arXiv subjects

D. Deras

Publications and source records attributed to D. Deras.

9 recordsLinked to original sources

arXiv2026

A Homogenized Catalogue of Variable Stars in the Globular Cluster M22: Membership, Physical Parameters, and Distance

We present a photometric analysis of the variable stars in the field of the globular cluster M22 (NGC 6656), using time-series photometry from the literature and from OGLE and Gaia-DR3 data. From a sample of 666 variable stars in the field, we identify 94 as cluster members, including 10 RRab, 17 RRc, 2 Type II Cepheids, 15 SX Phe, 15 SR stars and 22 eclipsing binaries. Fourteen of these members are not included in the Catalogue of Variable Stars in Globular Clusters. Through the Fourier decomposition of the RR Lyrae light curves, we derived $[\mathrm{Fe/H}] = -1.73 \pm 0.17$ dex and $d = 3.27 \pm 0.14$ kpc. Among the RRc stars, V15 shows an additional frequency with a frequency ratio $f_{0.61}/f_\mathrm{1O} = 0.6152$, consistent with the RR$_{0.61}$ phenomenon. Using P-L relations for member SX Phe stars, we obtained $d = 3.31 \pm 0.17$ kpc, consistent with the RR Lyrae distance. We constructed a CMD comprising only cluster members, which allows us to confirm the evolutionary status of the cluster's RR Lyrae stars. None of the fitted isochrones satisfactorily reproduces the CMD morphology, which is consistent with the presence of multiple stellar populations in M22.

astro-ph.SR↗

arXiv2026

Variable stars in the field of the Galactic globular cluster M71

M71 is a nearby, metal-rich globular cluster at low Galactic latitude, where field contamination and spatially variable extinction complicate colour-magnitude diagrams (CMDs) and the identification of cluster member variable stars. Our aims are (i) to construct a homogeneous census of variable stars in M71 by refining their periods and classifications and identifying new candidates, and (ii) to derive a decontaminated, differentially dereddened CMD to constrain its physical properties. We obtained Johnson-Kron-Cousins $VI$ time-series CCD photometry and reduced it using difference image analysis. Cluster membership was established from \textit{Gaia}~DR3 proper motions, and a differential-reddening correction was applied across the field of view. The resulting CMD, cleaned of field stars, was compared with tailored isochrones to estimate age ($12.9^{+0.9}_{-0.8}$ Gyr), metallicity ([Fe/H] =$-0.88^{+0.13}_{-0.15}$), mean reddening ($E(B-V)$ = $0.21 \pm 0.02$), and distance modulus ($(m-M)_{0}$ = $13.01 \pm 0.06$). Variable stars were identified using two complementary approaches: a periodogram-free string-length scan refined with phase dispersion minimisation, and a robust inter-site screening based on median statistics combined with a generalised Lomb-Scargle significance criterion. We identified 21 variable stars not previously reported in the Catalog of Variable Stars in Globular Clusters and provided their periods, amplitudes, classifications, membership status, and light curves. This combined strategy yields a consistent picture of M71, expanding its known variable-star population and confirming parameters typical of metal-rich Galactic disk globular clusters.

astro-ph.GA↗

arXiv2022

A new visit to the variable stars in M56 and its colour magnitude diagram structure

We present a $VI$ CCD photometric study and a membership analysis of the globular cluster M56 (NGC 6779). This produced a decontaminated CMD from field stars, which enabled a better confrontation with theoretical isochrones; Zero-Age Horizontal Branches (ZAHB) and post-ZAHB evolutionary tracks. Post He-flash evolutionary models with a He-core mass of 0.5 M$_\odot$ and envelopes of 0.04 - 0.18 M$_\odot$, cover the complete Horizontal Branch. Models with total mass $\sim$ 0.68 M$_\odot$ explain the RR Lyrae, while those with a mass $\sim$ 0.56 M$_\odot$ and a very subtle envelope, explain the Pop II cepheids with a progenitor in the blue tail of the HB. %Using the mean $(V-I)$ at minimum color curve for the RRab stars, we derived a value for the reddening of $E(B-V)$ = 0.26 $\pm$ 0.02. Based on the Fourier decomposition of the $V$ light curve of a single cluster member RRc star, we determined a metallicity of $\rm [Fe/H]_{ ZW}$ = -1.96 $\pm$ 0.09. Several independent distance determination approaches lead to a mean distance to M56 of $\big< d \big>$ = 9.4 $\pm$ 0.4 kpc. Finally, we report 5 new variables: 1 SX Phe, 3 EB, and 1 RRc.

astro-ph.SR↗

arXiv2022

A new photometric study of M14 (NGC 6402): An interpretation of the Horizontal Branch and beyond

We present a CCD $VI$ photometric study of the globular cluster M14. Particular attention is given to the variable stars. This allowed new classifications and cluster membership considerations. New variables are reported; 3 RRc, 18 SR and 1 SX Phe. The Fourier decomposition of RR Lyrae light curves lead to the mean cluster metallicity of [Fe/H]$_{\rm ZW}= -1.3 \pm 0.2$. Several independent methods yield a mean distance of $9.36 \pm 0.16$ kpc. A Colour-Magnitude diagram outlined by the cluster members enabled a matching with theoretical predictions of isochrones and zero-age horizontal branches, whose fitting to the observations is in good agreement with the above distance and metallicity. The Oosterhoff type of M14 is confirmed as Oo-int, and the pulsating mode distribution of RR Lyrae stars on the HB shows that the bimodal region of the instability strip is shared by RRab and RRc stars. By modelling the mass loss at the RGB after the He flash events, we were able to represent the blue tail of the HB, using a core mass of 0.48 $M_{\odot}$ and total masses of 0.52-0.55 $M_{\odot}$. A progenitor star on the MS of 0.84 $M_{\odot}$ reaches the HB in about 12.5 Gyrs, consonant with previous age determinations of the cluster. Type II Cepheids of M14 may be interpreted as products of post-HB evolution, driven by the complex processes involving the burning of the very thin low mass hydrogen and helium shells of these stars and their minuscule envelopes. No evidences were found in favor of M14 being of extragalactic origin.

astro-ph.SR↗

arXiv2020

CCD \emph{VI} time-series of the extremely metal poor globular cluster M92: revisiting its variable star population

We present an analysis of \emph{VI} CCD time-series photometry of the Oo II type globular cluster M92. The variable star population of the cluster is studied with the aim of revising their classifications, identifications, frequency spectra and to select indicators of the parental cluster metallicity and distance. The Fourier decomposition of RR Lyrae light curves lead to the estimation of mean [Fe/H]$_{\rm spec}=-2.20\pm 0.18$, and distance of $8.3\pm0.2$ kpc. Four new variables are reported; one RRd (V40), a multimode SX Phe (V41), a SR (V42) and one RRc (F1) that is most likely not a cluster member. The AC nature of V7 is confirmed. The double mode nature of the RRc star V11 is not confirmed and its amplitude modulations are most likely due to the Blazhko effect. Two modes are found in the known RRc variable V13. It is argued that the variable V30, previously classified as RRab is, in fact, an BL Her-type star not belonging to the cluster. Making use of the $Gaia$-DR2 proper motions, we identified 5012 stars in the field of the cluster, that are very likely cluster members, and for which we possess photometry, enabling the production of a refined Colour-Magnitude Diagram. This also allowed us to identify a few variable stars that do not belong to the cluster. The RR Lyrae pulsation modes on the HB are cleanly separated by the first overtone red edge, a common feature in all Oo II type clusters.

astro-ph.SR↗

arXiv2020

NGC 6712: the variable star population of a tidally disrupted globular cluster

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).

astro-ph.SR↗

arXiv2019

A new study of the variable star population in the Hercules globular cluster (M13; NGC 6205)

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.

astro-ph.SR↗

arXiv2018

Physical parameters of RR Lyrae stars in NGC 6171

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.

astro-ph.SR↗

arXiv2018

Period changes in the RR Lyrae stars of NGC 6171 (M107)

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.

astro-ph.SR↗