arXiv 1301.64802013-01-28astro-ph.SR

原行星状星云IRAS 18276-1431尘埃性质的研究

Investigation of dust properties of the proto-planetary nebula IRAS 18276-1431

K. Murakawa, H. Izumiura, R. D. Oudmaijer, L. T. Maud

备注：: 9 pages, 5 figures, accepted for publication in MNRAS
DOI：: 10.1093/mnras/stt118

摘要：

利用二维辐射转移模拟研究了富氧双极原行星状星云IRAS 18276-1431的星周尘埃壳层的性质。假设模型几何形状具有圆环和包络。通过比较SED和NIR强度和偏振数据与模型的尘埃和尘埃壳的参数的约束。在观测中，包层内的偏振度达到50 - 60%，在H和K_S波段几乎不变。偏振的这种弱波长依赖性可以用环面的粒度分布函数再现：0.05微米<= a，其中n（a）=a^{-（p=5.5）}exp（-a/{a_c=0.3微米}）。功率指数p明显比星际尘埃的陡峭。在其他一些PPN中也发现了类似的结果，这表明当尘埃颗粒形成时，研磨大颗粒的机制也可能起作用。光谱不透明度指数β被发现是0.6+/-0.5毫米通量。这个低的数值表明在环中存在大的尘埃颗粒。我们讨论两种可能的尘埃模型的环面。一个具有1.0微米<= a <= a_max= 5，000.0微米的尺寸分布函数，其中n（a）=a^{-（p=2.5）}，另一个是1.0微米<= a <= a_max= 10，000.0微米，其中n（a）=a^{-（p=3.5）}。前者的β值为0.633，但我们无法找到合理的几何参数来拟合红外中的SED。后者的β为1.12，但在宽波长范围内更好地再现了SED。用这个尘埃模型估算出的几何参数为：内外半径分别为30 AU和1000 AU，环质量为3.0M_sun。假设膨胀速度为15 km ^{-1}，环的形成时间和质量损失率分别为\sim 300 yrs和\sim 10^{-2}M_sun yr^{-1}。

英文摘要：

We investigate the circumstellar dust properties of the oxygen-rich bipolar proto-planetary nebula IRAS 18276-1431 by means of two-dimensional radiative transfer simulations of the circumstellar dust shell. The model geometry is assumed to have a torus and an envelope. The parameters of the dust and the dust shell are constrained by comparing the SED and NIR intensity and polarisation data with the models. The polarisation in the envelope reaches 50 -- 60 % and is nearly constant in the H and K_S bands in the observations. This weak wavelength dependence of the polarisation can be reproduced with a grain size distribution function for the torus: 0.05 micron <= a with n(a)=a^{-(p=5.5)}exp(-a/{a_c=0.3 micron}). The power index p is significantly steeper than that for interstellar dust. Similar results have also been found in some other PPNs and suggest that mechanisms that grind down large particles may also have acted when the dust particles formed. The spectral opacity index beta is found to be 0.6+/-0.5 from the millimeter fluxes. This low value indicates the presence of large dust grains in the torus. We discuss two possible dust models for the torus. One has a size distribution function of 1.0 micron <= a <= a_max=5,000.0 micron with n(a)=a^{-(p=2.5)} and the other is 1.0 micron <= a <= a_max=10,000.0 micron with n(a)=a^{-(p=3.5)}. The former has beta of 0.633, but we are not able to find reasonable geometry parameters to fit the SED in the infrared. The latter has beta of 1.12, but reproduces the SED better over a wide wavelength range. With this dust model, the geometric parameters are estimated as follows: the inner and outer radii are 30 AU and 1000 AU and the torus mass is 3.0 M_sun. Assuming an expansion velocity of 15 kms^{-1}, the torus formation time and mass-loss rate are found to be \sim300 yrs and \sim10^{-2}M_sun yr^{-1} respectively.

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