标题： 高致质量原恒星的非常紧凑的射电辐射。 II. 尘埃盘和电离吸积流 显示英文标题

标题： Very compact radio emission from high-mass protostars. II. Dust disks and ionized accretion flows

摘要： 本文报道了对高质量原恒星 W33A、AFGL 2591 和 NGC 7538 IRS9 在 43 GHz 频率下的成像观测，分辨率为 0.04" 和 0.6"。 在每种情况下，检测到弱（~毫焦耳）且紧凑（~100 AU）的发射，其形状呈拉长状（轴比 ~3）。 对于 AFGL 2591 和 NGC 7538 IRS9，发射呈现单峰特征，而对于亮度最高的源 W33A，则检测到一个由三个源组成的“小星团”。 这些源的推导出的大小、流量密度和宽带射电频谱支持最近的模型，在这些模型中，非常年轻的 O 型恒星周围的 HII 区的初始扩展被恒星引力所阻止。 在这些模型中，高质量恒星的吸积流起源于大规模分子包层，并在接近恒星时被电离。 这些模型能够重现我们对电离气体的观测结果，以及之前通过单碟亚毫米连续波和线映射得出的这些源分子包层在 10^3 至 10^4 AU 尺度上的结构。 对于 AFGL 2591，43 GHz 的流量密度也与从近红外“光斑”图像中看到的盘状尘埃发射一致。 然而，43 GHz 发射与大规模分子外流的一致性表明 AFGL 2591 和 NGC 7538 IRS9 的发射并非源自盘状结构。 相比之下，W33A 的外流似乎并未被束化。 结合之前关于 W3 IRS5 和 AFGL 2136 的案例研究，我们的结果表明，具有高达 ~10^5 L0 光度的恒星和恒星团的形成过程通过吸积进行，并产生类似于太阳型情况的束化外流，但具有“额外特性”，即吸积流在接近恒星时被电离。 超过 ~10^5 L0 时，会出现 HII 区簇，外流不再束化，这可能是由于原恒星或前恒星核心的合并所致。 显示英文摘要

摘要： This paper reports 43 GHz imaging of the high-mass protostars W33A, AFGL 2591 and NGC 7538 IRS9 at 0.04'' and 0.6'' resolution. In each case, weak (~mJy), compact (~100 AU) emission is detected, which has an elongated shape (axis ratio ~3). For AFGL 2591 and NGC 7538 IRS9, the emission is single-peaked, while for the highest luminosity source, W33A, a `mini-cluster' of three sources is detected. The derived sizes, flux densities, and broad-band radio spectra of the sources support recent models where the initial expansion of HII regions around very young O-type stars is prevented by stellar gravity. In these models, accretion flows onto high-mass stars originate in large-scale molecular envelopes and become ionized close to the star. These models reproduce our observations of ionized gas as well as the structure of the molecular envelopes of these sources on 10^3--10^4 AU scales derived previously from single-dish submillimeter continuum and line mapping. For AFGL 2591, the 43 GHz flux density is also consistent with dust emission from a disk seen in near-infrared `speckle' images. However, the alignment of the 43 GHz emission with the large-scale molecular outflow argues against an origin in a disk for AFGL 2591 and NGC 7538 IRS9. In contrast, the outflow from W33A does not appear to be collimated. Together with previously presented case studies of W3 IRS5 and AFGL 2136, our results indicate that the formation of stars and stellar clusters with luminosities up to ~10^5 L0 proceeds through accretion and produces collimated outflows as in the solar-type case, with the `additional feature' that the accretion flow becomes ionized close to the star. Above ~10^5 L0, clusters of HII regions appear, and outflows are no longer collimated, possibly as the result of mergers of protostars or pre-stellar cores.