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ESO's ADONIS AO System Observes Low-mass Eclipsing Binary Star in Orion

La Silla, Chile – October 8, 2001:   A series of very detailed images of a binary system of two young stars have been combined into a movie. In just 3 days, the stars swing around each other. As seen from the earth, they pass in front of each other twice during a full revolution, producing eclipses during which their combined brightness diminishes.
  A careful analysis of the orbital motions has now made it possible to deduce the masses of the two dancing stars. Both turn out to be about as heavy as our Sun.

But while the Sun is about 4500 million years old, these two stars are still in their infancy. They are located some 1500 light-years away in the Orion star-forming region and they probably formed just 10 million years ago.

This is the first time such an accurate determination of the stellar masses could be achieved for a young binary system of low-mass stars. The new result provides an important piece of information for our current understanding of how young stars evolve.

The observations were obtained by a team of astronomers from Italy and ESO using the ADaptive Optics Near Infrared System (ADONIS) on the 3.6-m telescope at the ESO La Silla Observatory.
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The object RXJ 0529.4+0041 is a very young eclipsing spectroscopic binary star. Eclipsing spectroscopic binaries represent true cornerstones for the determination of stellar masses, and as such they are fundamental for our understanding of stellar evolution. An eclipsing binary is one that is aligned almost exactly edge-on to the observer, so that the stars pass in front of each other from time to time. The light-curves of the eclipses, combined with spectroscopic measurements of the stellar velocities, allow not only the masses of the stars, but also their diameters and surface temperatures to be determined. Rather few eclipsing spectroscopic binaries are known, but they can be used to calibrate other, indirect methods to derive stellar parameters.

RXJ 0529.4+0041 was discovered by the X-ray satellite ROSAT, which detected the high-energy emission from the star's hot corona. Subsequent optical spectroscopy showed the object to be a young, low-mass spectroscopic binary system, and monitoring of the light curve showed that the system undergoes eclipses.

RXJ 0529.4+0041 is located in the Orion Nebula at a distance of about 1500 light-years. This is one of the nearest star-forming regions and almost all stars in this area are quite young. Spectroscopic observations soon confirmed that this binary system was no exception. The stars probably formed no more than 10 million years ago. In astronomical terms, they are "infant" stars.

High-resolution spectroscopic observations, mostly with the CORALIE spectrometer on the Swiss 1.2-m Leonard Euler telescope at the ESO La Silla Observatory, were used to determine the radial velocities of the stars. From these, a first determination of the orbital and stellar parameters was possible. The orbital period turned out to be short. The two stars swing around each other in just 3 days.

The short orbital period and the even shorter duration of the eclipses, only 6 hours, posed a real challenge for the observers. They decided to obtain further high-angular resolution observations with the ADaptive Optics Near Infrared System (ADONIS) on the 3.6-m telescope at the ESO La Silla Observatory. Luckily, early ADONIS images demonstrated that this binary stellar system has a third companion, sufficiently far away from the two others to be seen as a separate star by ADONIS. This unexpected bonus made it possible to monitor the light changes of the binary system in great detail, by using the third companion as a convenient "reference" star.

In December 2000 and January 2001, detailed ADONIS images of the RXJ 0529.4+0041 system were obtained in three near-infrared filters (the J-, H- and K-bands). ADONIS is equipped with the SHARP II camera and eliminates the adverse image-smearing effects of the atmospheric turbulence in real-time by means of a computer-controlled flexible mirror. As expected, the new, extremely sharp images of RXJ 0529.4+0041 greatly improved the achievable photometric precision. In particular, as the image of the third component was perfectly separated from the others, it did not "contaminate" the derived light curve of the eclipsing binary.
Laser Guide Star Adaptive Optics, Ageorges
The ADONIS images have been combined into a movie clip. On the image to the right, the left-hand panel shows the eclipsing binary system (the brighter object to the upper right – the light from the two stars merges into a single point of light) and the well separated third component (lower left), as they were recorded by ADONIS in the three different filter bands. As the two stars in the binary system move around each other in their orbits, eclipses occur and the brightness of the binary system changes.

The right-hand panel shows a build-up of the observed light curves for the binary system. It represents the brightness difference between binary system and the third object that shines with constant light. Both the primary, deeper and the secondary, less deep eclipses are clearly seen.

The primary eclipse was observed on December 8, 2000 and is here displayed at phase zero. During this minimum, the brightness of the binary system decreases by about 45% (0.4 magnitudes). The primary eclipse takes place when the
Eclipsing Binary: click to run movie A frame from the ADONIS movie of the RXJ 0529.4+0041 eclipsing binary stellar system, corresponding to the time near the end of the "secondary" eclipse.    >> The full movie.
Image:   © ESO
Science With Adaptive Optics, Brandner
smaller component blocks the light from the brighter and hotter star. The orbital motions of the two stars are illustrated by a computer-generated, animated sequence.

The secondary eclipse (at phase 0.5) dims the light from the system less; it occurs when the larger and brighter star almost completely (about 90%) hides its smaller companion. The second minimum was recorded on January 12, 2001. None of the eclipses is therefore "total".

A detailed analysis of these high-precision light curves allowed the astronomers to determine the orbits and hence, to perform an extremely accurate measurement of the fundamental stellar parameters for the two young stars of RXJ 0529.4+0041.

The star that is eclipsed during the primary eclipse (the "primary") is the more massive and also the hotter and brighter of the two stars. Its mass is 1.3 times that of our Sun. Its diameter is nearly 1.6 times larger than that of our Sun and the surface temperature is found to be a little more than 5000°C, or a few hundred degrees cooler than the Sun. The "secondary" star is slightly lighter than our Sun. Its weight is about 90% of that of the Sun and the diameter is 20% larger, while the surface temperature is 4000°C.

In fact, these two stars are still so young that most of their energy comes from the contraction process – the first phase during which they are formed from an interstellar cloud by this process is not yet over and they are still getting smaller. It is by this process that collapsing stars heat up enough to start nuclear burning. When infant stars in RXJ 0529.4+0041 eventually reach middle-age, their sizes will most likely also be quite similar to that of the Sun.

Few systems are known for which such precise determinations of the stellar parameters have ever been possible – and this binary system represents the first case where both the components are such young stars.

A detailed comparison of the derived stellar parameters with current models for the evolution of young stars shows fairly good agreement for the primary component. However, there are certain discrepancies in the case of the secondary component, showing that the current models for the early stages of lower-mass stars must still be refined.

Adaptive Optics in Astronomy, Roddier
E. Covino, S. Catalano, A. Frasca, E. Marilli, M. Fernández, J.M. Alcalá, C. Melo, R. Paladino, M.F. Sterzik
and B. Stelzer, "RXJ 0529.4+0041: a low-mass pre-main sequence eclipsing-spectroscopic binary,"
Astron. & Astrophys. 361, L49–L52 (2000)
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