Analytic Scattering and Refraction Models for Exoplanet Transit Spectra

Observations of exoplanet transit spectra are essential to understanding the physics and chemistry of distant worlds. The effects of opacity sources and many physical processes combine to set the shape of a transit spectrum. Two such key processes - refraction and cloud and/or haze forward scattering - have seen substantial recent study. However, models of these processes are typically complex, which prevents their incorporation into observational analyses and standard transit spectrum tools. In this work, we develop analytic expressions that allow for the efficient parameterization of forward scattering and refraction effects in transit spectra. We derive an effective slant optical depth that includes a correction for forward scattered light, and present an analytic form of this correction. We validate our correction against a full-physics transit spectrum model that includes scattering, and we explore the extent to which the omission of forward scattering effects may bias models. Also, we verify a common analytic expression for the location of a refractive boundary, which we express in terms of the maximum pressure probed in a transit spectrum. This expression is designed to be easily incorporated into existing tools, and we discuss how the detection of a refractive boundary could help indicate the background atmospheric composition by constraining the bulk refractivity of the atmosphere. Finally, we show that opacity from Rayleigh scattering and collision induced absorption will outweigh the effects of refraction for Jupiter-like atmospheres whose equilibrium temperatures are above 400-500 K.Comment: ApJ accepted; submitted Feb. 7, 201

Titan solar occultation observations reveal transit spectra of a hazy world

High altitude clouds and hazes are integral to understanding exoplanet observations, and are proposed to explain observed featureless transit spectra. However, it is difficult to make inferences from these data because of the need to disentangle effects of gas absorption from haze extinction. Here, we turn to the quintessential hazy world -- Titan -- to clarify how high altitude hazes influence transit spectra. We use solar occultation observations of Titan's atmosphere from the Visual and Infrared Mapping Spectrometer (VIMS) aboard NASA's Cassini spacecraft to generate transit spectra. Data span 0.88-5 microns at a resolution of 12-18 nm, with uncertainties typically smaller than 1%. Our approach exploits symmetry between occultations and transits, producing transit radius spectra that inherently include the effects of haze multiple scattering, refraction, and gas absorption. We use a simple model of haze extinction to explore how Titan's haze affects its transit spectrum. Our spectra show strong methane absorption features, and weaker features due to other gases. Most importantly, the data demonstrate that high altitude hazes can severely limit the atmospheric depths probed by transit spectra, bounding observations to pressures smaller than 0.1-10 mbar, depending on wavelength. Unlike the usual assumption made when modeling and interpreting transit observations of potentially hazy worlds, the slope set by haze in our spectra is not flat, and creates a variation in transit height whose magnitude is comparable to those from the strongest gaseous absorption features. These findings have important consequences for interpreting future exoplanet observations, including those from NASA's James Webb Space Telescope.Comment: Updated journal reference; data available via http://sites.google.com/site/tdrobinsonscience/science/tita

Enantioselective Total Synthesis of (–)-Myrifabral A and B

A catalytic enantioselective approach to the Myrioneuron alkaloids (−)-myrifabral A and (−)-myrifabral B is described. The synthesis was enabled by a palladium-catalyzed enantioselective allylic alkylation, that generates the C(10) all-carbon quaternary center. A key N-acyl iminium ion cyclization forged the cyclohexane fused tricyclic core, while vinyl boronate cross metathesis and oxidation afforded the lactol ring of (−)-myrifabral A. Adaptation of previously reported conditions allowed for the conversion of (−)-myrifabral A to (−)-myrifabral B

Inversion of spinning sound fields

A method is presented for the reconstruction of rotating monopole source distributions using acoustic pressures measured on a sideline parallel to the source axis. The method requires no \textit{a priori} assumptions about the source other than that its strength at the frequency of interest vary sinusoidally in azimuth on the source disc so that the radiated acoustic field is composed of a single circumferential mode. When multiple azimuthal modes are present, the acoustic field can be decomposed into azimuthal modes and the method applied to each mode in sequence. The method proceeds in two stages, first finding an intermediate line source derived from the source distribution and then inverting this line source to find the radial variation of source strength. A far-field form of the radiation integrals is derived, showing that the far field pressure is a band-limited Fourier transform of the line source, establishing a limit on the quality of source reconstruction which can be achieved using far-field measurements. The method is applied to simulated data representing wind-tunnel testing of a ducted rotor system (tip Mach number~0.74) and to control of noise from an automotive cooling fan (tip Mach number~0.14), studies which have appeared in the literature of source identification.Comment: Revised version of paper submitted to JASA; five more figures; expanded content with more discussion of error behaviour and relation to Nearfield Acoustical Holograph

Vasodilation to PTH 1-84 in bone resistance arteries of rats occurs via endothelium-dependent, rather than endothelium-independent signaling

Parathyroid hormone (PTH) is a potent vasodilator, causing systemic hypotension. Previous investigations concluded that vasodilation to PTH in a variety of vascular beds occurs via inhibition of L-type calcium channels in smooth muscle cells. Further, removal of the endothelium in aortic strips and tail arteries did not inhibit relaxation to PTH, suggesting that vasodilation in these vessels does not require nitric oxide (NO) or vascular endothelial cells (Pang et al., 1985; Nickols 1987; Nickols et al., 1986; Crass et al., 1988). We have previously shown that PTH 1-84 augments vasodilation to ~52% of maximum diameter in the femoral principal nutrient artery (PNA; the primary conduit for blood flow to long bones). Further, vasodilation was nearly obliterated with blockade of NO production with the endothelial nitric oxide synthase inhibitor L-NAME, suggesting that vasodilation in the bone vasculature occurs exclusively via NO-mediated signaling. Thus, to confirm these findings, the purpose of this study was to determine whether vasodilation to PTH 1-84 occurs in the absence of endothelial cells. METHODS: Right femoral PNAs were dissected from 4 month-old male Wistar rats (453 g; n=6), denuded (i.e., removed of the endothelial cells) and cannulated on glass micropipettes. PNAs were considered adequately denuded if they dilated ≤ 20 µm to a bolus dose of acetylcholine (5x10-5 M). Vasodilation to PTH 1-84 (10-13 – 10-8 M) was assessed in the presence of 1) PSS buffer, 2) PSS buffer with L-NAME, and 3) PSS buffer with L-NAME + indomethacin (Indo; a cyclooxygenase inhibitor). To ensure that the smooth muscle cells were not damaged during endothelial cell removal, endothelium-independent vasodilation to sodium nitroprusside (SNP; 10-10 – 10-4) was determined. RESULTS: Vasodilation to cumulative doses of PTH 1-84 peaked at 5% of maximum diameter. Inhibition of NO production with L-NAME completely inhibited vasodilation and caused a slight vasoconstriction, while combined inhibition with L-NAME + Indo did not constrict the femoral PNA; however, neither response differed from the PTH 1-84 response. Vasodilation to SNP rose to 69% of maximal diameter, indicating that smooth muscle cell responsiveness was not altered with endothelial denudation. DISCUSSION: These preliminary data support our contention that vasodilation of the bone resistance vasculature occurs via endothelium-dependent, rather than endothelium-independent signaling pathways. This novel finding suggests that, contrary to vasodilator mechanisms in blood vessels from other tissue beds, bone blood vessels require the vascular endothelial cell lining for vasodilation to PTH 1-84. Supported by NIH Grant 1R15AR062882-0

Earthshine as an Illumination Source at the Moon

Earthshine is the dominant source of natural illumination on the surface of the Moon during lunar night, and at locations within permanently shadowed regions that never receive direct sunlight. As such, earthshine may enable the exploration of areas of the Moon that are hidden from solar illumination. The heat flux from earthshine may also influence the transport and cold trapping of volatiles present in the very coldest areas. In this study, Earth's spectral radiance at the Moon is examined using a suite of Earth spectral models created using the Virtual Planetary Laboratory (VPL) three dimensional modeling capability. At the Moon, the broadband, hemispherical irradiance from Earth near 0 phase is approximately 0.15 watts per square meter, with comparable contributions from solar reflectance and thermal emission. Over the simulation timeframe, spanning two lunations, Earth's thermal irradiance changes less than a few mW per square meter as a result of cloud variability and the south-to-north motion of sub-observer position. In solar band, Earth's diurnally averaged light curve at phase angles < 60 degrees is well fit using a Henyey Greenstein integral phase function. At wavelengths > 0.7 microns, near the well known vegetation "red edge", Earth's reflected solar radiance shows significant diurnal modulation as a result of the longitudinal asymmetry in projected landmass, as well as from the distribution of clouds. A simple formulation with adjustable coefficients is presented for estimating Earth's hemispherical irradiance at the Moon as a function of wavelength, phase angle and sub-observer coordinates. It is demonstrated that earthshine is sufficiently bright to serve as a natural illumination source for optical measurements from the lunar surface.Comment: 27 pages, 15 figures, 1 tabl

The Value of Comparative Animal Research : Krogh’s Principle Facilitates Scientific Discoveries

There are no conflicts of interest to declare. This paper developed from the 2016 Early Career Impact Award from the Federation of Associations in Behavioral & Brain Sciences to TJS. TJS has received funding from The Leverhulme Trust. FJPE is in receipt of funding from the BBSRC (BB/M001555/1). The National Institutes of Health has funded RDF (NS 034950, NS093277, NIMH 087930), AGO (HD079573, IOS-1354760) and AMK (HD081959). BAA is an Arnold O. Beckman postdoctoral fellow.Peer reviewedPostprin