Asteroids’ reflectance from Gaia DR3: Artificial reddening at near-UV wavelengths

Abstract

Context. Observational and instrumental difficulties observing small bodies below 0.5 μm make this wavelength range poorly studied compared with the visible and near-infrared. Furthermore, the suitability of many commonly used solar analogues, essential in the computation of asteroid reflectances, is usually assessed only in visible wavelengths, while some of these objects show spectra that are quite different from the spectrum of the Sun at wavelengths below 0.55 μm. Stars HD 28099 (Hyades 64) and HD 186427 (16 Cyg B) are two well-studied solar analogues that instead present spectra that are also very similar to the spectrum of the Sun in the wavelength region between 0.36 and 0.55 μm. Aims. We aim to assess the suitability in the near-ultraviolet (NUV) region of the solar analogues selected by the team responsible for the asteroid reflectance included in Gaia Data Release 3 (DR3) and to suggest a correction (in the form of multiplicative factors) to be applied to the Gaia DR3 asteroid reflectance spectra to account for the differences with respect to the solar analogue Hyades 64. Methods. To compute the multiplicative factors, we calculated the ratio between the solar analogues used by Gaia DR3 and Hyades 64, and then we averaged and binned this ratio in the same way as the asteroid spectra in Gaia DR3. We also compared both the original and corrected Gaia asteroid spectra with observational data from the Eight Color Asteroid Survey (ECAS), one UV spectrum obtained with the Hubble Space Telescope (HST) and a set of blue-visible spectra obtained with the 3.6 m Telescopio Nazionale Galileo (TNG). By means of this comparison, we quantified the goodness of the obtained correction. Results. We find that the solar analogues selected for Gaia DR3 to compute the reflectance spectra of the asteroids of this data release have a systematically redder spectral slope at wavelengths shorter than 0.55 μm than Hyades 64. We find that no correction is needed in the red photometer (RP, between 0.7 and 1 μm), but a correction should be applied at wavelengths below 0.55 μm, that is in the blue photometer (BP). After applying the correction, we find a better agreement between Gaia DR3 spectra, ECAS, HST, and our set of ground-based observations with the TNG. Conclusions. Correcting the near-UV part of the asteroid reflectance spectra is very important for proper comparisons with laboratory spectra (minerals, meteorite samples, etc.) or to analyse quantitatively the UV absorption (which is particularly important to study hydration in primitive asteroids). The spectral behaviour at wavelengths below 0.5 μm of the selected solar analogues should be fully studied and taken into account for Gaia DR4.