Annapurni Subramaniam
Articles written in Journal of Astrophysics and Astronomy
Volume 34 Issue 2 June 2013 pp 175-192
Generation of a Near Infra-Red Guide Star Catalog for Thirty-Meter Telescope Observations
Smitha Subramanian Annapurni Subramaniam Luc Simard Kim Gillies A. N. Ramaprakash G. C. Anupama C. S. Stalin Swara Ravindranath B. Eswar Reddy
The requirements for the production of a near Infra-Red Guide Star Catalog (IRGSC) for Thirty Meter Telescope (TMT) observations are identified and presented. A methodology to compute the expected J band magnitude of stellar sources from their optical (𝑔, 𝑟 , 𝑖 ) magnitudes is developed. The computed and observed J magnitudes of sources in three test fields are compared and the methodology developed is found to be satisfactory for the magnitude range, JVega = 16–22 mag. From this analysis, we found that for the production of final TMT IRGSC (with a limiting magnitude of JVega = 22 mag), we need 𝑔, 𝑟, 𝑖 bands optical data which go up to 𝑖AB ∼ 23 mag. Fine tuning of the methodology developed, such as using Spectral Energy Distribution (SED) template fitting for optimal classification of stars in the fainter end, incorporating spectral libraries in the model, to reduce the scatter, and modification of the existing colour–temperature relation to increase the source density are planned for the subsequent phase of this work.
Volume 37 Issue 3 September 2016 Article ID 0024 Research Article
Smitha Subramanian Annapurni Subramaniam T Sivarani Luc Simard G. C. Anupama Kim Gillies A. N. Ramaprakash B. Eswar Reddy
The near-infrared instruments in the upcoming Thirty Meter Telescope (TMT) will be assisted by a multi conjugate Adaptive Optics (AO) system. For the efficient operation of the AO system, during observations, a near-infrared guide star catalog which goes as faint as 22 mag in ${\rm J}_{{\rm Vega}}$ band is essential and such a catalog does not exist. A methodology, based on stellar atmospheric models, to compute the expected near-infrared magnitudes of stellar sources from their optical magnitudes is developed. The method is applied and validated in JHKs bands for a magnitude range of ${\rm J}_{\rm{Vega}}$ 16--22 mag. The methodology is also applied and validated using the reference catalog of PAN STARRS. We verified that the properties of the final PAN STARRS optical catalog will satisfy the requirements of TMT IRGSC and will be one of the potential sources for the generation of the final catalog. In a broader context, this methodology is applicable for the generation of a guide star catalog for any existing/upcoming near-infrared telescopes.
Volume 38 Issue 1 March 2017 Article ID 0006 Research Article
Short-Term H$\alpha$ Line Variations in Classical Be Stars: 59 Cyg and OT Gem
K. T. Paul S. B. Shruthi Annapurni Subramaniam
We present the optical spectroscopic study of two classical Be stars, 59 Cyg and OT Gem obtained over a period of few months in 2009. We detected a rare triple-peak H$\alpha$ emission phase in 59 Cyg and a rapid decrease in the emission strength of H$\alpha$ in OT Gem, which are used to understand their circumstellar disks. We find that 59 Cyg is likely to be rapid rotator, rotating at a fractional critical rotation of $\sim$ 0.80. The radius of the H$\alpha$ emission region for 59 Cyg is estimated to be \( R_d/R_*\) $\sim$, assuming a Keplerian disk, suggesting that it has a large disk. We classify stars which have shown triple-peaks into two groups and find that the triple-peak emission in 59 Cyg is similar to $\zeta$ Tau. OT Gem is found to have a fractional critical rotation of $\sim$0.30, suggesting that it is either a slow rotator or viewed in low inclination. In OT Gem, we observed a large reduction in the radius of the H$\alpha$ emission region from $\sim$6.9 to $\sim$1.7 in a period of three months, along with the reduction in the emission strength. Our observations suggest that the disk is lost from outside to inside during this disk loss phase in OT Gem.
Volume 41 All articles Published: 11 December 2020 Article ID 0035
The horizontal branch morphology of the globular cluster NGC 1261 using AstroSat
SHARMILA RANI GAJENDRA PANDEY ANNAPURNI SUBRAMANIAM SNEHALALA SAHU N. KAMESWARA RAO
We present the results obtained from the UV photometry of the globular cluster NGC 1261 using far-UV (FUV) and near-UV (NUV) images acquired with the Ultraviolet Imaging Telescope (UVIT) onboard the AstroSat satellite. We utilized the UVIT data combined with HST, GAIA, and ground-based optical photometric data to construct the different UV colour-magnitude diagrams (CMDs). We detected blue HB (BHB), and two extreme HB (EHB) stars in FUV, whereas full HB, i.e., red HB (RHB), BHB as well as EHB is detected in NUV CMDs. The 2 EHB stars, identified in both NUV and FUV, are confirmedmembers of the cluster. The HB stars form a tight sequence in UV-optical CMDs, which is almost aligned with Padova isochrones. This study sheds light on the significance of UV imaging to probe the HB morphology in GCs.
Volume 41 All articles Published: 17 December 2020 Article ID 0045
ANNAPURNI SUBRAMANIAM SINDHU PANDEY VIKRANT V. JADHAV SNEHALATA SAHU
The blue straggler stars (BSSs) are main-sequence (MS) stars, which have evaded stellar evolution by acquiring mass while on the MS. The detection of extremely low mass (ELM) white dwarf (WD) companions to two BSSs and one yellow straggler star (YSS) from our earlier study using UVIT/AstroSat, aswell as WD companions to main-sequence stars (known as blue lurkers) suggest a good fraction of post-mass transfer binaries in M67. Using deeper UVIT observations, here we report the detection of another blue lurker in M67, with an ELM WD companion. The post-mass transfer systems with the presence of ELMWDs, including BSSs, are formed from Case A/B mass transfer and are unlikely to show any difference in surface abundances. We find a correlation between the temperature of the WD and the $\nu \sin i$ of the BSSs. We also find that the progenitors of the massive WDs are likely to belong to the hot and luminous group of BSSs in M67. The only detected BSS$+$WD system by UVIT in the globular cluster NGC 5466 has a normal WD and suggests that open cluster like environment might be present in the outskirts of low density globular clusters.
Volume 42 All articles Published: 15 June 2021 Article ID 0047 SCIENCE RESULTS
UOCS. V. UV study of the old open cluster NGC 188 using AstroSat
SHARMILA RANI ANNAPURNI SUBRAMANIAM SINDHU PANDEY SNEHALATA SAHU CHAYAN MONDAL GAJENDRA PANDEY
We present the UV photometry of the old open cluster NGC188 obtained using images acquired with Ultraviolet Imaging Telescope (UVIT) on board the AstroSat satellite, in two far-UV (FUV) and one near-UV (NUV) filters. UVIT data is utilised in combination with optical photometric data to construct the optical and UV colour-magnitude diagrams (CMDs). In the FUV images, we detect only hot and bright blue straggler stars (BSSs), one hot subdwarf, and one white dwarf (WD) candidate. In the NUV images, we detect members up to a faintness limit of $\sim$22 mag including 21 BSSs, 2 yellow straggler stars (YSSs), and one WD candidate. This study presents the first NUV-optical CMDs, and are overlaid with updated BaSTIIAC isochrones and WD cooling sequence, which are found to fit well to the observed CMDs. We use spectral energy distribution (SED) fitting to estimate the effective temperatures, radii, and luminosities of the UV-bright stars. We find the cluster to have an HB population with three stars ($T_{\rm eff}=4750–21000$ K). We also detect two yellow straggler stars, with one of them with UV excess connected to its binarity and X-ray emission.
Volume 42 All articles Published: 18 June 2021 Article ID 0050 SCIENCE RESULTS
A tale of two nearby dwarf irregular galaxies WLM and IC 2574: As revealed by UVIT
CHAYAN MONDAL ANNAPURNI SUBRAMANIAM KOSHY GEORGE
We present an ultra-violet study of two nearby dwarf irregular galaxies WLM and IC 2574, using the Far-UV and Near-UV data from the Ultra-Violet Imaging Telescope (UVIT). We used the F148W band Far-UV images and identified 180 and 782 young star-forming clumps in WLM and IC 2574, respectively. The identified clumps have sizes between 7–30 pc in WLM and 26–150 pc in IC 2574. We noticed more prominent hierarchical splitting in the structure of star-forming regions at different flux levels in IC 2574 than WLM. We found that the majority of the clumps have elongated shapes in the sky plane with ellipticity ($\varepsilon$) greater than 0.6 in both the galaxies. The major axis of the identified clumps is found to show no specific trend of orientation in IC 2574, whereas in WLM the majority are aligned along south-west to north-east direction. We estimated (F148W–N242W) colour for the clumps identified in WLM and noticed that theyounger ones (with (F148W–N242W) < $-$ 0:5) are smaller in size (<10 pc) and are located mostly in the southern half of the galaxy between galactocentric radii 0.4–0.8 kpc.
Volume 42 All articles Published: 19 June 2021 Article ID 0053 SCIENCE RESULTS
Study of dynamical status of the globular cluster NGC 1851 using ultraviolet imaging telescope
GAURAV SINGH R. K. S. YADAV SNEHALATA SAHU ANNAPURNI SUBRAMANIAM
We present the study of dynamical status of the globular cluster NGC 1851. A combination of multi-wavelength space and ground-based data sets are used for the present analysis. In order to select the genuine cluster members, we used the astro-photometric data available from HST and GAIA-DR2 catalogs. The BSS radial distribution of the cluster is plotted from the center of the cluster to the outskirts. The radial distribution of BSS shows a central peak, followed by a dip at the intermediate radii ($r_{\rm min}\sim 90''$) and a rising trend in the outskirts. We also estimated $A^+_{rh}$ parameter as $0.391 \pm 0.006$ to validate the findings of the radial distribution study. On the basis of the minima in the BSS radial distribution and the value of $A^+_{rh}$ parameter, we conclude that NGC 1851 belongs to
Volume 42 All articles Published: 2 July 2021 Article ID 0066 SCIENCE RESULTS
DEEPTHI S. PRABHU ANNAPURNI SUBRAMANIAM SNEHALATA SAHU
A recent study of hot (20,000 to 30,000 K) extreme horizontal branch (EHB) stars in globular clusters (GCs) has led to the discovery of their variability. It is suggested that this variability is driven by the projected rotation of magnetic spots on the stellar surfaces and is expected to have higher amplitudes atshorter wavelengths. Here, we present the analysis of such hot stars in the massive GC NGC 2808 using the UltraViolet Imaging Telescope (UVIT), aboard AstroSat. We use the UVIT data in combination with the
Volume 42 All articles Published: 30 July 2021 Article ID 0089 SCIENCE RESULTS
UOCS. IV. Discovery of diverse hot companions to blue stragglers in the old open cluster King 2
VIKRANT V. JADHAV SINDHU PANDEY ANNAPURNI SUBRAMANIAM RAM SAGAR
King 2, one of the oldest clusters in the Milky Way, with an age of $\sim$6 Gyr and distance of $\sim$5700 pc, has been observed with UVIT payload on the AstroSat. With membership information derived from
Volume 42 All articles Published: 3 September 2021 Article ID 0101 CORRECTION
DEEPTHI S. PRABHU ANNAPURNI SUBRAMANIAM SNEHALATA SAHU
Volume 42 All articles Published: 13 November 2021 Article ID 0109
Study of classical Be stars in open clusters older than 100 Myr
MADHU KASHYAP JAGADEESH BLESSON MATHEW K. T. PAUL GOURAV BANERJEE ANNAPURNI SUBRAMANIAM R. ARUN
We performed the slitless spectroscopic survey to identify classical Be (CBe) stars in openclusters older than 100 Myr. Observing a sample of 71 open clusters, we identified 13 CBe stars in 11 open clusters, one of, which (TYC 2679-432-1) is a new detection. The 13 CBe stars show both H$\alpha$ in emission and IR excess, which confirm that they possess gaseous circumstellar discs. Two more CBe stars are found toexhibit H$\alpha$ 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.
Volume 43, 2022
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