A SEARCH FOR CORONAL ACTIVITY AMONG TWO METAL-POOR SUBDWARFS AND ONE SUBGIANT

Abstract

A search has been made using the XMM-Newton satellite for coronal soft X-ray emission from HD 19445, HD 25329, and HD 140283, three Population II stars in the Galactic halo having metallicities of [Fe/H] ∼ -2. The program stars, consisting of two subdwarfs and one metal-poor subgiant, were pre-selected from ground-based observations to have He i λ10830 absorption lines with an equivalent width (EW) of 30 mÅ or more. If such stars follow a relation between He i EW and soft X-ray flux applicable to Population I dwarf stars, then they would be expected to have X-ray luminosities ∼ 5 x 10⁻⁷ times their bolometric luminosity, and as such would yield detectable sources in 20 ks exposures with the XMM-Newton EPIC-PN and MOS cameras. No detections were found in such exposures made with XMM-Newton. Upper limits to soft X-ray emission from the two program stars that have effective temperatures most similar to that of the Sun, namely HD 19445 and HD 140283, are comparable to the level of the quiet Sun. The star HD 25329, a cooler subdwarf, exhibits an upper limit similar to the Sun at maximum activity. These measurements suggest that coronal activity appears to decrease with age among the oldest G dwarfs, but K-M subdwarfs possibly have maintained a solar-like level of activity.