Modelling the 3D physical structure of astrophysical sources with GASS

The era of interferometric observations leads to the need of a more and more precise description of physical structures and dynamics of star-forming regions, from pre-stellar cores to protoplanetary discs. The molecular emission can be traced in multiple physical components such as infalling envelopes, outflows and protoplanetary discs. To compare with the observations, a precise and complex radiative transfer modelling of these regions is needed. We present GASS (Generator of Astrophysical Sources Structure), a code that allows us to generate the three-dimensional (3D) physical structure model of astrophysical sources. From the GASS graphical interface, the user easily creates different components such as spherical envelopes, outflows and discs. The physical properties of these components are modelled thanks to dedicated graphical interfaces that display various figures in order to help the user and facilitate the modelling task. For each component, the code randomly generates points in a 3D grid with a sample probability weighted by the molecular density. The created models can be used as the physical structure input for 3D radiative transfer codes to predict the molecular line or continuum emission. An analysis of the output hyper-spectral cube given by such radiative transfer code can be made directly in GASS using the various post-treatment options implemented, such as calculation of moments or convolution with a beam. This makes GASS well suited to model and analyse both interferometric and single-dish data. This paper is focused on the results given by the association of GASS and LIME, a 3D radiative transfer code, and we show that the complex geometry observed in star-forming regions can be adequately handled by GASS+LIME

High spectral resolution observations of HNC3 and HCCNC in the L1544 prestellar core

HCCNC and HNC3 are less commonly found isomers of cyanoacetylene, HC3N, a molecule that is widely found in diverse astronomical sources. We want to know if HNC3 is present in sources other than the dark cloud TMC-1 and how its abundance is relative to that of related molecules. We used the ASAI unbiased spectral survey at IRAM 30m towards the prototypical prestellar core L1544 to search for HNC3 and HCCNC which are by-product of the HC3NH+ recombination, previously detected in this source. We performed a combined analysis of published HNC3 microwave rest frequencies with thus far unpublished millimeter data because of issues with available rest frequency predictions. We determined new spectroscopic parameters for HNC3, produced new predictions and detected it towards L1544. We used a gas-grain chemical modelling to predict the abundances of N-species and compare with the observations. The modelled abundances are consistent with the observations, considering a late stage of the evolution of the prestellar core. However the calculated abundance of HNC3 was found 5-10 times higher than the observed one. The HC3N, HNC3 and HCCNC versus HC3NH+ ratios are compared in the TMC-1 dark cloud and the L1544 prestellar core.Comment: Accepted in MNRAS letters. 5 pages plus 2 additional pages for the on-line materia

First detection of the pre-biotic molecule glycolonitrile (HOCH2CN) in the interstellar medium

Theories of a pre-RNA world suggest that glycolonitrile (HOCH$_2$CN) is a key species in the process of ribonucleotide assembly, which is considered as a molecular precursor of nucleic acids. In this Letter, we report the first detection of this pre-biotic molecule in the interstellar medium (ISM) by using ALMA data obtained at frequencies between 86.5$\,$GHz and 266.5$\,$GHz toward the Solar-type protostar IRAS16293-2422 B. A total of 15 unblended transitions of HOCH$_2$CN were identified. Our analysis indicates the presence of a cold (T$\rm _{ex}$=24$\pm$8$\,$K) and a warm (T$\rm _{ex}$=158$\pm$38$\,$K) component meaning that this molecule is present in both the inner hot corino and the outer cold envelope of IRAS16293 B. The relative abundance with respect to H$_2$ is (6.5$\pm$0.6)$\times$10$^{-11}$ and $\geq$(6$\pm$2)$\times$10$^{-10}$ for the warm and cold components respectively. Our chemical modelling seems to underproduce the observed abundance for both the warm and cold component under various values of the cosmic-ray ionisation rate ($\zeta$). Key gas phase routes for the formation of this molecule might be missing in our chemical network.Comment: 5 pages, 4 figures, 2 tables, accepted in Monthly Notices of the Royal Astronomical Society Letter

The Fate of Formamide in a Fragmenting Protoplanetary Disk

Recent high-sensitivity observations carried out with the Atacama Large Millimeter Array have revealed the presence of complex organic molecules (COMs) such as methyl cyanide (CH3CN) and methanol (CH3OH) in relatively evolved protoplanetary discs. The behavior and abundance of COMs in earlier phases of disk evolution remain unclear. Here, we combine a smoothed particle hydrodynamics simulation of a fragmenting, gravitationally unstable disk with a gas-grain chemical code. We use this to investigate the evolution of formamide (NH2CHO), a prebiotic species, in both the disk and in the fragments that form within it. Our results show that formamide remains frozen onto grains in the majority of the disks where the temperatures are <100 K, with a predicted solid-phase abundance that matches those observed in comets. Formamide is present in the gas phase in three fragments as a result of the high temperatures (≥200 K), but remains in the solid phase in one colder (≤150 K) fragment. The timescale over which this occurs is comparable to the dust sedimentation timescales, suggesting that any rocky core that is formed would inherit their formamide content directly from the protosolar nebula

Carbon Nanotubes by a CVD Method. Part II: Formation of Nanotubes from (Mg, Fe)O Catalysts

The aim of this paper is to study the formation of carbon nanotubes (CNTs) from different Fe/MgO oxide powders that were prepared by combustion synthesis and characterized in detail in a companion paper. Depending on the synthesis conditions, several iron species are present in the starting oxides including Fe2+ ions, octahedral Fe3+ ions, Fe3+ clusters, and MgFe2O4-like nanoparticles. Upon reduction during heating at 5 °C/min up to 1000 °C in H2/CH4 of the oxide powders, the octahedral Fe3+ ions tend to form Fe2+ ions, which are not likely to be reduced to metallic iron whereas the MgFe2O4-like particles are directly reduced to metallic iron. The reduced phases are R-Fe, Fe3C, and ç-Fe-C. Fe3C appears as the postreaction phase involved in the formation of carbon filaments (CNTs and thick carbon nanofibers). Thick carbon nanofibers are formed from catalyst particles originating from poorly dispersed species (Fe3+ clusters and MgFe2O4-like particles). The nanofiber outer diameter is determined by the particle size. The reduction of the iron ions and clusters that are well dispersed in the MgO lattice leads to small catalytic particles (<5 nm), which tend to form SWNTS and DWNTs with an inner diameter close to 2 nm. Well-dispersed MgFe2O4-like particles can also be reduced to small metal particles with a narrow size distribution, producing SWNTs and DWNTs. The present results will help in tailoring oxide precursors for the controlled formation of CNTs

Carbon Nanotubes by a CVD Method. Part I: Synthesis and Characterization of the (Mg, Fe)O Catalysts

The controlled synthesis of carbon nanotubes by chemical vapor deposition requires tailored and wellcharacterized catalyst materials. We attempted to synthesize Mg1-xFexO oxide solid solutions by the combustion route, with the aim of performing a detailed investigation of the influence of the synthesis conditions (nitrate/urea ratio and the iron content) on the valency and distribution of the iron ions and phases. Notably, characterization of the catalyst materials is performed using 57Fe Mo¨ssbauer spectroscopy, X-ray diffraction, and electron microscopy. Several iron species are detected including Fe2+ ions substituting for Mg2+ in the MgO lattice, Fe3+ ions dispersed in the octahedral sites of MgO, different clusters of Fe3+ ions, and MgFe2O4-like nanoparticles. The dispersion of these species and the microstructure of the oxides are discussed. Powders markedly different from one another that may serve as model systems for further study are identified. The formation of carbon nanotubes upon reduction in a H2/CH4 gas atmosphere of the selected powders is reported in a companion paper

The ALMA Frontier Fields Survey. II. Multiwavelength Photometric analysis of 1.1 mm continuum sources in Abell 2744, MACSJ0416.1-2403 and MACSJ1149.5+2223

CONTEXT: The Hubble and Spitzer Space Telescope surveys of the Frontier Fields provide extremely deep images around six massive, strong-lensing clusters of galaxies. The ALMA Frontier Fields survey aims to cover the same fields at 1.1 mm, with maps reaching (unlensed) sensitivities of <70 μJy, in order to explore the properties of background dusty star-forming galaxies. AIMS: We report on the multi-wavelength photometric analysis of all 12 significantly detected (>5σ) sources in the first three Frontier Fields clusters observed by ALMA, based on data from Hubble and Spitzer, the Very Large Telescope and the Herschel Space Observatory. METHODS: We measure the total photometry in all available bands and determine the photometric redshifts and the physical properties of the counterparts via SED-fitting. In particular, we carefully estimate the far-infrared (FIR) photometry using 1.1 mm priors to limit the misidentification of blended FIR counterparts, which strongly affect some flux estimates in previous FIR catalogs. Due to the extremely red nature of these objects, we used a large range of parameters (e.g. 0.0 <Av< 20.0) and templates (including AGNs and ULIRGs models). RESULTS: We identify robust near-infrared (NIR) counterparts for all 11 sources with Ks detection, the majority of which are quite red, with eight having F814W − Ks ≳ 4 and five having F160W − [ 4.5 ] ≳ 3. From the FIR point of view, all our objects have zphot ~ 1–3, whereas based on the optical SED one object prefers a high-z solution (z ≥ 7). Five objects among our sample have spectroscopic redshifts from the GLASS survey for which we can reproduce their SEDs with existing templates. This verification confirms the validity of our photometric redshift methodology. The mean redshift of our sample is zphot = 1.99 ± 0.27. All 1.1 mm selected objects are massive (10.0 < log  [ M⋆(M⊙) ] < 11.5), with high star formation rates (⟨ log [ SFR(M⊙/ yr) ] ⟩ ≈ 1.6) and high dust contents (8.1 < log  [ Mdust(M⊙) ] < 8.8), consistent with previous ALMA surveys

Modeling the physical structure of star-forming regions with LIME, a 3D radiative transfer code

The ability to predict line emission is crucial in order to make a comparison with observations. From LTE to full radiative transfer codes, the goal is always to derive the most accurately possible the physical properties of the source. Non-LTE calculations can be very time consuming but are needed in most of the cases since many studied regions are far from LTE

HCO

Ionisation in the molecular environment of star-forming regions gives important clues on the chemical processes that take place in the environment of a young protostar. Ionisation can be traced with molecules like HCO+ and N2H+ and it regulates the formation of larger molecules such as complex organic molecules (COMs). The ionisation degree can also play an important role in the free-fall timescale of the protostar since it prevents the neutral material to collapse through collisions with ions and electrons bound to the magnetic field of the central protostar