• Volume 41, All articles

      Continuous Article Publishing mode

    • Acknowledgements

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    • The Periodic Table


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      In this article, a historical overview of the development of the Periodic Table has been sketched. After Mendeleev published his Periodic Table in 1869, 55 more elements have been discovered. Of these 55 elements, 35 are radioactive; most of them never existed on Earth earlier. The excitement of the discovery of these unstable elements has been emphasized in this article. In conclusion, the dynamicity of the Periodic Table and its future have been projected.

    • Status of nuclear physics behind nucleosynthesis processes: The role of exotic neutron-rich nuclei


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      We give a brief overview of the current status of important nuclear physics inputs, like reaction rates, in hydrostatic and explosive nucleosynthesis. Recently, it has been proposed that exotic neutron-rich nuclei play an important role even in the formation of heavy elements via the r-process. The main problemshere are identification, abundance estimation of seed nuclei in these processes, and their pathways. We will try to highlight how improved nuclear structure and reaction calculations can affect our present understanding of radiative capture rates of light-mass and medium-mass nuclei, which in turn can drasticallyinfluence the abundance of heavier-mass elements.

    • Oxygen abundances of carbon-enhanced stellar population in the halo


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      The large fraction of carbon-enhanced metal-poor (CEMP) stars at lower metallicities makes them an interesting class of objects to be probed further in greater detail. They show different abundance patterns of neutron-capture elements and based on that CEMP stars are further divided into four categories.Abundances of C, N and O, along with other elements, are required to understand the different nucleosynthetic origins of the subclasses and their progenitors. We studied nine bright carbon-enhanced stars from the Milky Way halo in a metallicity range from $-$0.8 to $-$2.5. They show enhancement in C, N, O and Ba and exhibit radial velocity variation. This indicates the presence of a binary companion which might have contributed to the enhanced carbon and s-process abundance through mass transfer during its asymptoticgiant-branch (AGB) phase of evolution. Their abundance pattern of C, N and O favors low-mass nature for their binary companion.

    • Evolution of lithium in low-mass giants: an observational perspective


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      The overabundance of lithium in low-mass red giants has been a topic of interest for over four decades. Low-mass stars are expected to destroy lithium gradually throughout their lifetimes. Against this expectation, about 1% of red giants in the Galaxy show anomalously large Li which, in the literature, areknown as lithium-rich giants. The advent of large-scale stellar surveys (LAMOST, GALAH, Kepler, Gaia) coupled with high-resolution spectra enabled to find important clues about Li enhancement origin in redgiants. These new studies suggest Li enhancement is mostly associated with the red clump region, post-Heflash. Here, we will describe our recent results along with current updates in the field.

    • Merged white dwarfs and nucleosynthesis


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      Orbital decay mechanisms argue that double white dwarf mergers are inevitable, but extremely rare. Whilst some mergers result in explosions, the survivors re-ignite helium and burn brightly for tens of thousands or millions of years. Candidate survivors include extreme helium stars, R CrB variables andvarious classes of helium-rich subluminous star. Nuclear waste on the survivors’ surfaces provides evidence of the stars’ nuclear history prior to and their nucleosynthesis during the merger. Extensive and deep spectroscopic surveys offer rich prospects for future discoveries.

    • i-Process nucleosynthesis: Observational evidences from CEMP stars


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      The surface chemical compositions of a large fraction of carbon-enhanced metal-poor (CEMP) stars, the so-called CEMP-r/s stars, are known to exhibit enhancement of both s-process and r-process elements. For these stars, the heavy-element abundances cannot be explained either by s-process or r-processnucleosynthesis alone, as the production sites of s-process and r-process elements are very different, and these two processes produce distinct abundance patterns. Thus, the observational evidence of the doubleenhancement seen in CEMP-r/s stars remains a puzzle as far as the origin of the elements is concerned. In this work, we have critically analysed the observed abundances of heavy elements in a sample of eight CEMP-r/s stars from the literature to trace the origin of the observed double enhancement. Towards this, we have conducted a parametric-model-based analysis to delineate the contributions of s-process and r-process nucleosynthesis to the observed elemental abundances. We have further examined if the i-process (intermediate-process) nucleosynthesis that occurs at high neutron density (${\rm n} \sim 10^{15}$ cm$^{-3}$) produced during proton ingestion from a H-rich envelope to the intershell region of an AGB star, which is capable of producing both r-process and s-process elements in a single stellar site, could explain the observed abundance patterns of the sample stars. Our analysis shows that the observed abundance patterns of the selected sample of CEMP-r/s stars could be fairly well reproduced using the i-process model yields.

    • Carbon-enhanced metal-poor stars enriched in s-process and r-process elements


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      We present an on-going project consisting of analysis of a sample of twenty-five metal-poor stars, most of them carbon-enriched and thus tagged carbon-enhanced metal-poor (CEMP) stars, observed with the high-resolution HERMES spectrograph mounted on the Mercator telescope (La Palma), the UVESspectrograph on VLT (ESO Chile), or the HIRES spectrograph on KECK (Hawaii). This sample consists of CEMP-s stars, which are CEMP stars enriched in slow-neutron-capture (s-process) elements, as well as CEMP-rs stars enriched with both s-process and rapid-neutron-capture (r-process) elements. We alsoincluded an r-process-enriched star for comparison purposes. The origin of the abundance differences between CEMP-s and CEMP-rs stars is presently unknown. It has been claimed that the i-process (intermediate nucleosynthesis process), whose site still remains to be identified, could better reproduce CEMP-rs abundances than the s-process. We aim at understanding whether the i-process and its putative site can reproduce the abundance pattern measured in CEMP-rs stars.

    • UVIT/AstroSat studies of blue straggler stars and post-mass transfer systems in star clusters: detection of one more blue lurker in M67


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      The blue straggler stars (BSSs) are main-sequence (MS) stars, which have evaded stellar evolution by acquiring mass while on the MS. The detection of extremely low mass (ELM) white dwarf (WD) companions to two BSSs and one yellow straggler star (YSS) from our earlier study using UVIT/AstroSat, aswell as WD companions to main-sequence stars (known as blue lurkers) suggest a good fraction of post-mass transfer binaries in M67. Using deeper UVIT observations, here we report the detection of another blue lurker in M67, with an ELM WD companion. The post-mass transfer systems with the presence of ELMWDs, including BSSs, are formed from Case A/B mass transfer and are unlikely to show any difference in surface abundances. We find a correlation between the temperature of the WD and the $\nu \sin i$ of the BSSs. We also find that the progenitors of the massive WDs are likely to belong to the hot and luminous group of BSSs in M67. The only detected BSS$+$WD system by UVIT in the globular cluster NGC 5466 has a normal WD and suggests that open cluster like environment might be present in the outskirts of low density globular clusters.

    • Recent advances in RV Tauri stars


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      The availability of multi-wavelength observations and parallaxes from the space missions and very comprehensive models of AGB evolution that include the accretion of matter from the circumbinary disc have strongly impacted our understanding of these enigmatic objects. The important developments made in the recent times are summarized here. The revised estimates of luminosities (derived from better-defined Spectral Energy Distributions (SEDs) and new distances from Gaia DR2) further support the opinion that RV Tauri stars containa mixture of post-AGB stars and post-RGB stars. Their locations in HR diagram also indicate that the instability strip (IS) of RV Tauri stars have a broader extension in the cooler edge than that of classical Cepheids. A newP-L relation has been calibrated for the galactic Cepheids which have a steeper slope than that derived for the Population II Cepheids and RV Tauri stars in Magellanic clouds . The most significant chemical peculiarity exhibited by RV Tauri stars and other post-AGB stars is the selective depletion of refractory elements that correlates with their condensation temperatures. A large range in the size of depletion as well as in the shapes of the depletion curves has been observed. Earlier models to explain this effect were mostly qualitative. Recentinvestigators model these depletions using evolutionary codes (e.g. MESA) to evolve stars in the post-AGB phase, while including accretion of metal-poor gas from circumbinary disc. These authors model the accretion rate onto a the binary post-AGB star from a viscously evolving disc for a range of initial accretion rates and disc masses. It is reported that large initial accretion rates and disc masses are required to explain the large depletion and saturated depletion curve that could extend the evolution time of post-AGB star. It is also proposed that theunsaturated depletion curve (with a plateau) are likely to be caused by post-RGB stars.

    • Recurrent novae: Single degenerate progenitors of Type Ia supernovae


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      Type Ia supernovae are the result of explosive thermonuclear burning in CO white dwarfs. The progenitors of the Ia supernovae are white dwarfs in an interacting binary system. The donor companion is either a degenerate star (white dwarf) or a non-degenerate star (e.g. red giant). Recurrent novae are interacting binaries with a massive white dwarf accreting from either a main sequence, slightly evolved, or a red giant star. The white dwarf in these systems is a massive, hot white dwarf, accreting at a high rate. Recurrent novae are thought to be the most promising single degenerate progenitors of Type Ia supernovae. Presented here are the properties of a few recurrent novae based on recent outbursts. The elemental abundances and their distribution in the ejected shell are discussed.

    • Post-AGB stars as tracers of the origin of elements in the universe


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      The chemical evolution of galaxies is governed by the chemical yields from stars, especially from Asymptotic Giant Branch (AGB) stars. This underlines the importance of understanding how AGB stars produce their elements by obtaining accurate stellar nucleosynthetic yields. Although AGB nucleosynthesis has general validity, critical uncertainties (such as the treatment of convective-driven mixing processes and mass loss) exist in current stellar models. Observations from post-Asymptotic Giant Branch (post-AGB) stars serve as excellent toolsto quantify the strongest discrepancies, and eliminate crucial uncertainties that hamper stellar modelling. Our recent studies of post-AGB stars have shown an intriguing chemical diversity that ranges from stars that are extremely enriched in carbon and $s$-process elements to the discovery of the first post-AGB star with no traces of carbon nor $s$-process elements. Additionally, AGB nucleosynthesis is significantly affected by a binary companion. These results reflect the complexity that surrounds the element production in AGB stars. In this review, Iwill briefly present the intriguing chemical diversity observed in post-AGB stars and its implications on element/isotope production in AGB stars and stellar nucleosynthetic yields.

    • Chemical composition of the solar surface


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      The Sun provides a standard reference against which we compare the chemical abundances found anywhere else in the Universe. Nevertheless, there is not a unique ‘solar’ composition, since the chemical abundances found in the solar interior, the photosphere, the upper atmosphere, or the solar wind,are not exactly the same. The composition of the solar photosphere, usually preferred as a reference, changes with time due to diffusion, convection, and probably accretion. In addition, we do not know the solar photospheric abundances, inferred from the analysis of the solar spectrum using model atmospheres, with high accuracy, and uncertainties for many elements exceed 25%. This article gives an overview of the methods and pitfalls of spectroscopic analysis, and discusses the chemistry of the Sun in the context of thesolar system.

    • Fluorine detection in hot extreme helium stars


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      The origin and evolution of hydrogen-deficient stars are not yet adequately understood. Their chemical peculiarities, along with hydrogen-deficiency, makes them stand out from the rest and sheds light on their possible origin. Severe fluorine enrichment (of the order of 800–8000) is one such characteristicfeature of a class of hydrogen deficient stars, mainly the RCBs (R Coronae Borealis stars) and cool EHes (Extreme Helium stars) which enforces their close connection. For hot EHes, this relationship with the cooler EHes, based on their fluorine abundance is unexplored. Here, first estimates of fluorine abundances in hot EHes are presented and discussed in the light of their cooler counterparts to try to establish an evolutionary connection. The relation between these fluorine estimates with the other elemental abundances observed in these stars plays a pivotal role to predict the formation and evolution of these exotic stars.

    • Investigation of the helium enhancement in a super lithium-rich giant HD 77361


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      In this work, the helium-enhancement (He-enhancement) in the lithium-rich (Li-rich) K-giant HD 77361 is investigated using the strengths of the MgH band and the Mg i lines. The detailed abundance analysis and also the synthesis of the MgH band and the Mg i lines has been carried out for HD 77361. One would expect, within uncertainities, same Mg abundance from both the MgH and Mg i lines. But, we found that Mg abundance derived from MgH lines is significantly less than the abundance from Mg i lines, and this difference cannot be reconciled by changing the stellar parameters within the uncertainties, implying He enhancement in star’s photosphere. The He enhancement in the atmospheres is estimated by using models of different He/H ratios so that both the lines, MgH as well as Mg i, return the same Mg abundance for the adopted model’s He/H ratio. We found ${\rm He}={\rm H }= 0.4 \pm 0.1$ as the value for HD 77361,the normal value of He$/$H $=$ 0.1. Knowing the amount of He-enhancement in the Li-rich giants is a strong clue for understanding the scenarios responsible for the Li and He enrichment. The analysis and results are discussed.

    • Galactic chemical evolution and chemical tagging with open clusters


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      The article presents the consolidated results drawn from the chemical composition studies of Reddy et al. (2012, 2013, 2015, 2016) and Reddy & Lambert (2019), who through the high-dispersion echelle spectra ($R = 60000$) of red giant members in a large sample of Galactic open clusters (OCs), derived stellar parameters and chemical abundances for 24 elements by either line equivalent widths or synthetic spectrum analyses. The focus of this article is on the issues with radial-metallicity distribution and the potential chemical tags offered by OCs. Results of these studies confirm the lack of an age–metallicity relation for OCs but argue that such a lack of trend for OCs arise from the limited coverage in metallicity compared to that of field stars which span a wide range in metallicity and age. Results demonstrate that the sample of clusters constituting a steep radial metallicity gradient of slope $-0.052 \pm 0.011$ dex kpc$^{-1}$ at $R_{\rm gc}$ < 12 kpc are younger than 1.5 Gyr and located close to the Galactic midplane ($|z|$ < 0.5 kpc). Whereas the clusters describing a shallow slope of $-0.015 \pm 0.007$ dex kpc$^{1}$ at $R_{\rm gc}$ > 12 kpc are relatively old with a striking spread in age and height above the midplane (0.5 < $|z|$ < 2.5 kpc). Results of these studies reveal that OCs and field stars yield consistent radial metallicity gradients if the comparison is limited to samples drawn from the similar vertical heights. The computation of Galactic orbits reveals that the outer disk OCs were actually born inward of 12 kpc but the orbital eccentricity has taken them to present locations very far from their birthplaces. Published results for OCs show that the abundances of the heavy elements La, Ce, Nd and Sm but not so obviously Y and Eu vary from one cluster to another across a sample all having about the solar metallicity. For La, Ce, Nd and Sm the amplitudes of the variations at solar metallicity scale approximately with the main s-process contribution to solar system material. Consideration of published abundances of field stars suggest that such a spread in heavy element abundances is present for the thin and thick disk stars of different metallicity. This result provides an opportunity to chemically tag stars by their heavy elements and to reconstruct dissolved open clusters from the field star population.

    • [Rb/Zr] ratio in Ba stars as a diagnostic of the companion AGB stellar mass


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      Understanding nucleosynthesis in and evolution of asymptotic giant branch (AGB) stars is of primary importance as these stars are the main producers of some of the key elements in the Universe. They are the predominant sites for slow-neutron-capture nucleosynthesis (s-process). The exact physical conditionsand nucleosynthetic processes occurring in the interior of AGB stars are not clearly understood, and that hinders better understanding of the contribution of these stars to the Galactic chemical enrichment. Extrinsic-variable stars that are known to have received products of AGB phase of evolution via binary masstransfer mechanisms are vital tools in tracing AGB nucleosynthesis. The [Rb/Zr] ratio is an important diagnostic to understand the average neutron density at the s-process site and provides important clues to the mass of companion AGB stars in binaries. In this work we have presented estimates of [Rb/Zr] ratios based on high-resolution spectroscopic analysis for a sample of Ba stars, and discussed how the ratio can be used to understand the characteristics of the AGB star. Results from an analysis based on a parametric model toconfirm the mass of the companion AGB star are also presented.

    • Abundances of neutron-capture elements in CH and carbon-enhanced metal-poor (CEMP) stars


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      All the elements heavier than Fe are produced either by the slow (-s) or rapid (-r) neutroncapture process. The neutron density prevailing in the stellar sites is one of the major factors that determines the type of neutron-capture processes. We present the results based on the estimates of corrected value of absolute carbon abundance, [C/N] ratio, carbon isotopic ratio and [hs/ls] ratio obtained from the highresolution spectral analysis of six stars that include both CH stars and CEMP stars. All the stars show enhancement of neutron-capture elements. Location of these objects in the A(C) vs. [Fe/H] diagram shows that they are Group I objects, with external origin of carbon and neutron-capture elements. Low values of carbon isotopic ratios estimated for these objects may also be attributed to some external sources. As the carbon isotopic ratio is a good indicator of mixing, we have used the estimates of ${}^{12}$C/${}^{13}$C ratios to examine the occurrence of mixing in the stars. While the object HD 30443 might have experienced an extra mixing process that usually occurs after red giant branch (RGB) bump for stars with log(L/L$_{\odot}$)>2.0, the remainingobjects do not show any evidence of having undergone any such mixing process. The higher values of [C/N] ratios obtained for these objects also indicate that none of these objects have experienced any strong internal mixing processes. Based on the estimated abundances of carbon and the neutron-capture elements, and the abundance ratios, we have classified the objects into different groups. While the objects HE 0110-0406, HD 30443 and CD-38 2151 are found to be CEMP-s stars, HE 0308-1612 and HD 176021 show characteristicproperties of CH stars with moderate enhancement of carbon. The object CD-28 1082 with enhancement of both r- and s-process elements is found to belong to the CEMP-r/s group.

    • The horizontal branch morphology of the globular cluster NGC 1261 using AstroSat


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      We present the results obtained from the UV photometry of the globular cluster NGC 1261 using far-UV (FUV) and near-UV (NUV) images acquired with the Ultraviolet Imaging Telescope (UVIT) onboard the AstroSat satellite. We utilized the UVIT data combined with HST, GAIA, and ground-based optical photometric data to construct the different UV colour-magnitude diagrams (CMDs). We detected blue HB (BHB), and two extreme HB (EHB) stars in FUV, whereas full HB, i.e., red HB (RHB), BHB as well as EHB is detected in NUV CMDs. The 2 EHB stars, identified in both NUV and FUV, are confirmedmembers of the cluster. The HB stars form a tight sequence in UV-optical CMDs, which is almost aligned with Padova isochrones. This study sheds light on the significance of UV imaging to probe the HB morphology in GCs.

    • On the cosmic origin of fluorine


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      The cosmic origin of fluorine, the ninth element of the periodic table, is still under debate. The reason for this fact is the large difficulties in observing stellar diagnostic lines, which can be used for the determination of the fluorine abundance in stars. Here we discuss some recent work on the chemical evolution of fluorine in the Milky Way and discuss the main contributors to the cosmic budget of fluorine.

    • Observations with the 3.6-meter Devasthal optical telescope


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      The 3.6-meter Indo–Belgian Devasthal optical telescope (DOT) has been used for optical and nearinfrared (NIR) observations of celestial objects. The telescope has detected stars of $B=24.5 \pm 0.2$, $R = 24.6 \pm 0.12$ and $g= 25.2 \pm 0.2$ mag in exposure times of 1200, 4320 and 3600 s respectively. In one hour of exposure time, a distant galaxy of $24.3 \pm 0.2$ mag and point sources of $\sim$25 mag have been detected in the SDSS $i$ band. The NIR observations show that stars up to $J = 20\pm 0.1$, $H = 18.8 \pm 0.1$ and $K = 18.2 \pm 0.1$ mag can be detected in effective exposure times of 500, 550 and 1000 s respectively. The $nbL$ band sources brighter than $\sim$9.2 mag and strong ($\geq$0.4 Jy) $PAH$ emitting sources like Sh 2-61 can also be observed with the 3.6-meter DOT. A binary star with angular separation of 0:$''$4 has been resolved by the telescope. Sky images with sub-arcsec angular resolutions are observed with the telescope at wavelengths ranging from optical to NIR for a good fraction of observing time. The on-site performance of the telescope is found to be at par with the performance of other, similar telescopes located elsewhere in the world. Owing to the advantage of its geographicallocation, the 3.6-meter DOT can provide optical and NIR observations for a number of frontline galactic and extra-galactic astrophysical research problems, including optical follow-up of GMRT and AstroSat sources and optical transient objects.

    • New measurements of cross sections and S-factors for $(p,\gamma)^3$He reaction at BBN energies


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      This communication is a summary of our measurements of cross sections and astrophysical S-factors for radiative proton capture on deuteron. The measurements are a part of a new program to study light-ion induced nuclear capture and inelastic scattering reactions relevant to nucleosynthesis and astrophysics. We are primarily interested in the capture reactions relevant to primordial or Big Bang Nucleosynthesis (BBN). Section 1 provides a brief and general overview of nuclear astrophysics and the primary experimental challenges in the measurements of cross sections and S-factors for reactions relevant to nuclear astrophysics. The next section discusses the significance of the $(p,\gamma)^3$He reaction in context of BBN and the need for generating a more precise data in the relevant energy window. The subsequent section is devoted to the experiment and results of measurements of cross sections and astrophysical S-factors for the $(p,\gamma)^3$He reaction at three new BBN energies. One salient features of the measurements is the use of large volumeLaBr$_3$:Ce scintillation detector for measurement of the capture $\gamma$-rays. To the best of our knowledge, capture $\gamma$-rays for this reaction had so far been measured with NaI(Tl) or HPGe detectors only. The detection efficiency of the detector has been measured experimentally for different monochromatic $\gamma$-ray energies. In ddition, realistic GEANT4 Monte Carlo simulations have been carried out to generate the response of thedetector for $\gamma$-ray energies of interest. The measured cross section and astrophysical $S(E)$-factor for the three incident proton energies are found to be in agreement with the overall trend of the global data set for the BBN region, reported in the literature. The measured S-factors are also found to be in agreement with recent microscopic calculations of Marcucci et al. (2016).

    • Editorial

      Gajendra Pandey

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    • A model for massless gravitons in radiation and matter-dominated universes


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      A toy model of a graviton is explored by considering the minimum amount of information gravitons could carry. The total entropy of the universe is calculated and compared to estimates from super massive black holes and massive models of the graviton. The running cosmological constant is calculated using the entropy relation previously computed and compared to its experimentally accepted value. Bothresults are quantified considering radiation and matter-dominated universes.

    • Sunspot interval prediction based on fuzzy information granulation and extreme learning machine


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      Sunspot prediction is an important task for space weather and solar physics. Traditional point forecast may not be sufficiently satisfactory and reliable. To quantify the uncertainty of point prediction, a hybrid interval prediction model has been proposed for sunspot forecasting. Three major steps are taken: (1)the complementary ensemble empirical mode decomposition (CEEMD), to decompose the sunspot sequence into a series of modal components, (2) the fuzzy information granulation (FIG), to extract the minimum, average and maximum value of each window, and (3) the extreme learning machine (ELM), to conduct point prediction and interval prediction, superimposing the prediction values of all components as the final forecast results. The empirical study focus on the 13-month smoothed monthly sunspot number recorded by SolarInfluences Data Analysis Center (SIDC) and show that the mixed model with the filtered CEEMD is more effective than the unfiltered one. It also enables us to track changes of the sunspot number with fast calculating speed and high accuracy both in point prediction and interval prediction.

    • On the BTZ black hole and the spinning cosmic string


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      We review the Banados–Teitelboim–Zanelli (BTZ) black hole solution in connection with the spinning string solution. We find a new exact solution, which can be related to the ($2+1$)-dimensional spinning point particle solution. There is no need for a cosmological constant, and so the solution can be upliftedto ($3 + 1$) dimensions. The exact solution in a conformally invariant gravity model, where the space–time is written as $g_{\mu\nu}=\omega^2\tilde{g}_{\mu\nu}$, is horizon free and has an ergo-circle, while $\tilde{g}_{\mu\nu}$ is the BTZ solution. The dilaton $\omega$ determines the scale of the model. It is conjectured that the conformally invariant non-vacuum BTZ solution will solve the boundary and causality problems which one encounters in spinning cosmic stringsolutions.

    • Detection of PAH and nbL features in planetary nebulae NGC 7027 and BD $+$30$^{\circ}$ 3639 with TIRCAM2 instrument on 3.6 m DOT


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      High resolution infrared imaging observations of the young planetary nebulae (PNe) NGC 7027 and BD $+$30$^{\circ}$ 3639, taken with the newly installed TIFR near infrared camera-II (TIRCAM2) on 3.6 m Devasthal optical telescope (DOT), ARIES, Nainital, are being reported. The images are acquired in J, H, K,polycyclic aromatic hydrocarbon (PAH) and narrow-band L (nbL) filters. The observations show emission from warm dust and PAHs in the circumstellar shells. The imaging of the two objects are among the first observations in PAH and nbL bands using TIRCAM2 on DOT. The NGC 7027 images in all bands showsimilar elliptical morphology with $\sim$6$^{\prime\prime}$.7 and $\sim$4$^{\prime\prime}$.5 semi-major and semi-minor axes. Considering size up to 10% of peak value the nebula extends upto 800 from the central star revealing a multipolar evolution. The relatively cooler BD $+$30$^{\circ}$ 3639 shows a rectangular-ring shaped nebula. In J and H bands it shows an angular diameter of $\sim$8$^{\prime\prime}$, while a smaller $\sim$6$^{\prime\prime}$.9 size is observed in K, PAH and nbL bands. The 3.28 $\mu$m emission indicates presence of PAHs at about 6000 and 5000 AU from the central stars in NGC 7027 and BD $+$30$^{\circ}$ 3639 respectively. Analysis suggests domination of neutral PAHs in BD $+$30$^{\circ}$ 3639, while in NGC 7027 there is higher ionization and more processed PAH population.

    • A comprehensive investigation of the variable overcontact system EH Cancri


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      Precise multi-color CCD-derived photometric data were obtained from EH Cnc at two sites during 2010, 2014 and 2018 wherein each epoch used a different instrument. This has provided a unique opportunity to investigate parameter uncertainty following Roche modeling of light curves optimized by differential corrections using the Wilson–Devinney code. Furthermore, new radial velocity data from EH Cnc presented in this study for the first time has produced absolute physical and geometric parameters for this A-subtype WUMa-type variable. Analysis of eclipse timing data confirms the presence of sinusoidallikeexcursions in the eclipse timing residuals. We address whether these are due to magnetic activity cycles, the so-called ‘‘Applegate effect’’, or related to a light-time effect (LiTE) resulting from the presence of a third gravitationally bound cool low mass white dwarf. A model using the PAdova & TRieste Stellar Evolution Code (PARSEC) has provided valuable insight about the evolutionary history of EH Cnc as a trinary system.

    • Prediction of meteoroid stream structure based on meteoroid fragmentation


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      Every day, large number of meteoroids enter the Earth’s atmosphere and deposit their mass either in atomic form or in ionic form depending on whether it has undergone ablation or fragmentation. The heavier meteoroids undergo fragmentation while the lighter ones are more prone to ablate. In this paper, wewould like to speculate meteoroid stream structure of Leonid meteor shower based on fragmenting meteoroids. A 23 revolutions old meteoroid trail left behind by the comet 55P/Tempel-Tuttle in the year 1213 AD, which instigated Leonid meteor shower in the year 2010 is considered for our study. We have calculatedmass of the meteoroids, echo durations and percentage of fragmentation. From the observed echo durations of meteoroids, estimated masses and from the percentage of fragmentation, we visualize the stream structure to be like the lighter particles wrapping up the heavier ones. The results we draw from these three different studies are matching with each other. To our knowledge, we are the first to speculate on the meteor stream structure based on fragmentation and making it a new tool in meteor stream evolution. Based on echo durations, it has been observed that 72% of the activity during the shower is contributed by lighter particles of the stream. It is found that about 20% of the meteoroids have undergone fragmentation indicating the minimal role of heavier particles (>10$^{-6}$ g) during Leonid Meteor Shower (LMS). The masses of the meteors are estimated to be in the range of 10$^{-10}$–10$^{-5}$ g.

    • On the structure of quasi-Keplerian accretion discs surrounding millisecond X-ray pulsars


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      In this study, we investigated the time-independent dynamics (disc structure, forces and torques) of a quasi-Keplerian disc around a millisecond pulsar (MSP) with an internal dynamo. We considered the disc around a MSP to be divided into the inner, middle and outer regions. By assuming that the disc matter flows in a quasi-Keplerian motion, we derived analytical equations for a complete structure (temperature, pressure, surface density, optical depth and magnetic field) of a quasi-Keplerian thin accretion disc, and the pressure gradient force (PGF). In our model, the MSP-disc interaction results into magnetic and material torques, such that for a given dynamo ($\varepsilon$) and quasi-Keplerian ($\xi$) parameter, we obtained enhanced spin-up and spin-down torques for a chosen star spin period. Results obtained reveal that PGF results into episodic torque reversals that contribute to spinning-up or spinning-down of a neutron star, mainly from the inner region. The possibility of a quasi-Keplerian disc is seen and these results can explain the observed spin variations in MSPs like SAX J1808.4-3658 and XTE J1814-338.

    • High-precision distance measurements with classical pulsating stars


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      Classical Cepheid and RR Lyrae variables are radially pulsating stars that trace young and old age stellar populations, respectively. These classical pulsating stars are the most sensitive probes for the precision stellar astrophysics and the extragalactic distance measurements. Despite their extensive use as standard candles thanks to their well-defined Period–Luminosity relations, distance measurements based on these objects suffer from their absolute primary calibrations, metallicity effects, and other systematicuncertainties. Here, I present a review of classical Cepheid, RR Lyrae and type II Cepheid variables starting with a historical introduction and describing their basic evolutionary and pulsational properties. I will focus on recent theoretical and observational efforts to establish absolute scale for these standard candles atmultiple wavelengths. The application of these classical pulsating stars to high-precision cosmic distance scale will be discussed along with observational systematics. I will summarize with an outlook for further improvements in our understanding of these classical pulsators in the upcoming era of extremely largetelescopes.

    • Quasar catalogue for the astrometric calibration of the forthcoming ILMT survey


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      Quasars are ideal targets to use for astrometric calibration of large scale astronomical surveys as they have negligible proper motion and parallax. The forthcoming 4-m International Liquid Mirror Telescope (ILMT) will survey the sky that covers a width of about 27$'$. To carry out astrometric calibration of theILMT observations, we aimed to compile a list of quasars with accurate equatorial coordinates and falling in the ILMT stripe. Towards this, we cross-correlated all the quasars that are known till the present date with the sources in the Gaia-DR2 catalogue, as the Gaia-DR2 sources have position uncertainties as small as a few milli arcsec (mas). We present here the results of this cross-correlation which is a catalogue of 6738 quasars that is suitable for astrometric calibration of the ILMT fields. In this work, we present this quasar catalogue. This catalogue of quasars can also be used to study quasar variability over diverse time scales when the ILMT starts its observations. While preparing this catalogue, we also confirmed that quasars in the ILMT stripe have proper motion and parallax lesser than 20 mas yr$^{-1}$ and 10 mas, respectively.

    • Approximate analytical solution for the propagation of shock waves in selfgravitating perfect gas via power series method: isothermal flow

      G. NATH

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      For the propagation of a shock (blast) in a self-gravitating perfect gas in case of spherical and cylindrical symmetry, an approximate analytical solution is investigated. The shock wave is considered to be a strong one, with the ratio $(C/V_S)^2$ to be a small quantity, where $C$ is the sound speed in an undisturbed medium and $V_S$ is the shock wave velocity. The initial density in the undisturbed medium is taken to be varying according to a power law. To obtain the approximate closed-form similarity solution, the flow variables are expanded in a power series of $(C/V_S)^2$. The first- and second-order approximations are discussed with the help of power series expansion. The analytical solutions are constructed for the first-order approximation. The distribution of the flow variables for first-order approximation in the flow field region behind the shock wave is shown in graphs for both the cylindrical and spherical geometries. The effect of flow parameters, namely, ambient density variation index $\alpha$, adiabatic exponent $\gamma$ and gravitational parameter $G_0$, are studied on the flow variables and on the total energy of disturbance in the case of the first approximation to the solutions. It is shown that the total energy of the disturbance in the flow field region behind the shock wave decreases with an increase in initial density variation index or adiabatic exponent, i.e. shock strength increases with increase in the value of adiabatic exponent or initial density variation index. A comparison is also made between the solutions obtained for non-gravitating and self-gravitating gases.

    • The southern stars identified in Indian astronomical catalogues


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      In our effort to decipher the words and identify the stars as cited in the text Sarvasiddh$\bar{a}$ntar$\bar{a}$jaby Nity$\bar{\rm a}$nanda of 15th century, we have found that most of the stars near the ecliptic belt are easily identifiable. The coordinate system used is Dhruvaka and Viksepa, which differ from the currently used ecliptic longitude–latitude and Right Ascension–declination coordinates. Most of the stars studied hitherto (Pai & Shylaja 2016, 2019; Shylaja & Pai Curr Sci 2018a, 2018b, 2019) were all in the northern hemisphere. It may be recalled that the name of the zodiacal sign (Pai & Shylaja 2016) for a group includes stars with all declinations – both north and south. For example, the group of Mesa includes the stars of Andromeda. The only two southern stars that were mentioned in the group of Gemini are Sirius and Canopus. Here, we study the stars grouped under Libra and Scorpio, which includes bright stars in the southern hemisphere. Theconfusion about the Dhruvaka (east–west coordinates) is discussed.

    • Effect of intense solar flares on TEC variation at low-latitude station Varanasi


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      The effect of intense solar flares on total electron content (TEC) variability during the declining phase of solar cycle-24 is studied at the low-latitude station at Varanasi, India (Geog. Lat. 25.31$^{\circ}$ N, Geog. Long. 82.97$^{\circ}$ E, Geomag. Lat. 16.54$^{\circ}$ N, Geomag. Long. 157.09$^{\circ}$ E). In the present paper, we have chosen the intense solar flares that occurred during 9–13 March 2015, 1–2 January 2016, 12–14 February 2016, 6–8 August 2016, and 6–8 September 2017 in the solar cycle-24 period for which the data is available. Our results showed significant enhancements in TEC up to the order of 15 TECU during and after the solar flare events. We have also given a brief account of solar flare effect in TEC with and without geomagnetic disturbances, local time effects (solar zenith angle effects) and changing the location of the solar active region. In a few cases, our results revealed a delay in TEC response during the flare peak time as well as recovery time.

    • Waves propagation in network and inter-network bright points channels between the chromosphere and transition regions with IRIS observations


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      The significance increase of the plasma temperature from the solar photosphere to the corona up to 1MK is still unresolved. One of the candidates for this issue is waves and instabilities in the solar plasma. It was suggested that the energy of these waves could heat the solar atmosphere to transition-region (TR) and corona. Despite many recent kind of researches about waves in the various solar dynamic structures, the reasons for the sudden rise of solar atmospheric temperature are still not enough. By using the Interface Region Imaging Spectrograph (IRIS) telescope observations, we investigate the waves in magnetic tubes such as UV bright points (BPs) at the boundary and inside the supper-granules. The Study of these dynamic structures, between the chromosphere and transition region, will impressively increase our information ofmass and energy transportation through interface region between the chromosphere and inner corona. The magnetic BPs are the faces of magnetic flux tubes that are associated with magnetic elements. The magnetic flux tubes expand upward and appear as bright grains that make up the chromospheric network. For investigating periodical behavior of these structures and various aspect of these oscillations, we analyze intensity oscillations individually in IRIS slit-jaw images at three filters included 2796 Å, 1400 Å and 1330 Å by the wavelet analysis method. The wavelet analysis of the intensity fluctuations of these points showed periods from 2 to 8 min, which are in order of the frequencies of the atmospheric pressure modes. For investigating wave propagation in these layers we also study the correlation of oscillations betweenchromosphere (2796 Å) and TR (1400 Å and 1330 Å) heights by cross wavelet method, and found that these heights correlate with periods between 2.5 and 5.5 min, with wave speeds from 30 to 200 km s$^{-1}$. Also, byextracting Si IV line, we determined the Doppler velocity of BPs in the network and inter-network as about $-$20 to $+$30 km s$^{-1}$ and $-$21 to $+$21 km s$^{-1}$, respectively. Our results suggest that the wave propagation along the magnetic flux tubes of upward-moving plasma in BPs have an important role in solar atmosphere heating.

    • Retraction Note to: Numerical simulation of inertial alfven waves to study localized structures and spectral index in auroral region


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      From a report based on an independent investigation by a senior fellow of the Indian Academy of Sciences, we find that there is evidence of academic misconduct in the paper titled, ‘‘Numerical simulation of inertial alfvenwaves to study localized structures and spectral index in auroral region’’ by Bheem Singh Jatav (https://doi.org/10.1007/s12036-018-9531-8; volume 39 article ID 37) published in Journal of Astrophysics and Astronomy. Therefore, the Indian Academy of Sciences has decided to withdraw the publication, on the grounds of academic misconduct by the author. The author has been notified and has agreed to the retraction.

    • Star sensor mounting for highly agile Earth imaging satellite


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      Star sensor (SS) is one of the most critical instrument on-board a spacecraft when it comes to attitude determination and control. Capability of providing precise attitude information makes it even more important for high resolution imaging satellite with stringent pointing requirements. At the same time, theseinstruments are highly sensitive to bright objects like Sun and Earth Albedo. In this paper, selection of star sensor mounting has been elaborately studied and presented for missions primarily performing highly agile Earth imaging. This paper addresses selection factors like Sun and Earth avoidance from bore-sight axis, orbit altitude and local time, angular separation between SS, sensitivity of SS on body axis and functional availability during imaging. Results are presented in the form of available mounting options and the way they satisfy geometrical constraints over a year.

    • A new method for clustering of boundary spectra


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      The stellar spectral data taken by LAMOST (Large Sky Area Multi-Object Fiber Spectroscopic Telescope) include multiple types, some of which that fall between two spectral classes, namely boundary spectra. Due to the massive and high dimensional nature of spectra data, it will take a lot of time and energyto cluster these spectra by manual operation alone. To address this problem, a new clustering method based on influence space is presented in this paper. First, we introduce the concept of influence space to reduce the amount of data involved in the operation, and reduce the dimension of the data by extracting the main feature lines. Second, a novel method for initial cluster center selection is applied. Next, based on the selected initial cluster centres, other spectra are clustered by running K-means algorithm on the whole data set. The experimental results indicate that the initial cluster centres obtained by this method are of higher quality and the problem of boundary spectra clustering is also well solved.

    • Bursts of gravitational waves due to crustquake from pulsars


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      We revisit here a possibility of generation of gravitational wave (GW) bursts due to a very quick change in the quadrupole moment (QM) of a deformed spheroidal pulsar as a result of crustquake. Since it was originally proposed as a possible explanation for sudden spin-up (glitch) of pulsars, the occurrence ofcrustquake and its various consequences have been studied and discussed quite often in the literature. Encouraged by recent development in gravitational wave (GW) astronomy, we re-investigate the role of crustquake in the emission of GWs. Assuming exponential decay of excitation caused by crustquake, wehave performed a Fourier analysis to estimate the GW strain amplitude $h(t)$, characteristic signal amplitude $h_c(f)$ and signal-to-noise ratio (SNR) of the burst for the Crab pulsar. For exotic quark stars, multifold enhancement of these quantities are expected, which might make quark star a potential source of gravitational waves. The absence of such bursts may put several constraints on pulsars and such hypothetical stars.

    • A search for variable stars in the four open star clusters


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      We present a CCD photometric survey for the search of variable stars in four open clusters namely Berkeley 69, King 5, King 7 and Berkeley 20. The time series observations were carried out for 1 and/or 2 nights for each of the clusters in the year 1998, which have led to identify nineteen variable stars in theseclusters. Out of these 19 variable stars, five stars show $\delta$ Scuti-like variability and two stars show W UMa type variability. In other stars, we could not find the periods, and hence the type of variability due to the lack of sufficient data. The periods of $\delta$ Scuti type stars are found to be in the range of 0.13–0.21 days, whereas the two stars in the cluster Berkeley 20, which showed W UMa type variability have orbital periods of 0.396 and 0.418 days, respectively. Using the Gaia data, the basic parameters of the clusters Berkeley 69, King 7 and King 5 are also revised. The age and reddening are estimated to be $0.79\pm 0.09$ Gyr and $0.68 \pm 0.03$ mag for Berkeley 69, $0.79\pm 0.09$ Gyr and $1.22 \pm 0.03$ mag for the cluster King 7 and $1.59\pm 0.19$ Gyr and $0.63\pm 0.02$ mag for the cluster King 5, respectively. The signature of mass segregation is found in the clusters King 7 and King 5.

    • $\Lambda$CDM cosmological models with quintessence in $f(R)$ theory of gravitation

      S. D. KATORE S. V. GORE

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      The work is the study of Bianchi type-I models with holographic dark energy in the framework of $f(R)$ theory of gravitation. The exact solutions of the field equations are deduced by considering the exponential and power-law volumetric expansion. Various physical and kinematical properties of the models are discussed. In addition to make our interpretation more clear for that we have taken the state finder diagnostic pair $\{r; s\}$ and cosmic jerk parameter $j(t)$ to characterize different phases of the universe. The holographic scalar fields of dark energy are reconstructed and quintessence potential for both the modelsdescribing accelerated expansion of the universe is also obtained.

    • Dust acoustic solitary waves in a five-component cometary plasma with charge variation


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      We studied dust acoustic solitary waves (DASW) in a five component cometary plasma by deriving the Kadomstev–Petiashvili (KP) equation. The five components consist of two components of electrons described by kappa distributions with different temperatures and spectral indices, a lighter (hydrogen)and a heavier (oxygen) ion component, both ion components are described by Maxwellian distributions. Dust particles, with varying charge, constitute the fifth component. The system supports rarefactive DASWs whose amplitudes are larger when the charges on the dust particles vary. The amplitudes alsoincrease with increasing $z_{d0}n_{d0}$ (product of equilibrium charge number and density of dust) and increasing ion densities. It, however, decreases with increasing spectral indices of the electrons.

    • H$\alpha$ full line spectropolarimetry as diagnostics of chromospheric magnetic field


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      Analysis of spectropolarimetric observations of two circular sunspots located close to disk centre in H$\alpha$ (6562.8 Å) and Fe i (6569.22 Å) is presented in this paper. The corresponding active region numbers areNOAA 10940 and NOAA 10941 referred to as AR1 and AR2, respectively. The vector magnetic field at the photosphere is derived through inversion of Stokes profiles of Fe I under Milne–Eddington atmospheric model. The chromospheric vector magnetic field is derived from H$\alpha$ Stokes profiles under weak-field approximation. Azimuthally averaged magnetic field as a function of radial distance from the centre of sunspot at the photosphere and chromosphere are studied. At the photosphere, the radial variation shows a well known behaviour that the total field and the line-of-sight (LOS) component monotonically decrease from centre to the edge of the sunspot and the transverse component initially increases, reaches a maximum close to half the sunspot radius and thendecreases. LOS and the transverse components become equal close to half the sunspot radius consistent with the earlier findings. At the chromosphere, all the components of the magnetic field decrease with the sunspot radius. However, the LOS component decreases monotonically whereas the transverse component decreases monotonically up to about 0.6 times the sunspot radius after which it reaches a constant value. Azimuthally averaged magnetic field gradient from photosphere to chromosphere is also presented here.

    • On the unified view of gamma-ray energy distribution of BL Lac objects and flat spectrum radio quasars

      F. C. ODO B. E. AROH

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      We study the distributions of $\gamma$-ray properties of a sample of $\gamma$-ray loud blazars taken from the third catalogue of blazars detected by Fermi-Large Area Telescope (Fermi-LAT).We compute the $\gamma$-ray dominance ($D_{\rm g}$) of the sample which includes 415 flat spectrum radio quasars (FSRQs) and 535 BL Lacertae objects (BL Lacs). We find that BL Lacs and FSRQs are highly dominated by $\gamma$-ray emission, which is consistent with diffuse high-energy neutrino flux associated with $\gamma$-ray loud blazars. The $\gamma$-ray dominance fairly scales with $\gamma$-ray luminosity ($r \sim +0.5$) in both BL Lacs and FSRQs, but shows little or no correlation ($r \leq 0.2$) with radio luminosity in either sample. BL Lacs and FSRQs occupy separate and parallel regions on the $D_{\rm g}$-luminosity plane. There is a fairly significant correlation ($r \sim 0.5$) between $\gamma$-ray dominance and frequency at synchrotron peak ($\nu_{\rm pk}$) in BL Lacs, which disappears in FSRQs. On the other hand, there is a tight correlation ($r \geq +0.8$) between $\gamma$-ray and radio luminosity with a smooth transition from BL Lacs at low luminosities to FSRQs at high luminosities. Nevertheless, the presence of few BL Lac-like FSRQs is noted. These results suggest that while there may be intrinsic differences between BL Lacs and FSRQs, some form of a unified scheme can also be relevant.

    • Review of pulsar timing array for gravitational wave research


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      Ongoing research on pulsar timing array (PTA) to detect gravitational radiation is reviewed. Here, we discuss the use of millisecond pulsars as a gravitational wave detector, the sources of gravitational radiation detectable by PTAs and the current status of PTA experiments pointing out the future possibilities.

    • Characteristics of SEPs during solar cycles 21–24


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      The study of the solar energetic particle events (SEPs) and their association with solar flares and other activities are very crucial to understand the space weather. Keeping this in view, here we present the study of SEPs (intensity $\geq$10 pfu) during the solar cycles 21–24 (1976–2017) in >10 MeV energy channels associated with solar flares. For our analysis, we have used the data from different instruments onboard SOHO satellite. We have examined the flare size, source location, coronal mass ejections (CMEs) characteristics of associated SEPs. About 31% and 69% of the SEPs were originated from the eastern and western solar hemisphere, respectively. The average CME speed and width were 1238 km s$^{−1}$ and 253$^{\circ}$, respectively. About58% SEPs were associated with halo CMEs and 42% of SEPs associated with CMEs width varying from 10$^{\circ}$ to 250$^{\circ}$, respectively.

    • Vilnius photometry and Gaia astrometry of Melotte 105


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      Archival Vilnius CCD photometric observations are presented for the heavily reddened star cluster Melotte 105, resulting in colour-magnitude diagrams and spectral class estimates. There is considerable lack of agreement between studies for reddening, age and distance for this cluster explaining why the archival data are being made available by this paper. The derived reddening $E(B − V) = 0.34 \pm 0.04$ mag and the distance $V − M_V = 12.9 \pm 0.3$ mag are directly from the Vilnius photometry. The Gaia Data Release 2(DR2) and Vilnius photometric data of the cluster were used to estimate the structural parameters of the cluster, probability of stellar membership in the cluster, the distance modulus and the cluster age. Lack of $Y$ bandobservations prevented determination of metal abundance. The values of the colour excess and distance module are determined by two different methods (i.e., Q and Zero Age Main Sequence or ZAMS methods). A distance modulus of $12.85 \pm 0.07$ mag was derived by ZAMS fitting, in good agreement with the above estimate. ZAMS fitting indicates a reddening of $0.403\pm 0.02$ mag, within two sigma of the estimate above. The cluster’s metallicity and age are estimated to be 0.24 dex and $240 \pm 25$ Myr, respectively. The derived mass function is in good agreement with the Salpeter slope. The cluster space velocity components (U, V,W) were determined as ($−3.90 \pm 3.34$, $−13.76 \pm 5.69$, $+3.45 \pm 0.41$) km/s. Perigalactic and apogalactic distances were obtained as $R_ p = 6.85$ and $R_a = 7.44$ kpc respectively. The maximum vertical distance from the Galactic plane was calculated as $Z_{\rm max} = 84$ pc and the eccentricity of the orbit was determined as $e = 0.042$.

    • Simulation of radiation field inside interplanetary spacecraft


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      A simulation of the radiation field inside a habitable module (a diameter of 6 m and length of 12 m) of a spacecraft generated by isotropic Galactic Cosmic Radiation (GCR) in deep interplanetary space is carried out for minimum and maximum solar activity using the FLUKA code. Protons, alpha-particles,deuterons, ${}^3$He, and nuclei with $Z$ > 2 are considered as primary GCR irradiating the spacecraft isotropically. The following particles are included in FLUKA radiation transport through the module shell (15 g/cm$^2$ of Al): protons, neutrons, $\gamma$-rays, electrons, $\pi^{\pm}$-mesons, $\mu^{\pm}$-mesons d, t, and nuclei from He to Ni. The inner particle spectra are needed to assess the astronaut’s radiation risk in a long-term interplanetary mission.

    • MOND, MONG, MORG as alternatives to dark matter and dark energy, and consequences for cosmic structures


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      In view of the negative results from various dark matter detection experiments, we propose alternate models by postulating a minimal field strength (minimal curvature) and also a minimal acceleration. These postulates lead to modified Newtonian dynamics, modified Newtonian gravity, and modified relativistic gravity. Through these postulates, we show that the observed flat rotation curves of galaxies can be accounted for. This can also set constraints on the size of large scale structures such as galaxies and clusters, which are consistent with observations. This minimum curvature in general relativity, and consequently the modification of the Hilbert action leads naturally to a term equivalent to cosmological constant. This model thus neatly ties up, with these two postulates: the observed flat rotation curves and the accelerated expansion of the universe.

    • New classification parameter of solar flares based on the maximum flux in soft X-rays and on duration of flare

      E. A. BRUEVICH

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      Solar flare activity is characterized by different classification systems, both in optical and X-ray ranges. The most generally accepted classifications of solar flares describe important parameters of flares such as the maximum of brightness of the flare in the optical range – $H_{\alpha}$ flare class (change from F to B), area of the flare in $H_{\alpha}$ (change from S for areas less than 2 square degrees to 4 for areas more than 24.7 square degrees) and the maximum amplitude of the soft X-ray (SXR)-flux in the band 0.1–0.8 nm ($F^{\rm max}_{0.1−0.8}$) – X-ray flares of classes from C to X. A new classification parameter of solar flares is proposed here – the X-ray index of flare XI, based on GOES measurements of solar radiation in the SXR-range. The XI-index has a clear physical interpretation associated with the total flare energy in the SXR-range. XI is easily calculated for each flare with the use of available GOES data. The XI-index can be used along with other geoeffective parameters of Solaractivity to assess both flares and coronal mass ejections that are connected with them.

    • A computational study of the properties of the quasi-perpendicular fast forward shock event during solar maximum


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      MHD shock waves and discontinuities are often observed in the solar wind. We studied a strong shock wave event on 7 June 2014, during a solar maximum. The properties of forward shock are investigated and all physical parameters of interplanetary shock are analyzed. The upstream parameters, such as Alfvenvelocity and sound speed are calculated, and the angle ($\theta$) between the upstream magnetic field direction and the shock normal is estimated. To determine the propagation speed of the shock, a full numerical solution ofshock adiabatic equation is carried out. The thermal pressure for electrons and ions are calculated and also the pressure ratio and the entropy change across the shock are estimated. The results show that the shock strengthis about 3.88 with $\theta \approx 72^{\circ}$ (a quasi-perpendicular case), and shock propagation velocity is about 678 km/s. The increase in the specific entropy is also evaluated.

    • The influence of the cometary particles dynamics on the activity of comets


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      The paper deals with the dynamics of particles which are on the surface of the cometary nucleus. The key point is to research the behavior of the particles as a result of local sublimation. Generally, we can describe three mechanisms related to the particles behavior. Relatively small particles (of the order ofmicrometers) are ejected into the atmosphere of the comet due to sublimation. Slightly larger particles (of the order of centimeters) can migrate across the comet’s surface towards the equator, while much larger rubbles ofcometary matter remain at rest. The angular width of the particles is the main factor influencing the migration time towards the equator of the comet. The measure of angular width and particles’ migration time is its sizeand coefficient of friction. The numerical calculations presented in the paper refer to a hypothetical comet X/P and the comet 67P/Churyumov–Gerasimenko.

  • Journal of Astrophysics and Astronomy | News

    • Continuous Article Publication

      Posted on January 27, 2016

      Since January 2016, the Journal of Astrophysics and Astronomy has moved to Continuous Article Publishing (CAP) mode. This means that each accepted article is being published immediately online with DOI and article citation ID with starting page number 1. Articles are also visible in Web of Science immediately. All these have helped shorten the publication time and have improved the visibility of the articles.

    • Editorial Note on Continuous Article Publication

      Posted on July 25, 2019

      Click here for Editorial Note on CAP Mode

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