The observed lifetime of protoplanetary disks

The above discussion provides the basis for using the infrared excess relative to the photospheric flux as a tool to detect primordial dust disks and determine the timescale over which they disperse. We should note that emission at infrared 

A dispersal timescale of lOMyr seems also to be characteristic of most low-mass stars even in clusters that have massive stars. Sicilia-Aguilar et al. (2005, 2006) surveyed two clusters in the Cep OB2 Association, Tr 37 and NGC 7160, that have or had a massive O star. They find that the fractions of low-mass stars with disks are similar to those in clusters with no massive stars (see also Fig. 9.2). This suggests that massive stars have only a limited effect on the dispersal of disks and hence on the formation of planets around most stars in a cluster. A similar result is found by Balog et al. (2007) in the NGC 2244 cluster that contains 7 O stars. Although there is a deficit of disks within 0.5 pc from the closest O stars, Balog et al. (2007) show that outside that radius, where most stars in the cluster are located, the disk fraction is comparable to the fraction of disks in clusters without a massive star. In conclusion, while massive stars can affect disk evolution in their immediate vicinity, most disks in the cluster remain unaffected. 

All the discussion above concentrated on young solar analogs for a direct comparison with the Solar System. It is interesting to note that the observed disk lifetime is strongly dependent on the stellar mass. Disks around intermediate-mass stars (M 1.5M ) dissipate in much less than lOMyr while disks around stars with masses similar to the Sun or smaller persist for longer times (Haisch et al. 2001a Sterzik et al. 2004 Hernandez et al. 2005 Carpenter et al. 2006 Lada et al. 2006 Currie et al. 2007 Hernandez et al. 2007 Riaz Gizis 2008 Merfn et al. 2008). 

The initial mass and lifetime of gas in circumstellar disks affect both the formation of giant planets as well as the formation of terrestrial planets. According to the widely accepted scenario of giant-planet formation, rocky cores need to reach several M before being able to accumulate a substantial amount of gas from the protoplanetary disk. Current models require from a few to 10 million years to form Jupiter-like planets at 5AU (see e.g. Lissauer Stevenson 2007), meaning that primordial 

In summary, current observations indicate that most circumstellar gas mass disperses quickly on a timescale similar to (or maybe even shorter than) the dustclearing timescale. Significant progress in this field is expected to occur in the next years with the launch of the Herschel Space Observatory. In particular, the 

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