202 research outputs found
3-D Models of Embedded High-Mass Stars: Effects of a Clumpy Circumstellar Medium
We use 3-D radiative transfer models to show the effects of clumpy
circumstellar material on the observed infrared colors of high mass stars
embedded in molecular clouds. We highlight differences between 3-D clumpy and
1-D smooth models which can affect the interpretation of data. We discuss
several important properties of the emergent spectral energy distribution
(SED): More near-infrared light (scattered and direct from the central source)
can escape than in smooth 1-D models. The near- and mid-infrared SED of the
same object can vary significantly with viewing angle, depending on the clump
geometry along the sightline. Even the wavelength-integrated flux can vary with
angle by more than a factor of two. Objects with the same average circumstellar
dust distribution can have very different near-and mid-IR SEDs depending on the
clump geometry and the proximity of the most massive clump to the central
source.
Although clumpiness can cause similar objects to have very different SEDs,
there are some observable trends. Near- and mid-infrared colors are sensitive
to the weighted average distance of clumps from the central source and to the
magnitude of clumpy density variations (smooth-to-clumpy ratio). Far-infrared
emission remains a robust measure of the total dust mass. We present simulated
SEDs, colors, and images for 2MASS and Spitzer filters. We compare to
observations of some UCHII regions and find that 3-D clumpy models fit better
than smooth models. In particular, clumpy models with fractal dimensions in the
range 2.3-2.8, smooth to clumpy ratios of <50%, and density distributions with
shallow average radial density profiles fit the SEDs best.Comment: accepted to ApJ; version with full-res figures:
http://www.astro.virginia.edu/~ri3e/clumpy3d.pd
Radio Astronomical Polarimetry and the Lorentz Group
In radio astronomy the polarimetric properties of radiation are often
modified during propagation and reception. Effects such as Faraday rotation,
receiver cross-talk, and differential amplification act to change the state of
polarized radiation. A general description of such transformations is useful
for the investigation of these effects and for the interpretation and
calibration of polarimetric observations. Such a description is provided by the
Lorentz group, which is intimately related to the transformation properties of
polarized radiation. In this paper the transformations that commonly arise in
radio astronomy are analyzed in the context of this group. This analysis is
then used to construct a model for the propagation and reception of radio
waves. The implications of this model for radio astronomical polarimetry are
discussed.Comment: 10 pages, accepted for publication in Astrophysical Journa
Mid-infrared interferometry of the massive young stellar object NGC3603 - IRS 9A
We present observations and models for one of these MYSO candidates, NGC3603
IRS 9A. Our goal is to investigate with infrared interferometry the structure
of IRS 9A on scales as small as 200AU, exploiting the fact that a cluster of O
and B stars has blown away much of the obscuring foreground dust and gas.
Observations in the N-band were carried out with the MIDI beam combiner
attached to the VLTI. Additional interferometric observations which probe the
structure of IRS 9A on larger scales were performed with an aperture mask
installed in the T-ReCS instrument of Gemini South. The spectral energy
distribution (SED) is constrained by the MIDI N-band spectrum and by data from
the Spitzer Space Telescope. Our efforts to model the structure and SED of IRS
9A range from simple geometrical models of the brightness distribution to one-
and two-dimensional radiative transfer computations. The target is resolved by
T-ReCS, with an equivalent (elliptical) Gaussian width of 330mas by 280mas
(2300 AU by 2000 AU). Despite this fact, a warm compact unresolved component
was detected by MIDI which is possibly associated with the inner regions of a
flattened dust distribution. Based on our interferometric data, no sign of
multiplicity was found on scales between about 200AU and 700AU projected
separation. A geometric model consisting of a warm (1000 K) ring (400 AU
diameter) and a cool (140 K) large envelope provides a good fit to the data. No
single model fitting all visibility and photometric data could be found, with
disk models performing better than spherical models. While the data are clearly
inconsistent with a spherical dust distribution they are insufficient to prove
the existence of a disk but rather hint at a more complex dust distribution.Comment: 8 pages, 11 figures. Accepted for publication in A&
The circumstellar disc in the Bok globule CB 26: Multi-wavelength observations and modelling of the dust disc and envelope
Circumstellar discs are expected to be the nursery of planets. Grain growth
within such discs is the first step in the planet formation process. The Bok
globule CB 26 harbours such a young disc. We present a detailed model of the
edge-on circumstellar disc and its envelope in the Bok globule CB 26. The model
is based on HST near-infrared maps in the I, J, H, and K bands, OVRO and SMA
radio maps at 1.1mm, 1.3mm and 2.7mm, and the spectral energy distribution
(SED) from 0.9 microns to 3mm. New photometric and spectroscopic data from the
Spitzer Space Telescope and the Caltech Submilimeter Observatory have been
obtained and are part of our analysis. Using the self-consistent radiative
transfer code MC3D, the model we construct is able to discriminate parameter
sets and dust properties of both its parts, namely envelope and disc. We find
that the disc has an inner hole with a radius of 45 +/- 5 AU. Based on a dust
model including silicate and graphite the maximum grain size needed to
reproduce the spectral millimetre index is 2.5 microns. Features seen in the
near-infrared images, dominated by scattered light, can be described as a
result of a rotating envelope. Successful employment of ISM dust in both the
disc and envelope hint that grain growth may not yet play a significant role
for the appearance of this system. A larger inner hole gives rise to the
assumption that CB 26 is a circumbinary disc.Comment: 18 pages, 15 figures, Accepted for publication in A&
Infrared Signatures of Protoplanetary Disk Evolution
We investigate the observational signatures of a straightforward evolutionary
scenario for protoplanetary disks in which the disk mass of small (50 micron)
particles decreases homologously with time, but the disk structure and stellar
parameters do not change. Our goal is to identify optimal infrared spectral
indicators of the existence of disks, their structure, and mass evolution that
may be tested with the upcoming SIRTF mission. We present simulated spectral
energy distributions and colors over a wide range of masses. The SED is most
sensitive to disk mass in the far-IR and longer wavelengths, which is already
known from millimeter and radio observations. As the disk mass decreases, the
excess emission of the disk over the stellar photosphere diminishes more
rapidly at the longest rather than at short wavelengths. At near-infrared
wavelengths, the disk remains optically thick to stellar radiation over a wide
range of disk mass, resulting in a slower decline in the SED in this spectral
regime. Therefore, near-IR excesses (K-L) provide a robust means of detecting
disks in star clusters down to 1E-7 solar masses, while the far-IR excess
probes the disk mass. Reducing the disk mass results in a clear progression in
color-color diagrams with low mass disks displaying the bluest colors. We
interpret color-color diagrams for Taurus-Auriga sources in the context of
decreasing disk mass.Comment: ApJ Accepte
Radiatively heated, protoplanetary discs with dead zones. I. Dust settling and thermal structure of discs around M stars
The irradiation of protoplanetary discs by central stars is the main heating
mechanism for discs, resulting in their flared geometric structure. In a series
of papers, we investigate the deep links between 2D self-consistent disc
structure and planetary migration in irradiated discs, focusing particularly on
those around M stars. In this first paper, we analyse the thermal structure of
discs that are irradiated by an M star by solving the radiative transfer
equation by means of a Monte Carlo code. Our simulations of irradiated
hydrostatic discs are realistic and self-consistent in that they include dust
settling with multiple grain sizes (N=15), the gravitational force of an
embedded planet on the disc, and the presence of a dead zone (a region with
very low levels of turbulence) within it. We show that dust settling drives the
temperature of the mid-plane from an distribution (well mixed dust
models) toward an . The dead zone, meanwhile, leaves a dusty wall at
its outer edge because dust settling in this region is enhanced compared to the
active turbulent disc at larger disc radii. The disc heating produced by this
irradiated wall provides a positive gradient region of the temperature in the
dead zone in front of the wall. This is crucially important for slowing
planetary migration because Lindblad torques are inversely proportional to the
disc temperature. Furthermore, we show that low turbulence of the dead zone is
self-consistently induced by dust settling, resulting in the Kelvin-Helmholtz
instability (KHI). We show that the strength of turbulence arising from the KHI
in the dead zone is .Comment: 19 pages, 20 figures, 3 tables, accepted for publication in MNRA
Neural correlates of enhanced visual short-term memory for angry faces: An fMRI study
Copyright: © 2008 Jackson et al.Background: Fluid and effective social communication requires that both face identity and emotional expression information are encoded and maintained in visual short-term memory (VSTM) to enable a coherent, ongoing picture of the world and its players. This appears to be of particular evolutionary importance when confronted with potentially threatening displays of emotion - previous research has shown better VSTM for angry versus happy or neutral face identities.Methodology/Principal Findings: Using functional magnetic resonance imaging, here we investigated the neural correlates of this angry face benefit in VSTM. Participants were shown between one and four to-be-remembered angry, happy, or neutral faces, and after a short retention delay they stated whether a single probe face had been present or not in the previous display. All faces in any one display expressed the same emotion, and the task required memory for face identity. We find enhanced VSTM for angry face identities and describe the right hemisphere brain network underpinning this effect, which involves the globus pallidus, superior temporal sulcus, and frontal lobe. Increased activity in the globus pallidus was significantly correlated with the angry benefit in VSTM. Areas modulated by emotion were distinct from those modulated by memory load.Conclusions/Significance: Our results provide evidence for a key role of the basal ganglia as an interface between emotion and cognition, supported by a frontal, temporal, and occipital network.The authors were supported by a Wellcome Trust grant (grant number 077185/Z/05/Z) and by BBSRC (UK) grant BBS/B/16178
Trisomy 21 induces pericentrosomal crowding delaying primary ciliogenesis and mouse cerebellar development.
Trisomy 21, the genetic cause of Down syndrome, disrupts primary cilia formation and function, in part through elevated Pericentrin, a centrosome protein encoded on chromosome 21. Yet how trisomy 21 and elevated Pericentrin disrupt cilia-related molecules and pathways, and the in vivo phenotypic relevance remain unclear. Utilizing ciliogenesis time course experiments combined with light microscopy and electron tomography, we reveal that chromosome 21 polyploidy elevates Pericentrin and microtubules away from the centrosome that corral MyosinVA and EHD1, delaying ciliary membrane delivery and mother centriole uncapping essential for ciliogenesis. If given enough time, trisomy 21 cells eventually ciliate, but these ciliated cells demonstrate persistent trafficking defects that reduce transition zone protein localization and decrease sonic hedgehog signaling in direct anticorrelation with Pericentrin levels. Consistent with cultured trisomy 21 cells, a mouse model of Down syndrome with elevated Pericentrin has fewer primary cilia in cerebellar granule neuron progenitors and thinner external granular layers at P4. Our work reveals that elevated Pericentrin from trisomy 21 disrupts multiple early steps of ciliogenesis and creates persistent trafficking defects in ciliated cells. This pericentrosomal crowding mechanism results in signaling deficiencies consistent with the neurological phenotypes found in individuals with Down syndrome
Star formation in Perseus: II. SEDs, classification and lifetimes
Working with the submillimetre continuum map of the Perseus molecular cloud
(Hatchell et al. 2005), we aimed to determine the evolutionary stage of each
submm core in Perseus, and investigate the lifetimes of these phases. We
compile spectral energy distributions (SEDs) from 2MASS, Spitzer IRAC,
Michelle, IRAS, SCUBA and Bolocam data. Sources are classified
starless/protostellar on the basis of infrared and/or outflow detections and
Class I/Class 0 on the basis of Tbol, Lbol/Lsmm and F_{3.6}/F_{850}. In order
to investigate the dependence of these evolutionary indicators on mass, we
construct radiative transfer models of Class 0 sources. Of the submm cores,
56/103 (54%) are confirmed protostars on the basis of infrared emission or
molecular outflows. Of these, 22 are classified Class 1 on the basis of three
evolutionary indicators, 34 are Class 0, and the remaining 47 are assumed
starless. Perseus contains a much greater fraction of Class 0 sources than
either Taurus or Rho Oph. Comparing the protostellar with the T Tauri
population, the lifetime of the protostellar phase in Perseus is 0.25-0.67 Myr
(95% confidence limits). The relative lifetime of the Class 0 and Class 1
phases are similar. We find that for the same source geometry but different
masses, evolutionary indicators such as Tbol vary their value. It is therefore
not always appropriate to use a fixed threshold to separate Class 0 and Class I
sources. More modelling is required to determine the observational
characteristics of the Class 0/Class I boundary over a range of masses.Comment: A&A accepted. 35 pages, 24 figures. Version with original figures
available at http://www.astro.ex.ac.uk/people/hatchell/publications.htm
Quantum Tomography under Prior Information
We provide a detailed analysis of the question: how many measurement settings
or outcomes are needed in order to identify a quantum system which is
constrained by prior information? We show that if the prior information
restricts the system to a set of lower dimensionality, then topological
obstructions can increase the required number of outcomes by a factor of two
over the number of real parameters needed to characterize the system.
Conversely, we show that almost every measurement becomes informationally
complete with respect to the constrained set if the number of outcomes exceeds
twice the Minkowski dimension of the set. We apply the obtained results to
determine the minimal number of outcomes of measurements which are
informationally complete with respect to states with rank constraints. In
particular, we show that 4d-4 measurement outcomes (POVM elements) is enough in
order to identify all pure states in a d-dimensional Hilbert space, and that
the minimal number is at most 2 log_2(d) smaller than this upper bound.Comment: v3: There was a mistake in the derived finer upper bound in Theorem
3. The corrected upper bound is +1 to the earlier versio
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