arXiv subjects

Harvey Liszt

Publications and source records attributed to Harvey Liszt.

At least 19 recordsLinked to original sources

arXiv2025

Spectral survey of the diffuse gas toward BL Lac in the Q band

The chemical composition of diffuse interstellar clouds is not fully established. They host an active chemistry despite their relatively low density and the ubiquitous presence of far-UV radiation. To further explore the chemical composition of diffuse clouds, we performed a spectral scan toward the bright radio source BL Lac in the Q band (from 32 to 50 GHz) using the Yebes 40m telescope. Yebes observations were performed interleaving Frequency Switching and Position Switching integrations toward BL Lac, using a spectral resolution of 38 kHz. The data have been reduced with the CLASS software. We achieved an unprecedented sensitivity on the continuum of 0.02 - 0.07 %, allowing the detection of very faint absorption features. We confirm previous detections of HCS+, C3H, C3H+, CH3CN and HC3N in diffuse clouds and report new detections of CCS, C4H, CH3CHO, H2CCO , HNCO and H2CS along the line of sight to BL Lac, with abundances relative to H2 from a few 10{-11} to a few 10{-10}. We compiled molecular detections toward diffuse clouds to obtain the chemical inventory of a typical diffuse interstellar cloud. The chemical inventory of diffuse interstellar clouds includes complex organic species with up to four heavy atoms. These species are efficiently formed in the diffuse interstellar gas and reach abundances similar to those measured in dense photodissociation regions, pointing to similar gas phase chemical processes.

astro-ph.GA↗

arXiv2025

CO, CS, HCO, HCO+, C2H, and HCN in the diffuse interstellar medium

Context. Radio frequency molecular absorption lines appear along sight lines with AV well below 1 mag, revealing the presence of H2 in diffuse gas even when 2.6mm CO emission is absent. Aims. We discuss absorption lines of HCO+, C2H, HCN, CS, and HCO in a larger sample (88 sight lines) than was available before. Methods. We observed millimeter-wave absorption at the IRAM and ALMA interferometers over the past 30 years and gathered the results for to compare with observations of HCO+ and CO emission taken at the ARO KP12m and IRAM 30m telescopes. Results: We detected HCO+ along 72 of 86 sightlines where it was observed, C2H along 53 of 76 sightlines and HCN along 38 of 57 sightlines. C2H is ubiquitous and N(C2H)/N(HCO+) increases at smaller EB-V and smaller N(HCO+) but C2H absorption is intrinsically weaker, decreasing the number of sightlines with low column density along which it was detected.The dense-gas tracer HCN was uniformly detected down to N(H2) = 10^20 cm-2 with little change in the relative abundance N(HCN)/N(HCO+) = 1.25. HCO was detected along only 4 of 46 sight lines that were newly observed at ALMA because its lines are intrinsically weak, but HCO is ubiquitous in the interstellar medium with N(HCO)/N(HCO+) = 1/3 or N(HCO)/N(H2) = 10^-9. The line widths of HCN features are (like those of CO) narrower than those of matching features observed in HCO+, and those of C2H are broader. HCO+ emission is commonly observed at log levels -2+/-0.3 dex with respect to CO emission.

astro-ph.GA↗

arXiv2024

A Search for 3-mm Molecular Absorption Line Transitions in the Magellanic Stream

The Magellanic Stream, a tidal tail of diffuse gas falling onto the Milky Way, formed by interactions between the Small and Large Magellanic Clouds, is primarily composed of neutral atomic hydrogen (HI). The deficiency of dust and the diffuse nature of the present gas make molecular formation rare and difficult, but if present, could lead to regions potentially suitable for star formation, thereby allowing us to probe conditions of star formation similar to those at high redshifts. We search for HCO$^+$, HCN, HNC, and C$_2$H using the highest sensitivity observations of molecular absorption data from the Atacama Large Millimeter Array to trace these regions, comparing with HI archival data to compare these environments in the Magellanic Stream to the HI column density threshold for molecular formation in the Milky Way. We also compare the line of sight locations with confirmed locations of stars, molecular hydrogen, and OI detections, though at higher sensitivities than the observations presented here. We find no detections to a 3$σ$ significance, despite four sightlines having column densities surpassing the threshold for molecular formation in the diffuse regions of the Milky Way. Here we present our calculations for the upper limits of the column densities of each of these molecular absorption lines, ranging from $3 \times 10^{10}$ to $1 \times 10^{13}$ cm$^{-2}$. The non-detection of HCO$^+$ suggests that at least one of the following is true: (i) $X_{HCO^+, \mathrm{MS}}$ is significantly lower than the Milky Way value; (ii) that the widespread diffuse molecular gas observed in the Milky Way's diffuse ISM does not have a direct analog in the MS; (iii) the HI-to-H$_2$ transition occurs in the MS at a higher surface density in the MS than in the LMC or SMC; or (iv) molecular gas exists in the MS, but only in small, dense clumps.

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arXiv2024

H I Kinematics and the E(B-V)/N(H I) ratio

The $λ21$cm H I emission that is used to trace the gas to dust ratio at high Galactic latitudes has contributions from material beyond the Milky Way disk, with uncertain and likely sub-Solar metallicity and dust content. These contributions can be isolated kinematically and their presence is clear for sightlines with small mean reddening $<$E(B-V)$>$ $\la$ 0.03 mag, which have mean ratios $<$N(H I)$>$/$<$E(B-V)$>$ that are 20-50\% above the high latitude Galactic average $<$N(H I)$>/<$E(B-V)$>=8.3\times10^{21}$cm$^{-2}$mag$^{-1}$. By mapping N(H I) and E(B-V) across H I High Velocity Cloud complexes and the Magellanic Clouds we show that the reddening of this kinematically-isolated gas is on average five times smaller per H I than the high latitude average. However, the aggregate contribution of this gas is small and $<$N(H I)$>/<$E(B-V)$>=8.3\times10^{21}$cm$^{-2}$mag$^{-1}$ is the appropriate value for Galactic gas seen at high latitude using the H I and reddening measures employed here and in our previous work.

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arXiv2024

Bias versus variance when fitting multi-species molecular lines with a non-LTE radiative transfer model

Robust radiative transfer techniques are requisite for efficiently extracting the physical and chemical information from molecular rotational lines.We study several hypotheses that enable robust estimations of the column densities and physical conditions when fitting one or two transitions per molecular species. We study the extent to which simplifying assumptions aimed at reducing the complexity of the problem introduce estimation biases and how to detect them.We focus on the CO and HCO+ isotopologues and analyze maps of a 50 square arcminutes field. We used the RADEX escape probability model to solve the statistical equilibrium equations and compute the emerging line profiles, assuming that all species coexist. Depending on the considered set of species, we also fixed the abundance ratio between some species and explored different values. We proposed a maximum likelihood estimator to infer the physical conditions and considered the effect of both the thermal noise and calibration uncertainty. We analyzed any potential biases induced by model misspecifications by comparing the results on the actual data for several sets of species and confirmed with Monte Carlo simulations. The variance of the estimations and the efficiency of the estimator were studied based on the Cram{é}r-Rao lower bound.Column densities can be estimated with 30% accuracy, while the best estimations of the volume density are found to be within a factor of two. Under the chosen model framework, the peak 12CO(1--0) is useful for constraining the kinetic temperature. The thermal pressure is better and more robustly estimated than the volume density and kinetic temperature separately. Analyzing CO and HCO+ isotopologues and fitting the full line profile are recommended practices with respect to detecting possible biases.Combining a non-local thermodynamic equilibrium model with a rigorous analysis of the accuracy allows us to obtain an efficient estimator and identify where the model is misspecified. We note that other combinations of molecular lines could be studied in the future.

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arXiv2024

H2CO and CS in diffuse clouds: Excitation and abundance

To provide constraints on the chemical processes responsible for the observed columns of organic species, we used NOEMA to observe the sight line toward NRAO150 in the 2mm spectral window. We targeted the low excitation lines of o-H2CO 2(1,1)-1(1,0) and p-H2CO 2(0,2)-1(0,1) as well as the nearby transitions of CS(3-2) and c-C3H2. We combined these data with previous observations to determine the excitation conditions, column densities, and abundances relative to H2 in the different velocity components. We performed non-LTE radiative transfer calculations including collision cross sections with ortho and para H2 and with electrons. New collision cross sections with electrons were computed for ortho and para formaldehyde. The c-C3H2 line profiles are very similar to those of HCO+ and CCH, while the CS absorption features are narrower and mostly concentrated in two main velocity components at V = -17 and -10 km/s. H2CO absorption lines present an intermediate pattern with absorption in all velocity components but larger opacities in the two main velocity components. The ortho-to-para ratios of H2CO and c-C3H2 are consistent with the statistical value of 3. While the excitation temperature of all c-C3H2 velocity components is consistent with the CMB, the two strong components detected in CS show a clear excess over the CMB indicating that CS resides at higher densities than other species along this particular sightline, n(H2) ~ 2500 cm-3 while n(H2) < 500 cm-3 for the other velocity components. We detected faint absorption from o-H213CO and C34S allowing us to derive isotopic ratios: o-H2CO/o-H213CO = 61 and C32S/C34S = 24. The excitation of the 4.8GHz line of formaldehyde is sensitive to the electron fraction and its excitation temperature is predicted to be lower than the CMB at low and moderate electron fractions, x(e)< 6E-5, and to rise above the CMB at high electron fractions, > 1e-4.

astro-ph.GA↗

arXiv2023

Deep learning denoising by dimension reduction: Application to the ORION-B line cubes

Context. The availability of large bandwidth receivers for millimeter radio telescopes allows the acquisition of position-position-frequency data cubes over a wide field of view and a broad frequency coverage. These cubes contain much information on the physical, chemical, and kinematical properties of the emitting gas. However, their large size coupled with inhomogenous signal-to-noise ratio (SNR) are major challenges for consistent analysis and interpretation.Aims. We search for a denoising method of the low SNR regions of the studied data cubes that would allow to recover the low SNR emission without distorting the signals with high SNR.Methods. We perform an in-depth data analysis of the 13 CO and C 17 O (1 -- 0) data cubes obtained as part of the ORION-B large program performed at the IRAM 30m telescope. We analyse the statistical properties of the noise and the evolution of the correlation of the signal in a given frequency channel with that of the adjacent channels. This allows us to propose significant improvements of typical autoassociative neural networks, often used to denoise hyperspectral Earth remote sensing data. Applying this method to the 13 CO (1 -- 0) cube, we compare the denoised data with those derived with the multiple Gaussian fitting algorithm ROHSA, considered as the state of the art procedure for data line cubes.Results. The nature of astronomical spectral data cubes is distinct from that of the hyperspectral data usually studied in the Earth remote sensing literature because the observed intensities become statistically independent beyond a short channel separation. This lack of redundancy in data has led us to adapt the method, notably by taking into account the sparsity of the signal along the spectral axis. The application of the proposed algorithm leads to an increase of the SNR in voxels with weak signal, while preserving the spectral shape of the data in high SNR voxels.Conclusions. The proposed algorithm that combines a detailed analysis of the noise statistics with an innovative autoencoder architecture is a promising path to denoise radio-astronomy line data cubes. In the future, exploring whether a better use of the spatial correlations of the noise may further improve the denoising performances seems a promising avenue. In addition,

astro-ph.IM↗

arXiv2023

The Dark Neutral Medium is (Mostly) Molecular Hydrogen

We acquired ALMA ground state absorption profiles of HCO+ and other molecules toward 33 extragalactic continuum sources seen toward the Galactic anticenter, deriving N(H2) = N(HCO+)/3x10^{-9}. We observed J=1-0 CO emission with the IRAM 30m in directions where HCO+ was newly detected. HCO+ absorption was detected in 28 of 33 new directions and CO emission along 19 of those 28. The 5 sightlines lacking detectable HCO+ have 3 times lower mean EBV and N(DNM). Binned in EBV, N(H2) and N(DNM) are strongly correlated and vary by factors of 50-100 over the observed range EBV~0.05-1 mag, while N(HI) varies by factors of only 2-3. On average N(DNM) and N(H2) are well matched, and detecting HCO+ absorption adds little/no H2 in excess of the previously inferred DNM. There are 5 cases where 2N(H2) < N(DNM)/2 indicates saturation of the HI emission. For sightlines with \WCO > 1 K-\kms the CO-H2 conversion factor N(H2)/\WCO\ = 2-3x10^{20}\pcc/K-\kms is higher than derived from studies of resolved clouds in gamma-rays. Our work sampled primarily atomic gas with a mean H2 fraction ~1/3, but the DNM is almost entirely molecular. CO fulfills its role as an H2 tracer when its emission is strong, but large-scale CO surveys are not sensitive to H2 columns associated with typical values N(DNM) = 2-6x10^{20}\pcc. Lower \XCO\ values from $γ$-ray studies arise in part from different definitions and usage. Sightlines with \WCO\ \ge 1 K-\kms\ represent 2/3 of the H2 detected in HCO+ and detecting 90% of the H2 would require detecting CO at levels \WCO\~0.2-0.3 K-\kms For full abstract see the paper

astro-ph.GA↗

arXiv2023

Molecular hydrogen and its proxies HCO$^+$ and CO in the diffuse interstellar medium

There is a robust polyatomic chemistry in diffuse, partially-molecular interstellar gas that is readily accessible in absorption at radio/mm/sub-mm wavelengths. Accurate column densities are derived owing to the weak internal excitation, so relative molecular abundances are well known with respect to each other but not with respect to H2. Here we consider the use of proxies for hydrogen column densities N(H2) and N(H) = N(HI)+2N(H2) based on measurements of HCO+ absorption and CO emission and absorption, and we compare these with results obtained by others when observing HI, H2 and CO toward stars and AGN. We consider the use of HCO+ as a proxy for H2 and show that the assumption of a relative abundance N(H2) = N(HCO+)/3x10^{-9} gives the same view of the atomic-molecular hydrogen transition that is seen in UV absorption toward stars. CO on the other hand shows differences between the radio and optical regimes because emission is always detected when N(\hcop) > 6x10^{11}\pcc or N(H2) > 2x10^20\pcc. Wide variations in the integrated CO {J=1-0} brightness W_CO and N(CO)/N(H2) imply equivalent variations in the CO-H2 conversion factor even while the ensemble mean is near the usual Galactic values. Gas/reddening ratios found in absorption toward stars, N(H)/E(B-V) = 6.2x10^21 H \pcc/mag overall or 6.8x10^21 H \pcc/mag for sightlines at E(B-V) <= 0.08 mag lacking H2 are well below the Galactic mean measured at low reddening and high Galactic latitude, 8.3x10^21 H \pcc/mag.

astro-ph.GA↗

arXiv2022

Gas kinematics around filamentary structures in the Orion B cloud

Understanding the initial properties of star-forming material and how they affect the star formation process is key. From an observational point of view, the feedback from young high-mass stars on future star formation properties is still poorly constrained. In the framework of the IRAM 30m ORION-B large program, we obtained observations of the translucent and moderately dense gas, which we used to analyze the kinematics over a field of 5 deg^2 around the filamentary structures. We used the ROHSA algorithm to decompose and de-noise the C18O(1-0) and 13CO(1-0) signals by taking the spatial coherence of the emission into account. We produced gas column density and mean velocity maps to estimate the relative orientation of their spatial gradients. We identified three cloud velocity layers at different systemic velocities and extracted the filaments in each velocity layer. The filaments are preferentially located in regions of low centroid velocity gradients. By comparing the relative orientation between the column density and velocity gradients of each layer from the ORION-B observations and synthetic observations from 3D kinematic toy models, we distinguish two types of behavior in the dynamics around filaments: (i) radial flows perpendicular to the filament axis that can be either inflows (increasing the filament mass) or outflows and (ii) longitudinal flows along the filament axis. The former case is seen in the Orion B data, while the latter is not identified. We have also identified asymmetrical flow patterns, usually associated with filaments located at the edge of an HII region. This is the first observational study to highlight feedback from HII regions on filament formation and, thus, on star formation in the Orion B cloud. This simple statistical method can be used for any molecular cloud to obtain coherent information on the kinematics.

astro-ph.GA↗

arXiv2021

Comparing absorption from classic tracers of the diffuse interstellar medium at optical and radio wavelengths along the same sightlines

% aims To compare information gained from radio and optical absorption line profiles from the diffuse interstellar medium along the same sightline % methods We compare new and existing 21cm HI and 3.4mm HCO+ profiles with profiles of the optical tracers CaII, NaI, and KI from an unpublished thesis of Tappe (2004) %results The atoms traced optically are all heavily depleted compared to a Solar abundance and only the integrated optical depths of HI and HCO+ correlate well with E(B-V). HCO+ is the species with by far the most limited kinematic distribution and the narrowest lines followed in order by KI, HI, NaI and CaII. CaII behaves separately in both column density and kinematics because it samples broader-lined warmer gas. Tracers of the cold neutral medium NaI, KI, HI and HCO+ share the same kinematic space statistically without correlating to nearly the same extent in abundance. N(NaI) and N(KI) are correlated, as are the integrated optical depths of HI and HCO+ but abundance correlations between optical and radio tracers are not seen. In the only direction with a measured CH+ profile, a 2 km/s velocity shift between CH+ and CH, usually interpreted as the sign of shocked gas, is mimicked in the shift of HI relative to HCO+. CH and HCO+ appear in the ratios N(CH)/N(HCO+)=14.6 and 21.1 along the two sightlines with optically-measured N(CH),compared with a mean of 12 determined previously at radio and submillimeter wavelengths.

astro-ph.GA↗

arXiv2021

CO+ as a probe of the origin of CO in diffuse interstellar clouds

The chemistry of the diffuse interstellar medium is driven by the combined influences of cosmic rays, ultraviolet (UV) radiation, and turbulence. Previously detected at the outer edges of photodissociation regions (PDRs) and formed from the reaction of C+ and OH, CO+ is the main chemical precursor of HCO+ and CO in a thermal, cosmic-ray, and UV-driven chemistry. Our aim was to test whether the thermal cosmic-ray and UV-driven chemistry is producing CO in diffuse interstellar molecular gas through the intermediate formation of CO+ We searched for CO+ absorption with the Atacama Large Millimeter Array (ALMA) toward two quasars with known Galactic foreground absorption from diffuse interstellar gas, J1717-3342 and J1744-3116, targeting the two strongest hyperfine components of the J=2-1 transition near 236 GHz. We could not detect CO+ but obtained sensitive upper limits toward both targets. The derived upper limits on the CO+ column densities represent about 4% of the HCO+ column densities. The corresponding upper limit on the CO+ abundance relative to H2 is <1.2 x 10^{-10}. The non-detection of CO+ confirms that HCO+ is mainly produced in the reaction between oxygen and carbon hydrides, CH2+ or CH3+ , induced by suprathermal processes, while CO+ and HOC+ result from reactions of C+ with OH and H2O. The densities required to form CO molecules at low extinction are consistent with this scheme.

astro-ph.GA↗

arXiv2020

Reconciling X-ray and 21cm HI absorption gas column densities toward obscured AGN

Hydrogen column densities inferred from X-ray absorption are typically 5 - 30 times larger than the neutral atomic hydrogen column densities derived from 21cm HI absorption toward radio-loud active galactic nuclei. Some part of the difference is ascribed to uncertainty in the spin temperature \Tsp\ = 100 K that is often used to convert 21cm HI HI absorption to N(HI). Here we propose another way to infer the gas column from HI absorption. In our Galaxy there is a nearly linear correlation between the inteferometrically-measured integrated 21cm HI absorption \WHI\ and reddening, \WHI\ $\propto$ \EBV$^{1.10}$ for \WHI\ $\ga 0.7$ \kms\ or \EBV\ $\ga 0.04$ mag. Scaling \EBV\ then provides the total gas column density N(H) from the same dust column that is responsible for optical obscuration and X-ray absorption, without calculating N(HI). Values of N(H) so derived typically exceed N(HI) by a factor 4 because the ubiquitous Galactic 21cm HI HI absorption samples only a portion of the interstellar gas. If the well-studied case of Hydra-A is a guide, even very large disparities in X-ray and 21cm HI gas column densities can be explained by resolving the core radiocontinuum and inferring N(H) from 21cm HI absorption. Milky Way conditions are often invoked in discussion of obscured AGN, so the empirical relationship seen in the Milky Way should be a relevant benchmark.

astro-ph.GA↗

arXiv2020

Quantitative inference of the $H_2$ column densities from 3 mm molecular emission: A case study towards Orion B

Molecular hydrogen being unobservable in cold molecular clouds, the column density measurements of molecular gas currently rely either on dust emission observation in the far-IR or on star counting. (Sub-)millimeter observations of numerous trace molecules are effective from ground based telescopes, but the relationships between the emission of one molecular line and the H2 column density (NH2) is non-linear and sensitive to excitation conditions, optical depths, abundance variations due to the underlying physico-chemistry. We aim to use multi-molecule line emission to infer NH2 from radio observations. We propose a data-driven approach to determine NH2 from radio molecular line observations. We use supervised machine learning methods (Random Forests) on wide-field hyperspectral IRAM-30m observations of the Orion B molecular cloud to train a predictor of NH2, using a limited set of molecular lines as input, and the Herschel-based dust-derived NH2 as ground truth output. For conditions similar to the Orion B molecular cloud, we obtain predictions of NH2 within a typical factor of 1.2 from the Herschel-based estimates. An analysis of the contributions of the different lines to the predictions show that the most important lines are $^{13}$CO(1-0), $^{12}$CO(1-0), C$^{18}$O(1-0), and HCO$^+$(1-0). A detailed analysis distinguishing between diffuse, translucent, filamentary, and dense core conditions show that the importance of these four lines depends on the regime, and that it is recommended to add the N$_2$H$^+$(1-0) and CH$_3$OH(20-10) lines for the prediction of NH2 in dense core conditions. This article opens a promising avenue to directly infer important physical parameters from the molecular line emission in the millimeter domain. The next step will be to try to infer several parameters simultaneously (e.g., NH2 and far-UV illumination field) to further test the method. [Abridged]

astro-ph.GA↗

arXiv2020

Tracers of the ionization fraction in dense and translucent gas: I. Automated exploitation of massive astrochemical model grids

The ionization fraction plays a key role in the physics and chemistry of the neutral interstellar medium, from controlling the coupling of the gas to the magnetic field to allowing fast ion-neutral reactions that drive interstellar chemistry. Most estimations of the ionization fraction have relied on deuterated species such as DCO+, whose detection is limited to dense cores representing an extremely small fraction of the volume of the giant molecular clouds they are part of. As large field-of-view hyperspectral maps become available, new tracers may be found. We search for the best observable tracers of the ionization fraction based on a grid of astrochemical models. We build grids of models that sample randomly a large space of physical conditions (unobservable quantities such as gas density, temperature, etc.) and compute the corresponding observables (line intensities, column densities) and the ionization fraction. We estimate the predictive power of each potential tracer by training a Random Forest model to predict the ionization fraction from that tracer, based on these model grids. In both translucent medium and cold dense medium conditions, several observable tracers with very good predictive power for the ionization fraction are found. Several tracers in cold dense medium conditions are found to be better and more widely applicable than the traditional DCO+/HCO+ ratio. We also provide simpler analytical fits for estimating the ionization fraction from the best tracers, and for estimating the associated uncertainties. We discuss the limitations of the present study and select a few recommended tracers in both types of conditions. The method presented here is very general and can be applied to the measurement of any other quantity of interest (cosmic ray flux, elemental abundances, etc.) from any type of model (PDR models, time-dependent chemical models, etc.). (abridged)

astro-ph.GA↗

arXiv2018

Chemical complexity in local diffuse and translucent clouds: ubiquitous l-C3H and CH3CN, a detection of HC3N and an upper limit on the abundance of CH2CN

We present Jansky Very Large Array observations of 20 - 37 GHz absorption lines from nearby Galactic diffuse molecular gas seen against four cosmologically-distant compact radio continuum sources. The main new observational results are that \linearC3H\ and \methCN\ are ubiqitous in the local diffuse molecular interstellar medium at \AV\ $\la 1$ while HC$_3$N was seen only toward B0415 at \AV\ $>$ 4 mag. The linear/cyclic ratio is much larger in C$_3$H than in C$_3$\HH\ and the ratio \methCN/HCN is enhanced compared to TMC-1, although not as much as toward the Horsehead Nebula. More consequentially, this work completes a long-term program assessing the abundances of small hydrocarbons (CH, \cch, linear and cyclic C$_3$H and C$_3$\HH, and \cfh\ and \cfhm) and the CN-bearing species (CN, HCN, HNC, HC$_3$N, HC$_5$N and CH$_3$CN): their systematics in diffuse molecular gas are presented in detail here. We also observed but did not strongly constrain the abundances of a few oxygen-bearing species, most prominently HNCO. We set limits on the column density of C\HH CN, such that the anion C\HH CN\m\ is only viable as a carrier of diffuse interstellar bands if the N(C\HH CN)/N(C\HH CN\m) abundance ratio is much smaller in this species than in any others for which the anion has been observed. We argue that complex organic molecules are not present in clouds meeting a reasonable definition of diffuse molecular gas, ie \AV\ $\la 1$ mag.

astro-ph.GA↗

arXiv2017

Clustering the Orion B giant molecular cloud based on its molecular emission

Previous attempts at segmenting molecular line maps of molecular clouds have focused on using position-position-velocity data cubes of a single line to separate the spatial components of the cloud. In contrast, wide field spectral imaging with large spectral bandwidth in the (sub)mm domain now allows to combine multiple molecular tracers to understand the different physical and chemical phases that constitute giant molecular clouds. We aim at using multiple tracers (sensitive to different physical processes) to segment a molecular cloud into physically/chemically similar regions (rather than spatially connected components). We use a machine learning clustering method (the Meanshift algorithm) to cluster pixels with similar molecular emission, ignoring spatial information. Simple radiative transfer models are used to interpret the astrophysical information uncovered by the clustering. A clustering analysis based only on the J=1-0 lines of 12CO, 13CO and C18O reveals distinct density/column density regimes (nH~100, 500, and >1000 cm-3), closely related to the usual definitions of diffuse, translucent and high-column-density regions. Adding two UV-sensitive tracers, the (1-0) lines of HCO+ and CN, allows us to distinguish two clearly distinct chemical regimes, characteristic of UV-illuminated and UV-shielded gas. The UV-illuminated regime shows overbright HCO+ and CN emission, which we relate to photochemical enrichment. We also find a tail of high CN/HCO+ intensity ratio in UV-illuminated regions. Finer distinctions in density classes (nH~7E3, and 4E4 cm-3) for the densest regions are also identified, likely related to the higher critical density of the CN and HCO+ (1-0) lines. The association of simultaneous multi-line, wide-field mapping and powerful machine learning methods such as the Meanshift algorithm reveals how to decode the complex information available in molecular tracers.

astro-ph.GA↗

arXiv2017

Dissecting the molecular structure of the Orion B cloud: Insight from Principal Component Analysis

Context. The combination of wideband receivers and spectrometers currently available in (sub-)millimeter observatories deliver wide- field hyperspectral imaging of the interstellar medium. Tens of spectral lines can be observed over degree wide fields in about fifty hours. This wealth of data calls for restating the physical questions about the interstellar medium in statistical terms. Aims. We aim at gaining information on the physical structure of the interstellar medium from a statistical analysis of many lines from different species over a large field of view, without requiring detailed radiative transfer or astrochemical modeling. Methods. We coupled a nonlinear rescaling of the data with one of the simplest multivariate analysis methods, namely the Principal Component Analysis, to decompose the observed signal into components that we interpret first qualitatively and then quantitatively based on our deep knowledge of the observed region and of the astrochemistry at play. Results. We identify 3 principal components, linear compositions of line brightness temperatures, that are correlated at various levels with the column density, the volume density and the UV radiation field. Conclusions. When sampling a sufficiently diverse mixture of physical parameters, it is possible to decompose the molecular emission in order to gain physical insight on the observed interstellar medium. This opens a new avenue for future studies of the interstellar medium.

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