Joel H. Kastner

The Structure of the Prototype Bipolar Protoplanetary Nebula CRL 2688 (Egg Nebula): Broadband, Polarimetric, and H2 Line Imaging with NICMOS on the Hubble Space Telescope

High-resolution near-infrared (1.65-2.1 μm) images and 2 μm polarimetric images of the inner 195 × 193 region of the bipolar protoplanetary nebula CRL 2688, taken with the newly installed Near-Infrared Camera and Multiobject Spectrometer (NICMOS) aboard the Hubble Space Telescope (HST), are reported. The NICMOS images reveal a wealth of structure also seen in HST Wide Field Planetary Camera 2 images but not detected in previous, ground-based near-infrared imaging studies. In particular, we detect a system of concentric arcs centered on, and twin searchlight beams emanating from, the obscured central star. The images also show two sharply bounded spindle-shaped polar cavities with point-symmetric structure, and the 2.122 μm H2 S(1) v = 1-0 emission-line image clearly resolves the sharp interface between the high-velocity outflow that produces these polar cavities and the surrounding slow outflow that forms the extended nebula. The H2 image also resolves the bright equatorial H2 emission into distinct components and elucidates their detailed morphologies. We have discovered, within the dark lane that bifurcates the bipolar lobes of CRL 2688, a compact source of unpolarized light. Our imaging polarimetry shows that this source is not the post-asymptotic giant branch star that illuminates the nebula; we conclude that the compact source is a companion star.

X-Ray Spectroscopy of the Nearby, Classical T Tauri Star TW Hydrae

We present ASCA and ROSAT X-ray observations of the classical T Tauri star TW Hya, the namesake of a small association that, at a distance of ~50 pc, represents the nearest known region of recent star formation. Analysis of ASCA and ROSAT spectra indicates characteristic temperatures of ~1.7 and ~9.7 MK for the X-ray-emitting region(s) of TW Hya, with emission lines of highly ionized Fe dominating the spectrum at energies of ~1 keV. The X-ray data show variations in X-ray flux on timescales of 1 hr as well as indications of changes in the X-ray-absorbing column on timescales of several years, suggesting that flares and variable obscuration are responsible for the large-amplitude optical variability of TW Hya on short and long timescales, respectively. Comparison with model calculations suggests that TW Hya produces sufficient hard X-ray flux to produce significant ionization of molecular gas within its circumstellar disk; such X-ray ionization may regulate both protoplanetary accretion and protoplanetary chemistry.

The Protostellar Origin of a CS Outflow in S68N

We have discovered a compact outflow of ≈ 80 (0.12 pc) total extent toward S68N in the Serpens molecular cloud, which we have imaged in CS. We present outflow images acquired with the Haystack 37 m antenna and from combined Haystack and interferometric data, the latter at 12×6 angular resolution. The large derived outflow momentum flux (~2×10 -->−4 M☉ km s-1 yr-1) is consistent with the high values found in the earliest protostellar stage (Bontemps et al.). We also present a new 450 μm continuum image of the S68N region at 7 resolution obtained with the Submillimetre Common User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope. We identify the source responsible for this outflow for the first time from the SCUBA image and report a new, more accurate position for S68N than has been used in previous investigations. We demonstrate spatial variability in the self-absorbed CS J=2→1 line profile shapes at 0.03 pc (18) scales, emphasizing the need for detailed radiative transfer modeling for disentangling the complicated source kinematics.

Infrared Spectroscopy and Imaging Polarimetry of the Disk around the T Tauri Star RNO 91

We present 3-5 microns spectra and a 2.2 microns polarimetric image of the T Tauri star RNO 91. We report the detection of three absorption bands centered at 3250 cm-1 (3.08 mum), 2139 cm-1 (4.68 mum), and 2165 cm-1 (4.62 mum) in spectra of RNO 91. These features are due to frozen H2O, CO, and possibly XCN along the line of sight toward RNO 91. Our 2187-2107 cm-1 spectrum of RNO 90, the only other T Tauri star in the dark cloud L43, does not show the CO or XCN absorption bands. By comparing our observed polarimetric image with modeled images of scattered light from bipolar nebulae or circumstellar disks as well as with known morphology of the RNO 91 bipolar outflow, we demonstrate that the reflection nebulosity seen in the near-infrared is most likely a circumstellar disklike structure with a radius of ˜1700 AU. The location of both RNO 90 and RNO 91 in front of or near the front of L43 suggests that the intracloud optical depths toward both stars are small and, therefore, that the frozen H2O, CO, and XCN molecules are located on grains in circumstellar material around RNO 91 at distances from the central star of perhaps 10-1700 AU. This frozen material may represent precometary grains orbiting RNO 91.

Direct Detection of the Mira at the Heart of OH 231.8+4.2

The evolved bipolar nebula OH 231.8+4.2 is host to a Mira variable star, QX Pup, at its core. We used the diffraction-limited near-infrared imaging (DLIRIM) system at Kitt Peak to obtain the first direct images of this star in the near-infrared. In subarcsecond resolution DLIRIM images at K (2.2 μm) and L (3.8 μm) the star is clearly detected and is well resolved from the surrounding nebulosity. The star lies midway between the lobes of the OH 231.8+4.2 reflection nebula, confirming previous inferences based on polarimetric imaging and nebular colors. In addition to betraying the location of the central Mira, these images reveal the detailed structure of OH 231.8+4.2, including a point symmetric system of jetlike features protruding from its polar lobes. The K - L color distribution of the nebula is consistent with reflection and extinction of near-infrared radiation from the Mira by dust grains larger than those characteristic of the interstellar medium. From the apparent K magnitude of the star, combined with its relatively well-determined distance (1300 pc) and the circumstellar extinction we infer from its measured K - L color, we estimate a mean absolute magnitude MK ~ -10.2, comparable to Mira variables of similar (~700 day) period in the Large Magellanic Cloud. In this and other respects, the central Mira appears remarkably normal given its position at the heart of such an unusual object.

The Onset of Molecular Hydrogen Emission from Proto-planetary Nebulae

This paper presents results obtained from high-resolution spectroscopy of the 2.1218 μm emission line of molecular hydrogen toward five bipolar pre-planetary nebulae. Our detections of IRAS 17441-2411 and AFGL 6815S bring to four, including AFGL 618 and AFGL 2688, the number of bipolar preplanetaries that have been detected as sources of H2 emission at 2.1218 μm. The spatially resolved H2 line profile of AFGL 6815S suggests that the bulk of the detected emission arises in an expanding molecular torus surrounding the central star. All of the H2-emitting pre-planetary nebulae are found at low galactic latitudes, consistent with previous results for planetary nebulae, and have central stars with intermediate or early spectral types. We were unable to detect H2 emission from the bipolar, post-main-sequence sources OH 231.8+4.2, IRAS 07131-0147, and IRAS 09371+1212, all of which have M-type central stars. These results suggest that the event that triggers the production of emission from shocked H2 occurs at an intermediate evolutionary epoch in the postasymptotic giant branch evolution of transition objects, after the generation of bipolar structure and before the nebular envelopes are ionized. From constraints imposed by the galactic distribution of H2-emitting planetaries, we estimate distances to the four pre-planetary nebulae that display H2 emission.

In Search of HL Tauri

We present infrared polarimetric images that demonstrate that the T Tauri star HL Tau is not seen in direct light at λ ≤ 2.2 μm. The position of the intensity centroid moves monotonically southwest from V (0.555 μm) through J (1.25 μm) and H (1.65 μm) to K (2.2 μm) wave bands. Furthermore, the mean position of the J, H, and K polarimetric centroids—which marks the position of the illuminating source of extensive reflection nebulosity—is displaced ~1 southwest of the position of the K-band intensity peak. This offset is in the same direction as the offset between the K-band intensity peak and the centroid of previously detected 3.6 cm emission. We conclude that, at optical and near-IR wavelengths, all of the observed radiation is scattered light from the circumstellar nebula. Our polarimetric images of HL Tau show a polarization disk at PA 134° and therefore support a model in which the projection of the equatorial plane of the protoplanetary disk is aligned at PA 134°. These results reinforce the contention that HL Tau is younger and more deeply embedded than typical T Tauri stars.

Diffraction-Limited 3.8 Micron Imaging of Protostellar Outflow Sources

We used the new high spatial resolution observing mode of the Kitt Peak near-IR Cryogenic Optical Bench on the 4 m telescope to detect and determine the locations of the powering sources of three protostellar outflows. Diffraction limited images at 3.8 μm of the cores of AFGL 437, L1287, and NGC 7129 demonstrate that, in each case, a young stellar object—WK 34, IRAS 00338+6312, and NGC 7129 PS 1, respectively—lies at the position predicted for the outflow source on the basis of near-IR polarimetric imaging at 2.2 μm. WK 34 and NGC 7129 PS 1 display nebulosity at 3.8 μm, confirming that each illuminates an outflow cavity. These data support the hypotheses that protostellar outflows are almost always associated with very young, heavily extincted sources, that associations of outflows with pre-main sequence stars are likely to be incorrect, and that infrared polarimetric imaging is a dependable tool for probing reflection nebulae to locate embedded young stellar objects.

Submillimeter Imaging of T Tauri’s Circumbinary Disk and the Discovery of a Protostar in Hind’s Nebula

We have obtained images at 2.2, 450, and 850 μm of the pre-main-sequence binary T Tauri and of Hinds Nebula, which is located less than 1 west of T Tauri. The three maps reveal that the nebulosity around T Tauri is extended, with an FWHM size of several hundred AU in thermal emission and more than 1000 AU in reflected light. Dust clearly connects the circumstellar nebula around T Tauri to the southwestern tip of Hinds Nebula, where we have identified a secondary intensity peak in the submillimeter maps. Calculations of the long-wavelength emissivity demonstrate that the dust emissivity index β is ~2 for the large nebula but is ~1.2 for the secondary intensity peak. These results for β suggest that we have found a class I protostar 30 southwest of T Tauri and that star formation is more active in the vicinity of T Tauri than previously had been thought. This protostar is detected directly at submillimeter wavelengths; however, polarization mapping at 2.2 μm demonstrates that the disk-shaped dust cloud around the protostar is illuminated externally at short wavelengths, making it similar to the Orion proplyds.

Water Ice in the Disk Around the Protostar AFGL 2136 IRS 1

The protostar AFGL 2136 IRS 1 illuminates the Juggler nebula and drives a massive molecular outflow. The line of sight toward AFGL 2136 also displays a rich spectrum of solid state absorption features. To investigate the distribution of ice-coated grains in the environment of IRS 1, we obtained narrowband images at six near-infrared wavelengths surrounding and including the 3.08 μm water ice feature. These images (specifically, a color map constructed from 2.14 and 3.60 μm images) provide vivid evidence for the presence of a disk around IRS 1 and appear to reveal, for the first time, the detailed distribution of water ice in the local environment of a protostar. Models of the 2-4 μm spectral energy distributions (SEDs) toward IRS 1 and the Juggler nebula suggest that the abundance of icy grains is larger along lines of sight through the disk than along lines of sight away from the equatorial plane of IRS 1. Thus, while we find evidence that icy grains exist throughout the cloud core containing AFGL 2136, it appears that the physical conditions in the circumstellar disk around IRS 1 are more conducive to ice mantle survival and/or growth than are physical conditions in its rarefied polar regions.