1,089 research outputs found
Evolution and appearance of Be stars in SMC clusters
Star clusters are privileged laboratories for studying the evolution of
massive stars (OB stars). One particularly interesting question concerns the
phases, during which the classical Be stars occur, which unlike HAe/Be stars,
are not pre-main sequence objects, nor supergiants. Rather, they are extremely
rapidly rotating B-type stars with a circumstellar decretion disk formed by
episodic ejections of matter from the central star. To study the impact of
mass, metallicity, and age on the Be phase, we observed SMC open clusters with
two different techniques: 1) with the ESO-WFI in its slitless mode, which
allowed us to find the brighter Be and other emission-line stars in 84 SMC open
clusters 2) with the VLT-FLAMES multi-fiber spectrograph in order to determine
accurately the evolutionary phases of Be stars in the Be-star rich SMC open
cluster NGC 330. Based on a comparison to the Milky Way, a model of Be stellar
evolution / appearance as a function of metallicity and mass / spectral type is
developed, involving the fractional critical rotation rate as a key parameter.Comment: Proceedings of the IAUS266 of the GA200
A hydrodynamic study of the circumstellar envelope of alpha Scorpii
Context: Both the absolute mass-loss rates and the mechanisms that drive the
mass loss of late-type supergiants are still not well known. Binaries such as
alpha Sco provide the most detailed empirical information about the winds of
these stars.
Aims: The goal was to improve the binary technique for the determination of
the mass-loss rate of alpha Sco A by including a realistic density distribution
and velocity field from hydrodynamic and plasma simulations.
Methods: We performed 3D hydrodynamic simulations of the circumstellar
envelope of alpha Sco in combination with plasma simulations accounting for the
heating, ionization, and excitation of the wind by the radiation of alpha Sco
B. These simulations served as the basis for an examination of circumstellar
absorption lines in the spectrum of alpha Sco B as well as of emission lines
from the Antares nebula.
Results: The present model of the extended envelope of alpha Sco reproduces
some of the structures that were observed in the circumstellar absorption lines
in the spectrum of alpha Sco B. Our theoretical density and velocity
distributions of the outflow deviate considerably from a spherically expanding
model, which was used in previous studies. This results in a higher mass-loss
rate of (2 +/- 0.5) x 10^-6 M_sun/yr. The hot H II region around the secondary
star induces an additional acceleration of the wind at large distances from the
primary, which is seen in absorption lines of Ti II and Cr II at -30 km/s.Comment: 12 pages, 14 figures, accepted for publication in A&
An improved sampling tube for in-duct fan sound measurement
A number of researchers have shown that the currently available commercial sampling tube (microphone turbulence screen) suffers from excessive self-noise, poor turbulence rejection and non-smooth frequency response. This paper describes the development of an improved sampling tube by Baade. In particular, it discusses the difficulties encountered by Halvarsson and Davy when measuring the pure tone frequency response of sampling tubes in an anechoic room. This research is still in progress, but results to date are presented. It is planned that the design resulting from this research will be included in ASHRAE Standard 68 and ISO 5136
A Sward Based Method to Estimate Herbage Selection of Grazing Dairy Cows
Diet selection of grazing animals is influenced by sward composition and vertical sward structure. Grazing studies were established in northern Germany (Kiel, Schleswig - Holstein state) to determine if selective grazing behaviour in a mixed sward can be measured by a sward based method. The hypothesis that active selection of different functional groups of forages can be documented by using the selection index (Figure 1, Hodgson, 1990) and regressive approaches vs time was tested
A spectropolarimetric view on the nature of the peculiar Type I SN 2005hk
We report two spectropolarimetric observations of SN 2005hk, which is a close
copy of the "very peculiar" SN 2002cx, showing low peak luminosity, slow
decline, high ionization near peak and an unusually low expansion velocity of
only about 7,000 km s^-1. Further to the data presented by Chornock et al.,
(2006), at -4 days before maximum, we present data of this object taken on 9
November 2005 (near maximum) and 23 November (+ two weeks) that show the
continuum and most of the spectral lines to be polarized at levels of about
0.2-0.3%. At both epochs the data corresponds to the Spectropolarimetric Type
D1. The general low level of line polarization suggests that the line forming
regions for most species observed in the spectrum have a similar shape to that
of the photosphere, which deviates from a spherical symmetry by <10%. In
comparison with spectropolarimetry of Type Ia and Core-collapse SNe at similar
epochs, we find that the properties of SN 2005hk are most similar to those of
Type Ia SNe. In particular, we find the low levels of continuum and line
polarization to indicate that the explosion mechanism is approximately
spherical, with homogeneous ejecta (unlike the chemically segregated ejecta of
CCSNe). We discuss the possibility that SN 2005hk was the result of the pure
deflagration of a white dwarf and note the issues concerning this
interpretation.Comment: ApJ accepted, uses emulateapj, 16 pages, 10 figures, figures 3 and 4
update
Spectropolarimetry of the Type Ia SN 2007sr Two Months After Maximum Light
We present late time spectropolarimetric observations of SN 2007sr, obtained
with the VLT telescope at ESO Paranal Observatory when the object was 63 days
after maximum light. The late time spectrum displays strong line polarization
in the CaII absorption features. SN 2007sr adds to the case of some normal Type
Ia SNe that show high line polarization or repolarization at late times, a fact
that might be connected with the presence of high velocity features at early
times
Not in my back yard! Sports stadia location and the property market
In recent years sports stadia have been built in the UK, not only for their intended sporting purpose but with the twin aim of stimulating economic and physical regeneration. However, proposals to locate stadia in urban areas often prompt a negative reaction from local communities, fearing a decline in property prices. This paper will use a case study of the Millennium Stadium in Cardiff and the City of Manchester Stadium to illustrate that in contrast to this widely held belief, sports stadia can actually enhance the value of residential property. Furthermore, it will argue that stadia also contribute indirectly to property value through the creation of pride, confidence and enhanced image of an area.</p
The Asymptotic Giant Branch of NGC 205: The Characteristics of Carbon Stars and M Giants Identified from JHK' Images
J, H, and K' images are used to investigate the asymptotic giant branch (AGB)
content of the Local Group dwarf elliptical galaxy NGC 205. The AGB on the (K,
H-K) and (K, J-K) color-magnitude diagrams consists of two sequences: a
near-vertical plume of giants with spectral types K and M, and a red arm
containing C stars. There are 320 C stars with M_bol 1.5
within 2 arcmin of the nucleus. C stars account for 10% of the integrated
luminosity of AGB stars brighter than M_bol = -3.75 near the center of NGC 205,
and this is in excellent agreement with what is measured in intermediate-age
clusters in the LMC. The most luminous AGB star has M_bol = -6.5, although
variability introduces an uncertainty of a few tenths of a magnitude when using
this as an estimate of the AGB-tip brightness. Comparisons with models suggest
that the brightest AGB stars formed within the past 0.1 Gyr, and that the
previous episode of star formation occured a few tenths of a Gyr earlier. These
results are consistent with star formation in NGC 205 being triggered by
interactions with M31. These data also demonstrate that near-infrared imaging
provides an efficient means of identifying C stars in nearby galaxies.Comment: 16 pages of text; 8 postscript figures; will appear in Ap
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