Frequency analyses over two or more seasons are presented for several pulsating sdB stars. These results indicate that amplitude variations in sdBV stars might be common and occur on timescales from ...less than a day to years. It is not clear whether such variations are a result of real amplitude modulation or of the fact that very close frequencies have not all been resolved. In either case, it would appear to be necessary to carry out reasonably extensive studies before one can claim to have resolved “all” frequencies, and to exhibit care before selecting frequencies for ‘
’ studies.
Darragh O'Donoghue died on 25 June 2015 following a short illness. He worked in the Department of Astronomy at the University of Cape Town (UCT) for the South African Astronomical Observatory (SAAO) ...and, most recently, for the Southern African Large Telescope (SALT). He was elected to the Fellowship of the Royal Society of South Africa in 2005. He is survived by the family he adored - his wife, Liz, and daughters, Andrea and Alexandra. He left a hole in many of our lives and a chasm in the small community of South African astronomy.
ABSTRACT
Peaking at 3.7 mag on 2020 July 11, YZ Ret was the second-brightest nova of the decade. The nova’s moderate proximity (2.7 kpc, from Gaia) provided an opportunity to explore its ...multiwavelength properties in great detail. Here, we report on YZ Ret as part of a long-term project to identify the physical mechanisms responsible for high-energy emission in classical novae. We use simultaneous Fermi/LAT and NuSTAR observations complemented by XMM–Newton X-ray grating spectroscopy to probe the physical parameters of the shocked ejecta and the nova-hosting white dwarf. The XMM–Newton observations revealed a supersoft X-ray emission which is dominated by emission lines of C v, C vi, N vi, N vii, and O viii rather than a blackbody-like continuum, suggesting CO-composition of the white dwarf in a high-inclination binary system. Fermi/LAT-detected YZ Ret for 15 d with the γ-ray spectrum best described by a power law with an exponential cut-off at 1.9 ± 0.6 GeV. In stark contrast with theoretical predictions and in keeping with previous NuSTAR observations of Fermi-detected classical novae (V5855 Sgr and V906 Car), the 3.5–78-keV X-ray emission is found to be two orders of magnitude fainter than the GeV emission. The X-ray emission observed by NuSTAR is consistent with a single-temperature thermal plasma model. We do not detect a non-thermal tail of the GeV emission expected to extend down to the NuSTAR band. NuSTAR observations continue to challenge theories of high-energy emission from shocks in novae.
We report an analysis of the first known β Cep pulsator observed by the Transiting Exoplanet Survey Satellite (TESS) mission, the runaway star PHL 346 = HN Aqr. The star, previously known as a singly ...periodic pulsator, has at least 34 oscillation modes excited, 12 of those in the g-mode domain and 22 p modes. Analysis of archival data implies that the amplitude and frequency of the dominant mode and the stellar radial velocity were variable over time. A binary nature would be inconsistent with the inferred ejection velocity from the Galactic disk of 420 km s−1, which is too large to be survivable by a runaway binary system. A kinematic analysis of the star results in an age constraint (23 1 Myr) that can be imposed on asteroseismic modeling and that can be used to remove degeneracies in the modeling process. Our attempts to match the excitation of the observed frequency spectrum resulted in pulsation models that were too young. Hence, asteroseismic studies of runaway pulsators can become vital not only in tracing the evolutionary history of such objects, but to understand the interior structure of massive stars in general. TESS is now opening up these stars for detailed asteroseismic investigation.
The post-outburst photometric behaviour of V838 Mon Crause, Lisa A.; Lawson, Warrick A.; Kilkenny, David ...
Monthly notices of the Royal Astronomical Society,
05/2003, Letnik:
341, Številka:
3
Journal Article
Recenzirano
Odprti dostop
The unusual eruptive variable discovered in Monoceros in 2002 January underwent dramatic photometric and spectroscopic changes in the months prior to its 2002 June–August conjunction with the Sun. ...Optical and infrared (IR) photometry obtained at the SAAO between 2002 January and June (JD 245 2280–440) is presented here in an analysis of the post-outburst behaviour of the star. The light curve indicated that three eruptions took place in 2002 January, February and March. South African Astronomical Observatory (SAAO) echelle spectra obtained in the week prior to the March maximum indicated the ejection of a new shell of material. JHKL photometry obtained during 2002 April showed the development of an IR excess owing to the formation of a dust shell. The shell appears to be largely responsible for the rapid fade in the optical flux during 2002 April–May (ΔV ã 6 mag within 3 weeks). Blueing of the optical colours during the decline is probably due either to the revealing of an emission-line region surrounding V838 Mon, or to the unveiling of the progenitor or a spatially close early-type star.
What Are the Hot R Coronae Borealis Stars? De Marco, Orsola; Clayton, Geoffrey C; Herwig, F ...
The Astronomical journal,
06/2002, Letnik:
123, Številka:
6
Journal Article
Resolving the 47 Tucanae Distance Problem Percival, Susan M; Salaris, Maurizio; van Wyk, Francois ...
The Astrophysical journal,
07/2002, Letnik:
573, Številka:
1
Journal Article
The post-outburst photometric behaviour of V838 Mon Crause, Lisa A.; Lawson, Warrick A.; Kilkenny, David ...
Monthly notices of the Royal Astronomical Society,
05/2003, Letnik:
341, Številka:
3
Journal Article
A survey of early-type stars at intermediate galactic latitudes was carried out in the southern hemisphere winters of 1970 and 1971. The observing programme was limited to negative declinations and ...covered a range in right ascension of approximately 12h to 20h. At the Royal Observatory, Cape Town, in 1970, UBV photoelectric measurements were made of 56 stars for which no UBV data existed and 20 stars which had been observed on one or two previous occasions, the intention being to obtain four separate measures of each star. In 19711 the Bochum University telescope at the E.S.O. site in Chile was used for H/3 photoelectric photometry of over 200 intermediate and high latitude stars. Shortly afterwards, spectra for radial velocity determination and MK classification were obtained with the two-prism spectrograph and 74" reflector of Radcliffe Observatory, Pretoria. Work was concentrated upon some 60 stars not previously observed with spectroscopic equipment and selected on the basis of blue colour or possible high luminosity from photometric considerations. A few southern standard stars and stars from earlier Radcliffe programmes were re-observed as control or overlap stars. Chapters II - IV describe the observational procedures and reduction methods. Tables in chapter V contain results from the 1970-71 programmes plus UBV and spectroscopic data for intermediate and high latitude stars from various other sources. The remaining chapters are concerned with analysis and discussion of the observations. Chapter VI summarises some optical and radio determinations of the spiral structure of the Galaxy and compares the spatial distribution of the programme stars with these results. The possibility that early-type stars may be formed well away from the galactic plane is considered by comparison of kinematic and evolutionary lifetimes of some stars at appreciable distances from the plane. In chapter VII, intermediate and high latitude stars are shown to participate in the differential rotation of the Galaxy and detailed analysis of the space motions of a number of high velocity stars leads to the conclusion that some may .have sufficient energy to escape from the galactic system. The radial velocities of interstellar Ca II lines are shown in chapter VIII to be as expected for material in the solar neighbourhood involved in differential galactic rotation. An apparent deviation from circular motion reported by observers investigating H II regions is also present in the Ca II gas. Constants in the cosecant equation of interstellar reddening are re-determined and show an apparently significant difference between northern and southern galactic hemispheres. Appendix I describes attempts to simulate the effect of filters in order to explain the curvature in the transformations from instrumental to standard photometric systems. Appendix II gives details of the method used to compute stellar space velocities from proper motions and radial velocities and includes a short Fortran IV programme which implements the operations described.
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