arXiv subjects

S. Cuadrado

Publications and source records attributed to S. Cuadrado.

17 recordsLinked to original sources

arXiv2025

PDRs4All. XII. FUV-driven formation of hydrocarbon radicals and their relation with PAHs

We present subarcsecond-resolution ALMA mosaics of the Orion Bar PDR in [CI] 609um, C2H (4-3), and C18O (3-2) emission lines complemented by JWST images of H2 and aromatic infrared band (AIB) emission. The rim of the Bar shows very corrugated structures made of small-scale H2 dissociation fronts (DFs). The [CI] 609 um emission peaks very close (~0.002 pc) to the main H2-emitting DFs, suggesting the presence of gas density gradients. These DFs are also bright and remarkably similar in C2H emission, which traces "hydrocarbon radical peaks" characterized by very high C2H abundances, reaching up to several x10^-7. The high abundance of C2H and of related hydrocarbon radicals, such as CH3, CH2, and CH, can be attributed to gas-phase reactions driven by elevated temperatures, the presence of C+ and C, and the reactivity of FUV-pumped H2. The hydrocarbon radical peaks roughly coincide with maxima of the 3.4/3.3 um AIB intensity ratio, a proxy for the aliphatic-to-aromatic content of PAHs. This implies that the conditions triggering the formation of simple hydrocarbons also favor the formation (and survival) of PAHs with aliphatic side groups, potentially via the contribution of bottom-up processes in which abundant hydrocarbon radicals react in situ with PAHs. Ahead of the DFs, in the atomic PDR zone (where [H]>>[H2]), the AIB emission is the brightest, but small PAHs and carbonaceous grains undergo photo-processing due to the stronger FUV field. Our detection of trace amounts of C2H in this zone may result from the photoerosion of these species. This study provides a spatially resolved view of the chemical stratification of key carbon carriers in a PDR. Overall, both bottom-up and top-down processes appear to link simple hydrocarbon molecules with PAHs in molecular clouds; however, the exact chemical pathways and their relative contributions remain to be quantified.

astro-ph.GA↗

arXiv2024

Multiline observations of hydrogen, helium, and carbon radio-recombination lines toward Orion A: A detailed dynamical study and direct determination of physical conditions

We present a study of hydrogen, helium, and carbon millimeter-wave radio-recombination lines (RRLs) toward ten representative positions throughout the Orion Nebula complex, using the Yebes 40m telescope in the Q band (31.3 GHz to 50.6 GHz) at an angular resolution of about $45\arcsec$ ($\sim$0.09\,pc). The observed positions include the Orion Nebula (M42) with the Orion Molecular Core 1, M43, and the Orion Molecular Core 3 bordering on NGC 1973, 1975, and 1977. While hydrogen and helium RRLs arise in the ionized gas surrounding the massive stars in the Orion Nebula complex, carbon RRLs stem from the neutral gas of the adjacent photo-dissociation regions (PDRs). The high velocity resolution ($0.3\,\mathrm{km\,s^{-1}}$) enables us to discern the detailed dynamics of the RRL emitting neutral and ionized gas. We compare the carbon RRLs with SOFIA/upGREAT observations of the [CII] $158\,\mu\mathrm{m}$ line and IRAM 30m observations of the $^{13}$CO (J=2-1) line. Using the [CII] and [$^{13}$CII] intensities with the carbon RRL intensities, we can infer physical conditions (electron temperature and electron density) in the PDR gas using non-LTE excitation models. Our observations are sensitive enough to detect faint lines toward two positions in OMC1, that may be attributed to RRLs of C$^+$ or O$^+$. In general, the RRL line widths of both the ionized and neutral gas, as well as the [CII] and $^{13}$CO line widths, are broader than thermal, indicating significant turbulence in the interstellar medium, which transitions from super-Alfv\'enic and subsonic in the ionized gas to sub-Alfv\'enic and supersonic in the molecular gas. At the scales probed by our observations, the turbulent pressure dominates the pressure balance in the neutral and molecular gas, while in the ionized gas the turbulent pressure is much smaller than the thermal pressure.

astro-ph.GA↗

arXiv2024

The OBDT-theta board: time digitization for the theta view of Drift Tubes chambers

We present the design and performance of the On-Board electronics for the Drift Tubes (OBDT) for the superlayer theta along the direction parallel to the beam-line, the new board built to substitute part of the CMS DT Muon on-detector electronics. The OBDT-theta is responsible for the time digitization of the DT chamber signals for the theta view, allowing further tracking and triggering of the barrel muons. It is also in charge of part of the slow-control of the DT chamber inner electronics in the theta view. Prototypes of the OBDT-theta board are under validation in different laboratories in CERN, as well as in demonstrator chambers installed in the CMS experiment. This allows evaluation of the full functionality of the boards in real conditions, showing very satisfactory results.

hep-ex↗

arXiv2022

The Double Chooz antineutrino detectors

This article describes the setup and performance of the near and far detectors in the Double Chooz experiment. The electron antineutrinos of the Chooz nuclear power plant were measured in two identically designed detectors with different average baselines of about 400 m and 1050 m from the two reactor cores. Over many years of data taking the neutrino signals were extracted from interactions in the detectors with the goal of measuring a fundamental parameter in the context of neutrino oscillation, the mixing angle θ13. The central part of the Double Chooz detectors was a main detector comprising four cylindrical volumes filled with organic liquids. From the inside towards the outside there were volumes containing gadolinium-loaded scintillator, gadolinium-free scintillator, a buffer oil and, optically separated, another liquid scintillator acting as veto system. Above this main detector an additional outer veto system using plastic scintillator strips was installed. The technologies developed in Double Chooz were inspiration for several other antineutrino detectors in the field. The detector design allowed implementation of efficient background rejection techniques including use of pulse shape information provided by the data acquisition system. The Double Chooz detectors featured remarkable stability, in particular for the detected photons, as well as high radiopurity of the detector components.

physics.ins-det↗

arXiv2021

Interstellar detection of the simplest aminocarbyne, H2NC: an ignored but abundant molecule

We report the first identification in space of H2NC, a high-energy isomer of H2CN that has been largely ignored in chemical and astrochemical studies. The observation of various unidentified lines around 72.2 GHz in the cold dark cloud L483 motivated the search for, and successful detection of, additional groups of lines in harmonic relation. After an exhaustive high-level ab initio screening of possible carriers, we confidently assign the unidentified lines to H2NC based on the good agreement between astronomical and theoretical spectroscopic parameters and sound spectroscopic and astrochemical arguments. The observed frequencies are used to precisely characterize the rotational spectrum of H2NC. This species is also detected in the cold dark cloud B1-b and the z=0.89 galaxy in front of the quasar PKS1830-211. We derive H2NC/H2CN abundance ratios of 1 in L483 and B1-b and 0.27 toward PKS1830-211. Neither H2NC nor H2CN are detected in the dark cloud TMC-1, which seriously questions a previous identification of H2CN in this source. We suggest that the H2NC/H2CN ratio behaves as the HNC/HCN ratio, with values close to one in cold dense clouds and below one in diffuse clouds. The reactions N + CH3 and C + NH3 emerge as strong candidates to produce H2NC in interstellar clouds. Further studies on these two reactions are needed to evaluate the yield of H2NC. Due to the small number of atoms involved, it should be feasible to constrain the chemistry behind H2NC and H2CN, just as it has been done for HNC and HCN, as this could allow to use the H2NC/H2CN ratio as a probe of chemical or physical conditions of the host clouds.

astro-ph.GA↗

arXiv2021

Submillimeter imaging of the Galactic Center starburst Sgr B2. Warm molecular, atomic, and ionized gas far from massive star-forming cores

We present 168 arcmin^2 spectral images of the Sgr B2 complex taken with Herschel/SPIRE-FTS. We detect ubiquitous emission from CO (up to J=12-11), H2O, [CI]492, 809 GHz, and [NII] 205 um lines. We also present maps of the SiO, N2H+, HCN, and HCO+ emission obtained with the IRAM30m telescope. The cloud environment dominates the emitted FIR (80%), H2O 752 GHz (60 %) mid-J CO (91%), and [CI] (93 %) luminosity. The region shows very extended [NII] emission (spatially correlated with the 24 and 70 um dust emission). The observed FIR luminosities imply G_0~10^3. The extended [CI] emission arises from a pervasive component of neutral gas with n_H~10^3 cm-3. The high ionization rates, produced by enhanced cosmic-ray (CR) fluxes, drive the gas heating to Tk~40-60 K. The mid-J CO emission arises from a similarly extended but more pressurized gas component (P_th~10^7 K cm-3). Specific regions of enhanced SiO emission and high CO-to-FIR intensity ratios (>10^-3) show mid-J CO emission compatible with shock models. A major difference compared to more quiescent star-forming clouds in the disk of our Galaxy is the extended nature of the SiO and N2H+ emission in Sgr B2. This can be explained by the presence of cloud-scale shocks, induced by cloud-cloud collisions and stellar feedback, and the much higher CR ionization rate (>10^-15 s-1) leading to overabundant H3+ and N2H+. Hence, Sgr B2 hosts a more extreme environment than star-forming regions in the disk of the Galaxy. As a usual template for extragalactic comparisons, Sgr B2 shows more similarities to ultra luminous infrared galaxies such as Arp 220, including a "deficit" in the [CI]/FIR and [NII]/FIR intensity ratios, than to pure starburst galaxies such as M82. However, it is the extended cloud environment, rather than the cores, that serves as a useful template when telescopes do not resolve such extended regions in galaxies.

astro-ph.GA↗

arXiv2021

Bottlenecks to interstellar sulfur chemistry: Sulfur-bearing hydrides in UV-illuminated gas and grains

Hydride molecules lie at the base of interstellar chemistry, but the synthesis of sulfuretted hydrides is poorly understood. Motivated by new observations of the Orion Bar PDR - 1'' resolution ALMA images of SH+; IRAM 30m detections of H2S, H2S34, and H2S33; H3S+ (upper limits); and SOFIA observations of SH - we perform a systematic study of the chemistry of S-bearing hydrides. We determine their column densities using coupled excitation, radiative transfer as well as chemical formation and destruction models. We revise some of the key gas-phase reactions that lead to their chemical synthesis. This includes ab initio quantum calculations of the vibrational-state-dependent reactions SH+ + H2 <-> H2S+ + H and S + H2 <-> SH + H. We find that reactions of UV-pumped H2 (v>1) with S+ explain the presence of SH+ in a high thermal-pressure gas component, P_th~10^8 cm^-3 K, close to the H2 dissociation front. However, subsequent hydrogen abstraction reactions of SH+, H2S+, and S with vibrationally excited H2, fail to ultimately explain the observed H2S column density (~2.5x10^14 cm^-2, with an ortho-to-para ratio of 2.9+/-0.3). To overcome these bottlenecks, we build PDR models that include a simple network of grain surface reactions leading to the formation of solid H2S (s-H2S). The higher adsorption binding energies of S and SH suggested by recent studies imply that S atoms adsorb on grains (and form s-H2S) at warmer dust temperatures and closer to the UV-illuminated edges of molecular clouds. Photodesorption and, to a lesser extent, chemical desorption, produce roughly the same H2S column density (a few 10^14 cm-^2) and abundance peak (a few 10^-8) nearly independently of n_H and G_0. This agrees with the observed H2S column density in the Orion Bar as well as at the edges of dark clouds without invoking substantial depletion of elemental sulfur abundances.

astro-ph.GA↗

arXiv2020

Molecular globules in the Veil bubble of Orion. IRAM 30m 12CO, 13CO, and C18O 2-1 expanded maps of Orion A

Strong winds and ultraviolet (UV) radiation from O-type stars disrupt and ionize their molecular core birthplaces, sweeping up material into parsec-size shells. Owing to dissociation by starlight, the thinnest shells are expected to host low molecular abundances and therefore little star formation. Here, we expand previous maps taken with the IRAM 30m telescope and present square-degree 12CO and 13CO (J=2-1) maps of the wind-driven "Veil bubble'' that surrounds the Trapezium cluster and its natal Orion molecular core (OMC). Although widespread and extended CO emission is largely absent from the Veil, we show that several CO "globules'' exist and are embedded in the [CII]158um-bright shell that confines the bubble. This includes the first detection of quiescent CO at negative LSR velocities in Orion. Given the harsh UV irradiation conditions in this translucent material, the detection of CO globules is surprising. These globules are small (R=7,100 AU), not massive (M=0.3M_Sun), and are moderately dense: n_ H=4x10^4 cm^-3 (median values). They are confined by the external pressure of the shell, P_ext/k~10^7 cm^-3 K, and are likely magnetically supported. They are either transient objects formed by instabilities or have detached from pre-existing molecular structures, sculpted by the passing shock associated with the expanding shell and by UV radiation from the Trapezium. Some represent the first stages in the formation of small pillars, others of isolated small globules. Although their masses do not suggest they will form stars, one globule matches the position of a known YSO. The lack of extended CO in the "Veil shell'' demonstrates that feedback from massive stars expels, agitates, and reprocesses most of the disrupted molecular cloud gas, thereby limiting the star-formation rate in the region. The presence of globules is a result of this feedback.

astro-ph.GA↗

arXiv2020

New molecular species at redshift z=0.89

We present the first detections of CH3SH, C3H+, C3N, HCOOH, CH2CHCN, and H2CN in an extragalactic source. Namely the spiral arm of a galaxy located at z=0.89 on the line of sight to the radio-loud quasar PKS 1830-211. OCS, SO2, and NH2CN were also detected, raising the total number of molecular species identified in that early time galaxy to 54, not counting isotopologues. The detections were made in absorption against the SW quasar image, at 2 kpc from the galaxy centre, over the course of a Q band spectral line survey made with the Yebes 40 m telescope (rest-frame frequencies: 58.7-93.5 GHz). We derived the rotational temperatures and column densities of those species, which are found to be subthermally excited. The molecular abundances, and in particular the large abundances of C3H+ and of several previously reported cations, are characteristic of diffuse or translucent clouds with enhanced UV radiation or strong shocks.

astro-ph.GA↗

arXiv2019

Direct estimation of electron density in the Orion Bar PDR from mm-wave carbon recombination lines

A significant fraction of the molecular gas in star-forming regions is irradiated by stellar UV photons. In these environments, the electron density (n_e) plays a critical role in the gas dynamics, chemistry, and collisional excitation of certain molecules. We determine n_e in the prototypical strongly irradiated photodissociation region (PDR), the Orion Bar, from the detection of new millimeter-wave carbon recombination lines (mmCRLs) and existing far-IR [13CII] hyperfine line observations. We detect 12 mmCRLs (including alpha, beta, and gamma transitions) observed with the IRAM 30m telescope, at ~25'' angular resolution, toward the H/H2 dissociation front (DF) of the Bar. We also present a mmCRL emission cut across the PDR. These lines trace the C+/C/CO gas transition layer. As the much lower frequency carbon radio recombination lines, mmCRLs arise from neutral PDR gas and not from ionized gas in the adjacent HII region. This is readily seen from their narrow line profiles (dv=2.6+/-0.4 km/s) and line peak LSR velocities (v_LSR=+10.7+/-0.2 km/s). Optically thin [13CII] hyperfine lines and molecular lines - emitted close to the DF by trace species such as reactive ions CO+ and HOC+ - show the same line profiles. We use non-LTE excitation models of [13CII] and mmCRLs and derive n_e = 60-100 cm^-3 and T_e = 500-600 K toward the DF. The inferred electron densities are high, up to an order of magnitude higher than previously thought. They provide a lower limit to the gas thermal pressure at the PDR edge without using molecular tracers. We obtain P_th > (2-4)x10^8 cm^-3 K assuming that the electron abundance is equal or lower than the gas-phase elemental abundance of carbon. Such elevated thermal pressures leave little room for magnetic pressure support and agree with a scenario in which the PDR photoevaporates.

astro-ph.GA↗

arXiv2017

The ALMA view of UV irradiated cloud edges: unexpected structures and processes

Far-UV photons (E<13.6 eV) from hot massive stars regulate, or at least influence, the heating, ionization, and chemistry of most of the neutral interstellar medium. Investigating the interaction between FUV radiation and interstellar matter thus plays an important role in astrochemistry. We have used ALMA to mosaic a small field of the Orion Bar where the critical transition from atomic to molecular gas takes place. These observations provide an unprecedented sharp view of this transition layer (~1" resolution or ~414 AU). The resulting images (so far in the rotational emission of CO, HCO+, H13CO+, SO+, SO, and reactive ions SH+ and HOC+) show the small-scale structure in gas density and temperature, and the steep abundance gradients. The images reveal a pattern of high-density substructures, photo-ablative gas flows and instabilities at the edge of the molecular cloud. We have also used the IRAM 30m telescope to carry out a line-survey of the illuminated edge of the Bar in the mm domain. Our observations reveal the presence of complex organic molecules that were not expected in such a harsh environment. In particular, we have reported the first detection of the unstable cis conformer of formic acid (HCOOH) in the ISM. The energy barrier to internal rotation (the conversion from trans to cis) is approximately 4827 cm-1. Hence, this detection is surprising. The low inferred trans-to-cis abundance ratio of 2.8+/-1.0 supports a photoswitching mechanism: a given conformer absorbs a FUV stellar photon that radiatively excites the molecule to electronic states above the interconversion barrier. Subsequent fluorescent decay leaves the molecule in a different conformer form. This mechanism, which we have specifically studied with ab initio quantum calculations, was not considered so far in astrochemistry although it can affect the structure of a variety of molecules in PDRs. (abridged)

astro-ph.GA↗

arXiv2017

Complex organic molecules in strongly UV-irradiated gas

We investigate the presence of COMs in strongly UV-irradiated interstellar molecular gas. We have carried out a complete millimetre line survey using the IRAM30m telescope towards the edge of the Orion Bar photodissociation region (PDR), close to the H2 dissociation front, a position irradiated by a very intense far-UV (FUV) radiation field. These observations have been complemented with 8.5 arcsec resolution maps of the H2CO 5(1,5)-4(1,4) and C18O 3-2 emission at 0.9 mm. Despite being a harsh environment, we detect more than 250 lines from COMs and related precursors: H2CO, CH3OH, HCO, H2CCO, CH3CHO, H2CS, HCOOH, CH3CN, CH2NH, HNCO, H13-2CO, and HC3N (in decreasing order of abundance). For each species, the large number of detected lines allowed us to accurately constrain their rotational temperatures (Trot) and column densities (N). Owing to subthermal excitation and intricate spectroscopy of some COMs (symmetric- and asymmetric-top molecules such as CH3CN and H2CO, respectively), a correct determination of N and Trot requires building rotational population diagrams of their rotational ladders separately. We also provide accurate upper limit abundances for chemically related molecules that might have been expected, but are not conclusively detected at the edge of the PDR (HDCO, CH3O, CH3NC, CH3CCH, CH3OCH3, HCOOCH3, CH3CH2OH, CH3CH2CN, and CH2CHCN). A non-LTE LVG excitation analysis for molecules with known collisional rate coefficients, suggests that some COMs arise from different PDR layers but we cannot resolve them spatially. In particular, H2CO and CH3CN survive in the extended gas directly exposed to the strong FUV flux (Tk = 150-250 K and Td > 60 K), whereas CH3OH only arises from denser and cooler gas clumps in the more shielded PDR interior (Tk = 40-50 K). We find a HCO/H2CO/CH3OH = 1/5/3 abundance ratio. These ratios are different from those inferred in hot cores and shocks.

astro-ph.GA↗

arXiv2017

Spatially resolved images of reactive ions in the Orion Bar

We report high angular resolution (4.9" x 3.0") images of reactive ions SH+, HOC+, and SO+ toward the Orion Bar photodissociation region (PDR). We used ALMA-ACA to map several rotational lines at 0.8 mm, complemented with multi-line observations obtained with the IRAM 30m telescope. The SH+ and HOC+ emission is restricted to a narrow layer of 2"- to 10"-width (~800 to 4000 AU depending on the assumed PDR geometry) that follows the vibrationally excited H2^* emission. Both ions efficiently form very close to the H/H2 transition zone, at a depth of A_V < 1 mag into the neutral cloud, where abundant C+, S+, and H2^* coexist. SO+ peaks slightly deeper into the cloud. The observed ions have low rotational temperatures (T_rot~10-30 K << T_k) and narrow line-widths (~2-3 km/s), a factor of ~2 narrower that those of the lighter reactive ion CH+. This is consistent with the higher reactivity and faster radiative pumping rates of CH+ compared to the heavier ions, which are driven relatively faster toward smaller velocity dispersion by elastic collisions and toward lower T_rot by inelastic collisions. We estimate column densities and average physical conditions from a non-LTE excitation model (n(H2)~10^5-10^6 cm^-3, n(e^-)~10 cm^-3, and T_k~200 K). Regardless of the excitation details, SH+ and HOC+ clearly trace the most exposed layers of the UV-irradiated molecular cloud surface, whereas SO+ arises from slightly more shielded layers.

astro-ph.GA↗

arXiv2016

Trans-cis molecular photoswitching in interstellar Space

As many organic molecules, formic acid (HCOOH) has two conformers (trans and cis). The energy barrier to internal conversion from trans to cis is much higher than the thermal energy available in molecular clouds. Thus, only the most stable conformer (trans) is expected to exist in detectable amounts. We report the first interstellar detection of cis-HCOOH. Its presence in ultraviolet (UV) irradiated gas exclusively (the Orion Bar photodissociation region), with a low trans-to-cis abundance ratio of 2.8+-1.0, supports a photoswitching mechanism: a given conformer absorbs a stellar photon that radiatively excites the molecule to electronic states above the interconversion barrier. Subsequent fluorescent decay leaves the molecule in a different conformer form. This mechanism, which we specifically study with ab initio quantum calculations, was not considered in Space before but likely induces structural changes of a variety of interstellar molecules submitted to UV radiation.

astro-ph.GA↗

arXiv2016

Compression and ablation of the photo-irradiated cloud the Orion Bar

The Orion Bar is the archetypal edge-on molecular cloud surface illuminated by strong ultraviolet radiation from nearby massive stars. Owing to the close distance to Orion (about 1,350 light-year), the effects of stellar feedback on the parental cloud can be studied in detail. Visible-light observations of the Bar(1) show that the transition between the hot ionised gas and the warm neutral atomic gas (the ionisation front) is spatially well separated from the transition from atomic to molecular gas (the dissociation front): about 15 arcseconds or 6,200 astronomical units (one astronomical unit is the Earth-Sun distance). Static equilibrium models(2,3) used to interpret previous far-infrared and radio observations of the neutral gas in the Bar(4,5,6) (typically at 10-20 arcsecond resolution) predict an inhomogeneous cloud structure consisting of dense clumps embedded in a lower density extended gas component. Here we report one-arcsecond-resolution millimetre-wave images that allow us to resolve the molecular cloud surface. In contrast to stationary model predictions(7,8,9), there is no appreciable offset between the peak of the H2 vibrational emission (delineating the H/H2 transition) and the edge of the observed CO and HCO+ emission. This implies that the H/H2 and C+/C/CO transition zones are very close. These observations reveal a fragmented ridge of high-density substructures, photoablative gas flows and instabilities at the molecular cloud surface. The results suggest that the cloud edge has been compressed by a high-pressure wave that currently moves into the molecular cloud. The images demonstrate that dynamical and nonequilibrium effects are important for the cloud evolution.

astro-ph.GA↗

arXiv2015

Velocity-resolved [CII] emission and [CII]/FIR Mapping along Orion with Herschel

We present the first 7.5'x11.5' velocity-resolved map of the [CII]158um line toward the Orion molecular cloud-1 (OMC-1) taken with the Herschel/HIFI instrument. In combination with far-infrared (FIR) photometric images and velocity-resolved maps of the H41alpha hydrogen recombination and CO J=2-1 lines, this data set provides an unprecedented view of the intricate small-scale kinematics of the ionized/PDR/molecular gas interfaces and of the radiative feedback from massive stars. The main contribution to the [CII] luminosity (~85%) is from the extended, FUV-illuminated face of the cloud G_0>500, n_H>5x10^3 cm^-3) and from dense PDRs (G_0~10^4, n_H~10^5 cm^-3) at the interface between OMC-1 and the HII region surrounding the Trapezium cluster. Around 15% of the [CII] emission arises from a different gas component without CO counterpart. The [CII] excitation, PDR gas turbulence, line opacity (from [13CII]) and role of the geometry of the illuminating stars with respect to the cloud are investigated. We construct maps of the [CII]/FIR and FIR/M_Gas ratios and show that [CII]/FIR decreases from the extended cloud component (10^-2-10^-3) to the more opaque star-forming cores (10^-3-10^-4). The lowest values are reminiscent of the "[CII] deficit" seen in local ultra-luminous IR galaxies hosting vigorous star formation. Spatial correlation analysis shows that the decreasing [CII]/FIR ratio correlates better with the column density of dust through the molecular cloud than with FIR/M_Gas. We conclude that the [CII] emitting column relative to the total dust column along each line of sight is responsible for the observed [CII]/FIR variations through the cloud.

astro-ph.GA↗

arXiv2014

The chemistry and spatial distribution of small hydrocarbons in UV-irradiated molecular clouds: the Orion Bar PDR

We study the spatial distribution and chemistry of small hydrocarbons in the Orion Bar PDR. We used the IRAM-30m telescope to carry out a millimetre line survey towards the Orion Bar edge, complemented with ~2'x2' maps of the C2H and c-C3H2 emission. We analyse the excitation of the detected hydrocarbons and constrain the physical conditions of the emitting regions with non-LTE radiative transfer models. We compare the inferred column densities with updated gas-phase photochemical models including 13CCH and C13CH isotopomer fractionation. ~40% of the lines in the survey arise from hydrocarbons (C2H, C4H, c-C3H2, c-C3H, C13CH, 13CCH, l-C3H and l-H2C3). We detect new lines from l-C3H+ and improve its rotational spectroscopic constants. Anions or deuterated hydrocarbons are not detected: [C2D]/[C2H]<0.2%, [C2H-]/[C2H]<0.007% and [C4H-]/[C4H]<0.05%. Our gas-phase models can reasonably match the observed column densities of most hydrocarbons (within factors <3). Since the observed spatial distribution of the C2H and c-C3H2 emission is similar but does not follow the PAH emission, we conclude that, in high UV-flux PDRs, photodestruction of PAHs is not a necessary requirement to explain the observed abundances of the smallest hydrocarbons. Instead, gas-phase endothermic reactions (or with barriers) between C+, radicals and H2 enhance the formation of simple hydrocarbons. Observations and models suggest that the [C2H]/[c-C3H2] ratio (~32 at the PDR edge) decreases with the UV field attenuation. The observed low cyclic-to-linear C3H column density ratio (<3) is consistent with a high electron abundance (Xe) PDR environment. In fact, the poorly constrained Xe gradient influences much of the hydrocarbon chemistry in the more UV-shielded gas. We propose that reactions of C2H isotopologues with 13C+ and H atoms can explain the observed [C13CH]/[13CCH]=1.4(0.1) fractionation level.

astro-ph.GA↗