David L. Lambert
Articles written in Journal of Astrophysics and Astronomy
Volume 8 Issue 1 March 1987 pp 51-56
One hundred and twentythree radial velocities for α Cyg are derived between December 1977 and October 1982. These photospheric velocities are derived from NI lines near 8700 å. Semiregular variations in radial velocities are present with periods of 7 to 20 days. The range of variation of 14.3 kms−1 observed in the present radial velocities of α Cyg is close to the sum of the amplitudes (10.44 kms−1) of all the pulsation periods from 7 to 101 days (Lucy 1976a) and is also approximately equal to micro and macro-turbulent velocities.
Volume 8 Issue 2 June 1987 pp 103-122
The abundances of the light (Na to Ca) elements in disc and halo stars are reviewed. New analyses are emphasized. Elements considered are the α-nuclei (Mg, Si, and Ca), and the odd-even nuclei (Na and Al, also25Mg and26Mg).
The α-nuclei are overabundant (relative to Fe) in the old disc and halo stars. Halo stars ([Fe/H] < —1.2) have [α/Fe] ∼0.3 with extreme halo ([Fe/H] ≲ −2.0) stars showing possibly higher overabundances. The scatter in [α/Fe] at a given [Fe/H] is small. To within the observational errors, the abundance patterns for Mg, Si, and Ca are identical.
For disc stars, the Na and Al abundances relative to Mg are almost independent of the [Fe/H]. Halo stars ([Fe/H] < −1) show [Na/Mg] < 0 and [AI/Mg] < 0, but the form of the mean relation and the scatter about the relation between [odd-even/Mg] and [Fe/H] remains uncertain.
Volume 11 Issue 4 December 1990 pp 475-490
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 13 Issue 4 December 1992 pp 307-318
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 15 Issue 1 March 1994 pp 47-67
This review presents a selection of recent highlights of observations of R Coronae Borealis variables. Emphasis is placed on an abundance analysis of a complete sample (18 stars) of the warm galactic RCBs. It is shown that 14 of the 18 have very similar compositions: the iron mass fraction ranges about a factor of 3 around the solar value (assuming C/He = 3%) but abundance ratios X/Fe for elements from Na to Ba show little variation. By contrast, the other 4 stars are deficient in iron but not in Na, Si, S and some other elements. With for example, [Si/Fe] ≃ 2, the quartet is indeed ‘peculiar’. One of the quartet, V854 Cen shows depletions of elements (other than CNO) similar to the depletions seen in interstellar medium corresponding to average log
Volume 17 Issue 1-2 June 1996 pp 23-40
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
The article presents the consolidated results drawn from the chemical composition studies of Reddy
Volume 41 All articles Published: 11 December 2020 Article ID 0040
The origin and evolution of hydrogen-deficient stars are not yet adequately understood. Their chemical peculiarities, along with hydrogen-deficiency, makes them stand out from the rest and sheds light on their possible origin. Severe fluorine enrichment (of the order of 800–8000) is one such characteristicfeature of a class of hydrogen deficient stars, mainly the RCBs (R Coronae Borealis stars) and cool EHes (Extreme Helium stars) which enforces their close connection. For hot EHes, this relationship with the cooler EHes, based on their fluorine abundance is unexplored. Here, first estimates of fluorine abundances in hot EHes are presented and discussed in the light of their cooler counterparts to try to establish an evolutionary connection. The relation between these fluorine estimates with the other elemental abundances observed in these stars plays a pivotal role to predict the formation and evolution of these exotic stars.
Volume 43, 2022
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