Mass Transfer by Stellar Wind

I review the process of mass transfer in a binary system through a stellar wind, with an emphasis on systems containing a red giant. I show how wind accretion in a binary system is different from the usually assumed Bondi-Hoyle approximation, first as far as the flow's structure is concerned, but most importantly, also for the mass accretion and specific angular momentum loss. This has important implications on the evolution of the orbital parameters. I also discuss the impact of wind accretion, on the chemical pollution and change in spin of the accreting star. The last section deals with observations and covers systems that most likely went through wind mass transfer: barium and related stars, symbiotic stars and central stars of planetary nebulae (CSPN). The most recent observations of cool CSPN progenitors of barium stars, as well as of carbon-rich post-common envelope systems, are providing unique constraints on the mass transfer processes.Comment: Chapter 7, in Ecology of Blue Straggler Stars, H.M.J. Boffin, G. Carraro & G. Beccari (Eds), Astrophysics and Space Science Library, Springe

3D Gasdynamic Modelling of the Changes in the Flow Structure During Transition From Quiescent to Active State in Symbiotic Stars

The results of 3D modelling of the flow structure in the classical symbiotic system Z~Andromedae are presented. Outbursts in systems of this type occur when the accretion rate exceeds the upper limit of the steady burning range. Therefore, in order to realize the transition from a quiescent to an active state it is necessary to find a mechanism able to sufficiently increase the accretion rate on a time scale typical to the duration of outburst development. Our calculations have confirmed the transition mechanism from quiescence to outburst in classic symbiotic systems suggested earlier on the basis of 2D calculations (Bisikalo et al, 2002). The analysis of our results have shown that for wind velocity of 20 km/s an accretion disc forms in the system. The accretion rate for the solution with the disc is ~22.5-25% of the mass loss rate of the donor, that is, ~4.5-5*10^(-8)Msun/yr for Z And. This value is in agreement with the steady burning range for white dwarf masses typically accepted for this system. When the wind velocity increases from 20 to 30 km/s the accretion disc is destroyed and the matter of the disc falls onto the accretor's surface. This process is followed by an approximately twofold accretion rate jump. The resulting accretion rate growth is sufficient for passing the upper limit of the steady burning range, thereby bringing the system into an active state. The time during which the accretion rate is above the steady burning value is in a very good agreement with observations. The analysis of the results presented here allows us to conclude that small variations in the donor's wind velocity can lead to the transition from the disc accretion to the wind accretion and, as a consequence, to the transition from quiescent to active state in classic symbiotic stars.Comment: 21 pages, 7 figure

Spectroscopic binaries among Hipparcos M giants I. Data, orbits, and intrinsic variations

This paper is a follow-up of the vast effort to collect radial velocity data for stars belonging to the Hipparcos survey. We aim at extending the orbital data available for binaries with M giant primaries. The data will be used in the companion papers of this series to (i) derive the binary frequency among M giants and compare it to that of K giants (Paper II), and (ii) analyse the eccentricity-period diagram and the mass-function distribution (Paper III). Keplerian solutions are fitted to radial-velocity data. However, for several stars, no satisfactory solution could be found, despite the fact that the radial-velocity standard deviation is larger than the instrumental error, because M giants suffer from intrinsic radial-velocity variations due to pulsations. We show that these intrinsic radial-velocity variations can be linked with both the average spectral-line width and the photometric variability. We present an extensive collection of spectroscopic orbits for M giants, with 12 new orbits, plus 17 from the literature. Moreover, to illustrate the fact that the large radial-velocity jitter present in Mira and semi-regular variables may easily be confused with orbital variations, we also present examples of pseudo-orbital variations (in S UMa, X Cnc and possibly in HD 115521, a former IAU radial-velocity standard). Because of this difficulty, M giants involving Mira variables were excluded from our monitored sample. We finally show that the majority of M giants detected as X-ray sources are actually binaries.Comment: 17 pages, 15 figures, accepted for publication in A&A, language editing changes onl

Abstracts from the NIHR INVOLVE Conference 2017

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Evaluation of surface treatments by self glaze, over glaze and a polishing agent on the porcelain surface modified with diamond rotary instrument-an in vitro study

The purpose of this study is to evaluate effect of a) Self glaze b) Over glaze and c) Polishing on porcelain surface which is subjected to grinding with a diamond rotary instrument. To test this theory 40 ceramic fused to metal disc samples were fabricated. The ceramic material used was Ivoclar Dsign. The discs were of 10mm X 3mm of which 2mm is metal substructure and 1mm is of porcelain. The specimens were divided into four groups of 10 samples each. The first group was left as fired or unglazed. The remaining three groups were subjected to surface modification with sintered diamond bur at 15000 rpm per minute with even pressure. Each of the three groups was treated with diamond polishing paste, self glaze and overglaze respectively. The surface of all the four groups were evaluated under optical microscope and scanning electron microscope. Optical microscope observation revealed that Group-I (as fired) had the surface of the specimens appearing to be pitted, rough with irregularities and there was no evidence of surface cracks. Group II (polished with diamond bur) had surface appeared to be very smooth

Wind accretion in binary stars

We present three-dimensional hydrodynamic calculations of mass transfer in an interacting binary system in which one component undergoes mass loss through a wind, and does so for various values of the mass ratio. The radius of the mass-losing star is taken to be half the size of its Roche lobe. Calculations are performed for gases with a ratio of specific heats $\gamma=5/3$. Mass loss is assumed to be mechanically, thermally, or radiatively driven.
We compute the specific angular momentum of gas escaping the system (lw) for these various cases. We show that lw does not reach a value higher than ∼1.2 for very low wind velocities and that it reaches the limiting case of a spherically symmetric wind for large wind velocities, for mass ratio smaller or equal to 1. For larger mass ratio, however, lw is larger than the expected limiting value. The value of lw depends slightly on the wind mechanism which modifies the relation between the wind velocity at the surface of the star and the velocity at the Roche lobe surface.
The specific angular momentum, lw, is large enough in a wide range of velocities to imply a shrinking of the system. This makes the symbiotic channel for Type Ia supernovae a plausible one and could also help explain the existence of Barium stars and other Peculiar Red Giants with orbital periods below, say, 1000 days.

Association of sleep duration and sleep quality with overweight/obesity among adolescents of Bangladesh: a multilevel analysis

Abstract Background Sleep deprivation is widely recognized as a potential contributor to childhood obesity. However, few studies have addressed this issue in low-income settings. The aim of this study was to determine the association of both sleep duration and sleep quality with overweight/obesity among adolescents of Bangladesh. Methods A cross-sectional study was conducted in four randomly selected schools in Gazipur, Bangladesh, from May to August 2019. Using a self-administered semi-structured questionnaire, data on sleep duration and sleep quality were collected from 1,044 adolescents between 13 and 17 years of age. The body mass indices of the study participants were evaluated using their objectively-assessed anthropometric measurements (weight and height). Multilevel logistic regression was used for data analysis. Results The prevalence of underweight, overweight and obesity in adolescents in this study were 14.9, 18 and 7.1%, respectively. More than 15% of the students reported sleep disturbance and poor sleep quality. After adjusting for confounders, reduced (<7 h/day) total sleep duration (OR=1.73, 95% CI=1.21-2.47), weekend sleep duration (OR=1.46, 95% CI=1.00-2.12), and night sleep duration (OR=1.55, 95% CI=1.06-2.28) were found to be significantly associated with overweight or obesity in Bangladeshi adolescents. Similarly, significant positive associations were evident between short duration of total sleep (OR=0.33, 95% CI=0.20-0.54), weekday sleep (OR=0.55, 95% CI=0.35-0.84), weekend sleep (OR=0.53, 95% CI=0.31-0.89), and night sleep (OR=0.56, 95% CI=0.36-0.87), and underweight in study participants. Adolescents with short sleep duration were found less likely to be underweight and more likely to be overweight/obese. Conclusions Study findings denoted short sleep duration to be associated with overweight/obesity and underweight among adolescents of Bangladesh. Adequate sleep may therefore serve as an effective obesity prevention strategy in the growing stages