Leicester University Physics Department

Current Research

Crossing into the substellar regime in Praesepe

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

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

Crossing into the substellar regime in Praesepe

We conducted a deep optical 2.6 deg 2 survey with near infrared 9NIR) follow-up measurements of the intermediate-aged Praesepe open cluster. The survey is complete to I c = 21.3, Z = 20.5, corresponding to M ~ 0.06 M solar assuming a cluster age of 0.5 Gyr. Using three to five passbands to constrain cluster membership, we identify 32 new low-mass cluster members, at least four of which are likely to be substellar. We used the low-mass census to trace the region where the sequence moves away from the NEXTGEN towards the DUSTY regime at T eff = 2200 K. In doing so, we identified four unresolved binaries, yielding a substellar binary fraction of ~ 30 %. The binary fractions appear to decrease below 0.1 M solar , in contrast to the rising fractions found around the Pleiades. A lack of late M dwarfs, thought to be due to a steepening in the mass-luminosity relation at these spectral types, was also observed. We compared the properties of this gap in the sequence to those observed in younger clusters. There is an over-density of faint sources in the region of the so-called subcluster (possibly an older, smaller cluster within Praesepe0, and we subsequently derived the luminosity and mass functions for the main Prasepe cluster. This revealed a turnover near the substellar boundary. We concluded that Rpr1 can be ruled out as a substellar member, and included in out paper astronomic measurements for low-mass Praesepe candidates.

Sky plot showing the positions of single candidates fainter than K = 15.0. Crosses, squares and asterisks represent single candidates in the magnitude bins K = 15-15.8, 15.8-16.3 and 16.3-17.0, respectively. Annuli of radius 0.5°, 1.0°, 1.5° and 2.0° from the centre of the cluster are also shown as dotted lines. The position of the excluded subcluster is represented by the solid line.

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.

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.