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Astrophysical Seminar (15 March 2018, 11:00)

Posted: 10/03/2018

1) Speaker: J. Murthy (Indian Institute of Astrophysics, Bangalore, India)

Title: "An evaluation of the UVIT performance"


We have studied the performance of the UVIT payload on Astrosat and derived a calibration of the FUV and NUV instruments on board. We find that the sensitivity of both the FUV and NUV channels is as expected from ground calibrations, with the FUV effective area about 35% and the NUV effective area about the same as that of GALEX. The PSF of the instrument is on the order of 1.2 -- 1.6 arcsec. I will also give an introduction to JUDE, a tool to extract scientifically useful data products from the raw data.

2) Speaker: R. Gupta (Inter-University Center for Astronomy & Astrophysics, Pune, India)

Title: "Classification of LAMOST DR3 FGK Spectra"


 The Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) is producing a very large database which consists of spectra from the Chinese 4-meter telescope. The LAMOST Data Release 3 (DR3; http://dr3.lamost.org/) contains 5,755,126 spectra out of which 5.4 million are stellar spectra. In the current preliminary work, we have used only the F, G and K spectral types of stars with S/N > 30. The data consists of 286,283 spectra which has been classified using Automated Supervised Neural Network Techniques (ANNs) where this forms the test set and training set consists of spectra from the Miles spectral library. The three ANN tools used were Tree, Forest and Neural and the best performance was seen for the Tree based classifier with a classification accuracy of 71.4% correct spectral types and a classification error of 4.61% sub-spectral types. The luminosity class is not known for the vast majority of LAMOST spectra, but our automated schemes provide this information along with a confidence estimate (with coresponding classification probabilities) for each spectra. Future work will involve (i) using more classification tools (ii) improve the classification accuracies and (iii) apply to future LAMOST data releases.


3) Speaker: H.P. Singh (Department of Physics & Astrophysics University of Delhi, India)

Title: "Distance Scale using NIR - PL Relations for Cepheids"


I shall present our recent work on new near-infrared (NIR) Cepheid period-Wesenheit (P-W) relations in the LMC using time-series observations from the Large Magellanic Cloud NIR Synoptic Survey. We also derive optical+NIR P-W relations using V and I magnitudes from the Optical Gravitational Lensing Experiment. We employ our new JHKs data to determine an independent distance to the LMC of mu_LMC =18.47 +/- 0.07 (statistical) mag, using an absolute calibration of the Galactic relations based on several distance determination methods and accounting for the intrinsic scatter of each technique. We also derive new NIR period-luminosity and Wesenheit relations for Cepheids in M31 using observations from the Panchromatic Hubble Andromeda Treasury survey. We use the absolute calibrations of the Galactic and LMC W_{J,H} relations to determine the distance modulus of M31, mu_M31= 24.46 +/- 0.20 mag. We apply a simultaneous fit to Cepheids in several Local Group galaxies covering a range of metallicities (7.7<12+log[O/H]< 8.6 dex) to determine a global slope of -3.244 +/- 0.016 mag dex-1 for the W_{J,Ks} relation and obtain robust distance estimates. Our distances are in good agreement with recent TRGB based distance estimates and we do not find any evidence for a metallicity dependence in the NIR P-W relations.

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