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HIP 78053
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Title: First evidence of external disc photoevaporation in a low mass star forming region: the case of IM Lup
Author: Thomas J. Haworth, Stefano Facchini, Cathie J. Clarke, L. Ilsedore Cleeves

We model the radiatively driven flow from IM Lup -- a large protoplanetary disc expected to be irradiated by only a weak external radiation field (at least 104 times lower than the UV field irradiating the Orion Nebula Cluster proplyds). We find that material at large radii (>400AU) in this disc is sufficiently weakly gravitationally bound that significant mass loss can be induced. Given the estimated values of the disc mass and accretion rate, the viscous timescale is long (~10Myr) so the main evolutionary behaviour for the first Myr of the disc's lifetime is truncation of the disc by photoevaporation, with only modest changes effected by viscosity. We also produce approximate synthetic observations of our models, finding substantial emission from the flow which can explain the CO halo observed about IM Lup out to \geq 1000AU. Solutions that are consistent with the extent of the observed CO emission generally imply that IM Lup is still in the process of having its disc outer radius truncated. We conclude that IM Lup is subject to substantial external photoevaporation, which raises the more general possibility that external irradiation of the largest discs can be of significant importance even in low mass star forming regions.

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RE: IM Lupi
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Title: The disk-bearing young star IM Lup
Authors: H. M. Guenther, S. P. Matt, J. H. M. M. Schmitt, M. Guedel, Z.-Y. Li, D. M. Burton

Classical T Tauri stars (CTTS) differ in their X-ray signatures from older pre-main sequence stars, e.g. weak-lined TTS (WTTS). CTTS show a soft excess and deviations from the low-density coronal limit in the He-like triplets. We test whether these features correlate with accretion or the presence of a disk by observing IM Lup, a disk-bearing object apparently in transition between CTTS and WTTS without obvious accretion. We analyse a Chandra grating spectrum and additional XMM-Newton data of IM Lup and accompanying optical spectra, some of them taken simultaneously to the X-ray observations. We fit the X-ray emission lines and decompose the Ha emission line in different components. In X-rays IM Lup has a bright and hot active corona, where elements of low first-ionisation potential are depleted. The He-like Ne IX triplet is in the low-density state, but due to the small number of counts a high-density scenario cannot be excluded on the 90% confidence level. In all X-ray properties IM Lup resembles a main-sequence star, but it is also compatible with CTTS signatures on the 90% confidence level, thus we cannot decide if the soft excess and deviations from the low-density coronal limit in the He-like triplets in CTTS require accretion or only the presence of a disk. IM Lup is chromospherically active, which explains most of the emission in Ha. Despite its low equivalent width, the complexity of the Ha line profile is reminiscent of CTTS. We present an estimate for the mass accretion rate of 10e-11 Msun/yr.

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Title: Probing dust grain evolution in IM Lupi's circumstellar disc. Multi-wavelength observations and modelling of the dust disc
Authors: C. Pinte, D.L. Padgett, F. Menard, K.R. Stapelfeldt, G. Schneider, J. Olofsson, O. Panic, J.C. Augereau, G. Duchene, J. Krist, K. Pontoppidan, M.D. Perrin, C.A. Grady, J. Kessler-Silacci, E.F. van Dishoeck, D. Lommen, M. Silverstone, D.C. Hines, S. Wolf, G.A. Blake, T. Henning, B. Stecklum

We present a panchromatic study, involving a multiple technique approach, of the circumstellar disc surrounding the T Tauri star IM Lupi (Sz 82). We have undertaken a comprehensive observational study of IM Lupi using photometry, spectroscopy, millimetre interferometry and multi-wavelength imaging. For the first time, the disc is resolved from optical and near-infrared wavelengths in scattered light, to the millimetre regime in thermal emission. Our data-set, in conjunction with existing photometric data, provides an extensive coverage of the spectral energy distribution, including a detailed spectrum of the silicate emission bands. We have performed a simultaneous modelling of the various observations, using the radiative transfer code MCFOST, and analysed a grid of models over a large fraction of the parameter space via Bayesian inference. We have constructed a model that can reproduce all of the observations of the disc. Our analysis illustrates the importance of combining a wide range of observations in order to fully constrain the disc model, with each observation providing a strong constraint only on some aspects of the disc structure and dust content. Quantitative evidence of dust evolution in the disc is obtained: grain growth up to millimetre-sized particles, vertical stratification of dust grains with micrometric grains close to the disc surface and larger grains which have settled towards the disc midplane, and possibly the formation of fluffy aggregates and/or ice mantles around grains.

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Position(2000): RA 15 56 09.23, Dec -37 56 05.9

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