304 research outputs found
Non-synchronous earthquake motion in bridges design
No full textThe study aims to further develop, with respect to previous findings, and validate structural design criteria which account for the effects of earthquakes spatial variability. In past works [Nuti, C. and Vanzi. I. (2004) & (2005); Carnevale, L. et al. (2010)] the two simplest forms of this problem were dealt with: differential displacements between two points belonging to the soil or to two single degree of freedom structures. Existing codes appear indeed improvable on this aspect. For the differential displacements of two points on the ground, these results are generalized with different response spectra and validated using (indeed a small set of) real recordings. For the experimental validation, the first obtained results point towards an acceptable agreement of model vs. experimental results [Tropeano, G. et al. (2011)]. In any case, results indicate that the design codes can be improved on this topic, both for the two points (e.g. simply supported decks) and the multiple points (e.g. continuous decks on multiple piers) cases
Non synchronous earthquake motion in bridges design
No full textThe study aims to further develop, with respect to previous findings, and validate structural design criteria which account for the effects of earthquakes spatial variability. In past works [Nuti, C. and Vanzi. I. (2004) & (2005); Carnevale, L. et al. (2010)] the two simplest forms of this problem were dealt with: differential displacements between two points belonging to the soil or to two single degree of freedom structures. Existing codes appear indeed improvable on this aspect. For the differential displacements of two points on the ground, these results are generalized with different response spectra and validated using (indeed a small set of) real recordings. For the experimental validation, the first obtained results point towards an acceptable agreement of model vs. experimental results [Tropeano, G. et al. (2011)]. In any case, results indicate that the design codes can be
improved on this topic, both for the two points (e.g. simply supported decks) and the multiple points (e.g. continuous decks on multiple piers) cases
Laser diode DC measurement protocols
No full textThe starting point is the acquisition of DC data, as usual for drawing the socalled LIV plot, as shown, where current I, optical power Pout and the externally applied voltage Vext are drawn together
The IMF and star formation history of the stellar clusters in the Vela D cloud
No full textAims.We present the results of a Near-Infrared deep photometric survey of a
sample of six embedded star clusters in the Vela-D molecular cloud,
all associated with luminous ( ) IRAS
sources. The clusters are unlikely to be older than a few 106
yrs, since all are still associated with molecular gas.Methods.We employed the fact that all clusters lie at the same distance and
were observed with the same instrumental setting to derive
their properties in a consistent way, being affected by the same
instrumental and observational biases.
We extracted the clusters' K Luminosity Functions and developed a
simple method to correct them for extinction, based on colour-magnitude
diagrams. The reliability of the method has been tested by
constructing synthetic clusters from theoretical tracks for pre-main
sequence stars and a standard Initial Mass Function.
The clusters' Initial Mass Functions have been derived from the
dereddened
K Luminosity Functions by adopting a set of pre-main sequence
evolutionary tracks and assuming coeval star formation.Results.All clusters are small ( members) and compact (radius
pc); their most massive stars are
intermediate-mass ( ) ones.
The dereddened K Luminosity Functions are likely to arise
from the same
distribution, suggesting that the selected clusters have quite
similar Initial Mass Functions and star formation histories.
The Initial Mass Functions are consistent with those
derived for field stars and clusters. Adding them
together we found that the “global” Initial Mass Function
appears steeper at the high-mass end and
exhibits a drop-off at . In fact,
a standard Initial Mass Function would
predict a star with within one of the clusters,
which is not found. Hence, either high-mass stars need larger
clusters
to be formed, or the Initial Mass Function of the single clusters is
steeper at the
high-mass end because of the physical conditions in the parental gas
Young stellar clusters in the Vela D molecular cloud
No full textIt is now well established by means of direct and indirect observations that most, if not all, stars are formed in groups rather than in isolation (Clarke, Bonnell & Hillenbrand 2000). An important result that strongly constrains theories of massive star and stellar cluster formation is that the stellar density of young stellar clusters seems to depend on the mass of the most massive star in the cluster. Low-mass stars are usually found to form in loose groups with typical densities of a few stars per cubic parsec (Gomez et al. 1993), while high-mass stars are found within dense stellar clusters of up to 104 stars per cubic parsec (e.g. the Orion Nebula Cluster, Hillenbrand & Hartmann 1998). To explain this different behaviour, it has been proposed that massive stars may form with a process that is drastically different from the standard accretion picture, e.g. by coalescence of lower mass seeds in a dense cluster environment. The transition between these two modes of formation should occur in the intermediate-mass regime, namely 2 <= M/M0 <= 15
Mid-IR observations of small stellar clusters surrounding Herbig AeBe stars
No full textWe present TIMMI2 observations at 10 mu m of 12 fields centered on known Herbig AeBe (HAeBe) stars. We detected sources (in addition to the HAeBe star) in five fields, three of which are associated with early B stars. Using complementary near-IR and optical data, we estimate that the detected sources are young, embedded (AV, ~ 5-24 mag), intermediate-mass objects ( ~ 1.5-3.5 Msun). In four objects the 10 mu m emission can be accounted for by a circumstellar disk, while in 2 cases (possibly 3) there is evidence that the objects are still in an earlier stage of evolution (Class I). These results confirm the association of the early B Herbig stars with rather rich clusters, as well as their very young age. If our estimates of the stellar parameters are confirmed, this would be the largest sample so far of embedded, intermediate-mass objects available for further studies
Characterization of Narrow Band Filters for Infrared Astronomy The Brgamma and H_2 filters
No full textWe present a characterization of two narrow band filters commonly used in Infrared Astronomy. Our study mainly quantifies the effect of temperature and tilt angle on the central wavelength and width of the transmission bands of such filters. This allows to evaluate some important effects that must be taken into account to have good quality astronomical images. Effects as the limitation of the field of view and the presence of OH sky lines in the final image are quantified
Testing circumstellar disk lifetimes in young embedded clusters associated with the Vela Molecular Ridge
No full textContext. The Vela Molecular Ridge hosts a number of young embedded star clusters at the same evolutionary stage.
Aims: We test whether the fraction of members with a circumstellar disk in a sample of clusters in the cloud D of the Vela Molecular Ridge, is consistent with relations derived for larger samples of star clusters with an age spread. In addition, we constrain the age of the young embedded star clusters associated with cloud D.
Methods: We carried out L (3.78 μm) photometry using images of six young embedded star clusters associated with cloud D of the Vela Molecular Ridge, taken with ISAAC at the VLT. These data are complemented with the available HKs photometry. The 6 clusters are of roughly the same size and appear to be at the same evolutionary stage. The fraction of stars with a circumstellar disk was measured in each cluster by counting the fraction of sources found to have a NIR excess in colour-colour (HKsL) diagrams.
Results: The L photometry allowed us to identify the NIR counterparts of the IRAS sources associated with the clusters. The fraction of stars with a circumstellar disk appears to be constant within the errors for the 6 clusters. There is a hint that this is lower for the most massive stars. The age of the clusters is constrained to be ∼ 1-2 Myr.
Conclusions: The fraction of stars with a circumstellar disk in the observed sample is consistent with the relations derived from larger samples of star clusters and with other age estimates for cloud D. The fraction may be lower for the most massive stars. Our results agree with a scenario where all intermediate and low-mass stars form with a disk, whose lifetime is shorter for higher mass stars
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