There is an intimate connection between the early solar nebula and the interstellar medium from which it formed. Thus, the study of current star-forming environments can tell us much about how we came to be. The combination of rapidly improving observational tools and increasingly sophisticated theory has provided, for the first time, a broad outline of the physical processes associated with the assembly of Sun-like stars and their attendant planetary systems. The figure below presents an overview of the scenario developed for the formation of a single, isolated low mass star from the collapse of a molecular cloud core. Despite these impressive gains, many crucial details are poorly understood. In addition, the scope of the problem of stellar and planetary formation has broadened drastically with the discovery of massive extra-solar planets (for up to the date information on this exciting research, go to the UC Berkeley/Carnegie or the Meudon exoplanet web sites) Furthermore, stars are by their nature gregarious. Whether they are part of a multiple system or members of an association, the majority of stars are born in environments that are considerably more complex than that outlined below. The characterization of star-forming regions therefore presents considerable challenges both observationally and theoretically.
A schematic view of the birth of low-mass stars (constructed by Mark McCaughrean at the Astrophysikalisches Institut Potsdam, similar figures have been compiled by M.R. Hogerheijde): (Top left) Dark cloud cores, roughly 1 pc in size, gradually contract until magnetic support is overcome and inside-out collapse begins at t=0. (Top right) For ~10,000-100,000 yr a phase of both high accretion and supersonic outflow occurs in deeply embedded protostars. (Bottom left) Gradual clearing by the outflow leaving only the young T Tauri star and a residual protoplanetary accretion disk, that, on time scales of 1-10 million years, leads to the formation of a mature planetary system (bottom right).
To this table chemistry does not arrive empty-handed. Indeed, observations of molecules play a pivotal role in understanding the physical and chemical evolution of star-forming molecular cloud cores and primitive solar systems. This occurs because the tremendous range in physical conditions and size scales that are present in young stellar objects (YSOs), where densities from 104 to >1013 molecules/cm3 and temperatures of 10 - 10,000 K exist over distances from a few stellar radii to many thousands of astronomical units (AU), is perhaps best probed by molecular spectroscopy. Over one hundred interstellar molecules are now known, and they also provide direct access to the velocity fields present in cloud cores with hundreds to thousands of magnitudes of extinction, while their abundances provide constraints on the internal and external radiation fields. A significant fraction of the molecules is condensed in icy mantles on dust grains, which contain important information on the temperature and irradiation history of the region. Finally, since chemistry controls critical physical parameters in star formation such as the fractional ionization and cooling of the gas, a detailed understanding of the chemical composition of the gas and dust surrounding young stars is important and interesting in its own right. For those interested in learning more, the Astrochemistry web site is a great place to start.
We are carrying out an observational study of the star and planet formation process using Caltech's extraordinary suite of telescopes, including the Owens Valley Radio Observatory (OVRO) Millimeter Array, the Caltech Submillimeter Observatory (CSO), and the Keck telescopes:
Specifically, we are using a variety of molecular line tracers and dust continuum emission to characterize the physical and chemical processes at work in objects ranging from deeply embedded protostars such as those in the NGC 1333 molecular cloud (IRAS 2, IRAS 4, etc.) to several million year old T Tauri (LkCa 15, GM Aur) and Herbig Ae stars (MWC 480, HD163296). The ties between these objects and the formation of planetary systems are tested by examining primitive solar system bodies such as Kuiper Belt Objects and, as they appear, comets (c.f. our studies of comets OVRO Hale-Bopp and C/1999 S4 LINEAR).
Our most recent work has centered on a characterization of the chemistry within the pre-planetary accretion disks around 1-5 Myr old T Tauri stars, most of which possess remnant accretion disks with masses of ~0.001-0.1 M(sun) and sizes of ~100 AU, comparable to those inferred for the primitive solar nebula. Such circumstellar disks can serve as models for the understanding of our own pre-solar nebula. The assessment of the chemical composition at each radius of the disk would provide valuable information (i.e. density, thermal history and composition) about the initial conditions in the planet-forming zones of the solar nebula and help determine the origin and evolution of primitive bodies such as comets and Kuiper Belt Objects. In addition, molecular emission studies of accretion disks can facillitate the quantification of gas-to-dust ratios and the timescales over which they are dissipated, disk properties of great importance for the process of planet formation.
Previous single dish work has detected emission from species such as HCN, CN, HNC, CS, ... and revealed that both ion-molecule chemistry and photon-dominated chemistry must contribute to the observed abundances, since ratios such as CN/HCN are too high to be accounted for by quiescent chemical models alone. A fuller understanding of the details of disk chemistry obviously requires interferometric images of several species in each important chemical family (C-, N-, O-, and S-bearing) with the kind of sensitivity, dynamic range, and high spatial resolution (<1-2´´) that has only recently become possible, and that were begun with the OVRO Millimeter Array over the last two years. Results from our study of the T Tauri star LkCa 15, summarized in the figures below, have revealed interesting morphological differences between different chemical families. CN and HCN, for example, showed maximum emission for both species at distances of a few hundred AU from the central star. This offset demands that the HCN and CN be formed in situ, and the location of the maxima may be related to the sublimation and UV processing of CO trapped in icy dust grain mantles (i.e. the CO "frost line"). The release of CO, but not water, drives a carbon-rich gas phase and grain mantle chemistry in certain disk models.
The observed chemical differentiation in LkCa 15 (and the Herbig Ae star HD 163296) opens the door to the detailed characterization of the physical and chemical processing in protoplanetary accretion disks. In order to reach quantitative conclusions, however, it is necessary to observe various species at higher resolution and to interpret them with detailed radiative transfer models. Both our observing programs and theoretical work are carried out in collaboration with Prof. Ewine van Dishoeck and her Molecular Astrophysics Group at the Sterrewacht Leiden. Since the observed structure is similar to the beam size, the true underlying morphology may be smaller still. Is the observed offset along the disk major axis consistent with a ring-like geometry? To help address this questions, detailed 2D radiative transfer calculations, outlined below, are underway. With the presently available sensitivity, the calculations do indeed reproduce the OVRO images; the emission along the disk minor axis is simply too weak to be detectable at present.
Observations with improved sensitivity and image quality are therefore critical, and the merging of the OVRO and BIMA millimeter arrays into a single instrument, the Combined Array for Research in Millimeter-wave Astronony, or CARMA, will be an important step towards this goal. The combined array will be situated at a new higher altitude site in the Inyo mountains of California near the present OVRO array. The site, Cedar Flat, and CARMA D-array layout are shown below. With much improved atmospheric transmission and 105 baselines, CARMA will possess nearly an order of magnitude more sensitivity and vastly enhanced imaging performance compared to the existing arrays. Groundbreaking for CARMA occurred on March 27th, 2004, and construction on the new site is underway.
Near the end of this decade, the Atacama Large Millimeter Array (or ALMA) will come into operation. This collaborative effort between the United States, Europe and Japan aims to construct a 64 element array of 12 meter telescopes at the extraordinarily dry Chajnantor plateau in northern Chile:
With the dramatic increase in collecting area and improved atmospheric conditions, ALMA will be able to image the dust and line emission from pre-planetary accretion disk in extraordinary detail. The simulation below, courtesy of Lee Mundy at the University of Maryland, illustrates well the expected capabilities of ALMA. The disk has a central hole 3 AU in radius, a roughly 2 AU wide cleared ring centered at 7 AU with soft edge, a protoplanet overdensity at 9 AU with a Gaussian FWHM of 2 AU and a factor of 9 overdensity, a protoplanet overdensity at 22 AU with a Gaussian FMWH of 1.5 AU and a factor of 6 overdensity, and a protoplanet overdensity at 37 AU with a Gaussian FWHM of 3 AU and a factor of 4 overdensity. Each of the protoplanets also has an underdensity of 10 percent in a ring at the same orbital radius. To test the image fidelity, the word ALMA written in the lower part of the disk with an overdensity of a factor of 2 on the underlying disk The leters are 12 pixels wide and 100 pixels high - that is roughly 4 AU by 35 AU. The word ALMA is also written in the upper part of the disk with an underdensity of 20 percent. The model images are at right, while the simulations from a single ALMA transit with perfect phase calibration but the estimated thermal noise level lie at left. The protoplanets are clearly visible, and chemical gradients such as those discussed above should be extremely well resolved!
Selected Recent Publications "Chemical Evolution of Star-Forming Regions" Ewine F. van Dishoeck & Geoffrey A. Blake 1998, Annual Reviews of Astron. Astrophys. 36, 317-68. (Link to PDF File, 851 kB)
"Sublimation from Icy Jets as a Probe of the Interstellar Volatile Content of Comets'' Geoffrey A. Blake, Chunhua Qi, Michiel R. Hogerheijde, Mark A. Gurwell, & Duane O. Muhleman 1999, Nature 398, 213. (Link to PDF File, 480 kB)
"ISO-SWS Detection of H2 Pure Rotational Lines from the GG Tau Binary System" W.F. Thi, Ewine F. van Dishoeck, Geoffrey A. Blake, G.J. van Zadelhoff, & Michiel R. Hogerheijde 1999, Ap.J.(Letters) 521, L63.
"Chemical Evolution of Protostellar Matter" William D. Langer, Ewine F. van Dishoeck, Edward A. Bergin, Alexander G.G.M. Tielens, Thangasamy Velusamy, & Douglas B. Whittet 2000, Protostars & Planets IV, V. Mannings, A.P. Boss, S.S. Russell, eds., pp. 29-58.
"Substantial Reservoirs of Molecular Gas in the Debris Disks around Young Stars" Wing-Fai Thi, Geoffrey A. Blake, Ewine F. van Dishoeck, Gerd-Jan van Zadelhoff, J. Horn, E.E. Becklin, V. Mannings, A.I. Sargent, M.E. van den Ancker, & A. Natta 2001, Nature 409, 60. (Go to Caltech press release, with reprints.)
"Submillimeter Lines from the Circumstellar Disks around Pre-Main Sequence Stars" Gerd-Jan van Zadelhoff, Ewine F. van Dishoeck, Wing-Fai Thi, & Geoffrey A. Blake 2001, Astron. Ap. 377, 566.
"High Resolution 4.7 µm Keck/NIRSPEC Spectra of Protostars. I: Ices and Infalling Gas in the Disk of L1489 IRS" A.C.A. Boogert, M.R. Hogerheijdge, & Geoffrey A. Blake 2002, Ap. J. 568, 761.
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