17 research outputs found
The record-breaking rotational braking of the He strong CP star HD 37776
We study the long-term light and spectral variations in the He-strong
magnetic chemically peculiar star HD 37776 (V901 Ori) to search for changes of
its 1.5387 d period in 1976-2007. We analyze all published photometric
observations and spectrophotometry in the HeI 4026 A line. The data were
supplmented with 506 new (U)VB observations obtained during the last 2
observing seasons, 66 estimates of HeI equivalent widths on 23 CFHT
spectrograms and 35 of the 6-m Zeeman spectrograms. All the 1895 particular
observations heve been processed simultaneously. We confirm the previously
suspected increase of the period in HD 37776 which is a record-breaking among
CP stars. The mean rate of the period increase during the last 31 years is
0.541+-0.020 s per year. We interpret this ongoing period increase as the
slowing down of the star's surface rotation due to momentum loss through events
and processes in its magnetosphere.Comment: 3 pages, 1 figure, Proceedings of the "CP#AP Workshop", Vienna,
10.-14.Sept.2007, eds.J.Ziznovsky, J.Zverko, E.Paunzen, M.Netopil, will be
published in Contrib. Astron. Obs. Skalnate Ples
In Search of Possible Associations between Planetary Nebulae and Open Clusters
We consider the possibility of cluster membership for 13 planetary nebulae
that are located in close proximity to open clusters lying in their lines of
sight. The short lifetimes and low sample size of intermediate-mass planetary
nebulae with respect to nearby open clusters conspire to reduce the probability
of observing a true association. Not surprisingly, line of sight coincidences
almost certainly exist for 7 of the 13 cases considered. Additional studies are
advocated, however, for 6 planetary nebula/open cluster coincidences in which a
physical association is not excluded by the available evidence, namely M
1-80/Berkeley 57, NGC 2438/NGC 2437, NGC 2452/NGC 2453, VBRC 2 & NGC 2899/IC
2488, and HeFa 1/NGC 6067. A number of additional potential associations
between planetary nebulae and open clusters is tabulated for reference
purposes. It is noteworthy that the strongest cases involve planetary nebulae
lying in cluster coronae, a feature also found for short-period cluster
Cepheids, which are themselves potential progenitors of planetary nebulae.Comment: Accepted for publication in PASP (December 2007
Photometry of ET Andromedae and pulsation of HD 219891
ET And is a binary system with a B9p(Si) star as the main component. We report on the photometric observing campaigns in 1988, 1989 and 1994 which confirmed the rotation period of 1(.)(d)618875 for ET And while refuting other published values. Furthermore, the controversial issue of pulsational stability of ET And is resolved since we have discovered pulsation for HD 219891, which was the main comparison star and sometimes exclusively used. The frequency of 10.0816 d(-1), a semi-amplitude of 2.5 mmag, T(eff) and M(v) suggest this comparison star to be a delta Scuti variable close to the blue border of the instability strip. The pulsational stability of ET And could be clearly established and hence no need exists to derive new driving mechanisms for stars between the classical instability strip and the region of slowly pulsating B-type (SPB) stars
A search for radial magnetic field gradients in Ap atmospheres
For the first time the possible presence of radial gradients of magnetic fields in the atmospheres of three magnetic Ap stars has been critically examined by measurements of the mean magnetic field modulus from spectral lines resolved into magnetically split components lying on the different sides of the Balmer jump. A number of useful diagnostic lines below and above the Balmer discontinuity, only slightly affected by blends, with simple doublet and triplet Zeeman pattern have been identified from the comparison between synthetic spectra computed with the SYNTHMAG code and the high resolution and S/N spectra obtained in unpolarized light with the ESO-VLT UVES spectrograph. For all three stars of our sample, HD 965, HD 116114 and 33 Lib (HD 137949), an increase of the magnetic field strength of the order of a few hundred Gauss has been detected bluewards of the Balmer discontinuity. These results should be taken into account in future modelling of the geometric structure of Ap star magnetic fields and the determination of the chemical abundances in Ap stars with strong magnetic fields
A-type stars: evolution, rotation and binarity
We discuss the internal structure of stars in the mass range 1.5 to 4 M[SUB]&sun;[/SUB] from the PMS to the subgiant phase with a particular emphasis on the convective core and the convective superficial layers. Different physical aspects are considered such as overshooting, treatment of convection, microscopic diffusion and rotation. Their influence on the internal structure and on the photospheric chemical abundances is briefly described