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 stars effective temperatu
re, 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.
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 prese
nt 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.
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 ar
ray 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
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 observatorys activities were re-oriented towards meteorology. The establishment of the observatory, the personalities involved and the final closing of the institution are described here.
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 a He white dwarf with a C-O white dwarf.
Neutral fluorine 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 to 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 neutral fluorine lines.
Some of the observational aspects related to the evolutionary status and dust production in R Cor Bor stars are discussed. Recent work regarding the surface abundances, stellar winds and evidence for dust production in these high luminosty hydrogen d
eficient stars are also reviewed. Possibility of the stellar winds being maintained by surface magnetic fields is also considered.
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 f
actors of 800 to 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.
Optical high-resolution spectra of the R Coronae Borealis star V CrA at light maximum and during minimum light arediscussed. Abundance analysis confirms previous results showing that V CrA has the composition of the small subclass of R Coronae Boreal
is (RCB) stars know as `minority RCBs, i.e., the Si/Fe and S/Fe ratios are 100 times their solar values. A notable novel result for RCBs is the detection of the 1-0 Swan system $^{12}$C$^{13}$C bandhead indicating that $^{13}$C is abundant: spectrum synthesis shows that $^{12}$C/$^{13}$C is about 3 to 4. Absorption line profiles are variable at maximum light with some lines showing evidence of splitting by about 10 km s$^{-1}$. A spectrum obtained as the star was recovering from a deep minimum shows the presence of cool C$_2$ molecules with a rotational temperature of about 1200K, a temperature suggestive of gas in which carbon is condensing into soot. The presence of rapidly outflowing gas is shown by blue-shifted absorption components of the Na {sc i} D and K {sc i} 7698 AA resonance lines.
