Jack G. Hills

Observation of contemporaneous optical radiation from a gamma-ray burst

The origin of γ-ray bursts (GRBs) has been enigmatic since their discovery. The situation improved dramatically in 1997, when the rapid availability of precise coordinates, for the bursts allowed the detection of faint optical and radio afterglows — optical spectra thus obtained have demonstrated conclusively that the bursts occur at cosmological distances. But, despite efforts by several groups, optical detection has not hitherto been achieved during the brief duration of a burst. Here we report the detection of bright optical emission from GRB990123 while the burst was still in progress. Our observations begin 22 seconds after the onset of the burst and show an increase in brightness by a factor of 14 during the first 25 seconds; the brightness then declines by a factor of 100, at which point (700 seconds after the burst onset) it falls below our detection threshold. The redshift of this burst, z ≈ 1.6 (refs 8, 9), implies a peak optical luminosity of 5× 1049 erg s−1. Optical emission from γ-ray bursts has been generally thought to take place at the shock fronts generated by interaction of the primary energy source with the surrounding medium, where the γ-rays might also be produced. The lack of a significant change in the γ-ray light curve when the optical emission develops suggests that the γ-rays are not produced at the shock front, but closer to the site of the original explosion.

Rotse all sky surveys for variable stars I: test fields

The Robotic Optical Transient Search Experiment I (ROTSE-I) experiment has generated CCD photometry for the entire northern sky in two epochs nightly since 1998 March. These sky patrol data are a powerful resource for studies of astrophysical transients. As a demonstration project, we present first results of a search for periodic variable stars derived from ROTSE-I observations. Variable identification, period determination, and type classification are conducted via automatic algorithms. In a set of nine ROTSE-I sky patrol fields covering roughly 2000 deg2, we identify 1781 periodic variable stars with mean magnitudes between mv = 10.0 and mv = 15.5. About 90% of these objects are newly identified as variable. Examples of many familiar types are presented. All classifications for this study have been manually confirmed. The selection criteria for this analysis have been conservatively defined and are known to be biased against some variable classes. This preliminary study includes only 5.6% of the total ROTSE-I sky coverage, suggesting that the full ROTSE-I variable catalog will include more than 32,000 periodic variable stars.

Three-dimensional hydrodynamical simulations of stellar collisions. I. Equal-mass main-sequence stars

Two distinct mass-loss mechanisms are noted in the present, fully three-dimensional calculations of collisions between identical stars. While strong shocks in nearly head-on collisions lead to high-velocity jets perpendicular to the collision axis, with increasing mass loss as impact velocity at infinity increases from zero to 2.3 times the escape velocity from the stellar surface, low velocity encounters lead to a sharp increase in mass loss at impact parameters that correspond to nearly-grazing collisions in a two-stage process. In the first stage, the two stars become gravitationally bound due to the encounters energy dissipation; these binary components then violently coalesce during subsequent periastron passage. 24 references.

Tsunami Produced by the Impacts of Small Asteroids

ABSTRACT: The fragmentation of a small asteroid in the atmosphere greatly increases its cross section for aerodynamic braking, so ground impact damage (craters, earthquakes, and tsunami) from a stone asteroid is nearly negligible if it is less than 200 meters in diameter. A larger one impacts the ground at nearly its velocity at the top of the atmosphere producing considerable impact damage. The protection offered by Earths atmosphere is insidious in that smaller, more frequent impactors such as Tunguska only produce air blast damage and leave no long‐term scars on the Earths surface, while objects 2.5 times larger than it, which hit every few thousand years, cause coherent destruction over many thousands of kilometers of coast. Smaller impactors give no qualitative warning of the enormous destruction wrought when an asteroid larger than the threshold diameter of 200 meters hits an ocean. A water wave generated by an impactor has a long range because it is two‐dimensional, so its height falls off inversely with distance from the impact. When the wave strikes a continental shelf, its speed decreases and its height increases to produce tsunamis. The average runup in height between a deep‐water wave and its tsunami is more than an order of magnitude. Tsunamis produce most of the damage from asteroids with diameters between 200 meters and 1 km. An impact anywhere in the Atlantic by an asteroid 400 meters in diameter would devastate the coasts on both sides of the ocean by tsunami over 100 meters high. An asteroid 5 km in diameter hitting in mid Atlantic would produce tsunami that would inundate the entire upper East Coast of the United States to the Appalachian Mountains. Studies of ocean sediments may be used to determine when coastal areas have been hit by tsunamis in the past. Tsunami debris has been found to be associated with the Cretaceous‐Tertiary impact and should be detectable for smaller impacts.

Prompt Optical Observations of Gamma-Ray Bursts

The Robotic Optical Transient Search Experiment (ROTSE) seeks to measure simultaneous and early afterglow optical emission from gamma-ray bursts (GRBs). A search for optical counterparts to six GRBs with localization errors of 1 deg2 or better produced no detections. The earliest limiting sensitivity is mROTSE>13.1 at 10.85 s (5 s exposure) after the gamma-ray rise, and the best limit is mROTSE>16.0 at 62 minutes (897 s exposure). These are the most stringent limits obtained for the GRB optical counterpart brightness in the first hour after the burst. Consideration of the gamma-ray fluence and peak flux for these bursts and for GRB 990123 indicates that there is not a strong positive correlation between optical flux and gamma-ray emission.

Damage from the impacts of small asteroids

Previous work is extended by using a model spherical atmosphere with a fitted density profile to find the damage done by an asteroid entering it at various zenith angles. At zenith angle 0° and a typical impact: velocity at the top of the atmosphere of V = 17.5 km s−1, the atmosphere absorbs more than half the kinetic energy of stony meteoroids with diameters, DM<230 m and iron meteoroids with DM<50 m. At zenith angle 45° the corresponding figures are 360 and 70 m while at 60° they are 500 and 100 m. For comets with V = 50 km s−1 the values are DM<1900 and 3000 m for 45 and 60°, respectively, using typical values of ablation, but they are much smaller if ablation is reduced. Only impactors with DM above these critical values are effective in producing ground impact damage: craters, earthquakes, and tsunami. Smaller impactors can still produce atmospheric blast waves. It is found that the area of destruction around the impact point in which the overpressure in the blast wave exceeds 4p.s.i. = 2.8 × 105 dyn cm−2, which is enough to knock over trees and destroy buildings. It is found that for chondritic asteroids entering at zenith angle 45° and an impact velocity at the top of the atmosphere of 17.5 km s−1 that it increases rapidly from zero for those less than 50 m in diameter (13.5 megatons) to about 2000 km2 for those 76 m in diameter (31 megatons). If we assume that a stony asteroid 100 m in diameter hits land about every 1000 years, we find that a 50 m diameter one (causing some blast damage) hits land every 125 years while a Tunguska size impactor occurs about every 400 years. If iron asteroids are about 3.5 per cent of the frequency of stony ones of the same size, they constitute most of the impactors that produce areas of blast damage of less than 300 km2. While the optical flux from a small asteroid such as Tunguska is enough to ignite pine forests, the blast from it goes beyond the radius within which the fire starts. The blast tends to blow out the fire, so it is likely that the impact will char the forest (as at Tunguska), but it will not produce a sustained fire.

A new way to make Thorne-Zytkow objects

We have found a new way to make Thorne-Zytkow objects, which are massive stars with degenerate neutron cores. The asymmetric kick given to the neutron star formed when the primary of a massive tight binary system explodes as a supernova sometimes has the appropriate direction and amplitude to place the newly formed neutron star into a bound orbit with a pericenter distance smaller than the radius of the secondary. Consequently, the neutron star becomes embedded in the secondary. Thorne-Zytkow objects are expected to look like extreme M-type supergiants, assuming that they can avoid a runaway neutrino instability. Accretion onto the embedded neutron star will produce either an isolated, spun-up neutron star (possibly a short-period pulsar) or a black hole. Whether neutron star or black hole remnants predominate depends on the lifetime of Thorne-Zytkow objects, the accretion rates involved, and the maximum neutron star mass, none of which are definitively understood.

Damage from comet-asteroid impacts with earth

Abstract Only a small fraction of the Earth-crossing asteroids, ECAs, have been found and cataloged. Uncataloged ECAs can hit the atmosphere of Earth without warning. Long-period comets may give as little as two months warning before impact. The damage ranges from fires and blastwaves from energy dissipation in the atmosphere to craters, earthquakes, and tsunami from ground impact. Tsunami damage is particularly severe. An asteroid 5–6km in diameter impacting in the mid Atlantic would cause substantial tsunami damage to both Europe and North America: the tsunami would run all the way to the Appalachians in the upper two-thirds of the United States and to the mountains of the Iberian Peninsula in Europe. Tsunami heights along the Iberian Peninsula can reach several hundred meters. The tsunami heights would be less in Northern Europe due to the shallow continental shelf in this region.

Rapid optical follow-up observations of SGR events with ROTSE-I

In order to observe nearly simultaneous emission from Gamma-ray Bursts (GRBs), the Robotic Optical Transient Search Experiment (ROTSE) receives triggers via the GRB Coordinates Network (GCN). Since beginning operations in March, 1998, ROTSE has also taken useful data for 10 SGR events: 8 from SGR 1900+14 and 2 from SGR 1806-20. We have searched for new or variable sources in the error regions of these SGRs and no optical counterparts were observed. Limits are in the range m_ROTSE ~ 12.5 - 15.5 during the period 20 seconds to 1 hour after the observed SGR events.

Meteoroids captured into Earth orbit by grazing atmospheric encounters

Abstract Some meteoroids, such as the one that produced the daytime fireball of August 10, 1972 that passed over the western United States and the European fireball of October 13, 1990, graze the atmosphere of Earth before returning to space (at reduced speed). Other grazing meteoroids, such as Peekskill, penetrate deeper into the atmosphere and lose enough energy to plunge to ground. It is evident that if a grazing meteoroid is within some critical range of closest approach distance and speed, it is captured into a gravitationally bound orbit around Earth. It must ultimately plunge to ground after further orbital dissipation in subsequent atmospheric passages unless the gravitational pull of the Moon and Sun or other intervention raise its perigee above the atmosphere. A spherical atmospheric model is used to integrate the passage of meteoroids in grazing atmospheric encounters. It is found that the corridor for capture narrows with increasing values of V∞, the approach velocity of the meteoroid prior to gravitational acceleration by Earth. As an example, if V∞= 5 km s−1, stony meteoroids with closest-approach distances of h = 40 km above the Earth are captured if their radii, R, are between 3 and 9 m while if V∞ = 15 km s−1 and h = 40 km, they are only captured if R is between 1.5 and 2m. Irons with V∞ = 5 km s−1 and h = 40 km, are captured if R is between 1 and 3.5 m, while if V∞ = 15 km s−1, they are captured if R is between 0.6 and 0.9 m. The cross section for orbital capture of iron meteoroids and small stony meteoroids is about 0.001 that for directly hitting Earth. Large stones are never captured except at very low impact velocities because of the large increase in drag resulting from fragmentation.