Abstract.Observations of the core of the massive cluster Cl 0024+1654, at a redshift z ∼ 0.39, were obtained with the Infrared Space Observatory using ISOCAM at 6.7 µm (hereafter 7 µm) and 14.3 µm (hereafter 15 µm). Thirty five sources were detected at 15 µm and thirteen of them are spectroscopically identified with cluster galaxies. The remaining sources consist of four stars, one quasar, one foreground galaxy, three background galaxies and thirteen sources with unknown redshift. The sources with unknown redshift are all likely to be background sources that are gravitationally lensed by the cluster. The spectral energy distributions (SEDs) of twelve cluster galaxies were fit from a selection of 20 models using the program GRASIL. The ISOCAM sources have best-fit SEDs typical of spiral or starburst models observed 1 Gyr after the main starburst event. The star formation rates were obtained for cluster members. The median infrared luminosity of the twelve cluster galaxies is ∼1.0 × 10 11 L , with 10 having infrared luminosity above 9 × 10 10 L , and so lying near or above the 1 × 10 11 L threshold for identification as a luminous infrared galaxy (LIRG). The [O ] star formation rates obtained for 3 cluster galaxies are one to two orders of magnitude lower than the infrared values, implying that most of the star formation is missed in the optical because it is enshrouded by dust in the starburst galaxy. The cluster galaxies in general are spatially more concentrated than those detected at 15 µm. However the velocity distributions of the two categories are comparable. The colour−magnitude diagramme is given for the galaxies within the ISOCAM map. Only 20% of the galaxies that are significantly bluer than the cluster main sequence were detected at 15 µm, to the limiting sensitivity recorded. The counterparts of about half of the 15 µm cluster sources are blue, luminous, star-forming systems and the type of galaxy that is usually associated with the Butcher-Oemler effect. HST images of these galaxies reveal a disturbed morphology with a tendency for an absence of nearby companions. Surprisingly the counterparts of the remaining 15 µm cluster galaxies lie on the main sequence of the colour-magnitude diagramme. However in HST images they all have nearby companions and appear to be involved in interactions and mergers. Dust obscuration may be a major cause of the 15 µm sources appearing on the cluster main sequence. The majority of the ISOCAM sources in the Butcher-Oemler region of the colourmagnitude diagram are best fit by spiral-type SEDs whereas post-starburst models are preferred on the main sequence, with the starburst event probably triggered by interaction with one or more galaxies. Finally, the mid-infrared results on Cl 0024+1654 are compared with four other clusters observed with ISOCAM. Scaling the LIRG count in Cl 0024+1654 to the clusters Abell 370, Abell 1689, Abell 2218 and Abell 2390 with reference to their virial radii, masses, distances, and the sky area scanned in each case, we compared the number o...

Discovering a selection principle and the origin of flavor symmetries from an ultraviolet completion of particle physics is an interesting open task. As a step in this direction, we classify all possible flavor symmetries of 4D massless spectra emerging from supersymmetric Abelian orbifold compactifications, including roto-translations and non-factorizable tori, for generic moduli values. Although these symmetries are valid in all string theories, we focus on the E 8 ×E 8 heterotic string. We perform the widest known search of E 8 ×E 8 Abelian orbifold compactifications, yielding over 121,000 models with MSSM-like features. About 75.4% of these models exhibit flavor symmetries containing D 4 factors and only nearly 1.2% have ∆(54) factors. The remaining models are furnished with purely Abelian flavor symmetries. Our findings suggest that, should particle phenomenology arise from such a heterotic orbifold, it could accommodate only one of these flavor symmetries.

Abstract.We have observed the cluster Abell 2218 (z = 0.175) with ISOCAM on board the Infrared Space Observatory using two filters, LW2 and LW3, with reference wavelengths of 6.7 and 14.3 µm, respectively. We detected 76 sources down to 54 and 121 µJy (50% completeness levels) at 6.7 and 14.3 µm, respectively. All these sources have visible optical counterparts. We have gathered optical and near-infrared magnitudes for 60 of the 67 non-stellar optical counterparts to the ISOCAM sources, as well as redshifts for 43 of them. We have obtained acceptable and well constrained fits to the observed spectral energy distributions (SEDs) of 41 of these sources, using the "GRASIL" models of Silva et al. (1998), and have determined their total infrared luminosities (L IR 's) and star formation rates (SFRs). The SEDs of 20 (out of 27) ISOCAM cluster members are best fit by models with negligible ongoing star formation, and no major episode of star formation in the last ∼1 Gyr. Their SEDs resemble those of 5−10 Gyr old early-type galaxies. A slightly higher, but still very mild, star-formation activity is found among the remaining cluster sources, which are mostly spirals. The median IR luminosity of the 27 ISOCAM cluster sources is L IR = 6 × 10 8 L . The ISOCAM-selected cluster galaxies have indistinguishable velocity and spatial distributions from those of the other cluster galaxies, and do not contribute significantly to the Butcher-Oemler effect. If A2218 is undergoing a merger, as suggested by some optical and X-ray analyses, then this merger does not seem to affect the mid-infrared properties of its galaxies. The SEDs of most ISOCAM-selected field sources are best fit by models with moderate ongoing star formation, with a significant fraction of their stellar mass formed in the last ∼1 Gyr. Their SEDs resemble those of massive star-forming spirals or starburst galaxies, observed close to the maximum of their star formation activity, but not necessarily during the short-lived starburst event. The median redshift of these field galaxies is z 0.6. Their L IR 's span almost two orders of magnitudes, from ∼10 10 L to ∼1012 L , with a median of 1.2 × 10 11 (eight of the 14 field sources are LIRGs). The SFRs of these 14 ISOCAM-selected field sources range from 2 to 125 M yr −1 , with a median value of 22 M yr −1 . We compare our findings with those obtained in other ISOCAM cluster and field surveys.

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