Andrea Isella

Large-Scale Asymmetries in the Transitional Disks of SAO 206462 and SR 21

We present Atacama Large Millimeter/submillimeter Array (ALMA) observations in the dust continuum (690 GHz, 0.45 mm) and ^(12)CO J = 6-5 spectral line emission of the transitional disks surrounding the stars SAO 206462 and SR 21. These ALMA observations resolve the dust-depleted disk cavities and extended gaseous disks, revealing large-scale asymmetries in the dust emission of both disks. We modeled these disk structures with a ring and an azimuthal Gaussian, where the azimuthal Gaussian is motivated by the steady-state vortex solution from Lyra & Lin. Compared to recent observations of HD 142527, Oph IRS 48, and LkHα 330, these are low-contrast (≾ 2) asymmetries. Nevertheless, a ring alone is not a good fit, and the addition of a vortex prescription describes these data much better. The asymmetric component encompasses 15% and 28% of the total disk emission in SAO 206462 and SR 21, respectively, which corresponds to a lower limit of 2 M_(Jup) of material within the asymmetry for both disks. Although the contrast in the dust asymmetry is low, we find that the turbulent velocity inside it must be large (~20% of the sound speed) in order to drive these azimuthally wide and radially narrow vortex-like structures. We obtain residuals from the ring and vortex fitting that are still significant, tracing non-axisymmetric emission in both disks. We compared these submillimeter observations with recently published H-band scattered light observations. For SR 21 the scattered light emission is distributed quite differently from the submillimeter continuum emission, while for SAO 206462 the submillimeter residuals are suggestive of spiral-like structure similar to the near-IR emission.

RINGED SUBSTRUCTURE AND A GAP AT 1 au IN THE NEAREST PROTOPLANETARY DISK

We present long-baseline Atacama Large Millimeter/submillimeter Array (ALMA) observations of the 870 micron continuum emission from the nearest gas-rich protoplanetary disk, around TW Hya, that trace millimeter-sized particles down to spatial scales as small as 1 AU (20 mas). These data reveal a series of concentric ring-shaped substructures in the form of bright zones and narrow dark annuli (1-6 AU) with modest contrasts (5-30%). We associate these features with concentrations of solids that have had their inward radial drift slowed or stopped, presumably at local gas pressure maxima. No significant non-axisymmetric structures are detected. Some of the observed features occur near temperatures that may be associated with the condensation fronts of major volatile species, but the relatively small brightness contrasts may also be a consequence of magnetized disk evolution (the so-called zonal flows). Other features, particularly a narrow dark annulus located only 1 AU from the star, could indicate interactions between the disk and young planets. These data signal that ordered substructures on ~AU scales can be common, fundamental factors in disk evolution, and that high resolution microwave imaging can help characterize them during the epoch of planet formation.

Investigating Planet Formation in Circumstellar Disks: CARMA Observations of Ry Tau and Dg Tau

(Abridged) We present CARMA observations of the thermal dust emission from the circumstellar disks around the young stars RYTau and DGTau at wavelengths of 1.3mm and 2.8mm. The angular resolution of the maps is as high as 0.15arcsec, or 20AU at the distance of the Taurus cloud, which is a factor of 2 higher than has been achieved to date at these wavelengths. The unprecedented detail of the resulting disk images enables us to address three important questions related to the formation of planets. (1) What is the radial distribution of the circumstellar dust? (2) Does the dust emission show any indication of gaps that might signify the presence of (proto-)planets? (3) Do the dust properties depend on the orbital radius? We find that modeling the disk surface density in terms of either a classical power law or the similarity solution for viscous disk evolution, reproduces the observations well. The 1.3mm image from RYTau shows two peaks separated by 0.2arcsec with a decline in the dust emission toward the stellar position, which is significant at about 2-4sigma. For both RYTau and DGTau, the dust emission at radii larger than 15 AU displays no significant deviation from an unperturbed viscous disk model. In particular, no radial gaps in the dust distribution are detected. Under reasonable assumptions, we exclude the presence of planets more massive than 5 Jupiter masses orbiting either star at distances between about 10 and 60 AU. The radial variation of the dust opacity slope, beta, was investigated by comparing the 1.3mm and 2.8mm observations. We find mean values of beta of 0.5 and 0.7 for DGTau and RYTau respectively. Variations in beta are smaller than 0.7 between 20 and 70 AU. These results confirm that the circumstellar dust throughout these disks differs significantly from dust in the interstellar medium.

Asymmetric features in the protoplanetary disk MWC 758

Context. The study of dynamical processes in protoplanetary disks is essential to understand planet formation. In this context, transition disks are prime targets because they are at an advanced stage of disk clearing and may harbor direct signatures of disk evolution. Aims. We aim to derive new constraints on the structure of the transition disk MWC 758, to detect non-axisymmetric features and understand their origin. Methods. We obtained infrared polarized intensity observations of the protoplanetary disk MWC 758 with SPHERE/VLT at 1.04 m to resolve scattered light at a smaller inner working angle (0.093 00 ) and a higher angular resolution (0.027 00 ) than previously achieved. Results. We observe polarized scattered light within 0.53 00 (148 au) down to the inner working angle (26 au) and detect distinct nonaxisymmetric features but no fully depleted cavity. The two small-scale spiral features that were previously detected with HiCIAO are resolved more clearly, and new features are identified, including two that are located at previously inaccessible radii close to the star. We present a model based on the spiral density wave theory with two planetary companions in circular orbits. The best model requires a high disk aspect ratio (H=r 0.20 at the planet locations) to account for the large pitch angles which implies a very warm disk. Conclusions. Our observations reveal the complex morphology of the disk MWC 758. To understand the origin of the detected features, the combination of high-resolution observations in the submillimeter with ALMA and detailed modeling is needed.

AN AZIMUTHAL ASYMMETRY IN THE LkHα 330 DISK

Theory predicts that giant planets and low mass stellar companions shape circumstellar disks by opening annular gaps in the gas and dust spatial distribution. For more than a decade it has been debated whether this is the dominant process that leads to the formation of transitional disks. In this paper, we present millimeter-wave interferometric observations of the transitional disk around the young intermediate mass star LkHα 330. These observations reveal a lopsided ring in the 1.3 mm dust thermal emission characterized by a radius of about 100 AU and an azimuthal intensity variation of a factor of two. By comparing the observations with a Gaussian parametric model, we find that the observed asymmetry is consistent with a circular arc, that extends azimuthally by about 90° and emits about 1/3 of the total continuum flux at 1.3 mm. Hydrodynamic simulations show that this structure is similar to the azimuthal asymmetries in the disk surface density that might be produced by the dynamical interaction with unseen low mass companions orbiting within 70 AU from the central star. We argue that such asymmetries might lead to azimuthal variations in the millimeter-wave dust opacity and in the dust temperature, which will also affect the millimeter-wave continuum emission. Alternative explanations for the observed asymmetry that do not require the presence of companions cannot be ruled out with the existing data. Further observations of both the dust and molecular gas emission are required to derive firm conclusions on the origin of the asymmetry observed in the LkHα 330 disk.

The origin of hydrogen line emission for five Herbig Ae/Be stars spatially resolved by VLTI/AMBER spectro-interferometry

Context. Accretion and outflow processes are of fundamental importance for our understanding of the formation of stars and planetary systems. To trace these processes, diagnostic spectral lines such as the Brγ 2.166 μm line are widely used, although due to a lack of spatial resolution, the origin of the line emission is still unclear. Aims. Employing the AU-scale spatial resolution which can be achieved with infrared long-baseline interferometry, we aim to distinguish between theoretical models which associate the Brγ line emission with mass infall (magnetospheric accretion, gaseous inner disks) or mass outflow processes (stellar winds, X-winds, or disk winds). Methods. Using the VLTI/AMBER instrument, we spatially and spectrally (λ/Δλ = 1500) resolved the inner (≾5 AU) environment of five Herbig Ae/Be stars (HD163296, HD104237, HD98922, MWC297, V921 Sco) in the Brγ emission line as well as in the adjacent continuum. From the measured wavelength-dependent visibilities, we derive the characteristic size of the continuum and Brγ line-emitting region. Additional information is provided by the closure phase, which we could measure both in the continuum wavelength regime (for four objects) as well as in the spectrally resolved Brγ emission line (for one object). The spectro-interferometric data is supplemented by archival and new VLT/ISAAC spectroscopy. Results. For all objects (except MWC297), we measure an increase of visibility within the Brγ emission line, indicating that the Brγ-emitting region in these objects is more compact than the dust sublimation radius. For HD98922, our quantitative analysis reveals that the line-emitting region is compact enough to be consistent with the magnetospheric accretion scenario. For HD163296, HD104237, MWC297, and V921 Sco we identify an extended stellar wind or a disk wind as the most likely line-emitting mechanism. Since the stars in our sample cover a wide range of stellar parameters, we also search for general trends and find that the size of the Brγ-emitting region does not seem to depend on the basic stellar parameters (such as the stellar luminosity), but correlates with spectroscopic properties, in particular with the Hα line profile shape. Conclusions. By performing the first high-resolution spectro-interferometric survey on Herbig Ae/Be stars, we find evidence for at least two distinct Brγ line-formation mechanisms. Most significant, stars with a P-Cygni Hα line profile and a high mass-accretion rate seem to show particularly compact Brγ-emitting regions (R_(Brγ)/R_(cont) < 0.2), while stars with a double-peaked or single-peaked Hα-line profile show a significantly more extended Brγ-emitting region (0.6 ≾ R_(Brγ)/R_(cont) ≾ 1.4), possibly tracing a stellar wind or a disk wind.

Disk and wind interaction in the young stellar object MWC 297 spatially resolved with VLTI/AMBER

The young stellar object MWC 297 is an embedded B1.5Ve star exhibiting strong hydrogen emission lines and a strong near-infrared continuum excess. This object has been observed with the VLT interferometer equipped with the AMBER instrument during its first commissioning run. VLTI/AMBER is currently the only near infrared interferometer which can observe spectrally dispersed visibilities. MWC 297 has been spatially resolved in the continuum with a visibility of

Gas density drops inside dust cavities of transitional disks around young stars observed with ALMA

0.50^{+0.08}_{-0.10}

Spiral density waves in a young protoplanetary disk.

as well as in the Brgamma emission line where the visibility decrease to a lower value of

On the structure of the transition disk around TW Hydrae

0.33\pm0.06