In the cores of galaxy clusters there is a population of lows-mass stellar systems such as dwarf Early-type galaxies, ultra-compact dwarf galaxies (UCDs) and ultra diffuse dwarf galaxies. We present here the photometric and morphological characterization of this population of objects using deep CFHT images of a sample of clusters belonging to the WINGS survey (0.04 < z < 0.06). We study only galaxies that are spectroscopically confirmed members of the cluster. The population of dwarfs ranges from ∽ 30% for the more rich clusters to ∽ 5-6% for the less rich ones.

We compared the number of lunar craters with diameters greater than 15 km with age less than 1.1 Gyr in the region of the Oceanus Procellarum with the estimates of the number of craters made based on the number of near-Earth objects and on the characteristic times elapsed before collisions of near-Earth objects with the Moon. Our estimates allow the increase of the number of near-Earth objects after a recent catastrophic disruption of a large main-belt asteroid. However, destruction of some old craters and variations in orbital distribution of near-Earth objects with time could allow that the mean number of near-Earth objects during the last billion years could be close to the present value.

Due to its unique 400-year duration, the sunspot number is a central reference for understanding the long-term evolution of solar activity and its influence on the Earth environment and climate. Here, we outline current data recovery work. For the sunspot number, we find historical evidence of a disruption in the source observers occurring in 1947–48. For the sunpot group number, recent data confirm the clear southern predominance of sunspots during the Maunder Minimum, while the umbra-penumbra ratio is similar to other epochs. For the Dalton minimum, newly recovered historical observations confirm a higher activity level than in a true Grand Minimum.

While the observed polarization maps of spatially resolved post-AGB objects usually require numerical modelling of radiative transfer, it is useful to have known analytical solutions of the polarized radiative transfer equation (PRTE) as benchmarks of computer codes in the simplest model cases. We consider two such solutions: cylindrically symmetric Green’s function for an infinite medium and cylindrically symmetric inner eigenfunctions of PRTE.

We demonstrate how pulse structures in Short gamma-ray bursts (SGRBs), coupled with observations of GRB/GW 170817A, constrain the geometries of dying HMXRB systems composed of merging neutron stars.

The next-generation radio telescopes such as LOFAR and SKA will give access to high time-resolution and high instantaneous sensitivity that can be exploited to study slow and fast transients over the whole radio window. The search for radio transients in large datasets also represents a new signal-processing challenge requiring efficient and robust signal reconstruction algorithms. Using sparse representations and the general ‘compressed sensing’ framework, we developed a 2D–1D algorithm based on the primal-dual splitting method. We have performed our sparse 2D–1D reconstruction on three-dimensional data sets containing either simulated or real radio transients, at various levels of SNR and integration times. This report presents a summary of the current level of performance of our method.

We present a brief summary of the results of a study of the effects of dynamical evolution on the stellar mass function of multiple-population globular clusters. Theoretical studies have predicted that the process of multiple-population cluster formation results in a system in which second-generation (2G) stars are initially more centrally concentrated than first-generation (1G) stars. In the study presented here, we have explored the implications of the initial differences between the 2G and 1G structural properties for the evolution of the local (measured at different distances from a cluster center) and global mass function. We have studied both systems in which 1G and 2G stars start with the same initial mass function (IMF) and systems in which 1G and 2G stars have different IMFs. Finally we have explored the evolution of the spatial mixing and found that the multiscale nature of the clusters studied leads to a dependence of the mixing rate on the stellar mass.

Ultra-diffuse galaxies (UDGs) have sizes comparable to the Milky Way and stellar masses of about 1/1000 of it. They attracted a lot of attention as possible “dark galaxies” heavily dominated by dark matter, however, no reliable dynamical mass estimates were done because of their extremely low surface brightness. We have recently found 13 gas free diffuse young (300–500 Myr) post-starburst galaxies (PSGs) without ongoing star formation in Coma and Abell 2147, which, should they continue to evolve passively, will become UDGs in 5 Gyr. We obtained deep spectroscopic observations for 11 diffuse PSGs and derived their internal kinematics and stellar population properties. All of them possess disk-like kinematics (substantial rotation, low stellar velocity dispersion) and likely experienced starburst episodes prior to the star formation quenching by ram pressure stripping. Our results suggest that at least some UDGs were “normal” intermediate to large-sized disk galaxies in the past, which were later quenched by dense environment.

We study how the void environment affects the chemical evolution of galaxies by comparing the metallicity of dwarf galaxies in voids with dwarf galaxies in denser regions. Using spectroscopic observations from SDSS DR7, we estimate oxygen and nitrogen abundances of 889 void dwarf galaxies and 672 dwarf galaxies in denser regions. A substitute for the [OII] λ3727 doublet is developed, permitting oxygen abundance estimates of SDSS dwarf galaxies at all redshifts with the direct method. We find that void dwarf galaxies have about the same oxygen abundances and slightly lower N/O ratios than dwarf galaxies in denser environments. The lower N/O ratios seen in void dwarf galaxies may indicate both delayed star formation and a dependence of cosmic downsizing on the large-scale environment. Similar oxygen abundances in the two dwarf galaxy populations might be evidence of larger ratios of dark matter halo mass to stellar mass in voids.

We present a scenario for the formation of super star clusters (with masses larger than 105 M⊙) in which multiple generations of star formation will occur. We stress that the gas left over (∼50%) from first generation (1G) star formation should be retained in such massive clusters (thanks to their deep potential wells, with escape speeds larger than 10 km/s) and be available for a second or even third generation of stars, with the basic HeCNONaMgAl chemical anomalies observed in globular clusters, the latter assumed to be the descendents of these super star clusters. One new feature of this model is the role of C+ cooling of the dense warm trapped neutral or ionized gas which defines a characteristic temperature of ∼100 K, leading to a second generation (2G) of stars with a top-heavy IMF (M > 5 M⊙). The ashes of the 2G very massive stars (VMS, M > 100 M⊙) sampled in this IMF quickly pollute and dilute the left-over pristine gas with their slow winds (that cannot escape the cluster), while the majority of massive stars develop fast winds (that actually can escape from the cluster). Meanwhile, much of the remaining dense T = 100 K gas contracts gravitationally in the massive cluster and may reach densities of the order of 109 cm−3, in which case the Jeans mass drops to about 0.2 M⊙ and leads to a substantial low-mass pre-MS 3G population (most likely on a very short timescale). In this way, we may solve both the mass budget and the excess Helium problem in proto-globular clusters, while also explaining the Na-O and Mg-Al anti-correlations resulting from hot H-burning of very massive stars at 45MK and 75MK, respectively.

RV Tauri stars are luminous population II Cepheids which show a characteristic light curve of alternating deep and shallow minima. There are 126 RV Tauri variables in our Galaxy. Using WISE [3.4]-[4.6], [12]-[22] diagram we show that Galactic RV Tauri stars show three main types of IR properties in their SEDs; disc-type, non-IR and uncertain, which does not show a clear characteristic in the SED. We also show that there is a strong correlation between disc-type SED and binarity (Gezer et al. 2015). RV Tauri stars were linked to post AGB stars in early studies (Jura 1986), however, recent studies show that their evolutionary nature is more complex than previously thought (Kamath & Van Winckel 2014, and Manick et al. 2018). In this study, we intentionally selected two RV Tauri stars, GK Car (disc-type) and GZ Nor (uncertain), with different IR characteristics to compare their chemical and photometric properties.

Observations of mergers of multi-compact object systems offer insights to the formation processes of massive stars in globular clusters. Simulations of stellar clusters, may be used to understand and interpret observations. Simulations generally adopt an Initial Mass Function (IMF) with a Salpeter slope at the high mass end, for the initial distribution of stellar masses. However, observations of the nearest high mass star forming regions point to the IMF at the high mass end being flatter than Salpeter, in regions where the stellar densities are high. We explore the impact of this on the formation rate of potential GW sources, estimated from standard considerations. Globular clusters being significant contributors to the ionization history of the universe, the results have implications for the same. It impacts our ability to explore the putative mass gap, between the upper limit for neutron star masses and the lower limit for black hole masses, also.

Self-Consistent 2D modelling of stellar wind interaction with the upper atmosphere of WASP-12b has been performed. The two case-scenarios of the planetary material escape and interaction with the stellar wind, namely the ‘blown by the wind’ (without the inclusion of tidal force) and ‘captured by the star’ (with the tidal force) have been modelled under different stellar XUV radiations and stellar wind parameters. In the first scenario, a shock is formed around the planet, and the planetary mass loss is controlled completely by the stellar radiation energy input. In the second scenario, the mass loss is mainly due to the gravitational interaction effects. The dynamics of MGII and related absorption were modelled with three sets of different stellar wind parameters and XUV flux values.

We present the first time-resolved spectroscopic observations, made with the SALT HRS instrument, of a rapidly oscillating Ap star. We used the instrument in the High Stability mode, with the fastest readout settings – a setup never previously used. Over a 2.5-hr track length, we obtained 280 spectra at 8-second integration times and a cadence of 30 seconds. The target, α Circini, is the brightest of the roAp stars, and thus provides an excellent opportunity to test the instrument. Previous time-resolved spectroscopic studies of this star have been conducted by Kurtz, Elkin & Mathys† with the VLT/UVES instrument, and by Mkrtichian & Hatzes‡ with the HARPS instrument on the ESO 3.6-m telescope. Those two studies provide us with benchmarks to compare the performance of SALT/HRS for this type of project. With the upcoming TESS mission, the ability to perform high-precision, time-resolved spectroscopy of pulsating stars will be key for the scientific output of SALT.

Analysis of historical records of eclipses of the Sun and Moon between 720 BC and AD 1600 gives a measure of the time difference, TT − UT = ΔT. The first derivative in time along a smooth curve fitted to the values of Δ T measures the changes in the length of the day (lod). The average rate of change of the lod is found to be significantly less than that expected on the basis of tidal friction. Fluctuations on a time-scale of centuries to millennia are mainly attributed to the effects of post-glacial uplift and core-mantle coupling.

The bright nearby binary α Centauri constitutes an excellent laboratory for testing stellar evolution models. The mass, radius, and luminosity of α Cen A and B are known to better than 1% accuracy thanks to recent interferometric and adaptive optical observations, and p-mode oscillations have been observed in both stars. We present new stellar models which fit simultaneously the classical and seismic observations, with particular emphasis on the convective mixing length parameter MLT – the adaptivity of which is necessary to fit the models to observations. The oscillation data provide an important constraint on the models, as the small frequency separation is sensitive to the composition gradient in the core of the stars, while the large frequency separation constrains the mean density of the stars, providing an independent check on the mass and radius.

The first visibility of the lunar crescent signals the start of a new month in the Islamic calendar. The eminent astronomer Ḥabash al-Ḥāsib sib developed a method of uncompromising complexity for predicting the visibility of the lunar crescent. He derived his threshold value from a moonwatch carried out at different places in Iraq on November 17th, 860 CE. We will allude to a few modern visibility criteria as well and highlight the uncertainties of today’s calculations when converting historical Arabic into Julian or Gregorian dates. Tables of first visibility of the lunar crescent for different locations are provided for the purpose of date conversions. Since the Islamic calendar was based on the observation of the lunar crescent, historical dates imply information on positive and negative sightings of the lunar crescent. From such information estimations of cloudiness in different regions of the Islamic world can be extracted.

We analyze the data from the 6 gravitational waves signals detected by LIGO through the lens of multifractal formalism using the MFDMA method, as well as shuffled and surrogate procedures. We identified two regimes of multifractality in the strain measure of the time series by examining long memory and the presence of nonlinearities. The moment used to divide the series into two parts separates these two regimes and can be interpreted as the moment of collision between the black holes. An empirical relationship between the variation in left side diversity and the chirp mass of each event was also determined.

We present an analysis of a sample of clusters of young stars in order to investigate the inherent properties of clustering and dynamic evolution of stellar components, based on fractal statistics. In addition, we present the application of new mathematical and numerical techniques with potentiality for use in models of filamentary structures.

We report an investigation of the properties of dwarf galaxies (Mr < -15) inside 26 clusters at z = 0.15 – 0.25, using the X-ray data from the Chandra archive, and optical images taken with Subaru Suprime-Cam. Our results include: 1. Investigation of the dwarf galaxy density distribution is sensitive to the background galaxies and the choice of colour selection of galaxies. 2. Cluster-centric dwarf-to-giant ratio is highly sensitive to the level of subtracted background galaxies. 3. A certain fraction of faint galaxies always remain undetected by the detection algorithm near the center of clusters, even after carefully treating the halo or extra diffuse light created by bright galaxies. The number of ‘undetected’ faint galaxies varies significantly from cluster to cluster, and even from pointing to pointing. 4. Dwarf galaxies extend up to 2 Mpc from the center in most clusters. Meanwhile, the distribution of blue dwarf galaxies extends more to the outside. 5. For a given colour, the spatial distributions of dwarf galaxies and giant galaxies become similar. Namely, the most of the radial distribution comes from the colour, rather than the size, of galaxies. 6. Relative to the NFW profile, all of the galaxy populations are showing a deficit near the cluster core (r < 0.3 Mpc). 7. The dwarf-to-giant ratio shows no variation against cluster measures such as the richness and X-ray luminosity, as well as various cluster X-ray characteristics related to possible dynamical status of clusters.