Detailed stellar occultation detection of Haumea and Namaka from IRTF

The study of trans-Neptunian binaries has uncovered a whole set of information about planetesimal formation and collisional evolution in the outer boundaries of our planetary system. In this context, obtaining the secondary’s orbit around the primary enables a calculation of the system’s mass, which combined with size estimates from other observational techniques allows for system density estimations—key to comprehend their composition and internal structure.

The stellar occultation observational technique is widely known by its ability to obtain accurate measurements of distant solar system small bodies and is highly effective in determining a binary system’s properties. The method involves high-cadence imaging of a background star while a small body passes in front of it, temporarily blocking the star's light as observed from a location within the path of the shadow. A positive detection provides a direct measurement of the object’s size and any surrounding structure. To date, the stellar occultation technique has been responsible for the discovery of ring-like features around the Centaurs Chariklo and Chiron [1, 2], and also the rings hosted by the trans-Neptunian objects Quaoar and Haumea [3, 4, 5]. However, in general, predicting when a trans-Neptunian object (TNO) produces a stellar occultation is challenging, and even more so when the occultation is produced by a TNO satellite. An accurate prediction requires both an accurate ephemeris for the primary object and a well-constrained determination of the secondary’s orbit around the primary.

In this work we present the prediction, observation, and preliminary results of a stellar occultation by Namaka, Haumea’s smallest known satellite. The prediction was performed using the JPL#124 and NIMA v12 ephemeris for Haumea. Meanwhile Namaka’s orbit solution was obtained from [6]. We did not consider possible small barycentric wobbles of Haumea and we assumed that the primary ephemeris refers to Haumea’s true center. We predicted that Namaka would occult the same G = 17.6 mag star after Haumea on March 16^th, 2025 (UT). Both stellar occultations were predicted to be visible only from Hawaiian telescopes and Namaka’s occultation was predicted to happen only ~6 minutes after Haumea’s occultation, providing an excellent opportunity to accurately measure their relative distances.

The event was recorded from the IRTF 3m telescope using the MORIS instrument with an exposure time of 0.3 seconds and 2×2 binning. The GPS pulse was set once per exposure to ensure that the image times were synchronized with universal time frame. After standard bias and sky flat calibrations, images were submitted to aperture photometry by using the PRAIA tool [7] and featuring concentric apertures of 3, 13, and 18 pixels for the stellar flux and background fluxes, respectively. Due to Haumea’s brightness contributions, a drop of only 45% was expected in the occultation light curve. Therefore, the occultation light curve was normalized to unity outside the event and to 0.55 for measurements during the events. The Stellar Occultation Reduction and Analysis (SORA) python library [8] was used to model the normalized data using the classic χ² test and obtain the star dis- and re-appearance times. Our results show positive chords of about 1260 km and 80 km in length for Haumea and Namaka, respectively. Additionally, the data were recorded in a perpendicular orientation regarding 2017’s observations, which can provide constraints regarding Haumea’s ring plane.

References

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[3]Ortiz, J. L., “The size, shape, density and ring of the dwarf planet Haumea from a stellar occultation”, Nature, v. 550, n. 7675, pp. 219–223, 2017. doi:10.1038/nature24051.

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[6]Proudfoot, B. C. N., et al., “Beyond Point Masses. III. Detecting Haumea's Nonspherical Gravitational Field”, The Planetary Science Journal, v. 5, n. 3, 2024. doi:10.3847/PSJ/ad26e9.

[7]Assafin, M., “Differential aperture photometry and digital coronagraphy with PRAIA”, Planetary and Space Science, v. 239, n. 105816, 2023. doi:10.1016/j.pss.2023.105816.

[8]Gomes-Júnior, A. R., et al., “SORA: Stellar occultation reduction and analysis”, Monthly Notices of the Royal Astronomical Society, v. 511, n. 1, pp. 1167–1181, 2022. doi:10.1093/mnras/stac032.