X-Shooter spectroscopy of young stellar objects III. Photospheric and chromospheric properties of Class III objects

TL;DR

This study uses X-Shooter spectra to analyze the photospheric and chromospheric properties of Class III young stellar objects, comparing chromospheric and coronal emissions and exploring activity diagnostics across different atmospheric layers.

Contribution

It provides detailed measurements of stellar parameters and activity indicators for Class III objects, and compares chromospheric and coronal emissions, offering new insights into stellar activity in young stars.

Findings

X-ray and Halpha luminosities are independent of effective temperature in early-M stars.

Coronal emission exceeds chromospheric emission by a factor of 2-5 in active pre-main sequence stars.

Flux ratios between emission lines depend smoothly on effective temperature.

Abstract

We analyzed X-Shooter/VLT spectra of 24 ClassIII sources from three nearby star-forming regions (sigmaOrionis, LupusIII, and TWHya). We determined the effective temperature, surface gravity, rotational velocity, and radial velocity by comparing the observed spectra with synthetic BT-Settl model spectra. We investigated in detail the emission lines emerging from the stellar chromospheres and combined these data with archival X-ray data to allow for a comparison between chromospheric and coronal emissions. Both X-ray and Halpha luminosity as measured in terms of the bolometric luminosity are independent of the effective temperature for early-M stars but decline toward the end of the spectral M sequence. For the saturated early-M stars the average emission level is almost one dex higher for X-rays than for Halpha: log(L_x/L_bol) = -2.85 +- 0.36 vs. log(L_Halpha/L_bol) = -3.72 +- 0.21. When all chromospheric emission lines (including the Balmer series up to H11, CaII HK, the CaII infrared triplet, and several HeI lines) are summed up the coronal flux still dominates that of the chromosphere, typically by a factor 2-5. Flux-flux relations between activity diagnostics that probe different atmospheric layers (from the lower chromosphere to the corona) separate our sample of active pre-main sequence stars from the bulk of field M dwarfs studied in the literature. Flux ratios between individual optical emission lines show a smooth dependence on the effective temperature. The Balmer decrements can roughly be reproduced by an NLTE radiative transfer model devised for another young star of similar age. Future, more complete chromospheric model grids can be tested against this data set.