Jupiter Trojans spectrophotometry using GAIA DR3

Introduction

Jupiter trojans have a red spectral behaviour typically associated with the presence of organics, which have been recently detected on Lucy mission Trojans targets thanks to JWST observations (1). More than 14500 Jupiter Trojans have been discovered. Their orbits are stable over the age of the Solar System, so their origin must date back to the early phase of the Solar System formation. They are supposed to be implanted TNOs captured by Jupiter during planetary migrations (2).

We present the spectral characterisation of Jupiter Trojans using data from the DR3 GAIA spectral catalogue and from the literature, the analysis of the taxonomy-spectral slope correlations with physical parameters, and the comparison with the outer Solar System bodies main properties.

Methods and data selection

The DR3 GAIA catalogue includes the spectroscopy of 60,518 Solar System Objects (3), and comprises 478 Jupiter Trojans. GAIA spectrophotometry is available in 16 spectrophotometric points covering the 0.33-1.08-micron range.

Among the Trojans observed by GAIA, we select those having a signal to noise ratio > 20, which is considered a reliable threshold for asteroid spectral classification in the visible range (4), and we visually inspected the spectral quality of those with a lower SNR, reducing the Trojans GAIA dataset to 320 objects. Some spectrophotometric points at the edges of the BP and BR photometers were discarded because affected by significant systematic errors, and the UV reflectance corrected using the factors reported by (5).

We taxonomically classified the Trojans using the Bus-Demeo and Mahlke classification scheme (6, 7), applying a chi-squared best fit between a given asteroid and the classes' mean reflectance spectra from these taxonomies, and visually inspecting the results of the fit. To enhance the statistical analysis, we have also included the visible spectra of Trojans available in the literature, mostly from (8, 9). The complete dataset of Trojans includes 519 objects, 291 in L4 and 228 in L5.

Results

The Trojan population is dominated by featureless asteroids with red spectral slopes, as already reported in the literature [8, 9]. In the Trojans sample here analysed, according to the Bus-Demeo Taxonomy [6], D-type asteroids dominate in both swarms (Fig. 1): 72.5% in L4 and 87.7% in L5. In addition to D-type, the L4 swarm also contains 15.5% X-type, 3.1% T-type, and 8.9% C-type, while the L5 swarm has 8.3% X-type, 1.3% T-type, and 2.6% C-type. Most of the X-type are featureless and dark asteroids that would be classified as P-type in the Tholen taxonomy.

The Mahlke taxonomy confirms the previous findings and allows to highlight the presence of very red asteroids, for which a new class, the Z-class, has been explicitly introduced (Fig. 1). In this taxonomy, the L4 swarm has 40.6% of Z-type, 31.6% of D-type, 10.0% of P-type, and an equal percentage, 8.9%, of X- and C-type. The L5 swarm has 45.6% of D-type and 42.1% of Z-type, 5.7% of P-type, 3.9% of X-type and only 2.6% of C-type.

The L4 swarm shows a higher spectral variability and a higher amount, by a factor of 2, of less spectrally red asteroids belonging to the C, P, and X classes.

The average spectral slope is of 8.45 ±0.21 (%/100nm) for the L4, and of 9.41 ±0.21 (%/100nm) for the L5 swarm. The small difference is due to the presence of the Eurybates family in L4. When excluding family members, the average slope of L4 and L5 Trojans are indistinguishable. The two swarms have very similar geometric albedo value (7.86 ± 0.15% and 7.35± 0.15% for the L4 and L5, respectively). The similarity in albedo and spectral slopes indicate a common origin for both swarms.

We explore also the correlation between the Trojans spectral slope with their size, albedo and orbital elements, and compare them to the properties of Kuiper belt objects, which are the probable source of Trojans. Trojans share similarities with the TNOs less red population (Fig 2), but have a distinct distribution in spectral slope, much narrower than TNOs. In the visible range, they lack the extremely red bodies observed in all the dynamically classes of TNOs, and which mainly dominate the cold classicals, known to have formed in situ. Trojans spectral slope distribution is closer to that of cometary nuclei, and to the less red Centaurs and SDO-Detached bodies. These dynamical classes, characterized by bodies with high inclination and eccentricities, may have supplied the Jupiter Trojans swarms during the planetary migration.

References: 1)Wong, I. & Brown, M. E. 2015, AJ, 150, 174; 2) Morbidelli A. et al., . 2005, Nature, 435, 462; 3) Tanga, P. et al. 2023, A&A, 674, A12; 4) Galinier et al. 2024 A&A, 683, L3 ; 5) Tinaut-Ruano_2023, A&A, 669, L14 ; 6) DeMeo, F. et al.  2009, Icarus, 202, 160; 7) Mahlke M. et al.  2022, A&A, 665, A26; 8) Fornasier, S. et al. 2007, Icarus, 190, 622–642 ; 9) Roig, F. Et al. 2008, A&A, 483, 911

Acknowledgement : This work has received support from France 2030 through the project named Académie Spatiale d'Île-de-France (https://academiespatiale.fr/) managed by the National Research Agency under bearing the reference ANR-23-CMAS-0041, as well as the Centre National d’Etude Spatial (CNES).

Fig 1: Pie charts showing the different Trojan taxonomic classes according to the Bus-DeMeo (top) andMahlke (bottom) classification schemes.

Fig 2. Comparison of the slopes of the Trojans (top plot) with those of the different dynamical classes of TNO, Centaurs and cometary nuclei.