Acceleration-Induced Nonlocality: Uniqueness of the Kernel

We consider the problem of uniqueness of the kernel in the nonlocal theory of accelerated observers. In a recent work, we showed that the convolution kernel is ruled out as it can lead to divergences for nonuniform accelerated motion. Here we determine the general form of bounded continuous kernels and use observational data regarding spin-rotation coupling to argue that the kinetic kernel given by $K(\tau ,\tau')=k(\tau')$ is the only physically acceptable solution.Comment: LaTeX file, 2 figures, 14 page

A Constraint on brown dwarf formation via ejection: radial variation of the stellar and substellar mass function of the young open cluster IC2391

Using the Wide Field Imager (WFI) at the ESO 2.2m telescope at La Silla and the CPAPIR camera at the CTIO 1.5m telescope at Cerro Tololo, we have performed an extensive, multiband photometric survey of the open cluster IC2391 (D~146pc, age~50Myr, solar metallicity). Here we present the results from our photometric survey and from a spectroscopic follow-up of the central part of the survey.Comment: 4 pages, 3 figures, to appear in the proceedings of the Cool Stars 15 conferenc

Phase-Field Simulation and Design of a Ferroelectric Nano-Generator

We study the behavior of ferroelectric material (BaTiO3) for the design of a nano-generator to convert mechanical into electrical energy. The investigations consider an electro-mechanical phase-field model with polarization as state variable. This widely accepted model has its origins in the work of and is fully developed by Landis and coworkers. We use a finite element model to simulate tetragonal regions of ferroelectric material sputtered on substrate. Different geometries as well as various mechanical and electrical boundary conditions are considered. The model parameters are normalized to achieve better computational conditions within the stiffness matrix. The major objective of this contribution is the fundamental understanding of domain switching caused by a cyclic electrical field. The corresponding hysteresis loops of the overall polarization cannot be achieved by using a two-dimensional model because the domain topologies evolve in three dimensions. The three-dimensional nature of the domain structure evolution is even true for flat regions or thin films. We show some examples of three-dimensional domain topologies, which are able to break energetically unfavorable symmetries. Finally, the computational model of a tetragonal nano-generator with dimensions 10 x 60 x 10 nm is presented. The specific ratio of height to width and the mounting on substrate is essential for its performance and principle of energy harvesting. We discuss the challenges and scopes of such a system.Aerospace Engineerin

Discovery of superthermal hydroxyl (OH) in the HH211 outflow

We present a 5-37 micron infrared spectrum obtained with the Spitzer Space Telescope toward the southeastern lobe of the young protostellar outflow HH211. The spectrum shows an extraordinary sequence of OH emission lines arising in highly excited rotational levels up to an energy E/k~28200K above the ground level. This is, to our knowledge, by far the highest rotational excitation of OH observed outside Earth. The spectrum also contains several pure rotational transitions of H2O (v=0), H2 (v=0) S(0) to S(7), HD (v=0) R(3) to R(6), and atomic fine-structure lines of [Fe II], [Si II], [Ne II], [S I], and [Cl I]. The origin of the highly excited OH emission is most likely the photodissociation of H2O by the UV radiation generated in the terminal outflow shock of HH211.Comment: ApJ Letters, in pres

The nature of the dense core population in the Pipe Nebula: A survey of NH3, CCS, and HC5N molecular line emission

Recent extinction studies of the Pipe Nebula (d=130 pc) reveal many cores spanning a range in mass from 0.2 to 20.4 Msun. These dense cores were identified via their high extinction and comprise a starless population in a very early stage of development. Here we present a survey of NH3 (1,1), NH3 (2,2), CCS (2_1,1_0), and HC5N (9,8) emission toward 46 of these cores. An atlas of the 2MASS extinction maps is also presented. In total, we detect 63% of the cores in NH3 (1,1) 22% in NH3 (2,2), 28% in CCS, and 9% in HC5N emission. We find the cores are associated with dense gas (~10^4 cm-3) with 9.5 < T_k < 17 K. Compared to C18O, we find the NH3 linewidths are systematically narrower, implying that the NH3 is tracing the dense component of the gas and that these cores are relatively quiescent. We find no correlation between core linewidth and size. The derived properties of the Pipe cores are similar to cores within other low-mass star-forming regions: the only differences are that the Pipe cores have weaker NH3 emision and most show no current star formation as evidenced by the lack of embedded infrared sources. Such weak NH3 emission could arise due to low column densities and abundances or reduced excitation due to relatively low core volume densities. Either alternative implies that the cores are relatively young. Thus, the Pipe cores represent an excellent sample of dense cores in which to study the initial conditions for star formation and the earliest stages of core formation and evolution.Comment: 35 pages, 10 figures (excluding the appendix). For the complete appendix contact [email protected]. Accepted for publication in ApJ

A multiwavelength study of embedded clusters in W5-east, NGC 7538, S235, S252 and S254-S258

articleWe present Spitzer, near-IR (NIR) and millimetre observations of the massive star-forming regions W5-east, S235, S252, S254-S258 and NGC 7538. Spitzer data is combined with NIR observations to identify and classify the young population while 12CO and 13CO observations are used to examine the parental molecular cloud. We detect in total 3021 young stellar objects (YSOs). Of those, 539 are classified as Class I, and 1186 as Class II sources. YSOs are distributed in groups surrounded by a more scattered population. Class I sources are more hierarchically organized than Class II and associated with the most dense molecular material. We identify in total 41 embedded clusters containing between 52 and 73 per cent of the YSOs. Clusters are in general non-virialized, turbulent and have star formation efficiencies between 5 and 50 per cent. We compare the physical properties of embedded clusters harbouring massive stars (MEC) and low-mass embedded clusters (LEC) and find that both groups follow similar correlations where the MEC are an extrapolation of the LEC. The mean separation between MEC members is smaller compared to the cluster Jeans length than for LEC members. These results are in agreement with a scenario where stars are formed in hierarchically distributed dusty filaments where fragmentation is mainly driven by turbulence for the more massive clusters. We find several young OB-type stars having IR-excess emission which may be due to the presence of an accretion disc.This work is based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, Caltech, under a contract with NASA. Support for this work was provided by NASA through a contract issued by JPL/Caltech. We also thank NOAO for their student thesis support. The Five College Radio Astronomy Observatory was supported by NSF grant AST 0540852. CB is supported by an RCUK Fellowship at the University of Exeter, UK. This work is based in part on the IRAC postBCD processing software ‘IRACPROC’ developed by Mike Schuster, Massimo Marengo and Brian Patten at the Smithsonian Astrophysical Observatory. This research used the facilities of the Canadian Astronomy Data Centre operated by the National Research Council of Canada with the support of the Canadian Space Agency. This research has made use of the NASA/ IPAC Infrared Science Archive, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. We thank the Spanish MINECO for funding support from grants CSD2009-00038, AYA2009-07304 and AYA2012-32032

The mass function of dense molecular cores and the origin of the IMF

Context: Stars form in the cold dense cores of interstellar molecular clouds and the detailed knowledge of the spectrum of masses of such cores is clearly a key for the understanding of the origin of the IMF. To date, observations have presented somewhat contradictory evidence relating to this issue. Aims: In this paper we propose to derive the mass function of a complete sample of dense molecular cores in a single cloud employing a robust method that uses uses extinction of background starlight to measure core masses and enables the reliable extension of such measurements to lower masses than previously possible. Methods: We use a map of near-infrared extinction in the nearby Pipe dark cloud to identify the population of dense cores in the cloud and measure their masses. Results: We identify 159 dense cores and construct the mass function for this population. We present the first robust evidence for a departure from a single power-law form in the mass function of a population of cores and find that this mass function is surprisingly similar in shape to the stellar IMF but scaled to a higher mass by a factor of about 3. This suggests that the distribution of stellar birth masses (IMF) is the direct product of the dense core mass function and a uniform star formation efficiency of 30%+/-10%, and that the stellar IMF may already be fixed during or before the earliest stages of core evolution. These results are consistent with previous dust continuum studies which suggested that the IMF directly originates from the core mass function. The typical density of ~10^4/cm^3 measured for the dense cores in this cloud suggests that the mass scale that characterizes the dense core mass function may be the result of a simple process of thermal (Jeans) fragmentation.Comment: A&A accepte

Gravitational Slingshot of Young Massive Stars in Orion

The Orion Nebula Cluster (ONC) is the nearest region of massive star formation and thus a crucial testing ground for theoretical models. Of particular interest amongst the ONC's ~1000 members are: \theta^1 Ori C, the most massive binary in the cluster with stars of masses 38 and 9 MSun (Kraus et al. 2009); the Becklin-Neugebauer (BN) object, a 30 km/s runaway star of ~8 MSun (Tan 2004); and the Kleinmann-Low (KL) nebula protostar, a highly-obscured, ~15 MSun object still accreting gas while also driving a powerful, apparently "explosive" outflow (Allen & Burton 1993). The unusual behavior of BN and KL is much debated: How did BN acquire its high velocity? How is this related to massive star formation in the KL nebula? Here we report the results of a systematic survey using ~ 10^7 numerical experiments of gravitational interactions of the \theta^1C and BN stars. We show that dynamical ejection of BN from this triple system at its observed velocity leaves behind a binary with total energy and eccentricity matching those observed for \theta^1C. Five other observed properties of \theta^C are also consistent with it having ejected BN and altogether we estimate there is only a <~ 10^{-5} probability that \theta^1C has these properties by chance. We conclude that BN was dynamically ejected from the \theta^1C system about 4,500 years ago. BN has then plowed through the KL massive-star-forming core within the last 1,000 years causing its recently-enhanced accretion and outflow activity.Comment: 16 pages, 9 figures, 1 table, accepted to Ap