SUMMARY OF CURRENT AND PREVIOUS RESEARCH


I am deeply interested in understanding the evolution of gas and dust around young stars and hope to further our knowledge of the origin and evolution of Solar Systems. I am currently a member of several Spitzer Space Telescope GTO teams, searching for new, dusty disks around nearby, young stars and determining the spatial distribution and the mineralogy of the constituent grains. 

Spitzer Searches for Dusty Circumstellar Disks:  Observations with the IRAS satellite serendipitously discovered that the main sequence A-type stars β Pictoris, Vega, and ε Eridani possess 60 μm fluxes 10 – 100 times larger than expected from the stellar photosphere alone. The discovery of infrared excess led to speculation that these stars possess circumstellar dust grains. Since the Poytning-Robertson Drag lifetime of the grains is a fraction of the stellar age, the grains must be replenished from a reservoir, such as collisions between parent bodies. The presence of a dusty disk around β Pic was dramatically confirmed when an edge-on disk was imaged in scattered light. While IRAS possessed the sensitivity to search comprehensively the environments around nearby, main sequence A-type stars for dust, it lacked the sensitivity to search comprehensively the environments around solar-type stars for dust.

I am leading several Spitzer GTO searches for circumstellar dust around nearby, young, solar-type main sequence stars using the MIPS photometer at 24 μm and 70 μm: (1) I am leading Mike Jura’s effort to search for warm terrestrial debris around 155 F-, G-, and K- type stars with estimated ages <100 Myr. The majority of objects in this sample (85%) are common proper motion members of nearby OB Associations identified with Hipparcos. (The first results from this survey are described in my research proposal and in Chen et al. 2004) The remaining objects are late-type stars in binary systems where the primary is a young, B-type main sequence star. (2) I am leading Mike Werner’s effort to search for dusty disks around 70 nearby, young stars which possess indicators of youth, such as high lithium abundance and/or strong chromospheric or coronaspheric activity, including AU Mic, whose disk was recently imaged in scattered light, and 7 TW Hydrae Association objects. (3) I am also leading Mike Jura’s effort to study, comprehensively, 157 low-luminosity, main sequence A-type stars, which were not detected by IRAS at wavelengths longer than 12 μm but may still possess infrared excesses. These studies are typically sensitive to the photospheres of all of the objects at 24 μm but are only sensitive to fluxes ~10 – 100 times greater than the photospheres at 70 μm depending on the brightness of the local, interstellar cirrus.

Spitzer Spectroscopic Studies of Dust and Gas: The enormous gain in sensitivity provided by Spitzer has enabled detailed studies of grain composition and grain size in ~10 Myr old transitional disks and debris disks using infrared spectroscopy. Within the past year, the Spitzer Infrared Spectrograph (IRS) Disks team, led by Dan Watson, has obtained infrared spectra of ~300 stars. I am leading the IRS Disks team’s TW Hydrae Association and Debris Disks efforts. We have obtained 5 – 40 μm spectra of ~115 debris disks around main sequence A-type stars, with ages between 100 and 300 Myr, and have not discovered any new crystalline or amorphous silicate features. The dust grains in these systems may be too cold or too large to produce the 10 μm and 20 μm silicate features. The non-detection of silicate features may be the result of selection effects. Our debris disk targets were selected for strong IRAS excesses, which typically peak at 60 μm because few main sequence stars possess strong 12 μm excesses. In a survey of 548 A-K dwarfs, Aumann & Probst (1991) were able to identify IRAS 12 μm excesses only with β Pic and ς Lep. Searches for silicate emission in the TW Hydrae Association has been similarly frustrating. We have obtained 5 – 40 μm spectra of ~15 objects in the 10 Myr old TW Hydrae Association. So far, only TW Hydrae, Hen 3-600, and HD 98800 appear to possess silicate emission features. TW Hydrae and Hen 3-600 had already been studied using ground-based observatories. The first 20 debris disk spectra and the TW Hydrae and Hen 3-600 spectra have been published in the Special Spitzer Issue of the Astrophysical Journal Supplement (Jura et al. 2004, Uchida et al 2004).

Spitzer IRS spectroscopy can also be used to search for molecular and atomic gases in disks. Whether ~10 Myr old circumstellar disks retain the bulk of their natal molecular gas is not known. ISO (R = 2000) observations of the H2 S(0) and S(1) lines toward the β Pic suggest that this system possesses 0.003 Mearth H2 with an excitation temperature, Tgas = 110 K. However, FUSE searches for H2 absorption in the Lyman band toward β Pic, which possesses a nearly edge-on disk, place upper limits on the column density of H2, N(H2) = 1018 cm-2, which is three orders of magnitude lower than is inferred from ISO. I am a member of the Fabulous 4 team that plans to study the famous debris disk systems β Pic, Fomalhaut, Vega, and ε Eridani in all Spitzer imaging and spectroscopic modes. We have obtained Spitzer IRS high-resolution (R ~ 600) 10 – 40 μm spectra of β Pic and searched for emission from H2 S(0) at 28.2 μm and S(1) at 17.0 μm, and [S I] at 25.2 μm and failed to detect any of these species. We rule out the previous H2 S(1) detection by several sigma and estimate that the disk possesses <3*10-7 Mearth H2 from our [S I] upper limits, assuming that the gas is not ionized, has an interstellar gas-phase sulfur abundance, and an excitation temperature, Tgas = 110 K. 

Multi-wavelength Disk Studies: Before joining the Spitzer GTO Teams, I conducted multi-wavelength studies of the circumstellar material around Herbig Ae/Be stars and young, main sequence stars that may be forming planets or may already possess planets. (1) I discovered that the dust around the nearby ~200 Myr debris disk system ς Lep is so close to the star (<6 AU) and that the lifetime of these dust grains is so short compared to the stellar lifetime, that the parent bodies in this system probably lie in a massive asteroid belt (Chen & Jura 2001). (2) I constructed a simple model for the gas around the debris disk system σ Her to show that collisions between parent bodies on unstable orbits around the binary system could liberate the gas and dust observed (Chen & Jura 2003a). (3) I used high-resolution mid-infrared imaging to constrain models for the dust geometry around the Herbig Ae star AB Aur (Chen & Jura 2003b). (4) I measured the gas:dust ratio in the circumstellar environment around the 8 Myr old resolved debris disk system HR 4796A and showed that it possesses too little hydrogen to form the atmosphere of a giant planet (Chen & Kamp 2004).

References
Chen, C. H., & Jura, M. 2001, ApJ, 560, L171
Chen, C. H., & Jura, M. 2003a, ApJ, 582, 443
Chen, C. H., & Jura, M. 2003b, ApJ, 591, 267
Chen, C. H., & Kamp, I. 2004, ApJ, 602, 985

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Last Updated March 31, 2005