A Spectral Comparison of Small Main Belt and Near-Earth V-types in the Near-Infrared

Introduction

V-type asteroids are not as common as those found in the C- and S- complexes in both near-Earth space and the main belt. In near-Earth space, V-types make up ~5% of the distribution (by number) [1], and the same is true for the main belt [2]. The vast majority of V-type asteroids in the main belt consist of the (4) Vesta dynamical asteroid family [3]. One study of interest is near-Earth V-types’ connection to the (4) Vesta family, which primarily consists of V-types. Its members are referred to as Vestoids. These two groups have been connected through orbital dynamics, due to Vestoids’ location in the inner main belt close to the ν6 resonance [4]. Near-Earth V-types and Vestoids have also been connected due to their similar compositions to each other and to howardite, eucrite, and diogenite (HED) meteorites [5,6]. The dynamical and compositional connections between these two groups gives us the opportunity to compare their spectra and make inferences on how the location of these asteroids affects their surface properties.

Spectra of V-type asteroids are quantified by computing band parameters of the 0.9-μm (Band I) and 1.9-μm (Band II) pyroxene absorptions. Previous work has found that near-Earth V-type spectra and main belt V-types have differences in their band parameters [7]. There are differences between the absorption band centers, with near-Earth V-types having band centers at longer wavelengths (average Band I center  = 0.935  ± 0.01 μm; average Band II center = 1.968  ± 0.032 μm) compared to main belt V-types (average Band I center  = 0.926  ± 0.01 μm; average Band II center = 1.946  ± 0.038 μm) [7]. The greatest difference is the Band I slope, which is the slope of the continuum line across the Band I absorption feature.  The Band I slope of main belt V-types has an average value of 0.66 ± 0.2 μm-1, while near-Earth V-types have a much lower average Band I slope of 0.23 ± 0.13 μm-1 [7]. The band parameters of the near-Earth V-types are much closer to those of eucrites (BI slope = 0.28 ± 0.09 μm-1), which are thought to have Vestoids as their parent bodies [8]. Both eucrites and near-Earth V-types have Band I slopes that are not as red as main belt V-types, which could be due to multiple factors, including space weathering [9] and regolith grain size [10]. 

Due to continuous collisions over time, smaller asteroids tend to have younger surfaces [11]. Near-Earth V-types are typically an order of magnitude smaller than most observed main belt V-types [1,12]. Therefore, we hypothesize that the cause of the band parameter discrepancy is the observational bias towards larger, older main belt V-types. If true, we would expect small main belt V-types to be spectrally similar to NEA V-types. 

 

Methods

Several main belt Vestoids in the size range of 1-3 km were observed during the Fall 2024 and Spring 2025 semesters using the SpeX spectrograph on NASA’s Infrared Telescope Facility (IRTF), which is a 3-m infrared telescope to measure their near-infrared spectra (prism, 0.7-2.5 um) [13]. Before each observation, the slit was rotated to match the parallactic angle of the target. All observations were aimed to be taken at an air mass less than  1.5 and spectra of local solar standard stars were taken to correct the asteroid spectra of telluric features. These solar standards were also used to remove the spectral slope caused by the Sun. The extraction and reduction of the data were done using the IDL SpeXtool package [14]. A Python program was written to perform the band parameter analysis, and the method used is described in [15]. For each asteroid, the Band I and II centers, the Band I slope, and the separation between the two bands were determined. We performed the same analysis on previously published data of both near-Earth and main belt V-type asteroids [1, 15, 16]. This analysis was done not only to validate the algorithm, but to be able to compare the near-Earth V-types to both large and small main belt V-types to look for any correlations with asteroid size. Main belt Vestoids in the size range of 4-6 km were also observed in the same manner, to identify any trends between Band I slope and asteroid size for V-types.

 

Results

The results show that there is potential correlation between Band I slope and asteroid size, rather than the asteroid’s dynamical class. This potential correlation is especially apparent for the smaller main belt V-types, which have Band I slopes that seem to overlap with those of near-Earth V-types of similar sizes. Our next steps for this project include obtaining spectra for main belt Vestoids <2 km in diameter. 

 

We will present the results of comparing the spectral parameters of near-Earth V-types to those of main belt Vestoids as a function of asteroid diameter and the implications of the results. 

 

References

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