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Register a new userAccretion through the inner hole of transitional disks: What happens to the dust?
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We study the effect of radiation pressure on the dust in the inner rim of transitional disks with large inner holes. In particular, we evaluate whether radiation pressure can be responsible for keeping the inner holes dust-free, while allowing gas accretion to proceed. This has been proposed in a paper by Chiang and Murray-Clay (2007, Nature Physics 3, p. 604) who explain the formation of these holes as an inside-out evacuation due to X- ray-triggered accretion of the innermost layer of the disk rim outside of the hole. We show that radiation pressure is clearly incapable of stopping dust from flowing into the hole because of dust pile-up and optical depth effects, and also because of viscous mixing. Other mechanisms need to be found to explain the persistence of the opacity hole in the presence of accretion, and we speculate on possible solutions.
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The formation of dust gaps in protoplanetary disks is one of the most important signposts of disk evolution and possibly the formation of planets. We aim to characterize the flaring disk structure around the Herbig Ae/Be stars HD 100453 and HD 34282. Their spectral energy distributions (SEDs) show an emission excess between 15-40{mu}m, but very weak (HD 100453) and no (HD 34282) signs of the 10 and 20 {mu}m amorphous silicate features. We investigate whether this implies the presence of large dust gaps. In this work, spatially resolved mid-infrared Q-band images taken with Gemini North/MICHELLE are investigated. We perform radiative transfer modeling and examine the radial distribution of dust. We simultaneously fit the Q-band images and SEDs of HD 100453 and HD 34282. Our solutions require that the inner-halos and outer-disks are likely separated by large dust gaps that are depleted wih respect to the outer disk by a factor of 1000 or more. The inner edges of the outer disks of HD 100453 and HD 34282 have temperatures of about $160 pm 10$ K and $60 pm 5$ K respectively. Because of the high surface brightnesses of these walls, they dominate the emission in the Q-band. Their radii are constrained at 20+2 AU and 92+31 AU, respectively. We conclude that, HD 100453 and HD 34282 likely have disk dust gaps and the upper limit on the dust mass in each gap is estimated to be about $10^{-7}$M$_{odot}$. We find that the locations and sizes of disk dust gaps are connected to the SED, as traced by the mid-infrared flux ratio F30/F13.5. We propose a new classification scheme for the Meeus groups (Meeus et al. 2001) based on the F30/F13.5 ratio. The absence of amorphous silicate features in the observed SEDs is caused by the depletion of small (smaller than 1 {mu}m) silicate dust at temperatures above 160 K, which could be related to the presence of a dust gap in that region of the disk.
LkCa 15 is an extensively studied star in the Taurus region known for its pre-transitional disk with a large inner cavity in dust continuum and normal gas accretion rate. The most popular hypothesis to explain the LkCa 15 data invokes one or more planets to carve out the inner cavity, while gas continues to flow across the gap from the outer disk onto the central star. We present spatially unresolved HCO+ J=4-3 observations of the LkCa 15 disk from the JCMT and model the data with the ProDiMo code. We find that: (1) HCO+ line-wings are clearly detected, certifying the presence of gas in the cavity within <50 AU of the star. (2) Reproducing the observed line-wing flux requires both a significant suppression of cavity dust (by a factor >10^4 compared to the ISM) and a substantial increase in the gas scale-height within the cavity (H_0/R_0 ~ 0.6). An ISM dust-to-gas ratio (d:g=10^-2) yields too little line-wing flux regardless of the scale-height or cavity gas geometry, while a smaller scale-height also under predicts the flux even with a reduced d:g. (3) The cavity gas mass is consistent with the surface density profile of the outer disk extended inwards to the sublimation radius (corresponding to mass M_d ~ 0.03 M_sun), and masses lower by a factor >10 appear to be ruled out.
In the past few years, several disks with inner holes that are empty of small dust grains have been detected and are known as transitional disks. Recently, Spitzer has identified a new class of pre-transitional disks with gaps; these objects have an optically thick inner disk separated from an optically thick outer disk by an optically thin disk gap. A near-infrared spectrum provided the first confirmation of a gap in the pre-transitional disk of LkCa 15 by verifying that the near-infrared excess emission in this object was due to an optically thick inner disk. Here we investigate the difference between the nature of the inner regions of transitional and pre-transitional disks using the same veiling-based technique to extract the near-infrared excess emission above the stellar photosphere. We show that the near-infrared excess emission of the previously identified pre-transitional disks of LkCa 15 and UX Tau A in Taurus as well as the newly identified pre-transitional disk of ROX 44 in Ophiuchus can be fit with an inner disk wall located at the dust destruction radius. We also model the broad-band SEDs of these objects, taking into account the effect of shadowing by the inner disk on the outer disk, considering the finite size of the star. The near-infrared excess continua of these three pre-transitional disks, which can be explained by optically thick inner disks, are significantly different from that of the transitional disks of GM Aur, whose near-infrared excess continuum can be reproduced by emission from sub-micron-sized optically thin dust, and DM Tau, whose near-infrared spectrum is consistent with a disk hole that is relatively free of small dust. The structure of pre-transitional disks may be a sign of young planets forming in these disks and future studies of pre-transitional disks will provide constraints to aid in theoretical modeling of planet formation.
Transitional disks (TDs) are thought to be a late evolutionary stage of protoplanetary disks with dust depleted inner regions. The mechanism responsible for this depletion is still under debate. To constrain the models it is mandatory to have a good understanding of the properties of the gas content of the inner disk. Using X-Shooter broad band -UV to NIR- medium resolution spectroscopy we derive the stellar, accretion, and wind properties of a sample of 22 TDs. The analysis of these properties allows us to put strong constraints on the gas content in a region very close to the star (<0.2 AU) which is not accessible with any other observational technique. We fit the spectra with a self-consistent procedure to derive simultaneously SpT,Av,and mass accretion rates (Macc) of the targets. From forbidden emission lines we derive the wind properties of the targets. Comparing our findings to values for cTTs, we find that Macc and wind properties of 80% of the TDs in our sample, which is strongly biased towards strongly accreting objects, are comparable to those of cTTs. Thus, there are (at least) some TDs with Macc compatible with those of cTTs, irrespective of the size of the dust inner hole.Only in 2 cases Macc are much lower, while the wind properties are similar. We do not see any strong trend of Macc with the size of the dust depleted cavity, nor with the presence of a dusty optically thick disk close to the star. In the TDs in our sample there is a gas rich inner disk with density similar to that of cTTs disks. At least for some TDs, the process responsible of the inner disk clearing should allow for a transfer of gas from the outer disk to the inner region. This should proceed at a rate that does not depend on the physical mechanism producing the gap seen in the dust emission and results in a gas density in the inner disk similar to that of unperturbed disks around stars of similar mass.
We study the orientation of accretion disks, traced by the position angle of the jet, relative to the dust disk major axis in a sample of 20 nearby Radio Galaxies. We find that the observed distribution of angles between the jet and dust disk major axis is consistent with jets homogeneously distributed over a polar cap of 77 degrees.
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