1,632 research outputs found

    Modeling giant extrasolar ring systems in eclipse and the case of J1407b: sculpting by exomoons?

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    The light curve of 1SWASP J140747.93-394542.6, a \sim16 Myr old star in the Sco-Cen OB association, underwent a complex series of deep eclipses that lasted 56 days, centered on April 2007. This light curve is interpreted as the transit of a giant ring system that is filling up a fraction of the Hill sphere of an unseen secondary companion, J1407b. We fit the light curve with a model of an azimuthally symmetric ring system, including spatial scales down to the temporal limit set by the star's diameter and relative velocity. The best ring model has 37 rings and extends out to a radius of 0.6 AU (90 million km), and the rings have an estimated total mass on the order of 100MMoon100 M_{Moon}. The ring system has one clearly defined gap at 0.4 AU (61 million km), which we hypothesize is being cleared out by a <0.8M< 0.8 M_{\oplus} exosatellite orbiting around J1407b. This eclipse and model implies that we are seeing a circumplanetary disk undergoing a dynamic transition to an exosatellite-sculpted ring structure and is one of the first seen outside our Solar system.Comment: 13 pages, 6 figures, 3 tables. Accepted for publication in ApJ. Data and computer code for model at: http://github.com/mkenworthy/exoring

    Infrared Variability of the Gliese 569B System

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    Gliese 569B is a multiple brown dwarf system whose exact nature has been the subject of several investigations over the past few years. Interpretation has partially relied on infra-red photometry and spectroscopy of the resolved components of the system. We present seeing limited Ks photometry over four nights, searching for variability in this young low mass substellar system. Our photometry is consistent with other reported photometry, and we report the tentative detection of several periodic signals consistent with rotational modulation due to spots on their surfaces. The five significant periods range from 2.90 hours to 12.8 hours with peak to peak variabilities from 28 mmag to 62 mmag in the Ks band. If both components are rotating with the shortest periods, then their rotation axes are not parallel with each other, and the rotation axis of the Bb component is not perpendicular to the Ba-Bb orbital plane. If Bb has one of the longer rotational periods, then the Bb rotation axis is consistent with being parallel to the orbital axis of the Ba-Bb system.Comment: 22 pages, 7 figures, accepted for publication in the Astrophysical Journa

    Spectrophotometry with a transmission grating for detecting faint occultations

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    High-precision spectrophotometry is highly desirable in detecting and characterizing close-in extrasolar planets to learn about their makeup and temperature. For such a goal, a modest-size telescope with a simple low-resolution spectroscopic instrument is potentially as good or better than a complex general purpose spectrograph since calibration and removal of systematic errors is expected to dominate. We use a transmission grating placed in front of an imaging CCD camera on Steward Observatory's Kuiper 1.5 m telescope to provide a high signal-to-noise, low dispersion visible spectrum of the star HD 209458. We attempt to detect the reflected light signal from the extra-solar planet HD 209458b by differencing the signal just before and after secondary occultation. We present a simple data reduction method and explore the limits of ground based low-dispersion spectrophotometry with a diffraction grating. Reflected light detection levels of 0.1% are achievable for 5000-7000A, too coarse for useful limits on ESPs but potentially useful for determining spectra of short-period binary systems with large (Delta m_vis=6) brightness ratios. Limits on the precison are set by variations in atmospheric seeing in the low-resolution spectrum. Calibration of this effect can be carried out by measurement of atmospheric parameters from the observations themselves, which may allow the precision to be limited by the noise due to photon statistics and atmospheric scintillation effects.Comment: 34 pages and 17 figures. Accepted for publication in PAS

    Feasibility of transit photometry of nearby debris discs

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    Dust in debris discs is constantly replenished by collisions between larger objects. In this paper, we investigate a method to detect these collisions. We generate models based on recent results on the Fomalhaut debris disc, where we simulate a background star transiting behind the disc, due to the proper motion of Fomalhaut. By simulating the expanding dust clouds caused by the collisions in the debris disc, we investigate whether it is possible to observe changes in the brightness of the background star. We conclude that in the case of the Fomalhaut debris disc, changes in the optical depth can be observed, with values of the optical depth ranging from 100.510^{-0.5} for the densest dust clouds to 10810^{-8} for the most diffuse clouds with respect to the background optical depth of 1.2×103\sim1.2\times10^{-3}.Comment: 19 pages, 15 figures, accepted for publication in MNRA

    Optimized Principal Component Analysis on Coronagraphic Images of the Fomalhaut System

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    We present the results of a study to optimize the principal component analysis (PCA) algorithm for planet detection, a new algorithm complementing ADI and LOCI for increasing the contrast achievable next to a bright star. The stellar PSF is constructed by removing linear combinations of principal components, allowing the flux from an extrasolar planet to shine through. The number of principal components used determines how well the stellar PSF is globally modelled. Using more principal components may decrease the number of speckles in the final image, but also increases the background noise. We apply PCA to Fomalhaut VLT NaCo images acquired at 4.05 micron with an apodized phase plate. We do not detect any companions, with a model dependent upper mass limit of 13-18 M_Jup from 4-10 AU. PCA achieves greater sensitivity than the LOCI algorithm for the Fomalhaut coronagraphic data by up to 1 magnitude. We make several adaptations to the PCA code and determine which of these prove the most effective at maximizing the signal-to-noise from a planet very close to its parent star. We demonstrate that optimizing the number of principal components used in PCA proves most effective for pulling out a planet signal.Comment: Accepted for publication in ApJ, 7 pages, 9 figure

    Characterising exo-ringsystems around fast-rotating stars using the Rossiter-McLaughlin effect

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    Planetary rings produce a distinct shape distortion in transit lightcurves. However, to accurately model such lightcurves the observations need to cover the entire transit, especially ingress and egress, as well as an out-of-transit baseline. Such observations can be challenging for long period planets, where the transits may last for over a day. Planetary rings will also impact the shape of absorption lines in the stellar spectrum, as the planet and rings cover different parts of the rotating star (the Rossiter-McLaughlin effect). These line-profile distortions depend on the size, structure, opacity, obliquity and sky projected angle of the ring system. For slow rotating stars, this mainly impacts the amplitude of the induced velocity shift, however, for fast rotating stars the large velocity gradient across the star allows the line distortion to be resolved, enabling direct determination of the ring parameters. We demonstrate that by modeling these distortions we can recover ring system parameters (sky-projected angle, obliquity and size) using only a small part of the transit. Substructure in the rings, e.g. gaps, can be recovered if the width of the features (δW\delta W) relative to the size of the star is similar to the intrinsic velocity resolution (set by the width of the local stellar profile, γ\gamma) relative to the stellar rotation velocity (vv sinii, i.e. δW/Rv\delta W / R_* \gtrsim vsinii/γ\gamma). This opens up a new way to study the ring systems around planets with long orbital periods, where observations of the full transit, covering the ingress and egress, are not always feasible.Comment: Accepted for publication in MNRA

    All NIRspec needs is HST/WFC3 pre-imaging? The use of Milky Way Stars in WFC3 Imaging to Register NIRspec MSA Observations

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    The James Webb Space Telescope (JWST) will be an exquisite new near-infrared observatory with imaging and multi-object spectroscopy through ESA's NIRspec instrument with its unique Micro-Shutter Array (MSA), allowing for slits to be positioned on astronomical targets by opening specific 0.002"-wide micro shutter doors. To ensure proper target acquisition, the on-sky position of the MSA needs to be verified before spectroscopic observations start. An onboard centroiding program registers the position of pre-identified guide stars in a Target Acquisition (TA) image, a short pre-spectroscopy exposure without dispersion (image mode) through the MSA with all shutters open. The outstanding issue is the availability of Galactic stars in the right luminosity range for TA relative to typical high redshift targets. We explore this here using the stars and z8z\sim8 candidate galaxies identified in the source extractor catalogs of Brightest of Reionizing Galaxies survey (BoRG[z8]), a pure-parallel program with Hubble Space Telescope Wide-Field Camera 3. We find that (a) a single WFC3 field contains enough Galactic stars to satisfy the NIRspec astrometry requirement (20 milli-arcseconds), provided its and the NIRspec TA's are mlim>24.5m_{lim}>24.5 AB in WFC3 F125W, (b) a single WFC3 image can therefore serve as the pre-image if need be, (c) a WFC3 mosaic and accompanying TA image satisfy the astrometry requirement at 23\sim23 AB mag in WFC3 F125W, (d) no specific Galactic latitude requires deeper TA imaging due to a lack of Galactic stars, and (e) a depth of 24\sim24 AB mag in WFC3 F125W is needed if a guide star in the same MSA quadrant as a target is required. We take the example of a BoRG identified z8z\sim8 candidate galaxy and require a Galactic star within 20" of it. In this case, a depth of 25.5 AB in F125W is required (with \sim97% confidence).Comment: 17 pages, 15 figures, to appear in the Journal of Astronomical Instrumentatio
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