| Summary: | 博士 === 國立中央大學 === 天文研究所 === 107 === 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.
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