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

Abstract

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 Group's 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.

Stellar Multiplicity

“…Extrapolating the observed frequency of visual companions assuming an overall Gaussian orbital period distribution, Gizis & Reid (1995) suggested that CF open cluster M⋆ 0.1 M ⊙ = 27 ± 16%. Photometric studies, which may be affected by a mass segregation selection effect, broadly support these results despite some significant scatter (Boudreault et al 2012, Converse & Stahler 2008, Lodieu et al 2007, Pinfield et al 2003.…”

The Hawaii Infrared Parallax Program. Ii. Young Ultracool Field Dwarfs* †

“…And thus the bifurcation of our int-g late-M sample may reflect the conflation of a younger population (e.g., 2MASSI J0019+4614 is a member of the ≈125 Myr AB Dor group) and an older one. An additional consideration is that the M7-M8 range is where Dobbie et al (2002) have suggested the mass-luminosity relation changes due to the onset of dust formation, as inferred from the luminosity function of young clusters (see also Pinfield et al 2003;Chappelle et al 2005). So the int-g bifurcation may also reflect dust opacity variations.…”

Spitzer discovery of very low luminosity objects

“…There is no proof that these BDs were ejected from forming protostellar systems. The large BD binary fraction of 15% (Martín et al 2003, Pinfield et al 2003) is furthermore hardly explained by ejection scenarios (Whitworth & Goodwin 2005). It thus appears plausible that at least some of the single brown dwarfs may have formed by processes similar to those by which normal stars form, i.e.…”