arXiv subjects

F. Paresce

Publications and source records attributed to F. Paresce.

21 records · Page 2Linked to original sources

arXiv2002

On the accuracy of the S/N estimates obtained with the exposure time calculator of the Wide Field Planetary Camera 2 on board the Hubble Space Telescope

We have studied the accuracy and reliability of the exposure time calculator (ETC) of the Wide Field Planetary Camera 2 (WFPC2) on board the Hubble Space Telescope (HST) with the objective of determining how well it represents actual observations and, therefore, how much confidence can be invested in it and in similar software tools. We have found, for example, that the ETC gives, in certain circumstances, very optimistic values for the signal-to-noise ratio (SNR) of point sources. These values overestimate by up to a factor of 2 the HST performance when simulations are needed to plan deep imaging observations, thus bearing serious implications on observing time allocation. For this particular case, we calculate the corrective factors to compute the appropriate SNR and detection limits and we show how these corrections vary with field crowding and sky background. We also compare the ETC of the WFPC2 with a more general ETC tool, which takes into account the real effects of pixel size and charge diffusion. Our analysis indicates that similar problems may afflict other ETCs in general showing the limits to which they are bound and the caution with which their results must be taken.

astro-ph↗

arXiv2001

VLT observations of the peculiar globular cluster NGC 6712, II: luminosity and mass functions

We have carried out extensive VLT-FORS1 observations covering a fair fraction of the intermediate metallicity globular cluster NGC 6712 in the V and R bands. We derive accurate colour-magnitude diagrammes (CMD) and luminosity functions (LFs) of the cluster main sequence (MS) for four overlapping fields extending from the centre of the cluster out to a radius of ~10', well beyond the nominal tidal radius, and for a control field at ~42' distance. The LFs extend from the cluster turn-off (TO) at M_R 4 to the point at which the incompleteness drops below 50% (corresponding to R 23 or M_R 7.5) for most fields studied. Cluster stars become indistinguishable from field stars at r 5'. The shape of the cluster's LF and its variation with distance from the centre in these ranges are well described by a standard multi-mass static model having the following parameters: core radius r_c=1', half-light radius r_hl=1.8', tidal radius r_t=5.2', concentration ratio c=0.7, and a power-law global mass function (MF) with index alpha 0.9 for masses smaller than 0.8 Msolar, i.e. for all detected MS stars, and alpha -2.35 for evolved objects. The MF obtained in this way is consistent with that found in a preliminary investigation of this cluster with the VLT Test Camera and confirms that this is the only globular cluster known so far for which the global MF drops with decreasing mass below the TO. Possible reasons for this unique characteristic are discussed with the most likely associated with its extreme vulnerability to tidal disruption.

astro-ph↗

arXiv1998

The mass function of M4 from near IR and optical HST observations

Deep images of the galactic globular cluster M4 taken at various locations with the NIC3 and the WFPC2 cameras on HST were used to derive detailed local optical and near IR luminosity functions. White dwarfs have been detected for the first time on a color sequence at constant luminosity in the F110W band. Transformation of the observed luminosity functions into mass functions via the most up to date theoretical mass luminosity relations currently available results in best fit local mass functions, that consist of a power-law with single exponent x = -0.8 for the inner regions and a two-segment power-law which rises with x = 1 down to M_{814}=8.5 and then drops all the way to the detection limit with x = -0.3 for the outer regions.

astro-ph↗