Hot white stars of spectral class A are among the brightest stars in the night sky. Sirius, Altair, and Vega are familiar examples of this type, lending their names to consumer products and often appearing in science fiction narratives. Despite their visibility, however, stars of this spectral class may represent less than 1% of the stellar population of the Milky Way Galaxy (Hillenbrand 2004, RECONS).

A-type stars range in mass from 1.6 MSOL to 3.0 MSOL (Johnson et al. 2007a), and in main sequence lifetime from 600 to 850 million years (Su et al. 2006, Sato et al. 2007). Thus they are considerably more massive than Sun-like stars, and their life spans are considerably shorter.

Both theory and observation indicate that these stars are at least as likely as Sun-like stars to harbor planetary systems. In fact, Kennedy and Kenyon argue that "the probability that a star has at least one gas giant increases linearly with stellar mass" up to a peak at 3 MSOL, the upper limit for A-type stars. They predict that more than 10% of stars in the mass range for A-type stars will host at least one giant planet (Kennedy & Kenyon 2008).

Unfortunately, the method that has proven most successful in detecting extrasolar planets – high-precision radial velocity measurements of stellar spectra – has been ineffective for spectral class A. Key characteristics of these stars, including their high temperatures and rapid rotation, make it very difficult to determine their radial velocities (Setiawan et al. 2005, Galland et al. 2005). Another successful method, photometric transit observation, is equally ineffective, but for a different reason. The large diameters of A-type stars make it highly problematic to detect the reduction in brightness caused by a Jupiter-sized planet transiting across one of them. To date, no such transits have been observed.

By process of elimination we are left with direct photographic imaging, a method that has been pursued around young Sun-like stars for more than a decade, with ambiguous results. Fortuitously, the youth and mass of A-type stars make direct imaging far more effective for objects in their immediate vicinities than for older, smaller stars like our Sun. Young planets are hotter and thus brighter than older planets, and massive stars are expected to host planets on wide orbits where they are easier to find.

Direct imaging has recently yielded its first fruits in the detection of candidate gas giant planets orbiting three A-type stars: Fomalhaut, Beta Pictoris, and HR 8799 (Kalas et al. 2008, Lagrange et al. 2008, Marois et al. 2008). At last we have good evidence that A stars can host planetary systems comparable to those of dimmer stars. In all cases, the imaged planets orbit their stars at much greater distances than the planets detected by radial velocity measurements – at least 100 AU in the case of Fomalhaut b, between 24 and 68 AU for the three proposed planets of HR 8799, and about 8 AU for the candidate around Beta Pictoris. These discoveries hint at a large population of gas giants hiding in the vicinity of the Milky Way’s bright stars.

Less direct methods have also captured a great deal of data on two phenomena closely related to the likelihood of A-star planets:

• Debris disks similar to the Asteroid and Kuiper Belts orbiting existing A stars
  
  
• Planetary systems of subgiant and giant stars whose progenitors were A stars

Much research has focused on debris disks, as discussed in the next several paragraphs (see also Debris Disk Systems). Indeed, all the A stars whose planets have been imaged originally attracted attention because they support such disks. Much less attention has been directed to the planetary systems of giant stars, since they are far less common than debris disk systems and have only recently been detected. Such "retired A stars" and their companions are discussed at the end of this chapter.

The study of A-star planets has a long way to go before it rivals the extensive data on Sun-like stars and M dwarfs. Notably, no numerical simulations of planetary formation around these stars have been conducted, so it is difficult to place current findings within a meaningful theoretical context. The evidence so far suggests that the planets of A stars will be extremely massive, and their orbits will be much wider than those of exoplanets around Sun-like stars. Hot Jupiters will be absent or rare. Comets and asteroids will be plentiful, and their collisions with orbiting giants will be frequent. Whether rocky planets like Earth can form in these environments remains unknown, and life-bearing worlds are not anticipated.

A stars with debris

Recent surveys have found that about one-third of A stars within a few hundred parsecs currently possess debris disks that are dusty enough to be detected by infrared observations (Su et al. 2006, Trilling et al. 2008). The investigators attribute this frequency to two factors. First, given their short lifetimes, A stars are younger on average than F, G, K, or M stars, and thus more likely to preserve large populations of colliding planetesimals. Second, A stars are brighter on average than stars of the more numerous spectral classes, providing the intense luminosity that enables their disks to be detected. Nevertheless, even this high rate of detections represents only a lower limit on the presence of debris around A stars. Many systems with quantities of dust comparable to those in the Solar System’s debris belts remain beyond the reach of current observing techniques.

By far the best-studied debris disks of any kind are those of three nearby A stars: Beta Pictoris, Fomalhaut, and Vega. As a group they are often called “Vega-type stars,” since Vega's disk was the first to be discovered (Aumann et al. 1984). Although all are quite young in astronomical terms, these stars have already completed a substantial portion of their time on the main sequence.

nearby A-type stars with debris disks

* Recently published sources vary considerably in their estimates of Vega's mass, ranging from 2.3 to 2.9 Msol.
Sources: Aufdenberg et al. 2006 ("First results from the CHARA Array VII: Long-baseline interferometric measurements of Vega"); Su et al. 2005 ("The Vega debris disk - a surprise from Spitzer"); Su et al. 2006 ("Debris disk evolution around A stars"); Wyatt et al. 2007b ("Steady-state evolution of debris disks around A stars"). See references.

Located at 19.3 parsecs (63 light years), Beta Pictoris is the youngest of the Vega trio, with an estimated age between 10 and 20 million years (Okamoto et al. 2004, Su et al. 2006). Published values for its mass range from 1.75 to 2.0 times Solar, and for its luminosity from 8 to 13 times Solar (Heap et al. 2000, Freistetter et al. 2007, Wyatt et al. 2007b). Also known as HD 39060, this star’s more familiar name designates the Beta Pictoris Moving Group, an association of young nearby stars of various spectral types that formed together in the same molecular cloud about 12 million years ago (Zuckerman & Song 2004). Another prominent member of this group is AU Microscopii, which is also surrounded by a dusty debris disk (Kalas et al. 2004).

Despite its youth, the Beta Pictoris disk has already lost its primordial gases, and any of its original fine-grained dust has been blasted to extreme distances. The heavier dust that we now observe results from the disintegration of comets and from collisions between planetesimals or asteroid-like bodies. Collisions are occurring throughout the disk, but most of the dust is produced in orbits beyond 100 AU.

Beta Pictoris is positioned so that we see its disk almost edge-on. Several studies are in broad agreement as to its morphology, which is quite complex (e.g., Heap et al. 2000, Wahhaj et al. 2003, Okamoto et al. 2004, Golimowski et al. 2006).

• Dust is relatively depleted within an astrocentric radius of 80 AU, although it displays a pattern of clumps and gaps throughout this region.
  
  
• Dust density increases from 80 AU until it reaches a plateau at about 100 AU (Golimowski et al. 2006).
  
  
• At about 135 AU, the disk flares to a much greater thickness, extending radially to distances in excess of 1000 AU while slowly attenuating in density (Golimowski et al. 2006).
  
  
• Calculations by Alice Quillen and colleagues indicate that the average diameter of colliding planetesimals at the “break radius” – the radius containing half of the debris belt’s mass – is about 180 km (110 mi). Their collisional cascade has been triggered by perturbations from still larger planetoids measuring at least 1000 km (620 mi) in diameter (Quillen et al. 2007).

Rather than a continuous field of dust and random debris, the Beta Pictoris disk is more accurately described as a series of rings separated by gaps, forming a composite structure. In fact, what we see is evidently two superimposed structures -- a primary disk (or ring formation) and a thinner, secondary formation, which is tilted at an angle of 5 degrees to the primary disk (Heap et al. 2000, Golimowski et al. 2006). Within the inner, dust-depleted region of the system, Wahhaj and colleagues discern a series of four rings, more or less evenly spaced with dust concentrations at radii of 14, 28, 52, and 82 AU. They further note that these rings are non-coplanar, some of them inclined at the angle of the primary disk and others at the angle of the secondary disk (Wahhaj et al. 2003). Okamoto and colleagues note slightly different but analogous peaks in dust emissions: at radii of 6 and 16 AU, flanking the "A ring" of Wahhaj and colleagues, and at 30 AU, corresponding to their "B ring" (Okamoto et al. 2004).

The ring-like morphology of the Beta Pictoris disk is frequently explained as an effect of recent planet formation around the star. Okamoto and colleagues hypothesize a giant planet orbiting at 12 AU to maintain the structure of their proposed rings at 6 and 16 AU. The ring structure would then be the result of mean motion resonances with this planet, on analogy with the mechanism by which Jupiter maintains the structure of the Asteroid Belt. Freistetter and colleagues have expanded on this analysis, arguing that the planet at 12 AU must have a mass between 2 and 5 MJUP. In addition, they propose two more planets to maintain the gap at 52 AU: a Super Saturn of 0.55 MJUP at 25 AU and a Neptune analog of 0.1 MJUP at 44 AU (Freistetter et al. 2007).

A team led by Anne-Marie Lagrange has recently imaged a planetary candidate within the Beta Pictoris disk (Lagrange et al. 2008). They identify it with the hypothetical planet at about 12 AU proposed by Okamoto, Freistetter, and colleagues. However, Lagrange's group estimate their candidate's mass as 8 MJUP and its present separation from the star as about 8 AU. Unfortunately, their observations span only a single epoch, making this candidate less secure than the ones proposed for Fomalhaut and HR 8977.

Fomalhaut (also Alpha Piscis Austrini and HD 216956) was for a long time the neglected middle child in the nearby trio of dusty A stars. That status changed dramatically with the announcement of its planetary companion, known by convention as Fomalhaut b. Located at 7.7 parsecs (25 light years), Fomalhaut is a relatively young star. Its age is given as 200 million years, its mass as 2.3 times Solar, and its luminosity as 16 times Solar (Chiang et al. 2008). Some sources argue that Fomalhaut is a sibling to Vega, both stars having formed in the molecular cloud that produced the Castor Moving Group, whose members include many other nearby stars (Barrado y Navascues 1998, Su et al. 2006).

The star’s debris disk is actually a well-defined ring extending from 133 AU to about 158 AU, for an approximate width of 25 AU. These modest dimensions contrast strongly with the extensive debris systems proposed for Beta Pictoris and Vega. Alice Quillen and colleagues (2007) calculate that the observed dust has been generated by collisions among planetesimals averaging 80 km (50 mi) in diameter, stirred by perturbations from still larger objects whose average size is 1000 km (620 mi). Both Quillen and Paul Kalas have argued for the presence of at least one planet to maintain the dust ring's sharp and eccentric inner edge (Kalas et al. 2005).

These predictions have been borne out by Hubble Space Telescope images of a co-moving object in just the right place (Kalas et al. 2008). Kalas' team conclude that this object is a gas giant with a minimum mass of 0.1 MJUP and a maximum mass of 3 MJUP, following a mildly eccentric orbit at a semimajor axis of 100-120 AU (Chiang et al. 2008). This remarkably wide orbit is about 4 times larger than the orbit of Neptune, the outermost planet of the Solar System. Kalas and colleagues consider it likely that additional planets exist on smaller orbits; the more inner planets, the smaller the maximum mass of Fomalhaut b must be (Chiang et al. 2008).

They conclude that Fomalhaut b formed through accretion of planetesimals (see Evolution of Planetary Systems) and suggest a variety of mechanisms to explain its present wide orbit: the planet may have formed in situ; it may have migrated outward from a smaller orbit through interactions with the system's primordial nebula; or it may have been scattered outward by perturbations from a more massive planet located closer to the star (Kalas et al. 2008).

Kalas and colleagues caution that their conclusions are still preliminary, pending further high-precision observations. One of their most intriguing speculations is that certain brightness anomalies in the data may be explained by the existence of an extensive and still-evolving system of rings and moons in orbit around Fomalhaut b, with a radius almost 10 times larger than the rings of Saturn (Chiang et al. 2008). So far, however, this attractive picture remains completely hypothetical.

Vega is the brightest of the nearby group of A stars with debris disks. Located 7.8 parsecs (25 light years) from our Sun, it is also known as Alpha Lyrae and HD 172167. Vega is considerably older and more massive than Beta Pictoris, but apparently quite similar to Fomalhaut, whose sibling it may be. Recent sources disagree on its basic characteristics. Estimates of the star's age range from 200 to 400 million years (Wyatt et al. 2007b, Su et al. 2005, Kaler 2007); estimates of its mass range from 2.3 to 2.9 times Solar, and of its luminosity from 37 to 58 times Solar (Aufdenberg et al. 2006, Isbell & Allen 2006, Su et al. 2005, Wyatt et al. 2007b).

For the last two parameters Aufdenberg and colleagues seem most reliable, providing a stellar mass of 2.3 MSOL (plus or minus 0.2) and a luminosity of 37 LSOL (plus or minus 3). The same group established that we observe Vega in a pole-on orientation, rotating in a period of only 12.5 hours. Such a rapid spin results in an equatorial diameter that is 20% wider than the polar diameter, creating a dimmer, cooler belt around the star's midsection (Aufdenberg et al. 2006).

Observations by the Spitzer Space Telescope indicate that Vega’s debris fields have an inner edge at 86 AU. The particle density peaks at about 100 AU and diminishes outward beyond 200 AU. Fine dust has been blown by stellar radiation to distances as great as 815 AU (all values from Su et al. 2005). This formation is explained as a ring of silicate and carbonaceous bodies rather like the asteroids in our own system, centered around 100 AU, with ever finer particles ranging steadily outward. On the basis of high-resolution imaging, Su and colleagues find that the debris is evenly distributed rather than exhibiting clumpiness, and that the belt itself is centered on the star, rather than offset. It is widely agreed that the source of the extreme dustiness now observed in Vega's debris field is the result of an astronomically recent collisional cascade involving large, presumably icy planetesimals, probably hundreds or even thousands of kilometers in diameter (Su et al. 2005; Wyatt et al. 2007b). These objects are very similar to those hypothesized by Quillen and colleagues to explain the debris around Fomalhaut (Quillen et al. 2007).

As with other dusty A stars, the inner clearance in Vega's debris disk is regarded as an indication that planets have formed within a few dozen AU of the star (Aumann et al. 1984, Wyatt et al. 2007b). Given the discovery of a gas giant shepherd for Fomalhaut’s dusty ring, the odds of a planetary system around Vega have become much more favorable.

retired A stars

Radial velocity searches have recently begun to detect planets around relatively massive subgiant and giant stars. As of November 2008, more than a dozen of these systems have been observed with host stars in the range of 1.6-3.0 MSOL. Because the transition to the giant phase involves little or no mass loss, the present values for these stars are an excellent indication that all of them once belonged to spectral class A (Johnson et al. 2007a). As a result, their companions provide our best available evidence on the configurations of planetary systems centered on A-type stars. Data for the most reliably constrained systems are summarized in the following table.