Sunetra Giridhar
Articles written in Journal of Astrophysics and Astronomy
Volume 4 Issue 2 June 1983 pp 75-107
Abundance analysis of selected cepheids and the galactic distribution of metallicity
We have determined the atmospheric abundances of selected Cepheids in order to study the large-scale chemical inhomogeneities across the galactic disk. The classical Cepheids were selected as probes to study the variation of metallicity in the galactic disk, because of their high intrinsic luminosity, small age and the existence of period-luminosity and period-age relationships. High dispersion spectra of programme stars WZ Sgr, X Sgr, ς Gem, T Mon and S V Mon were obtained using the 102-cm reflector of Kavalur Observatory. The atmospheric abundances were determined by theoretically synthesizing the selected portions of the stellar spectrum and comparing with the observed spectra. In order to compute the theoretical spectrum, the formal solution of the equation of radiative transfer was numerically evaluated with the simplifying assumptions of local thermodynamical equilibrium, plane-parallel geometry and hydrostatic equilibrium. These assumptions are reasonably good for the metallic lines of F-G supergiants and hence the observations were confined to the phases where Cepheids behave like nonvariable F-G supergiants.
The atmospheric abundances of iron-peak elements, Fe, Cr, Ti, Ca and heavier s-process elements Y, Ba, La, Ce, Sm were obtained by synthesizing a selected spectral region in the range 4330 Å — 4650 Å. We derive a radial abundance gradient for iron$$\frac{{d(Fe/H)}}{{dr_{gc} }} = - 0.056 \pm 0.08$$ for the region of galactic disk between 6.7 and 10.9 kpc from the galactic centre (assuming rgc = 8.5 kpc for the Sun). This value agrees with the one obtained from the general sample of Cepheids for which spectroscopic abundances are available, and also with the existing photometric determinations, but is shallower than the one derived by Luck (1982).
Abundances of the elements derived in the present investigation do not show any significant correlation with atomic number. Also the abundance ratio of s-process elements does not show any correlation with Fe. This lack of correlation for disk population stars shows the inadequacy of simple models of galactic chemical evolution and favours the infall models. Alternately, the evolution of [s/Fe] may be determined by the ratio of intermediate-mass stars (which contribute s-process nuclei) to high-mass stars (which contribute Fe peak nuclei). Thus the different behaviour of halo and disk population may indicate a difference in the mass spectrum of star formation.
Volume 7 Issue 2 June 1986 pp 83-98
Classical cepheids and the local chemical inhomogeneities in the galactic disc
Places of formation have been derived for a sample of 23 Cepheids with well-determined atmospheric abundances in an attempt to study the chemical inhomogeneities in the local interstellar medium. The abundance data available for the sample Cepheids is compiled and critically reviewed to adopt the most reliable esimates. We find that the most conspicuous irregularity in the metallicity is exhibited by stars that are born in the local arm or in the interarm region. We propose a scenario to explain these local variations in terms of supernova-induced star formation in interstellar gas enriched by massive stars formed in the density wave.
Volume 10 Issue 1 March 1989 pp 47-83
An internally consistent set of oscillator strengths for Fe I lines
Sunetra Giridhar A. Arellano Ferro
We present here an extensive list of internally consistent oscillator strengths for Fe I lines. We have compiled and critically reviewed the oscillator strengths existing in literature. An attempt has been made to scale the oscillator strengths derived by different workers to that of Blackwell and his collaborators and prepare a single consistent set of oscillator strengths.
We have calibrated a log
We also tabulate systematic corrections to be applied to the oscillator strengths of Kurucz & Peytremann (1975) for lines belonging to different multiplets
Volume 11 Issue 1 March 1990 pp 37-42
Emission spectrum of hot R OB star MV Sgr
N. Kameswara Rao L. Houziaux Sunetra Giridhar
The new spectroscopic observation of MV Sgr obtained at ESO in 1987 July shows enhanced emission lines of He I λ3889, [S
Volume 11 Issue 4 December 1990 pp 475-490
High resolution spectroscopy of R Coronae Borealis during the 1988— 1989 minimum
David L. Lambert N. Kameswara Rao Sunetra Giridhar
Moderate-resolution spectra of the C2 Swan 0-1 bandhead, the Na I D lines and the KI resonance lines near 7660 Å obtained at minimum light during the 1988–1989 decline of R CrB are discussed and interpreted in terms of a popular model for R CrB declines. High-resolution spectra obtained at maximum light show blue-shifted chromospheric emission in the cores of the Na I D and the Sc π 4246.8 Å lines
Volume 12 Issue 1 March 1991 pp 27-37
Abundance analysis of the long period southern cepheid RZ Vel
Sunetra Giridhar A. Arellano Ferro Aruna Goswami
The long period classical cepheid RZ Vel (HD 73502) is known to be a member of an OB association, Vel OB1 in Vela, and a high metallicity is ascribed to it by the photometric work of Eggen (1982). We have done an abundance analysis for this long period (
Volume 13 Issue 4 December 1992 pp 307-318
High resolution spectroscopy of the semi-regular variable LR Sco
Sunetra Giridhar N. Kameswara Rao David L. Lambert
A detailed spectroscopic investigation of LR Sco which was earlier misclassified as R CrB star is made. Atmospheric parameters and elemental abundances are determined using detailed depth-dependent model atmospheres and line synthesis technique. Most of the elements show near solar abundances.
The strength of circumstellar components seen in Na D lines are used to derive the mass loss rate. Another independent estimate of mass loss rate is made using the observed infrared flux from 1–100Μm. These two approaches lead to nearly the same value of mass loss rate when
Volume 17 Issue 1-2 June 1996 pp 23-40
Sunetra Giridhar N. Kameswara Rao David L. Lambert
Two high resolution spectra of the hot RCrB star DY Cen in the red region are compared. The photospheric absorption lines show a radial velocity variation of 12 kms-1 between 1989 July and 1992 May. Emission components to some CII lines present in 1989 are almost entirely absent in 1992. Nebular forbidden lines of [OI], [NII] and [SII] appear unchanged from 1989 to 1992
Volume 41 All articles Published: 11 December 2020 Article ID 0038
Galactic chemical evolution and chemical tagging with open clusters
ARUMALLA B. S. REDDY SUNETRA GIRIDHAR DAVID L. LAMBERT
The article presents the consolidated results drawn from the chemical composition studies of Reddy
Volume 41 All articles Published: 14 December 2020 Article ID 0044
Recent advances in RV Tauri stars
The availability of multi-wavelength observations and parallaxes from the space missions and very comprehensive models of AGB evolution that include the accretion of matter from the circumbinary disc have strongly impacted our understanding of these enigmatic objects. The important developments made in the recent times are summarized here. The revised estimates of luminosities (derived from better-defined Spectral Energy Distributions (SEDs) and new distances from Gaia DR2) further support the opinion that RV Tauri stars containa mixture of post-AGB stars and post-RGB stars. Their locations in HR diagram also indicate that the instability strip (IS) of RV Tauri stars have a broader extension in the cooler edge than that of classical Cepheids. A newP-L relation has been calibrated for the galactic Cepheids which have a steeper slope than that derived for the Population II Cepheids and RV Tauri stars in Magellanic clouds . The most significant chemical peculiarity exhibited by RV Tauri stars and other post-AGB stars is the selective depletion of refractory elements that correlates with their condensation temperatures. A large range in the size of depletion as well as in the shapes of the depletion curves has been observed. Earlier models to explain this effect were mostly qualitative. Recentinvestigators model these depletions using evolutionary codes (e.g. MESA) to evolve stars in the post-AGB phase, while including accretion of metal-poor gas from circumbinary disc. These authors model the accretion rate onto a the binary post-AGB star from a viscously evolving disc for a range of initial accretion rates and disc masses. It is reported that large initial accretion rates and disc masses are required to explain the large depletion and saturated depletion curve that could extend the evolution time of post-AGB star. It is also proposed that theunsaturated depletion curve (with a plateau) are likely to be caused by post-RGB stars.
Volume 43, 2022
All articles
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