V. V. Guzmán

The comet-like composition of a protoplanetary disk as revealed by complex cyanides

Observations of comets and asteroids show that the solar nebula that spawned our planetary system was rich in water and organic molecules. Bombardment brought these organics to the young Earth’s surface. Unlike asteroids, comets preserve a nearly pristine record of the solar nebula composition. The presence of cyanides in comets, including 0.01 per cent of methyl cyanide (CH3CN) with respect to water, is of special interest because of the importance of C–N bonds for abiotic amino acid synthesis. Comet-like compositions of simple and complex volatiles are found in protostars, and can readily be explained by a combination of gas-phase chemistry (to form, for example, HCN) and an active ice-phase chemistry on grain surfaces that advances complexity. Simple volatiles, including water and HCN, have been detected previously in solar nebula analogues, indicating that they survive disk formation or are re-formed in situ. It has hitherto been unclear whether the same holds for more complex organic molecules outside the solar nebula, given that recent observations show a marked change in the chemistry at the boundary between nascent envelopes and young disks due to accretion shocks. Here we report the detection of the complex cyanides CH3CN and HC3N (and HCN) in the protoplanetary disk around the young star MWC 480. We find that the abundance ratios of these nitrogen-bearing organics in the gas phase are similar to those in comets, which suggests an even higher relative abundance of complex cyanides in the disk ice. This implies that complex organics accompany simpler volatiles in protoplanetary disks, and that the rich organic chemistry of our solar nebula was not unique.

Chemical complexity in the Horsehead photodissociation region

The interstellar medium is known to be chemically complex. Organic molecules with up to 11 atoms have been detected in the interstellar medium, and are believed to be formed on the ices around dust grains. The ices can be released into the gas-phase either through thermal desorption, when a newly formed star heats the medium around it and completely evaporates the ices; or through non-thermal desorption mechanisms, such as photodesorption, when a single far-UV photon releases only a few molecules from the ices. The first mechanism dominates in hot cores, hot corinos and strongly UV-illuminated PDRs, while the second dominates in colder regions, such as low UV-field PDRs. This is the case of the Horsehead were dust temperatures are approximately eual to 20-30 K, and therefore offers a clean environment to investigate the role of photodesorption. We have carried out an unbiased spectral line survey at 3, 2 and 1mm with the IRAM-30m telescope in the Horsehead nebula, with an unprecedented combination of bandwidth, high spectral resolution and sensitivity. Two positions were observed: the warm PDR and a cold condensation shielded from the UV field (dense core), located just behind the PDR edge. We summarize our recently published results from this survey and present the first detection of the complex organic molecules HCOOH, CH2CO, CH3CHO and CH3CCH in a PDR. These species together with CH3CN present enhanced abundances in the PDR compared to the dense core. This suggests that photodesorption is an efficient mechanism to release complex molecules into the gas-phase in far-UV illuminated regions.

The IRAM-30 m line survey of the Horsehead PDR - III. High abundance of complex (iso-)nitrile molecules in UV-illuminated gas

Context. Complex (iso-)nitrile molecules, such as CH3CN and HC3N, are relatively easily detected in our Galaxy and in other galaxies. Aims: We aim at constraining their chemistry through observations of two positions in the Horsehead edge: the photo-dissociation region (PDR) and the dense, cold, and UV-shielded core just behind it. Methods: We systematically searched for lines of CH3CN, HC3N, C3N, and some of their isomers in our sensitive unbiased line survey at 3, 2, and 1 mm. We stacked the lines of C3N to improve the detectability of this species. We derived column densities and abundances through Bayesian analysis using a large velocity gradient radiative transfer model. Results: We report the first clear detection of CH3NC at millimeter wavelength. We detected 17 lines of CH3CN at the PDR and 6 at the dense core position, and we resolved its hyperfine structure for 3 lines. We detected 4 lines of HC3N, and C3N is clearly detected at the PDR position. We computed new electron collisional rate coefficients for CH3CN, andwe found that including electron excitation reduces the derived column density by 40% at the PDR position, where the electron density is 1-5 cm-3. While CH3CN is 30 times more abundant in the PDR (2.5 × 10-10) than in the dense core (8 × 10-12), HC3N has similar abundance at both positions (8 × 10-12). The isomeric ratio CH3NC/CH3CN is 0.15 ± 0.02. Conclusions: The significant amount of complex (iso-)nitrile molecule in the UV illuminated gas is puzzling as the photodissociation is expected to be efficient. This is all the more surprising in the case of CH3CN, which is 30 times more abundant in the PDR than in the dense core. In this case, pure gas phase chemistry cannot reproduce the amount of CH3CN observed in the UV-illuminated gas. We propose that CH3CN gas phase abundance is enhanced when ice mantles of grains are destroyed through photo-desorption or thermal-evaporation in PDRs, and through sputtering in shocks. Based on observations obtained with the IRAM-30 m telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain).Appendices are available in electronic form at http://www.aanda.org

The IRAM-30m line survey of the Horsehead PDR. I. CF+ as a tracer of C+ and as a measure of the fluorine abundance

C+ is a key species in the interstellar medium, but its 158 μm fine structure line cannot be observed from ground-based telescopes. Current models of fluorine chemistry predict that CF+ is the second-most important fluorine reservoir in regions where C+ is abundant. We detected the J = 1-0 and J = 2-1 rotational lines of CF+ with high signal-to-noise ratio toward the photo-dissociation region and dense core positions in the Horsehead. Using a rotational diagram analysis, we derive a column density of N(CF+) = (1.5-2.0) × 1012 cm-2. Because of the simple fluorine chemistry, the CF+ column density is proportional to the fluorine abundance. We thus infer the fluorine gas-phase abundance to be F/H = (0.6-1.5) × 10-8. Photochemical models indicate that CF+ is found in the layers where C+ is abundant. The emission arises in the UV-illuminated skin of the nebula, tracing the outermost cloud layers. Indeed, CF+ and C+ are the only species observed to date in the Horsehead with a double-peaked line profile caused by kinematics. We therefore propose that CF+, which is detectable from the ground, can be used as a proxy of the C+ layers.

AN OBSERVATIONAL INVESTIGATION OF THE IDENTITY OF B11244 (l-C3H + /C3H − )

Pety et al. have reported the detection of eight transitions of a closed-shell, linear molecule (B11244) in observations toward the Horsehead photodissociation region (PDR), which they attribute to the l-C_3H^+ cation. Recent high-level ab initio calculations have called this assignment into question; the anionic C_3H^– molecule has been suggested as a more likely candidate. Here, we examine observations of the Horsehead PDR, Sgr B2(N), TMC-1, and IRC+10216 in the context of both l-C_3H^+ and C_3H^–. We find no observational evidence of K_a = 1 lines, which should be present were the carrier indeed C_3H^–. Additionally, we find a strong anticorrelation between the presence of known molecular anions and B11244 in these regions. Finally, we discuss the formation and destruction chemistry of C_3H^– in the context of the physical conditions in the regions. Based on these results, we conclude there is little evidence to support the claim that the carrier is C_3H^–.

The hyperfine structure in the rotational spectrum of CF

Context. CF + has recently been detected in the Horsehead and Orion Bar photo-dissociation regions. The J = 1−0 line in the Horsehead is double-peaked in contrast to other millimeter lines. The origin of this double-peak profile may be kinematic or spectroscopic. Aims. We investigate the effect of hyperfine interactions due to the fluorine nucleus in CF + on the rotational transitions. Methods. We compute the fluorine spin rotation constant of CF + using high-level quantum chemical methods and determine the relative positions and intensities of each hyperfine component. This information is used to fit the theoretical hyperfine components to the observed CF + line profiles, thereby employing the hyperfine fitting method in GILDAS. Results. The fluorine spin rotation constant of CF + is 229.2 kHz. This way, the double-peaked CF + line profiles are well fitted by the hyperfine components predicted by the calculations. The unusually large hyperfine splitting of the CF + line therefore explains the shape of the lines detected in the Horsehead nebula, without invoking intricate kinematics in the UV-illuminated gas.

The distribution and chemistry of H2CO in the DM Tau protoplanetary disk

H2CO ice on dust grains is an important precursor of complex organic molecules (COMs). H2CO gas can be readily observed in protoplanetary disks and may be used to trace COM chemistry. However, its utility as a COM probe is currently limited by a lack of constraints on the relative contributions of two dierent formation pathways: on icy grain-surfaces and in the gas-phase. We use archival ALMA observations of the resolved distribution of H2CO emission in the disk around the young low-mass star DM Tau to assess the relative importance of these formation routes. The observed H2CO emission has a centrally peaked and radially broad brightness prole (extending out to 500 AU). We compare these observations with disk chemistry models with and without grain-surface formation reactions, and nd that both gas and grain-surface chemistry are necessary to explain the spatial distribution of the emission. Gas-phase H2CO production is responsible for the observed central peak, while grainsurface chemistry is required to reproduce the emission exterior to the CO snowline (where H2CO mainly forms through the hydrogenation of CO ice before being non-thermally desorbed). These observations demonstrate that both gas and grain-surface pathways contribute to the observed H2CO in disks, and that their relative contributions depend strongly on distance from the host star. Subject headings: Protoplanetary Disks, Astrochemistry, Circumstellar Matter, ISM: Molecules, Radio Lines: ISM

Turbulence and star formation efficiency in molecular clouds: solenoidal versus compressive motions in Orion B

Context. The nature of turbulence in molecular clouds is one of the key parameters that control star formation efficiency: compressive motions, as opposed to solenoidal motions, can trigger the collapse of cores, or mark the expansion of Hii regions. Aims. We try to observationally derive the fractions of momentum density (ρv) contained in the solenoidal and compressive modes of turbulence in the Orion B molecular cloud and relate these fractions to the star formation efficiency in the cloud. Methods. The implementation of a statistical method applied to a 13CO(J = 1−0) datacube obtained with the IRAM-30 m telescope, enables us to retrieve 3-dimensional quantities from the projected quantities provided by the observations, which yields an estimate of the compressive versus solenoidal ratio in various regions of the cloud. Results. Despite the Orion B molecular cloud being highly supersonic (mean Mach number ~ 6), the fractions of motion in each mode diverge significantly from equipartition. The cloud’s motions are, on average, mostly solenoidal (excess > 8% with respect to equipartition), which is consistent with its low star formation rate. On the other hand, the motions around the main star forming regions (NGC 2023 and NGC 2024) prove to be strongly compressive. Conclusions. We have successfully applied to observational data a method that has so far only been tested on simulations, and we have shown that there can be a strong intra-cloud variability of the compressive and solenoidal fractions, these fractions being in turn related to the star formation efficiency. This opens a new possibility for star formation diagnostics in galactic molecular clouds.

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.

Widespread galactic CF+ absorption: detection toward W49 with the Plateau de Bure Interferometer

To study the usefulness of \CFP\ as a tracer of the regions where C\p\ and \HH\ coexist in the interstellar medium. We used the Plateau de Bure Interferometer to synthesize \CFP\ J=1-0 absorption at 102.6 GHz toward the core of the distant HII region W49N at l = 43.2\degr, b=0.0\degr, and we modeled the fluorine chemistry in diffuse/translucent molecular gas. We detected \CFP\ absorption over a broad range of velocity showing that \CFP\ is widespread in the \HH-bearing Galactic disk gas. Originally detected in dense gas in the Orion Bar and Horsehead PDR, \CFP\ was subsequently detected in absorption from diffuse and translucent clouds seen toward \bll\ and 3C111. Here we showed that \CFP\ is distributed throughout the diffuse and translucent molecular disk gas with N(\CFP)/N(\HH)