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Gireesh C. Joshi

Publications and source records attributed to Gireesh C. Joshi.

15 recordsLinked to original sources

Photometric Search for variable stars in the field of two Northern open clusters, DOLIDGE 14 and NGC 1960

The aim of present work is extract and analyses the light curves of the stars in the field of two clusters, NGC 1960 and DOLIDZE 14. The photometric calibration is performed by comprehensive method of secondary standard transformation and differential photometry using two comparison stars per candidate variable star. The resultant light curves for each potential variable star are displayed and their period analyzed by two different methods. The period and classification of 18 discovered short periodic type variable stars of NGC 1960 are discussed, which consist of four known variable stars and fourteen new variable stars. In the case of DOLIDZE 14, four discovered variables consist of one miscellaneous, one rotational, two $binary$ type variable stars. In the case of NGC~1960, the 12 selected comparison stars appear to be likely candidate for long periodic variability and 4 stars may be standard stars. The variation in brightness of other twenty comparison stars is non-pulsating with an irregular pattern. Membership analysis of variable stars is performed using their distance, kinematic probability and location in $(U-B)$ vs $(B-V)$ TCD. C-M diagrams were constructed to confirm the evolutionary state of the new variable stars.

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Effect of Blue Component Stars in Parameters of NGC 6866

The open star clusters (OSC) are important tracers for understanding the Galactic evolution. The parametric study of these astronomical-objects is crucial task due to the appearing sequence of the members of OSC. These members are defined through the various approaches such as photometric, statistical, kinematics etc. In the present paper, we have been using the photometric colours of the identified stars for categorized them into the blue and red component groups and identification of these groups is possible through (B-V) vs V colour magnitude diagram (CMD). Furthermore, the influence/effect of these groups is also examined in the estimation of cluster parameters. The stellar enhancement of cluster NGC 6866 is found through the blue-component-stars (BCS) and the linear solution of best fitted values of King Models of the radial-densityprofile (RDP) gives the core radius as 5.22+/-0.29 arcmin. The good agreement of present estimated parameters of the cluster with the literature seems to be an effective evidence to consider BCS as the true representative of the cluster. The stellar distribution of the cluster shows continuous phenomena of the mass segregation. An effect of the incompleteness of photometric data is related to the mass-function slope values, which is found to be -3.80+/-0.11 and also shows the incremental nature with the incompleteness.

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Photometric and Spectroscopic Analysis of Five Am Stars

The photometric analysis of sample Am stars is carried out to determine the stellar characteristics and to constrain the stellar dynamics. The spectroscopic analysis of the studied Am stars confirms their general characteristics of Am stars. The available data on elemental abundances for HD 113878 and HD 118660 have shown different characteristics during different epochs of observations. The basic stellar parameters (mass, luminosity, radius, life time, distance, proper-motion, etc.) are also determined to identify the stellar habitat zones for earth like exoplanet. Such information is important to identify suitable planets for human settlement in the near future. In this connection, the tidal radius and boundaries of the habitable zone of each star have been computed to support the search of an extra-terrestrial life around them. Asteroseismic mass scale test shows greater stellar masses comparable to the solar mass.

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Stellar Variability with Photometric and Spectroscopic Analysis of five Am Field Stars

The spectroscopic and photometric analysis of sample Am stars are carried out to determine the stellar characteristics of each studied star. The CCD photometric analysis of HD 98851 and HD 207561 show clear evidence of pulsation variability of 1.55 hr and 5.8 min respectively. Similarly, a clear evidence of the photometric variability is also found for an Am star HD 73045 which is likely to be pulsating in nature with a period of about 36-min. We are also found dissimilar behaviour of elemental abundances of various ions for HD 113878 and HD 118660. The basic stellar parameters (mass, luminosity, radius, life time, distance, proper-motion etc.) are determined for each sample stars. The tidal radius and boundaries of habitable zone of each star are also computed to search the extra-terrestrial life. Asteroseismic mass scale test shows greater stellar masses compare to the solar mass.

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IC 361:- Near infrared and UBVRI photometric Analysis

We present here the detailed optical and infra-red photometric analysis of the open star cluster IC 361. On studying the radial density profile, radial extent of the cluster is found to be 8.0 +/- 0.5 arcmin. The basic physical parameters of the cluster such as E(B-V) = 0.56 +/- 0.10 mag, E(V-K) = 1.72+/-0.12 mag, log(Age)=9.10+/-0.05, and (m-M)0 = 12.54 +/-.05 mag are obtained using the color-color and color-magnitude diagrams. IC 361 is found to be located at a distance of 3.22 +/- 0.07 kpc. Using the archival proper motion catalogues, we estimate mean proper motions of IC 361 as 4.97+/-0.17 mas yr-1 and -5.80+/-0.18 mas yr-1 in the direction of RA and DEC, respectively. We derive the luminosity and mass functions for the cluster main sequence stars. The mass function slope is found to be -1.06+/-0.09 which is too low compare than Salpeter value.

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Test of depending frequencies of the variable Stars of OC: NGC 6866

The search of secondary pulsations is carried out to understand the possible relations among the known parameters of variables of the cluster, NGC 6866. These pulsations arise due to the various ongoing physical phenomena of the variables. Moreover, pulsations of the variables are identified through the visual inspection of their frequency-amplitude distributions. A total of 18 variables among the 28 known variables are showing the secondary pulsation modes. Furthermore, these pulsation modes do not occur for PV, EA, EB, Elliptical and semi-regular. In addition, the field variables seem to be the red-component-stars (RCS) for the studied cluster. The smoothness of the frequency-amplitude curves, signal to noise ratio and the significant limits are play major role for deciding the real peak or frequency values. We are not rejected those amplitude peak of parabolic patterns, for which, the amplitude is greater than then significant limit of variables. The weight of pulsation frequencies is given to be 0.5 for non full cyclic variation but the amplitude is greater than significant limit. Similarly, our present analysis does not support the HADS characteristics of previous known HADS stars within stellar cluster NGC 6866. We are also proposed to new correlation between the secondary pulsation modes (depending frequencies) and the absolute magnitude of known variables.

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Identification of Stellar Sequences in various Stellar Systems : ESO65-SC03, TEUTSCH 106, TURNER 6

The spatial morphological study of studied clusters is carried out through the identified probable members within them. The field stars decontamination is performed by the statistical cleaning approach (depends on the magnitude and colour of stars within the field and cluster regions). The CMRD (colour magnitude ratio diagram) approach is used to separate stellar sequences of the cluster systems. The age, distance and reddening of each cluster are estimated through the visual inspection of best fitted isochrone in colour magnitude diagrams(CMDs). The mean proper motion values of clusters are obtained through the extracted data from PPMXL and UCAC4 catalogs. Moreover, these values are varying according to the extracted data-set from these catalogues. This variation is occurred due to their different estimation efficiency of proper motions. The TCR (two colour ratio) and TCMR (two colour magnitude ratio) values of each cluster are determined by utilizing the WISE and PPMXL catalogues, these values are found abnormal for TEUTSCH 106. In addition, the TCMR values are similar to TCR values at longer wavelength, whereas both values are far away to each other at shorter wavelength. The fraction of young stellar objects (YSOs) is also computed for each cluster.

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Dynamical analysis of NGC 110: cluster of fainter stars or data fluctuation?

The stellar enhancement of the cluster NGC 110 is investigated in various optical and infrared (IR) bands. The radial density profile of the IR region does not show a stellar enhancement in the central region of the cluster. This stellar deficiency may be occurring by undetected fainter stars due to the contamination effect of massive stars. Since, our analysis is not indicating the stellar enhancement below 16.5 mag of I band, therefore the cluster is assumed to be a group of fainter stars. The proposed magnitude scatter factor would be an excellent tool to understand the characteristic of colour-scattering of stars. The most probable members do not coincide with the model isochronic fitting in the optical bands due to poor data quality of P P MXL catalogue. The different values of the mean proper motions are found for the fainter stars of the cluster and field regions, whereas similar values are obtained for radial zones of the cluster. The symmetrical distribution of fainter stars of the core are found around the best solution of isochrone. The mass function and mass segregation studies are not possible due to higher uncertainty of the photometric data. The number of the massive stars of the cluster region is low in comparison with the field region due to completed evolution life of the massive stars.

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Search of Secondary Pulsation Modes: Globular cluster (NGC 6496)

The Fourier-discrete-peridogram are used to identify pulsation modes in variables. We have found two pulsation modes in V1 and V2 among 13 new variables as described by Abbas et al.. The five variables V9 to V13 are not shown close to periodic values by analysis of the frequency distribution of multi-band data and also create difficulty to describe their varied nature. The multi-band periodic values of V1 and V6 are matched with known literature values. The scattering of the varied nature of secondary pulsation modes is eliminated by moving average methodology. The phase curve of secondary mode is found to be more smooth compared to a prominent mode of pulsation.

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Open star cluster: formation, parameters, membership and importance

We have been represented the collective information of estimation procedures of parameters of the open clusters and put them together for showing the importance of clusters to understand their role in stellar evolution phenomenon. Moreover, we have been discussed about analytic techniques to determine the structural and dynamical properties of galactic clusters. The members of clusters provide unique opportunity to determine their basic parameters such as: age, metallicity, distance, reddening etc. The membership probabilities of stars of clusters is assigned through the various approaches and each approach provides different number of probable members of the cluster. Here, we have been briefly discussed about various approaches to determine the stellar membership within clusters.

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Statistical analysis for precise estimation of structural properties of NGC 1960

The statistical analysis of gathered of astronomical objects (such as open cluster) are excellent tools to compute its parameters and to constrain the theory of its evolution. Here, we are represented detailed structural and membership analysis of an open cluster NGC 1960 through various statistical formulas and approaches. King empirical methods provide the information about the cluster extent as 5.2 +/- 0.4 pc. The exact identification of members of cluster is needed to precise estimation of its age, reddening, metallicity etc., therefore, to identify most probable members (MPMs), we adopted the combined approach of various statistical methods (kinematic, photometric and statistical) and 712 members are satisfied needed criteria of MPMs. The basic physical parameters of the cluster such as E(B-V)=0.23+/-0.02 mag, E(V-K)=1.05+/-0.03mag, log(Age)=7.35+/-0.05, and (m-M)=11.35+/-0.10 mag are obtained using the color-color and color-magnitude diagrams. NGC 1960 is found to be located at a distance of 1.34 +/- 0.03 kpc. Using the archival proper motion catalogues, we estimate mean proper motions of NGC 1960 as 0.83+/-0.26 mas yr-1 and -6.55+/-0.23 mas yr-1 in the direction of RA and DEC, respectively.

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Dynamical properties and search of variable stars: NGC 1960

The total-to-selective extinction RV in the direction of a cluster is found to be 3.12 +/- 0.2 (close to its normal value). We derive the luminosity and mass functions for the cluster main sequence stars. The mass function slope is found to be -2.29 +/- 0.20 which is close to Salpeter value. We find evidence of mass segregation process in the cluster which is not yet dynamically relaxed. We have performed time series photometric observations to detect variable stars within star cluster NGC 1960. The DAOPHOT-II package is utilized to estimate the apparent stellar magnitudes of stars. The secondary standardization method is applied to the transformation of these apparent magnitudes into standard values. The magnitude-time diagrams (light curves) of stars are constructed to identify possible variability nature within them. The stars, having sufficient magnitude variation with time, are considered to be variable stars and their period values have computed through PERIOD04 package. These periodic values of variables are used to construct their corresponding phase diagrams. Here, we are reporting short periodic variables through the photometric analysis of science frames of whole night observations. Their type and variability nature, have been prescribed on the basis of information about amplitude, period and shape of phase diagrams. The location of variables on colour-magnitude-diagram is effective to constrain the history of stellar evolution. Our present analysis indicates that the variability fraction of massive stars is found to be high in the comparison of lighter members.

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Dynamical Features of Open Star Cluster: DOLIDZE 14

The radial density profile (RDP) of cluster DOLIDZE 14 provides the radius of 9.8 arcmin, it is based on consideration of first plateau region of RDP. This value is less than the radius which comes through consideration of the second plateau region. The stars, which are inside of the cluster radius, are used to estimate the distance-modulus and E(I-R) by fitting theoretical isochrone on the colour-magnitude diagram (CMD). The best fitted stellar isochrone of solar matellacity provides these values as 11.15 mag and 0.25 mag, respectively. These results are used to estimate the mass-function slope of the cluster, which comes to be different that of Salpeter-Value. The enhancement of brighter stars does not found within the cluster region which indicates its old age. The Mass-Segregation phenomena of cluster is found between massive and lighter stars but not found in fainter stars, having magnitude greater than 15 mag in I-band. The relaxation-time is very little in the comparison of its age. The mean proper motion of cluster's radial zones is found to be high in the comparison of field region (12.77-15.18 arcmin). This is indicative of weak-gravitational bond among the stars of the cluster.

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Complex stellar system ESO65SC03: Open cluster or remnant?

We present a complete spatial and dynamical study of the poorly populated stellar system ESO65SC03. The radial distribution of the system gives a core and cluster radii of 1.10+/-0.63 arcmin and 5.36+/-0.24 arcmin, respectively. The surface number density profile (SNDP) does not show any clear enhancement of the surface stellar number density between the stars of the system and the field regions. We derive the optimum isochrone solution for a particular grid size in the colour-magnitude diagram (CMD) using the statistical cleaning procedure. Using the statistically cleaned CMDs, we find the distance modulus, (m-M)_0, and reddening, E({B-V}), of the system to be 11.8+/-0.2 mag and 0.45 mag, respectively. The mean proper motion of this system is -5.37+/-0.81 mas/yr and 0.31+/-0.40 in RA and DEC directions, respectively. The mean proper motion of this system is found to be almost similar to the field region. The mass function for the brighter stars is found to be too high for the system to be an open cluster. These combined results place constraints on whether stellar system ESO65SC03 is a possible open star cluster remnant (POCR) or an Asterism. Our understanding is that the ESO65SC03 is in a stage of POCR by loosing their main sequence stars in the dynamic evolution processes.

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Basic Parameters of Open Star Clusters DOLIDZE 14 and NGC 110 in Infrared bands

The basic physical parameters of a poorly studied open cluster NGC 110 and an unstudied open cluster DOLIDZE 14 are estimated in the present study using the archival PPMXL and WISE catalogues. The radius of both the clusters are estimated by fitting the modified King's empirical model on their stellar density profiles. The other basic parameters of the clusters such as distance, reddening, and age are obtained by visual fitting of the Marigo's solar metallicity isochrone on their IR colour-magnitude diagrams (CMDs). The mean-proper motion of the clusters are estimated through the individual proper motion of probable members identified through the dynamical and statistical methods. The archival catalogues (JHKW1W2) are constructed for both the clusters by compiling the extracted data from the PPMXL and WISE catalogues. The various colour-excesses, such as E(J-H), E(H-K) and E(W1-W2), are estimated using the best fit theoretical isochrone on the (J-H)-H, (H-K)-H and (W1-W2)-H CMDs, respectively. The ratios of various infrared colours of the clusters are obtained through their two-colour diagrams. We also identify the most probable members in these clusters by estimating spatial, kinematic and spatio-kinematic probabilities of stars within the cluster. A correlation between the E(H-K) and E(W1-W2) is also established.

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