Analysing the structure of the bulge with Mira variables

Abstract

Context. The Galactic bulge at latitude 4 < |b|(deg) < 10 was claimed to show an X-shape, which means that stellar density distributions along the line of sight have a double peak. However, this double peak is only observed with the red-clump population, and doubt has been cast on its use as a perfect standard candle. As such, a boxy bulge without an X-shape is not discarded. Aims. We aim to constrain the shape of the bulge making use of a different population: Mira variables from the new Optical Gravitational Lensing Experiment data release, OGLE-IV, with an average age of 9 Gyr. Methods. We analysed an area of the bulge far from the plane, where we fitted the density of the Miras with boxy bulge and X-shaped bulge models and calculated the probability of each model. Results. We find that the probability of a boxy bulge fitting the data is p = 0.19, whereas the probability for the X-shaped bulge is only p = 2.85 × 10−6 (equivalent to a tension of the model with the data of a 4.7σ level). Therefore, the boxy bulge model seems to be more appropriate for describing the Galactic bulge, although we cannot exclude any model with complete certainty.

A Three-dimensional Map of the Milky Way Using 66,000 Mira Variable Stars

“…They also used a simplified extinction and transformation of the I-band to the Kband mean magnitude and did not account for distance uncertainties, which are significant as we showed in this paper. A similar analysis has been carried out by Chrobáková et al (2022) based on the recently published Mira sample in Iwanek et al (2022). This paper (Chrobáková et al 2022) ignores distance uncertainties and uses simplified extinction and transformation of the I-band to the K-band mean magnitude.…”

“…A similar analysis has been carried out by Chrobáková et al (2022) based on the recently published Mira sample in Iwanek et al (2022). This paper (Chrobáková et al 2022) ignores distance uncertainties and uses simplified extinction and transformation of the I-band to the K-band mean magnitude. The authors came to a similar conclusion as López-Corredoira (2017), indicating that the boxy-bulge model is more favorable.…”