26,878 research outputs found
M4-18: The planetary nebula and its WC10 central star
We present a detailed analysis of the planetary nebula M4-18 (G146.7+07.6)
and its WC10-type Wolf-Rayet central star, based on high quality optical
spectroscopy (WHT/UES, INT/IDS, WIYN/DensPak) and imaging (HST/WFPC2). From a
non-LTE model atmosphere analysis of the stellar spectrum, we derive Teff=31kK,
log(Mdot/(Msun yr))=-6.05, v_inf=160 km/s and abundance number ratios of
H/He<0.5, C/He=0.60 and O/He=0.10. These parameters are remarkably similar to
He2-113 ([WC10]). Assuming an identical stellar mass to that determined by De
Marco et al. for He2-113, we obtain a distance of 6.8kpc to M4-18
(E(B-V)=0.55mag from nebular and stellar techniques). This implies that the
planetary nebula of M4-18 has a dynamical age of 3100 years, in contrast to
>270 years for He2-113. This is supported by the much higher electron density
of the latter. These observations may only be reconciled with evolutionary
predictions if [WC]-type stars exhibit a range in stellar masses.
Photo-ionization modelling of M4-18 is carried out using our stellar WR flux
distribution, together with blackbody and Kurucz energy distributions obtained
from Zanstra analyses. We conclude that the ionizing energy distribution from
the Wolf-Rayet model provides the best consistency with the observed nebular
properties, although discrepancies remain.Comment: 12 pages, 9 figures, accepted for MNRAS (latex uses mn.sty
Assessing Value for Money in PFI Projects: A Comparative Study of Practices in the UK and Italy
The Value for Money assessment is a critical process in procuring a Private Finance Initiative and it requires accurate ex-ante performance measurement methodologies. The British Government has set new requirements for evaluating VFM through a new assessment model composed of three main stages, namely: programme level assessment, project level assessment, and procurement level assessment. The objective of the new model for VFM assessment is to change the costly, inflexible and opaque side of PFIs in order to deliver cost-effective and improve the quality of public service provision. A theoretical analysis of the implementation of PFI shows the UK as the leading user of this procurement in Europe and Italy is the second. However, there is a disparity in the manner PFI is actually implemented in these two countries and especially how VFM is assessed. Aiming at underlying the best practices of this evaluation process for the most achievable VFM, this paper presents the new VFM assessment model of the UK and a suggestion for its potential application to the Italian PFI procurement process to improve outcomes therei
Two examples of non strictly convex large deviations
We present two examples of a large deviations principle where the rate
function is not strictly convex. This is motivated by a model used in
mathematical finance (the Heston model), and adds a new item to the zoology of
non strictly convex large deviations. For one of these examples, we show that
the rate function of the Cramer-type of large deviations coincides with that of
the Freidlin-Wentzell when contraction principles are applied.Comment: 11 page
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