83 research outputs found
Oscillatory convective modes in red giants: a possible explanation of the long secondary periods
We discuss properties of oscillatory convective modes in low-mass red giants,
and compare them with observed properties of the long secondary periods (LSPs)
of semi-regular red giant variables. Oscillatory convective modes are very
nonadiabatic g modes and they are present in luminous stars, such as red
giants with \log L/{\rm L}_\odot \ga 3. Finite amplitudes for these modes are
confined to the outermost nonadiabatic layers, where the radiative energy flux
is more important than the convective energy flux. The periods of oscillatory
convection modes increase with luminosity, and the growth times are comparable
to the oscillation periods. The LSPs of red giants in the Large Magellanic
Cloud (LMC) are observed to lie on a distinct period-luminosity sequence called
sequence D. This sequence D period-luminosity relation is roughly consistent
with the predictions for dipole oscillatory convective modes in AGB models if
we adopt a mixing length of 1.2 pressure scale height ().
However, the effective temperature of the red-giant sequence of the LMC is
consistent to models with , which predict periods too short by a
factor of two.Comment: 7 pages, 6 figures, accepted for publication in MNRA
Using broad-band photometry to examine the nature of long secondary periods in red giants
Long-term JHK light curves have recently become available for large numbers of the more luminous stars in the Small Magellanic Cloud (SMC). We have used these JHK light curves, along with OGLE (Optical Gravitational Lensing Experiment) V and I light curves, to examine the variability of a sample of luminous red giants in the SMC which show prominent long secondary periods (LSPs). The origin of the LSPs is currently unknown. In oxygen-rich stars, we found that while most broad-band colours (e.g. V − I) get redder when an oxygen-rich star dims during its LSP cycle, the J − K colour barely changes and sometimes becomes bluer.We interpret the J − K colour changes as being due to increasing water vapour absorption during declining light caused by the development of a layer of dense cool gas above the photosphere. This result and previous observations which indicate the development of a chromosphere between minimum to maximum light suggest that the LSP phenomenon is associated with the ejection of matter from the stellar photosphere near the beginning of light decline.We explore the possibility that broad-band light variations from the optical to the near-infrared regions can be explained by either dust absorption by ejected matter or large spots on a rotating stellar surface. However, neither model is capable of explaining the observed light variations in a variety of colour–magnitude diagrams.We conclude that some other mechanism is responsible for the light variations associated with LSPs in red giants
IRSF/SIRIUS JHKs near-infrared variable star survey in the Magellanic Clouds
We carried out a NIR variable star survey toward the Large and Small
Magellanic Clouds using the InfraRed Survey Facility (IRSF) at Sutherland,
South African Astronomical Observatory. This survey project was initiated in
Dember 2000, and since then, we kept monitoring a total area of 3 square
degrees along the LMC bar and also an area of 1 square degree around the center
of the SMC, sufficiently large to do statistical analysis and to make complete
catalog of variable red giants in the Magellanic Clouds. The detection limits
(S/N=10) of the survey are 17.0, 16.5 and 15.5 at J, H and Ks, respectively. In
this article, we present some results on infrared variables that are not
detected by the previous optical surveys. We show that they do not fall on the
standard period-Ks magnitude relation for Mira-type variables pulsating in the
fundamental mode.Comment: 3 pages, 2 figures. To appear in "Stellar Pulsation: Challenges for
Theory and Observation", Eds. J. Guzik and P. Bradle
A survey of T Tauri stars with AKARI toward the Taurus-Auriga region
Aims: We search new T Tauri star (TTS) candidates with the mid-infrared (MIR)
part of the AKARI All-Sky Survey at 9 and 18 um wavelengths. Methods: We used
the point source catalogue (PSC), obtained by the Infrared Camera (IRC) on
board AKARI. We combined the 2MASS PSC and the 3rd version of the USNO CCD
Astrograph Catalogue (UCAC) with the AKARI IRC-PSC, and surveyed 517 known TTSs
over a 1800-square-degree part of the Taurus-Auriga region to find criteria to
extract TTSs. We considered asymptotic giant branch (AGB) stars, post-AGB
stars, Planetary Nebulae (PNe), and galaxies, which have similar MIR colours,
to separate TTSs from these sources. Results: Of the 517 known TTSs, we
detected 133 sources with AKARI. Based on the colour-colour and
colour-magnitude diagrams made from the AKARI, 2MASS, and UCAC surveys, we
propose the criteria to extract TTS candidates from the AKARI All-Sky data. On
the basis of our criteria, we selected 176/14725 AKARI sources as TTS
candidates which are located around the Taurus-Auriga region. Comparing these
sources with SIMBAD, there are 148 previously identified sources including 115
Young Stellar Objects (YSOs), and 28 unidentified sources. Conclusions: Based
on SIMBAD identifications, we take the TTS-identification probability using our
criteria to be ~75 %. We find 28 TTS candidates, of which we expect 21 to be
confirmed once follow-up observations can be obtained. Although the probability
of ~75 % is not so high, it is affected by the completeness of the SIMBAD
database, and we can search for TTSs over the whole sky, over all star forming
regions.Comment: 12 pages, 9 figures, accepted for publication in A&
Galactic distributions of carbon- and oxygen-rich AGB stars revealed by the AKARI mid-infrared all-sky survey
Context: The environmental conditions for asympotic giant branch (AGB) stars
to reach the carbon-rich (C-rich) phase are important to understand the
evolutionary process of AGB stars. The difference between the spatial
distributions of C-rich and oxygen-rich (O-rich) AGB stars is essential for the
study of the Galactic structure and the chemical evolution of the interstellar
medium (ISM). Aims: We quantitatively investigate the spatial distributions of
C-rich and O-rich AGB stars in our Galaxy. We discuss the difference between
them and its origin. Methods: We classify a large number of AGB stars newly
detected by the AKARI id-infrared all-sky survey. In the color-color diagrams,
we define their occupation zones based on the locations of known objects. We
then obtain the spatial distributions of C-rich and O-rich AGB stars, assuming
that they have the same luminosity for a given mass-loss rate. Results: We find
that O-rich AGB stars are concentrated toward the Galactic center and that the
density decreases with Galactocentric distance, whereas C-rich AGB stars show a
relatively uniform distribution within about 8kpc of Sun. Conclusion: Our
result confirms the trends reported in previous studies and extends them to a
Galactic scale. We discuss the relations between our result, the Galactic
metallicity gradient, and the chemical evolution of the ISM in our Galaxy.Comment: 13 pages, 24 figures, Accepted for A&
Period-Magnitude relation of Mira-like variables in the Large Magellanic Cloud as a tool to understand circumstellar extinction
Near- to mid-infrared period-magnitude relations and also the
period-bolometric luminosity relation of OGLE-III Mira-like variables in the
LMC are derived. The relations have a kink, and the period at which the break
occurs is quantitatively obtained. There are many Mira-like variables whose
fluxes at the optical and the near-infrared wavebands are fainter than the ones
predicted by the period-magnitude relations. The deviation is due to the
circumstellar extinction, and the amount of the deviation is found to be
strongly correlated with near-infrared colors. The empirical formulae relating
the amount of the deviation and the near-infrared colors are derived. These
relations are useful to accurately calculate the distances to the dusty
Mira-like variables, because the dimmed fluxes due to the circumstellar
extinction can be estimated. In a manner analogous to the interstellar
extinction law, the ratios of deviations at any two different wavebands are
calculated. The ratios are found to change with the pulsation period,
indicating that the dust properties are subject to change as Mira-like
variables evolve.Comment: 8 pages, 5 figures and 4 tables, accepted for publication in MNRA
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