Multicolor Photometry of the Neptune Irregular Satellite Neso

Abstract

We report on time series photometry of the faint Neptune irregular satellite Neso. Observations in the V, R, and I pass-bands were performed in photometric conditions at the Cerro Paranal observatory using the instrument FORS2. Photometric calibration was secured using observatory standards. Magnitudes were extracted using the IRAF task qphot, and corrected for aperture using bright stars in the field. Photometric errors have been derived extracting photometry of stars with magnitudes comparable to Neso, and were found to be around 0.3−0.4 mag. Astrometry is derived using up to 5 stars in the satellite vicinity basing on the astrometric catalog USNO-B1. We expect these data will be useful to better constrain the poorly known Neso orbit (Brozovic & Jacobson 2017). The time coverage is not sufficient to construct a light curve and derive a meaningful rotational period. However, we could derive new estimates of apparent magnitudes, in particular in V and I passbands, and for the first time we could calculate Neso colors. Arithmetic averages yields V = 25.6 ± 0.3 from 12 exposures, R = 25.2 ± 0.2 from 13 exposures, and I = 24.5 ± 0.3 from from 25 exposures. The R averaged magnitude is in agreement with Brozović et al. (2011) who quote R = 25.2, the only available measure to date. From these measures of the averaged magnitudes, we derive averaged colors. We obtain V − I = 1.0 ± 0.4, R − I = 0.7 ± 0.4, and V − R = 0.3 ± 0.4, respectively. The color R − I appear to be slightly redder than the typical values for Centaurs and KBOs as reported in Peixinho et al. (2015), while the color V − I is in nice agreement with both populations (see Figure 1). Given the large error-bars in the averaged colors, it is a difficult task to assign Neso to any of the Peixinho et al. (2015) classes, although the data seem to suggest that we can rule out its membership in classes of resonant objects or Plutinos.