Blesson Mathew - Academia.edu (original) (raw)
Papers by Blesson Mathew
Research in Astronomy and Astrophysics
We performed the optical spectroscopy of 16 classical Be stars in 11 open clusters older than 100... more 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 Hα emission strength of most of the stars is less than 40 Å, in agreement with previous studies. Our analysis further suggests that one of the stars, [KW97] 35–12, might be a weak Hα emitter in nature, showing Hα equivalent width of −0.5 Å. 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 cl...
Monthly Notices of the Royal Astronomical Society
We study the formation and the kinematic evolution of the early-type Herbig Be star IL Cep and it... more We study the formation and the kinematic evolution of the early-type Herbig Be star IL Cep and its environment. The young star is a member of the Cep OB3 association, at a distance of 798 ± 9 pc, and has a ‘cavity’ associated with it. We found that the B0V star HD 216658, which is astrometrically associated with IL Cep, is at the centre of the cavity. From the evaluation of various pressure components created by HD 216658, it is established that the star is capable of creating the cavity. We identified 79 co-moving stars of IL Cep at 2-pc radius from the analysis of Gaia EDR3 astrometry. The transverse velocity analysis of the co-moving stars shows that they belong to two different populations associated with IL Cep and HD 216658, respectively. Further analysis confirms that all the stars in the IL Cep population are mostly coeval (∼0.1 Myr). Infrared photometry revealed that there are 26 Class II objects among the co-moving stars. The stars without circumstellar disc (Class III) ar...
Bulletin de la Société Royale des Sciences de Liège, 2019
Active galactic nuclei (AGN) are high luminosity sources powered by accretion of matter onto supe... more Active galactic nuclei (AGN) are high luminosity sources powered by accretion of matter onto super-massive black holes (SMBHs) located at the centres of galaxies. According to the Unification model of AGN, the SMBH is surrounded by a broad emission line region (BLR) and a dusty torus. It is difficult to study the extent of the dusty torus as the central region of AGN is not resolvable using any conventional imaging techniques available today. Though, current IR interferometric techniques could in principle resolve the torus in nearby AGN, it is very expensive and limited to few bright and nearby AGN. A more feasible alternative to the interferometric technique to find the extent of the dusty torus in AGN is the technique of reverberation mapping (RM). REMAP (REverbertion Mapping of AGN Program) is a long term photometric monitoring program being carried out using the 2 m Himalayan Chandra Telescope (HCT) operated by the Indian Institute of Astrophysics, Bangalore, aimed at measuring...
Research in Astronomy and Astrophysics, 2021
We present a catalog of 3339 hot emission-line stars (ELSs) identified from 451 695 O, B and A ty... more We present a catalog of 3339 hot emission-line stars (ELSs) identified from 451 695 O, B and A type spectra, provided by LAMOST Data Release 5 (DR5). We developed an automated Python routine that identified 5437 spectra having a peak between 6561 and 6568 Å. False detections and bad spectra were removed, leaving 4138 good emission-line spectra of 3339 unique ELSs. We re-estimated the spectral types of 3307 spectra as the LAMOST Stellar Parameter Pipeline (LASP) did not provide accurate spectral types for these emission-line spectra. As Herbig Ae/Be stars exhibit higher excess in near-infrared and mid-infrared wavelengths than classical Ae/Be stars, we relied on 2MASS and WISE photometry to distinguish them. Finally, we report 1089 classical Be, 233 classical Ae and 56 Herbig Ae/Be stars identified from LAMOST DR5. In addition, 928 B[em]/A[em] stars and 240 CAe/CBe potential candidates are identified. From our sample of 3339 hot ELSs, 2716 ELSs identified in this work do not have any...
The Astronomical Journal, 2020
Most (∼82%) of the over 4000 confirmed exoplanets known today orbit very close to their host star... more Most (∼82%) of the over 4000 confirmed exoplanets known today orbit very close to their host stars, within 0.5 au. Planets at such small orbital distances can result in significant interactions with their host stars, which can induce increased activity levels in them. In this work, we have searched for statistical evidence for Star-Planet Interactions (SPI) in the ultraviolet (UV) using the largest sample of 1355 GALEX detected host stars with confirmed exoplanets and making use of the improved host star parameters from Gaia DR2. From our analysis, we do not find any significant correlation between the UV activity of the host stars and their planetary properties. We further compared the UV properties of planet host stars to that of chromospherically active stars from the RAVE survey. Our results indicate that the enhancement in chromospheric activity of host stars due to star-planet interactions may not be significant enough to reflect in their near and far UV broad band flux.
Journal of Astrophysics and Astronomy, 2021
We performed the slitless spectroscopic survey to identify Classical (CBe) stars in open clusters... more We performed the slitless spectroscopic survey to identify Classical (CBe) stars in open clusters older than 100 Myr. Observing a sample of 71 open clusters, we identified 13 CBe stars in 11 open clusters, one of, which (TYC 2679-432-1) is a new detection. The 13 CBe stars show both Hα in emission and IR excess, which confirm that they possess gaseous circumstellar discs. Two more CBe stars are found to exhibit Hα in absorption for the first time, indicating that might be passing through disc-less episode presently. The spectral type estimation is done for all these 15 stars and it is noticed that they belong to B0.5-B8 type. Moreover, we found that the distribution of our sample is peaking near late B-types as expected.
Monthly Notices of the Royal Astronomical Society, 2020
ABSTRACTWe report results of the dust reverberation mapping (DRM) on the Seyfert 1 galaxy Z229–15... more ABSTRACTWe report results of the dust reverberation mapping (DRM) on the Seyfert 1 galaxy Z229–15 at z = 0.0273. Quasi-simultaneous photometric observations for a total of 48 epochs were acquired during the period 2017 July to 2018 December in B, V, J, H and Ks bands. The calculated spectral index (α) between B and V bands for each epoch was used to correct for the accretion disc (AD) component present in the infrared light curves. The observed α ranges between −0.99 and 1.03. Using cross-correlation function analysis we found significant time delays between the optical V and the AD corrected J, H and Ks light curves. The lags in the rest frame of the source are 12.52+10.00−9.5512.52^{+10.00}_{-9.55}12.52+10.00−9.55 d (between V and J), 15.63+5.05−5.1115.63^{+5.05}_{-5.11}15.63+5.05−5.11 d (between V and H) and 20.36+5.82−5.6820.36^{+5.82}_{-5.68}20.36+5.82−5.68 d (between V and Ks). Given the large error bars, these lags are consistent with each other. However, considering the lag between V and Ks bands to represent the inner edge of the dust torus, the torus in Z22...
Monthly Notices of the Royal Astronomical Society, 2021
Pre-main-sequence (PMS) stars evolve into main-sequence (MS) phase over a period of time. Interes... more 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 examine and attempt to better understand the stars in PMS to MS transition phase. The purpose of this 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 analysing the near- and mid-infrared excess and their location in the optical colour–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⊙, respectively. The TP candidates are found to possess rotational velocity and colour excess values in between CBe and HAeBe stars, which is reconfirmed by generating a set of synthetic samples using the machine learning approach.
Monthly Notices of the Royal Astronomical Society, 2020
The intermediate-mass Herbig Ae star V1787 Ori is a member of the L1641 star-forming region in th... more The intermediate-mass Herbig Ae star V1787 Ori is a member of the L1641 star-forming region in the Orion A molecular cloud. We report the detection of an M-type companion to V1787 Ori at a projected separation of 6.66 arcsec (corresponding to 2577 au), from the analysis of VLT/NACO adaptive optics Ks-band image. Using astrometric data from Gaia DR2, we show that V1787 Ori A and B share similar distance (d ∼ 387 pc) and proper motion, indicating that they are physically associated. We estimate the spectral type of V1787 Ori B to be M5 ± 2 from colour–spectral type calibration tables and template matching using SpeX spectral library. By fitting PARSEC models in the Pan-STARRS colour–magnitude diagram, we find that V1787 Ori B has an age of 8.1$^{+1.7}_{-1.5}$ Myr and a mass of 0.39$^{+0.02}_{-0.05}$ M⊙. We show that V1787 Ori is a pre-main-sequence wide binary system with a mass ratio of 0.23. Such a low-mass ratio system is rarely identified in Herbig Ae/Be binary systems. We conclud...
Journal of Astrophysics and Astronomy, 2020
Quasars are ideal targets to use for astrometric calibration of large scale astronomical surveys ... more Quasars are ideal targets to use for astrometric calibration of large scale astronomical surveys as they have negligible proper motion and parallax. The forthcoming 4-m International Liquid Mirror Telescope (ILMT) will survey the sky that covers a width of about 27'. To carry out astrometric calibration of the ILMT observations, we aimed to compile a list of quasars with accurate equatorial coordinates and falling in the ILMT stripe. Towards this, we cross-correlated all the quasars that are known till the present date with the sources in the Gaia-DR2 catalogue, as the Gaia-DR2 sources have position uncertainties as small as a few milli arcsec (mas). We present here the results of this cross-correlation which is a catalogue of 6738 quasars that is suitable for astrometric calibration of the ILMT fields. In this work, we present this quasar catalogue. This catalogue of quasars can also be used to study quasar variability over diverse time scales when the ILMT starts its observations. While preparing this catalogue, we also confirmed that quasars in the ILMT stripe have proper motion and parallax lesser than 20 mas yr −1 and 10 mas, respectively.
The Astrophysical Journal, 2018
We have investigated the role of a few prominent excitation mechanisms viz. collisional excitatio... more We have investigated the role of a few prominent excitation mechanisms viz. collisional excitation, recombination, continuum fluorescence, and Lyman beta fluorescence on the O Iline spectra in Herbig Ae/Be stars. The aim is to understand which of them is the central mechanism that explains the observed O Iline strengths. The study is based on an analysis of the observed optical spectra of 62 Herbig Ae/Be stars and near-infrared spectra of 17 Herbig Ae/Be stars. The strong correlation observed between the line fluxes of O Iλ8446 and O Iλ11287, as well as a high positive correlation between the line strengths of O Iλ8446 and Hα suggest that Lyman beta fluorescence is the dominant excitation mechanism for the formation of O Iemission lines in Herbig Ae/Be stars. Furthermore, from an analysis of the emission line fluxes of O Iλλ7774, 8446, and comparing the line ratios with those predicted by theoretical models, we assessed the contribution of collisional excitation in the formation of O Iemission lines.
New Astronomy, 2017
h i g h l i g h t s • We studied the NIR properties of Be star candidates in clusters in the LMC ... more h i g h l i g h t s • We studied the NIR properties of Be star candidates in clusters in the LMC and SMC. • New diagnostic area is suggested in the CCD for the Classical Be stars in the MCs.
We present preliminary results of the spectroscopic study of two Be Stars, 4 Her and 88 Her. The ... more We present preliminary results of the spectroscopic study of two Be Stars, 4 Her and 88 Her. The time series spectra of these Be stars were obtained using the 1.02m reflector of Vainu Bappu Observatory, Kavalur, India. We obtained a total of 41 spectra for 4 Her and 32 spectra for 88 Her during February to July 2009, with 1Å resolution at Hα. The rotational velocity v sini of the two stars were measured using HeI absorption lines (4026Å and 4471Å). The stars exhibit double peak Hα profile and their V/R variation is studied. Both the stars exhibit spectroscopic variations in the Hα emission line. We discuss their circumstellar environments based on the V/R variability. General Background on Classical BeStars • Decretion disk around the central Btype star is the source of emission lines seen in the spectra • Most common emission lines: Hα, Hβ, singlyionized metals like FeII and neutral Helium i.e., HeI • Types based on their emission line profiles: Poleon (Single peaked), Ordinary...
Research in Astronomy and Astrophysics, 2014
ABSTRACT Classical Be stars (CBe) are rapidly rotating (V_{{sin}i} ˜ 300 km s^{-1}) B-type stars ... more ABSTRACT Classical Be stars (CBe) are rapidly rotating (V_{{sin}i} ˜ 300 km s^{-1}) B-type stars with equatorial circumstellar decretion disks, the production of which is under debate. The Be phenomenon can be understood through the analysis of hydrogen recombination lines in the infrared spectral region. J-band spectroscopy of Be stars is one of the less studied fields in Be star research. We took J-band (11000 - 14000 Å) spectra of 34 CBe stars using the NICMOS instrument available with 1.2 m Mt. Abu infrared observatory. The spectra are interesting due to the presence of neutral oxygen (O I) lines, 11287 and 13165 Å, in addition to Paschen beta and Paschen gamma lines of hydrogen. The optical depth effects in the observed emission lines from the circumstellar disk have been deciphered through the line flux ratio analysis of the Paschen lines. The analysis of the O I lines has been used to understand the role of Lyman-beta fluorescence in the formation of these lines. Also, we used the spectra taken at different epochs separated over 1 year to study the variability in CBe stars.
Proceedings of the International Astronomical Union, 2010
Mennickent et al. (2002) presented a catalogue of 1056 Be star candidates in the Small Magellanic... more Mennickent et al. (2002) presented a catalogue of 1056 Be star candidates in the Small Magellanic cloud (SMC) by studying light curve variation using OGLE II data base. They classified these Be star candidates of the SMC in four categories: Type 1 stars showing outbursts (139 stars); Type 2 stars showing sudden luminosity jumps (154 stars); Type 3 stars showing periodic or near periodic variations (78 stars); Type 4 stars showing light curves similar to Galactic Be stars (658 stars). They suggested that Type 4 could be Be stars. On the other hand, they suggested that Type-3 stars may not be linked to the Be star phenomenon at all.
The Astrophysical Journal, 2012
The possibility of the Lyβ fluorescence mechanism being operational in classical Be stars and the... more The possibility of the Lyβ fluorescence mechanism being operational in classical Be stars and thereby contributing to the strength of the Oi 8446Å line has been recognized for long. However this supposition needs to be quantified by comparing observed and predicted Oi line ratios. In the present work, optical and near-infrared spectra of classical Be stars are presented. We analyse the observed strengths of the Oi 7774, 8446, 11287 and 13165Å lines which have been theoretically proposed as diagnostics for identifying the excitation mechanism. We have considered and examined the effects of Lyβ fluorescence, collisional excitation, recombination and continuum fluorescence on these Oi line strengths. From our analysis it appears that the Lyβ fluorescence process is indeed operative in Be stars.
The Astronomical Journal, 2013
Research in Astronomy and Astrophysics, 2012
Research in Astronomy and Astrophysics
We performed the optical spectroscopy of 16 classical Be stars in 11 open clusters older than 100... more 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 Hα emission strength of most of the stars is less than 40 Å, in agreement with previous studies. Our analysis further suggests that one of the stars, [KW97] 35–12, might be a weak Hα emitter in nature, showing Hα equivalent width of −0.5 Å. 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 cl...
Monthly Notices of the Royal Astronomical Society
We study the formation and the kinematic evolution of the early-type Herbig Be star IL Cep and it... more We study the formation and the kinematic evolution of the early-type Herbig Be star IL Cep and its environment. The young star is a member of the Cep OB3 association, at a distance of 798 ± 9 pc, and has a ‘cavity’ associated with it. We found that the B0V star HD 216658, which is astrometrically associated with IL Cep, is at the centre of the cavity. From the evaluation of various pressure components created by HD 216658, it is established that the star is capable of creating the cavity. We identified 79 co-moving stars of IL Cep at 2-pc radius from the analysis of Gaia EDR3 astrometry. The transverse velocity analysis of the co-moving stars shows that they belong to two different populations associated with IL Cep and HD 216658, respectively. Further analysis confirms that all the stars in the IL Cep population are mostly coeval (∼0.1 Myr). Infrared photometry revealed that there are 26 Class II objects among the co-moving stars. The stars without circumstellar disc (Class III) ar...
Bulletin de la Société Royale des Sciences de Liège, 2019
Active galactic nuclei (AGN) are high luminosity sources powered by accretion of matter onto supe... more Active galactic nuclei (AGN) are high luminosity sources powered by accretion of matter onto super-massive black holes (SMBHs) located at the centres of galaxies. According to the Unification model of AGN, the SMBH is surrounded by a broad emission line region (BLR) and a dusty torus. It is difficult to study the extent of the dusty torus as the central region of AGN is not resolvable using any conventional imaging techniques available today. Though, current IR interferometric techniques could in principle resolve the torus in nearby AGN, it is very expensive and limited to few bright and nearby AGN. A more feasible alternative to the interferometric technique to find the extent of the dusty torus in AGN is the technique of reverberation mapping (RM). REMAP (REverbertion Mapping of AGN Program) is a long term photometric monitoring program being carried out using the 2 m Himalayan Chandra Telescope (HCT) operated by the Indian Institute of Astrophysics, Bangalore, aimed at measuring...
Research in Astronomy and Astrophysics, 2021
We present a catalog of 3339 hot emission-line stars (ELSs) identified from 451 695 O, B and A ty... more We present a catalog of 3339 hot emission-line stars (ELSs) identified from 451 695 O, B and A type spectra, provided by LAMOST Data Release 5 (DR5). We developed an automated Python routine that identified 5437 spectra having a peak between 6561 and 6568 Å. False detections and bad spectra were removed, leaving 4138 good emission-line spectra of 3339 unique ELSs. We re-estimated the spectral types of 3307 spectra as the LAMOST Stellar Parameter Pipeline (LASP) did not provide accurate spectral types for these emission-line spectra. As Herbig Ae/Be stars exhibit higher excess in near-infrared and mid-infrared wavelengths than classical Ae/Be stars, we relied on 2MASS and WISE photometry to distinguish them. Finally, we report 1089 classical Be, 233 classical Ae and 56 Herbig Ae/Be stars identified from LAMOST DR5. In addition, 928 B[em]/A[em] stars and 240 CAe/CBe potential candidates are identified. From our sample of 3339 hot ELSs, 2716 ELSs identified in this work do not have any...
The Astronomical Journal, 2020
Most (∼82%) of the over 4000 confirmed exoplanets known today orbit very close to their host star... more Most (∼82%) of the over 4000 confirmed exoplanets known today orbit very close to their host stars, within 0.5 au. Planets at such small orbital distances can result in significant interactions with their host stars, which can induce increased activity levels in them. In this work, we have searched for statistical evidence for Star-Planet Interactions (SPI) in the ultraviolet (UV) using the largest sample of 1355 GALEX detected host stars with confirmed exoplanets and making use of the improved host star parameters from Gaia DR2. From our analysis, we do not find any significant correlation between the UV activity of the host stars and their planetary properties. We further compared the UV properties of planet host stars to that of chromospherically active stars from the RAVE survey. Our results indicate that the enhancement in chromospheric activity of host stars due to star-planet interactions may not be significant enough to reflect in their near and far UV broad band flux.
Journal of Astrophysics and Astronomy, 2021
We performed the slitless spectroscopic survey to identify Classical (CBe) stars in open clusters... more We performed the slitless spectroscopic survey to identify Classical (CBe) stars in open clusters older than 100 Myr. Observing a sample of 71 open clusters, we identified 13 CBe stars in 11 open clusters, one of, which (TYC 2679-432-1) is a new detection. The 13 CBe stars show both Hα in emission and IR excess, which confirm that they possess gaseous circumstellar discs. Two more CBe stars are found to exhibit Hα in absorption for the first time, indicating that might be passing through disc-less episode presently. The spectral type estimation is done for all these 15 stars and it is noticed that they belong to B0.5-B8 type. Moreover, we found that the distribution of our sample is peaking near late B-types as expected.
Monthly Notices of the Royal Astronomical Society, 2020
ABSTRACTWe report results of the dust reverberation mapping (DRM) on the Seyfert 1 galaxy Z229–15... more ABSTRACTWe report results of the dust reverberation mapping (DRM) on the Seyfert 1 galaxy Z229–15 at z = 0.0273. Quasi-simultaneous photometric observations for a total of 48 epochs were acquired during the period 2017 July to 2018 December in B, V, J, H and Ks bands. The calculated spectral index (α) between B and V bands for each epoch was used to correct for the accretion disc (AD) component present in the infrared light curves. The observed α ranges between −0.99 and 1.03. Using cross-correlation function analysis we found significant time delays between the optical V and the AD corrected J, H and Ks light curves. The lags in the rest frame of the source are 12.52+10.00−9.5512.52^{+10.00}_{-9.55}12.52+10.00−9.55 d (between V and J), 15.63+5.05−5.1115.63^{+5.05}_{-5.11}15.63+5.05−5.11 d (between V and H) and 20.36+5.82−5.6820.36^{+5.82}_{-5.68}20.36+5.82−5.68 d (between V and Ks). Given the large error bars, these lags are consistent with each other. However, considering the lag between V and Ks bands to represent the inner edge of the dust torus, the torus in Z22...
Monthly Notices of the Royal Astronomical Society, 2021
Pre-main-sequence (PMS) stars evolve into main-sequence (MS) phase over a period of time. Interes... more 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 examine and attempt to better understand the stars in PMS to MS transition phase. The purpose of this 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 analysing the near- and mid-infrared excess and their location in the optical colour–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⊙, respectively. The TP candidates are found to possess rotational velocity and colour excess values in between CBe and HAeBe stars, which is reconfirmed by generating a set of synthetic samples using the machine learning approach.
Monthly Notices of the Royal Astronomical Society, 2020
The intermediate-mass Herbig Ae star V1787 Ori is a member of the L1641 star-forming region in th... more The intermediate-mass Herbig Ae star V1787 Ori is a member of the L1641 star-forming region in the Orion A molecular cloud. We report the detection of an M-type companion to V1787 Ori at a projected separation of 6.66 arcsec (corresponding to 2577 au), from the analysis of VLT/NACO adaptive optics Ks-band image. Using astrometric data from Gaia DR2, we show that V1787 Ori A and B share similar distance (d ∼ 387 pc) and proper motion, indicating that they are physically associated. We estimate the spectral type of V1787 Ori B to be M5 ± 2 from colour–spectral type calibration tables and template matching using SpeX spectral library. By fitting PARSEC models in the Pan-STARRS colour–magnitude diagram, we find that V1787 Ori B has an age of 8.1$^{+1.7}_{-1.5}$ Myr and a mass of 0.39$^{+0.02}_{-0.05}$ M⊙. We show that V1787 Ori is a pre-main-sequence wide binary system with a mass ratio of 0.23. Such a low-mass ratio system is rarely identified in Herbig Ae/Be binary systems. We conclud...
Journal of Astrophysics and Astronomy, 2020
Quasars are ideal targets to use for astrometric calibration of large scale astronomical surveys ... more Quasars are ideal targets to use for astrometric calibration of large scale astronomical surveys as they have negligible proper motion and parallax. The forthcoming 4-m International Liquid Mirror Telescope (ILMT) will survey the sky that covers a width of about 27'. To carry out astrometric calibration of the ILMT observations, we aimed to compile a list of quasars with accurate equatorial coordinates and falling in the ILMT stripe. Towards this, we cross-correlated all the quasars that are known till the present date with the sources in the Gaia-DR2 catalogue, as the Gaia-DR2 sources have position uncertainties as small as a few milli arcsec (mas). We present here the results of this cross-correlation which is a catalogue of 6738 quasars that is suitable for astrometric calibration of the ILMT fields. In this work, we present this quasar catalogue. This catalogue of quasars can also be used to study quasar variability over diverse time scales when the ILMT starts its observations. While preparing this catalogue, we also confirmed that quasars in the ILMT stripe have proper motion and parallax lesser than 20 mas yr −1 and 10 mas, respectively.
The Astrophysical Journal, 2018
We have investigated the role of a few prominent excitation mechanisms viz. collisional excitatio... more We have investigated the role of a few prominent excitation mechanisms viz. collisional excitation, recombination, continuum fluorescence, and Lyman beta fluorescence on the O Iline spectra in Herbig Ae/Be stars. The aim is to understand which of them is the central mechanism that explains the observed O Iline strengths. The study is based on an analysis of the observed optical spectra of 62 Herbig Ae/Be stars and near-infrared spectra of 17 Herbig Ae/Be stars. The strong correlation observed between the line fluxes of O Iλ8446 and O Iλ11287, as well as a high positive correlation between the line strengths of O Iλ8446 and Hα suggest that Lyman beta fluorescence is the dominant excitation mechanism for the formation of O Iemission lines in Herbig Ae/Be stars. Furthermore, from an analysis of the emission line fluxes of O Iλλ7774, 8446, and comparing the line ratios with those predicted by theoretical models, we assessed the contribution of collisional excitation in the formation of O Iemission lines.
New Astronomy, 2017
h i g h l i g h t s • We studied the NIR properties of Be star candidates in clusters in the LMC ... more h i g h l i g h t s • We studied the NIR properties of Be star candidates in clusters in the LMC and SMC. • New diagnostic area is suggested in the CCD for the Classical Be stars in the MCs.
We present preliminary results of the spectroscopic study of two Be Stars, 4 Her and 88 Her. The ... more We present preliminary results of the spectroscopic study of two Be Stars, 4 Her and 88 Her. The time series spectra of these Be stars were obtained using the 1.02m reflector of Vainu Bappu Observatory, Kavalur, India. We obtained a total of 41 spectra for 4 Her and 32 spectra for 88 Her during February to July 2009, with 1Å resolution at Hα. The rotational velocity v sini of the two stars were measured using HeI absorption lines (4026Å and 4471Å). The stars exhibit double peak Hα profile and their V/R variation is studied. Both the stars exhibit spectroscopic variations in the Hα emission line. We discuss their circumstellar environments based on the V/R variability. General Background on Classical BeStars • Decretion disk around the central Btype star is the source of emission lines seen in the spectra • Most common emission lines: Hα, Hβ, singlyionized metals like FeII and neutral Helium i.e., HeI • Types based on their emission line profiles: Poleon (Single peaked), Ordinary...
Research in Astronomy and Astrophysics, 2014
ABSTRACT Classical Be stars (CBe) are rapidly rotating (V_{{sin}i} ˜ 300 km s^{-1}) B-type stars ... more ABSTRACT Classical Be stars (CBe) are rapidly rotating (V_{{sin}i} ˜ 300 km s^{-1}) B-type stars with equatorial circumstellar decretion disks, the production of which is under debate. The Be phenomenon can be understood through the analysis of hydrogen recombination lines in the infrared spectral region. J-band spectroscopy of Be stars is one of the less studied fields in Be star research. We took J-band (11000 - 14000 Å) spectra of 34 CBe stars using the NICMOS instrument available with 1.2 m Mt. Abu infrared observatory. The spectra are interesting due to the presence of neutral oxygen (O I) lines, 11287 and 13165 Å, in addition to Paschen beta and Paschen gamma lines of hydrogen. The optical depth effects in the observed emission lines from the circumstellar disk have been deciphered through the line flux ratio analysis of the Paschen lines. The analysis of the O I lines has been used to understand the role of Lyman-beta fluorescence in the formation of these lines. Also, we used the spectra taken at different epochs separated over 1 year to study the variability in CBe stars.
Proceedings of the International Astronomical Union, 2010
Mennickent et al. (2002) presented a catalogue of 1056 Be star candidates in the Small Magellanic... more Mennickent et al. (2002) presented a catalogue of 1056 Be star candidates in the Small Magellanic cloud (SMC) by studying light curve variation using OGLE II data base. They classified these Be star candidates of the SMC in four categories: Type 1 stars showing outbursts (139 stars); Type 2 stars showing sudden luminosity jumps (154 stars); Type 3 stars showing periodic or near periodic variations (78 stars); Type 4 stars showing light curves similar to Galactic Be stars (658 stars). They suggested that Type 4 could be Be stars. On the other hand, they suggested that Type-3 stars may not be linked to the Be star phenomenon at all.
The Astrophysical Journal, 2012
The possibility of the Lyβ fluorescence mechanism being operational in classical Be stars and the... more The possibility of the Lyβ fluorescence mechanism being operational in classical Be stars and thereby contributing to the strength of the Oi 8446Å line has been recognized for long. However this supposition needs to be quantified by comparing observed and predicted Oi line ratios. In the present work, optical and near-infrared spectra of classical Be stars are presented. We analyse the observed strengths of the Oi 7774, 8446, 11287 and 13165Å lines which have been theoretically proposed as diagnostics for identifying the excitation mechanism. We have considered and examined the effects of Lyβ fluorescence, collisional excitation, recombination and continuum fluorescence on these Oi line strengths. From our analysis it appears that the Lyβ fluorescence process is indeed operative in Be stars.
The Astronomical Journal, 2013
Research in Astronomy and Astrophysics, 2012