Stellar Activity on the Young Suns of Orion: COUP Observations of K5‐7 Pre–Main‐Sequence Stars

TLDR

The study analyzes 28 solar‑mass COUP sources observed for 13.2 days with Chandra to characterize the magnetic activity of young Sun analogs and understand the impact of solar X‑rays on the protoplanetary disk. The authors measured X‑ray fluxes, plasma temperatures, and flare rates, revealing that the stars spend ~70 % of their time in a steady, magnetically confined state with X‑ray luminosity ~0.03 % of the bolometric output, punctuated by weekly powerful flares (log L_x ≈ 30–32 erg s⁻¹) whose energy distribution follows dN/dE ∝ E⁻¹·⁷. These flares produce ionization rates of 6 × 10⁻⁹ s⁻¹ at 1 AU and energetic particle fluences sufficient to explain spallogenic short‑lived radionuclides observed in meteorites.

Abstract

In January 2003, the Chandra Orion Ultradeep Project (COUP) detected about 1400 young stars during a 13.2 day observation of the Orion Nebula Cluster (ONC). This paper studies a well-defined sample of 28 solar-mass COUP sources to characterize the magnetic activity of analogs of the young Sun and thereby to improve understanding of the effects of solar X-rays on the solar nebula during the era of planet formation. We find that active young Suns spend 70% of their time in a characteristic state with relatively constant flux and magnetically confined plasma with temperatures kT_2 = 2.1 * kT_1. During characteristic periods, the 0.5-8 keV X-ray luminosity is about 0.03% of the bolometric luminosity. One or two powerful flares per week with peak luminosities logL_x ~ 30-32 erg/s are typically superposed on this characteristic emission accompanied by heating of the hot plasma component from ~2.4 keV to ~7 keV at the flare peak. The energy distribution of flares superposed on the characteristic emission level follows the relationship dN/dE ~ E^-1.7. The flare rates are consistent with the production of sufficiently energetic protons to spawn a spallogenic origin of some important short-lived radionuclides found in ancient meteorites. The X-rays can ionize gas in the circumstellar disk at a rate of 6 10^-9 ionizations per second at 1 AU from the central star, orders of magnitude above cosmic ray ionization rates. The estimated energetic particle fluences are sufficient to a account for many isotopic anomalies observed in meteoritic inclusions.