- Surajit Sen’s Master’s thesis, titled “Detection of Variables Using Gravitational Microlensing and Plotting Colour Magnitude Diagram,” conducted at the Department of Physics and Astrophysics, Central University of Haryana, represents a significant study in observational astrophysics focusing on variable stars within the globular cluster NGC 4147.
- Introduction and Scientific ContextGlobular clusters are densely populated spherical groups of stars orbiting galaxies, often containing many variable stars that serve as vital tools for measuring cosmic distances and understanding stellar evolution. NGC 4147, the chosen target, is a globular cluster located in the constellation Coma Berenices. It was first discovered in 1784 by astronomer William Herschel and is known for its rich content of variable stars. The primary scientific goal of the thesis was to detect variable stars within this cluster through the technique of gravitational microlensing and to analyze their properties using light curves and color-magnitude diagrams.
- Gravitational Microlensing and Variable Stars
- Gravitational microlensing occurs when a massive foreground object acts as a gravitational lens, temporarily magnifying the light from a background star. This effect creates transient brightness variations detectable in highly precise photometric data. Especially intriguing are microlensing events caused by planetary objects, which produce asymmetric light curves revealing the presence and properties of these otherwise difficult-to-detect bodies.Variable stars, whose brightness fluctuates due to intrinsic and extrinsic factors, are broadly classified into pulsating variables (intrinsic) and eclipsing or rotating variables (extrinsic). Pulsating variables, including RR Lyrae stars commonly found in globular clusters, are particularly important in astronomy as they serve as standard candles for calculating accurate distances to celestial objects.
- Observations and Instrumentation
- For this study, observational data was acquired at the Indian Institute of Astrophysics using the 2.0-meter Himalayan Chandra Telescope (HCT) located at Hanle, Ladakh. Over 360 image frames were captured under I (infrared) and V (visual) bands with 100-second exposures for each frame. The CCD camera used had specific characteristics such as pixel scale, readout noise, and gain optimized for high-resolution astronomical imaging. These images provided raw data for subsequent analysis.
Data Reduction and Analysis Pipeline
- The raw CCD images contained both the astronomical signals and unwanted noise components, including bias signals, thermal noise, and cosmic ray artifacts. Surajit employed sophisticated image calibration techniques including bias frame subtraction, flat-field correction, and cosmic ray removal using the IRAF (Image Reduction and Analysis Facility) software.
- The analysis further utilized differential image analysis (DIA), implemented via the DIAPL2 package. This method involved subtracting a high-quality reference image from the individual calibrated frames to isolate brightness changes caused by variable objects within the cluster. The image subtraction process was meticulously performed by dividing the images into quadrants and applying pixel-to-pixel subtraction, enhancing the detectability of variable stars amid dense stellar populations.
- Astrometric calibration was applied to convert pixel coordinates of stars into celestial coordinates (Right Ascension and Declination), enabling cross-matching with catalogued variable stars. Differential photometry allowed the precise measurement of relative brightness changes across multiple observation dates, forming the basis for constructing detailed light curves.
- Results and Findings
- Light curves generated from the I and V filter data revealed variations in stellar brightness, with variable stars predominantly concentrated in the cluster’s densely populated central region. The study also produced a color-magnitude diagram (CMD), illustrating the relationship between stellar temperature and brightness, which is essential for classifying star populations and inferring their evolutionary status.
- The thesis systematically categorized the variable stars and identified their patterns of variability, period, and classification through comparisons with standard light curves. These findings contribute to the broader astrophysical understanding of stellar populations within globular clusters and the role of variable stars as distance indicators.
- Future Work
- Building on these results, the thesis proposes further analyses including refined astrometric calibrations, enhanced differential photometry accuracy, and the development of Hertzsprung-Russell diagrams for detailed classification of star types. Additionally, Surajit collaborated on ancillary research analyzing asteroid observations within another globular cluster dataset, illustrating versatility in astronomical data handling.
- Conclusion
- This comprehensive work combines observational astronomy, advanced data reduction techniques, and astrophysical theory to explore the dynamic nature of variable stars in NGC 4147. Surajit Sen’s thesis exemplifies rigorous methodological application and contributes valuable insights into stellar variability studies, microlensing phenomena, and astrophysical distance measurements.