New near-infrared spectra of (594913) 'Aylo'chaxnim, the first known asteroid orbiting inside Venus orbit

Introduction

Asteroid (594913) 'Aylo'chaxnim, formerly designated as 2020 AV₂, is the only one known to orbit inside Venus orbit. It was first observed by the Zwicky Transient Facility on January 4, 2020 (Bolin et al. 2020) and it may represent the largest member of a new population of small bodies with the aphelion distance smaller than 0.718 a.u., called Vatiras (Greenstreet et al. 2012). Because of its orbit, the surface of this asteroid is being constantly modified by the high temperature, by the strong solar wind irradiation that characterizes the innermost region of the Solar system, and by high-energy micrometeorite impacts. Thus, it represents an extreme case when compared with the near-Earth asteroids.

Based on the observations obtained on January 13, and 14, 2020, we found that 'Aylo'chaxnim has a wide spectral absorption band around 1 μm that represents the signature of an olivine-rich composition. The estimated absorption band center BIC = 1.08 ± 0.02 μm corresponds to a ferroan olivine mineralogy (Popescu et al. 2020 MNRAS). By comparing these spectra with the taxonomic types from Bus-DeMeo taxonomy, we found Sa types as the best fit for the (0.5 - 1.5 ) μm spectral interval. The classification as an S-complex asteroid was also reported by Bolin et al. (2020 EPSC) based on a visible spectrum obtained on January 23, 2020 with the Keck I telescope. They also reported a reddish surface with g-r=0.65 mag and r – i = 0.23 mag and i-z = 0.11 mag.

Here we report new near-infrared spectra, in the range 0.8-2.5 μm, obtained using the 3m NASA Infrared Telescope Facility (NASA IRTF). The observations were performed during August 2021, when the object was close to its maximum solar elongation of 40 deg.

Table 1. Observational circumstances for (594913) 'Aylo'chaxnim.

Observations

We used the SpeX instrument, with the PRISM mode and a slit of 0.8 x 15 arcsec. This configuration allowed us to cover the 0.82 – 2.5 μm spectral interval. The spectral images were acquired in the ABBA mode. Table 1 shows the observation log for the three sets of data. The object was observable at an air-mass of ~2.5-3 for about 30 min before the start of the morning twilight. We noticed a wide apparent magnitude variation between the nights.

In order to obtain the reflectance spectrum, the asteroid’s observed spectral data must be divided by that of a solar analog. We could observe only one solar analog, a G2V star, namely GSC 01881-01236, which was the best suited for our work in terms of apparent vicinity and time constraints. The data reduction was performed with the Spextool package (Cushing et al. 2004).

Fig.1 The spectral data obtained with IRTF/SpeX during August 2021 for (594913) 'Aylo'chaxnim. The gray points represent the data obtained as provided by Spextool (the pipeline used to reduce the spectral images). The red points were obtained by binning every 9 points from the original data (the gray ones). The observational details associated with S1, S2, S3 are described in Table 1. The data are normalized at 1 μm.

Results

The spectra obtained during the three nights are identical up to 1.3 μm. They also match the spectrum obtained in 2020 (Fig. 2). The S1 and S3 spectra are redder compared with S2, for wavelengths longer than 1.3 μm. A thermal tail is identifiable for S2.

Fig. 2 Comparison between the spectra obtained during January 2020 (Popescu et al. 2020), and those obtained during August 2021. All spectra are normalized to 1 μm.

The absorption band minima (BImin) is located at 1.07 +/- 0.015 μm for S2, and 1.09 +/-0.02 μm for S3 (we did not compute this band center for S1 due to its large error-bars). This result is consistent with the olivine rich composition proposed based on the 2020 observations. The S2, which has the best signal to noise ratio, shows a possible feature at 1.25-1.30 μm, which can also be attributed to olivine.

We used the S2 spectrum for comparison with the RELAB spectral database . Its shape is best matched by the CK meteorites (Cloutis et al. 2021). These are characterized by an olivine associated absorption band in the 1.05 μm region. Compared to pure olivine, CK spectra are darker, have a more subdued olivine absorption band, and are often more blue-sloped (Cloutis et al. 2012).

References

1. Bolin et al. 2020 , Minor Planet Electronic Circ., No. 2020-A99 (2020)

2. Bolin et al. 2020, the 14th Europlanet Science Congress 2020, abstract id. EPSC2020-482

3. Cloutis et al. 2012, Icarus, Volume 221, Issue 2, p. 911-924

4. Greenstreet S., Ngo H., Gladman B., 2012, Icarus, 217, 355

5. Popescu et al. 2020, MNRAS, Volume 496, Issue 3, pp. 3572-3581

Acknowledgments

The work of MP was supported by a grant of the Romanian National Authority for Scientific Research – UEFISCDI, project number PN-III-P1-1.1- TE-2019-1504. The work of MP, JdL, JL, is made in the framework of EU-funded project ”NEOROCKS - The NEO Rapid Observation, Characterization and Key Simulations project”, SU-SPACE-23-SEC-2019 from the Horizon 2020 - Work Programme 2018-2020, under grant agreement No 870403. The abstract make use of data published by the Relab Spectral Database