2,789 research outputs found
IR Dust Bubbles: Probing the Detailed Structure and Young Massive Stellar Populations of Galactic HII Regions
We present an analysis of wind-blown, parsec-sized, mid-infrared bubbles and
associated star-formation using GLIMPSE/IRAC, MIPSGAL/MIPS and MAGPIS/VLA
surveys. Three bubbles from the Churchwell et al. (2006) catalog were selected.
The relative distribution of the ionized gas (based on 20 cm emission), PAH
emission (based on 8 um, 5.8 um and lack of 4.5 um emission) and hot dust (24
um emission) are compared. At the center of each bubble there is a region
containing ionized gas and hot dust, surrounded by PAHs. We identify the likely
source(s) of the stellar wind and ionizing flux producing each bubble based
upon SED fitting to numerical hot stellar photosphere models. Candidate YSOs
are also identified using SED fitting, including several sites of possible
triggered star formation.Comment: 37 pages, 17 figure
VLTI/MIDI 10 micron interferometry of the forming massive star W33A
We report on resolved interferometric observations with VLTI/MIDI of the
massive young stellar object (MYSO) W33A. The MIDI observations deliver
spectrally dispersed visibilities with values between 0.03 and 0.06, for a
baseline of 45m over the wavelength range 8-13 micron. The visibilities
indicate that W33A has a FWHM size of approximately 120AU (0.030'') at 8 micron
which increases to 240AU at 13 micron, scales previously unexplored among
MYSOs. This observed trend is consistent with the temperature falling off with
distance. 1D dust radiative transfer models are simultaneously fit to the
visibility spectrum, the strong silicate feature and the shape of the mid
infrared spectral energy distribution (SED). For any powerlaw density
distribution, we find that the sizes (as implied by the visibilities) and the
stellar luminosity are incompatible. A reduction to a third of W33A's
previously adopted luminosity is required to match the visibilities; such a
reduction is consistent with new high resolution 70 micron data from Spitzer's
MIPSGAL survey. We obtain best fits for models with shallow dust density
distributions of r^(-0.5) and r^(-1.0) and for increased optical depth in the
silicate feature produced by decreasing the ISM ratio of graphite to silicates
and using optical grain properties by Ossenkopf et al. (1992).Comment: 4 pages, 4 figures. Accepted for ApJ letter
Constraints in the circumstellar density distribution of massive Young Stellar Objects
We use a Monte Carlo code to generate synthetic near-IR reflection nebulae that resemble those (normally associated with a bipolar outflow cavity) seen towards massive young stellar objects (YSOs). The 2D axi-symmetric calculations use an analytic expression for a flattened infalling rotating envelope with a bipolar cavity representing an outflow. We are interested in which aspects of the circumstellar density distribution can be constrained by observations of these reflection nebulae. We therefore keep the line of sight optical depth constant in the model grid, as this is often constrained independently by observations. It is found that envelopes with density distributions corresponding to mass infall rates of similar to10(-4) M-circle dot yr(-1) (for an envelope radius of 4700 AU) seen at an inclination angle of similar to45degrees approximately reproduce the morphology and extension of the sub-arcsecond nebulae observed in massive YSOs. Based on the flux ratio between the approaching and receding lobe of the nebula, we can constrain the system inclination angle. The cavity opening angle is well constrained from the nebula opening angle. Our simulations indicate that to constrain the outflow cavity shape and the degree of flattening in the envelope, near-IR imaging with higher resolution and dynamic range than speckle imaging in 4 m-class telescopes is needed. The radiative transfer code is also used to simulate the near-IR sub-arcsecond nebula seen in Mon R2 IRS3. We find indications of a shallower opacity law in this massive YSO than in the interstellar medium, or possibly a sharp drop in the envelope density distribution at distances of similar to1000 AU from the illuminating source.</p
Quicksort Revisited: Verifying Alternative Versions of Quicksort
© Springer International Publishing Switzerland 2016.We verify the correctness of a recursive version of Tony Hoare’s quicksort algorithm using the Hoare-logic based verification tool Dafny. We then develop a non-standard, iterative version which is based on a stack of pivot-locations rather than the standard stack of ranges. We outline an incomplete Dafny proof for the latter
Communicating Shared Resources: A Paradigm for Integrating Real-Time Specification and Implementation
The timed behavior of distributed real-time systems can be specified using a formalism called Communicating Shared Resources, or CSR. The underlying computation model of CSR is resource-based in which multiple resources execute synchronously, while processes assigned to the same resource are interleaved according to their priorities. CSR bridges the gap between an abstract computation model and implementation environments, but is too complex to be treated as a process algebra. We therefore give a calculus for CSR (CCSR), that provides the ability to perform equivalence proofs by syntactic manipulation. We illustrate how a CSR specification can be translated into the CCSR formalism using a periodic timed producer-consumer example, and how a translated CSR specification can be shown correct using syntactic manipulations
Even faster sorting of (not only) integers
In this paper we introduce RADULS2, the fastest parallel sorter based on
radix algorithm. It is optimized to process huge amounts of data making use of
modern multicore CPUs. The main novelties include: extremely optimized
algorithm for handling tiny arrays (up to about a hundred of records) that
could appear even billions times as subproblems to handle and improved
processing of larger subarrays with better use of non-temporal memory stores
The RMS Survey: Ammonia and water maser analysis of massive star forming regions
The Red MSX Source (RMS) survey has identified a sample of ~1200 massive
young stellar objects (MYSOs), compact and ultra compact HII regions from a
sample of ~2000 MSX and 2MASS colour selected sources. We have used the 100 m
Green Bank telescope to search for 22-24 GHz water maser and ammonia (1,1),
(2,2) and (3,3) emission towards ~600 RMS sources located within the northern
Galactic plane. We have identified 308 H2O masers which corresponds to an
overall detection rate of ~50%. Abridged: We detect ammonia emission towards
479 of these massive young stars, which corresponds to ~80%. Ammonia is an
excellent probe of high density gas allowing us to measure key parameters such
as gas temperatures, opacities, and column densities, as well as providing an
insight into the gas kinematics. The average kinetic temperature, FWHM line
width and total NH3 column density for the sample are approximately 22 K, 2
km/s and 2x10^{15} cm^{-2}, respectively. We find that the NH3 (1,1) line width
and kinetic temperature are correlated with luminosity and finding no
underlying dependence of these parameters on the evolutionary phase of the
embedded sources, we conclude that the observed trends in the derived
parameters are more likely to be due to the energy output of the central source
and/or the line width-clump mass relationship. The velocities of the peak H2O
masers and the NH3 emission are in excellent agreement with each other, which
would strongly suggest an association between the dense gas and the maser
emission. Moreover, we find the bolometric luminosity of the embedded source
and the isotropic luminosity of the H2O maser are also correlated. We conclude
from the correlations of the cloud and water maser velocities and the
bolometric and maser luminosity that there is a strong dynamical relationship
between the embedded young massive star and the H2O maser.Comment: 17 pages and 17 figures and 8 tables. Tables\,2 and 5 and full
versions of Figs. 3 and 7 are only available in electronic form at the CDS
via anonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5) or via
http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A
The RMS Survey: Far-Infrared Photometry of Young Massive Stars
Context: The Red MSX Source (RMS) survey is a multi-wavelength campaign of
follow-up observations of a colour-selected sample of candidate massive young
stellar objects (MYSOs) in the galactic plane. This survey is returning the
largest well-selected sample of MYSOs to date, while identifying other dust
contaminant sources with similar mid-infrared colours including a large number
of new ultra-compact (UC)HII regions. Aims:To measure the far-infrared (IR)
flux, which lies near the peak of the spectral energy distribution (SED) of
MYSOs and UCHII regions, so that, together with distance information, the
luminosity of these sources can be obtained. Methods:Less than 50% of RMS
sources are associated with IRAS point sources with detections at 60 micron and
100 micron, though the vast majority are visible in Spitzer MIPSGAL or IRAS
Galaxy Atlas (IGA) images. However, standard aperture photometry is not
appropriate for these data due to crowding of sources and strong spatially
variable far-IR background emission in the galactic plane. A new technique
using a 2-dimensional fit to the background in an annulus around each source is
therefore used to obtain far-IR photometry for young RMS sources.
Results:Far-IR fluxes are obtained for a total of 1113 RMS candidates
identified as young sources. Of these 734 have flux measurements using IGA 60
micron and 100 micron images and 724 using MIPSGAL 70 micron images, with 345
having measurements in both data sets.Comment: 10 pages, 10 figures, 2 Tables, accepted to A&A. A full version of
table 1 is available from the lead author or at the CDS upon publicatio
- …