1,118 research outputs found
Wavelength dependence of angular diameters of M giants: an observational perspective
We discuss the wavelength dependence of angular diameters of M giants from an
observational perspective. Observers cannot directly measure an optical-depth
radius for a star, despite this being a common theoretical definition. Instead,
they can use an interferometer to measure the square of the fringe visibility.
We present new plots of the wavelength-dependent centre-to-limb variation (CLV)
of intensity of the stellar disk as well as visibility for Mira and non-Mira M
giant models. We use the terms ``CLV spectra'' and ``visibility spectra'' for
these plots. We discuss a model-predicted extreme limb-darkening effect (also
called the narrow-bright-core effect) in very strong TiO bands which can lead
to a misinterpretation of the size of a star in these bands. We find no
evidence as yet that this effect occurs in real stars. Our CLV spectra can
explain the similarity in visibilities of R Dor (M8IIIe) that have been
observed recently despite the use of two different passbands. We compare
several observations with models and find the models generally under-estimate
the observed variation in visibility with wavelength. We present CLV and
visibility spectra for a model that is applicable to the M supergiant alpha
Ori.Comment: 16 pages with figures. Accepted by MNRA
Lithium Depletion Boundary in a Pre-Main Sequence Binary System
A lithium depletion boundary is detected in HIP 112312 (GJ 871.1 A and B), a
\~12 Myr old pre-main sequence binary system. A strong (EW 300 mA) Li 6708 A
absorption feature is seen at the secondary (~M4.5) while no Li 6708 A feature
is detected from the primary (~M4). The physical companionship of the two stars
is confirmed from common proper motions. Current theoretical pre-main sequence
evolutionary models cannot simultaneously match the observed colors,
brightnesses, and Li depletion patterns of this binary system. At the age upper
limit of 20 Myr, contemporary theoretical evolutionary models predict too slow
Li depletion. If true Li depletion is a faster process than predicted by
theoretical models, ages of open clusters (Pleiades, alpha Persei, and IC 2391)
estimated from the Li depletion boundary method are all overestimated. Because
of the importance of the open cluster age scale, development of self-consistent
theoretical models to match the HIP 112312 data is desirable.Comment: Accepted in ApJL. 5 pages total (3 tables, 3 figures
SkyMapper and the Southern Sky Survey - a resource for the southern sky
SkyMapper is amongst the first of a new generation of dedicated, wide-field
survey telescopes. The 1.3m SkyMapper telescope features a 5.7 square degree
field-of-view Cassegrain imager and will see first light in late 2007. The
primary goal of the facility is to conduct the Southern Sky Survey a six
colour, six epoch survey of the southern sky. The survey will provide
photometry for objects between 8th and 23rd magnitude with global photometric
accuracy of 0.03 magnitudes and astrometry to 50 mas. This will represent a
valuable scientific resource for the southern sky and in addition provide a
basis for photometric and astrometric calibration of imaging data.Comment: 6 pages, 4 figures, proceedings of ESO Calibration Workshop 200
The nearby eclipsing stellar system delta Velorum - I. Origin of the infrared excess from VISIR and NACO imaging
- Context: The triple stellar system delta Vel system presents a significant
infrared excess, whose origin is still being debated. A large infrared bow
shock has been discovered using Spitzer/MIPS observations. Although it appears
as a significant contributor to the measured IR excess, the possibility exists
that a circumstellar IR excess is present around the stars of the system. -
Aims: The objective of the present VISIR and NACO observations is to identify
whether one of the stars of the delta Vel system presents a circumstellar
photometric excess in the thermal IR domain and to quantify it. - Methods: We
observed delta Vel using the imaging modes of the ESO/VLT instruments VISIR (in
BURST mode) and NACO to resolve the A-B system (0.6" separation) and obtain the
photometry of each star. We also obtained one NACO photometry epoch precisely
at the primary (annular) eclipse of delta Vel Aa by Ab. - Results: Our
photometric measurements with NACO (2.17 mic), complemented by the existing
visible photometry allowed us to reconstruct the spectral energy distribution
of the three stars. We then compared the VISIR photometry (8.6-12.8 mic) to the
expected photospheric emission from the three stars at the corresponding
wavelengths. - Conclusions: We can exclude the presence of a circumstellar
thermal infrared excess around delta Vel A or B down to a few percent level.
This supports the conclusions of Gaspar et al. (2008) that the IR excess of
delta Vel has an interstellar origin, although a cold circumstellar disk could
still be present. In addition, we derive the spectral types of the three stars
Aa, Ab, and B (respectively A2IV, A4V and F8V), and we estimate the age of the
system around 400-500 Myr.Comment: 8 pages, 9 figures, A&A, in pres
Finding RR Lyrae Stars with SkyMapper: an Observational Test
One of the major science goals of the SkyMapper Survey of the Southern
Hemisphere sky is the determination of the shape and extent of the halo of the
Galaxy. In this paper we quantify the likely efficiency and completeness of the
survey as regards the detection of RR Lyrae variable stars, which are excellent
tracers of the halo stellar population. We have accomplished this via
observations of the RR Lyrae-rich globular cluster NGC 3201. We find that for
single epoch uvgri observations followed by two further epochs of g, r imaging,
as per the intended three-epoch survey strategy, we recover known RR Lyraes
with a completeness exceeding 90%. We also investigate boundaries in the
gravity-sensitive single-epoch two-color diagram that yield high completeness
and high efficiency (i.e., minimal contamination by non-RR Lyraes) and the
general usefulness of this diagram in separating populations.Comment: 10 pages, 5 figures, to appear in the Publications of the
Astronomical Society of Australia (PASA), published by Cambridge University
Pres
Low-Mass Pre-Main Sequence Stars in the Large Magellanic Cloud - III: Accretion Rates from HST-WFPC2 Observations
We have measured the present accretion rate of roughly 800 low-mass (~1-1.4
Mo) pre-Main Sequence stars in the field of Supernova 1987A in the Large
Magellanic Cloud (LMC, Z~0.3 Zo). It is the first time that this fundamental
parameter for star formation is determined for low-mass stars outside our
Galaxy. The Balmer continuum emission used to derive the accretion rate
positively correlates with the Halpha excess. Both these phenomena are believed
to originate from accretion from a circumstellar disk so that their
simultaneous detection provides an important confirmation of the pre-Main
Sequence nature of the Halpha and UV excess objects, which are likely to be the
LMC equivalent of Galactic Classical TTauri stars. The stars with statistically
significant excesses are measured to have accretion rates larger than
1.5x10^{-8}Mo/yr at an age of 12-16 Myrs. For comparison, the time scale for
disk dissipation observed in the Galaxy is of the order of 6 Myrs. Moreover,
the oldest Classical TTauri star known in the Milky Way (TW Hydrae, with 10
Myrs of age) has a measured accretion rate of only 5x10^{-10} Mo/yr, ie 30
times less than what we measure for stars at a comparable age in the LMC. Our
findings indicate that metallicity plays a major role in regulating the
formation of low-mass stars.Comment: Accepted for publication in the Astrophysical Journal (10 June 2004),
28 pages, 9 figures. Typo corrected in the abstract on 21 February 200
The Optical Emission from Gamma-ray Quasars
We present photometric observations of six radio-loud quasars that were
detected by the COMPTEL gamma-ray telescope. The data encompasses seven
wavebands in the optical and near-infrared. After correction for Galactic
extinction, we find a wide range in optical slopes. Two sources are as blue as
optically-selected quasars, and are likely to be dominated by the accretion
disc emission, while three others show colours consistent with a red
synchrotron component. We discuss the properties of the COMPTEL sample of
quasars, as well as the implications our observations have for multi-wavelength
modelling of gamma-ray quasars.Comment: 12 pages, 3 figures, accepted for publication in P.A.S.A; minor typos
correcte
Pre-recruitment abundance indices for eastern king prawn, blue swimmer crab and snapper in south-eastern Queensland
Fisheries Queensland scientists conduct an annual beam trawl survey in south-eastern Queensland to collect information on pre-recruit eastern king prawns, blue swimmer crabs and snapper. After a successful pilot study was completed in 2006, the survey commenced in 2007 and has been conducted every year since, except in 2016. The survey provides an independent estimate of pre-recruit abundance for this time series.
This report assessed the analysis methods and mathematical models used to estimate abundance indices for each species, to ensure the results are useful as empirical evidence in species sustainability assessments, periodic stock assessments and for fishery management purposes.
The report identifies the most appropriate mathematical models for estimating abundance indices, where and how the indices are currently used and opportunities to value add to the existing survey into the future
Stock assessment of the Queensland Gulf of Carpentaria Spanish mackerel (Scomberomorus commerson) fishery
Spanish mackerel (Scomberomorus commerson) is a pelagic species that forms genetic populations (stocks) around northern Australia. For this stock assessment, we have investigated fish harvested from eastern waters of the Gulf of Carpentaria, Queensland.
This stock assessment was set up as an age-structured model with a yearly time step and length-based selectivity. The annual data inputs included total fish harvest, standardised catch rates, length structures and conditional age at length data. The model used data from 1940 to 2018.
The assessment suggested the stock in 2018 was between 30 and 40 per cent unfished spawning biomass. The recommended biological catch was estimated at 21 t, which is below the 2018 harvest of 181 t. This initial catch estimate aims to allow the stock to rebuild to the target 60 per cent spawning biomass. Smaller harvests are estimated at first to be taken in early years, which increase through time as the fish population builds higher
Detection of lithium in nearby young late-M dwarfs
Late M-type dwarfs in the solar neighborhood include a mixture of very
low-mass stars and brown dwarfs which is difficult to disentangle due to the
lack of constraints on their age such as trigonometric parallax, lithium
detection and space velocity.
We search for young brown dwarf candidates among a sample of 28 nearby late-M
dwarfs with spectral types between M5.0 and M9.0, and we also search for debris
disks around three of them.
Based on theoretical models, we used the color , the -band absolute
magnitude and the detection of the Li I 6708 doublet line as a strong
constraint to estimate masses and ages of our targets. For the search of debris
disks, we observed three targets at submillimeter wavelength of 850 m.
We report here the first clear detections of lithium absorption in four
targets and a marginal detection in one target. Our mass estimates indicate
that two of them are young brown dwarfs, two are young brown dwarf candidates
and one is a young very low-mass star. The closest young field brown dwarf in
our sample at only 15 pc is an excellent benchmark for further studying
physical properties of brown dwarfs in the range 100150 Myr. We did not
detect any debris disks around three late-M dwarfs, and we estimated upper
limits to the dust mass of debris disks around them.Comment: 10 pages, 5 figures, accepted for publication in Astronomy and
Astrophysic
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