741 research outputs found
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
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
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
Oscillations in Arcturus from WIRE photometry
Observations of the red giant Arcturus (Alpha Boo) obtained with the star
tracker on the Wide Field Infrared Explorer (WIRE) satellite during a baseline
of 19 successive days in 2000 July-August are analysed. The amplitude spectrum
has a significant excess of power at low-frequencies. The highest peak is at
about 4.1 micro-Hz (2.8 d), which is in agreement with previous ground-based
radial velocity studies. The variability of Arcturus can be explained by sound
waves, but it is not clear whether these are coherent p-mode oscillations or a
single mode with a short life-time.Comment: 6 pages, 1 Latex file, 4 .eps figures, 2 .sty files, ApJL, 591, L151
See erratum (astro-ph/0308424
Dipolar modes in luminous red giants
Lots of information on solar-like oscillations in red giants has been
obtained thanks to observations with CoRoT and Kepler space telescopes. Data on
dipolar modes appear most interesting. We study properties of dipolar
oscillations in luminous red giants to explain mechanism of mode trapping in
the convective envelope and to assess what may be learned from the new data.
Equations for adiabatic oscillations are solved by numerical integration down
to the bottom of convective envelope, where the boundary condition is applied.
The condition is based on asymptotic decomposition of the fourth order system
into components describing a running wave and a uniform shift of radiative
core. If the luminosity of a red giant is sufficiently high, for instance at M
= 2 Msun greater than about 100 Lsun, the dipolar modes become effectively
trapped in the acoustic cavity, which covers the outer part of convective
envelope. Energy loss caused by gravity wave emission at the envelope base is a
secondary or negligible source of damping. Frequencies are insensitive to
structure of the deep interior.Comment: 10 pages, 7 figures, accepted for publication in Astronomy and
Astrophysic
Asteroseismology across the HR diagram
High precision spectroscopy provides essential information necessary to fully
exploit the opportunity of probing the internal structure of stars using
Asteroseismology. In this work we discuss how Asteroseismology combined with
High Precision Spectroscopy can establish a detailed view on stellar structure
and evolution of stars across the HR diagramme.Comment: 6 pages, 2 figures - to appear in Precision Spectroscopy in
Astrophysics, (Eds) L. Pasquini, M. Romaniello, N.C. Santos, and A. Correia,
ESO Astrophysics Symposia, 200
Period-luminosity relations of pulsating M giants in the solar neighbourhood and the Magellanic Clouds
We analyse the results of a 5.5-yr photometric campaign that monitored 247
southern, semi-regular variables with relatively precise Hipparcos parallaxes
to demonstrate an unambiguous detection of Red Giant Branch (RGB) pulsations in
the solar neighbourhood. We show that Sequence A' contains a mixture of AGB and
RGB stars, as indicated by a temperature related shift at the TRGB. Large
Magellanic Cloud (LMC) and Galactic sequences are compared in several ways to
show that the P-L sequence zero-points have a negligible metallicity
dependence. We describe a new method to determine absolute magnitudes from
pulsation periods and calibrate the LMC distance modulus using Hipparcos
parallaxes to find \mu (LMC) = 18.54 +- 0.03 mag. Several sources of systematic
error are discussed to explain discrepancies between the MACHO and OGLE
sequences in the LMC. We derive a relative distance modulus of the Small
Magellanic Cloud (SMC) relative to the LMC of \Delta \mu = 0.41 +- 0.02 mag. A
comparison of other pulsation properties, including period-amplitude and
luminosity-amplitude relations, confirms that RGB pulsation properties are
consistent and universal, indicating that the RGB sequences are suitable as
high-precision distance indicators. The M giants with the shortest periods
bridge the gap between G and K giant solar-like oscillations and M-giant
pulsation, revealing a smooth continuity as we ascend the giant branch.Comment: 12 pages, 17 figures, 1 table. Accepted for publication in MNRA
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