arXiv subjects

Ramkrishna Das

Publications and source records attributed to Ramkrishna Das.

At least 19 recordsLinked to original sources

arXiv2026

Optical Spectroscopy and Temporal Evolution of the Nova V1405 Cas

This paper presents the findings from our study of nova V1405~Cas over the first 1051 days after the outburst. The study includes an analysis of the photometric light curve evolution, along with a detailed spectroscopic evolution. Photometric analysis shows that the nova is a very slow nova, with a decline timescale of $t_2 \approx 165$ days. The mass of the white dwarf is calculated as $ M_{\rm WD} \sim 0.7\~M_{\odot} $. The secondary star is a low-mass main-sequence star, with a mass of $ M_{\rm sec} \approx 0.43\~M_{\odot} $. Spectral observations show initial dominance by Balmer emission lines accompanied by prominent P Cygni profiles during the first +339 days. These features disappeared in later epochs, being replaced by high-ionization coronal lines, indicating that the nova had transitioned to the coronal phases by day +371. To investigate the physical conditions of the ejecta and the central source, we conducted photoionization modeling using \textsc{cloudy}. Our model reveals a gradual increase in the temperature and luminosity of the system, and suggests that the ejecta were primarily composed of He, N, Fe, Ne, and Ca, with noticeable temporal variations in their relative abundances. The estimated mean ejected mass is approximately $1.10~\times~10^{-4}\~M_{\odot}$, which is relatively high and suggests a low-mass white dwarf. Our optical spectroscopic and photometric analyses, combined with detailed photoionization modeling, indicate that the white dwarf in Nova V1405 Cas is unlikely to be of the ONeMg type.

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arXiv2025

Dynamic Photometric Variability in Three Young Brown Dwarfs in Taurus: Detection of Optical Flares with TESS data

We present $I$-band time-series photometric variability studies of three known nearby ($\sim$ 140 pc) and young ( $\sim$ 1 Myr) brown dwarfs (BD) in the Taurus star-forming region in the Perseus Molecular Cloud. From 10 nights of observations over a time span of 10 years, with a typical run of 3 to 6 hours each night, we estimated that the BDs show unstable short-scale periodicity from 1.5 to 4.8 hours. Using the long-term photometry from the Transiting Exoplanet Survey Satellite (TESS), we have conducted a time-resolved variability analysis of CFHT-BD-Tau 3 and CFHT-BD-Tau 4, revealing orbital periods of $\sim$ 0.96 days and $\sim$ 3 days respectively, consistent with earlier studies. We also found two superflares in TESS sector 43 data for CFHT-BD-Tau 4 and estimated the flare energies as $7.09\times10^{35}$ erg and $3.75\times10^{36}$ erg. A magnetic field of $\sim3.39 ~kG$ is required to generate such flare energies on this BD. We performed spot modelling analysis on CFHT-BD-Tau 3 and CFHT-BD-Tau 4 to address the variability detected in the data using the package BASSMAN. Spectral energy distribution and infrared colours of the sources suggest that they have a sufficient amount of circumstellar material around them.

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arXiv2024

Fate and detectability of rare gas hydride ions in nova ejecta: A case study with nova templates

HeH$^+$ was the first heteronuclear molecule to form in the metal-free Universe after the Big Bang. The molecule gained significant attention following its first circumstellar detection in the young and dense planetary nebula NGC 7027. We target some hydride ions associated with the noble gases (HeH$^+$, ArH$^+$, and NeH$^+$) to investigate their formation in harsh environments like the nova outburst region. We use a photoionization modeling (based on previously published best-fit physical parameters) of the moderately fast ONe type nova, QU Vulpeculae 1984, and the CO type novae, RS Ophiuchi and V1716 Scorpii. Our steady-state modeling reveals a convincing amount of HeH$^+$, especially in the dense clump of RS Ophiuchi and V1716 Scorpii. The calculated upper limit on the surface brightness of HeH$^+$ transitions suggests that the James Webb Space Telescope (JWST) could detect some of them, particularly in sources like RS Ophiuchi and V1716 Scorpii, which have similar physical and chemical conditions and evolution. It must be clearly noted that the sources studied are used as templates, and not as targets for observations. The detection of these lines could be useful for determining the physical conditions in similar types of systems and for validating our predictions based on new electron-impact ro-vibrational collisional data at temperatures of up to 20,000 K.

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arXiv2024

Classification of Wolf Rayet stars using Ensemble-based Machine Learning algorithms

We develop a robust Machine Learning classifier model utilizing the eXtreme-Gradient Boosting (XGB) algorithm for improved classification of Galactic Wolf-Rayet (WR) stars based on Infrared (IR) colors and positional attributes. For our study, we choose an extensive dataset of 6555 stellar objects (from 2MASS and AllWISE data releases) lying in the Milky Way (MW) with available photometric magnitudes of different types including WR stars. Our XGB classifier model can accurately (with an 86\% detection rate) identify a sufficient number of WR stars against a large sample of non-WR sources. The XGB model outperforms other ensemble classifier models such as the Random Forest. Also, using the XGB algorithm, we develop a WR sub-type classifier model that can differentiate the WR subtypes from the non-WR sources with a high model accuracy ($>60\%$). Further, we apply both XGB-based models to a selection of 6457 stellar objects with unknown object types, detecting 58 new WR star candidates and predicting sub-types for 10 of them. The identified WR sources are mainly located in the Local spiral arm of the MW and mostly lie in the solar neighborhood.

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arXiv2024

Detection of high-frequency pulsation in WR 135: investigation of stellar wind dynamics

We report the detection of high-frequency pulsations in WR\,135 from short cadence (10\,minutes) optical photometric and spectroscopic time series surveys. The harmonics up to $6^{th}$ order are detected from the integrated photometric flux variations while the comparatively weaker $8^{th}$ harmonic is detected from the strengths of the emission lines. We investigate the driving source of the stratified winds of WR\,135 using the radiative transfer modeling code, CMFGEN, and find the physical conditions that can explain the propagation of such pulsations. From our study, we find that the optically thick sub-sonic layers of the atmosphere are close to the Eddington limit and are launched by the Fe-opacity. The outer optically thin super-sonic winds ($\tau_{ross}=0.1-0.01$) are launched by the He\,$\textsc{ii}$ and C\,$\textsc{iv}$ opacities. The stratified winds above the sonic point undergo velocity perturbation that can lead to clumps. In the optically thin supersonic winds, dense clumps of smaller size ($f_{VFF}=0.27-0.3$, where $f_{VFF}$ is the volume filling factor) pulsate with higher-order harmonics. The larger clumps ($f_{VFF}=0.2$) oscillate with lower-order harmonics of the pulsation and affect the overall wind variability.

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arXiv2024

Investigation of [KSF2015] 1381-19L, a WC9-type star in the high extinction Galactic region

We report a multi-wavelength study of the Wolf Rayet (WR) star: [KSF2015] 1381-19L, which is located in the solar metallicity region (Z=0.014) of the Milky Way Galaxy, strongly obscured by the interstellar dust. We perform a detailed characterization of the stellar atmosphere by fitting the spectral emission lines observed in the Optical and Near-InfraRed (NIR) bands, using CMFGEN. The best-fitted spectroscopic model indicates a highly luminous ($10^{5.89}L_{\odot}$) star with a larger radius ($15\,R_{\odot}$) and effective temperature, wind terminal velocity, and chemical composition similar to that of Galactic WC9-dusty (WC9d)-type stars. The atmospheric ionization structure shows coexisting ionization states of different elements, simultaneously affecting the opacity and thermal electron balance. Fitting of the spectral energy data (SED) reveals high interstellar optical extinction ($A_{V}=$ 8.87) while the IR extinction is found to be comparatively lower ($A_{K_{s}}=$ 0.98). We do not detect any excess emission at near-IR wavelengths due to dust. Upon comparison of our results with the GENEVA single star evolutionary models (Z=0.014), we identify the best possible progenitors ( a rotating star of $67\,M_{\odot}$ and a non-rotating star of $90\,M_{\odot}$).

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arXiv2024

Spectroscopic study of the Quiescent Stages in between the 2006 and 2021 outbursts of RS Ophiuchi

This paper presents a comprehensive spectroscopic analysis of the quiescent stage of the recurrent nova RS Ophiuchi between its 2006 and 2021 outbursts. The spectra shows prominent low-ionization emission features, including hydrogen, helium, iron emissions, and TiO absorption features. The \ion{H}{$\alpha$} and \ion{H}{$\beta$} lines showed double-peaked emission profiles, indicating that both originate from the accretion disc. The central peaks of the \ion{H}{$\alpha$} and \ion{H}{$\beta$} emission profiles exhibited subtle shifts towards the blue or red side, attributed to orbital motion and fluctuations in the accretion rate. Using the double-peak features observed in the \ion{H}{$\alpha$} and \ion{H}{$\beta$} lines, we have estimated the accretion disc size to be $ R_{AD} = 3.10 \pm 0.04 \times 10^{12} \, \text{cm} $. The \textsc{cloudy} photoionization code is employed to model the quiescent phase spectra, allowing us to study the evolution of various physical parameters such as temperature, luminosity, hydrogen density, elemental abundances, accreted mass, and accretion rate. The central ionizing sources exhibit temperatures in the range of $1.05 - 1.80~\times 10^4$ K and luminosities between $0.10 - 7.94~\times 10^{30}$ \ergs. The mean accretion rate, calculated from the model, is $\sim$ $1.25 \times 10^{-8} M_{\odot}$ yr$^{-1}$. The model results reveal that the accretion rate rose substantially in the later phase. The accreted mass in the 16 months, preceding the 2021 outburst exceeds 47\% of the critical mass, and more than 88\% of the critical mass was accreted in the last three years.

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arXiv2023

Study of the fastest classical nova, V1674 Her: Photoionization and Morpho-kinemetic model analysis

We present the results of the investigation of the nova V1674 Her (2021), recognised as the swiftest classical nova, with $t_2 \sim 0.90$ days. The distance to the nova is estimated to be 4.97 kpc. The mass and radius of the WD are calculated to be $\sim~1.36~M_\odot$ and $\sim 0.15~R_\oplus$, respectively. Over the course of one month following the outburst, V1674 Her traversed distinct phases -- pre-maxima, early decline, nebular, and coronal -- displaying a remarkably swift transformation. The nebular lines emerged on day 10.00, making it the classical nova with the earliest observed commencement to date. We modelled the observed optical spectrum using the photoionization code \textsc{cloudy}. From the best-fitting model we deduced different physical and chemical parameters associated withe the system. The temperature and luminosity of the central ionizing sources are found in the range of $1.99 - 2.34~\times 10^5$ K and $1.26 - 3.16~ \times 10^{38}$ \ergs, respectively. Elements such as He, O, N, and Ne are found to be overabundant compared to solar abundance in both the nebular and coronal phases. According to the model, Fe II abundance diminishes while Ne abundance increases, potentially elucidating the rare hybrid transition between Fe and He/N nova classes. The ejected mass across all epochs spanned from $3.42 - 7.04~ \times 10^{-5}~M_\odot$. Morpho-kinematic modelling utilising \textsc{shape} revealed that the nova V1674 Her possesses a bipolar structure with an equatorial ring at the centre and an inclination angle of i = 67$\pm$ 1.5$^{\circ}$.

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arXiv2023

Morpho-kinematic and photoionization models of the multipolar structures in planetary nebula NGC 6572

We have studied the planetary nebula (PN) NGC 6572 through 3D morpho-kinematic and photoionization modelling. The 3D morphology is reconstructed from the Hubble Space Telescope images in different narrow band filters and position-velocity spectra. The PN have a multipolar morphology consisting of highly collimated outflows. The nebular image show signatures of multiple lobes within a spiral-ring-like structure. The multipolar structure is modelled with two bipolar shells (axes ratios 5.5:1 and 3:1), having closed and opened lobes, respectively. A toroidal structure (radius:height = 1:3) surrounds the shells at the waist. The toroidal axis aligns with the major axes of the bipolar shells. Our study reveals the nebula to have a history of collimated polar outflow perpendicular to a higher density equatorial wind with the outflow seemingly have episodes of changing direction of ejection. We construct a photoionization model of NGC 6572 using the deep optical spectra obtained at the 2 m Himalayan Chandra Telescope. For the photoionization model, we configure the input shell geometry in form of a highly bipolar nebular shell with reference to the 3D morphology. Our photoionization model satisfactorily reproduces the observables. We estimate the nebular elemental abundances, and important characteristic parameters of the central star (e.g., effective temperature, luminosity, gravity, mass, etc.) and the nebula (e.g., hydrogen density profiles, radii, etc.). We compare the resolved H$\beta$, [O III], and [N II] profiles in the 4.2 m William Herschel Telescope with that from the photoionization model and find a good characteristic match.

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arXiv2022

Study of 2021 outburst of the recurrent nova RS Ophiuchi: Photoionization and morpho-kinematic modelling

We present the evolution of the optical spectra of the 2021 outburst of RS Ophiuchi (RS Oph) over about a month after the outburst. The spectral evolution is similar to the previous outbursts. Early spectra show prominent P Cygni profiles of hydrogen Balmer, \ion{Fe}{ii}, and \ion{He}{i} lines. The emission lines were very broad during the initial days, which later became narrower and sharper as the nova evolved. This is interpreted as the expanding shocked material into the winds of the red giant companion. We find that the nova ejecta expanded freely for $\sim 4$ days, and afterward, the shock velocity decreased monotonically with time as $v\propto t^{-0.6}$. The physical and chemical parameters associated with the system are derived using the photoionization code \textsc{cloudy}. The best-fit \textsc{cloudy} model shows the presence of a hot central white dwarf source with a roughly constant luminosity of $\sim$1.00 $\times$ 10$^{37}$ erg s$^{-1}$. The best-fit photoionization models yield absolute abundance values by number, relative to solar of He/H $\sim 1.4 - 1.9$, N/H = $70 - 95$, O/H = $0.60 - 2.60$, and Fe/H $\sim 1.0 - 1.9$ for the ejecta during the first month after the outburst. Nitrogen is found to be heavily overabundant in the ejecta. The ejected hydrogen shell mass of the system is estimated to be in the range of $3.54 - 3.83 \times 10^{-6} M_{\odot}$. The 3D morpho-kinematic modelling shows a bipolar morphology and an inclination angle of $i=30^{\circ}$ for the RS Oph binary system.

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arXiv2022

Compact planetary nebulae MaC 2-1 and Sp 4-1: Photoionization models and dust characteristics

We study the characteristics of planetary nebulae (PNe), MaC 2-1 and Sp 4-1. We use our optical spectra taken at 2 m Himalayan Chandra Telescope, Spitzer mid-infrared (mid-IR) spectra, HST images, and IR photometric data. These PNe have not been individually studied in details earlier. Both the PNe are in the low- to moderate-excitation class. MaC 2-1 shows the presence of silicon carbide (SiC) and magnesium sulphide (MgS) dust. Sp 4-1 hosts polycyclic aromatic hydrocarbon (PAH) molecules. We obtain plasma properties of the PNe from the optical and mid-IR emission line fluxes. We compute photoionization models of the PNe for self-consistent estimation of physical parameters associated with the central star and the nebula, including nebular abundances. From the modelling of the IR data, we obtain the characteristics of dust and molecules formed in the nebulae. From our study, we estimate that the progenitors of MaC 2-1 and Sp 4-1 had masses of 1.2 and 1.55 $M_{\sun}$, respectively, and both of them seem to have born in metal poor environment. Both are distant PNe, with the estimated distances of 16 and 18 kpc for MaC 2-1 and Sp 4-1, respectively.

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arXiv2021

Photoionization modeling of the dusty nova V1280 Scorpii

We perform photoionization modeling of the dusty nova V1280 Scorpii (V1280 Sco) with an aim to study the changes in the physical and chemical parameters. We model pre and post dust phase, optical and near-Infrared (NIR), spectra using the photoionization code \textsc{cloudy}, v.17.02, considering a two-component (low density and high density region) model. From the best-fit model, we find that the temperature and luminosity of the central ionizing source in the pre-dust phase are in the range 1.32 - 1.50 $\times 10^4$ K and 2.95 - 3.16 $\times 10^{36}$ ergs$^{-1}$, respectively, which increase to 1.58 - 1.62 $\times 10^4$ K and 3.23 - 3.31 $\times 10^{36}$ ergs$^{-1}$, respectively, in the post-dust phase. It is found that a very high hydrogen density ($\sim 10^{13} - 10^{14}$ cm$^{-3}$) is required for the generation of spectra properly. Dust condensation conditions are achieved at high ejecta density ($\sim 3.16 \times 10^{8}$cm$^{-3}$) and low temperature ($\sim$2000 K) in the outer region of the ejecta. It is found that a mixture of small (0.005 - 0.25$μ$m) amorphous carbon dust grains and large (0.03 - 3.0$μ$m) astrophysical silicate dust grains iis present n the ejecta in the post-dust phase. Our model yields very high elemental abundance values as C/H = 13.5 - 20, N/H = 250, O/H = 27 - 35, by number, relative to solar in the ejecta, during the pre-dust phase, which decrease in the post-dust phase.

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arXiv2021

Estimating $T_{\rm eff}$, radius and luminosity of M-dwarfs using high resolution optical and NIR spectral features

We estimate effective temperature ($T_{\rm eff}$), stellar radius, and luminosity for a sample of 271 M-dwarf stars (M0V-M7V) observed as a part of CARMENES (Calar Alto high-Resolution search for M dwarfs with Exo-earths with Near-infrared and optical Echelle Spectrographs) radial-velocity planet survey. For the first time, using the simultaneously observed high resolution (R$\sim90000$) spectra in the optical (0.52 - 0.96 $μ$m) and near-infrared (0.96 - 1.71 $μ$m) bands, we derive empirical calibration relationships to estimate the fundamental parameters of these low-mass stars. We select a sample of nearby and bright M-dwarfs as our calibrators for which the physical parameters are acquired from high-precision interferometric measurements. To identify the most suitable indicators of $T_{\rm eff}$, radius, and luminosity (log $L/L_{\odot}$), we inspect a range of spectral features and assess them for reliable correlations. We perform multivariate linear regression and find that the combination of pseudo equivalent widths and equivalent width ratios of the Ca II at 0.854 $μ$m and Ca II at 0.866 $μ$m lines in the optical and the Mg I line at 1.57 $μ$m in the NIR give the best fitting linear functional relations for the stellar parameters with root mean square errors (RMSE) of 99K, 0.06 $R_{\odot}$ and 0.22 dex respectively. We also explore and compare our results with literature values obtained using other different methods for the same sample of M dwarfs.

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arXiv2021

Spectroscopic and photometric monitoring of a poorly known high-luminous OH/IR star: IRAS 18278+0931

We present the time-dependent properties of a poorly known OH/IR star $-$ IRAS 18278+0931 (hereafter, IRAS 18+09) towards the Ophiuchus constellation. We have carried out long-term optical/near-infrared (NIR) photometric and spectroscopic observations to study the object. From optical $R$- and $I$-band light curves, the period of IRAS 18+09 is estimated to be 575 $\pm$ 30 days and the variability amplitudes range from $Δ$R $\sim$ 4.0 mag to $Δ$I $\sim$ 3.5 mag. From the standard Period-Luminosity (PL) relations, the distance ($D$) to the object, 4.0 $\pm$ 1.3 kpc, is estimated. Applying this distance in the radiative transfer model, the spectral energy distribution (SED) are constructed from multi-wavelength photometric and IRAS-LRS spectral data which provides the luminosity, optical depth, and gas mass-loss rate (MLR) of the object to be 9600 $\pm$ 500 $L_{\odot}$, 9.1 $\pm$ 0.6 at 0.55 $μ$m and 1.0$\times$10$^{-6}$ M$_\odot$ yr$^{-1}$, respectively. The current mass of the object infers in the range 1.0 $-$ 1.5 $M_\odot$ assuming solar metallicity. Notably, the temporal variation of atomic and molecular features (e.g., TiO, Na I, Ca I, CO, H$_2$O) over the pulsation cycle of the OH/IR star illustrates the sensitivity of the spectral features to the dynamical atmosphere as observed in pulsating AGB stars.

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arXiv2020

Fast Photometric Variability of Very Low Mass Stars in IC 348: Detection of Superflare in an M-dwarf

We present here optical I-band photometric variability study down to $\simeq$ 19 mag of a young ($\sim$2-3 Myr) star-forming region IC 348 in the Perseus molecular cloud. We aim to explore the fast rotation (in the time-scales of hours) in Very Low Mass stars (VLMs) including Brown Dwarfs (BDs). From a sample of 177 light-curves using our new I-band observations, we detect new photometric variability in 22 young M-dwarfs including 6 BDs, which are bonafide members in IC 348 and well-characterized in the spectral type of M-dwarfs. Out of 22 variables, 11 M dwarfs including one BD show hour-scale periodic variability in the period range 3.5 - 11 hours and rest are aperiodic in nature. Interestingly, an optical flare is detected in a young M2.75 dwarf in one night data on 20 December 2016. From the flare light curve, we estimate the emitted flared energy of 1.48 $\times$ 10$^{35}$ ergs. The observed flared energy with an uncertainty of tens of per cent is close to the super-flare range ($\sim$ 10$^{34}$ ergs), which is rarely observed in active M dwarfs.

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arXiv2020

Photoionization modelling of quiescence phase spectra of novae & symbiotic star

We present results of study, using observed and published spectra in optical region, of few novae (T CrB, GK Per, RS Oph, V3890 Sgr and V745 Sco) in their quiescence phase and a symbiotic star (BX Mon). Observations were made using the facilities available at 2m Himalayan Chandra Telescope (HCT). Generally, the spectra show prominent low ionization emission features of hydrogen, helium, iron and oxygen and TiO absorption features due to the cool secondary component; T CrB and GK Per show higher ionization lines. We used photoionization code CLOUDY to model these spectra. From the best-fit models, we have estimated the physical parameters, e.g., temperature, luminosity & hydrogen density; estimated elemental abundances and other parameters related to the system. By matching the spectra of various giants with the absorption features and from the best-fit, we determined the type of secondaries and also their contribution to the spectra.

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arXiv2020

Morphology and ionization characteristics of planetary nebulae PB 1 and PC 19

We present results of our study of two planetary nebulae (PNe), PB1 and PC 19. We use the optical spectra of these two PNe observed at 2 m Himalayan Chandra Telescope and also archival and literature data for the study. We use the morphokinematic code SHAPE to construct 3D morphologies of the PNe and the photoionization code CLOUDY to model the observed spectra. The 3D model of PB 1 consists of an elongated shell surrounded by a bipolar halo and that of PC 19 consists of an open lobed bipolar structure and a spiral filamentary pair. We analyze the ionization structure of the PNe by deriving several plasma parameters and by photoionization modelling. We estimate the elemental abundances of the the elements, He, C, N, O, Ne, S, Ar, and Cl, from our analysis. We find He, C and N abundances to be significantly higher in case of PB 1. We estimate different physical parameters of the central stars, namely effective temperature, luminosity and gravity, and of the nebula, namely hydrogen density profiles, radii, etc., from photoionization modelling. We estimate distances to the PNe as $\sim$4.3 kpc for PB 1 and as $\sim$5.6 kpc for PC 19 by fitting the photoionization models to absolute observed fluxes. Progenitor masses are estimated from theoretical evolutionary trajectories and are found to be $\sim$1.67 and $\sim$2.38 $M_{\odot}$ for PB 1 and PC 19, respectively.

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arXiv2020

Census of young stellar population in the Galactic H II region Sh2-242

We present here identification and characterization of the young stellar population associated with an active star-forming site Sh2-242. We used our own new optical imaging and spectroscopic observational data, as well as several archival catalogs, e.g., Pan-STARRS 1, $Gaia$ DR2, IPHAS, WIRCam, 2MASS, and $Spitzer$. Slit spectroscopic results confirm the classification of the main ionizing source BD+26 980 as an early-type star of spectral type B0.5 V. The spectrophotometric distance of the star is estimated as 2.08 $\pm$ 0.24 kpc, which confirms the source as a member of the cluster. An extinction map covering a large area (diameter $\sim$ 50') is generated with $H$ and $K$ photometry toward the region. From the map, three distinct locations of peak extinction complexes ($A_{V}$ $\simeq$ 7$-$17 mag) are identified for the very first time. Using the infrared color excess, a total of 33 Class I and 137 Class II young objects are classified within the region. The IPHAS photometry reveals classification of 36 H$α$ emitting sources, which might be class II objects. Among 36 H$α$ emitting sources, 5 are already identified using infrared excess emission. In total, 201 young objects are classified toward S242 from this study. The membership status of the young sources is further windowed with the inclusion of parallax from the $Gaia$ DR2 catalog. Using the optical and infrared color-magnitude diagrams, the young stellar objects are characterized with an average age of $\sim$ 1 Myr and the masses in the range 0.1$-$3.0 $M_\odot$. The census of the stellar content within the region is discussed using combined photometric and spectroscopic data.

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