Steinn Sigurdsson

Are There Unstable Planetary Systems around White Dwarfs

The presence of planets around solar-type stars suggests that many white dwarfs should have relic planetary systems. While planets closer than ~5 AU will most likely not survive the post-main-sequence lifetime of their parent star, any planet with semimajor axis greater than 5 AU will survive, and its semimajor axis will increase as the central star loses mass. Since the stability of adjacent orbits to mutual planet-planet perturbations depends on the ratio of the planet mass to the central stars mass, some planets in previously stable orbits around a star undergoing mass loss will become unstable. We show that when mass loss is slow, systems of two planets that are marginally stable can become unstable to close encounters, while for three planets the timescale for close approaches decreases significantly with increasing mass ratio. These processes could explain the presence of anomalous IR excesses around white dwarfs that cannot be explained by close companions, such as G29-38, and may also be an important factor in explaining the existence of DAZ white dwarfs. The onset of instability through changing mass ratios will also be a significant effect for planetary embryos gaining mass in protoplanetary disks.

A Lack of Planets in 47 Tucanae from a Hubble Space Telescope Search

We report results from a large Hubble Space Telescope project to observe a significant (~34,000) ensemble of main-sequence stars in the globular cluster 47 Tucanae with a goal of defining the frequency of inner orbit, gas giant planets. Simulations based on the characteristics of the 8.3 days of time series data in the F555W and F814W Wide Field Planetary Camera 2 (WFPC2) filters show that ~17 planets should be detected by photometric transit signals if the frequency of hot Jupiters found in the solar neighborhood is assumed to hold for 47 Tuc. The experiment provided high-quality data sufficient to detect planets. A full analysis of these WFPC2 data reveals ~75 variables, but no light curves resulted for which a convincing interpretation as a planet could be made. The planet frequency in 47 Tuc is at least an order of magnitude below that for the solar neighborhood. The cause of the absence of close-in planets in 47 Tuc is not yet known; presumably the low metallicity and/or crowding of 47 Tuc interfered with planet formation, with orbital evolution to close-in positions, or with planet survival.

Exotic Earths: forming habitable worlds with giant planet migration.

Close-in giant planets (e.g., “hot Jupiters”) are thought to form far from their host stars and migrate inward, through the terrestrial planet zone, via torques with a massive gaseous disk. Here we simulate terrestrial planet growth during and after giant planet migration. Several-Earth-mass planets also form interior to the migrating jovian planet, analogous to recently discovered “hot Earths.” Very-water-rich, Earth-mass planets form from surviving material outside the giant planets orbit, often in the habitable zone and with low orbital eccentricities. More than a third of the known systems of giant planets may harbor Earth-like planets.

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

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.

The spatial distribution of coalescing neutron star binaries: implications for gamma‐ray bursts

ABSTRA C T We find the distribution of coalescence times, birth rates, spatial velocities, and subsequent radial offsets of coalescing neutron stars (NSs) in various galactic potentials accounting for large asymmetric kicks introduced during a supernova. The birth rates of bound NS‐NS binaries are quite sensitive to the magnitude of the kick velocities but are, nevertheless, similar (,10 per galaxy per Myr) to previous population synthesis studies. The distribution of merger times since zero-age main sequence is, however, relatively insensitive to the choice of kick velocities. With a median merger time of ,10 8 yr, we find that compact binaries should closely trace the star formation rate in the Universe. In a range of plausible galactic potentials (with Mgalaxy * 3 · 10 10 M() the median radial offset of a NS‐NS merger is less than 10 kpc. At a redshift of z a 1 (with H0 a 65 km s ˇ1 Mpc ˇ1 and Q a 0:2), this means that half the coalescences should occur within ,1:3 arcsec from the host galaxy. In all but the most shallow potentials, 90 per cent of NS‐NS binaries merge within 30 kpc of the host. We find that although the spatial distribution of coalescing neutron star binaries is consistent with the close spatial association of known optical afterglows of gamma-ray bursts (GRBs) with faint galaxies, a non-negligible fraction (,15 per cent) of GRBs should occur well outside (*30 kpc) dwarf galaxy hosts. Extinction owing to dust in the host, projection of offsets, and a range in interstellar medium densities confound the true distribution of NS‐NS mergers around galaxies with an observable set of optical transients/galaxy offsets.

Formation of Earth-like Planets During and After Giant Planet Migration

Close-in giant planets are thought to have formed in the cold outer regions of planetary systems and migrated inward, passing through the orbital parameter space occupied by the terrestrial planets in our own solar system. We present dynamical simulations of the effects of a migrating giant planet on a disk of protoplanetary material and the subsequent evolution of the planetary system. We numerically investigate the dynamics of postmigration planetary systems over 200 million years using models with a single migrating giant planet, one migrating and one nonmigrating giant planet, and excluding the effects of a gas disk. Material that is shepherded in front of the migrating giant planet by moving mean motion resonances accretes into hot Earths, but survival of these bodies is strongly dependent on dynamical damping. Furthermore, a significant amount of material scattered outward by the giant planet survives in highly excited orbits; the orbits of these scattered bodies are then damped by gas drag and dynamical friction over the remaining accretion time. In all simulations Earth-mass planets accrete on approximately 100 Myr timescales, often with orbits in the habitable zone. These planets range in mass and water content, with both quantities increasing with the presence of a gas disk and decreasing with the presence of an outer giant planet. We use scaling arguments and previous results to derive a simple recipe that constrains which giant planet systems are able to form and harbor Earth-like planets in the habitable zone, demonstrating that roughly one-third of the known planetary systems are potentially habitable.

Three Modes of Metal-Enriched Star Formation in the Early Universe

Simulations of the formation of Population III (Pop III) stars suggest that they were much more massive than the Pop II and Pop I stars observed today. This is due to the collapse dynamics of metal-free gas, which is regulated by the radiative cooling of molecular hydrogen. We study how the collapse of gas clouds is altered by the addition of metals to the star-forming environment by performing a series of simulations of pre-enriched star formation at various metallicities. To make a clean comparison with metal-free star formation, we use initial conditions identical to a Pop III star formation simulation, with low ionization and no external radiation other than the cosmic microwave background (CMB). For metallicities below the critical metallicity, Z cr, collapse proceeds similar to the metal-free case, and only massive objects form. For metallicities well above Z cr, efficient cooling rapidly lowers the gas temperature to the temperature of the CMB. The gas is unable to radiatively cool below the CMB temperature, and becomes thermally stable. For high metallicities, Z 10?2.5 Z ?, this occurs early in the evolution of the gas cloud, when the density is still relatively low. The resulting cloud cores show little or no fragmentation, and would most likely form massive stars. If the metallicity is not vastly above Z cr, the cloud cools efficiently but does not reach the CMB temperature, and fragmentation into multiple objects occurs. We conclude that there were three distinct modes of star formation at high redshift (z 4): a primordial mode, producing massive stars (10s to 100s of M ?) at very low metallicities (Z 10?3.75 Z ?); a CMB-regulated mode, producing moderate mass (10s of M ?) stars at high metallicities (Z 10?2.5 Z ? at redshift z~ 15-20); and a low-mass (a few M ?) mode existing between these two metallicities. As the universe ages and the CMB temperature decreases, the range of the low-mass mode extends to higher metallicities, eventually becoming the only mode of star formation.

Binary-single star interactions in globular clusters

An extensive series of three-body interactions involving hard binaries and single stars are calculated by direct integration of ∼10 5 encounters. Unlike previous calculations, the stars have different mass ratios, chosen to be representative of the stellar and remnant population in a globular cluster, and most effort was devoted to the difficult case of hard binaries, the most likely primordial binaries to survive in a cluster. Graphs, tables and analytical fits to differential and integral cross sections for properties of different interaction channels are presented and discussed

Investigating stellar-mass black hole kicks

We investigate whether stellar-mass black holes have to receive natal kicks in order to explain the observed distribution of low-mass X-ray binaries containing black holes within our Galaxy. Such binaries are the product of binary evolution, where the massive primary has exploded forming a stellar-mass black hole, probably after a common envelope phase where the system contracted down to separations of the order of 10-30 R-circle dot. We perform population synthesis calculations of these binaries, applying both kicks due to supernova mass-loss and natal kicks due to the newly formed black hole. We then integrate the trajectories of the binary systems within the Galactic potential. We find that natal kicks are in fact necessary to reach the large distances above the Galactic plane achieved by some binaries. Further, we find that the distribution of natal kicks would seem to be similar to that of neutron stars, rather than one where the kick velocities are reduced by the ratio of black hole to neutron star mass (i.e. where the kicks have the same momentum). This result is somewhat surprising; in many pictures of stellar-mass black hole formation, one might have expected black holes to receive kicks having the same momentum (rather than the same speed) as those given to neutron stars. (Less)

Demonstration of on-sky calibration of astronomical spectra using a 25 GHz near-IR laser frequency comb

We describe and characterize a 25 GHz laser frequency comb based on a cavity-filtered erbium fiber mode-locked laser. The comb provides a uniform array of optical frequencies spanning 1450 nm to 1700 nm, and is stabilized by use of a global positioning system referenced atomic clock. This comb was deployed at the 9.2 m Hobby-Eberly telescope at the McDonald Observatory where it was used as a radial velocity calibration source for the fiber-fed Pathfinder near-infrared spectrograph. Stellar targets were observed in three echelle orders over four nights, and radial velocity precision of ∼10 m/s (∼6 MHz) was achieved from the comb-calibrated spectra.