arXiv subjects

S. Viti

Publications and source records attributed to S. Viti.

211 records · Page 12Linked to original sources

arXiv2005

Starburst Models For FIR/sub-mm/mm Line Emission. I. An Expanding Supershell Surrounding A Massive Star Cluster

The effect of a newly born star cluster inside a giant molecular cloud (GMC) is to produce a hot bubble and a thin, dense shell of interstellar gas and dust swept up by the H II expansion, strong stellar winds, and repeated supernova explosions. Lying at the inner side of the shell is the photodissociation region (PDR), the origin of much of the far-infrared/sub-millimeter/millimeter (FIR/sub-mm/mm) radiation from the interstellar medium (ISM). We present a model for the expanding shell at different stages of its expansion which predict mm/sub-mm and far-IR emission line intensities from a series of key molecular and atomic constituents in the shell. The kinematic properties of the swept-up shell predicted by our model are in very good agreement with the measurements of the supershell detected in the nearby starburst galaxy M 82. We compare the modeling results with the ratio-ratio plots of the FIR/sub-mm/mm line emission in the central 1.0 kpc region to investigate the mechanism of star forming activity in M 82. Our model has yielded appropriate gas densities, temperatures, and structure scales compared to those measured in M 82, and the total H2 content is compatible with the observations. This implies that the neutral ISM of the central star-forming region is a product of fragments of the evolving shells.

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arXiv2005

Chemical differentiation along the CepA-East outflows

We present the results of a multiline survey at mm-wavelengths of the Cepheus A star forming region. Four main flows have been identified: three pointing in the SW, NE, and SE directions and accelerating high density CS clumps. The fourth outflow, revealed by high-sensitivity HDO observations, is pointing towards South and is associated with conditions particularly favourable to a chemical enrichment. The analysis of the line profiles shows that the SiO molecule dominates at the highest velocities and at the highest excitation conditions, confirming its close association with shocks. H2S, SO2, and SO preferentially trace more quiescent regions than SiO, and in particular a lack of bright H2S emission at the highest velocities is found. OCS and H2CS emit at quite high velocities, where the abundances of three shock tracers like SiO, CH3OH, and HDO are higher. These results may indicate that H2S is not the only major sulphur carrier in the grain mantles, and that OCS and H2CS may probably play an important role on the grains; or that alternatively they rapidly form once the mantle is evaporated after the passage of a shock. Finally, the outflow peak emission has been compared with recent time-dependent sulphur chemistry models: the results indicate that, if associated with accurate measurements of the physical conditions, the CH3OH/H2CS column density ratio can be used as an effective chemical clock to date the age of shocked gas.

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arXiv2005

Hot Cores : Probes of High-Redshift Galaxies

The very high rates of second generation star formation detected and inferred in high redshift objects should be accompanied by intense millimetre-wave emission from hot core molecules. We calculate the molecular abundances likely to arise in hot cores associated with massive star formation at high redshift, using several independent models of metallicity in the early Universe. If the number of hot cores exceeds that in the Milky Way Galaxy by a factor of at least one thousand, then a wide range of molecules in high redshift hot cores should have detectable emission. It should be possible to distinguish between independent models for the production of metals and hence hot core molecules should be useful probes of star formation at high redshift.

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arXiv2005

Spectroscopic study of the long-period dust-producing WC7pd+O9 binary HD192641

We present the results of an optical spectroscopic study of the massive Wolf-Rayet binary WR137. These data cover the dust-formation maximum in 1997. Combining all available measurements of radial velocities, we derive, for the first time, a spectroscopic orbit with period 13.05 +/- 0.18 years. The resulting masses, adopting i=67degrees, are M(O)= 20 +/- 2 Mo and M(WR)= 4.4+/- 1.5 Mo. These appear, respectively, around normal and on the low side for the given spectral types. Analysis of the intense multi-site spectroscopic monitoring in 1999 shows that the CIII5696 and CIV5802/12 lines have the highest intrinsic variability levels. The periodogram analysis yields a small-amplitude modulation in the absorption troughs of the CIV5802/12 and HeI5876 lines with a period of 0.83 days, which could be related either to pulsations or large-scale rotating structures as seen in the WN4 star EZ Canis Majoris (WR6).Wavelet analysis of the strong emission lines of CIII5696 and CIV5802/12 enabled us to isolate and follow for several hours small structures associated with density enhancements within the wind of the Wolf-Rayet star. Cross-correlating the variability patterns seen in different lines, we find a weak but significant correlation between the varability in emission lines with different ionization potential, i.e. in lines formed at different distances from the WR stellar core. Adopting a beta wind-velocity law, from the motion of individual subpeaks we find beta around 5, which is significantly larger than the canonical value of 1 usually found in O-star winds.

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arXiv2005

The Molecular Condensations Ahead of Herbig-Haro Objects. III. Radiative and dynamical perturbations of the HH 2 condensation

We have carried out an extensive observational study (from BIMA data) and made a preliminary theoretical investigation of the molecular gas around HH2. The molecular maps show a very complex morphological, kinematical and chemical structure. The overall main conclusion of this work confirms the findings of Paper I and II, by demonstrating that in addition to the strong photochemical effects caused by penetration of the UV photons from HH2 into molecular cloud, a range of complex radiative and dynamical interactions occur. Thus, despite the apparent `quiescent' nature of the molecular cloud ahead of HH2, the kinematical properties observed within the field of view suggest that it is possibly being driven out by powerful winds from the VLA 1 protostar.

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arXiv2004

The chemistry of transient microstructure in the diffuse interstellar medium

Transient microstructure in the diffuse interstellar medium has been observed towards galactic and extragalactic sources for decades, usually in lines of atoms and ions, and, more recently, in molecular lines. Evidently, there is a molecular component to the transient microstructure. In this paper we explore the chemistry that may arise in such microstructure. We use a PDR code to model the conditions of relatively high density, low temperature, very low visual extinction and very short elapsed time that are appropriate for these objects. We find that there is a well defined region of parameter space where detectable abundances of molecular species might be found. The best matching models are those where the interstellar microstructure is young (<100 yr), small (~100 AU), and dense (>10^4 cm^-3).

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arXiv2004

Molecular abundance ratios as a tracer of accelerated collapse in regions of high mass star formation?

Recent observations suggest that the behaviour of tracer species such as N_2H+ and CS is significantly different in regions of high and low mass star formation. In the latter, N_2H+ is a good tracer of mass, while CS is not. Observations show the reverse to be true in high-mass star formation regions. We use a computational chemical model to show that the abundances of these and other species may be significantly altered by a period of accelerated collapse in high mass star forming regions. We suggest these results provide a potential explanation of the observations, and make predictions for the behaviour of other species.

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arXiv2004

Evaporation of ices near massive stars: models based on laboratory TPD data

Hot cores and their precursors contain an integrated record of the physics of the collapse process in the chemistry of the ices deposited during that collapse. In this paper, we present results from a new model of the chemistry near high mass stars in which the desorption of each species in the ice mixture is described as indicated by new experimental results obtained under conditions similar to those hot cores. Our models show that provided there is a monotonic increase in the temperature of the gas and dust surrounding the protostar, the changes in the chemical evolution of each species due to differential desorption are important. The species H$_2$S, SO, SO$_2$, OCS, H$_2$CS, CS, NS, CH$_3$OH, HCOOCH$_3$, CH$_2$CO, C$_2$H$_5$OH show a strong time dependence that may be a useful signature of time evolution in the warm-up phase as the star moves on to the Main Sequence. This preliminary study demonstrates the consequences of incorporating reliable TPD data into chemical models.

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arXiv2003

Atomic lines in infrared spectra for ultracool dwarfs

We provide a set of atomic lines which are suitable for the description of ultracool dwarf spectra from 10000 to 25000 Å. This atomic linelist was made using both synthetic spectra calculations and existing atlases of infrared spectra of Arcturus and Sunspot umbra. We present plots, which show the comparison of synthetic spectra and observed Arcturus and Sunspot umbral spectra for all atomic lines likely to be observable in high resolution infrared spectra.

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arXiv2003

Chemistry and clumpiness in planetary nebulae

We study the chemistry in the slow wind during the transition from AGB star to pre-planetary nebula (PPN) and planetary nebula (PN). We show that there is a very rich chemistry of degradation products created by photoprocessing, driven by the gradually hardening radiation field of the central star. Most of these products are, however, photodissociated during the PPN phase if the wind is smooth. By contrast, if the wind is clumpy, possibly because of clumpiness in the AGB atmosphere, then many of these degradation products survive into the PN phase. Thus, chemistry may be used to infer the existence of clumpiness in the AGB phase. We identify potential molecular tracers, and we note that, in the case of clumpiness, large molecules may survive the transport from the stellar atmosphere to the interstellar medium. We compare between our model results with observations of three objects at differing evolutionary stages: CRL618, NGC 7027 and the Helix nebula (NGC 7293).

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arXiv2002

Molecular gas in spiral galaxies: a new warm phase at large galactocentric distances?

There is now strong evidence suggesting that the 12CO J = 1-0 transition, widely used to trace H2 gas, significantly underestimates its mass in metal-poor regions. In spiral disks such regions are found in large galactocentric distances where we show that any unaccounted H2 gas phase is likely to be diffuse (~5-20 cm^-3) and warmer (T(kin) ~ 50-100 K) than the cool (T(kin) ~ 15-20 K) CO-luminous one. Moreover we find that a high value of the H2 formation rate on grains, suggested by recent observational work, can compensate for the reduction of the available grain surface in the metal-poor part of typical galactic disks and thus enhance this CO-poor H2 component which may be contributing significantly to the mass and pressure of spiral disks beyond their optical radius.

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arXiv2002

The Molecular Condensations Ahead of Herbig-Haro Objects. I. Multi-transition Observations of HH 2

We present a CSO and BIMA molecular line survey of the dense, quiescent molecular environment ahead of HH2. The molecular gas is cold, 13 K, and moderately dense, 3x10^5 cm^-3. A total of 14 species has been detected (including different isotopes and deuterated species). The relative abundances of the clump are compared with other dense molecular environments, including quiescent dark clouds, and active low and high mass star forming regions. This comparison confirms the peculiar chemical composition of the quiescent gas irradiated by the HH objects. Thus, from this comparison, we found that the HCO+, CH3OH and H2CO are strongly enhanced. SO and SO2 are weakly enhanced, whereas HCN and CS are underabundant. The CN abundance is within the range of value found in starless dark clouds, but it is low with respect to high mass star forming regions. Finally, the chemical composition of HH2 confirms the qualitative results of the Viti & Williams (1999) complex chemical model that follows the chemical behavior of a molecular clump irradiated by a HH object.

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arXiv2001

Star Formation Signatures in the Condensation Downstream of HH 80N

HH80N is one of the Herbig-Haro objects that have associated quiescent dense clumps. We report CO and CS BIMA observations that reveal star formation within the HH80N dense clump. The CO emission reveals clearly a bipolar molecular outflow centered on the dense clump. The CS emission traces a ring-like structure of radius ~0.24 pc. The CS kinematics shows that the ring is collapsing with an infall speed of ~0.6 km/s. The required mass to produce the collapse is in agreement with previous ammonia observations of the 20 solar mass core, which is embedded within the CS structure. However, we cannot discard that the ring structure is expanding driven by protostellar winds, if the CS abundance if unusually high and the CO momentum rate is much higher than that measured, due to inclination and optical depth effects. The properties of the molecular outflow and of the dense core suggest that it harbors a Class 0 object. There are also signatures of interaction of the HH 80/81/80N outflow with the dense gas. In particular it is possible that the HH 80/81/80N outflow has triggered or at least speed up the star formation in this region.

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