The chromospheric emission of solar-type stars in the young open clusters IC 2391 and IC 2602

TL;DR

This study measures chromospheric activity in young solar-type stars in clusters IC 2391 and IC 2602, revealing saturation in emission levels at high rotation rates and challenging the link between coronal supersaturation and magnetic dynamo efficiency.

Contribution

It provides the first detailed chromospheric emission analysis of stars in these young clusters, showing saturation without supersaturation in ultra-rapid rotators.

Findings

Chromospheric emission levels plateau at high rotation rates.

No evidence of supersaturation in chromospheric emission for ultra-rapid rotators.

Chromospheric saturation occurs independently of coronal supersaturation.

Abstract

In this paper we present chromospheric emission levels of the solar-type stars in the young open clusters IC 2391 and IC 2602. High resolution spectroscopic data were obtained for over 50 F, G, and K stars from these clusters over several observing campaigns using the University College London Echelle Spectrograph on the 3.9-m Anglo-Australian Telescope. Unlike older clusters, the majority (28/52) of the solar-type stars in the two clusters are rapid-rotators (vsini > 20 km/s) with five of the stars being classified as ultra-rapid rotators (vsini > 100 km/s). The emission levels in the Calcium infrared triplet lines were then used as a measure of the chromospheric activity of the stars. When plotted against Rossby number (NR) the star's chromospheric emission levels show a plateau in the emission for Log(NR) < -1.1 indicating chromospheric saturation similar to the coronal saturation seen in previously observed X-ray emission from the same stars. However, unlike the coronal emission, the chromospheric emission of the stars show little evidence of a reduction in emission (i.e. supersaturation) for the ultra-rapid rotators in the clusters. Thus we believe that coronal supersaturation is not the result of an overall decrease in magnetic dynamo efficiency for ultra-rapid rotators.