3,635 research outputs found

    Brown Dwarf Jets: Investigating the Universality of Jet Launching Mechanisms at the Lowest Masses

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    Recently it has become apparent that proto-stellar-like outflow activity extends to the brown dwarf (BD) mass regime. While the presence of accretion appears to be the common ingredient in all objects known to drive jets fundamental questions remain unanswered. The more prominent being the exact mechanism by which jets are launched, and whether this mechanism remains universal among such a diversity of sources and scales. To address these questions we have been investigating outflow activity in a sample of protostellar objects that differ considerably in mass and mass accretion rate. Central to this is our study of brown dwarf jets. To date Classical T Tauri stars (CTTS) have offered us the best touchstone for decoding the launching mechanism. Here we shall summarise what is understood so far of BD jets and the important constraints observations can place on models. We will focus on the comparison between jets driven by objects with central mass < 0.1M \odot and those driven by CTTSs. In particular we wish to understand how the the ratio of the mass outflow to accretion rate compares to what has been measured for CTTSs.Comment: Proceedings of IAU meeting 275, "Jets at All Scales

    Exploring Brown Dwarf Disks

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    We discuss the spectral energy distribution of three very low mass objects in Chamaeleon I for which ground-based spectroscopy and photometry as well as ISO measurements in the mid-infrared are available (Comeron et al. 2000; Persi et al. 2000). One of these stars (Cha Halpha1) is a bona-fide brown dwarf, with mass 0.04-0.05 Msun. We show that the observed emission is very well described by models of circumstellar disks identical to those associated to T Tauri stars, scaled down to keep the ratio of the disk-to-star mass constant and to the appropriate stellar parameters. This result provides a first indication that the formation mechanism of T Tauri stars (via core contraction and formation of an accretion disk) extends to objects in the brown dwarf mass range.Comment: 4 pages, 4 figures, A&A Letters in pres

    Diamonds in HD 97048

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    We present adaptive optics high angular resolution (\sim0\farcs1) spectroscopic observations in the 3 \mum region of the Herbig Ae/Be star HD 97048. For the first time, we spatially resolve the emission in the diamond features at 3.43 and 3.53 \mum and in the adjacent continuum. Using both the intensity profiles along the slit and reconstructed two-dimensional images of the object, we derive full-width at half-maximum sizes consistent with the predictions for a circumstellar disk seen pole-on. The diamond emission originates in the inner region (R15R \lesssim 15 AU) of the disk.Comment: ApJLetter, in pres

    Spatially resolved PAH emission in the inner disks of Herbig Ae/Be stars

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    We present adaptive optics high angular resolution (\sim0.1\arcsec) spectroscopic observations in the 3 \um region of eight well known Herbig Ae/Be stars with circumstellar disks. We detect the aromatic emission feature at 3.3 \um for four out of six of our objects with flared disks (HD 169142, HD 97048, HD 100453, HD 100546), some weaker additional features at 3.4 and 3.46 \mum and nanodiamond features at 3.43 and 3.53 \um in two of our flared object (HD 100546 and HD 97048 respectively). We also detect hydrogen recombination line at 3.74 \um in practically all objects. The emission in the polycyclic aromatic hydrocarbons (PAHs) feature at 3.3 \um, additional and nanodiamond features in the 3.4-3.5 \um region is, for the first time, spatially resolved in all the sources where the features are detected. The full-width at half-maximum sizes that we derive are typical of emission arising in a circumstellar disk. On the other hand, the continuum emission is unresolved, with the exception of HD 97048 where it is marginally resolved. We compare the observed spatial distribution of the 3.3 \mum PAH feature and the adjacent continuum to the predictions of a disk model that includes transiently heated small grains and PAHs in addition to large grains in thermal equilibrium \cite[]{habart2004a}. The model predicts that, as observed, the 3.3 \mum PAH emission feature is significantly broader than that of the adjacent continuum and that about 50% of its integrated intensity comes from a radius R<R< 30 AU. We find that the predicted brightness profiles reproduce very well the observed ones. This proves beyond doubt that the energetic 3.3 \mum PAH emission feature takes its origin in the inner disk regions.Comment: 7 figures, accepted to A&

    The Herschel view of circumstellar discs: a multi-wavelength study of Chamaeleon I

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    We present the results of a multi-wavelength study of circumstellar discs around 44 young stellar objects in the 3 Myr old nearby Chamaeleon I star-forming region. In particular, we explore the far-infrared/submm regime using Herschel fluxes. We show that Herschel fluxes at 160-500\,\mum can be used to derive robust estimates of the disc mass. The median disc mass is 0.005MM_{\odot} for a sample of 28 Class IIs and 0.006MM_{\odot} for 6 transition disks (TDs). The fraction of objects in Chamaeleon-I with at least the `minimum mass solar nebula' is 2-7%. This is consistent with previously published results for Taurus, IC348, \rho Oph. Diagrams of spectral slopes show the effect of specific evolutionary processes in circumstellar discs. Class II objects show a wide scatter that can be explained by dust settling. We identify a continuous trend from Class II to TDs. Including Herschel fluxes in this type of analysis highlights the diversity of TDs. We find that TDs are not significantly different to Class II discs in terms of far-infrared luminosity, disc mass or degree of dust settling. This indicates that inner dust clearing occurs independently from other evolutionary processes in the discs.Comment: 16 pages, 13 figures. Accepted for publication in MNRA

    X-Shooter study of accretion in \rho-Ophiucus: very low-mass stars and brown dwarfs

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    We present new VLT/X-Shooter optical and NIR spectra of a sample of 17 candidate young low-mass stars and BDs in the rho-Ophiucus cluster. We derived SpT and Av for all the targets, and then we determined their physical parameters. All the objects but one have M*<0.6 Msun, and 8 have mass below or close to the hydrogen-burning limit. Using the intensity of various emission lines present in their spectra, we determined the Lacc and Macc for all the objects. When compared with previous works targeting the same sample, we find that, in general, these objects are not as strongly accreting as previously reported, and we suggest that the reason is our more accurate estimate of the photospheric parameters. We also compare our findings with recent works in other slightly older star-forming regions to investigate possible differences in the accretion properties, but we find that the accretion properties for our targets have the same dependence on the stellar and substellar parameters as in the other regions. This leads us to conclude that we do not find evidence for a different dependence of Macc with M* when comparing low-mass stars and BDs. Moreover, we find a similar small (1 dex) scatter in the Macc-M* relation as in some of our recent works in other star-forming regions, and no significant differences in Macc due to different ages or properties of the regions. The latter result suffers, however, from low statistics and sample selection biases in the current studies. The small scatter in the Macc-M* correlation confirms that Macc in the literature based on uncertain photospheric parameters and single accretion indicators, such as the Ha width, can lead to a scatter that is unphysically large. Our studies show that only broadband spectroscopic surveys coupled with a detailed analysis of the photospheric and accretion properties allows us to properly study the evolution of disk accretion rates.Comment: accepted for publication in Astronomy & Astrophysics. Abstract shortened to fit arXiv constraint

    The Vault with Intertwined Arches in Castle of Racconigi: 3D Digital Reconstruction

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    The complex approach of Guarini to each discipline (Geometry, Architecture, Philosophy, Astronomy) finds important development in his method of implementing vaulted systems. The importance of this architectural element, which Guarini reminds as 鈥渢he main part of the buildings鈥, is reflected in the new taste of the civil architecture of the period. The design by Guarini for the vault in the Hall of Honor in Racconigi Castle is documented in a single drawing representing the hall鈥檚 cross-section. Guarini devotes particular attention to the drawing, both from the geometric and the representative point of view. Based on this drawing we want to propose, through the most recent digital modeling and visualization methods, a three-dimensional reconstruction of this unrealized work. The analysismethod involves an in-depth examination of: Guarini鈥檚 theory and his systematic approach to vaulted structures, linking Architecture and Geometry, the problems of design the artifacts, the comparison with similar shapes designed by the architect. Through a method of representation based on geometrical principles, the aim of this paper is to give evidence 鈥 and a three-dimensional visualization 鈥 of a case study documented by an archival source, already analyzed by historians
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