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Weighing the Coldest Brown Dwarfs

by Trent Dupuy, IfA Graduate Student

the brown dwarf binary HD130948BC

Infrared image of the dusty brown dwarf binary HD 130948BC. The binary is seen in the upper left and has a total mass about 11 percent the mass of the Sun. The binary is in orbit around a young Sun-like star, seen to the lower right. Image by T. Dupuy and M. Liu.

Brown dwarfs are free-floating objects that are not massive enough to achieve the high internal temperature and pressure needed to sustain nuclear fusion, which powers stars. So, unlike Sun-like stars, which shine steadily for billions of years, brown dwarfs cool and fade over their lifetime. Mass is the fundamental parameter that determines how their properties evolve over time.

IfA astronomer Michael Liu, University of Sydney astronomer Michael Ireland, and I have been monitoring the orbits of brown dwarf binary systems, in which one brown dwarf orbits another. We have recently determined the masses of two of these systems. Our findings show that theoretical predictions of how the properties of brown dwarfs evolve may have some problems. The implications of these results are far-reaching, since the same theoretical models are used to predict the properties of both brown dwarfs and gas-giant planets, including those found around other stars (exoplanets).

In the 17th century, Johannes Kepler showed that the total mass of any binary system could be determined by precisely measuring the orbit's size and how long it takes to complete one orbit. The same laws of motion apply whether the system being studied is a planet orbiting the Sun or one brown dwarf orbiting another.

Measuring the sizes and periods of orbits is challenging, patient work because brown dwarf binaries have tiny separations on the sky and orbit each other very slowly. Our observations span eight years, and our highest resolution images come from the 10-meter (400-inch) Keck Telescope, which is equipped with a powerful adaptive optics system that corrects for the blurring of astronomical images caused by turbulence in Earth's atmosphere. A person with vision as sharp as the Keck adaptive optics system would be able to read a magazine that was about a mile away. In fact, the positional accuracy achieved with such sharp images is equivalent to hitting a bull's-eye on a dartboard that is 8,000 miles away.

Brown dwarf binary 2MASS 1534-2952AB

Infrared image of the very low-temperature binary 2MASS 1534-2952AB, composed of two methane brown dwarfs. These are the coolest free-floating objects ever directly weighed outside the solar system. Image by M. Liu.

By regularly monitoring binaries with Keck adaptive optics and analyzing previous data obtained by the Hubble Space Telescope, we were able to precisely measure the sizes and periods of two binaries' orbits, and thereby determine their masses. One, known as 2MASS 1534-2952AB, is composed of two "methane" brown dwarfs, which have a temperature about 800 degrees F, about the same as a pizza oven. Each of these has a mass about 3 percent of the Sun's. The other binary system, HD 130948BC, is a pair of slightly warmer "dusty" brown dwarfs, each of which has a mass of about 5 percent the Sun's mass. These have a temperature of about 3,000 degrees, above the melting point of steel, yet still 7,000 degrees cooler than the Sun.

The binary system HD 130948BC is actually part of a triple system in which the primary component (HD 130948A) is a young, Sun-like star that is only 550-800 million years old. The only quantity more difficult to measure than mass for astronomical objects is their age. However, when a star is young, it displays phenomena, such as intense magnetic fields that superheat its upper atmosphere, that enable us to gauge its age accurately. HD 130948BC is thus the gold standard for testing the predictions of theoretical models since we have determined both its mass and age.

Theoretical models predict the energy output and temperature of brown dwarfs based on their mass and age, but our observations do not agree with these models. For example, 2MASS 1534-2952AB is somewhat cooler than expected given its current level of energy output, while HD 130948BC is somewhat warmer. Even more surprising is the profuse energy output of HD 130948BC, which is two to four times larger than predicted by theoretical models!

These puzzling results illustrate a significant gap in our understanding of very cool objects that do not generate their own internal energy, from brown dwarfs to exoplanets. Future progress in this area will require more data, and in the next few years, we plan to use the Keck Telescope and Hubble Space Telescope to make many more mass measurements of brown dwarfs.