Employing VI images of NGC 2419 acquired over 17 years, light curves for most of the known variables in the field of the cluster are produced. A cluster membership analysis for about 3100 stars in the cluster field with proper motions from Gaia-DR3 revealed the presence of member stars as far as 140 pc from the cluster center and enabled the construction of a cleaner CMD free of field stars. It was found that RRab and RRc stars share the inter-order region in the instability strip, which is unusual for OoII clusters. Theoretical considerations confirm that Pop II cepheids are descendants of extreme ZAHB blue tail stars with very thin envelopes of about 10% of the total mass. Member RR Lyrae stars were employed to calculate independent estimates of the mean cluster metallicity and distance; we found [Fe/H]UV = -1.90 ± 0.27 and D = 86.3 ± 5.0 kpc from the RRab and [Fe/H]UV = -1.88 ± 0.30 and D = 83.1 ± 8.1 kpc from the RRc light curves.

A 5-yr CCD photometric VI time series of NGC 7006 is employed to perform a detailed analysis of the known population of variable stars. In the process, we have corrected inconsistent classifications, sky coordinates and found 10 new cluster member variables. An independent reddening estimate with a value E(B-V) = 0.08 +/- 0.05 is made. Using Fourier decompositions of RR Lyrae light curves and well-established calibrations, the cluster mean metallicity and distance [Fe/H](ZW) = -1.53 +/- 0.15 and 41.2 +/- 1.4 kpc are estimated based on an extended sample of cluster member RRab stars. Using the Gaia DR3 data, we performed an extensive membership analysis that leads to a clean colour-magnitude diagram, and hence to the identification of variables that are likely field stars, and to considerations on the variable distribution in the horizontal branch (HB). A double-mode RR Lyrae and three CW stars are discussed. The origin of CW stars from precursors in the blue tail of the HB with very thin (similar to 0.06 +/- 0.01 M-circle dot) envelopes is argued. Our models indicate that the main-sequence predecessor of RR Lyrae stars had a mass of 0.82-0.85 M-circle dot and lost about 25-35 per cent of its mass during the red giant branch events before settling in the HB some 12-13.5 Gyr later.

A CCD VI imaging time-series over 11-year is employed to explore the light curves of stars in the field of Palomar 2. We discovered 20 RRab and 1 RRc variables. A revision of Gaia-DR3 data enabled us to identify 10 more variables and confirm the RRab nature of 6 of them and one RGB. The cluster membership is discussed, and 18 variables are most likely cluster members. The Fourier light curve decomposition for the 11 best quality light curves of cluster member stars leads to independent estimates of the cluster distance 27.2 +/- 1.8 kpc and [Fe/H] ZW = -1.39 +/- 0.55. We confirm the cluster as of the Oo I type.

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 contain a 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 new P−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. Recent investigators 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 the unsaturated depletion curve (with a plateau) are likely to be caused by post-RGB stars.

ABSTRACT Time-series VI CCD photometry of the globular cluster M10 (NGC 6254) is employed to perform a detailed identification, inspection of their light curves, their classification, and their cluster membership, of all the known variables reported up to 2018. The membership analysis is based on the $Gaia$-DR2 positions and proper motions. The metallicity of the cluster is estimated based on the sole RRc star known in the cluster. The Fourier decomposition of its light curve leads to [Fe/H]$_{\rm ZW}$ = $-1.59 \pm 0.23$ dex. The mean cluster distance, estimated by several independent methods, is $5.0 \pm 0.3$ kpc. A multiapproach search in a region of about 10$\times$10 arcmin$^2$ around the cluster revealed three new variables, one SX Phe (V35) and two sinusoidal variables on the red giant branch (RGB) of unclear classification (V36 and V37). Modelling the HB stars is very sensitive to the stellar hydrogen shell mass, which surrounds the 0.50 $\mathrm{ M}_{\odot }$ helium core. To match the full stretch of the HB population, a range of total mass of 0.56–0.62 $\mathrm{ M}_{\odot }$ is required. These models support a distance of 5.35 kpc and an age of about 13 Gyr, and hint to some individual variation of the mass-loss on the upper RGB, perhaps caused by the presence of closed magnetic field in red giants.

We present an analysis of $\emph{VI}$ CCD time series photometry of globular cluster NGC 6712. Our main goal is to study the variable star population as indicators of the cluster mean physical parameters. We employed the Fourier decomposition of RR Lyrae light curves to confirm that $[\rm Fe/H]_{UVES} = -1.0 \pm 0.05$ is a solid estimate. We estimated the reddening to the cluster as $E(B-V)$ = 0.35 $\pm$ 0.04 from the RRab stars colour curves. The distance to the cluster was estimated using three independent methods which yielded a weighted mean distance $\big < d \big>$ = 8.1 $\pm$ 0.2 kpc. The distribution of RRab and RRc stars on the HB shows a clear segregation around the first overtone red edge of the instability strip, which seems to be a common feature in OoI-type cluster with a very red horizontal branch. We carried out a membership analysis of 60,447 stars in our FoV using the data from $Gaia$-DR2 and found 1529 likely members; we possess the light curves of 1100 among the member stars. This allowed us to produce a clean colour-magnitude diagram, consistent with an age of 12 Gyrs, and enabled us to discover close unresolved contaminants for several variable stars. From the proper motion analysis we found evidence of non-member stars in the FoV of the cluster being tidally affected by the gravitational pull of the bulge of the Galaxy. We found that the RRab variable V6, shows a previously undetected Blazhko effect. Finally, we report sixteen new variables of the EW-type (9) and SR-type (7).

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 ± 0.011 dex kpc $$^{-1}$$ at R $$_\mathrm{gc}<$$ 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 ± 0.007 dex kpc $$^{-1}$$ at R $$_\mathrm{gc}>$$ 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.

We present new CCD VI photometry of the distant globular cluster Pal 13. Fourier decomposition of the light curves of the three cluster member RRab stars lead to estimations of [Fe/H] = - 1.65, and a distance of 23.67 +/- 0.57 kpc. Light and colour near minimum phases for RRab stars leads to an estimate of E(B - V) = 0.104 +/- 0.001. A V/(V - I) colour-magnitude diagram, built exclusively with likely star members, shows consistency with the above parameters and an age of 12 Gyrs. A search of variable stars in the field of view of our images revealed the variability of a red giant cluster member and of three probably non-member stars; two RRab stars and one W Virginis star or CW. The GAIA proper motions of member stars in Pal 13 show a significant scatter, consistent with the scenario of the cluster being tidally stripped.

We present the results from $VI$ CCD time-series photometry of the globular cluster M13 (NGC 6205). From the Fourier decomposition of the light curves of RRab and RRc stars we found an average metallicity of [Fe/H]zw = -1.58 $\pm$ 0.09. The distance to the cluster was estimated as 7.1 $\pm$ 0.1 kpc from independent methods related to the variable star families RR Lyrae, SX Phe and W Virginis, from the luminosity of the theoretical ZAHB and from the orbit solution of a newly discovered contact binary star. The RR Lyrae pulsation modes are segregated by the red edge of the first overtone instability strip in this OoII type cluster. A membership analysis of 52,800 stars in the field of the cluster is presented based on \textit{Gaia}-DR2 proper motions which enabled the recognition of 23,070 likely cluster members, for 7,630 of which we possess $VI$ photometry. The identification of member stars allowed the construction of a clean CMD and a proper ZAHB and isochrone fitting, consistent with a reddening, age and distance of 0.02 mag, 12.6 Gyrs and 7.1 kpc respectively. We report seven new variables; one RRc, two SX Phe stars, three SR and one contact binary. V31 presents double-mode nature and we confirm V36 as RRd. Fifteen variable star candidates are also reported. The analysis of eighteen stars in the field of the cluster, reported as RR Lyrae from the $Gaia$-DR2 data base reveals that at least seven are not variable. We noted the presence of a high velocity star in the field of the cluster.

We report an analysis of new $V$ and $I$ CCD time-series photometry of the globular cluster NGC 6934. Through the Fourier decomposition of the RR Lyrae light curves, the mean values of [Fe/H] and the distance of the cluster were estimated, we found; [Fe/H]$_{UVES}$=-1.48$\pm$0.14 and $d$=16.03$\pm$0.42 kpc, and [Fe/H]$_{UVES}$=-1.43$\pm$0.11 and $d$=15.91$\pm$0.39 kpc, from the calibrations of RRab and RRc stars respectively. Independent distance estimations from SX Phe and SR stars are also discussed. Individual absolute magnitudes, radii and masses are also reported for RR Lyrae stars. We found 12 new variables: 4 RRab, 3 SX Phe, 2 W Virginis (CW)and 3 semi-regular (SR). The inter-mode or either-or region in the instability strip is shared by the RRab and RRc stars. This characteristic, observed only in some OoI clusters and never seen in an OoII, is discussed in terms of mass distribution in the ZAHB.

We have calculated the physical parameters of the RR Lyrae stars in the globular cluster NGC 4147 via the Fourier decomposition of their light curves, using new data and well-established semi-empirical calibrations and zero points. The mean metallicity and distance estimated using the RRc stars are [Fe/H](zw) = -1.72 +/- 0.15 and 19.05 +/- 0.46 kpc, respectively. The star V18, whose variability has been previously in dispute, is confirmed to be a variable of the SR type with a period of about 24.8d, and it has been used to get an independent distance estimate of the cluster. It is observed that the RRab and RRc stars do not share the inter-mode region in the horizontal branch. The cluster can be classified as of intermediate Oosterhoff type. Its horizontal branch structure and metallicity make a good case for extragalactic origin. It follows the distribution of Oo I type globular clusters in the M-v - [Fe/H] plane, as depicted from the RRc stars.

We present an analysis of VI CCD time-series photometry of the globular cluster NGC 6171. The main goal is to determine individual physical parameters for single-mode RR Lyrae stars present in the field of view of our images, by means of light curve Fourier decomposition and well-established calibrations and zero-points. This leads to the estimation of the mean values of the metallicity and distance for the parental cluster. From the RRab stars we find [Fe/H]ZW = -1.33 +- 0.12 and a distance d = 5.3 +- 0.3 kpc and from the RRc stars we find [Fe/H]ZW = -1.02 +- 0.19 and a distance d = 5.3 +- 0.2 kpc. Independent methods, such as the P-L relations for RRab and SX Phe stars enable the estimation of a weighted average distance to the cluster of 5.4 +- 0.1 kpc. We confirm the amplitude modulations of the Blazhko type of five RRab and the non-membership of V21 to the cluster. The colour-magnitude diagram is consistent with an age of 11 Gyrs. The distribution of RRab and RRc stars seems well segregated around the first overtone red edge of the instability strip. This positions NGC 6171 among OoI type clusters where the pulsating modes are neatly separated in the HB. We report two new irregular variables of the Lb type.

The paper ‘The variable star population in the globular cluster NGC 6934’ was published in RMxAA, 54, 15 (2018). Several identifications of variable stars in the chart of Figure 9 are erroneous. The stars involved are V7 and V8, V94 and V95. The correct identifications of these stars are now shown in the present chart in Figure 1. For completeness we have now identified the candidate variable stars C2, C3, C4 and C5. The candidate C1 is out of the field of these charts. The equatorial coordinates of C2 and C3 in the original Table 2 are interchanged. In present Table 1 the correct coordinates are given. The rest of the entries of Table 2 of the original paper are correct.

Based on photometric data obtained between 1935 and 2017, $O-C$ diagrams were built for 22 RR Lyrae stars in the globular cluster NGC 6171, leading to the discovery of secular period changes in 4 variables for which we have calculated their period change rates $\beta$. In contrast we find that $82\%$ of the sample stars have stable periods over the last 82 years. For the stable period stars, the whole data base has been employed to refine their periods. Among the period changing stars, three (V10, V12 and V16) have decreasing periods larger than expected from stellar evolution. Despite these individual cases of significant period change rate, the golbal average of the measured period changes in the cluster is basically zero, in consonance with theoretical predictions for clusters with redder horizontal branches. The hitherto unpublished observations, now brought into public domain, are employed to calculate a set of times of maximum light which are used in the present analysis.

Hanle echelle spectrograph (HESP) is a high resolution, bench mounted, fiber-fed spectrograph at visible wavelengths. The instrument was recently installed at the 2m Himalayan Chandra Telescope (HCT), located at Indian Astronomical Observatory (IAO), Hanle at an altitude of 4500m. The telescope and the spectrograph are operated remotely from Bangalore,(similar to 3200km from Hanle), through a dedicated satellite link. HESP was designed and built by Kiwi Star Optics, Callaghan Innovation, New Zealand. The spectrograph has two spectral resolution modes (R=30000 & 60000). The low resolution mode uses a 100 micron fiber as a input slit and the high resolution mode is achieved using an image slicer. An R2 echelle grating, along with two cross dispersing prisms provide a continuous wavelength coverage between 350-1000nm. The spectrograph is enclosed in a thermally controlled environment and provides a stability of 200m/s during a night. A simultaneous thorium-argon calibration provides a radial velocity precision of 20m/s. Here, we present a design overview, performance and commissioning of the spectrograph.

SDSS J082625.70+612515.10 (V = 11.4; [Fe/H] = −3.1) and SDSS J134144.60+474128.90 (V = 12.4; [Fe/H] = −3.2) were observed with the SDSS 2.5m telescope as part of the SDSS MARVELS spectroscopic pre-survey and identified as extremely metal-poor (EMP; [Fe/H] < −3.0) stars during the high-resolution follow-up using the Hanle Echelle Spectrograph (HESP) on the 2.0-m Himalayan Chandra Telescope. In this paper, the first science results using HESP, we present a detailed analysis of their chemical abundances. Both stars exhibit under-abundances in their neutron capture elements, while one of them (SDSS J134144.60+474128.90) is clearly enhanced in carbon. Lithium was also detected in this star at a level of about A(Li) = 1.95. The spectra were obtained over a span of 6–24 months, and indicate that both stars could be members of binary systems. We compare the elemental abundances derived for these two stars along with other carbon-enhanced metal-poor (CEMP) and EMP stars, in order to understand the nature of their parent supernovae. We find that CEMP-no stars and EMP-dwarfs show a very similar trend in their lithium abundances at various metallicities. We also find indications of CEMP-no stars having larger abundances of Cr and Co at given metallicities compared to EMP stars.

As a continuation to the published work on model based calibration technique with HESP(Hanle Echelle Spectrograph) as a case study, in this paper we present the performance results of the technique. We also describe how the open parameters were chosen in the model for optimization, the glass data accuracy and handling the discrepancies. It is observed through simulations that the discrepancies in glass data can be identified but not quantifiable. So having an accurate glass data is important which is possible to obtain from the glass manufacturers. The model's performance in various aspects is presented using the ThAr calibration frames from HESP during its pre-shipment tests. Accuracy of model predictions and its wave length calibration comparison with conventional empirical fitting, the behaviour of open parameters in optimization, model's ability to track instrumental drifts in the spectrum and the double fibres performance were discussed. It is observed that the optimized model is able to predict to a high accuracy the drifts in the spectrum from environmental fluctuations. It is also observed that the pattern in the spectral drifts across the 2D spectrum which vary from image to image is predictable with the optimized model. We will also discuss the possible science cases where the model can contribute.

We describe and summarize the findings from our CCD time-series photometry of globular clusters (GCs) program and the use of difference image analysis (DIA) in the extraction of very precise light curves even in the crowded central regions down to $V\sim 19$ mag. We have discovered approximately 250 variable stars in a sample of 23 selected GCs and have, therefore, updated the census of variables in each system and in some cases we have actually completed it down to a certain magnitude. The absolute magnitude and [Fe/H] for each individual RR Lyrae is obtained via the Fourier decomposition of the light curve. An average of these parameters leads to the distance and metallicity of the host GCs. We have also calibrated the P-L relation for SX Phe stars which enables an independent calculation of the cluster distance. We present the mean [Fe/H], $M_V$ and distance for a group of selected GCs based exclusively on the RR Lyrae light curve Fourier decomposition technique and set on a rather unprecedented homogeneous scale. We also discuss the luminosity dependence of the horizontal branch (HB) via the $M_V$-[Fe/H] relation. We find that this relation should be considered separately for the RRab and RRc stars.