COS SCIENCE GOALS

Spectroscopy lies at the heart of astrophysical inference. Our understanding of the origin and evolution of the cosmos critically depends on our ability to make quantitative measurements of physical parameters such as the total mass, distribution, motions, temperatures, and composition of matter in the Universe. Detailed information on all of these properties can be gleaned from high-quality spectroscopic data. For distant objects, some of these properties (e.g., motions and composition) can only be measured through spectroscopy. Ultraviolet spectroscopy, in particular, provides some of the most fundamental diagnostic data necessary for discerning the physical characteristics of planets, stars, galaxies, and interstellar and intergalactic matter. The UV offers access to Lyman $\alpha$ at low to moderate redshift, high ionization stages of key diagnostic elements, and unique abundance information for both atomic and molecular species that cannot be obtained at other wavelengths. Such data are essential to draw a complete picture of the Universe.

COS will build on the scientific legacies of Copernicus, IUE, GHRS, FOS, STIS, FUSE, and, in the future, GALEX, giving HST the greatest possible grasp of faint UV targets, a capability perhaps not available from future space-based observatories for decades. COS will complement and extend the suite of HST instruments, ensuring that Hubble maintains a powerful UV spectroscopic capability from 2003 until the end of its mission.

The science goals of the COS GTO team address problems of fundamental importance in astrophysics and cosmology which require the moderate resolution and high throughput of COS, and four unique capabilities of HST: access to ultraviolet wavelengths, large collecting area, precise pointing stability, and excellent image quality. Our study is organized into three broad categories, united by the theme of cosmic origins: (1) the origin of large-scale structure and the intergalactic medium (IGM); (2) the formation, evolution, and ages of galaxies; and (3) the origins of stellar and planetary systems.

  

Figure 4: Number distributions of AGN observed by IUE brighter than a given flux limit. Top curve shows all IUE observations of AGN; lower curves show various redshift intervals. Using the medium-dispersion channels G130M, G160M, G185M, G225M, and G285M, COS can observe over ten times more targets with moderate spectral resolution than STIS. From http://casa.colorado.edu/~spenton/IUEAGN/FUSE.html.

Models for the formation of large-scale structure and the reionization of the IGM will be constrained by observing distant quasars to measure the He II Gunn-Peterson effect, the structure of the Lyman $\alpha$ forest, and the D/H ratio in primordial clouds. COS will be capable of obtaining moderate-resolution UV spectra of hundreds more quasars and AGN than existing UV instruments (Fig. 4). The COS database of absorption-line systems will have high enough spectral resolution and adequate S/N to determine accurate column densities, abundances, and kinematics of intergalactic matter at epochs when the first galaxies were formed and the first heavy elements were synthesized.

We will use COS to determine abundances and kinematics of hot gas in galaxy halos, the impact of violent starbursts and supernovae on interstellar and intergalactic environments, and the ages of globular clusters. The numerous quasar sight-lines accessible to COS will intersect hot galaxy halos over a large redshift range. COS spectra will constrain galaxy evolution models by mapping the production of metal-enriched gas through time. The large redshift coverage and high sensitivity provide access to numerous diagnostic features, such as C IV $\lambda\lambda$1550, O VI $\lambda\lambda1035$, and He II $\lambda$304 in a variety of high-redshift environments. COS will also observe nearby starbursting systems over a range of metallicity. These spectra will be used to model the chemical enrichment of the interstellar medium, and as templates for deriving the properties of high-z galaxies. COS UV spectra of horizontal-branch stars in globular clusters (Fig. 5) may allow significant refinement of globular cluster age estimates, which can be used to reconcile the ages of the oldest stars in galaxies with the age of the Universe derived from recent measurements of the Hubble constant and closure parameter.

  

Figure 5: Far-UV image of NGC 6752 ($\lambda _c$=1620 Å) obtained by the Ultraviolet Imaging Telescope (UIT). The 20-arcmin field of view shown encompasses nearly the entire cluster. Although NGC 6752 contains more than 10⁵ stars, nearly all of these stars are too cool to emit any UV light. The 330 stars seen in this UIT image are all HB stars, except for the over-exposed bright star 4' southwest of the cluster center, which is a foreground star. COS will provide follow-up spectroscopy of targets identified by UV imaging telescopes. From http://fondue.gsfc.nasa.gov/UIT/Astro2/.

The origins of stellar and planetary systems will be investigated by studying the physical processes and chemical abundances in the cold ISM. For the first time in the UV, COS will observe sight-lines toward hot, embedded stars that will probe dense, molecular regions where the star formation process begins (Fig. 6). Resolution R = 20,000 UV spectroscopy with the COS high-dispersion modes will provide detailed information on gas-phase atomic and molecular abundances in the regime where the gas has become largely molecular and grain mantles begin to grow by direct deposition from the gas phase. These processes signify the earliest phases of collapse and accretion of stellar and planetary systems. Low-dispersion spectroscopy can be used to probe the UV extinction properties of the dust at high A_V. We will also be able to search for structure in the UV extinction curve with unprecedented sensitivity, enabling us to determine whether the optical diffuse bands extend into the UV, and allowing us to search for specific features predicted to be formed by interstellar PAH molecules.

  

Figure 6: COS extends UV extinction studies by several magnitudes. Dashed lines represent limiting fluxes for t_exp = 20,000 sec and S/N = 10 per resolution element for the indicated spectroscopic modes. The points denote low-resolution flux measurements of stars, and the symbol types indicate their association with different extinction curves.

The strengths of COS for faint object UV spectroscopy of point sources are by their nature more applicable to cosmology and certain Galactic observations than planetary science. However, the high sensitivity of COS does open up new spectroscopic investigations by HST within the Solar System. COS data will provide clues to the conditions and composition of the outer solar nebula. The high sensitivity of COS will allow an order of magnitude more background stars to be observed in stellar occultation studies of planetary and cometary atmospheres. COS will break new ground with direct moderate-resolution UV observations of Pluto and Triton that will be used to detect fluorescence emission from volatile gases as these bodies both undergo rare seasonal changes during the first decade of the next century.

Our scientific program attacks fundamental topics in astronomy and astrophysics. However, our GTO program represents just a small fraction of the total observing time with COS that is available to the world-wide astronomical community. COS is an extremely potent instrument for a broad range of science problems. Most of the science to be pursued with COS will actually be accomplished by the Guest Observer (GO) community, tackling a very wide range of projects that address many astrophysical problems. Such problems may include: AGN monitoring campaigns; UV upturn in elliptical galaxies; UV monitoring of distant supernovae; monitoring of SN1987A as it impacts its circumstellar material; stellar winds and UV properties of massive stars in the LMC and SMC; cataclysmic variables and other high-energy systems with accretion disks; line emission from heated plasma in accretion columns in young stellar objects; UV continuum measurements to determine SEDs and bolometric corrections in YSOs with accretion disks; chromospheric emission from cool stars and the evolution of magnetospheric activity in young stars; planetary aurorae; emission characteristics of cometary comae; studies of diffuse objects where the goal is to reach the deepest possible UV flux levels, for example, detecting high-ionization (e.g., C IV) emission from the Milky Way halo, the outer blast wave of the Crab nebula, or shock waves in supernova remnants and Herbig-Haro jets.

Our investigation requires observations of very faint targets, taking full advantage of HST capabilities (large aperture, UV coatings, excellent pointing, and image quality). COS is optimized to observe faint UV sources (Fig. 7) with spectral resolution high enough to determine the physical conditions in a broad range of astrophysical environments. Its design meets programmatic requirements for reliability and redundancy, and its simplicity and efficient operation ensure a high science return. With these capabilities, we anticipate a high degree of interest in using COS throughout the world-wide astronomical community.

  

Figure 7: Summary of predicted exposure times to achieve S/N=10 per resolution element as a function of flux for the R $\geq$ 20,000 resolution COS G130M FUV grating, assuming the 2.5'' Primary Science Aperture is used. Example targets are noted along the horizontal axis above their corresponding UV fluxes. Exposure time estimates through the Bright Object Aperture are presented for bright fluxes at the right region of the plot.