The GALAH and TESS-HERMES surveys: high-resolution spectroscopy of luminous supergiants in the Magellanic Clouds and Bridge

TL;DR

This study reports high-resolution spectroscopic observations of luminous supergiants in the Magellanic Clouds, demonstrating that certain spectral features can effectively discriminate these stars despite current parameter estimation inaccuracies.

Contribution

It shows that The Cannon can identify key spectral features of luminous supergiants outside its training set, paving the way for improved future parameter determinations.

Findings

$v\sin i$ and certain abundances are effective discriminants

Spectroscopic parameters are currently inaccurate for these stars

Future expanded training sets could improve parameter accuracy

Abstract

We report the serendipitous observations of 571 luminous supergiants in the Magellanic Clouds by the spectroscopic GALAH and TESS-HERMES surveys: 434 stars in the Large Magellanic Cloud and 137 in the Small Magellanic Cloud. We also find one star that appears associated with structured star formation in the Magellanic Bridge. Both of these surveys are aimed at the local volume of the Galaxy but have simple, magnitude-limited selection functions that mean they include some observations of luminous extra-Galactic stars. The surveys determine stellar parameter and abundances using The Cannon, a data-driven generative modelling approach. In this work, we explore the results from The Cannon when it is fed the spectra of these intrinsically luminous supergiants in the Magellanic Clouds, which are well outside the normal bounds of The Cannon's training set. We find that, although the parameters are astrophysically incorrect, the $v\sin i$ and the abundances of lithium, barium, and magnesium are excellent discriminants of these stars. It shows that in the future, with an expanded training set, it should be possible to determine accurate values for these types of stars.