The Missing M Dwarfs

We present evidence that indicates the luminosity functions of star forming regions, open star clusters and the field dip between spectral types M7-M8. We believe that this is a result of dust formation in the atmospheres of objects in this effective temperature regime, causing a sharp local drop in the luminosity-mass relation. Current low mass stellar and substellar evolutionary models do not predict this, and if our interpretation is correct this result has important implications for investigations concerned with the mass functions of star-forming regions and young open star clusters. The hypothesized onset of dust formation may be modelled by assuming larger grain sizes than are currently used in the DUSTY calculations.

This deficit of very low mass stars suggests that brown dwarfs with spectral types later than M7-M8 may have higher masses than previously thought. The width of the luminosity function dip is observed to be greater in the younger s – and q –Orionis clusters than in either the Pleiades or the field populations, which is consistent with the interpretation that the luminosity mass relation changes sharply between spectral types M7-M8.

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 (M K ~= 5-7.5) are rather redder than the isochrones. We also identify this effect amongst alphaPer 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 DeltaI~ 0.5, DeltaJ~DeltaK~ 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 T eff > 4000 K. However, they are in reasonable agreement with the data at T eff ~ 3200 K. For T eff ~ 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 T eff values of ~2000 K. It is possible that dust formation does not begin until lower T eff 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, but 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 solar , in contrast to solar-type stars.

Figure 1. The two Pleiades BD candidate spectra (normalized to unity at 7500 Angstroms). The noisier spectra of BPL 45 has been binned by 2 pixel in this plot and offset for clarity.

A near-IR spectroscopic search for very-low-mass cool companions to notable DA white dwarfs.

There are many known examples of white dwarfs in binary systems with early-mid M dwarf companions, but few with late-M and cooler companions. We have undertaken a detailed near-IR spectroscopic analysis of eight notable white dwarfs, predominantly of southern declination. In each case the spectrum failed to reveal compelling evidence for the presence of a spatially unresolved, cool, late-type companion. Therefore, we have placed an approximate limit on the spectral-type of a putative companion to each degenerate. From these limits we conclude that if GD659, GD50, GD71 or WD2359-434 possesses an unresolved companion then most probably it is substellar in nature (less than 0.72 solar masses). Also, any late type-companion to RE J0457-280, RE J0623-374, RE J0723-274 or RE J2214-491 most likely has a mass less than 0.82 solar masses. If the unusual photospheric composition observed in these white dwarfs is due to weak accretion from a nearby late-type companion, the companion is very low-mass and cannot be detected by this study. These results do not contradict the previously noted deficit of very-low-mass stellar and brown dwarf companions to main sequence, F, G, K and early M-type primaries.