Browsing by Author "Nayak, Prasanta K."

Now showing 1 - 11 of 11
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    A chemodynamical analysis of bright metal-poor stars from the HESP-GOMPA survey - indications of a non-prevailing site for light r-process elements
    (2024) Bandyopadhyay, Avrajit; Beers, Timothy C.; Ezzeddine, Rana; Sivarani, Thirupathi; Nayak, Prasanta K.; Pandey, Jeewan C.; Saraf, Pallavi; Susmitha, Antony
    We present a comprehensive analysis of the detailed chemical abundances for a sample of 11 metal-poor, very metal-poor, and extremely metal-poor stars ([Fe/H] = -1.65 to [Fe/H] = -3.0) as part of the HESP-GOMPA (Galactic survey Of Metal Poor stArs) survey. The abundance determinations encompass a range of elements, including C, Na, Mg, Al, Si, Ca, Sc, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sr, and Ba, with a subset of the brighter objects allowing for the measurement of additional key elements. Notably, the abundance analysis of a relatively bright highly r-process-enhanced (r-II) star (SDSS J0019+3141) exhibits a predominantly main r-process signature and variations in the lighter r-process elements. Moreover, successful measurements of thorium in this star facilitate stellar age determinations. We find a consistent odd-even nucleosynthesis pattern in these stars, aligning with expectations for their respective metallicity levels, thus implicating Type II supernovae as potential progenitors. From the interplay between the light and heavy r-process elements, we infer a diminishing relative production of light r-process elements with increasing Type II supernova contributions, challenging the notion that Type II supernovae are the primary source of these light r-process elements in the early Milky Way. A chemodynamical analysis based on Gaia astrometric data and our derived abundances indicates that all but one of our program stars are likely to be of accreted origin. Additionally, our examination of alpha-poor stars underscores the occurrence of an early accretion event from a satellite on a prograde orbit, similar to that of the Galactic disc.
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    A uGMRT search for radio emission from planets around evolved stars
    (2024) Narang, Mayank; Puravankara, M.; Chandra, C. H. Ishwara; Banerjee, Bihan; Tyagi, Himanshu; Tamura, Motohide; Henning, Thomas; Mathew, Blesson; Lazio, Joseph; Surya, Arun; Nayak, Prasanta K.
    In this work, we present the results from a study using the Giant Meterwave Radio Telescope (GMRT) to search for radio emission from planets around three evolved stars namely alpha Tau, beta UMi, and beta Gem. Both alpha Tau and beta UMi host massive similar to 6 MJ mass planets at about similar to 1.4 au from the central star, while beta Gem is host to a 2.9 MJ mass planet at 1.7 au from the host star. We observe alpha Tau and beta UMi at two upgraded GMRT bands: band 3 (250-500 MHz) and band 4 (550-900 MHz). We also analysed the archival observations from beta Gem at 150 MHz from GMRT. We did not detect any radio signals from these systems. At 400 MHz, the 3 sigma upper limit is 87 mu Jy beam(-1) for alpha Tau b and 77.4 mu Jy beam(-1) for beta UMi b. From our observations at 650 MHz, we place a 3 sigma upper limit of 28.2 mu Jy beam(-1) for alpha Tau b and 33.6 mu Jy beam(-1) for beta UMi b. For beta Gem b, at 150 MHz, we place an upper limit of 2.5 mJy. At 400 and 650 MHz, our observations are the deepest radio images for any exoplanetary system.
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    Host-star Properties of Hot, Warm, and Cold Jupiters in the Solar Neighborhood from Gaia Data Release 3: Clues to Formation Pathways
    (2024) Banerjee, Bihan; Narang, Mayank; Manoj, P.; Henning, Thomas; Tyagi, Himanshu; Surya, Arun; Nayak, Prasanta K.; Tripathi, Mihir
    Giant planets exhibit diverse orbital properties, hinting at their distinct formation and dynamic histories. In this paper, using Gaia Data Release 3 (DR3), we investigate if and how the orbital properties of Jupiters are linked to their host star properties, particularly their metallicity and age. We obtain metallicities for main-sequence stars of spectral type F, G, and K, hosting hot, warm, and cold Jupiters with varying eccentricities. We compute the velocity dispersions of the host stars of these three groups using kinematic information from Gaia DR3 and obtain average ages using a velocity dispersion-age relation. We find that the host stars of hot Jupiters are relatively metal rich ([Fe/H] = 0.18 +/- 0.13) and young (median age of 3.97 +/- 0.51 Gyr) compared to the host stars of cold Jupiters in nearly circular orbits, which are relatively metal poor (0.03 +/- 0.18) and older (median age of 6.07 +/- 0.79 Gyr). The host stars of cold Jupiters in high-eccentricity orbits, on the other hand, show metallicities similar to those of the hosts of hot Jupiters, but are older, on average (median age of 6.25 +/- 0.92 Gyr). The similarity in metallicity between the hosts of hot Jupiters and the hosts of cold Jupiters in high-eccentricity orbits supports high-eccentricity migration as the potential origin of hot Jupiters, with the latter serving as the progenitors of hot Jupiters. However, the average age difference between them suggests that the older hot Jupiters may have been engulfed by their host star over timescales similar to 6 Gyr. This allows us to estimate the value of stellar tidal quality factor, Q * ' similar to 10 6 +/- 1 .
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    Hunting down white dwarf-main sequence binaries using multiwavelength observations
    (2024) Nayak, Prasanta K.; Ganguly, Anindya; Chatterjee, Sourav
    Identification of white dwarfs (WD) with main-sequence (MS) companions and characterization of their properties can put important constraints on our understanding of binary stellar evolution and guide the theoretical predictions for a wide range of interesting transient events relevant for, e.g., Rubin Observatory (LSST), ZTF, and LISA. In this study, we combine ultraviolet (UV) and optical color-magnitude diagrams (CMDs) to identify unresolved WD--MS binaries. In particular, we combine high-precision astrometric and photometric data in the optical from Gaia-DR3 and UV data from GALEX GR6/7 to identify 93 WD--MS candidates within 100 pc. Of these, 80 are newly identified. Using the Virtual Observatory SED Analyzer (VOSA) we fit the spectral energy distributions (SEDs) of all our candidates and derive stellar parameters, such as effective temperature, bolometric luminosity, and radius for both companions. We find that our identification method helps identify hotter and smaller WD companions (majority with >= 10,000 K and <= 0.02 R-circle dot) relative to the WDs identified by past surveys. We infer that these WDs are relatively more massive (median similar to 0.76M(circle dot)). We find that most of the MS companions in our binaries are of the K and M spectral types
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    Identifying the population of T-Tauri stars in Taurus: UV-optical synergy
    (2023) Nayak, Prasanta K.; Narang, Mayank; Puravankara, Manoj; Tyagi, Himanshu; Banerjee, Bihan; Sharma, Saurabh; Pandey, Rakesh; Surya, Arun; Mathew, Blesson; Arun, R.; Ujjwal, K.; Kartha, Sreeja S.
    With the third data release of the Gaia mission, Gaia DR3 with its precise photometry and astrometry, it is now possible to study the behavior of stars at a scale never seen before. In this paper, we developed new criteria to identify T-Tauri stars (TTS) candidates using UV and optical color-magnitude diagrams (CMDs) by combining the GALEX and Gaia surveys. We found 19 TTS candidates and five of them are newly identified TTS in the Taurus molecular cloud (TMC), not cataloged before as TMC members. For some of the TTS candidates, we also obtained optical spectra from several Indian telescopes. We also present the analysis of distance and proper motion of young stars in the Taurus using data from Gaia DR3. We found that the stars in Taurus show a bimodal distribution with distance, having peaks at 130.17(-1.24)(1.31) pc and 156.25(-5.00)(1.86) pc. The reason for this bimodality, we think, is due to the fact that different clouds in the TMC region are at different distances. We further showed that the two populations have similar ages and proper motion distribution. Using the Gaia DR3 CMD, we showed that the age of Taurus is consistent with 1 Myr.
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    Optical spectroscopy of Gaia detected protostars with DOT: Can we probe protostellar photospheres?
    (2023) Narang, Mayank; Manoj, P.; Tyagi, Himanshu; Nayak, Prasanta K.; Sharma, Saurabh; Surya, Arun; Banerjee, Bihan; Mathew, Blesson; Ghosh, Arpan; Verma, Aayushi
    Optical spectroscopy offers the most direct viewof the stellar properties and the accretion indicators. Standard accretion tracers, such as H beta, H alpha and Ca II triplet lines, and most photospheric features fall in the opticalwavelengths. However, these tracers are not readily observable from deeply embedded protostars because of the large line of sight extinction (A(v) similar to 50-100 mag) toward them. In some cases, however, it is possible to observe protostars at optical wavelengths if the outflow cavity is aligned along the line-of-sight that allows observations of the photosphere, or the envelope is very tenuous and thin, such that the extinction is low. In such cases, we not only detect these protostars at optical wavelengths, but also follow up spectroscopically. We have used the HOPS catalog (Furlan et al. in 2016) of protostars in Orion to search for optical counterparts for protostars in the Gaia DR3 survey. Out of the 330 protostars in the HOPS sample, an optical counterpart within 2 '' is detected for 62 of the protostars. For 17 out of 62 optically detected protostars, we obtained optical spectra (between 5500 and 8900 angstrom) using nt Object Spectrograph and Camera (ADFOSC) on the 3.6-m Devasthal Optical Telescope (DOT) and Hanle Faint Object Spectrograph Camera (HFOSC) on 2-m Himalayan Chandra Telescope (HCT). We detect strong photospheric features, such as the TiO bands in the spectra (of 4 protostars), hinting that photospheres can form early in the star-formation process. We further determined the spectral types of protostars, which show photospheres similar to a late M-type. Mass accretion rates derived for the protostars are similar to those found for T-Tauri stars, in the range of 10(-7)-10(-8) M-circle dot yr(-1).
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    Simultaneous Far-ultraviolet and Near-ultraviolet Observations of T Tauri Stars with UVIT/AstroSat: Probing the Accretion Process in Young Stars
    (2024) Nayak, Prasanta K.; Narang, Mayank; Manoj, P.; Gorti, Uma; Subramaniam, Annapurni; George, Nayana; Mondal, Chayan
    We present results from simultaneous far-ultraviolet (FUV) and near-ultraviolet (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 IR, we show that classical TTSs (CTTSs) emit significantly higher UV excess compared to weak-line TTSs (WTTSs). The equivalent blackbody temperatures corresponding to the UV excess in CTTSs (>10(4) K) are also found to be relatively higher than those in WTTSs (<9250 K). From the UV excess, we have reclassified two WTTSs (BS Tau and V836 Tau) as CTTSs, which has been supported by the follow-up optical spectroscopic study using the Himalayan Chandra Telescope, showing strong H alpha line emission. We find that CTTSs show strong excess emission in both the FUV (>10(7)) and NUV (>10(3)) bands, while WTTSs show strong excess only in the FUV (less than or similar to 10(5)), suggesting that excess emission in the 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 alpha luminosity (a secondary indicator of mass accretion) with a slope of 1.20 +/- 0.22 and intercept of 2.16 +/- 0.70.
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    Spatiotemporal map of star clusters in the Magellanic Clouds using Gaia: synchronized peaks and radial shrinkage of cluster formation
    (2024) Dhanush, S. R.; Subramaniam, A.; Nayak, Prasanta K.; Subramanian, S.
    We present a detailed view of cluster formation (CF) to trace the evolution and interaction history of the Magellanic Clouds (MCs) in the last 3.5 Gyr. Using the \textit{Gaia} DR3 data, we parameterized 1710 and 280 star clusters in the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC), where 847 and 113 clusters are newly characterized in the outer LMC and SMC, respectively. We estimated the age-extinction-metallicity-distance parameters using an automated fitting of the color-magnitude diagram (CMD) after field star removal, followed by an MCMC technique. We report a first-time detection of two synchronized CF peaks in the MCs at 1.5 +/- 0.12 Gyr and 800 +/- 60 Myr. We recommend that the choice of the metallicity (Z) values of isochrones for clusters with age <= 1 - 2 Gyr are Z(LMC) = 0.004 - 0.008 and Z(SMC) = 0.0016 - 0.004 for the LMC and SMC, respectively. We found evidence for spiral arms in the LMC, as traced by the cluster count profiles over the last 3.5 Gyr. The density maps provide evidence of ram-pressure stripping in the North-East of the LMC, a severe truncation of CF in the South of the LMC, and a radial shrinkage of CF in the SMC in the last 450 Myr. The last SMC-LMC interaction (similar to 150 Myr) resulted in a substantial CF in the North and Eastern SMC, with a marginal impact on the LMC. This study provides important insights into the CF episodes in the MCs and their connection to the LMC-SMC-MW interactions.
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    uGMRT Survey of EXoplanets Around M-dwarfs (GS-EXAM): Radio Observations of GJ 1151
    (2024) Narang, Mayank; Puravankara, Manoj; Vedantham, H. K.; Ishwara-Chandra, C. H.; De, Ayanabha; Tyagi, Himanshu; Banerjee, Bihan; Nayak, Prasanta K.; Surya, Arun; Shridharan, B.; Pathak, Vinod C.; Tripathi, Mihir
    Coherent radio emission with properties similar to planetary auroral signals has been reported from GJ 1151, a quiescent, slow-rotating mid-M star, by the LOFAR Two-meter (120-170 MHz) Sky Survey. The observed LOFAR emission is fairly bright at 0.89 mJy with 64% circular polarization, and the emission characteristics are consistent with the interaction between an Earth-sized planet with an orbital period of 1-5 days and the magnetic field of the host star. However, no short-period planet has been detected around GJ 1151. To confirm the reported radio emission caused by the putative planet around GJ 1151 and to investigate the nature of this emission, we carried out upgraded Giant Metrewave Radio Telescope observations of GJ 1151 at 150, 218, and 400 MHz over 33 hr across ten epochs. No emission was detected at any frequency. While at 150 and 218 MHz, nondetection could be due to the low sensitivity of our observations, at 400 MHz, the rms sensitivities achieved were sufficient to detect the emission observed with LOFAR at similar to 20 sigma level. Our findings suggest that the radio emission is highly time variable, likely influenced by the star-planet system's phase and the host star's magnetic field. Additional observations below 170 MHz, at more frequent epochs (as the periodicity of the emission is unknown), especially during periods of high stellar magnetic field strength, are needed to confirm the emission.
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    UV Spectral Characterization of Low-mass Stars with AstroSat UVIT for Exoplanet Applications: The Case Study of HIP 23309
    (2023) Ranjan, Sukrit; Nayak, Prasanta K.; Pineda, J. Sebastian; Narang, Mayank
    Characterizing rocky exoplanet atmospheres is a key goal of exoplanet science, but interpreting such observations will require understanding the stellar ultraviolet (UV) irradiation incident on the planet from its host star. Stellar UV mediates atmospheric escape, photochemistry, and planetary habitability, and observations of rocky exoplanets can only be understood in the context of the UV spectral energy distribution (SED) of their host stars. Particularly important are SEDs from observationally favorable but poorly understood low-mass M-dwarf stars, which are the only plausible targets for rocky planet atmospheric characterization for the next 1-2 decades. In this work, we explore the utility of AstroSat UltraViolet Imaging Telescope (UVIT) for the characterization of the UV SEDs of low-mass stars. We present observations of the nearby M0 star HIP 23309 in the far-UV (FUV) and near-UV (NUV) gratings of UVIT. Our FUV spectra are consistent with contemporaneous Hubble Space Telescope (HST) data and our NUV spectra are stable between orbits, suggesting UVIT is a viable tool for the characterization of the SEDs of low-mass stars. We apply our measured spectra to simulations of photochemistry and habitability for a hypothetical rocky planet orbiting HIP 23309 and elucidate the utility and limitations of UVIT in deriving UV SEDs of M-dwarf exoplanet hosts. Our work validates UVIT as a tool to complement HST in the characterization of exoplanet host stars and carries implications for its successor missions like INSIST.
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    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
    (2024) Hota, Sipra; Subramaniam, Annapurni; Nayak, Prasanta K.; Subramanian, Smitha
    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 similar to 76,800 far-ultraviolet (FUV) sources toward the SMC detected using the Ultra Violet Imaging Telescope onboard AstroSat. We created an FUV catalog with similar to 62,900 probable SMC members which predominantly comprise main-sequence, giant, and subgiant stars. We selected four 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 four stellar populations. The eastern region and the northeastern regions are mainly populated by Young 1, 2, and 3 stars. The central region predominantly has the Young 2 and 3 populations, whereas the SW has BL stars, and Young 2 and 3 stars. The 2D kinematic study using proper motion (PM) reveals that Young 2 and 3 populations show two kinematically distinct subpopulations 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 R.A. direction for stars younger than similar to 150 Myr located in the eastern region, with no significant disturbance along the decl.