Gajendra Pandey

Fluorine in R Coronae Borealis Stars

Neutral fluorine (F I) lines are identified in the optical spectra of several R Coronae Borealis stars (RCBs) at maximum light. These lines provide the first measurement of the fluorine abundance in these stars. Fluorine is enriched in some RCBs by factors of 800-8000 relative to its likely initial abundance. The overabundances of fluorine are evidence for the synthesis of fluorine. These results are discussed in the light of the scenario that RCBs are formed by accretion of an He white dwarf by a C-O white dwarf. Sakurais object (V4334 Sgr), a final He-shell flash product, shows no detectable F I lines.

An Analysis of Ultraviolet Spectra of Extreme Helium Stars and New Clues to Their Origins

Abundances of about 18 elements including the heavy elements Y and Zr are determined from Hubble Space Telescope Space Telescope Imaging Spectrograph ultraviolet spectra of seven extreme helium stars (EHes): LSE 78, BD +10 2179, V1920 Cyg, HD 124448, PV Tel, LS IV-1 2, and FQ Aqr. New optical spectra of BD +10 2179, V1920 Cyg, and HD 124448 were analyzed, and published line lists of LSE 78, HD 124448, and PV Tel were analyzed afresh. The abundance analyses are done using LTE line formation and LTE model atmospheres especially constructed for these EHes. The stellar parameters derived from an EHes UV spectrum are in satisfactory agreement with those derived from its optical spectrum. Adopted abundances for the seven EHes are from a combination of the UV and optical analyses. Published results for an additional 10 EHes provide abundances obtained in a nearly uniform manner for a total of 17 EHes, the largest sample on record. The initial metallicity of an EHe is indicated by the abundance of elements from Al to Ni; Fe is adopted to be the representative of initial metallicity. Iron abundances range from approximately solar to about 1/100 solar. Clues to EHe evolution are contained within the H, He, C, N, O, Y, and Zr abundances. Two novel results are (1) the O abundance for some stars is close to the predicted initial abundance yet the N abundance indicates almost complete conversion of initial C, N, and O to N by the CNO cycles; and (2) three of the seven stars with UV spectra show a strong enhancement of Y and Zr attributable to an s-process. The observed compositions are discussed in light of expectations from accretion of an He white dwarf by a C-O white dwarf. Qualitative agreement seems likely except that a problem may be presented by those stars in which the O abundance is close to the initial O abundance.

Neon And CNO Abundances For Extreme Helium Stars-A Non-LTE Analysis

A non-LTE (NLTE) abundance analysis was carried out for three extreme helium stars (EHes): BD+10? 2179, BD?9? 4395, and LS?IV+6??002, from their optical spectra with NLTE model atmospheres. NLTE TLUSTY model atmospheres were computed with H, He, C, N, O, and Ne treated in NLTE. Model atmosphere parameters were chosen from consideration of fits to observed He I line profiles and ionization equilibria of C and N ions. The program SYNSPEC was then used to determine the NLTE abundances for Ne as well as H, He, C, N, and O. LTE neon abundances from Ne I lines in the EHes: LSE?78, V1920?Cyg, HD?124448, and PV?Tel, are derived from published models and an estimate of the NLTE correction applied to obtain the NLTE Ne abundance. We show that the derived abundances of these key elements, including Ne, are well matched with semi-quantitative predictions for the EHe resulting from a cold merger (i.e., no nucleosynthesis during the merger) of an He white dwarf with a C-O white dwarf.

The Discovery of Fluorine in Cool Extreme Helium Stars

Neutral fluorine (F I) lines are identified in the optical spectra of cool extreme helium (EHe) stars. These are the first F I lines identified in a stars spectrum, and they provide the first measurement of fluorine abundances in EHe stars. The results show that fluorine is overabundant in EHe stars. The overabundance of fluorine provides us with evidence for the synthesis of fluorine in these stars, which is discussed in light of asymptotic giant branch evolution and the expectation from accretion of an He white dwarf by a C-O white dwarf.

Early time optical spectroscopy of supernova SN 1998S

We present medium-resolution optical spectra of the type II supernova SN 1998S obtained during the pre-maximum, maximum and early decline phases. The early decline spectra show complex Balmer line profiles: a narrow P-Cygni emission feature superimposed on a broad emission. The narrow emission has a redshift similar to that of the host galaxy, indicating its origin from a pre-supernova circumstellar material. The H α luminosity of the broad component implies a shell mass of ∼ 0.1

The hot γ Doradus and Maia stars

M_\odot

Variable circumstellar activity of V351 Orionis

for the supernova ejecta, while the luminosity of the narrow component implies a mass loss rate of

ON THE BINARY HELIUM STAR DY CENTAURI: CHEMICAL COMPOSITION AND EVOLUTIONARY STATE

10^{-4} M_\odot

Non-local Thermodynamic Equilibrium Abundance Analyses of the Extreme Helium Stars V652 Her and HD 144941

yr -1 for the precursor. The photospheric temperature of the supernova shell decreased from a value of

Are DY Persei Stars Cooler Cousins of R Coronae Borealis Stars

18 400\pm 2900