Optical spectroscopy is used to confirm membership for eight low-mass candidates in the young Beta Pic moving group (BPMG) via their radial velocities, chromospheric activity and kinematic ...parallaxes. We searched for the presence of the Li i 6708 Å resonance feature and combined the results with literature measurements of other BPMG members to find the age-dependent lithium depletion boundary (LDB) - the luminosity at which Li remains unburned in a coeval group. The LDB age of the BPMG is 21 ± 4 Myr and insensitive to the choice of low-mass evolutionary models. This age is more precise, likely to be more accurate, and much older than that commonly assumed for the BPMG. As a result, substellar and planetary companions of BPMG members will be more massive than previously thought.
Optical spectroscopic observations are reported for 24 and 23, nearby, proper-motion-selected M-dwarf candidate members of the Beta Pictoris and AB Doradus moving groups (BPMG and ABDMG). Using ...kinematic criteria, the presence of both Hα emission and high X-ray-to-bolometric luminosity, and position in absolute colour–magnitude diagrams, 10 and 6 of these candidates are confirmed as likely members of the BPMG and ABDMG, respectively. Equivalent widths or upper limits for the Li i 6708 Å line are reported and the lithium depletion boundary (LDB) age of the BPMG is revisited. Whilst non-magnetic evolutionary models still yield an estimated age of 21 ± 4 Myr, models that incorporate magnetic inhibition of convection imply an older age of 24 ± 4 Myr. A similar systematic increase would be inferred if the stars were 25 per cent covered by dark magnetic starspots. Since young, convective M-dwarfs are magnetically active and do have starspots, we suggest that the original LDB age estimate is a lower limit. The LDB age of the ABDMG is still poorly constrained – non-magnetic evolutionary models suggest an age in the range 35–150 Myr, which could be significantly tightened by new measurements for existing candidate members.
Abstract
We present a search for debris discs amongst M-dwarf members of nearby, young (5–150 Myr) moving groups (MGs) using infrared (IR) photometry, primarily from the Wide Infrared Survey Explorer ...(WISE). A catalogue of 100 MG M dwarfs that have suitable WISE data is compiled, and 19 of these are found to have significant IR excess emission at 22 μm. Our search is likely to be complete for discs where the ratio of flux from the disc to flux from the star fd/f* > 10−3. The spectral energy distributions are supplemented with Two-Micron All-Sky Survey (2MASS) photometry and data at longer wavelengths, and fitted with simple disc models to characterize the IR excesses. There is a bimodal distribution – 12 targets have W1 − W4 > 3, corresponding to fd/f* > 0.02, and are likely to be gas-rich, primordial discs. The remaining seven targets have W1 − W4 < 1 (fd/f* ≲ 10−3) and include three objects with previously known or suspected debris discs and four new debris disc candidates that are all members of the Beta Pic MG. All of the IR excesses are identified in stars that are likely members of MGs with age <30 Myr. The detected debris disc frequency falls from 13 ± 5 per cent to <7 per cent (at 95 per cent confidence) for objects younger or older than 30 Myr, respectively. This provides evidence for the evolution of debris discs on this time-scale and does not support models where the maximum of debris disc emission occurs much later in lower mass stars.
We present a kinematically-unbiased search to identify young, nearby low-mass members of kinematic moving groups (MGs). Objects with both rotation periods shorter than 5 d in the SuperWASP All-Sky ...Survey and X-ray counterparts in the ROSAT All-Sky Survey were chosen to create a catalogue of several thousand rapidly rotating, X-ray active FGK stars. These objects are expected to be either young single stars or tidally locked spectroscopic binaries. We obtained optical spectra for a sub-sample of 146 stars to determine their ages and kinematics, and in some cases repeat radial velocity measurements were used to identify binarity. 26 stars are found to have lithium abundances consistent with an age of ≤200 Myr, and show no evidence for binarity and in most cases measurements of H α and v sin i support their youthful status. Based on their youth, their radial velocities and estimates of their three-dimensional kinematics, we find 11 objects that may be members of known MGs, eight that do not appear associated with any young MG and a further seven that are close to the kinematics of the recently proposed ‘Octans-Near’ MG, and which may be the first members of this MG found in the Northern hemisphere. The initial search mechanism was ∼18 per cent efficient at identifying likely-single stars younger than 200 Myr, of which 80 per cent were early-K spectral types.
ABSTRACT
IC 4665 is one of only a dozen young open clusters with a ‘lithium depletion boundary’ (LDB) age. Using an astrometrically and spectroscopically filtered sample of cluster members, we show ...that both the positions of its low-mass stars in Gaia absolute colour–magnitude diagrams and the lithium depletion seen among its K- and early M-stars are discordant with the reported LDB age of $32^{+4}_{-5}$ Myr. Re-analysis of archival spectra suggests that the LDB of IC 4665 has not been detected and that the published LDB age should be interpreted as a lower limit. Empirical comparisons with similar data sets from other young clusters with better-established LDB ages indicate that IC 4665 is bracketed in age by the clusters IC 2602 and IC 2391 at 55 ± 3 Myr.
Abstract We present a software package designed to produce photometric lightcurves and measure rotation periods from full-frame images taken by the Transiting Exoplanet Survey Satellite (TESS), which ...we name “TESSILATOR”. TESSILATOR is the only publicly-available code that will run a full lightcurve and rotation period (Prot) analysis based on just a (list of) target identifier(s) or sky position(s) via a simple command-line prompt. This paper sets out to introduce the rationale for developing TESSILATOR, and then describes the methods, considerations and assumptions for: extracting photometry; dealing with potential contamination; accounting for natural and instrumental systematic effects; lightcurve normalisation and detrending; removing outliers and unreliable data; and finally, measuring the Prot value and several periodogram attributes. Our methods have been tuned specifically to optimise TESS lightcurves and are independent from the pipelines developed by the TESS Science Processing Operations Center, meaning TESSILATOR can, in principle, analyse any target across the entire celestial sphere. We compare TESSILATOR Prot measurements with TESS-SPOC-derived lightcurves of 1,560 (mainly FGKM-type) stars across four benchmark open clusters (Pisces-Eridanus, the Pleiades, the Hyades and Praesepe) and a sample of nearby field M-dwarfs. From a vetted subsample of 864 targets we find an excellent return of Prot matches for the first 3 open clusters (>85 per cent) and a moderate (∼60 per cent) match for the 700 Myr Praesepe and MEarth sample, which validates TESSILATOR as a tool for measuring Prot. The TESSILATOR code is available at https://github.com/alexbinks/tessilator.
ABSTRACT
In the last three decades several hundred nearby members of young stellar moving groups (MGs) have been identified, but there has been less systematic effort to quantify or characterize ...young stars that do not belong to previously identified MGs. Using a kinematically unbiased sample of 225 lithium-rich stars within 100 pc, we find that only 50 ± 10 per cent of young (≲125 Myr), low-mass (0.5 < M/M⊙ < 1.0) stars, are kinematically associated with known MGs. Whilst we find some evidence that five of the non-MG stars may be connected with the Lower Centaurus–Crux association, the rest form a kinematically ‘hotter’ population, much more broadly dispersed in velocity, and with no obvious concentrations in space. The mass distributions of the MG members and non-MG stars are similar, but the non-MG stars may be older on average. We briefly discuss several explanations for the origin of the non-MG population.
ABSTRACT
A growing disquiet has emerged in recent years that standard stellar models are at odds with observations of the colour–magnitude diagrams (CMDs) and lithium depletion patterns of pre-main ...sequence stars in clusters. In this work we select 1246 high probability K/M-type constituent members of five young open clusters (5–125 Myr) in the Gaia-ESO Survey to test a series of models that use standard input physics and others that incorporate surface magnetic fields or cool starspots. We find that: standard models provide systematically under-luminous isochrones for low-mass stars in the CMD and fail to predict Li-depletion of the right strength at the right colour; magnetic models provide better CMD fits with isochrones that are ∼1.5–2 times older, and provide better matches to Li depletion patterns. We investigate how rotation periods, most of which are determined here for the first time from Transiting Exoplanet Survey Satellite data, correlate with CMD position and Li. Among the K-stars in the older clusters we find the brightest and least Li-depleted are the fastest rotators, demonstrating the classic ‘Li-rotation connection’ for the first time at ∼35 Myr in NGC 2547, and finding some evidence that it exists in the early M-stars of NGC 2264 at $\lt 10\,$ Myr. However, the wide dispersion in Li depletion observed in fully convective M-dwarfs in the γ Vel cluster at ∼20 Myr appears not to be correlated with rotation and is challenging to explain without a very large (>10 Myr) age spread.
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