6 research outputs found
HST/ACS colour-magnitude diagrams of M31 globular clusters
With the aim of increasing the sample of M31 clusters for which a colour
magnitude diagram is available, we searched the HST archive for ACS images
containing objects included in the Revised Bologna Catalogue of M31 globular
clusters. Sixty-three such objects were found. We used the ACS images to
confirm or revise their classification and we obtained useful CMDs for 11 old
globular clusters and 6 luminous young clusters. We obtained simultaneous
estimates of the distance, reddening, and metallicity of old clusters by
comparing their observed field-decontaminated CMDs with a grid of template
clusters of the Milky Way. We estimated the age of the young clusters by
fitting with theoretical isochrones. For the old clusters, we found
metallicities in the range -0.4<=[Fe/H]<=-1.9, that generally agree with
existing spectroscopic extimates. At least four of them display a clear blue
HB, indicating ages >10 Gyr. All six candidate young clusters are found to have
ages <1Gyr. With the present work the total number of M31 GCs with reliable
optical CMD increases from 35 to 44 for the old clusters, and from 7 to 11 for
the young ones. The old clusters show similar characteristics to those of the
MW. We discuss the case of the cluster B407, with a metallicity [Fe/H] ~-0.6
and located at a large projected distance from the centre of M31 and from the
galaxy major axis. Metal-rich globulars at large galactocentric distances are
rare both in M31 and in the MW. B407, in addition, has a velocity in stark
contrast with the rotation pattern shared by the bulk of M31 clusters of
similar metallicity. This, along with other empirical evidence, supports the
hypothesis that the cluster is physically associated with a substructure in the
M31 halo that has been interpreted as the relic of a merging event.Comment: 19 pages, 14 figures, 6 tables. Accepted for publication on Astronomy
and Astrophysic
Influence of porosity on artificial deterioration of marble and limestone by heating
Testing of stone consolidants to be used on-site, as well as research on new consolidating products, requires suitable stone samples, with deteriorated but still uniform and controllable characteristics. Therefore, a new methodology to artificially deteriorate stone samples by heating, exploiting the anisotropic thermal deformation of calcite crystals, has recently been proposed. In this study, the heating effects on a variety of lithotypes was evaluated and the influence of porosity in determining the actual heating effectiveness was specifically investigated. One marble and four limestones, having comparable calcite amounts but very different porosity, were heated at 400 \ub0C for 1 hour. A systematic comparison between porosity, pore size distribution, water absorption, sorptivity and ultrasonic pulse velocity of unheated and heated samples was performed. The results of the study show that the initial stone porosity plays a very important role, as the modifications in microstructural, physical and mechanical properties are way less pronounced for increasing porosity. Heating was thus confirmed as a very promising artificial deterioration method, whose effectiveness in producing alterations that suitably resemble those actually experienced in the field depends on the initial porosity of the stone to be treated