THE PROJECT Speetro-Photometric
"SPICE"
Infrared Celestial Explorer
D. R O U A N , N. E P C H T E I N , M. D E M U I Z O N , F. L A C O M B E , P. P U G E T and D. T I P H E N E Ddpartement de Recherche Spatiale de l'Observatoire de Paris-Meudon (DESPA) F. B O U L A N G E R , X. D E S E R T , G. G U Y O T , L. D ' H E N D E C O U R T , J-M. L A M A R R E , A. L E G E R , F. P A J O T a n d J-L. P U G E T Institut d'Astrophysique Spatial - Orsay (IAS) E. C A U X , M. G I A R D and G. S E R R A Centre d'Etude Spatial des Rayonnements - Toulouse (CESR) C. C E S A R S K Y a n d L. V I G R O U X Service d'Astrophysique du CEA - Saclay (SAp) and A. O M O N T Institut d'Astrophysique de Paris (IAP)
A b s t r a c t . Proposed to both the French and the European Space Agency as one possible small mission, SPICE is a project for a dedicated small satellite for a near-IR spectroscopic all-sky survey. The instrument would cover the spectral range 1.8 - 3.6 #m, possibly extended to 1.8 - 7 #m, at a resolution of ~ 100, with pixels of 1 arc-min. The excellent sensitivity (0.02 MJy st-l) results from: i) the quasi-zero level of background due to the efficient passive cooling of the whole experiment; ii) the use of large format arrays; iii) the non-stop observing mode (drift-scanning). The spectral domain, complementing the one of ISO, partially opaque from the ground, is specially rich in spectral features tracing stars and all components of the Interstellar Medium (molecular, atomic and ionized gas, dust). With a cooling below 80 K of the focal instrument, then it becomes possible to consider doubling the spectral domain and to cover the whole 1.8 - 7 #rn range. K e y words: IR spectroscopy - survey - satellite - Interstellar Medium
1.
Scientific Objectives
W i t h r e s p e c t t o t h e m a n y all-sky a s t r o n o m i c a l surveys t h a t are c u r r e n t l y p l a n n e d , S P I C E would have the originality t o bring up t h e s p e c t r o s c o p i c d i m e n s i o n , a p o w e r f u l t o o l to p r o v i d e basic physical pieces o f i n f o r m a t i o n . At a s p e c t r a l r e s o l u t i o n of R ~ 100 and a spatial resolution of 1 arc-rain, t h e s u r v e y would cover t h e s p e c t r a l range, 1.8 - 3.6 # m (possibly e x t e n d e d to 3.6 - 7 # m ) whose a d v a n t a g e s are: i) richness in s p e c t r a l f e a t u r e s t r a c i n g s t a r s a n d t h e different c o m p o n e n t s of the I n t e r s t e l l a r M e d i u m ; ii) inclusion o f d o m a i n s c o m p l e t e l y blocked by t h e e a r t h a t m o s p h e r e , a n d coverage of a d o m a i n w h e r e t h e t h e r m a l emission of a g r o u n d e n v i r o n m e n t b e c o m e s Astrophysics and Space Science 217: 41--44, 1994. © 1994 Kluwer Academic Publishers. Printed in Belgium.
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prohibitive; iii) possibility of a deep probing in the galactic plane thank to the reduced dust opacity. Four main astrophysical domains would particularly benefit from SPICE: i) Study of galactic stellar populations and structure: A sample of 105 spectra of stars brighter than K = 12, (all the supergiants and most of the giants of our Galaxy) would provide a unique tool of investigation and classification for all types of stars and in particular for the most advanced ones (giant and supergiant stars, planetary nebulae): prolongation of spectrM classification towards the latest types, detection of peculiar stars and envelopes (C stars, detection of ice, PAH feature, emission lines, molecular hydrogen etc.), study of the chemical composition and the age of the clusters, spatial distribution of the different stellar sub-types as a function of the galactocentric distance. Combination with data of IRAS, DENIS and HIPPARCOS will greatly improve the knowledge of the luminosity function. ii) Galactic Interstellar Medium: Several components of the ISM will be studied: • Mapping in the 3.3 #m feature of the PAHs: the galactic plane emission would be detected with an excellent signal to noise ratio, even towards high latitude clouds. A thorough study of evolution with irradiance of the nature of the carbon matter in the ISM will be possible. • Ionized gas emission in the Paschen alpha line (1.88 #m): the intensity Paa is ~ 1/5 of Ha, so that the sensitivity of SPICE corresponds to a detection limit in emission measure of 25 pc c m - 6 which is better than the sensitivity of the Ha photographic survey but of course much deeper in the galactic plane. The diffuse emission of the ionized gas would be measured up to about 10 ° on each side of the plane and near OB associations. Other recombination lines (Brackett series) will allow in the brightest regions to measure the extinction. • Molecular hydrogen vibration lines emission: the galactic plane in the direction of the molecular ring will be detected. The study of the line ratioes emitted from different vibration levels should allow to tackle the problem of the origin of the emission, fluorescence of shocks, and of a still debated question: what fraction of the giant molecular clouds of our Galaxy forms massive stars ? iii) Nearby galaxies. The energy budget of galaxies can be thoroughly studied thanks to near infrared interstellar and stellar lines. The sensitivity of SPICE is such that all galaxies resolved by IRAS could be mapped in the lines with an angular scale of 1 arc-rain or better. In particular, the 3.3 #m feature and the H recombination lines, are powerful tools for the reconstruction of the history of our neighbour galaxies. iv) Diffuse extragalactic background. The diffuse background due to the integrated emission of galaxies, hampered by more intense background (zodiacal dust of the solar system in emission and absorption, unresolved stars) has not yet been detected. The question of the anisotropies of this background at better spatial resolution than DIRBE is essential for the study of primeval
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galaxies in the phase when they did not yet contain much dust and thus mainly radiated in the UV and visible (now shifted toward the infrared). With the implementation of a photometric mode of the instrument where the spectral resolution would be decreased to increase the signal, the expected brightness of the diffuse background would be detectable, resulting in a breakthrough in the study of galaxy formation. 2.
Mission and instrumental concept
Mission constraints: The orbit is a circular sun synchronous one, almost polar, of period ~ lh30 . Its advantages are: i) a regular scanning on sky, ii) a weak dose of radiations, iii) stability of sun and earth radiation that simplifies the straylight and thermal control. Reconstitution of position is done a posteriori on the stars detected. A small launcher like Pegasus is adequate (mass of 220 kg, volume of the fairing). The proposed duration of the mission is 6 years, with two phases: i) two years in the nominal mode with maximum spectral resolution and angular resolution; ii) four years with a spectral and angular resolution largely reduced (factor of 30 and 4 to 8 respectively) for the best sensitivity. Reduction of resolution can be performed simply by changing the read-out mode of the detector ("binning" of the CCD along both lines and rows), which is equivalent to use larger pixels without degrading the elementary read-out noise. Payload: The proposed payload consists in a telescope of 35 to 40 cm diameter feeding, through a fiber optics bundle, a spectrograph that forms on four 128 × 128 detector arrays the spectrum of each region of the sky limited by a sub-bundle of fibers. The fiber bundle is an anamorphoser that matches a rectangular area in the focal plane of the telescope and the entrance slit of the spectrograph. The spectrograph will probably use a grism, working in long slit mode, its camera optics is very fast (f/1.4) to provide a maximum luminosity; the resolving power is ~ 90 between 1.8 and 3.6 #m. To allow a significant integration time, the continuous drift of the sky image is compensated by transfering the charges in the detector, of the IR-CCD type, at the same velocity, avoiding any mechanism. Thanks to an efficient passive cooling of the all experiment (telescope, spectrograph, detectors), the thermal background radiation becomes negligible and the detectors could work without an on-board cryogenerator. An efficient on-board data compression (factor of three typically) and a large memory (1 Gbyte) are required. Fig. 1 sketches a possible configuration of the proposed instrument. The cold part of the instrument is contained in a cylindrical shaft and surrounded by cylindrical shields radiatively cooled. An obliquely cut cone-shaped screen at the top and internal baffles prevent the scattered or diffracted parasitic radiation issued from sun and earth. A temperature of T _< 80K could so be reached. The products released by the survey would be an atlas of spec-
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panneis
•:Solar Fiber optics bundle
I
Radiation shields
t i
Parabolic mirror j
f
l
~__SSpectrograph J
Platform
S Fig. 1.
Instrumental configuration
tra of previously identified sources (IRAS, DENIS, ISO) and a data base of spectra of the complete sky on a grid 1' × lq this base would contain ~ 150 millions spectra of 128 points each. Performances: Compared to the ground, the main gain would come from the lack of background emission ( I . ( 2 . 2 # m ) = 2 5 0 M g y s r -1 on ground). The sensitivity on extended sources (1 a) would then be 0.025 M J y vr -1, a figure that it is interesting to compare to the limit of 70 M J y sr -1 at 3 # m on ground due to background photon noise, leading to a gain of ~. 3000. On point sources the sensitivity will be ~ 5 m J y which is equiwlent to m K 13 and mL 12. Note that because of conservation of S × ~, the size of even a ground-based telescope could not be larger. =
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