arXiv 1808.02618: Earth and Planetary Astrophysics OSSOS: XIII. Fossilized Resonant Dropouts Imply Neptune's Migration was Grainy and Slow

Paper: OSSOS: XIII. Fossilized Resonant Dropouts Imply Neptune's Migration was Grainy and Slow
Authors: S. M. Lawler, R. E. Pike, N. Kaib, M. Alexandersen, M. T. Bannister, Y.-T. Chen, B. Gladman, S. Gwyn, J. J. Kavelaars, J.-M. Petit, K. Volk

Abstract: The migration of Neptune's resonances through the early Kuiper belt has left signatures of the migration timescale and mode in the distribution of small bodies in the outer Solar System. Here we analyze five published Neptune migration models in detail, focusing on the high pericenter distance (q) trans-Neptunian Objects (TNOs) near Neptune's mean-motion resonances. We focus on the TNOs near the 5:2 and 3:1 resonances, because they have large detected populations, are outside the main classical belt, and are relatively isolated from other strong resonances. We compare the observationally biased output from these dynamical models with the detected TNOs from the Outer Solar System Origins Survey, via its Survey Simulator. All of the four new OSSOS detections of high-q non-resonant TNOs are on the Sunward side of the 5:2 and 3:1 resonances. We show that even after accounting for observation biases, this asymmetric distribution cannot be drawn from a uniform distribution of TNOs at 2-sigma confidence. We find that the dynamical model that uses grainy slow Neptune migration provides the best match to the real TNO orbital data. However, due to extreme observational biases, we have very few high pericenter distance TNO discoveries with which to statistically constrain the models. We show that a deeper survey (to a limiting r-magnitude of 26.0) with a similar survey area to OSSOS could statistically distinguish between these five Neptune migration models. We speculate that the cycle of resonance sticking and Kozai oscillation within a resonance, followed by resonant dropout into this fossilized high-q population, could potentially explain all but the two very highest-q TNOs discovered to date.

My Comment: To me this is so very much the heart of astronomy science. We very rarely can do an experiment in the lab, so we have to do as much as possible with what we are able to observe. Our theories must be realized by careful simulation of what the physics say they do, and then must be carefull compared to well-characterized observations.

My Scrawling Notes:

arXiv:1808.01862: Yes, Aboriginal Australians Can and Did Discover the Variability of Betelgeuse

Paper: Yes, Aboriginal Australians Can and Did Discover the Variability of Betelgeuse
Authors: Bradley E. Schaefer

Abstract: Recently, a widely publicized claim has been made that the Aboriginal Australians discovered the variability of the red star Betelgeuse in the modern Orion, plus the variability of two other prominent red stars: Aldebaran and Antares. This result has excited the usual healthy skepticism, with questions about whether any untrained peoples can discover the variability and whether such a discovery is likely to be placed into lore and transmitted for long periods of time. Here, I am offering an independent evaluation, based on broad experience with naked-eye sky viewing and astro-history. I find that it is easy for inexperienced observers to detect the variability of Betelgeuse over its range in brightness from V = 0.0 to V = 1.3, for example in noticing from season-to-season that the star varies from significantly brighter than Procyon to being greatly fainter than Procyon. Further, indigenous peoples in the Southern Hemisphere inevitably kept watch on the prominent red star, so it is inevitable that the variability of Betelgeuse was discovered many times over during the last 65 millennia. The processes of placing this discovery into a cultural context (in this case, put into morality stories) and the faithful transmission for many millennia is confidently known for the Aboriginal Australians in particular. So this shows that the whole claim for a changing Betelgeuse in the Aboriginal Australian lore is both plausible and likely. Given that the discovery and transmission is easily possible, the real proof is that the Aboriginal lore gives an unambiguous statement that these stars do indeed vary in brightness, as collected by many ethnographers over a century ago from many Aboriginal groups. So I strongly conclude that the Aboriginal Australians could and did discover the variability of Betelgeuse, Aldebaran, and Antares.

My Comment: The title of this paper caught my eye, as a student was doing an astro-education project on the variability of Betelgeuse. She had various peoples, typically untrained observers, and children, observe Betelgeuse for variability, and the early results indicated that it was easily accomplished. This fact of it being an easily observable thing to "discover" was a central point in the paper. In no way is the claim of Aboriginal Australians discovering the variability of Betelgeuse "fantastic." This article tells a clear and direct narrative that lays out the evidence, analysis, and methodology involved, and show how to investigate claims of real cultural knowledge in the era of fantastical claims. 

My Scrawling Notes:

arXiv:1808.00609 - The excitation of a primordial cold asteroid belt as an outcome of the planetary instability

Paper: The excitation of a primordial cold asteroid belt as an outcome of the planetary instability
Authors: Rogerio Deienno, Andre Izidoro, Alessandro Morbidelli, Rodney S. Gomes, David Nesvorny, Sean N. Raymond

Abstract: The main asteroid belt (MB) is low in mass but dynamically excited. Here we propose a new mechanism to excite the MB during the giant planet ('Nice model') instability, which is expected to have featured repeated close encounters between Jupiter and one or more ice giants ('Jumping Jupiter' -- JJ). We show that, when Jupiter temporarily reaches a high enough level of excitation, both in eccentricity and inclination it induces strong forced vectors of eccentricity and inclination across the MB region. Because during the JJ instability Jupiter's orbit `jumps' around, the forced vectors keep changing both in magnitude and phase throughout the whole MB region. The entire cold primordial MB is thus excited as a natural outcome of the JJ instability. The level of such an excitation, however, is typically larger than the current orbital excitation observed in the MB. We show that the subsequent evolution of the Solar System is capable of reshaping the resultant over-excited MB to its present day orbital state, and that a strong mass depletion (∼90%) is associated to the JJ instability phase and its subsequent evolution throughout the age of the Solar System

My Comment: Solar System dynamics are what drew me into astronomy in the first place. More than any results of this paper, I find it absolutely stellar as it clearly explains what is being simulated, and why, and points the reader at additional sources of information on the particulars. In sense it manages to not only present the science that was done, but also how that science was accomplished.

My Scrawling Notes:

arXiv:1807.11496 - Gaia: Orion's Integral Shaped Filament is a Standing Wave

Paper: Gaia: Orion's Integral Shaped Filament is a Standing Wave
Authors: Amelia M. Stutz, Valentina I. Gonzalez-Lobos, Andrew Gould

Abstract: The Integral Shaped Filament (ISF) is the nearest molecular cloud with rapid star formation, including massive stars, and it is therefore a star-formation laboratory. We use Gaia parallaxes, to show that the distances to young Class II stars ('disks') projected along the spine of this filament are related to the gas radial velocity by

v=−Dτ+K;τ=4Myr,

where K is a constant. This implies that the ISF is a standing wave, which is consistent with the Stutz & Gould (2016) 'Slingshot' prediction. The τ=4Myr timescale is consistent with the 'Slingshot' picture that the Orion Nebula Cluster (ONC) is the third cluster to be violently split off from the Orion A cloud (following NGC 1981 and NGC 1987) at few-Myr intervals due to gravito-magnetic oscillations. We also present preliminary evidence that the truncation of the ISF is now taking place 16′ south of the ONC and is mediated by a torsional wave that is propagating south with a characteristic timescale τtorsion=0.5Myr, i.e. eight times shorter. The relation between these two wave phenomena is not presently understood.

My Comment: Dear students: this (one reason) why it is important to understand simple waves. 

My Scrawling Notes: