Universe Today

Image credit: Canadian Space Agency
MOST, Canada?s first space telescope, is shaking up the way astronomers think about stars — and putting a new spin on the life story of our own Sun — by allowing astronomers to see in unprecedented detail how stars shake and spin.

The first results from MOST, a Canadian Space Agency mission which was also the first scientific satellite to be launched by Canada in over 30 years, include the detection of a strong ?pulse? in a young adult star called eta Bootis, and a bad case of stellar acne and hyperactivity in a ?pre-teen? version of the Sun, kappa 1 Ceti. These data offer a unique perspective on what our own Sun may have been like in its youth.

?All this talk of stellar pulses and hyperactivity must sound like ER Meets Star Trek,? admitted MOST Mission Scientist Dr. Jaymie Matthews of the University of British Columbia, who presented the findings today in a keynote address to the annual meeting of the Canadian Astronomical Society in Winnipeg. ?But we really are doing diagnostic check-ups of stars at different points in their lives, by placing them under intensive observation for weeks at a time.?

Matthews made the presentation to a gathering of physicists, astrophysicists, and medical physicists at a unique conference of Canadian physics societies (CAP/CASCA/COMP/BSC CONGRESS 2004) hosted by the Department of Physics and Astronomy at the University of Manitoba in celebration of the Faculty of Science’s 100th anniversary.

These are ambitious results from a Canadian-built and -operated orbiting observatory which is no bigger than a suitcase but can monitor the brightnesses of stars with unmatched precision and thoroughness. MOST, which stands for Microvariability and Oscillations of STars, was launched into orbit last summer and has been collecting data for the last few months.

?MOST is a major advance in the way astronomers study stars, made possible by innovative Canadian technology,? noted Canadian Space Agency President, Dr. Marc Garneau. ?It is the world?s most precise light meter, capable of recording variations as small as one ten thousandth of a percent in the brightness of a star.?

How small is that?

?If all the lights in all the offices of the Empire State Building were on at night,? explains Dr. Garneau, ?you could dim the total light by 1/10,000th of a percent if you pulled down just one window blind by only one centimetre.?

From its vantage point in polar orbit, 820 km high, the tiny MOST space telescope can stare at stars without interruption for up to eight weeks. No other observatory or network of telescopes, including the Hubble, can do this. The unique combination of precision and time coverage enables MOST to look for subtle vibrations in stars that will reveal secrets hidden beneath their surfaces. It also gives MOST the best chance to detect light directly from planets outside our Solar System and study their atmospheres and weather.

MOST is a Canadian Space Agency mission. Dynacon Inc. of Mississauga, Ontario, is the prime contractor for the satellite and its operation, with the University of Toronto Institute for Aerospace Studies (UTIAS) as a major subcontractor. The University of British Columbia (UBC) is the main contractor for the instrument and scientific operations of the MOST mission. MOST is tracked and operated through a global network of ground stations located at UTIAS, UBC and the University of Vienna.

The MOST Canadian space telescope was launched from northern Russia in June 2003 aboard a former Soviet ICBM (Intercontinental Ballistic Missile) converted to peaceful use. Weighing only 54 kg, this suitcase-sized microsatellite is packed with a small telescope and electronic camera to study stellar variability.

One of its early targets was the star eta Bootis, a slightly more massive and younger version of the Sun. Astronomers had picked out this star as one of the best candidates for the new technique of ?asteroseismology? — using surface vibrations to probe the inside of a star, similar to how geophysicists use earthquake vibrations to probe the Earth?s core.

MOST monitored eta Bootis for 28 days without interruption, placing the star under a 24-hour scientific ?stake-out? that revealed behaviour that was hidden from the limited view possible for Earth-bound telescopes. Accumulating almost a quarter of a million individual measurements of this star, MOST reached a level of light-measuring precision at least 10 times better than the best ever achieved before from Earth or space.

The data reveal the star is vibrating, but at a pitch well below the range of human hearing. The stellar melody should allow the MOST team of scientists, including Dr. David Guenther of the Canadian Institute for Computational Astrophysics at St. Mary?s University, Halifax, to determine the age and structure of eta Bootis. ?We?re now in a position to explore new physics in stars, with observations like these,? said Dr. Guenther.

Before observing eta Bootis, while still in the shakedown phase of its mission, MOST was aimed for testing purposes at a fainter star called kappa 1 Ceti. Astronomers already suspected this was a younger version of our Sun, with an age of about 750 million years. The Sun?s age is about 4.5 billion years, and it?s just entering middle age. In terms of a human life, the Sun would be about 45 years old while kappa 1 Ceti would be eight years old ? barely a pre-teen.

Like many human kids, Kappa 1 Ceti is hyperactive, flaring up from time to time, and spinning with much more kinetic energy than sedate older stars like the Sun. It also has a severe case of acne — dark spots on its face which are much larger than those visible on the Sun’s surface. The MOST data, following Kappa 1 Ceti for 29 days, show in exquisite detail how the spots move across the visible side of the star as it spins once every nine days or so. And because a star is not solid, different parts of its gaseous surface spin at different rates. MOST has been able to measure this effect directly in a star other than the Sun for the first time. These results are being prepared for submission to The Astrophysical Journal.

Future targets for MOST include other stars representing the Sun at various stages in its life, and stars known to have giant planets. MOST is designed to be able to register the tiny changes in brightness that will occur as a planet orbits its parent star. The way in which the light changes will tell astronomers about the atmospheric composition of these mysterious worlds, and even if they have clouds.

?It?s like doing a weather report for a planet outside our Solar System,? says Dr. Jaymie Matthews, MOST Mission Scientist, of the University of British Columbia.

Original Source: UBC News Release

Image credit: ESO
Using ESO’s Very Large Telescope at Paranal and a suite of ground- and space-based telescopes in a four-year long study, an international team of astronomers has measured for the first time the mass of an ultra-cool star and its companion brown dwarf. The two stars form a binary system and orbit each other in about 10 years.

The team obtained high-resolution near-infrared images; on the ground, they defeated the blurring effect of the terrestrial atmosphere by means of adaptive optics techniques. By precisely determining the orbit projected on the sky, the astronomers were able to measure the total mass of the stars. Additional data and comparison with stellar models then yield the mass of each of the components.

The heavier of the two stars has a mass around 8.5% of the mass of the Sun and its brown dwarf companion is even lighter, only 6% of the solar mass. Both objects are relatively young with an age of about 500-1,000 million years.

These observations represent a decisive step towards the still missing calibration of stellar evolution models for very-low mass stars.

Telephone number star
Even though astronomers have found several hundreds of very low mass stars and brown dwarfs, the fundamental properties of these extreme objects, such as masses and surface temperatures, are still not well known. Within the cosmic zoo, these ultra-cool stars represent a class of “intermediate” objects between giant planets – like Jupiter – and “normal” stars less massive than our Sun, and to understand them well is therefore crucial to the field of stellar astrophysics.

The problem with these ultra-cool stars is that contrary to normal stars that burn hydrogen in their central core, no unique relation exists between the luminosity of the star and its mass. Indeed, luminosities and surface temperatures of ultra-cool dwarf stars depend both on their age and their mass. An older, somewhat more massive ultra-cool dwarf can thus have exactly the same temperature as a younger, less massive one.

It is therefore a basic goal of modern astrophysics to obtain independently the masses of an ultra-cool dwarf star. This is in principle possible by studying such objects that are members in a binary system.

This is precisely what an international team of astronomers has now done in a four-year long study of a binary stellar system with an ultra-cool dwarf star, using a plethora of top telescopic facilities, including ESO’s Very Large Telescope, as well as Keck I and Gemini North in Hawaii and also the Hubble Space Telescope. This system – with the telephone number name of 2MASSW J0746425+2000321 – is located at a distance of 40 light-years.

The astronomers used high-angular-resolution imaging to see both stars in the binary system and to measure their motion over a four-year period. However, this is more easily said than done, as the separation on the sky between the two stars is quite small: between 0.13 and 0.22 arcsec. This corresponds to the size of a 1-Euro coin, seen at a distance of about 25 km.

This separation is so small that it is normally not possible to differentiate the two stars due to the blurring effect of atmospheric turbulence (the “seeing”). It is therefore necessary to use the technique of adaptive optics. This wonderful method is based on the measurement of the image quality in real-time and sending corresponding corrective signals up to 100 times every second to a small deformable mirror, located in front of the detector. As the mirror continuously modifies its shape, the disturbing effect of the turbulence is neutralised. Applied at the VLT, this technique has resulted in images which are at least ten times sharper than the “seeing” and which therefore show many more details in the observed objects.

At the Very Large Telescope, the astronomers used the state-of-the-art adaptive optics NACO instrument. Says Herv? Bouy, principal author of the paper presenting the results described here: “NACO offers the possibility to work in the infrared and is therefore ideally suited for the study of ultra-cool stars, which emit most of their light in this wavelength range. With the combination of the high efficiency of NACO and the VLT, and the excellent atmospheric conditions prevailing at Paranal, we were able to achieve very sharp images of this binary stellar system, almost as good as if the telescope were located in space.”

Ultra-cool and on diet
During their four-year long study, seven different relative positions of the two components of the binary system were measured and Herv? Bouy and his co-workers were able to determine with good precision the stellar orbits. They find that the two stars revolve around each other once every 10 years and that their physical separation is only 2.5 times the distance of the Earth to the Sun – as astronomers say, 2.5 Astronomical Units. Using Kepler’s laws, it is then straightforward to derive the total mass of the system. The obtained value is less than 15 % of the mass of the Sun.

The astronomers then used the photometric data of each star obtained in several wavebands, as well as spectra obtained with the Hubble Space Telescope to study the two objects in more detail. Using the latest stellar models of the group of the Ecole Normale Sup?rieure de Lyon, they found that both stars have roughly the same surface temperature, around 1500 ?C (1800 K). For a star, this is ultra-cool indeed – by comparison, the surface temperature of the Sun is more than three times higher.

Using theoretical models, the team also found that the two stars are rather young (in astrophysical terms) – their age is between 500 and 1,000 million years only. The more massive of the two has a mass between 7.5 and 9.5% the mass of the Sun, while its companion has a mass between 5 and 7% of the solar mass.

Objects weighing less than about 7% of our Sun have been variously called “Brown Dwarfs”, “Failed Stars” or “Super Planets”. Indeed, since they have no sustained energy generation by thermal nuclear reactions in their interior, many of their properties are more similar to those of giant gas planets in our own solar system such as Jupiter, than to stars like the Sun.

The system 2MASSW J0746425+2000321 is thus apparently made up of a brown dwarf orbiting a slightly more massive ultra-cool dwarf star. It is a true “Rosetta stone” in the new field of low-mass stellar astrophysics and further studies will surely provide more valuable information about these objects in the transitional zone between stars and planets.

Original Source: ESO News Release