Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userSynthesis of the distribution function of ISN He
173
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
Interaction of the solar wind with interstellar matter involves, among other, charge exchange between interstellar neutral atoms and plasma, which results in the creation of secondary population of interstellar neutral (ISN) atoms. The secondary population of interstellar He was detected by Interstellar Boundary Explorer (IBEX), but interpretation of these measurements was mostly based on an approximation that the primary interstellar neutral population and the secondary population were non-interacting homogeneous Maxwell-Boltzmann functions in the outer heliosheath. We simulate the distribution function in the outer heliosheath and inside the heliopause using method of characteristics with statistical weights obtained from solutions of the production and loss equations for the secondary atoms due to charge-exchange collisions in the outer heliosheath. We show that the two-Maxwellian approximation for the distribution function of neutral He is not a good approximation within the outer heliosheath but a reasonable one inside the termination shock. This is due to a strong selection effect: the He atoms able to penetrate inside the termination shock are a small, peculiar subset of the entire secondary He population. Nevertheless, the two-Maxwellian approximation reproduces the density distribution of ISN He inside the termination shock well and enables a realistic reproduction of the orientation of the plane defined by the Suns velocity vector through the local interstellar matter and the vector of unperturbed interstellar magnetic field.
rate research
Read More
One crucial piece of information to study the origin of multiple stellar populations in globular clusters, is the range of initial helium abundances $Delta{Y}$ amongst the sub-populations hosted by each cluster. These estimates are commonly obtained by measuring the width in colour of the unevolved main sequence in an optical colour-magnitude-diagram. The measured colour spread is then compared with predictions from theoretical stellar isochrones with varying initial He abundances, to determine $Delta{Y}$. The availability of UV/optical magnitudes thanks to the {sl HST UV Legacy Survey of Galactic GCs} project, will allow the homogeneous determination of $Delta{Y}$ for a large Galactic globular cluster sample. From a theoretical point of view, accurate UV CMDs can efficiently disentangle the various sub-populations, and main sequence colour differences in the ACS $F606W-(F606W-F814W)$ diagram allow an estimate of $Delta{Y}$. We demonstrate that from a theoretical perspective the ($F606W-F814W$) colour is an extremely reliable He-abundance indicator. The derivative d$Y$/d($F606W-F814W$), computed at a fixed luminosity along the unevolved main sequence, is largely insensitive to the physical assumptions made in stellar model computations, being more sensitive to the choice of the bolometric correction scale, and is only slightly dependent on the adopted set of stellar models. From a theoretical point of view the ($F606W-F814W$) colour width of the cluster main sequence is therefore a robust diagnostic of the $Delta{Y}$ range.
We introduce a new dual power law (DPL) probability distribution function for the mass distribution of stellar and substellar objects at birth, otherwise known as the initial mass function (IMF). The model contains both deterministic and stochastic elements, and provides a unified framework within which to view the formation of brown dwarfs and stars resulting from an accretion process that starts from extremely low mass seeds. It does not depend upon a top down scenario of collapsing (Jeans) masses or an initial lognormal or otherwise IMF-like distribution of seed masses. Like the modified lognormal power law (MLP) distribution, the DPL distribution has a power law at the high mass end, as a result of exponential growth of mass coupled with equally likely stopping of accretion at any time interval. Unlike the MLP, a power law decay also appears at the low mass end of the IMF. This feature is closely connected to the accretion stopping probability rising from an initially low value up to a high value. This might be associated with physical effects of ejections sometimes (i.e., rarely) stopping accretion at early times followed by outflow driven accretion stopping at later times, with the transition happening at a critical time (therefore mass). Comparing the DPL to empirical data, the critical mass is close to the substellar mass limit, suggesting that the onset of nuclear fusion plays an important role in the subsequent accretion history of a young stellar object.
We present total infrared (IR) and ultraviolet (UV) luminosity functions derived from large representative samples of galaxies at z ~ 0, selected at IR and UV wavelengths from the IRAS IIFSCz catalogue, and the GALEX AIS respectively. We augment these with deep Spitzer and GALEX imaging of galaxies in the 11 Mpc Local Volume Legacy Survey (LVL), allowing us to extend these luminosity functions to lower luminosities (~10^6 L_sun), and providing good constraints on the slope of the luminosity function at the extreme faint end for the first time. Using conventional star formation prescriptions, we generate from our data the SFR distribution function for the local Universe. We find that it has a Schechter form, that the faint-end slope has a constant value (to the limits of our data) of {alpha} = -1.51 pm 0.08, and the characteristic SFR is 9.2 M_sun/yr. We also show the distribution function of the SFR volume density; we then use this to calculate a value for the total SFR volume density at z ~ 0 of 0.025 pm 0.0016 M_sun/yr/Mpc^-3, of which ~ 20% is occurring in starbursts. Decomposing the total star formation by infrared luminosity, it can be seen that 9 pm 1% is due to LIRGs, and 0.7 pm 0.2% is occuring in ULIRGs. By comparing UV and IR emission for galaxies in our sample, we also calculate the fraction of star formation occurring in dust obscured environments, and examine the distribution of dusty star formation: we find a very shallow slope at the highly extincted end, which may be attributable to line of sight orientation effects as well as conventional internal extinction.
About a century ago, Jeans (1919) discovered that if binary stars reach a state approximating energy equipartition, for example through many dynamical encounters that exchange energy, their eccentricity distribution can be described by : dN/de = 2e. This is referred to as the thermal eccentricity distribution, and has been widely used for initial conditions in theoretical investigations of binary stars. However, observations suggest that the eccentricity distributions of most observed binaries, and particularly those with masses < 5 Msun, are flatter than thermal and follow more closely to a uniform distribution. Nonetheless, it is often argued that dynamical interactions in a star cluster would quickly thermalize the binaries, which could justify imposing a thermal eccentricity distribution at birth for all binaries. In this paper we investigate the validity of this assumption. We develop our own rapid semi-analytic model for binary evolution in star clusters, and also compare with detailed N-body and Monte Carlo star cluster models. We show that, for nearly all binaries, dynamical encounters fail to convert an initially uniform eccentricity distribution to thermal within a star clusters lifetime. Thus, if a thermal eccentricity distribution is observed, it is likely imprinted upon formation rather than through subsequent long-term dynamical processing. Theoretical investigations that initialize all binaries with a thermal distribution will make incorrect predictions for the evolution of the binary population. Such models may overpredict the merger rate for binaries with modest orbital separations by a factor of about two.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
