Spectral Observations of BIG Objects. III

Results of spectral observations of 66 objects from the BIG (Byurakan IRAS Galaxies) sample made with the 1.93 m telescope at the Observatoire de Haute Provence (OHP, France) are presented. Emission lines are observed from 64 of the galaxies. The red shifts are determined, the radial velocities, distances, and absolute stellar magnitudes are calculated, the spectrum line parameters are determined, diagnostic diagrams are constructed, the objects are classified according to activity type, and their IR and far-IR luminosities are calculated. Of the 66 objects (corresponding to 61 IRAS sources), 6 are Sy2, 2 are LINERs, 8 are AGN (Sy2 or LINER), 10 are composite, 34 are HII, and 4 are Em of undetermined type. It is calculated that IRAS 07479+7832= BIG d141a is a ultraluminous IR galaxy (ULIG), and 21 are LIG. Spectra of several of the galaxies being studied are presented.     

A fundamental solution due to a periodic point force in the interior of an elastic half-space

The fundamental solution for a periodic point force in the interior of a three-dimensional, homogeneous, isotropic, elastic half-space is derived. The method of synthesis and superposition is employed to obtain the solution in the Laplace transform as well as the frequency domain. These correspond to the dynamic equivalent of Mindlin's static half-space point force solutions. It is reduced, for certain limiting conditions, to the dynamic equivalent of Boussinesq's and Cerruti's problems of a normal and tangential periodic point force respectively, on the boundary of a half-space. Also, static solutions of Mindlin, Boussinesq and Cerruti are recovered for small frequency parameters. Finally, results are presented and compared with other available solutions.     

Photometric Inference of the Corotation Radii of Ten Barred Galaxies

We have used photometric images of ten barred galaxies in the B and I bands to infer the geometrical and dynamical parameters of their bars: Their length, (R _bar), their strengths, (S _b), and their corotation radii, (R _CR).These parameters have been obtained studying azimuthal profiles from the B and I band images of the galaxies. We find that R _CR is in all cases slightly larger than R _bar, with mean values close to 1.2 R _bar. We have also found a dependence of the ratio R _CR/R _bar on S _bar. This revised version was published online in July 2006 with corrections to the Cover Date.     

Some New Observed Properties of Elliptical Galaxies

We suggest that elliptical galaxies, as stellar systems in a stage of quasi-equilibrium, may have a specific entropy. We use the Sérsic law to describe the light profile. The specific entropy (the Boltzmann–Gibbs definition) is then calculated assuming that the galaxy behaves as a spherical, isotropic, one-component system. We predict a relation between the three parameters of the Sérsic law linked to the specific entropy, defining a surface in the parameter space, an ‘entropic plane’. We have analysed a sample of simulated merging elliptical galaxies (virtual) and a sample of galaxies belonging to the Coma Cluster (real). Both virtual and realgalaxies are: 1) located in their own ‘entropic plane‘ and 2) in this plane, they are located on a straight line, indicating constant entropy: another physical property A careful examination of the value of the specific entropy indicates a very small increase in the specific entropy with the generation after merging (virtual sample). Although one cannot distinguish between various generations for real galaxies, the distribution of specific entropy in this sample is very similar to that in the virtual sample. This revised version was published online in July 2006 with corrections to the Cover Date.     

Investigation of Barred Galaxies. VII. Comparative Statistics of SB and SA Galaxies. Near and Mid-IR Regions

The near and mid-IR properties of barred and unbarred spiral galaxies are discussed on the basis of complete samples that were compiled earlier. The two types of spirals are shown not to differ from one another in emission power in the near and mid-IR ranges. Multiple regression and principal component analysis have been applied to investigate near and mid-IR properties of SB and SA galaxies, particularly their relation to X-ray and radio continuum emissions. There are definite differences between SB and SA spirals in the near and mid-IR. In the case of SB galaxies, the compactness of 10 m emission is closely related to the J - H color index, and the redder J - H color corresponds to relatively more extended emission at 10 m. It is assumed that these are caused by the bar itself, which stimulates enhanced star formation in a barred galaxy with respect to unbarred spiral.     

1998 UBV Light Curves of Eclipsing Binary AI Draconis and Absolute Parameters

New UBV photometry of Algol-Type eclipsing binary star AI Dra andthe absolute physical parameters of this system have been presented.The light curve analysis carried out by the method of differentialcorrections indicates that both components are inside their Roche-Lobes.From combining the photometric solution with spectroscopic data obtainedfrom velocity curve analysis, it has been found that the system consistof a main sequence primary and an evolved (subgiant) secondary.     

Porous interstellar grains

Recent observations of stellar composition suggest that elements in the Sun are significantly more abundant than in other stars. The reduction in the available element budget implies a drastic revision in current models of interstellar dust. Theoretical models are therefore exploring fluffy, porous physical structure for the grain material. Since a detailed exact treatment of extinction cross-sections is mandatory for a correct understanding of the nature of interstellar dust, we present a technique based on the multipole expansions of the electromagnetic field, which has proven to be general, flexible and powerful in treating scattering of light by porous, composite, arbitrarily shaped particles. The results of this study speak in favour of core–mantle structures characterized by the presence of porosities.     

On the second-overtone stability among Small Magellanic Cloud Cepheids

We present a new set of Cepheid, full amplitude, non-linear, convective models which are pulsationally unstable in the second overtone (SO). Hydrodynamical models were constructed by adopting a chemical composition typical of Cepheids in the Small Magellanic Cloud (SMC) and for stellar masses ranging from 3.25 to 4 M_⊙. Predicted φ ₂₁ Fourier parameters agree, within current uncertainties, with empirical data for pure first- and second-overtone variables as well as for first-/second-overtone (FO/SO) double-mode Cepheids collected by Udalski et al. in the SMC. On the other hand, predicted I -band amplitudes are systematically larger than the observed ones in the short-period range, but attain values that are closer to the empirical ones for log  P _SO≥−0.12 and log  P _FO≥0.1 . We also find, in agreement with empirical evidence, that the region within which both second and first overtones attain a stable limit cycle widens when moving towards lower luminosities. Moreover, predicted P _SO/ P _FO and P _FO/ P _F period ratios agree quite well with empirical period ratios for FO/SO and fundamental/FO double-mode SMC Cepheids.
Interestingly enough, current models support the evidence that pure SO Cepheids and SO components in FO/SO Cepheids are good distance indicators. In fact, we find that the fit of the predicted period–luminosity–colour ( V , V – I ) relation to empirical SMC data supplies a distance modulus ( DM ) of 19.11±0.08 mag . The same outcome applies to pure FO Cepheids and FO components in FO/SO Cepheids, and indeed we find DM =19.16±0.19 mag . Current distance estimates do not account for, within current uncertainties on photometry and reddening, the so-called short distance scale.     

Multiple stellar populations in the Sextans dwarf spheroidal galaxy?

We present wide-field     multiband ( BVI ) CCD photometry (down to     of the very low surface brightness dwarf spheroidal galaxy Sextans. In the derived colour–magnitude diagrams we find evidence suggesting the presence of multiple stellar populations in this dwarf spheroidal. In particular, we discover (i) a blue horizontal branch tail that appears to lie on a brighter sequence with respect to the prominent red horizontal branch and the RR Lyrae stars, very similar to what was found by Majewski et al. for the Sculptor dwarf spheroidal, (ii) hints of a bimodal distribution in colour of the red giant branch stars, (iii) a double red giant branch bump. All of these features suggest that (at least) two components are present in the old stellar population of this galaxy: the main one with     and a minor component around     . The similarity to the Sculptor case may indicate that multiple star formation episodes are also common in the most nearby dwarf spheroidals that ceased their star formation activity at very early epochs.