Browsing by Author "et"

Now showing 1 - 5 of 5

A 3D Automated Classification Scheme for the TAUVEX data pipeline
(Mon. Not. R. Astron. Soc, 2007-01-28) Bora, Archana; Gupta, Ranjan; Singh, Harinder P; et
In order to develop a pipeline for automated classification of stars to be observed by the TAUVEX ultraviolet space Telescope, we employ an artificial neural network (ANN) technique for classifying stars by using synthetic spectra in the UV region from 1250°A to 3220°A as the training set and International Ultraviolet Explorer (IUE) low resolution spectra as the test set. Both the data sets have been pre-processed to mimic the observations of the TAUVEX ultraviolet imager. We have successfully classified 229 stars from the IUE low resolution catalog to within 3-4 spectral sub-class using two different simulated training spectra, the TAUVEX spectra of 286 spectral types and UVBLUE spectra of 277 spectral types. Further, we have also been able to obtain the colour excess (i.e. E(B-V) in magnitude units) or the interstellar reddening for those IUE spectra which have known reddening to an accuracy of better than 0.1 magnitudes. It has been shown that even with the limitation of data from just photometric bands, ANNs have not only classified the stars, but also provided satisfactory estimates for interstellar extinction. The ANN based classification scheme has been successfully tested on the simulated TAUVEX data pipeline. It is expected that the same technique can be employed for data validation in the ultraviolet from the virtual observatories. Finally, the interstellar extinction estimated by applying the ANNs on the TAUVEX data base would provide an extensive extinction map for our galaxy and which could in turn be modeled for the dust distribution in the galaxy.
Filling Gaps in Indo-US Stellar Spectral Library using Principal Component Analysis
(Stellar Populations as Building Blocks of Galaxies Proceedings IAU Symposium, 2006-07-12) Singh, Harinder P; Singh, S. Jotin; Gupta, Ranjan; et
The Indo-US coud´e feed stellar spectral library (CFLIB) published recently by Valdes et al. (2004) contains spectra of 1273 stars in the spectral region 3460 to 9464 ˚A at a resolution of 1 ˚A. About 500 stars in this database have gaps ranging from a few ˚A to several tens of ˚A in this wavelength range. We use a variation of Principal Component Analysis (PCA) technique to fill gaps of up to 5˚A in a subset of spectra from the CFLIB. We hope to exploit the full potential of the scheme and attempt to fill larger gaps in stellar spectra in a subsequent study.
A Near-Infrared Stellar Spectral Library: II. K-Band Spectra
(Bull. Astr. Soc. India, 2007-05-31) Ranade, Ranade C; Singh, Harinder P.; Gupta, Ranjan; et
This paper is the second in the series of papers on near-infrared (NIR) stellar spectral library produced by reducing the observations carried out with 1.2 meter Gurushikhar Infrared Telescope (GIRT), at Mt. Abu, India using a NICMOS3 HgCdTe 256×256 NIR array based spectrometer. In paper I (Ranade et al. 2004), H-band spectra of 135 stars at a resolution of 16°A were presented. The K-band library being released now consists of 114 stars covering spectral types O7–M7 and luminosity classes I–V. The spectra have a moderate resolution of 22°A in the K band and have been continuum shape corrected to their respective effective temperatures. We hope to release the remaining J-band spectra soon. The complete H and K-Band library
Optical variability of radio-intermediate quasars
(2009-10-08) Goyal, Arti; Krishna, Gopal; Joshi, S; et
We report the results of our intensive intranight optical monitoring of 8 opti- cally bright ‘radio-intermediate quasars’ (RIQs) having flat or inverted radio spectra. The monitoring was carried out in R-band on 25 nights during 2005- 09. On each night only one RIQ was monitored for a minimum duration of ∼ 4 hours (the average being 5.2 hours per night). Using the CCD as an N-star photometer, an intranight optical variability (INOV) detection threshold of ∼ 1–2% was achieved for the densely sampled differential light curves (DLCs) de- rived from our data. These observations amount to a large increase over those reported hitherto for this rare and sparsely studied class of quasars which can, however, play an important role in understanding the link between the dominant varieties of powerful AGN, namely the radio-quiet quasars (RQQs), radio-loud quasars (RLQs) and blazars. Despite the probable presence of rela- tivistically boosted nuclear jets, inferred from their flat/inverted radio spectra, clear evidence for INOV in our extensive observations was detected only on one night. Also, flux variation between two consecutive nights was clearly seen for one of the RIQs. These results demonstrate that as a class, RIQs are much less extreme in nuclear activity compared to blazars. The availability in the literature of INOV data for another 2 RIQs conforming to our selection crite- ria allowed us to enlarge the sample to 10 RIQs (monitored on a total of 42 nights for a minimum duration of ∼ 4 hours per night). The absence of large 2 Goyal et al. amplitude INOV (ψ > 3%) persists in this enlarged sample. This extensive database has enabled us to arrive at the first estimate for the INOV Duty Cy- cle (DC) of RIQs. The DC is found to be small (∼ 9%), increasing to ∼ 14% if the two cases of ‘probable’ INOV are included. The corresponding value is known to be ∼ 60% for BL Lacs and ≈ 15% for both RLQs and RQQs, if they too are monitored for & 4 − 6 hours in each session. Our observations also provide information about the long-term optical variability (LTOV) of RIQs, which is found to be fairly common and reaches typical amplitudes of ≈ 0.1- mag. The light curves of these RIQs are briefly discussed in the context of a theoretical framework proposed earlier for linking this rare kind of quasars to the much better studied dominant classes of quasars.
Proposal for UV observations of star forming clouds
(Bull. Astr. Soc. India, 2007-03-02) Sen, A.K.; Mukai, T.; Gupta, R; et
The small, compact dark clouds (also known as Bok Globules), are undergoing gravitational collapse that can result in the production of low mass stars. Light from background stars is scattered in forward direction by magnetically aligned dichroic dust grains. The degree and direction of align- ment is proportional to the strength and direction of ambient magnetic ¯eld in the cloud. Background star polarimetry provides the technique to probe this ¯eld. In order to relate the physical conditions within the cloud to the back- ground star polarization and to know the dust properties, we need to determine E(B ¡V ) for each background star and relate it to the corresponding observed polarization (p). However, observed data, do not always show a correlation between polarization and extinction. Due to this the question arises whether the grains that produce polarization also produce observed extinction? The observation that the polarization is not related to the extinction, can be explained if polarization and extinction are caused by two di®erent grain populations. Polarization is mainly caused by short grains, whereas the extinction is caused by larger ones. Based on these ¯ndings, justi¯cations are made here to detect these small grains (0.0035-0.01 ¹m) in the star forming clouds through UV observations to be made by TAUVEX. These particles can be best detected through the UV observations, as they show far-UV excess and characteristic features of 2175 ºA bump. We propose imaging of these clouds through the three bandpass ¯lters of TAUVEX. We expect to resolve many unanswered questions associated with star forming clouds, through this set of proposed observations.