3,635 research outputs found
Brown Dwarf Jets: Investigating the Universality of Jet Launching Mechanisms at the Lowest Masses
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
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
We present adaptive optics high angular resolution (\sim0\farcs1)
spectroscopic observations in the 3 m 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 m 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 ( AU) of the disk.Comment: ApJLetter, in pres
Spatially resolved PAH emission in the inner disks of Herbig Ae/Be stars
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 m
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 m 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 m PAH emission feature is
significantly broader than that of the adjacent continuum and that about 50% of
its integrated intensity comes from a radius 30 AU. We find that the
predicted brightness profiles reproduce very well the observed ones. This
proves beyond doubt that the energetic 3.3 m 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
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 160-500m can be
used to derive robust estimates of the disc mass. The median disc mass is
0.005 for a sample of 28 Class IIs and 0.006 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, 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 -Ophiucus: very low-mass stars and brown dwarfs
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
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 “the main part of the buildings”, 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’s 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’s 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 – and a three-dimensional visualization – of a case study documented by an archival source, already analyzed by historians
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