1,993 research outputs found
Long secondary periods in variable red giants
âThe definitive version is available at www3.interscience.wiley.com '. Copyright Royal Astronomical Society. DOI: 10.1111/j.1365-2966.2009.15401.xWe present a study of a sample of Large Magellanic Cloud red giants exhibiting Long Secondary Periods (LSPs). We use radial velocities obtained from VLT spectral observations and MACHO and OGLE light curves to examine properties of the stars and to evaluate models for the cause of LSPs. This sample is much larger than the combined previous studies of Hinkle et al. and Wood, Olivier & Kawaler. Binary and pulsation models have enjoyed much support in recent years. Assuming stellar pulsation, we calculate from the velocity curves that the typical fractional radius change over an LSP cycle is greater than 30 per cent. This should lead to large changes in Teff that are not observed. Also, the small light amplitude of these stars seems inconsistent with the radius amplitude. We conclude that pulsation is not a likely explanation for the LSPs. The main alternative, physical movement of the star â binary motion â also has severe problems. If the velocity variations are due to binary motion, the distribution of the angle of periastron in our large sample of stars has a probability of 1.4 Ă 10â3 that it comes from randomly aligned binary orbits. In addition, we calculate a typical companion mass of 0.09 Mâ . Less than 1 per cent of low-mass main-sequence stars have companions near this mass (0.06â0.12 Mâ) whereas âŒ25â50 per cent of low-mass red giants end up with LSPs. We are unable to find a suitable model for the LSPs and conclude by listing their known properties.Peer reviewe
Variation in Environmental Parameters in Research and Aquaculture: Effects on Behaviour, Physiology and Cell Biology of Teleost Fish
Over the last few years the increasing use of fish as animal models in scientific research and the increased fish breeding for human consumption have stressed the need for more knowledge on the effect of variations in environmental parameters on fish biology and on the welfare of specimens used both in research and aquaculture contexts. Experimental evidence shows that environmental variations can affect fish biology at various levels, from the molecular to that of the population, sometimes in a different way depending on the species considered. In order to achieve reproducible results in experiments involving fish it is necessary to set and maintain all environmental parameters constant at the optimal value to guarantee the wellness of the animal. The effects of the variation in environmental parameters on the behaviour, physiology and cell biology of teleosts are here discussed in order to provide useful information for research based on fish models
Three aspects of red giant studies in the Magellanic Clouds
There are three important aspects concerning the study of the red giant and
in particular of the asymptotic giant branch (AGB) stars in the Magellanic
Clouds. These are: the surface distribution, the luminosity function and the
variability. The spatial distribution of AGB stars is an efficient tool to
study the structure of the galaxies and their metalicity by analysing the ratio
between carbon- and oxygen-rich AGB stars. The shape of the luminosity function
carries informations about the star formation rate in the Clouds and it can be
mathematically related to their history. Most AGB stars vary their magnitude in
a few to several hundred years time; the one epoch DENIS magnitudes for both
Large and Small Magellanic Cloud AGB stars outline the same relations as a
function of period.Comment: 8 pages, 6 figures, invited talk, to be published in: Mass-Losing
Pulsating Stars and their Circumstellar Matter, Y. Nakada & M. Honma (eds)
Kluwer ASSL serie
The AGB population of NGC 6822: distribution and the C/M ratio from JHK photometry
NGC 6822 is an irregular dwarf galaxy and part of the Local Group. Its close
proximity and apparent isolation provide a unique opportunity to study galactic
evolution without any obvious strong external influences. This paper aims to
study the spatial distribution of the asymptotic giant branch (AGB) population
and metallicity in NGC 6822. Using deep, high quality JHK photometry, taken
with WFCAM on UKIRT, carbon- and oxygen-rich AGB stars have been isolated. The
ratio between their number, the C/M ratio, has then been used to derive the
[Fe/H] abundance across the galaxy. The tip of the red giant branch is located
at K0 = 17.41 \pm 0.11 mag and the colour separation between carbon- and
oxygen-rich AGB stars is at (J - K)0 = 1.20 \pm 0.03 mag (i.e. (J - K)2MAS S
{\guillemotright} 1.28 mag). A C/M ratio of 0.62 \pm 0.03 has been derived in
the inner 4 kpc of the galaxy, which translates into an iron abundance of
[Fe/H] = -1.29\pm0.07 dex. Variations of these parameters were investigated as
a function of distance from the galaxy centre and azimuthal angle. The AGB
population of NGC 6822 has been detected out to a radius of 4 kpc giving a
diameter of 56 arcmin. It is metal-poor, but there is no obvious gradient in
metallicity with either radial distance from the centre or azimuthal angle. The
detected spread in the TRGB magnitude is consistent with that of a galaxy
surrounded by a halo of old stars. The C/M ratio has the potential to be a very
useful tool for the determination of metallicity in resolved galaxies but a
better calibration of the C/M vs. [Fe/H] relation and a better understanding of
the sensitivities of the C/M ratio to stellar selection criteria is first
required
On the usefulness of finding charts Or the runaway carbon stars of the Blanco & McCarthy field 37
We have been recently faced with the problem of cross--identifying stars
recorded in historical catalogues with those extracted from recent fully
digitized surveys (such as DENIS and 2MASS). Positions mentioned in the old
catalogues are frequently of poor precision, but are generally accompanied by
finding charts where the interesting objects are flagged. Those finding charts
are sometimes our only link with the accumulated knowledge of past literature.
While checking the identification of some of these objects in several
catalogues, we had the surprise to discover a number of discrepancies in recent
works.The main reason for these discrepancies was generally the blind
application of the smallest difference in position as the criterion to identify
sources from one historical catalogue to those in more recent surveys. In this
paper we give examples of such misidentifications, and show how we were able to
find and correct them.We present modern procedures to discover and solve
cross--identification problems, such as loading digitized images of the sky
through the Aladin service at CDS, and overlaying entries from historical
catalogues and modern surveys. We conclude that the use of good finding charts
still remains the ultimate (though time--consuming) tool to ascertain
cross--identifications in difficult cases.Comment: 4 pages, 1 figure, accepted by A&
The metallicity gradient as a tracer of history and structure : the Magellanic Clouds and M33 galaxies
Original article can be found at: http://www.aanda.org/ Copyright The European Southern Observatory (ESO) DOI: 10.1051/0004-6361/200912138Context. The stellar metallicity and its gradient place constraints on the formation and evolution of galaxies. Aims. This is a study of the metallicity gradient of the LMC, SMC and M33 galaxies derived from their asymptotic giant branch (AGB) stars. Methods. The [Fe/H] abundance was derived from the ratio between C- and M-type AGB stars and its variation analysed as a function of galactocentric distance. Galaxy structure parameters were adopted from the literature. Results. The metallicity of the LMC decreases linearly as â0.047±0.003 dex kpcâ1 out to âŒ8 kpc from the centre. In the SMC, [Fe/H] has a constant value of âŒâ1.25 ± 0.01 dex up to âŒ12 kpc. The gradient of the M33 disc, until âŒ9 kpc, is â0.078 ± 0.003 dex kpcâ1 while the outer disc/halo, out to âŒ25 kpc, has [Fe/H] ⌠â1.7 dex. Conclusions. The metallicity of the LMC, as traced by different populations, bears the signature of two major star forming episodes: the first one constituting a thick disc/halo population and the second one a thin disc and bar due to a close encounter with the Milky Way and SMC. The [Fe/H] of the recent episode supports an LMC origin for the Stream. The metallicity of the SMC supports star formation, âŒ3 Gyr ago, as triggered by LMC interaction and sustained by the bar in the outer region of the galaxy. The SMC [Fe/H] agrees with the present-day abundance in the Bridge and shows no significant gradient. The metallicity of M33 supports an âinsideoutâ disc formation via accretion of metal poor gas from the interstellar medium.Peer reviewe
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