arXiv subjects

Kuljeet Singh Saddal

Publications and source records attributed to Kuljeet Singh Saddal.

2 recordsLinked to original sources

arXiv2026

Stationary Resistive MHD Solutions Near an Isolated Three-Dimensional Magnetic Null

Magnetic null points define the magnetic field topology in stellar coronae, planetary magnetospheres, and astrospheres, and are commonly associated with magnetic reconnection. We investigate the local structure of stationary three-dimensional (3D) resistive, incompressible magnetohydrodynamic (MHD) flows in the vicinity of an isolated magnetic null and an isolated velocity stagnation point, assuming that the two nulls are colocated. The governing MHD equations are solved self-consistently using a first-order local expansion of the magnetic and velocity fields about the null. For any given magnetic-null configuration, the equations admit a two-parameter family of stationary solutions, comprising multiple solution branches with distinct velocity topologies. Along these families, the velocity-null polarity can change at bifurcation points where the flow becomes effectively 2.5D. A single magnetic null can therefore support positive-radial, negative-radial, and spiral stagnation flows, with qualitatively different inflow, outflow, and rotational properties. The associated pressure and resistivity distributions are likewise determined self-consistently and vary across the solution family, demonstrating that they are coupled to the flow topology rather than prescribed independently. The generic solutions do not require the velocity and magnetic separatrices to coincide, allowing plasma to cross the magnetic fan separatrix and providing a local reconnective flow. These results demonstrate that, in the presence of nonuniform resistivity, the magnetic topology alone does not uniquely determine the associated stationary flow, pressure, or resistivity structure around a 3D magnetic null, revealing the nonlinear feedback between all macroscopic variables and resistivity. The constant-resistivity field-aligned and spiral field-crossing solutions identified previously are recovered as special cases.

astro-ph.SR↗

arXiv2026

An Halpha survey of infrared bow-shocks around OB-type stars

Bow shocks (BSs) around massive stars are commonly identified in mid-infrared (MIR) surveys. In contrast, systematic optical investigations of these structures remain scarce with only a few possible detections reported so far. We performed a targeted Halpha imaging survey of 78 IR BS candidates compiled from literature catalogues. Observations were obtained with multiple ground-based facilities and complemented with archival MIR images from Spitzer and WISE. For the detected bow structures, geometrical parameters were measured, and the bows were classified based on the parameters of their central stars, the measured stand-off distances and adopted plausible ISM conditions Clear arc-shaped Halpha nebulae are detected in 15 objects, with one additional BS discovered serendipitously. A further 35 objects exhibit diffuse Halpha emission associated with complex background structures, while 28 show no detectable Halpha emission. The Halpha and IR morphologies generally trace the same large-scale structures, although most of the IR arcs lie slightly closer to the star and appear broader than their optical counterparts. We find that several systems are consistent with radiation-supported BSs, bow waves or dust waves, while others remain compatible with classical wind-supported BSs. This survey demonstrates that a fraction of IR BS candidates around OB stars also exhibit detectable ionised gas structures in Halpha, and both stellar wind momentum and radiation pressure can contribute to shaping these structures. No correlation is found between the detection of an Halpha BS and the stellar parameters, and the overlap in objects observed in Halpha and in the radio regime was too small for firm conclusions. Future spectroscopic observations will be required to determine the physical conditions and kinematics of the ionised gas and to further constrain the nature of these BS systems.

astro-ph.SR↗