- Email: [email protected]
New astronomy with the Odin satellite
Advances in Space Research 34 (2004) 504–510 www.elsevier.com/locate/asr
New astronomy with the Odin satellite Hjalmarson *, on behalf of the Odin Team Ake Centre for Astrophysics and Space Science, Onsala Space Observatoy, Chambers University of Technology, Onsala SE-439 92, Sweden Received 11 December 2002; received in revised form 19 March 2003; accepted 7 May 2003
Abstract The key Odin instrumental features are summarised, and the advantages vs Submillimeter Wave Astronomy Satellite (SWAS) are identified. We highlight results from OdinÕs first year of sub-millimetre and millimetre wave observations of H2 O, H2 18 O, NH3 , and O2 , focussing on a number of new results where OdinÕs high angular resolution, high frequency resolution, large spectrometer band widths, high sensitivity, or/and frequency tuning capability are crucial. We also show, by means of a quantitative comparison of Orion KL H2 O results, that the Odin and SWAS observational data sets are very consistently calibrated. Ó 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Odin spectroscopy satellite; Sub-millimetre wave observations; Gregorian telescope; Intersteller H2 O; O2 and NH3
1. Introduction The Odin sub-millimetre (submm) wave spectroscopy satellite – equipped with a high-precision offset Gregorian telescope of diameter 110 cm and a cryogenic submm/mm wave receiver package was launched on 20 February, 2001 from Svobodny in far-eastern Russia by a Start-1 rocket. The Odin project is a Swedish-led astronomy 50% aeronomy, 50% mission supported by space agencies and scientists in Canada, Finland, France and Sweden. The satellite was developed and is operated by the Swedish Space Corporation. The receiver package was integrated and optimised at Onsala Space Observatory, also responsible for the submm/mm data processing. The astronomy part of the mission is mainly focussing on observations of line emission from interstellar H2 O and O2 . A low-noise HEMT preamplifier at 119 GHz is used to improve OdinÕs O2 search sensitivity by more than an order of magnitude, compared to the search sensitivity of Odin and NASAÕs Submillimeter Wave Astronomy Satellite (SWAS) at 487 GHz. Odin is the second satellite for for submm line astronomy. The first one, SWAS was launched in December 1998 and since then has performed extensive *
Tel.: +46-31-7725-536; fax: +46-31-7725-590. E-mail address: [email protected] ( A. Hjalmarson). URL: http://www.snsb.se/eng_odin_intro.shtml.
observations of the ortho-H2 O (110 –101 ), 13 CO (5–4) and CI (3 P1 –3 P0 ) lines, using an off-axis Cassegrain telescope of size 54 68 cm (Melnick, 2004; Melnick et al., 2000a and subsequent papers in that ApJ issue). The estimated H2 O abundances in quiescent dense interstellar clouds are more than an order of magnitude lower than expected from pure gas-phase chemistry, most likely because of efficient H2 O sticking onto grain surfaces. Indeed, a large interstellar water abundance in terms of icy grain mantles has been discovered by ESAÕs Infrared Space Observatory (ISO: cf. van Dishoeck and Blake, 1998; Ehrenfreud and Charnley, 2000). Simultaneous deep SWAS searches for the O2 (32 –12 ) transition at 487 GHz have lead to low abundance limits in a number of sources (Goldsmith et al., 2000). A tentative detection of O2 in q Oph – presumably in an outflow wing – recently was presented by Goldsmith et al. (2002).
2. Odin’s observational characteristics vs SWAS Odin houses a 1.1-m diameter offset shaped Gregorian telescope. The surface RMS of the main and secondary reflector is <8 and <5 lm, respectively. The antenna FWHP beam-widths are 2.10 (12600 ) and 9.50 at 557 and 119 GHz, and the submm main beam efficiency, as estimated from Jupiter observations, is close to 90%. The pointing accuracy is observed to be better than 1000
0273-1177/$30 Ó 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2003.05.024
Hjalmarson / Advances in Space Research 34 (2004) 504–510 A. Table 1 Odin instrument benefits vs SWAS Property
Odin
SWAS
Antenna size Beam size (submm) Beam area ratio Sensitivity (submm) ratioa O2 search sensitivity ratiob Velocity resolution Bandwidth Frequency tuningc
110 cm 2.10 (12600 ) 3.4 >10 (>3.32 ) >10 0.08–1 km s1 100–1100 MHz 8 GHz
54 68 cm 3:30 4:50 – – – 1 km s1 350 MHz –
a
for small source, also considering the lower Odin system temperatures. b Odin at 119 GHz vs SWAS at 487 GHz. c for each Odin submm band.
(1r). The frequency range 541–581 GHz is covered by three tuneable Schottky mixers and a fourth Schottky mixer covers the band 486–504 GHz. A 119 GHz fixedtuned HEMT preamplifier has been installed to allow very sensitive searches for interstellar O2 . The receivers are actively cooled to 140 K and are all operated in single-sideband mode. The SSB submm system temperatures measured in-orbit are about 3300 K, while the 2.5 mm receiver has an SSB noise temperature of about 600 K. Any combination of four, three, or two front-end receivers (depending upon the available power and mode of operation) can be used in combination with two hybrid auto-correlation spectrometers (bandwidth range: 100–800 MHz; resolution range: 0.125–1 MHz) and an acousto-optical spectrometer, AOS (bandwidth: 1050 MHz; resolution: 1 MHz). The Odin attitude control system, attitude reconstruction capability, radiometer system, and data calibration are presented in some detail by Jacobsson et al. (2002a,b), Frisk et al. (2003), and Olberg et al. (2003), respectively. In Table 1 we compare the Odin instrument characteristics vs those of SWAS. Astronomical results will have to prove the importance of the Odin advantages in observational capability. However, a general statement in case of H2 O is that the observed interstellar emission regions are not very small compared to OdinÕs 2.10 antenna beam. The observed Odin/SWAS intensity ratios often are about 2, rather than 3.4 (as expected for very small sources). Hence the Odin H2 O sensitivity increase vs SWAS here is rather about 5 than >10 (for the same on-source integration time), if we also consider the lower Odin system temperatures. The conclusion that the dense cloud cores exhibit extended H2 O emission is quite important in itself. These results are based upon the fact that the Odin and SWAS data sets are very consistently calibrated, as will be demonstrated below (cf. Fig. 5). 3. New results from the first year of Odin observations We will here present highlights from the first year of Odin observations of searches for ortho-H2 O (110 –101 ),
505
ortho-H2 18 O (110 –101 ), ortho-NH3 (10 –00 ), and O2 (11 -10 ) signals from interstellar clouds and comets – cases where OdinÕs higher angular resolution, higher frequency resolution, larger spectrometer bandwidths, higher sensitivity or/and frequency tuning capability are important. 3.1. H2 O and H2 18 O in molecular clouds Fig. 1 displays a number of Odin spectra of the orthoH2 O (110 –101 ) line at 556.936 GHz – observed with the broad band (BW ¼ 1050 MHz), lower resolution AOS spectrometer. Simultaneously, a higher resolution hybrid auto-correlation spectrometer often has been used on a second mixer, also tuned to observe the water line. This also means an increase of sensitivity, and allows immediate verification of unexpected spectral features. Odin has observed H2 O in a number of molecular clouds and has mapped the H2 O emission in some selected regions. The results of our Orion KL mapping at 10 spacing as well as our detections of the ortho-H2 18 O (110 –101 ) line at 547.676 GHz at the cloud centre (outflow and quiescent gas) and from quiescent gas 20 south of the cloud centre are presented by Olofsson et al. (2003), and will be further discussed in the present paper. Our Odin mapping of the W3 region at 10 spacing (as discussed by Wilson et al., 2003), is able to separate the emissions from two nearby cloud cores, IRS5 and IRS4. While the former source is associated with an outflow and exhibits relatively broad self-absorbed lines, the latter one shows enhanced rather narrow H2 O emission (perhaps originating in a photon dominated region). The intriguing H2 O spectra observed by Odin in Galactic Centre directions – an intricate mixture of broad emission and narrow as well as broad absorption features from low excitation foreground gas (cf. Fig. 1(d)) are discussed by Sandqvist et al. (2003). 3.2. H2 O and H2 18 O in comets Fig. 1(c) displays the strong, very narrow H2 O line observed at 557 GHz from comet C/2001 A2 (LINEAR), here using the broad band acousto-optical spectrometer (AOS). Odin has mapped the water evaporation in several comets at high (80 m s1 ) spectral resolution (Lecacheux et al., 2003). A preliminary map at 10 spacing of the strong H2 O line from the recent bright comet 153P/2002 Cl (Ikeya-Zhang) is shown in Fig. 2, demonstrating that the emission region is extended with respect to the Odin 12600 antenna beam. Comet mapping in fact also has been an important ingredient in the Odin pointing verification. After 40 h of observations Odin was also able to detect the weak H2 18 O (110 –101 ) emission from comet Ikeya-Zhang. The observed H2 18 O intensity verifies the
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Hjalmarson / Advances in Space Research 34 (2004) 504–510 A.
Fig. 1. Sample Odin observations of H2 O in molecular clouds and comets: (a) High velocity H2 O outflow/shock spectrum in the core of the Orion KL molecular cloud. (b) Narrow H2 O line in the quiescent cloud 20 south of Orion KL. (c) Very narrow H2 O line from Comet C/2001 A2 (LINEAR). (d) Complicated mixture of H2 O emission and absorption towards Sgr A . (e) the broad H2 O emission line resulting from the IC443 supernova remnant shock. We have here also indicated the simultaneously used (on a second mixer also tuned to H2 O) auto-correlation spectrometer bands (allowing higher velocity resolution).
very high H2 O production rate of 26 1028 molecules s1 determined from the H2 O data, and is consistent with a 16 O/18 O isotope ratio ¼ 450 50, as discussed by Lecacheux et al. (2003). In our comet observations two mixers were tuned to H2 O, or H2 18 O, each connected to a high-resolution auto-correlation spectrometer (the selected bandwidth, 100 MHz, is in-
dicated in Fig. 1(c)), and one of them also to the broadband AOS. 3.3. H2 O and H2 18 O in the Orion KL molecular cloud core An Orion KL H2 O map at 10 spacing as observed by OdinÕs 2.10 antenna beam is displayed in Fig. 3. This
Hjalmarson / Advances in Space Research 34 (2004) 504–510 A.
Fig. 2. Preliminary Odin map at 10 spacing of the strong H2 O line from comet Ikeya-Zhang.
Fig. 3. Odin H2 O map at 10 spacing of the Orion KL region.
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Fig. 4. H2 O spectra from the Orion KL centre, as observed by Odin and SWAS (dashed; scaled by 2.4).
Odin result as well as detections of the ortho-H2 18 O (110 – 101 ) line at 547.676 GHz at the cloud centre (outflow and quiescent gas) and from quiescent gas 20 south of the outflow centre are presented by Olofsson et al. (2003), who also outline the physical and chemical conditions in the region and provide a number of relevant references. The Odin map centre H2 O spectrum is shown separately in Fig. 4. Here the H2 O emission dominantly appears to emanate in the well-known outflow/shock region. No narrow-line emission from the quiescent molecular cloud, or from the compact hot core source, is visible. The emission from the ‘‘blue-shifted’’ (with respect to the ambient cloud, ‘‘ridge’’, velocity of 8–10 km s1 ) part of the flow is considerably weaker than the ‘‘red-shifted’’ one. Here self-absorption in the optically thick outflow could play a role – just as observed by Odin in expanding cometary H2 O comae (Lecacheux et al., 2003). Such an effect has been observed and modelled in optically thick CO emisson from the mass-loss envelope around the carbon star IRC+10216 (cf. Crosas and Menten, 1997). Moreover, the abrupt H2 O brightness decrease at velocities blueward of about 10 km s1 is not an Odin instrument artifact, since this feature is clearly
Fig. 5. Comparison of the SWAS Orion KL H2 O spectrum (dashed) with the spectrum resulting from our convolution of OdinÕs H2 O map (Fig. 3) to the angular resolution of SWAS.
verified in our independently observed AOS spectrum (shown in Fig. 1(a)). Absorption by lower excitation H2 O foreground gas (in the molecular cloud envelope) at ambient cloud velocities here could play a role. In Fig. 4, the corresponding SWAS water spectrum also is shown (observed with a beam size of 3:30 4:50 ; Melnick et al., 2000b, kindly supplied in digital form). The SWAS spectrum here has been scaled by a factor of 2.4 to match the outflow line-wings observed by Odin. Two important facts are apparent from this comparison: (1) The very good match of the H2 O line-wings as observed by the two telescopes (after scaling the SWAS spectrum by a factor 2.4), which we can use to estimate the size of the outflow/shock region. If the scaling factor is interpreted as the Odin/SWAS beam filling ratio for a Gaussian source, this is consistent with an H2 O outflow FWHP size of circa 11000 . A similar size is estimated from our Odin H2 O mapping (cf. Olofsson et al., 2003). The H2 O outflow size observed by Odin verifies the very high H2 O abundance of 5 104 (vs H2 ) estimated by Melnick et al. (2000b) for the high-velocity, shocked gas component. (2) The larger SWAS beam appears to ‘‘pick up’’ ambient cloud H2 O emission not visible in the Odin spectrum. This latter fact easily can be verified by a convolution of the OdinÕs H2 O map of the Orion KL region (Fig. 3) to the angular resolution of SWAS. The resulting ‘‘SWAS-convolved’’ Odin map centre spectrum is shown in Fig. 5, together with the H2 O spectrum observed by SWAS. The two almost identical spectra also demonstrate the very consistent calibration of the Odin and SWAS H2 O data sets. The very strong H2 O signals, paired with OdinÕs very clean antenna beam with side-lobes well below 25 dB,
Fig. 6. Preliminary MEM-deconvolved Orion KL H2 O map, as observed with a 6000 antenna beam (see text).
Hjalmarson / Advances in Space Research 34 (2004) 504–510 A.
Fig. 7. H2 18 O spectra towards the Orion KL centre, observed by Odin and SWAS (gray; scaled by 3.3).
and the very precise attitude reconstruction (cf. Olofsson et al., 2003), has tempted us to try a MEM-type deconvolution of the position-velocity cube of OdinÕs Orion KL H2 O map. A very preliminary result, convolved with a 6000 Gaussian beam and here displayed at 6000 spacing, is shown in Fig. 6. While the FWHP size of the H2 O outflow/shock region is only about 6000 , more extended lower intensity H2 O outflow/shock emission is indeed present. Ultimately we hope (by using the much larger Odin position-velocity cube data set now available) to reach a ‘‘beam-size’’ of 4000 – similar to the antenna beam at 557 GHz of the future Herschel Space Observatory (Harwit, 2004). OdinÕs H2 18 O spectrum towards Orion KL is shown in Fig. 7, together with the corresponding SWAS spectrum (Melnick et al., 2000b). The latter H2 18 O spectrum here has been scaled by a factor 3.3 to fit the red wing of the Odin spectrum. This (beam filling) ratio is very close to the theoretical value for a very small source, 3.37, indicating an H2 18 O outflow size much smaller than that observed by Odin in
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case of H2 O. The broad SO2 (286;22 –285;23 ) line visible near 65 km s1 also nicely scales by a factor of 3.3, indicating a similarly small SO2 outflow size, which may be expected for this high energy line (Eu ¼ 475 K). We argue that the H2 18 O outflow emission – visible in a much narrower velocity range than is observed in the H2 O line must originate in the compact low-velocity flow and estimate an H2 O abundance of circa 105 compared to all H2 in this flow (Olofsson et al., 2003). This H2 O abundance is an order of magnitude lower than that earlier estimated for the shocked gas in the spatially more extended high-velocity flow. In Fig. 8 we show the H2 18 O spectrum observed by Odin in a quiescent cloud position, 20 south of the Orion KL outflow centre. This very probable detection allows us to estimate, for the first time by means of a (presumably) optically thin line, the ambient cloud H2 O abundance (cf. Olofsson et al., 2003). The resulting H2 O abundance is about 1 108 , in good agreement with the low abundance values derived from SWAS H2 O mapping (Snell et al., 2000). 3.4. First detections of the NH3 (10 –00 ) line Simultaneously with observations of H2 O, or H2 18 O, and searches for O2 , we also tuned one of our receivers to the ortho-NH3 (10 –00 ) line at 572 GHz. Larsson et al. (2003) present the detection of quiescent cloud as well as outflow NH3 (10 –00 ) emission in Orion KL cloud core, quiescent cloud emission 20 south, and the first detection of NH3 in the Orion Bar. The first detection of the NH3 (10 –00 ) line in a dark cloud is discussed by Liseau et al. (2003). 3.5. Deep odin searches for molecular oxygen Simultaneously with our submm wave observations of H2 O, H2 18 O, and NH3 , we have also been searching for the O2 (11 –00 ) at 118.750 GHz using our low noise HEMT receiver. In spite of some frustrating phaselocking problems, OdinÕs sensitive searches have led to very low O2 abundance limits, especially in cold dark clouds (<107 ; more than an order of magnitude below published SWAS limits; cf. Goldsmith et al., 2000), as reported by Pagani et al. (2003). Such low O2 abundance limits are very difficult to accommodate in current chemical models (cf. Goldsmith et al., 2002; Pagani et al., 2003).
Acknowledgements
Fig. 8. H2 18 O spectrum observed by Odin 20 south of the Orion KL outflow centre.
Generous financial support by Research Councils and Space Agencies in Canada, Finland, France, and Sweden is gratefully acknowledged. We also want to express our gratitude to the crews at the SSC Odin operations
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Hjalmarson / Advances in Space Research 34 (2004) 504–510 A.
centres in Kiruna and Solna for their skilful and dedicated work. The friendly attitude of Gary Melnick and the SWAS team, who upon our request supplied us with carefully reduced data for a number of targets, is greatly appreciated. Finally, the author wants to thank all Odin colleagues for their trust in him during good as well as bad times. References Crosas, M., Menten, K.M. Physical parameters of the IRC+10216 circumstellar envelope: new constraints from submillimeter observations. Astrophys. J. 483, 913–924, 1997. Ehrenfreud, P., Charnley, B.S. Organic molecules in the interstellar medium, comets, and meteorites: a voyage from dark clouds to the early earth. Annu. Rev. Astron. Astrophys. 38, 427–483, 2000. Frisk, U., Hagstr€ om, M., Ala-Laurinaho, J., et al. The Odin satellite, I. radiometer design and test. Astron. Astrophys. 402, L27–L34, 2003. Goldsmith, P.F., Melnick, G.J., Bergin, E.A., et al. O2 in interstellar molecular clouds. Astrophys. J. 539, L123–L127, 2000. Goldsmith, P.F., Li, D., Bergin, E.A., et al. Tentative detection of molecular oxygen in the rho Ophiuchi cloud. Astrophys. J. 576, 814–831, 2002. Harwit, M. The Herschel Mission. Adv. Space Res., this issue, 2004, doi:10.1016/j.asr.2003.03.026. Jacobsson, B., Nylund, M., Olsson, T., Vinterhav, E., The highperforming attitude control system of the scientific satellite Odin, IAC-02-A.P.13, World Space Congress, Houston, 2002. Jacobsson, B., Nylund, M., Olsson, T., et al., Odin Star tracker/gyro calibration and attitude reconstruction for the scientific satellite Odin – in flight results, IAC-02-A-4.01, World Space Congress, Houston, 2002.
Larsson, B., Liseau, R., Bergman, P., et al. First NH3 detection of the Orion Bar. Astron. Astrophys. 402, L69–L72, 2003. Lecacheux, A., Biver, N., Crovisier, J., et al. Observations of water in comets with Odin. Astron. Astrophys. 402, L55–L58, 2003. Liseau, R., Larsson, B., Brandeker, A., et al. First detection of NH3 (Jk ¼ 10 –00 ) from a low mass core: on the low ammonia abundance in the rho Oph A core. Astron. Astrophys. 402, L73–L76, 2003. Melnick, G.J. Submillimeter wave astronomy satellite science highlights. Adv. Space Res., this issue, 2004, doi:10.1016/ j.asr.2003.04.030. Melnick, G.J., Stauffer, J.R., Ashby, M.L.N., et al. The Submillimeter Wave Astronomy Satellite: science objectives and instrument description. Astrophys. J. 539, L77–L85, 2000a. Melnick, G.J., Ashby, M.L.N., Plume, R., et al. Observations of water vapor toward Orion BN/KL. AstrophysJ 539, L87–L91, 2000b. Olberg, M., Frisk, U., Lecacheux, A., et al. The Odin satellite, II. Radiometer data processing and calibration. Astron. Astrophys. 402, L35–L38, 2003. et al. Odin water Olofsson, A.O.H., Olofsson, G., Hjalmarson, A., mapping in the Orion KL region. Astron. Astrophys. 402, L47– L54, 2003. Pagani, L., Olofsson, A.O.H., Bergman, P., et al. Low upper limits on the O2 abundance from the Odin satellite. Astron. Astrophys. 402, L77–L81, 2003. Sandqvist, Aa., Bergman, P., Black, J.H., et al. Odin observations of H2 O in the Galactic Centre. Astron. Astrophys. 402, L63–L67, 2003. Snell, R.L., Howe, J.E., Ashby, M.L.N., et al. Submillimeter Wave Astronomy Observatory observations of extended water emission in Orion. Astrophys. J. 539, L93–L96, 2000. van Dishoeck, E.F., Blake, G.A. Chemical evolution of star-forming regions. Annu. Rev. Astron. Astrophys. 36, 317–368, 1998. Wilson, C.D., Mason, A., Gregersen, E., et al. Submillimeter emission from water in the W3 region. Astron. Astrophys. 402, L59–L62, 2003.
New astronomy with the Odin satellite Hjalmarson *, on behalf of the Odin Team Ake Centre for Astrophysics and Space Science, Onsala Space Observatoy, Chambers University of Technology, Onsala SE-439 92, Sweden Received 11 December 2002; received in revised form 19 March 2003; accepted 7 May 2003
Abstract The key Odin instrumental features are summarised, and the advantages vs Submillimeter Wave Astronomy Satellite (SWAS) are identified. We highlight results from OdinÕs first year of sub-millimetre and millimetre wave observations of H2 O, H2 18 O, NH3 , and O2 , focussing on a number of new results where OdinÕs high angular resolution, high frequency resolution, large spectrometer band widths, high sensitivity, or/and frequency tuning capability are crucial. We also show, by means of a quantitative comparison of Orion KL H2 O results, that the Odin and SWAS observational data sets are very consistently calibrated. Ó 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Odin spectroscopy satellite; Sub-millimetre wave observations; Gregorian telescope; Intersteller H2 O; O2 and NH3
1. Introduction The Odin sub-millimetre (submm) wave spectroscopy satellite – equipped with a high-precision offset Gregorian telescope of diameter 110 cm and a cryogenic submm/mm wave receiver package was launched on 20 February, 2001 from Svobodny in far-eastern Russia by a Start-1 rocket. The Odin project is a Swedish-led astronomy 50% aeronomy, 50% mission supported by space agencies and scientists in Canada, Finland, France and Sweden. The satellite was developed and is operated by the Swedish Space Corporation. The receiver package was integrated and optimised at Onsala Space Observatory, also responsible for the submm/mm data processing. The astronomy part of the mission is mainly focussing on observations of line emission from interstellar H2 O and O2 . A low-noise HEMT preamplifier at 119 GHz is used to improve OdinÕs O2 search sensitivity by more than an order of magnitude, compared to the search sensitivity of Odin and NASAÕs Submillimeter Wave Astronomy Satellite (SWAS) at 487 GHz. Odin is the second satellite for for submm line astronomy. The first one, SWAS was launched in December 1998 and since then has performed extensive *
Tel.: +46-31-7725-536; fax: +46-31-7725-590. E-mail address: [email protected] ( A. Hjalmarson). URL: http://www.snsb.se/eng_odin_intro.shtml.
observations of the ortho-H2 O (110 –101 ), 13 CO (5–4) and CI (3 P1 –3 P0 ) lines, using an off-axis Cassegrain telescope of size 54 68 cm (Melnick, 2004; Melnick et al., 2000a and subsequent papers in that ApJ issue). The estimated H2 O abundances in quiescent dense interstellar clouds are more than an order of magnitude lower than expected from pure gas-phase chemistry, most likely because of efficient H2 O sticking onto grain surfaces. Indeed, a large interstellar water abundance in terms of icy grain mantles has been discovered by ESAÕs Infrared Space Observatory (ISO: cf. van Dishoeck and Blake, 1998; Ehrenfreud and Charnley, 2000). Simultaneous deep SWAS searches for the O2 (32 –12 ) transition at 487 GHz have lead to low abundance limits in a number of sources (Goldsmith et al., 2000). A tentative detection of O2 in q Oph – presumably in an outflow wing – recently was presented by Goldsmith et al. (2002).
2. Odin’s observational characteristics vs SWAS Odin houses a 1.1-m diameter offset shaped Gregorian telescope. The surface RMS of the main and secondary reflector is <8 and <5 lm, respectively. The antenna FWHP beam-widths are 2.10 (12600 ) and 9.50 at 557 and 119 GHz, and the submm main beam efficiency, as estimated from Jupiter observations, is close to 90%. The pointing accuracy is observed to be better than 1000
0273-1177/$30 Ó 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2003.05.024
Hjalmarson / Advances in Space Research 34 (2004) 504–510 A. Table 1 Odin instrument benefits vs SWAS Property
Odin
SWAS
Antenna size Beam size (submm) Beam area ratio Sensitivity (submm) ratioa O2 search sensitivity ratiob Velocity resolution Bandwidth Frequency tuningc
110 cm 2.10 (12600 ) 3.4 >10 (>3.32 ) >10 0.08–1 km s1 100–1100 MHz 8 GHz
54 68 cm 3:30 4:50 – – – 1 km s1 350 MHz –
a
for small source, also considering the lower Odin system temperatures. b Odin at 119 GHz vs SWAS at 487 GHz. c for each Odin submm band.
(1r). The frequency range 541–581 GHz is covered by three tuneable Schottky mixers and a fourth Schottky mixer covers the band 486–504 GHz. A 119 GHz fixedtuned HEMT preamplifier has been installed to allow very sensitive searches for interstellar O2 . The receivers are actively cooled to 140 K and are all operated in single-sideband mode. The SSB submm system temperatures measured in-orbit are about 3300 K, while the 2.5 mm receiver has an SSB noise temperature of about 600 K. Any combination of four, three, or two front-end receivers (depending upon the available power and mode of operation) can be used in combination with two hybrid auto-correlation spectrometers (bandwidth range: 100–800 MHz; resolution range: 0.125–1 MHz) and an acousto-optical spectrometer, AOS (bandwidth: 1050 MHz; resolution: 1 MHz). The Odin attitude control system, attitude reconstruction capability, radiometer system, and data calibration are presented in some detail by Jacobsson et al. (2002a,b), Frisk et al. (2003), and Olberg et al. (2003), respectively. In Table 1 we compare the Odin instrument characteristics vs those of SWAS. Astronomical results will have to prove the importance of the Odin advantages in observational capability. However, a general statement in case of H2 O is that the observed interstellar emission regions are not very small compared to OdinÕs 2.10 antenna beam. The observed Odin/SWAS intensity ratios often are about 2, rather than 3.4 (as expected for very small sources). Hence the Odin H2 O sensitivity increase vs SWAS here is rather about 5 than >10 (for the same on-source integration time), if we also consider the lower Odin system temperatures. The conclusion that the dense cloud cores exhibit extended H2 O emission is quite important in itself. These results are based upon the fact that the Odin and SWAS data sets are very consistently calibrated, as will be demonstrated below (cf. Fig. 5). 3. New results from the first year of Odin observations We will here present highlights from the first year of Odin observations of searches for ortho-H2 O (110 –101 ),
505
ortho-H2 18 O (110 –101 ), ortho-NH3 (10 –00 ), and O2 (11 -10 ) signals from interstellar clouds and comets – cases where OdinÕs higher angular resolution, higher frequency resolution, larger spectrometer bandwidths, higher sensitivity or/and frequency tuning capability are important. 3.1. H2 O and H2 18 O in molecular clouds Fig. 1 displays a number of Odin spectra of the orthoH2 O (110 –101 ) line at 556.936 GHz – observed with the broad band (BW ¼ 1050 MHz), lower resolution AOS spectrometer. Simultaneously, a higher resolution hybrid auto-correlation spectrometer often has been used on a second mixer, also tuned to observe the water line. This also means an increase of sensitivity, and allows immediate verification of unexpected spectral features. Odin has observed H2 O in a number of molecular clouds and has mapped the H2 O emission in some selected regions. The results of our Orion KL mapping at 10 spacing as well as our detections of the ortho-H2 18 O (110 –101 ) line at 547.676 GHz at the cloud centre (outflow and quiescent gas) and from quiescent gas 20 south of the cloud centre are presented by Olofsson et al. (2003), and will be further discussed in the present paper. Our Odin mapping of the W3 region at 10 spacing (as discussed by Wilson et al., 2003), is able to separate the emissions from two nearby cloud cores, IRS5 and IRS4. While the former source is associated with an outflow and exhibits relatively broad self-absorbed lines, the latter one shows enhanced rather narrow H2 O emission (perhaps originating in a photon dominated region). The intriguing H2 O spectra observed by Odin in Galactic Centre directions – an intricate mixture of broad emission and narrow as well as broad absorption features from low excitation foreground gas (cf. Fig. 1(d)) are discussed by Sandqvist et al. (2003). 3.2. H2 O and H2 18 O in comets Fig. 1(c) displays the strong, very narrow H2 O line observed at 557 GHz from comet C/2001 A2 (LINEAR), here using the broad band acousto-optical spectrometer (AOS). Odin has mapped the water evaporation in several comets at high (80 m s1 ) spectral resolution (Lecacheux et al., 2003). A preliminary map at 10 spacing of the strong H2 O line from the recent bright comet 153P/2002 Cl (Ikeya-Zhang) is shown in Fig. 2, demonstrating that the emission region is extended with respect to the Odin 12600 antenna beam. Comet mapping in fact also has been an important ingredient in the Odin pointing verification. After 40 h of observations Odin was also able to detect the weak H2 18 O (110 –101 ) emission from comet Ikeya-Zhang. The observed H2 18 O intensity verifies the
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Fig. 1. Sample Odin observations of H2 O in molecular clouds and comets: (a) High velocity H2 O outflow/shock spectrum in the core of the Orion KL molecular cloud. (b) Narrow H2 O line in the quiescent cloud 20 south of Orion KL. (c) Very narrow H2 O line from Comet C/2001 A2 (LINEAR). (d) Complicated mixture of H2 O emission and absorption towards Sgr A . (e) the broad H2 O emission line resulting from the IC443 supernova remnant shock. We have here also indicated the simultaneously used (on a second mixer also tuned to H2 O) auto-correlation spectrometer bands (allowing higher velocity resolution).
very high H2 O production rate of 26 1028 molecules s1 determined from the H2 O data, and is consistent with a 16 O/18 O isotope ratio ¼ 450 50, as discussed by Lecacheux et al. (2003). In our comet observations two mixers were tuned to H2 O, or H2 18 O, each connected to a high-resolution auto-correlation spectrometer (the selected bandwidth, 100 MHz, is in-
dicated in Fig. 1(c)), and one of them also to the broadband AOS. 3.3. H2 O and H2 18 O in the Orion KL molecular cloud core An Orion KL H2 O map at 10 spacing as observed by OdinÕs 2.10 antenna beam is displayed in Fig. 3. This
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Fig. 2. Preliminary Odin map at 10 spacing of the strong H2 O line from comet Ikeya-Zhang.
Fig. 3. Odin H2 O map at 10 spacing of the Orion KL region.
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Fig. 4. H2 O spectra from the Orion KL centre, as observed by Odin and SWAS (dashed; scaled by 2.4).
Odin result as well as detections of the ortho-H2 18 O (110 – 101 ) line at 547.676 GHz at the cloud centre (outflow and quiescent gas) and from quiescent gas 20 south of the outflow centre are presented by Olofsson et al. (2003), who also outline the physical and chemical conditions in the region and provide a number of relevant references. The Odin map centre H2 O spectrum is shown separately in Fig. 4. Here the H2 O emission dominantly appears to emanate in the well-known outflow/shock region. No narrow-line emission from the quiescent molecular cloud, or from the compact hot core source, is visible. The emission from the ‘‘blue-shifted’’ (with respect to the ambient cloud, ‘‘ridge’’, velocity of 8–10 km s1 ) part of the flow is considerably weaker than the ‘‘red-shifted’’ one. Here self-absorption in the optically thick outflow could play a role – just as observed by Odin in expanding cometary H2 O comae (Lecacheux et al., 2003). Such an effect has been observed and modelled in optically thick CO emisson from the mass-loss envelope around the carbon star IRC+10216 (cf. Crosas and Menten, 1997). Moreover, the abrupt H2 O brightness decrease at velocities blueward of about 10 km s1 is not an Odin instrument artifact, since this feature is clearly
Fig. 5. Comparison of the SWAS Orion KL H2 O spectrum (dashed) with the spectrum resulting from our convolution of OdinÕs H2 O map (Fig. 3) to the angular resolution of SWAS.
verified in our independently observed AOS spectrum (shown in Fig. 1(a)). Absorption by lower excitation H2 O foreground gas (in the molecular cloud envelope) at ambient cloud velocities here could play a role. In Fig. 4, the corresponding SWAS water spectrum also is shown (observed with a beam size of 3:30 4:50 ; Melnick et al., 2000b, kindly supplied in digital form). The SWAS spectrum here has been scaled by a factor of 2.4 to match the outflow line-wings observed by Odin. Two important facts are apparent from this comparison: (1) The very good match of the H2 O line-wings as observed by the two telescopes (after scaling the SWAS spectrum by a factor 2.4), which we can use to estimate the size of the outflow/shock region. If the scaling factor is interpreted as the Odin/SWAS beam filling ratio for a Gaussian source, this is consistent with an H2 O outflow FWHP size of circa 11000 . A similar size is estimated from our Odin H2 O mapping (cf. Olofsson et al., 2003). The H2 O outflow size observed by Odin verifies the very high H2 O abundance of 5 104 (vs H2 ) estimated by Melnick et al. (2000b) for the high-velocity, shocked gas component. (2) The larger SWAS beam appears to ‘‘pick up’’ ambient cloud H2 O emission not visible in the Odin spectrum. This latter fact easily can be verified by a convolution of the OdinÕs H2 O map of the Orion KL region (Fig. 3) to the angular resolution of SWAS. The resulting ‘‘SWAS-convolved’’ Odin map centre spectrum is shown in Fig. 5, together with the H2 O spectrum observed by SWAS. The two almost identical spectra also demonstrate the very consistent calibration of the Odin and SWAS H2 O data sets. The very strong H2 O signals, paired with OdinÕs very clean antenna beam with side-lobes well below 25 dB,
Fig. 6. Preliminary MEM-deconvolved Orion KL H2 O map, as observed with a 6000 antenna beam (see text).
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Fig. 7. H2 18 O spectra towards the Orion KL centre, observed by Odin and SWAS (gray; scaled by 3.3).
and the very precise attitude reconstruction (cf. Olofsson et al., 2003), has tempted us to try a MEM-type deconvolution of the position-velocity cube of OdinÕs Orion KL H2 O map. A very preliminary result, convolved with a 6000 Gaussian beam and here displayed at 6000 spacing, is shown in Fig. 6. While the FWHP size of the H2 O outflow/shock region is only about 6000 , more extended lower intensity H2 O outflow/shock emission is indeed present. Ultimately we hope (by using the much larger Odin position-velocity cube data set now available) to reach a ‘‘beam-size’’ of 4000 – similar to the antenna beam at 557 GHz of the future Herschel Space Observatory (Harwit, 2004). OdinÕs H2 18 O spectrum towards Orion KL is shown in Fig. 7, together with the corresponding SWAS spectrum (Melnick et al., 2000b). The latter H2 18 O spectrum here has been scaled by a factor 3.3 to fit the red wing of the Odin spectrum. This (beam filling) ratio is very close to the theoretical value for a very small source, 3.37, indicating an H2 18 O outflow size much smaller than that observed by Odin in
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case of H2 O. The broad SO2 (286;22 –285;23 ) line visible near 65 km s1 also nicely scales by a factor of 3.3, indicating a similarly small SO2 outflow size, which may be expected for this high energy line (Eu ¼ 475 K). We argue that the H2 18 O outflow emission – visible in a much narrower velocity range than is observed in the H2 O line must originate in the compact low-velocity flow and estimate an H2 O abundance of circa 105 compared to all H2 in this flow (Olofsson et al., 2003). This H2 O abundance is an order of magnitude lower than that earlier estimated for the shocked gas in the spatially more extended high-velocity flow. In Fig. 8 we show the H2 18 O spectrum observed by Odin in a quiescent cloud position, 20 south of the Orion KL outflow centre. This very probable detection allows us to estimate, for the first time by means of a (presumably) optically thin line, the ambient cloud H2 O abundance (cf. Olofsson et al., 2003). The resulting H2 O abundance is about 1 108 , in good agreement with the low abundance values derived from SWAS H2 O mapping (Snell et al., 2000). 3.4. First detections of the NH3 (10 –00 ) line Simultaneously with observations of H2 O, or H2 18 O, and searches for O2 , we also tuned one of our receivers to the ortho-NH3 (10 –00 ) line at 572 GHz. Larsson et al. (2003) present the detection of quiescent cloud as well as outflow NH3 (10 –00 ) emission in Orion KL cloud core, quiescent cloud emission 20 south, and the first detection of NH3 in the Orion Bar. The first detection of the NH3 (10 –00 ) line in a dark cloud is discussed by Liseau et al. (2003). 3.5. Deep odin searches for molecular oxygen Simultaneously with our submm wave observations of H2 O, H2 18 O, and NH3 , we have also been searching for the O2 (11 –00 ) at 118.750 GHz using our low noise HEMT receiver. In spite of some frustrating phaselocking problems, OdinÕs sensitive searches have led to very low O2 abundance limits, especially in cold dark clouds (<107 ; more than an order of magnitude below published SWAS limits; cf. Goldsmith et al., 2000), as reported by Pagani et al. (2003). Such low O2 abundance limits are very difficult to accommodate in current chemical models (cf. Goldsmith et al., 2002; Pagani et al., 2003).
Acknowledgements
Fig. 8. H2 18 O spectrum observed by Odin 20 south of the Orion KL outflow centre.
Generous financial support by Research Councils and Space Agencies in Canada, Finland, France, and Sweden is gratefully acknowledged. We also want to express our gratitude to the crews at the SSC Odin operations
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centres in Kiruna and Solna for their skilful and dedicated work. The friendly attitude of Gary Melnick and the SWAS team, who upon our request supplied us with carefully reduced data for a number of targets, is greatly appreciated. Finally, the author wants to thank all Odin colleagues for their trust in him during good as well as bad times. References Crosas, M., Menten, K.M. Physical parameters of the IRC+10216 circumstellar envelope: new constraints from submillimeter observations. Astrophys. J. 483, 913–924, 1997. Ehrenfreud, P., Charnley, B.S. Organic molecules in the interstellar medium, comets, and meteorites: a voyage from dark clouds to the early earth. Annu. Rev. Astron. Astrophys. 38, 427–483, 2000. Frisk, U., Hagstr€ om, M., Ala-Laurinaho, J., et al. The Odin satellite, I. radiometer design and test. Astron. Astrophys. 402, L27–L34, 2003. Goldsmith, P.F., Melnick, G.J., Bergin, E.A., et al. O2 in interstellar molecular clouds. Astrophys. J. 539, L123–L127, 2000. Goldsmith, P.F., Li, D., Bergin, E.A., et al. Tentative detection of molecular oxygen in the rho Ophiuchi cloud. Astrophys. J. 576, 814–831, 2002. Harwit, M. The Herschel Mission. Adv. Space Res., this issue, 2004, doi:10.1016/j.asr.2003.03.026. Jacobsson, B., Nylund, M., Olsson, T., Vinterhav, E., The highperforming attitude control system of the scientific satellite Odin, IAC-02-A.P.13, World Space Congress, Houston, 2002. Jacobsson, B., Nylund, M., Olsson, T., et al., Odin Star tracker/gyro calibration and attitude reconstruction for the scientific satellite Odin – in flight results, IAC-02-A-4.01, World Space Congress, Houston, 2002.
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