Core D of Taurus Molecular Cloud 1 (TMC1-D)
Core D of Taurus Molecular Cloud 1 (TMC1-D)
In recent years, several lines of research have shown that cores believed to be pre-stellar have significant internal structure. The cores themselves, which have masses of one to a few times that of the Sun and a size of about 20,000 a.u. (comparable to the solar system's Oort comet cloud), were thought to be the last stage of condensation prior to proto- stellar collapse. However, the new studies show that these cores have internal structures with scales of 4000-8000 a.u. (30"-60" at the distance of the nearest active star-forming clouds). These structures show considerable differences in composition, suggesting that they evolve. Less than half of these structures (the statistics are not yet sufficient to determine a meaningful percentage) are smaller than 4000 a.u. The masses of the larger structures are typically 0.01-0.1 solar masses, which suggests that the statistics of coagulation play an important role in determining the efficiency of star formation and the typical sizes of stars (initial mass function).
Tau MC Cloud 1 cores C and D
The above integrated intensity CO image was made with a resolution of 100" using the AT&T Bell Labs 7-m antenna. The field is from 4h39m22s to 4h37m22s in right ascension and 25d25' to 25'55' in declination. The Goldstone observations shown in the first image are centered on core D, the bright region just below and to the left of the center.
The long-term goal of our research is to understand the evolution of these precursor objects, and thus the origin of stars and planetary systems, by obtaining statistical data on their density, temperature, internal motions, and chemical composition. This will require a major observational survey project consisting of both mapping a large number of cores in selected spectral lines, and comprehensive spectral searches in selected cores. Our near-term goal is to demonstrate the scientific potential and necessity of such a project by concentrating on a few typical examples. We are therefore making small maps in a few selected transitions of key molecules in order to determine the physical parameters of these objects.
The basic observation in this type of research is a spectrum such as the one shown below. Presented in this way, it shows the amount of emission for various velocities along the line of sight.
CCS spectrum of L1498
L1498, once thought to be a simple cloud core, has one of the narrowest spectral profiles known. The width of this CCS spectrum, 0.2 km/s, shows that there is very little motion in this region. By assembling spectra from various molecules taken at different positions we assembled this picture of L1498:
Composite map of molecular emission in L1498
The blue contours show that ammonia is concentrsted at the center of the core. Ammonia takes a relatively long time to form in molecular clouds, and so it shows that the gas at the center is older than the surrounding gas. The green contours are from a molecule CCS which does not survive long after its initial formation. Young gas surrounding old gas suggests that this core is in the process of assembling itself by gravitational attraction. Some of the structural details of the CCS distribution were revealed when we combined the DSN observations with those made of two parts of the core with the NRAO Very Large Array. The greyscale shows a shell-like or layered structure. Observations of CS made with the Owens Valley Millimeter Array show a similar layering a little closer to the center.
A later stage of star formation is shown in the core B335, in which the gas is collapsing onto a central protostar.
CCS spectrum of B335
This CCS spectrum shows a profile characteristic of infall. The gas at 8.5 km/s is on the near side of the core center, falling away from us towards the center. The core itself is moving away from us with a velocity of 8.4 km/s. The gas at 8.3 km/s is on the far side, falling towards us and the center, is somewhat weaker because attenuated by passage through the dense core. Because CCS is destroyed quickly, we did not expect that it would survive the trip from the outer envelope to the core center. Indeed, when we combined our Goldstone 70-m observations with those which we made with the VLA, the expected ring-like structure appeared.
Maps of B335 at three velocities
A popular report of the work on B335 appeared in the October 10, 1995, issue of the New York Times.
Because antennas cannot be packed together closely enough, interferometers arrays, such as the VLA and the OVRO and Haystack millimeter arrays, can observe objects larger than 30" only with great difficulty and low efficiency. Also, their dilute apertures are a serious handicap for the low emission levels from objects whose kinetic temperature is only 10 K. Although arrays are ideal for determining how stars form, our goal is to determine when and why stars form. For this, the most effective telescopes are large, single antennas having angular resolution in the range of 30"-60".
The 70-m antennas of the DSN have an angular resolution of 45" at 24 GHz and 30" at 34 GHz. The 34-m antennas have precise inner surfaces of 26-m diameter which provide beamsizes of 60" and 30" at 40 and 80 GHz respectively. We are working to expand on the antenna time available for astronomy in the DSN by developing automated procedures which will allow small scheduled and unexpected gaps in the spacecraft tracking and maintenance schedules to be used effectively. We believe that in this way, the DSN can provide significantly more antenna time than could be obtained at the few suitable radio astronomy observatories.
T. Velusamy, T. B. H. Kuiper, and W. D. Langer, CCS Observations of the Protostellar Envelope of B335, Astrophysical Journal, 451, L75-L78, 1995.
Langer, W.D., Velusamy, T., Kuiper, T.B.H., Levin, S., Olsen, E., Migenes, V. 1995, "Study of structure and small scale fragmentation in TMC 1", Astrophysical Journal, 451, 293-307.
Kuiper, T.B.H., Langer, W.D., Velusamy, T. 1996, Evolutionary Status of the Pre-Protostellar Core L1498, Astrophysical Journal, 468, 761-773..
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