Reconstructing the Last Major Merger of the Milky Way with the H3 Survey

Several lines of evidence suggest that the Milky Way underwent a major merger at z ∼ 2 with the Gaia-Sausage-Enceladus (GSE) galaxy. Here we use H3 Survey data to argue that GSE entered the Galaxy on a retrograde orbit based on a population of highly retrograde stars with chemistry similar to the largely radial GSE debris. We present the first tailored N-body simulations of the merger. From a grid of ≈500 simulations we find that a GSE with M ⋆ = 5 × 108 M ⊙, M DM = 2 × 1011 M ⊙ best matches the H3 data. This simulation shows that the retrograde stars are stripped from GSE’s outer disk early in the merger. Despite being selected purely on angular momenta and radial distributions, this simulation reproduces and explains the following phenomena: (i) the triaxial shape of the inner halo, whose major axis is at ≈35° to the plane and connects GSE’s apocenters; (ii) the Hercules-Aquila Cloud and the Virgo Overdensity, which arise due to apocenter pileup; and (iii) the 2 Gyr lag between the quenching of GSE and the truncation of the age distribution of the in situ halo, which tracks the lag between the first and final GSE pericenters. We make the following predictions: (i) the inner halo has a “double-break” density profile with breaks at both ≈15–18 kpc and 30 kpc, coincident with the GSE apocenters; and (ii) the outer halo has retrograde streams awaiting discovery at >30 kpc that contain ≈10% of GSE’s stars. The retrograde (radial) GSE debris originates from its outer (inner) disk—exploiting this trend, we reconstruct the stellar metallicity gradient of GSE (−0.04 ± 0.01 dex r 50 − 1 ). These simulations imply that GSE delivered ≈20% of the Milky Way’s present-day dark matter and ≈50% of its stellar halo.

The stellar occultations by the largest satellite of the dwarf planet Haumea, Hi'iaka

<p>Two stellar occultations by the largest satellite of the dwarf planet Haumea, Hi'iaka, were predicted to happen on April, 6th and 16th, 2021. Additional high accuracy astrometric analysis was carried out in order to refine the prediction for April 6th, using several telescopes in the 1.2-m to 2-m range, with the final shadow path crossing North Africa. We successfully detected the first event from TRAPPIST-North telescope at Oukaïmeden Observatory (Morocco). Although it was recorded from only one site, this first detection allowed us to improve the prediction for the second that crossed North America from East to West. We had a good success recording six positive detections and several negative detections that constrain the shape and size of the body. The light curves obtained from the different observatories provide the time at which the star disappears and reappears, which are translated into chords (the projected lines on the sky-plane as observed from each location). Additionally, we carried out a campaign to study Hi'iaka's rotational light-curve, studying the residuals of Haumea's rotational light-curve to a four-order Fourier fit. We obtained the rotational phases at the times of the occultations, which is critical for the analysis of the occultations, given that Hi’iaka is clearly non-spherical. Our preliminary results show that Hi'iaka indeed has a triaxial shape with a larger effective diameter than what has been published so far. The preliminary results and their implications will be discussed in this talk. </p>

Ring dynamics around the Centaur Chariklo and the dwarf planet Haumea: effects of high-order resonances

<p>Narrow and dense rings have been detected around the small Centaur body Chariklo (Braga-Ribas et al. 2014), as well as around the dwarf planet Haumea (Ortiz et al. 2017).</p> <p>Both objects have non-axisymmetric shapes that induce strong resonant effects between the rotating central body with spin rate <em>Ω </em>and the radial epicyclic motion of the ring particles, <em>κ</em>. These resonances include the classical Eccentric Lindblad Resonances (ELR), where <em>κ = m(n-Ω),</em> with <em>m</em> integer, <em>n </em>being the particle mean motion. These resonances create an exchange of angular momentum between the body and the collisional ring, clearing the corotation zone, pushing the inner disk onto the body and repelling the outer part outside of the outermost 1/2 ELR, where the particles complete one orbital revolution while the body executes two rotations, i.e. <em>n/Ω ~ </em>1/2 (Sicardy et al. 2019)</p> <p>Here I will focus on higher-order resonances. They may appear either by considering other resonances such as <em>n/Ω ~ </em>1/3, or the same resonance as above (<em>n/Ω ~ </em>1/2), but with a body that has a triaxial shape. In this case, the invariance of the potential under a rotation of<em> π</em> radians transforms the 1st-order 1/2 Lindblad Resonance into a 2nd order 2/4 resonance.</p> <p>Second-order resonances are of particular interest because they force a strong response of the particles near the resonance radius, in spite of the intrinsically small strength of their forcing terms. This stems from the topography of the associated resonant Hamiltonian, which possesses an unstable hyperbolic point at its origin.</p> <p>The width of the region where this strong response is expected will be discussed for both Chariklo's and Haumea's rings. The special case of the second-order 1/3 resonance will be discussed, as it appears that both ring systems are close to that resonance.</p> <p>This work is intended, among others, to pave the way for future collisional simulations of rings around non-axisymmetric bodies.</p> <div class="page" title="Page 1"> <div class="layoutArea"> <div class="column"> <p>Braga-Ribas et al., 2014, <em>Nature</em> <strong>508</strong>, 72<br />Ortiz et al., 2017, <em>Nature</em> <strong>550</strong>, 219<br />Sicardy et al., 2019, <em>Nature Astronomy</em> <strong>3</strong>, 146</p> <p>The work leading to these results has received funding from the European Research Council under the European Community's H2020 2014-2020 ERC Grant Agreement n°669416 "Lucky Star"</p> </div> </div> </div>

Triaxial models description of the low-lying properties in 192Os

Several typical triaxial models have been parallel addressed and applied to describe the energy values and [Formula: see text] transitional rates in the ground band and the [Formula: see text] band for [Formula: see text]Os. It is shown that the different triaxial model presents different triaxial dynamics but each of them can only succeed to explain part of the spectral properties of this nucleus, which indicates that the triaxial shape of [Formula: see text]Os may be more complicated than that reflected by an ideal triaxial model. In addition, the staggering signature in experiments hints that [Formula: see text]-rigid triaxiality should be more or less involved in [Formula: see text]Os.