Letter to the Editor

Analysing the structure of the bulge with Mira variables

¹ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
e-mail: zofiachrobakova@gmail.com
² Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain

Received: 25 August 2022
Accepted: 23 September 2022

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.