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.

In the infrared image (in the `J' band at 1.2 microns) in the this panel, the dark cloud L483 stands out against a Scutum starfield. Deep within, a purple glow locates one lobe of a flow emanating from the densest part of the cloud, completely hidden from view at even the longest infrared wavelengths accessible from the Earth's surface. At these wavelengths, we could never peer into the cloud and uncover the mechanisms of star formation. Radio techniques remedy this situation. Image courtesy of Gary Fuller.

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.

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


Gary Fuller


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.

On the left is the VLA image of emission in the (J,K)=(2,2) line of ammonia, a line which lies about 60K above zero energy, and about 40 K above the (1,1) line pictured above. On the right is the JCMT image at 450 microns, again taken with SCUBA at a resolution of 9". The bright protostar is now visible in both images, as we are selectively imaging warmer gas now, gas associated with the protostar.

Finally, L483 is shown with images from the National Radio Astronomy Observatory Very Large Array radiotelescope, and from the IRTF, a 4m telescope located atop Mauna Kea on Hawaii, using the NSFCAM infrared CCD array at 3.4 micron wavelength. On the right is the VLA image of emission in the (J,K)=(2,2) line of ammonia, and in the center the 450 micron image from SCUBA, shown more extensively above, and on the left is the infrared image at 3.4 microns. Note that the star is buried deep within the obscuring lane passing from northeast to southwest. On each side of the lane, some indication of indirect illumination by the central star, invisible in this image, can be seen. Interestingly, portions of the absorbing lane show up brightly in emission at 450 microns, but appear suppressed in the ammonia line. The yellow cross designates the position of a 6cm radio continuum source while the white cross locates the position of a water maser at VLSR=5.8 km/s.

Figures generated using WIP

Above is shown the spectral energy distribution of L483. It is characterized by a luminosity just below 20 solar luminosities and a bolometric temperature of 50K. The blackbody curve shown is calculated for a dust temperature of 30 K. About 0.4 solar masses of cool circumstellar dust apparently accompany the star.

The distribution of flux density (Jy) lies along the y-axis, as observed with SCUBA. Compared to that are flux densities computed from the densities in our infall model. Considering that the infall model was derived from ammonia observations alone, and not substantially constrained by the SCUBA observations, we consider the fit to be excellent.

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