Between Stars and Planets: the Missing Link

By Michael Liu and Trent Dupuy, Institute for Astronomy, University of Hawaii

Understanding what stars are made of and how they shine is a surprisingly recent discovery for humanity. The underlying cause is that the intense temperatures and pressures in their central regions are sufficient to fuse hydrogen into helium, and in the process energy is generated and released at a star’s surface as light. Only by the mid-twentieth century was it established that stars are these giant hydrogen fusion reactors, and astronomers were for the first time able to write down equations describing their hidden interiors. The energy output could be precisely predicted from these equations, and the relatively new theory of quantum physics provided the explanation for the properties of the light, or spectra, emitted from stars. These theoretical models of stars underlie most of modern astronomy research today and represent one of the great intellectual triumphs of the last century.

Despite these successes, understanding the inner workings of stars and planets remains one of astronomy’s fundamental challenges. Unlike stars, planets do not generate their own internal energy, as core hydrogen fusion is only possible for objects with a mass greater than about eight percent the mass of the Sun. (Jupiter, the most massive planet in the Solar System, has about 1/1000 of the Sun’s mass, and the Earth has about 1/300,000 the Sun’s mass.) This means that gas-giant planets like Jupiter are continually evolving over their lifetime, starting from an early hot, enlarged state and then cooling and shrinking with time. In addition, the gaseous atmospheres of planets are much colder than those of stars. Stars are hot enough to obliterate molecules and ionize the constituent atoms, creating a plasma fluid that is relatively simple to describe with basic physics. However, the colder atmospheres of planets harbor molecules and even clouds of particulate matter, or dust, both of which are much more difficult to predict theoretically.

At face value, stars and planets appear to be two very different classes of objects, separated by more than a factor of ten in mass and a factor of million in luminosity, and having radically different emergent spectra. In the 1960s, theorists Takenori Nakano and Shiv Kumar independently hypothesized a class of objects that are intermediates between planets and stars. The objects are now called brown dwarfs. These “missing links” were conceived of as objects with insufficient masses to reach the internal temperatures and pressures needed to become fusion reactors, yet they could be about 100 times more massive than the gas giants Jupiter and Saturn. As such, brown dwarfs would have surface temperatures between the coolest stars and the hottest planets. Even without internal energy generation, they could emit some light because residual heat from the time of their formation would be gradually released as they aged and cooled.

The first brown dwarf was conclusively identified in 1995, emitting infrared radiation like the coolest stars and yet its atmospheric properties were similar to the planet Jupiter, marked by a very low temperature and complex molecules. Since then, astronomers have identified hundreds of brown dwarfs within 100 light-years of Earth, including many with observations from the Keck Telescopes. In fact, we believe that nature produces about as many brown dwarfs as stars, but because brown dwarfs emit so little energy they are much harder to find in the sky. The coolest known brown dwarfs have surface temperatures comparable to the inside of a pizza oven (800 degrees Fahrenheit) more than 9,000 F cooler than the surface of the Sun. The study of brown dwarfs has become an extremely active area of research in the last decade, because they represent the lowest mass products of star formation and they can provide critical insights into the physics of low-temperature atmospheres.

Shortly after the first brown dwarfs were identified, observations with the Hubble Space Telescope found that some of them occur as binary systems, namely two brown dwarfs bound together through their mutual gravitational attraction and orbiting each other in a fashion similar to how the Earth orbits the Sun. Overall, about 15 percent of brown dwarfs are binaries. This is not surprising, since we know that about 50 percent of stars are binaries and we believe brown dwarfs form in a similar fashion to stars (though the reason as to why the brown dwarf binaries are rarer is a topic for another article).

Astronomy’s Sharpest Eyes

We have been conducting a long-term observational study of nearby brown dwarf binaries, with the dual goals of finding more of these systems and then carefully scrutinizing them to understand their complex physical properties. Such observations are very challenging, because nearly all brown dwarf binaries are separated by only a small angle on the sky. With ordinary astronomical imaging instruments, it is not possible to resolve the two components of the binaries.

To do this research, we require the unique capabilities of the Keck II Telescope. Since 2005, this telescope has been equipped with a powerful laser-guided adaptive optics (AO) system that corrects for the blurring of astronomical images caused by turbulence in Earth’s atmosphere. While astronomers have been using AO technology for nearly two decades on various telescopes, including on the Keck II Telescope since 1999, brown dwarfs have always been far too faint for traditional AO systems, which only work with bright stars. The Keck laser system creates an “artificial star” in the sky, which can then be pointed at the brown dwarfs to produce unprecedentedly sharp infrared images.

Keck II was the first large (eight to ten meter) ground-based telescope to deploy a laser-guided AO system, and this capability has been a transformational technology for many areas of astronomical research. The physics of light is such that the theoretical limit for the sharpness of an image from a telescope is inversely proportional to the diameter of the telescope’s primary mirror — bigger telescopes can make sharper images. However, the typical image quality of ground-based telescope is usually much worse, since the Earth’s turbulent atmosphere blurs light from stars. This problem can be overcome by equipping ground-based telescopes with AO. Since the Kecks are the largest optical/infrared telescopes anywhere in the world (or in space), Keck II AO produces the sharpest images ever achieved, three to four times sharper than those produced by Hubble. The images produced by Keck have an angular resolution as good as 1/20 of an arc second, about 1/40,000 the diameter of the full moon. If a person’s vision were as sharp as the Keck adaptive optics system, he 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. 

Two Are Better Than One

While appearing to be very simple systems, it turns out that brown dwarfs paired together provide remarkably useful laboratories for learning about the physics of low-temperature objects. The reason for this can be understood through an analogy with biology. Human beings occur in all different shapes, sizes, appearances, and personalities. One potent method for exploring the origins of these differences is by studying twins, namely two people born with the same genetic composition. Like an individual person, single, non-binary brown dwarfs free-floating in space are difficult to study since such objects possess a wide and unknown mix of ages, masses and compositions. However, for binary systems (“brown dwarf twins”), we believe that they formed together at the same time and out of the same natal gaseous material, which is useful information when studying these complex objects.

Furthermore, as first shown by Johannes Kepler in the 17th century, the total mass of any binary system can be determined by precisely measuring the orbit’s size and how long it takes for the two objects to complete one orbital cycle — the orbital period. Thus, binary brown dwarfs provide a unique opportunity to directly measure the masses of ultracool objects. In fact, mass is the fundamental parameter that governs the life-history of any free-floating object (star, planet, or brown dwarf). Over the last decade, astronomers have measured the energy outputs and temperatures for hundreds of brown dwarfs. However, the most important property of all — the mass — is the hardest one to measure.

Patience is a Virtue

Measuring masses of brown dwarf binaries is a challenging undertaking, requiring both technical sophistication (high-resolution imaging) and personal fortitude (specifically, patience). The former is needed because of the very small angular separations of brown dwarf binaries on the sky, and thus only very sharp images can precisely monitor the motion of the two components around each other. Patience is needed because the orbital motion is very slow. Typical orbital periods are estimated to be a decade of more.

In collaboration with Dr. Michael Ireland of the University of Sydney, we have been regularly monitoring about three dozen binaries with Keck adaptive optics since laser-guided AO became operational in 2005. The Hubble Space Telescope originally discovered many of our binaries, with observations dating as far back as the year 1998, but only a single epoch of imaging was obtained. Observing with Keck allows us to obtain many more epochs and with higher precision. Then, combining the Keck and Hubble datasets lets us precisely measure the size and duration of the orbits over a very long time baseline, thereby determining the masses of the binaries.

Thus far, our team has completed measuring the masses of two brown dwarf binaries. While two may not seem like a large number compared to the hundreds of brown dwarfs known, such precise measurements are very hard-earned, and each new result has great scientific value. We have already doubled the number of brown dwarfs with dynamical mass determinations, and in fact our results are the first two measurements ever for such cold objects.

One binary, known as 2MASS 1534-2952AB, is composed of two “methane” brown dwarfs, the coolest type of brown dwarf, which are characterized by the presence of methane gas in their atmospheres. This is the first mass measurement for this type of object. Our team found that the total mass of 2MASS 1534-2952AB is only six percent of the Sun’s mass, and each of its constituent brown dwarfs has a mass of about three percent of the Sun’s mass (about 30 times the mass of Jupiter). The other binary system, HD 130948BC, is a pair of slightly warmer “dusty” brown dwarfs with a total mass of only 11 percent of the Sun’s mass and individual masses of about 5.5 percent of the Sun’s mass.

The two binaries, located in the constellations of Libra (the Scales) and Bootes (the Herdsman), are about 45 and 60 light-years from Earth. The orbital configurations of the two binaries are very similar — both have orbital separations of about two astronomical units (AU), where one AU is the distance from Earth to the Sun or 93 million miles. This is somewhat larger than the 1.5 AU distance between Mars and the Sun. Their orbital periods are about 10-15 years compared with the two years it takes for Mars to orbit the Sun; the longer period is a direct result of the low masses of these objects, leading to much weaker mutual gravitational attraction than the Sun-Mars system.

Theoretical models attempt to predict the masses of brown dwarfs based on their energy output and temperature. But when we compared our mass measurements to the theoretical predictions, they did not agree. For example, the surface temperature of 2MASS 1534-2952AB was much cooler than expected given its current level of energy output, while HD 130948BC was much warmer.

Our study of HD 130948BC turned out to be especially puzzling. This brown dwarf binary system is actually part of a triple system in which the primary star (HD 130948A) is a young, Sun-like star that is about 600 million years old. The only quantity more difficult to measure than mass for astronomical objects is their age. However, when a Sun-like star is young, it displays energetic phenomena such as intense activity in its upper atmosphere that enables us to gauge its age accurately. Therefore, assuming the Sun-like star and brown dwarf binary formed at the same time (which is a very conservative assumption), we can use the Sun-like star as a “clock” to determine the age of its companion binary brown dwarf. Because of this unique set of information, HD 130948BC is now the gold standard for testing predictions of theoretical models of brown dwarfs. We find that the energy output theoretically predicted for the system seriously disagrees with observations: the system is emitting two to three times more energy than expected by models.

While theoretical predictions of brown dwarfs come close to matching our observations, something is obviously not quite right with the theory — either in determining the temperatures, in predicting the energy output, or perhaps both. This is a disturbing and potentially far-reaching result. These same theoretical models are used to infer the properties (masses, temperatures, and ages) of the hundreds of other known brown dwarfs that are not in binary systems. In addition, these models are used to predict the properties of gas-giant extrasolar planets found around other stars, such as those recently directly imaged around the stars Fomalhaut and HR 8799. Clearly, something is missing from our understanding of the coolest objects that do not generate their own internal energy, from brown dwarfs to exoplanets.

What Lies Ahead

Our work to date has been both thrilling and puzzling. Thrilling in the sense that the Keck II telescope has allowed us to carry out the most detailed studies of brown dwarf binaries to date, and yet puzzling in that our mass measurements have pointed to problems with our current understanding of the inner workings of low temperature objects. For us, these conundrums serve as inspiration to measure masses for more brown dwarfs in the coming years in order to better understand the successes and failings of current theory.

By 2010, the Keck I Telescope will also be equipped with a laser-guided AO system, which will be more advanced than the existing Keck II system. Looking ahead in the more distant future, however, Keck is now actively in the planning stages for its Next Generation Adaptive Optics system or NGAO. Working closely with astronomers and engineers throughout the Keck community, the new system will employ multiple laser guide stars to provide even sharper, more accurate images. It will even extend AO capability beyond the infrared to a much wider range of wavelengths. With these powerful new tools, we expect to make great leaps in understanding the inner workings of nature’s coolest, faintest objects. 

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