# The period-luminosity relation of red supergiants with Gaia DR2

## TL;DR

This study refines the period-luminosity relations of Galactic and LMC red supergiants using Gaia DR2 parallaxes and long-term photometry, revealing two main pulsation period groups and confirming consistency across different galaxies.

## Contribution

It provides improved P-L relations for red supergiants with detailed analysis of their pulsation periods using new Gaia data and extensive historical photometry.

## Key findings

- Two main pulsation period groups identified: 300-1000 days and 1000-8000 days.
- Galactic P-L relation aligns with those in the LMC and Andromeda.
- No clear continuity between red giant and red supergiant P-L sequences.

## Abstract

We revisit the K -band period-luminosity (P-L) relations of Galactic red supergiants using Gaia Data Release 2 parallaxes and up to 70 yr of photometry from AAVSO and ASAS campaigns. In addition, we examine 206 LMC red supergiants using 50 yr of photometric data from the Digitised Harvard Astronomical Plate Collection. We identified periods by computing power spectra and calculated the period-luminosity relations of our samples and compared them with the literature. Newly available data tighten the P-L relations substantially. Identified periods form two groups: one with periods of 300-1000 days, corresponding to pulsations, and another with Long Secondary Periods between 1000 and 8000 days. Among the 48 Galactic objects we find shorter periods in 25 stars and long secondary periods in 23 stars. In the LMC sample we identify 85 and 94 red supergiants with shorter and long secondary periods, respectively. The P-L relation of the Galactic red supergiants is in agreement with the red supergiants in both, the Large Magellanic Cloud and the Andromeda Galaxy. We and no clear continuity between the known red giant period-luminosity sequences, and the red supergiant sequences investigated here.