The Origins of the Ultramassive White Dwarf GD50

Original paper can be found at: http://www.astrosociety.org/pubs/cs/361.html Copyright ASPOn the basis of astrometric and spectroscopic data we argue that the ultramassive white dwarf GD 50 is associated with the star formation event that created the Pleiades and is potentially a former cluster member. Its cooling time (∼60Myrs) is consistent with it having evolved essentially as a single star from a M > 6M⊙ progenitor so there appears to be no need to invoke a white dwarf–white dwarf binary merger scenario to account for its existence. Our result may represent the first direct observational evidence that single star evolution can produce white dwarfs with M > 1.1M⊙, as predicted by some stellar evo- lutionary theories. Additionally, our findings may help towards alleviating the difficulties in reconciling the observed number of hot nearby ultramassive white dwarfs with the smaller number predicted by binary evolution models under the assumption that they are the products of white dwarf mergers

The Planetary Nebula Luminosity Function at the Dawn of Gaia

The [O III] 5007 Planetary Nebula Luminosity Function (PNLF) is an excellent extragalactic standard candle. In theory, the PNLF method should not work at all, since the luminosities of the brightest planetary nebulae (PNe) should be highly sensitive to the age of their host stellar population. Yet the method appears robust, as it consistently produces < 10% distances to galaxies of all Hubble types, from the earliest ellipticals to the latest-type spirals and irregulars. It is therefore uniquely suited for cross-checking the results of other techniques and finding small offsets between the Population I and Population II distance ladders. We review the calibration of the method and show that the zero points provided by Cepheids and the Tip of the Red Giant Branch are in excellent agreement. We then compare the results of the PNLF with those from Surface Brightness Fluctuation measurements, and show that, although both techniques agree in a relative sense, the latter method yields distances that are ~15% larger than those from the PNLF. We trace this discrepancy back to the calibration galaxies and argue that, due to a small systematic error associated with internal reddening, the true distance scale likely falls between the extremes of the two methods. We also demonstrate how PNLF measurements in the early-type galaxies that have hosted Type Ia supernovae can help calibrate the SN Ia maximum magnitude-rate of decline relation. Finally, we discuss how the results from space missions such as Kepler and Gaia can help our understanding of the PNLF phenomenon and improve our knowledge of the physics of local planetary nebulae.Comment: 12 pages, invited review at the conference "The Fundamental Cosmic Distance Scale: State of the Art and Gaia Perspective", to appear in Astrophysics and Space Scienc

The missing M dwarfs

Peer reviewe

Crossing into the substellar regime in Praesepe

Peer reviewe

A near-infrared spectroscopic detection of the brown dwarf in the post common envelope binary WD 0137-349

The definitive version is available at www.blackwell-synergy.com Copyright Blackwell Publishing DOI : 10.1111/j.1745-3933.2006.00242.xPeer reviewe

On the origin of the ultramassive white dwarf GD 50

The definitive version is available at www.blackwell-synergy.com Copyright Blackwell Publishing DOI : 10.1111/j.1745-3933.2006.00240.xPeer reviewe

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

The definitive version is available at www.blackwell-synergy.com '.--Copyright Blackwell Publishing DOI : 10.1046/j.1365-8711.2003.06630.xWe 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.Peer reviewe

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

Peer reviewe

Praesepe and the seven white dwarfs

Peer reviewe

Follow-up Observations of SPY White Dwarf + M-Dwarf Binaries

Original paper can be found at: http://www.astrosociety.org/pubs/cs/361.html Copyright ASPWe present the results of follow-up observations of white-dwarf + M-dwarf binaries identified using spectra obtained as part of the SPY survey. Spectra of the Hα region were obtained with the SPIRAL spectrograph on the AAT telescope. Of the eleven stars observed, seven are binaries with periods in the range 2.8 hours to 7.7 days. We also show that one of our targets, WD 0137−349, has a brown dwarf companion