Circumstellar environment of RX Puppis

The symbiotic Mira, RX Pup, shows long-term variations in its mean light level due to variable obscuration by circumstellar dust. The last increase in extinction towards the Mira, between 1995 and 2000, has been accompanied by large changes in the degree of polarization in the optical and red spectral range. The lack of any obvious associated changes in the position angle may indicate the polarization variations are driven by changes in the properties of the dust grains (e.g. variable quantity of dust and variable particle size distribution, due to dust grain formation and growth) rather than changes in the viewing geometry of the scattering region(s), e.g. due to the binary rotation.Comment: Paper presented at Torun 2000 conference on Post-AGB objects as a phase of stellar evolution; 8 pages, 3 figure

Substorm Onset Latitude and the Steadiness of Magnetospheric Convection

We study the role of substorms and steady magnetospheric convection (SMC) in magnetic flux transport in the magnetosphere, using observations of field‐aligned currents by the Active Magnetosphere and Planetary Electrodynamics Response Experiment. We identify two classes of substorm, with onsets above and below 65° magnetic latitude, which display different nightside field‐aligned current morphologies. We show that the low‐latitude onsets develop a poleward‐expanding auroral bulge, and identify these as substorms that manifest ionospheric convection‐braking in the auroral bulge region as suggested by Grocott et al. (2009, https://doi.org/10.5194/angeo-27-591-2009). We show that the high‐latitude substorms, which do not experience braking, can evolve into SMC events if the interplanetary magnetic field remains southward for a prolonged period following onset. We conclude that during periods of ongoing driving, the magnetosphere displays repeated substorm activity or SMC depending on the rate of driving and the open magnetic flux content of the magnetosphere prior to onset. We speculate that sawtooth events are an extreme case of repeated onsets and that substorms triggered by northward‐turnings of the interplanetary magnetic field mark the cessation of periods of SMC. Our results provide a new explanation for the differing modes of response of the terrestrial system to solar wind‐magnetosphere‐ionosphere coupling by invoking friction between the ionosphere and atmosphere.publishedVersio

Generalized Painleve-Gullstrand descriptions of Kerr-Newman black holes

Generalized Painleve-Gullstrand metrics are explicitly constructed for the Kerr-Newman family of charged rotating black holes. These descriptions are free of all coordinate singularities; moreover, unlike the Doran and other proposed metrics, an extra tunable function is introduced to ensure all variables in the metrics remain real for all values of the mass M, charge Q, angular momentum aM, and cosmological constant \Lambda > - 3/(a^2). To describe fermions in Kerr-Newman spacetimes, the stronger requirement of non-singular vierbein one-forms at the horizon(s) is imposed and coordinate singularities are eliminated by local Lorentz boosts. Other known vierbein fields of Kerr-Newman black holes are analysed and discussed; and it is revealed that some of these descriptions are actually not related by physical Lorentz transformations to the original Kerr-Newman expression in Boyer-Lindquist coordinates - which is the reason complex components appear (for certain ranges of the radial coordinate) in these metrics. As an application of our constructions the correct effective Hawking temperature for Kerr black holes is derived with the method of Parikh and Wilczek.Comment: 5 pages; extended to include application to derivation of Hawking radiation for Kerr black holes with Parikh-Wilczek metho

Sosa on Knowledge, Judgment and Guessing

In Chapter 3 of Judgment and Agency, Sosa (Judgment and Agency, 2015) explicates the concept of a fully apt performance. In the course of doing so, he draws from illustrative examples of practical performances and applies lessons drawn to the case of cognitive performances, and in particular, to the cognitive performance of judging. Sosa’s examples in the practical sphere are rich and instructive. But there is, I will argue, an interesting disanalogy between the practical and cognitive examples he relies on. Ultimately, I think the source of the disanalogy is a problematic picture of the cognitive performance of guessing and its connection to knowledge and defeat. Once this critical line of argument is advanced, an alternative picture of guessing, qua cognitive performance, is articulated, one which avoids the problems discussed, and yet remains compatible with Sosa’s broader framework

Bayesian Methods for Exoplanet Science

Exoplanet research is carried out at the limits of the capabilities of current telescopes and instruments. The studied signals are weak, and often embedded in complex systematics from instrumental, telluric, and astrophysical sources. Combining repeated observations of periodic events, simultaneous observations with multiple telescopes, different observation techniques, and existing information from theory and prior research can help to disentangle the systematics from the planetary signals, and offers synergistic advantages over analysing observations separately. Bayesian inference provides a self-consistent statistical framework that addresses both the necessity for complex systematics models, and the need to combine prior information and heterogeneous observations. This chapter offers a brief introduction to Bayesian inference in the context of exoplanet research, with focus on time series analysis, and finishes with an overview of a set of freely available programming libraries.Comment: Invited revie

Elgin on understanding:How does it involve know-how, endorsement and factivity?

In Chapter 3 of True Enough, Elgin (2017) outlines her view of objectual understanding, focusing largely on its non-factive nature and the extent to which a certain kind of know-how is required for the “grasping” component of understanding. I will explore four central issues that feature in this chapter, concentrating on (1) the role of know-how, (2) the concept of endorsement, (3) Elgin’s critique of the factivity constraint on understanding, and (4) how we might use aspects of Elgin’s framework to inform related debates on the norm of assertion