Near-ultraviolet to visible spectroscopy of the Themis and Polana-Eulalia complex families

TL;DR

This study characterizes the near-ultraviolet to visible reflectance spectra of primitive asteroids, focusing on the Themis and Polana-Eulalia families, and compares these with spectra of sample return mission targets Ryugu and Bennu.

Contribution

It provides new NUV-VIS spectral data for asteroid families and evaluates the suitability of Landolt's stars as solar analogs in this wavelength range.

Findings

Many Landolt's G stars are not suitable solar analogs in NUV.

Themis family shows deeper NUV absorption than Polana-Eulalia.

Spectral similarities suggest Ryugu and Bennu originate from the Polana-Eulalia complex.

Abstract

Aims. The aim of our study is to obtain the near-ultraviolet to visible (NUV-VIS, 0.35 - 0.95 micron) reflectance spectra of primitive asteroids with a focus on members of the Themis and Polana-Eulalia complex families. This characterization allows us to discuss the origin of two recent sample return mission target asteroids, (162173) Ryugu and (101955) Bennu. Methods. We obtain low-resolution visible spectra of target asteroids down to 0.35 micron using the telescopes located at the Roque de los Muchachos Observatory and revisit spectroscopic data that have already been published. We study the characteristics of the NUV-VIS reflectance spectra of primitive asteroids, focusing on data of the Themis family and the Polana-Eulalia family complex. Finally, we compare the NUV characteristics of these families with (162173) Ryugu and (101955) Bennu. In this work, we also study systematic effects due to the use of the five commonly used stars in Landolt's catalog as solar analogs to obtain the asteroid reflectance in the NUV wavelength range. We compare the spectra of five G-stars in Landolt's catalog with the spectrum of the well-studied solar analog Hyades 64, also observed on the same nights. Results. We find that many widely used Landolt's G-type stars are not solar analogs in the NUV wavelength spectral region and thus are not suitable for obtaining the reflectance spectra of asteroids. We also find that, even though the Themis family and the Polana-Eulalia family complex show a similar blueness at visible wavelengths, the NUV absorption of the Themis family is much deeper than that of the Polana-Eulalia family complex. We did not find significant differences between the New Polana and Eulalia families in terms of the NUV-VIS slope. (162173) Ryugu's and (101955) Bennu's spectral characteristics in the NUV-VIS overlap with those of the Polana-Eulalia family complex which implies that it.