WITNESSING PLANET FORMATION WITH ALMA AND THE ELTs

WITNESSING PLANET FORMATION WITH ALMA AND THE ELTs

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GMT TMT E-ELT

Lucas Cieza, IfA/U. of Hawaii

ABSTRACT:

Over the last 15 years, astronomers have discovered over 400 mature exoplanets in the solar neighborhood. However, the precise mechanisms through which planets are formed still remain largely unknown. The situation is particularly troubling for giant planets, for which two fundamentally dissimilar formation models exists, namely core accretion (Lissauer 1993) and gravitational instability (Boss 2000). Fortunately, and thanks to the unprecedented sensitivity and resolution of the Atacama Large Millimeter Array (ALMA) and the Extremely Large Telescopes (ELTs), we are now on the verge of being able to observe ongoing planet formation in primordial circumstellar disks. Here we review the capabilities of these new facilities, which will most likely revolutionize our current understanding of planet formation.



The targets: observing systems that are actively forming planets is the most promising path to progress in understanding the planet formation process. The so called “transition objects” (PMS stars with optically thin inner disks and optically thick outer disks) are currently the most likely sites of ongoing planet-formation. It is increasingly clear that many of these systems have recently formed giant planets still embedded within the primordial disk (Espaillat et al. 2008; Cieza et al. 2010). These systems, such as DM Tau, GM Aur, LkCa 15, and OPH TRAN 32 (see Fig. 1), are prime targets for detailed followup observations with ALMA and the ELTs.





ALMA (2012)

Wolf (2008)

ALMA simulation

Forming planet

2 AU

Fig. 3 ALMA (fifty four 12-m antennas,16 km max baselines). With 30 times the collecting area and resolution of the SMA, ALMA will provide images with 0.01’’ (~2 AU) resolution and allow the direct detection of forming planets.



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SMA 345 GHz image of OPH TRAN 32, reveals a close to edge-on disk with a

~40 AU inner hole.

35 AU

Fig 2. SMA (eight 6-m antennas, 0.5 km max baselines). With a 0.3’’ resolution, the SMA can currently resolve the inner holes of transition disks, constraining the size and sharpness of their holes.

Cieza et al. (in prep)



Cieza et al. (2010)

Fig 1. Optical to mm SED of OPH TRAN 32. The slope between 8.0 and 24 μm suggests the presence of sharp, dynamically carved inner hole. Yet, it appears to lack stellar companions (from AO observations). A significant amount of gas must be present within its

inner hole as the object is still accreting.

Transition disk Typical SED of CTTS disk

Planet formation science enabled by ALMA:

• High resolution (few AU scale) imaging of transition disks to constrain their structure and search for gaps, spiral density waves, and other indications of dynamical interactions with forming giant planets.

• Direct detection of forming giant planets to directly address the question of how and when giant planets form in circumstellar disks (see Figure 3).

• CO observations to investigate the dynamics of the gas and establish how turbulent circumstellar disks really are and whether they are conducive to gravitational instability.

• High-resolution, multi-wavelength continuous observations to establish the grain size distribution as a function of radius, which has important implications for grain growth and radial mixing.

• Survey of binary systems with IR excess to constrain the distribution of circumstellar material and establish which type of binary systems are conducive to planet formation.

Table 1. The physical resolution of the TMT as a function of wavelength and distance. The ELTs will be able to detect planets with separaration as small as ~5 AU at the distance of the Ophiuchus and Taurus SFRs (d =125-140 pc) and as small as ~2 pc at the distance of the TW Hydra association (d = 60 pc). Table taken from the TMT detailed science case: 2007.

Fig 4. Brightness ratio between the planet and the host star as a function of angular separation for different types of exoplanets. The detection limits of the Planet Formation Instrument planed for the TMT are shown as a red curve. The ELTs will be able to directly detect giant planets in nearby star-forming regions (SFRs) such as Taurus (d = 140 pc). Figure taken from the TMT Detailed Science Case: 2007.

Planet formation science enabled by the ELTs:

• Direct detection of self-luminous giant planets in nearby star-forming regions (d =125 -150 pc) using extreme Adaptive Optics instruments such as the Planet Formation Instrument (PFI) on the TMT and the Exo-Planet Imaging Camera and Spectrograph (EPICS) on the E-ELT (see Figure 4).

• Probing the dissipation timescale of the bulk of the gas in the planet formation regions of disks by observing tracers such as H2(S2) at 12.4 m and H2 (S1) at 17 m (see Table 2) using high-resolution mid-IR spectrographs such as MIRES on the TMT and METIS on the E-ELT. This is crucial to distinguish between competing planet formation theories (e.g., core accretion and gravitational instability).

• Study the distribution and dynamics of pre-biotic molecules, such as H2O, CH4, HCN, and C2H2 in the planet-forming regions of the disk (0.5 - 20 AU) using high-resolution near and mid-IR spectroscopy. These studies with the ELTs will complement similar chemistry studies of the outer disk that can be performed with ALMA.

References

• Boss, A. 2000, ApJL, 536, 101

• Cieza, L. et al. 2010, ApJ, 712, 925

• Espaillat, C. et al. 2008, ApJL, 682, 125,

• Lissauer J. 1993, ARAA, 31, 129

• Wolf, S. 2008, Ap&SS, 313. 109



Table 2. Limiting distances as a function of total disk mass detectable with TMT-MIRES observations of H2. Disk masses as small as 10-5 Msun

will be detectable up to 450 pc, the distance of the Orion Nebula Cluster. Table taken from the TMT detailed science case: 2007.

WITNESSING PLANET FORMATION WITH ALMA AND THE ELTs

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