9,218 research outputs found
Accretion in Evolved and Transitional Disks in Cep OB2: Looking for the Origin of the Inner Holes
We present accretion rates for a large number of solar-type stars in the Cep
OB2 region, based on U band observations. Our study comprises 95 members of the
~4 Myr-old cluster Tr 37 (including 20 "transition" objects; TO), as well as
the only CTTS in the ~12 Myr-old cluster NGC 7160. The stars show different
disk morphologies, with the majority of them having evolved and flattened
disks. The typical accretion rates are about one order of magnitude lower than
in regions aged 1-2 Myr, and we find no strong correlation between disk
morphology and accretion rates. Although half of the TO are not accreting, the
median accretion rates of normal CTTS and accreting "transition" disks are
similar (~3 10^{-9} and 2 10^{-9} Msun/yr, respectively). Comparison with other
regions suggests that the TO observed at different ages do not necessarily
represent the same type of objects, which is consistent with the fact that the
different processes that can lead to reduced IR excess/inner disk clearing
(e.g., binarity, dust coagulation/settling, photoevaporation, giant planet
formation) do not operate on the same timescales. Accreting TO in Tr 37 are
probably suffering strong dust coagulation/settling. Regarding the equally
large number of non-accreting TO in the region, other processes, like
photoevaporation, the presence of stellar/substellar companions, and/or giant
planet formation may account for their "transitional" SEDs and negligible
accretion rates.Comment: 37 pages, 5 figures, 6 tables Accepted by Ap
The low-mass stellar population in the young cluster Tr37: Disk evolution, accretion, and environment
We present a study of accretion and protoplanetary disks around M-type stars
in the 4 Myr-old cluster Tr37. With a well-studied solar-type population, Tr37
is a benchmark for disk evolution. We used low-resolution spectroscopy to
identify 141 members (78 new) and 64 probable members, mostly M-type stars.
H\alpha\ emission provides information about accretion. Optical, 2MASS,
Spitzer, and WISE data are used to trace the SEDs. We construct radiative
transfer models to explore the structures of full-disks, pre-transition,
transition, and dust-depleted disks. Including the new and previously known
members, we confirm that a substantial fraction (~2/5) of disks show signs of
evolution, either as radial dust evolution (transition/pre-transition disks) or
as a more global evolution (low small-dust masses, dust settling, and
weak/absent accretion signatures). Accretion is strongly dependent on the SED
type. About half of the transition objects are consistent with no accretion,
and dust-depleted disks have weak (or undetectable) accretion signatures,
especially among M-type stars. The analysis of accretion and disk structure
suggests a parallel evolution of dust and gas. We find several distinct classes
of evolved disks, based on SED type and accretion, pointing to different disk
dispersal mechanisms and probably different evolutionary paths. Dust depletion
and opening of inner holes appear to be independent processes: most transition
disks are not dust-depleted, and most dust-depleted disks do not require inner
holes. The differences in disk structure between M-type and solar-type stars in
Tr37 (4 Myr) are not as remarkable as in the young, sparse, Coronet cluster
(1-2 Myr), suggesting that other factors, like the environment/interactions,
are likely to play a role in the disk evolution and dispersal. Finally, we also
find some evidence of clumpy star formation or mini-clusters within Tr37.Comment: 21 pages, 16 figures, plus appendix with tables and figures. Accepted
by A&
Protostars and stars in the Coronet cluster: Age, evolution, and cluster structure
We present new optical spectroscopy with FLAMES/VLT, near-IR imaging with
HAWK-I/VLT, and 870 micron mapping with APEX/LABOCA of the Coronet cluster. The
optical data allow to estimate spectral types, extinction and the presence of
accretion in 6 more M-type members, in addition to the 12 that we had
previously studied. The submillimeter maps and near-IR data reveal the presence
of nebular structures and high extinction regions, which are in some cases
associated to known IR, optical, and X-ray sources. Most star formation is
associated to two elongated structures crossing in the central part of the
cluster. Placing all the 18 objects with known spectral types and extinction in
the HR diagram suggests that the cluster is younger than previously thought (<2
Myr, and probably ~0.5-1 Myr). The new age estimate is in agreement with the
evolutionary status of the various protostars in the region and with its
compactness (<1.3 pc across), but results in a conflict with the low disk and
accretion fraction (only 50-65% of low-mass stars appear to have protoplanetary
disks, and most transitional and homologously depleted disks are consistent
with no accretion) and with the evolutionary features observed in the mid-IR
spectra and spectral energy distributions of the disks.Comment: 30 pages, 9 figures, 6 tables, ApJ in pres
A Herschel view of IC 1396 A: Unveiling the different sequences of star formation
The IC1396A globule in the young cluster Tr37, hosting many young stars and
protostars, is assumed to be a site of triggered star formation. We mapped
IC1396A with Herschel/PACS at 70 and 160 micron. The Herschel maps trace in
great detail the very embedded protostellar objects and the structure of the
cloud. PACS data reveal a previously unknown Class 0 object (IC1396A-PACS-1)
located behind the ionization front. IC1396A-PACS-1 is not detectable with
Spitzer, but shows marginal X-ray emission. The data also allowed to study
three of the Class I intermediate-mass objects within the cloud. We derived
approximate cloud temperatures to study the effect and potential interactions
between the protostars and the cloud. The Class 0 object is associated with the
densest and colder part of IC1396A. Heating in the cloud is dominated by the
winds and radiation of the O6.5 star HD 206267 and, to a lesser extent, by the
effects of the Herbig Ae star V 390 Cep. The surroundings of the Class I and
Class II objects embedded in the cloud also appear warmer than the sourceless
areas, although most of the low-mass objects cannot be individually extracted
due to distance and beam dilution. The observations suggest that at least two
episodes of star formation have occurred in IC1396A. One would have originated
the known, ~1 Myr-old Class I and II objects in the cloud, and a new wave of
star formation would have produced the Class 0 source at the tip of the
brigth-rimmed cloud. From its location and properties, IC1396A-PACS-1 is
consistent with triggering via radiative driven implosion (RDI) induced by HD
206267. The mechanisms behind the formation of the more evolved population of
Class I/II/III objects in the cloud are uncertain. Heating of most of the
remaining cloud by Class I/Class II objects and by HD 206267 itself may
preclude further star formation in the region.Comment: Accepted by A&A, 9 pages, 5 figure
Design methodology of force feedback laws through helicopter control loop simulation
International audienceThe latest evolution of pilot controllers, referred to as ASSU (Active Side Sticks Units) provides static and dynamic tactile force (or haptic) feedback to the pilot at the grip. Combined with FBW (fly-by-wire), this promising technology has enhanced safety levels compared to the original mechanical linkage systems they have started to replace, while offering vast improved benefits in terms of carefree handling and pilot situational awareness. In the framework of a PhD thesis, the Information Processing and Systems Department (DTIS) of ONERA and SAFRAN Electronics & Defense have started a cooperation to evaluate the interest and the different possibilities offered by the ASSU technology to improve safety and handling qualities of rotary wing aircraft. Up to now, the design and tuning of these functions were essentially performed thanks to numerous simulator sessions or flight tests with pilots. More than just providing a set of values for the required parameters defining the cueing function (hopefully an optimal set of parameters), it is expected that the approach presented here would reduce the number of piloted simulation tests and associated difficulties of the availability of pilots, the significant amount of time and material resources. This paper describes the work done during the first half of the thesis. The main objective of this work is to develop a design methodology based on the simulation of the entire helicopter control loop (also including the pilot in some form) and enabling the definition and parameterization of cueing functions. Moreover, some objective criteria will be defined and used to design the force feedback laws, bringing additional means of evaluation and validation than the classical subjective rating scales
Outils basés simulation pour la conception d'une protection haptique sur l'axe de roulis pour hélicoptère
International audienceThe latest evolution of pilot controllers, referred to as ASSU (Active Side Sticks Units) provides static and dynamic tactile force (or haptic) feedback to the pilot at the grip. Combined with FBW (fly-by-wire), this promising technology has enhanced safety levels compared to the original mechanical linkage systems they have started to replace, while offering vast improved benefits in terms of carefree handling and pilot situational awareness. In the framework of a PhD thesis, the Information Processing and Systems Department (DTIS) of ONERA and SAFRAN Electronics & Defense have started a cooperation to evaluate the interest and the different possibilities offered by the ASSU technology to improve safety and handling qualities of rotary wing aircraft. Up to now, the design and tuning of these functions were essentially performed thanks to numerous simulator sessions or flight tests with pilots. More than just providing a set of values for the required parameters defining the cueing function (hopefully an optimal set of parameters), it is expected that the approach presented here would reduce the number of piloted simulation tests and associated difficulties of the availability of pilots, the significant amount of time and material resources. The main objective of this work is to develop a design methodology based on the simulation of the entire helicopter control loop (also including the pilot in some form) and enabling the definition and parameterization of cueing functions. Moreover, some objective criteria will be defined and used to design the force feedback laws, bringing additional means of evaluation and validation than the classical subjective rating scales
Méthodologie de conception de lois de retour d'efforts pour un mini-manche actif
International audienceSAFRAN Electronics & Defense and the Information and Systems Processing Department (DTIS) of ONERA have begun a cooperation to evaluate the interest and the methods of use of Active Side Stick Units (ASSU) to improve the safety and flight qualities of helicopters. This paper describes the work carried out to model an environment for simulation and evaluation of haptic feedback laws. An experiment, implemented in the simulator PycsHel at ONERA Salon de Provence, has brought some insight about the influence of ASSU's parameters on the detection of specific haptic feedbacks (Softstops). The results obtained will be added to the simulation model in order to allow the specification, optimal if possible, of the haptic cues.SAFRAN Electronique & Défense et le département Traitement de l'Information et Systèmes (DTIS) de l’ONERA ont entamé une coopération pour évaluer l'intérêt et les méthodes d’utilisation des mini-manches (ASSU: Active Side Stick Units) pour améliorer la sécurité et la qualité de vol des hélicoptères. Ce papier décrit les travaux menés pour modéliser un environnement de simulation et d'évaluation des lois de retour haptique. Une expérimentation, implémentée dans le simulateur PycsHel de l’ONERA Salon de Provence, a apporté un éclairage sur l’influence de paramètres de l’ASSU pour la détection de rretours haptiques spécifiques (SoftStops). Les résultats obtenus seront intégrés au modèle de simulation afin de permettre la spécification, optimale si possible, des signaux haptique
4-Holes in point sets
We consider a variant of a question of Erdos on the number of empty k-gons (k-holes) in a set of n points in the plane, where we allow the k-gons to be non-convex. We show bounds and structural results on maximizing and minimizing the number of general 4-holes, and maximizing the number of non-convex 4-holes. In particular, we show that for n >= 9, the maximum number of general 4-holes is ((pi)(4)); the minimum number of general 4-holes is at least 5/2 n(2) - circle minus(n); and the maximum number of non-convex 4-holes is at least 1/2 n(3) - circle minus(n(2) logn) and at most 1/2 n(3) - circle minus(n(2)). 2014 (c) Elsevier B.V. All rights reserved.Postprint (author’s final draft
Ubiquitin plays an atypical role in GPCR-induced p38 MAP kinase activation on endosomes.
Protease-activated receptor 1 (PAR1) is a G protein-coupled receptor (GPCR) for thrombin and promotes inflammatory responses through multiple pathways including p38 mitogen-activated protein kinase signaling. The mechanisms that govern PAR1-induced p38 activation remain unclear. Here, we define an atypical ubiquitin-dependent pathway for p38 activation used by PAR1 that regulates endothelial barrier permeability. Activated PAR1 K63-linked ubiquitination is mediated by the NEDD4-2 E3 ubiquitin ligase and initiated recruitment of transforming growth factor-β-activated protein kinase-1 binding protein-2 (TAB2). The ubiquitin-binding domain of TAB2 was essential for recruitment to PAR1-containing endosomes. TAB2 associated with TAB1, which induced p38 activation independent of MKK3 and MKK6. The P2Y1 purinergic GPCR also stimulated p38 activation via NEDD4-2-mediated ubiquitination and TAB1-TAB2. TAB1-TAB2-dependent p38 activation was critical for PAR1-promoted endothelial barrier permeability in vitro, and p38 signaling was required for PAR1-induced vascular leakage in vivo. These studies define an atypical ubiquitin-mediated signaling pathway used by a subset of GPCRs that regulates endosomal p38 signaling and endothelial barrier disruption
On -Gons and -Holes in Point Sets
We consider a variation of the classical Erd\H{o}s-Szekeres problems on the
existence and number of convex -gons and -holes (empty -gons) in a set
of points in the plane. Allowing the -gons to be non-convex, we show
bounds and structural results on maximizing and minimizing their numbers. Most
noteworthy, for any and sufficiently large , we give a quadratic lower
bound for the number of -holes, and show that this number is maximized by
sets in convex position
- …