arXiv subjects

Suman Bhattacharyya

Publications and source records attributed to Suman Bhattacharyya.

16 recordsLinked to original sources

arXiv2026

A multi-epoch spectral and photometric study to understand the nature of the interesting Be star candidate HD 249179

HD 249179, a B5 star with unclear classification as either a classical Be star or a high-mass X-ray binary (HMXB) system, is investigated using the first multi-epoch spectral and photometric analysis over 5.5 years (2017-2023). Optical spectroscopy from LAMOST, BeSS, and the Himalayan Chandra Telescope show large H-alpha variability (-17.6 to -3.2 angstrom, in six days, returning to -31.3 angstrom, in 2022) and the presence of Paschen and O I lines, confirming active circumstellar disc dynamics. H-alpha double peak profiles show weak V/R ratio variations 0.92--1.28 indicating one-armed density waves. TESS observations presents hybrid beta Cephei/slowly pulsating B-type variability with primary frequencies of 3.5 c/d and 1.5-1.6 c/d, and an outburst-like brightening with phase-dependent frequency evolution. Simultaneous anti-correlation between optical brightness and H$α$ equivalent width suggest moderate disc inclination. Gaia DR3 CMD analysis rules out HD 249179 as a BeXRB but instead places it in the classical Be-star region. The BeppoSAX survey, despite previous X-ray cataloging, found no emission above the sensitivity limits. Our analysis shows that HD 249179 is most likely a classical Be star with variable circumstellar disc activity and hybrid pulsations, and that there is no strong evidence for an HMXB classification.

astro-ph.SR↗

arXiv2025

Analysis of Forbidden Neon Emission lines in HAeBe Stars using Spitzer IRS spectra

We analyzed high-resolution mid-infrared spectra of 78 well-known Herbig Ae/Be (HAeBe) stars using Spitzer InfraRed Spectrograph data, focusing on the detection of [Ne {\sc II}] and [Ne {\sc III}] emission lines as indicators of ionized outflows or disk winds. Emission from [Ne {\sc II}] at 12.81 $μ$m or [Ne {\sc III}] at 15.55 $μ$m was identified in 25 sources, constituting the largest sample of HAeBe stars with these detected lines. Our analysis revealed a higher detection frequency of [Ne {\sc II}] in sources with lower relative accretion luminosity (L$_{acc}$/L$_*$ $<$ 0.1), suggesting a connection to the disk dispersal phase. We examined correlations between neon lines and various spectral features and investigated [Ne {\sc III}]-to-[Ne {\sc II}] line flux ratios to explore potential emission mechanisms. Neon emission is predominantly observed in Group I sources (75\%), where their flared disk geometry likely contributes to the observed emission, potentially originating from the irradiated disk atmosphere. Interestingly, we also find that Group II sources exhibit a higher median relative [Ne\,\textsc{ii}] line luminosity (L$_\mathrm{[Ne\,II]}$/L$_*$), suggesting enhanced photoevaporation rates possibly associated with their more settled disk structures. However, larger samples and higher-resolution spectra are required to confirm this trend definitively. The high detection rate of the [Fe {\sc II}] and [S {\sc III}] lines, commonly associated with EUV-dominated regions, alongside a [Ne {\sc III}]-to-[Ne {\sc II}] emission ratio greater than 0.1 in sources where both lines detected, suggests that EUV radiation is the primary driver of neon emission in our sample.

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arXiv2024

Spectroscopic study of late-type emission-line stars using the data from LAMOST DR6

Low-mass emission-line stars belong to various evolutionary stages, from pre-main-sequence young stars to evolved stars. In this work, we present a catalog of late-type (F0 to M9) emission-line stars from the LAMOST Data Release 6. Using the scipy package, we created a Python code that finds the emission peak at H-alpha in all late-type stellar spectra. A dataset of 38,152 late-type emission-line stars was obtained after a rigorous examination of the photometric quality flags and the signal-to-noise ratio of the spectra. Adopting well-known photometric and spectroscopic methods, we classified our sample into 438 infrared excess sources, 4,669 post-main-sequence candidates, 9,718 Fe/Ge/Ke sources, and 23,264 dMe sources. From a cross-match with known databases, we found that 29,222 sources, comprising 65 IR excess sources, 7,899 Fe/Ge/Ke stars, 17,533 dMe stars, and 3,725 PtMS candidates, are new detections. We measured the equivalent width of the major emission lines observed in the spectra of our sample of emission-line stars. Furthermore, the trend observed in the line strengths of major emission lines over the entire late-type spectral range is analyzed. We further classified the sample into 4 groups based on the presence of Hydrogen and Calcium emission lines. This work presents a large dataset of late-type emission-line stars, which can be used to study active phenomena in late-type stars.

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arXiv2024

Serendipitous detection of an intense X-ray flare in the weak-line T Tauri star KM Ori with SRG/eROSITA

Weak-line T Tauri stars (WTTS) exhibit X-ray flares, likely resulting from magnetic reconnection that heats the stellar plasma to very high temperatures. These flares are difficult to identify through targeted observations. Here, we report the serendipitous detection of the brightest X-ray flaring state of KM Ori in the eROSITA DR1 survey. Observations from SRG/eROSITA, Chandra X-ray Observatory, and XMM-Newton are analysed to assess the X-ray properties of KM Ori, thereby establishing its flaring state at the eROSITA epoch. The long-term (1999-2020) X-ray light curve generated for the Chandra observations confirmed that eROSITA captured the source at its highest X-ray flaring state recorded to date. Multi-instrument observations support the X-ray flaring state of the source, with time-averaged X-ray luminosity ($L_{0.2-5keV}$) reaching $\sim 1.9\times10^{32}{erg~s^{-1}}$ at the eROSITA epoch, marking it the brightest and possibly the longest flare observed to date. Such intense X-ray flares have been detected only in a few WTTS. The X-ray spectral analysis unveils the presence of multiple thermal plasma components at all epochs. The notably high luminosity ($L_{0.5-8~keV}\sim10^{32}{erg~s}^{-1}$), energy ($E_{0.5-8~keV}\sim10^{37}$erg), and the elevated emission measures of the thermal components in the eROSITA epoch indicate a superflare/megaflare state of KM Ori. Additionally, the H$α$ line equivalent width of $\sim$-5Åfrom our optical spectral analysis, combined with the lack of infrared excess in the spectral energy distribution, were used to re-confirm the WTTS (thin disk/disk-less) classification of the source. The long-duration flare of KM Ori observed by eROSITA indicates the possibility of a slow-rise top-flat flare. The detection demonstrates the potential of eROSITA to uncover such rare, transient events, thereby providing new insights into the X-ray activity of WTTS.

astro-ph.HE↗

arXiv2024

Detection of a new sample of Galactic White Dwarfs in the direction of the Small Magellanic Cloud

In this study, we demonstrate the efficacy of the Ultraviolet Imaging Telescope (UVIT) in identifying and characterizing white dwarfs (WDs) within the Milky Way Galaxy. Leveraging the UVIT point source catalogue towards SMC and crossmatching with Gaia DR3 data, we identified 43 single WDs (37 new detections), 13 new WD+main sequence (MS) candidates and 161 UV bright MS stars by analysing their Spectral Energy Distributions (SED). Using the WD evolutionary models, we determine the masses, effective temperatures, and cooling ages of these identified WDs. Masses of these WDs range from 0.2 to 1.3 M$_{\odot}$ and effective temperatures (T$_{eff}$) between 10000 K to 15000 K, with cooling ages spanning 0.1 to 2 Gyr. Notably, we detect hotter WDs compared to literature values, which is attributed to the sensitivity of UVIT. Furthermore, we report the detection of 20 new extremely low-mass (ELM) candidates from our analysis. Future spectroscopic studies of the ELM candidates will help us understand the formation scenarios of these exotic objects. Despite limitations in Gaia DR3 distance measurements for optically faint WDs, we provide a crude estimate of the WD space density within 1kpc as 1.3 $\times$ 10$^{-3}$ pc$^{-3}$, which is higher than previous estimates in the literature. Our results underscore the instrumental capabilities of UVIT and anticipate the forthcoming UV missions like INSIST for systematic WD discovery. Our methodology sets a precedent for future analyses in other UVIT fields to find more WDs and perform spectroscopic studies to verify their candidacy.

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arXiv2024

Insights on the Optical and Infrared Nature of MAXI J0709-159: Implications for High-Mass X-ray Binaries

In our previous study (Bhattacharyya et al., 2022), HD~54786, the optical counterpart of the MAXI J0709-159 system, was identified to be an evolved star, departing from the main sequence, based on comparisons with non-X-ray binary systems. In this paper, using color-magnitude diagram (CMD) analysis for High-Mass X-ray Binaries (HMXBs) and statistical t-tests, we found evidence supporting HD 54786's potential membership in both Be/X-ray binaries (BeXRBs) and supergaint X-ray binaries (SgXBs) populations of HMXBs. Hence, our study points towards dual optical characteristics of HD~54786, as an X-ray binary star and also belonging to a distinct evolutionary phase from BeXRB towards SgXB. Our further analysis suggests that MAXI J0709-159, associated with HD 54786, exhibits low-level activity during the current epoch and possesses a limited amount of circumstellar material. Although similarities with the previously studied BeXRB system LSI +61$^{\circ}$ 235 (Coe et al., 1994) are noted, continued monitoring and data collection are essential to fully comprehend the complexities of MAXI J0709-159 and its evolutionary trajectory within the realm of HMXBs.

astro-ph.SR↗

arXiv2024

Study of the Balmer decrements for Galactic classical Be stars using the Himalayan Chandra Telescope of India

In a recent study, Banerjee et al. (2021) produced an atlas of all major emission lines found in a large sample of 115 Galactic field Be stars using the 2-m Himalayan Chandra Telescope (HCT) facility located at Ladakh, India. This paper presents our further exploration of these stars to estimate the electron density in their discs. Our study using Balmer decrement values indicate that their discs are generally optically thick in nature with electron density (n_e) in their circumstellar envelopes (CEs) being in excess of 10^13 cm^-3 for around 65% of the stars. For another 19% stars, the average n_e in their discs probably range between 10^12 cm^-3 and 10^13 cm^-3. We noticed that the nature of the Hα and H\b{eta} line profiles might not influence the observed Balmer decrement values (i.e. D_34 and D_54) of the sample of stars. Interestingly, we also found that around 50% of the Be stars displaying D_34 greater than 2.7 are of earlier spectral types, i.e. within B0 -B3.

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arXiv2024

Optical Spectroscopy of Classical Be Stars in Old Open Clusters

We performed the optical spectroscopy of 16 classical Be stars in 11 open clusters older than 100 Myr. Ours is the first spectroscopic study of classical Be stars in open clusters older than 100 Myr. We found that the H alpha emission strength of most of the stars is less than 40 Angstrom, in agreement with previous studies. Our analysis further suggests that one of the stars, KW97 35 12, might be a weak H alpha emitter in nature, showing H alpha equivalent width of negative 0.5 Angstrom. Interestingly, we also found that the newly detected classical Be star LS III 47 37b might be a component of the possible visual binary system LS III 47 37, where the other companion is also a classical Be star. Hence, the present study indicates the possible detection of a binary Be system. Moreover, it is observed that all 16 stars exhibit a lesser number of emission lines compared to classical Be stars younger than 100 Myr. Furthermore, the spectral type distribution analysis of B type and classical Be stars for the selected clusters points out that the existence of CBe stars can depend on the spectral type distribution of B type stars present in these clusters.

astro-ph.SR↗

arXiv2023

New insights in the bubble wall of NGC 3324: intertwined sub-structures and a bipolar morphology uncovered by JWST

We report the discovery of intertwined/entangled sub-structures towards the bubble wall of NGC 3324 below a physical scale of 4500 AU, which is the sharp edge/ionization front/elongated structure traced at the interface between the HII region and the molecular cloud. The sharp edge appears wavy in the Spitzer 3.6-8.0 $μ$m images (resolution $\sim$2$''$). Star formation signatures have mostly been traced on one side of the ionization front, which lies on the molecular cloud's boundary. The James Webb Space Telescope's (JWST) near- and mid-infrared images (resolution $\sim$0.07$''$-0.7$''$) are employed to resolve the sharp edge, which has a curvature facing the exciting O-type stars. The elongated structures are associated with the 3.3 $μ$m polycyclic aromatic hydrocarbon (PAH) emission, the 4.05 $μ$m ionized emission, and the 4.693 $μ$m H$_{2}$ emission. However, the PAH-emitting structures are depicted between the other two. The H$_{2}$ emission reveals numerous intertwined sub-structures which are not prominently traced in the 3.3 $μ$m PAH emission. The separation between two sub-structures in the H$_{2}$ emission is $\sim$1.1$''$ or 2420 AU. The intertwined sub-structures are traced in the spatial areas associated with the neutral to H$_{2}$ transition zone, suggesting the origin of these structures by ``thin-shell'' instability. Furthermore, an arc-like feature traced in the Spitzer 3.6-8.0 $μ$m images is investigated as a bipolar HII region (extent $\sim$0.35 pc) at T$_\mathrm{d}$ $\sim$25-28~K using the JWST images. A massive star candidate VPHAS-OB1 #03518 seems to be responsible for the bipolar HII region.

astro-ph.GA↗

arXiv2023

Estimation of Stellar Parameters and Mass Accretion Rate of Classical T Tauri Stars from LAMOST DR6

Classical T Tauri stars are low-mass pre-main sequence stars with an active circumstellar environment. In this work we present the identification and study of 260 Classical T Tauri stars using LAMOST Data Release 6, among which 104 stars are newly identified. We distinguish Classical T Tauri stars from Giants and main-sequence dwarfs based on the log g values and the presence of H (alpha) emission line and infrared excess that arises from the circumstellar accretion disk. We estimated the mass and age of 210 stars using the Gaia color-magnitude diagram. The age is from 0.1 to 20 Myr, where 90% of the stars have age below 10 Myr and the mass ranges between 0.11 to 1.9 M(solar). From the measured H(alpha) equivalent widths, we homogeneously estimated the mass accretion rates for 172 stars, with most values ranging from 10^-7 to 10^-10 M(solar) yr^-1. The mass accretion rates are found to follow a power law distribution with the mass of the star, having a relation of the form Macc proportional to M(star)^1.43 +/- 0.26, in agreement with previous studies.

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arXiv2022

Study of the transient nature of classical Be stars using multi-epoch optical spectroscopy

Variability is a commonly observed property of classical Be stars (CBe) stars. In extreme cases, complete disappearance of the Hα emission line occurs, indicating a disc-less state in CBe stars. The disc-loss and reappearing phases can be identified by studying the Hα line profiles of CBe stars on a regular basis. In this paper, we present the study of a set of selected 9 bright CBe stars, in the wavelength range of 6200 - 6700 Å, to better understand their disc transient nature through continuous monitoring of their Hα line profile variations for 5 consecutive years (2015 -- 2019). Based on our observations, we suggest that 4 of the program stars (HD 4180, HD 142926, HD 164447 and HD 171780) are possibly undergoing disc-loss episodes, whereas one other star (HD 23302) might be passing through disc formation phase. The remaining 4 stars (HD 237056, HD 33357, HD 38708 and HD 60855) have shown signs of hosting a stable disc in recent epochs. Through visual inspection of the overall variation observed in the Hα EW for these stars, we classified them into groups of growing, stable and dissipating discs, respectively. Moreover, our comparative analysis using the BeSS database points out that the star HD 60855 has passed through a disc-less episode in 2008, with its disc formation happening probably over a timescale of only 2 months, between January and March 2008.

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arXiv2022

Emission line star catalogues post-Gaia DR3: A validation of Gaia DR3 data using LAMOST OBA emission catalogue

Gaia DR3 and further releases have the potential to identify and categorise new emission-line stars in the Galaxy. We perform a comprehensive validation of astrophysical parameters from Gaia DR3 with the spectroscopically estimated emission-line star parameters from LAMOST OBA emission catalogue. We compare different astrophysical parameters provided by Gaia DR3 with those estimated using LAMOST spectra. By using a larger sample of emission-line stars, we perform a global polynomial and piece-wise linear fit to update the empirical relation to convert Gaia DR3 pseudo-equivalent width to observed equivalent width, after removing the weak emitters from the analysis. We find that the emission-line source classifications given by DR3 is in reasonable agreement with the classification from LAMOST OBA emission catalogue. The astrophysical parameters estimated by esphs module from Gaia DR3 provides a better estimate when compared to gspphot and gspspec. A second-degree polynomial relation is provided along with piece-wise linear fit parameters for the equivalent width conversion. We notice that the LAMOST stars with weak Hα emission are not identified to be in emission from BP/RP spectra. This suggests that emission-line sources identified by Gaia DR3 is incomplete. In addition, Gaia DR3 provides valuable information about the binary and variable nature of a sample of emission-line stars.

astro-ph.GA↗

arXiv2022

Decoding the X-ray flare from MAXI J0709-159 using optical spectroscopy and multi-epoch photometry

We present a follow-up study on the recent detection of two X-ray flaring events by MAXI/GSC observations in soft and hard X-rays from MAXI J0709-159 in the direction of HD 54786 (LY CMa), on 2022 January 25. The X-ray luminosity during the flare was around 10^(37) erg/s (MAXI), which got reduced to 10^(32) erg/s (NuSTAR) after the flare. We took low-resolution spectra of HD 54786 from HCT and VBT facilities in India, on 2022 February 1 and 2. In addition to H-alpha emission, we found emission lines of He I in the optical spectrum of this star. By comparing our spectrum of the object with those from literature we found that He I lines show variability. Using photometric study we estimate that the star is having effective temperature of 20000 K. Although HD 54786 is reported as a supergiant in previous studies, our analysis favours it to be evolving off the main sequence in the Color-Magnitude Diagram. We could not detect any infrared excess, ruling out the possibility of IR emission from a dusty circumstellar disc. Our present study suggests that HD 54786 is a Be/X-ray binary system with a compact object companion, possibly a neutron star.

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arXiv2021

Identification of emission-line stars in transition phase from pre-main sequence to main sequence

Pre-main sequence (PMS) stars evolve into main sequence (MS) phase over a period of time. Interestingly, we found a scarcity of studies in existing literature that examines and attempts to better understand the stars in PMS to MS transition phase. The purpose of the present study is to detect such rare stars, which we named as 'Transition Phase' (TP) candidates - stars evolving from the PMS to the MS phase. We identified 98 TP candidates using photometric analysis of a sample of 2167 classical Be (CBe) and 225 Herbig Ae/Be (HAeBe) stars. This identification is done by analyzing the near- and mid-infrared excess and their location in the optical color-magnitude diagram. The age and mass of 58 of these TP candidates are determined to be between 0.1-5 Myr and 2-10.5 M$_\odot$, respectively. The TP candidates are found to possess rotational velocity and color excess values in between CBe and HAeBe stars, which is reconfirmed by generating a set of synthetic samples using the machine learning approach.

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arXiv2020

Identification of new Classical Ae stars in the Galaxy using LAMOST DR5

We report the first systematic study to identify and characterize a sample of classical Ae stars in the Galaxy. The spectra of these stars were retrieved from the A-star catalog using the Large sky Area Multi-Object fiber Spectroscopic Telescope (LAMOST) survey. We identified the emission-line stars in this catalog from which 159 are confirmed as classical Ae stars. This increases the sample of known classical Ae stars by about nine times from the previously identified 21 stars. The evolutionary phase of classical Ae stars in this study is confirmed from the relatively small mid- and far-infrared excess and from their location in the optical color-magnitude diagram. We estimated the spectral type using MILES spectral templates and identified Classical Ae stars beyond A3, for the first time. The prominent emission lines in the spectra within the wavelength range 3700 -- 9000 Å are identified and compared with the features present in classical Be stars. The Hα emission strength of the stars in our sample show a steady decrease from late-B type to Ae stars, suggesting that the disc size may be dependent on the spectral type. Interestingly, we noticed emission lines of Fe{\sc ii}, O{\sc i} and Paschen series in the spectrum of some classical Ae stars. These lines are supposed to fade out by late B-type and should not be present in Ae stars. Further studies, including spectra with better resolution, is needed to correlate these results with the rotation rates of classical Ae stars.

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arXiv2020

Optical spectroscopy of Galactic field Classical Be stars

In this study, we analyze the emission lines of different species present in 118 Galactic field classical Be stars in the wavelength range of 3800 - 9000 Å. We re-estimated the extinction parameter (A$_V$) for our sample stars using the newly available data from Gaia DR2 and suggest that it is important to consider A$_V$ while measuring the Balmer decrement (i.e. $D_{34}$ and $D_{54}$) values in classical Be stars. Subsequently, we estimated the Balmer decrement values for 105 program stars and found that $\approx$ 20\% of them show $D_{34}$ $\geq$ 2.7, implying that their circumstellar disc are generally optically thick in nature. One program star, HD 60855 shows H$α$ in absorption -- indicative of discless phase. From our analysis, we found that in classical Be stars, H$α$ emission equivalent width values are mostly lower than 40 Å, which agrees with that present in literature. Moreover, we noticed that a threshold value of $\sim$ 10 Å~of H$α$ emission equivalent width is necessary for Fe{\sc ii} emission to become visible. We also observed that emission line equivalent widths of H$α$, P14, Fe{\sc ii} 5169 and O{\sc i} 8446 Å~for our program stars tend to be more intense in earlier spectral types, peaking mostly near B1-B2. Furthermore, we explored various formation regions of Ca{\sc ii} emission lines around the circumstellar disc of classical Be stars. We suggest the possibility that Ca{\sc ii} triplet emission can originate either in the circumbinary disc or from the cooler outer regions of the disc, which might not be isothermal in nature.

astro-ph.SR↗