這篇碩士論文由我的兩篇論文以及與九州工大的合作項目組成。
尋找第九行星的摘要:
現今柯伊伯帶天體的觀測數據顯示這些小天體存在意料之外的軌道參數聚
集現象。目前最有可能的動力學解釋是在外太陽系中存在一顆未知的巨大行星──第九行星（Planet Nine）。然而，由於其距離極為遙遠，過去在可見光波段進行的大量搜尋仍未能成功發現它。我們的研究目標是在遠紅外線波段的觀測數據中尋找第九行星，因為它的黑體輻射峰值正位於此波段。我們使用迄今為止最靈敏的遠紅外線巡天資料──AKARI 來進行搜尋。與可見光波段搜尋不同的是，反射太陽光的亮度會隨太陽距離的四次方衰減（d^4），而熱輻射在紅外線波段只會隨距離平方衰減（d^2），因此紅外線觀測更適合偵測這類遙遠且晦暗的天體。我們在AKARI 的單次掃描偵測清單（Single Scan Detection List）中尋找移動中的天體。首先在Millholland 和Laughlin 於2017 年的N 體模擬所建議的潛在天球區域中搜尋：赤經（R.A.）：30◦ < R.A. < 50◦，赤緯（Dec.）：−20◦ < Dec. < 20◦。我們透過與9 個可見光與紅外線天體列表的交叉比對，排除已知天體；此外，為避免銀河捲雲（cirrus）造成的背景雜訊干擾，我們也剔除高背景輻射強度的區域。由於第九行星在數小時的時間尺度內在天
球上幾乎是靜止的，但在數月的時間尺度上會出現明顯移動，因此我們的主要策略是藉由AKARI 的多次單掃描找出那些在24 小時內靜止，但在六個月期間有明顯移動的緩慢移動天體。最後，我們也針對可能受到宇宙射線污染的點源進行審查。分析結果顯示，有兩個候選天體的位置與亮度皆落在第九行星的理論預測範圍內。這些候選點源值得進一步透過追蹤觀測來確認其是否為第九行星並研究其相關性質。
棕矮星數密度的摘要:
棕矮星是質量極低的「失敗恆星」，質量範圍介於13 到75 倍木星質量
之間，表面溫度低於2500 K。由於質量介於恆星與行星之間，棕矮星對於理解恆星與行星間的差異上扮演關鍵角色。然而，由於其本身非常黯淡，以往的搜尋工作只能侷限在太陽附近的區域（約20 秒差距內）。若要進一步了解初始恆星質量函數（IMF）中低質量部分的分布，尋找更遠處的棕矮星就成為一項重要課題。本研究利用詹姆斯 韋伯太空望遠鏡（JWST）於COSMOS-Web 區域拍攝的資料，探索位於數千秒差距（kpc）遠處的棕矮星，以期提升我們對它們物理特性的理解。JWST 卓越的靈敏度使我們能夠偵測到比過去全天空紅外線巡天中發現的棕矮星還要遠100 倍的天體，甚至遠至銀盤之外。此外，COSMOS-Web 的大範圍觀測區域，也讓我們能尋找到比過去JWST 小範圍研究更多的遙遠棕矮星。為了捕捉約2.7微米波段的強烈水氣與甲烷吸收光譜特徵，我們使用了兩項顏色選擇條件：F115W − F277W + 1 < F277W − F444W 以及F277W − F444W > 0.9。接著，利用軟體SExtractor 中的CLASS STAR、FLUX RADIUS 與SPREAD MODEL 參數挑選點狀天體，並對晦暗點源進行人工目視檢查，以排除可能的擴散源。最後，我們進行光譜能量分佈（SED）擬合與MCMC 模擬以推算其物理性質。本研究共發現25 顆T 型矮星候選天體與2 顆Y 型矮星候選天體，數量超越過往所有JWST 的棕矮星搜尋成果。這些天體距離地球約在0.3 到4 千秒差距之間。這些棕矮星候選天體的累積數量與標準的雙指數模型（double exponential model）預測相符。其中三顆候選天體也被哈伯太空望遠鏡（HST）所觀測到，其橫向速度分別為12 ± 5、12 ± 4 和17 ± 6 公里／秒。結合先前的研究成果揭示了JWST已為我們打開一扇研究銀盤與銀暈中棕矮星的新窗口。

This thesis is composed of two of my papers and a record of the cooperative work with KyuTech scientists. One paper is searching for Planet Nine, and the other is related to the Brown Dwarfs in the Milky Way.

Abstract of A Far-Infrared Search for Planet Nine Using AKARI All-Sky Survey:

An unusual orbital element clustering of Kuiper belt objects (KBOs) has been observed. The most promising dynamic solution is the presence of a giant planet in the outer Solar system, Planet Nine. However, due to its extreme distance, intensive searches in optical have not been successful. We aim to find Planet Nine in the far-infrared, where it has the peak of the black body radiation, using the most sensitive all-sky far-infrared survey to date, {\it AKARI}. In contrast to optical searches, where the energy of reflected sunlight decreases by $d^{4}$, thermal radiation in the infrared decreases with the square of the heliocentric distance $d^{2}$.
We search for moving objects in the AKARI Single Scan Detection List. We select sources from a promising region suggested by an N-body simulation from Millholland and Laughlin 2017: $30^{\circ}<$ R.A. $<50^{\circ}$ and $-20^{\circ}<$ Dec. $<20^{\circ}$. Known sources are excluded by cross-matching {\it AKARI} sources with 9 optical and infrared catalogues. Furthermore, we select sources with small background strength to avoid sources in the cirrus.
Since Planet Nine is stationary in a timescale of hours but moves on a monthly scale, our primary strategy is to select slowly moving objects that are stationary in 24 hours but not in six months, using multiple single scans by AKARI. The selected slowly moving {\it AKARI} sources are scrutinised for potential contamination from cosmic rays.
Our analysis reveals two possible Planet Nine candidates whose positions and flux are within the theoretical prediction ranges. These candidates warrant further investigation through follow-up observations to confirm the existence and properties of Planet Nine.

Abstract of Brown dwarf number density in the JWST COSMOS-Web field:

Brown dwarfs are failed stars with very low mass (13 to 75 Jupiter mass), and an effective temperature lower than 2500 K. Their mass range is between Jupiter and red dwarfs. Thus, they play a key role in understanding the gap in the mass function between stars and planets. However, due to their faint nature, previous searches are inevitably limited to the solar neighbourhood (20 pc). To improve our knowledge of the low mass part of the initial stellar mass function and the star formation history of the Milky Way, it is crucial to find more distant brown dwarfs.
Using James Webb Space Telescope (JWST) COSMOS-Web data, this study seeks to enhance our comprehension of the physical characteristics of brown dwarfs situated at a distance of kpc scale. The exceptional sensitivity of the JWST enables the detection of brown dwarfs that are up to 100 times more distant than those discovered in the earlier all-sky infrared surveys. The large area coverage of the JWST COSMOS-Web survey allows us to find more distant brown dwarfs than earlier JWST studies with smaller area coverages.
To capture prominent water absorption features around 2.7 $\mu$m, we apply two colour criteria, $\text{F115W}-\text{F277W}+1<\text{F277W}-\text{F444W}$ and $\text{F277W}-\text{F444W}>\,0.9$. We then select point sources by {\tt CLASS\_STAR}, {\tt FLUX\_RADIUS}, and {\tt SPREAD\_MODEL} criteria. Faint sources are visually checked to exclude possibly extended sources. We conduct SED fitting and MCMC simulations to determine their physical properties and associated uncertainties.
Our search reveals 25 T-dwarf candidates and 2 Y-dwarf candidates, more than any previous JWST brown dwarf searches. They are located from 0.3 kpc to 4 kpc away from the Earth.
The spatial number density of 900-1050 K dwarf is $(2.0\pm0.9) \times10^{-6}\text{ pc}^{-3}$, 1050-1200 K dwarf is $(1.2\pm0.7) \times10^{-6}\text{ pc}^{-3}$, and 1200-1350 K dwarf is $(4.4\pm1.3) \times10^{-6}\text{ pc}^{-3}$.
The cumulative number count of our brown dwarf candidates is consistent with the prediction from a standard double exponential model. Three of our brown dwarf candidates were detected by HST, with transverse velocities $12\pm5$ km s$^{-1}$, $12\pm4$ km s$^{-1}$, and $17\pm6$ km s$^{-1}$.
Along with earlier studies, the JWST has opened a new window of brown dwarf research in the Milky Way thick disk and halo.