Structure and Kinematics of the Circumstellar Gas on Few Hundred AU Scales near the L483 Protostar

First, L483 is shown with images from the National Radio Astronomy Observatory 12m radiotelescope, and optically. (NRAO).

Optical and radio views of The L483 Molecular Cloud.

The bottom panel shows an image of C18O J=1-->0 emission taken by the NRAO 12m radiotelescope. Here the interstellar gas glows with emission from the carbon monoxide molecules, which, mixed with dust, completely obscured the star-forming interior of the cloud in the previous image. Note how the gas glows most brightly where accretion onto a protostar warms the cloud. Only a dozen or so objects in the sky are thought to be as young as this protostar, which appears to be growing into a young star of about the same mass as the Sun. At this stage in its evolution, however, most of the matter which will eventually be incorporated into the star and its planets still lies in the surrounding cloud. Thus the L483 protostar offers us a chance to understand how interstellar gas and dust clouds form stars and planets.

Next, L483 is shown with images from the National Radio Astronomy Observatory Very Large Array radiotelescope, and from the James Clerk Maxwell Telescope, a 15m telescope located atop Mauna Kea on Hawaii, using the SCUBA submillimeter detector array.

Abstract submitted to the 198th meeting of the AAS:

The far-infrared source in L483 numbers among the youngest protostars. We report high angular resolution images of the submillimeter emission obtained with SCUBA on the JCMT, and high spatial and spectral resolution images of the NH(1,1) and (2,2) lines obtained with the VLA. These images suggest that the disk currently accretes matter from the surrounding cloud. The highest optical depth line is the main hyperfine component of the (1,1) line, which is much weaker than the satellite line near the FIR source. The velocity of this component shifts significantly blueward within 2000 AU of the FIR source. Such a shift signifies that material between the observer and the optically thick envelope of the protostellar core is moving away from the observer, or falling toward the protostar.

The infall solution found by Shu (1977) was used to model the region interior to an expansion wave which propagates outward from the center of a singular isothermal sphere. Ammonia spectra were then synthesized and matched to the observations through adjustments to the model. The modeled profiles show enhanced linewidth in the (2,2) line relative to the (1,1) line, as well as a shift of emergent spectra towards lower line center velocity, though not so large as observed. We conclude that the ammonia images demonstrate that infall occurs within the central 5000 AU of the core centered on L483-FIR, but that more elaborate models will be necessary to fit the kinematics of the region in detail.

Al Wootten
Wed Oct 22 15:40:19 EDT 1997

On the left is the VLA image, taken with 4" resolution, of emission in the (J,K)=(1,1) line of ammonia (NH₃). This image shows the location of cool gas molecules--velocity information has been averaged to give an overall view of the source. On the right is the JCMT image, at a wavelength of 850 microns, taken with the SCUBA instrument. The image on the right is emission from dust grains, with a spatial resolution of 15". The images differ somewhat, with the dust image dominated by a bright source. This source coincides with the position of emission from hot water molecules and free electrons, as seen in other VLA images. The bright source locates the protostar.

Why do the two images look dissimilar around the protostar? For one thing, the star undoubtedly heats its environment. When dust is heated, it glows more brightly. When ammonia is heated, the molecule emits more strongly at other frequencies. Therefore, in this low energy emission line, we are selecting out cool gas to image, with hot gas suppressed. For comparison, examine the image below, which shows the source in the light of an emission line which requires more energy to excite.

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