S. L. Casewell

New Praesepe white dwarfs and the initial mass–final mass relation

We report the spectroscopic confirmation of four further whi te dwarf members of Praesepe. This brings the total number of confirmed white dwarf me mbers to eleven making this the second largest collection of these objects in an open clu ster identified to date. This number is consistent with the high mass end of the initial mass function of Praesepe being Salpeter in form. Furthermore, it suggests that the bulk of Praesepe white dwarfs did not gain a substantial recoil kick velocity from possible asymmetries in thei r loss of mass during the asymptotic giant branch phase of evolution. By comparing our estimates of the effective temperatures and the surface gravities of WD0833+194, WD0840+190, WD0840+205 and WD0843+184 to modern theoretical evolutionary tracks we have derived their masses to be in the range 0.72 0.76M⊙ and their cooling ages �300Myrs. For an assumed cluster age of 625±50Myrs the infered progenitor masses are between 3.3 3.5M⊙. Examining these new data in the context of the initial mass-final mass relation we find that it can be adequately represented by a linear function (a0=0.289±0.051, a1=0.133±0.015) over the initial mass range 2.7M⊙ to 6M⊙. Assuming an extrapolation of this relation to larger initi al masses is valid and adopting a maximum white dwarf mass of 1.3M⊙, our results support a minimum mass for core-collapse supernovae progenitors in the range �6.8-8.6M⊙.

Fifteen new T dwarfs discovered in the UKIDSS Large Area Survey

We present the discovery of fifteen new T2.5-T7.5 dwarfs (with estimated distances between �24–93pc), identified in the first three main data releases of the UKIRT Infrared Deep Sky Survey. This brings the total number of T dwarfs discovered in the Large Area Survey (to date) to 28. These discoveries are confirmed by near infrared spectroscopy, from which we derive spectral types on the unified scheme of Burgasser et al. (2006). Seven of the new T dwarfs have spectral types of T2.5-T4.5, five have spectral types of T5-T5.5, one is a T6.5p, and two are T7-7.5. We assess spectral morphology and colours to identify T dwarfs in our sample that may have non-typical physical properties (by comparison to solar neighbourhood populations), and find that one of these new T dwarfs may be metal poor, three may have low surface gravity, and one may have high surface gravity. The colours of the full sample of LAS T dwarfs show a possible trend to bluer Y J with decreasing effective temperature, and some interesting colour changes in J H and z J (deserving further investigation) beyond T8. The LAS T dwarf sample from the first and second main data releases show good evidence for a consistent level of completion to J=19. By accounting for the main sources of incompleteness (selection, follow-up and spatial) as well as the effects of unresolved binarity and Malmquist bias, we estimate that there are 17±4 >T4 dwarfs in the J 619 volume of the LAS second data release. Comparing this to theoretical predictions is most consistent with a sub-stellar mass function exponent α between -1.0 and 0. This is consistent with the latest 2MASS/SDSS constraint (which is based on lower number statistics), and is significantly lower than the α � 1.0 suggested by L dwarf field populations, possibly a result of the lower mass range probed by the T dwarf class.

Eight new T4.5–T7.5 dwarfs discovered in the UKIDSS Large Area Survey Data Release 1

The definitive version is available at www.blackwell-synergy.com Copyright Blackwell Publishing DOI : 10.1111/j.1365-2966.2007.12023.x

High-resolution optical spectroscopy of Praesepe white dwarfs

We present the results of a high-resolution optical spectroscopic study of nine white dwarf candidate members of Praesepe undertaken with the VLT and Ultraviolet and Visual Echelle Spectrograph. We find, contrary to a number of previous studies, that WD0836+201 (LB390, EG59) and WD0837+199 (LB393, EG61) are magnetic and non-magnetic white dwarfs, respectively. Subsequently, we determine the radial velocities for the eight non-magnetic degenerates and provide compelling evidence that WD0837+185 is a radial velocity variable and possibly a double-degenerate system. We also find that our result for WD0837+218, in conjunction with its projected spatial location and position in initial mass–final mass space, argues it is more likely to be a field star than a cluster member. After eliminating these two white dwarfs, and WD0836+199 which has no clean Sloan Digital Sky Survey photometry, we use the remaining five stars to substantiate modern theoretical mass–radius relations for white dwarfs. In light of our new results, we re-examine the white dwarf members of Praesepe and use them to further constrain the initial mass–final mass relation (IFMR). We find a near-monotonic IFMR, which can still be adequately represented by simple linear function with only one outlier which may have formed from a blue straggler star.

Proper motion L and T dwarf candidate members of the Pleiades

We present the results of a deep optical–near-infrared (optical–NIR) multi-epoch survey covering 2.5 deg 2 of the Pleiades open star cluster to search for new very low-mass brown dwarf members. A significant (∼5 yr) epoch difference exists between the optical (CFH12k I -, Z -band) and NIR (UKIRT WFCAM J -band) observations. We construct I , I − Z and Z , Z − J colour–magnitude diagrams to select candidate cluster members. Proper motions are computed for all candidate members and compared to the background field objects to further refine the sample. We recover all known cluster members within the area of our survey. In addition, we have discovered nine new candidate brown dwarf cluster members. The seven faintest candidates have red Z − J colours and show blue NIR colours. These are consistent with being L- and T-type Pleiads. Theoretical models predict their masses to be around 11M Jup .

Origin of Electron Cyclotron Maser Induced Radio Emissions at Ultracool Dwarfs: Magnetosphere-Ionosphere Coupling Currents

A number of ultracool dwarfs emit circularly polarized radio waves generated by the electron cyclotron maser instability. In the solar system such radio is emitted from regions of strong auroral magnetic-field-aligned currents. We thus apply ideas developed for Jupiters magnetosphere, being a well-studied rotationally dominated analog in our solar system, to the case of fast-rotating UCDs. We explain the properties of the radio emission from UCDs by showing that it would arise from the electric currents resulting from an angular velocity shear in the fast-rotating magnetic field and plasma, i.e., by an extremely powerful analog of the process that causes Jupiters auroras. Such a velocity gradient indicates that these bodies interact significantly with their space environment, resulting in intense auroral emissions. These results strongly suggest that auroras occur on bodies outside our solar system.

A new detailed examination of white dwarfs in NGC 3532 and NGC 2287

We present the results of a photometric and spectroscopic study of the white dwarf candidate members of the intermediate age open clusters NGC 3532 and NGC 2287. Of the nine objects investigated, it is determined that six are probable members of the clusters, four in NGC 3532 and two in NGC 2287. For these six white dwarfs, we use our estimates of their cooling times together with the cluster ages to constrain the lifetimes and masses of their progenitor stars. We examine the location of these objects in initial mass–final mass space and find that they now provide no evidence for substantial scatter in initial mass–final mass relation (IFMR) as suggested by previous investigations. Instead, we demonstrate that, when combined with current data from other solar metallicity open clusters and the Sirius binary system, they hint at an IFMR that is steeper in the initial mass range 3 M⊙ ≲ M init ≲ 4 M⊙ than at progenitor masses immediately lower and higher than this. This form is generally consistent with the predictions of stellar evolutionary models and can aid population synthesis models in reproducing the relatively sharp drop observed at the high mass end of the main peak in the mass distribution of white dwarfs.

The nature of the close magnetic white dwarf + probable brown dwarf binary SDSS J121209.31+013627.7★

Submitted for publication in Monthly Notices of the Royal Astronomical Society by the Royal Astronomical Society and Blackwell Publishing.

GRB 060206 and the quandary of achromatic breaks in afterglow light curves

Gamma-ray burst afterglow observations in the Swift era have a perceived lack of achromatic jet breaks compared with the BeppoSAX era. We present our multi-wavelength analysis of GRB 060206 as an illustrative example of how inferences of jet breaks from optical and X-ray data might differ. The results of temporal and spectral analyses are compared, and attempts are made to fit the data within the context of the standard blast wave model. We find that while the break appears more pronounced in the optical and evidence for it from the X-ray alone is weak, the data are actually consistent with an achromatic break at about 16 h. This break and the light curves fit standard blast wave models, either as a jet break or as an injection break. As the pre-Swift sample of afterglows are dominated by optical observations, and in the Swift era most well-sampled light curves are in the X-ray, caution is needed when making a direct comparison between the two samples, and when making definite statements on the absence of achromatic breaks.

Two T dwarfs from the UKIDSS early data release

Context: We report on the first ultracool dwarf discoveries from the UKIRT Infrared Deep Sky Survey (UKIDSS) Large Area Survey Early Data Release (LAS EDR), in particular the discovery of T dwarfs which are fainter and more distant than those found using the 2MASS and SDSS surveys. Aims: We aim to show that our methodologies for searching the ~27 deg 2 of the LAS EDR are successful for finding both L and T dwarfs via cross-correlation with the Sloan Digital Sky Survey (SDSS) DR4 release. While the area searched so far is small, the numbers of objects found shows great promise for near-future releases of the LAS and great potential for finding large numbers of such dwarfs. Methods: Ultracool dwarfs are selected by combinations of their YJH ( K )