ALBERT

Description: We presented self-consistent disk models of T Tauri stars that include a parameterized treatment of dust settling and grain growth, building on techniques developed in a series of papers by D'Alessio et al. The models incorporate depleted distributions of dust in upper disk layers along with larger sized particles near the disk midplane, which are expected theoretically and, as we suggested earlier, are necessary to account for millimeter-wave emission, SEDs, scattered light images, and silicate emission features simultaneously. By comparing the models with recent mid- and near-IR observations, we find that the dust-to-gas mass ratio of small grains at the upper layers should be less than 10% of the standard value. The grains that have disappeared from the upper layers increase the dust-to-gas mass ratio of the disk interior; if those grains grow to maximum sizes of the order of millimeters during the settling process, then both the millimeter-wave fluxes and spectral slopes can be consistently explained. Depletion and growth of grains can also enhance the ionization of upper layers, increasing the possibility of the magnetorotational instability for driving disk accretion.

Description: Observations of T Tauri stars and young brown dwarfs suggest that the accretion rates of their disks scales roughly with the square of the central stellar mass. No dependence of accretion rate on stellar mass is predicted by the simplest version of the Gammie layered disk model, in which nonthermal ionization of upper disk layers allows accretion to occur via the magnetorotational instability. We show that a minor modification of Gaminie's model to include heating by irradiation from the central star yields a modest dependence of accretion on the mass of the central star. A purely viscous disk model could provide a strong dependence of accretion rate on stellar mass if the initial disk radius (before much viscous evolution has occurred) has a strong dependence on stellar mass. However, it is far from clear that at least the most massive pre-main-sequence disks can be totally magnetically activated by X-rays or cosmic rays. We suggest that a combination of effects are responsible for the observed dependence, with the lowest mass stars having the lowest mass disks, which can be thoroughly magnetically active, while the higher mass stars have higher mass disks that have layered accret,ion and relatively inactive or "dead" central zones at some radii. In such dead zones, we suggest that gravitational instabilities may play a role in allowing accretion to proceed. In this connection, we emphasize the uncertainty in disk masses derived from dust emission and argue that T Tauri disk masses have been systematically underestimated by conventional analyses. Furtlier study of accretion rates, especially in the lowest mass stars, would help to clarify the mechanisms of accretion in T Tauri stars.

Description: We presented Spitzer Infrared Spectrograph (IRS) observations of two objects of the Taurus population that show unambiguous signs of clea,ring in their inner disks. In one of the objects, DM Tau, the outer disk is truncated at 3 AU; this object is akin to another recently reported in Taurus, CoKu Tau/4, in that the inner disk region is free of small dust. Unlike CoKu Tau/4, however, this star is still accreting, so optically thin gas should still remain in the inner disk region. The other object, GM Aur, also accreting, has about 0.02 lunar masses of small dust in the inner disk region within about 5 AU, consistent with previous reports. However, the IRS spectrum clearly shows that the optically thick outer disk has an inner truncation at a much larger radius than previously suggested, of order 24 AU. These observations provide strong evidence for the presence of gaps in protoplanetary disks.

Description: We presented the infrared spectrum of the young binary system St 34 obtained with the Infrared Spectrograph (IRS) on the Spitzer Space Telescope. The IRS spectrum clearly shows excess dust emission, consistent with the suggestion of White & Hillenbrand that St 34 is accreting from a circumbinary disk. The disk emission of St 34 is low in comparison with the levels observed in typical T Tauri stars; silicate features at 10 and 20 microns are much weaker than typically seen in T Tauri stars; and excess emission is nearly absent at the shortest wavelengths observed. These features of the infrared spectrum suggest substantial grain growth (to eliminate silicate features) and possible settling of dust to the disk midplane (to reduce the continuum excess emission levels), along with a relatively evacuated inner disk, as expected due to gravitational perturbations by the binary system. Although the position of St 34 in the H-R diagram suggests an age of 8f Myr, assuming that it lies at the distance of the Taurus-Auriga molecular clouds, White & Hillenbrand could not detect any Li I absorption, which would indicate a Li depletion age of roughly 25 Myr or more. We suggest that St 34 is closer than the Taurus clouds by about 30-40 pc and has an age roughly consistent with Li depletion models. Such an advanced age would make St 34 the oldest known low-mass pre-main-sequence object with a dusty accretion disk. The persistence of optically thick dust emission well outside the binary orbit may indicate a failure to make giant planets that could effectively remove dust particles.

Description: We presented the results of an infrared imaging survey of Tr 37 and NGC 7160 using the IRAC and MIPS instruments on board the Spitzer Space Telescope. Our observations cover the wavelength range from 3.6 to 24 microns, allowing us to detect disk emission over a typical range of radii 0.1 to 20 AU from the central star. In Tr 37, with an age of about 4 Myr, about 48% of the low-mass stars exhibit detectable disk emission in the IRAC bands. Roughly 10% of the stars with disks may be "transition" objects, with essentially photospheric fluxes at wavelengths i 4.5 microns but with excesses at longer wavelengths, indicating an optically thin inner disk. The median optically thick disk emission in Tr 37 is lower than the corresponding median for stars in the younger Taurus region; the decrease in infrared excess is larger at 6-8 microns than at 24 microns, suggesting that grain growth and/or dust settling has proceeded faster at smaller disk radii, as expected on general theoretical grounds. Only about 4% of the low-mass stars in the 10 Myr old cluster NGC 7160 show detectable infrared disk emission. We also find evidence for 24 micron excesses around a few intermediate-mass stars, which may represent so-called "debris disk" systems. Our observations provided new constraints on disk evolution through an important age range.

Description: We report the design of a new application-specific integrated circuit (ASIC) for use in radio telescope correlators. It supports the construction of correlators for an arbitrarily large number of signals. The ASIC uses an intrinsically low-power architecture along with design techniques and a process that together result in unprecedentedly low power consumption. The design is flexible in that it can support telescopes with almost any number of antennas N. It is intended for use in an "FX" correlator, where a uniform filter bank breaks each signal into separate frequency channels prior to correlation.

Description: Radio telescopes that employ arrays of many antennas are in operation, and ever larger ones are being designed and proposed. Signals from the antennas are combined by cross-correlation. While the cost of most components of the telescope is proportional to the number of antennas N, the cost and power consumption of cross-correlationare proportional to N2 and dominate at sufficiently large N. Here we report the design of an integrated circuit (IC) that performs digital cross-correlations for arbitrarily many antennas in a power-efficient way. It uses an intrinsically low-power architecture in which the movement of data between devices is minimized. In a large system, each IC performs correlations for all pairs of antennas but for a portion of the telescope's bandwidth (the so-called "FX" structure). In our design, the correlations are performed in an array of 4096 complex multiply-accumulate (CMAC) units. This is sufficient to perform all correlations in parallel for 64 signals (N=32 antennas with 2 opposite-polarization signals per antenna). When N is larger, the input data are buffered in an on-chipmemory and the CMACs are re-used as many times as needed to compute all correlations. The design has been synthesized and simulated so as to obtain accurate estimates of the IC's size and power consumption. It isintended for fabrication in a 32 nm silicon-on-insulator process, where it will require less than 12mm2 of silicon area and achieve an energy efficiency of 1.76 to 3.3 pJ per CMAC operation, depending on the number of antennas. Operation has been analyzed in detail up to N = 4096. The system-level energy efficiency, including board-levelI/O, power supplies, and controls, is expected to be 5 to 7 pJ per CMAC operation. Existing correlators for the JVLA (N = 32) and ALMA (N = 64) telescopes achieve about 5000 pJ and 1000 pJ respectively usingapplication-specific ICs in older technologies. To our knowledge, the largest-N existing correlator is LEDA atN = 256; it uses GPUs built in 28 nm technology and achieves about 1000 pJ. Correlators being designed for the SKA telescopes (N = 128 and N = 512) using FPGAs in 16nm technology are predicted to achieve about 100 pJ.

Description: The INGV started its interest to extend the seismic monitoring network to the sea in 1995 with GEOSTAR (Geophysical and Oceanographic Station for Abyssal Research) project, coming out with the realization of the first multidisclipinary observatory for deep-sea monitoring [Favali et al. 2002]. At the end of 2004, the National Earthquake Center (CNT) of INGV decided to provide a pool of Ocean Bottom Seismometers to be employed as a submarine mobile network and to study submarine faults and volcanoes. This was possible thanks to an agreement between the INGV and the Italian National Civil Protection Department (DPC). On July 2006, the Gibilmanna OBS Lab, tested the first OBS prototype for nine days on the flat top of the Marsili submarine volcano [D’Anna et al. 2007] and in early 2007 other seven OBS’s were ready to be deployed on the seafloor. In May 2007, within the European project NERIES (activity NA6), the Gibilmanna OBS Lab of the INGV has deployed three Broad Band Ocean Bottom Seismometers (BBOBS) in the southern Ionian Sea at 3500-4000 meters of depth. This area has been chosen during the NERIES – “NA6-BBOBS net” meeting in Rome, on the 11th of September 2006 because at first, there are at the moment few seismological data [Scrocca et al., 2003] to construct a reliable model for the Ionian lithosphere and also the rate and features of the seismicity in the area between the Hyblean-Malta fault system and the accretionary prism of the Calabrian Arc are largely unknown [Catalano et al. 2002]. The Ionian Sea is indeed one of the most seismically active area in the Mediterranean region with several destructive earthquakes sometimes followed by tsunamis [Tinti et al. 2004]. The seismicity occurring in the Ionian basin is characterized by large location uncertainties due to the lack of seafloor seismic stations. In 2002, the quality of the seismic sensing and the location of earthquakes have been improved by the deployment of the real-time submarine observatory SN-1, about 25 km offshore Eastern Sicily [Sgroi et al, 2007]. However, the SN-1 location only allows to characterize the seismicity in the area offshore the eastern Sicily. Two of the three OBS’s were successfully recovered on the 2nd of February 2008; the last one was recovered on the 15th of March 2008 and another OBS was deployed on the same location to accomplish the continuous long-term seismic monitoring task (until May 2010) as planned in NERIES project.

Description: Istituto Nazionale di Geofisica e Vulcanologia - Centro Nazionale Terremoti

Description: Published

Description: 2.5. Laboratorio per lo sviluppo di sistemi di rilevamento sottomarini

Description: open

Description: Il Centro Nazionale Terremoti (CNT), in collaborazione con la sezione di Catania, ha progettato e realizzato un esperimento di sismica passiva nell’area Calabro–Peloritana il cui scopo è fornire nuovi dati sismici volti a comprendere come le dinamiche superficiali ed il processo di subduzione interagiscano tra loro, migliorando così la comprensione dei processi sismogenetici nella zona colpita dal terremoto del 1908 [Margheriti et al., 2008; http://dpc-s5.rm.ingv.it]. Con l’obiettivo di ridurre l’errore di localizzazione degli ipocentri degli eventi verificatisi nell’area interessata dal progetto, ad integrazione delle 30 stazioni della rete sismica nazionale già presenti nell’area in esame, l’esperimento ha previsto l’installazione di 15 stazioni della rete mobile e la deposizione di 5 OBS/H (Ocean Bottom Seismometer with Hydrophone), per un numero complessivo di 50 stazioni sismiche larga banda 3C coinvolte nel progetto. La campagna sismica a terra ha avuto inizio nell’ottobre 2007 e ad oggi (gennaio 2009), le stazioni sono ancora in funzione, mentre la deposizione dei cinque OBS/H è avvenuta tra il 15 e il 18 luglio 2008 ed il loro recupero è stato effettuato tra il 6 e il 7 novembre 2008. Gli OBS/H, progettati e realizzati presso l’OBS Lab di Gibilmanna, sono stati equipaggiati con sismometri Nanometrics Trillium 120p (120s - 175 Hz) e con sensori differenziali di pressione (Differential Pressure Gauge) Cox-Webb, con banda passante tra i 200s e i 2Hz. La base autolivellante sulla quale è installato il sensore sismico è stata realizzata anch’essa presso l’OBS Lab di Gibilmanna nei mesi intercorsi tra il recupero degli OBS impiegati nella prima campagna NERIES, avvenuto nel marzo 2008, e il luglio 2008, data della deposizione degli OBS del progetto “Messina 1908 – 2008”. La necessità di realizzare una nuova base autolivellante in tempi così brevi, è stata una diretta conseguenza dei risultati negativi ottenuti dalla base Nautilus in occasione della già citata campagna NERIES [D’Anna et al., 2008]: due sismometri su tre non si erano livellati nel range di ±0.2°, massimo tilt dinamico previsto per i Trillium 120p, provocando il non funzionamento degli stessi. Come meglio verrà analizzato nei paragrafi successivi, le problematiche affrontate nella realizzazione di questi dispositivi di livellamento sono state molteplici e di difficile soluzione. L’analisi preliminare dei dati ha evidenziato che soltanto uno dei cinque sensori sismici ha funzionato correttamente per l’intero periodo, mentre gli altri quattro hanno funzionato in media per circa 20 giorni. Causa di ciò, un rapido consumo delle batterie dovuto ad un livellamento sì compreso nel range di ±0.2° dall’orizzontale, condizione necessaria perché il sismometro sia in grado di rilevare eventi sismici, ma oltre il range di ±0.1°, condizione necessaria perché i consumi del Trillium120p si riducano da circa 2.5W ai 600mW nominali. I risultati ottenuti da questo esperimento, sono comunque da inquadrare in una successione degli eventi che ha fatto sì che lo sviluppo di questa nuova base autolivellante fosse condizionato da una certa urgenza: al CNT premeva presentare i dati raccolti dagli OBS al convegno “Scienza e società a 100 anni dal grande Terremoto”, che si sarebbe tenuto a Reggio Calabria dal 10 al 12 dicembre 2008 e visti gli insuccessi della base Nautilus nel precedente esperimento, ci si è trovati di fronte alla necessità di progettare e sviluppare un nuovo sistema di livellamento per i Trillium 120p nell’arco di tre mesi e mezzo. Queste, oltre a quelle di natura economica, le ragioni per cui non è stato possibile procedere secondo un iter che per noi, come per le aziende che operano nel settore marino, è uno standard: - Progettazione; - Realizzazione del prototipo; - Test in laboratorio (e. g. tavola vibrante) - Test in camera iperbarica; - Test in mare; - Produzione in serie; E’ nostra intenzione, in un prossimo futuro, portare avanti lo sviluppo di questa base autolivellante, con tempi e risorse finanziare ed umane più appropriate. Ciò che riportiamo in questo Rapporto Tecnico vuole essere una descrizione del lavoro sin qui svolto, anche se non conclusivo e risolutivo, ma che ci ha già permesso di individuare delle problematiche fondamentali la cui soluzione sarà oggetto di studi più approfonditi. Rimane positivo il fatto che le basi già realizzate riescono già da adesso a livellare automaticamente un sensore entro un range di ±0.2°. Gli OBS/H dell’INGV verranno nuovamente deposti con la stessa disposizione del progetto “Messina 1908 – 2008” nell’estate 2009, nell’ambito del progetto S5 finanziato dal Dipartimento della Protezione Civile. Per sopperire alle problematiche riscontrate nella marinizzazione del Trillium 120p, si è scelto di installare a bordo degli OBS dei sensori Guralp CMG40T-OBS (60s – 100 Hz), progettati per installazioni in mare sino a profondità di 6000 m è già dotati di una propria base autolivellante. Il motivo per il quale non si è utilizzato nelle passate campagne questo tipo di sensore è da addebitare unicamente al fatto che sui fondi DPC della convenzione 2005-2007 non è stato possibile inserire l’acquisto di questi specifici sensori, mentre erano disponibili i Trillium 120p.

Description: Istituto Nazionale di Geofisica e Vulcanologia

Description: Published

Description: 2.5. Laboratorio per lo sviluppo di sistemi di rilevamento sottomarini

Description: open

Description: In 2005, thanks to the 3-year agreement between Dipartimento Nazionale della Protezione Civile (DPC) and Istituto Nazionale di Geofisica e Vulcanologia (INGV) - Centro Nazionale Terremoti (CNT), the project of the first Italian “Ocean Bottom Seismometer with Hydrophone” (OBS/H) for long-term deployment was developed at the OBS Lab of the Gibilmanna Observatory (Sicily). The drawing of the instrument started in January 2005 and, after 18 months, the prototype was ready for test in laboratory, in shallow and deep water. Afterwards, the first OBS/H was tested during an oceanographic campaign on the Marsili submarine volcano, from the 10th to the 21st of July 2006.More than 1000 events of several kinds were recorded: 817 VTB (Volcano Tectonic events, B-type), 159 HF (High Frequency events), 53 SDE (Short Duration Event), 8 regional events localized by INGV land network, 10 not localized events, 1 teleseismic event an 2 rockfall events. The INGV OBS/H are equipped with: - Nanometrics Trillium 120p seismometers (theoretical flat response between 120s and 175 Hz) installed in a 17 inches glass sphere on a Nautilus gimbal for the leveling or Guralp CMG40T-OBS (flat response between 60s and 100 Hz); - Cox-Webb Differential Pressure Gauge (bandwidth 500s-2Hz) or OAS E-2PD hydrophone (0-5kHz); - 21 bits, 4 channels SEND Geolon-MLS digitizer with sampling frequency up to 200 Hz.

Description: Published

Description: 2.5. Laboratorio per lo sviluppo di sistemi di rilevamento sottomarini

Description: open