arXiv subjects

Geoffrey Bryden

Publications and source records attributed to Geoffrey Bryden.

At least 55 records · Page 3Linked to original sources

arXiv2019

The Wide Field Infrared Survey Telescope: 100 Hubbles for the 2020s

The Wide Field Infrared Survey Telescope (WFIRST) is a 2.4m space telescope with a 0.281 deg^2 field of view for near-IR imaging and slitless spectroscopy and a coronagraph designed for > 10^8 starlight suppresion. As background information for Astro2020 white papers, this article summarizes the current design and anticipated performance of WFIRST. While WFIRST does not have the UV imaging/spectroscopic capabilities of the Hubble Space Telescope, for wide field near-IR surveys WFIRST is hundreds of times more efficient. Some of the most ambitious multi-cycle HST Treasury programs could be executed as routine General Observer (GO) programs on WFIRST. The large area and time-domain surveys planned for the cosmology and exoplanet microlensing programs will produce extraordinarily rich data sets that enable an enormous range of Archival Research (AR) investigations. Requirements for the coronagraph are defined based on its status as a technology demonstration, but its expected performance will enable unprecedented observations of nearby giant exoplanets and circumstellar disks. WFIRST is currently in the Preliminary Design and Technology Completion phase (Phase B), on schedule for launch in 2025, with several of its critical components already in production.

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arXiv2018

The Origin of Heavy Element Content Trend in Giant Planets via Core Accretion

We explore the origin of the trend of heavy elements in observed massive exoplanets. Coupling of better measurements of mass ($M_p$) and radius of exoplanets with planet structure models enables estimating the total heavy element mass ($M_Z$) in these planets. The corresponding relation is characterized by a power-law profile, $M_Z \propto M_p^{3/5}$. We develop a simplified, but physically motivated analysis to investigate how the power-law profile can be produced under the current picture of planet formation. Making use of the existing semi-analytical formulae of accretion rates of pebbles and planetesimals, our analysis shows that the relation can be reproduced well if it traces the final stage of planet formation. In the stage, planets accrete solids from gapped planetesimal disks and gas accretion is limited by disk evolution. We also find that dust accretion accompanying with gas accretion does not contribute to $M_Z$ for planets with $M_p < 10^3 M_{\oplus}$. Our findings are broadly consistent with that of previous studies, yet we explicitly demonstrate how planetesimal dynamics is crucial for better understanding the relation. While our approach is simple, we can also reproduce the trend of a correlation between planet metallicity and $M_p$ that is obtained by detailed population synthesis calculations, when the same assumption is adopted. Our analysis suggests that pebble accretion would not play a direct role at the final stage of planet formation, whereas radial drift of pebbles might be important indirectly for metal enrichment of planets. Detailed numerical simulations and more observational data are required for confirming our analysis.

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arXiv2018

Additional Exoplanet Science Enabled by FINESSE

FINESSE, a NASA MIDEX mission in Step 2, would conduct a large, uniform survey of exoplanet atmospheres and create a statistically significant sample for comparative planetology. FINESSE's technical capabilities, driven by a scientific program to study planetary formation and climate mechanisms, enable additional exoplanet science presented here.

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arXiv2018

Kuiper Belt Analogues in Nearby M-type Planet-host Systems

We present the results of a Herschel survey of 21 late-type stars that host planets discovered by the radial velocity technique. The aims were to discover new disks in these systems and to search for any correlation between planet presence and disk properties. In addition to the known disk around GJ 581, we report the discovery of two new disks, in the GJ 433 and GJ 649 systems. Our sample therefore yields a disk detection rate of 14%, higher than the detection rate of 1.2% among our control sample of DEBRIS M-type stars with 98% confidence. Further analysis however shows that the disk sensitivity in the control sample is about a factor of two lower in fractional luminosity than for our survey, lowering the significance of any correlation between planet presence and disk brightness below 98%. In terms of their specific architectures, the disk around GJ 433 lies at a radius somewhere between 1 and 30au. The disk around GJ 649 lies somewhere between 6 and 30au, but is marginally resolved and appears more consistent with an edge-on inclination. In both cases the disks probably lie well beyond where the known planets reside (0.06-1.1au), but the lack of radial velocity sensitivity at larger separations allows for unseen Saturn-mass planets to orbit out to $\sim$5au, and more massive planets beyond 5au. The layout of these M-type systems appears similar to Sun-like star + disk systems with low-mass planets.

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arXiv2017

Forecasting the Impact of Stellar Activity on Transiting Exoplanet Spectra

Exoplanet host star activity, in the form of unocculted star spots or faculae, alters the observed transmission and emission spectra of the exoplanet. This effect can be exacerbated when combining data from different epochs if the stellar photosphere varies between observations due to activity. redHere we present a method to characterize and correct for relative changes due to stellar activity by exploiting multi-epoch ($\ge$2 visits/transits) observations to place them in a consistent reference frame. Using measurements from portions of the planet's orbit where negligible planet transmission or emission can be assumed, we determine changes to the stellar spectral amplitude. With the analytical methods described here, we predict the impact of stellar variability on transit observations. Supplementing these forecasts with Kepler-measured stellar variabilities for F-, G-, K-, and M-dwarfs, and predicted transit precisions by JWST's NIRISS, NIRCam, and MIRI, we conclude that stellar activity does not impact infrared transiting exoplanet observations of most presently-known or predicted TESS targets by current or near-future platforms, such as JWST.

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arXiv2015

Kuiper belt structure around nearby super-Earth host stars

We present new observations of the Kuiper belt analogues around HD 38858 and HD 20794, hosts of super-Earth mass planets within 1 au. As two of the four nearby G-type stars (with HD 69830 and 61 Vir) that form the basis of a possible correlation between low-mass planets and debris disc brightness, these systems are of particular interest. The disc around HD 38858 is well resolved with Herschel and we constrain the disc geometry and radial structure. We also present a probable JCMT sub-mm continuum detection of the disc and a CO J=2-1 upper limit. The disc around HD 20794 is much fainter and appears marginally resolved with Herschel, and is constrained to be less extended than the discs around 61 Vir and HD 38858. We also set limits on the radial location of hot dust recently detected around HD 20794 with near-IR interferometry. We present HARPS upper limits on unseen planets in these four systems, ruling out additional super-Earths within a few au, and Saturn-mass planets within 10 au. We consider the disc structure in the three systems with Kuiper belt analogues (HD 69830 has only a warm dust detection), concluding that 61 Vir and HD 38858 have greater radial disc extent than HD 20794. We speculate that the greater width is related to the greater minimum planet masses (10-20 $M_\oplus$ vs. 3-5 $M_\oplus$), arising from an eccentric planetesimal population analogous to the Solar System's scattered disc. We discuss alternative scenarios and possible means to distinguish among them.

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arXiv2014

Exo-zodi modelling for the Large Binocular Telescope Interferometer

Habitable zone dust levels are a key unknown that must be understood to ensure the success of future space missions to image Earth analogues around nearby stars. Current detection limits are several orders of magnitude above the level of the Solar System's Zodiacal cloud, so characterisation of the brightness distribution of exo-zodi down to much fainter levels is needed. To this end, the large Binocular Telescope Interferometer (LBTI) will detect thermal emission from habitable zone exo-zodi a few times brighter than Solar System levels. Here we present a modelling framework for interpreting LBTI observations, which yields dust levels from detections and upper limits that are then converted into predictions and upper limits for the scattered light surface brightness. We apply this model to the HOSTS survey sample of nearby stars; assuming a null depth uncertainty of 10$^{-4}$ the LBTI will be sensitive to dust a few times above the Solar System level around Sun-like stars, and to even lower dust levels for more massive stars.

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arXiv2014

A near-infrared interferometric survey of debris-disc stars. IV. An unbiased sample of 92 southern stars observed in H-band with VLTI/PIONIER

Context. Detecting and characterizing circumstellar dust is a way to study the architecture and evolution of planetary systems. Cold dust in debris disks only traces the outer regions. Warm and hot exozodiacal dust needs to be studied in order to trace regions close to the habitable zone. Aims. We aim to determine the prevalence and to constrain the properties of hot exozodiacal dust around nearby main-sequence stars. Methods. We search a magnitude limited (H < 5) sample of 92 stars for bright exozodiacal dust using our VLTI visitor instrument PIONIER in the H-band. We derive statistics of the detection rate with respect to parameters such as the stellar spectral type and age or the presence of a debris disk in the outer regions of the systems. We derive more robust statistics by combining our sample with the results from our CHARA/FLUOR survey in the K-band. In addition, our spectrally dispersed data allows us to put constraints on the emission mechanism and the dust properties in the detected systems. Results. We find an over-all detection rate of bright exozodiacal dust in the H-band of 11% (9 out of 85 targets) and three tentative detections. The detection rate decreases from early type to late type stars and increases with the age of the host star. We do not confirm the tentative correlation between the presence of cold and hot dust found in our earlier analysis of the FLUOR sample alone. Our spectrally dispersed data suggest that either the dust is extremely hot or the emission is dominated by the scattered light in most cases. The implications of our results for the target selection of future terrestrial planet finding missions using direct imaging are discussed.

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arXiv2014

Herschel/PACS photometry of transiting-planet host stars with candidate warm debris disks

Dust in debris disks is produced by colliding or evaporating planetesimals, remnants of the planet formation process. Warm dust disks, known by their emission at < 24 micron, are rare (4% of FGK main sequence stars) and especially interesting because they trace material in the region likely to host terrestrial planets, where the dust has a very short dynamical lifetime. Statistical analyses of the source counts of excesses as found with the mid-IR Wide Field Infrared Survey Explorer (WISE) suggest that warm-dust candidates found for the Kepler transiting-planet host-star candidates can be explained by extragalactic or galactic background emission aligned by chance with the target stars. These statistical analyses do not exclude the possibility that a given WISE excess could be due to a transient dust population associated with the target. Here we report Herschel/PACS 100 and 160 micron follow-up observations of a sample of Kepler and non-Kepler transiting-planet candidates' host stars, with candidate WISE warm debris disks, aimed at detecting a possible cold debris disk in any of them. No clear detections were found in any one of the objects at either wavelength. Our upper limits confirm that most objects in the sample do not have a massive debris disk like that in beta Pic. We also show that the planet-hosting star WASP-33 does not have a debris disk comparable to the one around eta Crv. Although the data cannot be used to rule out rare warm disks around the Kepler planet-hosting candidates, the lack of detections and the characteristics of neighboring emission found at far-IR wavelengths support an earlier result suggesting that most of the WISE-selected IR excesses around Kepler candidate host stars are likely due to either chance alignment with background IR-bright galaxies and/or to interstellar emission.

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arXiv2014

Disk Radii and Grain Sizes in Herschel-Resolved Debris Disks

(Abridged) The radii of debris disks and the sizes of their dust grains are tracers of the formation mechanisms and physical processes operating in these systems. We use a sample of 34 debris disks spatially resolved in various Herschel programs to constrain them. While we modeled disks with both warm and cold components, we focus our analysis only on the cold outer disks, i.e. Kuiper-belt analogs. The disk radii derived from the resolved images reveal a large dispersion, but no significant trend with the stellar luminosity, which argues against ice lines as a dominant player in setting the debris disk sizes. Fixing the disk radii to those inferred from the resolved images, we model the spectral energy distributions to determine the dust temperatures and the grain size distributions. While the dust temperature systematically increases towards earlier spectral types, its ratio to the blackbody temperature at the disk radius decreases with the stellar luminosity. This is explained by an increase of typical grain sizes towards more luminous stars. The sizes are compared to the radiation pressure blowout limit $s_\text{blow}$ that is proportional to the stellar luminosity-to-mass ratio and thus also increases towards earlier spectral classes. The grain sizes in the disks of G- to A-stars are inferred to be several times $s_\text{blow}$ at all stellar luminosities, in agreement with collisional models of debris disks. The sizes, measured in the units of $s_\text{blow}$, appear to decrease with the luminosity, which may be suggestive of the disk's stirring level increasing towards earlier-type stars.

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arXiv2013

The Debris Disk Explorer: a balloon-borne coronagraph for observing debris disks

The Debris Disk Explorer (DDX) is a proposed balloon-borne investigation of debris disks around nearby stars. Debris disks are analogs of the Asteroid Belt (mainly rocky) and Kuiper Belt (mainly icy) in our Solar System. DDX will measure the size, shape, brightness, and color of tens of disks. These measurements will enable us to place the Solar System in context. By imaging debris disks around nearby stars, DDX will reveal the presence of perturbing planets via their influence on disk structure, and explore the physics and history of debris disks by characterizing the size and composition of disk dust. The DDX instrument is a 0.75-m diameter off-axis telescope and a coronagraph carried by a stratospheric balloon. DDX will take high-resolution, multi-wavelength images of the debris disks around tens of nearby stars. Two flights are planned; an overnight test flight within the United States followed by a month-long science flight launched from New Zealand. The long flight will fully explore the set of known debris disks accessible only to DDX. It will achieve a raw contrast of 10^-7, with a processed contrast of 10^-8. A technology benefit of DDX is that operation in the near-space environment will raise the Technology Readiness Level of internal coronagraphs, deformable mirrors, and wavefront sensing and control, all potentially needed for a future space-based telescope for high-contrast exoplanet imaging.

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arXiv2012

Exploring the Effects of Stellar Rotation and Wind Clearing: Debris Disks Around F Stars

We have conducted a study of debris disks around F stars in order to explore correlations between rotation, stellar winds, and circumstellar disks. We obtained new 24 micron photometry from Spitzer's Multiband Imaging Photometer for Spitzer (MIPS) camera for a sample of 188 relatively nearby F dwarfs with various rotation rates and optical colors, and combined it with archival MIPS data for 66 more F stars, as well as Wide-field Infrared Survey Explorer (WISE) data for the entire sample, plus 9 more F stars. Based on the objects' K_s-[24] and [3.4]-[22] colors, we identify 22 stars in our sample as having 22 and/or 24 micron excesses above our detection limit, 13 of which are new discoveries. Our overall disk detection rate is 22/263, or 8%, consistent with previous determinations of disk fractions in the Solar neighborhood. While fast rotating stars are expected to have strong winds capable of efficiently removing dust, we find no correlation between rotational velocity and infrared excess. Similarly, we find no significant difference in excess detection rate between late-type F stars, which have convective surfaces, and early-type F stars, which have fully radiative envelopes. However, the essentially unknown range of ages in this sample may be washing out any effects relating rotation, winds, and disks.

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arXiv2012

Hubble Space Telescope Observations of the HD 202628 Debris Disk

A ring-shaped debris disk around the G2V star HD 202628 (d = 24.4 pc) was imaged in scattered light at visible wavelengths using the coronagraphic mode of the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. The ring is inclined by 64 degrees from face-on, based on the apparent major/minor axis ratio, with the major axis aligned along PA = 130 degrees. It has inner and outer radii (>50% maximum surface brightness) of 139 AU and 193 AU in the northwest ansae and 161 AU and 223 AU in the southeast (dr/r ~ 0.4). The maximum visible radial extent is ~254 AU. With a mean surface brightnesses of V ~ 24 mag per square arcsec, this is the faintest debris disk observed to date in reflected light. The center of the ring appears offset from the star by ~28 AU (deprojected). An ellipse fit to the inner edge has an eccentricity of 0.18 and a = 158 AU. This offset, along with the relatively sharp inner edge of the ring, suggests the influence of a planetary-mass companion. There is a strong similarity with the debris ring around Fomalhaut, though HD 202628 is a more mature star with an estimated age of about 2 Gyr. We also provide surface brightness limits for nine other stars in our study with strong Spitzer excesses around which no debris disks were detected in scattered light (HD 377, HD 7590, HD 38858, HD 45184, HD 73350, HD 135599, HD 145229, HD 187897, and HD 201219).

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arXiv2012

The Exozodiacal Dust Problem for Direct Observations of ExoEarths

Debris dust in the habitable zones of stars - otherwise known as exozodiacal dust - comes from extrasolar asteroids and comets and is thus an expected part of a planetary system. Background flux from the Solar System's zodiacal dust and the exozodiacal dust in the target system is likely to be the largest source of astrophysical noise in direct observations of terrestrial planets in the habitable zones of nearby stars. Furthermore, dust structures like clumps, thought to be produced by dynamical interactions with exoplanets, are a possible source of confusion. In this paper, we qualitatively assess the primary impact of exozodical dust on high-contrast direct imaging at optical wavelengths, such as would be performed with a coronagraph. Then we present the sensitivity of previous, current, and near-term facilities to thermal emission from debris dust at all distances from nearby solar-type stars, as well as our current knowledge of dust levels from recent surveys. Finally, we address the other method of detecting debris dust, through high-contrast imaging in scattered light. This method is currently far less sensitive than thermal emission observations, but provides high spatial resolution for studying dust structures. This paper represents the first report of NASA's Exoplanet Exploration Program Analysis Group (ExoPAG).

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arXiv2010

A Spitzer IRS Study of Debris Disks Around Planet-Host Stars

Since giant planets scatter planetesimals within a few tidal radii of their orbits, the locations of existing planetesimal belts indicate regions where giant planet formation failed in bygone protostellar disks. Infrared observations of circumstellar dust produced by colliding planetesimals are therefore powerful probes of the formation histories of known planets. Here we present new Spitzer IRS spectrophotometry of 111 Solar-type stars, including 105 planet hosts. Our observations reveal 11 debris disks, including two previously undetected debris disks orbiting HD 108874 and HD 130322. Combining our 32 micron spectrophotometry with previously published MIPS photometry, we find that the majority of debris disks around planet hosts have temperatures in the range 60 < T < 100 K. Assuming a dust temperature T = 70 K, which is representative of the nine debris disks detected by both IRS and MIPS, we find that debris rings surrounding Sunlike stars orbit between 15 and 240 AU, depending on the mean particle size. Our observations imply that the planets detected by radial-velocity searches formed within 240 AU of their parent stars. If any of the debris disks studied here have mostly large, blackbody emitting grains, their companion giant planets must have formed in a narrow region between the ice line and 15 AU.

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arXiv2010

HST and Spitzer Observations of the HD 207129 Debris Ring

A debris ring around the star HD 207129 (G0V; d = 16.0 pc) has been imaged in scattered visible light with the ACS coronagraph on the Hubble Space Telescope and in thermal emission using MIPS on the Spitzer Space Telescope at 70 microns (resolved) and 160 microns (unresolved). Spitzer IRS (7-35 microns) and MIPS (55-90 microns) spectrographs measured disk emission at >28 microns. In the HST image the disk appears as a ~30 AU wide ring with a mean radius of ~163 AU and is inclined by 60 degrees from pole-on. At 70 microns it appears partially resolved and is elongated in the same direction and with nearly the same size as seen with HST in scattered light. At 0.6 microns the ring shows no significant brightness asymmetry, implying little or no forward scattering by its constituent dust. With a mean surface brightness of V=23.7 mag per square arcsec, it is the faintest disk imaged to date in scattered light.

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arXiv2010

Locating the planetesimals belts in the multiple-planet systems HD 128311, HD 202206, HD 82943 and HR 8799

In addition to the Sun, six other stars are known to harbor multiple planets and debris disks: HD 69830, HD 38529, HD 128311, HD 202206, HD 82943 and HR 8799. In this paper we set constraints on the location of the dust-producing planetesimals around the latter four systems. We use a radiative transfer model to analyze the spectral energy distributions of the dust disks (including two new Spitzer IRS spectra presented in this paper), and a dynamical model to assess the long-term stability of the planetesimals' orbits. As members of a small group of stars that show evidence of harboring a multiple planets and planetesimals, their study can help us learn about the diversity of planetary systems.

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arXiv2010

Accurate Coordinates and 2MASS Cross-IDs for (Almost) All Gliese Catalog Stars

We provide precise J2000, epoch 2000 coordinates and cross-identifications to sources in the 2MASS point source catalog for nearly all stars in the Gliese, Gliese and Jahreiss, and Woolley catalogs of nearby stars. The only Gliese objects where we were not successful are two Gliese sources that are actually QSOs, two proposed companions to brighter stars which we believe do not exist, four stars included in one of the catalogs but identified there as only optical companions, one probable plate flaw, and two stars which simply remain un-recovered. For the 4251 recovered stars, 2693 have coordinates based on Hipparcos positions, 1549 have coordinates based on 2MASS data, and 9 have positions from other astrometric sources. All positions have been calculated at epoch 2000 using proper motions from the literature, which are also given here.

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