arXiv subjects

Annapurni Subramaniam

Publications and source records attributed to Annapurni Subramaniam.

At least 19 recordsLinked to original sources

arXiv2026

Investigating the young stellar populations and hierarchies in nearby galaxies with the UVIT. II. Presenting the properties of ~25,000 UV-detected star-forming clumps

Studying young stellar populations within galaxies can help refine our understanding of recent star formation in galaxies and their evolution. With this motivation, we present a catalog of ~25,000 recently formed (within 400 Myr) star-forming clumps (SFCs) in 17 morphologically diverse nearby galaxies, including 8 massive, classic spirals, 6 intermediate-mass, flocculent spirals, and 3 dwarf irregulars. We used far- and near-UV observations from the UltraViolet Imaging Telescope (UVIT), whose ~1.5" angular resolution and 28' field-of-view allow us to probe SFCs at a mean physical scale of ~54 parsec, within the full extent of our galaxies. We adopted a homogeneous SFC detection criterion, corrected for spatially varying dust attenuation (using 6" resolution A_V maps, made by combining FUV with archival infrared observations), and estimated the SFC ages by comparing the observed UV color-magnitude diagrams with Starburst99 simple stellar population models. Using our SFC catalog, we studied the age demographic of the recently formed stellar populations across different galaxy morphologies and observed age trends consistent with several well-known phenomena, such as the inside-out formation of disc galaxies, local gravitational instabilities leading to flocculent spiral arms, and the stochastic nature of star formation in dwarf galaxies. Leveraging full galaxy coverage and FUV data, our catalog complements existing optically-identified star cluster catalogs in the literature towards improving our understanding of star formation across a wide range of galaxy morphologies, masses, and environments. We make the SFC catalog and A_V maps of our 17 galaxies publicly available with this paper.

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arXiv2026

ROCKETS I: Investigating the Impact of the Rocket Effect on Two Nearby Young Open Clusters Encased in Infrared Bubbles

We introduce ROCKETS (Rocket-driven Cluster Kinematics & Triggered Star-formation), a Gaia based programme investigating kinematic fingerprints of stellar feedback or rocket effect in young star forming regions. We analyse two young clusters with O-type stars enclosed by mid-infrared bubbles, Collinder 69 and IC 1396 as a methodological demonstration on two nearby benchmark regions. Co-moving candidates are selected within an astrometric ellipse defined by the median and median absolute deviation of Class I/II YSOs with Gaia data. For each star, we compute a relative proper-motion angle (RPMA) relative to the ionising source. In both regions the RPMA distribution shows a strong excess at RPMA $<15^{\circ}$, corresponding to sources moving away from the respective ionising sources. We quantify this using the Rocket Effect Index (REI), finding $\mathrm{REI}\simeq 1.1$ for Collinder 69 and $\mathrm{REI}\simeq 1.7$ for IC 1396. When considering only Class I/II YSOs, the REI of both regions increases to $\mathrm{REI}\gtrsim 10$. Velocity-structure analysis of the sample shows that the RPMA $<15^\circ$ subset has stronger positive pairwise expansion than the full co-moving population, providing additional support for the rocket effect. The outward-moving stars are not uniformly distributed; instead, their position angles are strongly anisotropic and preferentially aligned towards the bright-rimmed clouds. Gaia colour-magnitude diagrams show no strong age offset between sources moving outward and literature cluster members. The observed kinematic signatures support feedback-driven acceleration of gas and triggered star formation preferentially towards higher density regions of expanding bubbles due to the rocket effect.

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arXiv2026

Investigating the Young Stellar Populations and Hierarchies in Nearby Galaxies with the UVIT. III. Evidence for a Largest Scale of Correlated Stellar Structures and a Non-universal Fractal Dimension

Scale-free turbulent motions, gravitational collapse and galactic dynamics govern galactic-scale, hierarchical organization of star formation (SF) within galaxies. Past studies suggest that properties of SF hierarchies depend upon host galaxy properties and interstellar medium (ISM) conditions. To characterize SF hierarchies, we performed two-point correlation function analysis on ~25000 UV-selected star-forming clumps (SFCs) identified in a morphologically diverse sample of 8 classic spirals, 6 flocculent spirals and 3 dwarf irregulars. We found that SF hierarchies in galaxies exhibit a maximum spatial scale -- the correlation length ($l_{\rm corr}$) -- largest scale up to which SF is spatially correlated, presumably owing to ISM turbulence. The $l_{\rm corr}$ values range from ~100 pc to 3.4 kpc and exhibit strong dependence on the galaxy's stellar mass, morphology and nature of spiral arms. This suggests that a galaxy's gravitational potential and spiral structure place an upper limit on the sizes of the largest, hierarchically structured SF complexes. Connecting $l_{\rm corr}$ with turbulence injection sources suggests that stellar feedback in dwarf irregulars, whereas disk instabilities and spiral structure in classic/flocculent spirals dominate towards sustaining their SF hierarchies up to the $l_{\rm corr}$ scale. These hierarchies disperse to near-random distributions on timescales ($T_{\rm dis}$) ranging from 20-160 Myr. The broad range of derived $l_{\rm corr}$, projected fractal dimension ($D_2$ $\in$ 0.71$-$1.73), and $T_{\rm dis}$ indicates a non-universal, galaxy-specific nature of SF hierarchies. In this work, full coverage of each galaxy's star-forming extent with the AstroSat-UltraViolet Imaging Telescope uniquely enabled us to connect global parameters of SF hierarchies with large-scale galaxy properties.

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arXiv2026

Star Clusters in the Ultraviolet

Ultraviolet (UV) observations provide a powerful window into the hot and evolved stellar populations that shape the structure, evolution and integrated light of star clusters. Because UV wavelengths are highly sensitive to massive main-sequence stars, blue straggler stars (BSS), extreme-horizontal branch (HB) stars, post-HB stars, interacting binaries and compact remnants, they probe key evolutionary processes that are inaccessible at optical and infrared wavelengths. This review synthesises five decades of UV studies of star clusters across the Milky Way, the Magellanic Clouds (MCs) and nearby galaxies, drawing on results from early space missions, wide-field surveys, and high-resolution imaging. In Galactic open clusters, UV studies have revealed compact companions$-$including white dwarfs, hot subdwarfs and stripped stars$-$and have established the mass-transfer origins of BSS, Blue Lurkers and yellow stragglers. In globular clusters, UV imaging has identified multiple stellar populations through UV-sensitive molecular bands, probed helium enrichment and mapped HB morphologies. Wide-field UVIT surveys have extended HST studies with homogeneous catalogues of HB and post-HB stars across entire clusters. In the MCs, UV observations have transformed our understanding of multiple populations, rotation-driven extended main-sequence turn-offs and the UV-dim phenomenon, while spectroscopic surveys have constrained massive-star evolution, stellar winds and binarity at low metallicity. UV mapping of the Magellanic Bridge has revealed ongoing massive-star formation in low-density tidal environments. Beyond the Local Group, UV studies of extragalactic clusters constrain star-formation histories, stellar feedback and population synthesis across galactic environments. Collectively, UV observations now form a cornerstone of star cluster astrophysics and will continue to do so with upcoming missions.

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arXiv2025

Nine years of UVIT: assessing sensitivity variation

The Ultra-Violet Imaging Telescope (UVIT) is one of the five payloads onboard the first Indian multiwavelength astronomical observatory, AstroSat, launched by the Indian Space Research Organisation on 28 September 2015. UVIT, designed for simultaneous imaging in the far-ultraviolet (FUV; 1300-1800 Å) and near-ultraviolet (NUV; 2000-3000 Å) channels, has completed nine years in orbit in 2024 despite the failure of the NUV channel in 2018. As the FUV optics is subject to possible reduction in sensitivity due to microscopic amounts of contaminants, we used the FUV data acquired by UVIT over the past nine years on the open cluster NGC 188 and the white dwarf HZ 4 to study sensitivity variations in the UVIT FUV channel. Our findings indicate no significant reduction in the sensitivity of the FUV channel over the last nine years, with no significant episodic variations due to unknown causes.

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arXiv2025

Tidal tails in open clusters: Morphology, binary fraction, dynamics, and rotation

Context: This research presents unsupervised machine learning and statistical methods to identify and analyze tidal tails in open star clusters using data from the Gaia DR3 catalog. Aims: We aim to identify member stars and to detect and analyze tidal tails in five open clusters, BH 164, Alessi 2, NGC 2281, NGC 2354, and M67, of ages between 60 Myr and 4 Gyr. These clusters were selected based on the previous evidence of extended tidal structures. Methods: We utilized machine learning algorithms such as Density-based Spatial Clustering of Applications with Noise (DBSCAN) and principal component analysis (PCA), along with statistical methods, to analyze the kinematic, photometric, and astrometric properties of stars. Key characteristics of tidal tails, including radial velocity, the color-magnitude diagram, and spatial projections in the tangent plane beyond the cluster's Jacobi radius, were used to detect them. We used N-body simulations to visualize and compare the observables with real data. Further analysis was done on the detected cluster and tail stars to study their internal dynamics and populations, including the binary fraction. We also applied the residual velocity method to detect rotational patterns in the clusters and their tails. Results: We identified tidal tails in all five clusters, with detected tails extending farther in some clusters and containing significantly more stars than previously reported (tails ranging from 40 to 100 pc, one to four times their Jacobi radius, with 100-200 tail stars). The luminosity functions of the tails and their parent clusters were generally consistent, and tails lacked massive stars. In general, the binary fraction was found to be higher in the tidal tails. Significant rotation was detected in M67 and NGC 2281 for the first time.

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arXiv2025

A kinematic and structural study of young open clusters in the Milky Way Galaxy using Gaia DR3 catalogue

We aim to identify the cluster members, estimate cluster properties, study the dynamical state of the clusters as a function of mass, trace the existence of dynamical effects in massive stars, and check for spatial patterns of members in young clusters. We studied 14 young open clusters located within 1 kpc using the data from Gaia DR3 with the membership estimated using the GMM method. The cluster parameters such as age, distance, metallicity, and extinction were estimated by fitting PARSEC isochrones to the CMDs. These clusters are found to have ages between 6-90 Myr, located between 334-910 pc, covering a mass range, of 0.13 to 13.77 solar mass. In five of these clusters, stars from F to M spectral type show increasing velocity dispersion, a signature for dynamical relaxation. We detect high proper motion for B and A-type stars, possible walkaway stars in the other five clusters, Alessi Teutsch 5, ASCC 16, ASCC 21, IC 2395, and NGC 6405. We demonstrate the existence of mass-dependent velocity dispersion in young clusters suggestive of dynamical relaxation. The typical range of transverse velocity dispersion is found to be 0.40 - 0.70 km/s for young clusters.

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arXiv2025

Tidal Tails and Their Dynamics in Open Clusters Using Gaia DR3

This research presents unsupervised machine learning and statistical techniques to detect and analyze tidal tails in five open clusters, BH 164, Alessi 2, NGC 2281, NGC 2354, and M67, with ages ranging from 60 Myr to 4 Gyr, using Gaia DR3 data. Color-magnitude diagram (CMD) matching, principal component analysis (PCA), and density-based clustering were used to detect the tails. Tidal tails were found in all clusters, extending 40 to 100 pc and containing 100 to 200 stars. Other statistical methods were used to study the stellar kinematics, photometry, and spatial distribution of the detected features. The tails had luminosity functions similar to their parent clusters, generally lacked massive stars, and showed higher binary fractions. Significant rotation was detected in M67 and NGC 2281 for the first time.

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arXiv2025

Detection of young massive white dwarfs in core-collapsed globular cluster NGC 362

Core-collapsed globular clusters are ideal targets to explore the presence of stellar collision products. Here, we have studied seventeen FUV bright white dwarf members in the globular cluster NGC 362 using data obtained from the Ultra Violet Imaging Telescope (UVIT) mounted on AstroSat and from HST. Multi-wavelength spectral energy distributions (SEDs) are analyzed using UV and optical data sets to characterize and determine the parameters of white dwarfs. Fourteen of the white dwarfs fit single-component SEDs well, while three showed a good fit with a two-component SED model,indicating a binary system comprising a white dwarf and a low-mass main-sequence star. The effective temperature, radius, luminosity, and mass of white dwarfs range between 22000 - 70000 K, 0.008 - 0.028 Rsun, 0.09 - 3.0 Lsun, and 0.30 - 1.13 Msun, respectively. The effective temperature, radius, luminosity, and mass of the companions (low-mass main-sequence stars) are 3500 - 3750 K, 0.150 - 0.234 Rsun,0.003 - 0.01 Lsun, and 0.14 - 0.24 Msun, respectively. The three binary systems (WD-MS), along with the massive WDs may have formed through dynamical processes that occurred during the core collapse of the cluster. This is the first evidence of a massive WD formation in a core-collapsed cluster, which is the missing link in the formation of a fast radio burst (FRB) progenitor in a globular cluster. This study provides evidence that NGC 362 hosts stellar systems that may evolve into exotic stars such as Type Ia supernovae, and/or FRBs in the future. *This is paper VI of the Globular Cluster UVIT Legacy Survey.

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arXiv2025

AstroSat/UVIT Study of NGC 663: First detection of Be+sdOB systems in a young star cluster

Be stars are rapidly rotating stars surrounded by a disc; however, the origin of these stars remains unclear. Mass and angular momentum transfer in close binaries account for the rapid rotation of a major fraction of Be stars, as supported by the previous detection of low-mass stripped companions to these stars. The stripped companions can be helium-burning subdwarf OB-type stars (sdOBs) and white dwarfs. The main objective of this study is to characterise the identified Be stars in the young open cluster NGC 663 and search for possible hot companions. We present the first ultraviolet (UV) photometric study of NGC 663 using far-UV and near-UV data from UVIT/AstroSat as a part of the UOCS series (XVIII). We identified 23 previously known Be stars in the cluster. Further, we utilised the spectral energy distribution fitting technique to derive the fundamental parameters and to search for UV-bright companions of the identified Be stars. Our study reveals that 19 out of 23 Be stars show a significant UV excess, indicating the presence of hot companions. Here, we report the first detection of high-mass sdOB companions to Be stars, with 69.5% of them found in binaries within a cluster, offering direct evidence of binary interactions. This study showcases the key role of binary interactions in the formation of Be stars in clusters and provides insights into massive star evolution.

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arXiv2025

UVIT Study of the Magellanic Clouds (U-SMAC). III. Hierarchical Star Formation in the Small Magellanic Cloud Regulated by Turbulence

The Small Magellanic Cloud (SMC), a satellite galaxy of the Milky Way, is an irregular dwarf galaxy exhibiting evidence of recent and ongoing star formation. We performed a spatial clustering analysis of far-ultraviolet stars in the SMC younger than 150 Myr using data from the Ultra Violet Imaging Telescope onboard AstroSat. We identified 236 young stellar structures as surface overdensities at different significance levels. The sizes of these structures range from a few parsecs to several hundred parsecs. Their irregular morphologies are characterized by a perimeter-area dimension, derived from the projected boundaries of the young stellar structures, of Dp = 1.46 +/- 0.4. The 2D fractal dimensions obtained from, respectively, the number-size relation and the size distribution are D2 = 1.64 +/- 0.03 and D2 = 1.31 +/- 0.16. These values indicate significant lumpiness among the young stellar structures. In addition, the surface density distribution of the identified structures follows a log-normal distribution. These features are strikingly similar to those of the turbulent interstellar medium, thus supporting the scenario of hierarchical star formation regulated by supersonic turbulence.

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arXiv2025

Orbits and vertical height distribution of 4006 open clusters in the Galactic disk using Gaia DR3

Open clusters (OCs) in the Galaxy are excellent probes for tracing the structure and evolution of the Galactic disk. We present an updated catalog of parameters for 1,145 OCs, estimated using the Gaia DR3 data earlier listed in Cantat-Gaudin et al. (2020). This sample is complemented by 3,677 OCs from the catalog by Hunt & Reffert (2023). Using the Galaxy potential and the space velocities, orbits of 4,006 OCs were computed. We provide a catalog with orbital parameters such as eccentricity, perigalactic and apogalactic distance, and the maximum vertical height traced by OCs from the Galactic disk. The OCs were found to be distributed between 5-16 kpc from the Galactic center, with older OCs showing a radially extended distribution. The low number of old OCs in the inner Solar circle region likely suggests their destruction in this area. We derive a quantitative expression for the dependency of the maximum vertical height (Z_max) OCs can reach with the cluster's age and Galactocentric radius for the first time. The young and intermediate-age OCs show similar values of Z_max till 9 kpc, with the latter group having higher values beyond. OCs older than 1 Gyr show larger values of Z_max at all Galactocentric radii and significantly larger values beyond 9 kpc. Higher values of Z_max are found in the third Galactic quadrant, suggesting a link between these higher values and the Galactic warp. This sample shows that young OCs are also involved in the diagonal ridge formation in the solar neighborhood.

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arXiv2024

Tracing Hierarchical Star Formation out to Kiloparsec Scales in Nearby Spiral Galaxies with UVIT

Molecular clouds fragment under the action of supersonic turbulence & gravity which results in a scale-free hierarchical distribution of star formation (SF) within galaxies. Recent studies suggest that the hierarchical distribution of SF in nearby galaxies shows a dependence on host galaxy properties. In this context, we study the nature of hierarchical SF from a few tens of pc up to several kpc in 4 nearby spiral galaxies NGC1566, NGC5194, NGC5457 & NGC7793, by leveraging the large FoV & high resolution FUV+NUV observations from the UltraViolet Imaging Telescope (UVIT). Using the two-point correlation function, we infer that the young star-forming clumps (SFCs) in the galaxies are arranged in a fractal-like hierarchical distribution, but only up to a maximum scale ($l_{corr}$) & it ranges from 0.5 kpc to 3.1 kpc. The flocculent spiral NGC7793 has $\sim$5 times smaller $l_{corr}$ than the 3 grand design spirals, possibly due to its lower mass, low pressure environment & lack of strong spiral arms. $l_{corr}$ being much smaller than the galaxy size suggests that the SF hierarchy does not extend to the full galaxy size & it is likely an effect set by multiple physical mechanisms in the galaxy. The hierarchical distribution of SFCs dissipates within 10 to 50 Myr, signifying their migration away from their birthplaces over time. Our results suggest that the global hierarchical properties of SF in galaxies are not universal & significant variations exist in the local & global hierarchy parameters of a galaxy. This study also demonstrates the capabilities of UVIT in characterizing the SF hierarchy in nearby galaxies. In the future, a bigger sample can be employed to further understand the role of large-scale galaxy properties (morphology, environment) & physical processes (feedback, turbulence, shear & ISM conditions) on determining the non-universal hierarchical properties of SF in galaxies.

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arXiv2024

UVIT Study of the MAgellanic Clouds (U-SMAC) II. A Far-UV catalog of the Small Magellanic Cloud: Morphology and Kinematics of young stellar population

The Small Magellanic Cloud (SMC) is an irregular dwarf galaxy that has recently undergone an interaction with the Large Magellanic Cloud. The young massive stars in the SMC formed in the disturbed low-metallicity environment are important targets in astrophysics. We present a catalog of $\sim$ 76,800 far ultraviolet (FUV) sources towards the SMC detected using the Ultra Violet Imaging Telescope (UVIT) onboard AstroSat. We created an FUV catalog with $\sim$ 62900 probable SMC members which predominantly comprise main-sequence, giant, and subgiant stars. We selected 4 young populations (Young 1, Young 2, Young 3, and Blue Loop (BL) stars) identified from the Gaia optical color-magnitude diagram to study the morphology and kinematics of the young SMC using this catalog. We detect a clumpy morphology with a broken bar, a shell-like structure, and the inner SMC Wing for the 4 stellar populations. The eastern region and the northeastern regions are mainly populated by Young 1, 2, and 3. The central region predominantly has the Young 2 and 3 populations, whereas the SW has BL stars, Young 2 and 3. The 2-D kinematic study using proper motion (PM) reveals that Young 2 and 3 populations show two kinematically distinct sub-populations with low and high PM dispersion, whereas the Young 1 and BL stars show two kinematically distinct populations with low dispersion. Our analysis points to a kinematic disturbance along the RA direction for stars younger than $\sim$ 150 Myr located in the eastern region, with no significant disturbance along the Declination.

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arXiv2024

A Partial Near-infrared Guide Star Catalog for Thirty Meter Telescope Operations

At first light, the Thirty Meter Telescope (TMT) near-infrared (NIR) instruments will be fed by a multiconjugate adaptive optics instrument known as the Narrow Field Infrared Adaptive Optics System (NFIRAOS). NFIRAOS will use six laser guide stars to sense atmospheric turbulence in a volume corresponding to a field of view of 2', but natural guide stars (NGSs) will be required to sense tip/tilt and focus. To achieve high sky coverage (50% at the north Galactic pole), the NFIRAOS client instruments use NIR on-instrument wavefront sensors that take advantage of the sharpening of the stars by NFIRAOS. A catalog of guide stars with NIR magnitudes as faint as 22 mag in the J band (Vega system), covering the TMT-observable sky, will be a critical resource for the efficient operation of NFIRAOS, and no such catalog currently exists. Hence, it is essential to develop such a catalog by computing the expected NIR magnitudes of stellar sources identified in deep optical sky surveys using their optical magnitudes. This paper discusses the generation of a partial NIR Guide Star Catalog (IRGSC), similar to the final IRGSC for TMT operations. The partial catalog is generated by applying stellar atmospheric models to the optical data of stellar sources from the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) optical data and then computing their expected NIR magnitudes. We validated the computed NIR magnitudes of the sources in some fields by using the available NIR data for those fields. We identified the remaining challenges of this approach. We outlined the path for producing the final IRGSC using the Pan-STARRS data. We have named the Python code to generate the IRGSC as irgsctool, which generates a list of NGS for a field using optical data from the Pan-STARRS 3pi survey and also a list of NGSs having observed NIR data from the UKIRT Infrared Deep Sky Survey if they are available.

astro-ph.IM↗

arXiv2024

Discovery of a barium blue straggler star in M67 and sighting of its WD companion*

We report the discovery of a barium blue straggler star (BSS) in M67, exhibiting enhancements in slow neutron-capture ($s$-) process elements. Spectroscopic analysis of two BSSs (WOCS\,9005 \& WOCS\,1020) and 4 stars located near the main-sequence turn-off using GALAH spectra, showed that WOCS\,9005 has a significantly high abundance of the s-process elements ([Ba/Fe] = 0.75$\pm$0.08, [Y/Fe] = 1.09$\pm$0.07, [La/Fe] = 0.65$\pm$0.06). The BSS (WOCS\,9005) is a spectroscopic binary with a known period, eccentricity and a suspected white dwarf (WD) companion with a kinematic mass of 0.5 M$_\odot$. The first `sighting' of the WD in this barium BSS is achieved through multi-wavelength spectral energy distribution (SED) with the crucial far-UV data from the UVIT/\textit{AstroSat}. The parameters of the hot and cool companions are derived using binary fits of the SED using two combinations of models, yielding a WD with T$_{eff}$ in the range 9750--15250 K. Considering the kinematic mass limit, the cooling age of the WD is estimated as $\sim$ 60 Myr. The observed enhancements are attributed to a mass transfer (MT) from a companion asymptotic giant branch star, now a WD. We estimate the accreted mass to be 0.15 M$_{\odot}$, through wind accretion, which increased the envelope mass from 0.45 M$_{\odot}$. The detection of chemical enhancement, as well as the sighting of WD in this system have been possible due to the recent MT in this binary, as suggested by the young WD.

astro-ph.SR↗

arXiv2024

UVIT Study of the MAgellanic Clouds (U-SMAC) I. Recent star formation history and kinematics of the Shell region in the North-Eastern Small Magellanic Cloud

The interactions between the Magellanic Clouds significantly affect the shape and distribution of the young stellar population, particularly in the periphery of the Small Magellanic Cloud (SMC). We present the first far-UV (FUV) map of the north-east SMC-Shell region using the Ultra Violet Imaging Telescope (UVIT) onboard AstroSat. The detected FUV stars are combined with Gaia Early Data Release 3 data to create a FUV-optical catalog of ~ 14,400 stars. FUV-optical colour-magnitude diagrams are used along with isochrones to estimate the stellar ages. The detected stars are formed in multiple episodes. We identified two episodes of star formation (~ 60 and ~ 260 Myr ago) where the episode at ~ 260 Myr is linked to the recent interaction with the Large Magellanic Cloud (LMC) and the episode at ~ 60 Myr is linked to the pericentric passage of the SMC around our Galaxy. The median proper motion (PM) and velocity dispersion are found to be similar to the SMC main body, indicating that this region has not experienced significant tidal effects. The FUV stellar surface density and the dispersion in PM suggest an extent of the inner SMC in the north-east direction to be around 2.2 deg. We detect arm-like and arc-like structures in the FUV stellar density map, and their kinematics appear to be similar to the SMC main body. These extended outer features are the spatial stellar overdensities formed over multiple episodes of star formation, but without apparent kinematic distinction.

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arXiv2024

Simultaneous FUV and NUV observations of T Tauri stars with UVIT/AstroSat: probing accretion process in young stars

We present results from simultaneous FUV and NUV observations of T-Tauri stars (TTSs) in the Taurus molecular cloud with UVIT/AstroSat. This is the very first UVIT study of TTSs. From the spectral energy distribution of TTSs from FUV to near-IR, we show that classical TTSs (CTTSs) emit significantly higher UV excess compared to weak-line TTSs (WTTSs). The equivalent black-body temperatures corresponding to the UV excess in CTTSs ($>10^4$ K) are also found to be relatively higher than that in WTTSs ($<9250$ K). From the UV excess, we have re-classified two WTTSs (BS Tau, V836 Tau) as CTTSs, which has been supported by the follow-up optical spectroscopic study using the Himalayan Chandra Telescope (HCT), showing strong H$α$ line emission. We find that CTTSs show strong excess emission in both FUV ($>$10$^7$) and NUV ($>$10$^3$) bands, while WTTSs show strong excess only in the FUV ($\lesssim$10$^5$), suggesting that excess emission in NUV can be used as a tool to classify the TTSs. We also find a linear correlation between UV luminosity (a primary indicator of mass accretion) and H$α$ luminosity (a secondary indicator of mass accretion) with a slope of 1.20$\pm$0.22 and intercept of 2.16$\pm$0.70.

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