4,767 research outputs found
Mid-Infrared Variability of protostars in IC 1396A
We have used Spitzer/IRAC to conduct a photometric monitoring program of the
IC1396A dark globule in order to study the mid-IR (3.6 - 8 micron) variability
of the heavily embedded Young Stellar Objects (YSOs) present in that area. We
obtained light curves covering a 14 day timespan with a twice daily cadence for
69 YSOs, and continuous light curves with approximately 12 second cadence over
7 hours for 38 YSOs. Typical accuracies for our relative photometry were 1-2%
for the long timespan data and a few mmag, corresponding to less than 0.5%, for
the 7 hour continuous "staring-mode" data. More than half of the YSOs showed
detectable variability, with amplitudes from ~0.05 mag to ~0.2 mag. About
thirty percent of the YSOs showed quasi-sinusoidal light curve shapes with
apparent periods from 5-12 days and light curve amplitudes approximately
independent of wavelength over the IRAC bandpasses. We have constructed models
which simulate the time dependent spectral energy distributions of Class I and
I I YSOs in order to attempt to explain these light curves. Based on these
models, the apparently periodic light curves are best explained by YSO models
where one or two high latitude photospheric spots heat the inner wall of the
circumstellar disk, and where we view the disk at fairly large inclination
angle. Disk inhomogeneities, such as increasing the height where the accretion
funnel flows to the stellar hotspot, enhances the light curve modulations. The
other YSOs in our sample show a range of light curve shapes, some of which are
probably due to varying accretion rate or disk shadowing events. One star,
IC1396A-47, shows a 3.5 hour periodic light curve; this object may be a PMS
Delta Scuti star
A Disk Shadow Around the Young Star ASR 41 in NGC 1333
We present images of the young stellar object ASR 41 in the NGC 1333 star
forming region at the wavelengths of H_alpha and [SII] and in the I, J, H, and
K-bands.
ASR 41 has the near-infrared morphology of an edge-on disk object, but
appears an order of magnitude larger than typical systems of this kind.
We also present detailed models of the scattering and radiative transfer in
systems consisting of a young star surrounded by a proto-planetary disk, and
the whole system being embedded in either an infalling envelope or a uniform
molecular cloud. The best fit to the observed morphology can be achieved with a
disk of approx. 200 AU diameter, immersed in a low density cloud. The low cloud
density is necessary to stay below the sub-mm flux upper limits and to preserve
the shadow cast by the disk via single scattering.
The results demonstrate that ASR 41 is probably not inherently different from
typical edge-on disk objects, and that its large apparent size is due to the
shadow of a much smaller disk being projected into the surrounding dusty
molecular material
The Relationship of Immune Status to the Efficacy of Immunotherapy in Preventing Tumour Recurrence in Mice
The immunotherapeutic value of tumour extracts or B.C.G. in preventing either the occurrence of primary tumours or the recurrence of tumours in surgically resected animals has been examined. A transplantable methylcholanthrene induced tumour in DBA/2J mice was used. Neither tumour extract nor chemically modified extract was effective in preventing tumour growth in immunized animals, even though the mice demonstrated measurable levels of cell mediated tumour immunity at the time of tumour challenge. The frequency of tumour recurrence after resection of small tumours (about 1·0 g) was significantly lowered by treatment of the mice with a combination of B.C.G. and either modified or unmodified tumour extract. The frequency of recurrence after resection of large tumours (about 2·5 g) was not affected by any form of immunotherapy although the survival time of treated animals was significantly prolonged. The immunological status of animals with small and large tumours was examined and it was shown that mice with 1·0 g tumours have unimpaired mitogen responsiveness and measurable tumour specific immunity, whereas mice bearing large tumours (2·5 g) have a markedly impaired immune system
Effect of certain combinations of wall contouring and design exit velocity distribution on prediction of turbine-nozzle mass flow
Herschel Observations of a Newly Discovered UX Ori Star in the Large Magellanic Cloud
The LMC star, SSTISAGE1C J050756.44-703453.9, was first noticed during a
survey of EROS-2 lightcurves for stars with large irregular brightness
variations typical of the R Coronae Borealis (RCB) class. However, the visible
spectrum showing emission lines including the Balmer and Paschen series as well
as many Fe II lines is emphatically not that of an RCB star. This star has all
of the characteristics of a typical UX Ori star. It has a spectral type of
approximately A2 and has excited an H II region in its vicinity. However, if it
is an LMC member, then it is very luminous for a Herbig Ae/Be star. It shows
irregular drops in brightness of up to 2 mag, and displays the reddening and
"blueing" typical of this class of stars. Its spectrum, showing a combination
of emission and absorption lines, is typical of a UX Ori star that is in a
decline caused by obscuration from the circumstellar dust. SSTISAGE1C
J050756.44-703453.9 has a strong IR excess and significant emission is present
out to 500 micron. Monte Carlo radiative transfer modeling of the SED requires
that SSTISAGE1C J050756.44-703453.9 has both a dusty disk as well as a large
extended diffuse envelope to fit both the mid- and far-IR dust emission. This
star is a new member of the UX Ori subclass of the Herbig Ae/Be stars and only
the second such star to be discovered in the LMC.Comment: ApJ, in press. 9 pages, 5 figure
The youngest massive protostars in the Large Magellanic Cloud
We demonstrate the unique capabilities of Herschel to study very young
luminous extragalactic young stellar objects (YSOs) by analyzing a central
strip of the Large Magellanic Cloud obtained through the HERITAGE Science
Demonstration Program. We combine PACS 100 and 160, and SPIRE 250, 350, and 500
microns photometry with 2MASS (1.25-2.17 microns) and Spitzer IRAC and MIPS
(3.6-70 microns) to construct complete spectral energy distributions (SEDs) of
compact sources. From these, we identify 207 candidate embedded YSOs in the
observed region, ~40% never-before identified. We discuss their position in
far-infrared color-magnitude space, comparing with previously studied,
spectroscopically confirmed YSOs and maser emission. All have red colors
indicating massive cool envelopes and great youth. We analyze four example
YSOs, determining their physical properties by fitting their SEDs with
radiative transfer models. Fitting full SEDs including the Herschel data
requires us to increase the size and mass of envelopes included in the models.
This implies higher accretion rates (greater than or equal to 0.0001 M_sun/yr),
in agreement with previous outflow studies of high-mass protostars. Our results
show that Herschel provides reliable longwave SEDs of large samples of
high-mass YSOs; discovers the youngest YSOs whose SEDs peak in Herschel bands;
and constrains the physical properties and evolutionary stages of YSOs more
precisely than was previously possible.Comment: Main text: 4 pages, 3 figures, 1 table; Online material: 3 figures, 1
table; to appear in the A&A Herschel Special Issu
Reconstructing a Simple Polytope from its Graph
Blind and Mani (1987) proved that the entire combinatorial structure (the
vertex-facet incidences) of a simple convex polytope is determined by its
abstract graph. Their proof is not constructive. Kalai (1988) found a short,
elegant, and algorithmic proof of that result. However, his algorithm has
always exponential running time. We show that the problem to reconstruct the
vertex-facet incidences of a simple polytope P from its graph can be formulated
as a combinatorial optimization problem that is strongly dual to the problem of
finding an abstract objective function on P (i.e., a shelling order of the
facets of the dual polytope of P). Thereby, we derive polynomial certificates
for both the vertex-facet incidences as well as for the abstract objective
functions in terms of the graph of P. The paper is a variation on joint work
with Michael Joswig and Friederike Koerner (2001).Comment: 14 page
- …