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This document presents the results of WP1400 “Input Star Catalogue for the SPICA mission” study conducted by the OATo Team for Thales Alenia Space Italia (TASI) within the “FGS Feasibility Consolidation for SPICA Mission” project commissioned by ESA. SPICA is a proposed medium-class mission (M5) to be launched in 2035 and aimed to study galaxy and planet formation by means of spectral mapping and imaging in the wavelength range of 12–230 microns. Here, we present a feasibility study for the construction of the FGS ISC that shall provide astrometry and photometry for reference sources suitable for the SPICA operations. We assume a FGS with a FOV of 5’ x 5’ and a sensitivity in three possible spectral ranges: 0.8 – 2.5 μm (NIR), 4 – 27 μm (MIR), and 17 – 36 μm (MIR-FIR). For the NIR case, we also consider a larger size of 20’ x 20’. We investigate four astronomical optical-infrared catalogues, namely Gaia DR2, 2MASS, CatWISE, AllWISE and discuss the feasibility of an ISC that shall provide at least five reference sources with a 95% probability over the whole sky. Given the stringent astrometric requirement, Gaia DR2 and its future releases appear the only catalogues that can provide enough sources with errors, \sigma_\alpha, \sigma_\delta, smaller than 10 mas on the positions propagated at the reference epoch J2035.5. We adopt barycentric positions because the correction for the annual parallactic motion affects less than 1% of Gaia DR2 sources. The detector QE convolved with the telescope transmission will define the instrumental FGS passband, and magnitudes, mFGS, shall be computed by means of color transformations based on stellar spectra libraries. Here, we show that sufficiently precise transformations from the optical Gaia passbands (G, GBP, GRP) can be constructed to infer the source brightness for a FGS that operates in the NIR wavelengths. Although 2MASS magnitudes (J, H, Ks) should produce a more accurate transformation to a NIR FGS passband, we exclude this approach because of the insufficient number of 2MASS sources at high galactic latitudes. Therefore, we estimated J magnitudes for all the Gaia DR2 sources down to the magnitude limit J ~ 20mag and computed statistics inside a test region of 137 square degrees towards the NGP, which is the most critical area in the sky where the counts decrease to the minimum. In order to validate our analysis with respect the real distribution, we also run Montecarlo simulations based on 10,000 and 5000 FOVs of 5’ x 5 and 20’x20, respectively, randomly distributed over the NGP test region. For the 5’ x 5’ FOV, we show that a NIR ISC based on Gaia DR2 only does not comply with the astrometric requirement of 10 mas. Anyway, the 95% probability to detect at least 5 sources within a FOV of 5’x5’ can be met if we relax the astrometric requirement to 20 mas. In this case, the mean number of sources within the 5’ x 5’ FOV increases to \simeq 9.6 while the probability to detect less than five stars decreases to 4.81%. These results improve even more if we assume the astrometric accuracy of Gaia DR3, whose proper motions and parallaxes will be more precise of a factor 0.5 and 0.8, respectively, with respect to Gaia DR2. The mean density of sources with position errors less than 20 mas will then increase of a 36% factor up to \simeq 13.1 and the probability to detect less than five stars decreases to 0.55%. Unfortunately, the mean density of sources with position errors less than 10 mas does not slightly comply with the input requirement, as the probability to detect less than five stars is 5.91%. Of course, if the large 20’ x 20’ FOV were adopted, the results improve dramatically. In fact, we estimate that Gaia DR2 sources with J < 13.4mag are able to guarantee an average number of fifteen stars with position errors less than 10 mas within the 20’ x 20’ FOV, which implies a Poissonian probability to detect less than five stars smaller than 0.1%. In our opinion, the NIR ISC derived from Gaia DR3 and its future releases is the preferred option for SPICA, thanks to its higher density of sources with accurate astrometry and more precise angular resolution. However, a MIR ISC can also be produced by cross-matching the Gaia sources with an auxiliary catalogue that provides the multi-band photometry necessary to estimate the magnitudes in FGS passband, mFGS. In this case, a fainter magnitude limit is necessary to compensate the incompleteness of the very red objects in the Gaia catalogue. Fortunately, deep all-sky catalogues exist in the MIR with a sufficient number of sources and we tested a hypothetical catalogue, constructed from a cross-match with Gaia and the AllWISE catalogue. We show that, if we assume the astrometric precision expected from Gaia DR3, the 5’x5’ FOV is able to guarantee an average number of 10 sources with magnitudes brighter than W3=12.8 and position errors less than 20 mas. In particular, our Montecarlo analysis gives a 98% probability for detecting at least 5 sources down to the magnitude W3=13 in the lowest density region towards the NGP. Conversely, the astrometric accuracy expected from Gaia DR3 is not able to comply with the stringent astrometric requirement, even if we consider the full catalogue down to W3 \simeq13.5, as the probability to detect less than five stars with position errors less than 10 mas is 9.42%. Finally we point out that this study only considers the current source catalogues, including Gaia DR3 that will been released next year. Future planned releases of the Gaia catalogue will have much smaller uncertainties, as well as being at an epoch that is closer to that of the SPICA mission. Indeed, we can conclude from this study that it will be necessary to use either Gaia DR4 or the last Gaia data release, expected sometime before 2028, as an input catalogue for the SPICA ISC. |
