不規則形狀海外天體掩星繞射曲線特性研究

Abstract

外海王星天體(Trans-Neptunian Objects ，TNO)的探索和認識，是現代天文學家很感興趣的一個課題。掩星法是目前唯一研究典型彗星大小TNO之方法，地面上掩星訊號會受到被遮掩星光的盤面大小、光譜種類、TNO的形狀大小、和距離遠近等因素所影響。因此，我們能夠藉由地面所觀測到的陰影，去了解到遠方TNO的大小，形狀、分布、距離，甚至是背景星光的資訊。過去對於TNO繞射的研究，模型基礎相當的單純，主要設定為規則形狀的模擬(圓形、方形等)，但就目前我們所了解，絕大多數典型彗星大小的TNO是不規則形狀的。 為了得到更接近真實掩星現象的模擬，本文中我們考慮背景星光的光譜，TAOS望遠鏡CCD(SI-800)的量子效率、及實際觀測時使用的濾鏡(TAOS 5564A)，並且以不規則形狀TNO所產生的繞射圖案為討論的主軸。在計算上，我們所使用的理論基礎為邊界繞射波理論(boundary diffraction waves)，此理論能夠以線積分的計算取代傳統面積分的計算，這將使我們研究繞射圖案時的計算效率大大的提升。我們的研究成果將會有助於掩星現象之相關研究及TAOS計劃(中美掩星計劃)對於TNO的認識。

Serendipitous occultation is currently the only way to study the comet-size Trans-Neptunian Objects (TNOs). The occultation signal is basically restricted by the angular size of a target star, spectrum, the shape and size of TNOs and the distance from earth to TNOs. One will be able to derive the size, shape, population, and even its distance through the study of TNOs’ shadow on the ground. It was focused on the analysis of the diffraction pattern of regular objects in most of the previous works. Though, a comet-size TNO is believed to be irregular in shape. Considering the spectral types of the background stars together with the spectral response of a CCD camera and a band-pass filter, we analyze the diffraction patterns by TNOs of various shapes. The calculation in this paper bases on the theory of‘Boundary Diffraction waves’. This theory helps us to replace the surface-integral with line-integral. With this theory, we are able to study the statistical properties of these diffraction patterns. Our result should be helpful in the interpretation of the result of a TNO occultation surveys, such as the Taiwan-America Occultation Survey (TAOS) project. It can also be used in the related occultation simulation so that a more complete picture can be obtained.