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Planet-Forming Disks

Disk Formation

The initial chemical and physical properties of disks are important for the subsequent evolution and the formation of planetary systems. The size and properties of disks as they are assembled are key properties that can be investigated with ALMA at high angular resolution. One example is shown in Figure 4.1 for the TMC-1A young protostar.



Figure 4.1: ALMA observations of TMC-1A showing the complex structure of the inner infalling envelope and the outflowing material. ALMA angular resolution can help resolve the kinematics of the inner envelope and reveal the forming disk. Credit: ALMA (ESO/NAOJ/NRAO), Aso et al. (2015).


Detailed observations of young protostellar systems with ALMA have revealed the chemical signatures of the shock at the interface between the infalling envelope and the forming protoplanetary disk. The drastic change of the chemistry at the interface between the envelope and disk are believed to be caused by localized heating at the centrifugal barrier (see Figure 4.2).



Figure 4.2: Left: Schematic illustration of the infalling, rotating envelope around a protostar. The gas cannot go inside the centrifugal barrier due to the centrifugal force. The observer is on the left-hand side, looking at the envelope in an edge-on configuration. Middle: The highest and lowest velocities of the infalling-rotating gas calculated from the model. The emissions of the colored closed circles come from the corresponding colored closed circles in the left panel. Right: The highest and lowest velocities shown in the middle panel are superposed on the position-velocity diagram of cyclic-C3H2. The observation of cyclic-C3H2 shows a beautiful agreement with the model. The cyclic-C3H2. molecules completely disappear at a radius of ~100 AU (the centrifugal barrier) from the protostar, whereas SO appears at the radius. SO preferentially exists in the ring located at the centrifugal barrier. Figures from Sakai et al. (2014).



Aso et al. 2015, ApJ, 812, 27

Sakai et al. 2014, Nature, 507, 78


Disk surveys

ALMA sensitivity allows for the first time true demographical studies of the disk properties in nearby star forming regions. Several regions have been surveyed thus far with ages ranging from 1 to 10 Myr (see Figure 4.3). The emerging trend is that the evolution of the dust content in disks confirm the expectations from near infrared surveys that most disks exhaust most of the material available for planet formation in the first few million years. The detailed mass evolution and evolution of dust properties are still under investigation, including the potential hints for a differential evolution as a function of the stellar mass.



Figure 4.3: Gallery of dust continuum images of disks in the Lupus (left; Ansdell et al. 2016) and Upper Scorpius (right; Barenfeld et al. 2016) star forming regions.


ALMA spectral line observations of disks in various star forming regions show a deficit in the luminosity of CO and its isotopologues when compared to the dust continuum luminosity. These observations suggest that disks have a very low gas to dust ratio (3-30 by mass; see Figure 4.4) based on disk models that incorporate CO excitation, photodissociation and condensation. These results are at odds with other gas mass estimates and suggest that there are still large uncertainties in our understanding of the carbon budget in disks.



Figure 4.4: Top: Dust masses for disks in the Lupus star forming region. Middle: Gas masses from modeling of CO isotopologues. Bottom: Apparent gas to dust ratio. Figure from Miotello et al. (2016).



Ansdell et al. 2016, ApJ, 828, 46

Barenfeld et al. 2016, ApJ, 827, 142

Miotello et al. 2016, A&A, in press (arXiv:1612.01538)


Structure of planet forming disks

One of the most stunning results from ALMA have been the discovery of gaps and asymmetries in the dust distribution in planet forming disks (see Figure 4.5). At this time it is still unclear whether a phase transition of the major volatiles are responsible for the symmetric gaps and bright rings in the continuum images of disks, or if the effects of photoevaporation of the disk are dominating, or whether planetesimals, planetary cores and gas giants are responsible. In some cases the presence of planets seems unavoidable, especially in some of the so-called Transition Disks.



Figure 4.5: Gallery of high angular resolution continuum observations of planet forming disks obtained with ALMA. From left to right and from top to bottom: TW Hya (Andrews et al. 2016), V883 Ori (Cieza et al. (2016), HD 163296 (Isella et al. 2016), HL Tau (ALMA Partnership et al. 2015), Elias 2-27 (Pérez et al. 2016), and HD 142527 (Kataoka et al. 2016). Credits: S. Andrews, L. Cieza, A. Isella, A. Kataoka, B. Saxton (NRAO/AUI/NSF), and ALMA (ESO/NAOJ/NRAO).

ALMA sensitivity also allows, for the first time, detailed studies of the chemical complexity of protoplanetary disks (see Figure 4.6). The transition of CO from the gas phase (in the inner warmer regions of the disk) to the solid state (in the outer cooler regions) has the effect that the destruction of certain molecules is suppressed where CO is not available in the gas. The prime molecular tracer of this effect has been shown to be N2H+(see Figure 4.6). Also, for the first time, complex molecules like CH3CN and CH3OH, have been detected in disks (Öberg et al. 2015, Walsh et al. 2016), indicating that planet forming disks share the chemical complexity observed in Solar System icy bodies and potentially a similar chemistry that led to the formation of a life supporting biosphere on Earth.


Figure 4.6: ALMA image of the dust, CO, and N2H+ emission toward the disk around the star Tw Hya. The red dashed circle marks the onset of CO freeze-out according to astrochemical theory, and thus marks the CO snow line in the disk midplane. Figure from Qi et al. (2013).



ALMA Partnership et al. 2015, ApJ, 808, L3

Andrews et al. 2016, ApJ, 820, L40

Cieza et al. 2016, Nature, 535, 258

Isella et al. 2016, PhysRevLett, 117, 1101

Kataoka et al. 2016, ApJ, 831, L12

Pérez et al. 2016, Science, 353,1519

Qi et al. 2013, Science, 341, 630

Walsh et al. 2016, ApJ, 823, L10


Debris disks

ALMA has detected molecular gas in several debris disks, despite the fact that molecular gas as expected to be short lived in these systems. The currently favored explanation is that fresh gaseous material is constantly replenished in these systems through shattering of icy bodies. In particular, the observations of the nearby young star Beta Pictoris have revealed an asymmetric distribution of gas with most of the emission associated with a clump of gas (see Figure 4.7). The mass and location of the molecular gas is consistent with being produced by the recent destruction of an icy body with a mass similar to Mars.



Figure 4.7: ALMA image of carbon monoxide around Beta Pictoris (top) that has been deprojected (bottom) to simulate a view looking down on the system, revealing the large concentration of gas in its outer reaches (Dent et al. 2014). For comparison, orbits within the Solar System are shown for scale. Credit: ALMA (ESO/NAOJ/NRAO) and NASA's Goddard Space Flight Center/F. Reddy.



Dent et al. 2014, Science, 343, 1490