Structural relaxation in a supercooled molecular liquid

We perform molecular-dynamics simulations of a molecular system in supercooled states for different values of inertia parameters to provide evidence that the long-time dynamics depends only on the equilibrium structure. This observation is consistent with the prediction of the mode-coupling theory for the glass transition and with the hypothesis that the potential energy-landscape controls the slow dynamics. We also find that dynamical properties at intermediate wavenumber depend on the spatial correlation of the molecule's geometrical center.Comment: 7 pages, 4 figures, Europhys. Lett. in pres

A Chandra X-ray study of the young star cluster NGC 6231: low-mass population and initial mass function

NGC6231 is a massive young star cluster, near the center of the Sco OB1 association. While its OB members are well studied, its low-mass population has received little attention. We present high-spatial resolution Chandra ACIS-I X-ray data, where we detect 1613 point X-ray sources. Our main aim is to clarify global properties of NGC6231 down to low masses through a detailed membership assessment, and to study the cluster stars' spatial distribution, the origin of their X-ray emission, the cluster age and formation history, and initial mass function. We use X-ray data, complemented by optical/IR data, to establish cluster membership. The spatial distribution of different stellar subgroups also provides highly significant constraints on cluster membership, as does the distribution of X-ray hardness. We perform spectral modeling of group-stacked X-ray source spectra. We find a large cluster population down to ~0.3 Msun (complete to ~1 Msun), with minimal non-member contamination, with a definite age spread (1-8 Myrs) for the low-mass PMS stars. We argue that low-mass cluster stars also constitute the majority of the few hundreds unidentified X-ray sources. We find mass segregation for the most massive stars. The fraction of circumstellar-disk bearing members is found to be ~5%. Photoevaporation of disks under the action of massive stars is suggested by the spatial distribution of the IR-excess stars. We also find strong Halpha emission in 9% of cluster PMS stars. The dependence of X-ray properties on mass, stellar structure, and age agrees with extrapolations based on other young clusters. The cluster initial mass function, computed over ~2 dex in mass, has a slope Gamma~-1.14. The total mass of cluster members above 1 Msun is 2280 Msun, and the inferred total mass is 4380 Msun. We also study the peculiar, hard X-ray spectrum of the Wolf-Rayet star WR79.Comment: 25 pages, 36 figures, accepted for publication on Astronomy and Astrophysic

Time evolution of X-ray coronal activity in PMS stars; a possible relation with the evolution of accretion disks

We investigate the evolution of X-ray stellar activity from the age of the youngest known star forming regions (SFR), < 1Myr, to about 100 Myr, i.e. the zero age main sequence (ZAMS) for a ~ 1M_sun star. We consider five SFR of varying age (Rho Ophiuchi, the Orion Nebula Cluster, NGC 2264, Chamaeleon I, and Eta Chamaeleontis) and two young clusters (the Pleiades and NGC 2516). Optical and X-ray data for these regions are retrieved both from archival observations and recent literature, and reanalyzed here in a consistent manner so to minimize systematic differences in the results. We study trends of L_X and L_X/L_bol as a function of stellar mass and association age. For low mass stars (M < 1M_sun) we observe an increase in L_X/L_bol in the first 3-4 Myr and a subsequent leveling off at the saturation level (L_X/L_bol ~ -3). Slowly evolving very low mass stars then retain saturated levels down to the oldest ages here considered, while for higher mass stars activity begins to decline at some age after ~10^7 years. We find our data consistent with the following tentative picture: low mass PMS stars with no circumstellar accretion disk have saturated activity, consistently with the activity-Rossby number relation derived for MS stars. Accretion and/or the presence of disks somehow lowers the observed activity levels; disk dissipation and/or the decrease of mass accretion rate in the first few Myrs of PMS evolution is therefore responsible for the observed increase of L_X/L_bol with time.Comment: 17 pages, 8 figures, accepted for publication in A&

Spatial distribution of X-ray emitting ejecta in Tycho's SNR: indications of shocked Titanium

Young supernova remnants show a characteristic ejecta-dominated X-ray emission that allows us to probe the products of the explosive nucleosynthesis processes and to ascertain important information about the physics of the supernova explosions. Hard X-ray observations have recently revealed the radioactive decay lines of 44Ti at ~67.9 keV and ~78.4 keV in the Tycho's SNR. We here analyze the set of XMM-Newton archive observations of the Tycho's SNR. We produce equivalent width maps of the Fe K and Ca XIX emission lines and find indications for a stratification of the abundances of these elements and significant anisotropies. We then perform a spatially resolved spectral analysis by identifying five different regions characterized by high/low values of the Fe K equivalent width. We find that the spatial distribution of the Fe K emission is correlated with that of the Cr XXII. We also detect the Ti K-line complex in the spectra extracted from the two regions with the highest values of the Fe and Cr equivalent widths. The Ti line emissions remains undetected in regions where the Fe and Cr equivalent widths are low. Our results indicate that the post-shock Ti is spatially co-located with other iron-peak nuclei in Tycho's SNR, in agreement with the predictions of multi-D models of Type Ia supernovae.Comment: Accepted for publication in Ap

Investigation of the relation between local diffusivity and local inherent structures in the Kob-Andersen Lennard-Jones model

We analyze one thousand independent equilibrium trajectories of a system of 155 Lennard Jones particles to separate in a model-free approach the role of temperature and the role of the explored potential energy landscape basin depth in the particle dynamics. We show that the diffusion coefficient $D$ can be estimated as a sum over over contributions of the sampled basins, establishing a connection between thermodynamics and dynamics in the potential energy landscape framework. We provide evidence that the observed non-linearity in the relation between local diffusion and basin depth is responsible for the peculiar dynamic behavior observed in supercooled states and provide an interpretation for the presence of dynamic heterogeneities.Comment: minor text changes, references adde

Il termine dell'expulsio del conduttore per mancato pagamento del canone nella locatio di aedes e di fundi

il lavoro \ue8 volto a fornire una spiegazione al dato apparentemente contraddittorio delle fonti, secondo le quali, il potere del locatore di espellere il conduttore per mancato pagamento del canone \ue8 talvolta immediato, tal altra subordinato al decorso di un biennio dalla scadenza del termine di pagamento della mercede. La soluzione proposta \ue8 quella di distinguere la locazione delle aedes da quella dei fundi: nel primo caso si tollerarono ritardi di pagamento della mercede fino a due anni; nel secondo caso, la giurisprudenza legittim\uf2 il locatore all'expulsio immediata del conduttore inadempiente all'obbligo di pagare il canone

Res acta e potere magistratuale di interrompere una legis actio irregolare

l'articolo concerne la ricostruzione della nozione di res acta nelle legis actiones e del connesso problema dell'individuazione del momento a partire dal quale si verificavano gli effetti preclusivi nelle legis actiones. La ricerca \ue8 inoltre utile a rispondere positivamente alla domanda se il magistrato giusdicente potesse fermare in iure una legis actio caratterizzata da errori o omissioni da parte dell'attore sancendo, in questo modo, contro di lui la perdita della lit