I. Tilling

The University of Edinburgh, Edinburgh, SCT, United Kingdom

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Publications (17)20.77 Total impact

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    ABSTRACT: GASPS is a far-infrared line and continuum survey of protoplanetary and young debris disks using PACS on the Herschel Space Observatory. The survey includes [OI] at 63 microns, as well as 70, 100 and 160um continuum, with the brightest objects also studied in [OI]145um, [CII]157um, H2O and CO. Targets included T Tauri stars and debris disks in 7 nearby young associations, and a sample of isolated Herbig AeBe stars. The aim was to study the global gas and dust content in a wide disk sample, systemically comparing the results with models. In this paper we review the main aims, target selection and observing strategy. We show initial results, including line identifications, sources detected, and a first statistical study. [OI]63um was the brightest line in most objects, by a factor of ~10. Detection rates were 49%, including 100% of HAeBe stars and 43% of T Tauri stars. Comparison with published dust masses show a dust threshold for [OI]63um detection of ~1e-5 M_solar. Normalising to 140pc distance, 32% with mass 1e-6 - 1e-5 M_solar, and a small number with lower mass were also detected. This is consistent with moderate UV excess and disk flaring. In most cases, continuum and line emission is spatially and spectrally unresolved, suggesting disk emission. ~10 objects were resolved, likely from outflows. Detection rates in [OI]145um, [CII]157um and CO J=18-17 were 20-40%, but [CII] was not correlated with disk mass, suggesting it arises instead from a compact envelope. [OI] detection rates in T Tauri associations of ages 0.3-4Myr were ~50%. ~2 stars were detectable in associations of 5-20Myr, with no detections in associations of age >20Myr. Comparing with the total number of young stars, and assuming a ISM-like gas/dust ratio, this indicates that ~18% of stars retain a gas-rich disk of total mass >1M_Jupiter for 1-4Myr, 1-7% keep such disks for 5-10Myr, and none remain beyond 10-20Myr.
    06/2013;
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    ABSTRACT: We present detailed model fits to observations of the disc around the Herbig Ae star HD 163296. This well-studied object has an age of ~ 4 Myr, with evidence of a circumstellar disc extending out to ~ 540AU. We use the radiation thermo-chemical disc code ProDiMo to model the gas and dust in the circumstellar disc of HD 163296, and attempt to determine the disc properties by fitting to observational line and continuum data. These include new Herschel/PACS observations obtained as part of the open-time key program GASPS (Gas in Protoplanetary Systems), consisting of a detection of the [OI]63mic line and upper limits for several other far infrared lines. We complement this with continuum data and ground-based observations of the 12CO 3-2, 2-1 and 13CO J=1-0 line transitions, as well as the H2 S(1) transition. We explore the effects of stellar ultraviolet variability and dust settling on the line emission, and on the derived disc properties. Our fitting efforts lead to derived gas/dust ratios in the range 9-100, depending on the assumptions made. We note that the line fluxes are sensitive in general to the degree of dust settling in the disc, with an increase in line flux for settled models. This is most pronounced in lines which are formed in the warm gas in the inner disc, but the low excitation molecular lines are also affected. This has serious implications for attempts to derive the disc gas mass from line observations. We derive fractional PAH abundances between 0.007 and 0.04 relative to ISM levels. Using a stellar and UV excess input spectrum based on a detailed analysis of observations, we find that the all observations are consistent with the previously assumed disc geometry.
    Astronomy and Astrophysics 11/2011; · 5.08 Impact Factor
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    ABSTRACT: Aims. We want to understand the chemistry and physics of disks on the basis of a large unbiased and statistically relevant grid of disk models. One of the main goals is to explore the diagnostic power of various gas emission lines and line ratios for deriving main disk parameters such as the gas mass. Methods. We explore the results of the DENT grid (Disk Evolution with Neat Theory) that consists of 300 000 disk models with 11 free parameters. Through a statistical analysis, we search for correlations and trends in an effort to find tools for disk diagnostic. Results. All calculated quantities like species masses, temperatures, continuum and line fluxes differ by several orders of magnitude across the entire parameter space. The broad distribution of these quantities as a function of input parameters shows the limitation of using a prototype T Tauri or Herbig Ae/Be disk model. The statistical analysis of the DENT grid shows that CO gas is rarely the dominant carbon reservoir in disks. Models with large inner radii (10 times the dust condensation radius) and/or shallow surface density gradients lack massive gas phase water reservoirs. Also, 60% of the disks have gas temperatures averaged over the oxygen mass in the range between 15 and 70 K; the average gas temperatures for CO and O differ by less than a factor two. Studying the observational diagnostics, the [CII] 158 \mum fine structure line flux is very sensitive to the stellar UV flux and presence of a UV excess and it traces the outer disk radius (Rout). In the submm, the CO low J rotational lines also trace Rout. Low [OI] 63/145 line ratios (< a few) can be explained with cool atomic O gas in the uppermost surface layers leading to self-absorption in the 63 \mum line; this occurs mostly for massive non-flaring, settled disk models without UV excess. ... abbreviated
    Astronomy and Astrophysics 03/2011; 532. · 5.08 Impact Factor
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    ABSTRACT: The Herschel GASPS key program is a survey of the gas phase of protoplanetary discs, targeting 240 objects which cover a large range of ages, spectral types, and disc properties. To interpret this large quantity of data and initiate self-consistent analyses of the gas and dust properties of protoplanetary discs, we have combined the capabilities of the radiative transfer code MCFOST with the gas thermal balance and chemistry code ProDiMo to compute a grid of ≈300 000 disc models (DENT). We present a comparison of the first Herschel/GASPS line and continuum data with the predictions from the DENT grid of models. Our objective is to test some of the main trends already identified in the DENT grid, as well as to define better empirical diagnostics to estimate the total gas mass of protoplanetary discs. Photospheric UV radiation appears to be the dominant gas-heating mechanism for Herbig stars, whereas UV excess and/or X-rays emission dominates for T Tauri stars. The DENT grid reveals the complexity in the analysis of far-IR lines and the difficulty to invert these observations into physical quantities. The combination of Herschel line observations with continuum data and/or with rotational lines in the (sub-)millimetre regime, in particular CO lines, is required for a detailed characterisation of the physical and chemical properties of circumstellar discs.
    Astronomy and Astrophysics. 07/2010;
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    ABSTRACT: Context. Circumstellar discs are ubiquitous around young stars, but rapidly dissipate their gas and dust on timescales of a few Myr. The Herschel Space Observatory allows for the study of the warm disc atmosphere, using far-infrared spectroscopy to measure gas content and excitation conditions, and far-IR photometry to constrain the dust distribution. Aims: We aim to detect and characterize the gas content of circumstellar discs in four targets as part of the Herschel science demonstration phase. Methods: We carried out sensitive medium resolution spectroscopy and high sensitivity photometry at λ ~ 60-190 μm using the Photodetector Array Camera and Spectrometer instrument on the Herschel Space Observatory. Results: We detect [OI] 63 μm emission from the young stars HD 169142, TW Hydrae, and RECX 15, but not HD 181327. No other lines, including [CII] 158 and [OI] 145, are significantly detected. All four stars are detected in photometry at 70 and 160 μm. Extensive models are presented in associated papers.
    Astronomy and Astrophysics. 07/2010;
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    ABSTRACT: We have combined the thermo-chemical disc code ProDiMo with the Monte Carlo radiative transfer code MCFOST to calculate a grid of ∼300 000 circumstellar disc models, systematically varying 11 stellar, disc and dust parameters including the total disc mass, several disc shape parameters and the dust-to-gas ratio. For each model, dust continuum and line radiative transfer calculations are carried out for 29 far-infrared, sub-mm and mm lines of [O i], [C ii], 12CO and o/p-H2O under five inclinations. The grid allows us to study the influence of the input parameters on the observables, to make statistical predictions for different types of circumstellar discs and to find systematic trends and correlations between the parameters, the continuum fluxes and the line fluxes. The model grid, comprising the calculated disc temperature and chemical structures, the computed spectral energy distributions, line fluxes and profiles, will be used in particular for the data interpretation of the HERSCHEL open time-key program GASPS. The calculated line fluxes show a strong dependence on the assumed ultraviolet excess of the central star and on the disc flaring. The fraction of models predicting [O i] and [C ii] fine-structure lines fluxes above HERSCHEL/PACS and SPICA/SAFARI detection limits is calculated as a function of disc mass. The possibility of deriving the disc gas mass from line observations is discussed.
    Monthly Notices of the Royal Astronomical Society Letters 06/2010; 405(1):L26 - L30. · 5.52 Impact Factor
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    ABSTRACT: Context - Circumstellar discs are ubiquitous around young stars, but rapidly dissipate their gas and dust on timescales of a few Myr. The Herschel space observatory allows for the study of the warm disc atmosphere, using far-infrared spectroscopy to measure gas content and excitation conditions, and far-IR photometry to constrain the dust distribution. Aims - We aim to detect and characterize the gas content of circumstellar discs in four targets as part of the Herschel science demonstration phase. Methods - We carried out sensitive medium resolution spectroscopy and high sensitivity photometry at lambda ~60-190 micron using the Photodetector Array Camera and Spectrometer instrument on the Herschel space observatory. Results - We detect [OI] 63 micron emission from the young stars HD 169142, TW Hydrae, and RECX 15, but not HD 181327. No other lines, including [CII] 158 and [OI] 145, are significantly detected. All four stars are detected in photometry at 70 and 160 micron. Extensive models are presented in associated papers. Comment: Comments: 5 pages, 2 Postscript figures, accepted for publication in the A&A Herschel special issue
    05/2010;
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    ABSTRACT: Planets are formed in disks around young stars. With an age of ~10 Myr
    Astronomy and Astrophysics, v.518, id.L125 (2010). 05/2010;
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    http://dx.doi.org/10.1051/0004-6361/201014595. 01/2010;
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    ABSTRACT: In an effort to simultaneously study the gas and dust components of the disc surrounding the young Herbig Ae star HD 169142, we present far-IR observations obtained with the PACS instrument onboard the Herschel Space Observatory. This work is part of the open time key program GASPS, which is aimed at studying the evolution of protoplanetary discs. To constrain the gas properties in the outer disc, we observed the star at several key gas-lines, including [OI] 63.2 and 145.5 μm, [CII] 157.7 μm, CO 72.8 and 90.2 μm, and o-H2O 78.7 and 179.5 μm. We only detect the [OI] 63.2 μm line in our spectra, and derive upper limits for the other lines. We complement our data set with PACS photometry and 12/13CO data obtained with the Submillimeter Array. Furthermore, we derive accurate stellar parameters from optical spectra and UV to mm photometry. We model the dust continuum with the 3D radiative transfer code MCFOST and use this model as an input to analyse the gas lines with the thermo-chemical code ProDiMo. Our dataset is consistent with a simple model in which the gas and dust are well-mixed in a disc with a continuous structure between 20 and 200 AU, but this is not a unique solution. Our modelling effort allows us to constrain the gas-to-dust mass ratio as well as the relative abundance of the PAHs in the disc by simultaneously fitting the lines of several species that originate in different regions. Our results are inconsistent with a gas-poor disc with a large UV excess; a gas mass of 5.0 ± 2.0 × 10-3 Mȯ is still present in this disc, in agreement with earlier CO observations. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
    01/2010;
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    ABSTRACT: Methods. We use the recently developed disk code ProDiMo to calculate the physico-chemical structure of protoplanetary disks and apply the Monte-Carlo line radiative transfer code RATRAN to predict observable line profiles and fluxes. We consider a series of Herbig Ae type disk models ranging from 10^-6 M_Sun to 2.2 10^-2 M_Sun (between 0.5 and 700 AU) to discuss the dependency of the line fluxes and ratios on disk mass for otherwise fixed disk parameters. Results. We find the [CII] 157.7 mum line to originate in LTE from the surface layers of the disk, where Tg > Td . The total emission is dominated by surface area and hence depends strongly on disk outer radius. The [OI] lines can be very bright (> 10^-16 W/m^2) and form in slightly deeper and closer regions under non-LTE conditions. The high-excitation [OI] 145.5 mum line, which has a larger critical density, decreases more rapidly with disk mass than the 63.2 mum line. Therefore, the [OI] 63.2 mum/145.5 mum ratio is a promising disk mass indicator, especially as it is independent of disk outer radius for Rout > 200 AU. CO is abundant only in deeper layers A_V >~ 0.05. For too low disk masses (M_disk <~10^-4 M_Sun) the dust starts to become transparent, and CO is almost completely photo-dissociated. For masses larger than that the lines are an excellent independent tracer of disk outer radius and can break the outer radius degeneracy in the [OI] 63.2 mum/[CII]157.7 mum line ratio. Conclusions. The far-IR fine-structure lines of [CII] and [OI] observable with Herschel provide a promising tool to measure the disk gas mass, although they are mainly generated in the atomic surface layers. In spatially unresolved observations, none of these lines carry much information about the inner, possibly hot regions < 30 AU. Comment: accepted for publication in A&A
    Astronomy and Astrophysics 11/2009; · 5.08 Impact Factor
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    ABSTRACT: Context. Circumstellar discs are ubiquitous around young stars, but rapidly dissipate their gas and dust on timescales of a few Myr. The Herschel Space Observatory allows for the study of the warm disc atmosphere, using far-infrared spectroscopy to measure gas content and excitation conditions, and far-IR photometry to constrain the dust distribution. Aims. We aim to detect and characterize the gas content of circumstellar discs in four targets as part of the Herschel science demonstration phase. Methods. We carried out sensitive medium resolution spectroscopy and high sensitivity photometry at gimel similar to 60-190 mu m using the Photodetector Array Camera and Spectrometer instrument on the Herschel Space Observatory. Results. We detect [OI] 63 mu m emission from the young stars HD 169142, TW Hydrae, and RECX 15, but not HD 181327. No other lines, including [CII] 158 and [OI] 145, are significantly detected. All four stars are detected in photometry at 70 and 160 mu m. Extensive models are presented in associated papers.
    ASTRONOMY & ASTROPHYSICS. 518.
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    ABSTRACT: In an effort to simultaneously study the gas and dust components of the disc surrounding the young Herbig Ae star HD 169142, we present far-IR observations obtained with the PACS instrument onboard the ensuremath Herschelensuremath<?iensuremath> Space Observatory. This work is part of the open time key program GASPS, which is aimed at studying the evolution of protoplanetary discs. To constrain the gas properties in the outer disc, we observed the star at several key gas-lines, including ?OI? 63.2 and 145.5 ensuremathmum, ?CII? 157.7 ensuremathmum, CO 72.8 and 90.2 ensuremathmum, and o-Hensuremath 2ensuremath<?subensuremath>O 78.7 and 179.5 ensuremathmum. We only detect the ?OI? 63.2 ensuremathmum line in our spectra, and derive upper limits for the other lines. We complement our data set with PACS photometry and ensuremath 12?13ensuremath<?supensuremath>CO data obtained with the Submillimeter Array. Furthermore, we derive accurate stellar parameters from optical spectra and UV to mm photometry. We model the dust continuum with the 3D radiative transfer code MCFOST and use this model as an input to analyse the gas lines with the thermo-chemical code PRODIMO. Our dataset is consistent with a simple model in which the gas and dust are well-mixed in a disc with a continuous structure between 20 and 200 AU, but this is not a unique solution. Our modelling effort allows us to constrain the gas-to-dust mass ratio as well as the relative abundance of the PAHs in the disc by simultaneously fitting the lines of several species that originate in different regions. Our results are inconsistent with a gas-poor disc with a large UV excess; a gas mass of 5.0 $pm$ 2.0 $times$ 10ensuremath -3ensuremath<?supensuremath>Mensuremath ?ensuremath<?subensuremath> is still present in this disc, in agreement with earlier CO observations.
    Astronomy and Astrophysics. 518:L124?1-L123?5.
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    ABSTRACT: Planets are formed in disks around young stars. With an age of textttchar12610 Myr, TW Hya is one of the nearest T Tauri stars that is still surrounded by a relatively massive disk. In addition a large number of molecules has been found in the TW Hya disk, making TW Hya the perfect test case in a large survey of disks with ensuremath Herschelensuremath<?iensuremath>?PACS to directly study their gaseous component. We aim to constrain the gas and dust mass of the circumstellar disk around TW Hya. We observed the fine-structure lines of ?O I? and ?C II? as part of the open-time large program GASPS. We complement this with continuum data and ground-based ensuremath 12ensuremath<?supensuremath>CO 3?2 and ensuremath 13ensuremath<?supensuremath>CO 3?2 observations. We simultaneously model the continuum and the line fluxes with the 3D Monte-Carlo code ensuremath MCFOSTensuremath<?iensuremath> and the thermo-chemical code ensuremath ProDiMoensuremath<?iensuremath> to derive the gas and dust masses. We detect the ?O I? line at 63 ensuremathmum. The other lines that were observed, ?O I? at 145 ensuremathmum and ?C II? at 157 ensuremathmum, are not detected. No extended emission has been found. Preliminary modeling of the photometric and line data assuming ?ensuremath 12ensuremath<?supensuremath>CO? ??ensuremath 13ensuremath<?supensuremath>CO? = 69 suggests a dust mass for grains with radius ensuremath<1 mm of textttchar1261.9 $times$ 10ensuremath -4ensuremath<?supensuremath> Mensuremath ?ensuremath<?subensuremath> (total solid mass of 3 $times$ 10ensuremath -3ensuremath<?supensuremath> Mensuremath ?ensuremath<?subensuremath>) and a gas mass of (0.5?5) $times$ 10ensuremath -3ensuremath<?supensuremath> Mensuremath ?ensuremath<?subensuremath>. The gas-to-dust mass may be lower than the standard interstellar value of 100.
    Astronomy and Astrophysics. 518:L125?1-L125?6.
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    ABSTRACT: Context. Circumstellar discs are ubiquitous around young stars, but rapidly dissipate their gas and dust on timescales of a few Myr. The Herschel Space Observatory allows for the study of the warm disc atmosphere, using far-infrared spectroscopy to measure gas content and excitation conditions, and far-IR photometry to constrain the dust distribution. Aims: We aim to detect and characterize the gas content of circumstellar discs in four targets as part of the Herschel science demonstration phase. Methods: We carried out sensitive medium resolution spectroscopy and high sensitivity photometry at λ ~ 60-190 μm using the Photodetector Array Camera and Spectrometer instrument on the Herschel Space Observatory. Results: We detect [OI] 63 μm emission from the young stars HD 169142, TW Hydrae, and RECX 15, but not HD 181327. No other lines, including [CII] 158 and [OI] 145, are significantly detected. All four stars are detected in photometry at 70 and 160 μm. Extensive models are presented in associated papers. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
    Astronomy and Astrophysics, v.518, id.L127 (2010).
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    http://dx.doi.org/10.1051/0004-6361/201014591.