Determine Column Densities Research Articles

Bayesian analysis of the molecular emission and dust continuum of protoplanetary disks

Context. The MIRI instrument on board the James Webb Space Telescope probes the chemistry and dust mineralogy of the inner regions of protoplanetary disks. The observed spectra are unprecedented in their detail and reveal a rich chemistry with strong diversity between objects. This complicates interpretations that are mainly based on manual continuum subtraction and 0D slab models. Aims. We investigate the physical conditions under which the gas emits in protoplanetary disks. Based on MIRI spectra, we apply a full Bayesian analysis that provides the posterior distributions of dust and molecular properties, such as column densities and emission temperatures. Methods. To do so, we introduced the Dust Continuum Kit with Line emission from Gas (DuCKLinG), a Python-based model simultaneously describing the molecular line emission and the dust continuum of protoplanetary disks without large computational cost. The model describes the dust continuum emission by dust models with precomputed dust opacities. The molecular emission is based on LTE slab models but from extended radial ranges with gradients in column densities and emission temperatures. We compare the model to observations using Bayesian analysis with linear regression techniques to reduce the dimension of the parameter space. We benchmarked this model to a complex thermo-chemical ProDiMo model of AATau and fit the MIRI spectrum of GW Lup. The latter allowed for a comparison to the previous results obtained with single slab models and hand-fitted continuum. Results. We successfully decrease the computational time of the fitting method by a factor of 80 by eliminating linear parameters, such as the emission areas, from the Bayesian run. This approach does not significantly change the retrieved molecular parameters, and only the calculated errors on the optically thin dust masses slightly decrease. For an AA Tau ProDiMo mock observation, we find that the retrieved molecular conditions from DuCKLinG (column densities from 3 × 1018 cm−2 to 4 × 1020 cm−2, radial range from 0.2 au to 1.2 au, and temperature range from about 200 K to 400 K) fall within the true values from ProDiMo (column densities between 4 × 1017 cm-2 to 5 × 1020 cm−2, radial extent 0.1 au to 6.6 au, and temperature range from about 120 to 1000 K). The smaller DuCKLinG ranges can be explained by the relative flux contributions of the different parts of ProDiMo. The parameter posterior of GW Lup reinforces previously found results. The previously determined column densities fall within the retrieved ranges in this study for all examined molecules (CO2, H2O, HCN, and C2H2). Similar overlap is found for the temperatures with only the temperature range of HCN (from 570−60+60 to 750−70+90 K) not including the previously found value (875 K). This discrepancy may be due to the simultaneous fitting of all molecules compared to the step-by-step fitting of the previous study. There is statistically significant evidence for radial temperature and column density gradients for H2O and CO2 compared to the constant temperature and column density assumed in the 0D slab models. Additionally, HCN and C2H2 emit from a small region with near constant conditions. Due to the small selected wavelength range 13.6–16.3 µm, the dust properties are not well constrained for GW Lup. DuCKL inG can become an important tool to analyse the molecular emission and dust mineralogy of large samples based on JWST /MIRI spectra in an automated way.

Hyperfine excitation of NH and ND by molecular hydrogen

The NH and ND radicals are of key importance in the comprehension of nitrogen chemistry and the enhancement of deuterated molecules in the interstellar medium. Observations by space telescopes yield spectra that can resolve the fine and hyperfine structure of these radicals, a consequence of the electronic and magnetic interactions of nitrogen, hydrogen, and deuterium nuclei. Accurate rate coefficients, induced by collisions with H2, are required to interpret spectra of these radicals. We report the first rate coefficients for fine and hyperfine transitions of NH and ND in collision with both ortho- and para-H2. Based on a recent four-dimensional potential energy surface, fine-structure resolved cross sections and rate coefficients are computed with the time-independent close-coupling method over a temperature range of 5–300 K. Our calculations include the first 25 energy levels of NH and ND. Hyperfine resolved cross sections and rate coefficients are determined using the infinite-order sudden (IOS) approximation between 5 and 200 K for NH and 100 K for ND. We consider the first 71 and 105 energy levels of NH and ND, respectively. General propensity rules are discussed. We found a significant isotopic substitution effect in the rate coefficients. In addition, the rate coefficients for collisions with H2 are larger than those with He by a factor of up to 5, leading to lower critical densities for collisional excitation with H2 than He. The impact of the new set of collisional data has been investigated in simple radiative transfer models of the NH emission seen toward the Orion Bar and the ejecta of the η Carinae binary star. We observed significant differences by a factor of 5 between the presently determined column densities for NH compared to those from the literature using He as a collider.

Protostellar Interferometric Line Survey of the Cygnus-X region (PILS-Cygnus)

Context. Molecular lines are commonly detected towards protostellar sources. However, to get a better understanding of the chemistry of these sources we need unbiased molecular surveys over a wide frequency range for as many sources as possible to shed light on the origin of this chemistry, particularly any influence from the external environment. Aims. We present results from the PILS-Cygnus survey of ten intermediate- to high-mass protostellar sources in the nearby Cygnus-X complex, through high angular resolution interferometric observations over a wide frequency range. Methods. Using the Submillimeter Array (SMA), a spectral line survey of ten sources was performed in the frequency range 329–361 GHz, with an angular resolution of ~1″.5, or ~2000 AU at a source distance of 1.3 kpc from the Sun. Spectral modelling was performed to identify molecular emission and determine column densities and excitation temperatures for each source. Emission maps were made to study the morphology of emission. Finally, emission properties were compared across the sample. Results. We detect CH3OH towards nine of the ten sources, with CH3OCH3 and CH3OCHO towards three sources. We further detect CH3CN towards four sources. Towards five sources the chemistry is spatially differentiated, meaning that different species peak at different positions and are offset from the peak continuum emission. Low levels of deuteration are detected towards four sources in HDO emission, whereas deuterated complex organic molecule emission is detected towards one source (CH2DOH towards N63). The chemical properties of each source do not correlate with their position in the Cygnus-X complex, nor do the distance or direction to the nearest OB associations. However, the five sources located in the DR21 filament do appear to show less line emission compared to the five sources outside the filament. Conclusions. This work shows how important wide frequency coverage observations are combined with high angular resolution observations for studying the protostellar environment. Furthermore, based on the ten sources observed here, the external environment appears to only play a minor role in setting the chemical environment on these small scales (<2000 AU).

The Distribution of Metallicities in the Local Galactic Interstellar Medium* *Based in part on observations made with the NASA/ESA Hubble Space Telescope, obtained from the MAST data archive at the Space Telescope Science Institute. The STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Based in part on data obtained from the

In this investigation, we present an analysis of the metallicity distribution that pertains to neutral gas in the local Galactic interstellar medium (ISM). We derive relative ISM metallicities for a sample of 84 sight lines probing diffuse atomic and molecular gas within 4 kpc of the Sun. Our analysis is based in large part on column density measurements reported in the literature for 22 different elements that are commonly studied in interstellar clouds. We supplement the literature data with new column density determinations for certain key elements and several individual sight lines important to our analysis. Our methodology involves comparing the relative gas-phase abundances of many different elements for a given sight line to simultaneously determine the strength of dust depletion in that direction and the overall metallicity offset. We find that many sight lines probe multiple distinct gas regions with different depletion properties, which complicates the metallicity analysis. Nevertheless, our results provide clear evidence that the dispersion in the metallicities of neutral interstellar clouds in the solar neighborhood is small (∼0.10 dex) and only slightly larger than the typical measurement uncertainties. We find no evidence for the existence of very low metallicity gas (as has recently been reported by De Cia et al.) along any of the 84 sight lines in our sample. Our results are consistent with a local Galactic ISM that is well mixed and chemically homogeneous.

A major asymmetric ice trap in a planet-forming disk

The complex organic molecules (COMs) detected in star-forming regions are the precursors of the prebiotic molecules that can lead to the emergence of life. By studying COMs in more evolved protoplanetary disks we can gain a better understanding of how they are incorporated into planets. This paper presents ALMA band 7 observations of the dust and ice trap in the protoplanetary disk around Oph IRS 48. We report the first detection of dimethyl ether (CH3OCH3) in a planet-forming disk and a tentative detection of methyl formate (CH3OCHO). We determined column densities for the detected molecules and upper limits on non-detected species using the CASSIS spectral analysis tool. The inferred column densities of CH3OCH3 and CH3OCHO with respect to methanol (CH3OH) are of order unity, indicating unusually high abundances of these species compared to other environments. Alternatively, the 12CH3OH emission is optically thick and beam diluted, implying a higher CH3OH column density and a smaller emitting area than originally thought. The presence of these complex molecules can be explained by thermal ice sublimation, where the dust cavity edge is heated by irradiation and the full volatile ice content is observable in the gas phase. This work confirms the presence of oxygen-bearing molecules more complex than CH3OH in protoplanetary disks for the first time. It also shows that it is indeed possible to trace the full interstellar journey of COMs across the different evolutionary stages of star, disk, and planet formation.

Signatures of UV radiation in low-mass protostars

Context. Ultraviolet radiation (UV) influences the physics and chemistry of star-forming regions, but its properties and significance in the immediate surroundings of low-mass protostars are still poorly understood. Aims. Our aim is to extend the use of the CN/HCN ratio, already established for high-mass protostars, to the low-mass regime to trace and characterize the UV field around low-mass protostars on ~0.6 × 0.6 pc scales. Methods. We present 5′ × 5′ maps of the Serpens Main Cloud encompassing ten protostars observed with the EMIR receiver at the IRAM 30 m telescope in CN 1–0, HCN 1–0, CS 3–2, and some of their isotopologs. The radiative-transfer code RADEX and the chemical model Nahoon were used to determine column densities of molecules, gas temperature and density, and the UV field strength, G0. Results. The spatial distribution of HCN and CS are closely correlated with CO 6–5 emission, that traces outflows. The CN emission is extended from the central protostars to their immediate surroundings also tracing outflows, likely as a product of HCN photodissociation. The ratio of CN to HCN total column densities ranges from ~1 to 12 corresponding to G0 ≈ 101–103 for gas densities and temperatures typical for outflows of low-mass protostars. Conclusions. UV radiation associated with protostars and their outflows is indirectly identified in a significant part of the Serpens Main low-mass star-forming region. Its strength is consistent with the values obtained from the OH and H2O ratios observed with Herschel and compared with models of UV-illuminated shocks. From a chemical viewpoint, the CN to HCN ratio is an excellent tracer of UV fields around low- and intermediate-mass star-forming regions.

Protostellar Interferometric Line Survey of the Cygnus X region (PILS-Cygnus)

Context. Complex organic molecules (COMs) are commonly detected in and near star-forming regions. However, the dominant process in the release of these COMs from the icy grains – where they predominately form – to the gas phase is still an open question. Aims. We investigate the origin of COM emission in a high-mass protostellar source, CygX-N30 MM1, through high-angular-resolution interferometric observations over a continuous broad frequency range. Methods. We used 32 GHz Submillimeter Array observations with continuous frequency coverage from 329 to 361 GHz at an angular resolution of ~1″ to do a line survey and obtain a chemical inventory of the source. The line emission in the frequency range was used to determine column densities and excitation temperatures for the COMs. We also mapped out the intensity distribution of the different species. Results. We identified approximately 400 lines that can be attributed to 29 different molecular species and their isotopologues. We find that the molecular peak emission is along a linear gradient, and coincides with the axis of red- and blueshifted H2CO and CS emission. Chemical differentiation is detected along this gradient, with the O-bearing molecular species peaking towards one component of the system and the N- and S-bearing species peaking towards the other. The chemical gradient is offset from but parallel to the axis through the two continuum sources. The inferred column densities and excitation temperatures are compared to other sources where COMs are abundant. Only one deuterated molecule is detected, HDO, while an upper limit for CH2DOH is derived, leading to a D/H ratio of <0.1%. Conclusions. We conclude that the origin of the observed COM emission is probably a combination of the young stellar sources along with accretion of infalling material onto a disc-like structure surrounding a young protostar and located close to one of the continuum sources. This disc and protostar are associated with the O-bearing molecular species, while the S- and N-bearing species on the other hand are associated with the other continuum core, which is probably a protostar that is slightly more evolved than the other component of the system. The low D/H ratio likely reflects a pre-stellar phase where the COMs formed on the ices at warm temperatures (~30 K), where the deuterium fractionation would have been inefficient. The observations and results presented here demonstrate the importance of good frequency coverage and high angular resolution when disentangling the origin of COM emission.

Far infrared laser, millimeter-wave and microwave Microwave stark, millimeter wave, far infrared, infrared and far infrared laser spectroscopy of methanol-OD ([formula omitted]): Precise dipole moment and millimeter-wave atlas of astrophysical significance

In this report accurate microwave (MW) Stark effect measurements have been performed to obtain precise dipole moment and precise zero-field microwave frequencies of singly deuterated quasi symmetric CH3OD methanol species. This species is of significance to radio astronomers to study the hydrogen-deuterium exchange in interstellar space in cold clouds to study basic chemical processes in other worlds. The telescope facilities at Atacama Large millimeter Array (ALMA), the facilities at Institute for Radio Astronomy in the Millimeter Range (IRAM), and the NASA's floating Stratospheric Observatory for Infrared Astronomy (SOFIA), etc. routinely gauge the sky in the millimeter-wave (MMW) to infrared spectral (IR) regions (Drozdovskaya et al., 2019; Faure et al., 2015; Neill et al., 2013; Peng et al., 2012; Taquet et al., 2011; Whittet et al., 2011; Parise et al., 2004, 2002, 2002, 2002, 2003, 2003, 2003; Johns et al., 1998; Charnley et al., 1997). The laboratory back up of the millimeter region of CH3OD is of importance to astronomers for such studies as the determination of column densities and the deuteration process. In the present study, the MMW spectra previously recorded with unprecedented accuracy (Mukhopadhyay and Bo Duan, 2000; Mukhopadhyay et al., 1997; Wang et al., 2001; Mukhopadhyay, 1999; Mukhopadhyay and Bo Duan, 2000; Walsh et al., 2000) are presented at atlas form with minute details to aid such detection. The dipole results also should aid to find the strength of individual lines. In the IR region, the off-axis deuteration pushes the of the absorption bands of the molecule to the lower region, particularly the increased asymmetry of the molecule spreads the COD- bending and CO stretching bands in a wide frequency range making them more accessible to strong CO2 lasers resulting in preferential population transfer to excited states of the molecule. Thus making this species a prolific source of coherent laser emission radiation in the scarce FIR region (Landsberg, 1980; Gastaud et al., 1980; Kon et al., 1984; Pereira et al., 1994; Telles et al., 1999; Lundsgaard et al., 1968; Dijkerman and Ruitenberg, 1969; Amano, 1981; Lees et al., 2013; Petersen, 2020). In this work, a large number of CO2 laser pump/FIR laser systems have been identified with specific quantum numbers and predict many new lines. Schematic diagrams have been presented for the emission and prediction process along with important experimental diagnostic information about relative polarization. The assignments will aid the optimization of laser output and the predictions will enable new coherent light sources in the FIR region.

Cause and effects of the massive star formation in Messier 8 East

Context. Messier 8 (M8), one of the brightest H II regions in our Galaxy, is powered by massive O-type stars and is associated with recent and ongoing massive star formation. Two prominent massive star-forming regions associated with M8 are M8-Main, the particularly bright part of the large-scale H II region (mainly) ionized by the stellar system Herschel 36 (Her 36) and M8 East (M8 E), which is mainly powered by a deeply embedded young stellar object (YSO), the bright infrared (IR) source M8E-IR. Aims. We study the interaction of the massive star-forming region M8 E with its surroundings using observations of assorted diffuse and dense gas tracers that allow quantifying the kinetic temperatures and volume densities in this region. With a multiwavelength view of M8 E, we investigate the cause of star formation. Moreover, we compare the star-forming environments of M8-Main and M8 E, based on their physical conditions and the abundances of the various observed species toward them. Methods. We used the Institut de Radioastronomía Millimétrica 30 m telescope to perform an imaging spectroscopy survey of the ~1 pc scale molecular environment of M8E-IR and also performed deep integrations toward the source itself. We imaged and analyzed data for the J = 1 → 0 rotational transitions of 12CO, 13CO, N2H+, HCN, H13CN, HCO+, H13CO+, HNC, and HN13C observed for the first time toward M8 E. To visualize the distribution of the dense and diffuse gas in M8 E, we compared our velocity-integrated intensity maps of 12CO, 13CO, and N2H+ with ancillary data taken at IR and submillimeter wavelengths. We used techniques that assume local thermodynamic equilibrium (LTE) and non-LTE to determine column densities of the observed species and constrain the physical conditions of the gas that causes their emission. Examining the class 0/ I and class II YSO populations in M8 E, allows us to explore the observed ionization front (IF) as seen in the high resolution Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) 8 μm emission image. The difference between the ages of the YSOs and their distribution in M8 E were used to estimate the speed of the IF. Results. We find that 12CO probes the warm diffuse gas also traced by the GLIMPSE 8 μm emission, while N2H+ traces the cool and dense gas following the emission distribution of the APEX Telescope Large Area Survey of the Galaxy 870 μm dust continuum. We find that the star-formation in M8 E appears to be triggered by the earlier formed stellar cluster NGC 6530, which powers an H II region giving rise to an IF that is moving at a speed ≥0.26 km s−1 across M8 E. Based on our qualitative and quantitative analysis, the J = 1 → 0 transition lines of N2H+ and HN13C appear to be more direct tracers of dense molecular gas than the J = 1 → 0 transition lines of HCN and HCO+. We derive temperatures of 80 and 30 K for the warm and cool gas components, respectively, and constrain the H2 volume densities to be in the range of 104–106 cm−3. Comparison of the observed abundances of various species reflects the fact that M8 E is at an earlier stage of massive star formation than M8-Main.

Very high-resolution synchrotron radiation far-infrared (FIR) spectrum of methanol‑D2 (CHD2OH) & millimeter-wave (MMW) measurements involving highly excited torsional vibrational rotational states, and identification of optically pumped FIR laser lines

Synchrotron radiation-based very high-resolution far-infrared (FIR) spectrum recorded at the Canadian Light Source (CSL) of doubly deuterated asymmetric methanol molecule (CHD2OH) have been analyzed for transitions associated with highly excited torsional-vibration–rotation levels. Complimentary millimeter-wave (MMW) transition frequencies have been measured using a backward wave oscillator based spectrometer at the Ohio State University. A new and simplified model has been proposed to treat the Hamiltonian Eigenvalues which do not require a large number of parameters with uncertain physical significance. A systematic approach has been laid out on how to proceed in order to obtain self-consistent and rigorously confirmed assignment of an extremely complicated spectrum arising due interaction with other states. New measurements in the MMW region should facilitate astrophysical determination of column densities of this abundant molecular species in cold interstellar clouds, especially in star-forming regions. The scarcity of MMW and the FIR laboratory database severely hindered the astrophysical work in the past. All the FIR assignments have been rigorously confirmed by closed combination relations (CR) using the MMW measurements and/or through ground state CRs from fundamental vibrational band analysis. These FIR transitions are presented in the form an atlas of about 900 lines. As a byproduct of the measurements, it was possible to put forward tentative assignments of optically pumped FIR laser lines.

The excitation temperature of the CH 3335-MHz line

ABSTRACT Molecular hydrogen is the main constituent of dense molecular clouds, but is expected to also be a dominant constituent in many environments where CO can no longer be seen, the so-called ‘CO-dark molecular gas’. Based on comparisons of ultraviolet spectroscopy of H2 and optical line observations (4300 Å), CH is a prime candidate to trace H2. Since the optical line (and the UV lines at 3143, 3890, and 3878 Å) require bright background sources (and the CH N = 2←1 ground state rotation line at 149 µm requires space-based, or stratospheric, observations), the hyperfine structure transition at 3335 MHz is a potentially important tool for probing the CO-dark molecular gas. However, the excitation of this transition is complicated, and has often been found to be inverted, making column density determinations uncertain. To clarify the potential use of the 3.3-GHz line as a proxy for H2, we have observed the CH 3335-MHz line with the Arecibo 305-m radio telescope along 16 lines of sight towards stars with existing measurements of the 4300-Å line. By comparing the CH column densities from optical and UV absorption lines to the CH radio emission line, we can derive the excitation temperature (Tex) of the 3335-MHz transition. We obtain a wide range of excitation temperatures for nine lines of sight, including some with |Tex| < 5 K. The common assumption that Tex for the 3335-MHz line is always much larger than the background temperature (Tbg) is not always warranted and can lead to significant errors in the value of N(CH).

Concerning the asymmetric top rotational partition function in astronomical spectroscopy

The partition function plays an important role in determining column densities from emission lines in molecular clouds, since under typical conditions the two are linearly related. Here, we focus on successively applying quantum and centrifugal distortion correction terms to the classical rotational partition function for asymmetric top molecules, in an effort to uncover the relative importance of each term. We do this for example asymmetric top molecules that contain from one to four heavy atoms (water, methanimine, vinyl alcohol, and glycolaldehyde), at temperatures relevant to interstellar molecular clouds. Particular attention is paid to methanimine, for which it was recently attested that the value of the classical partition function significantly differs from the numerically summed (exact) value at several hundred Kelvin [Sharma et al., Astron. Nachrichten. 338, 125 (2017)]. We show that the classical values are in fact within 1% of the exact values from ~50 K to well above 400 K; the addition of three simple correction terms reduces the error to ≲0.001% (down to 10 K).

Unveiling the chemistry of interstellar CH

Context. The methylidyne radical CH is commonly used as a proxy for molecular hydrogen in the cold, neutral phase of the interstellar medium. The optical spectroscopy of CH is limited by interstellar extinction, whereas far-infrared observations provide an integral view through the Galaxy. While the HF ground state absorption, another H2 proxy in diffuse gas, frequently suffers from saturation, CH remains transparent both in spiral-arm crossings and high-mass star forming regions, turning this light hydride into a universal surrogate for H2. However, in slow shocks and in regions dissipating turbulence its abundance is expected to be enhanced by an endothermic production path, and the idea of a “canonical” CH abundance needs to be addressed. Aim. The N = 2 ← 1 ground state transition of CH at λ149 μm has become accessible to high-resolution spectroscopy thanks to the German Receiver for Astronomy at Terahertz Frequencies (GREAT) aboard the Stratospheric Observatory for Infrared Astronomy (SOFIA). Its unsaturated absorption and the absence of emission from the star forming regions makes it an ideal candidate for the determination of column densities with a minimum of assumptions. Here we present an analysis of four sightlines towards distant Galactic star forming regions, whose hot cores emit a strong far-infrared dust continuum serving as background signal. Moreover, if combined with the sub-millimeter line of CH at λ560 μm , environments forming massive stars can be analyzed. For this we present a case study on the “proto-Trapezium” cluster W3 IRS5. Methods. While we confirm the global correlation between the column densities of HF and those of CH, both in arm and interarm regions, clear signposts of an over-abundance of CH are observed towards lower densities. However, a significant correlation between the column densities of CH and HF remains. A characterization of the hot cores in the W3 IRS5 proto-cluster and its envelope demonstrates that the sub-millimeter/far-infrared lines of CH reliably trace not only diffuse but also dense, molecular gas. Results. In diffuse gas, at lower densities a quiescent ion-neutral chemistry alone cannot account for the observed abundance of CH. Unlike the production of HF, for CH+ and CH, vortices forming in turbulent, diffuse gas may be the setting for an enhanced production path. However, CH remains a valuable tracer for molecular gas in environments reaching from diffuse clouds to sites of high-mass star formation.

Chemical segregation in hot cores with disk candidates

In the study of high-mass star formation, hot cores are empirically defined stages where chemically rich emission is detected toward a massive YSO. It is unknown whether the physical origin of this emission is a disk, inner envelope, or outflow cavity wall and whether the hot core stage is common to all massive stars. We investigate the chemical make up of several hot molecular cores to determine physical and chemical structure. We use high spectral and spatial resolution Cycle 0 ALMA observations to determine how this stage fits into the formation sequence of a high mass star. We observed the G35.20-0.74N and G35.03+0.35 hot cores at 350 GHz. We analyzed spectra and maps from four continuum peaks (A, B1, B2 and B3) in G35.20, separated by 1000-2000 AU, and one continuum peak in G35.03. We made all possible line identifications across 8 GHz of spectral windows of molecular emission lines and determined column densities and temperatures for as many as 35 species assuming local thermodynamic equilibrium. In comparing the spectra of the four peaks, we find each has a distinct chemical composition expressed in over 400 different transitions. In G35.20, B1 and B2 contain oxygen- and sulfur-bearing organic and inorganic species but few nitrogen-bearing species whereas A and B3 are strong sources of O, S, and N-bearing species (especially those with the CN-bond). CH$_2$DCN is clearly detected in A and B3 with D/H ratios of 8 and 13$\%$, respectively, but is much weaker at B1 and undetected at B2. No deuterated species are detected in G35.03, but similar molecular abundances to G35.20 were found in other species. We also find co-spatial emission of HNCO and NH$_2$CHO in both sources indicating a strong chemical link between the two species. The chemical segregation between N-bearing organic species and others in G35.20 suggests the presence of multiple protostars, surrounded by a disk or torus.

Millimetre molecular lines in Planck cold clumps

AbstractGalactic cold clumps have been identified from the Planck data (Planck Collaboration, 2011a, 2011b, 2015) as 10 342 cold (7 - 19 K) sources that stand out against a warmer environment, with the Early Cold Cores as a subsample of 915 most reliable detections. There is CO emission associated with the Planck Cold Clumps (PCCs), which has been observed with ground-based radio telescopes at higher resolution (Wu et al. 2012, Liu et al. 2014). A subset of PCCs have also been observed with Herschel at higher resolution (Juvela et al. 2012).A southern sub-sample of the PCCs has been observed with the Mopra 22-m telescope to study the molecular gas. The Mopra telescope has 3-mm, 7-mm and 12-mm bands, with broadband correlator configuration 8-GHz wide with 0.27-MHz channels, or multiple zoom bands 137-MHz wide with 33-KHz channels, within the 8 GHz.During the 2013 southern winter season we observed 10 clumps. This included observations in the 3-mm band of 12CO, 13CO and C18O and lines around 89 GHz (e.g. HCN, HCO+ and HNC), in the 7-mm band (e.g. CS) and in the 12-mm band (e.g. NH3). These observations were heterogenous, with sources selected by LST in gaps between observations of other projects, and band chosen by weather (i.e. in conditions unsuitable for higher frequencies, lower frequency bands were observed). During the 2014 season we observed 34 positions in 22 clumps, with zoom mode observations of lines around 89 GHz. This was a more well-defined sample of sources.The mapping of the CO lines shows good spatial correlation of the CO with the dust column density The CO isotoplogues show high optical depth in 12CO and 13CO. The lines of HCN, HCO+ and HNC are weak, but detected in many of the 2014 sample. We are modelling the line results to determine column densities, excitation temperatures and abundances, using tools such as radex (van der Tak et al. 2007).

How an improved implementation of H2self-shielding influences the formation of massive stars and black holes

High redshift quasars at z>6 have masses up to ~$10^9$ M$_\odot$. One of the pathways to their formation includes direct collapse of gas, forming a supermassive star, precursor of the black hole seed. The conditions for direct collapse are more easily achievable in metal-free haloes, where atomic hydrogen cooling operates and molecular hydrogen (H2) formation is inhibited by a strong external UV flux. Above a certain value of UV flux (J_crit), the gas in a halo collapses isothermally at ~$10^4$ K and provides the conditions for supermassive star formation. However, H2 can self-shield, reducing the effect of photodissociation. So far, most numerical studies used the local Jeans length to calculate the column densities for self-shielding. We implement an improved method for the determination of column densities in 3D simulations and analyse its effect on the value of J_crit. This new method captures the gas geometry and velocity field and enables us to properly determine the direction-dependent self-shielding factor of H2 against photodissociating radiation. We find a value of J_crit that is a factor of two smaller than with the Jeans approach (~2000 J_21 vs. ~4000 J_21). The main reason for this difference is the strong directional dependence of the H2 column density. With this lower value of J_crit, the number of haloes exposed to a flux >J_crit is larger by more than an order of magnitude compared to previous studies. This may translate into a similar enhancement in the predicted number density of black hole seeds.

Thermal degradation of Poly(methyl methacrylate) with a 1.064 μm Nd:YAG laser in a buoyant flow

The laser radiative oxidative decomposition of poly(methyl methacrylate) (PMMA) and the evolved decomposition products in the gas-phase are investigated using imaging Fourier transform infrared spectroscopy (IFTS). The spatial and temporal evolution of the gas plume in air was investigated with adequate spatial (0.81 mm2 per pixel) and spectral resolutions (2 cm−1). Surfaces of black PMMA samples were irradiated from 4 to 22 W/cm2 with a cw 1.064 μm Nd:YAG laser. Strong spectral emission of methyl methacrylate (MMA) was observed in the infrared. Spatial maps of plume temperature and MMA column density were obtained from modeling the observed spectra by assuming a homogeneous single-plume radiative transfer model (RTM). A spectral model was used to compute the gas emissivity from an experimentally measured, interpolated and extrapolated MMA absorption coefficient database. In addition, the spectral radiance from the irradiated PMMA surface was fitted with Planck's distribution to obtain temporal and spatial surface temperature profiles. The peak signal-to-noise exceeded 50:1, allowing plume temperature and MMA column density determinations with low statistical errors. Laser irradiated PMMA reached a steady surface temperature of 613.9 ± 0.8 K and a peak gas-phase temperature of 700 ± 19 K at 22 W/cm2. The reported statistical uncertainties for all the results are defined as the half-width of the 95% confidence interval and do not include systematic errors associated with the assumption of a homogeneous plume or the effects of turbulence. As laser intensity increased, gas temperature decreased at the surface-boundary layer. A simplified thermal analysis was developed to understand the wavelength dependent surface heating rates from using both CO2 and Nd:YAG lasers. An Arrhenius plot of MMA formation at the surface for a single pixel was compared with established kinetics models. At surface temperatures of 450–600 K, an effective activation energy of 30.83 ± 8.29 kJ/mol was obtained, consistent with surface desorption of the monomer.

An efficient radiative cooling approximation for use in hydrodynamic simulations

To make relevant predictions about observable emission, hydrodynamical simulation codes must employ schemes that account for radiative losses, but the large dimensionality of accurate radiative transfer schemes is often prohibitive. Stamatellos and collaborators introduced a scheme for smoothed particle hydrodynamics (SPH) simulations based on the notion of polytropic pseudo-clouds that uses only local quantities to estimate cooling rates. The computational approach is extremely efficient and works well in cases close to spherical symmetry, such as in star formation problems. Unfortunately, the method, which takes the local gravitational potential as an input, can be inaccurate when applied to non-spherical configurations, limiting its usefulness when studying disks or stellar collisions, among other situations of interest. Here, we introduce the "pressure scale height method," which incorporates the fluid pressure scale height into the determination of column densities and cooling rates, and show that it produces more accurate results across a wide range of physical scenarios while retaining the computational efficiency of the original method. The tested models include spherical polytropes as well as disks with specified density and temperature profiles. We focus on applying our techniques within an SPH code, although our method can be implemented within any particle-based Lagrangian or grid-based Eulerian hydrodynamic scheme. Our new method may be applied in a broad range of situations, including within the realm of stellar interactions, collisions, and mergers.

Gas-phase plume from laser-irradiated fiberglass-reinforced polymers via imaging fourier transform spectroscopy.

Emissive plumes from laser-irradiated fiberglass-reinforced polymers (FRP) were investigated using a mid-infrared imaging Fourier transform spectrometer, operating at fast framing rates (50 kHz imagery and 2.5 Hz hyperspectral imagery) with adequate spatial (0.81 mm(2) per pixel) and spectral resolution (2 cm(-1)). Fiberglass-reinforced polymer targets were irradiated with a 1064 nm continuous wave neodymium-doped yttrium aluminum garnet (Nd:YAG) laser for 60 s at 100 W in air. Strong emissions from H(2)O, CO, CO(2), and hydrocarbons were observed between 1800 and 5000 cm(-1). A single-layer radiative transfer model was developed for the spectral region from 2000 to 2400 cm(-1) to estimate spatial maps of temperature and column densities of CO and CO(2) from the hyperspectral imagery. The spectral model was used to compute the absorption cross sections of CO and CO(2) using spectral line parameters from the high-temperature extension of the HITRAN. The analysis of pre-combustion spectra yields effective temperatures rising from ambient to 1200 K and suddenly increasing to 1515 K upon combustion. The peak signal-to-noise ratio for a single spectrum exceeds 60:1, enabling temperature and column density determinations with low statistical error. For example, the spectral analysis for a single pixel within a single frame yields an effective temperature of 1019 ± 6 K, and CO and CO(2) column densities of 1.14 ± 0.05 and 1.11 ± 0.03 × 10(18) molec/cm(2), respectively. Systematic errors associated with the radiative transfer model dominate, yielding effective temperatures with uncertainties of >100 K and column densities to within a factor of 2-3. Hydrocarbon emission at 2800 to 3200 cm(-1) is well correlated with CO column density.

Diagnostics of carbon arc plasma under formation of carbon-encapsulated iron nanoparticles by optical emission and absorption spectroscopy

Absorption and emission spectra pertaining to the electronic transition a 3Πu–d 3Πg (0–0 band) of C2 molecule as present in the carbon arc discharge zone are analysed to determine the temperature and C2 radical content in function of carbon–iron anode composition. One of the objectives was to explore the use of emission and absorption, both approximate, methods for diagnostics of carbon arc plasma used for synthesis of carbon-encapsulated iron nanoparticles. The theory behind both methods was exploited to estimate the errors resulting from both approximate methods. The results obtained showed that the uncertainty in the determination of average temperatures and C2 column densities should not exceed ±3% and 10%, respectively. Contrary to these predictions, the experimental values of temperature and C2 column density determined from the plasma radiation significantly exceeded those resulting from the absorption measurements. It is concluded that some non-equilibrium state is between the ground (a 3Πu) and excited (d 3Πg) electronic states in carbon arc plasma under consideration. The column density distributions of non-excited C2 radical were transformed into radial concentration distributions, and then to carbon vapour pressures assuming thermodynamic equilibrium between carbon species. Both quantities rise with Fe content in the anode. Additionally, clear relationship between the carbon vapour pressure and the yield of carbon encapsulated iron nanoparticles synthesis was found.