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All articles by
Joshi Pankaj S. | Joshi V. | Joshi G. C. | Joshi Nalini | Joshi J. | Joshi Kirti | Joshi C. | Joshi B. C. | Joshi Bhuwan | Joshi Gauri

Yogesh V. | Yogesh | Yogesh Budharam

C David J. Fernández | C David J Fernández | C Susanth | c Aleksandar Ili\' | c Marko Kosti\' | c Stevan Pilipovi\' | C Jiji | C Swathi | C | C Dharmani Bhaveshkumar

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Variable stars in the field of intermediate-age open cluster NGC 559

Joshi Yogesh C., John Ancy Anna, Maurya Jayanand, Panchal Alaxendra, Kumar Brijesh, Joshi Santosh
15 Sep 2020 astro-ph.SR astro-ph.GA arxiv.org/abs/2009.06997

This work presents the first long-term photometric variability survey of the intermediate-age open cluster NGC 559. Time-series V band photometric observations on 40 nights taken over more than three years with three different telescopes are analyzed to search for variable stars in the cluster. We investigate the data for the periodicity analysis and reveal 70 variable stars including 67 periodic variables in the target field, all of them are newly discovered. The membership analysis of the periodic variables reveal that 30 of them belong to the cluster and remaining 37 are identified as field variables. Out of the 67 periodic variables, 48 are short-period (P<1 day) variables and 19 are long-period (P>1 day) variables. The variable stars have periodicity between 3 hours to 41 days and their brightness ranges from V = 10.9 to 19.3 mag. The periodic variables belonging to the cluster are then classified into different variability types on the basis of observational properties such as shape of the light curves, periods, amplitudes, as well as their positions in the Hertzsprung-Russell (H-R) diagram. As a result, we identify one Algol type eclipsing binary, one possible blue straggler star, 3 slowly pulsating B type stars, 5 rotational variables, 11 non-pulsating variables, 2 FKCOM variables and remaining 7 are characterized as miscellaneous variables. We also identify three Eclipsing Binary stars (EBs) belonging to the field star population. The PHOEBE package is used to analyse the light curve of all four EBs in order to determine the parameters of the binary systems such as masses, temperatures and radii.

Photometric, kinematic and variability study in the young open cluster NGC 1960

Joshi Yogesh C., Maurya Jayanand, John Ancy Anna, Panchal Alaxender, Joshi Santosh, Kumar Brijesh
13 Jan 2020 astro-ph.SR astro-ph.GA arxiv.org/abs/2001.04068

We present a comprehensive photometric analysis of a young open cluster NGC 1960 (M36) along with the long-term variability study of this cluster. Based on the kinematic data of Gaia DR2, the membership probabilities of 3871 stars are ascertained in the cluster field among which 262 stars are found to be cluster members. Considering the kinematic and trigonometric measurements of the cluster members, we estimate a mean cluster parallax of 0.86+/-0.05 mas and mean proper motions of mu_RA = -0.143+/-0.008 mas/yr, mu_Dec = -3.395+/-0.008 mas/yr. We obtain basic parameters of the cluster such as E(B-V) = 0.24+/-0.02 mag, log(Age/yr)=7.44+/-0.02, and distance = 1.17+/-0.06 kpc. The mass function slope in the cluster for the stars in the mass range of 0.72-7.32 M_solar is found to be \gamma = -1.26+/-0.19. We find that mass segregation is still taking place in the cluster which is yet to be dynamically relaxed. This work also presents first high-precision variability survey in the central 13'x13' among which 72 are periodic variables. Among them, 59 are short-period (P<1 day)and 13 are long-period (P>1 day). The variable stars have V magnitudes ranging between 9.1 to 19.4 mag and periods between 41 minutes to 10.74 days. On the basis of their locations in the H-R diagram, periods and characteristic light curves, the 20 periodic variables belong to the cluster. We classified them as 2 delta-Scuti, 3 gamma-Dor, 2 slowly pulsating B stars, 5 rotational variables, 2 non-pulsating B stars and 6 as miscellaneous variables.

A Long-term photometric variability and spectroscopic study of luminous blue variable AF And in M31

Joshi Yogesh C., Sharma Kaushal, Gangopadhyay Anjasha, Gokhale Rishikesh, Misra Kuntal
05 Aug 2019 astro-ph.SR arxiv.org/abs/1908.01893

We present photometric and spectroscopic analysis of the Hubble Sandage variable AF And in M31. The data has been taken under the Nainital Microlensing Survey during 1998-2002 and follow-up observations were carried out until 2011. During this period, photometric observations in Cousins R and I bands were obtained for 169 nights spanning over about 5000 days. AF And has shown a prominent outburst around mid-January in 1999 followed by a gradual decrease in brightness of about 1.5 mag in the next 3 years with a declining rate of ~0.0015 mag/day leading to a quiescent phase at the end of 2001. After lying low for about 9 years, AF And again went through a secondary outburst phase in late 2010 with an amplitude of 0.44 mag where it lasted for one year before fading back to its quiescence phase. The spectroscopic observations of AF And show prominent Balmer and He I emission lines along with the comparatively weaker FeII and [FeII] emissions. Asymmetric emission line profiles in its spectrum imply the mass loss rate of about 2.2x10^{-4} solar mass per yr through the stellar winds in the photosphere. Using SED fitting, we find the photospheric temperature of 33,000+/-3000 K during the visual minimum. Using a weak P Cygni profile of HeI emission line, the wind terminal velocity for AF And is found to be around 280-300 km/s.

Reddening map and recent star formation in the Magellanic Clouds based on OGLE IV Cepheids

Joshi Yogesh C., Panchal Alaxender
11 Jun 2019 astro-ph.GA arxiv.org/abs/1906.04481

In the present study, we examine reddening distribution across the LMC and SMC through largest data on Classical Cepheids provided by the OGLE Phase IV survey. The V and I band photometric data of 2476 fundamental mode (FU) and 1775 first overtone mode (FO) Cepheids in the LMC and 2753 FU and 1793 FO Cepheids in the SMC are analyzed for their Period-Luminosity (P-L) relations. We convert period of FO Cepheids to corresponding period of FU Cepheids before combining the two modes of Cepheids. The reddening analysis is performed on 133 segments covering a total area of about 154.6 deg^2 in the LMC and 136 segments covering a total area of about 31.3 deg^2 in the SMC. By comparing with well calibrated P-L relations of these two galaxies, we determine reddening E(V-I) in each segment. Using reddening values in different segments across the LMC and SMC, reddening maps are constructed. We find clumpy structures in the reddening distributions of the LMC and SMC. From the reddening map of the LMC, highest reddening of E(V-I) = 0.466 mag is traced in the region centered at RA ~ 85.13 deg, DEC ~ -69.34 deg which is in close vicinity of the star forming HII region 30 Doradus. In the SMC, maximum reddening of E(V-I) = 0.189 mag is detected in the region centered at RA ~ 12.10 deg, DEC ~ -73.07 deg. The mean reddening values in the LMC are estimated as E(V-I) = 0.113+/-0.060 mag and E(B-V) = 0.091+/-0.050 mag; and that in the SMC are E(V-I) = 0.049+/-0.070 mag and E(B-V) = 0.038+/-0.053 mag. The period-age relations are used to derive the age of the Cepheid populations in the LMC and SMC. We investigate age and spatio-temporal distributions of Cepheids to understand the recent star formation history in the Magellanic Clouds (MCs) and found an evidence of a common enhanced Cepheid population in the MCs at around 200 Myr ago which appears to have occurred due to close encounter between the two clouds.

VR CCD photometry of variable stars in globular cluster NGC 4147

Lata Sneh, Pandey A. K., Pandey J. C., Yadav R. K. S., Pandey Shashi B., Gupta Aashish, Bangia Tarun, Chand Hum, Jaiswar Mukesh K., Joshi Yogesh C.
20 May 2019 astro-ph.SR arxiv.org/abs/1905.08249

We present results of a search for variable stars in a region of the globular cluster NGC 4147 based on photometric observations with 4k x 4k CCD imager mounted at the axial port of the recently installed 3.6 m Devasthal optical telescope at Aryabhatta Research Institute of Observational Sciences, Nainital, India. We performed time series photometry of NGC 4147 in V and R bands, and identified 42 periodic variables in the region of NGC 4147, 28 of which have been detected for the first time. Seventeen variable stars are located within the half light radius $\lesssim$ 0.48 arcmin, of which 10 stars are newly identified variables. Two of 10 variables are located within the core radius $\lesssim$ 0.09 arcmin. Based on the location in the $V/(V-R)$ colour magnitude diagram and variability characteristics, 7, 8, 5 and 1 newly identified probable member variables are classified as RRc, EA/E, EW and SX Phe, respectively. The metallicity of NGC 4147 estimated from light curves of RRab and RRc stars with the help of Fourier decomposition is found to be characteristics of Oosterhoff II. The distance derived using light curves of RRab stars is consistent with that obtained from the observed $V/(V-R)$ colour-magnitude diagram.

Probing nearby Galactic structure through open star clusters

Joshi Yogesh C.
28 Jul 2017 astro-ph.GA astro-ph.SR arxiv.org/abs/1707.09125

Based on the most complete sample of Galactic open star clusters up to 1.8 kpc, we performed statistical analysis of the distribution of open cluster parameters in order to understand the Galactic structure. The geometrical characteristics of a large number of open clusters enable us to determine solar offset and scale height and distribution of reddening material in the Galactic neighbourhood.

Long-term photometric study of a faint W UMa binary in the direction of M31

Joshi Yogesh C., Jagirdar Rukmini
28 Jul 2017 astro-ph.SR arxiv.org/abs/1707.09124

We carry out a re-analysis of the photometric data in Cousin RI bands which was taken under the Nainital Microlensing Survey during 1998 to 2002 with the aim to detect gravitational microlensing events in the direction of M31. Here, we do photometric analysis of a faint W UMa binary CSS_J004259.3+410629 identified in the target field. The orbital period of this star is found to be 0.266402+/-0.000018 day. The photometric mass ratio, q, is found to be 0.28+/-0.01. The photometric light curves are investigated using the Wilson-Devinney code and absolute parameters are determined using empirical relations which provide masses and radii of the binary as M1 = 1.19+/-0.09 Msun, M2 = 0.33+/-0.02 Msun and R1 = 1.02+/-0.04 Rsun, R2 = 0.58+/-0.08 Rsun based on the Rc band data. Almost similar values are found by analysing Ic band data. From the photometric light curve examination, the star is understood to be a low mass-ratio over-contact binary of A-subtype with a high fill-out factor of about 47%. The binary system is found to be located approximately at a distance of 2.64+/-0.03 kpc having a separation of 2.01+/-0.05 Rsun between the two components.

Investigation of open clusters based on IPHAS and APASS survey data

Dambis Andrei, Glushkova Elena, Berdnikov Leonid, Joshi Yogesh C., Pandey Anil K.
13 Nov 2016 astro-ph.GA arxiv.org/abs/1611.04110

We adapt the classical Q-method based on a reddening-free parameter constructed from three passband magnitudes to thwe filter set of IPHAS survey and combine it with the maximum-likelihood-based cluster parameter estimator by Naylor and Jeffries (2006) to determine the extinction, heliocentric distances, and ages of young open clusters using Halpha and ri data. Themethod is also adapted for the case of signific ant variations of extinction across the cluster rfield. Our technique is validated by comparing the colour excesses, disdtances, and ages determined in this study with the most bona fide values reported for the 18 well-studied young open clusters in the past, and a fairly good agreement is found between our extinction and distance estimnates and earlier published results. although our age estimates are not very consistent with those published by other authors. We also show that individual extinction values can be determined rather accurately for stars with (r-i)0>0.1. Our results open up a prospect fpr determining a uniform set of parameters for northern clusters based on homogeneous photometric data, and for searching for new, hitherto undiscovered open clusters.

Study of open clusters within 1.8 kpc and understanding the Galactic structure

Joshi Yogesh C., Dambis Andrei, Pandey Anil K., Joshi Santosh
21 Jun 2016 astro-ph.GA astro-ph.SR arxiv.org/abs/1606.06425

Based on an almost complete sample of Galactic open star clusters within 1.8 kpc, we perform a comprehensive statistical analysis of various cluster parameters like spatial position, age, size, mass and extinction in order to understand the general properties of the open cluster system in the Galaxy and the Galactic structure. Based on the distribution of 1241 open clusters about the Galactic plane and in different age bins, we find the average Galactic scale height as Zh = 60+/-2 pc for the youngest cluster population having Age <700 Myr, however, it increases up to 64+/-2 pc when we also include older population of clusters. The solar offset is found to be 6.2+/-1.1 pc above the formal Galactic plane. We derive a local mass density of \rho_0 = 0.090+/-0.005 Msun/pc^3 and found a negligibly small amount of dark matter in the solar neighbourhood. The reddening in the direction of clusters suggests a strong correlation with their vertical distance from the Galactic plane having a respective slope of dE(B-V)/dz = 0.40+/-0.04 and 0.42+/-0.05 mag/kpc below and above the GP. We observe a linear mass-radius and mass-age relations in the open clusters and derive a slope of dR/d(logM) = 2.08+/-0.10 and d(logM)/d(logT) = -0.36+/-0.05,respectively.

The hot Gamma-Doradus and Maia stars

Balona L. A., Engelbrecht C. A., Joshi Yogesh C., Joshi S., Sharma K., Semenko E., Pandey G., Chakradhari N. K., Mkrtichian David, Hema B. P.
21 Jun 2016 astro-ph.SR arxiv.org/abs/1606.06426

The hot $\gamma$~Doradus stars have multiple low frequencies characteristic of $\gamma$~Dor or SPB variables, but are located between the red edge of the SPB and the blue edge of the $\gamma$~Dor instability strips where all low-frequency modes are stable in current models of these stars. Though $\delta$~Sct stars also have low frequencies, there is no sign of high frequencies in hot $\gamma$~Dor stars. We obtained spectra to refine the locations of some of these stars in the H-R diagram and conclude that these are, indeed, anomalous pulsating stars. The Maia variables have multiple high frequencies characteristic of $\beta$~Cep and $\delta$~Sct stars, but lie between the red edge of the $\beta$~Cep and the blue edge of the $\delta$~Sct instability strips. We compile a list of all Maia candidates and obtain spectra of two of these stars. Again, it seems likely that these are anomalous pulsating stars which are currently not understood.