Summary
A series of very detailed images of a binary system of two young stars
have been combined into a movie.
In merely 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 [1] using the ADaptive Optics Near Infrared System (ADONIS) on the
3.6-m telescope at the ESO La Silla Observatory.
Binary stars and stellar masses
Since some time, astronomers have noted that most stars seem to form in
binary or multiple systems. This is quite fortunate, as the study of binary
stars is the only way in which it is possible to measure directly one of
the most fundamental quantities of a star, its mass.
The mass of a star determines its fate. Massive stars (with masses more than
50 times that of the Sun) lead a glorious, but short life. They are hot and
very luminous and exhaust their energy supply in just a few million years.
At the other end of the scale, low-mass stars like the Sun are more
economical with their resources. Being cooler and dimmer, they are able to
shine for billions of years [2].
But although the mass determines the fate of a star, it is not a trivial
matter to measure this crucial parameter. In fact, it can only be determined
directly if the star happens to be gravitationally bound to another star in
a binary stellar system. Observations of the orbital motions of the two
stars as they circle each other allows to “weigh” them, and also provide
other important information, e.g. about their sizes and temperatures.
Orbital motions
The understanding of orbital motions has a long history in astronomy. The
basic laws of Johannes Kepler (1571-1630) are still used to calculate the
masses of orbiting objects, in the solar system as well as in binary stellar
systems. However, while the observations of the motion of the nine planets
and moons have allowed us to measure quite accurately the masses of objects
in our vicinity, the information needed to “weigh” the binary stellar
systems is not that easy to obtain. As a result, the mass estimates of the
stars in binary systems are often rather uncertain.
A main problem is that the individual stars in many binary systems can not
be visually separated, even in the best telescopes. The information about
the orbit may then come from the motions of the stars, if these are
revealed by spectroscopic observations of the combined light (such systems
are referred to as “spectroscopic binaries”).
If absorption lines from both components are present in the spectrum, the
measured wavelength of these double lines will shift periodically back and
forth. This is the well-known Doppler effect and it directly reflects the
changing velocities of the stars, as they move along their orbits and
periodically approach and recede from the observer. Such spectroscopic
observations therefore allow to measure the orbital velocities of the
stars. It is exactly the same technique that is used to study and weigh
extra-solar planets orbiting other stars [3].
However, this method has an important limitation. From the spectroscopical
observations alone, it is only possible to deduce limits on the masses, as
the inclination of orbits to the line-of-sight is usually unknown. The
masses derived in this way (for stars as well as for exoplanets) are
therefore only lower limits on the actual masses.
Eclipsing Binaries
However, fortunate observational circumstances sometimes allow to obtain all
information about the stellar orbits. If a binary system is viewed (almost
exactly) edge-on, the stars may pass in front of each other from time to
time. Astronomers refer to this phenomenon as an “eclipse” and speak about
an “eclipsing binary”.
The effect is similar to a “solar” eclipse as seen on the Earth, whenever
the Moon passes in front of the Sun. Like the Moon blocks the sunlight,
less light is received from the eclipsed star and thus the combined light
from the binary system decreases during the eclipse. The way this happens
(astronomers speak about the system’s “lightcurve”) then provides the
additional information about the inclination of the orbit that is needed to
determine exactly the stellar masses in a “spectroscopic” binary system.
Very accurate values for the stellar diameters and the surface temperatures
of the two stars can also be deduced. In short, when a full set of
observations is available, it is possible to give a comprehensive
description of an eclipsing binary system and its components.
Eclipsing, spectroscopic binaries thus represent true cornerstones for the
determination of stellar masses, and as such they are fundamental for our
understanding of stellar evolution. Rather few such systems are known, but
they can also be used to check (“calibrate”) other, indirect methods to
derive stellar parameters.
It is on this background that the first discovery of an eclipsing binary
system with two young, solar-like stars is of great interest.
The Orion Binary
Young stars are not so easy to find. One way is to look for their
high-energy emission from a hot corona, created by their enhanced magnetic
activity. The object RXJ 0529.4+0041 was first discovered in this way by
the X-ray satellite ROSAT.
Subsequent optical spectroscopy showed this object to be a young, low-mass
spectroscopic binary system. And when a team of astronomers [1] used a
91-cm telescope at the Serra La Nave observing station on the slope of the
Etna volcano (Sicily) to monitor the light curve, they also discovered that
this system undergoes eclipses.
All data confirm that 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 the binary system was no exception. In
particular, fairly strong absorption lines of the fragile element Lithium
[4] were detected in both of the binary stars. As Lithium is known to be
rapidly destroyed in stars, the finding of a relatively high content of
this element implies that the stars must indeed be young. They were
probably formed no more than 10 million years ago, i.e., 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. This also means they must be very close to each other
(but still entirely detached from each other) — the detailed analysis
showed that the distance between the two components is only 12 solar radii,
or a little more than 8 million kilometres. If you would image yourself
standing on the surface of the smaller star, the disk of the companion star
would extend some 15 in the sky. This is 30 times larger than our view of
the Sun!
ADONIS observations
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. Most fortunately, 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.
Caption: Six individual frames from the ADONIS movie of the RXJ
0529.4+0041 eclipsing, binary stellar system, corresponding to the
time around the “primary” and “secondary” eclipses, respectively. For
a detailed explanation, read the text.ESO PR Video Clip 06/01 (150 frames/00:06 min)
[MPEG Video; 512 x 448 pix; 871 k]ESO Video Clip 06/01 shows the ADONIS images of the RXJ 0529.4+0041
eclipsing, binary stellar system, as recorded in three near-infrared
filters (J, H, and K; to the left), with the observed light-curves (top)
and a graphical representation of the system during a full orbit, as
it would look like to a nearby observer. More details in the text.
The ADONIS images have been combined into an instructive movie (PR Video
Clip 06/01). The left-hand panel shows the eclipsing binary system (it is
the upper right and brighter of the two objects; the light from the two
stars merge into a single point of light) and the well visible 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 clearly changes — it may help to play the movie several times to
see this more clearly. For reference, the Universal Time (UT) and the
orbital phase (increasing from 0 to 1 during a full revolution) are
continuously displayed in the movie.
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 well visible. 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
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”.
The stellar parameters
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, i.e., about 2.6×10^30 kg [2]. Its diameter is
nearly 1.6 times larger than that of our Sun (i.e., about 2.2 million km)
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 (1.8×10^30
kg) and the diameter is 20% larger (about 1.7 million km), while the
surface temperature is 4000 degrees.
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.
The significance of RXJ 0529.4+0041
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.
More information
Part of the results described in this press release are described in more
detail in a scientific article (“RXJ 0529.4+0041: a low-mass pre-main
sequence eclipsing-spectroscopic binary” by E. Covino et al.) that has been
published in the European research journal Astronomy & Astrophysics (Vol.
361, p. 49).
Notes
[1] The team consists of Elvira Covino (Principal Investigator), Juan M.
Alcalá, Rosita Paladino (all Osservatorio Astronomico di Capodimonte,
Napoli, Italy), Antonio Frasca, Santo Catalano, Ettore Marilli (all
Osservatorio Astrofisico di Catania, Italy) and Michael Sterzik (ESO-Chile).
[2] One solar mass corresponds to 1.99×10^30 kg, or about 330,000 times the
mass of the Earth. The Sun is about 4500 million years old and its total
lifetime is of the order of 12-13,000 million years. It is an interesting
thought that if the Sun would have been somewhat heavier, its total lifetime
might have been too short for living organisms to develop on the Earth. In
fact, the biological evolution that ultimately lead to the emergence of
human beings apparently lasted about 4 billion years; this corresponds to
the total lifetime of a star that is only about 20% heavier than the Sun.
Note also the current ESO-ESA CERN educational programme on “Life in the
Universe”.
[3] In the case of exoplanets, the planet itself is not visible, but the
spectral lines from the star are seen to wobble due to the gravitational
influence of the planet, cf. ESO PR 07/01.
[4] Several ESO Press Releases concern observations of the element Lithium
in stars, e.g., PR 03/99 (in a giant star), PR 08/00 (in a metal-poor star)
and PR 10/01 (from a “swallowed” exoplanet).
Contacts
Elvira Covino
Osservatorio Astronomico di Capodimonte
Napoli, Italy
Tel.: +39 081 5575581
E-Mail: covino@astro.na.it
Juan M. Alcalá
Osservatorio Astronomico di Capodimonte
Napoli, Italy
Tel.: +39 081 5575479
E-Mail: alcala@astro.na.it
Antonio Frasca
Osservatorio Astrofisico di Catania
Catania, Italy
Tel.: + 39 095 7332240
E-Mail: afr@ct.astro.it
Michael Sterzik
ESO-Santiago
Chile
Tel.: +56 2 228 5006
E-Mail: msterzik@eso.org
