PROFILE OF AN ESO KEY PROGRAMME Standard Stars for the Infrared Space Observatory, ISO 1 2 1 3 N.S. VAN DER BUEK , P. BOUCHET , H.J. HABING , M. JOURDAIN OE MUIZON , 1, 5 D. E. BLACKWELL 4, B. GUSTAFSSON , P. L. HAMMERSLEy6, M.F. KESSLER~ T.L. UMS, 9 J. MANFROID , L. METCALFE~ A. SALAMA 7 1Leiden Observatory, the Nether/ands; 2 ESO, La Silla; 3Paris Observatory, France; 4 University ofOxford, Eng/and; 5Uppsa/a Observatory, Sweden; 6/AC, Tenerife, Spain; 7ESA-ESTEC, The Nether/ands; 8/mperia/ College, London, Eng/and; 9/nstitut d'Astrophysique de Liege, Be/gium The Infrared Space Observatory (ISO) volves the use both of internal calibra­ servational programmes, but also es­ is a European Space Agency (ESA) tion sources and a range of astronomi­ tablished a collaboration with Black­ satellite to be launched in 1995. Operat­ cal reference sources (i. e. stars and as­ well's group (Oxford) and with Gustafs­ ing at wavelengths ranging from 2.5 to teroids for the photometrie calibration son's group (Uppsala) to carry out the 200 ~lm (Kessler, 1992), ISO will be a and planetary nebulae or HII regions for theoretical part of the project: determin­ unique facility with wh ich to explore the the spectroscopic calibration). ing fundamental parameters of the stars universe. Its targets will range from ob­ A full description of the plans for the and modeling their far-infrared spectra. jects in the solar system, at one ex­ in-flight calibration of the ISO instru­ The goal of the working group is to treme, to distant extragalactic sourees, ments can be found in the "ISO In Orbit deliver a database of standard stars and at the other extreme. ISO will operate as Calibration Requirements Document", fluxes to the ISO Science Operations an observatory with 65% of its observ­ wh ich is regularly updated by ESA in Team weil before the launch of ISO. ing time open to the general astronomi­ consultation with the instrument con­ cal community. Observations will be sortia. Selecting the Stars selected by proposal submission and peer review. For an efficient calibration of observa­ Stars as Calibrators, the ESO Key The satellite will carry on board a 60­ tions by ISO, i. e. to minimize the slewing Programme cm telescope and four scientific instru­ time of the telescope, there should be at ments, mounted inside a 2300-litre The most suitable sources for the least 1 standard star per hundred superfluid Helium cryostat and operat­ photometrie calibration of ISOCAM and square degrees. In other words, the set ing at a temperature of around 3 K. The the shorter wavelength region of of standard stars for ISO has to contain instruments are: ISOPHOT are stars with well-known at least 400 stars, evenly spread over 1. ISOCAM, a two-channel camera, monochromatic fluxes. Stars can also the sky. operating between 2.5 and 17 Ilm, be used as photometrie standards for In fact, several conditions have to be with a 32 x 32-element detector array SWS and for the short wavelength re­ met by a set of standard stars for ISO. in each channel; gion of LWS, and for correlating SWS Since they will be used as standards for 2. ISOPHOT, a broad-band multi-filter and LWS spectra. However, a different instruments of ISO, the stars photo-polarimeter, covering the homogeneous set of standard stars should cover a wide range of mag­ wavelengths between 2 and 200 Ilm, suitable for wavelengths up to at least nitudes as weil as a wide range of spec­ with a low-resolution spectrometer, 50 ~lm does not exist! tral types. In summary, the setected covering the wavelength ranges 2.5 to The ESO Key Programme, "Infrared stars should be: 5 Ilm and 6 to 12 Ilm; Standards for ISO", is a first step to­ 1. non variable stars; 3. SWS, the short wavelength spec­ wards the setting up of such a system of 2. single stars; trometer, offering resolutions in the standard stars. 3. stars without an infrared excess; range 1000 to 20,000, for In order to make it possible to use 4. brighter than K = 12, and fainter than wavelengths between 2.4 and 44 Ilm; stars as calibrators up to these K = 0; 4. LWS, the long wavelength spec­ wavelengths, their far-infrared f1uxes 5. evenly distributed over spectral type trometer, offering resolutions in the must be known, on the basis of photo­ and magnitude; range 100 to 10,000, for wavelengths metrie and spectroscopic data obtained 6. homogeneously spread over the sky. between 45 and 180 Ilm. from the ground in combination with The ESO Key Programme covers the These instruments are being built by stellar model atmospheres. The aim of observations for the southern hemi­ four independent consortia from ESA this Key Programme is to obtain near­ sphere. We selected stars from the in­ member States, using national funding. infrared (NIR) and mid-infrared (MIR, at frared standard star lists of ESO ESA is responsible for the development 10 and 20 Ilm) photometrie data and NIR (Bouchet et al., 1991), SAAO (Carter, and launch of the satellite, and for the spectroscopic data of the stars selected 1990), AAO (Allen and Cragg, 1983), and observatory operations, which end in the southern hemisphere. Similar CTIO (Elias and Frogel, 1983). We ex­ when the liquid Helium cryogen is ex­ efforts are being undertaken in the tended the sampie by selecting stars hausted, i. e. after a baseline lifetime of northern hemisphere by the IAC from the Bright Star Catalogue (Hoffleit, at least 18 months. (Tenerife) and Imperial College (Lon­ 1982) and the Henry Draper Catalogue. One of the major concerns for such a don). The project, as a whole, runs We used both catalogues and the mission is the calibration of the instru­ under auspices of the ISO Ground Hipparcos Input Catalogue to discard ments and thus of the scientific data Based Preparatory Programme working multiple and variable stars. In addition, products. In addition to the pre-Iaunch group (Jourdain de Muizon and Habing, we used the IRAS catalogue to check on-ground calibration and characteriza­ 1992), wh ich was formed on the initia­ that the spectra of the selected stars do tion of the instruments, the observatory tive of the ISO Science Team. This not show an infrared excess. The sam­ must be calibrated in-flight. This in- working group not only initiated the ob- pie contains 300 stars (see Fig. 1), of 28 - the existing set of standard stars for broad-band NIR photometry will be extended with standard stars having K magnitudes of up to 12; 0 0 - a system of standard stars for o~ 0 narrow-band NIR photometry will be 0 0 . 0 00 0" 0 0 defined; 0 0 .. .. • .1' • - a system of standard stars for MIR ; 0 . •• 0 · 'j> ~m 0 .. photometry at 10 and 20 will be '\, . .. .. · 0 . .. 0' ·0 <I' · .. set up. 6 0 · . 0 · 0 . "'. We reduee the photometrie data with ~ -30 · -30 . ·. IR SNOPY, a reduetion programme . 8· ~ . ci . 0 g S . i ·8 available at La Silla. For the narrow­ -0" : I. · .. ... · # . .. 00 .. ... :° '" ..- band NIR photometry and the MIR pho­ . fSlo o ' . tometry this is a preliminary reduetion '" · 0 . o QO CI 0. only: the sets of standard stars first have -60 60 to be established. Therefore we will rede . 8 · .. the reduetion, when all photometrie data have been eolleeted, using a "global method", namely the one developed by Manfroid (1985). This method skips en­ -90 90 24 20 16 12 B 0 tirely the eolour-transformation prob­ 1'.6. (2000.0) lem, i. e., only zero points are eomputed, Figure 1: Distribution of the stars over the southern hemisphere. In the present sampie only instead of eomplete eolour transforma­ very few stars have declinations smaller than 75°. We will therefore add some stars to the tions. The method is ideally suited for sampie which are located in the polar region. setting up a new photometrie system. For a large data set this method pro­ vides aeeuraeies better than 0.01 mag for the broad-band NIR photometry, and whieh the 200 most suitable stars will predieted by theoretieal atmosphere 0.005 mag for the narrow-band NIR be used as southern standard stars for models. photometry. For the MIR photometry ISO. Clearly this key programme will yield aeeuraeies better than 0.01 mag in Ni several by-produets, whieh are useful and N2, 0.02 in N3, and 0.05 at 00 ean for ground-based work: be reaehed. Photometry For the entire set of stars we are aequiring J,H,K and L broad-band pho- Table 1: The Royal Standard Stars tometry as weil as narrow-band NIR photometry. The narrow-band filters are Spectral type RA (2000) Dec (2000) CVF filters eentred on 1.58 ~lm (H band), 2.16 ~lm (Bry), 2.22 ~m (K band), A-type stars 2.29 ~m (CO) and 3.70 ~m (L band). The HR2421, YGem, Alhena AOIV 06 37 42.7 +16 23 57 use of sueh narrow-band filters im­ HR2491, a CMa, Sirius A1Vm 06 45 08.9 -16 42 58 proves the aeeuraey of the photometry HR3314, C Hya AOV 08 25 39.6 -03 54 23 in two ways: 1. the narrow-band filters HR7069, 111 Her A5111 18 47 01.2 +18 10 53 have pass-bands weil within the atmo­ HR7557, a Aql, Altair A7V 19 50 46.9 +08 52 06 spherie windows: their profiles are not defined by the edges of the atmospherie Early F-type stars windows, and are therefore not ehanged HR0740, 0 Ceti F41V 02 32 05.1 -15 14 41 by variations in the atmospherie trans­ HR5570, 16 Lib FOV 14 57 10.9 -04 20 47 pareney; 2.