764 research outputs found
Disc orientations in pre-main-sequence multiple systems. A study in southern star formation regions
Classical T Tauri stars are encircled by accretion discs most of the time
unresolved by conventional imaging observation. However, numerical simulations
show that unresolved aperture linear polarimetry can be used to extract
information about the geometry of the immediate circumstellar medium that
scatter the starlight. Monin, Menard & Duchene (1998) previously suggested that
polarimetry can be used to trace the relative orientation of discs in young
binary systems in order to shed light on the stellar and planet formation
process. In this paper, we report on new VLT/FORS1 optical linear polarisation
measurements of 23 southern binaries spanning a range of separation from 0.8''
to 10''. In each field, the polarisation of the central binary is extracted, as
well as the polarisation of nearby stars in order to estimate the local
interstellar polarisation. We find that, in general, the linear polarisation
vectors of individual components in binary systems tend to be parallel to each
other. The amplitude of their polarisations are also correlated. These findings
are in agreement with our previous work and extend the trend to smaller
separations. They are also similar to other studies, e.g., Donar et al. 1999;
Jensen et al. 2000, 2004; Wolf et al. 2001. However, we also find a few systems
showing large differences in polarisation level, possibly indicating different
inclinations to the line-of-sight for their discs.Comment: 13 pages, 11 figures, accepted in Astronomy and Astrophysics.
accepted in Astronomy and Astrophysics (A&A
Discovery of the Fomalhaut C debris disc
Fomalhaut is one of the most interesting and well studied nearby stars,
hosting at least one planet, a spectacular debris ring, and two distant
low-mass stellar companions (TW PsA and LP 876-10, a.k.a. Fomalhaut B & C). We
observed both companions with Herschel, and while no disc was detected around
the secondary, TW PsA, we have discovered the second debris disc in the
Fomalhaut system, around LP 876-10. This detection is only the second case of
two debris discs seen in a multiple system, both of which are relatively wide
(3000 AU for HD 223352/40 and 158 kAU [0.77 pc] for Fomalhaut/LP
876-10). The disc is cool (24K) and relatively bright, with a fractional
luminosity , and represents the rare
observation of a debris disc around an M dwarf. Further work should attempt to
find if the presence of two discs in the Fomalhaut system is coincidental,
perhaps simply due to the relatively young system age of 440 Myr, or if the
stellar components have dynamically interacted and the system is even more
complex than it currently appears.Comment: Published in MNRAS Letters. Merry Xma
A layered edge-on circumstellar disk around HK Tau B
We present the first high angular resolution 1.4mm and 2.7mm continuum maps
of the T Tauri binary system HK Tau obtained with the Plateau de Bure
Interferometer. The contributions of both components are well disentangled at
1.4mm and the star previously known to host an edge-on circumstellar disk, HK
Tau B, is elongated along the disk's major axis. The optically bright primary
dominates the thermal emission from the system at both wavelengths, confirming
that it also has its own circumstellar disk. Its non-detection in scattered
light images indicates that the two disks in this binary system are not
parallel. Our data further indicate that the circumprimary disk is probably
significantly smaller than the circumsecondary disk.
We model the millimeter thermal emission from the circumstellar disk
surrounding HK Tau B. We show that the disk mass derived from scattered light
images cannot reproduce the 1.4mm emission using opacities of the same
population of submicron dust grains. However, grain growth alone cannot match
all the observed properties of this disk. We propose that this disk contains
three separate layers: two thin outer surfaces which contain dust grains that
are very similar to those of the ISM, and a disk interior which is relatively
massive and/or has experienced limited grain growth with the largest grains
significantly smaller than 1mm. Such a structure could naturally result from
dust settling in a protoplanetary disk.Comment: Accepted fopr publication in A&A, 8 pages, 1 embedded figur
Multiple protostellar systems. I. A deep near infrared survey of Taurus and Ophiuchus protostellar objects
(Abridged) We performed a deep infrared imaging survey of 63 embedded young
stellar objects (YSOs) located in the Taurus and Ophiuchus clouds to search for
companions. The sample includes Class I and flat infrared spectrum protostellar
objects. We find 17 companions physically bound to 15 YSOs with angular
separations in the range 0.8-10" (110-1400 AU) and derive a companion star
fraction of 23+/-9 % and 29+/-7 % for embedded YSOs in Taurus and Ophiuchus,
respectively. In spite of different properties of the clouds and especially of
the prestellar cores, the fraction of wide companions, 27+/-6 % for the
combined sample, is identical in the two star-forming regions. This suggests
that the frequency and properties of wide multiple protostellar systems are not
very sensitive to specific initial conditions. Comparing the companion star
fraction of the youngest YSOs still surrounded by extended envelopes to that of
more evolved YSOs, we find evidence for a possible evolution of the fraction of
wide multiple systems, which seems to decrease by a factor of about 2 on a
timescale of about 10^5 yr. Somewhat contrary to model predictions, we do not
find evidence for a sub-clustering of embedded sources at this stage on a scale
of a few 100 AU that could be related to the formation of small-N protostellar
clusters. Possible interpretations for this discrepancy are discussed.Comment: Accepted for publication in Astronomy & Astrophysics; 16 pages, 3
embedded figures, 1 JPEG figur
Gas and dust in the Beta Pictoris Moving Group as seen by the Herschel Space Observatory
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.Comment: 12 pages, 7 figures, 6 table
Molecular excitation in the Eagle nebula's fingers
Context: The M16 nebula is a relatively nearby Hii region, powered by O stars
from the open cluster NGC 6611, which borders to a Giant Molecular Cloud.
Radiation from these hot stars has sculpted columns of dense obscuring material
on a few arcmin scales. The interface between these pillars and the hot ionised
medium provides a textbook example of a Photodissociation Region (PDR).
Aims: To constrain the physical conditions of the atomic and molecular
material with submillimeter spectroscopic observations.
Methods: We used the APEX submillimeter telescope to map a ~3'x3' region in
the CO J=3-2, 4-3 and 7-6 rotational lines, and a subregion in atomic carbon
lines. We also observed C18O(3-2) and CO(7-6) with longer integrations on five
peaks found in the CO(3-2) map. The large scale structure of the pillars is
derived from the molecular lines' emission distribution. We estimate the
magnitude of the velocity gradient at the tips of the pillars and use LVG
modelling to constrain their densities and temperatures. Excitation
temperatures and carbon column densities are derived from the atomic carbon
lines.
Results: The atomic carbon lines are optically thin and excitation
temperatures are of order 60 K to 100 K, well consistent with observations of
other Hii region-molecular cloud interfaces. We derive somewhat lower
temperatures from the CO line ratios, of order 40 K. The Ci/CO ratio is around
0.1 at the fingers tips.Comment: 4 pages, APEX A&A special issue, accepte
The EU and Asia within an evolving global order: what is Europe? Where is Asia?
The papers in this special edition are a very small selection from those presented at the EU-NESCA (Network of European Studies Centres in Asia) conference on "the EU and East Asia within an Evolving Global Order: Ideas, Actors and Processes" in November 2008 in Brussels. The conference was the culmination of three years of research activity involving workshops and conferences bringing together scholars from both regions primarily to discuss relations between Europe and Asia, perceptions of Europe in Asia, and the relationship between the European regional project and emerging regional forms in Asia. But although this was the last of the three major conferences organised by the consortium, it in many ways represented a starting point rather than the end; an opportunity to reflect on the conclusions of the first phase of collaboration and point towards new and continuing research agendas for the future
Triplicity and Physical Characteristics of Asteroid (216) Kleopatra
To take full advantage of the September 2008 opposition passage of the M-type
asteroid (216) Kleopatra, we have used near-infrared adaptive optics (AO)
imaging with the W.M. Keck II telescope to capture unprecedented high
resolution images of this unusual asteroid. Our AO observations with the W.M.
Keck II telescope, combined with Spitzer/IRS spectroscopic observations and
past stellar occultations, confirm the value of its IRAS radiometric radius of
67.5 km as well as its dog-bone shape suggested by earlier radar observations.
Our Keck AO observations revealed the presence of two small satellites in orbit
about Kleopatra (see Marchis et al., 2008). Accurate measurements of the
satellite orbits over a full month enabled us to determine the total mass of
the system to be 4.64+/-0.02 10^18 Kg. This translates into a bulk density of
3.6 +/-0.4 g/cm3, which implies a macroscopic porosity for Kleopatra of ~
30-50%, typical of a rubble-pile asteroid. From these physical characteristics
we measured its specific angular momentum, very close to that of a spinning
equilibrium dumbbell.Comment: 35 pages, 3 Tables, 9 Figures. In press to Icaru
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