M-dwarfs

Zwerg, m. dwarf, -welle, /. midget wave, dwarf wave, microwave,... 

The coolest stars with just enough mass to fuse hydrogen are the M-dwarfs (see chapter 3). Two new classes of brown dwarfs have been added to the cool end of the stellar spectrum. The L-dwarfs (1,300 to 2,000 Kelvin) are slightly cooler and less massive than M-dwarfs. T-dwarfs are cooler and lighter than the L-dwarfs. Both of these new dwarfs cannot sustain hydrogen fusion. Researchers have recently discovered hundreds of T-dwarfs and tens of L-dwarfs. Even the cool T-dwarfs may have magnetic fields that create occasional stellar flares. [For more information, see Linda Rowan, Cooler dwarf stars, Science 289(5480) 697 (August 4, 2000).]... 

HZ is defined as the orbital area around a star where a planet can sustain liquid water at the surface. Several studies have attempted to define the HZ as a function of stellar type (e.g. F, G, K, M dwarf stars) and time (see e.g. 46, 47, and references therein). The HZ is of interest because it is widely believed that liquid water is necessary for the genesis of (recognizable) life. The particular emphasis of the planned space missions is to search for signs of life on extrasolar Earth-like planets via spectroscopy. Atmospheric compounds such as O2, O3, N2O, CH4, and CH3CI are considered biomarkers, and their spectroscopic detection in a terrestrial-type atmosphere, particularly O2 or O3 found together with a reduced gas such as CH4, would suggest life (48, 49). Detection of CO2 would indicate that the planet is indeed a terrestrial-type planet... 

An empirical value for A was established by comparison of objects at atomic and stellar levels, likely to be correlated, such as the radius of a Rydberg atom and the orbit of the planet Jupiter. To correlate mass scales the hydrogen atom and an M dwarf star of mass 0.15M are assumed analogous. The derived values of... 

Bonfils X et al (2011) The HARPS search for southern extra-solar planets XXXI. The M-dwarf sample, A A, 2011arXivllll.5019B... 

Rauer H, Gebauer S, Paris PV, Cabrera J, Godolt M, Grenfell JL, Belu A, Selsis F, Hedelt P, Schreier F (2011) Potential biosignatures in super-Earth atmospheres. I. Spectral appearance of super-Earths around M dwarfs. Astron Astrophys 529, id.A8 2011... 

Segura A, Kasting JE, Meadows V, Cohen M, Scalo J, Crisp D, Butler RAH, Tinetti G (2005) Biosignatures from Earth-like planets around M Dwarfs. Astrobiology 5 706-725... 

M-dwarf stars have small radii. A planet with 2 Earth radii orbiting around such a star in the habitable zone (here defined as the region were liquid surface water may exist on a planet) will produce a 0.5% drop of luminosity of the star during transit and can be detected even with small aperture telescopes (Irwin et al., 2009 [170]). 

Barlow et al., 1996 [17], investigated the rich far IR water vapor spectrum of the star W Hya (Fig. 7.9). This object is a cool (2700 K) M-dwarf at a distance of 130 pc. It has a high mass loss (about 10 M0yr ). Water vapor seems to be the major coolant and the H2O/H2 abundance is 8 x 10 " for r < 4.5 x 10 " cm and 3 x 10 " at large radii. Maercker et al., 2008 [210] found high amounts of H2O in the circumstellar envelope of that star and they speculate that evaporation of icy cometary or planetary bodies might be an effective ongoing mechanism in such systems. 

Irwin, J., Charbonneau, D., Nutzman, P, Falco, E. The MEarth project searching fortransiting habitable super-Earth planets around nearby M-dwarfs. In Stempels, E. (ed.) American Institute of Physics Conference Series, vol. 1094, pp. 445-448 (2009)... 

The simplest interpretation is that there is no nuclear M dwarf enhancement in NGC 4472 but there is a strong radial [Na/Fe] gradient. However, a flat CO gradient also could be produced by a combination of a radial M D/G gradient and [C/H] or [0/H] gradients. 

Operating Costs Power cost for a continuous thickener is an almost insignificant item. For example, a unit thickener 60 m (200 ft) in diameter with a torque rating of 1.0 MN-m (8.8 Mlbf in) will normally require 12 kW (16 hp). The low power consumption is due to the very slow rotative speeds. Normally, a mechanism vi l be designed for a peripheral speed of about 9 m/min (0.5 ft/s), which corresponds to only 3 r/h for a 60-m (200-ft) unit. This low speed also means veiy low maintenance costs. Operating labor is low because little attention is normally required after initial operation has balanced the feed and underflow. If chemicals are required for flocculation, the chemical cost frequently dwarfs all other operating costs. 

The evolution of a. star after it leaves the red-giant phase depends to some extent on its mass. If it is not more than about 1.4 M it may contract appreciably again and then enter an oscillatory phase of its life before becoming a white dwarf (p. 7). When core contraction following helium and carbon depletion raises the temperature above I0 K the y-ray.s in the stellar assembly become sufficiently energetic to promote the (endothermic) reaction Ne(y,a) 0. The a-paiticle released can penetrate the coulomb barrier of other neon nuclei to form " Mg in a strongly exothermic reaction ... 

The theoretical framework developed above is valid in the electric dipole approximation. In this context, it is assumed that the nonlinear polarization Ps 2a)) is reduced to the electric dipole contribution as given in Eq. (1). This assumption is only valid if the surface susceptibility tensor co, m) is large enough to dwarf the contribution from higher... 

Pearl C., Cervantes M., Chan M., Ho U., et al. (2000). Evidence for a mate-attracting chemosignal in the dwarf African clawed frog Hymenochirus spp. Horm Behav 38, 67-74. 

Li, S., Crenshaw, E. B., Rawson, E. J., Simmons, D. M., Swanson, L. W and Rosenfeld, M. G. (1990). Dwarf locus mutants lacking three pituitary cell types result from mutations in the POU-domain gene pit-1. Nature 347 528-533. 

Middle-sized stars, between about 1 and 8 M , undergo complicated mixing processes and mass loss in advanced stages of evolution, culminating in the ejection of a planetary nebula while the core becomes a white dwarf. Such stars are important sources of fresh carbon, nitrogen and heavy elements formed by the slow neutron capture (s-) process (see Chapter 6). Finally, small stars below 1 M have lifetimes comparable to the age of the Universe and contribute little to chemical enrichment or gas recycling and merely serve to lock up material. 

A further effect during evolution up the AGB is mass loss through stellar winds, at an increasing rate as the star increases in luminosity and radius and becomes unstable to pulsations which drive a super-wind in the case of intermediate-mass stars. For stars with an initial mass below some limit, which may be of order 6 M , the wind evaporates the hydrogen-rich envelope before the CO core has reached the Chandrasekhar limiting mass (see Section 5.4.3), the increase in luminosity ceases and the star contracts at constant luminosity, eventually becoming a white dwarf (Figs. 5.15, 5.19). A computed relation between initial stellar mass and the final white-dwarf mass is shown in Fig. 5.21. 

Fig. 5.24. Maximum temperatures and densities calculated for an outward propagating deflagration front in a model (model W7) of an SN la explosion from accretion on to a CO white dwarf with initial mass 1 M at a rate of 4 x 10-8 M yr 1. Zones of different burning conditions are indicated. After Thielemann, Nomoto and Yokoi (1986). Courtesy Ken-ichi Nomoto.

The site of the r-process is also not clear, but it seems that the conditions needed to reproduce Solar-System r-process abundances may hold in the hot bubble caused by neutrino winds in the immediate surroundings of a nascent neutron star in the early stages of a supernova explosion (see Fig. 6.10). Circumstantial evidence from Galactic chemical evolution supports an origin in low-mass Type II supernovae, maybe around 10 M (Mathews, Bazan Cowan 1992 Pagel Tautvaisiene 1995). Another possibility is the neutrino-driven wind from a neutron star formed by the accretion-induced collapse of a white dwarf in a binary system (Woosley Baron 1992) leading to a silent supernova (Nomoto 1986). In stars with extreme metal-deficiency, the heavy elements sometimes display an abundance pattern characteristic of the r-process with little or no contribution from the s-process, and the... 

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