Both the probes of gaseous disks discussed thus far—the in situ and indirect probes— can be used to explore the evolutionary status of certain classes of young stars in order to gain insights into the processes governing planet formation. One class of stars that is of interest is the transitional T Tauri stars. These are T Tauri stars that typically have photospheric colors at short wavelengths (optical and near-infrared) and strong continuum excesses at longer wavelengths. Examples of this class include TW Hya, DM Tau, GM Aur, and CoKu Tau/4 (Calvet et al. 2002; Rice et al. 2003; Bergin et al. 2004; D'Alessio et al. 2005; Calvet et al. 2005b). These systems make up a significant fraction (-10%) of T Tauri stars (e.g., Skrutskie et al. 1990; McCabe et al. 2006).
The deficit of emission at intermediate wavelengths in the spectral energy distributions (SEDs) of these sources suggests that the inner region of the disk has become optically thin in the continuum. The fact that these systems show the presence of outer disks, but not inner disks, has been taken as evidence that disks dissipate from the inside out (e.g., Skrutskie et al. 1990). The relatively small fraction of sources that are found to reside in this state is often interpreted as evidence that disks dissipate on a short timescale, compared to the mean lifetime of T Tauri stars, i.e., <1 Myr. However, other interpretations are possible. For example, a fraction of -10% is comparable to the fraction of nearby stars that are found to harbor giant planets. So these sources may have taken a particular evolutionary path—one that perhaps involves the formation of giant planets?
Indeed, the spectral energy distribution that is observed for transitional T Tauri stars could arise in several ways. One possibility is that a giant planet has formed in the inner disk and has dynamically cleared the inner disk region of gas and dust (e.g., Skrutskie et al. 1990; Marsh & Mahoney 1992; Rice et al. 2003). Another possibility is that the system might be in a more primitive state of planet formation. That is, perhaps plan-etesimals, protoplanets, or planetary cores have formed, thereby rendering the inner disk optically thin, but a gaseous envelope has not yet been accreted, and so a significant gaseous reservoir remains in the inner disk.
Yet another possibility is that the inner disk has been cleared of gas and dust, not by planet formation, but through the combination of viscous accretion and photoevap-oration, as described for example in the "UV Switch" model of Clarke et al. 2001. In this model, UV photons from the star photoevaporate away the outer disk, reducing its ability to resupply the material in the inner disk through viscous accretion. This eventually causes the inner disk to decouple from the outer disk, whereupon it drains away onto the star. Thus, there are three plausible explanations for the SEDs of transitional T Tauri stars. If we could determine which of the three scenarios is valid for a sample of transitional T Tauri stars of known ages, this would provide a way of constraining the timescales for either forming planetary cores, or forming planetary cores and accreting gaseous envelopes, or for dissipating disks.
We could take a look at one particular source as a case study to see what we might learn. TW Hya is a good example. In their study of the SED for the source, Calvet et al. (2002) estimated a total column density of 32gcm~2 at 20 AU for TW Hya, based on the properties of the dust emission. If we look at the Clark et al. model, we find that the inner and outer disks decouple at -10 Myr, approximately the age of TW Hya, but at a column density at 20 AU that is only 0.1g cm~2—much smaller than the column density inferred for TW Hya. Thus, in the case of TW Hya, the outer disk appears to be far too massive for the "UV Switch" model to explain the observed structure of the SED.
What about the other two possibilities, that either giant planet or planetary core formation is responsible for the observed SED? Here, a measurement of the gas content of the inner disk might provide a useful discriminant. Several detections of gas have been made in the inner disk of this source (Section 4.3). So gas is known to be present, although models of the thermal-chemical structure of the inner disk are as yet unable to convert these measurements into total gas masses or column densities. In the interim, we might ask: What constraints can stellar accretion rates place on the gas content of the inner disk in this system?
Estimates of the stellar accretion rate for TW Hya range from 5 x 10~10-5 x 10~9 Mq yr_1 (Muzerolle et al. 2000; Alencar & Basri 2000). If a = 0.01, as is often assumed, this range of accretion rates corresponds to column densities of 1 AU of E = 5-50 gcm~2. This is much less than the minimum mass solar nebula column density of ^1000gcm~2 at 1 AU, and consistent with the idea that the gas in the inner disk has been dissipated by the formation of a giant planet. However, a lower value of a = 0.0003 has been invoked by Calvet et al. (2002) to explain various properties of the outer disk in the system. If the same value of a applies to the inner disk, the implied gas column density at 1 AU is E = 100-1000 gcm~2, more similar to the column density in the minimum mass solar nebula, and more consistent with planetary core formation than giant planet formation. Determining which of these is the appropriate interpretation would help us to understand whether the age of the TW Hya system (~10 Myr) is the characteristic time for the assembly of a planetary core, or the time that is needed to go a step further and accrete a gaseous envelope. Measurements of the gas content in such systems using in situ diagnostics can help to resolve this issue.
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