arXiv subjects

Jagadheep D. Pandian

Publications and source records attributed to Jagadheep D. Pandian.

17 recordsLinked to original sources

arXiv2026

The dynamical environment of the high-mass star-forming region G28.288$-$0.364

Massive stars form within deeply embedded dust cores, and the development of hypercompact and ultracompact H II regions characterizes their early evolution. Identifying and analyzing such regions is essential for understanding the physical processes that govern massive star formation and the transitions between early evolutionary stages. In this article, we investigate the physical and kinematic properties of the massive star-forming region G28.288$-$0.364 to constrain the evolutionary stages of embedded H II regions and the surrounding cores. We analyze multiwavelength observations, including uGMRT radio continuum data; archival continuum and radio recombination line data from the GLOSTAR-D survey; high-angular-resolution ALMA Band 3 radio recombination line and 1.36-mm dust continuum data from the ALMAGAL survey; and complementary molecular line tracers. We derive spectral indices, measure linewidths and velocities of the radio recombination line emission, identify compact dust cores using dendrogram analysis, and estimate their physical properties. The radio continuum emission exhibits a positive spectral index, consistent with partially optically thick free-free emission. High-resolution observations resolve the ionized gas into two distinct components with physical sizes of $\sim$ 0.06 pc, and their RRL linewidths ($\sim$ 37 and 32 km s$^{-1}$, respectively) indicate that one component is in a transitional stage between hypercompact and ultracompact H II regions, while the other is more evolved. The 1.36-mm dust continuum data reveal five dust cores with surface densities consistent with the theoretical threshold for massive star formation. In summary, these results highlight the complex and sequential nature of massive star formation in clustered environments and demonstrate the importance of high angular resolution observations for resolving the early evolution of massive stars.

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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.

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arXiv2026

Detection of non-thermal radio emission components from the Orion Nebula: stellar jets, cloud collision or feedback from stellar winds?

The Orion Nebula is the closest high-mass star-forming region, making it an ideal laboratory to investigate physical processes in complex star-forming environments. At radio frequencies, the dominant emission mechanisms are thermal bremsstrahlung and non-thermal synchrotron. HII regions typically emit thermal radiation tracing the ionised gas; however, detecting and characterising non-thermal emission can provide insights into magnetic fields and the energy distribution of relativistic particles in star-forming regions. We have utilised the upgraded Giant Metrewave Radio Telescope (uGMRT) to study radio emission in the Extended Orion Nebula (EON) region. We present results from wide-band interferometric observations using uGMRT bands 3 and 4, probing a frequency range not covered by other sensitive radio interferometers. We produced deep continuum images with RMS noise levels of $\sim400\,μ$Jy~beam$^{-1}$ in band 3 and $\sim200\,μ$Jy~beam$^{-1}$ in band 4. We further generated in-band and broad-band spectral index maps using these images. To establish the robustness of the spectral index measurements, we conducted a detailed analysis using simulated uGMRT data. From the continuum spectral index analysis, we report the unambiguous presence of non-thermal radio emission in the EON region. To investigate its plausible origin, we correlated our results with multiwavelength observations, identifying a strong association between non-thermal emission and outflows from young stellar objects, while also exploring alternative explanations. In future, reliable broad-band radio spectral index measurements, together with dedicated multiwavelength observations, will be invaluable for resolving the origin of non-thermal emission in the Orion Nebula and other star-forming regions.

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arXiv2025

Research on the Interstellar Medium and Star Formation in the Galaxy: An Indian Perspective

Although the star formation process has been studied for decades, many important aspects of the physics involved remain unsolved. Recent advancement of instrumentation in the infrared, far-infrared and sub-millimetre wavelength regimes have contributed to a significantly improved understanding of processes in the interstellar medium (ISM) leading to star formation. The future of research on the ISM and star formation looks exciting with instruments like the JWST, ALMA, etc., already contributing to the topic by gathering high-resolution high-sensitivity data and with several larger ground- and space-bound facilities either being planned or constructed. India has a sizable number of astronomers engaged in research on topics related to the ISM and star formation. In this white paper invited by the Astronomical Society of India to prepare a vision document for Indian astronomy, we review the Indian contributions to the global understanding of the star formation process and suggest areas that require focused efforts both in creating observing facilities and in theoretical front in India, in order to improve the impact of our research in the coming decades.

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arXiv2024

A multi-wavelength study of Galactic H II regions with extended emission

H II regions are the signposts of massive ($M\geq\,8\,M_\odot$) star-forming sites in our Galaxy. It has been observed that the ionizing photon rate inferred from the radio continuum emission of H II regions is significantly lower ($\sim$ 90%) than that inferred from far-infrared fluxes measured by IRAS. This discrepancy in the ionizing photon rates may arise due to there being significant amounts of dust within the H II regions or the presence of extended emission that is undetected by high-resolution radio interferometric observations. Here, we study a sample of eight compact and ultracompact H II regions with extended emission to explore its role in resolving the discrepancy. We have used observations at the uGMRT (1.25-1.45 GHz) and data from the GLOSTAR survey (4-8 GHz) to estimate the ionizing photon rate from the radio continuum emission. We have also estimated the ionizing photon rate from the infrared luminosity by fitting a spectral energy distribution function to the infrared data from the GLIMPSE, MIPSGAL, and Hi-GAL surveys. The excellent sensitivity of the radio observations to extended emission allows us to investigate the actual fraction of ionizing photons that are absorbed by dust in compact and ultracompact H II regions. Barring one source, we find a direct association between the radio continuum emission from the compact and diffuse components of the H II region. Our study shows that the ionizing photon rates estimated using the radio and infrared data are within reasonable agreement (5-28%) if we include the extended emission. We also find multiple candidate ionizing stars in all our sources, and the ionizing photon rates from the radio observations and candidate stars are in reasonable agreement.

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arXiv2024

Mass assembly in massive star formation: a fragmentation study of ATLASGAL clumps

The mass assembly in star forming regions arises from the hierarchical structure in molecular clouds in tandem with fragmentation at different scales. In this paper, we present a study of the fragmentation of massive clumps covering a range of evolutionary states, selected from the ATLASGAL survey, using the compact configuration of the Submillimeter Array. The observations reveal a wide diversity in the fragmentation properties with about 60% of the sources showing limited to no fragmentation at the 2" scale, or a physical scale of 0.015 - 0.09 pc. We also find several examples where the cores detected with the Submillimeter array are significantly offset from the clump potential suggesting that initial fragmentation does not result in the formation of a large number of Jeans mass fragments. The fraction of the clump mass that is in compact structures is seen to increase with source evolution. We also see a significant correlation between the maximum mass of a fragment and the bolometric luminosity of the parent clump. These suggest that massive star formation proceeds through clump fed core accretion with the initial fragmentation being dependent on the density structure of the clumps and/or magnetic fields.

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arXiv2023

Metrewave Galactic Plane with the uGMRT (MeGaPluG) Survey: Lessons from the Pilot Study

Context. The advent of wide-band receiver systems on interferometer arrays enables one to undertake high-sensitivity and high-resolution radio continuum surveys of the Galactic plane in a reasonable amount of telescope time. However, to date, there are only a few such studies of the first quadrant of the Milky Way that have been carried out at frequencies below 1 GHz. The Giant Metrewave Radio Telescope (GMRT) has recently upgraded its receivers with wide-band capabilities (now called the uGMRT) and provides a good opportunity to conduct high resolution surveys, while also being sensitive to the extended structures. Aims. We wish to assess the feasibility of conducting a large-scale snapshot survey, the Metrewave Galactic Plane with the uGMRT Survey (MeGaPluG), to simultaneously map extended sources and compact objects at an angular resolution lower than $10''$ and a point source sensitivity of 0.15 mJy/beam. Methods. We performed an unbiased survey of a small portion of the Galactic plane, covering the W43/W44 regions ($l=29^\circ-35^\circ$ and $|b|<1^\circ$) in two frequency bands: 300$-$500 MHz and 550$-$750 MHz. The 200 MHz wide-band receivers on the uGMRT are employed to observe the target field in several pointings, spending nearly 14 minutes on each pointing in two separate scans. We developed an automated pipeline for the calibration, and a semi-automated self-calibration procedure is used to image each pointing using multi-scale CLEAN and outlier fields. Results. We produced continuum mosaics of the surveyed region at a final common resolution of $25''$ in the two bands that have central frequencies of 400 MHz and 650 MHz, with a point source sensitivity better than 5 mJy/beam. We plan to cover a larger footprint of the Galactic plane in the near future based on the lessons learnt from this study. (Abridged)

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arXiv2022

A multiwavelength study of the W33 Main ultracompact HII region

The dynamics of ionized gas around the W33 Main ultracompact HII region is studied using observations of hydrogen radio recombination lines and a detailed multiwavelength characterization of the massive star-forming region W33 Main is performed. We used the Giant Meterwave Radio Telescope (GMRT) to observe the H167$α$ recombination line at 1.4 GHz at an angular resolution of 10 arcsec, and Karl. G. Jansky Very Large Array (VLA) data acquired in the GLOSTAR survey to study the dynamics of ionized gas. We also observed the radio continuum at 1.4 GHz and 610 MHz with the GMRT and used GLOSTAR 4$-$8 GHz continuum data to characterize the nature of the radio emission. In addition, archival data from submillimeter to near-infrared wavelengths were used to study the dust emission and identify YSOs in the W33 Main star-forming region. The radio recombination lines were detected at good signal to noise in the GLOSTAR data, while the H167$α$ radio recombination line was marginally detected with the GMRT. The spectral index of radio emission in the region determined from GMRT and GLOSTAR shows the emission to be thermal in the entire region. Along with W33 Main, an arc-shaped diffuse continuum source, G12.81$-$0.22, was detected with the GMRT data. The GLOSTAR recombination line data reveal a velocity gradient across W33 Main and G12.81$-$0.22. The electron temperature is found to be 6343 K and 4843 K in W33 Main and G12.81$-$0.22, respectively. The physical properties of the W33 Main molecular clump were derived by modeling the dust emission using data from the ATLASGAL and Hi-GAL surveys and they are consistent with the region being a relatively evolved site of massive star formation. The gas dynamics and physical properties of G12.81$-$0.22 are consistent with the HII region being in an evolved phase and its expansion on account of the pressure difference is slowing down.

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arXiv2021

Gas dynamics in the star forming region G18.148$-$0.283: Is it a manifestation of two colliding molecular clouds?

We report the results obtained from a multi-wavelength study of the HII region, G18.148$-$0.283, using the upgraded Giant Metre-wave Radio Telescope (uGMRT) at 1350 MHz along with other archival data. In addition to the radio continuum emission, we have detected the H169$α$ and H170$α$ radio recombination lines towards G18.148$-$0.283 using a correlator bandwidth of 100 MHz. The moment-1 map of the ionized gas reveals a velocity gradient of approximately 10 km s$^{-1}$ across the radio continuum peaks. The $^{12}$CO ($J$=3$-$2) molecular line data from the COHRS survey also shows the presence of two velocity components that are very close to the velocities detected in the ionized gas. The spectrum and position-velocity diagram from CO emission reveal molecular gas at an intermediate velocity range bridging the velocity components. We see mid-infrared absorption and far-infrared emission establishing the presence of a filamentary infrared dark cloud, the extent of which includes the targeted HII region. The magnetic field inferred from dust polarization is perpendicular to the filament within the HII region. We have also identified two O9 stars and 30 young stellar objects towards the target using data from the 2MASS, UKIDSS, and GLIMPSE surveys. Cumulatively, this suggests that the region is the site of a cloud-cloud collision that has triggered massive star formation and subsequent formation of an HII region.

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arXiv2021

Chemical environments of 6.7 GHz methanol maser sources

6.7 GHz methanol masers are the brightest of class II methanol masers that are regarded as excellent signposts in the formation of young massive stars. We present here a molecular line study of 68 6.7 GHz methanol maser hosts chosen from the MMB catalogue, that have MALT90 data available. We performed (1) pixel-by-pixel study of 9 methanol maser sources that have high signal-to-noise ratio and (2) statistical study taking into account the entire 68 sources. We estimated the molecular column densities and abundances of N$_2$H$^+$(1-0), HCO$^+$(1-0), HCN(1-0) and HNC(1-0) lines. The derived abundances are found to be in congruence with the typical values found towards high mass star forming regions. We derived the column density and abundance ratios between these molecular species as an attempt to unveil the evolutionary stage of methanol maser sources. We found the column density and abundance ratio of HCN to HNC to increase and that of N$_2$H$^+$ to HCO$^+$ to decline with source evolution, as suggested by the chemical models. The HCN/HNC, N$_2$H$^+$/HCO$^+$, HNC/HCO$^+$ and N$_2$H$^+$/HNC ratios of the methanol maser sources are consistent with them being at a later evolutionary state than quiescent phase and possibly protostellar phase, but at an earlier stage than HII regions and PDRs.

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arXiv2021

A global view on star formation: The GLOSTAR Galactic plane survey III. 6.7 GHz methanol maser survey in Cygnus X

The Cygnus X complex is covered by the Global View of Star Formation in the Milky Way (GLOSTAR) survey, an unbiased radio-wavelength Galactic plane survey, in 4--8 GHz continuum radiation and several spectral lines. The GLOSTAR survey observed the 6.7~GHz transition of methanol (CH$_3$OH), an exclusive tracer of high-mass young stellar objects. Using the Very Large Array in both the B and D configurations, we observed an area in Cygnus~X of $7^{\rm o}\times3^{\rm o}$ in size and simultaneously covered the methanol line and the continuum, allowing cross-registration. We detected thirteen sources with Class~II methanol maser emission and one source with methanol absorption. Two methanol maser sources are newly detected; in addition, we found four new velocity components associated with known masers. Five masers are concentrated in the DR21 ridge and W75N. We determined the characteristics of the detected masers and investigated the association with infrared, (sub)millimeter, and radio continuum emission. All maser sources are associated with (sub)millimeter dust continuum emission, which is consistent with the picture of masers tracing regions in an active stage of star formation. On the other hand, only five masers ($38\pm17\%$) have radio continuum counterparts seen with GLOSTAR within $\sim$1$''$, testifying to their youth. Comparing the distributions of the bolometric luminosity and the luminosity-to-mass ratio of cores that host 6.7~GHz methanol masers with those of the full core population, we identified lower limits $L_{\rm Bol}\sim200~L_\odot$ and $L_{\rm Bol}/M_{\rm core}\sim1~L_\odot~M^{-1}_\odot$ for a dust source to host maser emission.

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arXiv2019

Probing the early phases of high mass star formation with 6.7 GHz methanol masers

Methanol masers at 6.7 GHz are the brightest of class II methanol masers and have been found exclusively towards massive star forming regions. These masers can thus be used as a unique tool to probe the early phases of massive star formation. We present here a study of the spectral energy distributions of 320 6.7 GHz methanol masers chosen from the MMB catalogue, which fall in the Hi-GAL range ($|l| \le 60^{\circ}$, $|b| \le 1^{\circ}$). The spectral energy distributions are constructed from $870 - 70~μ$m using data from the ATLASGAL and Hi-GAL surveys. The emission from cold dust is modelled by a single grey body component fit. We estimate the clump properties such as mass, FIR luminosity and column density using the best fit parameters of the SED fits. Considering the Kauffman criteria for massive star formation, we find that all but a few maser hosts have the potential to harbour at least one high mass star. The physical properties of the methanol maser hosts are also discussed. The evolutionary stages of 6.7 GHz maser sources, explored using the mass luminosity diagram, suggests that they are predominantly associated with high mass stars with the majority being in the accretion phase. However, we observe a small number of sources that could possibly be related to intermediate or low-mass stars.

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arXiv2012

HII Regions, Embedded Protostars, and Starless Cores in Sharpless 2-157

We present arcsecond resolution 1.4mm observations of the high mass star forming region, Sharpless 2-157, that reveal the cool dust associated with the first stages of star formation. These data are compared with archival images at optical, infrared, and radio wavelengths, and complemented with new arcsecond resolution mid-infrared data. We identify a dusty young HII region, numerous infrared sources within the cluster envelope, and four starless condensations. Three of the cores lie in a line to the south of the cluster peak, but the most massive one is right at the center and associated with a jumble of bright radio and infrared sources. This presents an interesting juxtaposition of high and low mass star formation within the same cluster which we compare with similar observations of other high mass star forming regions and discuss in the context of cluster formation theory.

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arXiv2011

Searching for new hypercompact HII regions

Hypercompact (HC) HII regions are, by nature, very young HII regions, associated with the earliest stages of massive star formation. They may represent the transition phase as an early B-type star grows into an O-type star. Unfortunately, so few HCHII regions are presently known that their general attributes and defining characteristics are based on small number statistics. A larger sample is needed for detailed studies and good statistics. Class II methanol masers are one of the best indicators of the early stages of massive star formation. Using the Arecibo Methanol Maser Galactic Plane Survey - the most sensitive blind survey for 6.7 GHz methanol masers to date - we selected 24 HCHII region candidates. We made EVLA continuum observations at 3.6 and 1.3 cm to search for HCHII regions associated with these masers. We identified six potential HCHII regions in our sample based on the presence of optically thick free-free emission. Overall, we find that 30% of the methanol masers have an associated centimeter radio continuum source (separation less than 0.1 pc), which is in general agreement with previous studies.

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arXiv2007

The Arecibo Methanol Maser Galactic Plane Survey - II: Statistical and Multi-wavelength Counterpart Analysis

We present an analysis of the properties of the 6.7 GHz methanol maser sample detected in the Arecibo Methanol Maser Galactic Plane Survey. The distribution of the masers in the Galaxy, and statistics of their multi-wavelength counterparts is consistent with the hypothesis of 6.7 GHz maser emission being associated with massive young stellar objects. Using the detection statistics of our survey, we estimate the minimum number of methanol masers in the Galaxy to be 1275. The l-v diagram of the sample shows the tangent point of the Carina-Sagittarius spiral arm to be around 49.6 degrees, and suggests occurrence of massive star formation along the extension of the Crux-Scutum arm. A Gaussian component analysis of the maser spectra shows the mean line-width to be 0.38 km/s which is more than a factor of two larger than what has been reported in the literature. We also find no evidence that faint methanol masers have different properties than those of their bright counterparts.

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arXiv2007

The Arecibo Methanol Maser Galactic Plane Survey--I: Data

We present the results of an unbiased survey for 6.7 GHz methanol masers in the Galactic plane carried out using the 305 m Arecibo radio telescope. A total of 18.2 square degrees was surveyed with uniform sampling at 35.2 deg < l < 53.7 deg, |b| < 0.41 deg. The large collecting area of Arecibo and the sensitive C-Band High receiver allowed the survey to be complete at the level of 0.27 Jy making this the most sensitive blind survey carried out to date. We detected a total of 86 sources, 48 of which are new detections. Most of the new detections have a peak flux density below 2 Jy. Many methanol masers are clustered, reflecting the formation of massive stars in clusters.

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arXiv2006

First results from the Arecibo Galactic HI Survey: the disk/halo interface region in the outer Galaxy

The consortium for Galactic studies with the Arecibo L-band Feed Array (ALFA) is conducting a neutral hydrogen (HI) survey of the whole Arecibo sky (declination range from -1 to 38 degrees), with high angular (3.5') and velocity resolution (0.2 km/s). The precursor observations with ALFA of a region in the Galactic anti-center reveal numerous isolated, small (a few pc in size), and cold (T_k<400 K) HI clouds at low negative velocities, distinctly separated from the HI disk emission (`low-velocity clouds', LVCs). These clouds are most likely located in the transition region between the Galactic disk and halo (at scale heights of 60--900 pc), yet they have properties of typical cold neutral clouds. LVCs are colder and, most likely, smaller and less massive than Lockman's clouds in the disk/halo interface region of the inner Galaxy. Our observations demonstrate that the cloudy structure of the interface region is most likely a general phenomenon, not restricted to the inner Galaxy. LVCs have sizes and radial velocities in agreement with the expectations for clouds formed in low-temperature fountain flows, although we measure a factor of ten higher HI column densities. Alternatively, LVCs could represent the final stages of the infalling intergalactic material in the on-going construction of the Galaxy. In the same dataset at higher negative velocities, we have discovered a `companion' HI cloud located 50' southwest of CHVC186+19-114. CHVC186+19-114 is a typical compact high velocity cloud (HVC) with a well-defined core/envelope structure. The companion cloud has a diameter of only 7'x9', and is one of the smallest HVCs known, most likely stripped from the main cloud through the interactions with the halo medium.

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