The Science of Sungrazers, Sunskirters, and Other Near-Sun Comets

This review addresses our current understanding of comets that venture close to the Sun, and are hence exposed to much more extreme conditions than comets that are typically studied from Earth. The extreme solar heating and plasma environments that these objects encounter change many aspects of their behaviour, thus yielding valuable information on both the comets themselves that complements other data we have on primitive solar system bodies, as well as on the near-solar environment which they traverse. We propose clear definitions for these comets: We use the term near-Sun comets to encompass all objects that pass sunward of the perihelion distance of planet Mercury (0.307 AU). Sunskirters are defined as objects that pass within 33 solar radii of the Sun’s centre, equal to half of Mercury’s perihelion distance, and the commonly-used phrase sungrazers to be objects that reach perihelion within 3.45 solar radii, i.e. the fluid Roche limit. Finally, comets with orbits that intersect the solar photosphere are termed sundivers. We summarize past studies of these objects, as well as the instruments and facilities used to study them, including space-based platforms that have led to a recent revolution in the quantity and quality of relevant observations. Relevant comet populations are described, including the Kreutz, Marsden, Kracht, and Meyer groups, near-Sun asteroids, and a brief discussion of their origins. The importance of light curves and the clues they provide on cometary composition are emphasized, together with what information has been gleaned about nucleus parameters, including the sizes and masses of objects and their families, and their tensile strengths. The physical processes occurring at these objects are considered in some detail, including the disruption of nuclei, sublimation, and ionisation, and we consider the mass, momentum, and energy loss of comets in the corona and those that venture to lower altitudes. The different components of comae and tails are described, including dust, neutral and ionised gases, their chemical reactions, and their contributions to the near-Sun environment. Comet-solar wind interactions are discussed, including the use of comets as probes of solar wind and coronal conditions in their vicinities. We address the relevance of work on comets near the Sun to similar objects orbiting other stars, and conclude with a discussion of future directions for the field and the planned ground- and space-based facilities that will allow us to address those science topics

Diurnal variation in stature: is stretching the answer?

AIMS To investigate the extent and timing of diurnal variation in stature and to examine the effectiveness of the stretched technique in reducing the loss in height. SETTING A Southampton school. DESIGN Fifty three children, divided into two groups, were measured by two independent auxologists using a Leicester height measure. Each child was measured four times, at 0900, 1100, 1300, and 1500, using both an unstretched and a stretched technique. OUTCOME MEASURES Height loss after each of the three time intervals for both unstretched and stretched modes. RESULTS There was a clear decrease in stature during the morning, but no further loss occurred after the subjects had been up for around six hours. The mean height losses for the unstretched (stretched) modes were 0.31 cm (0.34 cm) and 0.20 cm (0.23 cm) for the periods 0900 to 1100 and 1100 to 1300, respectively, but only 0.045 cm (-0.019 cm) from 1300 to 1500. Stretching did not reduce the effects of diurnal variation, but significantly affected the recorded height by an average of 0.28 cm. There was no significant difference in reproducibility using either technique (SD 0.30 cm stretched v 0.31 cm unstretched). CONCLUSIONS Diurnal variation in stature may substantially affect the reliability of height data and careful consideration should be given to the timing of repeat measurements. As most height loss occurs in the morning, afternoon clinic appointments would be preferable. The standard stretched technique does not appear to reduce diurnal variation, nor does it affect precision. Measurements made using an unstretched method are recommended to avoid interobserver differences, known to occur where different observers are used

Sample size determination for constructing a constant width confidence interval for a binomial success probability

This article determines the sample size required to construct a confidence interval for the success probability of an event with preassigned width 2d and confidence level 1??. For ?=0.10,0.05,0.01 and d=0.01(0.01)0.05, the sample size is tabulated and the corresponding confidence interval defined. For other values of ? and d, Fortran 77 programs are available from the authors

Pubertal growth of the short normal girl.

Objectives: To determine the timing, magnitude and duration of the pubertal spurt for short normal and average height girls, to compare these with Tanner's standard and to investigate predictors of pubertal growth.Methods: The growth of 46 short normal and 55 control girls, identified at school entry, was monitored throughout puberty. Height and weight were measured at 6-month intervals from which body mass index (BMI) was derived. Annual velocities were calculated and used to estimate the age and magnitude of peak height velocity (PHV). Age of menarche was recorded to the nearest month. Parents provided information on the child's medical and social history.Results: The mean age at PHV, the magnitude of PHV and age at menarche were similar for both groups and close to Tanner's 50th centile values. Pre-pubertal BMI predicted age at menarche for short and control girls, accounting for 17% of the variance. There was a tendency for early maturing girls of average stature to have greater PHV. However, this relationship was not observed in short girls, nor did any other variable, genetic or environmental, predict the timing or magnitude of their pubertal spurt.Conclusions: Delayed puberty in short normal girls is unlikely and their growth during puberty is comparable to girls of average height. The pubertal variables measured remain close to Tanner's original standards for both groups, suggesting the lack of a secular trend towards earlier puberty in girls. The onset of menstruation is influenced by pre-pubertal BMI. However, the clinician should be aware that short normal girls have normal pubertal growth and that no genetic or environmental variable can predict the timing or magnitude of their growth spurt

Hormones and handedness

Background: The Wessex Growth Study has monitored the growth and psychological development of short normal (SN) and average height control subjects since they entered school in 1985/1986. During psychometric testing, we found that 25% SN compared to 9% control subjects wrote with their left hand. The short group also attained significantly lower scores on measures of IQ and attainment and displayed less internalisation of control. Laterality, however, is thought to be influenced by the intrauterine environment and has been associated with pubertal delay. At recruitment, short children had a relatively low birth weight, delayed bone age and were more likely than controls to be short for family.Objectives: To determine if birth conditions were associated with lateral preference and whether laterality could account for the differences found during the psychometric assessment or predict pubertal timing of SN children.Methods: Subjects were classified as right- (RH) or left-handed (LH) according to the writing hand and the data were investigated examining the effect of handedness and stature.Results: RH and LH SN children were no more likely to suffer birth complications than those of average height. Psychometric testing did not reveal any significant differences between RH and LH SN children and their patterns of growth appeared to be similar. However, both RH and LH SN children scored less well on tests of cognitive ability and analyses of covariance revealed significant gender/handedness effects for both the timing of puberty and final height.Conclusions: The increase in left-handedness among SN children did not appear to be related to adverse birth conditions, but it may be that the hormones responsible for growth and development also play some part in brain laterality and cognitive development