WITNESSING PLANET FORMATION WITH ALMA AND THE ELTs

1
WITNESSING PLANET FORMATION WITH ALMA AND THE
ELTs
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.
Lucas Cieza, IfA/U. of Hawaii
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.
TMT
E-ELT
GMT
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.
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.

• 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.

• 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.

• 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, ApSS, 313. 109