arXiv:1807.08728 -- P/2017 S5: Another Active Asteroid Associated with the Theobalda Family

Paper: P/2017 S5: Another Active Asteroid Associated with the Theobalda Family
Authors: Bojan Novakovic

Abstract: In this note we have shown that a newly discovered comet P/2017 S5 (ATLAS), that moves around the Sun in an asteroid-like orbit, is a member of the Theobalda asteroid family.

My Comment: Super short letter, but packed full of references giving a clear "how you do this science" outline. Love it. One of my favorite experiences with an undergrad research student was when she serendipitously found a candidate active asteroid while working with me on a comet project. Turns out it (most likely) wasn't but working out that puzzle involved her getting time on a 3.5m telescope(!), and having her work praised by Jocelyn Bell Burnell(!!).

My Scrawling Notes:

arXiv:1807.07557 -- WASP-128b: a transiting brown dwarf in the dynamical-tide regime

Paper: WASP-128b: a transiting brown dwarf in the dynamical-tide regime
Authors: V. Hodžić, et al.

Abstract: Massive companions in close orbits around G dwarfs are thought to undergo rapid orbital decay due to runaway tidal dissipation. We report here the discovery of WASP-128b, a brown dwarf discovered by the WASP survey transiting a G0V host on a 2.2d orbit, where the measured stellar rotation rate places the companion in a regime where tidal interaction is dominated by dynamical tides. Under the assumption of dynamical equilibrium, we derive a value of the stellar tidal quality factor logQ′⋆=6.96±0.19. A combined analysis of ground-based photometry and high-resolution spectroscopy reveals a mass and radius of the host, M⋆=1.16±0.04M⊙, R⋆=1.16±0.02R⊙, and for the companion, Mb=37.5±0.8MJup, Rb=0.94±0.02RJup, placing WASP-128b in the driest parts of the brown dwarf desert, and suggesting a mild inflation for its age. We estimate a remaining lifetime for WASP-128b similar to that of some ultra-short period massive hot Jupiters.

My Comment: I honestly didn't know (or had forgotten) that brown dwarf companions were very rare around solarish stars. I enjoyed the mix of observations, dynamic theory, and computational modelling all nicely fit together.

My Scrawling Notes:

arXiv:1807.06368 -- Planetary Nebulae distances in GAIA DR2

Paper: Planetary Nebulae distances in GAIA DR2
Authors: Stefan Kimeswenger, Daniela Barría

Abstract: Context: Planetary Nebula distance scales often suffer for model dependent solutions. Model independent trigonometric parallaxes have been rare. Space based trigonometric parallaxes are now available for a larger sample using the second data release of GAIA. 

Aims: We aim to derive a high quality approach for selection criteria of trigonometric parallaxes for planetary nebulae and discuss possible caveats and restrictions in the use of this data release. 

Methods: A few hundred sources from previous distance scale surveys were manually cross identified with data from the second GAIA data release (DR2) as coordinate based matching does not work reliable. The data are compared with the results of previous distance scales and to the results of a recent similar study, which was using the first data release GAIA DR1. 

Results: While the few available previous ground based and HST trigonometric parallaxes match perfectly to the new data sets, older statistical distance scales, reaching larger distances, do show small systematic differences. Restricting to those central stars, were photometric colors of GAIA show a negligible contamination by the surrounding nebula, the difference is negligible for radio flux based statistical distances, while those derived from H-alpha surface brightness still show minor differences. The DR2 study significantly improves the previous recalibration of the statistical distance scales using DR1/TGAS.

My Comment: Had not really though about it much before, but planetary nebula being big ol' clouds of gas are really tricky to measure distances to, if one is lucky you can find an embedded point-source that isn't suffering observationally from the surrounding gas. GAIA DR2, which is redefining our knowledge of our galaxy right now actually gives negative(!) parallaxes for many of these objects because space is hard.

My Scrawling Notes: (color and thumb avoidance fail today!)

arXiv:1807.05965 -- The Cow: discovery of a luminous, hot and rapidly evolving transient

Paper: The Cow: discovery of a luminous, hot and rapidly evolving transient
Authors: S. J. Prentice, et al.

Abstract: We present the ATLAS discovery and initial analysis of the first 18 days of the unusual transient event, ATLAS18qqn/AT2018cow. It is characeterized by a high peak luminosity (∼1.7×1044 erg s−1), rapidly evolving light curves (>5 mag rise in ∼3 days), hot blackbody spectra, peaking at ∼27000 K that are relatively featureless and unchanging over the first two weeks. The bolometric light curve cannot be powered by radioactive decay under realistic assumptions. The detection of high-energy emission may suggest a central engine as the powering source. Using a magnetar model, we estimated an ejected mass of 0.1−0.4 M⊙, which lies between that of low-energy core-collapse events and the kilonova, AT2017gfo. The spectra of AT2018cow showed a number of shallow features overlying a blackbody continuum. The spectra cooled rapidly from 27000 to 15000 K in just over 2 weeks but the positions of shallow bumps in the spectra did not evolve, suggesting that they are produced in a shell or are potentially emission features. Using spectral modelling, we tentatively identify some features as being due to He I and He II and rule out that the features in the spectra are due to most elements up to and including the Fe-group. The presence of r-process elements cannot be ruled out. If these lines are due to He, then we suggest a low-mass star with residual He as a potential progenitor. Alternatively, models of magnetars formed in neutron-star mergers give plausible matches to the data. 

My Comment: Weird things that go bump in the night are always neat. This object seems to offer some unique challenges (spectra) in terms of matching what it is. Best guess being a magnetar-like object. The combination of Cow and magnetar is honestly why this paper was chosen. Moooooo.

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arXiv:1807.02960 -- Outer solar system possibly shaped by a stellar fly-by

Paper: Outer solar system possibly shaped by a stellar fly-by
Authors: Susanne Pfalzner, Asmita Bhandare, Kirsten Vincke, Pedro Lacerda

Abstract: The planets of our solar system formed from a gas-dust disk. However, there are some properties of the solar system that are peculiar in this context. First, the cumulative mass of all objects beyond Neptune (TNOs) is only a fraction of what one would expect. Second, unlike the planets themselves, the TNOs do not orbit on coplanar, circular orbits around the Sun, but move mostly on inclined, eccentric orbits and are distributed in a complex way. This implies that some process restructured the outer solar system after its formation. However, some of TNOs, referred to as Sednoids, move outside the zone of influence of the planets. Thus external forces must have played an important part in the restructuring of the outer solar system. The study presented here shows that a close fly-by of a neighbouring star can simultaneously lead to the observed lower mass density outside 30 AU and excite the TNOs onto eccentric, inclined orbits, including the family of Sednoids. In the past it was estimated that such close fly-bys are rare during the relevant development stage. However, our numerical simulations show that such a scenario is much more likely than previously anticipated. A fly-by also naturally explains the puzzling fact that Neptune has a higher mass than Uranus. Our simulations suggest that many additional Sednoids at high inclinations still await discovery, perhaps including bodies like the postulated planet X.

My Comment: Fun discussion of solar system shaping mechanism and conducted further work to show plausibility of a fly-by during very early (10Myr) Solar System. Got me thinking about the Tisserand parameter with respect to Neptune, and once again am very happy to have astro-twitter folks willing to point papers and ideas out to me!

My Scrawling Notes:

Further investigation of changes in cometary rotation: arXiv:1806.11158

Paper: Further investigation of changes in cometary rotation
Authors: Beatrice E. A. Mueller, Nalin H. Samarasinha

Abstract: Samarasinha & Mueller (2013) related changes of cometary rotation to other physical parameters for four Jupiter family comets defining a parameter X  , which is approximately constant within a factor of two irrespective of the active fraction of a comet. Two additional comets are added to this sample in this paper and the claim of a nearly constant parameter X  for these six comets is confirmed, albeit with a larger scatter. Taking the geometric mean of X  for all the comets above excluding 2P/Encke (as X  for each comet was determined with respect to that of 2P/Encke), the expected changes in the rotation periods for a sample of 24 periodic comets are derived. We identify comets from this sample that are most likely to show observationally detectable changes in their rotation periods. Using this sample and including the six comets used to determine X  , we find a correlation between the parameter ζ  (i.e. the total water production per unit surface area per orbit approximated by that inside of 4 au) and the perihelion distance q  ; specifically we derive ζ  ∝  q −0.8   and provide a theoretical basis for this in Appendix A. This relationship between ζ  and q  enables ready comparisons of activity due to insolation between comets. Additionally, a relationship between the nuclear radius R  and the rotation period P  is found. Specifically, we find that on average smaller nuclei have smaller rotation periods compared to the rotation periods of larger nuclei. This is consistent with expectations for rotational evolution and spin-up of comet nuclei, providing strong observational evidence for sublimation-driven rotational changes in comets.

My Comment: The empirical law given seems to work on a factor of a few, which is better than the order of magnitude spanning effects being modeled. This paper does not justify its creation, with the details being presented in an earlier work from the authors in 2013.

My Scrawling Notes:

Special cases : moons, rings, comets, trojans; arXiv:1806.10032

Paper: Special cases: moons, rings, comets, trojans
Authors: Juan Cabrera, Maria Fernandez Jimenez, Antonio Garcia Munoz, Jean Schneider
Abstract: Non-planetary bodies provide valuable insight into our current under- standing of planetary formation and evolution. Although these objects are challeng- ing to detect and characterize, the potential information to be drawn from them has motivated various searches through a number of techniques. Here, we briefly review the current status in the search of moons, rings, comets, and trojans in exoplanet systems and suggest what future discoveries may occur in the near future. .

My Comment: Nice quick review - actually an (intro?) chapter for a larger review. Although posted to astro-ph today, the article is only current through mid 2017, and makes many references to the other chapters of the larger review series. Takeaway: we are in an era where smaller than planetary objects (rings, comets, moons, etc) may potentially be found around other star systems, but it is going to be a monumental task to cleanly separate an unambiguous signal of such from various systematics of the observations, and the fickleness of stars.

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