Joseph H. Rhee

Characterization of Dusty Debris Disks: the IRAS and Hipparcos Catalogs

Dusty debris disks around main-sequence stars are signposts for the existence of planetesimals and exoplanets. From cross-correlating Hipparcos stars with the IRAS catalogs, we identify 146 stars within 120 pc of Earth that show excess emission at 60 μm. This search took special precautions to avoid false positives. Our sample is reasonably well distributed from late B to early K-type stars, but it contains very few later type stars. Even though IRAS flew more than 20 years ago and many astronomers have cross-correlated its catalogs with stellar catalogs, we were still able to newly identify debris disks at as many as 33 main-sequence stars; of these, 32 are within 100 pc of Earth. The power of an all-sky survey satellite like IRAS is evident when comparing our 33 new debris disks with the total of only 22 dusty debris disk stars first detected with the more sensitive, but pointed, satellite ISO. Our investigation focuses on the mass, dimensions, and evolution of dusty debris disks.

Warm Dust in the Terrestrial Planet Zone of a Sun-like Pleiades Star: Collisions between Planetary Embryos?

Only a few solar-type main-sequence stars are known to be orbited by warm dust particles; the most extreme is the G0 field star BD +20 307 that emits ~4% of its energy at mid-infrared wavelengths. We report the identification of a similarly dusty star HD 23514, an F6-type member of the Pleiades. A strong mid-IR silicate emission feature indicates the presence of small warm dust particles, but with the primary flux density peak at the nonstandard wavelength of ~9 μm. The existence of so much dust within an AU or so of these stars is not easily accounted for given the very brief lifetime in orbit of small particles. The apparent absence of very hot (≳1000 K) dust at both stars suggests the possible presence of a planet closer to the stars than the dust. The observed frequency of the BD +20 307/HD 23514 phenomenon indicates that the mass equivalent of Earths Moon must be converted, via collisions of massive bodies, to tiny dust particles that find their way to the terrestrial planet zone during the first few hundred million years of the life of many (most?) Sun-like stars. Identification of these two dusty systems among youthful nearby solar-type stars suggests that terrestrial planet formation is common.

THE AGE OF THE HD 15407 SYSTEM AND THE EPOCH OF FINAL CATASTROPHIC MASS ACCRETION ONTO TERRESTRIAL PLANETS AROUND SUN-LIKE STARS

From optical spectroscopic measurements we determine that the HD 15407 binary system is ~80?Myr old. The primary star, HD 15407A (spectral type F5 V), exhibits strong mid-infrared excess emission indicative of a recent catastrophic collision between rocky planetary embryos or planets in its inner planetary system. The synthesis of all known stars with large quantities of dust in their terrestrial planet zone indicates that for stars of roughly solar mass this warm dust phenomenon occurs at ages between 30 and 100?Myr. In contrast, for stars of a few solar masses, the dominant era of the final assembling of rocky planets occurs earlier, between 10 and 30?Myr age. The incidence of the warm dust phenomenon, when compared against models for the formation of rocky terrestrial-like bodies, implies that rocky planet formation in the terrestrial planet zone around Sun-like stars is common.

Rapid disappearance of a warm, dusty circumstellar disk

Stars form with gaseous and dusty circumstellar envelopes, which rapidly settle into disks that eventually give rise to planetary systems. Understanding the process by which these disks evolve is paramount in developing an accurate theory of planet formation that can account for the variety of planetary systems discovered so far. The formation of Earth-like planets through collisional accumulation of rocky objects within a disk has mainly been explored in theoretical and computational work in which post-collision ejecta evolution typically is ignored, although recent work has considered the fate of such material. Here we report observations of a young, Sun-like star (TYC 8241 2652 1) where infrared flux from post-collisional ejecta has decreased drastically, by a factor of about 30, over a period of less than two years. The star seems to have gone from hosting substantial quantities of dusty ejecta, in a region analogous to where the rocky planets orbit in the Solar System, to retaining at most a meagre amount of cooler dust. Such a phase of rapid ejecta evolution has not been previously predicted or observed, and no currently available physical model satisfactorily explains the observations.

The Origin of the Silicate Emission Features in the Seyfert 2 Galaxy, NGC 2110

The unified model of active galactic nuclei (AGN) predicts silicate emission features at 10 and 18 microns in type 1 AGN, and such features have now been observed in objects ranging from distant QSOs to nearby LINERs. More surprising, however, is the detection of silicate emission in a few type 2 AGN. By combining Gemini and Spitzer mid-infrared imaging and spectroscopy of NGC 2110, the closest known Seyfert 2 galaxy with silicate emission features, we can constrain the location of the silicate emitting region to within 32 pc of the nucleus. This is the strongest constraint yet on the size of the silicate emitting region in a Seyfert galaxy of any type. While this result is consistent with a narrow line region origin for the emission, comparison with clumpy torus models demonstrates that emission from an edge-on torus can also explain the silicate emission features and 2-20 micron spectral energy distribution of this object. In many of the best-fitting models the torus has only a small number of clouds along the line of sight, and does not extend far above the equatorial plane. Extended silicate-emitting regions may well be present in AGN, but this work establishes that emission from the torus itself is also a viable option for the origin of silicate emission features in active galaxies of both type 1 and type 2.

The first spatially resolved MID-infrared spectra of NGC 1068 obtained at diffraction-limited resolution with the keck i telescope long wavelength spectrometer

We present spatially resolved mid-infrared (mid-IR) spectra of NGC 1068 with a diffraction-limited resolution of 025 using the Long Wavelength Spectrometer (LWS) at the Keck I telescope. The mid-IR image of NGC 1068 is extended along the north-south direction. Previous imaging studies have shown that the extended regions are located inside the ionization cones, indicating that the mid-IR emission arises perhaps from the inner regions of the narrow-line clouds instead of the proposed dusty torus itself. The spatially resolved mid-IR spectra were obtained at two different slit position angles, +80 and -130 across the elongated regions in the mid-IR. From these spectra, we found only weak silicate absorption toward the northern extended regions but strong absorption in the nucleus and the southern extended regions. This is consistent with a model of a slightly inclined cold obscuring torus that covers much of the southern regions but is behind the northern extension. While a detailed analysis of the spectra requires a radiative transfer model, the lack of silicate emission from the northern extended regions prompts us to consider a dual dust population model as one of the possible explanations in which a different dust population exists in the ionization cones compared to that in the dusty torus. Dust inside the ionization cones may lack small silicate grains, giving rise to only a featureless continuum in the northern extended regions, while dust in the dusty torus has plenty of small silicate grains to produce the strong silicate absorption lines toward the nucleus and the southern extended regions.

A SUBSTANTIAL DUST DISK SURROUNDING AN ACTIVELY ACCRETING FIRST-ASCENT GIANT STAR

We report identification of the first unambiguous example of what appears to be a new class of first-ascent giant stars that are actively accreting gas and dust and that are surrounded by substantial dusty disks. These old stars, who are nearing the end of their lives, are experiencing a rebirth into characteristics typically associated with newborn stars. The F2-type first-ascent giant star TYC 4144 329 2 is in a wide separation binary system with an otherwise normal G8 IV star, TYC 4144 329 1. From Keck near-infrared imaging and high-resolution spectroscopy, we are able to determine that these two stars are ~1 Gyr old and reside at a distance of ~550 pc. One possible explanation for the origin of the accreting material is common-envelope interaction with a low-mass stellar or substellar companion. The gaseous and dusty material around TYC 4144 329 2, as it is similar to the primordial disks observed around young classical T Tauri stars, could potentially give rise to a new generation of planets and/or planetesimals.

Gas and Dust Associated with the Strange, Isolated Star BP Piscium

We have carried out a multiwavelength observational campaign demonstrating some of the remarkable properties of the infrared-bright variable star BP Psc. Surrounded by a compact dusty, gaseous disk, this little-studied late G (or early K) type star emits about 75% of its detected energy flux at infrared wavelengths. Evidence for accretion of gas in conjunction with narrow bipolar jets and Herbig-Haro objects is apparently consistent with classification of BP Psc as a pre-main-sequence star, as postulated in most previous studies. If young, then BP Psc would be one of the nearest and oldest known classical T Tauri stars. However, such an evolutionary classification encounters various problems that are absent or much less severe if BP Psc is instead a luminosity class III post-main-sequence star. In this case, it would be the first known example of a first-ascent giant surrounded by a massive molecular disk with accompanying rapid gas accretion and prominent jets and HH objects. In this model, the genesis of the massive dusty gaseous disk could be a consequence of the envelopment of a low-mass companion star. Properties in the disk may be conducive to the current formation of planets, a gigayear or more after the formation of BP Psc itself.

Copious amounts of hot and cold dust orbiting the main sequence A-type stars HD 131488 and HD 121191

We report two new dramatically dusty main sequence stars: HD 131488 (A1 V) and HD 121191 (A8 V). HD 131488 is found to have substantial amounts of dust in its terrestrial planet zone (L {sub IR}/L {sub bol} ≈ 4 × 10{sup –3}), cooler dust farther out in its planetary system, and an unusual mid-infrared spectral feature. HD 121191 shows terrestrial planet zone dust (L {sub IR}/L {sub bol} ≈ 2.3 × 10{sup –3}), hints of cooler dust, and shares the unusual mid-infrared spectral shape identified in HD 131488. These two stars belong to sub-groups of the Scorpius-Centaurus OB association and have ages of ∼10 Myr. HD 131488 and HD 121191 are the dustiest main sequence A-type stars currently known. Early-type stars that host substantial inner planetary system dust are thus far found only within the age range of 5-20 Myr.

Probing the Dust Obscuration in Seyfert Galaxies Using Infrared Spectroscopy

We present near-infrared spectroscopy of four Seyfert 1 galaxies, one Seyfert 1.9 galaxy, and one Seyfert 2 galaxy, obtained using the Gemini infrared camera at the Lick Observatory 3 m Shane telescope. With the unique design of the Gemini camera, full J- and K-band spectra were taken simultaneously through the same slit. This produced a more accurate comparison of hydrogen recombination line fluxes than previous infrared studies. We have also used template galaxies to remove stellar features from the infrared spectra and produced more accurate measurements of the weak Brγ feature. We concentrate on the velocity-resolved ratio of Paβ/Brγ and its implication for dust within the central active galactic nucleus (AGN). For our Seyfert 1s, the line ratios of Paβ/Brγ are not only comparable in both broad- and narrow-line regions but also consistent with case B recombination, confirming that the ratio Paβ/Brγ is less affected by collisional effects than optical lines and is a good indicator of dust extinction up to Av ~ 10 in AGN. Using the same diagnostic the Seyfert 1.9 galaxy, NGC 2992, has an extinction of Av = 4.03 ± 0.20 to the broad-line region while the narrow-line region is unobscured. Assuming that Seyfert 1.9 galaxy has the same broad-line region that Seyfert 1 galaxies have, we conclude that the obscuring dust is located between the broad-line regions and the narrow-line regions, consistent with the AGN unification model. The Seyfert 2 galaxy, NGC 5929, consistent with dust being present on larger scales, shows significant obscuration to the narrow-line region. These data are part of a larger program to measure many spectral properties of Seyfert galaxies.