550 research outputs found
A new look at NICMOS transmission spectroscopy of HD189733, GJ-436 and XO-1: no conclusive evidence for molecular features
We present a re-analysis of archival HST/NICMOS transmission spectroscopy of
three exoplanet systems; HD 189733, GJ-436 and XO-1. Detections of several
molecules, including H20, CH4 and CO2, have been claimed for HD 189733 and
XO-1, but similarly sized features are attributed to systematic noise for
GJ-436. The data consist of time-series grism spectra covering a planetary
transit. After extracting light curves in independent wavelength channels, we
use a linear decorrelation technique account for instrumental systematics
(which is becoming standard in the field), and measure the planet-to-star
radius ratio as a function of wavelength. For HD 189733, the uncertainties in
the transmission spectrum are significantly larger than those previously
reported. We also find the transmission spectrum is considerably altered when
using different out-of-transit orbits to remove the systematics, when some
parameters are left out of the decorrelation procedure, or when we perform the
decorrelation with quadratic functions rather than linear functions. Given that
there is no physical reason to believe the baseline flux should be modelled as
a linear function of any particular set of parameters, we interpret this as
evidence that the linear decorrelation technique is not a robust method to
remove systematic effects from the light curves for each wavelength channel.
For XO-1, the parameters measured to decorrelate the light curves would require
extrapolation to the in-transit orbit to remove the systematics, and we cannot
reproduce the previously reported results. We conclude that the resulting
NICMOS transmission spectra are too dependent on the method used to remove
systematics to be considered robust detections of molecular species in
planetary atmospheres, although the presence of these molecules is not ruled
out.Comment: 17 pages, 28 figures, accepted in MNRA
Charge-based supercapacitor storage estimation for indoor sub-mW photovoltaic energy harvesting powered wireless sensor nodes
Supercapacitors offer an attractive energy storage solution for lifetime “fit and forget” photovoltaic (PV) energy harvesting powered wireless sensor nodes for internet of things (IoT) applications. Whilst their low storage capacity is not an issue for sub-mW PV applications, energy loss in the charge redistribution process is a concern. Currently there is no effective method to estimate the storage of the supercapacitor in IoT applications for optimal performance with sub-mW input. The existing energy-based method requires supercapacitor model parameters to be obtained and the initial charge state to be determined, consequently it is not suitable for practical applications. This paper defines a charge-based method, which can directly evaluate supercapacitor’s storage with straightforward calculations. Time constant analysis and experimental tests demonstrate that with the newly proposed method the manufacturer-specified tiny leakage current, although measured long after post-charge (e.g. 72 hours), can be directly used, making the storage estimation for a supercapacitor in IoT applications as simple as that for an ordinary capacitor. In addition, the demonstrated tiny leakage current at the required energy storage for a sub-mW PV powered IoT application enables a supercapacitor alone to be employed as the storage mechanism, thus achieving lifetime battery-replacementfree, self-powered IoT nodes
Comparison of single-layer and double-layer anti-reflection coatings using laser-induced damage threshold and photothermal common-path interferometry
The dielectric thin-film coating on high-power optical components is often the weakest region and will fail at elevated optical fluences. A comparison of single-layer coatings of ZrO2, LiF, Ta2O5, SiN, and SiO2 along with anti-reflection (AR) coatings optimized at 1064 nm comprised of ZrO2 and Ta2O5 was made, and the results of photothermal common-path interferometry (PCI) and a laser-induced damage threshold (LIDT) are presented here. The coatings were grown by radio frequency (RF) sputtering, pulsed direct-current (DC) sputtering, ion-assisted electron beam evaporation (IAD), and thermal evaporation. Test regimes for LIDT used pulse durations of 9.6 ns at 100 Hz for 1000-on-1 and 1-on-1 regimes at 1064 nm for single-layer and AR coatings, and 20 ns at 20 Hz for a 200-on-1 regime to compare the //ZrO2/SiO2 AR coating
HST/STIS Lyman-alpha observations of the quiet M dwarf GJ436: Predictions for the exospheric transit signature of the hot neptune GJ436b
Lyman-alpha (Lya) emission of neutral hydrogen (1215.67 Angstr\"om) is the
main contributor to the ultraviolet flux of low-mass stars such as M dwarfs. It
is also the main light source used in studies of the evaporating upper
atmospheres of transiting extrasolar planets with ultraviolet transmission
spectroscopy. However, there are very few observations of the Lya emissions of
quiet M dwarfs, and none exist for those hosting exoplanets. Here, we present
Lya observations of the hot-neptune host star GJ436 with the Hubble Space
Telescope Imaging Spectrograph (HST/STIS). We detect bright emission in the
first resolved and high quality spectrum of a quiet M dwarf at Lya. Using an
energy diagram for exoplanets and an N-body particle simulation, this detection
enables the possible exospheric signature of the hot neptune to be estimated as
a ~11% absorption in the Lya stellar emission, for a typical mass-loss rate of
10^10 g/s. The atmosphere of the planet GJ436b is found to be stable to
evaporation, and should be readily observable with HST. We also derive a
correlation between X-ray and Lya emissions for M dwarfs. This correlation will
be useful for predicting the evaporation signatures of planets transiting other
quiet M dwarfs.Comment: 8 pages, 8 figures, 2 tables. Accepted for publication in Astronomy &
Astrophysic
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