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1.2 Rocks and ices in the Galaxy- studies of how and when rocks and ices are made that could coalesce to form planets (Becklin, Ghez, Jura, Mclean, Morris, Shuping, Zuckerman)

1.2.1 Grain growth in young stellar systems

Figure 1.2.2. Schematic of a typical protoplanetary disk in Orion. Ultraviolet (UV) radiation from a nearby massive star eats away at the protoplanetary disk surrounding a young star creating a bubble of warm gas. The outer portions of the gas bubble are then heated and removed by energetic UV radiation. Material falling from the disk onto the central protostar fuels twin gas jets. Artwork courtesy of Space Telescope Science Institute.

Most, if not all, young solar-like stars are surrounded by circumstellar disks prior to planet formation. Indeed, it is these disks that provide the basic building blocks for future planetary systems. The ways in which sub-µm size particles of dust in the interstellar medium eventually accumulate into kilometer sized, asteroid-like, planetesimals, which in turn aggregate to form rocky planets, are poorly understood. The process starts in the disks. Understanding the time scales and regions of significant grain growth in disks that surround young stars would enhance our understanding of how rocky planets form, and might be used as a tool for identifying those systems with proclivity for terrestrial planet formation.

Some observational and theoretical research on grain growth from sub-micron specks to millimeter size particles has been carried out (e.g., Pollack et al. 1994). But many more direct measurements of circumstellar disks are needed. Ghez and others are engaged in identifying the earliest stages of planet formation (i.e., dust coagulation) in regions surrounding million year old T Tauri stars. Their search for grain evolution focuses on infrared thermal emissivities and scattering/polarization properties which change as grains grow in size. By comparing infrared images between 1 and 10 µm, obtained with the Keck telescope and Hubble Space Telescope, Ghez has recently demonstrated the existence of grains substantially larger than interstellar in the disk that encircles the T Tauri star GG Tau (McCabe et al. 2003).

During the coming years Ghez and coworkers will build on this early success by performing similar observations at mid-infrared wavelengths of disks surrounding T Tauri stars of various ages. By correlating the extent of grain growth with age, this program of observations should paint a clearer picture of time scales for grain growth around solar-like stars. The program will require observing sessions at the Keck observatory for the next few years.

Ghez's research focuses on the nearest regions of current star formation such as the Taurus dark clouds. While proximity is obviously a virtue when one is investigating planetary system size phenomena, study of more distant youthful star clusters also has advantages. For example, most stars are thought to form in clusters containing many hundreds of stars, including some that are much more massive than our Sun. The closest, well studied, such region is the Orion Nebula Cluster.

Grain growth up to a few µm has been inferred from studies of transmitted light through a circumstellar disk seen in silhouette against the Orion Nebula. But, unlike forming stars in Taurus which are far from any massive, luminous stars, disks in Orion are being evaporated by intense ultraviolet light from four high-mass stars at the cluster's center (Figure 1.2.1). Models indicate that these disks can be evaporated away on timescales of about a million years. The key question is whether or not planets can form before the disks are destroyed by the UV radiation field. Evidence of grain growth in the Orion disks is especially important in addressing this question.

Silhouette and scattered light images reveal the nature of grains in the outer edges of the protostellar disk while thermal radiation from the mid-infrared through radio wavelengths traces grains in the midplane. Morris and Shuping plan to continue their studies of Orion using existing data available from HST archives coupled with new observations of: (1) silhouettes and scattered light in the near- and mid-IR (at Keck Observatory); and (2) thermal emission from the mid-IR through the far-IR, sub-millimeter and radio wavelengths (using Keck, SIRTF, SOFIA, and various radio observatories). The planned observations will require approximately five to 10 nights to complete, and a few years to analyze and publish.

1.2.2 Detecting asteroids and comets in extrasolar systems – precursors to rocky planets elsewhere in the Galaxy

Figure 1.2.2. Image of a dusty debris disk around the bright star Fomalhaut obtained by lead team member Zuckerman and outside collaborators using the SCUBA camera at JCMT, Mauna Kea Observatory . The image, at 0.45 mm wavelength, shows a non-uniform distribution implying the existence of a planet that shepherds the debris.

Asteroids and comets are composed of rocks and volatile ices. Since cosmochemical studies of meteorites demonstrate convincingly that planets of the inner Solar System were made from similar objects, the presence of asteroids and/or comets in other stellar systems may point toward the existence also of rocky planets. Although the asteroidal and cometary building blocks of planets are too small to be directly detected around other stars, their presence can be inferred indirectly, as a program of astronomy-based research that will permit indirect detection of these objects.

In our Solar System, asteroids are eroded by mutual collisions while comets disintegrate by passing near the Sun. Microscopic dust particles from these disrupted parent bodies are subsequently distributed throughout the inner solar system. The zodiacal light is produced by sunlight scattering off these dust particles while absorption and reemission of sunlight by this material generates infrared emission. Dust particles near Earth have a typical lifetime of about 100,000 years before spiraling into the Sun under the operation of the Poynting Robertson effect (photon drag).

In 1983, the IRAS satellite discovered dust orbiting many main sequence stars, including the very bright star Vega. Analogous to our own Solar System, it is thought that this dust results from the disruption of parent bodies (e.g., Zuckerman 2001). Within 2 AU of the Sun, the total mass of dust is about 2x10¹⁷ g (Ney 1982). Around stars like Pictoris (age ~12 Myrs) and Fomalhaut (age ~200 Myr), the total mass of dust may be 10²⁵ g (Zuckerman and Becklin 1993), but as shown by direct imaging in infrared, optical and submillimeter wavelengths and as illustrated in Figure 1.2.2, dust in some systems is detected as far as 100 AU from the central star (Holland et al. 1998, Weinberger et al. 1999, Weinberger et al. 2002). Typically, this corresponds to location in the Kuiper Belt of comets and large icy bodies in our Solar System. At least one star, Lepus, appears to be encircled by an asteroid belt with about 200 times the mass of the asteroids in the Solar System (Chen and Jura 2001).

One of the most significant consequences of the initial IRAS discovery, is the realization that the dusty disks around main sequence stars usually show non-axisymmetric structure (e.g., Zuckerman 2001, Holland et al 2003 and Figure 1.2.2). The submillimeter SCUBA camera at the James Clerk Maxwell Telescope (JCMT) at Mauna Kea Observatory has been the most successful instrument in imaging non-axisymmetric disks around main sequence stars including some of the best known (e.g., Vega and Fomalhaut). The most plausible cause of such structure is the gravitational field of planets of substantial mass with semi-major axes as large as that of Neptune and even larger (e.g., Ozernoy et al 2000). This is the first observational evidence, albeit somewhat indirect, for the existence of planets in such wide orbits. During the coming years, Zuckerman expects to continue his fruitful SCUBA collaboration with astronomers from the United Kingdom.

With a variety of instruments, including the 10 meter Keck telescopes, the HST, SIRTF (launch April 2003) and SOFIA (first light late 2004), Jura, Zuckerman, Becklin, and Hansen introduce further studies of debris dust derived from comets and asteroids around main sequence stars with the specific goal of learning more about the formation and evolution of planets in the context of what we know about our Solar System.

For example, for comparisons with the Solar System, the minimum mass, M_PB, of the parent bodies of debris can be obtained with the expression,

where L_IR is the observed infrared luminosity of the dust, t_age is the age of the star, and c is the speed of light (Chen and Jura 2001). Observation of stars with different ages enables study of parent body mass as a function of time (see Spangler et al. 2001), thus enabling a comparison with what is known about evolutionary time scales in our Solar System as derived from studies of meteorites.

With low resolution spectra obtainable with SIRTF, Jura and colleagues expect to learn about the origin and evolution of the particles that comprise the debris. Grains spiraling inwards under the action of the Poynting Robertson effect produce a spectrum which varies as n^-1 independent of the grain size (Jura et al. 1998). Comparison of the data with simple models based on this result will enable Jura et al. to infer where the particles are formed (perhaps in the equivalent of our Kuiper Belt) and whether they spiral all the way into the star or are stopped - as might occur if there is accretion onto a Jovian-mass planet.

New observations made by the UCLA lead team relating to the mass and dynamics of dust vs. central star age will be compared with models for early Solar System evolution by consultations between Jura and colleagues and lead-team cosmochemists McKeegan, Wasson, and Young. The result will be a synthesis of what is known about our Solar System formation in the context of the formation of rocky materials around other stars.

Comparisons between the evolution of our Solar System and that of debris disks around other stars can be taken further. For example, we have the capacity to search for the equivalent around other stars of the era of the Late Heavy Bombardment inferred to have occurred within the Solar System within the first ~800 million years. The development of life in the Solar System is thought to have been delayed by this bombardment. Are such events common in planetary systems? Elsewhere, are they comparable in magnitude to the event(s) recorded in the inner Solar System?

IRAS could only begin to address such questions because its sensitivity limited meaningful observations mostly to stars with twice or more the mass of the Sun. Soon SIRTF will enable astronomers, including Jura and Zuckerman, to investigate stars with masses comparable to that of the Sun. Also, in the Solar System, comets and asteroids produce about 10⁶ g s^-1 of dust which then spirals into the Sun under the operation of the Poynting Robertson effect (Ney1982). SIRTF will be sufficiently sensitive that Jura and coworkers will be able to search for similar dust-production rates around nearby, low mass, main sequence stars of various ages. Do such stars experience asteriodal grinding and comet disruption at the same rate as in the Solar System? Through a new program of SIRTF observations of debris disks, the lead-team members will determine whether there are similarities between our Solar System and other regions where tell-tale signs (albeit indirect) of rock and ice formation are present.

1.2.3 Searches for signs of life’s essential chemical constituents surrounding young stars

The importance for astrobiology of debris disks surrounding other stars goes beyond garnering indirect evidence for planet-forming processes; the nature of disk material can be constrained from its spectroscopic features. Are there signatures of organics that potentially could be precursors to life? Major organic spectral features are seen in comets in the infrared at 3.3 to 3.4 µm and 5.5 to 8 µm. The former can be studied from the ground with, for example, the Keck telescope. The latter feature must be studied from space (with SIRTF) or from the stratosphere (with SOFIA).

Because of his position as Chief Scientist and Director Designate for SOFIA, lead team member Becklin will concentrate his future efforts on SOFIA. He is currently working with the NASA Ames Astrobiology lab group to assure that the correct filters and spectrometers become available soon after initiation of SOFIA flights.

In addition, UCLA is building a camera and spectrometer for the 1 to 5 µm region. This camera, called FLITECAM (lead team member McLean is the FLITECAM PI), is missing GRISM (GRISM stands for "grating prism") spectrometers in the critical region from 3.0 to 5.0 µm. With these GRISMs we will be able to investigate organic spectral features around 3.3 µm with greater sensitivity than from ground-based telescopes. In addition we will be able to observe the primary carbon containing molecules CO and CO₂, which together with H₂O, are essential to the chemistry that could lead to life.

Becklin receommends the purchase the appropriate GRISMs that will enable him to utilize SOFIA for observations relevant to astrobiology. McLean has obtained price quotes from three US vendors for the two necessary GRISMs in the 3 to 5 µm range; typical costs are included in the budget.

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