Charles M. Telesco

How Observations of Circumstellar Disk Asymmetries Can Reveal Hidden Planets: Pericenter Glow and Its Application to the HR 4796 Disk

Recent images of the disks of dust around the young stars HR 4796A and Fomalhaut show, in each case, a double-lobed feature that may be asymmetric (one lobe may be brighter than the other). A symmetric double-lobed structure is that expected from a disk of dust with a central hole that is observed nearly edge-on (i.e., close to the plane of the disk). This paper shows how the gravitational influence of a second body in the system with an eccentric orbit would cause a brightness asymmetry in such a disk by imposing a forced eccentricity on the orbits of the constituent dust particles, thus shifting the center of symmetry of the disk away from the star and causing the dust near the forced pericenter of the perturbed disk to glow. Dynamic modeling of the HR 4796 disk shows that its ~5% brightness asymmetry could be the result of a forced eccentricity as small as 0.02 imposed on the disk by either the binary companion HR 4796B or by an unseen planet close to the inner edge of the disk. Since it is likely that a forced eccentricity of 0.01 or higher would be imposed on a disk in a system in which there are planets but no binary companion, the corresponding asymmetry in the disks structure could serve as a sensitive indicator of these planets that might otherwise remain undetected.

A Dust Disk Surrounding the Young A Star HR 4796A

We report the codiscovery of the spatially resolved dust disk of the Vega-like star HR 4796A. Images of the thermal dust emission at λ=18 μm show an elongated structure approximately 200 AU in diameter surrounding the central A0V star. The position angle of the disk (30° ± 10°) is consistent to the position angle of the M companion star (225°), suggesting that the disk-binary system is being seen nearly along its orbital plane. The surface brightness distribution of the disk is consistent with the presence of an inner disk hole of approximately 50 AU radius, as was originally suggested by Jura et al. on the basis of the infrared spectrum. HR 4796 is a unique system among the Vega-like or β Pictoris stars in that the M star companion (a weak-emission T Tauri star) shows that the system is relatively young, ~8 ± 3 Myr. The inner disk hole may provide evidence for coagulation of dust into larger bodies on a timescale similar to that suggested for planet formation in the solar system.

The Infrared Nuclear Emission of Seyfert Galaxies on Parsec Scales: Testing the Clumpy Torus Models

We present subarcsecond resolution mid-infrared (mid-IR) photometry in the wavelength range from 8 to 20 μm of 18 Seyfert galaxies, reporting high spatial resolution nuclear fluxes for the entire sample. We construct spectral energy distributions (SEDs) that the active galactic nucleus (AGN) dominates, relatively uncontaminated by starlight, adding near-IR measurements from the literature at similar angular resolution. We find that the IR SEDs of intermediate-type Seyferts are flatter and present higher 10 to 18 μm ratios than those of Seyfert 2 galaxies. We fit the individual SEDs with clumpy dusty torus models using the in-house-developed BayesClumpy tool. We find that the clumpy models reproduce the high spatial resolution measurements. Regardless of the Seyfert type, even with high spatial resolution data, near- to mid-IR SED fitting poorly constrains the radial extent of the torus. For the Seyfert 2 galaxies, we find that edge-on geometries are more probable than face-on views, with a number of clouds along equatorial rays of N 0 = 5-15. The 10 μm silicate feature is generally modeled in shallow absorption. For the intermediate-type Seyferts, N 0 and the inclination angle of the torus are lower than those of the Seyfert 2 nuclei, with the silicate feature appearing in weak emission or absent. The columns of material responsible for the X-ray absorption are larger than those inferred from the model fits for most of the galaxies, which is consistent with X-ray absorbing gas being located within the dust sublimation radius, whereas the mid-IR flux arises from an area farther from the accretion disk. The fits yield both the bolometric luminosity of the intrinsic AGN and the torus-integrated luminosity, from which we derive the reprocessing efficiency of the torus. In the models, the outer radial extent of the torus scales with the AGN luminosity, and we find the tori to be confined to scales less than 5 pc.

Mid-infrared images of β Pictoris and the possible role of planetesimal collisions in the central disk

When viewed in optical starlight scattered by dust, the nearly edge-on debris disk surrounding the A5V star β Pictoris (distance 19.3 pc; ref. 1) extends farther than 1,450 au from the star. Its large-scale complexity has been well characterized, but the detailed structure of the disks central ∼200-au region has remained elusive. This region is of special interest, because planets may have formed there during the stars 10–20-million-year lifetime, perhaps resulting in both the observed tilt of 4.6 degrees relative to the large-scale main disk and the partial clearing of the innermost dust. A peculiarity of the central disk (also possibly related to the presence of planets) is the asymmetry in the brightness of the ‘wings’, in which the southwestern wing is brighter and more extended at 12 µm than the northeastern wing. Here we present thermal infrared images of the central disk that imply that the brightness asymmetry results from the presence of a bright clump composed of particles that may differ in size from dust elsewhere in the disk. We suggest that this clump results from the collisional grinding of resonantly trapped planetesimals or the cataclysmic break-up of a planetesimal.

A Protocometary Cloud around HR 4796A

We report both ROSAT observations and ground-based 10.8 ?m imaging of the wide binary HR 4796, which consists of a main-sequence A-type star with a large amount of circumstellar dust, HR 4796A, and, at a separation of 77, a pre-main-sequence M-type companion, HR 4796B. From the ROSAT data, we find that the X-ray emission is centered on HR 4796B, with LX/Lbol ? 3 ? 10-4. The 10.8 ?m flux, which arises from HR 4796A, displays an excess over the photospheric emission of 0.08 ? 0.02 Jy, a result consistent with the previous characterization of the emission from the circumstellar dust in the wavelength range 12 ?m ? ? ? 100 ?m as a 110 K blackbody. The Hipparcos data can be used to argue that the three main-sequence A-type stars in the Bright Star Catalogue with LIR/Lbol > 10-3 (HR 4796A, ? Pic, and 49 Cet) all have low luminosities for their colors. We argue that approximately 20% of all A-type stars pass through an early phase where they possess an amount of circumstellar dust comparable to that found around HR 4796A or ? Pic. In order to explain the result that the grain emission can be approximated by a 110 K blackbody, we propose that the circumstellar grains are largely composed of ice particles with a typical radius near 100 ?m, and that they sublimate rapidly when they are closer than ~35 AU to HR 4796A. This swarm of ice particles might be a protocometary cloud.

Investigating Disk Evolution: A High Spatial Resolution Mid-Infrared Survey of T Tauri Stars

We present a high spatial resolution, 10-20 μm survey of 65 T Tauri binary stars in Taurus, Ophiuchus, and Corona Australis using the Keck 10 m telescopes. Designed to probe the inner ~1 AU region of the circumstellar disks around the individual stellar components in these binary systems, this study increases the number of binaries with spatially resolved measurements at 10 μm by a factor of ~5. Combined with resolved near-infrared photometry and spectroscopic accretion diagnostics, we find that ~10% of stars with a mid-infrared excess do not appear to be accreting. In contrast to an actively accreting disk system, these passive disks have significantly lower near-infrared colors that are, in most cases, consistent with photospheric emission, suggesting the presence of an inner disk hole. In addition, there appears to be a spectral type/mass dependence associated with the presence of a passive disk, with all passive disks occurring around M-type stars. The presence of a passive disk does not appear to be related to the fact that these objects are in visual binary systems; the passive disk systems span the entire range of binary separations present in the sample, and a similar fraction of passive disks is observed in a sample of single stars. The possibility that the passive disks are caused by the presence of an as yet undetected companion at a small separation (0.3-3 AU) is possible for any individual system; however, it cannot account for the spectral type dependence of the passive disk sample as a whole. We propose that these passive disks represent a subset of T Tauri stars that are undergoing significant disk evolution. The fraction of observed passive disks and the observed spectral type dependence can both be explained by models of disk evolution that include disk photoevaporation from the central star.

Deep 10 and 18 Micron Imaging of the HR 4796A Circumstellar Disk: Transient Dust Particles and Tentative Evidence for a Brightness Asymmetry

We present new 10.8 and 18.2 km images of HR 4796A, a young A0 V star that was recently dis- covered to have a spectacular, nearly edge-on, circumstellar disk prominent at D20 km (Jayawardhana and coworkers ; Koerner and coworkers). These new images, obtained with OSCIR (the University of Florida Observatory Spectrometer/Camera for the Infrared) at Keck II, show that the disks size at 10 km is comparable to its size at 18 km. Therefore, the 18 kmemitting dust may also emit some, or all, of the 10 km radiation. Using these multiwavelength images, we determine a ii characteristic ˇˇ diameter of 2¨3 km for the mid-infraredemitting dust particles if they are spherical and composed of astronomical silicates. Particles this small are expected to be blown out of the system by radiation pressure in a few hundred years, and therefore these particles are unlikely to be primordial. Rather, as inferred in a com- panion paper (Wyatt and coworkers), they are probably products of collisions that dominate both the creation and the destruction of dust in the HR 4796A disk. Dynamical modeling of the disk, the details of which are presented in the companion paper, indicates that the disk surface density is relatively sharply peaked near 70 AU, which agrees with the mean annular radius deduced by Schneider and coworkers from their NICMOS images. Interior to 70 AU, the model density drops steeply by a factor of 2 between 70 and 60 AU, falling to zero by 45 AU, which corresponds to the edge of the previously discovered central hole ; in the context of the dynamical models, this ii soft ˇˇ edge for the central hole occurs because the dust particle orbits are noncircular. The optical depth of mid-infraredemitting dust in the hole is D3% of the optical depth in the disk, and the hole is therefore relatively very empty. We present evidence (D1.8p signi—cance) for a brightness asymmetry that may result from the presence of the hole and the gravitational perturbation of the disk particle orbits by the low-mass stellar companion or a planet. This ii pericenter glow,ˇˇ which must still be con—rmed, results from a very small (a few AU) shift of the disks center of symmetry relative to the central star HR 7496A ; one side of the inner bound- ary of the annulus is shifted toward HR 4796A, thereby becoming warmer and more infrared-emitting. The possible detection of pericenter glow implies that the detection of even complex dynamical eUects of planets on disks is within reach. Subject headings : circumstellar matterinfrared : starsstars : individual (HR 4796A)

Resolved Mid-Infrared Emission in the Narrow-Line Region of NGC 4151

We present subarcsecond-resolution mid-infrared images of NGC 4151 at 10.8 and 18.2 μm. These images were taken with the University of Florida mid-IR camera/spectrometer OSCIR at the Gemini North 8 m telescope. We resolve emission at both 10.8 and 18.2 μm, extending ~35 across at a P.A. of ~60°. This coincides with the narrow-line region of NGC 4151, as observed in [O III] by the Hubble Space Telescope. The most likely explanation for this extended mid-IR emission is dust in the narrow-line region heated by a central engine. We find no extended emission associated with the proposed torus and place an upper limit on its mid-IR size of 35 pc.

GatirCam: Gemini mid-infrared imager

The University of Florida is developing the mid-IR imager, called GatirCam, to be used primarily, but not solely, at the southern hemisphere Gemini telescope at Cerro Pachon, Chile. Key features of GatirCam are its fully reflective optics, its very high mechanical rigidity, and the fact that the associated electronics are very similar to those is in use successfully on similar instrumentation. Design studies for GatirCam indicate that it will meet or exceed all critical requirements of image quality and performance. A low-resolution spectroscopic mode is also currently under consideration for implementation in GatirCam.

A luminous 3 kiloparsec infrared disk in NGC 1068

A 10 micron map of the Seyfert galaxy NGC 1068 and airborne measurements of its angular extent in the far-infrared are presented. It is shown that the infrared emission originates primarily from two physically distinct regions; approximately half of the total infrared luminosity of 3 x 10 to the 11th solar luminosities is associated with the Seyfert nucleus and half with a 3 kpc (35 arc sec) diameter disk surrounding it. It is argued that the disk component of infrared emission originates from an extended but heavily obscured burst of star formation which resembles those seen in some non-Seyfert galaxies. This high-luminosity disk is distinguished more by its large size than by its high surface brightness. On the basis of current evidence it cannot be concluded that the high disk luminosity in NGC 1068 is causally related to its Seyfert activity.