One Night of Many
By Ryan Wyatt
On Monday, I described a night of observing at Chile’s Las Campanas Observatory, and yesterday, I gave some background on the objects known as Y dwarfs—the coldest and least massive category of stellar wannabes known as brown dwarfs. I’m tagging along with two researchers—Jackie Faherty, an NSF fellow at the Universidad de Chile, and Chris Tinney, a professor at the University of New South Wales—to describe some of their research on these objects.
Most importantly, the two astronomers are chasing a short “target list” of Y dwarfs, making careful measurements to determine their distances and velocities—where they are, and where they’re going. One aspect of this research could make headlines quite easily, if one of these brown dwarfs turns out to lie closer to the Sun than the nearest star, Proxima Centauri (but not so close as the fabled Nemesis object proposed as a resident of the outer Solar System). Such a discovery would beat out even the recent announcement of “the closest star system found in a century,” and indeed, Faherty and Tinney used some of their observing time to take a look at that object to learn more about it.
To accomplish their program, Faherty and Tinney use the principal of stellar parallax, or the apparent shift of an object as the Earth travels around the Sun over the course of a year. “Parallaxes offer the most reliable distance estimates we have in astronomy. They take care and time but the scientific return is enormous,” says Faherty. Forgive me as I digress for the rest of the paragraph, and feel free to skip to the beginning of the next… Typically, when people teach about parallax (even in three detailed videos), they treat it as a two-dimensional problem, possibly because the trigonometry is easiest to draw on the surface of a blackboard or a video screen. In reality, Earth‘s orbit and distant stars represent a three-dimensional relationship that includes a lot more information than a perceived back-and-forth movement of an object observed in six-month intervals. Over the course of a year, a stars path actually inscribes a tiny ellipse on the sky as perceived from different points in (elliptical, BTW) orbit around the Sun. I won’t go into details, but it turns out that astronomers are trying to measure the size of these elliptical movements (they already know the shape based on the star’s position in our sky), a much more manageable task than measuring a linear shift in six-month increments.
Nonetheless, measuring parallaxes requires numerous precise determinations of a star’s position on the sky over multiple years, and it only works for the objects quite nearby in astronomical terms. Luckily, these Y dwarfs are some of the closest things outside our Solar System (cf. that previous comment about hoping to find a brown dwarf closer than the nearest star), so they exhibit enormous parallaxes.
Of course, Y dwarfs also happen to be quite faint, which makes the observations trickier, and they also don’t sit still. The classic parallax description assumes that the star remains motionless relative to us (a good assumption for most stars), but some nearby stars zip along on their own trajectory through three-dimensional space. Any movement not toward or away from us shows up as the star’s proper motion, or apparent motion on the sky from year to year, turning the neat ellipse described above into a squiggly path across the sky.
But that turns out to be a bonus! The proper motion of the star can suggest a relationship to other stars nearby. Stars that move together probably share a common origin, so a Y dwarf’s association with such a “moving group” (e.g., the ones associated with Ursa Major or Beta Pictoris) gives a clear indication of its age. You can’t determine the age of most lone stars, but a group of stars that share a common origin show color and temperature relationships that allow for an accurate estimate of the group’s collective age.
Age provides a critical benchmark for understanding Y dwarfs. As Faherty describes it:
In a nutshell, brown dwarfs lack the nice relationship that exists for stars whereby you can get an idea of the mass if you know its temperature. Anything goes for objects below 3,000 Kelvin. Without an age you might be studying an old low-mass star, a cool brown dwarf, or a hot planet. In each case, their light fingerprint would appear the same.
Faherty and Tinney will return to these Y dwarfs for many nights over the next several years, catching them at the right times to refine estimates of their positions and velocities, and figuring out how these objects fit into the awkward conceptual space between stars and planets. They started their parallax program in March of 2012, and they published their first paper from the campaign last fall.
Eventually, their observations will lead to a clearer understanding of these exotic objects, and these studies also pave the way to understanding the hundreds upon hundreds of planets we’re finding orbiting other stars. Faherty summarizes:
We are at an interesting crossroad with brown dwarf science. We’ve found the objects with temperatures that inch up next to Jupiter. They are faint but they are sitting out in space all by their lonesome waiting to be studied in detail. Y dwarfs with parallaxes will be a critical key to understanding the composition of exoplanets.
Thus, each night of observing plays a role in piecing together a much larger puzzle, revealing a picture of objects that reside along a continuum from planet to brown dwarf to star.
By the way, if you’re in San Francisco and looking for something tomorrow night, come to NightLife! At the 6:30 planetarium show, I’ll talk about “Color of the Cosmos” and describe how astronomers don’t see (or talk about) color the way most people do. I might even mention brown dwarfs…
Ryan Wyatt is the director of the Morrison Planetarium and Science Visualization at the California Academy of Sciences.
Image credit: Karl Schultz