Search

We report the detection of color gradients in six massive (stellar mass (Mstar) > 1010 M...) and passively evolving (specific star formation rate <10-11 yr-1) galaxies at redshift 1.3 < z < 2.5 identified in the Hubble Ultra Deep Field using ultra-deep Hubble Space Telescope (HST) Advanced Camera for Surveys and WFC3/IR images. After carefully matching the different point-spread functions, we obtain color maps and multi-band optical/near-IR photometry (BVizYJH) in concentric annuli, from the smallest resolved radial distance (≈1.7 kpc) up to several times the H-band effective radius. We find that the inner regions of these galaxies have redder rest-frame UV-optical colors (U - V, U - B, and B - V) than the outer parts. The slopes of the color gradient have no obvious dependence on the redshift and on the stellar mass of the galaxies. They do mildly depend, however, on the overall dust obscuration (E(B - V)) and rest-frame (U - V) color, with more obscured or redder galaxies having steeper color gradients. The z ~ 2 color gradients are also steeper than those of local early-type ones. The gradient of a single parameter (age, extinction, or metallicity) cannot fully explain the observed color gradients. Fitting the spatially resolved HST seven-band photometry to stellar population synthesis models, we find that, regardless of assumptions on the metallicity gradient, the redder inner regions of the galaxies have slightly higher dust obscuration than the bluer outer regions, implying that dust partly contributes to the observed color gradients, although the magnitude depends on the assumed extinction law. Due to the age-metallicity degeneracy, the derived age gradient depends on the assumptions for the metallicity gradient. We discuss the implications of a number of assumptions for metallicity gradients on the formation and evolution of these galaxies. We find that the evolution of the mass-size relationship from z ~ 2 to the present cannot be driven by in situ extended star formation, which implies that accretion or merger is mostly responsible for the growth of their stellar mass and size. The lack of a correlation between the strength of the color gradient and the stellar mass argues against the metallicity gradient predicted by the monolithic-collapse scenario, which would require significant major mergers to evolve into the one observed at the present. [ABSTRACT FROM AUTHOR]

We have carried out a survey for CO J= 1-0 and J= 2-1 emission in the 260 early-type galaxies of the volume-limited ATLAS sample, with the goal of connecting their star formation and assembly histories to their cold gas content. This is the largest volume-limited CO survey of its kind and is the first to include many Virgo cluster members. Sample members are dynamically hot galaxies with a median stellar mass ∼3 × 10 M; they are selected by their morphology rather than colour, and the bulk of them lie on the red sequence. The overall CO detection rate is 56/259 = 0.22 ± 0.03, with no dependence on the K luminosity and only a modest dependence on the dynamical mass. There are a dozen CO detections among the Virgo cluster members; statistical analysis of their H mass distributions and their dynamical status within the cluster shows that the cluster's influence on their molecular masses is subtle at best, even though (unlike spirals) they seem to be virialized within the cluster. We suggest that the cluster members have retained their molecular gas through several Gyr residences in the cluster. There are also a few extremely CO-rich early-type galaxies with H masses ≳10 M and these are in low-density environments. We do find a significant trend between the molecular content and stellar specific angular momentum. The galaxies of low angular momentum also have low CO detection rates, suggesting that their formation processes were more effective at destroying the molecular gas or preventing its re-accretion. We speculate on the implications of these data for the formation of various subclasses of early-type galaxies. [ABSTRACT FROM AUTHOR]

Type Ia supernovae (SNe Ia) spectra are compared using the coefficient of the largest wavelet scale in their decomposition. Two distinct subgroups have been identified, and their occurrence is discussed with regards to the use of SNe Ia as cosmological probes. Apart from the group of normal SNe, another trend characterized by intrinsically redder colours consists of many different SN events, which exhibit diverse properties. These include the interaction with the circumstellar material and the existence of a specific shell structure in or surrounding the SN ejecta or super-Chandrasekhar mass progenitors. Compared with normal objects, these SNe may violate the standard width-luminosity correction. This could influence the cosmological results if these are all calibrated equally, as their fraction among SNe Ia is not negligible when performing precision cosmology. Using the largest wavelet scale coefficient in combination with long-baseline colours, we show how to disentangle the SN intrinsic colour from the part that corresponds to the reddening as a result of dust extinction in the host galaxy in the colour parameter c. We discuss how the intrinsic colour differences may explain the different reddening laws for two subsamples. There are wavelength intervals for which the measured largest scale coefficient is invariant to the additional extinction applied to a spectrum. A combination of wavelet coefficients measured in different wavelength intervals can be used to develop a technique that allows for the estimation of extinction. [ABSTRACT FROM AUTHOR]

We present a search for galaxies at using the latest Hubble Space Telescope/Wide Field Camera 3 (WFC3) near-infrared data, based on the Lyman-break technique. We search for galaxies which have large colours (the '-drops') on account of the Lyman α forest absorption, and with colours inconsistent with being low-redshift contaminants. We identify 24 candidates at redshift (15 are robust and a further nine more marginal but consistent with being high redshift) over an area of arcmin. Previous searches for -drops with WFC3 have focused only on the Hubble Ultra Deep Field, and our larger survey (involving two other nearby deep fields and a wider area survey) has trebled the number of robust -drop candidates. For the first time, we have sufficient galaxies to fit both and of the UV Schechter luminosity function. There is evidence for evolution in this luminosity function from to , in the sense that there are fewer UV-bright galaxies at , consistent with an evolution mainly in . The candidate galaxies we detect have insufficient ionizing flux to reionize the Universe, and it is probable that galaxies below our detection limit provide a significant UV contribution. The faint-end slope, α, is not well constrained. However, adopting a similar faint-end slope to that determined at () and a Salpeter initial mass function (IMF), then the ionizing photon budget still falls short if , even integrating down to . A steeper faint-end slope or a low-metallicity population (or a top-heavy IMF) might still provide sufficient photons for star-forming galaxies to reionize the Universe, but confirmation of this might have to await the James Webb Space Telescope. [ABSTRACT FROM AUTHOR]

Following on from our discovery of a significant population of M31 outer halo globular clusters (GCs), and updates to the Revised Bologna Catalogue of M31 GCs, we investigate the GC system of M31 out to an unprecedented radius (≈120 kpc). We derive various ensemble properties, including the magnitude, colour and metallicity distributions, as well as the GC number density profile. One of our most significant findings is evidence for a flattening in the radial GC number density profile in the outer halo. Intriguingly, this occurs at a galactocentric radius of (∼30 kpc) which is the radius at which the underlying stellar halo surface density has also been shown to flatten. The GCs which lie beyond this radius are remarkably uniform in terms of their blue ( colours, consistent with them belonging to an ancient population with little to no metallicity gradient. Structural parameters are also derived for a sample of 13 newly discovered extended clusters (ECs), and we find the lowest luminosity ECs have magnitudes and sizes similar to Palomar-type GCs in the Milky Way halo. We argue that our findings provide strong support for a scenario in which a significant fraction of the outer halo GC population of M31 has been accreted. [ABSTRACT FROM AUTHOR]

We present the results of a 5 month photometric time-series survey for stellar rotation periods combined with a 4 year radial-velocity survey for membership and binarity in the 220 Myr open cluster M34. We report surface rotation periods for 120 stars, 83 of which are kinematic and photometric late-type cluster members. A comparison to previous work serves to illustrate the importance of high-cadence long baseline photometric observations and membership information. The new M34 periods are less biased against slow rotation and cleaned for non-members. The rotation periods of the cluster members span over more than an order of magnitude from 0.5 days up to 11.5 days, and trace two distinct rotational sequences-fast (C) and moderate-to-slow (1)-in the color-period diagram. The sequences represent two different states (fast and slow) in the rotational evolution of the late-type cluster members. We use the color-period diagrams for M34 and for younger and older clusters to estimate the timescale for the transition from the C to the sequence and find ≲150 Myr, ~150-300 Myr, and ~300-600 Myr for G, early-mid K, and late K dwarfs, respectively. The small number of stars in the gap between C and I suggests a quick transition. We estimate a lower limit on the maximum spin-down rate (dP/dt) during this transition to be ~0.06 days Myr-1 and ~-0.08 days Myr-1 for early and late K dwarfs, respectively. We compare the I sequence rotation periods in M34 and the Hyades for G and K dwarfs and find that K dwarfs spin down slower than the Skumanich 唅 rate. We determine a gyrochronology age of 240 Myr for M34. The gyro-age has a small formal uncertainty of 2% which reflects the tight I sequence in the M34 color-period diagram. We measure the effect of cluster age uncertainties on the gyrochronology age for M34 and find the resulting error on the gyro-age to be consistent with the ~15% error estimate for the technique in general. We use the M34 I sequence to redetermine the coefficients in the expression for rotational dependence on color used in gyrochronology. Finally, we propose that stability in the phase, shape, and amplitude of the photometric variability for the 120 rotators over the ~5 month duration of our survey is due to spot generation at active stellar longitudes. [ABSTRACT FROM AUTHOR]

The potential of combining Adaptive Optics (AO) and Lucky Imaging (LI) to achieve high-precision astrometry and differential photometry in the optical is investigated by conducting observations of the close 0.1 arcsec brown dwarf binary GJ569Bab. We took 50 000 -band images with our LI instrument FastCam attached to NAOMI, the 4.2-m William Herschel Telescope (WHT) AO facility. In order to extract the most of the astrometry and photometry of the GJ569Bab system we have resorted to a PSF fitting technique using the primary star GJ569A as a suitable PSF reference which exhibits an -band magnitude of . The AO+LI observations at WHT were able to resolve the binary system GJ569Bab located at arcsec from GJ569A. We measure a separation of mas and -band magnitudes of and and colours of 2.72 0.08 and 2.83 0.08 for the Ba and Bb components, respectively. Our study rules out the presence of any other companion to GJ569A down to magnitude at distances larger than 1 arcsec. The colours measured are consistent with M8.5-M9 spectral types for the Ba and Bb components. The available dynamical, photometric and spectroscopic data are consistent with a binary system with Ba being slightly (10-20 per cent) more massive than Bb. We obtain new orbital parameters which are in good agreement with those in the literature. [ABSTRACT FROM AUTHOR]

We present the results of photometric surveys for stellar rotation in the Hyades and in Praesepe, using data obtained as part of the SuperWASP exoplanetary transit-search programme. We determined accurate rotation periods for more than 120 sources whose cluster membership was confirmed by common proper motion and colour-magnitude fits to the clusters' isochrones. This allowed us to determine the effect of magnetic braking on a wide range of spectral types for expected ages of 600 Myr for the Hyades and Praesepe. Both clusters show a tight and nearly linear relation between colour and rotation period in the F, G and K spectral range. This confirms that loss of angular momentum was significant enough that stars with strongly different initial rotation rates have converged to the same rotation period for a given mass, by the ages of Hyades and Praesepe. In the case of the Hyades, our colour-period sequence extends well into the M dwarf regime and shows a steep increase in the scatter of the colour-period relation, with identification of numerous rapid rotators from 0.5 M down to the lowest masses probed by our survey (0.25 M). This provides crucial constraints on the rotational braking time-scales and further clears the way to use gyrochronology as an accurate age measurement tool for main-sequence stars. [ABSTRACT FROM AUTHOR]

We study the relationship between galaxy colour, stellar mass and local galaxy density in a deep near-infrared imaging survey up to a redshift of using the GOODS (Great Observatories Origins Deep Survey) NICMOS (Near Infrared Camera and Multi-Object Spectrometer) Survey (GNS). The GNS is a deep near-infrared Hubble Space Telescope survey imaging a total of 45 arcmin of the GOODS fields, reaching a stellar mass completeness limit of at . Using these data, we measure galaxy local densities based on galaxy counts within a fixed aperture, as well as the distance to the third, fifth and seventh nearest neighbour. We compare the average rest-frame colour and fraction of blue galaxies in different local densities and at different stellar masses. We find a strong correlation between colour and stellar mass at all redshifts up to . Massive red galaxies are already in place at at the expected location of the red sequence in the colour-magnitude diagram, although they are star forming. We do not find a strong correlation between colour and local density; however, there may be evidence that the highest overdensities are populated by a higher fraction of blue galaxies than average or underdense areas. This could be indicating that the colour-density relation at high redshift is reversed with respect to lower redshifts (), where higher densities are found to have lower blue fractions. Our data suggest that the possible higher blue fraction at extreme overdensities might be due to a lack of massive red galaxies at the highest local densities. [ABSTRACT FROM AUTHOR]

We derive the near- to mid-infrared period-magnitude relations and the period-bolometric luminosity relation of OGLE-III Mira-like variables in the Large Magellanic Cloud. The relations have a kink, and the period at which the break occurs is quantitatively obtained. There are many Mira-like variables whose fluxes at the optical and near-infrared wavebands are fainter than those predicted by the period-magnitude relations. The deviation is a result of the circumstellar extinction, and the amount of deviation is found to be strongly correlated with near-infrared colours. We derive the empirical formulae relating the amount of deviation and the near-infrared colours. These relations are useful to accurately calculate the distances to the dusty Mira-like variables, because the dimmed fluxes resulting from the circumstellar extinction can be estimated. In a manner analogous to the interstellar extinction law, the ratios of deviations at any two different wavebands are calculated. The ratios are found to change with the pulsation period, indicating that the dust properties are subject to change as Mira-like variables evolve. [ABSTRACT FROM AUTHOR]

Star-galaxy classification is one of the most fundamental data-processing tasks in survey astronomy and a critical starting point for the scientific exploitation of survey data. Star-galaxy classification for bright sources can be done with almost complete reliability, but for the numerous sources close to a survey's detection limit each image encodes only limited morphological information about the source. In this regime, from which many of the new scientific discoveries are likely to come, it is vital to utilize all the available information about a source, both from multiple measurements and from prior knowledge about the star and galaxy populations. This also makes it clear that it is more useful and realistic to provide classification probabilities than decisive classifications. All these desiderata can be met by adopting a Bayesian approach to star-galaxy classification, and we develop a very general formalism for doing so. An immediate implication of applying Bayes's theorem to this problem is that it is formally impossible to combine morphological measurements in different bands without using colour information as well; however, we develop several approximations that disregard colour information as much as possible. The resultant scheme is applied to data from the UKIRT Infrared Deep Sky Survey (UKIDSS) and tested by comparing the results to deep Sloan Digital Sky Survey (SDSS) Stripe 82 measurements of the same sources. The Bayesian classification probabilities obtained from the UKIDSS data agree well with the deep SDSS classifications both overall (a mismatch rate of 0.022 compared to 0.044 for the UKIDSS pipeline classifier) and close to the UKIDSS detection limit (a mismatch rate of 0.068 compared to 0.075 for the UKIDSS pipeline classifier). The Bayesian formalism developed here can be applied to improve the reliability of any star-galaxy classification schemes based on the measured values of morphology statistics alone. [ABSTRACT FROM AUTHOR]

This is the third paper of a series in which we present new measurements of the observed rates of supernovae (SNe) in the local Universe, determined from the Lick Observatory Supernova Search (LOSS). We have considered a sample of SNe and used an optimal subsample of 726 SNe (274 SNe Ia, 116 SNe Ibc and 324 SNe II) to determine our rates. We study the trend of the rates as a function of a few quantities available for our galaxy sample, such as luminosity in the and bands, stellar mass and morphological class. We discuss different choices (SN samples, input SN luminosity functions, inclination correction factors) and their effect on the rates and their uncertainties. A comparison between our SN rates and the published measurements shows that they are consistent with each other to within the uncertainties when the rate calculations are done in the same manner. Nevertheless, our data demonstrate that the rates cannot be adequately described by a single parameter using either galaxy Hubble types or colours. A secondary parameter in galaxy 'size', expressed by luminosity or stellar mass, is needed to adequately describe the rates in the rate-size relation: the galaxies of smaller sizes have higher SN rates per unit mass or per unit luminosity. The trends of the SN rates in galaxies of different Hubble types and colours are discussed. We examine possible causes for the rate-size relation. Physically, such a relation for the core-collapse SNe is probably linked to the correlation between the specific star-formation rate and the galaxy sizes, but it is not clear whether the same link can be established for SNe Ia. We discuss the two-component ('tardy' and 'prompt') model for SN Ia rates, and find that the SN Ia rates in young stellar populations might have a strong correlation with the core-collapse SN rates. We derive volumetric rates for the different SN types [e.g. for SNe Ia, a rate of SN Mpc yr at redshift 0] and compare them to the measurements at different redshifts. Finally, we estimate the SN rate for the Milky Way Galaxy to be SNe per century (with a systematic uncertainty of a factor of ), consistent with published SN rates based on several different techniques. [ABSTRACT FROM AUTHOR]

The dynamical processes that lead to open cluster disruption cause its mass to decrease. To investigate such processes from the observational point of view, it is important to identify open cluster remnants (OCRs), which are intrinsically poorly populated. Due to their nature, distinguishing them from field star fluctuations is still an unresolved issue. In this work, we developed a statistical diagnostic tool to distinguish poorly populated star concentrations from background field fluctuations. We use 2MASS photometry to explore one of the conditions required for a stellar group to be a physical group: to produce distinct sequences in a colour-magnitude diagram (CMD). We use automated tools to (i) derive the limiting radius; (ii) decontaminate the field and assign membership probabilities; (iii) fit isochrones; and (iv) compare object and field CMDs, considering the isochrone solution, in order to verify the similarity. If the object cannot be statistically considered as a field fluctuation, we derive its probable age, distance modulus, reddening and uncertainties in a self-consistent way. As a test, we apply the tool to open clusters and comparison fields. Finally, we study the OCR candidates DoDz 6, NGC 272, ESO 435 SC48 and ESO 325 SC15. The tool is optimized to treat these low-statistic objects and to separate the best OCR candidates for studies on kinematics and chemical composition. The study of the possible OCRs will certainly provide a deep understanding of OCR properties and constraints for theoretical models, including insights into the evolution of open clusters and dissolution rates. [ABSTRACT FROM AUTHOR]

The evolutionary path followed by a galaxy shapes its internal structure, and, in particular, its internal colour variation. We present a study of the internal colour variation within galaxies from the Seventh Data Release of the Sloan Digital Sky Survey (SDSS DR7). We statistically study the connection between the internal colour variation and global galactic properties, looking for hints of the recent galactic evolution. Considering only galaxies with good photometry and spectral measurements, we define four luminosity-threshold samples within the redshift range , each containing more than 48 000 galaxies. Colour gradients are calculated for these galaxies from the surface brightness measurements provided by the SDSS DR7. Possible systematic effects in their determination have been analysed. We find that, on average, galaxies have redder cores than their external parts. We also find that it is more likely to find steep colour gradients among late-type galaxies. This result holds for a range of classifications based on both morphological and spectral characteristics. In fact, our results relate, on average, steep colour gradients to a higher presence of young stars within a galaxy. Our results also suggest that nuclear activity is a marginal driver for creating steep colour gradients in massive galaxies. We have selected pairs of interacting galaxies, with a separation of 5 arcmin, in projected radius, and a difference in redshift of , finding that they present steeper gradients than the average population, skewed towards bluer cores. Our analysis implies that colour gradients can be useful for selecting galaxies that have suffered a recent (minor) burst of star formation. [ABSTRACT FROM AUTHOR]

Observations show that nearly all star clusters and stellar populations contain blue straggler stars (BSs). BSs in a cluster can significantly enhance the integrated spectrum of the host population, preferentially at short wavelengths, and render it much bluer in photometric colours. Current theoretical simple stellar population (SSP) models constructed within the traditional framework of single and binary stellar evolution cannot fully account for the impact of these objects on the integrated spectral properties of stellar populations. Using conventional SSP models without taking into account BS contributions may significantly underestimate a cluster's age and/or metallicity, simply because one has to balance the observed bluer colours (or a bluer spectrum) with a younger age and/or a lower metallicity. Therefore, inclusion of BS contributions in SSP models is an important and necessary improvement for population synthesis and its applications. Here, we present a new set of SSP models, which include BS contributions based on our analysis of individual star clusters. The models cover the wavelength range from 91 Å to 160 μm, ages from 0.1 to 20 Gyr and metallicities (solar) and 0.05. We use the observed integrated spectra of several Magellanic Cloud star clusters to cross-check and validate our models. The results show that the age predictions from our models are closer to those from isochrone fitting in the clusters' colour-magnitude diagrams compared to age predictions based on standard SSP models. [ABSTRACT FROM AUTHOR]

We present a study characterizing the environments of galaxies in the redshift range of based on data from the POWIR near-infrared imaging and DEEP2 spectroscopic redshift surveys, down to a stellar mass of . Galaxy environments are measured in terms of nearest-neighbour densities as well as fixed aperture densities and kinematical and dynamical parameters of neighbour galaxies within a radius of Mpc. We disentangle the correlations between galaxy stellar mass, galaxy colour and galaxy environment, using not only galaxy number densities, but also other environmental characteristics such as velocity dispersion, mean harmonic radius and crossing time. We find that galaxy colour and the fraction of blue galaxies depends very strongly on stellar mass at , while a weak additional dependence on local number densities is in place at lower redshifts . This environmental influence is most visible in the colours of intermediate-mass galaxies , whereas colours of lower- and higher-mass galaxies remain largely unchanged with redshift and environment. At a fixed stellar mass, the colour-density relation almost disappears, while the colour-stellar mass relation is present at all local densities. We find a weak correlation between stellar mass and environment at intermediate redshifts, which contributes to the overall colour-density relation. We furthermore do not find a significant correlation between galaxy colour and virial mass, i.e. parent dark matter halo mass. Galaxy stellar mass thus appears to be the crucial defining parameter for intrinsic galaxy properties such as ongoing star formation and colour. [ABSTRACT FROM AUTHOR]

ABSTRACT We assembled a catalogue of bright, hot subdwarf and white dwarf stars extracted from a joint ultraviolet, optical and infrared source list. The selection is secured using colour criteria that correlate well with effective temperatures Teff≳ 12 000 K. We built a NUV− V versus V − J diagram for ≳60 000 bright sources using the Galaxy Evolution Explorer ( GALEX) NUV magnitude ( NUV < 14), the associated Guide Star Catalog (GSC2.3.2) photographic quick- V magnitude and the Two-Micron All-Sky Survey J and H magnitudes. This distillation process delivered a catalogue of ≈700 sources with NUV− V < 0.5 comprising ∼160 known hot subdwarf stars and another ∼60 known white dwarf stars. A reduced proper-motion diagram built using the proper-motion measurements extracted from the Naval Observatory Merged Astrometric Dataset allowed us to identify an additional ∼120 new hot subdwarf candidates and ∼10 hot white dwarf candidates. We present a spectroscopic study of a subset of 52 subdwarfs, 48 of them analysed here for the first time, and with nine objects brighter than V ∼ 12. Our sample of spectroscopically confirmed hot subdwarfs comprises 10 sdO-type stars and 42 sdB-type stars suitable for pulsation and binary studies. We also present a study of 50 known white dwarfs selected in the GALEX survey and six new white dwarfs from our catalogue of subluminous candidates. Ultraviolet, optical and infrared synthetic magnitudes employed in the selection and analysis of white dwarf stars are listed in appendix. [ABSTRACT FROM AUTHOR]

Abstract: Using the 2m telescope of the Turingia State Observatory at Tauten-berg (TLS), imaging observations in 3 wavebands (Hα, R and I) are performed in the 16 fields in the Aquila Rift region. The observed fields cover about 7 square degrees. Excluding the 3 fields with unqualified data, the photometrical analysis is made for the remaining 13 fields, from which point sources are identified, and finally 7 Hα emission-line star candidates are identified by color-color diagrams. The 7 candidates are located in five fields. Three of them are located near the Galactic plane, while the galactic latitudes of the rest are greater than 4°. The 2M ASS counterparts of the point sources are identified, and the properties of the 7 Hα emission-line star candidates are further analyzed by using the two-color diagrams. It is found that the near-infrared radiation from these Hα emission-line star candidates has no obvious infrared excess, one of them even falls on the main-sequence branch. This indicates that the Hα-emissive young stellar objects (YSOs) are not always accompanied with the infrared excess, and that the results of the Hα emission line observation and the infrared excess observation are mutually supplemented. If the 7 Hα emission-line star candidates are regarded as YSO candidates, then the number of YSOs in the Aquila Rift region is quite small. The further confirmation of these candidates needs subsequent spectral observations. [ABSTRACT FROM AUTHOR]

Abstract: A new intrinsic-colour calibration is presented for the uvby-β photometric system, making use of re-calibrated Hipparcos parallaxes and published reddening maps. This new calibration for , our Eq. , has been based upon stars with in the photometric catalogues of , , provides a small dispersion, , and has a positive “standard” coefficient, which is not too different from the coefficients of and of . For 61 stars with spectra from CASPEC, UVES/VLT, and FIES/NOT databases, without detectable Na I lines, the average reddening value shows that any zero-point correction to our intrinsic-colour equation must be minuscule. [Copyright &y& Elsevier]

We present the spectroscopic study of 5 Herbig Ae/Be stars in young open clusters. These are identified from a survey of emission-line stars in young open clusters. The Herbig Ae/Be stars are found to show a linear correlation in Hα equivalent width versus reddening corrected (H-Ks) colour plot, with a clear offset from the distribution of Classical Be stars. The candidates are found to show near infrared excess which was revealed through de-reddened (J-H) versus (H-Ks) colour-colour diagram and spectral energy distribution. From optical/near-IR photometry and spectroscopy, we suggest that Bochum 6-1, IC 1590-1 and NGC 6823-1 are Herbig Be while IC 1590-2 and NGC 7380-4 are Herbig Ae candidates. Bochum 6-1 is an interesting Herbig B[e] star with a high Hα equivalent width of -206 Å, which is the highest among the surveyed stars. We combined the optical and near-IR photometry to estimate the duration of star formation in the clusters, Bochum 6, IC 1590, NGC 6823 and NGC 7380. We found ongoing star formation in all these clusters, with an appreciable number of pre-main sequence stars. The age of these Herbig Ae/Be stars, estimated using pre-main sequence isochrones, were found to range between 0.25-3 Myr. IC 1590 is found to be an interesting young cluster (~ 4 Myr) with 3 emission stars, each belonging to Herbig Ae, Herbig Be and Classical Be respectively. All the four clusters studied here were found to be forming stars for the last 10 Myr. [ABSTRACT FROM AUTHOR]

We present and discuss the results of our long-term JHKLM photometry for two RV Tau stars (R Sge and RV Tau) and the yellow supergiant V1027 Cyg, a candidate for protoplanetary nebulae. The amplitude of the infrared brightness variations in R Sge and RV Tau over fourteen years of observations was 09−1 m; the infrared brightness variations in V1027 Cyg over eighteen years did not exceed 025. The infrared brightness and color of R Sge fluctuated about their gradually changing mean values; the infrared brightness variations agree with a period of 70.77 days. The periodic J brightness and J-H color variations in R Sge can be explained by temperature pulsations with Δ T ≤ 200 K and radial pulsations with [Δ R/R] ≤ 0.2. From 1995 to 2008, the mean J brightness of RV Tau increased, while its mean J-H color index decreased; the variations in the mean J brightness can be associated mainly with stellar temperature variations; a periodic component with P = 78.73 days is observed in the infrared brightness and color fluctuations. The variations in the mean J brightness and J-H color index of the supergiant V1027 Cyg over eighteen years of observations did not exceed a few hundredths of a magnitude; both temperature and radial pulsations may be present in the observed J brightness variations. The most probable period of the infrared brightness fluctuations in V1027 Cyg is 237 ± 2 days. The dust shell of R Sge may consist of two layers with grain temperatures of ∼1000 and ∼700 K; the optical depth at 1.25 µm is ∼0.02 and ∼0.24, respectively. The grain temperatures in the circumstellar dust shells of the supergiants RV Tau and V1027 Cyg are ∼600 K (RV Tau) and ∼700 K (V1027 Cyg). Their optical depths at 1.25 µm are ∼0.24 (RV Tau) and ∼0.008 (V1027 Cyg). [ABSTRACT FROM AUTHOR]

Using the Sloan Digital Sky Survey Data Release 4 group catalogue of Yang et al., we investigate sizes, concentrations, colour gradients and surface brightness profiles of central and satellite galaxies. We compare central and satellite galaxies at fixed stellar mass, in order to disentangle environmental from stellar mass dependencies. Early- and late-type galaxies are defined according to concentration. We find that at fixed stellar mass, late-type satellite galaxies have smaller radii and larger concentrations than late-type central galaxies. No such differences are found for early-type galaxies. We have also constructed surface brightness and colour profiles for the central and satellite galaxies in our sample. We find that late-type satellite galaxies have a lower surface brightness and redder colours than late-type central galaxies. We show that all observed differences between satellite and central galaxies can be explained by a simple fading model, in which the star formation in the disc decreases over time-scales of 2–3 Gyr after a galaxy becomes a satellite. Processes that induce strong morphological changes (e.g. harassment) and processes that strip the galaxy of its entire interstellar medium need not to be invoked in order to explain the environmental dependencies we find. [ABSTRACT FROM AUTHOR]

This paper presents the initial results of a multi-site photometric programme to examine the extraordinary behaviour displayed by 18 R Coronae Borealis (RCB) stars in the Magellanic Clouds (MCs). RCB stars exhibit a unique variability whereby they undergo rapid declines of up to several magnitudes. These are thought to be caused by the formation of dust in the stellar environment which reduces the brightness. The monitoring programme comprised the collection of UBVRIphotometric data using five telescopes located at three different southern hemisphere longitudes (Las Campanas Observatory in Chile, Mount John University Observatory in New Zealand, and the Southern African Large Telescope, SALT, in South Africa). Examination of the data acquired in the Vand Ifilters resulted in the identification of a total of 18 RCB declines occurring in four stars. Construction of colour–magnitude diagrams (Vversus V– I), during the recovery to maximum light were undertaken in order to study the unique colour behaviour associated with the RCB declines. The combined recovery slope for the four stars was determined to be 3.37 ± 0.24, which is similar to the value of 3.1 ± 0.1 calculated for galactic RCB stars (Skuljan et al. 2003). These results may imply that the nature of the dust (i.e. the particle size) is similar in both our Galaxy and the MCs. [ABSTRACT FROM AUTHOR]

The nature of the source G25.5+0.2 is open to considerable uncertainty, and it has variously been identified as being a young stellar outflow, an H ii region, a young supernova remnant and a planetary nebula. We present further Two-Micron All-Sky Survey (2MASS), Mid-course Space Experiment (MSX) and Spitzer near- (NIR) and mid-infrared (MIR) photometry and mapping in an attempt to clarify the properties of the source. It is apparent that the morphology varies appreciably with MIR wavelength and indicates the presence of a bipolar structure and probable circum-nebular torus. It is also noted that the 2MASS stellar photometry is quite disparate from that of earlier NIR measures and implies an appreciable variation in the brightness and colour of the central star. These two factors, the nuclear variability and bipolarity, combine to suggest the G25.5+0.2 may be identifiable as a symbiotic outflow. The nebular continuum peaks at wavelengths , and may be fitted using a fall-off in grain emissivity, together with a model in which the grains have temperatures and 130 K. An analysis of dust/gas mass ratios also implies that , depending upon the types of grains and temperatures involved. Such a range is typical of what is observed in planetary nebulae and symbiotic outflows. [ABSTRACT FROM AUTHOR]

We have studied the variability of the H β line and the adjacent continuum in the spectrum of the Seyfert galaxy Ark 120, based on spectral observations of the galaxy’s nucleus obtained in the Crimea in 1992–2005, supplemented by published data for 1988–1996. Irregular variability on various timescales (years to days) can be accompanied by periodic brightness variations in both the continuum and the H β line, with a period of P ∼ 430 days and an amplitude of Δ m ∼ 0.2 m in the continuum, which were traced for more than 13 cycles. In total, in 1988–2005, the flux variations in the line lag those in the continuum by 55 ± 9 days if calculated from the peak of the cross correlation function, or by 72 ± 7 days, if calculated from the centroid of the CCF. The delay is correlated with the continuum brightness, increasing when the continuum flux increases. The H β line profiles indicate both a high degree of diversity and the presence of features that recur after various extended time intervals. Analysis of the evolution of the differences between each individual normalized line profile and the mean normalized profile indicates systematic motion of excesses relative to the average profile from negative to positive radial velocities. In contrast, parts of the H β line with low radiation relative to the mean normalized profile evolve in the opposite direction (from the red to the blue H β wing). This pattern is also typical for the rotating broad-line region, if this region has the form of a disk. The rotation period exceeds 9000–10000 days, or 25–27 years. The size of the broad-line region calculated form this period corresponds to a reverberation time of no fewer than 30 days, consistent with the results of cross-correlation analysis. [ABSTRACT FROM AUTHOR]

For this paper, we collected all infrared carbon stars (IRCSs) known so far from the literature and identified the 2MASS counterparts of all IRCSs. Using 2MASS, IRAS and ISO SWS data, we investigate the infrared properties of IRCSs. We find that the infrared colors and temperatures of IRCSs—not only in the IRAS region but also in the near infrared—are between those for visual carbon stars and extreme carbon stars. The results in this paper strongly support the suggestion that the sequence of visual carbon stars → infrared carbon stars → extreme carbon stars is the evolutionary sequence in the AGB phase for carbon-rich stars. In addition, using the ISO SWS data, we find that an evolutionary sequence also exists within the IRCS stage. [ABSTRACT FROM AUTHOR]

Abstract: 15257 face-on disk galaxies are selected from the second group of data released by the Sloan Digital Sky Survey, and their color-magnitude relation is studied statistically. It is shown that the three colors g − r, r − i and r − z are each closely correlated with the r-band absolute magnitude, i.e., the brighter the galaxies, the redder the colors, and the smaller the dispersion of color indexes. The implications of the observed color-magnitude relations on the star formation history of disk galaxies are briefly discussed. [Copyright &y& Elsevier]

The luminosities and effective temperatures, as well as the whole bolometric light curves of non-linear convective RR Lyrae models with , are transformed into the Sloan Digital Sky Survey (SDSS) photometric system. The obtained ugriz light curves, mean magnitudes and colours, pulsation amplitudes and colour–colour loops are shown and analytical relations connecting pulsational to intrinsic stellar parameters, similar to the ones currently used in the Johnson–Cousins filters, are derived. Finally, the behaviour in the colour–colour planes is compared with available observations in the literature and possible systematic uncertainties affecting this comparison are discussed. [ABSTRACT FROM AUTHOR]

We selected from the VLA FIRST survey a sample of 94 objects with star-like counterparts in the Sloan Digital Sky Survey (SDSS), and with APM POSS-I colour , i.e. consistent with their being high-redshift quasars. 78 of the 94 candidates can be classified spectroscopically on the basis of either published data (mainly SDSS) or the observations presented here. The fractions of QSOs (51 out of 78) and redshift QSOs (23 out of 78, 29 per cent) are comparable to those found in other photometric searches for high-redshift QSOs. We confirm that selecting colour ensures inclusion of all QSOs with . The fraction of QSOs with broad absorption lines (BALs) is 27 ± 10 per cent (7/26) and the estimated BAL fraction for radio-loud QSOs is at least as high as for optically selected QSOs (∼13 per cent). Both the high BAL fraction and the high fraction of low-ionization BALs among BALs (four to five out of seven) in our sample, compared to previous work, are likely due to the red colour selection . The space density of radio-loud QSOs in the range with and is . Adopting a radio-loud fraction of 13.4 ± 3 per cent, this corresponds to , in substantial agreement with the cumulative luminosity function of SDSS QSOs in Fan et al. We note the unusual flat-spectrum radio-luminous QSO FIRST 1413+4505 , which shows strong associated Lyα absorption (rest-frame equivalent width ∼40 Å) and an extreme observed luminosity, . [ABSTRACT FROM AUTHOR]

We analyse how the properties of galaxies in groups identified in the Sloan Digital Sky Survey depend on the environment. In particular, we study the relationship between galaxy properties and group mass and group-centric distance. Among the galaxy properties we have considered here, we find that the colour is the most predictive parameter for group mass, while the most predictive pair of properties are colour and r-band absolute magnitude. Regarding the position inside the systems, the colour is the best tracer of group-centric distance and the most predictive pair of properties are colour and spectral type taken together. These results remain unchanged when a subsample of high-mass groups is analysed. The same happens if the brightest group galaxies are excluded. [ABSTRACT FROM AUTHOR]

We revisit the bimodal distribution of the galaxy population commonly seen in the local universe. Here, we address the bimodality observed in galaxy properties in terms of spectral synthesis products, such as mean stellar ages and stellar masses, derived from the application of this powerful method to a volume-limited sample, with magnitude limit cut-off , containing about 50 000 luminous galaxies from the Sloan Digital Sky Survey (SDSS) Data Release 2 (DR2). In addition, galaxies are classified according to their emission-line properties in three distinct spectral classes: star-forming galaxies, with young stellar populations; passive galaxies, dominated by old stellar populations; and hosts of active nuclei, which comprise a mix of young and old stellar populations. We show that the extremes of the distribution of some galaxy properties, essentially galaxy colours, 4000 Å break index and mean stellar ages, are associated to star-forming galaxies at one side, and passive galaxies at another. We find that the mean light-weighted stellar age of galaxies is directly responsible for the bimodality seen in the galaxy population. The stellar mass, in this view, has an additional role since most of the star-forming galaxies present in the local universe are low-mass galaxies. Our results also give support to the existence of a ‘downsizing’ in galaxy formation, where massive galaxies seen nowadays have stellar populations formed at early times. [ABSTRACT FROM AUTHOR]

Distances to nine dark globules are determined by a method using optical ( VRI) and near-infrared (near-IR) ( JHK) photometry of stars projected towards the field containing the globules. In this method, we compute intrinsic colour indices of stars projected towards the direction of the globule by dereddening the observed colour indices using various trial values of extinction and a standard extinction law. These computed intrinsic colour indices for each star are then compared with the intrinsic colour indices of normal main-sequence stars and a spectral type is assigned to the star for which the computed colour indices best match with the standard intrinsic colour indices. Distances ( d) to the stars are determined using the and absolute magnitude corresponding to the spectral types thus obtained. A distance versus extinction plot is made and the distance at which undergoes a sharp rise is taken to be the distance to the globule. All the clouds studied in this work are in the distance range 160–400 pc. The estimated distances to dark globules LDN 544, LDN 549, LDN 567, LDN 543, LDN 1113, LDN 1031, LDN 1225, LDN 1252 and LDN 1257 are and 250 ± 50 pc, respectively. Using the distances determined, we have estimated the masses of the globules and the far-IR luminosity of the IRAS sources associated with them. The mass of the clouds studied are in the range . [ABSTRACT FROM AUTHOR]

The near-infrared (NIR) extinction power-law index (β) and its uncertainty is derived from three different techniques based on star counts, colour excess and a combination of them. We have applied these methods to Two Micron All Sky Survey (2MASS) data to determine maps of β and NIR extinction of the small cloud IC 1396 W. The combination of star counts and colour excess results in the most reliable method to determine β. It is found that the use of the correct β map to transform colour excess values into extinction is fundamental for column density profile analysis of clouds. We describe how artificial photometric data, based on the model of stellar population synthesis of the Galaxy, can be used to estimate uncertainties and derive systematic effects of the extinction methods presented here. We find that all colour excess based extinction determination methods are subject to small but systematic offsets, which do not affect the star counting technique. These offsets occur since stars seen through a cloud do not represent the same population as stars in an extinction-free control field. [ABSTRACT FROM AUTHOR]

In this paper we present charge-coupled device (CCD) images in the Johnson B and V and Kron–Cousins I passbands for the previously unstudied open cluster NGC 5288. The sample consists of 15 688 stars reaching down to V∼ 20.5. The cluster appears to have a relatively small but conspicuous nucleus and a low-density extended coronal region. Star counts carried out in 25 × 25 pixel2 boxes distributed throughout the whole observed field allowed us to estimate the angular core and corona radii as ∼1.3 and 6.3 arcmin, respectively. Our analysis suggests that NGC 5288 is moderately young and probably more metal-rich than the Sun. Adopting the theoretical metal content Z= 0.040, which provides the best global fit, we derive an age of . Simultaneously, we have obtained colour excesses E( B− V) = 0.75 and E( V− I) = 0.95 and an apparent distance modulus . The law of interstellar extinction in the cluster direction is found to be normal. NGC 5288 is located at 2.1 ± 0.3 kpc from the Sun beyond the Carina spiral feature and ∼7.4 kpc from the Galactic Centre. The cluster metallicity seems to be compatible with the cluster position in the Galaxy, given the recognized radial abundance gradient in the disc. For the first time, in this paper we determine the basic parameters for the open cluster NGC 5381, situated in the same direction as NGC 5288. This determination was reached by using CCD VI data published almost a decade ago by Pietrzyński et al. (1997) for NGC 5381. The properties of some open clusters aligned along the line of sight of NGC 5288 are examined. The properties of clusters of similar ages to NGC 5288 are also looked into. Evidence is presented that these did not form mainly along the spiral arms but rather in the thin Galactic disc ( Z∼±100 pc). [ABSTRACT FROM AUTHOR]

Galaxy colours are determined for two samples of 6C and 3CR radio sources at z∼ 1, differing by a factor of ∼6 in radio power. Corrections are made for emission-line contamination and the presence of any nuclear point source, and the data analysed as a function of both redshift and the radio source properties. The galaxy colours are remarkably similar for the two populations, and the ultraviolet excess evolves with radio source size similarly in both samples, despite the fact that the alignment effect is more extensive for the more powerful 3CR radio galaxies. These results seem to suggest that the alignment effect at these redshifts does not scale strongly with radio power, and is instead more closely dependent on galaxy mass (which is statistically comparable for the two samples). However, it is likely that the presence of relatively young (≲several times 108-yr-old) stellar populations has considerably contaminated the K-band flux of these systems, particularly in the case of the more powerful 3CR sources, which are ∼0.5 mag more luminous than the predictions of passive evolution models at z∼ 1. The higher luminosity of the 3CR alignment effect is balanced by emission at longer wavelengths, thereby leading to comparable colours for the two samples. [ABSTRACT FROM AUTHOR]

We compile a sample of Sun-like stars with accurate effective temperatures, metallicities and colours (from the ultraviolet to the near-infrared). A crucial improvement is that the effective temperature scale of the stars has recently been established as both accurate and precise through direct measurement of angular diameters obtained with stellar interferometers. We fit the colours as a function of effective temperature and metallicity, and derive colour estimates for the Sun in the Johnson–Cousins, Tycho, Strömgren, 2MASS and SDSS photometric systems. For , we favour the ‘red’ colour 0.64 versus the ‘blue’ colour 0.62 of other recent papers, but both values are consistent within the errors; we ascribe the difference to the selection of Sun-like stars versus interpolation of wider colour– Teff–metallicity relations. [ABSTRACT FROM AUTHOR]

The colors of globular clusters in most large elliptical galaxies are bimodal. This is generally taken as evidence for the presence of two cluster subpopulations that have different geneses. However, here we find that, because of the nonlinear nature of the metallicity-to-color transformation, a coeval group of old clusters with a unimodat metallicity spread can exhibit color bimodality. The models of cluster colors indicate that horizontal-branch stars are the main drivers behind the empirical nonlinearity. We show that the scenario gives simple and cohesive explanations for all the key observations and could simplify theories of elliptical galaxy formation. [ABSTRACT FROM AUTHOR]

The colours of high-redshift Type II quasi-stellar objects (QSOs) are synthesized from observations of moderate-redshift systems. It is shown that Type II QSOs are comparable to starbursts at matching the colours of z850-dropouts and i775-drops in the Hubble UltraDeep Field, and more naturally account for the bluest objects detected. Type II QSOs may also account for some of the i775-drops detected in the Great Observatories Origins Deep Survey (GOODS) fields. It is shown that by combining imaging data from the Hubble Space Telescope and the James Webb Space Telescope, it will be possible to clearly separate Type II QSOs from Type I QSOs and starbursts based on their colours. Similarly, it is shown that the United Kingdom Infrared Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS) ZYJ filters may be used to discriminate high-redshift Type II QSOs from other objects. If Type II QSOs are prevalent at high redshifts, then active galactic nuclei (AGNs) may be major contributors to the re-ionization of the intergalactic medium. [ABSTRACT FROM AUTHOR]

We substantiate the conclusion that the unusual color variability found previously in some eruptive stars is typical of a broad class of nonstationary objects, manifests itself over a wide temperature range (from B0 to K3), and can be regarded as a new type of stellar variability.© 2005 Pleiades Publishing, Inc. [ABSTRACT FROM AUTHOR]

Relationships between two interstellar reddening-free quantities, the photometric parameter Q and the Balmer jump D, are derived for main sequence O9-G0 stars. We compare our average values of the Balmer jumps for main sequence stars with a theoretical model of the Balmer jump as a function of the effective temperature for main sequence stars. The theoretical and observational data, as well as our calculations, are shown to correlate well. [ABSTRACT FROM AUTHOR]

Some features of a sample of galaxies from the Second Byurakan Survey are discussed. Most of them are small galaxies with star formation. It is shown that different types of galaxies are in the Survey, with high, medium, and low levels of excitation and continua ranging from blue to relatively red. Many of the galaxies have absorption lines in their spectra; their continua are formed by stars in later spectral classes. [ABSTRACT FROM AUTHOR]

A total of 6083 photoelectric observations in the BVI C system, obtained between February and April 2002, are presented for 122 Cepheids located in the southern hemisphere. The main function of the photometry is to provide data that can be used to establish new epochs of maximum brightness for the Cepheids as input for the scientific study of their period changes, as well as to establish current light elements for the variables. The primary cause of period changes in Cepheids is evolution through the instability strip. [ABSTRACT FROM AUTHOR]

Utilizing the Two Micron All Sky Survey (2MASS) All-Sky Data Release Catalog, we have retrieved useful near-IR J, H and KS magnitudes for more than 800 hot subdwarfs (sdO and sdB stars) drawn from the Catalogue of Spectroscopically Identified Hot Subdwarfs (Kilkenny, Heber, and Drilling, 1988, 1992). This sample size greatly exceeds previous studies of hot subdwarfs. We find that of the hot subdwarfs in Kilkenny et al., ∼40% in a magnitude-limited sample have colors that are consistent with the presence of an unresolved late-type (FGK) companion. Binary stars are over-represented in a magnitude-limited sample. In an approximately volume-limited sample the fraction of composite-color binaries is ∼25%. [ABSTRACT FROM AUTHOR]

The utility of pulsations for the investigation of the structure and evolution of subdwarf B stars is considerable. However, the small number of detected modes generally limits the potential for a traditional seismological analysis such as that carried out for the Sun. Therefore, it is crucial to acquire additional primary data to characterise the stellar oscillations more completely. We review recent studies of radial velocity amplitudes and introduce new multi-colour photometry of small-amplitude sdBV stars. We also discuss a set of models for radial velocity and colour variations and demonstrate how these may be used to infer the spherical and azimuthal degrees of observed pulsations. [ABSTRACT FROM AUTHOR]

We combine a grid of high-resolution theoretical intensity spectra with simulated non-radially pulsating stellar surfaces for a variety of pulsation modes. These are used to simulate the light curves in a number of photometric pass bands. We attempt to provide diagnostics for identifying the modes in pulsating sdB stars KPD 2109+4401 and HS 0039+4302. [ABSTRACT FROM AUTHOR]

We have calibrated and combined an extensive set of BVI observations of M67 to produce a colour–magnitude diagram of stars measured with high relative precision. We have selected stars that are most likely to be single-star members of the cluster using proper motion, radial velocity, and variability information from the literature, and an examination of the most probable colour–magnitude diagram locations of unresolved stellar blends. We have made detailed comparisons of our photometry of the selected stars with theoretical models, and discuss the most notable discrepancies. Observations of M67 turn-off stars are a severe test of algorithms attempting to describe convective cores in the limit of small extent, and we find strong evidence of a ‘hook’ just fainter than the turn-off gap. The stars in M67 support assertions that the degree of convective core overshooting decreases to zero for stars with masses in the range , but that the degree of overshoot is smaller than currently used in published isochrones. We also verify that all current theoretical models for the lower main sequence (with the exception of ) are too blue for , even when the sequences are shifted to match M67 near the of the Sun, probably due to a combination of problems with transformations and realistic surface boundary conditions for models. Finally, we identify a subset of cluster members with unusual photometry (candidate red giant binaries, blue straggler stars, and triple systems) deserving of further study. [ABSTRACT FROM AUTHOR]

We present high-time-resolution multicolour observations of the quiescent soft X-ray transient V404 Cyg obtained with ULTRACAM. Superimposed on the ellipsoidal modulation of the secondary star are large flares on time-scales of a few hours, as well as several distinct rapid flares on time-scales of tens of minutes. The rapid flares, most of which show further variability and unresolved peaks, cover shorter time-scales than those reported in previous observations. The power density spectrum of the 5-s time-resolution data shows a quasi-periodic oscillation (QPO) feature at 0.78 mHz (=21.5 min). Assuming this periodicity represents the Keplerian period at the transition between the thin and advective disc regions, we determine the transition radius. We discuss the possible origins for the QPO feature in the context of the advection-dominated accretion flow model. We determine the colour of the large flares and find that the i′-band flux per unit frequency interval is larger than that in the g′ band. The colour is consistent with optically thin gas with a temperature of ∼8000 K arising from a region with an equivalent blackbody radius of at least , which covers 3 per cent of the surface of the accretion disc. Our timing and spectral analysis results support the idea that the rapid flares (i.e. the QPO feature) most likely arise from regions near the transition radius. [ABSTRACT FROM AUTHOR]

We show that the color index of a quasar image and the second time derivative of the intensity should be correlated if variations of the amplification coefficient for gravitational microlensing are smooth. We estimate an upper limit for the size of the emitting region of the quasar associated with the gravitational lens Q2237 + 0305 based on simultaneous observations made in 1997 by the OGLE group in the V band and at the Maıdanak Observatory in the R band.© 2002 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]

We extend the mass-halo formalism for analytically generating power spectra to allow for the different clustering behaviour observed in galaxy subpopulations. Although applicable to other separations, we concentrate our methods on a simple separation by rest-frame colour into ‘red’ and ‘blue’ subpopulations through modifications to the (M) relations and halo distribution functions for each of the subpopulations. This sort of separation is within the capabilities of the current generations of simulations as well as galaxy surveys, suggesting a potentially powerful observational constraint for current and future simulations. In anticipation of this, we demonstrate the sensitivity of the resulting power spectra to the choice of model parameters. [ABSTRACT FROM AUTHOR]

We report on UBVRI CCD photometry of two overlapping fields in the region of the intermediate-age open cluster NGC 2158 down to V=21 . By analysing colour–colour (CC) and colour–magnitude diagrams (CMDs) we infer a reddening E(B-V)=0.55±0.10 , a distance of 3600±400 pc , and an age of about 2 Gyr. Synthetic CMDs constructed with these parameters (but fixing E(B-V)=0.60 and [Fe/H]=-0.60) , and including binaries, field contamination and photometric errors, yield a good description of the observed CMD. The elongated shape of the clump of red giants in the CMD is interpreted as resulting from a differential reddening of about ΔE(B-V)=0.06 across the cluster, in the direction perpendicular to the Galactic plane. NGC 2158 turns out to be an intermediate-age open cluster with an anomalously low metal content. The combination of these parameters, together with the analysis of the cluster orbit, suggests that the cluster belongs to the old thin disc population. [ABSTRACT FROM AUTHOR]