Correlation between X-ray luminosity and the deviations from the main-sequence colour-diagram for stellar X-ray sources

Published photometry in Sgri is used to derive the period-luminosity relation for Mira variables in yellow and in blue light. The observed dispersion about the adopted relation is comparable to that of RR Lyrae-stars around their mean luminosity. This suggests that the Mira star period-luminosity relation has a relatively small dispersion and is, therefore, a powerful tool for astronomical distance determination. The calibration of the Mira period-luminosity relation indicates that Glass and Feast may have overestimated the absorption in the direction of Sgri and NGC 6522.     

Investigation of the correlation between morphology and luminosity for two classes of main galaxies

Using an apparent-magnitude limited Main galaxy sample of the Sloan Digital Sky Survey Data Release 7(SDSS DR7), we investigate the correlation between morphologies and luminosity for the Main galaxy sample. Our Main galaxy sample is divided into two classes: Main galaxies only with TARGET_GALAXY flag (bestPrimtarget = 64), and ones also with other flags. It is found that for the second class Main galaxies, the early-type proportion monotonously increases with increasing luminosity nearly in the whole luminosity region. But for the first class Main galaxies, the early-type proportion increases with increasing luminosity only within a certain luminosity region (−22.2 < M _r  < −19.8). In the high luminosity region (M _r  < −22.2), the early-type proportion of the first class Main galaxies even decreases dramatically with increasing luminosity. We also analyze the correlation between morphologies and luminosity of galaxies around the peak of the redshift distribution ( 0.07 ≤ z ≤ 0.08 ). In such a narrow redshift region, we still observe strong correlation between morphologies and luminosity, which shows that this correlation is fundamental.     

Statistical confirmation of the UV spectral classification system based on IUE Low-Dispersion stellar spectra

In theIUE Low-Dispersion Spectra Reference Atlas (Hecket al., 1984a), a new spectral classification system specific to the UV had to be introduced because of the lack of one-to-one correspondence between the UV and visible ranges. It was elaborated from a classical morphological approach (Jaschek and Jaschek, 1984).This paper presents an independent confirmation of the correctness of this system. A statistical methodology working in a multidimensional parametric space has been applied to variables expressing, as objectively as possible, the information contained in the continuum and the spectral features of a set of stellar low-dispersion IUE spectra. This was done through, on the one hand, an asymmetry coefficient describing the continuum shape and empirically corrected for the interstellar reddening, and, on the other hand, the intensities of sixty objectively selected lines.These line intensities have been weighted in a way we called the variable Procrustean bed method because, contrary to a standard weighting where a variable is weighted in the same way for all the individuals of a sample, the spectral variables were weighted here according to the asymmetry coefficient which varies with the star at hand. The statistical algorithm consisted of a Principal-Component Analysis followed by a Cluster Analysis.The choice of the lines used for the morphological approach in the Atlas is shown to be correct. With respect to the UV classification system introduced in the atlas, the groups constructed by the cluster analysis display good homogeneity and discrimination for spectral types and luminosity classes, especially in the early spectral types which are well represented in the sample used for this study. The UV standard stars can be found in the neighbourhood of the barycenters of the groups. Moreover, the methodology developed here could be used in a later stage to predict UV spectral classifications.Based on observations collected by the International Ultraviolet Explorer (IUE) at the European Space Agency Villafranca Satellite Tracking Station (VILSPA) and on IUE data retrieved from the VILSPA data bank.     

Relationship between radio luminosity and optical surface brightness of spiral galaxies

It is shown that the radio luminosity of spiral galaxies correlates with their optical surface brightness. Galaxies with high surface brightness have stronger radio emission, on the average. Their ratio of radio and optical luminosities is also higher than for galaxies with lower surface brightness.     

Broad line and multi-wave luminosity relations in Fermi FSRQs

We study broad line and multi-wave luminosity relations for 80 flat-spectrum radio quasars (FSRQs) detected by Fermi LAT. Our results are as follows: for FSRQs, the correlations between logL _γ and logL _BLR, between logL _X and logL _BLR, between logL _O and logL _BLR, between logL _R and logL _BLR are significant; the correlation between logL _IR and logL _BLR (P=0.08) is not significant, but might be refereed as a “trend” of significant correlation. These results support a close link between jet formation and accretion disk, and the L _γ -L _BLR correlation suggests that the radiation mechanism of the γ-ray emission in FSRQs is likely to be inverse Compton scattering of seed photons from BLR or outflowing BLR.     

Infrared helium–hydrogen line ratios as a measure of stellar effective temperature

We have observed a large sample of compact planetary nebulae in the near-infrared to determine how the 2¹P–2¹S He  i line at 2.058 μm varies as a function of stellar effective temperature, T _eff. The ratio of this line with H  i Br γ at 2.166 μm has often been used as a measure of the highest T _eff present in a stellar cluster, and hence of whether there is a cut-off in the stellar initial mass function at high masses. However, recent photoionization modelling has revealed that the behaviour of this line is more complex than previously anticipated. Our work shows that in most aspects the photoionization models are correct. In particular, we confirm the weakening of the 2¹P–2¹S line as T _eff increases beyond 40 000 K. However, in many cases the model underpredicts the observed ratio when we consider the detailed physical conditions in the individual planetary nebulae. Furthermore, there is evidence that there is still significant 2¹P–2¹S He  i line emission even in the planetary nebulae with very hot     central stars. It is clear from our work that this ratio cannot be considered as a reliable measure of effective temperature on its own.     

On the relation between radius,luminosity, surface brightness,and surface density in elliptical galaxies

We have analyzed luminosity profiles of E galaxies studied by Strom and Strom in six clusters of galaxies. We have found a relationship between radius, luminosity, and surface brightness for galaxies in each of the clusters. A dependence of the zero point of the relation with the local projected density of galaxies is likewise found:r _{e proj} ^–0.14 L ^0.445 I _e ^–0.413 . This relationship implies (i) that there is not a universal luminosity profile for elliptical galaxies, (ii) the environmental variation of radius is larger than that produced by mergers of galaxies, (iii) distance to a galaxy can be estimated from apparent magnitude, surface brightness, angular size, and apparent local projected density of galaxies.     

On the influence of density and temperature fluctuations on the formation of spectral lines in stellar atmospheres

A method taking into account the influence of temperature and density flucuations generated by the velocity field in stellar atmospheres on the formation of spectral lines is presented. The influenced line profile is derived by exchanging the values in a static atmosphere by a mean value and a fluctuating one. The correlations are calculated with the help of the well-know hydrodynamic eqs. It results, that in normal stellar atmospheres the visual lines are only very weakly influenced by such fluctuations due to the small values of the gradients of the pressure and density and of the velocity dispersion.     

Period–luminosity relations for type II Cepheids and their application

JHK _s magnitudes corrected to mean intensity are estimated for Large Magellanic Cloud (LMC) type II Cepheids in the OGLE-III survey the third phase of the Optical Gravitational Lensing Experiment (OGLE). Period–luminosity (PL) relations are derived in JHK _s as well as in a reddening-free VI parameter. Within the uncertainties, the BL Her stars  ( P < 4 d)  and the W Vir stars (   P = 4  to 20 d) are colinear in these PL relations. The slopes of the infrared relations agree with those found previously for type II Cepheids in globular clusters within the uncertainties. Using the pulsation parallaxes of V553 Cen and SW Tau, the data lead to an LMC modulus uncorrected for any metallicity effects of  18.46 ± 0.10  mag. The type II Cepheids in the second-parameter globular cluster, NGC 6441, show a PL( VI ) relation of the same slope as that in the LMC, and this leads to a cluster distance modulus of  15.46 ± 0.11  mag, confirming the hypothesis that the RR Lyrae variables in this cluster are overluminous for their metallicity. It is suggested that the Galactic variable κ Pavonis is a member of the peculiar W Vir class found by the OGLE-III group in the LMC. Low-resolution spectra of OGLE-III type II Cepheids with   P > 20  d (RV Tau stars) show that a high proportion have TiO bands; only one has been found showing C₂. The LMC RV Tau stars, as a group, are not colinear with the shorter period type II Cepheids in the infrared PL relations in marked contrast to such stars in globular clusters. Other differences between LMC, globular cluster and Galactic field type II Cepheids are noted in period distribution and infrared colours.     

The stellar temperature scale for stars of spectral types from O8 to F6 and the standard deviation of the MK spectral classification

Empirical effective temperatures of 211 early-type stars found in a previous investigation (Kontizas and Theodossiou, 1980; Theodossiou, 1985) are combined with the effective temperatures of other 313 early-type stars found from literature. From these effective temperatures of a total number of 524 early-type stars of spectral types from O8 to F6 we derive a new stellar temperature scale and the standard deviation of the MK spectral classification.     

Approximating the solar/stellar energy generation rate,luminosity gradient,and the hydrogen content

In this paper we wish to develop an analytical technique for modelling the energy generation rate and the luminosity gradient for a one solar mass star like the Sun. Following the MDV method for approximating the solar/stellar structure (Doorish, 1989) we derive dimensionless variables for the energy generation rate and luminosity which are used in an expansion equation (on the order of a simple general confluent hypergeometric function). Modelling the luminosity of the Sun is complicated due to the nature of the energy generation rate. For this reason, we attempt to first derive approximation of the energy generation rate. The resulting solutions bring about a model of the luminosity gradient and the corresponding energy generation rate for the proton-proton cycle. Also, we are able to derive, analytically, the hydrogen content from the center of the star to the surface using concepts derived from the energy generation rate.Presented at the 2nd UN/ESA Workshop, held in Bogotá, Colombia, 9–13 November, 1992.     

Inclination- and dust-corrected galaxy parameters: bulge-to-disc ratios and size–luminosity relations

While galactic bulges may contain no significant dust of their own, the dust within galaxy discs can strongly attenuate the light from their embedded bulges. Furthermore, such dust inhibits the ability of observationally determined inclination corrections to recover intrinsic (i.e. dust-free) galaxy parameters. Using the sophisticated 3D radiative transfer model of Popescu et al. and Tuffs et al., together with the recent determination of the average face-on opacity by Driver et al. in nearby disc galaxies, we provide simple equations to correct (observed) disc central surface brightness and scalelengths for the effects of both inclination and dust in the B , V , I , J and K passbands. We then collate and homogenize various literature data sets and determine the typical intrinsic scalelengths, central surface brightness and magnitudes of galaxy discs as a function of morphological type. All galaxies have been carefully modelled in their respective papers with a Sérsic   R ^{1/ n}   bulge plus an exponential disc. Using the bulge magnitude corrections from Driver et al., we additionally derive the average, dust-corrected, bulge-to-disc flux ratio as a function of galaxy type. With values typically less than 1/3, this places somewhat uncomfortable constraints on some current semi-analytic simulations. Typical bulge sizes, profile shapes, surface brightness and deprojected densities are provided. Finally, given the two-component nature of disc galaxies, we present luminosity–size and (surface brightness)–size diagrams for discs and bulges. We also show that the distribution of elliptical galaxies in the luminosity–size diagram is not linear but strongly curved.     

Gravity and effective temperature distribution on the surface of contact binary systems

The aim of this work is to study the gravity and the effective temperature distribution on the outer equipotential surface of a contact binary. In particular, the lines of constant g, (the iso‐g curves), are computed and plotted on the outer surface C_s of contact configurations. Since the gravity distribution would also specify the effective temperature distribution on the binary's common photosphere through the gravity darkening effect, these lines correspond to isothermals too. Applications have been made to two contact binaries; namely, to AW UMa and OO Aql, two systems with very different mass ratios. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Symbiotic stars with spectral indication of bipolar ejection and stellar wind

Four out of a total of five symbiotic systems whose optical spectral lines contain satellite components indicating collimated ejection have an additional absorption of P Cyg type with different velocity. These systems are Z And, Hen 3‐1341, StHα 190, and BF Cyg. It is shown that the line profiles of every of these systems can be interpreted in the framework of a model of a collimated stellar wind from the hot compact component proposed initially to interpret the line profiles of Z And, observed during its last active phase after 2000. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Relationship between pulsars and stellar associations

The spatial distribution of pulsars and stellar associations in the solar neighborhood is investigated on the basis of recent data. The distribution of young pulsars and stellar associations indicates fragments of the spiral structure of the galaxy. The similar spatial distributions of young pulsars and stellar associations can be considered as confirmation of the hypothesis that pulsars originate in stellar associations. Translated fromAstrofizika, Vol. 40, No. 1, pp. 60–65, February, 1997.