Santosh Joshi - Research
The main domain of my primary research lie in the areas of optical astronomy involving time series analysis of variable stellar sources such as: Asteroseismology of Pulsating Stars Time-resolved Photometric Analysis of Chemical Peculiar Stars Chemical Abundance Analysis of the Variable Stars Census of the Variable Stars in Open and Intermediate Age Star Clusters Intra-night Variability of Blazars Though, I am involved in many research fields of astronomy and astrophysics but in the following sub-sections, I have mention only some of front-line research areas where I am participating actively :
Fast Photometric Variability in Chemically Peculiar Stars
We have been conducting an Indo-South African bilateral project The Nainital-Cape Survey" since 1999 between ARIES Nainital and South African Astronomical Observatory (SAAO), South Africa. In last fifteen years, more than 300 sample stars were examined for the pulsations, making it one of the longest ground-based surveys for pulsation in CP stars in terms of time span and sample size. The main aim of this survey is to find new pulsating Ap and Am stars in Northern and Southern hemispheres and set-up test bed to perform the asteroseismic study. Under this project, we discov- ered rapid oscillation (also known as rapidly oscillating Ap star) in an Ap star HD12098 and Scuti type pulsation in seven Am stars namely HD13038, HD13079, HD25515 HD98851, HD102480, HD113878 and HD118660. The more detail information on the Indo-South African project can be access through the URL http://www.aries.res.in/cns/ This survey programme has been financially supported by DST Govt. of India and NRF, South Africa.
High-Resolution Spectroscopy and Spectro-Polarimetry of Pulsating CP stars
For the detailed spectroscopic study of CP stars, in year 2008 we initiated an observing programme in the collaboration of the Russian astronomers. To achieve our goals, the high-resolution spec- troscopic observations of the sample Ap and Am stars discovered under the The Nainital-Cape Survey" were carried out from 6.0-m Russian telescope of Special Astrophysical Observatory (SAO), Russia. Such observations are very useful to study the pulsation phase of individual spectral lines and their locations in the chemically stratified atmosphere. The radial velocity (RV) measurements of individual spectral line in peculiar stars enable us to measure the pulsational phase and amplitude of the stellar oscillations as a function of the atmospheric height. The time resolved radial velocity observations were also used to confirm the variability detected through the photometric observations. The combined photometric and spectroscopic analysis of these stars have been used in estimating the basic physical parameters such as elective temperature, surface gravity, magnetic field strength and presence of chemical elements in their atmosphere. This project was initiated in the collaboration of astronomers from Russia and was supported by DST Govt. of India and Russian Academy of Science (RAS), Russia under the ILTP and RFBR programs.
Census of the Variable Stars in Open and Intermediate Age Star Clusters
Open star clusters are excellent tracers to understand astrophysical problems concerning to star formation and stellar evolution. In order to investigate the occurrence of pulsation in A- and F- type stars in different galactic environments, we took an initiative in year 2008 to search for the photometric variability in a well defined set of young and intermediate age open star clusters. We selected sample of 25 open star clusters of different age, metallicity and galacto-centric position where probabilities of finding A-type pulsators are relatively high. For an example, using ground based time-series photometric observations of an open star cluster NGC6866, we discovered 28 new variables stars. Our photometric observations were also used for the estimation of the basic physical parameters and kinematic study of the clusters under study. The photometric analysis of the other clusters are in progress and we are expecting more interesting results from the data we have collected 2 using the observing facilities of ARIES. This project is being carried out in the collaboration of astronomers from India, Russia and South Africa.
Study of Pulsational Variability and Rotational Activities using Kepler Data
We have been also studying the open star clusters and field stars using combined space and ground based time-series photometric data obtained on various facilities at different epochs.
Open Star Clusters
After the launch of the Kepler mission on 07 March 2009, enormous of the data have been downloaded at the NASA MAST science center. Though the primary objective of Kepler mission was to find the earth like planets in habitable zones in Cygnus and Lyra regions but this mission has also observed thousands of stars to probe the interior of the stars and study the stellar evolution using asteroseismic technique. Coincidentally one of the cluster NGC6866 observed extensively using 1.04-m telescope of ARIES also falls on the Kepler field. Using the time-series data extracted from the Kepler archive data base, we discovered 31 Scuti, 8 Doradus, 23 red giants with solar-like oscillations, 4 eclipsing binaries and 106 stars showing rotational modulation indicative of star-spots. The combined space and ground based data were also used to disentangle the cluster members to the non-members for estimation of age, reddening in the direction of cluster, distance and other parameters used to study the morphology of the cluster.
Orbital solutions for new spectroscopic systems in Kepler database
This project intend to perform high-resolution spectroscopy for a selection of AF-type candidate hybrid variable stars detected by the Kepler mission. The proposal's objectives are: 1) to complete the phase coverage of the orbits by monitoring the long-term behavior in radial velocity (RV) of the new systems and 2) to determine accurate stellar atmospheric properties and v sini for each of the components. Here, we propose to extend the time base of the spectroscopic data for 13 new and suspected long-period SB1/2/3 systems recently observed with the Hermes spectrograph. The immediate goal of the HCT observations is to analyse and model the composite spectra in order to derive reliable orbital periods and solutions, as well as to improve upon the component properties of these systems. This project is being carried out in the collaboration of astronomers from India and Belgium under the Belgo-India Network on Astronomy and Astrophysics.
We have studied the time-series photometric analysis of about 15000 field stars observed by the Kepler space mission. We measured rotation periods of 2270 active stars as a function of spectral type and studied the distribution of their amplitudes. We examined the period-color relationship for rotational variables and found that this relationship holds for early-F to early A-type stars. In- terestingly, we found that this relation is not consistent with the period-age-mass (PtM) relation that was previously anticipated. Through the visual inspection of the light curves and periodograms, we confirmed the solar-like oscillations in 16 red giant stars and identified 75 additional solar-like oscillations. The basic parameters such as masses, radii and luminosities of these solar-like oscilla- tors are estimated using the asteroseismic scaling relation. We have characterized several non-radial pulsating variables as Sct, Dor and hybrid stars. The relationship between two newly constructed observable, energy" and efficiency" for these stars have also been reported. Our results support that the present observational instability strip should be extended towards the red end of instability strip to accommodate the Sct and Dor variables. 3 This project is being carried out in the collaboration of astronomers from India, Belgium and South Africa.
Low-resolution Spectroscopy of the Kepler Stars
For the detailed study of a star one need to estimate the basic parameters such as elective tem- perature, gravity because these parameters acts as an input for the stellar models. We used the low-resolution spectroscopic observations of the Kepler stars from 6.0-m telescope at SAO, Russia, 2.0-m telescope of HCT India and the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) at China. All the spectra were analysed using ULySS (University of Lyon spectroscopic software) as well as standardized routines in IRAF package. On comparison of two data sets analysed using two different techniques, we found that the low-resolution spectroscopic data can be also used to estimated the basic parameters such as elective temperature, gravity etc. This work is being carried out under the collaboration of astronomers from India, South Africa and Belgium.
Intra-night Variability of Blazar
Rapid light variations are seen in many Quasars and almost in all the Blazars. The amount of vari- ability in different classes of active galactic nuclei has been studied extensively but many questions remain unanswered. To characterize the intra-night optical variability(INOV) of TeV Blazars and Quasars, we monitored a well defined sets of objects in optical bands. These observations play an important role in understanding the link between the dominant varieties of powerful active galactic nucleus and comparison of the characteristics of the two major sub-classes of TeV Blazars. The physical processes responsible for producing INOV of Blazars continue to be debated. For simulta- neous measurements on intra-night variability on different time-scale we used various Indian optical telescopes for photometry and the KANATA telescope of Japan for imaging polarimetry. Besides the quiescent slowly varying emission, the Blazars display ares on time scales as short as minutes, we have assumed a two component model responsible for producing a long term and short term aring trends. This work is being carried out in the collaborations among the astronomers from India, Japan and Poland.