Taurus Molecular Cloud Research Articles

Studying the chemical and kinematical structures of dense cores TMC-1C, L1544, and TMC-1 in the Taurus molecular cloud using CCS and NH3 observations

ABSTRACT The measurement of chemical and kinematic structures in pre-stellar cores is essential for better understanding of the star-formation process. Here, we study three pre-stellar cores (TMC-1C, L1544, and TMC-1) of the Taurus molecular cloud by means of the thioxoethenylidene (CCS) radical and ammonia (NH3) molecule observed with the Karl G. Jansky Very Large Array telescope in the D, C, and CNB configurations. Our main results are based on the CCS observation of the TMC-1C core, showing that complex structures are present. A spatial offset relative to dust emission is observed in the CCS radical. Across a wide region around the dust peak, inward motion is found through the CCS radical. We have calculated the infall velocity and measured the turbulence inside the core. The turbulence is found to be subsonic. We obtain that the virial parameter α is < 1. Thus, thermal and non-thermal motions cannot prevent the collapse. Spatial incoherence of the CCS and NH3 is observed from the integrated intensity maps in these cores, suggesting that these molecules trace different environments in the cores. We compare the integrated flux densities of CCS with previous single-dish data and find that a small amount of flux is recovered in the interferometric observations, indicating the presence of significant diffuse emission in favourable conditions for producing CCS.

Superresolution trends in the ALMA Taurus survey: structured inner discs and compact discs

ABSTRACT The 1.33-mm survey of protoplanetary discs in the Taurus molecular cloud found annular gaps and rings to be common in extended sources (≳ 55au), when their 1D visibility distributions were fit parametrically. We first demonstrate the advantages and limitations of non-parametric visibility fits for data at the survey’s 0.12-arcsec resolution. Then we use the non-parametric model in Frankenstein (frank) to identify new substructure in three compact and seven extended sources. Among the new features, we identify three trends: a higher occurrence rate of substructure in the survey’s compact discs than previously seen, underresolved (potentially azimuthally asymmetric) substructure in the innermost disc of extended sources, and a ‘shoulder’ on the trailing edge of a ring in discs with strong depletion at small radii. Noting the shoulder morphology is present in multiple discs observed at higher resolution, we postulate it is tracing a common physical mechanism. We further demonstrate how a superresolution frank brightness profile is useful in motivating an accurate parametric model, using the highly structured source DL Tau in which frank finds two new rings. Finally, we show that sparse (u, v) plane sampling may be masking the presence of substructure in several additional compact survey sources.

A Search for Heterocycles in GOTHAM Observations of TMC-1.

We have conducted an extensive search for nitrogen-, oxygen-, and sulfur-bearing heterocycles toward Taurus Molecular Cloud 1 (TMC-1) using the deep, broadband centimeter-wavelength spectral line survey of the region from the GOTHAM large project on the Green Bank Telescope. Despite their ubiquity in terrestrial chemistry, and the confirmed presence of a number of cyclic and polycyclic hydrocarbon species in the source, we find no evidence for the presence of any heterocyclic species. Here, we report the derived upper limits on the column densities of these molecules obtained by Markov Chain Monte Carlo (MCMC) analysis and compare this approach to traditional single-line upper limit measurements. We further hypothesize why these molecules are absent in our data, how they might form in interstellar space, and the nature of observations that would be needed to secure their detection.

From Molecules with a Planar Tetracoordinate Carbon to an Astronomically Known C5H2 Carbene

Ethynylcyclopropenylidene (2), an isomer of C5H2, is a known molecule in the laboratory and has recently been identified in Taurus Molecular Cloud-1 (TMC-1). Using high-level coupled-cluster methods up to the CCSDT(Q)/CBS level of theory, it is shown that two isomers of C5H2 with a planar tetracoordinate carbon (ptC) atom, (SP-4)-spiro[2.2]pent-1,4-dien-1,4-diyl (11) and (SP-4)-spiro[2.2]pent-1,4-dien-1,5-diyl (13), serve as the reactive intermediates for the formation of 2. Here, a theoretical connection has been established between molecules containing ptC atoms (11 and 13) and a molecule (2) that is present nearly 430 light years away, thus providing evidence for the existence of ptC species in the interstellar medium. The reaction pathways connecting the transition states and the reactants and products have been confirmed by intrinsic reaction coordinate calculations at the CCSDT(Q)/CBS//B3LYP-D3BJ/cc-pVTZ level. While isomer 11 is non-polar (μ = 0), isomers 2 and 13 are polar, with dipole moment values of 3.52 and 5.17 Debye at the CCSD(T)/cc-pVTZ level. Therefore, 13 is also a suitable candidate for both laboratory and radioastronomical studies.

Chemical Variations Across the TMC-1 Boundary: Molecular Tracers from the Translucent Phase to the Dense Phase

Abstract We investigated the chemical evolutions of gas-phase and grain-surface species across the Taurus molecular cloud-1 (TMC-1) filament from the translucent phase to the dense phase. By comparing observations with modeling results from an up-to-date chemical network, we examined the conversion processes for the carbon-, oxygen-, nitrogen-, and sulfur-bearing species, i.e., from their initial atomic form to their main molecular reservoir form both in the gas phase and on the grain surface. The conversion processes were found to depend on the species and A V . The effect of initial carbon-to-oxygen elemental abundances ratio (C/O) by varying O on the chemistry was explored, and an initial carbon elemental abundance of 2.5 × 10−4 and a C/O ratio of 0.5 could best reproduce the abundances of most observed molecules at TMC-1 CP, where more than 90 molecules have been identified. Based on the TMC-1 condition, we predicted a varied grain ice composition during the evolutions of molecular clouds, with H2O ice as the dominant ice composition at A V > 4 mag, CO2 ice as the dominant ice composition at A V <4 mag, while CO ice severely decreased at A V around 4–5 mag.

Velocity Anisotropy in Self-gravitating Molecular Clouds. II. Observation

Abstract The view on velocity structures in molecular clouds and their relationship with magnetic fields (B field) has evolved during the past decade from almost no correlation to highly parallel. Our numerical simulations suggest a more nuanced picture: Depending on whether the self-gravity is dynamically dominant, the velocity field can be governed by either contraction (at high densities) or turbulence (at low densities), and their anisotropies will tend to be either perpendicular or parallel, respectively, to the B fields. High-density regions are always embedded in the low-density fore/background, so the velocity behaviors from lines of sight (LOSs) with high column densities will be a mixture of orthogonal anisotropies, which can be hard to interpret and necessitates zooming in onto certain LOS scales to better characterize localized behaviors. We tested and confirmed the above prediction with CO observations of the Taurus molecular cloud.

The width of Herschel filaments varies with distance

Context. Filamentary structures in nearby molecular clouds have been found to exhibit a characteristic width of 0.1 pc, as observed in dust emission. Understanding the origin of this universal width has become a topic of central importance in the study of molecular cloud structure and the early stages of star formation. Aims. We investigate how the recovered widths of filaments depend on the distance from the observer by using previously published results from the Herschel Gould Belt Survey. Methods. We obtained updated estimates on the distances to nearby molecular clouds observed with Herschel by using recent results based on 3D dust extinction mapping and Gaia. We examined the widths of filaments from individual clouds separately, as opposed to treating them as a single population. We used these per-cloud filament widths to search for signs of variation amongst the clouds of the previously published study. Results. We find a significant dependence of the mean per-cloud filament width with distance. The distribution of mean filament widths for nearby clouds is incompatible with that of farther away clouds. The mean per-cloud widths scale with distance approximately as 4−5 times the beam size. We examine the effects of resolution by performing a convergence study of a filament profile in the Herschel image of the Taurus Molecular Cloud. We find that resolution can severely affect the shapes of radial profiles over the observed range of distances. Conclusions. We conclude that the data are inconsistent with 0.1 pc being the universal characteristic width of filaments.

Gas phase Elemental abundances in Molecular cloudS (GEMS) V. Methanol in Taurus

Context. Methanol, one of the simplest complex organic molecules in the interstellar medium, has been shown to be present and extended in cold environments such as starless cores. Studying the physical conditions at which CH3OH starts its efficient formation is important to understand the development of molecular complexity in star-forming regions. Aims. We aim to study methanol emission across several starless cores and investigate the physical conditions at which methanol starts to be efficiently formed, as well as how the physical structure of the cores and their surrounding environment affect its distribution. Methods. Methanol and C18O emission lines at 3 mm have been observed with the IRAM 30 m telescope within the large programme Gas phase Elemental abundances in Molecular CloudS towards 66 positions across 12 starless cores in the Taurus Molecular Cloud. A non-LTE (local thermodynamic equilibrium) radiative transfer code was used to compute the column densities in all positions. We then used state-of-the-art chemical models to reproduce our observations. Results. We have computed N(CH3OH)/N(C18O) column density ratios for all the observed offsets, and the following two different behaviours can be recognised: the cores where the ratio peaks at the dust peak and the cores where the ratio peaks with a slight offset with respect to the dust peak (~10 000 AU). We suggest that the cause of this behaviour is the irradiation on the cores due to protostars nearby which accelerate energetic particles along their outflows. The chemical models, which do not take irradiation variations into account, can reproduce the overall observed column density of methanol fairly well, but they cannot reproduce the two different radial profiles observed. Conclusions. We confirm the substantial effect of the environment on the distribution of methanol in starless cores. We suggest that the clumpy medium generated by protostellar outflows might cause a more efficient penetration of the interstellar radiation field in the molecular cloud and have an impact on the distribution of methanol in starless cores. Additional experimental and theoretical work is needed to reproduce the distribution of methanol across starless cores.

Multi-scale Dust Polarization and Spiral-like Stokes-I Residual in the Class I Protostellar System TMC-1A

Abstract We have observed the Class I protostar TMC-1A in the Taurus molecular cloud using the Submillimeter Array (SMA) and the Atacama Large Millimeter/submillimeter Array (ALMA) in the linearly polarized 1.3 mm continuum emission at angular resolutions of ∼3″ and ∼0.3″, respectively. The ALMA observations also include CO, 13CO, and C18O J = 2−1 spectral lines. The SMA observations trace magnetic fields on the 1000 au scale, the directions of which are neither parallel nor perpendicular to the outflow direction. Applying the Davis–Chandrasekhar–Fermi method to the SMA polarization angle dispersion, we estimate a field strength in the TMC-1A envelope of 1–5 mG. It is consistent with the field strength needed to reduce the radial infall velocity to the observed value, which is substantially less than the local freefall velocity. The ALMA polarization observations consist of two distinct components—a central component and a north/south component. The central component shows polarization directions in the disk minor axis to be azimuthal, suggesting dust self-scattering in the TMC-1A disk. The north/south component is located along the outflow axis and the polarization directions are aligned with the outflow direction. We discuss possible origins of this polarization structure, including grain alignment by a toroidal magnetic field and mechanical alignment by the gaseous outflow. In addition, we discover a spiral-like residual in the total intensity (Stokes I) for the first time. The C18O emission suggests that material in the spiral-like structure is infalling at a speed that is 20% of the local Keplerian speed.

The Per-Tau Shell: A Giant Star-forming Spherical Shell Revealed by 3D Dust Observations

Abstract A major question in the field of star formation is how molecular clouds form out of the diffuse interstellar medium (ISM). Recent advances in 3D dust mapping are revolutionizing our view of the structure of the ISM. Using the highest-resolution 3D dust map to date, we explore the structure of a nearby star-forming region, which includes the well-known Perseus and Taurus molecular clouds. We reveal an extended near-spherical shell, 156 pc in diameter (hereafter called the “Per-Tau Shell”), in which the Perseus and Taurus clouds are embedded. We also find a large ring structure at the location of Taurus (hereafter called the “Tau Ring”). We discuss a formation scenario for the Per-Tau Shell, in which previous stellar and supernova feedback events formed a large expanding shell, where the swept-up ISM has condensed to form both the shell and the Perseus and Taurus molecular clouds within it. We present auxiliary observations of H i, Hα, 26Al, and X-rays that further support this scenario, and estimate the Per-Tau Shell’s age to be ≈6–22 Myrs. The Per-Tau shell offers the first 3D observational view of a phenomenon long-hypothesized theoretically, molecular cloud formation and star formation triggered by previous stellar and supernova feedback.

Evolutionary view through the starless cores in Taurus

Context. The chemical and physical evolution of starless and pre-stellar cores are of paramount importance to understanding the process of star formation. The Taurus Molecular Cloud cores TMC 1-C and TMC 1-CP share similar initial conditions and provide an excellent opportunity to understand the evolution of the pre-stellar core phase. Aims. We investigated the evolutionary stage of starless cores based on observations towards the prototypical dark cores TMC 1-C and TMC 1-CP. Methods. We mapped the prototypical dark cores TMC 1-C and TMC 1-CP in the CS 3 → 2, C34S 3 → 2, 13CS 2 → 1, DCN 1 → 0, DCN 2 → 1, DNC 1 → 0, DNC 2 → 1, DN13C 1 → 0, DN13C 2 → 1, N2H+ 1 → 0, and N2D+ 1 → 0 transitions. We performed a multi-transitional study of CS and its isotopologs, DCN, and DNC lines to characterize the physical and chemical properties of these cores. We studied their chemistry using the state-of-the-art gas-grain chemical code NAUTILUS and pseudo time-dependent models to determine their evolutionary stage. Results. The central nH volume density, the N2H+ column density, and the abundances of deuterated species are higher in TMC 1-C than in TMC 1-CP, yielding a higher N2H+ deuterium fraction in TMC 1-C, thus indicating a later evolutionary stage for TMC 1-C. The chemical modeling with pseudo time-dependent models and their radiative transfer are in agreement with this statement, allowing us to estimate a collapse timescale of ~1 Myr for TMC 1-C. Models with a younger collapse scenario or a collapse slowed down by a magnetic support are found to more closely reproduce the observations towards TMC 1-CP. Conclusions. Observational diagnostics seem to indicate that TMC 1-C is in a later evolutionary stage than TMC 1-CP, with a chemical age ~1 Myr. TMC 1-C shows signs of being an evolved core at the onset of star formation, while TMC 1-CP appears to be in an earlier evolutionary stage due to a more recent formation or, alternatively, a collapse slowed down by a magnetic support.

The evolution of carbon-chain chemistry from prestellar to protostellar cores in Taurus Molecular Cloud

AbstractThe discovery of abundant carbon-chain molecules in protostellar cores motivates the development of the warm carbon-chain chemistry. To understand the role of warm carbon-chain chemistry in star-forming regions, we studied C2H and c-C3H2 in 15 embedded protostars in the Taurus molecular cloud, whose evolutionary stages range from prestellar to Class I/II, using data from the Submillimeter Telescope (SMT). We calculated the excitation temperature, column density, and abundance of C2H and c-C3H2 in each source. We compared those properties with evolutionary indicators of the protostars. We also estimated the kinetic temperature using RADEX. Finally, we compared the abundance of C2H and c-C3H2 in our survey with that in the survey of protostellar cores in the Perseus molecular cloud. While we are unable to identify new WCCCs, our results suggest that the abundances of C2H and c-C3H2 could be an indicator to find WCCC candidates.

Machine Learning of Interstellar Chemical Inventories

Abstract The characterization of interstellar chemical inventories provides valuable insight into the chemical and physical processes in astrophysical sources. The discovery of new interstellar molecules becomes increasingly difficult as the number of viable species grows combinatorially, even when considering only the most thermodynamically stable. In this work, we present a novel approach for understanding and modeling interstellar chemical inventories by combining methodologies from cheminformatics and machine learning. Using multidimensional vector representations of molecules obtained through unsupervised machine learning, we show that identification of candidates for astrochemical study can be achieved through quantitative measures of chemical similarity in this vector space, highlighting molecules that are most similar to those already known in the interstellar medium. Furthermore, we show that simple, supervised learning regressors are capable of reproducing the abundances of entire chemical inventories, and predict the abundance of not-yet-seen molecules. As a proof-of-concept, we have developed and applied this discovery pipeline to the chemical inventory of a well-known dark molecular cloud, the Taurus Molecular Cloud 1, one of the most chemically rich regions of space known to date. In this paper, we discuss the implications and new insights machine learning explorations of chemical space can provide in astrochemistry.

Two-component Magnetic Field along the Line of Sight to the Perseus Molecular Cloud: Contribution of the Foreground Taurus Molecular Cloud

Abstract Optical stellar polarimetry in the Perseus molecular cloud direction is known to show a fully mixed bimodal distribution of position angles across the cloud. We study the Gaia trigonometric distances to each of these stars and reveal that the two components in position angles trace two different dust clouds along the line of sight. One component, which shows a polarization angle of −37.°6 ± 35.°2 and a higher polarization fraction of 2.0 ± 1.7 %, primarily traces the Perseus molecular cloud at a distance of 300 pc. The other component, which shows a polarization angle of +66.°8 ± 19.°1 and a lower polarization fraction of 0.8 ± 0.6 %, traces a foreground cloud at a distance of 150 pc. The foreground cloud is faint, with a maximum visual extinction of ≤1 mag. We identify that foreground cloud as the outer edge of the Taurus molecular cloud. Between the Perseus and Taurus molecular clouds, we identify a lower-density ellipsoidal dust cavity with a size of 100–160 pc. This dust cavity is located at l = 170°, b = −20°, and d = 240 pc, which corresponds to an HI shell generally associated with the Per OB2 association. The two-component polarization signature observed toward the Perseus molecular cloud can therefore be explained by a combination of the plane-of-sky orientations of the magnetic field both at the front and at the back of this dust cavity.

The Transition from Diffuse Molecular Gas to Molecular Cloud Material in Taurus

Abstract We study four lines of sight that probe the transition from diffuse molecular gas to molecular cloud material in Taurus. Measurements of atomic and molecular absorption are used to infer the distribution of species and the physical conditions toward stars behind the Taurus Molecular Cloud (TMC). New high-resolution spectra at visible and near-IR wavelengths of interstellar Ca ii, Ca i, K i, CH, CH+, C2, CN, and CO toward HD 28975 and HD 29647 are combined with data at visible wavelengths and published CO results from ultraviolet measurements for HD 27778 and HD 30122. Gas densities and temperatures are inferred from C2, CN, and CO excitation and CN chemistry. Our results for HD 29647 are noteworthy because the CO column density is 1018 cm−2 while C2 and CO excitation reveals a temperature of 10 K and a density of ∼1000 cm−3, more like conditions found in dark molecular clouds. Similar results arise from our chemical analysis for CN through reactions involving observations of CH, C2, and NH. Enhanced potassium depletion and a reduced CH/H2 column density ratio also suggest the presence of a dark cloud. The directions toward HD 27778 and HD 30122 probe molecule-rich diffuse clouds, which can be considered CO-dark gas, while the sight line toward HD 28975 represents an intermediate case. Maps of dust temperature help refine the description of the material along the four sight lines and provide an estimate of the distance between HD 29647 and a clump in the TMC. An appendix provides results for the direction toward HD 26571; this star also probes diffuse molecular gas.

Toward a 3D kinetic tomography of Taurus clouds. I. Linking neutral potassium and dust

Context. Gaia parallaxes and photometric measurements open a three-dimensional (3D) era for the Milky Way, including its interstellar (IS) matter. Three-dimensional Galactic dust distributions are constructed in various ways, based on Gaia data and photometric or spectroscopic surveys. Aims. The assignment of radial motions to IS dust structures seen in 3D, or 3D kinetic tomography, would be a valuable tool allowing one to connect the structures to emission lines of the associated gas, which are now measured at increasingly higher spectral and angular resolutions, and rich in information on physical and chemical processes. To this end, one of the potential techniques is to establish a link between dust clouds and Doppler velocities of absorption lines imprinted in stellar spectra by the gas associated with the dust. This requires a relatively close correlation between the absorber column and the dust opacity. We have investigated the link between the strength of interstellar K I absorption and the opacity of the dust in front of stars in the Taurus area, and we have tested the feasibility of assigning velocities to 3D dust clouds on the basis of K I absorption data. Methods. We have obtained high spectral resolution and high signal-to-noise spectra of 58 early-type stars in the direction of the Taurus, Perseus, and California molecular clouds. We have developed a new, dual interstellar and telluric profile-fitting technique to extract the interstellar K I λλ 7665, 7699 Å absorption lines from stellar spectra and applied it to the new data and to archived spectra of 58 additional targets. In parallel, we have updated 3D dust maps reconstructed through the inversion of individual stellar light extinctions. To do so, we supplemented the catalog of extinction estimates based on Gaia and 2MASS photometry with recently published extinction catalogs based on stellar spectroscopic surveys. We used the 3D map and the set of velocity components seen in absorption to assign radial velocities to the dust clouds distributed along their paths in the most consistent way. Results. We illustrate our profile-fitting technique and present the K I velocity structure of the dense ISM along the paths to all targets. As a validation test of the dust map, we show comparisons between distances to several reconstructed clouds with recent distance assignments based on different techniques. Target star extinctions estimated by integration in the 3D map are compared with their K I 7699 Å absorptions and the degree of correlation is found comparable to the one between the same K I line and the total hydrogen column for stars distributed over the sky that are part of a published high resolution survey. We show images of the updated dust distribution in a series of vertical planes in the Galactic longitude interval 150–182.5° and our estimated assignments of radial velocities to the opaque regions. Most clearly defined K I absorptions may be assigned to a dense dust cloud between the Sun and the target star. It appeared relatively straightforward to find a velocity pattern consistent will all absorptions and ensuring coherence between adjacent lines of sight, at the exception of a few weak lines. We compare our results with recent determinations of the velocities of several clouds and find good agreement. These results demonstrate that the extinction-K I relationship is tight enough to allow one to link the radial velocity of the K I lines to the dust clouds seen in 3D and that their combination may be a valuable tool in building a 3D kinetic structure of the dense ISM. We discuss limitations and perspectives for this technique.

Molecular dynamics reveals formation path of benzonitrile and other molecules in conditions relevant to the interstellar medium

Polycyclic aromatic hydrocarbons and polycyclic aromatic nitrogen heterocycles are believed to be widespread in different areas of the interstellar medium. However, the astronomical detection of specific aromatic molecules is extremely challenging. As a result, only a few aromatic molecules have been identified, and very little is known about how they are formed in different areas of the interstellar medium. Recently, McGuire et al. [Science 359, 202-205 (2018)] detected the simple aromatic molecule benzonitrile in Taurus Molecular Cloud-1. Although benzonitrile has been observed, the molecular mechanism for its formation is still unknown. In this study, we use quantum chemistry and ab initio molecular dynamics to model ionization processes of van der Waals clusters containing cyanoacetylene and acetylene molecules. We demonstrate computationally that the clusters' ionization leads to molecular formation. For pure cyanoacetylene clusters, we observe bond formation among two and three monomer units, whereas in mixed clusters, bond formation can also occur in up to four units. We show that the large amount of energy available to the system after ionization can lead to barrier crossing and the formation of complex molecules. Our study reveals the rich chemistry that is observed upon ionization of the clusters, with a wide variety of molecules being formed. Benzonitrile is among the observed molecules, and we study the potential energy path for its formation. These results also offer insights that can guide astronomers in their search for aromatic molecules in the interstellar medium.

Identifying PAHs in space

Astrochemistry Midinfrared spectroscopy has shown that polycyclic aromatic hydrocarbons (PAHs) are abundant in many astronomical objects, but this technique cannot determine which specific PAH molecules are present. Radio astronomy could provide individual identifications if the molecule is sufficiently abundant and has a large dipole moment, but PAHs are expected to produce large numbers of very weak lines. McGuire et al. performed a stacking and matched filter analysis to search for PAHs in radio observations of TMC-1, located within the interstellar Taurus Molecular Cloud. They identified emission from two isomers of the small PAH cyanonapthalene, two fused benzene rings with a CN group attached. Science , this issue p. [1265][1] [1]: /lookup/doi/10.1126/science.abb7535

Interstellar Detection of 2-cyanocyclopentadiene, C5H5CN, a Second Five-membered Ring toward TMC-1

Abstract Using radio observations with the Green Bank Telescope, evidence has now been found for a second five-membered ring in the dense cloud Taurus Molecular Cloud-1 (TMC-1). Based on additional observations of an ongoing, large-scale, high-sensitivity spectral line survey (GOTHAM) at centimeter wavelengths toward this source, we have used a combination of spectral stacking, Markov Chain Monte Carlo (MCMC), and matched filtering techniques to detect 2-cyanocyclopentadiene, a low-lying isomer of 1-cyanocyclopentadiene, which was recently discovered there by the same methods. The new observational data also yield a considerably improved detection significance for the more stable isomer and evidence for several individual transitions between 23–32 GHz. Through our MCMC analysis, we derive cospatial, total column densities of 8.3 × 1011 and 1.9 × 1011 cm−2 for 1- and 2-cyanocyclopentadiene, respectively, corresponding to a ratio of ∼4.4 favoring the former. The derived abundance ratios point toward a common formation pathway—most likely being cyanation of cyclopentadiene by analogy to benzonitrile.

Chemical compositions of five Planck cold clumps

Aims. Interstellar molecules form early in the evolutionary sequence of interstellar material that eventually forms stars and planets. To understand this evolutionary sequence, it is important to characterize the chemical composition of its first steps. Methods. In this paper, we present the result of a 2 and 3 mm survey of five cold clumps identified by the Planck mission. We carried out a radiative transfer analysis on the detected lines in order to put some constraints on the physical conditions within the cores and on the molecular column densities. We also performed chemical models to reproduce the observed abundances in each source using the gas-grain model Nautilus. Results. Twelve molecules were detected: H2CO, CS, SO, NO, HNO, HCO+, HCN, HNC, CN, CCH, CH3OH, and CO. Here, CCH is the only carbon chain we detected in two sources. Radiative transfer analyses of HCN, SO, CS, and CO were performed to constrain the physical conditions of each cloud with limited success. The sources have a density larger than 104 cm−3 and a temperature lower than 15 K. The derived species column densities are not very sensitive to the uncertainties in the physical conditions, within a factor of 2. The different sources seem to present significant chemical differences with species abundances spreading over one order of magnitude. The chemical composition of these clumps is poorer than the one of Taurus Molecular Cloud 1 Cyanopolyyne Peak (TMC-1 CP) cold core. Our chemical model reproduces the observational abundances and upper limits for 79–83% of the species in our sources. The ‘best’ times for our sources seem to be smaller than those of TMC-1, indicating that our sources may be less evolved and explaining the smaller abundances and the numerous non-detections. Also, CS and HCN are always overestimated by our models.