The unusual photometric variability of the PMS star GM Cep

TL;DR

This study presents six years of UBVRI photometric data on the PMS star GM Cep, revealing large amplitude variability, deep minima, and color reversal phenomena, with spectral analysis indicating active accretion and T Tauri characteristics.

Contribution

It provides long-term photometric and spectral observations of GM Cep, highlighting its variability patterns and accretion properties, contributing to understanding PMS star behaviors.

Findings

Large amplitude variability (~2.3 mag) observed over six years.

Color reversal during brightness minima typical of UX Ori variables.

Spectral analysis shows active accretion with a broad H-alpha emission line.

Abstract

Results from UBVRI photometric observations of the pre-main sequence star GM Cep obtained in the period April 2011 - August 2014 are reported in the paper. Presented data are a continuation of our photometric monitoring of the star started in 2008. GM Cep is located in the field of the young open cluster Trumpler 37 and over the past years it has been an object of intense photometric and spectral studies. The star shows a strong photometric variability interpreted as a possible outburst from EXor type in previous studies. Our photometric data for a period of over six years show a large amplitude variability (Delta V ~ 2.3 mag.) and several deep minimums in brightness are observed. The analysis of the collected multicolor photometric data shows the typical of UX Ori variables a color reversal during the minimums in brightness. The observed decreases in brightness have a different shape, and evidences of periodicity are not detected. At the same time, high amplitude rapid variations in brightness typical for the classical T Tauri stars also present on the light curve of GM Cep. The spectrum of GM Cep shows the typical of classical T Tauri stars wide H/alpha emission line and absorption lines of some metals. We calculate the outer radius of the H/alpha emitting region as 10.4 +/-0.5 Rsun and the accretion rate as 1.8 x 10 E-7 Msun/yr.