Observational indicators of the transition from fully convective stars to stars with radiative cores

TL;DR

This paper investigates the transition from fully convective stars to stars with radiative cores, identifying observational indicators such as spectral type, X-ray luminosity, and rotation rate changes, with implications for stellar evolution understanding.

Contribution

It introduces new observational markers for the convective to radiative core transition and analyzes their persistence across different stellar ages.

Findings

A predicted dearth of G-type stars due to transition characteristics.

A photometric gap centered on radiative core size peak at certain masses.

Persistent changes in X-ray luminosity and magnetic fields beyond 20 Myrs.

Abstract

We present a discussion of the similarities and key differences between the transition onto (at the turn-on) and away from (at the turn-off) the main sequence, the latter termed the Hertzsprung gap. Using a set of model isochrones and adopting an initial mass function leads us to predict a dearth of G-type stars for any star forming region. This is caused by the (relatively) constant spectral type at which the transition from a fully convective star to a star with a radiative core begins. We also present analysis of the details of this transition in the ONC. In particular we show that a gap in the photometric and spectral type distributions is centred on, and a change in the fractional X-ray luminosity and rotation rate distribution occurs approximately at, the position of a peak in radiative core size as a function of mass. Whilst photometric signatures of this transition are lost at ages over ~20 Myrs, we show that changes in fractional X-ray luminosity and magnetic field configuration persist to older ages. Analysis of literature data show that the mass at which the change in fractional X-ray luminosity is observed decreases with age.