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Magnetic stars, 2004, 212-215

Averaged stellar effective magnetic fields. A catalog
Bychkov V.D.
1 2 3

1,2

, Bychkova L.V.1 , Madej J.

3

Special Astrophysical Observatory of the Russian AS, Nizhnij Arkhyz 369167, Russia, Isaac Newton Institute of Chile, SAO Branch, Nizhnij Arkhyz, 369167 Russia Warsaw University Observatory, Al. Ujazdowskie 4, 00-478 Warsaw, Poland

Abstract. This pap er presents a description of a catalog containing averaged quadratic effective magnetic fields Be for 596 main sequence and giant stars. The catalog is based on 12076 measurements of the stellar effective (or mean longitudinal) magnetic field strengths Be , which were compiled from the available literature.

1

Intro duction

Research on stellar magnetic fields is among the most important issues in both observational and theoretical astrophysics. The first measurements of magnetic fields in stars were made more than 50 years ago (Babcock & Burd 1952). Since that time, both the number of magnetic field measurements and the number of investigated stars have grown enormously. Therefore we decided to collect and present in some homogeneous form all the published magnetic field measurements. Similar efforts were made previously, but they were based on much less numerous sets of measurements (Brown et al. 1981; Borra et al. 1983; Glagolevskij et al. 1986; Bychkov 1990, Bychkov et al. 1990). The above compilations have been essential for our understanding of the magnetic field strength and structure in stellar atmospheres, and their generation and time evolution in stellar interiors. Taking into account the large increase in the accumulated observational material, we believe that new research of this kind is necessary and fully justified. The catalog presented below does not include either isolated degenerate stars (cooling neutron stars and most white dwarfs), or degenerate stars in interacting binaries. Only a few of the brightest white dwarfs are present in the catalog.

2

Stellar effective magnetic fields

The differential contribution dBe to the effective magnetic field of a star is defined as the area-weighted projection of the local vector of the magnetic field B loc onto the line of sight. The local monochromatic intensity I of outgoing radiation is also a weighting factor in that pro jection. The effective (or mean longitudinal) magnetic field Be is the weighted mean value, integrated over the visible stellar disc
2 /2

B Be =
0 0

loc

cos I () sin cos d d , I () sin cos d d (1)

2 /2 0 0

where denotes the angle between the local vector B loc and the direction towards the observer. The variable denotes the colatitude angle, and stands for the azimuthal angle of the angular integration. The above definition assumes a simplified situation, in which the Be is determined at a single discrete frequency only (Madej 1983). In general, the specific intensity of radiation I () depends strongly on the frequency of radiation , and exhibits various limb-darkening relations for different . Therefore the value of the effective magnetic field B e
c Sp ecial Astrophysical Observatory of the Russian AS, 2004


AVERAGED STELLAR EFFECTIVE MAGNETIC FIELDS. A CATALOG

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is also a frequency dependent quantity, when measured for the given magnetic field configuration of a star. Dependence of Be on frequency was always ignored in previous papers. In most magnetic stars the values of Be change periodically with the rotational phase of the star. Values of Be can be either positive or negative. Moreover, it is possible that a star with strong magnetic field can momentarily exhibit Be = 0, depending on the aspect. Therefore it is useful to characterize the magnetic properties of various stars by the averaged quadratic effective magnetic field B e , which is always positive (Borra et al. 1983).

3

Averaging pro cedure

For a series of Be measurements, we define 1 n 1 n
n 1/2 2 ei 1/2

B

e

=

B
i=1 n

,

(2)



e

=


i=1

2 ei

,

(3)

where Bei denotes the i-th measurement of the effective magnetic field, and n is the total number of observations for a given star. The variable ei is the standard error of Bei , and e is the rms standard error of Be . The value of 2 /n (given per single degree of freedom) allows one to judge whether a series of B ei for a given star represents a reliable detection of a nonzero effective magnetic field, or whether this series is rather the result of random noise 2 /n = 1 n
n i=1 2 Bei 2. ei

(4)

This method for averaging the individual Be measurements of a magnetic star was introduced by Borra et al. (1983) to study magnetic properties of He-weak stars. This evaluation of B e is particularly useful to study stars with low or high noise Be observations, where full magnetic curves cannot yet be constructed. Borra et al. (1983) have pointed out that the value of Be gives an estimate of the amplitude of the Be variations of a given star, provided that this amplitude is substantially larger than e .

4

The catalog

The basic and most extensive Table A.1 presents the full catalog of stars for which we performed computations of the quadratic Be averages. For convenience, these stars are ordered according to their HD number. Successive rows of Table A.1 give: HD number (or BD number in case of faint stars), spectral type, number N of magnetic observations, value of Be in G, standard deviation in G, value of 2 /n, method of Be determination (abbreviations are explained at the bottom of Table A.1), and numbers referring to papers where we found the original magnetic field measurements. Cross-references between these numbers and the original papers are also given at the bottom of Table A.1. Table A.1 contains magnetic data on a total of 596 stars of various spectral types. One can easily see that in the case of many stars listed there, the value of Be is approximately equal or smaller than e , which usually means that detection of the magnetic field itself is highly uncertain. Unfortunately, the extensive set of available Be is not satisfactory. It exhibits very strong selection effects, since during the recent 50 years observers were mostly interested in measuring strong stellar magnetic fields exhibited by many hot chemically peculiar stars on the upper main sequence (dwarfs of B-A-F type). Existing measurements of Be fields in stars of other spectral classes are much more scarce. Both figures convincingly imply that during the recent 50 years efforts of observers measuring stellar magnetic fields were very strongly concentrated on investigation of chemically peculiar A stars.


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BYCHKOV, BYCHKOVA, MADEJ

100

80

Namber of Stars

60

40

20

B0
10

A0
20

F0
30

G0
40

K0
50

M0
60

Spectral type

Figure 1: Number of stars which have magnetic field Be measured vs. spectral type.

1200 1000 800 600 400 200

Namber of Measurements

10 B0

20 A0

30 F0

40 G0

50 K0

60 M0

Spectral type

Figure 2: Total number of published Be measurements vs. spectral type for al l stars col lected in our catalog.


AVERAGED STELLAR EFFECTIVE MAGNETIC FIELDS. A CATALOG

215

5

Summary

We present an extensive list of the averaged quadratic effective magnetic fields B e for main sequence and giant stars. Individual Be observations were compiled from the available literature, and were further processed to obtain a homogeneous set of averaged effective magnetic fields. We consider our averaged values of Be as a reasonable representative measure of the field strength in the atmosphere of a given star. This is because the value of Be results directly from the observed effective magnetic field strengths Be and is a strictly model-independent quantity. Parameter Be is a single scalar parameter which describes the magnetic field of a star also if the number of individual Be is low or the Be observations are noisy.
Acknowledgements. We are grateful to J.D. Landstreet, the referee, for his criticism and numerous suggestions which helped us to improve this paper. Our research is based on data compiled and posted in the SIMBAD, ADS, and CDS databases. We acknowledge support from the Polish Committee for Scientific Research, grant No. 2 P03D 021 22.

References
Babcock H.W., Burd S., 1952, Astrophys. J., 116, 8 Borra E.F., Landstreet J.D., Thompson I., 1983, Astrophys. J. Suppl. Ser., 53, 151 Borra E.F., Edwards G., Mayor M., 1984, Astrophys. J., 284, 211 Brown D.N., Landstreet J.D., Thompson I.A., 1981, 23-rd Liege Coll., 195 Bychkov V.D., 1990, Mitteilungen des Karl-Schwarzschild-Observatoriums Tautenburg, No. 125, 13 Bychkov V.D., Glagolevskij Yu.V., El'kin V.G., Kopylova F.G., Naidenov I.D., Romanyuk I.I., Chunakova N.M., Shtol' V.G., 1990, Bull. Spec. Astrophys. Obs., No. bf 30, 78 Bychkov V.D., Bychkova L.V., Madej J., 2003, Astron. Astrophys., 407, 631 Madej J., 1983, Acta Astron., 33, 1

---------------- The catalog has been published in Astron. Astrophys. 407, pp.631-642 (2003). Table A.1 and its references are available only in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/407/631 .