149 research outputs found
Ultra stable frequency distribution system
A system is presented for synchronizing a signal at a remotely located slave station with the phase and frequency of a signal generated at a master station. The signal transmitted at the master station and received by the slave station provides compensation for the phase shift caused by the transmission path delays between the master and slave station. The slave station transmits a signal to the master station at a frequency that is different from the frequency of the signal being transmitted by the master station. The signal transmitted by the slave station is received by the master station while the master station transmitter is off. The signal transmitted by the master station is received by the slave station while the slave station transmitter is off
Carbon stars in the IRTS survey
We have identified 139 cool carbon stars in the near-infrared
spectro-photometric survey of the InfraRed Telescope in Space (IRTS) from the
conspicuous presence of molecular absorption bands at 1.8, 3.1 and 3.8 microns.
Among them 14 are new, bright (K ~ 4.0-7.0), carbon stars. We find a trend
relating the 3.1 microns band strength to the K-L' color index, which is known
to correlate with mass-loss rate. This could be an effect of a relation between
the depth of the 3.1 microns feature and the degree of development of the
extended stellar atmosphere where dust starts to form.Comment: accepted by the PASP; December 7, 200
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
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The occultation events of the Herbig Ae/Be star V1247 Ori
Aims: I study new deep (DeltaV ~ 1.20-1.65 mag) occultation events of the
delta Scuti, Herbig Ae/Be star V1247 Ori in the Ori OB1 b association. Methods:
I use the V-band ASAS light curve of V1247 Ori, which covers the last nine
years, together with photometric data in the near-ultraviolet, visible, near-,
and far-infrared taken from the literature. I carry out a periodogram analysis
of the "cleaned" light curve and construct the spectral energy distribution of
the star. Results: The star V1247 Ori is interesting for the study of the UX
Orionis phenomenon, in which Herbig Ae/Be stars are occulted by their
protoplanetary discs, for three reasons: brightness (V ~ 9.85 mag), large
infrared excess at 20-100 mum (F_60 ~ 10 Jy), and photometric stability out of
occultation (sigma(V) ~ 0.02 mag), which may help to determine the location and
spatial structure of the occulting disc clumps.Comment: A&A Letters, in pres
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
Investigating potential planetary nebula/cluster pairs
Fundamental parameters characterizing the end-state of intermediate-mass
stars may be constrained by discovering planetary nebulae (PNe) in open
clusters (OCs). Cluster membership may be exploited to establish the distance,
luminosity, age, and physical size for PNe, and the intrinsic luminosity and
mass of its central star. Four potential PN-OC associations were investigated,
to assess the cluster membership for the PNe. Radial velocities were measured
from intermediate-resolution optical spectra, complemented with previous
estimates in the literature. When the radial velocity study supported the PN/OC
association, we analyzed if other parameters (e.g., age, distance, reddening,
central star brightness) were consistent with this conclusion. Our measurements
imply that the PNe VBe3 and HeFa1 are not members of the OCs NGC5999 and
NGC6067, respectively, and likely belong to the background bulge population.
Conversely, consistent radial velocities indicate that NGC2452/NGC2453 could be
associated, but our results are not conclusive and additional observations are
warranted. Finally, we demonstrate that all the available information point to
He2-86 being a young, highly internally obscured PN member of NGC4463. New
near-infrared photometry acquired via the Vista Variables in the Via Lactea ESO
public survey was used in tandem with existing UBV photometry to measure the
distance, reddening, and age of NGC4463, finding d=1.55+-0.10 kpc,
E(B-V)=0.41+-0.02, and tau=65+-10 Myr, respectively. The same values should be
adopted for the PN if the proposed cluster membership will be confirmed.Comment: Accepted for publication in A&
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 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 Origin of Massive O-type Field Stars. Part I: A Search for Clusters
We present a study aimed at clarifying the birthplace for 43 massive O-type
field stars. In this first paper we present the observational part: a search
for stellar clusters near the target stars. We derive stellar density maps at
two different resolving scales, viz. 0.25pc and 1.0pc from NTT and TNG imaging
and the 2MASS catalogue. These scales are typical for cluster sizes. The main
result is that the large majority of the O-type field population are isolated
stars: only 12% (5 out of 43) of the O-type field stars is found to harbour a
small-scale stellar cluster. We review the literature and aim at characterizing
the stellar field of each O-type field star with the emphasis on star formation
and the presence of known young stellar clusters. An analysis of the result of
this paper and a discussion of the O-type field population as products of a
dynamical ejection event is presented in an accompanying paper.Comment: 22 pages, accepted for publication in A&
Analysis of Galactic late-type O dwarfs: more constraints on the weak wind problem
We have investigated the stellar and wind properties of a sample of five
late-type O dwarfs in order to address the weak wind problem. A grid of TLUSTY
models was used to obtain the stellar parameters, and the wind parameters were
determined by using the CMFGEN code. We found that the spectra have mainly a
photospheric origin. A weak wind signature is seen in CIV 1549, from where
mass-loss rates consistent with previous CMFGEN results regarding O8-9V stars
were obtained. A discrepancy of roughly 2 orders of magnitude is found between
these mass-loss rates and the values predicted by theory (Mdot(Vink)),
confirming a breakdown or a steepening of the modified wind momentum-luminosity
relation at log L/Lsun < 5.2. We have estimated the carbon abundance for the
stars of our sample and concluded that its uncertainty cannot cause the weak
wind problem. Upper limits on Mdot were established for all objects using lines
of different ions, namely, PV 1118,28, CIII 1176, NV 1239,43, Si IV 1394,03,
and NIV 1718. All the values obtained are also in disagreement with theoretical
predictions, bringing support to the reality of weak winds. Together with CIV
1549, the use of NV 1239,43 results in the lowest mass-loss rates: the upper
limits indicate that Mdot must be less than about -1.0 dex Mdot(Vink).
Regarding the other transitions, the upper limits still point to low rates:
Mdot must be less than about dex Mdot(Vink). We have studied
the behavior of the Halpha line with different mass-loss rates. We have also
explored ways to fit the observed spectra with Mdot(Vink). By using large
amounts of X-rays, we verified that few wind emissions take place, as in weak
winds. However, unrealistic X-rays luminosities had to be used (log Lx/Lbol >
-3.5) (abridged).Comment: A&A, accepte
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