New Blue and Red Variable Stars in NGC 247

TL;DR

This study identifies 15 new variable stars in NGC 247 using Gemini GMOS and archival images, revealing diverse types including Cepheids, semi-regular, LBVs, and binary candidates, and analyzes their variability patterns.

Contribution

It reports the discovery and classification of new variable stars in NGC 247, including the first identification of LBV-like variables and their placement on the CMD, expanding understanding of stellar variability in this galaxy.

Findings

15 new variable stars identified in NGC 247.

LBV candidates form a distinct sequence on the CMD.

The frequency of LBV-like variability is approximately 0.2 over seven months.

Abstract

Images recorded with the Gemini Multi-Object Spectrograph (GMOS) on Gemini South are combined with archival images from other facilities to search for variable stars in the southern portion of the nearby disk galaxy NGC 247. Fifteen new periodic and non-periodic variables are identified. These include three Cepheids with periods < 25 days, four semi-regular variables, one of which shows light variations similar to those of R CrB stars, five variables with intrinsic visible/red brightnesses and colors that are similar to those of luminous blue variables (LBVs), and three fainter blue variables, one of which may be a non-eclipsing close binary system. The S Doradus instability strip defines the upper envelope of a distinct sequence of objects on the (i', g'-i') color-magnitude diagram (CMD) of NGC 247. The frequency of variability with an amplitude > 0.1 magnitude in the part of the CMD that contains LBVs over the seven month period when the GMOS images were recorded is ~ 0.2. The light curve of the B[e] supergiant J004702.18--204739.9, which is among the brightest stars in NGC 247, is also examined. Low amplitude variations on day-to-day timescales are found, coupled with a systematic trend in mean brightness over a six month time interval.