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Register a new userHunting for planets in the HL Tau disk
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Authors
L. Testi
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Recent ALMA images of HL Tau show gaps in the dusty disk that may be caused by planetary bodies. Given the young age of this system, if confirmed, this finding would imply very short timescales for planet formation, probably in a gravitationally unstable disk. To test this scenario, we searched for young planets by means of direct imaging in the L-band using the Large Binocular Telescope Interferometer mid-infrared camera. At the location of two prominent dips in the dust distribution at ~70AU (~0.5) from the central star we reach a contrast level of ~7.5mag. We did not detect any point source at the location of the rings. Using evolutionary models we derive upper limits of ~10-15MJup at <=0.5-1Ma for the possible planets. With these sensitivity limits we should have been able to detect companions sufficiently massive to open full gaps in the disk. The structures detected at mm-wavelengths could be gaps in the distributions of large grains on the disk midplane, caused by planets not massive enough to fully open gaps. Future ALMA observations of the molecular gas density profile and kinematics as well as higher contrast infrared observations may be able to provide a definitive answer.
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We model the ALMA and VLA millimeter radial profiles of the disk around HL Tau to constrain the properties of the dust grains. We adopt the disk evolutionary models of Lynden-Bell & Pringle and calculate their temperature and density structure and emission. These disks are heated by the internal viscosity and irradiated by the central star and a warm envelope. We consider a dust size distribution $n(a) da propto a^{-3.5} da $, and vary the maximum grain size in the atmosphere and the midplane, $a_{rm max}=100 mu$m, 1 mm, and 1cm. We also include dust settling and vary the dust-to-gas mass ratio from 1 to 9 times the ISM value. We find that the models that can fit the observed level of emission along the profiles at all wavelengths have an atmosphere with a maximum grain size $a_{rm max} = 100 mu$m, and a midplane with $a_{rm max}=1$ cm. The disk substructure, with a deficit of emission in the gaps, can be due to dust properties in these regions that are different from those in the rings. We test an opacity effect (different $a_{rm max}$) and a dust mass deficit (smaller dust-to-gas mass ratio) in the gaps. We find that the emission profiles are better reproduced by models with a dust deficit in the gaps, although a combined effect is also possible. These models have a global dust-to-gas mass ratio twice the ISM value, needed to reach the level of emission of the 7.8 mm VLA profile.
The protoplanetary disk around HL Tau is so far the youngest candidate of planet formation, and it is still embedded in a protostellar envelope with a size of thousands of au. In this work, we study the gas kinematics in the envelope and its possible influence on the embedded disk. We present our new ALMA cycle 3 observational results of HL Tau in the 13CO (2-1) and C18O (2-1) emission at resolutions of 0.8 (110 au), and we compare the observed velocity pattern with models of different kinds of gas motions. Both the 13CO and C18O emission lines show a central compact component with a size of 2 (280 au), which traces the protoplanetary disk. The disk is clearly resolved and shows a Keplerian motion, from which the protostellar mass of HL Tau is estimated to be 1.8+/-0.3 M$_odot$, assuming the inclination angle of the disk to be 47 deg from the plane of the sky. The 13CO emission shows two arc structures with sizes of 1000-2000 au and masses of 3E-3 M$_odot$ connected to the central disk. One is blueshifted and stretches from the northeast to the northwest, and the other is redshifted and stretches from the southwest to the southeast. We find that simple kinematical models of infalling and (counter-)rotating flattened envelopes cannot fully explain the observed velocity patterns in the arc structures. The gas kinematics of the arc structures can be better explained with three-dimensional infalling or outflowing motions. Nevertheless, the observed velocity in the northwestern part of the blueshifted arc structure is ~60-70% higher than the expected free-fall velocity. We discuss two possible origins of the arc structures: (1) infalling flows externally compressed by an expanding shell driven by XZ Tau and (2) outflowing gas clumps caused by gravitational instabilities in the protoplanetary disk around HL Tau.
The first long-baseline ALMA campaign resolved the disk around the young star HL Tau into a number of axisymmetric bright and dark rings. Despite the very young age of HL Tau these structures have been interpreted as signatures for the presence of (proto)planets. The ALMA images triggered numerous theoretical studies based on disk-planet interactions, magnetically driven disk structures, and grain evolution. Of special interest are the inner parts of disks, where terrestrial planets are expected to form. However, the emission from these regions in HL Tau turned out to be optically thick at all ALMA wavelengths, preventing the derivation of surface density profiles and grain size distributions. Here, we present the most sensitive images of HL Tau obtained to date with the Karl G. Jansky Very Large Array at 7.0 mm wavelength with a spatial resolution comparable to the ALMA images. At this long wavelength the dust emission from HL Tau is optically thin, allowing a comprehensive study of the inner disk. We obtain a total disk dust mass of 0.001 - 0.003 Msun, depending on the assumed opacity and disk temperature. Our optically thin data also indicate fast grain growth, fragmentation, and formation of dense clumps in the inner densest parts of the disk. Our results suggest that the HL Tau disk may be actually in a very early stage of planetary formation, with planets not already formed in the gaps but in the process of future formation in the bright rings.
The recent advent of spatially resolved mm- and cm-wavelength polarimetry in protostellar accretion discs could help clarify the role of magnetic fields in the angular momentum transport in these systems. The best case to date is that of HL~Tau, where the inability to produce a good fit to the 1.25-mm data with a combination of vertical and azimuthal magnetic field components was interpreted as implying that centrifugally driven winds (CDWs) are probably not a significant transport mechanism on the $sim 10^2,$au scale probed by the observations. Using synthetic polarization maps of heuristic single-field-component discs and of a post-processed simulation of a wind-driving disc, we demonstrate that a much better fit to the data can be obtained if the radial field component, a hallmark of the CDW mechanism, dominates in the polarized emission region. A similar inference was previously made in modelling the far-infrared polarization map of the pc-scale dust ring in the Galactic centre. To reconcile this interpretation with theoretical models of protostellar discs, which indicate that the wind is launched from a comparatively high elevation above the mid-plane, we propose that most of the polarized emission originates -- with a high ($ga 10$%) intrinsic degree of polarization -- in small ($la 0.1,$mm) grains that remain suspended above the mid-plane, and that the bulk of the mm-wavelength emission is produced -- with low intrinsic polarization -- by larger grains that have settled to the mid-plane.
We present results of high-resolution imaging toward HL Tau by the Combined Array for Research in Millimeter-wave Astronomy (CARMA). We have obtained 1.3 and 2.7 mm dust continua with an angular resolution down to 0.13 arc second. Through model fitting to the two wavelength data simultaneously in Bayesian inference using a flared viscous accretion disk model, we estimate the physical properties of HL Tau, such as density distribution, dust opacity spectral index, disk mass, disk size, inclination angle, position angle, and disk thickness. HL Tau has a circumstellar disk mass of 0.13 solar mass, a characteristic radius of 79 AU, an inclination of 40 degree, and a position angle of 136 degree. Although a thin disk model is preferred by our two wavelength data, a thick disk model is needed to explain the high mid- and far-infrared emission of the HL Tau spectral energy distribution. This could imply large dust grains settled down on the mid plane with fine dust grains mixed with gas. The HL Tau disk is likely gravitationally unstable and can be fragmented between 50 and 100 AU of radius. However, we did not detect dust thermal continuum supporting the protoplanet candidate claimed by a previous study using observations of the Very Large Array at 1.3 cm.
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