| 摘要: | 我們利用測光與偏振來研究年輕星體的變化。目標涵蓋T Tauri星、Herbig A/B型發射星和UX\,Ori(UXor)型態星。UXor是T Tauri和Herbig A/B型發射星的一種次型態分類，這類恆星偶爾顯現出環星塵埃的遮擋現象。我們主要研究的目標GM Cephei屬於這類型態變星，位在年齡約4百萬年的Trumpler 37疏散星團內，距離約850秒差距，恆星展現顯著的紅外超量，發射線光譜還有太陽閃焰的活動。我們從2008年至2019年利用多波段光度監測偵測到： (1)一個大約3.43天左右的週期變化，這個週期變化可能來自於表面星斑的旋轉； (2)因為星球吸積而造成偶爾亮度變亮的現象； (3)因環星塵埃消光而造成不規律的小程度亮度減弱事件； (4)大程度的亮度減弱事件，每次影響時間長達數個月，大約每兩年會重複發生，但並沒有特定週期。這個恆星因大尺寸塵埃而經歷了一般的星際紅化，亦即亮度變暗而變紅，但在亮度最暗時顯示了一個不尋常的藍化效應，也就是亮度變暗而變藍。對於短時間長度的變暗事件(小於50天)，其最大消光程度正比於
事件的時間長度，此效應是來自於不同大小塵埃團的消光所造成。對於長時間長度的變暗事件(大於100天)，其最大消光大約保持在變暗1.5個星等左右，與時間長度沒有特定關聯，顯現長時間長度事件是因為鍊狀結構或螺旋狀結構的塵埃團橫越星體所造成。而對恆星的偏極化監測發現其在可見光偏振程度約有3至8百分比，而偏振程度與恆星亮度有一個緩慢的反向相關的變化。藉由不同時間觀測到的恆星亮度與偏極化程度，我們將之拆解成偏極化光流量與非偏極化光流量，使我們可以對那些與年輕恆星或散射包層有交互作用的環星物質的尺寸與距離加以限制。那些造成掩星的塵埃團塊以鍊狀、環狀或螺旋狀的形式存在於環星盤上，近期已在年輕星球內利用紅外散射光的影像或者是次毫米望波段對帶有熱源塵埃的觀測偵測到此類結構。
此外，我們執行了一個小規模的偏極化巡天，總共觀測35個T Tauri星和15個Herbig A/B型發射星。以我們TRIPOL儀器的優勢，我們有能力去研究年輕星體的偏極化變化以及波長的的相關性。我們發現有一半的目標星體展現了偏極化強度小於1.5百分比，而恆星數量隨著偏極化程度上升而遞減。一般而言，相較於T Tauri星，Herbig A/B型發射星傾向顯示較高的偏極化程度，因為它們擁有較多數量塵埃在它們的環星盤上，而有些Herbig A/B型發射星周圍還留有恆星形成後殘存的雲氣。在我們的樣本裡，大約有60百分比的T Tauri星與80百分比的Herbig A/B型發射星顯示有偏極化強度或角度的變化而偏極化變化的來源可能是因為 (1)在環星盤最內側的結構發生變化； (2)吸積的活動像是星球熱斑； (3)來自於環星物質的消光效應。UXor是顯現環星物質消光效應的主要型態，其展示了偏極化強度與亮度的反向相關的關係。我們觀測了五個UXors (UX Ori、 RR Tau、 BM And、 BF Ori和RZ Psc)發現了同樣的反向相關的特性。但我們也發現到某些資料點稍微小於模型的預測，認為其可能有著與GM Cep有著相同的機制，亦即除了恆星光其散射光也被環星物質所遮擋而減少。另外，我們利用TRIPOL觀測了一個延展的目標R Mon。因為雲氣散射來自恆星的光而呈現中心對稱的偏極化向量結構也被我們再次確認。而我們的多次觀測發現在3年內其向量強度發生變化，有可能來自於近距離的物質遮擋而
造成了遠處雲氣接收到不同強度的恆星光所造成。;We used photometry and polarimetry to study the young stellar variability. The targets include T Tauri stars, Herbig Ae/Be stars, and UX Ori type stars. UX Orionis stars, or UXors, are a sub-type of Herbig Ae/Be or T Tauri stars exhibiting sporadic occultation of stellar light by circumstellar dust. Our main target, GM Cephei is such a UXor in the young (~4Myr) open cluster Trumpler 37 at a distance of ~850pc, showing prominent infrared excess, emission-line spectra, and flare activity. Our multiband photometric monitoring from 2008 to 2019 detects (1) a ~3.43 day period, likely arising from rotational modulation by surface starspots, (2)sporadic brightening on time scales of days due to young stellar accretion, (3)irregular minor flux drops due to circumstellar dust extinction, and (4)major flux drops, each lasting for a couple of months, with a recurrence time, but not exactly periodic, of about two years. The star experiences normal reddening by large grains, i.e., redder when dimmer, but exhibits an unusual "blueing" phenomenon in that the star turns blue near brightness minima. The maximum extinction during relatively short (lasting less than about 50 day) events, is proportional to the duration, a consequence of varying clump sizes. For longer events, the extinction is independent of duration, suggestive of a transverse string distribution of clumps. Polarization monitoring indicates an optical polarization varying ~3-8 percent, with the level anticorrelated with the slow brightness change. Temporal variation of the unpolarized and polarized light, after decomposition of the total flux, sets constraints on the size and orbital distance of the circumstellar clumps in the interplay with the young star and scattering envelope. These transiting clumps are edge-on manifestations of the ring- or spiral-like structures found recently in young stars with imaging in infrared of scattered light, or in submillimeter of thermalized dust emission.
In addition, we carried out a small scale of polarmetric survey to 35 T Tauri and 15 Herbig Ae/Be stars. With the advantage of our TRIPOL instrument, we are able to study the polarimetric variability and wavelength dependence of these YSOs. We found that half of our targets present P<1.5%, and ratio becomes lower for larger P. In general, HAEBE stars tend to show higher polarization than T Tauri stars because they contain abundant in dust in the circumstellar disk, and some of them also contain the nebular cloud near the star. In our sample, up to 60% of T Tauri and ~80% of HAEBE stars show polarimetric variability either in P or angle, where the polarimetric variability probably arise from the (1)structure changes at innermost region; (2)accreting activity like hot spots; and (3)extinction events from circumstellar cloud. UXor is the main type to show extinction events by its circumstellar clouds and display the anti-correlation between polarization and brightness. We observed 5 UXors (UX Ori, RR Tau, BM And, BF Ori, and RZ Psc) and found the similar conclusion with anti-correlation. But we found that in some case we detected a slightly lower polarization than the prediction of model, suggesting that these stars may also have similar mechanism as GM Cep with reducing the polarized component during the occultation events as well. In addition, we observed a extending source, R Mon, by using TRIPOL. The centrosymmetric pattern of polarization vectors due to single scattering from the central star has been revealed again by this work. Our multiepoch observation detected a pattern changes within 3 years interval, possibly induced by the
occultation events at inner part so that the reflecting cloud at far side receive different intensities of directly stellar light. |
