The Gaia-ESO Survey: A new diagnostic for accretion and outflow activity in the young cluster NGC 2264

Abstract

Context. NGC 2264 is a young cluster whose accretion properties can be investigated in detail by taking advantage of the FLAMES data in the context of the Gaia-ESO Survey. In fact, the analysis of the Hα emission line profile can provide us with information about the accretion and ejection activity of young stars. However, a strong nebular emission that contributes to the Hα emission can alter the profiles, with consequences for their physical interpretation. Aims. Our study is aimed at investigating the accretion and ejection properties of NGC 2264 by applying a proper treatment of the sky contribution to the Hα and forbidden emission lines (FELs; [SII] and [NII] doublets). Methods. We developed a tool, the OHαNA-method, to handle the strong nebular contribution and spectra with spurious profiles of the Hα and FELs, namely altered Hα profiles or absorption features artificially created where emission lines (FELs) are expected. We derived the quantitative measurements of relevant parameters to describe the accretion and ejection processes in young members of NGC 2264, focusing on reliable quantities derived from the width of the lines, which is relatively unaffected by the nebular emission, unlike the intensity peak, which can be altered significantly. Results. We derive the quantitative measurements related to the Hα emission line and discuss the comparison between the original and sky-subtracted spectra. We thus reveal possible profile alterations with consequences for their physical interpretation. Furthermore, we show the analysis of the variability for multi-epoch observations, also deriving the velocity of the infalling and outflowing plasma from the wings of the broad Hα emission line (in accreting stars). We also explore the mass accretion rate versus full width at zero intensity of the Hα line, namely Ṁ versus FWZI(Hα), a correlation based on the width of the emission line, which is expected to be more robust with respect to any measurement derived from the peak (e.g., Hα10%) and possibly altered by the nebular contribution. Conclusions. We are able to ascertain that more than 20% of the confirmed accretors, which have already been identified in NGC 2264, are affected by the alteration of their line profiles due to the contribution of the nebular emission. Therefore, this is an important issue to consider when investigating accretion and ejection processes in young stellar clusters. While a small fraction of spectra can be unequivocally classified as either unaffected by nebular emission or dominated by nebular emission, the majority (> 90%) represent intermediate cases whose spectral features have to be investigated in detail to derive reliable measurements of the relevant parameters and their physical implications.

PENELLOPE: The ESO data legacy program to complement the Hubble UV Legacy Library of Young Stars (ULLYSES)

“…G.3-G.13. Some of this variability in the line profile is similar to what has been observed by other authors (e.g., Jayawardhana et al 2006;Biazzo et al 2012;Fang et al 2013;Sousa et al 2016;Bonito et al 2020). A detailed analysis of the kinematic variations traced by the shape of the emission lines, as well as by the profiles of the forbidden lines, will be subject of future works.…”

The Gaia-ESO survey: Constraining evolutionary models and ages for young low mass stars with measurements of lithium depletion and rotation

“…The older ages inferred from magnetic models have obvious implications for the empirical estimation of timescales associated with the evolution of PMS stars and their circumstellar environments, which are usually calibrated using groups of stars with assigned isochronal ages. These include the timescales associated with accretion processes for the youngest clusters (Bonito et al 2020), the lifetimes of protoplanetary disks (Carpenter et al 2006;Ribas et al 2015;Li & Xiao 2016) dissipation rates of debris disks (Binks & Jeffries 2017;Silverberg et al 2020) and the angular momentum evolution for Solar-type and low-mass stars in clusters (Gallet & Bouvier 2013;Amard et al 2019). The clusters studied in this work are at the forefront of near-future exoplanet surveys (Kastner et al 2019), allowing astronomers to empirically test models of planetary formation and evolution in the first ∼ 100 Myr.…”

Mapping the Galactic Metallicity Gradient with Open Clusters: The State-of-the-Art and Future Challenges

“…It provides stellar parameters, metallicity, and elemental abundances, radial velocities, and additional products, such as gravity index, chromospheric activity tracers, mass accretion rate diagnostics, and veiling. Several works in recent years have been devoted to the study of open clusters observed by the GES, among many we recall the latest ones: Bertelli Motta et al [76], Magrini et al [77,78,79], Prisinzano et al [80], Hatzidimitriou et al [81], Casali et al [82,83], Baratella et al [84], Randich et al [85], Jackson et al [86], Bonito et al [87], Gutiérrez Albarrán et al [88], Semenova et al [89], Binks et al [90]. In the following sections, we adopt the average abundances from IDR6 for the member stars of 57 clusters with ages ≥ 120 Myr published in Magrini et al [79].…”