arXiv subjects

S. Muneer

Publications and source records attributed to S. Muneer.

44 records · Page 3Linked to original sources

arXiv2015

A beam-displacement prism based, three band stellar photo-polarimeter

A new astronomical photo-polarimeter that can measure linear polarization of point sources simultaneously in three spectral bands was designed and built in Indian Institute of Astrophysics. The polarimeter has a Calcite beam-displacement prism as the analyzer. The ordinary and extra-ordinary emerging beams in each spectral band are quasi-simultaneously detected by the same photomultiplier by using a high speed rotating chopper. The effective chopping frequency can be set to as high as 200 Hz. A rotating superachromatic Pancharatnam halfwave plate is used to modulate the light incident on the analyzer. The spectral bands are isolated using appropriate dichroic and glass filters. A detailed analysis shows that the reduction of 50% in the efficiency of the polarimeter because of the fact that the intensities of the two beams are measured alternately is partly compensated by the reduced time to be spent on the observation of the sky background. The position angle of polarization produced by the Glan-Taylor prism in the light path is found to be slightly wavelength dependent, indicating that the fixed super-achromatic halfwave plate in the beam does not fully compensate for the variation in the position angle of the effective optical axis of the rotating plate. However, the total amplitude of variation in the U-I spectral region is only 0.92 degree. The polarization efficiency is also found to be wavelength-dependent with a total amplitude of 0.271% in the U-I region; its mean value is 99.211%. The instrumental polarization is found to be very low. It is nearly constant in the V-I spectral region (~0.04%), and apparently, it increases slightly towards the ultraviolet. The observations of polarized stars show that the agreement between the measured polarization values and those available in the literature to be excellent.

astro-ph.IM↗

arXiv2015

A multi-spectral band stellar photo-polarimeter

We designed and built a new astronomical photo-polarimeter that can measure linear polarization simultaneously in three spectral bands. It has a Calcite beamdisplacement prism as the analyzer. The ordinary and extra-ordinary emerging beams in each spectral bands are quasi-simultaneously detected by the same photomultiplier by using a high speed rotating chopper. A rotating superachromatic Pancharatnam halfwave plate is used to modulate the light incident on the analyzer. The spectral bands are isolated using appropriate dichroic and glass filters. We show that the reduction of 50% in the efficiency of the polarimeter because of the fact that the intensities of the two beams are measured alternately is partly compensated by the reduced time to be spent on the observation of the sky background. The use of a beam-displacement prism as the analyzer completely removes the polarization of background skylight, which is a major source of error during moonlit nights, especially, in the case of faint stars. The field trials that were carried out by observing several polarized and unpolarized stars show the performance of the polarimeter to be satisfactory.

astro-ph.IM↗

arXiv2015

New variables in M5 (NGC 5904) and some identification corrections

We report twelve variables not previously detected in the globular cluster M5 (NGC 5904); one SX Phe and eleven semi-regular variables (SR). Their identifications, equatorial coordinates, ephemerides, and light curves are given. Furthermore, we have explored the light curves of a group of stars whose variability has not been confirmed and that are marked as probable non- variables in the CVSGC. Finally, we offer detailed identifications for some of the known variables in crowded regions that were misidentified in previous studies. We shall also address the cases of the cataclysmic variable or U Gem type V101 and of the variable blue straggler V159.

astro-ph.SR↗

arXiv2014

Spectroscopic analysis of four post-AGB candidates

We have done a detailed abundance analysis of four unexplored candidate post- Asymptotic Giant Branch(AGB) stars IRAS 13110 - 6629, IRAS 17579 - 3121, IRAS 18321 - 1401 and IRAS 18489 - 0629 using high resolution spectra. We have constructed Spectral Energy Distributions (SED) for these objects using the existing photometric data combined with infrared (IR) fluxes. For all sample stars, the SEDs exhibit double peaked energy distribution with well separated IR peaks showing the presence of dusty circumstellar material. The CNO abundances indicate the production of N via CN cycling, but observed [C/Fe] indicates the mixing of carbon produced by He burning by third dredge up although C/O ratio remains less that 1. A moderate DG effect is clearly seen for IRAS 18489 - 0629 and IRAS 17579 - 3121 while a large scatter observed in depletion plots for IRAS 18321 - 1401 and IRAS 13110 - 6629 indicate the presence of other processes affecting the observed abundance pattern.

astro-ph.SR↗

arXiv2013

Identification of metal-poor stars using the artificial neural network

Identification of metal-poor stars among field stars is extremely useful for studying the structure and evolution of the Galaxy and of external galaxies. We search for metal-poor stars using the artificial neural network (ANN) and extend its usage to determine absolute magnitudes. We have constructed a library of 167 medium-resolution stellar spectra (R ~ 1200) covering the stellar temperature range of 4200 to 8000 K, log g range of 0.5 to 5.0, and [Fe/H] range of -3.0 to +0.3 dex. This empirical spectral library was used to train ANNs, yielding an accuracy of 0.3 dex in [Fe/H], 200 K in temperature, and 0.3 dex in log g. We found that the independent calibrations of near-solar metallicity stars and metal-poor stars decreases the errors in T_eff and log g by nearly a factor of two. We calculated T_eff, log g, and [Fe/H] on a consistent scale for a large number of field stars and candidate metal-poor stars. We extended the application of this method to the calibration of absolute magnitudes using nearby stars with well-estimated parallaxes. A better calibration accuracy for M_V could be obtained by training separate ANNs for cool, warm, and metal-poor stars. The current accuracy of M_V calibration is (+-)0.3 mag. A list of newly identified metal-poor stars is presented. The M_V calibration procedure developed here is reddening-independent and hence may serve as a powerful tool in studying galactic structure.

astro-ph.SR↗

arXiv2012

The Stellar parametrization using Artificial Neural Network

An update on recent methods for automated stellar parametrization is given. We present preliminary results of the ongoing program for rapid parametrization of field stars using medium resolution spectra obtained using Vainu Bappu Telescope at VBO, Kavalur, India. We have used Artificial Neural Network for estimating temperature, gravity, metallicity and absolute magnitude of the field stars. The network for each parameter is trained independently using a large number of calibrating stars. The trained network is used for estimating atmospheric parameters of unexplored field stars.

astro-ph.SR↗

arXiv2006

Evolution of emission line activity in intermediate mass young stars

We present optical spectra of 45 intermediate mass Herbig Ae/Be stars. Together with the multi-epoch spectroscopic and photometric data compiled for a large sample of these stars and ages estimated for individual stars by using pre-main sequence evolutionary tracks, we have studied the evolution of emission line activity in them. We find that, on average, the H_alpha emission line strength decreases with increasing stellar age in HAeBe stars, indicating that the accretion activity gradually declines during the PMS phase. This would hint at a relatively long-lived (a few Myr) process being responsible for the cessation of accretion in Herbig Ae/Be stars. We also find that the accretion activity in these stars drops substantially by ~ 3 Myr. This is comparable to the timescale in which most intermediate mass stars are thought to lose their inner disks, suggesting that inner disks in intermediate mass stars are dissipated rapidly after the accretion activity has fallen below a certain level. We, further find a relatively tight correlation between strength of the emission line and near-infrared excess due to inner disks in HAeBe stars, indicating that the disks around Herbig Ae/Be stars cannot be entirely passive. We suggest that this correlation can be understood within the frame work of the puffed-up inner rim disk models if the radiation from the accretion shock is also responsible for the disk heating.

astro-ph↗

arXiv2006

Photometric and spectroscopic evolution of the type IIP supernova SN 2004et

We present optical photometry and spectroscopy of the type IIP supernova SN 2004et that occurred in the nearby galaxy NGC 6946. The observations span a time range of 8 days to 541 days after explosion. The late time bolometric luminosity and the H$α$ luminosity in the nebular phase indicate that $0.06\pm0.02{M_\odot}$ of $^{56}$Ni was synthesised during the explosion. The plateau luminosity, its duration and the expansion velocity of the supernova at the middle of the plateau indicate an explosion energy of $E_{\rm{exp}}= 1.20^{+0.38}_{-0.30} \times 10^{51}$ ergs. The late time light curve and the evolution of the [OI] and H$α$ emission line profiles indicate the possibility of an early dust formation in the supernova ejecta. The luminosity of [OI] 6300, 6364 Ådoublet, before the dust formation phase, is found to be comparable to that of SN 1987A at similar epochs, impling an oxygen mass in the range $1.5 - 2{M_\odot}$, and a main sequence mass of $20{M_\odot}$ for the progenitor.

astro-ph↗