arXiv subjects

Santosh Joshi

Publications and source records attributed to Santosh Joshi.

62 records · Page 4Linked to original sources

arXiv2013

Population I Cepheids and star formation history of the Large Magellanic Cloud

In this paper we study the Cepheids distribution in the Large Magellanic Cloud (LMC) as a function of their ages using data from the OGLE III photometric catalogue. To determine age of the Pop I Cepheids, we derived a period-age (PA) relationship using the Cepheids found in the LMC star clusters. We find two peaks in the period distribution at logP =0.49+/-0.01 and logP =0.28+/-0.01 days which correspond to fundamental and first overtone pulsation modes, respectively. Ages of the Cepheids are used to understand star formation scenario in the LMC in last 30-600 Myr. The age distribution of the LMC Cepheids is found to have a peak at log(Age)=8.2+/-0.1. This suggests that major star formation event took place at about 125-200 Myr ago which may have been triggered by a close encounter between the SMC and the LMC. Cepheids are found to be asymmetrically distributed throughout the LMC and many of them lie in clumpy structures along the bar. The frequency distribution of Cepheids suggests that most of the clumps are located to the eastern side of the LMC optical center.

astro-ph.GA↗

arXiv2011

Discovery of the first giant double radio relic in a galaxy cluster found in the PLANCK Sunyaev-Zel'dovich cluster survey: PLCK G287.0+32.9

We report the discovery of large scale diffuse non-thermal radio emission in PLCK G287.0+32.9, an exceptionally hot (T ~ 13 keV), massive and luminous galaxy cluster, strongly detected by the PLANCK satellite in a recent, all-sky blind search for new clusters through Sunyaev-Zel'dovich effect. Giant Metrewave Radio telescope 150 MHz and Very Large Array 1.4 GHz radio data reveal a pair of giant (>1 Mpc) "arc" shaped peripheral radio-relics (signatures of shock waves) of unprecedented scale (linear separation ~4.4 Mpc at redshift 0.39), located at distances from the cluster center which are about 0.7 and 1.3 of the cluster's virial radius. Another possible giant relic and a radio-halo is detected closer to the cluster center. These relic sources are unique "signposts" of extremely energetic mergers and shocks (both ongoing and past), that are assembling and heating up this very massive galaxy cluster. They are also a probe of the filamentary cosmic-web structure beyond the cluster virial radius. Optical imaging with the IUCAA 2 meter telescope and XMM-Newton X-ray data confirm a very rich galaxy cluster with a morphologically disturbed core region, suggesting a dynamically perturbed merging system.

astro-ph.CO↗

arXiv2011

Photometric Study and Detection of Variable Stars in the Open Clusters - I : NGC 6866

We present results of a variability search in the intermediate-age open cluster NGC 6866 from 29 nights over two observing seasons. We find 28 periodic variables, of which 19 are newly identified. The periods of these variables, which have V magnitudes from 11.5 to 19.3 mag, range from ~ 48 min to 37 d. We detected several delta-Scuti stars, some of which are of high amplitude, as well as gamma-Doradus, rotational variables and eclipsing binaries. In order to study the physical properties of the cluster, we obtained UBVRI photometry of all the stars on a good photometric night. The radial distribution of the stellar surface density shows that the cluster has a radial extent of about 7 arcmin (~3 pc) with a peak density of 5.7+/-0.7 stars/arcmin^2 at the cluster center. The colour-colour diagram indicates a reddening of E(B-V) = 0.10 mag towards NGC 6866. A distance of ~ 1.47 kpc and an age of ~ 630 Myr is estimated from the colour-magnitude diagram using the theoretical isochrones of solar metallicity.

astro-ph.SR↗

arXiv2009

A giant radio jet of very unusual polarization in a single-lobed radio galaxy

We report the discovery of a very unusual, predominantly one-sided radio galaxy CGCG049-033. Its radio jet, the largest detected so far, emits strongly polarized synchrotron radiation and can be traced all the way from the galactic nucleus to the hot spot located ~440 kpc away. This jet emanates from an extremely massive black-hole (mass > 10^9 times solar mass) and forms a strikingly compact radio lobe. To a surface brightness contrast of at least 20 no radio lobe is detected on the side of the counter-jet, which is similar to the main jet in brightness upto the scale of tens of kpc. Thus, contrary to the nearly universal trend, the brightness asymmetry in this radio galaxy increases with distance from the nucleus. With several unusual properties, including a predominantly toroidal magnetic field, this Fanaroff-Riley type II (FR-II) mega-jet is an exceptionally useful laboratory for testing the role of magnetic field in jet stabilization and radio lobe formation.

astro-ph.CO↗

arXiv2008

Superdisks in Radio Galaxies: Jet-Wind Interactions

Taking a clue from their sharp-edged (strip-like) morphology observed in several cases, a new mechanism is proposed for the formation of the emission gaps seen between the radio lobes of many powerful extragalactic double radio sources. Canonical understanding of the radio gaps invokes either blocking of the back-flowing lobe plasma by the denser interstellar medium (ISM) of the host galaxy, or "squeezing" of the radio bridge in the middle through buoyancy force exerted by either the ISM or the surrounding intra-cluster medium (ICM). These pictures encounter difficulties in explaining situations where the sharp-edged radio gaps associated with non-cluster radio galaxies have widths running into several tens (even hundreds) of kiloparsecs. More particularly, the required dense high-pressure ISM/ICM is likely to be lacking at least in the case of high-redshift radio galaxies. We propose here that radio emission gaps in at least such cases could arise from a dynamical interaction between the powerful thermal wind outflowing from the active galactic nucleus and the back-flowing synchrotron plasma in the two radio lobes, which occurs once the rapidly advancing jets have crossed out of the wind zone into the intergalactic medium. A simple analytical scheme is presented to explore the plausibility of the side-ways confinement of the thermal wind by the radio lobe pair, which would "freeze" pancake shaped conduits in the space, along which the hot, metal enriched wind from the AGN can escape (roughly orthogonal to the radio axis). Some other possible consequences of this scenario are pointed out.

astro-ph↗

arXiv2007

A giant radio jet ejected by an ultramassive black hole in a single-lobed radio galaxy

We report the discovery of a very unusual, highly asymmetric radio galaxy whose radio jet, the largest yet detected, emits strongly polarized synchrotron radiation and can be traced all the way from the galactic nucleus to the hot spot located ~440 kpc away. This jet emanates from an extremely massive black hole (>10^9 solar mass) and forms a strikingly compact radio lobe. No radio lobe is detected on the side of the counterjet, even though it is similar to the main jet in brightness up to a scale of tens of kiloparsecs. Thus, contrary to the nearly universal trend, the brightness asymmetry in this radio galaxy increases with distance from the nucleus. With several unusual properties, including a predominantly toroidal magnetic field, this Fanaroff-Riley type II megajet is an exceptionally useful laboratory for testing the role of magnetic field in jet stabilization and radio lobe formation.

astro-ph↗

arXiv2006

The Nainital-Cape Survey -- II:Report for pulsation in five chemically peculiar A-type stars and presentation of 140 null results

To search photometric variability in chemically peculiar A type stars in the northern hemisphere. High-speed photometric observations of Ap and Am star candidates have been carried out from ARIES (Manora Peak, Nainital) using a three-channel fast photometer attached to the ARIES 104-cm Sampurnanand telescope. This paper presents three new variables: HD 113878, HD 118660 and HD 207561. During the time span of the survey (1999 December to 2004 January) pulsations of the $δ$ Sct type were also found for the two evolved Am stars HD 102480 and HD 98851, as reported in Joshi et al. (2002, 2003). Additionally, we present 140 null results of the survey for this time span. The star HD 113878 pulsates with a period of 2.31 hr, which is typical of $δ$ Sct stars. HD 118660 exhibits multi-periodic variability with a prominent period of nearly 1 hr. These periods need to be investigated and make HD 118660 a particularly interesting target for further observations. For HD 207561, a star classified as Am, a probable pulsation with a period of 6 min was found in the light curves obtained on two consecutive nights. Both HD 102480 and HD 98851 exhibit unusual alternating high and low amplitude maxima, with a period ratio of 2:1. The analysis of the null results confirms the photometric quality of the Nainital

astro-ph↗

arXiv2003

Discovery of unusual pulsations in the cool, evolved Am stars HD 98851 and HD 102480

The chemically peculiar (CP) stars HD 98851 and HD 102480 have been discovered to be unusual pulsators during the ``Naini Tal Cape Survey'' programme to search for pulsational variability in CP stars. Time series photometric and spectroscopic observations of these newly discovered stars are reported here. Fourier analyses of the time series photometry reveal that HD 98851 is pulsating mainly with frequencies 0.208 mHz and 0.103 mHz, and HD 102480 is pulsating with frequencies 0.107 mHz, 0.156 mHz and 0.198 mHz. The frequency identifications are all subject to 1 d$^{-1}$ cycle count ambiguities. We have matched the observed low resolution spectra of HD 98851 and HD 102480 in the range 3500-7400 Åwith theoretical synthetic spectra using Kurucz models with solar metallicity and a micro-turbulent velocity 2 km s$^{-1}$. These yield $T_{eff}=7000\pm250$ K, log $g=3.5 \pm 0.5$ for HD 98851 and $T_{eff} = 6750 \pm 250$ K, log $g = 3.0 \pm 0.5$ for HD 102480. We determined the equivalent H-line spectral class of these stars to be F1 IV and F3 III/IV, respectively. A comparison of the location of HD 98851 and HD 102480 in the HR diagram with theoretical stellar evolutionary tracks indicates that both stars are about 1-Gyr-old, 2-$M_{\odot}$ stars that lie towards the red edge of the $δ$ Sct instability strip. We conclude that HD 98851 and HD 102480 are cool, evolved Am pulsators. The light curves of these pulsating stars have alternating high and low amplitudes, nearly harmonic (or sub-harmonic) period ratios, high pulsational overtones and Am spectral types. This is unusual for both Am and $δ$ Sct pulsators, making these stars interesting objects.

astro-ph↗