103 research outputs found
Crack detection in a rotating shaft using artificial neural networks and PSD characterisation
Peer reviewedPostprin
A Spitzer view of protoplanetary disks in the gamma Velorum cluster
We present new Spitzer Space Telescope observations of stars in the young ~5
Myr gamma Velorum stellar cluster. Combining optical and 2MASS photometry, we
have selected 579 stars as candidate members of the cluster. With the addition
of the Spitzer mid-infrared data, we have identified 5 debris disks around
A-type stars, and 5-6 debris disks around solar-type stars, indicating that the
strong radiation field in the cluster does not completely suppress the
production of planetesimals in the disks of cluster members. However, we find
some evidence that the frequency of circumstellar primordial disks is lower,
and the IR flux excesses are smaller than for disks around stellar populations
with similar ages. This could be evidence for a relatively fast dissipation of
circumstellar dust by the strong radiation field from the highest mass star(s)
in the cluster. Another possibility is that gamma Velorum stellar cluster is
slightly older than reported ages and the the low frequency of primordial disks
reflects the fast disk dissipation observed at ~5 Myr.Comment: Accepted for publication in the Astrophysical Journal: 32 pages; 11
Figure
Post-AGB stars with hot circumstellar dust: binarity of the low-amplitude pulsators
While the first binary post-AGB stars were serendipitously discovered, the
distinct characteristics of their Spectral Energy Distribution (SED) allowed us
to launch a more systematic search for binaries. We selected post-AGB objects
which show a broad dust excess often starting already at H or K, pointing to
the presence of a gravitationally bound dusty disc in the system. We started a
very extensive multi-wavelength study of those systems and here we report on
our radial velocity and photometric monitoring results for six stars of early F
type, which are pulsators of small amplitude. To determine the radial velocity
of low signal-to-noise time-series, we constructed dedicated auto-correlation
masks. The radial velocity variations were subjected to detailed analysis to
differentiate between pulsational variability and variability due to orbital
motion. Finally orbital minimalisation was performed to constrain the orbital
elements. All of the six objects are binaries, with orbital periods ranging
from 120 to 1800 days. Five systems have non-circular orbits. The mass
functions range from 0.004 to 0.57 solar mass and the companions are likely
unevolved objects of (very) low initial mass. We argue that these binaries must
have been subject to severe binary interaction when the primary was a cool
supergiant. Although the origin of the circumstellar disc is not well
understood, the disc is generally believed to be formed during this strong
interaction phase. The eccentric orbits of these highly evolved objects remain
poorly understood. With the measured orbits and mass functions we conclude that
the circumbinary discs seem to have a major impact on the evolution of a
significant fraction of binary systems.Comment: 13 pages, 15 figures, accepted for Astronomy and Astrophysic
Stellar contents and star formation in the young star cluster Be 59
We present CCD photometry of the young open cluster Be 59 with the
aim to study the star formation scenario in the cluster. The radial extent of
the cluster is found to be 10 arcmin (2.9 pc). The interstellar
extinction in the cluster region varies between 1.4 to 1.8 mag.
The ratio of total-to-selective extinction in the cluster region is estimated
as . The distance of the cluster is found to be kpc.
Using near-infrared colours and slitless spectroscopy, we have identified young
stellar objects (YSOs) in the open cluster Be 59 region. The ages of these YSOs
range between Myr to 2 Myr, whereas the mean age of the massive
stars in the cluster region is found to be 2 Myr. There is evidence for
second generation star formation outside the boundary of the cluster, which may
be triggered by massive stars in the cluster. The slope of the initial mass
function, , in the mass range is found to be
which is shallower than the Salpeter value (-1.35), whereas in
the mass range the slope is almost flat. The slope of
the K-band luminosity function is estimated as , which is smaller
than the average value (0.4) reported for young embedded clusters.
Approximately 32% of H emission stars of Be 59 exhibit NIR excess
indicating that inner disks of the T-Tauri star (TTS) population have not
dissipated. The MSX and IRAS-HIRES images around the cluster region are also
used to study the emission from unidentified infrared bands and to estimate the
spatial distribution of optical depth of warm and cold interstellar dust.Comment: Accepted for the publication in MNRAS, 39 pages, 26 Figures, 8 Table
Broad-band Optical Polarimetric Studies toward the Galactic young star cluster Be 59
We present multiwavelength optical linear polarimetric observations of 69
stars toward the young open cluster Be 59. The observations reveal the presence
of three dust layers located at the distances of \sim300, \sim500 and \sim700
pc. The dust layers produce a total polarization Pv \sim 5.5 per cent. The mean
values of polarization and polarization angles due to the dust layers are found
to increase systematically with distance. We show that polarimetry in
combination with the (U - B) - (B - V) colour-colour diagram yields a better
identification of cluster members. The polarization measurements suggest that
the polarization due the intra-cluster medium is \sim 2.2 per cent. An
anomalous reddening law exists for the cluster region, indicating a relatively
larger grain size than that in the diffuse ISM. The spatial variation of the
polarization and E(B - V) is found to increase with radial distance from the
cluster center, whereas the {\theta}v and {\lambda}max are found to decrease
with increasing radial distance from the cluster center. About 40 per cent of
cluster members show the signatures of either intrinsic polarization or
rotation in their polarization angles. There is an indication that the star
light of the cluster members might have been depolarized because of non-uniform
alignment of dust grains in the foreground dust layers and in the intra-cluster
medium.Comment: 22 pages, 14 figures and 7 tables, Accepted in MNRA
The structure and regulation of the Irish equine industries: Links to considerations of equine welfare
The equine industries in Ireland are vibrant and growing. They are broadly classified into two sectors: Thoroughbred racing, and sports and leisure. This paper describes these sectors in terms of governance, education and training in equine welfare, and available data concerning horse numbers, identification, traceability and disposal. Animal welfare, and specifically equine welfare, has received increasing attention internationally. There is general acceptance of concepts such as animal needs and persons' responsibilities toward animals in their care, as expressed in the 'Five Freedoms'. As yet, little has been published on standards of equine welfare pertaining to Ireland, or on measures to address welfare issues here. This paper highlights the central role of horse identification and legal registration of ownership to safeguard the health and welfare of horses
The Evolutionary Status of Be Stars: Results from a Photometric Study of Southern Open Clusters
Be stars are a class of rapidly rotating B stars with circumstellar disks
that cause Balmer and other line emission. There are three possible reasons for
the rapid rotation of Be stars: they may have been born as rapid rotators, spun
up by binary mass transfer, or spun up during the main-sequence (MS) evolution
of B stars. To test the various formation scenarios, we have conducted a
photometric survey of 55 open clusters in the southern sky. Of these, five
clusters are probably not physically associated groups and our results for two
other clusters are not reliable, but we identify 52 definite Be stars and an
additional 129 Be candidates in the remaining clusters. We use our results to
examine the age and evolutionary dependence of the Be phenomenon. We find an
overall increase in the fraction of Be stars with age until 100 Myr, and Be
stars are most common among the brightest, most massive B-type stars above the
zero-age MS (ZAMS). We show that a spin-up phase at the terminal-age MS (TAMS)
cannot produce the observed distribution of Be stars, but up to 73% of the Be
stars detected may have been spun-up by binary mass transfer. Most of the
remaining Be stars were likely rapid rotators at birth.
Previous studies have suggested that low metallicity and high cluster density
may also favor Be star formation. Our results indicate a possible increase in
the fraction of Be stars with increasing cluster distance from the Galactic
center (in environments of decreasing metallicity). However, the trend is not
significant and could be ruled out due to the intrinsic scatter in our data. We
also find no relationship between the fraction of Be stars and cluster density.Comment: 44 pages, accepted by ApJS (minor changes to introduction, expanded
conclusions per referee comments
The relation between the most-massive star and its parental star cluster mass
We present a thorough literature study of the most-massive star, m_max, in
several young star clusters in order to assess whether or not star clusters are
populated from the stellar initial mass function (IMF) by random sampling over
the mass range 0.01 < m < 150 M_sol without being constrained by the cluster
mass, M_ecl. The data reveal a partition of the sample into lowest mass objects
(M_ecl < 10^2 M_sol), moderate mass clusters (10^2 M_sol < M_ecl < 10^3 M_sol)
and rich clusters above 10^3 M_sol. Additionally, there is a plateau of a
constant maximal star mass (m_max ~ 25 M_sol) for clusters with masses between
10^3 M_sol and 4 10^3 M_sol. Statistical tests of this data set reveal that the
hypothesis of random sampling from the IMF between 0.01 and 150 M_sol is highly
unlikely for star clusters more massive than 10^2 M_sol with a probability of p
~ 2 10^-7 for the objects with M_ecl between 10^2 M_sol and 10^3 M_sol and p ~
3 10^-9 for the more massive star clusters. Also, the spread of m_max values at
a given M_ecl is smaller than expected from random sampling. We suggest that
the basic physical process able to explain this dependence of stellar inventory
of a star cluster on its mass may be the interplay between stellar feedback and
the binding energy of the cluster-forming molecular cloud core. Given these
results, it would follow that an integrated galactic initial mass function
(IGIMF) sampled from such clusters would automatically be steeper in comparison
to the IMF within individual star clusters.Comment: 26 pages, 13 figures, 3 tables, accepted for publication in MNRA
Total and CO-reactive heme content of actinorhizal nodules and the roots of some non-nodulated plants
The concentration of total and CO-reactive heme was measured in actinorhizal nodules from six different genera. This gave the upper limit to hemoglobin concentration in these nodules. Quantitative extraction of CO-reactive heme was achieved under anaerobic conditions in a buffer equilibrated with CO and containing Triton X-100. The concentration of CO-reactive heme in nodules of Casuarina and Myrica was approximately half of that found in legume nodules, whereas in Comptonia, Alnus and Ceanothus the concentrations of heme were about 10 times lower than in legume nodules. There was no detectable CO-reactive heme in Datisca nodules, but low concentrations were detected in roots of all non-nodulating plants examined, including Zea mays . Difference spectra of CO treated minus dithionite-reduced extracts displayed similar wavelengths of maximal and minimal light absorption for all extracts, and were consistent with those of a hemoglobin. The concentration of CO-reactive heme was not correlated to the degree to which CO inhibited nitrogenase activity nor was it affected by reducing the oxygen concentration in the rooting zone. However, there was a positive correlation between heme concentration and suberization or lignification of the walls of infected host cells. These observations demonstrate that, unlike legume nodules, high concentrations of heme or hemoglobin are not needed for active nitrogen fixation in most actinorhizal nodules. Nonetheless, a significant amount of CO-reactive heme is found in the nodules of Alnus, Comptonia, and Ceanothus, and in the roots of Zea mays . The identity and function of this heme is unknown.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43460/1/11104_2006_Article_BF02370943.pd
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