How Magnetic Activity Alters What We Learn from Stellar Spectra

Abstract

Magnetic fields and stellar spots can alter the equivalent widths of absorption lines in stellar spectra, varying during the activity cycle. This also influences the information that we derive through spectroscopic analysis. In this study, we analyze high-resolution spectra of 211 sunlike stars observed at different phases of their activity cycles, in order to investigate how stellar activity affects the spectroscopic determination of stellar parameters and chemical abundances. We observe that the equivalent widths of lines can increase as a function of the activity index log R during the stellar cycle, which also produces an artificial growth of the stellar microturbulence and a decrease in effective temperature and metallicity. This effect is visible for stars with activity indexes log R (i.e., younger than 4–5 Gyr), and it is more significant at higher activity levels. These results have fundamental implications on several topics in astrophysics that are discussed in the paper, including stellar nucleosynthesis, chemical tagging, the study of Galactic chemical evolution, chemically anomalous stars, the structure of the Milky Way disk, stellar formation rates, photoevaporation of circumstellar disks, and planet hunting.

The GALAH survey: Chemical homogeneity of the Orion complex

“…Some trends might be even more pronounced, as we are dealing with PMS stars, which might not have model spectra calculated as carefully and rigorously as main sequence stars. Chromospheric activity (Carter 1989;Yana Galarza et al 2019) and strong magnetic fields (Basri et al 1992;Johns-Krull et al 1999;Spina et al 2020) are known to influence PMS stars significantly. (iv) Biased photometric temperature and gravity could have an effect as well, although it must be minor, as this is the only effect we thoroughly analysed.…”

The GAPS Programme at TNG

“…Moreover, these authors demonstrate that such variations of EWs depend on the optical depth of line formation and, marginally, on the Landè g L factor, which measures the sensitivity of a spectral line to magnetic fields. Spina et al (2020) conducted the same study as Yana Galarza et al (2019) to a sample of 211 solar-analogue stars observed with High Accuracy Radial velocity Planet Searcher (HARPS) and find similar results. Yana Galarza et al (2019), Baratella et al (2020), and Spina et al (2020) demonstrate how iron lines forming in the upper layers of the photosphere of young stars can yield higher abundances due to the possible influence of the more intense chromospheric or photospheric magnetic fields.…”

“…Even though we are aware of the issues related to the spectroscopic analysis of young and cool stars and we are starting to shed light on the topic, we still lack a definitive solution. However, we can overcome these problems with strategic choices of the line list to use in the analysis, for example with a refined selection based on the EWs (Spina et al 2020) or using new approaches (Baratella et al 2020).…”

The GALAH+ survey: Third data release

“…Souto et al 2018;Gao et al 2018;Bertelli Motta et al 2018;Souto et al 2019;Liu et al 2019;Semenova et al 2020) or stellar activity (e.g. Spina et al 2020b). Furthermore, astrophysical abundance trends, like anti-correlations of light elements (see Bastian & Lardo 2018, and references therein), have also been found in globular clusters and are partially hard to disentangle from other abundance trends, e.g.…”

“…We stress that our stellar parameters for the youngest stars (below 0.5 Gyr) are likely unreliable. This is caused by our analysis setup with an isochrone grid selection favouring older stars, tying mic to an empirical relation and estimating stellar parameters mainly from iron lines (Baratella et al 2020), but also neglecting stellar rotation, possible stellar activity and magnetic fields (Spina et al 2020b) which can alter the shape of stellar lines quite drastically.…”