arXiv subjects

Ekta Sharma

Publications and source records attributed to Ekta Sharma.

13 recordsLinked to original sources

arXiv2025

The JCMT BISTRO-3 Survey: Variation of magnetic field orientations on parsec and sub-parsec scales in the massive star-forming region G28.34+0.06

Magnetic fields play a significant role in star-forming processes on core to clump scales. We investigate magnetic field orientations and strengths in the massive star-forming clump P2 within the filamentary infrared dark cloud G28.34+0.06 using dust polarization observations made using SCUBA-2/POL-2 on the James Clerk Maxwell Telescope as part of the B-field In STar-forming Region Observations (BISTRO) survey. We compare the magnetic field orientations at the clump scale of ~2 parsecs from these JCMT observations with those at the core scale of ~0.2 parsecs from archival ALMA data, finding that the magnetic field orientations on these two different scales are perpendicular to one another. We estimate the distribution of magnetic field strengths, which range from 50 to 430 {\mu}G over the clump. The region forming the core shows the highest magnetic field strength. We also obtain the distribution of mass-to-flux ratios across the clump. In the region surrounding the core, the mass-to-flux ratio is larger than 1, which indicates the magnetic field strength is insufficient to support the region against gravitational collapse. Therefore, the change in the magnetic field orientation from clump to core scales may be the result of gravitational collapse, with the field being pulled inward along with the flow of material under gravity.

astro-ph.GA↗

arXiv2025

The Molecular and Atomic Hydrogen Gas Content of the Bo\"otes Void galaxy CG 910

Void galaxies are located in the most underdense environments of the Universe, where the number density of galaxies is extremely low. They are, hence, good targets for studying the secular evolution of galaxies and the slow buildup of stellar mass through star formation. To date, very little is known about their cold gas content, both molecular (H$_2$) gas and atomic hydrogen (HI) gas. We present CO (1--0) observations of the H$_2$ gas disk in CG 910, which lies in the Bo\"otes void, one of the largest nearby voids at relatively low redshifts (z$\sim$0.04-0.05). We used the Combined Array for Research in Research in Millimeter Astronomy (CARMA) to study the CO(1-0) distribution and gas kinematics in CG 910. We also carried out atomic hydrogen observations of the galaxy using the Robert C. Byrd Green Bank Telescope (GBT). The CARMA CO(1-0) observations reveal a molecular gas disk of mass, M(H$_{2}$) $\sim$12.0$\pm$1.1$\times$10$^{9}$M$_{\odot}$ and diameter 7 kpc. The CO velocity field shows a regularly rotating disk with a flat rotation velocity of 256 kms$^{-1}$ with no clear signatures of interaction or gas accretion. This is possibly the first CO (1-0) map of a void galaxy, and hence, important for understanding the molecular gas distribution and kinematics in void galaxies. The GBT observations reveal a HI disk of mass, M(HI) $\sim$3.1$\pm$0.8$\times$10$^{9}$M$_{\odot}$, which is relatively small compared to its stellar mass of M$_{\star}\sim$21.5$\times$10$^{9}M_{\odot}$. The total gas mass fraction, (M(H$_2$)+M(HI))/$M_{\star}$ and the atomic gas mass fraction, M(HI)/M$_{\star}$ for CG 910 are 0.70 and 0.14, respectively. We conclude that CG 910 has a regularly rotating but massive molecular gas disk. The lower atomic gas mass fraction and star formation rate indicate a longer gas depletion timescale, confirming that CG 910 is slowly evolving compared to normal disk galaxies.

astro-ph.GA↗

arXiv2025

Imprints of Stellar Feedback on Magnetic Fields in the Iris Nebula NGC 7023

We present 850 $\mu$m polarized continuum observations carried out with the POL-2 polarimeter mounted on the James Clerk Maxwell Telescope (JCMT) towards NGC 7023 located in the Cepheus Flare region. NGC 7023 is a reflection nebula powered by a Herbig Ae Be star HD 200775 and also identified as a hub in the hub-filament cloud, LDN 1172/1174. We detect submillimetre emission well towards the northern (identified as C1) and the eastern region of the reflection nebula. We investigated the polarization structure and the magnetic field (B-field) morphology, which is found to be curved and follows the clump morphology. The comparison of the B-field morphology at the clump scales ($\sim$0.02 pc) with that of the envelope ($\sim$0.5 pc) suggests that the field lines are not preserved from envelope to clump scales, implying that an external factor may be responsible for disturbing the B-field structure. We estimated a magnetic field strength of 179$\pm$50 $\mu$G in the starless core, 121$\pm$34 $\mu$G in the protostellar core with a class I source and 150$\pm$42 $\mu$G in the protostellar core with a class II source using the $N_{2}H^{+}$ (1-0) line observed with 13.7 m single dish radio telescope at Taeduk Radio Astronomy Observatory (TRAO). The stability analysis using these B-field strengths gives magnetically sub-critical values, while the magnetic, gravitational, and outflow kinetic energies are roughly balanced. We also suggest that the reordering of the magnetic field lines could be due to the interaction with the already evolved high-velocity outflow gas around the central star, which hints at the presence of outflow feedback.

astro-ph.GA↗

arXiv2025

The JCMT BISTRO Survey: Unveiling the Magnetic Fields around Galactic Center

We acquired 450 {\mu}m and 850 {\mu}m dust continuum polarization observations toward the inner region of the Central Molecular Zone (CMZ) as part of the B-Fields In Star-Forming Region Observations (BISTRO) survey using the POL-2 polarimeter on the James Clerk Maxwell Telescope. These observations encompassed three dense structures: the 20 km s{^{-1}} cloud (20MC), 50 km s{^{-1}} cloud (50MC), and circumnuclear disk (CND). Our aim is to investigate the magnetic field morphology and strength in the inner region of the CMZ using polarized dust continuum and the Davis-Chandrasekhar-Fermi method. The magnetic field morphology is highly ordered in all three dense regions. The plane-of-sky magnetic field strengths are {\sim}1 mG for the 20MC and the 50MC, and {\sim}2 mG for the CND. We compare the energy contributions of turbulence, gravity, and thermal motion with that of the magnetic field using the plasma {\beta}, mass-to-flux ratio, and Alfv\'en Mach number. The outcomes reveal the magnetic field stands out as the predominant factor within the inner region of the CMZ. The dominance of the magnetic field may explain the low star-forming rate in the CMZ. We further investigate the dust grain alignment efficiency by exploring the relationship between polarization fraction and total intensity. The results suggest that dust grains are well aligned with the magnetic fields.

astro-ph.GA↗

arXiv2025

Foam stabilization in salt solutions : the role of capillary drainage and Marangoni stresses

The long-standing question of why foaming is easier in seawater than in freshwater remains unresolved. In this study, we address this issue through precise interferometry single bubble experiments, demonstrating that the theory proposed by G. Marrucci (1969) provides a compelling explanation. Electrolyte solutions with varying concentrations of phosphate salts were used to study film formation and drainage, with thickness tracked by interferometry. In deionized water, bubbles rupture within seconds due to repaid dimple collapse. However, in phosphate salt solutions, bubbles persisted for several minutes. While surface tension gradients from evaporation-driven salt concentration gradients have been thought to create Marangoni stresses, our results show that despite film thinning being capillary-dominated, Marangoni-driven influx can be observed. Marrucci's theory explains this by showing that an increased interfacial area as the film thins, leads to higher salt concentration in the film due to Gibbs surface excess. This concentration gradient induces Marangoni stresses, causing flow reversal, increased film thickness, and enhanced foam stability. We show that Marrucci's theory has been incorrectly dismissed, and the predicted critical heights where fluid influx occurs closely match our findings and other studies using sodium chloride. Additionally, we extend the theory's applicability to foam films in non-aqueous film mixtures, highlighting its broader relevance.

physics.flu-dyn↗

arXiv2024

The JCMT BISTRO Survey: The magnetised evolution of star-forming cores in the Ophiuchus Molecular Cloud interpreted using Histograms of Relative Orientation

The relationship between B-field orientation and density structure in molecular clouds is often assessed using the Histogram of Relative Orientations (HRO). We perform a plane-of-the-sky geometrical analysis of projected B-fields, by interpreting HROs in dense, spheroidal, prestellar and protostellar cores. We use James Clerk Maxwell Telescope (JCMT) POL-2 850 $\mu$m polarisation maps and Herschel column density maps to study dense cores in the Ophiuchus molecular cloud complex. We construct two-dimensional core models, assuming Plummer column density profiles and modelling both linear and hourglass B-fields. We find high-aspect-ratio ellipsoidal cores produce strong HRO signals, as measured using the shape parameter $\xi$. Cores with linear fields oriented $< 45^{\circ}$ from their minor axis produce constant HROs with $-1 < \xi < 0$, indicating fields are preferentially parallel to column density gradients. Fields parallel to the core minor axis produce the most negative value of $\xi$. For low-aspect-ratio cores, $\xi \approx 0$ for linear fields. Hourglass fields produce a minimum in $\xi$ at intermediate densities in all cases, converging to the minor-axis-parallel linear field value at high and low column densities. We create HROs for six dense cores in Ophiuchus. $\rho$ Oph A and IRAS 16293 have high aspect ratios and preferentially negative HROs, consistent with moderately strong-field behaviour. $\rho$ Oph C, L1689A and L1689B have low aspect ratios, and $\xi \approx 0$. $\rho$ Oph B is too complex to be modelled using a simple spheroidal field geometry. We see no signature of hourglass fields, agreeing with previous findings that dense cores generally exhibit linear fields on these size scales.

astro-ph.GA↗

arXiv2022

Core orientations and magnetic fields in isolated molecular clouds

Molecular clouds are sites of star formation. Magnetic fields are believed to play an important role in their dynamics and shaping morphology. We aim to study any possible correlation that might exist between the magnetic fields orientation inside the clouds and the magnetic fields at envelope scales and their connection with respect to the observed morphology of the selected clouds. We examine the magnetic field orientation towards the clouds L1512, L1523, L1333, L1521E, L1544, L1517, L1780, and L183 using optical and \textit{Planck} polarization observations. We also found the correlation between the ambient magnetic field and core orientations derived using \textit{Astrodendrogram} on the \textit{Herschel} 250 $μ$m data. We find that the magnetic fields derived from optical and \textit{Planck} agree with each other. The derived magnetic fields are aligned along the observed emission of each cloud as seen in \textit{Herschel} 250 $μ$m data. We also find that the relative orientation between the cores and the magnetic fields is random. This lack of correlation may arise due to the fact that the core orientation could also be influenced by the different magnetization within individual clouds at higher densities or the feedback effects which may vary from cloud to cloud. The estimated magnetic field strength and the mass-to-flux ratio suggest that all the clouds are in a magnetically critical state except L1333, L1521E, and L183 where the cloud envelope could be strongly supported by the magnetic field lines.

astro-ph.GA↗

arXiv2021

The structure and characteristic scales of the HI gas in galactic disks

The spatial distribution of the HI gas in galaxies holds important clues on the physical processes that shape the structure and dynamics of the interstellar medium (ISM). In this work, we quantify the structure of the HI gas in a sample of 33 nearby galaxies taken from the THINGS Survey using the delta-variance spectrum. The THINGS galaxies display a large diversity in their spectra, however, there are a number of recurrent features. In many galaxies, we observe a bump in the spectrum on scales of a few to several hundred pc. We find the characteristic scales associated with the bump to be correlated with galactic SFR for values of the SFR > 0.5 M$_{sol}$ yr$^{-1}$ and also with the median size of the HI shells detected in those galaxies. On larger scales, we observe the existence of two self-similar regimes. The first one, on intermediate scales is shallow and the power law that describes this regime has an exponent in the range [0.1-1] with a mean value of 0.55 which is compatible with the density field being generated by supersonic turbulence in the cold phase of the HI gas. The second power law is steeper, with a range of exponents between [0.5-1.5] and a mean value of 1.5. These values are associated with subsonic turbulence which is characteristic of the warm phase of the HI gas. The spatial scale at which the transition between the two regimes occurs is found to be $\approx 0.5 R_{25}$ which is similar to the size of the molecular disk in the THINGS galaxies. Overall, our results suggest that on scales < $0.5 R_{25}$, the structure of the ISM is affected by the effects of supernova explosions. On larger scales (> 0.5 $R_{25}$), stellar feedback has no significant impact, and the structure of the ISM is determined by large scale processes that govern the dynamics of the gas in the warm neutral medium such as the flaring of the HI disk and the effects of ram pressure stripping.

astro-ph.GA↗

arXiv2021

Cloud Motion and magnetic fields: Four clouds in the Cepheus Flare region

The Cepheus Flare region consists of a group of dark cloud complexes that are currently active in star formation. The aim of this work is to estimate the motion of four clouds, L1147/1158, L1172/1174, L1228 and L1251 located at relatively high Galactic latitude (b $\gt$ 14$^{\circ}$) in the Cepheus Flare region. We study the relationship between the motion of the cloud with respect to the magnetic field and the clump orientations with respect to both the magnetic field and the motion. We estimated the motion of the molecular clouds using the proper motion and the distance estimates of the young stellar objects (YSOs) associated with them using the Gaia EDR3 data. By assuming that the YSOs are associated with the clouds and share the same velocity, the projected direction of motion of the clouds are estimated. We estimated a distance of 371$\pm$22 pc for L1228 and 340$\pm$ 7 pc for L1251 implying that all four complexes are located at almost the same distance. Assuming that both the clouds and YSOs are kinematically coupled, we estimated the projected direction of motion of the clouds using the proper motions of the YSOs. All the clouds in motion are making an offset of $\sim$ 30$^{\circ}$ with respect to the ambient magnetic fields except in L1172/1174 where the offset is $\sim$ 45$^{\circ}$. In L1147/1158, the starless clumps are found to be oriented predominantly parallel to the magnetic fields while prestellar clumps show random distribution. In L1172/1174, L1228 and L1251,the clumps are oriented randomly with respect to magnetic field. With respect to the motion of the clouds, there is a marginal trend that the starless clumps are oriented more parallel in L1147/1158 and L1172/1174. In L1228, the clumps major axis are oriented more randomly. In L1251, we find a bimodal trend in case of starless clumps.

astro-ph.GA↗

arXiv2021

Tracing the magnetic field morphology of the LDN 1172/1174 cloud complex

The LDN 1172/1174 cloud complex in the Cepheus Flare region presents a hub-filament structure with the reflection nebula, NGC 7023, illuminated by a Herbig Be star, HD 200775, which consists of the hub with a $\sim$5 pc long narrow filament attached to it. Formation of a sparse cluster of low- and intermediate-mass stars is presently taking place in the hub. The aim of this work is to map the magnetic field geometry of LDN 1172/1174 to understand the role played by the field lines in the formation of the molecular cloud. We made R-band polarization measurements of 249 stars projected on the entire LDN 1172/1174 cloud complex to map the geometry of the magnetic field of this region. The magnetic field geometry constructed from our R-band polarization measurements is found to be parallel to the elongated structure inferred from the column density distribution of the cloud produced using the Herschel images. Our R-band polarization measurements are found to be in good agreement with those obtained from Planck. There is evidence of a possible distortion of the magnetic fields toward the northwestern part of the cloud by HD 200775. The magnetic field strength is estimated as $\sim$30 $μ$G. The estimated star formation rate (SFR)/mass of 2.0$\pm$1.3 \%Myr$^{-1}$ and 0.4$\pm$0.3 \%Myr$^{-1}$ for LDN 1172/1174 and the neighboring cloud complex, LDN 1147/1158, respectively, are found to be consistent with the mean SFR/mass found for the clouds with magnetic field orientations parallel and perpendicular to their elongated structures, respectively. These results support earlier findings that the clouds with magnetic field lines parallel to their long axes seem to have higher SFRs compared to those with the magnetic field orientation perpendicular to the cloud elongation.

astro-ph.GA↗

arXiv2020

Distance, magnetic field and kinematics of a filamentary cloud LDN 1157

LDN 1157, is one of the several clouds situated in the cloud complex, LDN 1147/1158, represents a coma-shaped morphology with a well-collimated bipolar outflow emanating from a Class 0 protostar, LDN 1157-mm. The main goals of this work are (a) to map the inter-cloud magnetic field (ICMF) geometry of the region surrounding LDN 1157 to investigate its relationship with the cloud morphology, with the outflow direction and with the core magnetic field (CMF) geometry inferred from the mm- and sub-mm polarization results from the literature, and (b) to investigate the kinematic structure of the cloud. We carried out R-band polarization observations of the stars projected on the cloud to map the pc-scale magnetic field geometry and made spectroscopic observations of the entire cloud in 12CO, C18O and N2H+ (J=1-0) lines to investigate its kinematic structure. We obtained a distance of 340$\pm$3 pc to the LDN 1147/1158, complex based on the Gaia DR2 parallaxes and proper motion values of the three YSOs associated with the complex. A single filament of $\sim1.2$ pc in length and $\sim0.09$ pc in width is found to run all along the coma-shaped cloud. Based on the relationships between the ICMF, CMF, filament orientations, outflow direction, and the presence of an hour-glass morphology of the magnetic field, it is likely that the magnetic field had played an important role in the star formation process in LDN 1157. Combining the proper motions of the YSOs and the radial velocity of LDN 1147/1158 and another complex LDN 1172/1174 which is situated $\sim2$\dgr~east of it, we found that both the complexes are moving collectively toward the Galactic plane. The filamentary morphology of the east-west segment of LDN 1157 may have formed as a result of mass lost by ablation due to the interaction of the moving cloud with the ambient interstellar medium.

astro-ph.SR↗

arXiv2020

A census of young stellar population associated with the Herbig Be star HD 200775

The region surrounding the well-known reflection nebula, NGC 7023, illuminated by a Herbig Be star, HD 200775, located in the dark cloud L1174 is studied in this work. Based on the distances and proper motion values from Gaia DR2 of 20 previously known young stellar object candidates, we obtained a distance of 335$\pm$11 pc to the cloud complex L1172/1174. Using polarization measurements of the stars projected on the cloud complex, we show additional evidence for the cloud to be at $\sim$ 335 pc distance. Using this distance and proper motion values of the YSO candidates, we searched for additional comoving sources in the vicinity of HD 200775 and found 20 new sources which show low infrared excess emission and are of age $\sim$ 1 Myr. Among these, 10 YSO candidates and 4 newly identified comoving sources are found to show X-ray emission. Three of the four new sources for which we have obtained optical spectra show H$α$ in emission. About 80% of the total sources are found within $\sim$ 1 pc distance from HD 200775. Spatial correlation of some of the YSO candidates with the Herschel dust column density peaks suggests that star formation is still active in the region and may have been triggered by HD 200775.

astro-ph.SR↗

arXiv2019

First Sub-pc Sale Mapping of Magnetic Fields in the Vicinity of a Very Low Luminosity Object, L1521F-IRS

L1521F is found to be forming multiple cores and it is cited as an example of the densest core with an embedded VeLLO in a highly dynamical environment. We present the core-scale magnetic fields (B-fields) in the near vicinity of the VeLLO L1521F-IRS using submm polarization measurements at 850$~μ$m using JCMT POL-2. This is the first attempt to use high-sensitivity observations to map the sub-parsec scale B-fields in a core with a VeLLO. The B-fields are ordered and very well connected to the parsec-scale field geometry seen in our earlier optical polarization observations and the large-scale structure seen in Planck dust polarization. The core scale B-field strength estimated using Davis-Chandrasekhar-Fermi relation is $\rm 330\pm100~μ$G which is more than ten times of the value we obtained in the envelope (envelope in this paper is "core envelope"). This indicates that B-fields are getting stronger on smaller scales. The magnetic energies are found to be 1 to 2 orders of magnitude higher than non-thermal kinetic energies in the envelope and core. This suggests that magnetic fields are more important than turbulence in the energy budget of L1521F. The mass-to-flux ratio of 2.3$\pm$0.7 suggests that the core is magnetically-supercritical. The degree of polarization is steadily decreasing towards the denser part of the core with a power law slope of -0.86.

astro-ph.GA↗