Erratum: "Oscillations in Arcturus from WIRE photometry" (ApJ, 591, L151 [2003])
The value of the mode lifetime given in the first paragraph of the Discussion should be divided by 2pi and is therefore 2.0 days rather than 13 days.
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arXiv subjects
Hans Kjeldsen
Publications and source records attributed to Hans Kjeldsen.
Solar-like Oscillations (Invited Review)
The five-minute oscillations in the Sun have provided a wealth of information about the solar interior. After many attempts, positive detections of similar oscillations in solar-type stars have now been made. This review discusses the properties of solar-like oscillations, the methods used to observe them and the results on individual stars. We conclude that the study of solar-like oscillations from the ground and space has an exciting future.
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Evidence for Solar-like Oscillations in Arcturus (Alpha Boo)
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 power spectrum has a significant excess of power at low-frequencies. The highest peak is at about 4.1 micro-Hz, which is in agreement with the ground-based radial velocity and photometry study of Belmonte et al. (1990a; 1990b). The variability of Arcturus can be explained by sound waves, but it is not clear whether these are coherent p-mode oscillations.
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Evidence for solar-like oscillations in beta Hydri
We have made a clear detection of excess power, providing strong evidence for solar-like oscillations in the G2 subgiant beta Hyi. We observed this star over five nights with the UCLES echelle spectrograph on the 3.9-m Anglo-Australian Telescope, using an iodine absorption cell as a velocity reference. The time series of 1196 velocity measurements shows an rms scatter of 3.30 m/s, and the mean noise level in the amplitude spectrum at frequencies above 0.5 mHz is 0.11 m/s. We see a clear excess of power centred at 1.0 mHz, with peak amplitudes of about 0.5 m/s, in agreement with expectations for this star. Fitting the asymptotic relation to the power spectrum indicates the most likely value for the large separation is 56.2 microHz, also in good agreement with the known properties of beta Hyi.
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Current status of Asteroseismology
Oscillation frequencies are the most accurate properties one can measure for a star, potentially allowing detailed tests of stellar models and evolution theories. We briefly review asteroseismology for two classes of stars. In delta Scuti variables, the main problems are the identification of the observed modes and the theoretical treatment of rotation. In solar-like stars the main difficulty is the tiny amplitudes, but credible detections are now being made. These confirm that stars are oscillating at the approximately the expected levels, but suggest that amplitudes scale as 1/g rather than L/M. We also stress the importance of multi-site campaigns. Several space missions will be launched over the coming years, promising an exciting future for asteroseismology.
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The MONS Star Trackers
The MONS satellite will have two Star Trackers to sense the spacecraft attitude, and we plan to use them as scientific instruments to perform high-precision photometry of thousands of stars. We briefly describe the current plans for the Star Trackers and their expected capabilities.
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Time-series Spectroscopy of EC14026 Stars: Preliminary Results
We have obtained time-series spectra for the pulsating hot subdwarf (sdB) PG1605+072. Previous time-series photometry of this star has shown maximum amplitude variations of about 0.1 mag and at least 50 periods. The pulsator has the largest amplitude and longest periods of all the pulsating sdBs (or EC14026 stars) discovered so far, and appears to be unusual in its class. Preliminary results of a search for velocity variations are presented here. With these variations, along with equivalent width measurements, oscillation mode identification should be possible.
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A search for solar-like oscillations in alpha Cen A
We have been using a new method to search for solar-like oscillations that involves measuring temperature changes via their effect on the equivalent widths of the Balmer hydrogen lines. We observed alpha Cen A over six nights in 1995 with the 3.9-metre Anglo-Australian Telescope and the European Southern Observatory's 3.6-metre telescope in Chile. Oscillations were not detected, with an upper limit only slightly higher than the expected signal.
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Solar-like oscillations: the search goes on
Despite numerous attempts and many hours of telescope time, there has so far been no confirmed detection of solar-like oscillations in any star except the Sun. We review recent efforts, with particular emphasis on the technique of monitoring equivalent widths of Balmer lines, including a description of the steps in data reduction.
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Hipparcos parallaxes for eta Boo and kappa_2 Boo: two successes for asteroseismology
The release of the Hipparcos catalogue provides an opportunity to check results from asteroseismology. Here we show that Hipparcos parallaxes for two stars are in good agreement with oscillation results. For eta Boo, a probable detection of solar-like oscillations by Kjeldsen at al. (1994; astro-ph/9411016) gave a frequency splitting in good agreement with the Hipparcos luminosity. For kappa_2 Boo, a delta Scuti variable, the Hipparcos parallax agrees well with the distance derived by Frandsen et al. (1995).
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Observing solar-like oscillations
We review techniques for measuring stellar oscillations in solar-type stars. Despite great efforts, no unambiguous detections have been made. A new method, based on monitoring the equivalent widths of strong lines, shows promise but is yet to be confirmed. We also discuss several subtleties, such as the need to correct for CCD non-linearities and the importance of data weighting.
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Solar-like oscillations in eta Boo
We have observed evidence for $p$-mode oscillations in the G0 IV star etaBoo (V = 2.68). This represents the first clear evidence of solar-like oscillations in a star other than the Sun. We used a new technique which measures fluctuations in the temperature of the star via their effect on the equivalent widths of the Balmer lines. The observations were obtained over six nights with the 2.5 m Nordic Optical Telescope on La Palma and consist of 12684 low-dispersion spectra. In the power spectrum of the equivalent-width measurements, we find an excess of power at frequencies around 850 microHz (period 20 minutes) which consists of a regular series of peaks with a spacing of $Δν=40.3$ microHz. We identify thirteen oscillation modes, with frequency separations in agreement with theoretical expectations. Similar observations of the daytime sky show the five-minute solar oscillations at the expected frequencies.
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