arXiv subjects

Param Rekhi

Publications and source records attributed to Param Rekhi.

5 recordsLinked to original sources

arXiv2026

Mapping the Landscape of M Dwarf X-ray Flares: New Discoveries in Context

We report the discovery of 11 X-ray flares from 7 M dwarfs previously unknown to exhibit flaring activity, by cross-matching eROSITA observations of bright, nearby M dwarfs with the Chandra telescope archive. To analyze the properties of these flares in a broader context, we compile the sample of all reported X-ray flares from the 15 M dwarfs identified as flaring in the literature. We use this combined sample to derive constraints on the X-ray flare frequency distributions of M0-M6 stars. The average flare occurrence rate we measure is $\sim 10^{-1}\,\rm ks^{-1}$ (corresponding to $\sim 9$ flares per day). The X-ray flares in this sample span energies from $10^{29}\,\rm erg$ to $10^{33}\,\rm erg$ and exhibit a strong correlation between flare strength and duration. The flare properties we characterize include their durations, flux and temperature enhancements, and temporal asymmetries. Using these results and recent simulations of flare-driven atmospheric escape, we derive an upper limit on the time required for habitable Earth-like planets orbiting these M dwarfs to completely lose their atmospheres: 0.5-30 Myr.

astro-ph.SR↗

arXiv2026

IK Pegasi and the Double-merger Path to Type Ia Supernovae

Recent Gaia astrometry has revealed thousands of main-sequence + white-dwarf binaries (MS+WD) at separations of ~0.1-10 au, including a subset hosting unusually massive (>~0.8 Msun) WDs. We argue that s-process enrichment in the non-degenerate companion provides a powerful diagnostic for identifying WDs that formed via mergers in hierarchical triple systems. For a massive WD, standard single-star evolution requires a massive (>~4 Msun) progenitor, yet such progenitors produce negligible s-process yields. We define IK Peg-type systems as those exhibiting this mass-yield tension: barium-enhanced companions orbiting WDs too massive to have descended from efficient s-process producers. The well-known system IK Peg exemplifies this class. Applying this framework to published spectroscopic data reveals several additional candidates, and we estimate that a few dozen such systems should exist in the current Gaia sample. If these systems trace inner-binary mergers in primordial triples, they represent observable intermediate stages toward eventual Type Ia supernovae via the double-merger pathway, as predicted by recent population-synthesis models.

astro-ph.SR↗

arXiv2026

Gaia Barium Dwarfs and Their Ostensibly Ordinary Counterparts

The recently identified Gaia population of main-sequence--white dwarf (MS+WD) binaries at separations of ${\sim}\,1~{\rm AU}$, often with moderate eccentricities, is not readily reproduced by binary population synthesis models. Barium stars represent a closely related population whose enrichment in $s$-process elements both confirms the presence of a WD companion and attests to past binary interaction. It also indicates that mass transfer occurred at least during the late- and post-AGB phases of the WD progenitor, when $s$-process elements are dredged up. In this work, we further explore the connection between the astrometrically identified Gaia MS+WD binaries and the classical barium star population. To achieve this, we used high-resolution FEROS spectroscopy to measure abundances for 30 Gaia DR3 non-single-star binaries, identifying 10 as Ba-enriched. Together with our recent analysis of archival GALAH data, this yields a sample of 38 barium dwarfs with dynamically measured WD masses, compared to only 6 previously known systems with known WD masses at these separations. We find that, in cases where metallicity is sufficiently low to facilitate efficient $s$-process production, barium and yttrium enrichment is often detected. This enrichment is also identified in eccentric systems, suggesting that post-AGB mass transfer mechanisms are capable of pumping eccentricity into the orbit or occur without erasing it. Our results indicate that the Gaia MS+WD binaries trace the population from which barium stars emerge. Treating the large Gaia-discovered population as an extension of known $s$-process enriched dwarfs opens an avenue to empirically constrain their formation and evolution.

astro-ph.SR↗

arXiv2025

A Census of NUV M-Dwarf Flares Using Archival GALEX Data and the gPhoton2 Pipeline

M-dwarfs are common stellar hosts of habitable-zone exoplanets. NUV radiation can severely impact the atmospheric and surface conditions of such planets, making characterization of NUV flaring activity a key aspect in determining habitability. We use archival data from the GALEX and XMM-Newton telescopes to study the flaring activity of M-dwarfs in the NUV. The GALEX observations form the most extensive dataset of M-dwarfs in the NUV to date, with exploitation of this data possible due to the new gPhoton2 pipeline. We run a dedicated algorithm to detect flares in the pipeline produced lightcurves and find some of the most energetic flares observed to date within the NUV bandpass, with energies of $\sim10^{34}$ ergs. Using GALEX data, we constrain flare frequency distributions for stars from M0 to M6 in the NUV up to $10^5\,$s in equivalent duration and $10^{34}$ ergs in energy, orders of magnitude above any previous study in the UV. We speculate the combined effect of NUV luminosities and flare rates of M4 and later stars could potentially allow abiogenesis on their habitable zone exoplanets. As a counterpoint, we estimate that the high frequencies of energetic UV flares and associated coronal mass ejections would inhibit the formation of an ozone layer, possibly preventing genesis of complex Earth-like lifeforms due to sterilizing levels of surface UV radiation. We also provide a framework for future observations of M-dwarfs with ULTRASAT, a wide FoV NUV telescope to be launched in 2027.

astro-ph.SR↗

arXiv2024

Ba Enrichment in Gaia MS+WD Binaries: Tracing $s$-Process Element Production

A large population of intermediate-separation binaries, consisting of a main-sequence (MS) star and a white dwarf (WD), has recently emerged from Gaia's third data release (DR3), posing challenges to current models of binary evolution. Here we examine the $s$-process element abundances in these systems using data from GALAH DR3. Following refined sample analysis with parameter estimates based on GALAH spectra, we find a distinct locus where enhanced $s$-process elements depend on both the WD mass and metallicity, consistent with loci identified in previous asymptotic giant branch (AGB) nucleosynthesis studies with higher $s$-process yields. Notably, these enhanced abundances show no correlation with the systems' orbital parameters, supporting a history of accretion in intermediate-separation MS+WD systems. Consequently, our results form a direct observational evidence of a connection between AGB masses and $s$-process yields. We conclude by showing that the GALAH DR3 survey includes numerous Ba dwarf stars, within and beyond the mass range covered in our current sample, which can further elucidate $s$-process element distributions in MS+WD binaries.

astro-ph.SR↗