635 research outputs found
The evolution of the Mira variable R Hydrae
The Mira variable R Hydrae is well known for its declining period, which Wood
& Zarro (1981) attributed to a possible recent thermal pulse. Here we
investigate the long-term period evolution, covering 340 years, going back to
its discovery in AD 1662. Wavelets are used to determine both the period and
semi-amplitude. We show that the period decreased linearly between 1770 and
1950; since 1950 the period has stabilized at 385 days. The semi-amplitude
closely follows the period evolution. Detailed analysis of the oldest data
shows that before 1770 the period was about 495 days. We find no evidence for
an increasing period during this time as found by Wood & Zarro. IRAS data shows
that the mass loss dropped dramatically around AD 1750. The decline agrees with
the mass-loss formalism from Vassiliadis & Wood, but is much larger than
predicted by the Bloecker mass-loss law. An outer detached IRAS shell suggests
that R Hya has experienced such mass-loss interruptions before. The period
evolution can be explained by a thermal pulse occuring around AD 1600, or by an
non-linear instability leading to an internal relaxation of the stellar
structure. The elapsed time between the mass-loss decline giving rise to the
outer detached shell, and the recent event, of approximately 5000 yr suggests
that only one of these events could be due to a thermal pulse. Further
monitoring of R Hya is recommended, as both models make strong predictions for
the future period evolution. R Hya-type events, on time scales of 10^2-10^3 yr,
could provide part of the explanation for the rings seen around some AGB and
post-AGB stars.Comment: 13 pages. MNRAS, accepted for publicatio
Mode switching in the nearby Mira-like variable R Doradus
We discuss visual observations spanning nearly 70 years of the nearby
semiregular variable R Doradus. Using wavelet analysis, we show that the star
switches back and forth between two pulsation modes having periods of 332 days
and about 175 days, the latter with much smaller amplitude. Comparison with
model calculations suggests that the two modes are the first and third radial
overtone, with the physical diameter of the star making fundamental mode
pulsation unlikely. The mode changes occur on a timescale of about 1000 d,
which is too rapid be related to a change in the overall thermal structure of
the star and may instead be related to weak chaos.
The Hipparcos distance to R Dor is 62.4 +/- 2.8 pc which, taken with its
dominant 332-day period, places it exactly on the period-luminosity relation of
Miras in the Large Magellanic Cloud. Our results imply first overtone pulsation
for all Miras which fall on the P-L relation. We argue that semiregular
variables with long periods may largely be a subset of Miras and should be
included in studies of Mira behaviour. The semiregulars may contain the
immediate evolutionary Mira progenitors, or stars may alternate between periods
of semiregular and Mira behaviour.Comment: 12 pages, latex with figures, accepted by MNRA
Solar-like oscillations in the metal-poor subgiant nu Indi: II. Acoustic spectrum and mode lifetime
Convection in stars excites resonant acoustic waves which depend on the sound
speed inside the star, which in turn depends on properties of the stellar
interior. Therefore, asteroseismology is an unrivaled method to probe the
internal structure of a star. We made a seismic study of the metal-poor
subgiant star nu Indi with the goal of constraining its interior structure. Our
study is based on a time series of 1201 radial velocity measurements spread
over 14 nights obtained from two sites, Siding Spring Observatory in Australia
and ESO La Silla Observatory in Chile. The power spectrum of the high precision
velocity time series clearly presents several identifiable peaks between 200
and 500 uHz showing regularity with a large and small spacing of 25.14 +- 0.09
uHz and 2.96 +- 0.22 uHz at 330 uHz. Thirteen individual modes have been
identified with amplitudes in the range 53 to 173 cm/s. The mode damping time
is estimated to be about 16 days (1-sigma range between 9 and 50 days),
substantially longer than in other stars like the Sun, the alpha Cen system or
the giant xi Hya.Comment: 5 pages, 7 figures, A&A accepte
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
The {\gamma} Dor stars as revealed by Kepler : A key to reveal deep-layer rotation in A and F stars
The {\gamma} Dor pulsating stars present high-order gravity modes, which make
them important targets in the intermediate-and low-mass main-sequence region of
the Hertzsprung-Russell diagram. Whilst we have only access to rotation in the
envelope of the Sun, the g modes of {\gamma} Dor stars can in principle deliver
us constraints on the inner layers. With the puzzling discovery of unexpectedly
low rotation rates in the core of red giants, the {\gamma} Dor stars appear now
as unique targets to explore internal angular momentum transport in the
progenitors of red giants. Yet, the {\gamma} Dor pulsations remain hard to
detect from the ground for their periods are close to 1 day. While the CoRoT
space mission first revealed intriguing frequency spectra, the almost
uninterrupted 4-year photometry from the Kepler mission eventually shed a new
light on them. It revealed regularities in the spectra, expected to bear
signature of physical processes, including rotation, in the shear layers close
to the convective core. We present here the first results of our effort to
derive exploitable seismic diagnosis for mid- to fast rotators among {\gamma}
Dor stars. We confirm their potential to explore the rotation history of this
early phase of stellar evolution.Comment: 4 pages, 1 figure, proceedings of the 22nd Los Alamos Stellar
Pulsation Conference, "Wide-field variability surveys: a 21st-century
perspective" held in San Pedro de Atacama, Chile, Nov. 28-Dec. 2, 201
Multi-wavelength visibility measurements of the red giant R Doradus
We present visibility measurements of the nearby Mira-like star R Doradus
taken over a wide range of wavelengths (650--990 nm). The observations were
made using MAPPIT (Masked APerture-Plane Interference Telescope), an
interferometer operating at the 3.9-m Anglo-Australian Telescope. We used a
slit to mask the telescope aperture and prism to disperse the interference
pattern in wavelength. We observed in R Dor strong decreases in visibility
within the TiO absorption bands. The results are in general agreement with
theory but differ in detail, suggesting that further work is needed to refine
the theoretical models.Comment: 8 pages; SPIE Conf. 4006 "Interferometry in Optical Astronomy
KIC 10080943: a binary star with two γ Doradus/δ Scuti hybrid pulsators. Analysis of the g modes
We use 4 yr of Kepler photometry to study the non-eclipsing spectroscopic binary KIC 10080943. We find both components to be γ Doradus/δ Scuti hybrids, which pulsate in both p and g modes. We present an analysis of the g modes, which is complicated by the fact that the two sets of l = 1 modes partially overlap in the frequency spectrum. Nevertheless, it is possible to disentangle them by identifying rotationally split doublets from one component and triplets from the other. The identification is helped by the presence of additive combina- tion frequencies in the spectrum that involve the doublets but not the triplets. The rotational splittings of the multiplets imply core rotation periods of about 11 and 7 d in the two stars. One of the stars also shows evidence of l = 2 modes
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