"The adventure is not to see new things but to see things with new eyes."

- Claude Bernard

For the latest research updates, check my publications in NASA ADS.

Current Work

HST COS Ultraviolet spectroscopic survey of DA white dwarfs

Under the Hubble COS snapshot survey program (P.I. Boris Gaensicke), we have carried out a far-ultraviolet spectroscopy survey of 311 DA white dwarfs (WDs) with Cosmic Origins Spectrograph (COS) of HST with the main aim of searching for signatures of planetary debris in intermediate temperature white dwarfs (12,000-30,000 K). I am currently analyzing the HST/COS high-resolution ultraviolet (UV) spectroscopy survey data of DA (hydrogen-dominant atmospheres) white dwarfs (WDs). The atmospheric parameters of WDs, such as effective temperature and surface gravity, are extensively studied using optical spectroscopic and photometric observations; however, there are fewer studies using UV spectroscopy.

The main motivation of this work is to search for planetary debris around young white dwarfs, estimate the abundances using UV spectroscopy, check their variation with WD parameters (mass, cooling age, etc.), provide a comparative analysis of the atmospheric parameters determined from UV and optical estimates from different studies, etc. Recently, we have also utilised this spectroscopic sample to understand the local interstellar medium, study the UV extinction correlation with 3D dust-extinction maps, and estimate the bias in the optical extinction.

Interesting results from HST COS snaphsot survey of white dwarfs:

1. Sahu et al. 2025a: Discovery of a hot white dwarf merger remnant using the ultraviolet capability of HST. Check out the associated press release by Warwick, NASA, and ESA.

2. Sahu et al. 2025b: Discovery of an icy and nitrogen-rich extrasolar planetesimal. Check out the associated press release by Warwick, NASA, and ESA.

3. Sahu et al. 2024: Ultraviolet extinction correlation with 3D dust maps using white dwarfs.

4. Sahu et al 2023: An HST COS ultraviolet spectroscopic survey of 311 DA white dwarfs. I. Fundamental parameters and comparative studies

5. Williams, J., Gaensicke, B., Sahu, S. et al. 2025: Measurements of three exo-planetesimal compositions: a planetary core, a chondritic body, and an icy Kuiper belt analogue

UV fit (in red) to HST COS spectra of DA white dwarfs

The HST COS Survey catalogue of 311 DA white dwarfs with their basic parameters are available via Vizier/CDS.

I am also a science team member of the 4MOST survey team (sub-survey S3) working on the 100 parsec survey sample. We are interested in obtaining the low-resolution spectra of main-sequence stars (up to spectral types M) covering the wavelength range 370–950 nm, which will be useful for studying their basic properties.

Ph.D. Work

Hot stellar populations in Globular Clusters

My thesis work was based on studying the properties of hot stellar populations in Globular clusters (GCs) by combining AstroSat/UVIT with Gaia and HST observations. We mainly focused on constructing the UV-optical color-magnitude diagrams to understand the role of multiple stellar populations in the morphology of horizontal branch stars (NGC1851, NGC 288), radial distribution of blue straggler stars (BSSs), etc. We also studied the spectral energy distributions to derive the properties and identify hot companions to exotic objects such as BSSs (WD companion to a BSS in NGC 5466).

To further expand our knowledge of GCs in UV, we carried out FUV legacy survey of GCs with UVIT (GlobULeS) with deep exposures (~10 Ksec) and provided a catalog in FUV with membership probabilities covering the entire cluster regions.

Sahu et al. 2022: Globular Cluster UVIT Legacy Survey (GlobULeS) - I. FUV-optical colour-magnitude diagrams for eight globular clusters

The Globules survey Catalog is available via Vizier/CDS.

Link to the thesis

Stacked FUV-optical Color-Magnitude diagrams of 11 GCs

Press releases from AstroSat/UVIT:

A star relives its youth while dancing with its wizened companion in NGC 5466, Astrosat Picture of the Month #018, March 2019.
Hot Ultraviolet stars in the Globular Cluster NGC 288, Astrosat Picture of the Month #014, November 2018.
NGC 1851: One cluster, two populations, Astrosat Picture of the Month #003, November 2017.