Fisica

In the writer’s opinion the frequently used expression “scientific theatre ” is actually an ambiguous term, since it refers to a number of widely differing experiences. For this reason, the word “theatre ” is preferred in this paper when referring to those events which take place in institutionalised settings, while the term “theatrical communic-action ” is deemed more appropriate when referring to those taking place in settings devoted to scientific communication, such as museums. After a brief historical analysis of the various approaches of theatre to science and some examples of plays in Italy and abroad, the writer investigates how theatre can produce effective scientific communication in museums. Theatrical action can bring out the value of the exhibits of a museum, while creating a new way of experiencing the exhibitions. Theatrical actions link education and entertainment, consequently becoming a highly effective didactic instrument. The advantages of theatre are briefly out...

We consider the general problem of fitting a parametric density model to discrete observations, taken to follow a non-homogeneous Poisson point process. This class of models is very common, and can be used to describe many astrophysical processes, including the distribution of protostars in molecular clouds. We give the expression for the likelihood of a given spatial density distribution of protostars and apply it to infer the most probable dependence of the protostellar surface density on the gas surface density. Finally, we apply this general technique to model the distribution of protostars in the Orion molecular cloud and robustly derive the local star formation scaling (Schmidt) law for a molecular cloud. We find that in this cloud the protostellar surface density, Σ YSO , is directly proportional to the square gas column density, here expressed as infrared extinction in the K-band, A K : more precisely, Σ YSO = (1.65 ± 0.19) (A K /mag) 2.03 ± 0.15 stars pc −2 .

We consider from a general point of view the problem of determining the extinction in dense molecular clouds. We use a rigorous statistical approach to characterize the properties of the most widely used optical and infrared techniques, namely the star count and the color excess methods. We propose a new maximum-likelihood method that takes advantage of both star counts and star colors to provide an optimal estimate of the extinction. Detailed numerical simulations show that our method performs optimally under a wide range of conditions and, in particular, is significantly superior to the standard techniques for clouds with high column-densities and affected by contamination by foreground stars.

We generalize the technique of Lada et al. (1994) to map dust column density through a molecular cloud (NICE) to an optimized multi-band technique (NICER) that can be applied to any multi-band survey of molecular clouds. We present a first application to a ~625 deg^2 subset of the Two Micron All Sky Survey (2MASS) data and show that when compared to NICE, the optimized NICER technique (i) achieves the same extinction peak values, (ii) improves the noise variance of the map by a factor of 2 and (iii) is able to reach 3 \sigma dust extinction measurements as low as A_V = 0.5 magnitudes, better than or equivalent to classical optical star count techniques and below the threshold column density for the formation of CO, the brightest H2 tracer in radio-spectroscopy techniques. The application of the NICER techniques to near-infrared data obtained with a 8 meter-class telescope with a state-of-the-art NIR camera, such as the VLT-ISAAC combination, will be able to achieve dynamic ranges from below 10^21 protons cm^-2 to over 10^23 protons cm^-2 (A_V in the range [0.3, 60]) and spatial resolutions < 10'', making use of a single and straightforward column density tracer, extinction by interstellar dust.

We present an analysis of wide-field infrared extinction maps of a region in Perseus just north of the Taurus-Auriga dark cloud complex. From this analysis we have identified a massive, nearby, but previously unrecognized, giant molecular cloud (GMC). From comparison of foreground star counts with Galactic models we derive a distance of 450 +/- 23 parsecs to the cloud. At this distance the cloud extends over roughly 80 pc and has a mass of approximately 10^5 solar masses, rivaling the Orion (A) Molecular Cloud as the largest and most massive GMC in the solar neighborhood. Although surprisingly similar in mass and size to the more famous Orion Molecular Cloud (OMC) the newly recognized cloud displays significantly less star formation activity with more than an order of magnitude fewer young stellar objects than found in the OMC, suggesting that both the level of star formation and perhaps the star formation rate in this cloud are an order of magnitude or more lower than in the OMC. Analysis of extinction maps of both clouds shows that the new cloud contains only 10% the amount of high extinction (A_K > 1.0 mag) material as is found in the OMC. This, in turn, suggests that the level of star formation activity and perhaps the star formation rate in these two clouds may be directly proportional to the total amount of high extinction material and presumably high density gas within them and that there might be a density threshold for star formation on the order of a few times 10^4 hydrogen molecules per cc.

We combine extinction maps from the Two Micron All Sky Survey (2MASS) with Hipparcos and Tycho parallaxes to obtain reliable and high-precision estimates of the distance to the Ophiuchus and Lupus dark complexes. Our analysis, based on a rigorous maximumlikelihood approach, shows that the ρ-Ophiuchi cloud is located at (119 ± 6) pc and the Lupus complex is located at (155 ± 8) pc; in addition, we are able to put constraints on the thickness of the clouds and on their orientation on the sky (both these effects are not included in the error estimate quoted above). For Ophiuchus, we find some evidence that the streamers are closer to us than the core. The method applied in this paper is currently limited to nearby molecular clouds, but it will find many natural applications in the GAIA-era, when it will be possible to pin down the distance and three-dimensional structure of virtually every molecular cloud in the Galaxy.

A little more than a decade has passed since the advent of large format infrared array cameras opened a new window on molecular cloud research. This powerful observational tool has enabled dust extinction and column density maps of molecular clouds to be constructed with unprecedented precision, depth and angular resolution. Near-infrared extinction studies can achieve column density dynamic ranges of 0.3 < AV < 40 magnitudes (6 x 10 20 < N < 10 23 cm −2 ), allowing with one simple tracer a nearly complete description of the density structure of a cloud free from the uncertainties that typically plague measurements derived from radio spectroscopy and dust emission. This has led in recent years to an empirical characterization of the evolutionary status of dense cores based on the shapes of their radial column density profiles and revealed the best examples in nature of Bonnor-Ebert spheres. Wide-field infrared extinction mapping of large cloud complexes provides the most complete inventory of cloudy material that can be derived from observations. Such studies enable the measurement of the mass function of dense cores within a cloud, a critical piece of information for developing an understanding of the origin of the stellar IMF. Comparison with radio spectroscopic data has allowed detailed chemical structure studies of starless cores and provided some of the clearest evidence for differential depletion of molecular species in cold gaseous configurations. Recent studies have demonstrated the feasibility of infrared extinction mapping of GMCs in external galaxies, enabling the fundamental measurement of the GMC mass function in these systems. In this contribution we review recent results arising from this powerful technique, ranging from studies of Bok globules to local GMCs, to GMCs in external galaxies.

We present an overview of data available for the Ophiuchus and Perseus molecular clouds from Phase I of the COMPLETE Survey of Star-Forming Regions. This survey provides a range of data complementary to the Spitzer Legacy Program ''From Molecular Cores to Planet Forming Disks.'' Phase I includes the following: extinction maps derived from the Two Micron All Sky Survey (2MASS) near-infrared data using the NICER algorithm; extinction and temperature maps derived from IRAS 60 and 100 m emission; H i maps of atomic gas; 12 CO and 13 CO maps of molecular gas; and submillimeter continuum images of emission from dust in dense cores. Not unexpectedly, the morphology of the regions appears quite different depending on the column density tracer that is used, with IRAS tracing mainly warmer dust and CO being biased by chemical, excitation, and optical depth effects. Histograms of column density distribution are presented, showing that extinction as derived from 2MASS NICER gives the closest match to a lognormal distribution, as is predicted by numerical simulations. All the data presented in this paper, and links to more detailed publications on their implications, are publicly available at the COMPLETE Web site.

In this paper we present the results of a systematic investigation of an entire population of starless dust cores within a single molecular cloud. Analysis of extinction data shows the cores to be dense objects characterized by a narrow range of density. Analysis of C18O and NH3 molecular-line observations reveals very narrow lines. The non-thermal velocity dispersions measured in both these tracers are found to be subsonic for the large majority of the cores and show no correlation with core mass (or size). Thermal pressure is thus the dominate source of internal gas pressure and support for most of the core population. The total internal gas pressures of the cores are found to be roughly independent of core mass over the entire range of the core mass function (CMF) indicating that the cores are in pressure equilibrium with an external source of pressure. This external pressure is most likely provided by the weight of the surrounding Pipe cloud within which the cores are embedded. Most of the cores appear to be pressure confined, gravitationally unbound entities whose nature, structure and future evolution are determined by only a few physical factors which include self-gravity, the fundamental processes of thermal physics and the simple requirement of pressure equilibrium with the surrounding environment. The observed core properties likely constitute the initial conditions for star formation in dense gas. The entire core population is found to be characterized by a single critical Bonnor-Ebert mass. This mass coincides with the characteristic mass of the Pipe CMF indicating that most cores formed in the cloud are near critical stability. This suggests that the mass function of cores (and the IMF) has its origin in the physical process of thermal fragmentation in a pressurized medium.

We report here the phase-sensitive measurem ents of two-photon uorescence polarizatio n anisotropy (FPA) on highly diluted solutions. We rst describe the characteriza tion of the response of the two-photon microscope to the light polarization and its test by mean s of measurem ents of the FPA on rhodamine 6G as a function of the viscosity. Further, we report the study of the FPA at high dilutions on a globular protein, the beta-lactoglobulin B (BLG) labeled with Alexa 532. The number of m olecules (from 0.4 to 17 molecules per excitation volume) is m easured by means of uorescence correlation spectroscopy (FCS). The average rotational and translational diffusion coef cients measured with the FPA and FCS methods are in good agreem ent with the protein size and do not show a substantial dependence on the protein concen tration, despite the very low signal (ù 3 times the backgroun d) observed for highly diluted solutions.

We com pare two m ethods for the measurement of the excitation volume of an optical inverted m icroscope for two-photon excited uorescence spectro scopy. The measure of the decay tim e of thē uorescence autocorrelation function gives the particles' diffusion tim e in the excitation volume, and from an a priori knowledge of the diffusion coef® cient we can estim ate the excitation volume. O n the other hand, we can count the average number of particles in the excitation volume by analyzing the histogram of the¯uorescen ce photon counting, and from the knowledge of the¯uorophores' concen tration we obtain the excitation volume. We show that the measurem ents of the excitation volume based on the analysis of the correlation function are very close, for a wide range of excitation volumes (0.1 fL , V EX C , 6 fL), to those obtained with no a priori assumptions from the analysis of the photon counting histogram s. Moreover, we suggest that the study of the relation between the average number of particles in the excitation volume and the nominal concentration can be a valuable tool in verifying the good alignment of the optical setup.

Micro- and nanoscale objects with anisotropic shape are key components of a variety of biological systems and inert complex materials, and represent fundamental building blocks of novel self-assembly strategies. The time scale of their thermal motion is set by their translational and rotational diffusion coefficients, whose measurement may become difficult for relatively large particles with small optical contrast. Here we show that Dark Field Differential Dynamic Microscopy is the ideal tool for probing the roto-translational Brownian motion of shape anisotropic particles. We demonstrate our approach by successful application to aqueous dispersions of non-motile bacteria and of colloidal aggregates of spherical particles.

Micro- and nanoscale objects with anisotropic shape are key components of a variety of biological systems and inert complex materials, and represent fundamental building blocks of novel self-assembly strategies. The time scale of their thermal motion is set by their translational and rotational diffusion coefficients, whose measurement may become difficult for relatively large particles with small optical contrast. Here we show that Dark Field Differential Dynamic Microscopy is the ideal tool for probing the roto-translational Brownian motion of shape anisotropic particles. We demonstrate our approach by successful application to aqueous dispersions of non-motile bacteria and of colloidal aggregates of spherical particles.