Skewed Distributions and Opposite Velocity Gradients of Submillimeter
  Molecular Lines in Low-Mass Protostellar Envelopes
  We have made mapping observations of L1551 IRS 5, L1551NE, L723, and L43 and
single-point observations of IRAS 16293-2422 in the submillimeter CS (J = 7-6)
and HCN (J = 4-3) lines with ASTE. Including our previous ASTE observations of
L483 and B335, we found a clear linear correlation between the source
bolometric luminosities and the total integrated intensities of the
submillimeter lines (I_CS ~L_bol^0.92). The combined ASTE + SMA CS (7-6) image
of L1551 IRS 5 exhibits an extended (~2000 AU) component tracing the associated
reflection nebula at the west and southwest, as well as a compact (< 500 AU)
component centered on the protostellar position. The emission peaks of the CS
and HCN emissions in L1551 NE are not located at the protostellar position but
offset (~1400 AU) toward the associated reflection nebula at the west. With the
statistical analyses, we confirmed the opposite velocity gradients of the CS
(7-6) emission to those of the millimeter lines along the outflow direction,
which we reported in our early paper. The magnitudes of the submillimeter
velocity gradients are estimated to be (9.7\pm1.7) \times 10-3 km s-1 arcsec-1
in L1551 IRS 5 and (7.6\pm2.4) \times 10-3 km s-1 arcsec-1 in L483. We suggest
that the "skewed" submillimeter molecular emissions toward the associated
reflection nebulae at a few thousands AU scale trace the warm (> 40 K) walls of
the envelope cavities, excavated by the associated outflows and irradiated by
the central protostars directly. The opposite velocity gradients along the
outflow direction likely reflect the dispersing gas motion at the wall of the
cavity in the envelopes perpendicular to the outflow.
