Mass accretion

Since most (if not all) low-metallicity objects that are currently observed in the halo are not in the AGB phase, material enriched in carbon and the s-process elements is assumed to have accreted from the companion AGB stars, which have already evolved to faint white dwarfs, to the surface of the surviving companion. This scenario is the same as that applied to classical CH stars [4], Unfortunately, long-term radial velocity monitoring has been obtained for only a limited number of objects a clear binarity signature has been established for six objects in our sample to date. However, there exists additional support for the mass-accretion scenario for the Ba-rich CEMP stars. Fig. lb shows [C/H] as a function of luminosity roughly estimated from the effective temperature... 

It should be noted that at least three of the Ba-rich stars in our sample exhibit no clear variation in their radial velocities over the last 8-10 years. Either their periods are quite long, or the mass-accretion scenario may not apply to these objects. Further investigation of the binarity for these objects is clearly required. Ba-normal stars The other nine objects in our sample have relatively low Ba abundances ( — 1.0 < [Ba/Fe] < —0.5). These values are typical in metal-deficient stars that show no carbon excess ([C/Fe]< +0.5), hence the scenario of carbon enrichment by AGB stars cannot be simply applied to these stars. 

Here p is the stellar dipole magnetic moment and M is the mass accretion rate through the disk. For this formula we assume a purely dipolar field higher multipoles weaken the dependence on M because the field is effectively stiffen... 

In our scenario, we consider a purely hadronic star whose central pressure is increasing due to spin-down or due to mass accretion, e.g., from the material left by the supernova explosion (fallback disc), from a companion star or from the interstellar medium. As the central pressure exceeds the threshold value Pq at static transition point, a virtual drop of quark matter in the Q -phase can be formed in the central region of the star. As soon as a real drop of Q -matter is formed, it will grow very rapidly and the original Hadronic Star will be converted to and Hybrid Star or to a Strange Star, depending on the detail of... 

To summarize, in the present scenario pure hadronic stars having a central pressure larger than the static transition pressure for the formation of the Q -phase are metastable to the decay (conversion) to a more compact stellar configuration in which deconfined quark matter is present (i. e., HyS or SS). These metastable HS have a mean-life time which is related to the nucleation time to form the first critical-size drop of deconfined matter in their interior (the actual mean-life time of the HS will depend on the mass accretion or on the spin-down rate which modifies the nucleation time via an explicit time dependence of the stellar central pressure). We define as critical mass Mcr of the metastable HS, the value of the gravitational mass for which the nucleation time is equal to one year Mcr = Miis t = lyr). Pure hadronic stars with Mh > Mcr are very unlikely to be observed. Mcr plays the role of an effective maximum mass for the hadronic branch of compact stars. While the Oppenheimer-Volkov maximum mass Mhs,max (Oppenheimer Volkov 1939) is determined by the overall stiffness of the EOS for hadronic matter, the value of Mcr will depend in addition on the bulk properties of the EOS for quark matter and on the properties at the interface between the confined and deconfined phases of matter (e.g., the surface tension a). 

Love, S. G. and Brownlee, D. E. (1993). A direct measurement of the terrestrial mass accretion rate of cosmic dust. Science, 262, 550-3. 

Rapid rotation and mass-loss connection in Be stars was suggested for the first time by Struve(1931), which necessarily leads to the requirement of break-up velocity in Be stars. However, Vsin i statistics suggests almost all Be stars are well below the break-up velocity. The additional forces have been searched for so far i.e., stellar wind, magnetic field, mass accretion, and so on. At the moment, none of them can succeed in explaining the episodic mass-loss in Be stars. 

The evolution of the carbon abundance at the surface of both components of a mass-exchanging (Algol-type) binary is examined (fig. 1). Distinction is made between case B and case AB (fig. 2) of mass transfer, in view of the different timescales involved. In the mass accreting component thermohaline mixing is adopted when matter with decreasing hydrogen abundance is deposited on the surface. 

We have thus established that (in principle at least)DN can metamorphose into CN. The remaining question is therefore, do CN become DN du- ring some phases of their evolution Before attempting to answer this question, we shall examine some problems with the mass accretion rates deduced from observations of CN systems. 

Patterson (1984) and Warner (1987) attempted to deduce the mass accretion rates, M, in CN systems. The values which they obtained present some problems,which we shall now briefly discuss. 

Table 2.2 shows timescales required to increase the mass by given mass increments and average mass-infall rates during these periods for three different final masses of low-mass stars. The formation of the initial stellar embryo requires about one free-fall time and is very short. The subsequent addition of 80% of the final mass requires about four free-fall times, i.e. is rather rapid. The mass accretion rate... 

All young stellar objects show indications for stellar winds and outflows. These phenomena are always observed to occur in systems that undergo mass accretion that interacts with magnetic fields and rotation. They are not limited to star formation but are also observed in other cases, e.g. during accretion onto central black holes in galaxies. 

Viscous evolution and mass accretion through the disk... 

While the viscous model for the evolution of protoplanetary disks has had some success in matching some of the general properties of protoplanetary disks, such as the observed mass accretion rates and effective temperatures, the exact source of the viscosity remains the subject of ongoing studies. Currently, the most popular candidates for driving the mass transport in protoplanetary disks are the magneto-rotational instability (MRI) and gravitational instability. A third candidate, shear instability, has also been proposed based on laboratory experiments of rotating fluids (Richard Zahn 1999), but questions remain as to whether these results can be extended to the scale of protoplanetary disks. 

Table 8.1 Typical stellar and disk parameters for young stars. The columns show typical spectral types at i Myr, effective temperatures, luminosity, disk masses, and mass accretion rates.

Figure 9.3 Mass accretion rates versus the age of the stellar group. The age error bars represent typical uncertainties, while die accretion rate error bars are the maximum and minimum values measured in each region. In addition to the data presented in Calvet et al. (2005), we have included the mass accretion rates from p Ophiuchi (Natta et al. 2006), and from a Orionis (Gatti et al. 2008). Above the plot we show a comparison to the formation timescale of CAIs, chondrules, and the moon lapetus in the Solar System (see Sections 9.3.1 and 9.3.2).

FU Orionis outbursts the brightening of an FU Orionis star. Such outbursts are thought to occur when the mass accretion rate through the accretion disk increases by orders of magnitude. 

Table 22.5 Burial rates of nitrogen Rates were calculated using several methods, assuming accretion has kept up with relative sea level rise (SLR), a budget of organic matter inputs and losses (OM budget), mass accretion rates using radiometric dating techniques ( Cs or Pb), or net particulate nitrogen (net PN) budgets to the marsh or in tidal creeks. GSW stands for Great Sippewissett Marsh...

Mass accretion rates—episodicity and outbursts. 04.3.1.2 Temperature profiles from spectral energy distributions... 

Observations of UV excesses and emission hnes in young stars are used to estimate the rate at which matter from an unseen disk is accreting onto the central protostar (Calvet et al., 2000). To fuel this accretion, matter from the initial cloud core infalls onto the protostar s disk. In general, these two mass accretion rates are not the same. During the earliest phases of evolution, the disk accretes mass from its infalhng envelope at a rate of about a solar mass or less per 10 yr, with this... 

Figure 4 Midplane temperature as a function of heliocentric radius for a solar nebula with varying mass (inside 10 AU) undergoing mass accretion at a rate of a solar mass in —0.1-1 Myr, compared to various cosmochemical constraints, and the results of a viscous accretion disk model (dashed line) with a mass of 0.24 solar masses (source Boss, 1998).

Magnetic interactions between the Sun and the disk gave rise to powerful winds which ejected material in jets directed along open magnetic field fines out of the plane of the disk. The amount of material lost in this wind was perhaps 30-50% of the mass accreted by the Sun. Some sohd particles entrained in the wind would have fallen back into the disk, possibly many AU from the Sun (Shu et al., 1988). 

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