H. Olofsson

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.

W hya through the eye of odin : Satellite observations of circumstellar submillimetre H2O line emission

We present Odin observations of the AGB star W Hya in the ground-state transition of ortho-H2O, 110 − 101 ,a t 557 GHz. The line is clearly of circumstellar origin. Radiative transfer modelling of the water lines observed by Odin and ISO results in a mass-loss rate of (2.5 ± 0.5) × 10 −7 Myr −1 , and a circumstellar H2O abundance of (2.0 ± 1.0) × 10 −3 . The inferred mass-loss rate is consistent with that obtained from modelling the circumstellar CO radio line emission, and also with that obtained from the dust emission modelling combined with a dynamical model for the outflow. The very high water abundance, higher than the cosmic oxygen abundance, can be explained by invoking an injection of excess water from evaporating icy bodies in the system. The required extra mass of water is quite small, on the order of ∼0.1 M⊕.

A HIFI view on circumstellar H2O in M-type AGB stars: radiative transfer, velocity profiles, and H2O line cooling

We aim to constrain the temperature and velocity structures, and H2O abundances in the winds of a sample of M-type AGB stars. We further aim to determine the effect of H2O line cooling on the energy balance in the inner circumstellar envelope. We use two radiative-transfer codes to model molecular emission lines of CO and H2O towards four M-type AGB stars. We focus on spectrally resolved observations of CO and H2O from HIFI. The observations are complemented by ground-based CO observations, and spectrally unresolved CO and H2O observations with PAC. The observed line profiles constrain the velocity structure throughout the circumstellar envelopes (CSEs), while the CO intensities constrain the temperature structure in the CSEs. The H2O observations constrain the o-H2O and p-H2O abundances relative to H2. Finally, the radiative-transfer modelling allows to solve the energy balance in the CSE, in principle including also H2O line cooling. The fits to the line profiles only set moderate constraints on the velocity profile, indicating shallower acceleration profiles in the winds of M-type AGB stars than predicted by dynamical models, while the CO observations effectively constrain the temperature structure. Including H2O line cooling in the energy balance was only possible for the low-mass-loss-rate objects in the sample, and required an ad hoc adjustment of the dust velocity profile in order to counteract extreme cooling in the inner CSE. H2O line cooling was therefore excluded from the models. The constraints set on the temperature profile by the CO lines nevertheless allowed us to derive H2O abundances. The derived H2O abundances confirm previous estimates and are consistent with chemical models. However, the uncertainties in the derived abundances are relatively large, in particular for p-H2O, and consequently the derived o/p-H2O ratios are not well constrained.

A new 3 mm band receiver for the Onsala 20 m antenna

A new receiver for the Onsala 20 m antenna with the possibility of being equipped with 3 mm and 4 mm bands has been built and the 3 mm channel has been commissioned during the Spring 2014. For single-dish operation, the receiver uses an innovative on-source/off-source optical switch. In combination with additional optical components and within the same optical layout, the switch provides two calibration loads (for the 3 mm and 4 mm channels), sideband rejection measurement, and tuning possibilities. The optical layout of the receiver employs all cold (4 K) offset elliptical mirrors for both channels, whereas the on-off switch employs flat mirrors only. The 3 mm channel employs a sideband separation (2SB) dual polarization receiver with orthomode transducer (OMT), 4-8 GHz intermediate frequency (IF), x? 2pol x? upper and lower sidebands (USB? +? LSB). The cryostat has four optical windows made of high density polyethylene (HDPE) with anti-reflection corrugations, two for the signal and two for each frequency band cold load. The cryostat uses a two-stage cryocooler produced by Sumitomo HI? RDK? 408D2 with anti-vibration suspension of the cold-head to minimize impact of the vibrations on the receiver stability. The local oscillator (LO) system is based on a Gunn oscillator with aphase lock loop (PLL) and four mechanical tuners for broadband operation, providing independently tunable LO power for each polarization. This paper provides a technical description of the receiver and its technology and could be useful for instrumentation engineers and observers using the Onsala 20 m telescope.

H12CN and H13CN excitation analysis in the circumstellar outflow of R Sculptoris

The (CO)-C-12/(CO)-C-13 isotopologue ratio in the circumstellar envelope (CSE) of asymptotic giant branch (AGB) stars has been extensively used as the tracer of the photospheric C-12/C-13 ratio. However, spatially-resolved ALMA observations of R Scl, a carbon rich AGB star, have shown that the (CO)-C-12/(CO)-C-13 ratio is not consistent over the entire CSE. Hence, it can not necessarily be used as a tracer of the C-12/C-13 ratio. The most likely hypothesis to explain the observed discrepancy between the (CO)-C-12/(CO)-C-13 and C-12/C-13 ratios is CO isotopologue selective photodissociation by UV radiation. Unlike the CO isotopologue ratio, the HCN isotopologue ratio is not a ff ected by UV radiation. Therefore, HCN isotopologue ratios can be used as the tracer of the atomic C ratio in UV irradiated regions. Aims. We aim to present ALMA observations of (HCN)-C-13(4-3) and APEX observations of (HCN)-C-12(2-1), (HCN)-C-13(2-1, 3-2) towards R Scl. These new data, combined with previously published observations, are used to determine abundances, ratio, and the sizes of line-emitting regions of the aforementioned HCN isotopologues. Methods. We have performed a detailed non-LTE excitation analysis of circumstellar (HCN)-C-12(J = 1-0, 2-1, 3-2, 4-3) and (HCN)-C-13(J = 2-1, 3-2, 4-3) line emission around R Scl using a radiative transfer code based on the accelerated lambda iteration (ALI) method. The spatial extent of the molecular distribution for both isotopologues is constrained based on the spatially resolved (HCN)-C-13(4-3) ALMA observations. Results. We find fractional abundances of (HCN)-C-12/H2 = (5.0 +/- 2.0) x 10(-5) and (HCN)-C-13/H-2 = (1.9 +/- 0.4) x 10(-6) in the inner wind (r <= (2.0 +/- 0.25) x 10(15) cm) of R Scl. The derived circumstellar isotopologue ratio of (HCN)-C-12/(HCN)-C-13 = 26.3 +/- 11.9 is consistent with the photospheric ratio of C-12/C-13 similar to 19 +/- 6. Conclusions. We show that the circumstellar (HCN)-C-12/(HCN)-C-13 ratio traces the photospheric C-12/C-13 ratio. Hence, contrary to the (CO)-C-12/(CO)-C-13 ratio, the (HCN)-C-12/(HCN)-C-13 ratio is not a ff ected by UV radiation. These results support the previously proposed explanation that CO isotopologue selective-shielding is the main factor responsible for the observed discrepancy between (CO)-C-12/(CO)-C-13 and (CO)-C-12/(CO)-C-13 ratios in the inner CSE of R Scl. This indicates that UV radiation impacts on the CO isotopologue ratio. This study shows how important is to have high-resolution data on molecular line brightness distribution in order to perform a proper radiative transfer modelling.

Sulphur-bearing molecules in AGB stars: II. Abundances and distributions of CS and SiS★

Context. Sulphur has long been known to form different molecules depending on the chemical composition of its environment. More recently, the sulphur-bearing molecules SO and H2S have been shown to behave differently in oxygen-rich asymptotic giant branch (AGB) circumstellar envelopes of different densities. Aims. By surveying a diverse sample of AGB stars for CS and SiS emission, we aim to determine in which environments these sulphur bearing molecules most readily occur. We include sources with a range of mass-loss rates and carbon-rich, oxygen-rich, and mixed S-type chemistries. Where these molecules are detected, we aim to determine their CS and SiS abundances. Methods. We surveyed 20 AGB stars of different chemical types using the APEX telescope, and combined this with an IRAM 30 m and APEX survey of CS and SiS emission towards over 30 S-type stars. For those stars with detections, we performed radiative transfer modelling to determine abundances and abundance distributions. Results. We detect CS towards all the surveyed carbon stars, some S-type stars, and the highest mass-loss rate oxygen-rich stars, (M > 5 x 10(-6) M-circle dot yr(-1)). SiS is detected towards the highest mass-loss rate sources of all chemical types (M >= similar to 8 x 10(-7) M-circle dot yr(-1)). We find CS peak fractional abundances ranging from similar to 4 x 10(-7) to similar to 2 x 10(-5) for the carbon stars, from similar to 3 x 10(-8) to similar to 1 x 10(-7) for the oxygen-rich stars, and from similar to 1 x 10(-7) to similar to 8 x 10(-6) for the S-type stars. We find SiS peak fractional abundances ranging from similar to 9 x 10(-6) to similar to 2 x 10(-5) for the carbon stars, from similar to 5 x 10(-7) to similar to 2 x 10(-6) for the oxygen-rich stars, and from similar to 2 x 10(-7) to similar to 2 x 10(-6) for the S-type stars. Conclusions. Overall, we find that wind density plays an important role in determining the chemical composition of AGB circumstellar envelopes. It is seen that for oxygen-rich AGB stars both CS and SiS are detected only in the highest density circumstellar envelopes and their abundances are generally lower than for carbon-rich AGB stars by around an order of magnitude. For carbon-rich and S-type stars SiS was also only detected in the highest density circumstellar envelopes, while CS was detected consistently in all surveyed carbon stars and sporadically among the S-type stars.

Circumstellar environment of the M-type AGB star R Doradus: APEX spectral scan at 159.0–368.5 GHz★★★

Context. Our current insights into the circumstellar chemistry of asymptotic giant branch (AGB) stars are largely based on studies of carbon-rich stars and stars with high mass-loss rates. Aims. In order to expand the current molecular inventory of evolved stars we present a spectral scan of the nearby, oxygen-rich star R Dor, a star with a low mass-loss rate (similar to 2 x 10(-7) M-circle dot yr(-1)). Methods. We carried out a spectral scan in the frequency ranges 159.0-321.5 GHz and 338.5-368.5 GHz (wavelength range 0.8-1.9 mm) using the SEPIA/Band-5 and SHeFI instruments on the APEX telescope and we compare it to previous surveys, including one of the oxygen-rich AGB star IK Tau, which has a high mass-loss rate (similar to 5 x 10(-6) M-circle dot yr(-1)). Results. The spectrum of R Dor is dominated by emission lines of SO2 and the different isotopologues of SiO. We also detect CO, H2O, HCN, CN, PO, PN, SO, and tentatively TiO2, AlO, and NaCl. Sixteen out of approximately 320 spectral features remain unidentified. Among these is a strong but previously unknown maser at 354.2 GHz, which we suggest could pertain to H2SiO, silanone. With the exception of one, none of these unidentified lines are found in a similarly sensitive survey of IK Tau performed with the IRAM 30m telescope. We present radiative transfer models for five isotopologues of SiO ((SiO)-Si-28, (SiO)-Si-29, (SiO)-Si-30, (SiO)-O-17, (SiO)-O-18), providing constraints on their fractional abundance and radial extent. We derive isotopic ratios for C, O, Si, and S and estimate that, based on our results for O-17/O-18, R Dor likely had an initial mass in the range 1.3-1.6 M-circle dot, in agreement with earlier findings based on models of H2O line emission. From the presence of spectral features recurring in many of the measured thermal and maser emission lines we tentatively identify up to five kinematical components in the outflow of R Dor, indicating deviations from a smooth, spherical wind.

Multi-line detection of O-2 toward rho Ophiuchi A

Models of pure gas-phase chemistry in well-shielded regions of molecular clouds predict relatively high levels of molecular oxygen, O2, and water, H2O. Contrary to expectation, the space missions SWAS and Odin found only very small amounts of water vapour and essentially no O2 in the dense star-forming interstellar medium. Only toward rho Oph A did Odin detect a weak line of O2 at 119 GHz in a beam size of 10 arcmin. A larger telescope aperture such as that of the Herschel Space Observatory is required to resolve the O2 emission and to pinpoint its origin. We use the Heterodyne Instrument for the Far Infrared aboard Herschel to obtain high resolution O2 spectra toward selected positions in rho Oph A. These data are analysed using standard techniques for O2 excitation and compared to recent PDR-like chemical cloud models. The 487.2GHz line was clearly detected toward all three observed positions in rho Oph A. In addition, an oversampled map of the 773.8GHz transition revealed the detection of the line in only half of the observed area. Based on their ratios, the temperature of the O2 emitting gas appears to vary quite substantially, with warm gas (> 50 K) adjacent to a much colder region, where temperatures are below 30 K. The exploited models predict O2 column densities to be sensitive to the prevailing dust temperatures, but rather insensitive to the temperatures of the gas. In agreement with these model, the observationally determined O2 column densities seem not to depend strongly on the derived gas temperatures, but fall into the range N(O2) = (3 to >6)e15/cm^2. Beam averaged O2 abundances are about 5e-8 relative to H2. Combining the HIFI data with earlier Odin observations yields a source size at 119 GHz of about 4 - 5 arcmin, encompassing the entire rho Oph A core.