Using the Hubble Space Telescope (HST), astronomers Brian Wood and Jeffrey Linsky of the University of Colorado in Boulder, Colorado and Gary Zank and Hans Müller of the Bartol Research Institute in Newark, Delaware have obtained the first measurements of hot ionized gas blown out by stars like the Sun. Although the solar wind and its influence on the outer atmosphere of the Earth and other planets has been studied for years, we now have evidence that other stars with ages and masses roughly similar to the Sun also have winds that can influence the atmospheres of their planets. These results are being presented today at the American astronomical Society meeting in San Diego, California.

The solar wind of high speed electrons and ions sweeping by the Earth is the cause of such phenomena as the Northern Lights, the pointing of comet tails away from the Sun, and changes in the upper atmosphere of the Earth. Astronomers have long speculated that other middle-aged stars with masses similar to the Sun should also have hot ionized winds, but these winds are very difficult to see against the bright glare of the star and have not previously been detected. Other types of more massive winds have previously been detected from stars that are very different from the Sun, but these winds are accelerated by very different processes.

Brian Wood and Jeffrey Linsky working in JILA on the University of Colorado campus, and Hans-Reinhard Müller and Gary Zank working at the Bartol Research Institute on the University of Delaware campus have developed a new technique that is sensitive enough to detect and measure the properties of weak solar-like winds. The technique uses the very strong Lyman-alpha spectral line of hydrogen, observable in ultraviolet spectra of nearby stars. The presence of a stellar wind is indicated by a subtle absorption signature in the Lyman-alpha line.

The warm outer layer of a star, usually called the chromosphere, emits bright emission lines primarily in the ultraviolet part of the star's spectrum. One of the brightest of these emission lines is the Lyman-alpha line of atomic hydrogen. The Space Telescope Imaging Spectrograph on the Hubble Space Telescope has observed this emission line from many nearby stars. The central part of this line is always dark, because atomic hydrogen in the interstellar gas towards a star absorbs this part of the line. The rest of the bright line not absorbed by interstellar hydrogen contains the unique information on the stellar wind.

The amount of the Lyman-alpha line absorbed by interstellar hydrogen can be accurately predicted from the observed interstellar absorption in lines of other chemical elements such as magnesium and iron. The predicted absorption by interstellar hydrogen explains most but not all of the absorption in the hydrogen line of nearby stars. Additional hydrogen absorbers must be present, but where are they located?

Theoretical studies have provided the answer. The solar wind does not expand forever. It eventually collides with interstellar atomic hydrogen and protons at a distance of 100 to 200 times further away from the Sun than the Earth's orbit. When the solar wind and interstellar gas collide, the result is an accumulation of hot atomic hydrogen that is slowed down from the speed of the incoming interstellar gas. This material produces additional absorption in the stellar Lyman-alpha line, and the decrease in velocity of this material means that this additional absorption is Doppler shifted to the "red" side of the interstellar absorption, in agreement with observations.

If stars like the Sun also have hot ionized winds, then these winds should also collide with the interstellar gas. Since we are observing this collision from outside the region rather than inside, the excess absorption from this region should be Doppler shifted to the "blue" side of the line rather than the red side where the solar wind is apparent.

The application of this theory is presented in a paper to be published in the January 20, 2001 issue of The Astrophysical Journal Letters by Wood, Linsky, Müller, and Zank. They compare observations of stars in the closest star system to Earth, the Alpha Centauri system. Alpha Centauri itself is a binary star 4.39 light years from the Sun consisting of a near twin of the Sun and a somewhat cooler star. Alpha Centauri has a very distant companion star called Proxima Centauri located 4.22 light years from the Sun. Proxima Centauri is an M dwarf star, which is much cooler in effective temperature than the Sun and much lower in mass.

Spectrographs on the Hubble Space Telescope have observed both Alpha Centauri and Proxima Centauri. Both stars show Lyman-alpha emission lines with dark absorption by interstellar atomic hydrogen. Both also show the same additional absorption on the red side of the line produced by hydrogen where the solar wind and the interstellar gas collide.

The most interesting aspect of the data is on the blue side of the interstellar absorption where the theory predicts additional absorption from the collision of the stellar wind with the interstellar gas. Here a large difference is observed. Toward Alpha Centauri additional absorption is in fact observed which indicates that the strength of the wind from each of the two stars in the binary system is about the same as that of the Sun. On the other hand, no detectable additional absorption is observed toward Proxima Centauri, indicating that its wind has less than 20 percent of the mass loss of the solar wind.

Comparison of the Lyman alpha line spectrum of Alpha Centauri B (green histogram), the cooler star in the binary system, and Proxima Centauri (red histogram). The inferred interstellar absorption by hydrogen and deuterium is shown as a green dashed line. The Alpha Centauri and Proxima Centauri data agree very well on the red side of the hydrogen absorption produced in the region where the solar wind and interstellar gas collide, but on the blue side the Proxima Centauri data do not show the excess Lyman alpha absorption seen toward Alpha Centauri. The blue lines show the predicted excess absorption by gas where the stellar wind and interstellar gas collide. Four models assuming four different mass loss rates are shown. The twice solar mass loss rate fits the Alpha Centauri spectrum reasonably well, whereas the 0.2 times the solar mass loss rate is an upper limit for Proxima Centauri. This material was presented to the American Astronomical Society meeting in San Diego, CA on January 8, 2001 and will appear in the January 20, 2001 issue of the The Astrophysical Journal Letters.

FIGURE CREDIT:
Brian Wood, Jeffrey Linsky, Gary Zank, Hans Müller

The conclusion from this study is that the winds of nearby middle-aged stars with mass similar to the Sun can be measured for the first time. The two solar-like stars in the Alpha Centauri binary have winds with properties similar to the Sun. The solar wind is therefore not unique and may be a good prototype for the winds of many other stars of similar mass and age to the Sun. On the other hand, stars much cooler and less massive than the Sun, like the M dwarf Proxima Centauri, have weaker winds than the Sun. Since this type of star flares very often, it has been speculated that such stars may be losing a large amount of mass. The new data suggests that this is not the case. Analysis of HST spectra of other stars will test the analysis techniques and determine the range of stars that have solar-like winds.

JILA is a joint research institute of the University of Colorado and the National Institute of Standards and Technology in Boulder Colorado. This research is supported by NASA.

For more information:

Dr. Brian Wood (303-492-7820; woodb@casa.colorado.edu)
Dr. Jeffrey linsky (303-492-7838; jlinsky@jila.colorado.edu)
Dr. Gary Zank (909-787-; gary.zank@ucr.edu)
Dr. Hans Müller (603-646-2738; hans.mueller@ucr.edu)

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