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Time-Series Photometric Detection and Physical Characterization of Variable Stars in Four Intermediate- to Old-Age Galactic Open Clusters

K. Belwal, D. Bisht, Ing-Guey Jiang, Mohit Singh Bisht, A. Raj, S. K. Chakrabarti, D. Bhowmick

Abstract

We present a ground-based time-series photometric study of stellar variability in four intermediate- to old-age open clusters NGC 2192, NGC 2266, NGC 2509, and IC 1369 based on high-cadence Cousins R-band observations obtained with the 0.6 m VASISTHA telescope at the IERCOO observatory. The monitoring campaign comprises more than 34 h of time-series data, providing sensitivity to short-period variability on timescales of 0.02-2 d. We identified between 190 and 290 probable members in each cluster using a Gaussian Mixture Model. Structural parameters were derived from radial density profiles fitted with King models. Fundamental parameters were further constrained using color-magnitude diagram analysis with PARSEC isochrones, yielding ages of 0.3-1.6 Gyr and distances of 2.5-3.9 kpc. From the time-series photometry, we identify four new variable stars and seven previously uncharacterized periodic variables, including δ Scuti and γ Doradus pulsators, as well as rotational variables. The detected variables exhibit periods between 0.12-0.90 d, with R-band amplitudes ranging from 0.01 to 0.20 mag. Periods were determined using Lomb-Scargle analysis of calibrated light curves. For a subset of variables, spectral energy distribution fitting was performed to derive effective temperatures (4300-10 000 K), radii (1.3-46 R_{\odot}), and luminosities (2-100 L_{\odot}), enabling reliable placement on the Hertzsprung-Russell diagram. We present PHOEBE light-curve modelling of the W UMa-type eclipsing binary Gaia DR3 2164531610149292288 in IC 1369, deriving its physical parameters and providing the first detailed characterization beyond its previously reported variability. These results demonstrate that combining dense-cadence ground-based observations with Gaia astrometry provides a reliable approach for identifying and characterizing variable stars in OCs.

Time-Series Photometric Detection and Physical Characterization of Variable Stars in Four Intermediate- to Old-Age Galactic Open Clusters

Abstract

We present a ground-based time-series photometric study of stellar variability in four intermediate- to old-age open clusters NGC 2192, NGC 2266, NGC 2509, and IC 1369 based on high-cadence Cousins R-band observations obtained with the 0.6 m VASISTHA telescope at the IERCOO observatory. The monitoring campaign comprises more than 34 h of time-series data, providing sensitivity to short-period variability on timescales of 0.02-2 d. We identified between 190 and 290 probable members in each cluster using a Gaussian Mixture Model. Structural parameters were derived from radial density profiles fitted with King models. Fundamental parameters were further constrained using color-magnitude diagram analysis with PARSEC isochrones, yielding ages of 0.3-1.6 Gyr and distances of 2.5-3.9 kpc. From the time-series photometry, we identify four new variable stars and seven previously uncharacterized periodic variables, including δ Scuti and γ Doradus pulsators, as well as rotational variables. The detected variables exhibit periods between 0.12-0.90 d, with R-band amplitudes ranging from 0.01 to 0.20 mag. Periods were determined using Lomb-Scargle analysis of calibrated light curves. For a subset of variables, spectral energy distribution fitting was performed to derive effective temperatures (4300-10 000 K), radii (1.3-46 R_{\odot}), and luminosities (2-100 L_{\odot}), enabling reliable placement on the Hertzsprung-Russell diagram. We present PHOEBE light-curve modelling of the W UMa-type eclipsing binary Gaia DR3 2164531610149292288 in IC 1369, deriving its physical parameters and providing the first detailed characterization beyond its previously reported variability. These results demonstrate that combining dense-cadence ground-based observations with Gaia astrometry provides a reliable approach for identifying and characterizing variable stars in OCs.
Paper Structure (22 sections, 1 equation, 12 figures, 4 tables)

This paper contains 22 sections, 1 equation, 12 figures, 4 tables.

Table of Contents

  1. Introduction
  2. Data Used
  3. Ground-Based Time-Series Photometry
  4. Gaia DR3 Astrometric and Photometric Data
  5. Multi-wavelength Photometric Data
  6. Membership Probability
  7. Structural Parameters
  8. Fundamental Parameters of the Clusters
  9. Distances of clusters from trigonometric parallaxes
  10. Color Magnitude Diagram
  11. Identification and Characterization of Variable Stars
  12. SED-Based Stellar Parameters and Hertzsprung–Russell Diagram
  13. Classification of the Variable Stars
  14. Non Pulsating
  15. $\delta$ Scuti Variables
  16. ...and 7 more sections

Figures (12)

  • Figure 1: $R$-band finding charts of the observed fields for the OCs NGC 2192, NGC 2266, NGC 2509, and IC 1369. The images are representative frames from the time-series photometric observations used in this study. Newly identified variable stars are marked with white circles and labelled by their IDs. Yellow plus symbols indicate the cluster centres. The inner red circle denotes the cluster core radius, while the outer blue circle indicates the cluster (limiting) radius, both estimated in the present analysis. The orientation is north up and east to the left, with right ascension and declination shown in degrees.
  • Figure 2: Mean photometric uncertainties as a function of stellar brightness in the $R$ band for the clusters under study. Each panel shows the variation of photometric error with $R$-band magnitude. The plots indicate that the uncertainty remains below $\sim$0.01 mag for the brighter stars and gradually increases up to $\sim$0.15–0.20 mag toward the fainter end. The magnitude distribution for NGC 2509 appears truncated at the faint end compared to the other clusters. This effect arises from differences in observing conditions and the shorter total integration time for this cluster, which limits the detection of faint sources.
  • Figure 3: Spatial, kinematic, and parallax distributions of the stars in the field of IC 1369. The grey points represent all sources within the adopted search radius, while the red points denote the stars identified as cluster members. The panels illustrate (left) the spatial concentration of members, (middle) their tight grouping in proper-motion space, and (right) the parallax histogram, which shows a distinct peak corresponding to the cluster. This figure shows IC 1369 as a representative example of the procedure applied to all four clusters analyzed in this study.
  • Figure 4: Radial density profiles for the OCs NGC 2192, NGC 2266, NGC 2509 , and IC 1369. The red points with error bars represent the observed stellar density distribution, while the blue dashed curves show the best-fitting three-parameter King (1962) model. The horizontal dashed lines denote the background field stellar density, and the vertical dashed lines mark the core radii. The derived core radii are $1.70$ arcmin for NGC 2192, $1.20$ arcmin for NGC 2266,$1.35$ arcmin for NGC 2509, and $2.22$ arcmin for IC 1369.
  • Figure 5: Color--magnitude diagrams (CMDs) of the OCs NGC 2192, NGC 2266, NGC 2509, and IC 1369, shown in the $G$ versus $(\mathrm{BP}-\mathrm{RP})$ plane. Grey points represent stars within the estimated cluster radius, while red filled circles denote the probable cluster members selected using astrometric and photometric criteria. The PARSEC isochrones plotted correspond to three ages for each cluster, with the central value providing the best fit and the younger and older values illustrating the age uncertainty. The adopted ages (in $\log[\mathrm{age/yr}]$) are 9.00, 9.05, and 9.10 for NGC 2192; 8.85, 8.95, and 9.05 for NGC 2266; 9.05, 9.15, and 9.25 for NGC 2509; and 8.30, 8.40, and 8.50 for IC 1369. These CMD-based fits provide the fundamental cluster parameters used throughout this work. In addition, the stars marked by blue boxes and triangles are found to be consistent with the BSSs identified by jadhav2021highrain2021new, with two BSSs located in NGC 2192 and two in NGC 2266.
  • ...and 7 more figures