On The Radii of Ap Stars

1976 ◽  
Vol 32 ◽  
pp. 49-55 ◽  
Author(s):  
F.A. Catalano ◽  
G. Strazzulla
Keyword(s):  
Observational Data ◽  
Line Width ◽  
Main Sequence ◽  
Main Sequence Stars ◽  
Ap Stars

SummaryFrom the analysis of the observational data of about 100 Ap stars, the radii have been computed under the assumption that Ap are main sequence stars. Radii range from 1.4 to 4.9 solar units. These values are all compatible with the Deutsch's period versus line-width relation.

The Peculiar A Stars: Atmospheric Parameters and Abundance Peculiarities

1976 ◽  
Vol 32 ◽  
pp. 255-274 ◽  
Author(s):  
M. Hack
Keyword(s):  
Main Sequence ◽  
Atmospheric Parameters ◽  
Main Sequence Stars ◽  
Ap Stars ◽  
Large Groups

We have known for more than 40 years (Morgan 1933) that a group of stars having abnormal spectra exists among the main sequence stars included between B5 and F2. They represent at least 15% of the normal stars with the same color and luminosity.They can be divided in two large groups and their properties are summarized in Tables 1 and 2. They are the Ap stars (including the various subgroups, Mn stars, Si stars, Sr-Cr-Euor cool Ap stars) and the Am, Fm stars.

scholarly journals The Rotation of the Ap Stars From the Point of View of the Rigid Rotator Model

1970 ◽  
Vol 4 ◽  
pp. 254-263
Author(s):  
George W. Preston
Keyword(s):  
Distribution Function ◽  
Main Sequence ◽  
Point Of View ◽  
Main Sequence Stars ◽  
Rigid Rotator ◽  
Long Period ◽  
Rotator Model ◽  
Rotational Distribution ◽  
The Mean ◽  
Ap Stars

AbstractDeutsch’s period vs. line-width relation for the periodic Ap stars is re-examined from the point of view of the rigid rotator with the aid of recently determined values of v sin i for these objects. The agreement between computed rotational velocities and observed values of v sin i is satisfactory if the radii of the Ap stars are twice those of zero-age main sequence stars of the same color. The computed rotational velocities also agree with the mean rotational velocities of all Ap stars if suitable allowances are made for observational limitations. However, a difficulty arises in connection with the number of ‘long-period’ stars that have been discovered. Their small computed rotational velocities and their frequency are such that they cannot be regarded as part of any rotational distribution function that describes normal stars or even most Ap stars. It is concluded that if the periods of these objects are rotational periods, then a powerful rotational deceleration mechanism must be operative in at least some Ap stars, primarily those of the SrCrEu group.

scholarly journals Abundance and Magnetic Field Geometries of Helium-Strong and Helium-Weak Stars

1988 ◽  
Vol 132 ◽  
pp. 309-312
Author(s):  
David A. Bohlender ◽  
J. D. Landstreet
Keyword(s):  
Magnetic Field ◽  
High Temperature ◽  
Magnetic Fields ◽  
Main Sequence ◽  
Line Profiles ◽  
Main Sequence Stars ◽  
Hot Stars ◽  
Surface Magnetic Field ◽  
Peculiar Stars ◽  
Ap Stars

The helium-weak and helium-strong stars are main sequence stars with anomalously weak and strong helium lines for their spectral types respectively. Many members of the two classes have strong, globally ordered magnetic fields (Thompson and Landstreet 1985; Bohlender et al. 1987) and are currently thought to represent high temperature extensions of the Ap stars. In collaboration with C. T. Bolton (U. of Toronto), we have obtained high S/N phase resolved spectra of several stars using the coudé reticon detector at CFHT. One of the principle goals of this work is to determine abundance and surface magnetic field geometries of several helium peculiar stars with large, well-determined effective fields. We employ a line synthesis program (Landstreet 1987) that incorporates the effects of surface magnetic fields and non-uniform abundances on the observed line profiles of a star. Since these stars are rapid rotators the surface magnetic field strength must be inferred from differential magnetic intensification of lines with different magnetic sensitivities. Of the few lines with suitable strengths in these hot stars we have decided that the Si III multiplet 2 lines are best suited for this aspect of our investigation. We have also modelled the unblended He I line λ4437, ignoring magnetic effects for the time being. Individual results are discussed below.

scholarly journals Spectroscopy and Geography of Magnetic Ap Stars: Implications for Structure

1991 ◽  
Vol 145 ◽  
pp. 161-171
Author(s):  
J. D. Landstreet
Keyword(s):  
Large Scale ◽  
Main Sequence ◽  
Meridional Circulation ◽  
Field Structure ◽  
Surface Chemical ◽  
Main Sequence Stars ◽  
Chemical Abundances ◽  
B Stars ◽  
Uniform Distributions ◽  
Ap Stars

Some main sequence A and B stars have strong, ordered magnetic fields. These Ap and Bp stars usually have anomalous chemical abundances, and often rather non-uniform distributions of at least some elements (e.g. He, Ca, Ti, Cr) over their surfaces. Maps of magnetic field structure may provide a means of testing theories of how large coherent fields form, and of probing large-scale hydrodynamic flows (meridional circulation) inside stars. Maps of distributions of various elements can help to elucidate the mechanisms, such as diffusion under competing influences of gravity and radiation, turbulence, meridional circulation, mass loss, and perhaps accretion from the interstellar medium, that lead to the distinctive abundances and surface abundance distributions. This kind of mapping is important as an aid to understanding how the Ap and Bp stars develop. It is even more important because the processes involved in producing Ap and Bp stars probably have significant effects on surface chemical abundances of “normal” upper main sequence stars, and so understanding the relevant physics is essential to correctly relating observed surface chemistry to stellar and galactic evolution. In this paper, efforts to map field and abundance structures in Ap stars are reviewed, and some of the principal results obtained thus far are discussed.

scholarly journals The search for magnetic fields in mercury-manganese stars

2010 ◽  
Vol 6 (S272) ◽  
pp. 202-203
Author(s):  
Vitalii Makaganiuk ◽  
Oleg Kochukhov ◽  
Nikolai Piskunov ◽  
Sandra V. Jeffers ◽  
Christopher M. Johns-Krull ◽  
...  
Keyword(s):  
Magnetic Fields ◽  
Large Scale ◽  
Main Sequence ◽  
Chemical Elements ◽  
Field Measurements ◽  
Atomic Diffusion ◽  
Main Sequence Stars ◽  
Weak Magnetic Fields ◽  
Ap Stars ◽  
Spot Formation

AbstractMercury-manganese (HgMn) stars were considered to be non-magnetic, showing no evidence of surface spots. However, recent investigations revealed that some stars in this class possess an inhomogeneous distribution of chemical elements on their surfaces. According to our current understanding, the most probable mechanism of spot formation involves magnetic fields. Taking the advantage of a newly-built polarimeter attached to the HARPS spectrometer at the ESO 3.6m-telescope, we performed a high-precision spectropolarimetric survey of a large group of HgMn stars. The main purpose of this study was to find out how typical it is for HgMn stars to have weak magnetic fields. We report no magnetic field detection for any of the studied objects, with a typical precision of the longitudinal field measurements of 10 G and down to 1 Gauss for some of the stars. We conclude that HgMn stars lack large-scale magnetic fields typical of spotted magnetic Ap stars and probably lack any fields capable of creating and sustaining chemical spots. Our study confirms that alongside the magnetically altered atomic diffusion, there must be other structure formation mechanism operating in the atmospheres of late-B main sequence stars.

scholarly journals The Boundary between the Magnetic Fields of Ap Stars and the Fields of Solar-type Stars

1991 ◽  
Vol 130 ◽  
pp. 342-346
Author(s):  
John D. Landstreet
Keyword(s):  
Magnetic Fields ◽  
Main Sequence ◽  
Convection Zone ◽  
Frequency Of Occurrence ◽  
Main Sequence Stars ◽  
Chemically Peculiar ◽  
Solar Type ◽  
F Stars ◽  
Ap Stars

AbstractThe boundary between Ap-type magnetic fields and the magnetic fields of solar-type stars occurs near Te ~ 7000K, about where deep envelope convection develops in main sequence stars. This seems natural for solar-type stars, in which the field is generated by the convection zone. However, among magnetic Ap stars the frequency of occurrence declines from about 10% of all A stars near A0 to about 1% near F0. It is not clear what produces this decline in frequency, but the convection zone is probably not responsible. In fact, it seems likely that if global fossil fields occur in main sequence F stars, such fields should be detectable even if the stars having them are not chemically peculiar.

scholarly journals Hα and OI Photometry of Upper Main Sequence Stars with Anomalous Abundances

1986 ◽  
Vol 90 ◽  
pp. 195-198 ◽  
Author(s):  
Eugenio E. ◽  
Mendoza V.
Keyword(s):  
Main Sequence ◽  
Main Sequence Stars ◽  
Abundance Anomalies ◽  
Ap Stars

AbstractAdditional α(16)Λ(9)-photometry of upper main sequence stars with anomalous abundances confirm our previous results, namely, Ap stars are neatly separated from normal main sequence stars. Furthermore, Ap stars are located on three different zones of the α(16)Λ(9)-plane, according to their abundance anomalies to form three photometric groups, (i) Si stars, (ii) Hg,Mn stars, and (iii) Cr,Eu,Sr stars.

Chromospheric activity as a function of age in main-sequence stars

1966 ◽  
Vol 24 ◽  
pp. 40-43
Author(s):  
O. C. Wilson ◽  
A. Skumanich
Keyword(s):  
Main Sequence ◽  
The Sun ◽  
Main Sequence Stars ◽  
Tentative Conclusion ◽  
Chromospheric Activity ◽  
General Field ◽  
Large Clusters

Evidence previously presented by one of the authors (1) suggests strongly that chromospheric activity decreases with age in main sequence stars. This tentative conclusion rests principally upon a comparison of the members of large clusters (Hyades, Praesepe, Pleiades) with non-cluster objects in the general field, including the Sun. It is at least conceivable, however, that cluster and non-cluster stars might differ in some fundamental fashion which could influence the degree of chromospheric activity, and that the observed differences in chromospheric activity would then be attributable to the circumstances of stellar origin rather than to age.

Pre–Main-Sequence Stars in the Young Galactic Cluster IC 4996: A CCD Photometric Study

1998 ◽  
Vol 116 (4) ◽  
pp. 1801-1809 ◽  
Author(s):  
Antonio J. Delgado ◽  
Emilio J. Alfaro ◽  
André Moitinho ◽  
José Franco
Keyword(s):  
Main Sequence ◽  
Photometric Study ◽  
Main Sequence Stars ◽  
Galactic Cluster
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