GG Tau: the ringworld and beyond Dutrey, Anne; Di Folco, Emmanuel; Beck, Tracy ...
The Astronomy and astrophysics review,
12/2016, Letnik:
24, Številka:
1
Journal Article
Recenzirano
In binary stellar systems, exoplanet searches have revealed planetary mass companions orbiting both in circumstellar and in circumbinary orbits. Modelling studies suggest increased dynamical ...complexity around the young stars that form such systems. Circumstellar and circumbinary disks likely exhibit different physical conditions for planet formation, which also depends on the stellar separation. Although binaries and higher order multiple stars are relatively common in nearby star-forming regions, surprisingly few systems with circumbinary distributions of proto-planetary material have been found. With its spectacular ring of dust and gas encircling the central triple star, one such system, GG Tau A, has become a unique laboratory for investigating the physics of circumsystem gas and dust evolution. We review here its physical properties.
Observations of multiple rotational transitions from a single molecule allow for unparalleled constraints on the physical conditions of the emitting region. We present an analysis of CS in TW Hya ...using the J = 7-6, 5-4 and 3-2 transitions imaged at ∼0 5 spatial resolution, resulting in a temperature and column density profile of the CS emission region extending out to 230 au, far beyond previous measurements. In addition, the 15 kHz resolution of the observations and the ability to directly estimate the temperature of the CS emitting gas, allow for one of the most sensitive searches for turbulent broadening in a disk to date. Limits of vturb 0.1cs can be placed across the entire radius of the disk. We are able to place strict limits of the local H2 density due to the collisional excitations of the observed transitions. From these we find that a minimum disk mass of 3 × 10−4 MSun is required to be consistent with the CS excitation conditions and can uniquely constrain the gas surface density profile in the outer disk.
Abstract
We report observations of polarized line and continuum emission from the disk of TW Hya using the Atacama Large Millimeter/submillimeter Array. We target three emission lines,
12
CO (3–2),
...13
CO (3–2), and CS (7–6), to search for linear polarization due to the Goldreich–Kylafis effect, while simultaneously tracing the continuum polarization morphology at 332 GHz (900
μ
m), achieving a spatial resolution of 0.″5 (30 au). We detect linear polarization in the dust continuum emission; the polarization position angles show an azimuthal morphology, and the median polarization fraction is ∼0.2%, comparable to previous, lower frequency observations. Adopting a “shift-and-stack” technique to boost the sensitivity of the data, combined with a linear combination of the
Q
and
U
components to account for their azimuthal dependence, we detect weak linear polarization of
12
CO and
13
CO line emission at a ∼10
σ
and ∼5
σ
significance, respectively. The polarization was detected in the line wings, reaching a peak polarization fraction of ∼5% and ∼3% for the two molecules between disk radii of 0.″5 and 1″. The sign of the polarization was found to flip from the blueshifted side of the emission to the redshifted side, suggesting a complex, asymmetric polarization morphology. Polarization is not robustly detected for the CS emission; however, a tentative signal, comparable in morphology to that found for the
12
CO and
13
CO emission, is found at a ≲3
σ
significance. We are able to reconstruct a polarization morphology, consistent with the azimuthally averaged profiles, under the assumption that this is also azimuthally symmetric, which can be compared with future higher-sensitivity observations.
We report the results of ALMA observations of a protoplanetary disk surrounding the Herbig Ae star AB Aurigae. We obtained high-resolution (0 1; 14 au) images in 12CO J = 2 − 1 emission and in the ...dust continuum at the wavelength of 1.3 mm. The continuum emission is detected at the center and at the ring with a radius (r) of ∼120 au. The CO emission is dominated by two prominent spirals within the dust ring. These spirals are trailing and appear to be about 4 times brighter than their surrounding medium. Their kinematics is consistent with Keplerian rotation at an inclination of 23°. The apparent two-arm-spiral pattern is best explained by tidal disturbances created by an unseen companion located at r of 60-80 au, with dust confined in the pressure bumps created outside this companion orbit. An additional companion at r of 30 au, coinciding with the peak CO brightness and a large pitch angle of the spiral, would help to explain the overall emptiness of the cavity. Alternative mechanisms to excite the spirals are discussed. The origin of the large pitch angle detected here remains puzzling.
The formation of planets around binary stars may be more difficult than around single stars. In a close binary star (with a separation of less than a hundred astronomical units), theory predicts the ...presence of circumstellar disks around each star, and an outer circumbinary disk surrounding a gravitationally cleared inner cavity around the stars. Given that the inner disks are depleted by accretion onto the stars on timescales of a few thousand years, any replenishing material must be transferred from the outer reservoir to fuel planet formation (which occurs on timescales of about one million years). Gas flowing through disk cavities has been detected in single star systems. A circumbinary disk was discovered around the young low-mass binary system GG Tau A (ref. 7), which has recently been shown to be a hierarchical triple system. It has one large inner disk around the single star, GG Tau Aa, and shows small amounts of shocked hydrogen gas residing within the central cavity, but other than a single weak detection, the distribution of cold gas in this cavity or in any other binary or multiple star system has not hitherto been determined. Here we report imaging of gas fragments emitting radiation characteristic of carbon monoxide within the GG Tau A cavity. From the kinematics we conclude that the flow appears capable of sustaining the inner disk (around GG Tau Aa) beyond the accretion lifetime, leaving time for planet formation to occur there. These results show the complexity of planet formation around multiple stars and confirm the general picture predicted by numerical simulations.
ABSTRACT We conducted IRAM-30 m C18O (2-1) and SMA 1.3 mm continuum 12CO (2-1) and C18O (2-1) observations toward the Class 0/I protostar L1455 IRS1 in Perseus. The IRAM-30 m C18O results show IRS1 ...in a dense 0.05 pc core with a mass of 0.54 M , connecting to a filamentary structure. Inside the dense core, compact components of 350 au and 1500 au are detected in the SMA 1.3 mm continuum and C18O, with a velocity gradient in the latter one perpendicular to a bipolar outflow in 12CO, likely tracing a rotational motion. We measure a rotational velocity profile that becomes shallower at a turning radius of ∼200 au, which is approximately the radius of the 1.3 mm continuum component. These results hint at the presence of a Keplerian disk with a radius <200 au around L1455 IRS1 with a protostellar mass of about 0.28 M . We derive a core rotation that is about one order of magnitude faster than expected. A significant velocity gradient along a filament toward IRS1 indicates that this filament is dynamically important, providing a gas reservoir and possibly responsible for the faster-than-average core rotation. Previous polarimetric observations show a magnetic field aligned with the outflow axis and perpendicular to the associated filament on a 0.1 pc scale, while on the inner 1000 au scale, the field becomes perpendicular to the outflow axis. This change in magnetic field orientations is consistent with our estimated increase in rotational energy from large to small scales that overcomes the magnetic field energy, wrapping the field lines and aligning them with the disk velocity gradient. These results are discussed in the context of the interplay between filament, magnetic field, and gas kinematics from large to small scales. Possible emerging trends are explored with a sample of 8 Class 0/I protostars.
Abstract
We report Atacama Large Millimeter/submillimeter Array (ALMA) polarization observations at 3 and 0.9 mm toward the GG Tau A system. In the ring, the percentage is relatively homogeneous at 3 ...mm, being 1.2%, while it exhibits a clear radial variation at 0.9 mm with a mean increasing from 0.6% to 2.8% toward larger radius (
r
). The polarization orientation at
r
> 1.″85 appears nearly azimuthal at both wavelengths. At
r
< 1.″85, the pattern remains azimuthal at 3 mm but becomes radial at 0.9 mm. The dust self-scattering model with
a
max
of 1 mm could reproduce the observed polarization orientation and percentage at 0.9 mm, but the expected polarization percentage at 3 mm would be 0.2%, much smaller than the detected 1.2%. Dust alignment with poloidal magnetic field could qualitatively reproduce the flip in polarization at
r
< 1.″85 and also the detected polarization percentage. A closer inspection of the nearly azimuthal pattern reveals that polarization orientations are systematically deviating by −9.°0 ± 1.°2 from the tangent of the orbit ellipses. This deviation agrees with the direction of the spiral pattern observed in the near-infrared, but it is unclear how dust grains could be aligned along such spirals. For the scenario where the −9° deviation (−7.°3 after considering the inclination effect) measures the radial component of the dust drift motion, the expected inward drifting velocity would be ∼12.8% of the Keplerian speed, a factor of 2.8 larger than the theoretical predictions. Possible additional interpretations of the polarization are discussed, but there is no single mechanism that could explain the detected polarization simultaneously.
Molecular line emission from protoplanetary disks is a powerful tool to constrain their physical and chemical structure. Nevertheless, only a few molecules have been detected in disks so far. We take ...advantage of the enhanced capabilities of the IRAM 30m telescope by using the new broad band correlator (FTS) to search for so far undetected molecules in the protoplanetary disks surrounding the TTauri stars DM Tau, GO Tau, LkCa 15 and the Herbig Ae star MWC 480. We report the first detection of HC3N at 5 sigma in the GO Tau and MWC 480 disks with the IRAM 30-m, and in the LkCa 15 disk (5 sigma), using the IRAM array, with derived column densities of the order of 10^{12}cm^{-2}. We also obtain stringent upper limits on CCS (N < 1.5 x 10^{12} cm^{-3}). We discuss the observational results by comparing them to column densities derived from existing chemical disk models (computed using the chemical code Nautilus) and based on previous nitrogen and sulfur-bearing molecule observations. The observed column densities of HC3N are typically two orders of magnitude lower than the existing predictions and appear to be lower in the presence of strong UV flux, suggesting that the molecular chemistry is sensitive to the UV penetration through the disk. The CCS upper limits reinforce our model with low elemental abundance of sulfur derived from other sulfur-bearing molecules (CS, H2S and SO).
ABSTRACT We aim to unveil the observational imprint of physical mechanisms that govern planetary formation in the young, multiple system GG Tau A. We present ALMA observations of 12CO and 13CO 3-2 ...and 0.9 mm continuum emission with 0 35 resolution. The 12CO 3-2 emission, found within the cavity of the circumternary dust ring (at radius <180 au) where no 13CO emission is detected, confirms the presence of CO gas near the circumstellar disk of GG Tau Aa. The outer disk and the recently detected hot spot lying at the outer edge of the dust ring are mapped both in 12CO and 13CO. The gas emission in the outer disk can be radially decomposed as a series of slightly overlapping Gaussian rings, suggesting the presence of unresolved gaps or dips. The dip closest to the disk center lies at a radius very close to the hot spot location at ∼250-260 au. The CO excitation conditions indicate that the outer disk remains in the shadow of the ring. The hot spot probably results from local heating processes. The two latter points reinforce the hypothesis that the hot spot is created by an embedded proto-planet shepherding the outer disk.
Recent exo-planetary surveys reveal that planets can orbit and survive around binary stars. This suggests that some fraction of young binary systems which possess massive circumbinary (CB) disks may ...be in the midst of planet formation. However, there are very few CB disks detected. The streaming gas accreting from the CB ring toward the CS disks and possible outflows are also identified and resolved. The SO emission is found to be at the bases of the streaming shocks. Our results suggest that the UY Aur system is undergoing an active accretion phase from the CB disk to the CS disks. The UY Aur B might also be a binary system, making the UY Aur a triple system.
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