J. Cernicharo

Atmospheric transmission at microwaves (ATM): an improved model for millimeter/submillimeter applications

We present a model of the longwave atmospheric spectrum that improves in many respects widely used older models such as the microwave propagation model (MPM), since it is based on broadband measurements and calculations. According to our data, the model is fully applicable from 0 to 2 THz while including lines up to 10 THz. Its primary goal is to simulate the millimeter/submillimeter region accessible from the ground (frequencies up to /spl sim/2 THz at most, with a few windows between 1 and 2 THz accessible only under exceptional conditions at very dry sites). Line-by-line calculations of the absorption are performed using a line database generated from the latest available spectroscopic constants for all relevant atmospheric species. The collisional line widths are obtained from published laboratory data. The excess of absorption in the longwave range that cannot be explained by the line spectrum is modeled by introducing two different continuum-like terms based on FTS measurements between 170 and 1100 GHz: collision-induced absorption of the dry atmosphere due to transient dipoles in symmetric molecules (N/sub 2/ and O/sub 2/) and continuum-like water vapor opacity. All H/sub 2/O lines up to 10 THz are included in order to correctly account for the entire H/sub 2/O far-wing opacity below 2 THz for a given line-shape. Hence, this contribution does not need to be part of a pseudocontinuum term below that frequency cutoff (still necessary, as shown in this paper) in contrast to other models used to date. Phase delays near H/sub 2/O and O/sub 2/ resonances are also important for ground-based astronomy since they affect interferometric phase. The frequency-dependent dispersive phase delay function is formally related to the absorption line shape via the Kramers-Kronig dispersion theory, and this relation has been used for modeling those delays. Precise calculations of phase delays are essential for the future Atacama large millimeter array (ALMA) project. A software package called atmospheric transmission at microwaves (ATM) has been developed to provide the radioastronomy and aeronomy communities with an updated tool to compute the atmospheric spectrum in clear-sky conditions for various scientific applications. We use this model to provide detailed simulations of atmospheric transmission and phase dispersion for several sites suitable for submillimeter astronomy.

In-orbit performance of Herschel-HIFI

Aims. In this paper the calibration and in-orbit performance of the Heterodyne Instrument for the Far-Infrared (HIFI) is described. Methods. The calibration of HIFI is based on a combination of ground and in-flight tests. Dedicated ground tests to determine those instrument parameters that can only be measured accurately using controlled laboratory stimuli were carried out in the instrument level test (ILT) campaign. Special in-flight tests during the commissioning phase (CoP) and performance verification (PV) allowed the determination of the remaining instrument parameters. The various instrument observing modes, as specified in astronomical observation templates (AOTs), were validated in parallel during PV by observing selected celestial sources. Results. The initial calibration and in-orbit performance of HIFI has been established. A first estimate of the calibration budget is given. The overall in-flight instrument performance agrees with the original specification. Issues remain at only a few frequencies.

Water in Star-forming Regions with the Herschel Space Observatory (WISH). I. Overview of Key Program and First Results

Water In Star-forming regions with Herschel (WISH) is a key program on the Herschel Space Observatory designed to probe the physical and chemical structures of young stellar objects using water and related molecules and to follow the water abundance from collapsing clouds to planet-forming disks. About 80 sources are targeted, covering a wide ranee of luminosities-from low ( 10(5) L-circle dot)-and a wide range of evolutionary stages-from cold prestellar cores to warm protostellar envelopes and outflows to disks around young stars. Both the HIFI and PACS instruments are used to observe a variety of lines of H2O, (H2O)-O-18 and chemically related species at the source position and in small maps around the protostars and selected outflow positions. In addition, high-frequency lines of CO, (CO)-C-13, and (CO)-O-18 are obtained with Herschel and are complemented by ground-based observations of dust continuum, HDO, CO and its isotopologs, and other molecules to ensure a self-consistent data set for analysis. An overview of the scientific motivation and observational strategy of the program is given, together with the modeling approach and analysis tools that have been developed. Initial science results are presented. These include a lack of water in cold gas at abundances that are lower than most predictions, strong water emission from shocks in protostellar environments, the importance of UV radiation in heating the gas along outflow walls across the full range of luminosities, and surprisingly widespread detection of the chemically related hydrides OH+ and H2O+ in outflows and foreground gas. Quantitative estimates of the energy budget indicate that H2O is generally not the dominant coolant in the warm dense gas associated with protostars. Very deep limits on the cold gaseous water reservoir in the outer regions of protoplanetary disks are obtained that have profound implications for our understanding of grain growth and mixing in disks.

Laboratory and Astronomical Detection of the Negative Molecular Ion C3N

The negative molecular ion C3N− has been detected at millimeter wavelengths in a low-pressure laboratory discharge, and then with frequencies derived from the laboratory data in the molecular envelope of IRC+10216. Spectroscopic constants derived from laboratory measurements of 12 transitions between 97 and 378 GHz allow the rotational spectrum to be calculated well into the submillimeter-wave band to 0.03 km s−1 or better in equivalent radial velocity. Four transitions of C3N− were detected in IRC+10216 with the IRAM 30 m telescope at precisely the frequencies calculated from the laboratory measurements. The column density of C3N− is 0.5% that of C3N, or approximately 20 times greater than that of C4H− relative to C4H. The C3N− abundance in IRC+10216 is compared with a chemical model calculation by Petrie & Herbst. An upper limit in TMC-1 for C3N− relative to C3N (<0.8%) and a limit for C4H− relative to C4H (<0.004%) that is 5 times lower than that found in IRC+10216, were obtained from observations with the NRAO 100 m Green Bank Telescope (GBT). The fairly high concentration of C3N− achieved in the laboratory implies that other molecular anions containing the CN group may be within reach.

Astonomical and laboratory detection of the SiC radical

Laboratory and space observations of a number of mm-wave rotation lines in the previously unobserved 3Pi electronic ground state of the SiC radical are discussed. Laboratory-derived frequencies, accurate to better than 0.1 ppm, are used to obtain a highly precise determination of the fine structure, rotational, centrifugal distortion, and Lambda-doubling constants of the SiC ground state. It is found that SiC is appreciably extended toward IRC+10216, with a diameter of at least 54 arcsec. 11 refs.

Detection of C5N− and Vibrationally Excited C6H in IRC +10216*

We report the detection in the envelope of the C-rich star IRC +10216 of four series of lines with harmonically related frequencies: B1389, B1390, B1394 and B1401. The four series must arise from linear molecules with mass and size close to those of C6H and C5N. Three of the series have half-integer rotational quantum numbers; we assign them to the 2Delta and 2Sigma vibronic states of C6H in its lowest (v_11) bending mode. The fourth series, B1389, has integer J with no evidence of fine or hyperfine structure; it has a rotational constant of 1388.860(2) MHz and a centrifugal distortion constant of 33(1) Hz; it is almost certainly the C5N- anion.

Are PAHs precursors of small hydrocarbons in Photo-Dissociation Regions? The Horsehead case

We present maps at high spatial and spectral resolution in emission lines of CCH, c-C3H2, C4H, 12CO and C18O of the edge of the Horsehead nebula obtained with the IRAM Plateau de Bure Interferometer (PdBI). The edge of the Horsehead nebula is a one-dimensional Photo-Dissociation Region (PDR) viewed almost edge-on. All hydrocarbons are detected at high signal-to-noise ratio in the PDR where intense emission is seen both in the H2 ro-vibrational lines and in the PAH mid-infrared bands. C18O peaks farther away from the cloud edge. Our observations demonstrate that CCH, c-C3H2 and C4H are present in UV-irradiated molecular gas, with abundances nearly as high as in dense, well-shielded molecular cores. PDR models i) need a large density gradient at the PDR edge to correctly reproduce the offset between the hydrocarbons and H2 peaks; and ii) fail to reproduce the hydrocarbon abundances. We propose that a new formation path of carbon chains, in addition to gas phase chemistry, should be considered in PDRs: because of intense UV-irradiation, large aromatic molecules and small carbon grains may fragment and feed the interstellar medium with small carbon clusters and molecules in significant amounts.

Astronomical identification of CN-, the smallest observed molecular anion

We present the first astronomical detection of a diatomic negative ion, the cyanide anion CN-, as well as quantum mechanical calculations of the excitation of this anion through collisions with para-H2. CN- is identified through the observation of the J = 2-1 and J = 3-2 rotational transitions in the C-star envelope IRC +10216 with the IRAM 30-m telescope. The U-shaped line profiles indicate that CN-, like the large anion C6H-, is formed in the outer regions of the envelope. Chemical and excitation model calculations suggest that this species forms from the reaction of large carbon anions with N atoms, rather than from the radiative attachment of an electron to CN, as is the case for large molecular anions. The unexpectedly large abundance derived for CN-, 0.25 % relative to CN, makes likely its detection in other astronomical sources. A parallel search for the small anion C2H- remains so far unconclusive, despite the previous tentative identification of the J = 1-0 rotational transition. The abundance of C2H- in IRC +10216 is found to be vanishingly small, < 0.0014 % relative to C2H.

Interstellar OH+, H2O+ and H3O+ along the sight-line to G10.6–0.4

We report the detection of absorption lines by the reactive ions OH + ,H 2O + and H3O + along the line of sight to the submillimeter continuum source G10.6−0.4 (W31C). We used the Herschel HIFI instrument in dual beam switch mode to observe the ground state rotational transitions of OH + at 971 GHz, H2O + at 1115 and 607 GHz, and H3O + at 984 GHz. The resultant spectra show deep absorption over a broad velocity range that originates in the interstellar matter along the line of sight to G10.6−0.4 as well as in the molecular gas directly associated with that source. The OH + spectrum reaches saturation over most velocities corresponding to the foreground gas, while the opacity of the H2O + lines remains lower than 1 in the same velocity range, and the H3O + line shows only weak absorption. For LSR velocities between 7 and 50 kms −1 we estimate total column densities of N(OH + ) ≥ 2.5 × 10 14 cm −2 , N(H2O + ) ∼6 × 10 13 cm −2 and N(H3O + ) ∼4.0 × 10 13 cm −2 . These detections confirm the role of O + and OH + in initiating the oxygen chemistry in diffuse molecular gas and strengthen our understanding of the gas phase production of water. The high ratio of the OH + by the H2O + column density implies that these species predominantly trace low-density gas with a small fraction of

Astronomical detection of H2CCC

H2CCC, an isomer of the widely distributed interstellar ring C3H2, has been detected in TMC-1 and possibly IRC + 10216 with the IRAM 30 m telescope, following a recent laboratory determination of the rotational spectrum of this new type of highly polar carbon chain. The rotational temperature of H2CCC in TMC-1, like that of other highly polar molecules in this source, is very low: 4-6 K; the column density is also fairly low: (2.5 + or - 0.5) x 10 to the 12th/sq cm, slightly more than 1 percent that of the cyclic isomer. 16 refs.