arXiv subjects

Ramlal Unnikrishnan

Publications and source records attributed to Ramlal Unnikrishnan.

4 recordsLinked to original sources

arXiv2026

The Impact and Environment of Massive Stars and Stellar Clusters

Massive stars and stellar clusters shape galactic evolution through powerful feedback mechanisms including radiation pressure, photoionization, stellar winds, and cosmic ray acceleration. However, their impact remains poorly understood due to observational challenges: they are rare, distant on average, and deeply embedded within dense, dusty environments. Radio observations provide a unique window into these processes, as radio emission penetrates obscuring material and traces both thermal free-free emission from ionized gas and non-thermal synchrotron emission from shocks and particle acceleration. The Square Kilometre Array (SKA) will revolutionize massive star studies through unprecedented sensitivity and angular resolution. SKA observations will enable detailed characterization of hierarchical structures within HII regions, measurements of physical conditions through hydrogen, helium, and carbon radio recombination lines (RRLs), and detection of non-thermal emission from cosmic ray acceleration in star-forming regions. SKA will permit systematic measurements of stellar wind mass-loss rates, studies of photoionized gas kinematics and dynamics, and exploration of photodissociation regions surrounding ultracompact HII regions. Additionally, magnetic field strengths can be probed through Zeeman effect observations of RRLs. This chapter discusses the current understanding of massive stars and stellar clusters and their feedback processes. We highlight how SKA observations will advance our knowledge of massive star formation, stellar winds, hierarchical structures in HII regions, cosmic ray acceleration, and magnetic field regulation of star formation - providing crucial insights into feedback mechanisms governing the structure and evolution of the Milky Way and galaxies.

astro-ph.GA↗

arXiv2025

An empirical view of the extended atmosphere and inner envelope of the asymptotic giant branch star R Doradus. II. Constraining the dust properties with radiative transfer modelling

Mass loss in oxygen-rich asymptotic giant branch (AGB) stars remains poorly understood, as the dust detected around them appears too transparent to drive winds through absorption alone. The current paradigm invokes outflows driven by photon scattering on relatively large grains ($\sim0.3\,μ$m), but whether such grains exist in sufficient quantities remains uncertain. We test whether the dust around the oxygen-rich AGB star R~Doradus can drive its wind by combining polarimetric constraints, elemental abundance limits, and force-balance calculations. We examine Fe-free silicates (MgSiO$_3$), alumina (Al$_2$O$_3$), and Fe-bearing silicates (MgFeSiO$_4$) to evaluate whether any species can produce enough radiative pressure under realistic conditions. Using high-angular-resolution polarimetric observations from SPHERE/ZIMPOL at the VLT, we model the circumstellar dust with RADMC-3D and explore a broad parameter space in grain size, density structure, and wavelength-dependent stellar radius. For models consistent with the observations, we assess wind feasibility using updated gas-density profiles, elemental depletion constraints, and radiation-pressure thresholds. Although several dust configurations reproduce the observed scattering patterns, none generate sufficient radiative force at realistic gas-to-dust ratios, even under maximal elemental depletion. Our results for R~Doradus indicate that photon scattering on dust cannot by itself launch the wind, implying that additional mechanisms must contribute.

astro-ph.SR↗

arXiv2023

The heart of Sakurai's Object revealed by ALMA

We present high angular-resolution observations of Sakurai's object using the Atacama Large Millimeter Array, shedding new light on its morpho-kinematical structure. The millimetre continuum emission, observed at an angular resolution of 20 milliarcsec (corresponding to 70 AU), reveals a bright compact central component whose spectral index indicates that it composed of amorphous carbon dust. Based on these findings, we conclude that this emission traces the previously suggested dust disc observed in mid-infrared observations. Therefore, our observations provide the first direct imaging of such a disc. The H$^{12}$CN($J$=4$\rightarrow$3) line emission, observed at an angular resolution of 300 milliarcsec (corresponding to 1000 AU), displays bipolar structure with a north-south velocity gradient. From the position-velocity diagram of this emission we identify the presence of an expanding disc and a bipolar molecular outflow. The inclination of the disc is determined to be $i$=72$^\circ$. The derived values for the de-projected expansion velocity and the radius of the disc are $v_{\rm exp}$=53 km s$^{-1}$ and $R$=277 AU, respectively. On the other hand, the de-projected expansion velocity of the bipolar outflow detected in the H$^{12}$CN($J$=4$\rightarrow$3) emission of approximately 1000 km s$^{-1}$. We propose that the molecular outflow has an hourglass morphology with an opening angle of around 60$^{\circ}$. Our observations unambiguously show that an equatorial disc and bipolar outflows formed in Sakurai's object in less than 30 years after the born-again event occurred, providing important constraints for future modelling efforts of this phenomenon.

astro-ph.SR↗

arXiv2022

First images of the molecular gas around a born-again star revealed by ALMA

Born-again stars allow probing stellar evolution in human timescales and provide the most promising path for the formation of hydrogen-deficient post-asymptotic giant branch objects, but their cold and molecular components remain poorly explored. Here we present ALMA observations of V605 Aql that unveil for the first time the spatio-kinematic distribution of the molecular material associated to a born-again star. Both the continuum and molecular line emission exhibit a clumpy ring-like structure with a total extent of $\approx$1$^{\prime\prime}$ in diameter. The bulk of the molecular emission is interpreted as being produced in a radially-expanding disk-like structure with an expansion velocity v$_{\rm exp}$$\sim$90 km s$^{-1}$ and an inclination $i$$\approx$60$^{\circ}$ with respect to the line-of-sight. The observations also reveal a compact high-velocity component, v$_{\rm exp}$$\sim$280 km s$^{-1}$, that is aligned perpendicularly to the expanding disk. This component is interpreted as a bipolar outflow with a kinematical age $τ$$\lesssim$20 yr, which could either be material that is currently being ejected from V605 Aql, or it is being dragged from the inner parts of the disk by a stellar wind. The dust mass of the disk is in the range $M_{\rm dust}$$\sim$0.2-8$\times$10$^{-3}$ M$_{\odot}$, depending on the dust absorption coefficient. The mass of the CO is $M_{\rm CO}$$\approx$1.1$\times10^{-5}$ $M_{\odot}$, which is more than three orders of magnitude larger than the mass of the other detected molecules. We estimate a $^{12}$C/$^{13}$C ratio of 5.6$\pm$0.6, which is consistent with the single stellar evolution scenario in which the star experienced a very late thermal pulse instead of a nova-like event as previously suggested.

astro-ph.GA↗