Constraining the structure of the non-spherical pre-protostellar core L1544

Abstract

We present a study of the pre-protostellar core L1544. A series of self-consistent, three-dimensional continuum radiative transfer models is constructed. The outputs of these models are convolved with appropriate telescope beam responses, and compared with existing SCUBA data. The resulting comparison allows us to constrain the structure of L1544. We find that the source is well-fitted by a prolate spheroid, having an ellipsoidal power-law density distribution of index m ∼ 2(1.75 < m < 2.25) in to at least r ∼ 1600 au. For r < 1600 au, the data are consistent with either an extension of the power law to smaller radii, or a flattened (Bonner-Ebert-like) density distribution. Furthermore, we find an optical depth along the short axis at 1300 μm of τ 1300,short = 5 x 10 -3 (2 x 10 -3 < τ 1300,short < 8 x 10 -3 ), a central luminosity L* =0(<10 -3 L ○. ), a long-axis diameter D = 0.1 pc [0.08 < D(pc) < 0.16; 16000 < D(au) < 32 000], an axis ratio q = 2 (1.7 < q < 2.5), and an external interstellar radiation field (ISRF) defined by Mathis et al. to within 50 per cent. The outer diameter and axis ratio may each be somewhat larger due to potential on-source chopping in the observations, and the projection of the long axis on to the plane of the sky. While these results are similar to those inferred directly from observations or spherical modelling due to the source transparency at submillimetre wavelengths, we infer a smaller size, lower mass, and higher optical depth/column density, exposed to a stronger external radiation field than previously assumed. Finally, we find that both the spectral energy distribution (SED) and surface brightness distribution are necessary to constrain the source properties in this way, and even a modest variation in X 2 can significantly alter the fit quality.