arXiv subjects

Jeremy Bailey

Publications and source records attributed to Jeremy Bailey.

At least 55 records · Page 3Linked to original sources

arXiv2016

The polarisation of HD 189733

We present linear polarization observations of the exoplanet system HD 189733 made with the HIgh Precision Polarimetric Instrument (HIPPI) on the Anglo-Australian Telescope (AAT). The observations have higher precision than any previously reported for this object. They do not show the large amplitude polarization variations reported by Berdyugina et al. 2008 and Berdyugina et al. 2011. Our results are consistent with polarization data presented by Wiktorowicz et al. 2015. A formal least squares fit of a Rayleigh-Lambert model yields a polarization amplitude of 29.4 +/- 15.6 parts-per-million. We observe a background constant level of polarization of ~ 55-70 ppm, which is a little higher than expected for interstellar polarization at the distance of HD 189733.

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arXiv2016

Evidence for Reflected Light from the Most Eccentric Exoplanet Known

Planets in highly eccentric orbits form a class of objects not seen within our Solar System. The most extreme case known amongst these objects is the planet orbiting HD~20782, with an orbital period of 597~days and an eccentricity of 0.96. Here we present new data and analysis for this system as part of the Transit Ephemeris Refinement and Monitoring Survey (TERMS). We obtained CHIRON spectra to perform an independent estimation of the fundamental stellar parameters. New radial velocities from AAT and PARAS observations during periastron passage greatly improve our knowledge of the eccentric nature of the orbit. The combined analysis of our Keplerian orbital and Hipparcos astrometry show that the inclination of the planetary orbit is $> 1.22\degr$, ruling out stellar masses for the companion. Our long-term robotic photometry show that the star is extremely stable over long timescales. Photometric monitoring of the star during predicted transit and periastron times using MOST rule out a transit of the planet and reveal evidence of phase variations during periastron. These possible photometric phase variations may be caused by reflected light from the planet's atmosphere and the dramatic change in star--planet separation surrounding the periastron passage.

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arXiv2015

Empirical line lists and absorption cross sections for methane at high temperature

Hot methane is found in many "cool" sub-stellar astronomical sources including brown dwarfs and exoplanets, as well as in combustion environments on Earth. We report on the first high-resolution laboratory absorption spectra of hot methane at temperatures up to 1200 K. Our observations are compared to the latest theoretical spectral predictions and recent brown dwarf spectra. The expectation that millions of weak absorption lines combine to form a continuum, not seen at room temperature, is confirmed. Our high-resolution transmittance spectra account for both the emission and absorption of methane at elevated temperatures. From these spectra, we obtain an empirical line list and continuum that is able to account for the absorption of methane in high temperature environments at both high and low resolution. Great advances have recently been made in the theoretical prediction of hot methane, and our experimental measurements highlight the progress made and the problems that still remain.

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arXiv2015

The linear polarisation of southern bright stars measured at the parts-per-million level

We report observations of the linear polarisation of a sample of 50 nearby southern bright stars measured to a median sensitivity of $\sim$4.4 $\times 10^{-6}$. We find larger polarisations and more highly polarised stars than in the previous PlanetPol survey of northern bright stars. This is attributed to a dustier interstellar medium in the mid-plane of the Galaxy, together with a population containing more B-type stars leading to more intrinsically polarised stars, as well as using a wavelength more sensitive to intrinsic polarisation in late-type giants. Significant polarisation had been identified for only six stars in the survey group previously, whereas we are now able to deduce intrinsic polarigenic mechanisms for more than twenty. The four most highly polarised stars in the sample are the four classical Be stars ($α$ Eri, $α$ Col, $η$ Cen and $α$ Ara). For the three of these objects resolved by interferometry, the position angles are consistent with the orientation of the circumstellar disc determined. We find significant intrinsic polarisation in most B stars in the sample; amongst these are a number of close binaries and an unusual binary debris disk system. However these circumstances do not account for the high polarisations of all the B stars in the sample and other polarigenic mechanisms are explored. Intrinsic polarisation is also apparent in several late type giants which can be attributed to either close, hot circumstellar dust or bright spots in the photosphere of these stars. Aside from a handful of notable debris disk systems, the majority of A to K type stars show polarisation levels consistent with interstellar polarisation.

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arXiv2015

Atmospheric Modelling for Neptune's Methane D/H Ratio - Preliminary Results

The ratio of deuterium to hydrogen (D/H ratio) of Solar System bodies is an important clue to their formation histories. Here we fit a Neptunian atmospheric model to Gemini Near Infrared Spectrograph (GNIRS) high spectral resolution observations and determine the D/H ratio in methane absorption in the infrared H-band ($\sim$ 1.6 μm). The model was derived using our radiative transfer software VSTAR (Versatile Software for the Transfer of Atmospheric Radiation) and atmospheric fitting software ATMOF (ATMOspheric Fitting). The methane line list used for this work has only become available in the last few years, enabling a refinement of earlier estimates. We identify a bright region on the planetary disc and find it to correspond to an optically thick lower cloud. Our preliminary determination of CH$_{\rm 3}$D/CH$_{\rm 4}$ is 3.0$\times10^{-4}$, which is in line with the recent determination of Irwin et al. (2014) of 3.0$^{+1.0}_{-0.9}\sim\times10^{-4}$, made using the same model parameters and line list but different observational data. This supports evidence that the proto-solar ice D/H ratio of Neptune is much less than that of the comets, and suggests Neptune formed inside its present orbit.

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arXiv2015

A high-sensitivity polarimeter using a ferro-electric liquid crystal modulator

We describe the HIgh Precision Polarimetric Instrument (HIPPI), a polarimeter built at UNSW Australia and used on the Anglo-Australian Telescope (AAT). HIPPI is an aperture polarimeter using a ferro-electric liquid crystal modulator. HIPPI measures the linear polarization of starlight with a sensitivity in fractional polarization of ~4 x 10$^{-6}$ on low polarization objects and a precision of better than 0.01% on highly polarized stars. The detectors have a high dynamic range allowing observations of the brightest stars in the sky as well as much fainter objects. The telescope polarization of the AAT is found to be 48 $\pm$ 5 x 10$^{-6}$ in the g' band.

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arXiv2014

The Dawes Review 3: The Atmospheres of Extrasolar Planets and Brown Dwarfs

The last few years has seen a dramatic increase in the number of exoplanets known and in the range of methods for characterising their atmospheric properties. At the same time, new discoveries of increasingly cooler brown dwarfs have pushed down their temperature range which now extends down to Y-dwarfs of <300 K. Modelling of these atmospheres has required the development of new techniques to deal with the molecular chemistry and clouds in these objects. The atmospheres of brown dwarfs are relatively well understood, but some problems remain, in particular the behavior of clouds at the L/T transition. Observational data for exoplanet atmosphere characterization is largely limited to giant exoplanets that are hot because they are near to their star (hot Jupiters) or because they are young and still cooling. For these planets there is good evidence for the presence of CO and H2O absorptions in the IR. Sodium absorption is observed in a number of objects. Reflected light measurements show that some giant exoplanets are very dark, indicating a cloud free atmosphere. However, there is also good evidence for clouds and haze in some other planets. It is also well established that some highly irradiated planets have inflated radii, though the mechanism for this inflation is not yet clear. Some other issues in the composition and structure of giant exoplanet atmospheres such as the occurence of inverted temperature structures, the presence or absence of CO2 and CH4, and the occurrence of high C/O ratios are still the subject of investigation and debate.

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arXiv2014

The spectrum of hot methane in astronomical objects using a comprehensive computed line list

Hot methane spectra are important in environments ranging from flames to the atmospheres of cool stars and exoplanets. A new spectroscopic line list, 10to10, for $^{12}$CH$_4$ containing almost 10 billion transitions is presented. This comprehensive line list covers a broad spectroscopic range and is applicable for temperatures up to 1 500 K. Previous methane data are incomplete leading to underestimated opacities at short wavelengths and elevated temperatures. Use of 10to10 in models of the bright T4.5 brown dwarf 2MASS 0559-14 leads to significantly better agreement with observations and in studies of the hot Jupiter exoplanet HD 189733b leads to up to a twentifold increase in methane abundance. It is demonstrated that proper inclusion of the huge increase in hot transitions which are important at elevated temperatures is crucial for accurate characterizations of atmospheres of brown dwarfs and exoplanets, especially when observed in the near-infrared.

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arXiv2013

Atmospheric Modelling for the Removal of Telluric Features from Infrared Planetary Spectra

The effects of telluric absorption on infrared spectra present a problem for the observer. Strong molecular absorptions from species whose concentrations vary with time can be particularly challenging to remove precisely. Yet removing these effects is key to accurately determining the composition of many astronomical objects, planetary atmospheres in particular. Here we present a method for removing telluric effects based on a modelling approach. The method relies only on observations usually made by the planetary astronomer, and so is directly comparable with current techniques. We use the modelling approach to process observations made of Jupiter, and Saturnian moon Titan and compare the results with those of the standard telluric division technique, finding the modelling approach to have distinct advantages even in conditions regarded as ideal for telluric division.

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arXiv2013

Examining the broadband emission spectrum of WASP-19b: A new z band eclipse detection

WASP-19b is one of the most irradiated hot-Jupiters known. Its secondary eclipse is the deepest of all transiting planets, and has been measured in multiple optical and infrared bands. We obtained a z band eclipse observation, with measured depth of 0.080 +/- 0.029 %, using the 2m Faulkes Telescope South, that is consistent with the results of previous observations. We combine our measurement of the z band eclipse with previous observations to explore atmosphere models of WASP-19b that are consistent with the its broadband spectrum. We use the VSTAR radiative transfer code to examine the effect of varying pressure-temperature profiles and C/O abundance ratios on the emission spectrum of the planet. We find models with super-solar carbon enrichment best match the observations, consistent with previous model retrieval studies. We also include upper atmosphere haze as another dimension in the interpretation of exoplanet emission spectra, and find that particles <0.5 micron in size are unlikely to be present in WASP-19b.

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arXiv2013

VSTAR Modelling of the Infrared Spectrum of Uranus

We modelled the H-band spectrum obtained with the Infrared Imaging Spectrograph (IRIS2) at the Anglo-Australian Telescope in order to infer the cloud structure and composition of the atmosphere of the planet between 0.1 and 11 bar. Such modelling can be used to derive the D/H ratio in the atmosphere of Uranus, which is an important diagnostic of the conditions in early history of the planetary formation. We describe here our modelling technique based on the Versatile Software for Transfer of Atmospheric Radiation (VSTAR) (Bailey2012a). Since the infrared spectrum of Uranus is dominated by absorption from methane, the accuracy of the models is limited largely by the quality of the low temperature, methane line databases used. Our modelling includes the latest laboratory line data for methane described in Bailey (2011). The parameters of this model will be applied in the future to model our observations of GNIRS high resolution spectra of Uranus and derive the D/H ratio in the 1.58 micron window based on absorption in the deuterated methane band in this region.

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arXiv2013

Observations of the D/H ratio in Methane in the atmosphere of Saturn's moon, Titan - where did the Saturnian system form?

The details of the Solar system's formation are still heavily debated. Questions remain about the formation locations of the giant planets, and the degree to which volatile material was mixed throughout the proto-planetary system. One diagnostic which offers great promise in helping to unravel the history of planet formation is the study of the level of deuteration in various Solar system bodies. For example, the D/H ratio of methane in the atmosphere of Titan can be used as a diagnostic of the initial conditions of the solar nebula within the region of giant planet formation, and can help us to determine where Titan (and, by extension, the Saturnian system) accreted its volatile material. The level of Titanian deuteration also has implications for both the sources and long term evolution of Titan's atmospheric composition. We present the results of observations taken in the 1.58 microns window using the NIFS spectrometer on the Gemini telescope, and model our data using the VSTAR line--by--line transfer model, which yields a D/H ratio for Titan's atmosphere of 143+/-16) x 10^{-6} [1]. We are currently in the process of modeling the Gemini high resolution GNIRS spectra using new sets of line parameters derived for methane in the region between 1.2-1.7 microns [2].

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arXiv2013

Mariner 2 and its Legacy: 50 Years on

Fifty years ago, NASA's Jet Propulsion Laboratory (JPL) built and flew the first successful spacecraft to another planet: Mariner 2 to Venus. This paper discusses the context of this mission at a crucial phase in the space race between the USA and USSR and its results and legacy. As its first major success, Mariner 2 helped to cement JPL's position as a centre for robotic planetary exploration. Mariner 2 successfully solved the scientific problem of the high temperature observed for Venus by ground-based radio telescopes. It also pioneered new techniques for observing the atmosphere of a planet from space, which were subsequently developed into the microwave sounding and infrared sounding techniques for observing the Earth atmosphere. Today these techniques provide some of the most important data for constraining weather forecasting models, as well as a key series of data on the Earth's changing climate.

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arXiv2012

Signals embedded in the radial velocity noise. Periodic variations in the tau Ceti velocities

The abilities of radial velocity exoplanet surveys to detect the lowest-mass extra-solar planets are currently limited by a combination of instrument precision, lack of data, and "jitter". Jitter is a general term for any unknown features in the noise, and reflects a lack of detailed knowledge of stellar physics (asteroseismology, starspots, magnetic cycles, granulation, and other stellar surface phenomena), as well as the possible underestimation of instrument noise. We study an extensive set of radial velocities for the star HD 10700 ($τ$ Ceti) to determine the properties of the jitter arising from stellar surface inhomogeneities, activity, and telescope-instrument systems, and perform a comprehensive search for planetary signals in the radial velocities. We perform Bayesian comparisons of statistical models describing the radial velocity data to quantify the number of significant signals and the magnitude and properties of the excess noise in the data. We reach our goal by adding artificial signals to the "flat" radial velocity data of HD 10700 and by seeing which one of our statistical noise models receives the greatest posterior probabilities while still being able to extract the artificial signals correctly from the data. We utilise various noise components to assess properties of the noise in the data and analyse the HARPS, AAPS, and HIRES data for HD 10700 to quantify these properties and search for previously unknown low-amplitude Keplerian signals. ...

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arXiv2011

Modelling the Spectra of Planets, Brown Dwarfs and Stars using VSTAR

We describe a new software package capable of predicting the spectra of solar-system planets, exoplanets, brown dwarfs and cool stars. The Versatile Software for Transfer of Atmospheric Radiation (VSTAR) code combines a line-by-line approach to molecular and atomic absorption with a full multiple scattering treatment of radiative transfer. VSTAR is a modular system incorporating an ionization and chemical equilibrium model, a comprehensive treatment of spectral line absorption using a database of more than 2.9 billion spectral lines, a scattering package and a radiative transfer module. We test the methods by comparison with other models and benchmark calculations. We present examples of the use of VSTAR to model the spectra of terrestrial and giant planet in our own solar system, brown dwarfs and cool stars.

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arXiv2011

The Distribution of Carbon Monoxide in the Lower Atmosphere of Venus

We have obtained spatially resolved near-infrared spectroscopy of the Venus nightside on 15 nights over three observing seasons. We use the depth of the CO absorption band at 2.3 microns to map the two-dimensional distribution of CO across both hemispheres. Radiative transfer models are used to relate the measured CO band depth to the volume mixing ratio of CO. The results confirm previous investigations in showing a general trend of increased CO abundances at around 60 degrees latitude north and south as compared with the equatorial regions. Observations taken over a few nights generally show very similar CO distributions, but significant changes are apparent over longer periods. In past studies it has been assumed that the CO latitudinal variation occurs near 35 km altitude, at which K-band sensitivity to CO is greatest. By modeling the detailed spectrum of the excess CO at high latitudes we show that it occurs at altitudes around 45 km, much higher than has previously been assumed, and that there cannot be significant contribution from levels of 36 km or lower. We suggest that this is most likely due to downwelling of CO-rich gas from the upper atmosphere at these latitudes, with the CO being removed by around 40 km through chemical processes such as the reaction with sulfur trioxide.

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arXiv2011

The Near-IR Spectrum of Titan Modeled with an Improved Methane Line List

We have obtained spatially resolved spectra of Titan in the near-infrared J, H and K bands at a resolving power of ~5000 using the near-infrared integral field spectrometer (NIFS) on the Gemini North 8m telescope. Using recent data from the Cassini/Huygens mission on the atmospheric composition and surface and aerosol properties, we develop a multiple-scattering radiative transfer model for the Titan atmosphere. The Titan spectrum at these wavelengths is dominated by absorption due to methane with a series of strong absorption band systems separated by window regions where the surface of Titan can be seen. We use a line-by-line approach to derive the methane absorption coefficients. The methane spectrum is only accurately represented in standard line lists down to ~2.1 μm. However, by making use of recent laboratory data and modeling of the methane spectrum we are able to construct a new line list that can be used down to 1.3 μm. The new line list allows us to generate spectra that are a good match to the observations at all wavelengths longer than 1.3 μm and allow us to model regions, such as the 1.55 μm window that could not be studied usefully with previous line lists such as HITRAN 2008. We point out the importance of the far-wing line shape of strong methane lines in determining the shape of the methane windows. Line shapes with Lorentzian, and sub-Lorentzian regions are needed to match the shape of the windows, but different shape parameters are needed for the 1.55 μm and 2 μm windows. After the methane lines are modelled our observations are sensitive to additional absorptions, and we use the data in the 1.55 μm region to determine a D/H ratio of 1.77 \pm 0.20 x 10-4, and a CO mixing ratio of 50 \pm 11 ppmv. In the 2 μm window we detect absorption features that can be identified with the ν5+3ν6 and 2ν3+2ν6 bands of CH3D.

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arXiv2011

Modelling the near-IR spectra of Jupiter using line-by-line methods

We have obtained long-slit, infrared spectra of Jupiter with the Anglo Australian Telescope in the K and H bands at a resolving power of 2260. Using a line-by-line, radiative transfer model with the latest, improved spectral line data for methane and ammonia, we derive a model of the zonal characteristics in the atmosphere of this giant planet. We fit our model to the spectra of the zones and belts visible at 2.1 μm using different distributions of cloud opacities. The modeled spectra for each region match observations remarkably well at K band and in low pressure regions at the H band. Our results for the upper deck cloud distribution are consistent with previous models (Banfield et al.1998) fitted to low resolution, grism spectra. The ability to obtain and model high resolution planetary spectra in order to search for weakly absorbing atmospheric constituents can provide better constraints on the chemical composition of planetary atmospheres.

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