived period ratio is close to that observed in SX Phoenicis itself, suggesting that these stars are pulsating in the fundamental and the first-overtone radial modes. Using the McNamara (1997) period-luminosity relation we have estimated the apparent distance modulus to NGC 3201 to be 14.08$\pm0.06\pm0.1$mag.Comment: 10 pages, requires mn2e.cls,contact the first author at [email protected] for high-resolution figure

Twisting commutative algebraic groups

If $V$ is a commutative algebraic group over a field $k$, $O$ is a commutative ring that acts on $V$, and $I$ is a finitely generated free $O$-module with a right action of the absolute Galois group of $k$, then there is a commutative algebraic group $I \otimes_O V$ over $k$, which is a twist of a power of $V$. These group varieties have applications to cryptography (in the cases of abelian varieties and algebraic tori over finite fields) and to the arithmetic of abelian varieties over number fields. For purposes of such applications we devote this article to making explicit this tensor product construction and its basic properties.Comment: To appear in Journal of Algebra. Minor changes from original versio

Anharmonic Torsional Stiffness of DNA Revealed under Small External Torques

DNA supercoiling plays an important role in a variety of cellular processes. The torsional stress related with supercoiling may be also involved in gene regulation through the local structure and dynamics of the double helix. To check this possibility steady torsional stress was applied to DNA in the course of all-atom molecular dynamics simulations. It is found that small static untwisting significantly reduces the torsional persistence length ($l_t$) of GC-alternating DNA. For the AT-alternating sequence a smaller effect of the opposite sign is observed. As a result, the measured $l_t$ values are similar under zero stress, but diverge with untwisting. The effect is traced to sequence-specific asymmetry of local torsional fluctuations, and it should be small in long random DNA due to compensation. In contrast, the stiffness of special short sequences can vary significantly, which gives a simple possibility of gene regulation via probabilities of strong fluctuations. These results have important implications for the role of local DNA twisting in complexes with transcription factors.Comment: 8 pages, 5 figures, to appear in Phys. Rev. Let

NGC 4654: polarized radio continuum emission as a diagnostic tool for a galaxy--cluster interaction

A recent comparison between deep VLA HI observations and dynamical models of the Virgo cluster spiral galaxy NGC 4654 has shown that only a model involving a combination of a tidal interaction and ram pressure can reproduce the data. Deep radio polarization studies, together with detailed MHD modeling, can independently verify those conclusions, that are based on HI observations and dynamical models. We performed deep polarized radio-continuum observations of the Virgo cluster spiral galaxy NGC 4654 with the Effelsberg 100m telescope at 8.35 GHz and the VLA at 4.85 GHz. Detailed 3D MHD simulations were made to determine the large-scale magnetic field and the emission distribution of the polarized radio continuum in the model, during the galaxy evolution within the cluster environment. This direct comparison between the observed and simulated polarized radio continuum emission corroborates the earlier results, that the galaxy had a recent rapid close encounter with NGC 4639 and is undergoing weak ram pressure by the intracluster medium. This combination of deep radio polarization studies and detailed MHD modeling thus gives us unique insight into the interactions of a galaxy with its cluster environment. It represents a diagnostic tool that is complementary to deep HI observations.Comment: Corrected galaxy name in captions of figures (1 & 2

Are There Topological Black Hole Solitons in String Theory?

We point out that the celebrated Hawking effect of quantum instability of black holes seems to be related to a nonperturbative effect in string theory. Studying quantum dynamics of strings in the gravitational background of black holes we find classical instability due to emission of massless string excitations. The topology of a black hole seems to play a fundamental role in developing the string theory classical instability due to the effect of sigma model instantons. We argue that string theory allows for a qualitative description of black holes with very small masses and it predicts topological solitons with quantized spectrum of masses. These solitons would not decay into string massless excitations but could be pair created and may annihilate also. Semiclassical mass quantization of topological solitons in string theory is based on the argument showing existence of nontrivial zeros of beta function of the renormalization group.Comment: 12 pages, TeX, requires phyzzx.tex, published in Gen. Rel. Grav. 19 (1987) 1173; comment added on December 18, 199

Comparison of FDMA and CDMA for second generation land-mobile satellite communications

Code Division Multiple Access (CDMA) and Frequency Division Multiple Access (FDMA) (both analog and digital) systems capacities are compared on the basis of identical link availabilities and physical propagation models. Parameters are optimized for a bandwidth limited, multibeam environment. For CDMA, the benefits of voice activated carriers, antenna discrimination, polarization reuse, return link power control and multipath suppression are included in the analysis. For FDMA, the advantages of bandwidth efficient modulation/coding combinations, voice activated carriers, polarization reuse, beam placement, and frequency staggering were taken into account