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Teresa C. Teixeira

Other affiliations: Queen Mary University of London
Bio: Teresa C. Teixeira is an academic researcher from University of Porto. The author has contributed to research in topics: Solar-like oscillations & Subgiant. The author has an hindex of 5, co-authored 7 publications receiving 468 citations. Previous affiliations of Teresa C. Teixeira include Queen Mary University of London.

Papers
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Journal ArticleDOI
TL;DR: In this article, the discovery of solar-like oscillations in a giant star was confirmed by using the CORALIE spectrograph attached to the 1.2m Swiss Euler telescope.
Abstract: We report the firm discovery of solar-like oscillations in a giant star. We monitored the star xi Hya (G7III) continuously during one month with the CORALIE spectrograph attached to the 1.2m Swiss Euler telescope. The 433 high-precision radial-velocity measurements clearly reveal multiple oscillation frequencies in the range 50 - 130 uHz, corresponding to periods between 2.0 and 5.5 hours. The amplitudes of the strongest modes are slightly smaller than 2 m/s. Current model calculations are compatible with the detected modes.

151 citations

Journal ArticleDOI
TL;DR: In this paper, the HARPS and UCLES spectrographs were combined with those obtained five years earlier to identify 28 oscillation modes, and a detailed comparison of the mixed l ¼ 1 modes with theoretical models should allow a precise estimate of the age of the star.
Abstract: We have observed oscillations in the nearby G2 subgiant starHyi using high-precision velocity observations obtained over more than a week with the HARPS and UCLES spectrographs. The oscillation frequencies show a regularcombstructure,asexpectedforsolar-likeoscillations,butwithseverall ¼ 1modesbeingstronglyaffectedby avoided crossings. These data, combined with those we obtained five years earlier, allow us to identify 28 oscillation modes.Byscalingthelarge-frequencyseparationfromtheSun,wemeasurethemeandensityofHyitoanaccuracy of 0.6%. The amplitudes of the oscillations are about 2.5 times solar and the mode lifetime is 2.3 days. A detailed comparison of the mixed l ¼ 1 modes with theoretical models should allow a precise estimate of the age of the star.

113 citations

Journal ArticleDOI
TL;DR: In this paper, the authors observed oscillations in the nearby G2 subgiant star beta Hyi using high-precision velocity observations obtained over more than a week with the HARPS and UCLES spectrographs.
Abstract: We have observed oscillations in the nearby G2 subgiant star beta Hyi using high-precision velocity observations obtained over more than a week with the HARPS and UCLES spectrographs. The oscillation frequencies show a regular comb structure, as expected for solar-like oscillations, but with several l=1 modes being strongly affected by avoided crossings. The data, combined with those we obtained five years earlier, allow us to identify 28 oscillation modes. By scaling the large frequency separation from the Sun, we measure the mean density of beta Hyi to an accuracy of 0.6%. The amplitudes of the oscillations are about 2.5 times solar and the mode lifetime is 2.3 d. A detailed comparison of the mixed l=1 modes with theoretical models should allow a precise estimate of the age of the star.

100 citations

Journal ArticleDOI
TL;DR: In this paper, the authors used HARPS to measure oscillations in the low-mass star tau Cet. Although the data were compromised by instrumental noise, they were able to extract the main features of the oscillations.
Abstract: We used HARPS to measure oscillations in the low-mass star tau Cet. Although the data were compromised by instrumental noise, we have been able to extract the main features of the oscillations. We found tau Cet to oscillate with an amplitude that is about half that of the Sun, and with a mode lifetime that is slightly shorter than solar. The large frequency separation is 169 muHz, and we have identified modes with degrees 0, 1, 2, and 3. We used the frequencies to estimate the mean density of the star to an accuracy of 0.45% which, combined with the interferometric radius, gives a mass of 0.783 +/- 0.012 M_sun (1.6%).

79 citations

Journal ArticleDOI
TL;DR: In this article, velocity variations in the pulsating sdB star PG 1605+072 were detected at the same frequencies found from photometry and have amplitudes of up to 14 km/s for H$\beta$.
Abstract: We report the detection of velocity variations in the pulsating sdB star, PG 1605+072. Oscillations are detected at the same frequencies found from photometry and have amplitudes of up to 14 km/s for H$\beta$. The strongest oscillation found in previous photometric observations is not evident in our spectroscopy or photometry, and may be absent due to beating of closely spaced modes. Phase differences between spectroscopy and B magnitude photometry imply that maximum brightness occurs not long after maximum radius. We have also found evidence of variation in the observed amplitudes of five Balmer lines, with a decrease in amplitude of the strongest mode blueward from H$\beta$. This effect is not expected and a longer time-series will be needed to clarify it.

39 citations


Cited by
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Journal ArticleDOI
TL;DR: A review of the current state of the field can be found in this article, with a particular emphasis on recent advances provided by the Kepler and COROT (Convection, Rotation & Planetary Transits) space missions.
Abstract: We are entering a golden era for stellar physics driven by satellite and telescope observations of unprecedented quality and scope. New insights on stellar evolution and stellar interiors physics are being made possible by asteroseismology, the study of stars by the observation of natural, resonant oscillations. Asteroseismology is proving to be particularly significant for the study of solar-type and red-giant stars. These stars show rich spectra of solar-like oscillations, which are excited and intrinsically damped by turbulence in the outermost layers of the convective envelopes. In this review we discuss the current state of the field, with a particular emphasis on recent advances provided by the Kepler and COROT (Convection, Rotation & Planetary Transits) space missions and the wider significance to astronomy of the results from asteroseismology, such as stellar populations studies and exoplanet studies.

499 citations

Journal ArticleDOI
TL;DR: The discovery of an sdO star unbound to the Galaxy, potential SN Ia progenitors and probably a hidden population of neutron stars or black hole companions have great impact on astrophysics at large.
Abstract: Hot subdwarf stars (sdBs, sdOs) are core helium-burning stars at the blue end of the horizontal branch or have evolved even beyond that stage. They are found in all Galactic stellar populations and are sufficiently common to account for the UV-upturn of early-type galaxies. About half of the sdBs reside in close binaries; companions are white dwarfs or low-mass main-sequence stars. Binary population-synthesis models explain naturally the actual sdB binary fractions of field and globular cluster stars as well as of He-sdOs if white-dwarf mergers are considered. Hot helium flashes explain the chemical composition of He-sdOs. Asteroseismology of a dozen pulsating sdB stars allowed determination of their masses and detection of a planet to V391 Peg. The discoveries of an sdO star unbound to the Galaxy, potential SN Ia progenitors and probably a hidden population of neutron stars or black hole companions have great impact on astrophysics at large.

439 citations

Journal ArticleDOI
TL;DR: In this paper, the authors used data for the Sun to derive an empirical correction for the near-surface offset, which they then applied to three other stars (α Cen A, α Cen B, and β Hyi).
Abstract: In helioseismology, there is a well-known offset between observed and computed oscillation frequencies. This offset is known to arise from improper modeling of the near-surface layers of the Sun, and a similar effect must occur for models of other stars. Such an effect impedes progress in asteroseismology, which involves comparing observed oscillation frequencies with those calculated from theoretical models. Here, we use data for the Sun to derive an empirical correction for the near-surface offset, which we then apply to three other stars (α Cen A, α Cen B, and β Hyi). The method appears to give good results, in particular providing an accurate estimate of the mean density of each star.

339 citations

Journal ArticleDOI
21 May 2009-Nature
TL;DR: The presence of radial and non-radial oscillations in more than 300 giant stars is reported, finding giant stars with equally spaced frequency peaks in the Fourier spectrum of the time series, as well as giants for which the spectrum seems to be more complex.
Abstract: When main-sequence stars like the Sun near the end of their life, they expand to become oscillating red giants. Such evolved stars could in principle provide stringent tests of stellar theory via analysis of radial and non-radial stellar oscillations. Until now it has been unclear whether non-radial modes are observable at all in red giants. De Ridder et al. now report the presence of both radial and non-radial oscillations in over 300 giant stars. For some red giants, mode lifetimes are of the order of a month. Current stellar evolution theory cannot account for these observations. Towards the end of their lives, stars like the Sun expand greatly to become red giant stars that oscillate. Such evolved stars could provide stringent tests of stellar theory through the analysis of radial and non-radial stellar oscillations. Here, the presence of such oscillations in more than 300 giant stars is reported, with mode lifetimes of some of the giants in the order of a month. Towards the end of their lives, stars like the Sun greatly expand to become red giant stars. Such evolved stars could provide stringent tests of stellar theory, as many uncertainties of the internal stellar structure accumulate with age. Important examples are convective overshooting and rotational mixing during the central hydrogen-burning phase, which determine the mass of the helium core, but which are not well understood1. In principle, analysis of radial and non-radial stellar oscillations can be used to constrain the mass of the helium core. Although all giants are expected to oscillate2, it has hitherto been unclear whether non-radial modes are observable at all in red giants, or whether the oscillation modes have a short or a long mode lifetime3,4,5,6,7, which determines the observational precision of the frequencies. Here we report the presence of radial and non-radial oscillations in more than 300 giant stars. For at least some of the giants, the mode lifetimes are of the order of a month. We observe giant stars with equally spaced frequency peaks in the Fourier spectrum of the time series, as well as giants for which the spectrum seems to be more complex. No satisfactory theoretical explanation currently exists for our observations.

325 citations

Journal ArticleDOI
TL;DR: In this article, the authors extracted accurate seismic parameters from the Kepler time series and used them to infer asteroseismic fundamental parameters from scaling relations and a comparison with red-giant models.
Abstract: Context. Clear power excess in a frequency range typical for solar-type oscillations in red giants has been detected in more than 1000 stars, which have been observed during the first 138 days of the science operation of the NASA Kepler satellite. This sample includes stars in a wide mass and radius range with spectral types G and K, extending in luminosity from the bottom of the giant branch up to high-luminous red giants, including the red bump and clump. The high-precision asteroseismic observations with Kepler provide a perfect source for testing stellar structure and evolutionary models, as well as investigating the stellar population in our Galaxy. Aims. We aim to extract accurate seismic parameters from the Kepler time series and use them to infer asteroseismic fundamental parameters from scaling relations and a comparison with red-giant models. Methods. We fit a global model to the observed power density spectra, which allows us to accurately estimate the granulation background signal and the global oscillation parameters, such as the frequency of maximum oscillation power. We find regular patterns of radial and non-radial oscillation modes and use a new technique to automatically identify the mode degree and the characteristic frequency separations between consecutive modes of the same spherical degree. In most cases, we can also measure the small separation between l = 0, 1, and 2 modes. Subsequently, the seismic parameters are used to estimate stellar masses and radii and to place the stars in an H-R diagram by using an extensive grid of stellar models that covers a wide parameter range. Using Bayesian techniques throughout our entire analysis allows us to determine reliable uncertainties for all parameters. Results. We provide accurate seismic parameters and their uncertainties for a large sample of red giants and determine their astero

295 citations