The RAdial Velocity Experiment (RAVE) is a medium-resolution (R ~ 7500) spectroscopic survey of the Milky Way that has already obtained over half a million stellar spectra. They present a randomly selected magnitude-limited sample, so it is important to use a reliable and automated classification scheme that identifies normal single stars and discovers different types of peculiar stars. To this end, we present a morphological classification of ~350, 000 RAVE survey stellar spectra using locally linear embedding, a dimensionality reduction method that enables representing the complex spectral morphology in a low-dimensional projected space while still preserving the properties of the local neighborhoods of spectra. We find that the majority of all spectra in the database (~ 90%-95%) belong to normal single stars, but there is also a significant population of several types of peculiars. Among them, the most populated groups are those of various types of spectroscopic binary and chromospherically active stars. Both of them include several thousands of spectra. Particularly the latter group offers significant further investigation opportunities since activity of stars is a known proxy of stellar ages. Applying the same classification procedure to the sample of normal single stars alone shows that the shape of the projected manifold in two-dimensional space correlates with stellar temperature, surface gravity, and metallicity.
COBISS.SI-ID: 393601
In this paper, we present a sample of ten short gamma-ray bursts (GRBs) with a robust redshift determination, discovered by the Swift satellite up to 2011 January. We measure their X-ray absorbing column densities and collect data on the host galaxy offsets. We find evidence for intrinsic absorption and no correlation between the intrinsic absorbing column density and the projected offset of the GRB from its host galaxy centre. We find that the properties in the gamma regime of short GRBs with 'bright' and 'faint' X-ray afterglow likely disfavour different prompt emission mechanisms. The host galaxy offset and GRB duration do not correlate. Instead, there is a hint of anticorrelation between the effective radius normalized host galaxy offset and T90. Finally, we examine the properties of short GRBs with short-lived and long-lived X-ray afterglows, finding that some short GRBs with short-lived X-ray afterglows have their optical afterglow detected. In light of this, the X-ray afterglow duration does not seem to be a unique indicator of a specific progenitor and/or environment for short GRBs.
COBISS.SI-ID: 397953
This paper deals with the large-scale inertio-gravity (IG) wave energy in the operational ECMWF analyses in July 2007. Energy percentages of the IG waves obtained from the standard-pressure-level data are compared to those derived from various discretizations of the model-level data. The results show a small albeit systematic increase of the IG energy percentage as the vertical level density increases from the standard-pressure levels toward the model-level density; the small relative change is explained by the sufficient vertical resolution to resolve the large-scale IG waves in the tropics that make the majority of the global IG energy on large scales. A relatively larger increase of the IG energy is obtained when the mesospheric model levels are included; however, the analyses at these levels in July 2007 are less reliable. Furthermore, two numerical methods for the normal-mode function (NMF) decomposition are shown to provide similar results. The decomposition of atmospheric analyses into the NMF series is proposed as a tool to analyze the spatial and temporal variations of the large-scale equatorial waves and their role in global energetics.
COBISS.SI-ID: 285609
This paper reports recent advances in understanding of dynamical aspects of the tropical data assimilation. In contrast with the mid-latitudes, there is no a well-defined approach for the tropical data assimilation in numerical weather prediction (NWP) community which has traditionally been concentrated on the midlatitude analysis problem. In particular, the impact of the equatorial Rossby, inertio-gravity, and mixed Rossby-gravity waves on the tropical forecast-error covariances is difficult to quantify. Various tropical waves are characterized by different couplings between the mass field and the wind field. The average mixture of these waves, built into the background-error covariance matrix for data assimilation provides analysis increments which appear nearly univariate even though they result from the advanced multivariate assimilation methodology. This applies to both dry and moist idealized tropical systems as well as to a 4D-Var NWP assimilation system.
COBISS.SI-ID: 2408292
We introduce a generalized scaling law, M_tot = 10^K A^a B^b, to look for the minimum scatter in reconstructing the total mass of hydrodynamically simulated X-ray galaxy clusters, given gas mass M_gas, luminosity L and temperature T. We find a locus in the plane of the logarithmic slopes a and b of the scaling relations where the scatter in mass is minimized. This locus corresponds to b_M=-3/2a_M+ 3/2 and b_L=-2a_L+ 3/2 for A=M_gas and L, respectively, and B=T. Along these axes, all the known scaling relations can be identified (at different levels of scatter), plus a new one defined as M_tot ∝ (LT)^(1/2). Simple formula to evaluate the expected evolution with redshift in the self-similar scenario is provided. In this scenario, no evolution of the scaling relations is predicted for the cases (b_M = 0, a_M= 1) and (b_L = 7/2, a_L=-1), respectively. Once the single quantities are normalized to the average values of the sample under considerations, the normalizations K corresponding to the region with minimum scatter are very close to zero. The combination of these relations allows one to reduce the number of free parameters of the fitting function that relates X-ray observables to the total mass and includes the self-similar redshift evolution.
COBISS.SI-ID: 386177