Chapter 2

Chapter 2, The Science Case

Section 2.4: Planet Formation Environments

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2.4.1 SCIENCE OVERVIEW

The study of planet formation environments is a central part of our search for an understanding of the origin of the Earth and solar system. Indeed, our motivation to study planet formation environments is all the more intense today given the discovery of planets outside the solar system. One of the most intriguing results of searches for extra-solar planets is the discovery that the planet formation process gives rise to considerable diversity. Precision radial velocity studies have uncovered the unexpected existence of Jupiter-mass planets that span a much larger range in mass (0.2 - 17 MJ), orbital radii (0.04 AU < a < 4AU), and eccentricity (0 < e < 0.93) than is covered by the planets in our solar system (e.g., exoplanets.org; see Figure 1). The unexpected diversity in the properties of extra-solar planets challenges traditional theories of planet formation and highlights familiar questions: What are the protoplanetary disk conditions that lead to the formation of planetary systems? How common is the formation of solar systems like our own?

Figure 1  The unexpected diversity of extra-solar planetary systems, as illustrated by the range in orbital radii for a subset of the known extra-solar planetary systems. (Figure courtesy of Geoff Marcy.)

Traditionally, theories of giant planet formation have focused on explaining the growth of planets under the protoplanetary disk conditions expected at 5 AU, the current orbital radius of Jupiter. However, the large spread in the observed orbital radii of extra-solar planets may be an indication that giant planets actually form over a range of disk radii and physical conditions. Indeed, it is probably inaccurate to assume that planets formed where they are now observed to be because dynamical effects, such as orbital migration due to tidal interactions between the disk and protoplanet and dynamical scattering between planets following disk dissipation, are likely to significantly alter planetary systems. Dynamical studies (e.g., Lin & Papaloizou; Lin & Ida)1,2 indicate that these processes can alter the masses (via mergers), orbital radii, and eccentricities of planets, and reduce the number of surviving planets (via mergers and ejection). In the light of these recent developments, planet formation now appears to be a much more complex process than was originally envisioned. It may well be that the observed diversity of planetary systems is a manifestation of the multitude of physical processes at work, and the interactions between these processes, in the formation of planetary systems.

The likely complexity of the planet formation process emphasizes the need for direct observational study of young disk systems (less than or approximately 1 Myr) in order to develop a reliable theory of planet formation. The needed measurements are measurements of the environmental (e.g., density, temperature, column density) conditions under which planets form, observational constraints on the efficiency of a variety physical processes (e.g., grain growth, orbital migration), and a census of young (1-10 Myr) planetary systems. We would ideally hope to measure masses, orbital radii, and eccentricities of planetary companions for comparison with the properties of older (several Gyr old) systems in order to begin to chart out the evolution of planetary systems.

Indirect Detection of Forming Protoplanets: For the latter set of measurements, it may be difficult to detect young planets directly during the epoch of their formation (i.e., when a substantial circumstellar disk is still present). For example, the much larger emitting area of the disk compared to the planet may make it difficult to detect the planet in the glare of the disk. Thus, we may have to rely on a more indirect, dynamical signature of the presence of planetary companions. One such signature is dynamical impact of the forming planet on its parent disk. Dynamical theory predicts that as a giant planet forms, tidal interactions between the planet and disk clear a region in the disk (a "gap") within which the planet orbits (see Figure 2).1,3 Because the width of the gap depends on planetary mass, measurements of the location and width of gaps in protoplanetary disks can ultimately provide us with a method of inferring both the masses and orbital radii of planets at their epoch of formation.

Figure 2  As shown in this simulation (Geoff Bryden, personal communication), young planets are expected to carve out low column density 'gaps' in their parent disks. Observations of gaps may provide an indirect means of detecting young planets and inferring their formation masses and orbital radii.

We might hope to detect such gaps either through high angular resolution imaging or by using spectroscopy. Large protoplanets may produce gaps large enough to be detected in images of disks seen in scattered light (at 1-2.5m) from the central star (Section 2.5). Much smaller gaps produced by lower mass protoplanets can, in principle, be detected over a range of radii using spectroscopic techniques described in the next paragraph. With a spectroscopic approach, it is also possible to search for planets forming at smaller orbital radii than are accessible with direct imaging techniques. One would ideally hope to combine high-resolution imaging and spectroscopy in order to obtain a more complete picture of the morphology and dynamics of planet formation environments. If we wish to carry out spectroscopy of in situ emission from the disk with a 30-m GSMT, this capability is probably most applicable to the 10m region, i.e., orbital radii > 10 AU at a distance of 140 pc (Section 2.5).

High-Resolution Infrared Spectroscopy: Observations in the thermal infrared (4-30m) are ideal for both the spectroscopic detection of disk gaps and the study of planet formation environments at radial distances < 5 AU, because the Planck function for disk material at these radii peaks in the mid-infrared. At the warm temperatures (100-2000 K) and high densities of disks at < 5 AU, molecules are expected to be abundant in the gas phase, and sufficiently excited to produce a rich ro-vibrational and rotational spectrum. A high-resolution spectroscopic approach offers several advantages, including the ability to resolve individual lines, which improves the detectability of weak features, and to provide the kinematic information by which the emitting region can be located in the disk. From the measurement of multiple resolved line profiles, physical properties such as temperatures, densities, and column densities can be determined as a function of disk radius.

Abundant molecules such as CO, H2O, and H2 have transitions in the mid-infrared, and can be used to trace the structure and dynamics of disks. Mid-infrared transitions of rarer molecules (e.g., hydrocarbons and nitrogen-bearing molecules) can be used to probe the chemistry of disks and thereby constrain the history of chemical processing in disks. Because measurements of disk chemistry provide an observational context in which to interpret cometary abundances, which carry the fossil record of the conditions in the solar nebula at its formation, the comparison of disk and cometary abundances may provide important clues to the origin of our solar system.

Figure 3  Infrared spectral line diagnostics of disks (e.g., Najita et al.)(ref 4).  These diagnostics, the CO overtone (Delta v = 2, 2.3um), CO fundamental (v = 1-0, 4.6um), and the ro-vibrational water lines (K-band) probe the structure of disks at r less than or approximately 1 AU, i.e., the terrestrial planet region of the solar system. Longer wavelength diagnostics are expected to probe disk radii r > 1 AU.

An Opportunity for the GSMT: Despite the tremendous potential of thermal infrared spectroscopy to probe planet formation environments, this spectral region has remained largely unexplored due to the severe sensitivity limitations imposed by large thermal backgrounds and strong telluric absorption. Because JWST is not expected to have high spectral resolution capability in the mid-infrared, the GSMT, with its high sensitivity, has the opportunity to make the first detailed studies of the dynamics, chemistry, and physical structure of planet formation environments within 5 AU.

Figure 4  CO fundamental (v = 1-0) emission from the young spectroscopic binary DQ Tau observed at R = 20,000. (ref 5) The line widths and relative line strengths locate the emission in the region of the gap cleared by the binary. The GSMT will give us the sensitivity to extend these studies to probe gaps created by lower, planetary mass companions.

Work to date on high-resolution IR spectroscopy of disks has focused on developing the specific gas phase diagnostics with which to probe disks over this range of radii. Thus far it has been shown that the CO overtone (2.3m), CO fundamental (4.6m), and H2O (K-band) ro-vibrational lines can be used to probe the kinematics and physical structure of disks at radii less than or approximately 1 AU (see Figure 3),4 i.e., in what is today the terrestrial planet region of the solar system. By extending these studies to the mid-infrared (e.g., H2 and H2O lines) with the GSMT, it should be possible to extend these studies to the larger distances traditionally considered in planet formation theories (less than or approximately 5 AU).

Of particular relevance are the results for CO fundamental line observations of pre-main sequence (PMS) spectroscopic binaries where attempts are being made to study the dynamical impact of higher mass (stellar) companions on disks. Spectroscopy with CSHELL/IRTF at R = 20,000 demonstrates that the line profiles and relative line strengths of the CO fundamental lines are consistent with emission from residual gas in the gap created by the binary (see Figure 4).5 Higher signal-to-noise line profiles, now becoming available with high-resolution spectrographs such as NIRSPEC/Keck and Phoenix/Gemini, will enable more definite conclusions about the gas kinematics in these systems and in systems with lower, potentially planetary mass companions.

Jupiter-mass companions in particular will produce much smaller gaps, ~ 0.3 AU wide at an orbital distance of 1 AU (e.g., Takeuchi et al.).3 The resulting spectral energy distribution (SED) will be indistinguishable from that of a disk without a gap. Even with the high photometric accuracy and continuous wavelength coverage of SIRTF (Space Infrared Telescope Facility) or JWST (James Webb Space Telescope) observations of disks in the mid-infrared, such small gaps will be impossible to detect given the ambiguities involved in interpreting SEDs. Our simulations indicate that 8-10-m telescopes potentially have the sensitivity to spectroscopically detect gaps created by Jupiter-mass companions in the nearest star forming regions. Thus, this approach can be tested out using existing facilities. The role of the GSMT will be to extend these results to a larger, statistically significant (and necessarily more distant) sample of young planet forming systems in order to measure the demographics of forming planetary systems.

An Example GSMT Program: A Census of Young Planets: The requirement that we probe the same range of orbital radii (r < 5 AU), sampled by precision radial velocity searches for planetary companions, sets the required velocity resolution. Because projected disk rotational velocities at 1 AU around approximately solar-mass young stars are ~ 10 km s-1, we require a spectral resolution R approximately 100,000. In order to address the evolution of planetary systems, we need to study a large sample of protoplanets whose demographics can be compared against those of known planets around main sequence stars. If the incidence of protoplanets at orbital radii < 5 AU is the same as the detection rate of extra-solar planets from precision radial velocity searches (~ 10%??), we need to survey, for example, ~1000 planet forming systems (e.g., T Tauri stars) in order to obtain a sample of ~ 100 protoplanets; That is, we need to observe to the distance of Orion (480 pc; see Figure 5), where the number of candidate objects with M (5) < 10 is sufficient to enable this survey. Sampling to this distance also provides access to a variety of star forming environments, from loose associations to dense clusters like the Trapezium.

Figure 5  Simulated molecular line emission (R = 100,000) from a disk gap created by the formation of a 1 M-sub-J planet at an orbital radius of 1 AU from a T Tauri star at the distance of Orion, viewed at an inclination of 30 degrees. The gap is filled with residual gas with a surface density of less than or approximately 0.1 g cm^-2 and an excitation temperature of ~ 350 K. At this column density, the infrared dust continuum should be optically thin, given the likely agglomeration of grains into larger bodies. Because the projected emitting area of the gap is small, the lines will be weak compared to the T Tauri continuum, but detectable at high signal-to-noise with the GSMT.

Mid-infrared H2O Lines: The mid-infrared rotational lines of H2O are expected to be a good probe of gas at radii greater than or equal to 1 AU due to their smaller critical densities and lower energy levels as compared to the CO fundamental lines. H2O is expected to be abundant in protoplanetary disks at temperatures above the ice condensation temperature (~ 150K), and has numerous mid-infrared transitions which can be used to probe disk physical conditions. As with the CO fundamental lines, telluric absorption is a significant concern. Previous work on the CO fundamental lines shows that this challenge can be overcome by using high spectral resolution to resolve the telluric line, and by observing targets at appropriate times of the year when their radial velocities shift the emission from the source out of the telluric absorption line.

Figure 6  Pure rotational H2 emission from the young binary GG Tau, as observed by ISO.(ref 6) GSMT observations of the 17 um line can be used to map the rate of decay in the disk gas mass as a function of age in order to constrain the timescale for giant planet formation.

Rotational H2 Lines: Wetherill has suggested that the development of life on the Earth may be a consequence of the existence of Jupiter, because Jupiter probably cleared the inner solar system of planetesimals that would otherwise have impacted the Earth at a damagingly high rate. Thus, understanding the formation of giant planets is critical to understanding the origin of the Earth and our solar system. One of the important measurements in this regard is measuring the gas dissipation timescale of disks in order to constrain the giant planet formation timescale and, by extension, theories of giant planet formation. Molecular hydrogen observation of disks (e.g., the 17 m S(1) line) is one of the best diagnostics for this purpose. Not only it is the dominant mass constituent of disks, but depletion onto grains is expected to be insignificant, and the rotational transitions remain optically thin over large column densities. The pure rotational lines have been detected by ISO in young (T Tauri) stars (e.g., GG Tau-Thi et al.; see Figure 6)7, as well as older transitional objects (e.g., beta Pic; Thi et al.).6 These observations appear to indicate that the gaseous components of disks required for the formation of giant planets can survive for much longer (~ 20 Myr compared to less than or approximately 1 Myr) than had been indicated indirectly on the basis of infrared excesses and millimeter wavelength CO measurements. SIRTF will extend these studies by measuring spectrally unresolved H2 emission line strengths for a larger number of systems.

What will remain unclear even after the SIRTF measurements are made is where in the disk the gas resides, and whether it resides in the region in which planets are believed to form (~ 5 AU) or at much larger distances (> 10 AU). Thus, the role of the GSMT would be to measure the orbital radii from which the H2 emission originates using high-resolution spectroscopy. The use of high-resolution spectroscopy will also allow the detection of much weaker line emission and the measurement of lower gas masses at least as low as 1MJ.

2.4.2 SENSITIVITY ESTIMATES

The relative gains in sensitivity for high-resolution thermal infrared spectroscopy of a 30-m GSMT over 8-10m ground-based telescopes is shown in Figure 7 (Carr, personal communication). For completeness, the GSMT performance is also compared with that of a 6.5-m JWST, were JWST to be equipped with a similar high-resolution spectrograph, although at present no such capability is planned. The comparison assumes diffraction-limited images, with the slit width matched to the image width at each wavelength, and a resolution of R = 100,000 (2-pixel). The detector performance is assumed to be comparable to that required by JWST (i.e., MIR: 10 e- readnoise, 1 e-/s dark; NIR: 4 e- readnoise, 0.02 e-/s dark). The throughput is assumed to be 0.35 (NIR) and 0.20 (MIR) with a slit throughput of 0.75. The telescope temperature is assumed to be 273 K (ground-based) and 60 K (JWST), with 3% emissivity in all cases. For the GSMT, the additional case of 10% emissivity is also considered.

Figure 7  Estimated sensitivity for high-resolution thermal IR spectroscopy of a 30-m GSMT compared with a 6.5-m NGST and the 8-m Gemini telescope. The comparison is made in terms of the limiting flux at which s/n = 100 (per pixel) is reached in 1 hr at R = 100,000. The two GSMT curves correspond to emissivities of 3% and 10%.

FacilityEmissivityTaurusOrion20,000s
8-m Gemini 3%450 s 12 hr ---
6.5m JWST 3%310 s 3800 s 1.0 kpc
30-m GSMT 10%18 s 700 s 1.0 kpc
30-m GSMT 3%15 s 360 s 1.5 kpc
Table 3 Limiting Time or Distance

These sensitivities can be used to estimate our ability to study protoplanet formation in different star forming regions. For example, in order to study protoplanet formation using CO fundamental emission, the integration times shown in Table 1 are required. The physical situation is assumed to be that modeled in Figure 5, but placed at the distance of the Taurus (140 pc) and Orion (480 pc) molecular clouds. The last column of the table shows the limiting distance to which T Tauri stars can be observed in 20,000s of integration. The T Tauri continuum is assumed to be 0.45Jy at the distance of Taurus, and in all cases s/n = 300 on the continuum is assumed to be the requirement to detect and measure the line profile. This comparison shows that the GSMT can effectively survey star forming regions out to > 1 kpc, whereas a ground-based 8-m telescope is restricted to star forming regions < 300 pc. Given the much larger number and variety of star forming regions at 1 kpc distance compared to < 300 pc, only the GSMT will be able to carry out a statistically significant census of young protoplanets using the techniques described in this section.

2.4.3 INSTRUMENTATION REQUIRED

The key to enabling these critical science programs are higher resolution mid-IR spectrographs fed by an adaptive secondary capable of delivering high Strehl images in the mid-IR (see Section 4.7).

2.4.4 REFERENCES

  1. Lin, D. & Papaloizou, J. "On the tidal interaction between protostellar disks and companions", in Protostars and Planets III, ed. Levy & Lunine, 749 (1993)

  2. Lin, D. & Ida, S. "On the Origin of Massive Eccentric Planets". ApJ 477, 781 (1997)

  3. Takeuchi, T., Miyama, S.M., & Lin, D.N.C. "Gap Formation in Protoplanetary Disks". ApJ 460, 832 (1996)

  4. Najita, J. R.; Edwards, S.; Basri, G.; Carr, J. "Spectroscopy of Inner Protoplanetary Disks and the Star-Disk Interface" in Protostars and Planets IV, ed. V. Mannings, A.P. Boss, S.S. Russell, 457 (2000)

  5. Carr, J. S.; Mathieu, R. D.; Najita, J. R. "Evidence for Residual Material in Accretion Disk Gaps: CO Fundamental Emission from the T Tauri Spectroscopic Binary DQ Tauri". ApJ 551, 454 (2001)

  6. Thi, W.F., Blake, G.A., van Dishoeck, E.F., van Zadelhoff, G.J., Horn, J.M.M., Becklin, E.E., Mannings, V., Sargent, A.I., van den Ancker, M.E., & Natta, A. "Substantial reservoirs of molecular hydrogen in the debris disks around young stars". Nature, 409, 60 (2001)

  7. Thi, W.F., van Dishoeck, E.F., Blake, G.A., van Zadelhoff, G.J., Hogerheijde, M.R. "Detection of H2 Pure Rotational Line Emission from the GG Tauri Binary System". ApJ 521, L63 (1999)


November 2002