HD 172167    HR 7001    alfa Lyra    HIP 91262
RA (2000) = 18 36 56.3364       Dec (2000) = +38 47 01.291
SpT = A0V    V = 0.03 mag    d = 7.756 pc
Proper Motion (mas/yr) = +201.03 +287.47

This image of Vega essentially traces the thermal emission of dust. Notice that the black star represents the location of the star. The white peak to the northeast of the star represents the peak of dust emission.

Vega is the brightest star in the Northern Hemisphere and in 1983 was the first nearby star observed to possess orbiting solid material.  Researchers using data from IRAS (Infrared Astronomical Satellite) deduced that the dust was in a ring approximately 80 AU from the star with grains larger than 10 microns in diameter.

The 850 micron image confirms extended thermal emission from dust around the star. The big surprise is that the dust doesn't appear to be an azimuthally symmetric ring because there is a distinct peak of thermal emission 5.9" East and 7.1" North of the star (a minimum distance of 72 AU).

Dent et al. (2000) model the SCUBA image above with a dust ring that has inner and outer radii of 80 AU and 120 AU, respectively. Because the ring is viewed 15 degrees away from face-on, the vertical thickness of the disk is projected into the sky, and cannot be easily measured. From analysis of the spectral energy distribution (SED) the grain temperature is 80 K, with characteristic size 70 microns, though a range of size between 30 and 200 microns is consistent with the data. The lack of significant short-wavelength excess thermal emission suggests that 10 micron sized grains must have cumulative mass <10 percent that of the 70 micron grains. The total dust mass is estimated between 0.2-0.7 lunar masses.

The Dent et al. (2000) constraints on dust size and location can be used to calculate the lifetime of grains. They find that Poynting-Robertson drag would destroy 70 micron sized grains at 80 AU in 20 Myr, but grain-grain collisions at 80 AU would destroy these grains in 500,000 yr. Therefore collisions are the most significant process that destroys grains. If the age of Vega is 300 Myr, then the 70 micron grains observed with SCUBA have been replenished 600 times over. The source of replenishment is thought to be collisions of larger dust grains or planetesimals. These objects are a parent population of primordial bodies that can survive up to the present age of the star. Dent et al. estimate that objects larger than 0.38 cm in size replenish the observed 70 micron grains.

Basic facts about Vega:

1) age is estimated to be between 310 Myr and 390 Myr

2) structure is a ring with inner and outer radii of 80 and 120 AU, respectively.

3) characteristic grain size is 70 microns, with characteristic temperature 80 K.

4) total dust mass estimates between 0.2-0.7 lunar mass, i.e. (1.5-5.1)x10^25 g or (7.5-26)x10^-9 M_sun

5) morphology is significantly asymmetric

6) ring is seen near face-on from our point of view. This orientation is also derived from Vega's low projected rotational velocity of 22 km/s.