arXiv subjects

Serena Viti

Publications and source records attributed to Serena Viti.

At least 109 records · Page 6Linked to original sources

arXiv2014

A high resolution study of complex organic molecules in hot cores

We present the results of a line identification analysis using data from the IRAM Plateau de Bure Inferferometer, focusing on six massive star-forming hot cores: G31.41+0.31, G29.96-0.02, G19.61-0.23, G10.62-0.38, G24.78+0.08A1 and G24.78+0.08A2. We identify several transitions of vibrationally excited methyl formate (HCOOCH$_3$) for the first time in these objects as well as transitions of other complex molecules, including ethyl cyanide (C$_2$H$_5$CN), and isocyanic acid (HNCO). We also postulate a detection of one transition of glycolaldehyde (CH$_2$(OH)CHO) in two new hot cores. We find G29.96-0.02, G19.61-0.23, G24.78+0.08A1 and 24.78+0.08A2 to be chemically very similar. G31.41+0.31, however, is chemically different: it manifests a larger chemical inventory and has significantly larger column densities. We suggest that it may represent a different evolutionary stage to the other hot cores in the sample, or it may surround a star with a higher mass. We derive column densities for methyl formate in G31.41+0.31, using the rotation diagram method, of $\times$10$^{17}$ cm$^{-2}$ and a T$_{rot}$ of $\sim$170 K. For G29.96-0.02, G24.78+0.08A1 and G24.78+0.08A2, glycolaldehyde, methyl formate and methyl cyanide all seem to trace the same material and peak at roughly the same position towards the dust emission peak. For G31.41+0.31, however, glycolaldehyde shows a different distribution to methyl formate and methyl cyanide and seems to trace the densest, most compact inner part of hot cores.

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arXiv2014

Deuterium chemistry of dense gas in the vicinity of low-mass and massive star forming regions

The standard interstellar ratio of deuterium to hydrogen (D/H) atoms is $\sim 1.5 \times 10^{-5}$. However, the deuterium fractionation is in fact found to be enhanced, to different degrees, in cold, dark cores, hot cores around massive star forming regions, lukewarm cores, and warm cores ({\it hereafter}, hot corinos) around low-mass star forming regions. In this paper, we investigate the overall differences in the deuterium chemistry between hot cores and hot corinos. We have modelled the chemistry of dense gas around low-mass and massive star forming regions using a gas-grain chemical model. We investigate the influence of varying the core density, the depletion efficiency of gaseous species on to dust grains, the collapse mode and the final mass of the protostar on the chemical evolution of star forming regions. We find that the deuterium chemistry is, in general, most sensitive to variations of the depletion efficiency on to grain surfaces, in agreement with observations. In addition, the results showed that the chemistry is more sensitive to changes in the final density of the collapsing core in hot cores than in hot corinos. Finally, we find that ratios of deuterated sulphur bearing species in dense gas around hot cores and corinos may be good evolutionary indicators in a similar way as their non deuterated counterparts.

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arXiv2014

Detectability of Glycine in Solar-type System Precursors

Glycine (NH2CH2COOH) is the simplest amino acid relevant for life. Its detection in the interstellar medium is key to understand the formation mechanisms of pre-biotic molecules and their subsequent delivery onto planetary systems. Glycine has extensively been searched for toward hot molecular cores, although these studies did not yield any firm detection. In contrast to hot cores, low-mass star forming regions, and in particular their earliest stages represented by cold pre-stellar cores, may be better suited for the detection of glycine as well as more relevant for the study of pre-biotic chemistry in young Solar System analogs. We present 1D spherically symmetric radiative transfer calculations of the glycine emission expected to arise from the low-mass pre-stellar core L1544. Water vapour has recently been reported toward this core, indicating that a small fraction of the grain mantles in L1544 (~0.5%) has been injected into the gas phase. Assuming that glycine is photo-desorbed together with water in L1544, and considering a solid abundance of glycine on ices of ~1E-4 with respect to water, our calculations reveal that several glycine lines between 67 GHz and 80 GHz have peak intensities larger than 10 mK. These results show for the first time that glycine could reach detectable levels in cold objects such as L1544. This opens up the possibility to detect glycine, and other pre-biotic species, at the coldest and earliest stages in the formation of Solar-type systems with near-future instrumentation such as the Band 2 receivers of ALMA.

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arXiv2014

An Alternative Accurate Tracer of Molecular Clouds: The "$X_{\rm CI}$-Factor"

We explore the utility of CI as an alternative high-fidelity gas mass tracer for Galactic molecular clouds. We evaluate the X$_{\rm CI}$-factor for the 609 $μ$m carbon line, the analog of the CO X-factor, which is the ratio of the H$_2$ column density to the integrated $^{12}$CO(1-0) line intensity. We use 3D-PDR to post-process hydrodynamic simulations of turbulent, star-forming clouds. We compare the emission of CI and CO for model clouds irradiated by 1 and 10 times the average background and demonstrate that CI is a comparable or superior tracer of the molecular gas distribution for column densities up to $6 \times 10^{23}$ cm$^{-2}$. Our results hold for both reduced and full chemical networks. For our fiducial Galactic cloud we derive an average $X_{\rm CO}$ of $3.0\times 10^{20}$ cm$^{-2}$K$^{-1}$km$^{-1}$s and $X_{\rm CI}$ of $1.1\times 10^{21}$ cm$^{-2}$K$^{-1}$km$^{-1}$s.

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arXiv2013

Molecules in the Circumnuclear Disk of the Galactic Center

Within a few parsecs around the central Black Hole Sgr A*, chemistry in the dense molecular cloud material of the circumnuclear disk (CND) can be affected by many energetic phenomena such as high UV-flux from the massive central star cluster, X-rays from Sgr A*, shock waves, and an enhanced cosmic-ray flux. Recently, spectroscopic surveys with the IRAM 30 meter and the APEX 12 meter telescopes of substantial parts of the 80--500 GHz frequency range were made toward selected positions in and near the CND. These datasets contain lines from the molecules HCN, HCO$^+$, HNC, CS, SO, SiO, CN, H$_2$CO, HC$_3$N, N$_2$H$^+$, H$_3$O$^+$ and others. We conduct Large Velocity Gradient analyses to obtain column densities and total hydrogen densities, $n$, for each species in molecular clouds located in the southwest lobe of CND. The data for the above mentioned molecules indicate 10$^5\,$cm$^{-3} \lesssim n <10^6\,$cm$^{-3}$, which shows that the CND is tidally unstable. The derived chemical composition is compared with a chemical model calculated using the UCL\_CHEM code that includes gas and grain reactions, and the effects of shock waves. Models are run for varying shock velocities, cosmic-ray ionization rates, and number densities. The resulting chemical composition is fitted best to an extremely high value of cosmic-ray ionization rate $ζ\sim 10^{-14}\,$s$^{-1}$, 3 orders of magnitude higher than the value in regular Galactic molecular clouds, if the pre-shock density is $n=10^5\,$cm$^{-3}$.

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arXiv2013

Glycolaldehyde Formation via the Dimerisation of the Formyl Radical

Glycolaldehyde, the simplest monosaccharide sugar, has recently been detected in low- and high-mass star-forming cores. Following on from our previous investigation into glycolaldehyde formation (Woods et al. 2012, ApJ, 750, 19), we now consider a further mechanism for the formation of glycolaldehyde that involves the dimerisation of the formyl radical, HCO. Quantum mechanical investigation of the HCO dimerisation process upon an ice surface is predicted to be barrierless and therefore fast. In an astrophysical context, we show that this mechanism can be very efficient in star-forming cores. It is limited by the availability of the formyl radical, but models suggest that only very small amounts of CO are required to be converted to HCO to meet the observational constraints.

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arXiv2013

Modeling the Atomic-to-Molecular Transition and Chemical Distributions of Turbulent Star-Forming Clouds

We use 3D-PDR, a three-dimensional astrochemistry code for modeling photodissociation regions (PDRs), to post-process hydrodynamic simulations of turbulent, star-forming clouds. We focus on the transition from atomic to molecular gas, with specific attention to the formation and distribution of H, C+, C, H2 and CO. First, we demonstrate that the details of the cloud chemistry and our conclusions are insensitive to the simulation spatial resolution, to the resolution at the cloud edge, and to the ray angular resolution. We then investigate the effect of geometry and simulation parameters on chemical abundances and find weak dependence on cloud morphology as dictated by gravity and turbulent Mach number. For a uniform external radiation field, we find similar distributions to those derived using a one-dimensional PDR code. However, we demonstrate that a three-dimensional treatment is necessary for a spatially varying external field, and we caution against using one-dimensional treatments for non-symmetric problems. We compare our results with the work of Glover et al. (2010), who self-consistently followed the time evolution of molecule formation in hydrodynamic simulations using a reduced chemical network. In general, we find good agreement with this in situ approach for C and CO abundances. However, the temperature and H2 abundances are discrepant in the boundary regions (Av < 5), which is due to the different number of rays used by the two approaches.

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arXiv2013

An improved chemical scheme for the reactions of atomic oxygen and simple unsaturated hydrocarbons - implications for star-forming regions

Recent laboratory experiments have demonstrated that, even though contribution from other reaction channels cannot be neglected, unsaturated hydrocarbons easily break their multiple C-C bonds to form CO after their interactions with atomic oxygen. Here we present an upgraded chemical modelling including a revision of the reactions between oxygen atoms and small unsaturated hydrocarbons for different astrochemical environments. A first conclusion is that towards hot cores/corinos atomic oxygen easily degrades unsaturated hydrocarbons directly to CO or to its precursor species (such as HCCO or HCO) and destroys the double or triple bond of alkenes and alkynes. Therefore, environments rich in atomic oxygen at a relatively high temperature are not expected to be rich in large unsaturated hydrocarbons or polycyclic aromatic hydrocarbons. On the contrary, in O-poor and C-rich objects, hydrocarbon growth can occur to a large extent. In addition, new radical species, namely ketyl and vinoxy radicals, generated from other reaction channels can influence the abundances of hydrocarbons towards hot cores. We, therefore, suggest they should be included in the available databases. Hydrocarbon column densities are calculated in the 10(13)-10(15) cm(-2) range, in good agreement with their observed values, despite the small number of data currently published in the literature.

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arXiv2012

Small-scale structure in the interstellar medium: time-varying interstellar absorption towards κ Velorum

Ultra-high spectral resolution observations of time-varying interstellar absorption towards κ Vel are reported, using the Ultra-High Resolution Facility on the Anglo-Australian Telescope. Detections of interstellar Ca I, Ca II, K I, Na I and CH are obtained, whilst an upper limit on the column density is reported for C_2. The results show continued increases in column densities of K I and Ca I since observations ~ 4 yr earlier, as the transverse motion of the star carried it ~ 10 AU perpendicular to the line of sight. Line profile models are fitted to the spectra and two main narrow components (A & B) are identified for all species except CH. The column density N(K I) is found to have increased by 82 +10-9 % between 1994 and 2006, whilst N(Ca I) is found to have increased by 32 +- 5 % over the shorter period of 2002-2006. The line widths are used to constrain the kinetic temperature to T_k,A < 671 +18-17 K and T_k,B < 114 +15-14 K. Electron densities are determined from the Ca I / Ca II ratio, which in turn place lower limits on the total number density of n_A > 7 * 10^3 cm^-3 and n_B > 2 * 10^4 cm^-3. Calcium depletions are estimated from the Ca I / K I ratio. Comparison with the chemical models of Bell et al. (2005) confirms the high number density, with n = 5 * 10^4 cm^-3 for the best-fitting model. The first measurements of diffuse interstellar bands (DIBs) towards this star are made at two epochs, but only an upper limit of < 40 % is placed on their variation over ~ 9 years. The DIBs are unusually weak for the measured E(B-V) and appear to exhibit similar behaviour to that seen in Orion. The ratio of equivalent widths of the λ5780 to λ5797 DIBs is amongst the highest known, which may indicate that the carrier of λ5797 is more sensitive to UV radiation than to local density.

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arXiv2012

A lambda=3 mm molecular line survey of NGC1068. Chemical signatures of an AGN environment

We aimed to study the molecular composition of the interstellar medium (ISM) surrounding an Active Galactic Nucleus (AGN), by making an inventory of molecular species and their abundances, as well as to establish a chemical differentiation between starburst galaxies and AGN. We used the IRAM-30 m telescope to observe the central 1.5-2 kpc region of NGC1068, covering the frequencies between 86.2 GHz and 115.6 GHz. Using Boltzmann diagrams, we calculated the column densities of the detected molecules. We used a chemical model to reproduce the abundances found in the AGN, to determine the origin of each detected species, and to test the influence of UV fields, cosmic rays, and shocks on the ISM. We identified 24 different molecular species and isotopologues, among which HC3N, SO, N2H+, CH3CN, NS, 13CN, and HN13C are detected for the first time in NGC1068. We obtained the upper limits to the isotopic ratios 12C/13C=49, 16O/18O=177 and 32S/34S=5. Our chemical models suggest that the chemistry in the nucleus of NGC1068 is strongly influenced by cosmic rays, although high values of both cosmic rays and far ultraviolet (FUV) radiation fields also explain well the observations. The gas in the nucleus of NGC1068 has a different chemical composition as compared to starburst galaxies. The distinct physical processes dominating galaxy nuclei (e.g. C-shocks, UV fields, X-rays, cosmic rays) leave clear imprints in the chemistry of the gas, which allow to characterise the nucleus activity by its molecular abundances.

astro-ph.CO↗

arXiv2012

The influence of cosmic rays in the circumnuclear molecular gas of NGC1068

We surveyed the circumnuclear disk of the Seyfert galaxy NGC1068 between the frequencies 86.2 GHz and 115.6 GHz, and identified 17 different molecules. Using a time and depth dependent chemical model we reproduced the observational results, and show that the column densities of most of the species are better reproduced if the molecular gas is heavily pervaded by a high cosmic ray ionization rate of about 1000 times that of the Milky Way. We discuss how molecules in the NGC1068 nucleus may be influenced by this external radiation, as well as by UV radiation fields.

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arXiv2012

On the formation of glycolaldehyde in dense molecular cores

Glycolaldehyde is a simple monosaccharide sugar linked to prebiotic chemistry. Recently it was detected in a molecular core in the star-forming region G31.41+0.31 at a reasonably high abundance. We investigate the formation of glycolaldehyde at 10K to determine whether it can form efficiently under typical dense core conditions. Using an astrochemical model, we test five different reaction mechanisms that have been proposed in the astrophysical literature, finding that a gas-phase formation route is unlikely. Of the grain-surface formation routes, only two are efficient enough at very low temperatures to produce sufficient glycolaldehyde to match the observational estimates, with the mechanism culminating in CH3OH + HCO being favoured. However, when we consider the feasibility of these mechanisms from a reaction chemistry perspective, the second grain-surface route looks more promising, H3CO + HCO.

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arXiv2012

Chemical complexity in NGC1068

We aimed to study the chemistry of the circumnuclear molecular gas of NGC1068, and to compare it with those of the starburst galaxies M82 and NGC253. Using the IRAM-30m telescope, we observed the inner 2 kpc of NGC1068 between 86.2 GHz and 115.6 GHz. We identified 35 spectral features, corresponding to 24 different molecular species. Among them, HC3N, SO, N2H+, CH3CN, NS, 13CN, and HN13C are detected for the first time in NGC1068. Assuming local thermodynamic equilibrium (LTE), we calculated the column densities of the detected molecules, as well as the upper limits to the column densities of some undetected species. The comparison among the chemistries of NGC1068, M82, and NGC253, suggests that, apart from X-rays, shocks also determine the chemistry of NGC1068. We propose the column density ratio between CH3CCH and HC3N as a prime indicator of the imprints of starburst and AGN environments in the circumnuclear interstellar medium. This ratio is, at least, 64 times larger in M82 than in NGC1068, and, at least, 4 times larger in NGC253 than in NGC1068. Finally, we used the UCL_CHEM and UCL_PDR chemical codes to constrain the origin of the species, as well as to test the influence of UV radiation fields and cosmic rays on the observed abundances.

astro-ph.CO↗

arXiv2011

Multi-line spectral imaging of dense cores in the Lupus molecular cloud

The molecular clouds Lupus 1, 3 and 4 were mapped with the Mopra telescope at 3 and 12 mm. Emission lines from high density molecular tracers were detected, i.e. NH$_3$ (1,1), NH$_3$ (2,2), N$_2$H$^+$ (1-0), HC$_3$N (3-2), HC$_3$N (10-9), CS (2-1), CH$_3$OH (2$_0-1_0$)A$^+$ and CH$_3$OH (2$_{-1}-1_{-1}$)E. Velocity gradients of more than 1 km s$^{-1}$ are present in Lupus 1 and 3 and multiple gas components are present in these clouds along some lines of sight. Lupus 1 is the cloud richest in high density cores, 8 cores were detected in it, 5 cores were detected in Lupus 3 and only 2 in Lupus 4. The intensity of the three species HC$_3$N, NH$_3$ and N$_2$H$^+$ changes significantly in the various cores: cores that are brighter in HC$_3$N are fainter or undetected in NH$_3$ and N$_2$H$^+$ and vice versa. We found that the column density ratios HC$_3$N/N$_2$H$^+$ and HC$_3$N/NH$_3$ change by one order of magnitude between the cores, indicating that also the chemical abundance of these species is different. The time dependent chemical code that we used to model our cores shows that the HC$_3$N/N$_2$H$^+$ and HC$_3$N/NH$_3$ ratios decrease with time therefore the observed column density of these species can be used as an indicator of the chemical evolution of dense cores. On this base we classified 5 out of 8 cores in Lupus 1 and 1 out of 5 cores in Lupus 3 as very young protostars or prestellar cores. Comparing the millimetre cores population with the population of the more evolved young stellar objects identified in the Spitzer surveys, we conclude that in Lupus 3 the bulk of the star formation activity has already passed and only a moderate number of stars are still forming. On the contrary, in Lupus 1 star formation is on-going and several dense cores are still in the pre--/proto--stellar phase. Lupus 4 is at an intermediate stage, with a smaller number of individual objects.

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arXiv2011

Chemistry of dense clumps near moving Herbig-Haro objects

Localised regions of enhanced emission from HCO+, NH3 and other species near Herbig-Haro objects (HHOs) have been interpreted as arising in a photochemistry stimulated by the HHO radiation on high density quiescent clumps in molecular clouds. Static models of this process have been successful in accounting for the variety of molecular species arising ahead of the jet; however recent observations show that the enhanced molecular emission is widespread along the jet as well as ahead. Hence, a realistic model must take into account the movement of the radiation field past the clump. It was previously unclear as to whether the short interaction time between the clump and the HHO in a moving source model would allow molecules such as HCO+ to reach high enough levels, and to survive for long enough to be observed. In this work we model a moving radiation source that approaches and passes a clump. The chemical picture is qualitatively unchanged by the addition of the moving source, strengthening the idea that enhancements are due to evaporation of molecules from dust grains. In addition, in the case of several molecules, the enhanced emission regions are longer-lived. Some photochemically-induced species, including methanol, are expected to maintain high abundances for ~10,000 years.

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arXiv2011

A study of methyl formate in astrochemical environments

Several complex organic molecules are routinely detected in high abundances towards hot cores and hot corinos. For many of them, their paths of formation in space are uncertain, as gas phase reactions alone seem to be insufficient. In this paper, we investigate a possible solid-phase route of formation for methyl formate (HCOOCH3). We use a chemical model updated with recent results from an experiment where simulated grain surfaces were irradiated with 200 keV protons at 16 K, to simulate the effects of cosmic ray irradiation on grain surfaces. We find that this model may be sufficient to reproduce the observed methyl formate in dark clouds, but not that found in hot cores and corinos.

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arXiv2011

Herschel Measurements of Molecular Oxygen in Orion

We report observations of three rotational transitions of molecular oxygen (O2) in emission from the H2 Peak 1 position of vibrationally excited molecular hydrogen in Orion. We observed the 487 GHz, 774 GHz, and 1121 GHz lines using HIFI on the Herschel Space Observatory, having velocities of 11 km s-1 to 12 km s-1 and widths of 3 km s-1. The beam-averaged column density is N(O2) = 6.5\times1016 cm-2, and assuming that the source has an equal beam filling factor for all transitions (beam widths 44, 28, and 19"), the relative line intensities imply a kinetic temperature between 65 K and 120 K. The fractional abundance of O2 relative to H2 is 0.3 - 7.3\times10-6. The unusual velocity suggests an association with a ~ 5" diameter source, denoted Peak A, the Western Clump, or MF4. The mass of this source is ~ 10 M\odot and the dust temperature is \geq 150 K. Our preferred explanation of the enhanced O2 abundance is that dust grains in this region are sufficiently warm (T \geq 100 K) to desorb water ice and thus keep a significant fraction of elemental oxygen in the gas phase, with a significant fraction as O2. For this small source, the line ratios require a temperature \geq 180 K. The inferred O2 column density \simeq 5\times1018 cm-2 can be produced in Peak A, having N(H2) \simeq 4\times1024 cm-2. An alternative mechanism is a low-velocity (10 to 15 km s-1) C-shock, which can produce N(O2) up to 1017 cm-2.

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arXiv2011

Extreme Cosmic-Ray-Dominated-Regions: a new paradigm for high star formation density events in the Universe

We examine in detail the recent proposal that extreme Cosmic-Ray-Dominated-Regions (CRDRs) characterize the ISM of galaxies during events of high-density star formation, fundamentally altering its initial conditions (Papadopoulos 2010). Solving the coupled chemical and thermal state equations for dense UV-shielded gas reveals that the large cosmic ray energy densities in such systems (U_{CR} (few)x(10^3-10^4) U_{CR,Gal}) will indeed raise the minimum temperature of this phase (where the initial conditions of star formation are set) from ~10K (as in the Milky Way) to (50-100)K. Moreover in such extreme CRDRs the gas temperature remains fully decoupled from that of the dust, with T_{kin} >> T_{dust}, even at high densities (n(H_2)~10^5--10^6 cm^{-3}), quite unlike CRDRs in the Milky Way where T_k T_{dust} when n(H_2) >= 10^5 cm^{-3}. These dramatically different star formation initial conditions will: a) boost the Jeans mass of UV-shielded gas regions by factors of ~10--100 with respect to those in quiescent or less extreme star forming systems, and b) "erase" the so-called inflection point of the effective equation of state (EOS) of molecular gas. Both these effects occur across the entire density range of typical molecular clouds, and may represent {\it a new paradigm for all high-density star formation in the Universe}, with cosmic rays as the key driving mechanism, operating efficiently even in the high dust extinction environments of extreme starbursts...

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