Skewed Distributions and Opposite Velocity Gradients of Submillimeter Molecular Lines in Low-Mass Protostellar Envelopes

Abstract

Abstract We have made mapping observations of L 1551 IRS 5, L 1551 NE, L 723, and L 43 and single-point observations of IRAS 16293$-$2422 in the submillimeter CS ($J$$=$ 7–6) and HCN ($J$$=$ 4–3) lines with Atacama Submillimeter Telescope Experiment (ASTE). Using the present data and our previous ASTE observations of L 483 and B 335 together, we found a definite correlation between source bolometric luminosities and total integrated intensities of the submillimeter lines ($I_{\rm CS}$$\propto$$L_{\rm bol}^{0.92}$). The combined ASTE $+$ Submillimeter Array CS (7–6) image of L 1551 IRS 5 exhibits an extended ($\sim$ 2000 AU) component tracing the associated reflection nebula at west and southwest, as well as a compact ($\lesssim$ 500 AU) component centered on the protostellar position. The peaks of the CS and HCN emissions in L 1551 NE are not located at the protostellar position but offset ($\sim$ 1400 AU) toward the associated reflection nebula at west. By statistical analyses, we confirmed the velocity gradients of the CS (7–6) emission that are opposite 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$\ \pm\ $1.7) $\times$ 10$^{-3}\ $km s$^{-1}\ $arcsec$^{-1}\ $ in L 1551 IRS 5 and (7.6$\ \pm\ $2.4) $\times$ 10$^{-3}\ $km s$^{-1}\ $arcsec$^{-1}\ $ in L 483. We suggest that the “skewed” submillimeter molecular emissions toward the associated reflection nebulae at a few thousand AU scale trace the warm ($\gtrsim$ 40 K) walls of the envelope cavities, excavated by the associated outflows and directly irradiated by the central protostars. The opposite velocity gradients along the outflow direction likely reflect the dispersing gas motion at the wall of the cavity in envelopes perpendicular to the outflow.