Thursday, March 28, 2013

Vega’s Asteroid Belt

Our solar system consists of a warm-and-cold-belt architecture. The warm belt corresponds to the asteroid belt and the cold belt corresponds to the Kuiper belt. Both belts are separated by a large gap that is populated by the giant planets - Jupiter, Saturn, Uranus and Neptune. The asteroid belt has a characteristic temperature of ~170 K and is located between the orbits of Mars and Jupiter. The more outlying Kuiper belt has a characteristic temperature of ~50 K and it extends beyond the orbit of Neptune. It is worth considering if such warm-and-cold-belt architectures also exist around other stars, if they might be common and if such belts may serve as clues to the existence of planets.

Spectroscopic studies were performed using the infrared spectrograph instrument on NASA’s Spitzer space telescope on Vega - a relatively nearby star about 25 light-years away. The observations revealed a mid-infrared excess which corresponds to material with a characteristic temperature of ~170 K. This indicates the presence of an asteroid belt at a distance of ~14 AU around Vega, analogous to the asteroid belt in our solar system. Since Vega is 37 times more luminous than our Sun, the asteroid belt around Vega is located much further out from the star than the asteroid belt in our solar system which is ~2.7 AU from our Sun.

In addition to an asteroid belt, Vega is also known to have a cold belt exterior to the asteroid belt. This cold belt is likely to be analogous to our solar system’s Kuiper belt and it reveals itself as an excess in the far-infrared, corresponding to material having a characteristic temperature of ~50 K. This suggests that Vega possesses a warm-and-cold-belt architecture like our Sun.

Observations have also revealed the presence of hot dust in the close vicinity of Vega. The dust shows up as an excess of short-infrared radiation corresponding to a characteristic temperature of ~1500 K. Based on this temperature, the dust is expected to exist within a distance of ~0.2 AU from Vega. The dust is proposed to be nano-size metal oxides coming from the sublimation of silicate-rich planetesimals. These nano-size metal oxides become charged either via the photoelectric effect or the stellar wind. Once charged, they become trapped in the star’s magnetic field at close proximity to the star.

Similar to the asteroid belt in our solar system, the asteroid belt around Vega is also located at the water-frost line. During the formation of planets in a protoplanetary disk, the water-frost line demarks a boundary around a star where temperatures become cool enough for grains of water ice to form. As a result, the density of solid particles in a protoplanetary disk increases abruptly by a factor of a few, beginning at the water-frost line and extending outwards. In addition to longer dynamical timescales, the increase in the amount of solid material favours the formation of giant planets exterior to the water-frost line. Our solar system is a good example where 4 giant planets exist beyond the asteroid belt. Besides our Sun, another example is a young star called HR 8799 which has a similar two-belt configuration with 4 giant planets between the inner and outer belts. These giant planets dominate the dynamics of the inner and outer belts, and help keep the large gap between the two belts relatively clear.

The large gap separating the warm and cold belts around Vega hints that Vega could be surrounded by multiple undiscovered planets. In fact, a number of searches have been conducted to look for planets around Vega. Although no planets have been discovered around Vega so far, these searches have placed strong limits on the maximum mass that planets around Vega could have. This limiting mass is no more than a few times the mass of Jupiter. As a result, the presence of one or two very massive planets with several times the mass of Jupiter is unlikely to be responsible for the large gap separating the inner and outer belts around Vega. Instead, the presence of multiple Jupiter-mass planets is sufficient to maintain the gap and keep within the planetary mass constrains. Better observations will be required to detect the presence of these planets.

Vega has an inner warm belt and an outer cold belt. The two belts are separated by a large gap that is likely to be populated by multiple planets. Besides our Sun, Vega and HR 8799, a number of other stars are also known to posses warm-and-cold-belt architectures. These stars include Fomalhaut and Epsilon Eridani.

Su et al. (2013), “Asteroid Belts in Debris Disk Twins: VEGA and FOMALHAUT”, arXiv:1301.1331 [astro-ph.EP]