# Diagnosing the Clumpy Protoplanetary Disk of the UXor Type Young Star GM Cephei

## TL;DR

This study presents a detailed 10-year photometric and polarimetric analysis of the UXor star GM Cephei, revealing complex circumstellar dust clump dynamics, unusual blueing during brightness minima, and implications for disk structures in young stellar objects.

## Contribution

It provides new insights into the size, distribution, and behavior of circumstellar dust clumps around GM Cephei, linking observational phenomena to disk structures like rings or spirals.

## Key findings

- Detection of a ~3.43-day rotational modulation.
- Observation of irregular flux drops due to dust extinction.
- Identification of blueing phenomenon near brightness minima.

## Abstract

UX Orionis stars (UXors) are Herbig Ae/Be or T Tauri stars exhibiting sporadic occultation of stellar light by circumstellar dust. GM\,Cephei is such a UXor in the young ($\sim4$~Myr) open cluster Trumpler\,37, showing prominent infrared excess, emission-line spectra, and flare activity. Our photometric monitoring (2008--2018) detects (1)~an $\sim$3.43~day period, likely arising from rotational modulation by surface starspots, (2)~sporadic brightening on time scales of days due to 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, though 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 $\leq 50$~days) 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 $\sim3\%$--8$\%$, with the level anticorrelated with the slow brightness change. Temporal variation of the unpolarized and polarized light 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.