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Next Generation Space Telescope

Ad-Hoc Science Working Group Design Reference Mission Proposal

Explorations in Astrobiology: Evolution of Organic Matter from the ISM to Planetary Systems
Program contacts: Scienti c category: Instruments: Days of observation: Tom Greene, Lou Allamandola, Scott Sandford OTHER: Astrobiology MIR SPEC, OTHER: CORONAGRAPH FOR DISKS 72

Abstract
We propose to observe the mid-IR spectroscopic signatures of biologically important organic molecules in molecular clouds, protostellar envelopes, planet forming systems, and planetary debris disks. In addition to yielding a more complete census of organic materials in the ISM, these observations will provide insightinto how these molecules are physically and chemically processed into the chemical precursors of life in the circumstellar environments and planetary systems around young stars. Understanding the nature, sources, and evolution of this matter in solar systems is a ma jor ob jective of the NASA Astrobiology program. NGST is uniquely suited to this observational task. It is the only planned observatory sensitive enough to sample multiple lines of sight in nearly any galactic molecular cloud or resolve the spatial distribution of pre-biotic organic matter in protostellar envelopes and planetary disks.


ASWG DRM Proposal Explorations in Astrobiology: Evolution of Organic Matter from the ISM to Planetary Systems

Observing Summary:
Target 100 DARK CLOUD LOS ABSORPTIONS 50 PROTOSTELLAR ENVELOPES 25TTAURI DISKS 25 PIC DISKS

RA Dec mAB Con guration mode Days gal plane gal plane NAB 15 , 20 MIR SPEC R3000 12 gal plane gal plane NAB 15 , 20 MIR SPEC R3000 gal plane gal plane NAB 15 gal plane gal plane NAB 15 MIR SPEC R3000 MIR SPEC R1000 Grand total days 12 6 42 72

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ASWG DRM Proposal Explorations in Astrobiology: Evolution of Organic Matter from the ISM to Planetary Systems

Scienti c Ob jectives
The biogenic elements needed for the formation of life, including C, N, and O, are created via nucleosynthesis in stars. Within the circumstellar shells of carbon-rich giants, some of these elements combine with each other and H to form simple molecules. Once incorporated in dense clouds and ultimately into circumstellar disks of young stellar planetary systems, these molecules are processed into more complex ones, including some of pre-biotic interest. This processing is caused byambient UV radiation in the cloud and also radiation from the young central stars of pre-planetary systems. This hypothesis is supported by IR spectroscopic evidence for energetic processing as well as the detection of a wide variety of complex organic molecules in dense clouds and in comets. Thus it is likely that complex organic molecules were delivered to the young Earth and maywell be common in extra-solar planetary systems. This picture for the origin and evolution of the chemical precursors to life has emerged and come into focus over the past several decades. Combined laboratory and observational astronomical studies have recently shown that: i the ma jor components of ices in dense molecular clouds are usually H2O, CH3OH, CO, CO2, and H2 e.g., Allamandola et al. 1988; Tielens et al. 1991; Sandford et al. 1993; d'Hendecourt et al. 1999; ii Polycyclic Aromatic Hydrocarbons PAHs are the largest source of accessible carbon in the Universe Allamandola, Hudgins, & Sandford 1999 and upon modi cation in the ISM are biochemically active Bernstein et al. 1999; Dwek et al. 1997; iii the organic molecules frozen on dust grains can be changed into new compounds by protostellar radiation elds Bernstein et al. 1995, 1999; and iv a large variety of organic molecular ices have been detected in recent comets and on the surfaces of the outer planets and on Kuiper belt ob jects Brown et al.1999; Crovosier et al. 1999 . Although this is a very intriguing scenario for the chemical evolution of biological precursors, there is a large dose of speculation involved. This is because tremendous gaps exist in understanding how prebiotically important molecules evolve from the quiescent ISM to protostellar envelopes to planetary disks to planetary systems. For example, NH3 ammonia and H2CO formaldehyde are very interesting prebiotic molecules since they can be chemically combined to produce amino acids. Both NH3 and H2CO are known to be abundantin the interstellar gas, and there is evidence for both frozen in ices Schutte et al. 1995, Lacy et al. 1998. Ices on grains in the circumstellar disks around young stars are the most likely place for NH3 and H2CO to combine to form amino acids, especially since there is evidence for gas-phase abundance enhancements attributed to grain mantle vaporization in protostellar environments. However, we cannot predict the amount of amino acids or understand the conditions that produce them until we carefully measure the abundance of frozen NH3 and H2CO along the same lines-of-sight in molecular clouds. Conducting a basic inventory of prebiotic and organic molecular ices in quiescent clouds is one part of the work to be done in this area. To truly understand the evolution of interstellar carbon into solar system ices, we must also measure the abundances of organic molecular species as a function of location in quiescent dark clouds, protostellar envelopes Figure 1, pre-planetary disks, and debris disks. These are the most important ob jects for studying the evolution of biogenic 3


ASWG DRM Proposal Explorations in Astrobiology: Evolution of Organic Matter from the ISM to Planetary Systems

Figure 1: ISO SWS absorption spectrum of the massive protostar W33A from Gibb et al. 1999. A rich variety of organic ices are seen in absorption in its protostellar envelope. This ob ject has a luminosity L ' 3 104 L , and its ux is at least 4 orders of magnitude greater than that of the background stars, low-mass protostars, and circumstellar disks which are the targets of this proposed study. NGST will also be able to spatially resolve the envelopes and disks of nearby protostars.

molecules during the formation of solar systems like ours. When combined with laboratory data and knowledge of local radiation environments, these measurements will reveal the detailed physical and chemical processes by which organic compounds are formed and modi ed in these systems. Recent laboratory studies as cited above have shown that biologically importantchemical evolution of organic compounds is likely to occur on dust grains around young stars. However, to date we have very little observational astronomical data which show which of the possible reactions actually do occur. We must acquire such data to determine what organic compounds mayhave been present on the young Earth, which ones were likely delivered by comets, and where life is most likely to form in extra-solar planetary systems. These are ma jor goals of the new NASA Astrobiology initiative, and the required astronomical observations are a ma jor ob jective on the NASA Astrobiology Roadmap.

References

Allamandola, L. J., Sandford, S. A., & Valero, G. J. 1988, Icarus, 76, 225 Allamandola, L. J., Hudgins, D., & Sandford 1999, ApJ 511, L115 4


ASWG DRM Proposal Explorations in Astrobiology: Evolution of Organic Matter from the ISM to Planetary Systems Bernstein, M. P., Sandford, S. A., Allamandola, L. J., Chang, S. & Scharberg, M. A. 1995, ApJ, 454, 327 Brown, R. H., Cruikshank, D. P., Pendleton, Y. , & Veeder, G. J. 1999, ApJL, in press Bernstein, M. P., et al. 1999, Science 283, 1135 Crovosier, J., Brooke, T. Y., Hanner, M.S., Keller, H.U., Lamy, P.L., Altieri, B., BockleeMorvan, D., Jorda, L., Leech, K., & Lellouch, E., 1996, A&A, 315, L385 d'Hendecourt, L.B., Jourdain de Muizon, M., Dartois, E., Demyk, K., Ehrenfreund, P., & Heras, A., 1999, ISO Observations of Protostars: The Relevance of Laboratory Ice Simulations", in . . . Dwek E., et al. 1997, ApJ 475, 565 Gibb, E., Whittet, D., et al. 1999, in preparation Lacy, J. H., Fara ji, H., Sandford, S. A., & Allamandola, L. J. 1998, ApJL, 501, L105 Sandford, S. A., Allamandola, L. J., & Geballe, T. R. 1993, Science, 262, 400 Schutte, W.A., Gerakines, P.A., Geballe, T.R., van Dishoek, E.F., & Greenberg, J.M., 1995, A&A 309, 633 Tielens, A. G. G. M., Tokunaga, A. T., Geballe, T. R., & Baas, F. 1991, ApJ, 381, 181

NGST Uniqueness Relationship to Other Facilities
NGST is uniquely suited to make these observations. The spectral signatures of organic molecules and amino acids are primarily in the 3 20 mwavelength region, with the 5 8 m range being particularly important for identi cations. Most of this region is obscured from ground-based observation by atmospheric H2O, CO2, and other terrestrial organic and inorganic molecules. Therefore observation from airborne or spaceborne observatories is required. ISO was only sensitive enough to observe the most luminous protostars in the galaxy Figure 1 and did not measure absorptions in the spectra of low-mass protostars, stars seen through quiescent dark clouds, or circumstellar disks ob jects which are most important for studying the chemical evolution of biogenic molecules on earth-like planets. Furthermore, even the best signal-to-noise ratios S N 100 currently obtained by ISO in the important5 8 m region preclude detection of 1 features, the level at which some very important absorptions are expected. SOFIA will not have the sensitivity or spatial resolution required for these observations. The high temperature of the SOFIA telescope limits its sensitivity to the point where it is not capable of observing a single known line-of-sight absorption through a quiescent dark cloud, and it will be capable of measuring organic ice absorption spectra for only the brightest protostars F 1 Jy. The spatial resolution of SOFIA will also be approximately 1005, : about the size of protostellar disks and inner protostellar envelopes in nearby dark clouds, so it will be incapable of studying the evolution of material as a function of position in these systems. Thus, while SOFIA will make important inroads in this area, it will be incapable of achieving the goals of surveying the entire evolution of organic material as it is processed from quiescent dark clouds through protostars through planetary systems. 5


ASWG DRM Proposal Explorations in Astrobiology: Evolution of Organic Matter from the ISM to Planetary Systems A large-aperture, cold space observatory is necessary to make these required observations. NGST is the only planned US or international mission which will be sensitive enough to probe the organic absorption spectra of galactic molecular clouds along many lines of sight and to detect weak organic ice absorptions in the debris disks around young stars. The di raction-limit of NGST about 0002 in the mid-IR will provide spatial resolution of : approximately 10 AU in nearby debris disk systems e.g. Pic and TW Hya stars and approximately 30 AU in the envelopes of nearby protostars. This sensitivity and resolution are required for meeting the goals of this survey so that key astrobiological questions are probed and answered.

Observing Strategy

We wish to conduct a systematic spectroscopic study of organic molecular ices in quiescent dark clouds, protostellar envelopes, pre-planetary disks, and post-planetary disks. R=3000 observations over 5 12 m are required to identify molecular species and to determine in what matrices they are embedded. High sensitivity and dynamic range are required to detect weak absorptions against both extinguished background stars and weak disks with bright protostars in or near the slit. A multi-order, echelle-type long-slit 5 , 1000 spectrograph would be ideal for these moderate-resolution, wide-bandwidth, spatially-resolved observations. We shall inventory the molecular ices characteristic of quiescent regions of approximately 10 dark clouds within 1 kpc by observing weak 1 molecular absorptions seen against background stars over approximately 10 lines of sight within each cloud. A reasonably bright background star would be NAB = 15 mag through a dark cloud, and its 1 organic ice absorption should be measured at S N=100 for adequate compound identi cations. This is equivalent to S N = 104 on the continuum, requiring an exposure time of 104 s at R=3000. Therefore a total of 12 days observing time is required to complete this phase of the pro ject. The next observational phase requires spatially-resolved spectra of Class 0 and I protostars. These observations shall show the di erences in compounds present in the dense, radiation-rich protostellar regions versus those in the quiescent portions of dense clouds and will characterize the photo-chemical evolution of organic molecular species in protostellar envelopes. We expect to see the onset of such processing at distances of approximately 500 AU 5" in nearby regions from the protostars, where we expect to encounter continuum ux surface densities of approximately NAB = 15 mag per spatial resolution element, also leading to exposure times of 104 s per ob ject. An inventory of ices in 5 bright protostars in 10 nearby clouds would also require 12 days of observation if 2 slit orientations 10" long slit were observed per ob ject. The nal set of observations will study the types and distribution of organic compounds in the pre-planetary T Tauri and post-planetary debris disks around young stars. These environments are most interesting and relevant for the formation of biologically important molecules, so we seek to study 25 ob jects of eachtype. We estimate that the T Tauri disks will also have an average ux NAB = 15 mag per spatial resolution element at a distance 6


ASWG DRM Proposal Explorations in Astrobiology: Evolution of Organic Matter from the ISM to Planetary Systems of 100 AU from their central stars. Observing 25 of these ob jects shall require 3 6 days, depending on whether disk orientations are known and whether random slit orientations are possible. Observations of debris disks will be most di cult due to their unknown and very low surface densities, but we estimate that the required sensitivity will be a factor of 5 lower than for T Tauri disks. Therefore 1.7 days exposure time will be required to observe each ob ject at a reduced resolution of R=1000 adequate for identi cations, requiring a total of 42 days for this sample. Minimum Spatial Resolution: 190 mas at 6 m Minimum Spectral Resolution: 1000 at 6 m Minimum obs time with same orientation: 1days

Special Requirements

Precursor Supporting Observations
Ground-based =1 , 4m and SOFIA =2 , 25m spectra will greatly leverage the scienti c value of this program by extending the sample to brighter ob jects. TPF will also provide important complementary information on the disks of extra-solar systems; it will provide much higher spatial resolution in disks at the expense of spectral resolution. All of these facilities will provide a very powerful arsenal, but NGST is clearly the key observatory for conducting this uni ed scienti c program.

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