Search

Identifying and characterizing young stars in the Solar neighbourhood is essential to find and describe planets in the early stages of their evolution. This work seeks to identify nearby young stars showing a lithium 6708 Å absorption line in the GALAH survey. A robust, data-driven approach is used to search for corresponding templates in the pool of 434 215 measured dwarf spectra in the survey. It enables a model-free search for best-matching spectral templates for all stars, including M dwarfs with strong molecular absorption bands. 3147 stars have been found to have measurable lithium: 1408 G and 892 K0–K5 dwarfs (EW(Li) > 0.1 Å), 335 K5–K9 (>0.07 Å) and 512 M0–M4 dwarfs (>0.05 Å). Stars with such lithium features are used to investigate the possibility of searching for young stars above the main sequence based merely on their parallaxes and broad-band photometry. The selection of young stars above the main sequence is highly effective for M dwarfs, moderately effective for K dwarfs and ineffective for G dwarfs. Using a combination of the lithium information and the complete 6D kinematics from Gaia and GALAH, 305 new candidate moving group members have been found, 123 of which belong to the Scorpius–Centaurus association, 36 to the Pleiades and 25 to the Hyades clusters. [ABSTRACT FROM AUTHOR]

RAVE, the unbiased magnitude limited survey of southern sky stars, contained 456,676 medium-resolution spectra at the time of our analysis. Spectra cover the Ca II infrared triplet (IRT) range, which is a known indicator of chromospheric activity. Our previous work classified all spectra using locally linear embedding. It identified 53,347 cases with a suggested emission component in calcium lines. Here, we use a spectral subtraction technique to measure the properties of this emission. Synthetic templates are replaced by the observed spectra of non-active stars to bypass the difficult computations of non-local thermal equilibrium profiles of the line cores and stellar parameter dependence. We derive both the equivalent width of the excess emission for each calcium line on a 5 Å wide interval and their sum EWIRT for ∼44,000 candidate active dwarf stars with signal-to-noise ratio >20, with no cuts on the basis of the source of their emission flux. From these, ∼14,000 show a detectable chromospheric flux with at least a 2σ confidence level. Our set of active stars vastly enlarges previously known samples. Atmospheric parameters and, in some cases, radial velocities of active stars derived from automatic pipelines suffer from systematic shifts due to their shallower calcium lines. We re-estimate the effective temperature, metallicity, and radial velocities for candidate active stars. The overall distribution of activity levels shows a bimodal shape, with the first peak coinciding with non-active stars and the second with the pre-main-sequence cases. The catalog will be made publicly available with the next RAVE public data releases. [ABSTRACT FROM AUTHOR]

We present a qualitative characterization of activity levels of a large database of ~44,000 candidate RAVE stars (unbiased, magnitude limited medium resolution survey) that show chromospheric emission in the Ca II infrared triplet and this vastly enlarges previously known samples. Our main motivation to study these stars is the anti-correlation of chromospheric activity and stellar ages that could be calibrated using stellar clusters with known ages. Locally linear embedding used for a morphological classification of spectra revealed 53,347 cases with a suggested emission component in the calcium lines. We analyzed a subsample of ~44,000 stars with S/N>20 using a spectral subtraction technique where observed reference spectra of inactive stars were used as templates instead of synthetic ones. Both the equivalent width of the excess emission for each calcium line and their sum is derived for all candidate active stars with no respect to the origin of their emission flux. ~17,800 spectra show a detectable chromospheric flux with at least 2 σ confidence level. The overall distribution of activity levels shows a bimodal shape, with the first peak coinciding with inactive stars and the second with the pre-main-sequence cases. [ABSTRACT FROM PUBLISHER]