A. Hjalmarson

The Odin satellite - I. Radiometer design and test

The Sub-millimetre and Millimetre Radiometer (SMR) is the main instrument on the Swedish, Canadian, Finnish and French spacecraft Odin. It consists of a 1.1 metre diameter telescope with four tuneable heterodyne receivers covering the ranges 486-504 GHz and 541-581 GHz, and one fixed at 118.75 GHz together with backends that provide spectral resolution from 150 kHz to 1 MHz. This Letter describes the Odin radiometer, its operation and performance with the data processing and calibration described in Paper II.

Survey Observations of c-C2H4O and CH3CHO toward Massive Star-forming Regions

In order to clarify the formation mechanisms of ethylene oxide (cyclic-C2H4O, hereafter c-C2H4O) and its structural isomer acetaldehyde (CH3CHO), we carried out survey observations of these two molecules toward 20 massive star-forming regions and two dark clouds. CH3CHO and c-C2H4O were detected in 10 massive star-forming regions, and CH3CHO was also detected in five others. The column densities and the rotational temperatures were derived using the rotation diagram method. The column densities of these molecules were derived to be (0.1-3.3) × 1014 and (0.2-5.0) × 1014 cm-2 for c-C2H4O and CH3CHO, respectively. The fractional abundances with respect to H2 are X(c-C2H4O) = 4 × 10-11 to 6 × 10-10 and X(CH3CHO) = 7 × 10-12 to 3 × 10-9. We also detected several transitions of methanol (CH3OH), ethanol (C2H5OH), dimethyl ether [(CH3)2O], methyl formate (HCOOCH3), formic acid (HCOOH), vinyl cyanide (C2H3CN), and ethyl cyanide (C2H5CN). Comparing the abundances of the detected molecules with physical conditions of each source, we found that the abundances of most of the molecules except for c-C2H4O and CH3CHO increase along with the dust temperature of each source. On the other hand, the abundances of c-C2H4O and CH3CHO show little correlation with the dust temperature. The rotation temperatures of c-C2H4O, CH3CHO, and HCOOH are low (10-40 K) in all sources in spite of the fact that the gas kinetic temperature greatly varies from cloud to cloud. This may indicate that the line emission from each molecular species is excited in regions with different physical conditions. We performed pseudo-time-dependent chemical reaction simulations based on pure gas-phase reactions and found that the calculated abundances of observed molecules decreased when the gas kinetic temperature was raised. We investigated the relationship between the column density of C2H5OH and that of the C2H4O group (c-C2H4O + CH3CHO) because C2H5OH is believed to be a precursor of c-C2H4O and CH3CHO in the gas-phase chemistry scheme. If this hypothesis is correct, it is expected that the column density of C2H5OH is related to that of the C2H4O group. We found that the column density of the C2H4O group is high in sources where the column density of C2H5OH is high. This result is consistent with the above-mentioned hypothesis. We also investigated the relationships between the column densities of several organic species [CH3OH, C2H5OH, (CH3)2O, HCOOCH3, C2H3CN, and C2H5CN] and the luminosity-to-mass ratio, LIR/M, in OMC-1, W51A, and Sgr B2(N). We found that the column densities of these molecules are high in sources where LIR/M is high. Since LIR/M is believed to be a measure of the star formation rate per unit mass, it indicates that the column densities of these molecules become higher in sources where high star formation activity leads to a higher dust temperature. This strongly suggests that the formation of these molecules involves processes on the dust grains and subsequent sublimation to the gas phase, where they can be observed.

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 Three-Position Spectral Line Survey of Sagittarius B2 between 218 and 263 GHz. I. The Observational Data

We have surveyed the frequency band 218.30-263.55 GHz toward the core positions N and M and the quiescent cloud position NW in the Sgr B2 molecular cloud using the Swedish-ESO Submillimetre Telescope. In total 1730, 660, and 110 lines were detected in N, M, and NW, respectively, and 42 different molecular species were identified. The number of unidentified lines are 337, 51, and eight. Toward the N source, spectral line emission constitutes 22% of the total detected flux in the observed band, and complex organic molecules are the main contributors. Toward M, 14% of the broadband flux is caused by lines, and SO2 is here the dominant source of emission. NW is relatively poor in spectral lines and continuum. In this paper we present the spectra together with tables of suggested line identifications.

Submillimetre observations of comets with Odin: 2001–2005

The Odin satellite, launched in February 2001, is equipped with a 1.1-m submillimetre telescope. Odin was used to observe the 557 GHz line of water with high spectral resolution in 12 comets between 2001 and 2005. Line shapes and spatial mapping provide information on the anisotropy of the outgassing and constraints on water excitation, enabling accurate measurements of the water production rate. Five comets were regularly observed over periods of more than one month to monitor the variation of their water outgassing rate with heliocentric distance. Observing campaigns have been generally coordinated with ground-based observations of molecular lines at Nancay, CSO or IRAM 30-m telescopes to obtain molecular abundances relative to water. Thanks to Odins frequency coverage, it was also possible to detect the H218O 548 GHz line, first in comet 153P/Ikeya Zhang in April 2002 [Lecacheux, A., Biver, N., Crovisier, J. et al., 2003, Observations of water in comets with Odin. Astron. Astrophys. 402, L55 L58.] and then in comets C/2002 T7 (LINEAR), C/2001 Q4 (NEAT) and C/2004 Q2 (Machholz). The 16O/18O isotopic ratio (≈450) is consistent with the terrestrial value. Ammonia has been searched for in three comets through its J=1 0 line at 572 GHz and was tentatively detected in C/2001 Q4 and C/2002 T7. The derived abundances of NH3 relative to water are 0.5% and 0.3%, respectively, similar to values obtained in other comets with different techniques.

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 ...

Chemical Abundances in Molecular Clouds

At present approximately 70 interstellar molecules are known. We discuss methods for determining chemical abundances in interstellar clouds and present results for the best studied regions, which include the “spiral arm” clouds seen towards distant continuum sources, quiescent dark and giant clouds, and the gas in regions of active star formation. For many simple molecules abundances are rather uniform over a range of densities and temperatures in quiescent clouds, in accord with gas phase, ion-molecule chemical models. Some striking chemical differences do exist both within and among clouds, however, particularly in star-forming regions. This chapter is organized as follows: 1. Introduction 2. Measurement of the Chemical Composition of Interstellar Clouds 2.1. Introduction 2.2. Optically Thin Emission 2.3. Determination of Total Molecular Column Density 2.4. Multi-Transition Studies 2.5. Optically Thick Emission 2.6. Optical Depth Determination 2.7. Cloud Structure and Molecular Abundances 3. Results 3.1. Identified Interstellar Molecules 3.2. Individual Molecular Clouds 3.2.1. Orion KL 3.2.2. Sgr B2 3.2.3. TMC-1/L134N 3.2.4. Spiral Arm Clouds 3.3. Chemical Abundances 4. Interpretation 4.1. General Uniformities in Abundance 4.2. Chemical Differences Among Clouds 4.2.1. Dark Clouds 4.2.2. Giant Molecular Clouds 4.2.3. Influences of Cloud Temperature 4.3. Chemical Differences Within Clouds 4.3.1. L134N(L183) 4.3.2. Sgr B2 4.3.3. Orion KL 4.3.4. TMC-1 5. Conclusions

The detection of interstellar methylcyanoacetylene

A new interstellar molecule, methylcyanoacetylene (CH3C3N), has been detected in the molecular cloud TMC-1. The J = 8 --> 7, J = 7 --> 6, J = 6 --> 5, and J = 5 --> 4 transitions have been observed. For the first three of these, both the K = 0 and K = 1 components are present, while for J = 5 --> 4, only the K = 0 line has been detected. The observed frequencies were calculated by assuming a value of radial velocity VLSR = 5.8 km s-1 for TMC-1, typical of other molecules in the cloud. All observed frequencies are within 10 kHz of the calculated frequencies, which are based on the 1982 laboratory constants of Moises et al., so the identification is secure. The lines are broadened by hyperfine splitting, and the J = 5 --> 4, K = 0 transition shows incipient resolution into three hyperfine components. The rotational temperature determined for these observations is quite low, with 2.7 K < or = Trot < or = 4 K. the total column density is approximately 5 x 10(12) cm-2.

Observations of the Circumstellar Water 110→101 and Ammonia 10→00 Lines in IRC +10216 by the Odin Satellite

Submillimeter lines of H2O and NH3 have been detected in the carbon star IRC +10216 (CW Leo) with the Odin submillimeter satellite. The detection of the J = 110 → 101 557 GHz line of ortho-H2O confirms the earlier detection in the same source with SWAS. The detection of the JK = 10 → 00 572 GHz line represents the first observation of the ground-state rotational transition of NH3 in a stellar envelope. By fitting a molecular line transfer model to the observed lines, we derive an ortho-H2O abundance of 2.4 × 10-6, which is consistent with estimates from the SWAS observation. The derived ortho-NH3 abundance of 1 × 10-6 relative to H2 is significantly higher than those derived from 24 GHz inversion transitions and is slightly higher than those from vibrational transitions in the infrared band. The high H2O and NH3 abundances in the carbon-rich star IRC +10216 underscore shortcomings in the conventional gas-phase LTE and non-LTE chemical models.