A. C. Katsiyannis

Brown dwarfs and low-mass stars in the Pleiades and Praesepe: membership and binarity

We present near-infrared J-, H- and K-band photometry and optical spectroscopy of low-mass star and brown dwarf (BD) candidates in the Pleiades and Praesepe open clusters. We flag non-members from their position in K, I−K and J, J−K colour–magnitude diagrams (CMDs), and J−H, H−K two-colour diagrams. In general, the dust-free NextGen model isochrones of the Lyon Group fit the K, I−K CMDs well for stars with I−K∼ 1.5–3.5. However, Pleiades stars with K≃ 10.5–13 (MK≃ 5–7.5) are rather redder than the isochrones. We also identify this effect amongst αPer sources from the literature, but find no evidence of it for field stars from the literature. The NextGen isochrones fit the J, J−K CMDs of both clusters very well in this photometric range. It is possible that the I−K colour of youthful stars is affected by the presence of magnetic activity. The Lyon Groups Dusty isochrones fit both K, I−K and K, J−K Pleiades CMDs well for I−K≃ 4.3–6/J−K≃ 1.1–1.4. In between these colour ranges the Pleiades cluster sequence comprises three portions. Starting at the bluer side, there is a gap where very few sources are found (the gap size is ΔI∼ 0.5, ΔJ∼ΔK∼ 0.3), probably resulting from a sharp local drop in the magnitude–mass relation. Then the sequence is quite flat from I−K∼ 3.5–4. Finally, the sequence turns over and drops down to join the Dusty isochrone. We also compare model atmosphere colours to the two-colour diagrams of the clusters. The NextGen models are seen to be ∼0.1 too blue in H−K and ∼0.1 too red in J−H for Teff > 4000 K. However, they are in reasonable agreement with the data at Teff∼ 3200 K. For Teff∼ 2800–3150 K, the colours of Pleiades and Praesepe sources are significantly different, where Praesepe sources are ∼0.1 bluer in J−H and up to ∼0.1 redder in H−K. These differences could result from gravity-sensitive molecular opacities. Cooler Praesepe sources then agree well with the dusty models, suggesting that dust is beginning to form in Praesepe sources around 2500 K. However, Pleiades sources remain consistent with the NextGen models (and inconsistent with the dusty models) down to Teff values of ∼2000 K. It is possible that dust formation does not begin until lower Teff values in sources with lower surface gravities (and hence lower atmospheric pressures). We also identify unresolved binaries in both clusters, and estimate mass ratios (q) for Pleiades BDs. Most of these have q > 0.7, however, 3/18 appear to have lower q values. We determine the binary fraction (BF) for numerous mass ranges in each cluster, and find that it is generally rising towards lower masses. We find a BD BF of 50+11−10 per cent. We also find some evidence suggesting that the BF–q distribution is flat for 0.5–0.35 M⊙, in contrast to solar-type stars.

Discovery of the benchmark metal‐poor T8 dwarf BD +01° 2920B

We have searched the WISE first data release for widely separated (610,000AU) late T dwarf companions to Hipparcos and Gliese stars. We have discovered a new binary system containing a K-band suppressed T8p dwarf WISEP J1423+0116 and the mildly metal poor ([Fe/H]= 0.38±0.06) primary BD+01 2920 (Hip 70319), a G1 dwarf at a distance of 17.2pc. This new benchmark has Teff=680±55K and a mass of 20 50MJup. Its spectral properties are well modelled except for known discrepancies in the Y and K bands. Based on the well determined metallicity of its companion, the properties of BD+01 2920B imply that the currently known T dwarfs are dominated by young lowmass objects. We also present an accurate proper motion for the T8.5 dwarf WISEP J075003.84+272544.8.

Extreme-Ultraviolet Emission Lines of S X in Solar Flare and Active Region Spectra

R-matrix calculations of electron impact excitation rates in N-like S X are used to derive theoretical emission-line intensity ratios involving 2s22p3-2s2p4 transitions in the 189-265 A wavelength range. A comparison of these with observational data for solar flares and active regions, obtained with the Naval Research Laboratorys S082A spectrograph on board Skylab and the Solar EUV Rocket Telescope and Spectrograph, reveals that many of the S X lines in the spectra are badly blended with emission features from other species. However, the intensity ratios I(228.70 A)/I(264.24 A) and I(228.70 A)/I(259.49 A) are found to provide useful electron density diagnostics for flares, although the latter cannot be employed for active regions, because of blending of the 259.49 A line with an unidentified transition in these solar features.