L77 THE SUBMILLIMETER WAVE ASTRONOMY ... - IOPscience

The Astrophysical Journal, 539:L77–L85, 2000 August 20 q 2000. The American Astronomical Society. All rights reserved. Printed in U.S.A.

THE SUBMILLIMETER WAVE ASTRONOMY SATELLITE: SCIENCE OBJECTIVES AND INSTRUMENT DESCRIPTION Gary J. Melnick,1 John R. Stauffer,1 Matthew L. N. Ashby,1 Edwin A. Bergin,1 Gordon Chin,2 Neal R. Erickson,3 Paul F. Goldsmith,4 Martin Harwit,5 John E. Howe,3 Steven C. Kleiner,1 David G. Koch,6 David A. Neufeld,7 Brian M. Patten,1 Rene´ Plume,1 Rudolf Schieder,8 Ronald L. Snell,3 Volker Tolls,1 Zhong Wang,1 Gisbert Winnewisser,8 and Yun Fei Zhang1 Received 1999 December 9; accepted 2000 June 20; published 2000 August 16

ABSTRACT The Submillimeter Wave Astronomy Satellite (SWAS), launched in 1998 December, is a NASA mission dedicated to the study of star formation through direct measurements of (1) molecular cloud composition and chemistry, (2) the cooling mechanisms that facilitate cloud collapse, and (3) the large-scale structure of the UV-illuminated cloud surfaces. To achieve these goals, SWAS is conducting pointed observations of dense [n(H 2 ) 1 10 3 cm23] molecular clouds throughout our Galaxy in either the ground state or a low-lying transition of five astrophysically important species: H2O, H218 O , O2, C i, and 13CO. By observing these lines SWAS is (1) testing long-standing theories that predict that these species are the dominant coolants of molecular clouds during the early stages of their collapse to form stars and planets and (2) supplying previously missing information about the abundance of key species central to the chemical models of dense interstellar gas. SWAS carries two independent Schottky barrier diode mixers—passively cooled to ∼175 K—coupled to a 54 # 68 cm off-axis Cassegrain antenna with an aggregate surface error ∼11 mm rms. During its baseline 3 yr mission, SWAS is observing giant and dark cloud cores with the goal of detecting or setting an upper limit on the water and molecular oxygen abundance of 3 # 1026 (relative to H2). In addition, advantage is being taken of SWAS’s relatively large beam size of 39. 3 # 49. 5 at 553 GHz and 39. 5 # 59. 0 at 490 GHz to obtain large-area (∼17 # 17 ) maps of giant and dark clouds in the 13CO and C i lines. With the use of a 1.4 GHz bandwidth acousto-optical spectrometer, SWAS has the ability to simultaneously observe either the H2O, O2, C i, and 13 CO lines or the H218 O, O2, and C i lines. All measurements are being conducted with a velocity resolution less than 1 km s21. Subject headings: ISM: clouds — stars: formation — submillimeter more readily than gas rich in atoms and molecules having only high-lying first excited states, such as O i, or molecules with no dipole moment, such as O2. The goal of the Submillimeter Wave Astronomy Satellite (SWAS) mission is to investigate these various aspects of star formation by focusing on a few key species. Table 1 lists the five species observed by SWAS in order of transition frequency. Figure 1 shows the relevant portion of the energy-level diagrams for O2, C i, 13CO, and ortho-H216 O along with several other transitions within these species of astronomical interest. By observing O2, H2O, and H218O, SWAS addresses two questions central to our understanding of molecular cloud chemistry and thermal balance: (1) Where is all of the oxygen in the interstellar medium? and (2) Are water and, in some cases, O2 dominant cloud coolants? By observing C i and 13CO, SWAS is able to study the UV-illuminated surfaces of molecular clouds. The relative strength and distribution of these two species offer important insights into the clumpiness of the gas, the stratification of atomic and molecular gas, and the gas temperatures. The observing strategy for SWAS is twofold: (1) to establish the presence of, or set a scientifically interesting abundance upper limit on H2O and O2, and (2) to map the large-scale distributions of C i and 13CO. Since little is known about the H2O distribution and even less is known about the O2 abundance and distribution, more than half of the mission will be devoted to searching for and mapping the distributions of these species. The remainder of the 3 yr baseline mission will be dedicated to conducting large-scale (∼17 # 17) C i and 13CO mapping toward approximately 20 specifically interesting clouds. Section 2 of this Letter briefly reviews the science motivations for the mission. Section 3 contains an overview of the

1. INTRODUCTION

The process of star formation is tightly linked to the chemistry, composition, structure, and thermal balance within molecular clouds. Gas-phase and grain mantle chemical reactions build complex molecules, while structural factors, such as the presence of strong shocks and permeability to UV radiation, contribute to the destruction of molecules. Atoms and molecules can also be effectively removed from the gas phase through adsorption onto dust grains, a means of depletion that is believed to be particularly efficient in the cold (T & 30 K) dense [n(H 2 ) * 10 5 cm23] gas typical of many interstellar cloud cores. The balance of these processes determines the composition of the gas and thus the abundances of atoms and molecules available to cool interstellar clouds. This is of crucial importance to the star formation process because molecular cloud cores can only undergo gravitational collapse insofar as gravitational potential energy can be radiated away. Consequently, cores possessing a high abundance of species with low-lying (E/k & 30 K) easily excited transitions, such as C i, H2O, and CO, will cool more efficiently and facilitate collapse 1 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138. 2 NASA Goddard Space Flight Center, Greenbelt, MD 20771. 3 Five College Radio Astronomy Observatory, University of Massachusetts, Lederle Graduate Research Tower, Amherst, MA 01003. 4 National Astronomy and Ionosphere Center, Department of Astronomy, Cornell University, Space Sciences Building, Ithaca, NY 14853-6801. 5 511 H Street SW, Washington, DC 20024-2725; also Cornell University. 6 NASA Ames Research Center, Moffett Field, CA 94035. 7 Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218. 8 I. Physikalisches Institut, Universita¨t zu Ko¨ln, Zu¨lpicher Strasse 77, D-50937 Ko¨ln, Germany.

L77

L78

SWAS OBJECTIVES AND INSTRUMENT DESCRIPTION

Vol. 539

TABLE 1 Spectral Lines Observed by SWAS

Species

Transition

Energy above Ground State (E/k) (K)

O2 . . . . . . . . . C i ......... H218O . . . . . . 13 CO . . . . . . . H2O . . . . . . .

3, 1–3, 2 3 P1–3P0a 110–101a J p 5–4 110–101a

26 24 26 79 27

Frequency (GHz)

Wavelength (mm)

Critical Density (cm 23)

487.249 492.161 547.676 550.926 556.936

615.276 609.134 547.390 544.161 538.289

103 104 109 b 3 # 105 109 b

a

Ground-state transition. The critical density for H2O will likely be less than this value by a factor of 102–104 as a result of significant radiation trapping in this line. The critical density for H218O could be reduced by a factor of 1.5–50 because of the same effect. b

SWAS instrument and spacecraft, while § 4 describes the mission profile. Section 5 presents the measured on-orbit performance, and finally, § 6 summarizes the data products and the data distribution plans. 2. SCIENCE OBJECTIVES

The distribution of oxygen and carbon in atomic and molecular form in the gas phase and on the surfaces of grains is a critical unknown in understanding the chemistry, the ionization structure, and the thermal balance of interstellar clouds, and hence in predicting their evolutionary course. The abundances of the more complex carbon-containing interstellar molecules are directly responsive to the fraction of the carbon that is available as neutral atoms, and because of the different chemical reactivities of O, O2, and H2O, the overall molecular composition is sensitive to the oxygen distribution. Thermally, the importance of oxygen- and carbon-bearing species is magnified because none of the most abundant species—H, H2, and He—possess strong transitions from the ground state having

energies equivalent to less than 500 K, making them insignificant gas coolants within quiescent molecular clouds. However, among the possible molecules containing oxygen and carbon there are many that, if present in sufficient quantity, could be powerful gas coolants. For these reasons, understanding the oxygen and carbon chemistry and establishing the abundance of oxygen- and carbon-bearing species has been an important goal of both theoretical and observational studies. 2.1. Reservoirs of Gas-Phase Oxygen Although a complete inventory of chemical species is not available for any source, millimeter and submillimeter spectral line surveys toward a few sources (e.g., Orion BN/KL and Sgr B2) have detected thousands of lines from approximately 100 distinct chemical species (see Sutton et al. 1995; Schilke, Phillips, & Mehringer 1999). However, the sum of the abundances of the oxygen-bearing species thus detected appears to account for less than 20% of the elemental abundance of oxygen found in the solar system. Unfortunately, the repositories

Fig. 1.—Relevant portions of the energy-level diagrams for O2, C i, 13 CO, and ortho-H216O . The energy-level diagram for ortho-H218O is similar in structure to that of ortho-H216O. Several other transitions of astronomical interest are shown as dashed lines.

No. 2, 2000

MELNICK ET AL.

L79

Fig. 2.—Cooling rates per H2 molecule for H2O, CO, hydrides (other than H2O and H2), O2, oxygen atoms, and all other molecules, and total cooling rate for gas temperatures of 40 and 100 K (after Neufeld et al. 1995).

of interstellar oxygen that could account for this discrepancy—O i, H2O and O2, and H2O and O2 ices—have spectral signatures that lie at frequencies where the terrestrial atmospheric absorption prevents their study from ground-based telescopes, even at mountaintop sites. (Several H2O maser lines have been observed from the ground, but it is difficult to derive reliable abundances from these transitions.) This limitation has been overcome, in part, by the use of airborne and space-based telescopes. For example, using the Goddard High Resolution Spectrometer on board the Hubble Space Telescope, Meyer, Jura, & Cardelli (1998) have observed interstellar O i l1356 absorption toward 13 stars and infer that the total abundance of oxygen (gas plus grains) is homogeneous in the vicinity of the Sun and about two-thirds that of the solar value. Unfortunately, these measurements sample mostly lowdensity gas along the line of sight to these stars and say little about the reservoirs of oxygen within dense clouds. Spectrometers on board the Kuiper Airborne Observatory (KAO) and the Infrared Space Observatory (ISO) have been used to obtain 3 O i 63 mm (3P– P2) absorption-line measurements against bright 1 far-infrared continuum sources (e.g., Poglitsch et al. 1996; Kraemer, Jackson, & Lane 1998; Baluteau et al. 1997; Caux et al. 1999). These observations suggest that O i may be abundant in cold clouds, constituting more than 10% and perhaps as much as 90% of the solar oxygen abundance. Relatively few lines of sight have been sampled to date, and interpretation remains controversial. Spectrometers on board ISO have also been used to study gas-phase H2O along with H2O and O2 ices. However, because ISO operated at wavelengths shortward of 200 mm and was only able to detect water vapor transitions that lie more than 80 K above the ground state, it was primarily sensitive to gas warmer than that found throughout the bulk of dense molecular clouds. Thus, in most cases, it is hard to know whether the ISO-determined water abundances are representative of such clouds or if these results are affected by water liberated from grain mantles or chemical processing not favored at lower tem-

peratures. Similarly, the ISO band (2.5–200 mm) contained many O2 transitions. However, these transitions all lie more than 180 K above the ground state, and within molecular clouds the column density in any one of these lines would be expected to be quite small and undetectable by ISO. ISO studies of interstellar ices suggest that H2O ice may account for as much as 5%–25% of the elemental oxygen abundance in star-forming regions and quiescent clouds (see Schutte 1999 and references therein), whereas less than 6% of the elemental abundance of oxygen appears to be in the form of O2 ice (Vandenbussche et al. 1999). SWAS permits the opportunity to observe for the first time transitions of gaseous H2O and O2 whose energy above the ground state (see Table 1) is well matched to the temperatures typical of molecular clouds. Moreover, SWAS allows examination of a large number of lines of sight throughout the Galaxy. In addition, the &1 km s21 spectral resolution of SWAS (vs. *10 km s21 for ISO) allows SWAS to measure line profiles sufficiently well to distinguish between emission from outflows and shocked gas and more quiescent regions. 2.2. Thermal Balance in Molecular Clouds Theoretical studies of the thermal balance in dense [n(H 2 ) * 10 3 cm23] molecular clouds indicate that under a wide range of conditions the equilibrium gas temperature within such clouds is determined by the balance between heating processes and cooling in radiative transitions of interstellar molecules (see Goldsmith & Langer 1978; Neufeld, Lepp, & Melnick 1995). In particular, prior to the SWAS mission, it was predicted that for the range of conditions that would apply to a cloud core beginning its collapse [i.e., 10 K ≤ T ≤ 200 K and 10 3 cm23 ≤ n(H 2 ) ≤ 10 10 cm23], H2O, CO, and O i would be the dominant gas coolants. This model, being tested by SWAS, is illustrated in Figure 2, which shows the most important coolants for gas temperatures of 40 and 100 K. At higher temperatures (T 1 200 K) and densities [n(H 2 ) 1

L80


Vol. 539

Fig. 3.—Block diagram of the SWAS submillimeter signal detection system

107 cm23], H2O is predicted to assume an even greater share of the cooling, since the H2 cooling rate is quenched by collisional de-excitation and CO, with its smaller dipole moment (than H2O), becomes less effective. Within the outer UV-illuminated cloud surface, C1, C0, and CO are predicted (and observed) to be the dominant coolants. By observing H2O, C i, and 13CO, SWAS is being used to measure and map these primary gas coolants in clouds throughout the Galaxy. 2.3. The UV-illuminated Surfaces of Molecular Clouds Prior to 1980, models of dense interstellar clouds suggested that external illumination by the ambient Galactic UV field would create thin surface layers of ionized and atomic gas before the cloud interior became predominantly molecular. Strong and extended C1 and C0 emission, first observed from the NASA Lear Jet Observatory and the KAO (e.g., Russell et al. 1980; Phillips et al. 1980), highlighted the effect UV radiation can have on a cloud and dispelled the notion that molecular clouds can be understood as large homogeneous structures; these observations demonstrated conclusively that molecular clouds are clumped and porous to UV radiation. Both ground-based and Cosmic Background Explorer (COBE) follow-up studies in the 492 GHz C i line as well as mediumJ (3 ≤ J ≤ 8) CO lines have confirmed that emission in these lines is widespread and that such photon-dominated regions (PDRs) can constitute a large fraction of molecular clouds. While COBE’s low spectral resolution (0.45 cm21) and large beam size (77) were well suited for studying the very large scale diffuse line emission over the sky, COBE was ill suited for studying the structure of PDRs within individual cloud complexes. Alternately, 10–15 m diameter ground-based submillimeter telescopes with beam sizes of ≤150 have difficulty surveying extended cloud complexes because of the large number of positions to be sampled. To solve this problem, several smaller ground-based telescopes have been dedicated to the task of mapping in the C i line (see Plume, Jaffe, & Keene 1994; Ingalls et al. 1997; Ikeda et al. 1999). Nonetheless, these facilities are still limited by meteorological conditions that vary during the course of making a large map. For this reason,

calibration corrections over the area of a large map can be uncertain, and observing low surface brightness emission can be extremely difficult or impossible. As an adjunct to the primary lines of interest—O2 and H2O—it is possible to observe the C i 492 GHz transition (in the upper sideband of the O2 receiver) and the 13CO J p 5–4 551 GHz transition (in the lower sideband of the H2O receiver). With its ∼49 beam size and the high system stability possible in space, SWAS is very well suited to map large areas of the sky in the C i and 13CO J p 5–4 lines. Given its critical density of only ∼1000 cm23 and an upper level energy of only 23.6 K above the ground state, C i is easily excitable wherever neutral carbon exists. It is therefore a good tracer of the total extent of PDRs. In contrast, the 13CO J p 5–4 line arises from a state that is 79 K above the ground state and has a critical density of ∼105 cm23. Hence, this line preferentially traces the warm, dense gas in PDRs. 3. INSTRUMENT DESCRIPTION

The instrument portion of the satellite is composed of (1) the telescope, (2) two heterodyne receivers, (3) an acoustooptical spectrometer (AOS), (4) the thermal control system, (5) the instrument control electronics, (6) the star tracker, and (7) the instrument structure. The spacecraft portion of the satellite is composed of (1) the attitude control system (ACS), (2) the solar arrays and power regulating hardware, (3) the onboard command, data, and ACS computer, (4) the solid-state memory for data recording, and (5) all data uplink and downlink receivers and transmitters. As a result of the passive cooling design, described below, and the use of reaction wheels and torque rods to maneuver the spacecraft, SWAS carries no expendables. Therefore, barring the failure of a critical component, SWAS’s ultimate lifetime is limited only by orbit decay, i.e., between 6 and 10 yr depending on the severity of 2001 solar maximum. This section focuses mainly on a description of the instrument hardware. A more complete description of the spacecraft can be found in Watzin (1994). A block diagram of the signal detection subsystem, elements of which are described below, is shown in Figure 3, and a

No. 2, 2000

MELNICK ET AL.

summary of key instrument parameters is given in Table 2. The SWAS optical subsystem consists of a 54 # 68 cm diameter f/0.76 # f/0.59 off-axis Cassegrain primary mirror, a secondary mirror, a secondary mirror chopping mechanism, a calibration load, and a calibration load flip mirror. Both the primary and secondary mirrors are made of aluminum. The combined measured rms surface accuracy, which is dominated by imperfections in the primary mirror (the secondary mirror surface having been diamond-turned and being of optical quality), is 11 mm within the inner 80% (by area) of the primary mirror and increases to about 15 5 3 mm at the outer edge. For sources that appear pointlike within the SWAS beam, a chopping secondary mirror mechanism is available for beam switching 89. 5 on the sky along a fixed axis coinciding with the smaller dimension of each beam (see Table 2). The chopping rate is selectable among three options: 2 Hz, 14 Hz, and off. The 2 Hz rate is used in conjunction with the total power continuum detectors associated with each receiver channel and is synchronized with their readout. The 14 Hz rate is used in conjunction with spectroscopic observations and is synchronized with the readout rate of the AOS. When the chopping mechanism is off, the incoming beam is on-axis. The nodding mode is employed when the spacecraft (and not the chopper) is used to move the telescope over spatial distances greater than 89. 5, as is required for extended sources. The SWAS spacecraft has been designed to enable spatial nodding over distances as large as 37 with a slew-and-settle time of ≤15 s. Smaller angle nods require less time. A blackbody radiator at ∼270 K thermally connected to the instrument structure is used in combination with blank-sky measurements to periodically measure the system noise temperature. This oversized calibration load is viewed by engaging a flip mirror (see Fig. 3). The detection system consists of two independent second harmonic Schottky diode mixers, operating in orthogonal linear polarizations, pumped by frequency-tripled InP Gunn oscillators. The receiver 1 oscillator frequency is 81.5 GHz, which is used to observe O2 at 487 GHz and C i at 492 GHz in the lower and upper sidebands, respectively. The receiver 2 oscillator frequency is 92.3 GHz, which is used to observe 13CO at 551 GHz and H2O at 557 GHz in the lower and upper sidebands, respectively. A fifth line, the ground-state transition of orthoH218 O at 548 GHz, can be observed by tuning receiver 2 slightly beyond its nominal operating range, which results in a system noise temperature about 2 times higher. Thus, the choice exists to simultaneously observe either the O2, C i, 13CO, and H2O lines or, when measurements of isotopic water are desired, the O2, C i, and H218 O lines. The cooled front end (CFE) consists of input optics, a mixer, a tripler, and an intermediate-frequency (IF) high electron mobility transistor amplifier for each of the two receiver channels. The CFE is passively cooled to an on-orbit temperature between 171 and 177 K, depending on spacecraft orientation and the longer term variations in the Earth-Sun-SWAS geometry owing to orbit precession. Through a combination of thermal design and observing strategy, temperature changes for these components have been kept to less than 1 K within a 24 hr period and less than 0.15 K within an hour. The output of each receiver is down-converted and diplexed with a resulting bandwidth per channel of 700 MHz and an input band to the AOS of 1.4–2.8 GHz. The SWAS spectrometer is a single AOS with 1400 1 MHz channels (about 1700 kHz resolution bandwidth and about 2100 kHz equivalent noise bandwidth per channel). This yields a velocity channel spacing of approximately 0.6 km s21 and a

L81 TABLE 2 SWAS Instrument Summary Parameter

Value

Telescope . . . . . . . . . . . . . . . . . . . . . . . . . Aperture efficiency . . . . . . . . . . . . . . . Main-beam efficiency . . . . . . . . . . . . Beam size: 490 GHz . . . . . . . . . . . . . . . . . . . . . . . 553 GHz . . . . . . . . . . . . . . . . . . . . . . . Mirror surface accuracy . . . . . . . . . . Absolute pointing accuracy . . . . . Jitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Receiver type . . . . . . . . . . . . . . . . . . . . . Receiver temperature . . . . . . . . . . . . . Operating frequencies: Receiver 1: O2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . C i ........................... Receiver 2: H2O . . . . . . . . . . . . . . . . . . . . . . . . . . 13 CO . . . . . . . . . . . . . . . . . . . . . . . . . H218O . . . . . . . . . . . . . . . . . . . . . . . . Receiver noise temperature: Receiver 1 (DSB) . . . . . . . . . . . . . Receiver 2 (DSB) . . . . . . . . . . . . .

54 # 68 cm diameter off-axis Cassegrain 66% 90%

Back-end spectrometer . . . . . . . . . . . Velocity resolution . . . . . . . . . . . . . . . Orbit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mission lifetime . . . . . . . . . . . . . . . . . .

39. 5 # 59. 0 39. 3 # 49. 5 ! l/50 total error at 557 GHz ≤50 (1 j) ≤50 (1 j) Schottky barrier diode harmonic mixers 171–177 K (passively cooled) 487 GHz 492 GHz 557 GHz 551 GHz 548 GHz 2500 K 2200 K 4000 K (H218O) 1.4 GHz bandwidth (⇔840 km s21) AOS 0.6 km s21 650 km; 707 inclination ≥3 yr

total bandwidth of 840 km s21, or about 210 km s21 per line. The SWAS AOS incorporates redundant Hitachi 780 nm laser diodes, a Marconi LiNbO3 Bragg cell, and a Thomson 1728 pixel linear CCD. The digitized output of the CCD is coaveraged within the AOS electronics unit and forwarded, via the instrument control electronics, to the onboard solid-state memory within the spacecraft for later transmission to the ground. To ensure that the SWAS spectral lines are centered in the AOS, regardless of their v LSR within the Galaxy, the Gunn oscillators are tunable in steps of 2 MHz over a frequency range of 550 MHz about their nominal (unmultiplied) frequency. This tuning range translates to an observational frequency range of 5300 MHz at 490 and 553 GHz, or 5182 km s21 in receiver 1 and 5164 km s21 in receiver 2, in commandable steps of 7.3 and 6.6 km s21, respectively. (Because of the relatively coarse local oscillator steps, Doppler corrections due to spacecraft motion are made in software; see § 6.) The frequency tuning parameters are stored in a command table on board the SWAS spacecraft together with other receiver settings. In this way, the receiver setup parameters for each source can be uplinked in a very condensed form by referring only to the appropriate index number of the table entries. As mentioned above, SWAS also carries two broadband (650 MHz) total power continuum detectors, one associated with each receiver. The continuum detectors are used primarily in chopped mode when observing pointlike sources, such as planets, for (1) verifying the co-alignment of the two submillimeter beams, (2) verifying the co-alignment of the submillimeter beams and the star tracker, and (3) establishing flux calibration. The thermal control subsystem consists of three Winston cone passive radiators, two equipment plate radiators (part of the outer shell structure), the instrument shell (part of which also serves as a sunshade), a Goretex cover over the telescope aperture, selected surface finishes on various parts and assemblies, and multilayer insulation blankets to provide additional thermal isolation where required. The cold radiators, located

L82


Vol. 539

Fig. 4.—Cutaway view of the SWAS instrument. In the foreground are the three Winston cone shades, at the base of which are the cold radiator plates. These radiators cool the CFE (mixers and first IF amplifier stages) to a temperature of ∼175 K. The CFE and the structure that holds the secondary mirror are located directly behind the radiators. The remainder of the electronics and the AOS components are mounted onto a baseplate under the primary mirror. The baseplate mounts to the top of the spacecraft. A Goretex cover over the primary mirror, which serves to protect the system against inadvertent excursions within the Sun avoidance angle, is not shown.

at the base of each Winston cone, view only dark sky during normal mission operations (i.e., avoiding Earth and Sun radiation), thus cooling the CFE to which they are thermally connected, to ∼171–177 K. The equipment plate radiators, located on opposite sides of the instrument, maintain a nominal operating temperature environment of ∼167C for all electronic boxes mounted to the instrument baseplate. The Goretex cover, located over the telescope aperture, protects the telescope and receivers in the event of the Sun traversing the field of view during launch and early orbit operations or of anomalous spacecraft behavior during the mission. A cutaway view of the instrument is shown in Figure 4. The SWAS submillimeter receivers were built by Millitech Corporation; the AOS was built by the University of Cologne, Germany; and the optics, cooling radiators, star tracker, and instrument structure as well as the thermal design, systems integration, and testing were the responsibility of Ball Aerospace Systems Group. The Smithsonian Astrophysical Observatory had overall responsibility for the instrument and provided oversight for all aspects of the instrument design, construction, and testing. The SWAS spacecraft was built by NASA Goddard Space Flight Center (GSFC). 4. ORBIT AND OBSERVATIONS

SWAS was launched on 1998 December 5 into a 650 km, 707 inclination circular orbit from which it conducts observations in a “point and integrate” mode. As SWAS orbits the Earth, sources become visible outside of the 757 Sun and 457

Earth limb avoidance angles. SWAS initially acquires and then inertially points toward a source until either a higher priority source becomes available or an avoidance angle is approached. SWAS typically observes two to four sources per 97 minute orbit. For purposes of mapping or long integrations, SWAS can return to a given source on successive orbits, accumulating as much as ∼10 hr of observing time on a single source per day. Since the beginning of the mission, SWAS has spent an average of 85% of the time taking data on astronomical sources (vs. slewing between sources or sitting at waypoints waiting for a source to rise). This high observing efficiency is achieved through a combination of a rapid slewing capability—an average of 17 per second—and a star tracker whose 87 field of view and sensitivity to stars as faint as 6 mag enables SWAS to acquire sources over the entire sky. Observations are scheduled in blocks of 1 week. Since fluxcalibrated spectra are produced daily (see § 6) and represent observations no older than 48 hr, weekly planning generally relies heavily on the results of the previous week. In this way, the SWAS mission is very flexible and seeks to avoid the inefficiencies of scheduling either too much or too little time on a given source. A 1 week observing timeline is generated in seven segments at the SWAS Science Operations Center (SOC) at the Harvard-Smithsonian Center for Astrophysics, after which the entire timeline is electronically forwarded to GSFC, and one segment is uplinked daily. With the exception of a few point sources—notably stars and planets—all observations have been conducted in nodded

No. 2, 2000

MELNICK ET AL.

L83

Fig. 5.—Far-field beam pattern for the 490 GHz (left) and the 553 GHz (right) receivers derived from raster scans of Jupiter. The contour levels are 21, 23, 25, 27, and 29 dB relative to the peak.

(vs. chopping) mode. This approach ensures that good reference positions are always used and, because there is no change in the optical path between on-source and off-source reference observations, the spectral baselines are generally very flat. Reference positions can be up to 37 from the on-source position in any direction and are selected for each source to coincide with the closest position exhibiting no detectable 12CO J p 1–0 emission. On-orbit tests indicate that the receiver-AOS system is radiometrically stable over timescales of ∼160 s. Based on these results, nodding between on- and off-source positions is conservatively set at every 50 s, yielding a total on 1 off 1 slew time of approximately 115 s. 5. ON-ORBIT PERFORMANCE

SWAS was not designed to use state-of-the-art SIS mixers operating at 4 K because mission weight and volume constraints prohibited SWAS from accommodating stored cryogens along with a primary mirror greater than 0.5 m in diameter and because power limitations ruled out the use of mechanical coolers. To compensate for this loss in sensitivity, care was taken to ensure that the optics were as efficient as possible, that sources of spurious noise were suppressed, and that temperaturesensitive components, such as the CFE, dual-IF, and AOS were always kept stable to better than 0.15 K hr21. These measures resulted in long integrations times being possible for all observations (see discussion below). At the on-orbit operating temperatures of 171–177 K, the measured system noise temperatures are approximately 2500 K (double sideband [DSB]) in receiver 1 (490 GHz) and 2200 K (DSB) in receiver 2 (553 GHz). These values have been quite stable; noise temperature measurements made in 1998 December, 1999 May, and 1999 November agree to within 1%. The prelaunch pointing and jitter goals for SWAS were 380 and 190 (1 j), respectively, corresponding to one-fifth and one-tenth of the diameter of the smallest dimension of the SWAS 553 GHz beam. On-orbit tests show that SWAS’s absolute pointing ability is approximately 50 (1 j) with a comparable value for the jitter. Figure 5 shows the 490 and 553 GHz far-field beam patterns

as determined by raster scanning Jupiter across the telescope. The beams are symmetrical and elliptical (as designed), with no evidence for vignetting or other distortions. The beam FWHMs listed in Table 2 match those predicted by diffraction theory to within 5% for the SWAS telescope with an 11 dB Gaussian edge taper. Strip scans of Jupiter 5109 across the beam minor axes and 5129 across the beam major axes demonstrate that the sidelobes’ suppression exceeds the 215 dB design goal. These observations confirm the results of prelaunch instrument testing, which showed the highest sidelobe to be suppressed by ∼217 dB with all other sidelobes below 230 dB out to 159 from the beam centers (the limit of these measurements). Beam centroiding on Jupiter showed that the 490 and 553 GHz beam centers are co-aligned to less than 50, or about 1/40th of the FWHM of the minor axis of the 553 GHz beam. Using Mars as a primary calibrator, a receiver 1 aperture efficiency of 66% and a main-beam efficiency of 90% were determined. It is estimated that these efficiencies have an associated error of 55%. As a result of the presence of a Martian water absorption feature in the receiver 2 band (see Gurwell et al. 2000), there is no direct calibration for this channel. However, observations of the Moon enabled crosscalibration of the 490 and 553 GHz bands, and these data are consistent with equivalent efficiencies in both receiver channels within the stated errors. Based on several different measurements, a sideband ratio of 1 : 1 (to within an error of a few percent) is inferred in both receivers. First, prelaunch tests using an absorption cell directly established a sideband ratio of 1 : 1 in each receiver. At the same time, noise temperature measurements were made. Subsequent to launch, repeated determinations of the noise temperature in each receiver agree with prelaunch values to within ∼3%, implying no significant change in system performance. Second, receiver 2 has been used to obtain both an H2O and H218 O spectrum toward Sgr B2 (see Neufeld et al. 2000). At LSR velocities not believed to be associated with the poorly understood excitation conditions within Sgr B2, the H2O signal strength drops to 0.5 of the signal strength outside of the absorption feature at every velocity where there is evidence for

L84


Vol. 539

Fig. 6.—Measured decrease in spectral noise vs. integration time for spectral channels devoid of line emission. The dotted line in each panel represents the theoretical performance for each receiver.

H218 O absorption. This is consistent with 100% absorption in the H2O line and a sideband ratio in receiver 2 of 1 : 1. To date, no such absorption features have been found in the receiver 1 band. However, observations of the C i line strength made in the upper sideband of receiver 1 agree to within 10% with measurements obtained using ground-based telescopes with comparable beam sizes (e.g., Plume et al. 1999). Similarly, the CH3OH A 242–41 transition (n0 p 486.941 GHz) was detected in the lower sideband and the 41–30 transition (n0 p 492.279 GHz) was detected in the upper sideband of receiver 1 toward Orion BN/KL. The ratio of antenna temperatures of these lines, 0.3, is consistent with the physical conditions in this hot, dense molecular core (n H 2 . 10 5.5 ; Tk . 100 K) and further suggests that the prelaunch-measured receiver 1 sideband ratio of 1 : 1 still pertains. One of the most challenging goals of the SWAS instrument design was to ensure that all sources of noise and instability be kept low enough to enable integrations of up to 30 hr during which the spectral noise would be reduced in accordance with the radiometer equation (i.e., ∝ 1/Îtime). On-orbit measurements demonstrate that this goal has been met and exceeded. As shown in Figure 6, on-source integration times of 80 hr in receiver 1 and 50 hr in receiver 2 show a continuing decrease in spectral noise in agreement with theory (receiver 1 has more integration time than receiver 2 only because the integration time on receiver 2 is split between observing 13CO/H2O and H218 O). At an integration time of 80 hr there is as yet no evidence that a noise floor has been reached, the resulting baselines remain flat, and no spurious lines due to leakage from on-board oscillators have been seen. We have also analyzed the statistics of the on-orbit instrumental noise and found it to be very nearly Gaussian. Our analysis involved fitting spectral lines at 10,000 randomly chosen spectral positions within portions of the band that are believed to be devoid of astronomical features. In each case, we subtracted a baseline using a fourth-order polynomial fit and then fitted a narrow Gaussian line with a fixed width Dv FWHM p 3 km s21, a constrained position, and a peak antenna temperature Tpk, optimized to fit the data using a least-squares method. The value of Tpk is equally likely to be positive or

negative. We then compared the fitted line strengths with the standard error in the line strength, j(Tpk ) p j0 Nef21/2 f , where j0 is the rms noise in the spectrometer channels and Nef f is the effective number of channels within the fitted line. If the spectrometer channels were entirely uncorrelated, the effective number of channels for a Gaussian line would be 0.752Dv FWHM /Dvch, where Dvch is the channel width in velocity units. For the SWAS AOS spectrometer, Neff is reduced by a factor ∼2.3 as a result of correlations between adjacent and nearly adjacent spectrometer channels. The result of our analysis is 10,000 values of the ratio R p Tpk /j(Tpk), the distribution of which we investigated. In Figure 7 we show the fraction of all our fits for which FRF

Fig. 7.—Statistics of the instrumental noise, obtained from 10,000 line fits to SWAS spectral scans that are devoid of astronomical features. Solid lines show the fraction of those fits for which the ratio R of the best-fit line strength to the standard error in the line strength has an absolute value FRF greater than R0. Results are shown separately for positive (solid line) and negative (dotted line) best-fit line strengths. The dashed line shows the expected fraction for a perfectly Gaussian distribution with a mean of zero and a variance of unity.

No. 2, 2000

MELNICK ET AL.

exceeds some value R0, both for positive (solid line) and negative (dotted line) values of R. Also shown (dashed line) is the expected fraction for a Gaussian distribution with mean zero and variance unity. Figure 7 demonstrates that the noise is very nearly Gaussian out to R 0 ∼ 3, at which point small number statistics resulting from our use of only 10,000 line fits start to become evident. Thus, our analysis of the noise characteristics of the instrument suggests that when SWAS spectra reveal the presence of a weak emission or absorption feature, the probability that it is spurious can accurately be estimated using Gaussian statistics. 6. DATA PRODUCTS AND DATA DISTRIBUTION PLANS

The output of the back-end AOS, containing all of the simultaneously observed spectral lines (see § 3), is read out every 10 ms, coaveraged for 2 s (i.e., 200 spectra), and each 2 s coaverage is stored on board. These data, along with engineering and housekeeping data, are downlinked to a NASAcontrolled ground station twice per day. The two downlink data sets are checked for transmission errors, time-ordered, and merged at GSFC, then transferred to SAO once each day. Upon receipt of the data at the SOC, the data are automatically reformatted into separate FITS files for each observation and into a separate engineering database. The IRAF-based SWAS pipeline data reduction package then processes the data through several steps: aligning the spectra in velocity space to correct for Doppler shifts induced by spacecraft motion around the Earth; separately coaveraging all 2 s on-source and off-source scans for an object; performing the on minus off subtraction for adjacent on/off pairs; folding in the appropriate calibration load, zero signal, and blank-sky measurements; checking the frequency calibration using data from the onboard comb-line generator; and creating flux- and frequency-calibrated spectra.

L85

Since all observations are performed using the standard chopper wheel calibration method, the results are presented on the TA∗ scale (Kutner & Ulich 1981). This scale corrects for atmospheric attenuation (which is not a factor for SWAS), most of the ohmic loss (which is expected to be very small), and rearward spillover and scattering. For a source that fills the main beam of the antenna, the main-beam efficiency (see § 5) relates TA∗ to the main-beam brightness temperature Tmb. In addition to automatic checks, each day’s coaveraged calibrated spectra are quality-checked by eye and then added to the SWAS data archive within which all spectra of a given object are merged to produce single up-to-date coaveraged spectra. The data archive contains all of the coaveraged spectra organized into multiextension FITS files. An IRAF-based archive is maintained, with separate extensions for each of the five SWAS spectral lines for each source. In order to take advantage of the powerful analysis and presentation tools contained in the widely available radio spectral line analysis package CLASS, a separate set of archive spectra are stored as CLASSFITS files. SWAS is a principal investigator–class mission and, thus, no formal guest investigator program exists. However, following a proprietary period lasting 18 months after launch and onorbit commissioning, the first 6 months of SWAS data was made available to the general community via a Web-based system set up by the National Space Science Data Center. The first installment of SWAS data was available in 2000 June. Additional data will be added to this archive in 6 month increments. This work was supported by NASA contract NAS5-30702. R. Schieder and G. Winnewisser would like to acknowledge the generous support provided by the DLR through grants 50 0090 090 and 50 0099 011.

REFERENCES Baluteau, J.-P., et al. 1997, A&A, 322, L33 Caux, E., et al. 1999, A&A, 347, L1 Goldsmith, P. F., & Langer, W. D. 1978, ApJ, 222, 881 Gurwell, M. A., et al. 2000, ApJ, 539, L143 Ikeda, M., et al. 1999, ApJ, 527, L59 Ingalls, J. G., Chamberlin, R. A., Bania, T. M., Jackson, J. M., Lane, A. P., & Stark, A. A. 1997, ApJ, 479, 296 Kraemer, K., Jackson, J. M., & Lane, A. P. 1998, ApJ, 503, 785 Kutner, M. L., & Ulich, B. L. 1981, ApJ, 250, 341 Meyer, D. M., Jura, M., & Cardelli, J. A. 1998, ApJ, 493, 222 Neufeld, D. A., et al. 2000, ApJ, 539, L111 Neufeld, D. A., Lepp, S., & Melnick, G. J. 1995, ApJS, 100, 132 Phillips, T. G., Huggins, P. J., Kuiper, T. B. H., & Miller, R. E. 1980, ApJ, 238, L103 Plume, R., Jaffe, D. T., & Keene, J. 1994, ApJ, 425, L49

Plume, R., Jaffe, D. T., Tatematsu, K., Evans, N. J., II, & Keene, J. 1999, ApJ, 512, 768 Poglitsch, A., Herrmann, F., Genzel, R., Madden, S. C., Nikola, T., Timmermann, R., Geis, N., & Stacey, G. J. 1996, ApJ, 462, L43 Russell, R. W., Melnick, G., Gull, G. E., & Harwit, M. 1980, ApJ, 240, L99 Schilke, P., Phillips, T. G., & Mehringer, D. M. 1999, in The Physics and Chemistry of the Interstellar Medium, ed. V. Ossenkopf, J. Stutzki, & G. Winnewisser (Herdecke: GCA), 330 Schutte, W. A. 1999, in Laboratory Astrophysics and Space Research, ed. P. Ehrenfreund et al. (Dordrecht: Kluwer), 69 Sutton, E. C., Peng, R., Danchi, W. C., Jaminet, P. A., Sandell, G., & Russell, A. P. G. 1995, ApJS, 97, 455 Vandenbussche, B., et al. 1999, A&A, 346, L57 Watzin, J. G. 1994, Space Technology, 14(4), 269