Herschel/SPIRE photometry of targeted and serendipitously observed asteroids

The Herschel Space Observatory observed with its PACS and SPIRE instruments a diverse selection of astronomical objects, including Solar System objects (SSOs), either in dedicated measurements or serendipitously. The infrared observations of asteroids helped scientists to determine their physical and thermal properties. In addition, PACS and SPIRE observations of selected large asteroids were used for a wide range of calibration aspects. Due to their strong thermal emission, SSOs may sporadically also contaminate the photometric measurements of other sources. Racero et al. (2022) presented ESASky tools to find these moving targets in Herschel data products. As a first step, we focused on the flux extraction of these serendipitous observations of SSOs in Herschel/PACS measurements at 70, 100 and 160 μm. We obtain far-infrared photometry on the PACS maps, either by extracting the flux densities from the existing standard data products, or via re-reducing the PACS maps in the co-moving frame of the target. Based on these scientifically very important far-IR flux densities, in combination with already published IR detections (Szak ́ats et al., 2020), we perform radiometric studies to determine the objects’ physical and thermal properties with unprecedented accuracy.

A natural continuation of our current work is the extension to SPIRE maps at 250, 350, and 500 μm, with a two-fold aim. First, to find serendipitous asteroids, extract new submm flux densities and flag sources in the SPIRE Point Source Catalog (Schulz et al., 2018) for possible contamination. In addition, the dedicated SPIRE observations of asteroids for various calibration activities are not yet published. As the SPIRE absolute photometry is connected to planet models, these asteroid flux densities are of great value and will put strong constraints on their submm emissivity properties. All SPIRE asteroid measurements were reprocessed and calibrated with the latest pipeline. The final maps are then used to extract high quality IR/submm flux densities. The flux densities are then added to the publicly available SBNAF Infrared Database (Szakáts et al., 2020).

We collected the Herschel/SPIRE targeted asteroid measurements, and we went through the list of SPIRE observations with potential serendipitous asteroid detections (from Racero et al. 2022). We only considered those, where the isvisible flag was ’true’. In a first step we verified that the photometry via the point source calibrated maps and the SSO calibrated maps are in agreement. This is important because all the dedicated measurements were reprocessed in the object co-moving frame and made available in the Herschel Science Archive, but not the serendipitous ones. Our tests confirmed that the two maps give the same flux densities within the error bars, so there is no need to reprocess the serendipitous maps into the co-moving frame. In a second part of the project, after making all flux densities available in the IR database, we perform radiometric
studies for individual asteroids. Depending on the a priori knowledge for a given object, we distinguish between the following five categories: (1) mission targets, well-known/-studied objects; (2) objects with good-quality spin-shape solution and multi-epoch/-mission thermal measurements; (3) good-quality spin-shape solution, but very limited or no thermal measurements available; (4) no good-quality spin-shape solution, but sufficient multi-epoch/-mission thermal measurements available; (5)
no spin-shape solution and very limited thermal measurements available. In each of these five categories, the SPIRE (and PACS) flux densities are important to obtain the object’s emissivity properties, to constrain the thermal properties of the surface or to establish basic radiometric size-albedo solutions.

We will present an overview of the new far-IR and sub-mm measurements (for about 17 dedicated program asteroids and several different main-belt asteroids), the extracted flux densities, and applications for the different categories.

Figure 1: The SPIRE maps for asteroid (10) Hygiea at 250, 350, and 500 μm, taken from a dedicated calibration sequence, and processed in the object’s co-moving frame.

 

Figure 2: The SPIRE maps of the serendipitously seen asteroid (464) Megaira (marked by the green circles) at 250, 350, and 500 μm.