arXiv subjects

N. Kameswara Rao

Publications and source records attributed to N. Kameswara Rao.

At least 37 records · Page 2Linked to original sources

arXiv2017

Unveiling Vela - Time Variability of Interstellar Lines in the Direction of the Vela Supernova Remnant II. Na D and Ca II

In a survey conducted between 2011-12 of interstellar Na I D line profiles in the direction of the Vela supernova remnant, a few lines of sight showed dramatic changes in low velocity absorption components with respect to profiles from 1993-1994 reported by Cha & Sembach. Three stars - HD 63578, HD 68217 and HD 76161 showed large decrease in strength over the 1993-2012 interval. HD 68217 and HD 76161 are associated with the Vela SNR whereas HD 63578 is associated with $γ^2$ Velorum wind bubble. Here, we present high spectral resolution observations of Ca II K lines obtained with the Southern African Large Telescope (SALT) towards these three stars along with simultaneous observations of Na I D lines. These new spectra confirm that the Na D interstellar absorption weakened drastically between 1993-1994 and 2011-2012 but show for the first time that the Ca II K line is unchanged between 1993-1994 and 2015. This remarkable contrast between the behaviour of Na D and Ca II K line absorption lines is a puzzle concerning gas presumably affected by the outflow from the SNR and the wind from $γ^2$ Velorum.

astro-ph.SR↗

arXiv2016

In-orbit Performance of UVIT and First Results

The performance of the ultraviolet telescope (UVIT) on-board ASTROSAT is reported. The performance in orbit is also compared with estimates made from the calibrations done on the ground. The sensitivity is found to be within ~15% of the estimates, and the spatial resolution in the NUV is found to exceed significantly the design value of 1.8 arcsec and it is marginally better in the FUV. Images obtained from UVIT are presented to illustrate the details revealed by the high spatial resolution. The potential of multi-band observations in the ultraviolet with high spatial resolution is illustrated by some results.

astro-ph.IM↗

arXiv2016

In-orbit Performance of UVIT on ASTROSAT

We present the in-orbit performance and the first results from the ultra-violet Imaging telescope (UVIT) on ASTROSAT. UVIT consists of two identical 38cm coaligned telescopes, one for the FUV channel (130-180nm) and the other for the NUV (200-300nm) and VIS (320-550nm) channels, with a field of view of 28 $arcmin$. The FUV and the NUV detectors are operated in the high gain photon counting mode whereas the VIS detector is operated in the low gain integration mode. The FUV and NUV channels have filters and gratings, whereas the VIS channel has filters. The ASTROSAT was launched on 28th September 2015. The performance verification of UVIT was carried out after the opening of the UVIT doors on 30th November 2015, till the end of March 2016 within the allotted time of 50 days for calibration. All the on-board systems were found to be working satisfactorily. During the PV phase, the UVIT observed several calibration sources to characterise the instrument and a few objects to demonstrate the capability of the UVIT. The resolution of the UVIT was found to be about 1.4 - 1.7 $arcsec$ in the FUV and NUV. The sensitivity in various filters were calibrated using standard stars (white dwarfs), to estimate the zero-point magnitudes as well as the flux conversion factor. The gratings were also calibrated to estimate their resolution as well as effective area. The sensitivity of the filters were found to be reduced up to 15\% with respect to the ground calibrations. The sensitivity variation is monitored on a monthly basis. UVIT is all set to roll out science results with its imaging capability with good resolution and large field of view, capability to sample the UV spectral region using different filters and capability to perform variability studies in the UV.

astro-ph.IM↗

arXiv2015

Unveiling Vela - Time Variability of Na I D lines in the Direction of the Vela Supernova Remnant

High-resolution spectral profiles of Na I D lines from the interstellar medium towards 64 stars in the direction of the Vela supernova remnant are presented. This survey conducted mostly between 2011-12 complements an earlier survey of the same stars by Cha & Sembach done in the 1993-96 period. The interval of 15 to 18 years provides a base line to search for changes in the interstellar profiles. Dramatic disappearance of strong absorption components at low radial velocity is seen towards three stars - HD 63578, HD 68217, HD 76161 - over 15-18 years; HD 68217 and HD 76161 are associated with the Vela SNR but HD 63578 is likely associated with the wind bubble of g2 Velorum. The vanishing of these cold neutral clouds in the short time of 15 to 18 years needs some explanation. Other changes are seen in high-velocity Na D components.

astro-ph.SR↗

arXiv2014

Mid-Infrared Variations of R Coronae Borealis Stars

Mid-infrared photometry of R Coronae Borealis stars obtained from various satellites from IRAS to WISE has been utilized in studying the variations of the circumstellar dust's contributions to the spectral energy distribution of these stars. The variation of the fractional coverage (R) of dust clouds and their blackbody temperatures (T$_d$) have been used in trying to understand the dust cloud evolution over the three decades spanned by the satellite observations. In particular, it is shown that a prediction R $ \propto T_d^4$ developed in this paper is satisfied, especially by those stars for which a single collection of cloud dominates the IR fluxes. Correlations of R with photospheric abundance and luminosity of the stars are explored.

astro-ph.SR↗

arXiv2014

A search for diffuse bands in fullerene planetary nebulae: evidence of diffuse circumstellar bands

Large fullerenes and fullerene-based molecules have been proposed as carriers of diffuse interstellar bands (DIBs). The recent detection of the most common fullerenes (C60 and C70) around some planetary nebulae (PNe) now enable us to study the DIBs towards fullerene-rich space environments. We search DIBs in the optical spectra towards three fullerene-containing PNe (Tc 1, M 1-20, and IC 418). Special attention is given to DIBs which are found to be unusually intense towards these fullerene sources. In particular, an unusually strong 4428A absorption feature is a common charateristic of fullerene PNe. Similar to Tc 1, the strongest optical bands of neutral C60 are not detected towards IC 418. Our high-quality (S/N > 300) spectra for PN Tc 1, together with its large radial velocity, permit us to search for the presence of diffuse bands of circumstellar origin, which we refer to as diffuse circumstellar bands (DCBs). We report the first tentative detection of two DCBs at 4428 and 5780 A in the fullerene-rich circumstellar environment around the PN Tc 1. Laboratory and theoretical studies of fullerenes in their multifarious manifestations (carbon onions, fullerene clusters, or even complex species formed by fullerenes and other molecules like PAHs or metals) may help solve the mystery of some of the diffuse band carriers.

astro-ph.SR↗

arXiv2014

On the binary helium star DY Centauri: Chemical composition and evolutionary state

DY Cen has shown a steady fading of its visual light by about 1 magnitude in the last 40 years suggesting a secular increase in its effective temperature. We have conducted non-LTE and LTE abundance analyses to determine the star's effective temperature, surface gravity, and chemical composition using high-resolution spectra obtained over two decades. The derived stellar parameters for three epochs suggest that DY Cen has evolved at a constant luminosity and has become hotter by about 5000 K in 23 years. We show that the derived abundances remain unchanged for the three epochs. The derived abundances of the key elements, including F and Ne, are as observed for the extreme helium stars resulting from a merger of an He white dwarf with a C-O white dwarf. Thus, DY Cen by chemical composition appears to be also a product of a merger of two white dwarfs. This appearance seems to be at odds with the recent suggestion that DY Cen is a single-lined spectroscopic binary.

astro-ph.SR↗

arXiv2014

High-resolution optical spectroscopy of the R Coronae Borealis star V532 Ophiuchi at maximum light

High-resolution optical spectra of the R Coronae Borealis (RCB) star V532 Oph at light maximum are discussed. The absolute visual magnitude M_V of the star is found to be -4.9 \pm 0.5. The elemental abundances suggest the star belongs to the majority class of RCB stars but is among the most O-poor of this class with mild enhancements of heavy elements Y, Zr, Ba and La. The C_2 Swan bands are weak in V532 Oph relative to R CrB. Other aspects of the high-resolution spectrum confirm that V532 Oph is representative of majority RCBs, i.e., the radial velocity is variable, circumstellar material is present and the photosphere feeds a high-velocity stellar wind.

astro-ph.SR↗

arXiv2014

Bhavnagar Telescope: the most widely travelled telescope in the country

In the last decade of the 19th century Maharaja Takhtasingji Observatory was built at Poona (1888-1912) under the supervision of K.D.Naegamavala, with the grant from Maharaja of Bhavnagar (from where the name Bhavnagar Telescope must have originated. The story of this telescope from its inception to the current status is traced. IIA Archives has been extensively used to resource information for this note.

physics.hist-ph↗

arXiv2014

Charles Michie Smith - founder of the Kodaikanal (Solar Physics) Observatory and beginnings of physical astronomy in India

It has been about 115 years since the establishment of Kodaikanal Observatory as an extension of the original Madras Observatory, which has evolved into the present Indian Institute of Astrophysics at Bangalore. It is also the first mountain observatory in India. Charles Michie Smith was the man who selected the site, established the observatory and directed it for the first 12 years. He was also the man who recruited John Evershed, discoverer of the famous Evershed effect and established Kodaikanal Observatory as a major centre for solar physics. Michie Smith, the person and the establishment of Kodaikanal Observatory are discussed here in the context of the early studies in physical astronomy (observational astrophysics) in India.

physics.hist-ph↗

arXiv2013

Dust around R Coronae Borealis stars: II. Infrared emission features in a H-poor environment

Residual Spitzer/IRS spectra for a sample of 31 R Coronae Borealis (RCB) stars are presented and discussed in terms of narrow emission features superimposed on the quasi-blackbody continuous infrared emission. A broad ~6-10 um dust emission complex is seen in the RCBs showing an extreme H-deficiency. A secondary and much weaker ~11.5-15 um broad emission feature is detected in a few RCBs with the strongest ~6-10 um dust complex. The Spitzer infrared spectra reveal for the first time the structure within the ~6-10 um dust complex, showing the presence of strong C-C stretching modes at ~6.3 and 8.1 um as well as of other dust features at ~5.9, 6.9, and 7.3 um, which are attributable to amorphous carbonaceous solids with little or no hydrogen. The few RCBs with only moderate H-deficiencies display the classical 'unidentified infrared bands (UIRs)' and mid-infrared features from fullerene-related molecules. In general, the characteristics of the RCB infrared emission features are not correlated with the stellar and circumstellar properties, suggesting that the RCB dust features may not be dependent on the present physical conditions around RCB stars. The only exception seems to be the central wavelength of the 6.3 um feature, which is blue-shifted in those RCBs showing also the UIRs, i.e., the RCBs with the smallest H deficiency.

astro-ph.SR↗

arXiv2013

The changing nebula around the hot R Coronae Borealis star DY Centauri

Among the distinguishing characteristics of the remarkable hot R Coronae Borealis star DY Cen, which was recently found to be a spectroscopic binary, is the presence of nebular forbidden lines in its optical spectrum. A compilation of photometry from 1970 to the present suggests that the star has evolved to higher effective temperatures. Comparison of spectra from 2010 with earlier spectra show that between 2003 and 2010, the 6717 and 6730 A emission lines of [S II] underwent a dramatic change in their fluxes suggesting an increase in the nebula's electron density of 290 cm-3 to 3140 cm-3 from 1989 to 2010 while the stellar temperature increased from 19500 K to 25000 K. The nebular radius is about 0.02 pc, 60000 times bigger than the semimajor axis of DY Cen binary system. Rapid changes of stellar temperature and its response by the nebula demonstrate stellar evolution in action.

astro-ph.SR↗

arXiv2012

The hot R Coronae Borealis star DY Centauri is a binary

The remarkable hot R Coronae Borealis star DY Cen is revealed to be the first and only binary system to be found among the R Coronae Borealis (RCB) stars and their likely relatives, including the Extreme Helium stars and the hydrogen-deficient carbon stars. Radial velocity determinations from 1982-2010 have shown DY Cen is a single-lined spectroscopic binary in an eccentric orbit with a period of 39.67 days. It is also one of the hottest and most H-rich member of the class of RCB stars. The system may have evolved from a common-envelope to its current form.

astro-ph.SR↗

arXiv2012

High-resolution optical spectroscopy of DY Cen: diffuse interstellar bands in a proto-fullerene circumstellar environment?

We search high-resolution and high-quality VLT/UVES optical spectra of the hot R Coronae Borealis (RCB) star DY Cen for electronic transitions of the C60 molecule and diffuse interstellar bands (DIBs). We report the non-detection of the strongest C60 electronic transitions (e.g., those at ~3760, 3980, and 4024 A). Absence of C60 absorption bands may support recent laboratory results, which show that the ~7.0, 8.5, 17.4, and 18.8 um emission features seen in DY Cen - and other similar objects with PAH-like dominated IR spectra - are attributable to proto-fullerenes or fullerene precursors rather than to C60. DIBs towards DY Cen are normal for its reddening; the only exception is the DIB at 6284 A (possibly also the 7223A DIB) that is found to be unusually strong. We also report the detection of a new broad (FWHM~2 A) and unidentified feature centered at ~4000 A. We suggest that this new band may be related to the circumstellar proto-fullerenes seen at infrared wavelengths.

astro-ph.SR↗

arXiv2011

Nebula around R Corona Borealis

The star R Corona Borealis (R CrB) shows forbidden lines of [O II], [N II], and [S II] during the deep minimum when the star is fainter by about 8 to 9 magnitudes from normal brightness, suggesting the presence of nebular material around it. We present low and high spectral resolution observations of these lines during the ongoing deep minimum of R CrB, which started in July 2007. These emission lines show double peaks with a separation of about 170 km/s. The line ratios of [S II] and [O II] suggest an electron density of about 100 cm$^{-3}$. We discuss the physical conditions and possible origins of this low density gas. These forbidden lines have also been seen in other R Coronae Borealis stars during their deep light minima and this is a general characteristic of these stars, which might have some relevance to their origins.

astro-ph.SR↗

arXiv2011

The Astronomical Significance of 'Nilurallu', The Megalithic Stone Alignment at Muradoddiin Andhra Pradesh, India

The stone alignment 'Nilurallu' at Murardoddi is a megalithic monument containing standing stones of 12 to 16 feet high that are arranged somewhat in a squarish pattern. This is one of the stone alignments listed by Allchin (1956) as a non-sepulchal array that might have some astronomical connotations. This impressive stone alignment seems to be similar to that at Vibhuthihalli, that was studied earlier, but constructed with much larger stones. The observations conducted by us show that the rows of stones are aligned to the directions of sunrise (and sunset) on calendrically-important events, like equinoxes and solstices. In contrast to Vibhuthihalli, the shadows of stones provide a means of measuring shorter intervals of time. Key words: Observational astronomy, megalithic astronomy, stone alignments, equinoxes, solstices, sunrises

physics.pop-ph↗

arXiv2011

Dust around R Coronae Borealis stars: I. Spitzer/IRS observations

Spitzer/IRS spectra from 5 to 37 um for a complete sample of 31 R Coronae Borealis stars (RCBs) are presented. These spectra are combined with optical and near-infrared photometry of each RCB at maximum light to compile a spectral energy distribution (SED). The SEDs are fitted with blackbody flux distributions and estimates made of the ratio of the infrared flux from circumstellar dust to the flux emitted by the star. Comparisons for 29 of the 31 stars are made with the IRAS fluxes from three decades earlier: Spitzer and IRAS fluxes at 12 um and 25 um are essentially equal for all but a minority of the sample. For this minority, the IRAS to Spitzer flux ratio exceeds a factor of three. The outliers are suggested to be stars where formation of a dust cloud or dust puff is a rare event. A single puff ejected prior to the IRAS observations may have been reobserved by Spitzer as a cooler puff at a greater distance from the RCB. RCBs which experience more frequent optical declines have, in general, a circumstellar environment containing puffs subtending a larger solid angle at the star and a quasi-constant infrared flux. Yet, the estimated subtended solid angles and the blackbody temperatures of the dust show a systematic evolution to lower solid angles and cooler temperatures in the interval between IRAS and Spitzer. Dust emission by these RCBs and those in the LMC is similar in terms of total 24 um luminosity and [8.0]-[24.0] color index.

astro-ph.SR↗

arXiv2011

Early Pioneers of Telescopic Astronomy in India: G.V.Juggarow and His Observatory

G.V.Juggarow was one of the early pioneers of observational astronomy in India who built his own observatory in 1840 at Vizagapatnam. His legacy was continued by his son-in-law A.V.Nursing Row till 1892, his daughter till 1894, Madras Government till 1898, and his grandson till it became inactive in early 1900s. Observations of comets, planetary transits, stellar occultations etc have been continued along with meteorological observations. Celestial photography was also started at the observatory. After 1898 the observatory's activities were re-oriented towards meteorology. The establishment of the observatory, the personalities involved and the final closing of the institution are described here.

astro-ph.IM↗