Astron. Astrophys. 327, 758-770 (1997)

6. Conclusion

Motivated by an earlier suggestion, that the 22 GHz maser emission toward the center of the dark cloud L 1287 originates near an accretion disk around a deeply embedded YSO instead of within the associated molecular outflow, the concept of disk-impinging-clumps has been investigated.

Derived color temperatures for the far infrared source IRAS 00338+6312, located near the center of L 1287, as well as results from infrared polarimetric imaging and radio continuum observations obtained by others support the interpretation that IRAS 00338+6312 is not identical with the nearby FUors RNO 1B/C, but is a still younger and more deeply embedded Class I object. In the case of a low mass protostar, its (far infrared) luminosity is consistent with the accretion luminosity required for a FUor during an outburst.

The clumpy structure of the molecular cloud gas forms a possible reservoir for small high density clumps. If the LSR-velocity gradient derived from the NH₃ observations is interpreted in terms of large scale cloud rotation, angular momentum conservation allows to estimate an outer accretion disk radius on the order of 10³ AU, comparable with the size scale found for circumstellar disks around T Tau stars. The large scale rotational direction of the cloud agrees with that of the suspected accretion disk. The estimated age of the protostar ( 10⁵ yrs) is compatible with the lower boundary for T Tau stars, indicating that the embedded YSO reaches the end of its embedded/accretion dominated stage.

Consequently, the anticipated pre-T Tau phase of the YSO would be in accordance with somewhat smaller disk radii (than for T Tau stars) as required by the proposed disk-impinging-clump model. However, within the limitations of this concept, disk radii already in excess of about 10² AU would lead to unacceptably large disk masses. The model restrictions are mainly based upon the assumption of a standard (viscous) disk and a simplified description of the clump-disk collision. More detailed models and observational constraints are necessary and might eventually lead to less stringent requirements.

If the suspected accretion disk is described through the disk structure equations for a standard viscous accretion disk, temperature and density at the maximum radius of observed maser emission ( 35 AU) yield 100 K and 10¹¹ , indicating that sufficiently dense clumps impinging onto the disk will form a shock-compressed layers of clump matter. The density and temperature in such a layer forms an appropriate environment for the excitation of water molecules to emit 22 GHz maser radiation. Although the estimated shock velocities are within the limits required for non-dissociating J-shocks, the formation of C-shocks seem likely as well. The velocity field of the disk approaching clumps is approximately characterized by free-fall parabolic orbits. A simplified view of the hydrodynamics at impact suggest that the shock-compressed layers only maintain their radial and rotational velocity components. Based upon this approximation, comparison of the modeled masers' position-velocity distribution shows a remarkable agreement with the observational results obtained earlier.

Identifying the variability timescale of the 22 GHz masers in L 1287 (H₂ O) with the timescale required to maintain the shock front in a representative disk-impinging-clump, typical clump sizes and shock-compressed layers on the order of 0.1 AU and 0.01 AU are derived, respectively. From the observed flux density of the maser lines, optical depths are derived that are consistent with unsaturated maser emission. Estimated inversion efficiencies of 10^-7 are easily attainable with simple collisional pumping schemes.

The concept of accretion-disk-impinging clumps seems to be an alternative interpretation to the nature of 22 GHz maser emission in L 1287 (H₂ O) that is likely to explain the high angular resolution observational results in a simple straightforward fashion.

© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998
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