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Evidence for Disk Photoevaporation Driven by the Central Star

The lifetime of isolated protoplanetary disks is thought to be set by the combination of viscous accretion and photoevaporation driven by stellar high-energy photons. Observational evidence for magnetospheric accretion in young sun-like stars is robust. Here we report the first observational evidence for disk photoevaporation driven by the central star. We acquired high-resolution (R~30,000) spectra of the [NeII] 12.81 micron line from 7 circumstellar disks using VISIR on Melipal/VLT. We show that the 3 transition disks in the sample all have [NeII] line profiles consistent with those predicted by a photoevaporative flow driven by stellar extreme UV photons. The ~6 km/s blue-shift of the line from the almost face-on disk of TW Hya is clearly inconsistent with emission from a static disk atmosphere and convincingly points to the presence of a photoevaporative wind. We do not detect any [NeII] line close to the stellar velocity from the sample of classical optically thick (non-transition) disks. We conclude that most of the spectrally unresolved [NeII] emission in these less evolved systems arises from jets/outflows rather than from the disk. The pattern of the [NeII] detections and non-detections suggests that extreme UV-driven photoevaporation starts only at a later stage in the disk evolution.


Introduction
It is well established that most ∼1 Myr-old stars are surrounded by relatively massive optically thick dust disks. By an age of ∼10 Myr only a few percent of sun-like stars still retain an optically thick dust disk (e.g. Hernández et al. 2007) and many intermediate-mass stars might already harbor second-generation dust disks (e.g. Currie et al. 2008). This fast clearing of primordial dust is in agreement with the short formation timescales of chondrules, asteroids, and planets in the Solar System (Pascucci & Tachibana 2009).
Although much less is known about the evolution of the gas component, there are at least three observables pointing to a similarly fast dispersal timescale: a) an order of magnitude lower accretion rate for the few stars still accreting at an age of ∼10 Myr in comparison to ∼1 Myr-old stars Lawson et al. 2004); b) gas mass upper limits of less than 0.1 M Jup in disks around non-accreting stars (Hollenbach et al. 2005;Pascucci et al. 2006); and c) upper limits on the H 2 -to-dust ratio of less than 10 in two ∼ 12 Myr-old edge-on disks (Lecavelier des Etangs et al. 2001;Roberge et al. 2005).
Which are the physical mechanisms clearing out primordial disks? Models of protoplanetary disk evolution suggest that most of the disk mass is cleared out by viscous evolution (accretion of gas onto the central star) and photoevaporation driven by the central star (e.g., Clarke et al. 2001;Alexander et al. 2006b;Gorti & Hollenbach most of the [Ne ii] emission originates in jets/outflows with neon atoms partly ionized by stellar X-rays. Our aim here is to enlarge the sample of spectrally resolved [Ne ii] lines and understand in which systems [Ne ii] emission originates in a disk. In the following subsections we describe our observational campaign and the data reduction (Sects. 2.1, 2.2, 2.3).
Observations were carried out with the high-resolution (R∼30,000) spectrograph VISIR mounted on the VLT telescope Melipal (Sect. 2.2 for more details). In addition to the already published Spitzer/IRS spectra, we reduced and present here new archival IRS spectra that aid the interpretation of the VISIR observations (Sect. 2.3).

Target Selection
Because of the lower-sensitivity of VISIR with respect to Spitzer/IRS, we restricted our sample to disks with bright/unresolved [Ne ii] lines (Spitzer fluxes greater than 10 −14 erg cm −2 s −1 ). In addition, we selected disks brighter than ∼50 mJy in the continuum because target acquisition for fainter objects becomes challenging with VISIR. The 6 targets we selected for this campaign are TW Hya, CS Cha, VW Cha, T Cha, Sz 73, and Sz 102 (see Table 1 and 4 for their main properties). At the beginning of the second night we also observed the double-lined spectroscopic binary HD 34700A, which is the brightest infrared T Tauri couple in a quadruple system . Although HD 34700A did not have a published [Ne ii] line detection, its near-and mid-infrared spectra present unusually strong PAH emission bands (e.g., Smith et al. 2004), possibly hinting to a high stellar far-UV flux (e.g., Geers et al. 2006), which may be indicative of a high EUV flux.
The systems we selected can be grouped into three categories based on their broad-band spectral energy distribution (SED). VW Cha, Sz 73, and Sz 102 have significant excess emission relative to the photospheric flux from near-through to far-infrared wavelengths (Gauvin & Strom 1992;Hughes et al. 1994;Kessler-Silacci et al. 2006). Such broad SEDs can be well reproduced by radially continuous optically thick dust disks, extending from a few stellar radii out to hundreds of AU. TW Hya, CS Cha, and T Cha are classified as transition disks in the literature. Their SEDs present a strongly reduced (or lack of) near-infrared excess emission but large mid-and far-infrared emission. Detailed modeling of their SEDs points to relatively large inner dust cavities almost devoid of sub-micron-and micron-sized dust grains: ∼1-4 AU for TW Hya (Calvet et al. 2002;Ratzka et al. 2007), ∼43 AU for CS Cha , and ∼15 AU for T Cha (Brown et al. 2007).
There is no evidence of gas inner holes in these systems: TW Hya is accreting disk gas at a rate of ∼ 5 × 10 −10 M ⊙ /yr , the spectroscopic binary CS Cha at < 10 −8 M ⊙ /yr ) 1 , while the small (< 10Å) but variable Hα equivalent width from T Cha suggests low-level and possibly episodic accretion ).
Finally, the SED of HD 34700A has little excess emission at all wavelengths produced by a tenuous dust disk, possibly a debris disk (Sylvester & Skinner 1996).

Observations
We performed long-slit high-resolution spectroscopy with the spectrograph VISIR mounted on the VLT telescope Melipal (Lagage et al. 2004). The observations were executed on 20, 21, and 22 February 2008. VISIR has a 256 × 256 BIB detector array with an image scale of 0. ′′ 127/pixel. We acquired all targets with the PAH2-NEII filter. We used a slit width of 0. ′′ 4 centered at 12.81µm to detect and spectrally resolve the [Ne ii] ( 2 P 3/2 − 2 P 1/2 ) 1 the estimate from Espaillat et al. (2007) should be considered only an upper limit be-fine-structure line at 12.81355µm (Yamada et al. 1985). With this configuration we covered the spectral region between ∼ 12.79 and 12.83 µm with a resolution of R ∼ 30, 000 as measured from first light data (Käufl 2006) and from the FWHM of 5 narrow O 3 sky lines from our spectra. This resolution corresponds to 3 VISIR pixels or ∼10 km/s in velocity scale.
We applied the standard chopping/nodding technique to suppress the mid-infrared background. For each target the slit was positioned in the default North-South orientation and chopping/nodding was done along the slit with a throw of 8 ′′ . Total on-source exposure times per science target were 1 h or longer. To correct the spectra for telluric absorption and to obtain an absolute flux calibration, we observed standard stars immediately before or after the science target and at a similar airmass. A summary of the observations is presented in Table 2. the Half-Cycle frames are now cleaned by interpolation with neighboring pixels. In parallel, a reference frame of the infrared background, not corrected for chopping or nodding, is also created from the first Half-Cycle of the first nod position. The nodded images as well as the infrared background image are corrected for the optical distortion, which is known analytically, and are shifted and added to form the final combined image using the offsets stored in the FITS header (the nodding cycle sequence used for VISR is AnBnBnAn). The last step before extraction is to fold the two negative beams into the central positive beam.
The spectrum is then extracted following the optimal extraction method by Horne (1986). In brief, the image is collapsed in the spectral dispersion direction to create a source profile, which is normalized and then expanded in spectral direction to obtain a weight map. The science frame is multiplied by the weight map and the spectrum is obtained by summing in spatial direction. This extraction method works well for sources where, in addition to narrow emission line(s), the continuum emission is also detected. For sources like CS Cha, where only a narrow emission line is detected (but not the continuum, see Fig. 1), the optimal extraction method fails. For this source and its calibrators we have performed the standard full aperture extraction (same weights within a defined spatial range of pixels). We tested that an aperture of 10 pixels gives the best S/N in the extracted spectra.
-9 - The wavelength calibration is done using the collapsed background frame in spatial direction and cross-correlating it to a synthetic model spectrum of the atmosphere. The dispersion relation is well approximated by a first order polynomial and the offset in pixels that maximizes the cross-correlation can be determined to an accuracy of 0.01 pixel (∼0.03 km/s). We also compared the observed peak position of two photospheric absorption lines in the spectra of the K5 III star HD 136422 and the K4.5 III star HD 139127 to those from the MARCS model atmosphere of a K5 III star (HR 6705, the spectrum was kindly provided by L. Decin). This exercise demonstrates that the peak centroids can be determined to a lower accuracy, varying from a tenth up to a few km/s (see also Table 3).
All steps described above have been applied to both the target and its standard star(s).
In addition, the extracted spectrum of the standard star was flux calibrated using the model flux from the VISIR standard star catalog. From this calibration we created a spectral response function (Jy/ADU) that we applied at the wavelengths of the science spectrum.
This step removes most of the "fringing" present in the VISIR spectra (the flux modulation with wavelengths has a peak to peak amplitude of about 15% and is found to be stable over periods of at least several months, van Boekel et al. 2009).
Spitzer/IRS spectra. As mentioned in Sect. 2.1, all targets except HD 34700 have published low-(R∼105) or high-resolution (R∼600) IRS spectra with bright [Ne ii] emission lines (see Table 1). We have reduced the archival IRS low-resolution spectrum of HD 34700 and report no detection of the [Ne ii] line. In addition, we have reduced archival high-resolution IRS spectra for TW Hya and CS Cha obtained at different epochs than the already published spectra by Ratzka et al. (2007) and Espaillat et al. (2007). These spectra were acquired as part of the Spitzer GO program 30300 (PI, J. Najita) and are presented here to aid the interpretation of the VISIR results (Sect. 4). The data reduction of the IRS high-and low-resolution spectra followed the procedure outlined in Pascucci et al. (2006Pascucci et al. ( , 2007Pascucci et al. ( , 2008 and Bouwman et al. (2008). The [Ne ii] line fluxes from these new spectra are also reported in Table 1 (first entries in column 5).

Results
We detect and spectrally resolve [Ne ii] emission lines from 4 out of 7 targets, specifically TW Hya, CS Cha, T Cha, and Sz 73. The [Ne ii] emission detected with VISIR is found to be comparable to the angular resolution (∼0.5 ′′ ) estimated from both the mid-infrared continuum of our targets and from the telluric standards observed before or after the targets. The profiles of the lines are consistent with a single Gaussian profile (see Fig. 2). Table 3 provides the peak centroids (in the stellocentric frame), FWHMs, and fluxes of the [Ne ii] lines computed assuming a Gaussian profile and a first-order polynomial for the continuum. In the case of non-detections (see Fig. 3), we fit a first-order polynomial within ±100 km/s of the star velocity. Table 3  is also variable. More details on the line and continuum variability of TW Hya will be presented in an upcoming paper (Najita et al in prep.). The important result for the interpretation of these VISIR spectra is that changes of ∼30% in the line and/or in the continuum are possible even among the class of transition disks.
We do not detect the [Ne ii] emission line in the Spitzer low-resolution spectrum of HD 34700A. This star is surrounded by a tenuous dust disk and is thus more similar to the optically thin dust disks studied by Pascucci et al. (2006) rather than to the protoplanetary disks presented in this paper. The [Ne ii] non-detection suggests that very little or no primordial gas is left in the disk of HD 34700A ), corroborating its identification as a debris disk.
In the following, we discuss in more detail the 6 protoplanetary disks with spectrally unresolved [Ne ii] lines in their Spitzer spectra. We separate transition objects from radially continuous dust disks because their VISIR spectra appear remarkably different.

[Ne ii] emission from the disk of transition objects
For the three transition disks TW Hya, CS Cha, and T Cha we detect and spectrally resolve [Ne ii] lines located near the stellar velocity. In the case of CS Cha we detect only the emission line but not the continuum. 2 . For all sources [Ne ii] line fluxes from VISIR spectra are lower than those measured from Spitzer spectra of factors between ∼0.5-0.8. One possibility is that the [Ne ii] emission is extended beyond the 0.4 ′′ slit adopted in these observations. Spatially extended [Ne ii] emission can certainly arise in outflow sources, as demonstrated by van Boekel et al. (2009) and by our sample of optically thick dust disks (see Sect. 5). However, no jets have been reported toward TW Hya and T Cha (Azevedo et al. 2007;Alcala et al. 1993) and only a compact (8-10 mas) outflow in Hα has been detected toward CS Cha (Takami et al. 2003). In Sect. 4.1 we also show that jets/outflows are not likely to be the main source of [Ne ii] emission in transition objects. Another explanation for the Spitzer-VLT flux difference is line and/or continuum variability. At least for CS Cha and TW Hya we know that line/continuum variability measured from 2-epochs of Spitzer spectra is ∼30%, similar to the [Ne ii] flux differences from the VLT and Spitzer spectra. We should also keep in mind that at the distance of TW Hya and T Cha (Table 4) our slit just covers out to ∼10 AU from the central stars. As The most important result from the VISIR spectra is that the measured [Ne ii] lines of transition disks are relatively narrow (ranging from ∼14 km/s to ∼40 km/s) and peaked near the stellar velocity (see Table 3). These line characteristics are consistent with a disk origin for the emission 3 . We note that TW Hya, whose disk is almost seen face-on (4 ± 1 • , Pontoppidan et al. 2008), has the narrower line width. The inclinations of the disks around CS Cha and T Cha are not well constrained but likely closer to edge-on. Espaillat et al. (2007) successfully model the SED of CS Cha with a disk viewed at an inclination of 60 • . T Cha has a high projectional velocity suggesting that it is viewed almost edge-on   Alexander (2008) showed that the velocity structure of a photoevaporative disk wind results in a line profile that is different from that produced by a bound disk atmosphere. In particular, when a disk is viewed face-on the vertical component of the wind velocity leads to a profile that is about ∼10km/s in width (broader than the thermal line width) and blueshifted by ∼6-7 km/s. When a disk is viewed closer to edge-on, line profiles become broader (∼ 30 − 40km/s), are double peaked, dominated by Keplerian rotation, and centered at the stellar velocity.  Table 4 and assuming a stellar ionizing flux of 10 41 photons/s. There is an almost perfect match between the observed and predicted [Ne ii] profile from the disk of TW Hya. The almost face-on disk of TW Hya has a blueshifted [Ne ii] line fully consistent with the emission originating in a photoevaporative wind moving toward the observer (Alexander 2008). A static disk atmosphere would result in a line emission centered at the stellar velocity, clearly inconsistent with our observations. Thus, we consider this result as the first strong evidence for a disk being photoevaporated by its central star.
The [Ne ii] line profiles of CS Cha and T Cha are broadly consistent with predictions from the photoevaporative disk wind model (Fig. 4), but there are also a few discrepancies, especially for the spectroscopic binary CS Cha. For this binary the predicted [Ne ii] line is broader than observed. This could result from a disk viewed at less than 45 • inclination, but the emission should be slightly more blue-shifted than observed.
In the simplest assumption that neon atoms are ionized only by stellar EUV photons,  Alexander (2008) keeps this fraction constant to 1/3 while more detailed gas models suggest that it could be as high as 1 ). The ionizing rates we compute from the [Ne ii] lines seem plausible for accreting T Tauri stars. For TW Hya far-UV observations suggest values of 7 × 10 39 or 5 × 10 41 depending on how much EUV emission from the accretion shock is attenuated by the accretion streams (Herczeg 2007).
As a next step we can use the values of Φ, in combination with star and disk masses, to estimate the disk wind rates and the disk lifetimes if EUV-driven photoevaporation would be the only disk dispersal mechanism. Using the results from Font et al. (2004) we find disk wind rates ranging from ∼ 2 to 5 × 10 −10 M ⊙ /yr. At these rates only the relatively low-mass disk of T Cha (0.003 M ⊙ , Lommen et al. 2008) could be photoevaporated in ∼10 Myr. The disks of TW Hya and CS Cha would disperse in several tens of millions of years. However, a more realistic estimate for the disk lifetime should include on-going mass loss onto the star via viscous accretion. In addition, disk lifetimes can be further reduced once the viscous accretion infall rate falls below the photoevaporation rate. At this point a gap opens in the gas disk and the direct EUV flux from the star is expected to disperse the entire disk in only ∼ 10 5 yr. As we discuss in Sect. 6, at least two of our transition objects may be on the verge of opening an inner gap in the gas disk.
Finally, it is worth commenting on the extension of the [Ne ii] emission in the photoevaporative disk models. Calculations from Alexander (priv. comm.) show that more than 90% of the [Ne ii] emission arises inside ∼ 2 × 9(M ⋆ /M ⊙ ) AU. These radial distances are covered by our 0.4 ′′ slit for CS Cha, are comparable to half the slit width for TW Hya, and smaller by a factor of 2 for T Cha. This shows that we certainly missed some [Ne ii] flux (especially from the disks of TW Hya and T Cha) even in the case that ionization is solely from EUV photons. Stellar X-rays could contribute to the gas ionization at larger disk radii (Glassgold et al. 2007) and the X-ray heated gas might also participate in the photoevaporative flow (Ercolano et al. 2009). If X-rays substantially contribute to the [Ne ii] fluxes then the stellar ionizing fluxes we calculated above should be viewed only as upper limits. It would be extremely interesting in the future to compare the observed [Ne ii] profiles to those predicted by photoevaporating X-ray-heated gas disks.

Can jets explain the [Ne ii] emission from transition disks?
The observations of the T Tau complex from van Boekel et al. (2009) demonstrate that [Ne ii] emission can also originate from shocks generated by protostellar outflows/jets. Because of the high ionization potential of neon atoms (21.6 eV), substantial ionization can be only produced by fast shocks with speeds ≥ 70 km/s for typical pre-shock densities of 10 4 cm −3 (also called J shocks, Hollenbach & McKee 1989). For this type of strong radiative shocks the shocked gas moves at almost the shock velocity. van Boekel et al. (2009) argue that these high velocities may be reached in the outflow from the T Tau S source which has a sight velocity of only ∼40 km/s, but it is likely to be in the plane of the sky. We now show that it is very unlikely that jets/outflows could explain the [Ne ii] emission of transition disks. First, let us consider the case of TW Hya. Its disk is seen almost face-on so we would expect any jet/outflow to be almost in the direction of the observer and the sight velocity to be close to the actual shock velocity. The sight velocity of the [Ne ii] emission from TW Hya is just -6 km/s and the line is relatively narrow (deconvolved FWHM of ∼10 km/s). If this velocity is representative for the shock velocity as predicted from J shock models, it would be unable to appreciably ionize neon atoms. Corroborating this assertion,  show that the [Ne ii] line luminosity produced by the postshock region of a radiative shock depends linearly on:Ṁ w v 2 s , whereṀ w is the protostellar mass wind loss rate and v s is the shock velocity. The wind mass loss rate scales with the stellar mass accretion rate asṀ w ≃ 0.01 − 0.1 ×Ṁ acc (Hartigan et al. 1995;White & Hillenbrand 2004). Given the low stellar accretion rates of transition disks, the [Ne ii] fluxes reported in Table 3 could be reached only with shock velocities ranging from several hundreds of km/s to thousands of km/s, clearly inconsistent with the small velocity shifts and relatively small FWHM of the observed [Ne ii] lines. Finally, line widths from shocked gas are expected to be quite broad (several tens of km/s) regardless of the observer viewing angle (e.g., Hartigan et al. 1987), thus shocked gas cannot reproduce the observed trend between the FWHM of [Ne ii] lines and the disk inclination. Although shocks generated by jets/outflows are unlikely to explain the [Ne ii] emission from transition disks, they may dominate the [Ne ii] emission from less-evolved optically thick dust disks (see Guedel et al. 2009 and next Section).  (Bally et al. 2006;Krautter 1986;Graham & Heyer 1988). Since we did not change the default slit orientation of VISIR (PA=0 • ) we acquired spectra in a direction perpendicular to the known outflows. Thus, our non-detections can be reconciled with the Spitzer observations if most of the [Ne ii] emission arises in jets/outflows, which can be verified with future high-resolution spectra covering the region of the known outflows.

On the onset of centrally-driven photoevaporation
Viscous accretion is thought to be the dominant disk dispersal mechanism but alone cannot explain the relatively short ( 10 Myr) disk dispersal timescales inferred from observations (e.g. review by Dullemond et al. 2007). High-energy stellar photons can heat and ionize the disk surface and induce photoevaporation (i.e. disk mass loss) from outer disk regions. Exactly when photoevaporation becomes an important disk dispersal mechanism and how much disk mass it can remove is still a matter of debate. FUV and X-ray photons can penetrate much larger column densities of gas than stellar EUV photons. As a consequence, they are expected to drive efficient photoevaporation from the early times of disk evolution at rates that may exceed the EUV photoevaporation rate by -18one or even two orders of magnitudes Ercolano et al. 2009). Stellar EUV photons start to penetrate the disk wind, and thus reach the disk surface, only when accretion rates fall below ∼ 10 −8 M ⊙ /yr, corresponding to hydrogen column density screens of N(H) 10 17 cm −2 ). Although EUV photons likely produce little mass loss at all stages ), they can help to quickly (∼ 10 5 yr) disperse remaining disk gas once stellar accretion rates fall below a few 10 −10 M ⊙ /yr (Alexander et al. 2006a).
Recently, Currie et al. (2009) have argued that the large number of evolved dust disks in the 5 Myr-old cluster NGC 2362 is inconsistent with predictions from the standard UV photoevaporation model, in which EUV-driven photoevaporation follows viscous accretion (Clarke et al. 2001;Alexander et al. 2006b). Indeed, these models predict a paucity of evolved disks because once the accretion inflow rates fall below the photoevaporation rate the disk dispersal is fast (∼ 10 5 yr). However, model predictions are directly relevant only to the evolution of the gas disk component. In order to translate the gas evolution into dust evolution, Alexander et al. (2006b) made the simplistic assumption that the dust is coupled to the gas throughout the entire disk evolution. This is certainly unrealistic because grain growth, observed in almost all protoplanetary disks (e.g. Natta et al. 2007), results in dust settling, i.e. decoupling of the dust from the gas disk (Dullemond & Dominik 2004). A more direct way to test photoevaporation models is to constrain the evolution of the gas disk through observations of gas lines tracing the accreting as well as the possibly photoevaporating material. In what follows we will take this approach to speculate on the onset of photoevaporation.
We start by discussing our sample of probably less evolved classical optically thick disks with radially continuous dust distribution. The Hα equivalent widths from these three systems are greater than 50Å Guenther et al. 2007), indicating -19that the central stars are accreting nebular gas. This is corroborated by the detection of outflows and large near-infrared excess emission, both characteristic to actively accreting disks (Hartigan et al. 1995). In these systems we do not find evidence for on-going disk photoevaporation. The [Ne ii] line flux upper limits at the stellar velocity imply that not only EUV photons but also stellar X-rays are efficiently screened by the protostellar wind and accretion columns, requiring large gas column densities (N H 10 22 cm −2 , e.g. Guedel et al. 2007). If column densities exceed ∼ 10 24 cm −2 , FUV photons are also significantly attenuated ). For lower column densities, photoevaporation driven by FUV photons could be occurring and we would not detect it because FUV photons have energies lower than the ionization potential of neon atoms (21.6 eV). These systems may be at the same evolutionary stage as the T Tau complex.
Recently, Najita et al. (2009) reported spectrally resolved [Ne ii] lines from two disks in the Taurus-Auriga star-forming region: AA Tau, a classical optically thick disk whose star presents periodic accretion rate bursts (Bouvier et al. 2007); and GM Aur, a transition disk whose central star is accreting at a level comparable to that of classical T Tauri stars (10 −8 M ⊙ /yr, Gullbring et al. 1998). The [Ne ii] lines are found to be centered at the stellar velocity suggesting that the emission originates from a static disk atmosphere very likely heated by stellar X-rays. However, the measured fluxes are factors of 2 and 3 lower than the spectrally unresolved fluxes from Spitzer spectra. One possible explanation is that most of the undetected [Ne ii] emission originates far from the central stars, in ambient gas shocked by a jet/outflow. This seems very plausible for AA Tau, whose optical spectrum has forbidden emission lines typical of high-velocity jets (e.g. [NII] 6530Å, Hirth et al. 1997) and a jet has also been imaged with HST/STIS and Goddard Fabry Perot spectrograph (Cox et al. 2005). The [Ne ii] emission reported by Najita et al. (2009) does not seem to trace photoevaporating gas in these two systems, possibly because X-ray heated gas producing [Ne ii] emission is slightly cooler (∼5,000 K) and thus more bound than the EUV-heated gas.
Finally, the transition disks presented here have [Ne ii] line profiles consistent with emission from a photoevaporative disk wind indicating on-going photoevaporation at this evolutionary stage. In the case of TW Hya, the stellar accretion rate is close to the limit where EUV-driven photoevaporation could start opening a gap in the gas disk. The small but varying Hα equivalent width of T Cha suggests that this transition object may also be on the verge of clearing a gas hole. Interestingly, TW Hya and T Cha are also thought to be relatively old, older than 5 Myr (Webb et al. 1999;van den Ancker et al. 1998, see also Table 1). CS Cha is the youngest among our transition disks. Its upper limit to the stellar accretion rate is close to what is needed for EUV photons to start penetrating the disk wind, placing it at an earlier evolutionary stage than TW Hya and T Cha (though the evolution of the inner disk is certainly affected by presence of the stellar companion).
In overall, these results support the evolutionary picture in which EUV-driven photoevaporation occurs at later times in the disk evolution and the mass loss rates can be only modest. We should also note that if stellar X-rays substantially contribute to the  x-axis gives the velocity in the stellocentric frame (see Table 1 for the stellar heliocentric radial velocity). On top of the continuum emission we overplot the best Gaussian fits to the data (dashed lines). In the case of CS Cha the continuum emission is not detected (see also    Table 4. Models have been degraded to the spectral resolution of VISIR and scaled by the factors reported in the last column of Table 4 to match the observed and predicted [Ne ii] line luminosities. The black thick line below the [Ne ii] emission shows the total uncertainty in the observed [Ne ii] peak position. Note the excellent match between the observed and predicted line profile from the almost face-on disk of TW Hya suggesting that the [Ne ii] emission originates from a photoevaporative disk  (Cutri et al. 2003).
b Stellar heliocentric radial velocity * These stars are part of multiple stellar systems. HD 34700A is a spectroscopic binary and has two visual companions . CS Cha was recently found to be a long-period spectroscopic binary . VW Cha has a visual companion at ∼0.7 ′′ (Brandner et al. 1996).