One-loop renormalization in a toy model of Horava-Lifshitz gravity

We present a one loop calculation in the context of Horava-Lifshitz gravity. Due to the complexity of the calculation in the full theory we focus here on the study of a toy model, namely the conformal reduction of the z=2 projectable theory in 2+1 dimensions. For this value of the dimension there are no gravitons, hence the conformal mode is the only physical degree of freedom, and thus we expect our toy model to lead to qualitatively correct answers regarding the perturbative renormalization of the full theory. We find that Newton's constant (dimensionless in Horava-Lifshitz gravity) is asymptotically free. However, the DeWitt supermetric approaches its Weyl invariant form with the same speed and the effective interaction coupling remains constant along the flow. In other words, the would-be asymptotic freedom associated to the running Newton's constant is exactly balanced by the strong coupling of the scalar mode as the Weyl invariant limit is approached. We conclude that in such model the UV limit is singular at one loop order, and we argue that a similar phenomenon can be expected in the full theory, even in higher dimensions

GRBs with optical afterglow and known redshift: a statistical study

We present a correlation between two intrinsic parameters of GRB optical afterglows. These are the isotropic luminosity at the maximum of the light curve (Lpeak) and the time-integrated isotropic energy (Eiso) radiated after the observed maximum. We test the correlation between the logarithms of (Eiso) and (Lpeak) and finally we value the effect of the different samples of GRBs in according with the first optical observation reduced to proper time.Comment: To be published in the proceedings of the conference "SWIFT and GRBs: Unveiling the Relativistic Universe", Venice, June 5-9, 200

Metal abundances in extremely distant Galactic old open clusters. II. Berkeley 22 and Berkeley 66

We report on high resolution spectroscopy of four giant stars in the Galactic old open clusters Berkeley~22 and Berkeley~66 obtained with HIRES at the Keck telescope. We find that $[Fe/H]=-0.32\pm0.19$ and $[Fe/H]=-0.48\pm0.24$ for Berkeley~22 and Berkeley~66, respectively. Based on these data, we first revise the fundamental parameters of the clusters, and then discuss them in the context of the Galactic disk radial abundance gradient. We found that both clusters nicely obey the most updated estimate of the slope of the gradient from \citet{fri02} and are genuine Galactic disk objects.Comment: 20 pages, 6 eps figures, accepted for publication in the Astronomical Journa

A Photometric Investigation of the GRB970228 Afterglow and the Associated Nebulosity

We carefully analyze the WFPC2 and STIS images of GRB970228. We measure magnitudes for the GRB970228 point source component in the WFPC2 images of $V=26.20^{+0.14}_{-0.13}$, $I_c=23.94^{+0.10}_{-0.09}$ and $V=26.52^{+0.16}_{-0.18}$, $I_c=24.31^{+0.15}_{-0.11}$ on March 26 and April 7, respectively; and $R_c=27.09^{+0.14}_{-0.14}$ on September 4 in the STIS image. For the extended component, we measure magnitudes of $R_c=25.48^{+0.22}_{-0.20}$ in the combined WFPC2 images and $R_c=25.54^{+0.33}_{-0.22}$ in the STIS image, which are consistent with no variation. This value is fainter than previously reported (Galama et al. 98) and modifies the previously assumed magnitudes for the optical transient when it faded to a level where the extended source component contribution was not negligible, alleviating the discrepancy to a power-law temporal behavior. We also measure a color of $V_{606}-I_{814}=-0.18^{+0.51}_{-0.61}$ for the extended source component. Taking into account the extinction measured in this field (Castander & Lamb 1998), this color implies that the extended source is most likely a galaxy with ongoing star formation.Comment: 21 pages, including 8 figures. Submitted to Ap

ASCA and BeppoSAX observations of the peculiar X-ray source 4U1700+24/HD154791

The X-ray source 4U1700+24/HD154791 is one of the few galactic sources whose counterpart is an evolved M star. In X-rays the source shows extreme erratic variability and a complex and variable spectrum. While this strongly suggests accretion onto a compact object, no clear diagnosis of binarity was done up to now. We report on ASCA and BeppoSAX X-ray broad band observations of this source and on ground optical observations from the Loiano 1.5 m telescope.Comment: 5 pages, 4 figures, uses aipproc.sty, to appear in Proceedings of the Fifth Compton Symposiu

Near-infrared Spectral Features in Single-aged Stellar Populations

Synthetic spectra for single-aged stellar populations of metallicities [M/H] = -0.5, 0.0 and +0.5, ages = 3 to 17 Gyrs, and initial mass function exponents x = 0.1 to 2.0 were built in the wavelength range 6000-10200 Angstrons. For such we have employed the grid of synthetic spectra described in Schiavon & Barbuy (1999), computed for the stellar parameters 2500 <= Teff <= 6000 K, -0.5 <= log g <= 5.0, [M/H] = -0.5, 0.0 and +0.5, and [alpha/Fe] = 0.0, together with the isochrones by Bertelli et al. (1994) and Baraffe et al. (1998). The behavior of the features NaI8190, CaII8662, TiO6600 and FeH9900 in the integrated spectra of single stellar populations were studied in terms of metallicity, initial mass function and age variations. The main conclusions are that the NaI doublet is an IMF-sensitive feature, which is however sensitive also to metallicity and age, whereas TiO, CaII and FeH are very sensitive to metallicity and essentially insensitive to IMF and age.Comment: 13 pages + 7 figures, ApJ accepte