A high geometric albedo and small size of the Haumea cluster member (24835) 1995 SM55 from a stellar occultation and photometric observations

TL;DR

The study targets (24835) 1995 SM55, a Haumea cluster member, to determine its projected size, absolute magnitude, and geometric albedo through a 2024 stellar occultation and extensive photometry. Seven positive chords yield an instantaneous projected limb well described by an ellipse with $ a' = 104.3 \pm 0.4 $ km and $ b' = 83.5 \pm 0.5 $ km, giving $ D_{eq,A} = 186.7 \pm 1.8 $ km; locking this with the photometric results, the absolute magnitude is $ H_V = 4.55 \pm 0.03 $, $ V-R = 0.37 \pm 0.05 $, and $ \beta = 0.04 \pm 0.02 $ mag/deg. Combining the occultation size with the photometry and a rotational-phase correction, the geometric albedo is $ p_V = 0.80 \pm 0.04 $, among the highest measured for a TNO and consistent with Haumea cluster membership. The data imply a nonhydrostatic, triaxial ellipsoid with axis estimates $ a \approx 106 $, $ b \approx 92 $, and $ c \approx 76 $ km and an aspect angle $ \psi \approx 52^\circ $, yielding a 3D diameter $ D_{eq} \approx 181 \pm 12 $ km; hydrostatic equilibrium would require unrealistically low densities at this size. The work also delivers high-precision occultation-based astrometry to improve orbital predictions for future occultations. Overall, the results refine the physical properties of 1995 SM55 and reinforce its association with the Haumea cluster, while revealing surface albedo contrasts among cluster members and Haumea itself.

Abstract

Trans-Neptunian objects (TNOs) are among the most ancient bodies of the solar system. Understanding their physical properties is key to constraining their origin and the evolution of the outer regions beyond Neptune. Stellar occultations provide highly accurate size and shape information. (24835) 1995 SM55 is one of the few members of the Haumea cluster and thus of particular interest. We aimed to determine its projected size, absolute magnitude, and geometric albedo, and to compare these with Haumea. A stellar occultation on 25 February 2024 was observed from five sites, with seven positive detections and 33 negative chords. An elliptical fit to the occultation chords yields semi-axes of $(104.3 \pm 0.4) \times (83.5 \pm 0.5)$ km, giving an area-equivalent diameter of $186.7 \pm 1.8$ km, smaller than the 250 km upper limit from Herschel thermal data. Photometry provides an absolute magnitude $H_V = 4.55 \pm 0.03$, a phase slope of $0.04 \pm 0.02$ mag/deg, and a $V-R = 0.37 \pm 0.05$. The rotational variability has an amplitude $Δm = 0.05$ mag, but the period remains uncertain. Combining occultation and photometry, we derive a geometric albedo $p_V = 0.80 \pm 0.04$, one of the highest values measured for a TNO. This value is slightly higher than that of Haumea, consistent with the interpretation that 1995 SM55 belongs to the Haumea cluster.

Figures (7)

• Figure 1: This map displays the ground track from our refined prediction, which incorporated astrometry acquired at the 1.5m Sierra Nevada telescope, the 1.2m Calar Alto telescope, and the 2-m Liverpool telescope. The blue lines mark the boundaries of the body's shadow, calculated assuming a spherical shape with a diameter $D = 398$ km. The central path is indicated by a green line. Red lines illustrate the 3-sigma uncertainties associated with the prediction. The map also shows the observation sites: green markers denote positive detections, red markers indicate negative detections (i.e., 'misses'), blue markers signify planned but unexecuted observations, and yellow markers represent locations with adverse weather. The purple marker indicates technical problems. Map credit: https://www.openstreetmap.org.
• Figure 2: Occultation light curves from the various instruments that successfully registered the event. The light curves (flux versus time) are normalized to one, with an arbitrary offset applied for enhanced clarity. Dots represent the observational data, while the lines correspond to the model as described in Section 2. The Chalin light curve (pink) is presented with an enlarged view in the inset.
• Figure 3: Left Panel: The colored segments correspond to positive chords obtained from the sites indicated in the legend. Red segments illustrate the $1\sigma$ uncertainties originating from errors in ingress and egress times. Note that the large uncertainties come from the fact that the detector was at readout when the reappearance of the star happened. Continuous grey lines denote locations where negative data were obtained. Right panel: This panel displays the elliptical fit to the chords from the 2024 February 25 occultation. This fit characterizes the limb of 1995 SM$_\textrm{55}$ as projected onto the sky plane, defined by the $(f,g)$ axes with origin on NIMA v10 ephemeris, at the moment of the occultation. The two chords obtained using the T100 and RTT150 telescopes at the Türkiye National Observatories are graphically indistinguishable in these plots, as are those from the two telescopes at Wise Observatory. The grey shaded area represents the $3\sigma$-uncertainty region of the derived ellipse.
• Figure 4: Grazing light curve at Chalin (black dots) together with a model fit (red line) as described in the text. The light curve (flux versus time) clearly shows diffraction spikes at both disappearance and reappearance.
• Figure 5: Reduced magnitude $m_V(1,1,\alpha)$ plotted against the phase angle $\alpha$. A total of 649 observations, obtained with the 2-m Liverpool telescope, the 1.5-m telescope at Sierra Nevada Observatory, and the 1.2-m telescope at Calar Alto Observatory, were analyzed. This plot was constructed after applying a sigma-clip rejection for outliers and selecting images with a signal-to-noise ratio (SNR) greater than 30.