The Size, Shape, Albedo, Density, and Atmospheric Limit of Transneptunian Object (50000) Quaoar from Multi-chord Stellar Occultations

We present results derived from the first multi-chord stellar occultations by the transneptunian object (50000) Quaoar, observed on 2011 May 4 and 2012 February 17, and from a single-chord occultation observed on 2012 October 15. If the timing of the five chords obtained in 2011 were correct, then Quaoar would possess topographic features (crater or mountain) that would be too large for a body of this mass. An alternative model consists in applying time shifts to some chords to account for possible timing errors. Satisfactory elliptical fits to the chords are then possible, yielding an equivalent radius R [SUB]equiv[/SUB] = 555 ± 2.5 km and geometric visual albedo p[SUB]V[/SUB] = 0.109 ± 0.007. Assuming that Quaoar is a Maclaurin spheroid with an indeterminate polar aspect angle, we derive a true oblateness of \epsilon = 0.087^{+0.0268}_{-0.0175}, an equatorial radius of 569^{+24}_{-17} km, and a density of 1.99 ± 0.46 g cm[SUP]–3[/SUP]. The orientation of our preferred solution in the plane of the sky implies that Quaoar's satellite Weywot cannot have an equatorial orbit. Finally, we detect no global atmosphere around Quaoar, considering a pressure upper limit of about 20 nbar for a pure methane atmosphere.Peer reviewe

Refined physical parameters for Chariklo’s body and rings from stellar occultations observed between 2013 and 2020

Context. The Centaur (10199) Chariklo has the first ring system discovered around a small object. It was first observed using stellar occultation in 2013. Stellar occultations allow sizes and shapes to be determined with kilometre accuracy, and provide the characteristics of the occulting object and its vicinity. Aims. Using stellar occultations observed between 2017 and 2020, our aim is to constrain the physical parameters of Chariklo and its rings. We also determine the structure of the rings, and obtain precise astrometrical positions of Chariklo. Methods. We predicted and organised several observational campaigns of stellar occultations by Chariklo. Occultation light curves were measured from the datasets, from which ingress and egress times, and the ring widths and opacity values were obtained. These measurements, combined with results from previous works, allow us to obtain significant constraints on Chariklo's shape and ring structure. Results. We characterise Chariklo's ring system (C1R and C2R), and obtain radii and pole orientations that are consistent with, but more accurate than, results from previous occultations. We confirm the detection of W-shaped structures within C1R and an evident variation in radial width. The observed width ranges between 4.8 and 9.1 km with a mean value of 6.5 km. One dual observation (visible and red) does not reveal any differences in the C1R opacity profiles, indicating a ring particle size larger than a few microns. The C1R ring eccentricity is found to be smaller than 0.022 (3σ), and its width variations may indicate an eccentricity higher than ~0.005. We fit a tri-axial shape to Chariklo's detections over 11 occultations, and determine that Chariklo is consistent with an ellipsoid with semi-axes of 143.8-1.5+1.4, 135.2-2.8+1.4, and 99.1-2.7+5.4 km. Ultimately, we provided seven astrometric positions at a milliarcsecond accuracy level, based on Gaia EDR3, and use it to improve Chariklo's ephemeris

The relationship between body size, pupal thoracic horn development and dissolved oxygen in Chironomini (Diptera: Chironomidae)

The aim of this study was to analyze the relationship between the surface development of pupal thoracic horn and adaptation to low oxygen concentration in different species of chironomids. A total of 773 samples of larvae were collected in lakes and streams together with measurements of dissolved oxygen and collection of pupal exuviae. The area of the base of the thoracic horn (ring organ, RO) and abdomen length (ABD) of pupae were measured in forty-eight species of Chironomini. RO was signifi cantly correlated with ABD, therefore, the ratio of ring organ/abdomen length (ROA) was calculated to provide a measure less dependent on body size. Chironomus plumosus Linnaeus, Chironomus riparius Meigen and Glyptotendipes pallens (Meigen) had the highest ROA, Microtendipes britteni (Edwards), Paralauterborniella nigrohalteralis (Malloch) and Pseudochironomus prasinatus (Staeger) had the lowest ROA. ROA was still correlated with ABD, so residuals from the regression line (RROA) between RO and ABD were also calculated. An examination of the residuals revealed that some species (C. riparius , Cladopelma virescens (Meigen) and Einfeldia pagana (Meigen)) had a RO larger, whereas other species (M. britteni, P. prasinatus and Stenochironomus gibbus (Fabricius)) had a RO smaller than predicted by regression with ABD. RO was signifi cantly correlated with the 25th percentile of dissolved oxygen (PCTL) measured in conjunction with benthic macroinvertebrates samples. Hemoglobin concentrations measured in six species were not correlated with ABD and RO, a high hemoglobin content was measured both in small species with a small RO (Polypedilum nubeculosum (Meigen)) and in large species with a large RO (G. pallens). The ring organ is proposed as a morphological trait useful to indicate oxygen concentration in the environment, while other factors such as hemoglobin content are also likely to affect the species ability to respond to oxygen shortage

Ecological assessment of lakes using macroinvertebrates in the regions of Lombardia and Trentino, project outline and preliminary results

Macroinvertebrates are one of biological elements required to be monitored by the Water Framework Directive. Unfortunately, there are currently no working assessment systems in use for lakes based on benthic invertebrates in Europe. This presentation introduces a rasearch project started in 2005 based on 12 lakes in the regions of Lombardia and Trentino. The sampling design in based on 3 transects per lake with each transect being sampled in the littoral, sub-littoral and profundal zones (2 replicates per site). Preliminary results are presented. It is hoped that the results will allow the research to lead to a more extensive survey of interest in a broader European context