A. I. Sargent

Resolved Inner Disks around Herbig Ae/Be Stars

We have observed 14 Herbig Ae/Be (HAEBE) sources with the long-baseline near-IR Palomar Testbed Interferometer. All except two sources are resolved at 2.2 μm, with angular sizes generally 5 mas. We determine the size scales and orientations of the 2.2 μm emission using various models: uniform disks, Gaussians, uniform rings, flat accretion disks with inner holes, and flared disks with puffed-up inner rims. Although it is difficult to distinguish different radial distributions, we are able to place firm constraints on the inclinations of most sources; seven objects display significantly inclined morphologies. The inner disk inclinations derived from our near-IR data are generally compatible with the outer disk geometries inferred from millimeter interferometric observations, implying that HAEBE disks are not significantly warped. Using the derived inner disk sizes and inclinations, we compute the spectral energy distributions (SEDs) for two simple physical disk models and compare these with observed SEDs compiled from the literature and new near-IR photometry. While geometrically flat accretion disk models are consistent with the data for the earliest spectral types in our sample (MWC 297, V1685 Cyg, and MWC 1080), the later type sources are explained better through models incorporating puffed-up inner disk walls. The different inner disk geometries may indicate different accretion mechanisms for early- and late-type HAEBE stars.

DUSTY DEBRIS AROUND SOLAR-TYPE STARS: TEMPORAL DISK EVOLUTION

Using ISO-ISOPHOT, we carried out a survey of almost 150 stars to search for evidence of emission from dust orbiting young main-sequence stars, both in clusters and isolated systems. Over half of the detections are new examples of dusty stellar systems and demonstrate that such dust can be detected around numerous stars older than a few times 106 yr. Fluxes at 60 and either 90 or 100 μm for the new excess sources together with improved fluxes for a number of IRAS-identified sources are presented. Analysis of the excess luminosity relative to the stellar photosphere shows a systematic decline of this excess with stellar age consistent with a power-law index of -2.

Dust and gas in the core of Arp 220 (IC 4553)

New higher sensitivity and higher angular resolution data are reported for the CO emission and adjacent 2.7 mm continuum in the ultraluminous IR galaxy Arp 220 are reported. Two emission components are identified, an extended component containing one-third of the flux and a compact, nuclear component containing two-thirds of the flux. The extended component has a size of 2.5 x 6.5 kpc; the major axis ia approximately aligned with the optical dust lane. The nuclear source has a radius of 315 pc. Velocity gradients with the same sense of rotation are seen in both components. The mass of gas in the central concentration is estimated at 25 billion solar masses. A correlation is found between the total luminosity-to-gas mass ratio and the central gas surface density. A central dust-embedded active nucleus is the favored mechanism for luminosity generation in Arp 220. 30 refs.

Observations of T Tauri disks at sub-AU radii: Implications for magnetospheric accretion and planet formation

We determine inner disk sizes and temperatures for four solar-type (1-2 M?) classical T Tauri stars, AS 207A, V2508 Oph, AS 205A, and PX Vul, using 2.2 ?m observations from the Keck Interferometer. Nearly contemporaneous near-IR adaptive optics imaging photometry, optical photometry, and high-dispersion optical spectroscopy are used to distinguish contributions from the inner disks and central stars in the interferometric observations. In addition, the spectroscopic and photometric data provide estimates of stellar properties, mass accretion rates, and disk corotation radii. We model our interferometric and photometric data in the context of geometrically flat accretion disk models with inner holes, and flared disks with puffed-up inner walls. Models incorporating puffed-up inner disk walls generally provide better fits to the data, similar to previous results for higher mass Herbig Ae stars. Our measured inner disk sizes are larger than disk truncation radii predicted by magnetospheric accretion models, with larger discrepancies for sources with higher mass accretion rates. We suggest that our measured sizes correspond to dust sublimation radii, and that optically thin gaseous material may extend farther inward to the magnetospheric truncation radii. Finally, our inner disk measurements constrain the location of terrestrial planet formation as well as potential mechanisms for halting giant planet migration.

Near-Infrared Interferometric Measurements of Herbig Ae/Be Stars

We have observed the Herbig Ae/Be sources AB Aur, VV Ser, V1685 Cyg (BD +40°4124), AS 442, and MWC 1080 with the Palomar Testbed Interferometer, obtaining the longest baseline near-IR interferometric observations of this class of objects. All of the sources are resolved at 2.2 μm with angular size scales generally 5 mas, consistent with the only previous near-IR interferometric measurements of Herbig Ae/Be stars, by Millan-Gabet and collaborators. We determine the angular size scales and orientations predicted by uniform-disk, Gaussian, ring, and accretion disk models. Although it is difficult to distinguish different radial distributions, we are able to place firm constraints on the inclinations of these models, and our measurements are the first that show evidence for significantly inclined morphologies. In addition, the derived angular sizes for the early-type Herbig Be stars in our sample, V1685 Cyg and MWC 1080, agree reasonably well with those predicted by the face-on accretion disk models used by Hillenbrand and collaborators to explain observed spectral energy distributions. In contrast, our data for the later-type sources AB Aur, VV Ser, and AS 442 are somewhat inconsistent with these models and may be explained better through the puffed-up inner disk models of Dullemond and collaborators.

The Spitzer c2d Survey of Large, Nearby, Interstellar Clouds. VII. Ophiuchus Observed with MIPS

We present maps of 14.4 deg^2 of the Ophiuchus dark clouds observed by the Spitzer Space Telescope Multiband Imaging Photometer for Spitzer (MIPS). These high-quality maps depict both numerous point sources and extended dust emission within the star-forming and non–star-forming portions of these clouds. Using PSF-fitting photometry, we detect 5779 sources at 24 μm and 81 sources at 70 μm at the 10 σ level of significance. Three hundred twenty-three candidate young stellar objects (YSOs) were identified according to their positions on the MIPS/2MASS K versus color-magnitude diagrams, as compared to 24 μm detections in the SWIRE extragalactic survey. We find that more than half of the YSO candidates, and almost all those with protostellar Class I spectral energy distributions, are confined to the known cluster and aggregates.

The Spitzer c2d Survey of Large, Nearby, Interstellar Clouds. VI. Perseus Observed with MIPS

We present observations of 10.6 deg^2 of the Perseus molecular cloud at 24, 70, and 160 μm with Spitzer MIPS. The images show prominent, complex extended emission dominated by illuminating B stars on the east side of the cloud and by cold filaments of 160 μm emission on the west side. Of 3950 point sources identified at 24 μm, 1141 have 2MASS counterparts. A quarter of these populate regions of the K_s versus K_s - [24] diagram that are distinct from stellar photospheres and background galaxies and thus are likely to be cloud members with infrared excess. Nearly half (46%) of these 24 μm excess sources are distributed outside the IC 348 and NGC 1333 clusters. A significant number of IRAS PSC objects are not recovered by Spitzer MIPS, most often because the IRAS objects were confused by bright nebulosity. The intercluster region contains several tightly clumped (r ~ 0.1 pc) young stellar aggregates whose members exhibit a wide variety of infrared SEDs characteristic of different circumstellar environments. This could be explained by a significant age spread among the aggregate members, or if the members formed at the same time, a remarkably rapid circumstellar evolution would be required to account for the association of Class I and Class III sources at ages ≲1 Myr. We highlight important results for the HH 211 flow, where the bow shocks are detected at both 24 and 70 μm, and for the debris disk candidate BD +31 643, where the MIPS data show the linear nebulosity to be an unrelated interstellar feature. Our data, mosaics, and catalogs are available at the Spitzer Science Archive for use by interested members of the community.

Substantial reservoirs of molecular hydrogen in the debris disks around young stars

Circumstellar accretion disks transfer matter from molecular clouds to young stars and to the sites of planet formation. The disks observed around pre-main-sequence stars have properties consistent with those expected for the pre-solar nebula from which our own Solar System formed 4.5 Gyr ago. But the ‘debris’ disks that encircle more than 15% of nearby main-sequence stars appear to have very small amounts of gas, based on observations of the tracer molecule carbon monoxide: these observations have yielded gas/dust ratios much less than 0.1, whereas the interstellar value is about 100 (ref. 9). Here we report observations of the lowest rotational transitions of molecular hydrogen (H2) that reveal large quantities of gas in the debris disks around the stars β Pictoris, 49 Ceti and HD135344. The gas masses calculated from the data are several hundreds to a thousand times greater than those estimated from the CO observations, and yield gas/dust ratios of the same order as the interstellar value.

Outflows and Luminous YSOs: A New Perspective on the G192.16 Massive Bipolar Outflow

We have obtained new observations of the massive molecular outflow in G192.16-3.82 in CO(J = 1-0) line and 3 mm continuum using the Owens Valley Radio Observatory millimeter-wave array. We have also imaged the outflow in the near-infrared J, H, and K bands and 2.12 μm H2 at Lowell Observatory and in the Ks band at the Apache Point Observatory. A large-field Palomar image shows that the outflow structure may extend more than 4 pc from the young stellar object (YSO), making it one of the largest known Galactic outflows. There is approximately 80 M☉ of molecular material in the high-velocity flow, and the mass flow rate is ~5 × 10-4 M☉ yr-1. The flow appears to be driven by an early-B star that is surrounded by approximately 17 M☉ of material. A biconical infrared reflection nebula is centered near the millimeter continuum peak, and shock-excited H2 emission is detected at the brightest peak in the K-band reflection nebula. H2 emission is also detected just beyond the highest velocity gas in the blueshifted CO outflow. The slope of the mass spectrum is approximately -2 at velocities below 15 km s-1 and decreases to as low as -8 at higher velocities. This is significantly steeper than in low-mass outflows, suggesting that the powering mechanism is less efficient at accelerating material in the flow. Alternatively, the outflow may have built up a substantial reservoir of low-velocity gas that steepens the mass spectrum. The observation of a wide outflow opening angle (~60°) and limb-brightened shell surrounding the high-velocity gas, together with shock-excited H2 emission and large bow shocks that extend up to 4 pc from the YSO, are consistent with the presence of both a poorly collimated disk-wind and a jet.

The Spitzer c2d Survey of Large, Nearby, Interstellar Clouds. 6. Perseus Observed with MIPS

We present observations of 10.6 deg^2 of the Perseus molecular cloud at 24, 70, and 160 μm with Spitzer MIPS. The images show prominent, complex extended emission dominated by illuminating B stars on the east side of the cloud and by cold filaments of 160 μm emission on the west side. Of 3950 point sources identified at 24 μm, 1141 have 2MASS counterparts. A quarter of these populate regions of the K_s versus K_s - [24] diagram that are distinct from stellar photospheres and background galaxies and thus are likely to be cloud members with infrared excess. Nearly half (46%) of these 24 μm excess sources are distributed outside the IC 348 and NGC 1333 clusters. A significant number of IRAS PSC objects are not recovered by Spitzer MIPS, most often because the IRAS objects were confused by bright nebulosity. The intercluster region contains several tightly clumped (r ~ 0.1 pc) young stellar aggregates whose members exhibit a wide variety of infrared SEDs characteristic of different circumstellar environments. This could be explained by a significant age spread among the aggregate members, or if the members formed at the same time, a remarkably rapid circumstellar evolution would be required to account for the association of Class I and Class III sources at ages ≲1 Myr. We highlight important results for the HH 211 flow, where the bow shocks are detected at both 24 and 70 μm, and for the debris disk candidate BD +31 643, where the MIPS data show the linear nebulosity to be an unrelated interstellar feature. Our data, mosaics, and catalogs are available at the Spitzer Science Archive for use by interested members of the community.