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Register a new userGas and dust in the Beta Pictoris Moving Group as seen by the Herschel Space Observatory
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Added by
Pablo Rivi\\`ere-Marichalar
Publication date
2014
fields
Physics
and research's language is
English
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Context. Debris discs are thought to be formed through the collisional grinding of planetesimals, and can be considered as the outcome of planet formation. Understanding the properties of gas and dust in debris discs can help us to comprehend the architecture of extrasolar planetary systems. Herschel Space Observatory far-infrared (IR) photometry and spectroscopy have provided a valuable dataset for the study of debris discs gas and dust composition. This paper is part of a series of papers devoted to the study of Herschel PACS observations of young stellar associations. Aims. This work aims at studying the properties of discs in the Beta Pictoris Moving Group (BPMG) through far-IR PACS observations of dust and gas. Methods. We obtained Herschel-PACS far-IR photometric observations at 70, 100 and 160 microns of 19 BPMG members, together with spectroscopic observations of four of them. Spectroscopic observations were centred at 63.18 microns and 157 microns, aiming to detect [OI] and [CII] emission. We incorporated the new far-IR observations in the SED of BPMG members and fitted modified blackbody models to better characterise the dust content. Results. We have detected far-IR excess emission toward nine BPMG members, including the first detection of an IR excess toward HD 29391.The star HD 172555, shows [OI] emission, while HD 181296, shows [CII] emission, expanding the short list of debris discs with a gas detection. No debris disc in BPMG is detected in both [OI] and [CII]. The discs show dust temperatures in the range 55 to 264 K, with low dust masses (6.6*10^{-5} MEarth to 0.2 MEarth) and radii from blackbody models in the range 3 to 82 AU. All the objects with a gas detection are early spectral type stars with a hot dust component.
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Context. Circumstellar discs are the places where planets form, therefore knowledge of their evolution is crucial for our understanding of planet formation. The Herschel Space Observatory is providing valuable data for studying disc systems, thanks to its sensitivity and wavelength coverage. This paper is one of several devoted to analysing and modelling Herschel-PACS observations of various young stellar associations from the GASPS Open Time Key Programme. Aims. The aim of this paper is to elucidate the gas and dust properties of circumstellar discs in the 10 Myr TW Hya Association (TWA) using new far-infrared (IR) imaging and spectroscopy from Herschel-PACS. Methods. We obtained far-IR photometric data at 70, 100, and 160 microns of 14 TWA members; spectroscopic observations centred on the [OI] line at 63.18 microns were also obtained for 9 of the 14. The new photometry for each star was incorporated into its full spectral energy distribution (SED). Results. We detected excess IR emission that is characteristic of circumstellar discs from five TWA members, and computed upper limits for another nine. Two TWA members (TWA 01 and TWA 04B) also show [OI] emission at 63.18 microns. Discs in the TWA association display a variety of properties, with a wide range of dust masses and inner radii, based on modified blackbody modelling. Both transitional and debris discs are found in the sample. Models for sources with a detected IR excess give dust masses in the range from 0.15 Msun to 63 Msun.
Jeffries & Binks (2014) and Malo et al. (2014) have recently reported Li depletion boundary (LDB) ages for the {beta} Pictoris moving group (BPMG) which are twice as old as the oft-cited kinematic age of $sim$12 Myr. In this study we present (1) a new evaluation of the internal kinematics of the BPMG using the revised Hipparcos astrometry and best available published radial velocities, and assess whether a useful kinematic age can be derived, and (2) derive an isochronal age based on the placement of the A-, F- and G-type stars in the colour-magnitude diagram (CMD). We explore the kinematics of the BPMG looking at velocity trends along Galactic axes, and conducting traceback analyses assuming linear trajectories, epicyclic orbit approximation, and orbit integration using a realistic gravitational potential. None of the methodologies yield a kinematic age with small uncertainties using modern velocity data. Expansion in the Galactic X and Y directions is significant only at the 1.7{sigma} and 2.7{sigma} levels, and together yields an overall kinematic age with a wide range (13-58 Myr; 95 per cent CL). The A-type members are all on the zero age-main-sequence, suggestive of an age of $>$20Myr, and the loci of the CMD positions for the late-F- and G-type pre-main-sequence BPMG members have a median isochronal age of 22 Myr ($pm$ 3 Myr stat., $pm$ 1 Myr sys.) when considering four sets of modern theoretical isochrones. The results from recent LDB and isochronal age analyses are now in agreement with a median BPMG age of 23 $pm$ 3 Myr (overall 1{sigma} uncertainty, including $pm$2 Myr statistical and $pm$2 Myr systematic uncertainties).
We obtained Herschel PACS and SPIRE images of the thermal emission of the debris disk around the A5V star {beta} Pic. The disk is well resolved in the PACS filters at 70, 100, and 160 {mu}m. The surface brightness profiles between 70 and 160 {mu}m show no significant asymmetries along the disk, and are compatible with 90% of the emission between 70 and 160 {mu}m originating in a region closer than 200 AU to the star. Although only marginally resolving the debris disk, the maps obtained in the SPIRE 250 - 500 {mu}m filters provide full-disk photometry, completing the SED over a few octaves in wavelength that had been previously inaccessible. The small far-infrared spectral index ({beta} = 0.34) indicates that the grain size distribution in the inner disk (<200AU) is inconsistent with a local collisional equilibrium. The size distribution is either modified by non-equilibrium effects, or exhibits a wavy pattern, caused by an under-abundance of impactors which have been removed by radiation pressure.
We have obtained Spitzer IRS 5.5 - 35 micron spectroscopy of the debris disk around beta Pictoris. In addition to the 10 micron silicate emission feature originally observed from the ground, we also detect the crystalline silicate emission bands at 28 micron and 33.5 micron. This is the first time that the silicate bands at wavelengths longer than 10 micron have ever been seen in the beta Pictoris disk. The observed dust emission is well reproduced by a dust model consisting of fluffy cometary and crystalline olivine aggregates. We searched for line emission from molecular hydrogen and atomic [S I], Fe II, and Si II gas but detected none. We place a 3 sigma upper limit of <17 Earth masses on the H2 S(1) gas mass, assuming an excitation temperature of Tex = 100 K. This suggests that there is less gas in this system than is required to form the envelope of Jupiter. We hypothesize that some of the atomic Na I gas observed in Keplerian rotation around beta Pictoris may be produced by photon-stimulated desorption from circumstellar dust grains.
Context: The $beta$ Pictoris moving group is one of the most well-known young associations in the solar neighbourhood and several members are known to host circumstellar discs, planets, and comets. Measuring its age with precision is basic to study several astrophysical processes such as planet formation and disc evolution which are strongly age dependent. Aims: We aim to determine a precise and accurate dynamical traceback age for the $beta$ Pictoris moving group. Methods: Our sample combines the extremely precise Gaia DR2 astrometry with ground-based radial velocities measured in an homogeneous manner. We use an updated version of our algorithm to determine dynamical ages. The new approach takes into account a robust estimate of the spatial and kinematic covariance matrices of the association to improve the sample selection process and to perform the traceback analysis. Results: We estimate a dynamical age of $18.5_{-2.4}^{+2.0}$ Myr for the $beta$ Pictoris moving group. We investigated the spatial substructure of the association at birth time and we propose the existence of a core of stars more concentrated. We also provide precise radial velocity measurements for 81 members of $beta$ Pic, including ten stars with the first determination of their radial velocities. Conclusions: Our dynamical traceback age is three times more precise than previous traceback age estimates and, more important, for the first time, reconciles the traceback age with the most recent estimates of other dynamical, lithium depletion boundary, and isochronal ages. This has been possible thanks to the excellent astrometric and spectroscopic precisions, the homogeneity of our sample, and the detailed analysis of binaries and membership.
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