Abstract
Evidence is mounting that the small bodies of our Solar system, such as comets and asteroids, have at least partially inherited their chemical composition from the first phases of the Solar ...system formation. It then appears that the molecular complexity of these small bodies is most likely related to the earliest stages of star formation. It is therefore important to characterize and to understand how the chemical evolution changes with solar-type protostellar evolution. We present here the Large Program ‘Astrochemical Surveys At IRAM’ (ASAI). Its goal is to carry out unbiased millimetre line surveys between 80 and 272 GHz of a sample of 10 template sources, which fully cover the first stages of the formation process of solar-type stars, from pre-stellar cores to the late protostellar phase. In this paper, we present an overview of the surveys and results obtained from the analysis of the 3 mm band observations. The number of detected main isotopic species barely varies with the evolutionary stage and is found to be very similar to that of massive star-forming regions. The molecular content in O- and C-bearing species allows us to define two chemical classes of envelopes, whose composition is dominated by either (a) a rich content in O-rich complex organic molecules, associated with hot corino sources, or (b) a rich content in hydrocarbons, typical of warm carbon-chain chemistry sources. Overall, a high chemical richness is found to be present already in the initial phases of solar-type star formation.
We investigate the presence of complex organic molecules (COMs) in strongly UV-irradiated interstellar molecular gas. We have carried out a complete millimetre (mm) line survey using the IRAM 30 m ...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′′ resolution maps of the H2CO JKa,Kc = 51,5 → 41,4 and C18O J = 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, H213CO, 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. The inferred column densities are in the 1011–1013 cm-2 range. 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-thermodynamic equilibrium 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). The non-detection of HDCO towards the PDR edge is consistent with the minor role of pure gas-phase deuteration at very high temperatures. We find a HCO/H2CO/CH3OH ≃ 1/5/3 abundance ratio. These ratios are different from those inferred in hot cores and shocks. Taking into account the elevated gas and dust temperatures at the edge of the Bar (mostly mantle-free grains), we suggest the following scenarios for the formation of COMs: (i) hot gas-phase reactions not included in current models; (ii) warm grain-surface chemistry; or (iii) the PDR dynamics is such that COMs or precursors formed in cold icy grains deeper inside the molecular cloud desorb and advect into the PDR.
During their late pulsating phase, AGB stars expel most of their mass in the form of massive dusty envelopes, an event that largely controls the composition of interstellar matter. The envelopes, ...however, are distant and opaque to visible and NIR radiation: their structure remains poorly known and the mass-loss process poorly understood. Millimeter-wave interferometry, which combines the advantages of longer wavelength, high angular resolution and very high spectral resolution is the optimal investigative tool for this purpose. Mm waves pass through dust with almost no attenuation. Their spectrum is rich in molecular lines and hosts the fundamental lines of the ubiquitous CO molecule, allowing a tomographic reconstruction of the envelope structure. The circumstellar envelope IRC +10 216 and its central star, the C-rich TP-AGB star closest to the Sun, are the best objects for such an investigation. Two years ago, we reported the first detailed study of the CO(2–1) line emission in that envelope, made with the IRAM 30-m telescope. It revealed a series of dense gas shells, expanding at a uniform radial velocity. The limited resolution of the telescope (HPBW 11″) did not allow us to resolve the shell structure. We now report much higher angular resolution observations of CO(2–1), CO(1–0), CN(2–1) and C4H(24–23) made with the SMA, PdB and ALMA interferometers (with synthesized half-power beamwidths of 3″, 1″ and 0.3″, respectively). Although the envelope appears much more intricate at high resolution than with an 11″ beam, its prevailing structure remains a pattern of thin, nearly concentric shells. The average separation between the brightest CO shells is 16″ in the outer envelope, where it appears remarkably constant. Closer to the star (<40″), the shell pattern is denser and less regular, showing intermediary arcs. Outside the small (r< 0.3′′) dust formation zone, the gas appears to expand radially at a constant velocity, 14.5 km s-1, with small turbulent motions. Based on that property, we have reconstructed the 3D structure of the outer envelope and have derived the gas temperature and density radial profiles in the inner (r< 25′′) envelope. The shell-intershell density contrast is found to be typically 3. The over-dense shells have spherical or slightly oblate shapes and typically extend over a few steradians, implying isotropic mass loss. The regular spacing of shells in the outer envelope supports the model of a binary star system with a period of 700 yr and a near face-on elliptical orbit. The companion fly-by triggers enhanced episodes of mass loss near periastron. The densification of the shell pattern observed in the central part of the envelope suggests a more complex scenario for the last few thousand years.
The interpretation of molecular line surveys at millimetre and submillimetre wavelengths towards astrophysical objects requires a good knowledge of the spectroscopy of the molecules present in the ...gas phase, and of the basic collisional processes between them and molecular and atomic Hydrogen, Helium, and electrons. From the chemical point of view, the modeling of these data requires a large amount of laboratory information concerning the reactivity of these species in the gas phase and in the ice mantles of dust grains. In this contribution I will focus on the problems to interpret line surveys and I will discuss how we will have to proceed in the new era opened by ALMA where sensitivity and angular resolution will surpass by nearly a factor 10 those of existing facilities, and where the spectral confusion limit could be reached towards many sources.
Abstract
The shock L1157-B1 driven by the low-mass protostar L1157-mm is a unique environment to investigate the chemical enrichment due to molecules released from dust grains. IRAM-30m and Plateau ...de Bure Interferometer observations allow a census of Si-bearing molecules in L1157-B1. We detect SiO and its isotopologues and, for the first time in a shock, SiS. The strong gradient of the SiO/SiS abundance ratio across the shock (from ≥180 to ∼25) points to a different chemical origin of the two species. SiO peaks where the jet impacts the cavity walls (SiO/H2 ∼ 10−6), indicating that SiO is directly released from grains or rapidly formed from released Si in the strong shock occurring at this location. In contrast, SiS is only detected at the head of the cavity opened by previous ejection events (SiS/H2 ∼ 2 × 10−8). This suggests that SiS is not directly released from the grain cores but instead should be formed through slow gas-phase processes using part of the released silicon. This finding shows that Si-bearing molecules can be useful to distinguish regions where grains or gas-phase chemistry dominates.
We report the detection in space of a new molecular species that has been characterized spectroscopically and fully identified from astrophysical data. The observations were carried out with the IRAM ...30 m telescope. The molecule is ubiquitous as its transition has been found in cold molecular clouds, prestellar cores, and shocks. However, it is not found in the hot cores of Orion-KL and in the carbon-rich evolved star IRC+10216. Three rotational transitions in perfect harmonic relation have been identified in the prestellar core B1b. The molecule has a 1 electronic ground state and its transition presents the hyperfine structure characteristic of a molecule containing a nucleus with spin 1. A careful analysis of possible carriers shows that the best candidate is NS+. The derived rotational constant agrees within 0.3%-0.7% with ab initio calculations. NS+ was also produced in the laboratory to unambiguously validate the astrophysical assignment. The observed rotational frequencies and determined molecular constants confirm the discovery of the nitrogen sulfide cation in space. The chemistry of NS+ and related nitrogen-bearing species has been analyzed by means of a time-dependent gas-phase model. The model reproduces well the observed NS/NS+ abundance ratio, in the range 30-50, and indicates that NS+ is formed by reactions of the neutral atoms N and S with the cations SH+ and NH+, respectively.
Aims. The HIFI instrument onboard Herschel has allowed high spectral resolution and sensitive observations of ground-state transitions of three molecular ions: the methylidyne cation CH+, its ...isotopologue 13CH+, and sulfanylium SH+. Because of their unique chemical properties, a comparative analysis of these cations provides essential clues to the link between the chemistry and dynamics of the diffuse interstellar medium. Methods. The CH+, 13CH+, and SH+ lines are observed in absorption towards the distant high-mass star-forming regions (SFRs) DR21(OH), G34.3+0.1, W31C, W33A, W49N, and W51, and towards two sources close to the Galactic centre, SgrB2(N) and SgrA*+50. All sight lines sample the diffuse interstellar matter along pathlengths of several kiloparsecs across the Galactic Plane. In order to compare the velocity structure of each species, the observed line profiles were deconvolved from the hyperfine structure of the SH+ transition and the CH+, 13CH+, and SH+ spectra were independently decomposed into Gaussian velocity components. To analyse the chemical composition of the foreground gas, all spectra were divided, in a second step, into velocity intervals over which the CH+, 13CH+, and SH+ column densities and abundances were derived. Results. SH+ is detected along all observed lines of sight, with a velocity structure close to that of CH+ and 13CH+. The linewidth distributions of the CH+, SH+, and 13CH+ Gaussian components are found to be similar. These distributions have the same mean (⟨Δυ⟩ ~ 4.2 km s-1) and standard deviation (σ(Δυ) ~ 1.5 km s-1). This mean value is also close to that of the linewidth distribution of the CH+ visible transitions detected in the solar neighbourhood. We show that the lack of absorption components narrower than 2 km s-1 is not an artefact caused by noise: the CH+, 13CH+, and SH+ line profiles are therefore statistically broader than those of most species detected in absorption in diffuse interstellar gas (e.g. HCO+, CH, or CN). The SH+/CH+ column density ratio observed in the components located away from the Galactic centre spans two orders of magnitude and correlates with the CH+ abundance. Conversely, the ratio observed in the components close to the Galactic centre varies over less than one order of magnitude with no apparent correlation with the CH+ abundance. The observed dynamical and chemical properties of SH+ and CH+ are proposed to trace the ubiquitous process of turbulent dissipation, in shocks or shears, in the diffuse ISM and the specific environment of the Galactic centre regions.
ABSTRACT In recent years, ultra-low temperature chemical kinetic experiments have demonstrated that some gas-phase reactions are much faster than was previously thought. One example is the reaction ...between OH and CH3OH, which has recently been found to be accelerated at low temperatures yielding CH3O as its main product. This finding raised the question of whether or not the CH3O observed in the dense core Barnard 1b could be formed by the gas-phase reaction of CH3OH and OH. Several chemical models including this reaction and grain-surface processes have been developed to explain the observed abundance of CH3O, but they have met with little success. Here, we report for the first time the rate coefficients for the gas-phase reaction of OH and CH3OH down to a temperature of 22 K, which is very close to the temperature in cold interstellar clouds. Two independent experimental set-ups based on the supersonic gas expansion technique coupled to the pulsed laser photolysis laser-induced fluorescence technique were used to determine the rate coefficients in the temperature range 22-64 K. The temperature dependence obtained in this work can be expressed as k(22-64 K) = ( 3.6 0.1 ) × 10 − 12 ( T / 300 K ) − ( 1.0 0.2 ) cm3 molecule−1 s−1. Implementing this expression in a chemical model of a cold, dense cloud results in CH3O/CH3OH abundance ratios similar to or slightly lower than the value of ∼3 × 10−3 observed in Barnard 1b. This finding confirms that the gas-phase reaction between OH and CH3OH is an important contributor to the formation of interstellar CH3O. The role of grain-surface processes in the formation of CH3O, although it cannot be fully neglected, remains controversial.
Thermally-pulsating AGB stars provide three-fourths of the matter returned to the interstellar medium. The mass and chemical composition of their ejecta largely control the chemical evolution of ...galaxies. Yet, both the mass loss process and the gas chemical composition remain poorly understood. We present maps of the extended 12CO and 13CO emissions in IRC+10216, the envelope of CW Leo, the high mass loss star the closest to the Sun. IRC+10216 is nearly spherical and expands radially with a velocity of 14.5 km s-1. The observations were made On-the-Fly with the IRAM 30 m telescope; their sensibility, calibration, and angular resolution are far higher than all previous studies. The telescope resolution at λ = 1.3 mm (11′′ HPBW) corresponds to an expansion time of 500 yr. The CO emission consists of a centrally peaked pedestal and a series of bright, nearly spherical shells. It peaks on CW Leo and remains relatively strong up to rphot = 180′′. Further out the emission becomes very weak and vanishes as CO gets photodissociated. As CO is the best tracer of the gas up to rphot, the maps show the mass loss history in the last 8000 yr. The bright CO shells denote over-dense regions. They show that the mass loss process is highly variable on timescales of hundreds of years. The new data, however, do not support previous claims of a strong decrease of the average mass loss in the last few thousand years. The over-dense shells are not perfectly concentric and extend farther to the N-NW. The typical shell separation is 800−1000 yr in the middle of the envelope, but seems to increase outwards. The shell-intershell brightness contrast is ≥3. All those key features can be accounted for if CW Leo has a companion star with a period ≃800 yr that increases the mass loss rate when it comes close to periastron. Higher angular resolution observations are needed to fully resolve the dense shells and measure the density contrast. The latter plays an essential role in our understanding of the envelope chemistry.
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