Pondering the nature of an ultracool object in Orion
Posted: May 22, 2002

A new substellar object, named SOri70, has been discovered near the young star Sigma Orionis. Is it a young planet, or a wandering old brown dwarf in the line of sight? This is a question to be discussed by the astronomers attending the International Astronomical Union Symposium on Brown Dwarfs this week on the Big Island of Hawaii.

An image of the Sigma Orionis region. The multiple star Sigma Orionis, which is visible with the naked eye, is at the center. A box indicates the position of the planet candidate, which is only 8.7 arcminutes from the star. The image was taken from the Digital Sky Survey and has a size of 23 x 22 square arcminutes. The inset shows the infrared image obtained at the William Herschel Telescope by Dr. Victor Bejar and Prof. Eduardo Martin.
Deep sky images and follow-up spectroscopy obtained by an international team of astronomers revealed this extremely cool and dim object close to the multiple stellar system Sigma Orionis. The astronomers made the observations with large telescopes in Hawaii and the Canary Islands.

Since the acute visual observations of Sir William Herschel in the eighteenth century, astronomers have noted a clustering of stars in a region of the sky of about the size of the full moon surrounding the hot star Sigma Orionis. Many X-ray emitting low-mass stars in this cluster were found by Scott Wolk and Fred Walter of SUNY at Stony Brook. Several brown dwarfs in this region were revealed by some of the members of the team that today reports on the discovery of the coolest and faintest object ever seen around Sigma Orionis.

The story of how SOri70 was found includes two of the world's most powerful telescopes separated by more than 8,000 miles and about 4 years of international collaborative effort. It is an example of the complicated work that is needed to hunt for the elusive brown dwarfs and extrasolar planets.

In December 1998, team members Victor Bejar and Eduardo Martin pointed one of the world's largest optical telescopes, the 10-meter Keck I on Mauna Kea (Hawaii), at several fields around Sigma Orionis and obtained CCD images of unprecedented sensitivity for this region of the sky. They found several extremely faint red objects, but they did not have enough information to determine their basic properties. They had to wait patiently for a chance to obtain additional data. It came when they used an infrared camera at the William Herschel Telescope in La Palma (Canary Islands) in November 2000.

One of the objects turned out to have blue infrared colors despite being very red at optical wavelengths, a unique signature of the coolest known dwarfs. The unusual colors of these dwarfs are explained by the presence of methane in their atmospheres, which is a gas that can be present only at temperatures lower than about 1,200 degrees Kelvin (about 900 degrees Celsius or 1650 degrees Fahrenheit). An object of this temperature must have a mass smaller than a star.

The intriguing object was observed once more with the Keck I telescope in December 2001 by team member Maria Rosa Zapatero Osorio of the Laboratory for Fundamental Astrophysics in Madrid, Spain. These observations confirmed spectroscopically the presence of methane in the object, which unambiguously classifies it as a brown dwarf or planet. If the object is located at the same distance as the Sigma Orionis system (1,150 light-years from Earth), it should have an age between 1 and 8 million years and a mass close to that of Jupiter, the largest planet in the Solar System.

However, the distance to the object is not known yet; it will take the sharp imaging capabilities of the Hubble Space Telescope to determine it. There is about a 20% probability that SOri70 is a wandering old brown dwarf that happens to be in the direction of the Sigma Orionis, but is actually closer to Earth.

If the new ultracool dwarf is related to the Sigma Orionis system, it would be the lowest mass extrasolar object imaged to date. Because it would be located more than 180,000 astronomical units from Sigma Orionis (more than 36,000 times the Jupiter-Sun distance), it would challenge our ideas about the formation of extrasolar giant planets.

Four members of the science team are attending the International Astronomical Union Symposium on Brown Dwarfs on the Big Island of Hawaii during this week, namely, Dr. David Barrado y Navascues (Laboratory for Fundamental Astrophysics in Madrid, Spain), Mr. Jose Antonio Caballero (Instituto de Astrofisica de Canarias), Prof. Eduardo Martin (University of Hawaii Institute for Astronomy) and Dr. Maria Rosa Zapatero Osorio (Laboratory for Fundamental Astrophysics in Madrid, Spain). Other team members include Dr. Victor Bejar (Instituto de Astrofisica de Canarias), Dr. Joachim Eisloeffel (Thueringer Landessternwarte Tautenburg, Germany), Dr. Reinhold Mundt (Max Plank Institut fur Astronomie, Heidelberg, Germany), and Dr. Rafael Rebolo (Instituto de Astrofisica de Canarias).

Members of the press and news media are invited to attend all sessions of the Symposium at no cost. They are asked to check in at the Conference Registration Desk at the Outrigger Waikoloa Beach Hotel in order to obtain Symposium materials and other information of interest.

The Institute for Astronomy at the University of Hawaii conducts research into galaxies, cosmology, stars, planets, and the Sun. Its faculty and staff are also involved in astronomy education, deep space missions, and in the development and management of the observatories on Haleakala and Mauna Kea.

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