arXiv subjects

C. Rossi

Publications and source records attributed to C. Rossi.

32 records · Page 2Linked to original sources

arXiv2012

A combined ultrasonic flow meter and binary vapour mixture analyzer for the ATLAS silicon tracker

An upgrade to the ATLAS silicon tracker cooling control system may require a change from C3F8 (octafluoro-propane) evaporative coolant to a blend containing 10-25% of C2F6 (hexafluoro-ethane). Such a change will reduce the evaporation temperature to assure thermal stability following radiation damage accumulated at full LHC luminosity. Central to this upgrade is a new ultrasonic instrument in which sound transit times are continuously measured in opposite directions in flowing gas at known temperature and pressure to deduce the C3F8/C2F6 flow rate and mixture composition. The instrument and its Supervisory, Control and Data Acquisition (SCADA) software are described in this paper. Several geometries for the instrument are in use or under evaluation. An instrument with a pinched axial geometry intended for analysis and measurement of moderate flow rates has demonstrated a mixture resolution of 3.10-3 for C3F8/C2F6 molar mixtures with 20%C2F6, and a flow resolution of 2% of full scale for mass flows up to 30gs-1. In mixtures of widely-differing molecular weight (mw), higher mixture precision is possible: a sensitivity of <5.10-5 to leaks of C3F8 into part of the ATLAS tracker nitrogen envelope (mw difference 160) has been seen. An instrument with an angled sound path geometry has been developed for use at high fluorocarbon mass flow rates of around 1.2 kgs-1 - corresponding to full flow in a new 60kW thermosiphon recirculator under construction for the ATLAS silicon tracker. Extensive computational fluid dynamics studies were performed to determine the preferred geometry (ultrasonic transducer spacing and placement, together with the sound crossing angle with respect to the vapour flow direction). A prototype with 45deg crossing angle has demonstrated a flow resolution of 1.9% of full scale for linear flow velocities up to 15 ms-1. The instrument has many potential applications.

physics.ins-det↗

arXiv2012

New Bright Carbon Stars Found In The DFBS. (Research note, submitted to Astrophysics)

Carbon rich stars of Population II, such as CH giants, can provide direct information on the role of low to intermediate mass stars of the Halo in early Galactic evolution. Moreover, accurate knowledge of the CH stellar population is a critical requirement for building up scenarios for early Galactic chemical evolution. The first list of the faint high latitude C stars (FHLCs), found in the Digitized First Byurakan Survey {http://byurakan.phys.uniroma1.it and http://www.aras.am/Dfbs/dfbs.html} (DFBS) is given in a paper recently submitted for publication in Astronomy ad Astrophysics. In the present work, we report the recent discovery of two additional CH type stars (not previously catalogued), namely DFBS J075331.98+190344.3 and DFBS J111422.94+091442.7, detected on the DFBS plates with help of the image analysis softwares (FITSView and SAOImage DS9). Medium resolution spectra confirm the C rich nature for both of them. Using infrared color magnitudes relationship, we estimated the distances and K band absolute magnitudes of the new objects.

astro-ph.SR↗

arXiv2012

A New Fast Silicon Photomultiplier Photometer

The realization of low-cost instruments with high technical performance is a goal which deserves some efforts in an epoch of fast technological developments: indeed such instruments can be easily reproduced and therefore allow to open new research programs in several Observatories. We realized a fast optical photometer based on the SiPM technology, using commercially available modules. Using low-cost components we have developed a custom electronic chain to extract the signal produced by a commercial MPPC module produced by Hamamatsu, in order to obtain sub millisecond sampling of the light curve of astronomical sources, typically pulsars. In the early February 2011 we observed the Crab Pulsar at the Cassini telescope with our prototype photometer, deriving its period, power spectrum and shape of its light curve in very good agreement with the results obtained in the past with other instruments.

astro-ph.IM↗

arXiv2011

Revised classification of the SBS carbon star candidates including the discovery of a new emission line dwarf carbon star

Faint high latitude carbon stars are rare objects commonly thought to be distant, luminous giants. For this reason they are often used to probe the structure of the Galactic halo; however more accurate investigation of photometric and spectroscopic surveys has revealed an increasing percentage of nearby objects with luminosities of main sequence stars. Aiming at clarifyin the nature of the ten carbon star candidates present in the General Catalog of the Second Byurakan Survey we analyzed new optical spectra and photometry and used astronomical databases available on the web. We verified that two stars are N-type giants already confirmed by other surveys. We found that four candidates are M type stars and confirmed the carbon nature of the remaining four stars; the characteristics of three of them are consistent with an early CH giant type. The fourth candidate, SBS1310+561 identified with a high proper motion star, is a rare type of dwarf carbon showing emission lines in its optical spectrum. We estimated absolute magnitudes and distances to the dwarf carbon and the three CH stars. Our limited sample confirmed the increasing evidence that spectroscopy or colour alone are not conclusive luminosity discriminants for CH$-$type carbon stars .

astro-ph.SR↗

arXiv2010

The Extreme LBV Star GR290 (Romano's Star) in M33. Optical Spectrophotometric Monitoring

We study the long term, S Dor-type variability and the present hot phase of the LBV star GR290 (Romano's Star) in M33 in order to investigate possible links between the LBV and WNL stages of very massive stars. We use intermediate resolution spectra, obtained with WHT in December 2008, when GR290 was at minimum (V = 18.6), as well as new low resolution spectra and B V R I photometry obtained with the Loiano and Cima Ekar telescopes during 2007-2010. We identify more than 80 emission lines in the 3100-10000 A range, belonging to different species and to forbidden transitions. Many lines, especially the HeI triplets, show a P Cygni profile with an a-e radial velocity difference from -300 to -500 km/s. The shape of the 4630-4713 A emission blend and of other emission lines resembles that of WN9 stars; the blend deconvolution shows that the HeII 4686 A has a strong broad component with FWHM \simeq 1700 km/s. During 2003-2010 the star underwent large spectral variations, best seen in the 4630-4686 A emission feature. Using the late-WN spectral types of Crowther & Smith (1997), GR290 apparently varied between the WN11 and WN8-9 spectral types, the hotter being the star the fainter its visual magnitude. This spectrum-visual luminosity anticorrelation of GR290 is reminiscent of the behaviour of the best studied LBVs. During the 2008 minimum we find a significant decrease in bolometric luminosity, which could be attributed to absorption by newly formed circumstellar matter. We suggest that, presently, the broad 4686 A line and the optical continuum are formed in a central WR region, while the narrow emission line spectrum originate in an extended, slowly expanding envelope, that is composed by matter ejected during previous high luminosity phases, and ionized by the central nucleus. GR290 could have just entered in a phase preceeding the transition from the LBV state to late WN type.

astro-ph.SR↗

arXiv2009

On the nature of the prototype LBV AG Carinae I. Fundamental parameters during visual minimum phases and changes in the bolometric luminosity during the S-Dor cycle

We present a detailed spectroscopic analysis of the luminous blue variable AG Carinae during the last two visual minimum phases of its S-Dor cycle (1985-1990 and 2000-2003). The analysis reveals an overabundance of He, N, and Na, and a depletion of H, C, and O, on the surface of AG Car, indicating the presence of CNO-processed material. Furthermore, the ratio N/O is higher on the stellar surface than in the nebula. We found that the minimum phases of AG Car are not equal to each other, since we derived a noticeable difference between the maximum effective temperature achieved during 1985-1990 (22,800 K) and 2000-2001 (17,000 K). While the wind terminal velocity was 300 km/s in 1985-1990, it was as low as 105 km/s in 2001. The mass-loss rate, however, was lower from 1985-1990 (1.5 x 10^(-5) Msun/yr) than from 2000-2001 (3.7 x 10^(-5) Msun/yr). We found that the wind of AG Car is significantly clumped (f=0.10 - 0.25) and that clumps must be formed deep in the wind. We derived a bolometric luminosity of 1.5 x 10^6 Lsun during both minimum phases which, contrary to the common assumption, decreases to 1.0 x 10^6 Lsun as the star moves towards maximum flux in the V band. Assuming that the decrease in the bolometric luminosity of AG Car is due to the energy used to expand the outer layers of the star (Lamers 1995), we found that the expanding layers contain roughly 0.6 - 2 Msun. Such an amount of mass is an order of magnitude lower than the nebular mass around AG Car, but is comparable to the nebular mass found around lower-luminosity LBVs and to that of the Little Homunculus of Eta Car. If such a large amount of mass is indeed involved in the S Dor-type variability, we speculate that such instability could be a failed Giant Eruption, with several solar masses never becoming unbound from the star.(abridged)

astro-ph.SR↗

arXiv2007

The 2006 hot phase of Romano's star (GR 290) in M33

Understanding the nature of the instabilities of LBVs is important to understand the late evolutionary stages of very massive stars. We investigate the long term, S Dor-type variability of the luminous blue variable GR290 (Romano's star) in M33, and its 2006 minimum phase. New spectroscopic and photometric data taken in November and December 2006 were employed in conjunction with already published data on GR290 to derive the physical structure of GR290 in different phases and the time scale of the variability. We find that by the end of 2006, GR 290 had reached the deepest visual minimum so far recorded. Its present spectrum resembles closely that of the Of/WN9 stars, and is the hottest so far recorded in this star (and in any LBV as well), while its visual brightness decreased by about 1.4 mag. This first spectroscopic record of GR290 during a minimum phase confirms that, similarly to AG Car and other LBVs, the star is subject to ample S Dor-type variations, being hotter at minimum, suggesting that the variations take place at constant bolometric luminosity.

astro-ph↗

arXiv2005

The Long Term Optical Variability of the BL Lac object S5 0716+714: Evidence for a Precessing Jet

We present the historic light curve of the BL Lac object S5 0716+714, spanning the time interval from 1953 to 2003, built using Asiago archive plates and our recent CCD observations, together with literature data. The source shows an evident long term variability, over which well known short term variations are superposed. In particular, in the period from 1961 to 1983 the mean brightness of S5 0716+714 remained significantly fainter than that observed after 1994. Assuming a constant variation rate of the mean magnitude we can estimate a value of about 0.11 magnitude/year. The simultaneous occurrence of decreasing ejection velocities of superluminal moving components in the jet reported by Bach et al. (2005) suggests that both phenomena are related to the change of the direction of the jet to the line of sight from about 5 to 0.7 degrees for an approximately constant bulk Lorentz factor of about 12. A simple explanation is that of a precessing relativistic jet, which should presently be close to the smallest orientation angle. One can therefore expect in the next ten years a decrease of the mean brightness of about 1 magnitude.

astro-ph↗

arXiv2004

3C 345: the historical light curve (1967-1990) from the digitized plates of the Asiago Observatory

In the frame of a large project to digitize the plate archives of the Italian and Vatican Astronomical Observatories, we have already performed the digitization of all available plates of the field of the quasar 3C345. The plates, approximately 100, were taken with the three telescopes of the Asiago Observatory (122 cm, 182 cm, 67/90 cm Schmidt Telescope) in the period from 1967 to 1990. We present here essentially new data, mostly in the B band, about the variability of 3C 345 and also of other four objects (3 quasars and the active galaxy NGC 6212) in the same field, in that period. Beyond the well known 3C 345 itself, also the other three quasars show variability, with a range of 2.0 mag for Q1 and Q2, 1 mag for Q3. The low level variability detected for the nucleus of NGC 6212 is more suspicious, and should be confirmed by linear detector data.

astro-ph↗

arXiv2004

A study of the expanding envelope of Nova V1974 Cyg 1992 based on IUE high resolution spectroscopy

We have carried out a detailed analysis of the IUE archival high resolution spectra of the classical nova V1974 Cyg 1992. In addition to the P Cygni and emission lines, two shortward shifted absorption systems are present. Evidence is given that these absorptions originate in two separate expanding shells, outside the wind layers where the emission lines are formed. The outer main shell, containing most of the matter ejected at outburst, produces the so-called ``principal absorption line system'', and the inner faster moving second shell produces the so-called ``diffuse--enhanced absorption line system''. The outflow velocity of the two shells increases exponentially with time reaching a value of about 1750 km/s and 2900 km/s, respectively. We suggest that the acceleration of the shells is the result of increasing line-radiation pressure due to the UV-brightening of the star as the effective radius decreases. Around day 60 the second shell has overtaken the slower moving principal system shell, and merged with it. This explains: the sudden disappearance of the diffuse line system near that date, the upward jump of 240 km/s in velocity of the principal system and the first detection of hard X-ray emission on day 63. This velocity jump indicates that the main shell is about 4 times more massive than the second shell. The deceleration suffered by the diffuse-enhanced system after the shock provides a shock temperature of about 1.6 KeV, in fairly good agreement with the temperature of the observed hard X-ray emission.

astro-ph↗

arXiv2004

The high energy X-ray tail of Eta Car revealed by BeppoSAX

We report on the June 2000 long (100 ks) BeppoSAX exposure that has unveiled above 10 keV a new very high energy component of the X-ray spectrum of Eta Car extending to at least 50 keV. We find that the 2-150 keV spectrum is best reproduced by a thermal + non-thermal model. The thermal component dominates the 2-10 keV spectral range with kT_h=5.5 +/- 0.3 keV and log NH_h=22.68 +/- 0.01. The spectrum displays a prominent iron emission line centred at 6.70 keV. Its equivalent width of 0.94 keV, if produced by the thermal source, gives a slightly sub-solar iron abundance ([Fe/H]=-0.15 +/- 0.02). The high energy tail above 10 keV is best fitted by a power law with a photon index of 2.42 +/- 0.04. The integrated 13-150 keV luminosity of ~12 L-sun is comparable to that of the 2-10 keV thermal component. The present result can be explained, in the Eta Car binary star scenario, by Comptonisation of low frequency radiation by high energy electrons, probably generated in the colliding wind shock front, or in instabilities in the wind of the S Dor primary star. It is possible that the high energy tail had largely weakened near the minimum of the 5.53 yr cycle. With respect to the thermal component, it probably has a longer recovering time like that of the highest excitation optical emission lines. Both features can be associated with the large absorption measured by BeppoSAX at phase 0.05.

astro-ph↗

arXiv2004

3C 345: The Historical Light Curve (1966-1991) from the digitized plates of the Asiago Observatory

In the frame of a large project to digitize the plate archives of the Italian and Vatican Astronomical Observatory, we have already performed the digitization of all available plates of the field of 3C345. The plates, approximately 100, were taken with the three telescopes of the Asiago Observatory (122 cm, 180 cm, 67/90 Schmidt Telescope) in the period from 1966 to 1991. Many of them were not analyzed till now, so we present here essentially new data about the variability of 3C 345 and also of other six quasars in the same field, in that period.

astro-ph↗

arXiv2000

The FaBrE Project at Trieste

A program to design a high brilliance electron source suitable for a short wavelength Linac-based FEL is presented. The goal of the project is to develop a multi-cell integrated photoinjector capable of delivering 1 nC bunches with emittance below 1 mm mrad. This will be the first step toward a possible development of a IV generation light source test facility based on the existing Trieste Linac. For this purpose a common program between Sincrotrone Trieste and INFN-Milano has been undertaken. Here a brief description of the program and the first results of the RF Gun electromagnetic structure with the beam dynamics on the ELETTRA Linac are presented.

physics.acc-ph↗

arXiv1993

AG Carinae III. The 1990 hot phase of the star and the physical structure of the circumstellar environment

Long slit spectra of the region around the Luminous Blue Variable AG Car obtained in June 1990, just before the onset of its new brightening phase, indicate that AG Car was in the hottest phase so far recorded. The spectrum shows strong broad emissions of He II 468.6 nm, and [Fe III] 465.8-470.1 nm, and He I, N II, Si III, Al III and Fe III with a P Cygni profile. We have also identified weak lines of C II and O II. A faint continuum emission has been detected within the whole ring nebula. We also find that the spectrum of some regions is partially due to scattered star's light. This provides evidence of the presence of dust grains, and indicates that even in present times dust is condensing from AG Car wind. The sharp outer edge of the ring suggest the presence of a shock front. We have also identified an extended low density H II $halo$, which should be associated with the residual of the stellar wind of a previous cooler evolutionary phase of AG Car. In the halo the H$α$/[N II] ratio is strengthened with respect to the ring nebula, possibly because of a discontinuity in the N abundance. We suggest that part of the star's reddening originates in the ring nebula and diffuse halo, which could alter the present estimates of the star's distance. The extracted spectrograms are available as computer files on request to rossi@astrom.astro.it. Article in press in A\&A.

astro-ph↗