C. M. Telesco

The Silicates in the Disk of beta Pictoris

We have obtained intermediate-resolution (R≃50) infrared (2.6-13.5 μm) spectra of the particles in the circumstellar disk of β Pic. The silicate dust feature near 10 μm is broader and contains more structure than interstellar and most circumstellar emission features. The silicate feature in β Pic is remarkably similar to those in comets Halley, Bradfield 1987s, and Levy 1990 XX which have emission features characteristic of crystalline silicates. This result supports the inference based on IRAS results that cometary bodies resupply the grains in the β Pic disk. Detailed models of the dust disk and grains are used to derive plausible disk temperature and density gradients

Infrared mapping of M82 : a starburst in an edge-on barred galaxy

New 1-30-micron images of the starburst galaxy M82 are presented. The appearance of M82 at 2 microns indicates that a bar about 1 kpc long is located at its center. This bar may dramatically influence the gas dynamics and may have caused the formation of the central gas and dust toroid associated with the luminous episode of star formation. The mid-IR maps define the structure of the toroid out to 30 microns and show that the extent of the emitting region is at least 1.3 kpc. The absorption optical depth determined from 1-2 micron colors corresponds to A(v) less than about 8 and is distributed in a ridge spanning the starburst region and closely resembling the distribution of H I optical depth. Finally, the contribution that very small grains may make to the 10-micron emission is considered, and it is concluded, by comparison to Galactic H II regions, that their mid-IR emission is significant.

Detection of silicates in the 51 Ophiuchi system

We have detected a prominent 10 μm silicate emission feature in 51 Oph, a B9.5 Ve star with circumstellar emission. 51 Oph shows the Vega phenomenon, probable disk emission from a main-sequence star. We obtained narrow-band spectrophotometry (Δλ≃1 μm) of the inner 5″ diameter region. The emission feature is more than 25 times stronger than the photospheric flux at 10 μm. We estimate that the temperatures of grains within 180 AU of the star range between 400 and 1000 K. The total flux density at the peak of the emission feature is ∼1.4 times the underlying dust continuum, implying particle sizes smaller than ∼8 μm

The far-infrared structure of the luminous interacting galaxy Arp 299

The strongly interacting galaxies NGC 3690 and IC 694 (or Arp 299) exhibit extensive star formation, a large infrared luminosity, and possible nuclear activity. To determine the origin of the infrared luminosity in this system, high-resolution far-infrared profiles have been obtained in which two distinct sources can clearly be discerned; these observations reveal that 60 percent of the far-infrared luminosity comes from an unresolved source in the center of IC 694, with the remaining 40 percent originating in a compact source in NGC 3690. A new high-resolution 20-micron image of the Arp 299 system is compared with the far-infrared profiles, revealing that NGC 3690 is substantially hotter than IC 694 in the thermal infrared; the observed temperature gradient is opposite to that which is expected if the putative active nucleus in IC 694 generates a significant fraction of the total luminosity. 28 refs.

The stellar velocity dispersion in the nucleus of M82

We observed the 2-0 (C-12)O absorption bandhead from integrated starlight from the nuclear bulge of the starburst galaxy M82. The velocity dispersion of the stellar component in the central arcsecond is significantly larger than that of the ionized gas in the same region, implying that the stars and gas represent different kinematic systems. The M/L(K) in this nuclear region is essentially identical to that in our own nuclear bulge, but is much higher than in the starburst disk of M82 at larger radii. This implies that the near-infrared continuum from the central r is less than 7.5 pc of M82 is dominated by a nuclear bulge population that predates the enveloping starburst. This work is a first step in separating these two distinct populations in the central region of this well-observed starburst galaxy.

Thermal-infrared and visual imaging of comet Giacobini-Zinner

Thermal-infrared and visual images obtained simultaneously of comet Giacobini-Zinner on UT 1985 August 4.4 are presented. The 10.8-micron map, which spans several arc minutes with 8-arcsec resolution, is the first ground-based infrared image of a comet. Considerable spatial structure is evident in the map, including a curved dust tail consisting primarily of dust grains larger than 100 microns. The width of this large-grain tail implies that these grains depart the nucleus with velocities less than 3 m/s. The distribution of grain albedo is determined for the central 1 arcmin. It is speculated that an observed increase in the albedo by a factor of 2 with distance from the nucleus and prolonged radius vector results from the confinement to the orbital plane of large fluffy grains previously considered for GZ by others; the lower albedo of this dust could result from multiple internal scattering increasing the probability of absorption of incident light. It is estimated that dust was departing the nucleus at the rate of about 8 x 10 to the 5th g/s, comparable to that derived for many other comets. 29 references.

High spatial resolution mapping of the Cepheus A region at 20, 50, and 100 microns

The Cepheus A region at 20 microns has been mapped, and a bright, unresolved central source with a peak flux of 41 Jy into a 4.0 arcsec x 4.3 arcsec beam has been detected. One-dimensional slit scans have been obtained in two perpendicular directions across Cepheus A at 50 and 100 microns using a high spatial-resolution sampling technique. The temperature and optical depth profile of the far-IR emission are determined, and a peak temperature of 46 K and peak 100 micron optical depth of 0.4 are derived. Maximum entropy deconvolution of the far-IR scans indicate that there is a bright central core with clumpy extended emission. Within errors the position of the 50/100 micron peak is the same as the 20-micron source; this is interpreted as indicating that the 20 micron radiation represents the short-wavelength emission from the core of the dust cloud seen in the far-IR. It is concluded that an embedded source provides the major source of luminosity in the Cepheus A region. 23 refs.

Infrared imaging of the coma of Comet Wilson

Abstract Thermal infrared images of Comet Wilson were obtained on three consecutive days starting on 1987 March 13 UT. The brightness of the nuclear condensation did not vary appreciably during the observations, in sharp contrast with the behavior observed in Comet Halley. A dynamical analysis of the structure of the dust coma and tail indicates that the particle size distribution in Comet Wilson is clearly different from that observed in Comet Giacobini-Zinner but closer to that in Comet Halley. The dust ejection was nonisotropic with most of the activity concentrated within about a 60°-wide area east of the subsolar point. This result is interpreted as evidence for prograde rotation of the nucleus. We speculate that Wilson, which is a new comet, has not yet developed a dust mantle. The absence of such a mantle would explain the lack of short time scale (Halley-like) variability and the constant position, with respect to the subsolar point, of the area of maximum dust ejection on all 3 days.

Albedo maps of comets P/Halley and P/Giacobini-Zinner

Near-simultaneous infrared and visual images of P/Halley are combined to create a map of the spatial variation of average albedo (which is generally a factor of four larger than the geometric albedo defined by Hanner et al., 1981). The map is compared with a similar map of P/Giacobini-Zinner (P/G-Z) obtained previously. P/G-Z showed a minimum in albedo of 0.07 with an increase by a factor of 2 over an angular distance of 30″. The lowest albedos in P/G-Z were offset from the nucleus in the anti-sunward direction, coincident with a dust tail observed in the infrared. The P/Halley albedos are higher than those found for P/G-Z, ranging from 0.2–0.4, but both comets have relatively low albedo in the anti-sunward direction (along the tail). We attribute the albedo distribution to large, dark, fluffy grains confined to the orbital plane close to the nucleus. The high albedo values in P/Halley may be due to enhanced flux in the visual image because of the comet’s very small phase angle.

Small grains in M82: a dusty halo surrounding the starburst

The well-known starburst in M82 at 10.8, 19.2, and 30 microns has been mapped. Scans along the galactic plane which are derived from these data reveal marked color variations with position. These color variations are correlated with the UV energy density in a manner explicable in terms of a heated mixture of dust grains extending to very small sizes. Many of the small grains may have been destroyed in the most intense regions of the starburst, but they have survived in a halo where the UV energy density is too low to destroy them but still high enough to significantly heat them. Implications of the results for interpreting IR continuum emission from starburst galaxies are considered. 22 refs.