| 摘要: | 年輕的 T型棕矮星預測僅有數個木星質量,發現這些年輕棕矮星對於了解行星質量 天體的 起源與早期演化具有關鍵意義,但目前尚未在恆星形成區中發現可靠的 T型 棕矮星。本研究的主要目的,就是搜尋並確認這些年輕 T型棕矮星的存在。 根據已 知的棕矮星,我們建立了一組利用顏色篩選 T型棕矮星的經驗方法,可以有效 的辨 識 T型棕矮星的甲烷吸收與低溫特徵。 在蛇夫座恆星形成區中,這個方法選出了 28顆年輕 T型棕矮星的候選星。 在沒有假設任何理論模型的條件下,這些由顏色挑 選的候選星大致符合 COND或 BT-Settl 大氣模型所預測的等年齡線。不過,受限 於觀測資料的波長範圍與信噪比,背景的 AGNs、 QSOs、與背景星汙染無法完全 從候選星中排除。 後續的光譜觀測中,六顆候選星取得了近紅外光譜,其中兩顆有 明顯的甲烷吸收特徵。我們因此確認這兩顆候選星為 T型棕矮星。這是目前在恆星 形成區中觀察到光譜型最冷的天體,依照理論模型的預測,這兩個 T型棕矮星 約在 1到 2個木星質量之間,也是目前已知質量最低的年輕天體。;We present a list of T dwarf candidates in the dark cloud L1688 in the ρOph star-forming region. These candidates are selected with infrared colors sensitive to T dwarf characteristics of methane absorptions and of cool atmospheres. The 1.6 μm methane feature is diagnosed by on-off imaging using an H-band and an intermediate-band methane filter, calibrated to a set of known brown dwarfs of M, L, and T types in the field. Another methane feature at 3.3 μm is traced with the Spitzer/IRAC [3.6] − [4.5] color. For cool atmospheres, the H − [4.5], and K − [4.5] colors are utilized. With an additional criterion of mid-infrared brightness to elim- inate extragalactic interlopers, a total of 28 T dwarf candidates have been identi- fied. A comprehensive assessment was conducted to estimate the level of contam- ination of our sample by young stellar variability, by extragalactic sources sharing the same color behavior, or by foreground T dwarfs. Though extragalactic sources may contribute up to about half of the false positives, our candidates show close spatial association with the dark cloud, rather than randomly distributed as a back- ground population would have been. Furthermore, even though our candidates are not selected a priori by a color-magnitude relation, they mostly follow the 1 Myr isochrones, ascertaining their youth.
In the Gemini South/FLAMINGOS-2 follow-up spectroscopic observations, we found evidential 1.6 μm methane absorption in the spectra of two T dwarf candidates, Oph- T3 and Oph-T17. The spectral morphologies of Oph-T3 were similar with T0/T1 standards, consisting with the classification based on spectral indices. Oph-T17 is similar with T3/T4 standards in H band but L8 to T1 in K band. The discovery validate our method of candidate selection. According to theoritical models, the masses of the two T dwarfs would be arount 1 to 2 Jupiter mass with temperatures around 1000 to 700K. We have been granted observing time on the VLT/SINFONI for multi-object spectroscopy. In addition to confirm more T dwarf candidates, the other goal is to understand the nature of their close companions. Once confirmed, these objects will be the lowest mass binaries and be the benchmark systems on the field of planetary mass objects. |
