A. O. H. Olofsson

Low upper limits on the O2 abundance from the Odin satellite

For the first time, a search has been conducted in our Galaxy for the 119 GHz transition connecting to the ground state of O2, using the Odin satellite. Equipped with a sensitive 3 mm receiver (Tsy ...

A spectral line survey of Orion KL in the bands 486-492 and 541-577 GHz with the Odin satellite - II. Data analysis

Aims. We investigate the physical and chemical conditions in a typical star forming region, including an unbiased search for new molecules in a spectral region previously unobserved. Methods. Due to its proximity, the Orion KL region offers a unique laboratory of molecular astrophysics in a chemically rich, massive star forming region. Several ground-based spectral line surveys have been made, but due to the absorption by water and oxygen, the terrestrial atmosphere is completely opaque at frequencies around 487 and 557 GHz. To cover these frequencies we used the Odin satellite to perform a spectral line survey in the frequency ranges 486−492 GHz and 541−577 GHz, filling the gaps between previous spectral scans. Odin’s high main beam efficiency, ηmb = 0.9, and observations performed outside the atmosphere make our intensity scale very well determined. Results. We observed 280 spectral lines from 38 molecules including isotopologues, and, in addition, 64 unidentified lines. A few U-lines have interesting frequency coincidences such as ND and the anion SH − . The beam-averaged emission is dominated by CO, H2O, SO2 ,S O, 13 CO and CH3OH. Species with the largest number of lines are CH3OH, (CH3)2O, SO2, 13 CH3OH, CH3CN and NO. Six water lines are detected including the ground state rotational transition 11,0–10,1 of o-H2O, its isotopologues o-H 18 Oa nd o-H 17 O, the Hot Core tracing p-H2O transition 62,4–71,7 ,a nd the 2 0,2–11,1 transition of HDO. Other lines of special interest are the 10–0 0 transition of NH3 and its isotopologue 15 NH3. Isotopologue abundance ratios of D/H, 12 C/ 13 C, 32 S/ 34 S, 34 S/ 33 S, and 18 O/ 17 O are estimated. The temperatures, column densities and abundances in the various subregions are estimated, and we find very high gas-phase abundances of H2O, NH3 ,S O 2, SO, NO, and CH3OH. A comparison with the ice inventory of ISO sheds new light on the origin of the abundant gas-phase molecules.

The Odin satellite - II. Radiometer data processing and calibration

The radiometer on-board the Odin satellite comprises four different sub-mm receivers covering the 486-581 GHz frequency range and one fixed frequency 119 GHz receiver. Two auto-correlators and one ...

Nitrogen hydrides in interstellar gas - II. Analysis of Herschel/HIFI observations towards W49N and G10.6 − 0.4 (W31C)

As a part of the Herschel key programme PRISMAS, we have used the Herschel-HIFI instrument to observe interstellar nitrogen hydrides along the sight-lines towards eight high-mass star-forming regions in order to elucidate the production pathways leading to nitrogen-bearing species in diffuse gas. Here, we report observations towards W49N of the NH N = 1 - 0, J = 2 - 1, and J = 1 - 0, ortho-NH2 N_(Ka, K_c) J = 1_(1,1) 3/2 - 0_(0,0) 1/2, ortho-NH3 J_K = 1_0 - 0_0 and 2_0 - 1_0, para-NH3 J_K = 2_1 - 1_1 transitions, and unsuccessful searches for NH+. All detections show absorption by foreground material over a wide range of velocities, as well as absorption associated directly with the hot-core source itself. As in the previously published observations towards G10.6-0.4, the NH, NH2 and NH3 spectra towards W49N show strikingly similar and non-saturated absorption features. We decompose the absorption of the foreground material towards W49N into different velocity components in order to investigate whether the relative abundances vary among the velocity components, and, in addition, we re-analyse the absorption lines towards G10.6-0.4 in the same manner. Abundances, with respect to molecular hydrogen, in each velocity component are estimated using CH, which is found to correlate with H2 in the solar neighbourhood diffuse gas. The analysis points to a co-existence of the nitrogen hydrides in diffuse or translucent interstellar gas with a high molecular fraction. Towards both sources, we find that NH is always at least as abundant as both o-NH2 and o-NH3, in sharp contrast to previous results for dark clouds. We find relatively constant N(NH)/N(o-NH3) and N(o-NH2)/N(o-NH3) ratios with mean values of 3.2 and 1.9 towards W49N, and 5.4 and 2.2 towards G10.6-0.4, respectively. The mean abundance of o-NH4 is ~2x10^-9 towards both sources. The nitrogen hydrides also show linear correlations with CN and HNC towards both sources, and looser correlations with CH. The upper limits on the NH+ abundance indicate column densities < 2 - 14 % of N(NH), which is in contrast to the behaviour of the abundances of CH+ and OH+ relative to the values determined for the corresponding neutrals CH and OH. Surprisingly low values of the ammonia ortho-to-para ratio are found in both sources, ~0.5 - 0.7 +- 0.1, in the strongest absorption components. This result cannot be explained by current models as we had expected to find a value of unity or higher.

On the Ubiquity of Molecular Anions in the Dense Interstellar Medium

Results are presented from a survey for molecular anions in seven nearby Galactic star-forming cores and molecular clouds. The hydrocarbon anion C6H- is detected in all seven target sources, including four sources where no anions have been previously detected: L1172, L1389, L1495B, and TMC-1C. The C6H-/C6H column density ratio is greater than or similar to 1.0% in every source, with a mean value of 3.0% (and standard deviation 0.92%). Combined with previous detections, our results show that anions are ubiquitous in dense clouds wherever C6H is present. The C6H-/C6H ratio is found to show a positive correlation with molecular hydrogen number density, and with the apparent age of the cloud. We also report the first detection of C4H- in TMC-1 (at 4.8 sigma confidence), and derive an anion-to-neutral ratio C4H-/C4H = (1.2 +/- 0.4) x 10(-5)(= 0.0012% +/- 0.0004%). Such a low value compared with C6H- highlights the need for a revised radiative electron attachment rate for C4H. Chemical model calculations show that the observed C4H- could be produced as a result of reactions of oxygen atoms with C5H- and C6H-.

Herschel observations of the Herbig-Haro objects HH52-54

Context. The emission from Herbig-Haro objects and supersonic molecular outflows is understood as cooling radiation behind shocks, which are initiated by a (proto-)stellar wind or jet. Within a given object, one often observes both dissociative (J-type) and non-dissociative (C-type) shocks, owing to the collective effects of internally varying shock velocities. Aims. We aim at the observational estimation of the relative contribution to the cooling by CO and H2O, as this provides decisive information for understanding the oxygen chemistry behind interstellar shock waves. Methods. The high sensitivity of HIFI, in combination with its high spectral resolution capability, allowed us to trace the H2O outflow wings at an unprecedented signal-to-noise ratio. From the observation of spectrally resolved H2O and CO lines in the HH5254 system, both from space and from the ground, we arrived at the spatial and velocity distribution of the molecular outflow gas. Solving the statistical equilibrium and non-LTE radiative transfer equations provides us with estimates of the physical parameters of this gas, including the cooling rate ratios of the species. The radiative transfer is based on an accelerated lambda iteration code, where we use the fact that variable shock strengths, distributed along the front, are naturally implied by a curved surface. Results. Based on observations of CO and H2O spectral lines, we conclude that the emission is confined to the HH54 region. The quantitative analysis of our observations favours a ratio of the CO-to-H2O-cooling-rate 1. Formally, we derived the ratio Λ(CO)/Λ(o-H2O) = 10, which is in good agreement with earlier determination of 7 based on ISO-LWS observations. From the bestfit model to the CO emission, we arrive at an H2O abundance close to 1 × 10−5. The line profiles exhibit two components, one that is triangular and another that is a superposed, additional feature. This additional feature is likely to find its origin in a region that is smaller than the beam where the ortho-water abundance is smaller than in the quiescent gas. Conclusions. Comparison with recent shock models indicate that a planar shock cannot easily explain the observed line strengths and triangular line profiles. We conclude that the geometry can play an important role. Although abundances support a scenario where J-type shocks are present, higher cooling rate ratios are derived than predicted by these types of shocks.

A spectral line survey of Orion KL in the bands 486–492 and 541–577 GHz with the Odin satellite - I. The observational data

Aims. Spectral line surveys are useful since they allow identification of new molecules and new lines in uniformly calibrated data sets. The subsequent multi-transition analysis will provide improved knowledge of molecular abundances, cloud temperatures and densities, and may also reveal previously unsuspected blends of molecular lines, which otherwise may lead to erroneous conclusions. Nonetheless, large portions of the sub-millimetre spectral regime remain unexplored due to severe absorptions by H2 Oa nd O 2 in the terrestrial atmosphere. The purpose of the measurements presented here is to cover wavelength regions at and around 0.55 mm – regions largely unobservable from the ground. Methods. Using the Odin astronomy/aeronomy satellite, we performed the first spectral survey of the Orion KL molecular cloud core in the bands 486–492 and 541–576 GHz with rather uniform sensitivity (22–25 mK baseline noise). Odin’s 1.1 m size telescope, equipped with four cryo-cooled tuneable mixers connected to broad band spectrometers, was used in a satellite position-switching mode. Two mixers simultaneously observed different 1.1 GHz bands using frequency steps of 0.5 GHz (25 h each). An on-source integration time of 20 h was achieved for most bands. The entire campaign consumed ∼1100 orbits, each containing one hour of serviceable astro-observation. Results. We identified 280 spectral lines from 38 known interstellar molecules (including isotopologues) having intensities in the range 80 to 0.05 K. An additional 64 weak lines remain unidentified. Apart from the ground state rotational 11,0–10,1 transitions of ortho-H2O, H 18 Oa nd H 17 O, the high energy 62,4–71,7 line of para-H2 O( Eu = 867 K) and the HDO(20,2–11,1) line have been observed, as well as the 10–01 lines from NH3 and its rare isotopologue 15 NH3. We suggest assignments for some unidentified features, notably the new interstellar molecules ND and SH − . Severe blends have been detected in the line wings of the H 18 O, H 17 Oa nd 13 CO lines changing the true linewidths of the outflow emission.

Upper limits to interstellar NH+ and para-NH2− abundances - Herschel-HIFI observations towards Sgr B2 (M) and G10.6−0.4 (W31C)

The understanding of interstellar nitrogen chemistry has improved significantly with recent results from the Herschel Space Observatory. To set even better constraints, we report here on deep searches for the NH^+ ground state rotational transition J = 1.5−0.5 of the ^2Π_(1/2) lower spin ladder, with fine-structure transitions at 1013 and 1019 GHz, and the para-NH_2^− 1_(1,1)−0_(0,0) rotational transition at 934 GHz towards Sgr B2 (M) and G10.6−0.4 (W31C) using the Herschel Heterodyne Instrument for the Far-Infrared (HIFI). No clear detections of NH^+ are made and the derived upper limits relative to the total number of hydrogen nuclei are ≲2 × 10^(-12) and ≲7 × 10^(-13) in the Sgr B2 (M) molecular envelope and in the G10.6−0.4 molecular cloud, respectively. The searches are, however, complicated by the fact that the 1 013 GHz transition lies only −2.5 km s^(-1) from a CH_2NH line, which is seen in absorption in Sgr B2 (M), and that the hyperfine structure components in the 1019 GHz transition are spread over 134 km s^(-1). Searches for the so far undetected NH_2^− anion turned out to be unfruitful towards G10.6−0.4, while the para-NH_2^− 1_(1,1)−0_(0,0) transition was tentatively detected towards Sgr B2 (M) at a velocity of 19 km s^(-1). Assuming that the absorption occurs at the nominal source velocity of +64 km s^(-1), the rest frequency would be 933.996 GHz, offset by 141 MHz from our estimated value. Using this feature as an upper limit, we found N(p-NH_2^−) ≲4 × 10^(11) cm^(-2), which implies an abundance of ≲8 × 10^(-13) in the Sgr B2 (M) molecular envelope. The upper limits for both species in the diffuse line-of-sight gas are less than 0.1 to 2% of the values found for NH, NH_2, and NH_3 towards both sources, and the abundance limits are ≲2−4 × 10^(-11). An updated pseudo time-dependent chemical model with constant physical conditions, including both gas-phase and surface chemistry, predicts an NH^+ abundance a few times lower than our present upper limits in diffuse gas and under typical Sgr B2 (M) envelope conditions. The NH_2^− abundance is predicted to be several orders of magnitudes lower than our observed limits, hence not supporting our tentative detection. Thus, while NH_2^− may be very difficult to detect in interstellar space, it could, on the other hand, be possible to detect NH^+ in regions where the ionisation rates of H_2 and N are greatly enhanced.

Water and ammonia abundances in S140 with the Odin satellite

We investigate the effect of the physical environment on water and ammonia abundances across the S140 photodissociation region (PDR) with an embedded outflow. We used the Odin satellite to obtain strip maps of the ground-state rotational transitions of ortho-water and ortho-ammonia, as well as CO(5-4) and 13co(5-4) across the PDR, and H_2^18O in the central position. A physi-chemical inhomogeneous PDR model was used to compute the temperature and abundance distributions for water, ammonia, and CO. A multi-zone escape probability method then calculated the level populations and intensity distributions. These results are compared to a homogeneous model computed with an enhanced version of the RADEX code. H_2O, NH_3, and ^13CO show emission from an extended PDR with a narrow line width of ~3 km/s. Like CO, the water line profile is dominated by outflow emission, but mainly in the red wing. Even though CO shows strong self-absorption, no signs of self-absorption are seen in the water line. The H_2^18O molecule is not detected. The PDR model suggests that the water emission arises mainly from the surfaces of optically thick, high-density clumps with n(H_2)>10^6 cm^-3 and a clump water abundance, with respect to H_2, of 5*10^-8. The mean water abundance in the PDR is 5*10^-9 and between ~4*10^-8 - 4*10^-7 in the outflow derived from a simple two-level approximation. The RADEX model points to a somewhat higher average PDR water abundance of 1*10^-8. At low temperatures deep in the cloud, the water emission is weaker, likely due to adsorption onto dust grains, while ammonia is still abundant. Ammonia is also observed in the extended clumpy PDR, likely from the same high density and warm clumps as water. The average ammonia abundance is about the same as for water: 4*10^-9 and 8*10^-9 given by the PDR model and RADEX, respectively. The differences between the models most likely arise from uncertainties in density,beam-filling, and volume-filling of clumps. The similarity of water and ammonia PDR emission is also seen in the almost identical line profiles observed close to the bright rim. Around the central position, ammonia also shows some outflow emission, although weaker than water in the red wing. Predictions of the H_2O 1(1,0)-1(0,1) and 1(1,1)-0(0,0) antenna temperatures across the PDR are estimated with our PDR model for the forthcoming observations with the Herschel Space Observatory.

On the accretion process in a high-mass star forming region - A multitransitional THz Herschel-HIFI study of ammonia toward G34.26+0.15

[Abridged] Our aim is to explore the gas dynamics and the accretion process in the early phase of high-mass star formation. The inward motion of molecular gas in the massive star forming region G34.26+0.15 is investigated by using high-resolution profiles of seven transitions of ammonia at THz frequencies observed with Herschel-HIFI. The shapes and intensities of these lines are interpreted in terms of radiative transfer models of a spherical, collapsing molecular envelope. An accelerated Lambda Iteration (ALI) method is used to compute the models. The seven ammonia lines show mixed absorption and emission with inverse P-Cygni-type profiles that suggest infall onto the central source. A trend toward absorption at increasingly higher velocities for higher excitation transitions is clearly seen in the line profiles. The