X-Shooter spectroscopy of young stellar objects: V - Slow winds in T Tauri stars

Disks around T Tauri stars are known to lose mass, as best shown by the profiles of forbidden emission lines of low ionization species. At least two separate kinematic components have been identified, one characterised by velocity shifts of tens to hundreds km/s (HVC) and one with much lower velocity of few km/s (LVC). The HVC are convincingly associated to the emission of jets, but the origin of the LVC is still unknown. In this paper we analyze the forbidden line spectrum of a sample of 44 mostly low mass young stars in Lupus and $\sigma$-Ori observed with the X-Shooter ESO spectrometer. We detect forbidden line emission of [OI], [OII], [SII], [NI], and [NII], and characterize the line profiles as LVC, blue-shifted HVC and red-shifted HVC. We focus our study on the LVC. We show that there is a good correlation between line luminosity and both L$_{star}$ and the accretion luminosity (or the mass-accretion rate) over a large interval of values (L$_{star}$ $\sim 10^{-2} - 1$ L$_\odot$; L$_{acc}$ $\sim 10^{-5} - 10^{-1}$ L$_\odot$; $\dot M_{acc}$ $\sim 10^{-11} - 10^{-7}$ M$_\odot$/yr). The lines show the presence of a slow wind ($V_{peak}10^8$ cm$^{-3}$), warm (T$\sim 5000-10000$ K), mostly neutral. We estimate the mass of the emitting gas and provide a value for the maximum volume it occupies. Both quantities increase steeply with the stellar mass, from $\sim 10^{-12}$ M$_\odot$ and $\sim 0.01$ AU$^3$ for M$_{star}$$\sim 0.1$ M$_\odot$, to $\sim 3 \times 10^{-10}$ M$_\odot$ and $\sim 1$ AU$^3$ for M$_{star}$$\sim 1$ M$_\odot$, respectively. These results provide quite stringent constraints to wind models in low mass young stars, that need to be explored further

Discrepancy bounds for normal numbers generated by necklaces in arbitrary base

Mordechay B. Levin has constructed a number $\lambda$ which is normal in base 2, and such that the sequence $(\left\{2^n \lambda\right\})_{n=0,1,2,\ldots}$ has very small discrepancy $D_N$. Indeed we have $N\cdot D_N = \mathcal{O} \left(\left(\log N\right)^2\right)$. This construction technique of Levin was generalized by Becher and Carton, who generated normal numbers via perfect nested necklaces, and they showed that for these normal numbers the same upper discrepancy estimate holds as for the special example of Levin. In this paper now we derive an upper discrepancy bound for so-called semi-perfect nested necklaces and show that for the Levin's normal number in arbitrary prime base $p$ this upper bound for the discrepancy is best possible, i.e., $N\cdot D_N \geq c\left(\log N\right)^2$ with $c>0$ for infinitely many $N$. This result generalizes a previous result where we ensured for the special example of Levin for the base $p=2$, that $N\cdot D_N =O( \left(\log N\right)^2)$ is best possible in $N$. So far it is known by a celebrated result of Schmidt that for any sequence in $[0,1)$, $N\cdot D_N\geq c \log N$ with $c>0$ for infinitely many $N$. So there is a gap of a $\log N$ factor in the question, what is the best order for the discrepancy in $N$ that can be achieved for a normal number. Our result for Levin's normal number in any prime base on the one hand might support the guess that $O( \left(\log N\right)^2)$ is the best order in $N$ that can be achieved by a normal number, while generalizing the class of known normal numbers by introducing e.g. semi-perfect necklaces on the other hand might help for the search of normal numbers that satisfy smaller discrepancy bounds in $N$ than $N\cdot D_N=O( \left(\log N\right)^2)$.Comment: 29 page

Signs of magnetic accretion in X-ray pulsars

The spin-down mechanism of accreting neutron stars is discussed with an application to one of the best studied X-ray pulsars GX 301-2. We show that the maximum possible spin-down torque applied to a neutron star from the accretion flow can be evaluated as $K_{\rm sd}^{\rm (t)} = \mu^2/(r_{\rm m} r_{\rm cor})^{3/2}$. The spin-down rate of the neutron star in GX 301-2 can be explained provided the magnetospheric radius of the neutron star is smaller than its canonical value. We calculate the magnetospheric radius considering the mass-transfer in the binary system in the frame of the magnetic accretion scenario suggested by V.F. Shvartsman. The spin-down rate of the neutron star expected within this approach is in a good agreement with that derived from observations of GX 301-2.Comment: accepted for publication in Astronomy Report

A LeVeque-type Lower Bound for Discrepancy

A sharp lower bound for discrepancy on R / Z is derived that resembles the upper bound due to LeVeque. An analogous bound is proved for discrepancy on Rk / Zk. These are discussed in the more general context of the discrepancy of probablity measures. As applications, the bounds are applied to Kronecker sequences and to a random walk on the torus

Lyman Alpha Emitter Evolution in the Reionization Epoch

Combining cosmological SPH simulations with a previously developed Lyman Alpha production/transmission model and the Early Reionization Model (ERM, reionization ends at redshift z~7), we obtain Lyman Alpha and UV Luminosity Functions (LFs) for Lyman Alpha Emitters (LAEs) for redshifts between 5.7 and 7.6. Matching model results to observations at z~5.7 requires escape fractions of Lyman Alpha, f_alpha=0.3, and UV (non-ionizing) continuum photons, f_c=0.22, corresponding to a color excess, E(B-V)=0.15. We find that (i) f_c increases towards higher redshifts, due the decreasing mean dust content of galaxies, (ii) the evolution of f_alpha/f_c hints at the dust content of the ISM becoming progressively inhomogeneous/clumped with decreasing redshift. The clustering photoionization boost is important during the initial reionization phases but has little effect on the Lyman Alpha LF for a highly ionized IGM. Halo (stellar) masses are in the range 10.0 < \log M_h < 11.8 (8.1 < \log M_* < 10.4) with M_h \propto M_*^{0.64}. The star formation rates are between 3-120 solar masses per year, mass-weighted mean ages are greater than 20 Myr at all redshifts, while the mean stellar metallicity increases from Z=0.12 to 0.22 solar metallicity from z~7.6 to z~5.7; both age and metallicity positively correlate with stellar mass. The brightest LAEs are all characterized by large star formation rates and intermediate ages (~200 Myr), while objects in the faint end of the Lyman Alpha LF show large age and star formation rate spreads. With no more free parameters, the Spectral Energy Distributions of three LAE at z~5.7 observed by Lai et al. (2007) are well reproduced by an intermediate age (182-220 Myr) stellar population and the above E(B-V) value.Comment: 13 pages, 9 figures, accepted to MNRA