Brad M. S. Hansen

THE PULSAR KICK VELOCITY DISTRIBUTION

We analyse the sample of pulsar proper motions, taking detailed account of the selection effects of the original surveys. We treat censored data using survival statistics. From a comparison of our results with Monte Carlo simulations, we find that the mean birth speed of a pulsar is 250-300 km/s, rather than the 450 km/s foundby Lyne & Lorimer (1994). The resultant distribution is consistent with a maxwellian with dispersion

The Initial-Final Mass Relation: Direct Constraints at the Low-Mass End* **

\sigma_v = 190 km/s

The Chemical Composition of an Extrasolar Minor Planet

. Despite the large birth velocities, we find that the pulsars with long characteristic ages show the asymmetric drift, indicating that they are dynamically old. These pulsars may result from the low velocity tail of the younger population, although modified by their origin in binaries and by evolution in the galactic potential.

The Phase-Dependent Infrared Brightness of the Extrasolar Planet ʊ Andromedae b

The initial-final mass relation represents a mapping between the mass of a white dwarf remnant and the mass that the hydrogen-burning main-sequence star that created it once had. The empirical relation thus far has been constrained using a sample of ~40 stars in young open clusters, ranging in initial mass from ~2.75 to 7 -->M?, and shows a general trend that connects higher mass main-sequence stars with higher mass white dwarfs. In this paper, we present CFHT CFH12K photometric and Keck LRIS multiobject spectroscopic observations of a sample of 22 white dwarfs in two older open clusters, NGC 7789 ( -->t = 1.4 Gyr) and NGC 6819 ( -->t = 2.5 Gyr). We measure masses for the highest signal-to-noise ratio spectra by fitting the Balmer lines to atmosphere models and place the first direct constraints on the low-mass end of the initial-final mass relation. Our results indicate that the observed general trend at higher masses continues down to low masses, with -->Minitial = 1.6 M? main-sequence stars forming -->Mfinal = 0.54 M? white dwarfs. When added to our new data from the very old cluster NGC 6791, the relation is extended down to -->Minitial = 1.16 M? (corresponding to -->Mfinal = 0.53 M?). This extension of the relation represents a fourfold increase in the total number of hydrogen-burning stars for which the integrated mass loss can now be calculated from empirical data, assuming a Salpeter initial mass function. The new leverage at the low-mass end is used to derive a purely empirical initial-final mass relation. The sample of white dwarfs in these clusters also shows several interesting systems that we discuss further: a DB (helium) white dwarf, a magnetic white dwarf, a DAB (mixed hydrogen/helium atmosphere or a double degenerate DA+DB) white dwarf(s), and two possible equal-mass DA double degenerate binary systems.

Cooling models for old white dwarfs

We report the relative abundances of 17 elements in the atmosphere of the white dwarf star GD 362, material that, very probably, was contained previously in a large asteroid or asteroids with composition similar to the Earth-Moon system. The asteroid may have once been part of a larger parent body not unlike one of the terrestrial planets of our solar system.

The White Dwarf Cooling Sequence of the Globular Cluster Messier 4

The star υ Andromedae is orbited by three known planets, the innermost of which has an orbital period of 4.617 days and a mass at least 0.69 that of Jupiter. This planet is close enough to its host star that the radiation it absorbs overwhelms its internal heat losses. Here, we present the 24-micrometer light curve of this system, obtained with the Spitzer Space Telescope. It shows a variation in phase with the orbital motion of the innermost planet, demonstrating that such planets possess distinct hot substellar (day) and cold antistellar (night) faces.

A Young White Dwarf Companion to Pulsar B1620-26: Evidence for Early Planet Formation

We present new white dwarf cooling models that incorporate an accurate outer boundary condition based on new opacity and detailed radiative transfer calculations. We —nd that helium-atmosphere dwarfs cool considerably faster than has previously been claimed, while old hydrogen-atmosphere dwarfs will deviate signi—cantly from blackbody appearance. We use our new models to derive age limits for the Galactic disk. We —nd that the Liebert, Dahn, & Monet luminosity function yields an age of only 6 Gyr if it is complete to stated limits. However, age estimates of individual dwarfs and the luminosity function of Oswalt et al. are both consistent with disk ages as large as D11 Gyr. We have also used our models to place constraints on white dwarf dark matter in the Galactic halo. We —nd that previous attempts using inadequate cooling models were too severe and that direct detection limits allow a halo that is 11 Gyr old. If the halo is composed solely of helium-atmosphere dwarfs, the lower age limit is only 7.5 Gyr. We also demonstrate the importance of studying the cooling sequences of white dwarfs in globular clusters. Subject headings: Galaxy: fundamental parametersGalaxy: halosolar neighborhoodstars: evolutionstars: fundamental parameters

TESTING IN SITU ASSEMBLY WITH THE KEPLER PLANET CANDIDATE SAMPLE

We present the white dwarf sequence of the globular cluster M4, based on a 123 orbit Hubble Space Telescope exposure, with a limiting magnitude of V ~ 30 and I ~ 28. The white dwarf luminosity function rises sharply for I > 25.5, consistent with the behavior expected for a burst population. The white dwarfs of M4 extend to approximately 2.5 mag fainter than the peak of the local Galactic disk white dwarf luminosity function. This demonstrates a clear and significant age difference between the Galactic disk and the halo globular cluster M4. Using the same standard white dwarf models to fit each luminosity function yields ages of 7.3 ± 1.5 Gyr for the disk and 12.7 ± 0.7 Gyr for M4 (2 σ statistical errors).

Two Classes of Hot Jupiters

The pulsar B1620-26 has two companions, one of stellar mass and one of planetary mass. We detected the stellar companion with the use of Hubble Space Telescope observations. The color and magnitude of the stellar companion indicate that it is an undermassive white dwarf (0.34 ± 0.04 solar mass) of age 480 × 106 ± 140 × 106 years. This places a constraint on the recent history of this triple system and supports a scenario in which the current configuration arose through a dynamical exchange interaction in the cluster core. This implies that planets may be relatively common in low-metallicity globular clusters and that planet formation is more widespread and has happened earlier than previously believed.

Stellar Evolution in NGC 6791: Mass Loss on the Red Giant Branch and the Formation of Low-Mass White Dwarfs* **

We present a Monte Carlo model for the structure of low-mass (total mass <25 M ?) planetary systems that form by the in situ gravitational assembly of planetary embryos into final planets. Our model includes distributions of mass, eccentricity, inclination, and period spacing that are based on the simulation of a disk of 20 M ?, forming planets around a solar-mass star, and assuming a power-law surface density distribution that drops with distance a as a ?1.5. The output of the Monte Carlo model is then subjected to the selection effects that mimic the observations of a transiting planet search such as that performed by the Kepler satellite. The resulting comparison of the output to the properties of the observed sample yields an encouraging agreement in terms of the relative frequencies of multiple-planet systems and the distribution of the mutual inclinations when moderate tidal circularization is taken into account. The broad features of the period distribution and radius distribution can also be matched within this framework, although the model underpredicts the distribution of small period ratios. This likely indicates that some dissipation is still required in the formation process. The most striking deviation between the model and observations is in the ratio of single to multiple systems in that there are roughly 50% more single-planet candidates observed than are produced in any model population. This suggests that some systems must suffer additional attrition to reduce the number of planets or increase the range of inclinations.