Computer Science – Sound
Scientific paper
May 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002eso..pres...10.&link_type=abstract
ESO Press Release, 05/2002
Computer Science
Sound
Scientific paper
First Observations of Solar-type Oscillations in a Star Very Different from the Sun
Summary
About 30 years ago, astronomers realised that the Sun resonates like a giant musical instrument with well-defined periods (frequencies). It forms a sort of large, spherical organ pipe. The energy that excites these sound waves comes from the turbulent region just below the Sun's visible surface.
Observations of the solar sound waves (known as " helioseismology ") have resulted in enormous progress in the exploration of the interior of the Sun, otherwise hidden from view. As is the case on Earth, seismic techniques can be applied and the detailed interpretation of the observed oscillation periods has provided quite accurate information about the structure and motions inside the Sun, our central star.
It has now also become possible to apply this technique to some solar-type stars. The first observations concerned the northern star eta Bootis (cf. ESO PR 16/94 ). Last year, extensive and much more accurate observations with the 1.2-m Swiss telescope at the ESO La Silla Observatory proved that Alpha Centauri , a solar "twin", behaves very much like the Sun (cf. ESO PR 15/01 ), and that some of the periods are quite similar to those in the Sun.
These new observational data were of a superb quality, and that study marked a true break-through in the new research field of " asteroseismology " (seismology of the stars) for solar-type stars. But what about other types of stars, for instance those that are much larger than the Sun?
Based on an extremely intensive observing project with the same telescope, an international group of astronomers [1] has found that the giant star xi Hya ("xi" is the small greek letter [2]; "Hya" is an abbreviation of "Hydrae") behaves like a giant sub-ultra-bass instrument . This star is located in the constellation Hydra (the Water-Monster) at a distance of 130 light-years, it has a radius about 10 times that of the Sun and its luminosity is about 60 times larger.
The new observations demonstrate that xi Hya oscillates with several periods of around 3 hours. xi Hya is now approaching the end of its life - it is about to expand its outer envelope and to become a "red giant star" . It is quite different from stars like the Sun, which are only halfway through their active life. xi Hya is considerably more massive than any other star in which solar-like oscillations have so far been detected.
This observational feat allows to study for the first time with seismic techniques the interior of such a highly evolved star. It paves the way for similar studies of different types of stars. A new chapter of stellar astrophysics is now opening as asteroseismology establishes itself as an ingenious method that is able to revolutionise our detailed understanding of stellar interiors and the overall evolution of stars .
PR Photo 13a/02 : Oscillation frequencies in the Giant Star xi Hya PR Photo 13b/02 : Non-radial oscillations of xi Hya (computer graphics) PR Audio Clip 01/02 : Listen to the sound of xi Hya (RealMedia and MP3) The difficult art of asteroseismology
Helioseismology (seismology of the Sun) is based on measurements of the changing radial velocity of the solar upper atmospheric layers (the "surface") by means of the well-known Doppler effect, as this surface moves up and down during acoustic oscillations. The corresponding amplitudes are very small, with velocities of up to 15 - 20 cm/sec, and the typical period is around 5 minutes. Therefore the phenomenon was first known as the "five-minute oscillations".
Intensity measurements have also been tried, but the noise level is larger than for velocity data due to the presence of "granulation" (moving cells of hot gas) on the solar surface.
In the case of larger and brighter stars like the giant stars, the corresponding amplitudes and periods increase. For instance, theoretical predictions for the giant star xi Hya have indicated that velocity amplitudes of about 7 m/sec and periods of the order of 3 - 4 hours could be expected.
Observations of such oscillations are much more difficult, because the demands on the performance of the spectrograph increase dramatically, as this timescale is similar to that of variations of conditions in the Earth's atmosphere during the observing night.
Spurious instrumental effects, like mechanical flexure, would be detrimental to such demanding observations. However, the experience from the search for exoplanets orbiting other stars - by observing the periodic change in velocity of the parent star due to the weak pull of the orbiting planet over even longer timescales - has proven to be very useful. Indeed, asteroseismology has benefitted greatly from the development of accurate techniques now employed in the search for exoplanets . The observations of the giant star xi Hya
An international team of astronomers [1] observed xi Hya with the Swiss 1.2-m Euler telescope at the ESO La Silla Observatory (Chile). They used the CORALIE spectrograph, which is well known for numerous discoveries of exoplanets (cf. PR 07/01 ), and recently for the detection of 7-min acoustic oscillations in the solar-twin star Alpha Centauri A (cf. PR 15/01 ).
The same technique that delivered superb observations of Alpha Centauri A was employed to investigate the oscillations of xi Hya . The sound waves make the surface of the star oscillate periodically in and out, and the CORALIE spectrograph measures the velocities of the up-down motion.
As xi Hya is a giant, these waves need more time to propagate through the stellar interior up to the stellar surface than they do in a solar-like star. Thus, the generated oscillations of the surface are slower.
An observing campaign lasting no less than one full month, taking about two measurements every hour was necessary to detect the tiny movements of the surface of xi Hya .
The detected oscillations have periods of about 3 hours, and have speeds of only up to 2 metres per second . This is somewhat smaller than expected, but the predictions for these amplitudes were very uncertain as the conditions in xi Hya are so very different from those in the Sun. First results for xi Hya
ESO PR Photo 13a/02
ESO PR Photo 13a/02
[Preview - JPEG: 492 x 400 pix - 68k] [Normal - JPEG: 983 x 800 pix - 168k]
Caption : PR Photo 13a/02 shows the "frequency spectrum" of the giant star xi Hya , as deduced on the basis of extensive velocity measurements with the 1.2-m Leonhard Euler telescope at the ESO La Silla Observatory (Chile). The abscissa unit is microHertz; 100 µHz corresponds to a period of 10,000 seconds (2.78 hours).
PR Audio Clip 01/02 : Listen to the sound of xi Hya ! This 15-sec audio clip was produced by mixing the 16 strongest frequencies in the observed sound spectrum ( PR Photo 13a/02 ) with the correct, relative amplitudes. In order to render the signal audible, all frequencies were multiplied by a factor of one million. Note that quality loudspeakers are required to fully appreciate this rich and complex signal, especially the underlying bass tones. Several beat frequencies are obviously present. Available in RealMedia (requires RealPlayer software) and MP3 (264k) formats.
PR Photo 13a/02 shows the frequency spectrum of xi Hya , based on these extensive observations. The "power peaks" indicate the frequencies of the oscillation of the stellar atmosphere. The broad distribution means that several different sound waves are clearly present. This is the first time such a spectrum has ever been obtained for a giant star.
A first analysis showed the presence of about one dozen significant frequencies and correspondingly, periods . Among those, four have amplitudes above 1 metre per second. In addition to these twelve frequencies, others appear to have been detected as well, but with less certainty and their reality must be confirmed by a subsequent, more detailed study.
The "sound of xi Hya" has been synthesized in PR Audio Clip 01/02 . Stellar models
A good model of the star is necessary before the observed oscillation frequencies (periods) can be properly interpreted.
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