- Paper Title: Asteroid Belts in Debris Disk Twins: Vega and Fomalhaut
- Authors: Kate Y. L. Su, George H. Rieke, Renu Malhotra, Karl R. Stapelfeldt, A. Meredith Hughes, Amy Bonsor, David J. Wilner, Zoltan Balog, Dan M. Watson, Michael W. Werner, Karl A. Misselt
- First Author’s Affiliation: Steward Observatory, University of Arizona
- Journal: ApJ (accepted)
One of the major questions that drives exoplanet research is whether the architecture of the Solar System is unique or commonplace. So far, this question has been hard to answer because the detectable planets are the oddballs and the extreme cases. But Su et al. recently uncovered evidence that the Solar System may not be alone after all. They observed two nearby stellar systems, Vega and Fomalhaut, and discovered that they also harbor belts of rocky material, similar to the Solar System’s asteroid belt.
Vega, a bright star in the northern hemisphere, is surrounded by a disk of rocks and dust known as a debris disk. Infrared telescopes, such as Spitzer and Herschel, can observe the thermal emission from small dust grains in these disks thanks to the large surface area of the dust grains but are unable to detect the larger bodies. However, the presence of dust indicates that large km-sized bodies, called planetesimals, must also be present: several mechanisms work to remove dust grains quickly (such as radiation pressure or Poynting-Robertson drag), so they must be continually replenished by collisions of larger bodies.
Astronomers use infrared observations to determine the temperature of the dust, which depends on its distance from the star. This method is used to find the disk location, even if it cannot be directly imaged. Su et al. used this technique for the disk around Vega, which was already known to have a colder outer disk (T~50 K, shown in Figure 1). Su et al. discovered that there was a second disk, much closer to the star, mirroring the architecture of the Solar System.
The need for precise stellar models
Debris disks were first discovered when stars like Vega turned out to be much brighter in the infrared than they should be. We now know that this infrared excess, known at the time as the “Vega Phenomenon”, is due to thermal emission from dust surrounding the star. But to determine the brightness of the dust (and therefore the dust properties), we need to know exactly how bright the star should be. To do this, Su et al. collected stellar data in the near-IR (2.2-8 μm), where no excess had been seen. They fit a stellar model to the data, and extrapolated out to longer wavelengths.
Discovery of the Asteroid Belt
With the knowledge of the stellar brightness in the infrared, the authors subtracted the stellar signal from the images shown in Figure 1. They found that there was still an excess signal close to the center that could not be explained by the outer belt. There must be another belt close to the star.
Figure 2 shows the stellar subtracted spectra of Vega and three other systems (Eps Eri, Fomalhaut, and HR 8799). The spectra are fit with 2 components: a cold outer disk seen in the resolved images and a warm inner belt at a temperature of 170 Kelvin. This is just above the temperature at at which water ice transitions from a solid state to a gas state. This also happens to be the temperature of the Solar System’s asteroid belt.
Origin of the warm dust
There are two main possibilities to explain the origin of the warm dust. The first is that dust from the outer belt leaked inwards. There are several drag mechanisms to explain this motion, but they all predict that the dust would keep moving, all the way into the star. This is not supported by the observations. The other possible origin is a second planestimal belt located closer to the star, similar to our asteroid belt. This scenario is supported by the low brightness and the presence of large grains (seen in radio observations) that would not be affected by drag mechanisms.
Any formation theory for the Vega System has to explain 1) how two separate belts were created and 2) how the gap remains dust free. The most plausible explanation is the presence of multiple planets in the gap. These planets would play the same role as the four giant planets in the Solar System, removing dust grains and preventing the formation of another belt in the gap.
Vega as an analog to the Solar System
This discovery of a warm asteroid belt makes Vega look like a scaled up version of the Solar System. Vega’s two belts have similar proportions to the asteroid belt and Kuiper belt, but they are more massive and farther from their star. But this difference is actually not too surprising, and the Vega system may actually be more similar to the Solar System than it seems at first glance.
The star Vega is much more massive than our sun, and evidence suggests that more massive stars have more massive disks. Massive stars are also much brighter, which affects the temperature of the dust. So even though Vega’s asteroid belt is farther from its star than ours is from the Sun, they both have similar temperatures. This indicates that these disks have a temperature dependent formation mechanism. This may be related to giant planet formation, which is easier at colder temperatures where there is more ice present.