Photometry of serendipitously observed asteroids with Herschel/PACS

Aside from a limited number of dedicated observations of solar system small bodies (Müller et al. 2018), asteroids have randomly been captured in many PACS and SPIRE maps with the Herschel Space Observatory, which operated from 2009 to 2013. Given the still limited number of infrared measurements of asteroids, these incidental detections offer valuable data for modeling asteroid thermal emission and deriving their fundamental physical properties.

In our work, we first searched for serendipitously observed asteroids in measurements taken with the PACS instrument aboard the Herschel Space Observatory. The workflow began by retrieving all available Level 2 scan map observations in FITS format from the Herschel Science Archive (HSA) for each OBSID.

Next, using the ephemd program (Pál et al. 2018), we identified small Solar System bodies that could have potentially been visible in each map at the time it was taken—that is, during the instrument's scan of the corresponding sky area. We considered the start, mid, and end times of each observation to determine whether a potential asteroid entered or exited the field of view during the scan. Predicted positions were then translated into pixel coordinates and checked against the map coverage. If at least one of the three predicted positions fell within the map boundaries, we treated it as a positive detection; otherwise, the object was excluded from further analysis.

This initial selection was followed by a filtering step based on detectability. For each candidate asteroid, we ran a NEATM model and retained only those with an optimistic predicted flux density above 100 mJy at 70 μm. This threshold reflects a practical detection limit, as fainter sources would not be reliably observable.

For the photometry, the asteroid’s precise position was determined using the "fine time" of the scan. As a consistency check, we compared our final candidate list with the one presented by Racero et al. (2022), and found good agreement with their Table B.1. Minor discrepancies were mostly due to our algorithm excluding targets located too close to the map edges to allow for reliable photometry.

At each predicted position, we performed centroid fitting followed by aperture photometry in the Herschel Interactive Processing Environment (HIPE). To estimate the photometric uncertainty, we placed six additional apertures of the same size around the target aperture to sample the background. The standard deviation of these background measurements was used as the one-sigma error estimate (Klaas et al. 2018). Since we applied multiple apertures per source, we subsequently selected the most suitable one for each target.

This process yielded 364 new flux density measurements for 144 unique asteroids. We are currently categorizing these asteroids into five groups based on the availability of shape models and thermal data. The categories are as follows:

1) Mission targets and well-studied objects – These include well-known asteroids such as (1) Ceres and (20) Massalia. For these, we will compare our serendipitous fluxes with predictions based on published model solutions.

2) Objects with shape models and a sufficient number of multi-mission thermal measurements – For these asteroids, we will perform detailed thermophysical modeling to derive high-quality size and albedo estimates, and to constrain thermal inertia and surface roughness.

3) Objects with shape models but limited or no thermal measurements – In these cases, we will utilize the available spin and shape models and generate radiometric solutions using all combined thermal data.

4) Objects without shape models but with sufficient thermal measurements – Here, simple spherical models will be applied. If the spin axis and rotation period are unknown, we will use the NEATM model to derive size-albedo solutions, either by combining all thermal data or analyzing different datasets separately to identify potential shape effects.

5) Objects lacking both shape models and significant thermal measurements – For these asteroids, we will derive basic size and albedo estimates from the available data. If necessary, we will consult the WISE catalog to supplement the thermal measurements.

In our presentation, we show the thermal emission model results from categories 4 and 5.

The resulting flux density values will be made available through the SBNAF Infrared Database (Szakáts et al. 2020).

We plan to publish the full results in an upcoming paper (Szakáts et al. 2025, in prep).

References:
Müller, T. G., Marciniak, A., Kiss, Cs., et al. 2018, Advances in Space Research, 62, 2326

Pál, A., Molnár, L., & Kiss, C. 2018, PASP, 130, 114503

Racero, E., Giordano, F., Carry, B., et al. 2022, A&A, 659, A38

Klaas, U., Balog, Z., Nielbock, M., et al. 2018, A&A, 613, A40

Szakáts, R., Müller, T., Alí-Lagoa, V., et al. 2020, A&A, 635, A54