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Physics of Magnetic Stars, 2007, pp. 105­108

Magnetization of stars vs. effective temperature
V.D. Bychkov
1 2 3

1,3

, L.V. Bychkova1 , J. Madej

2

Special Astrophysical Observatory of the Russian AS, Nizhnij Arkhyz 369167, Russia Warsaw University Observatory, Al. Ujazdowskie 4, 00-478 Warsaw, Poland Stavropol State University, ul. Pushkina 1, Stavropol, 355009 Russia

Abstract. In this pap er we present results of our search for some general signatures of magnetic fields in Ap stars. Key words: stars: magnetic fields ­ stars: chemically p eculiar

1

Intro duction

On the basis of all the available published magnetic measurements for Ap stars we have found a few relations describing the frequency of o ccurrence of the sp ecific magnetic field intensity for all typ es of chemical p eculiarities (Bychkov et al. 2003a). Subsequently, we renormalized these relations assuming that the extremely `non-magnetic' Am stars represent the reference level, and we estimated the relative value of `magnetization' (M) for Ap stars of various typ es of p eculiarity (Bychkov et al. 2003b). At the same time we credibly separated all chemically p eculiar stars into two groups: they either are `weakly'-magnetic ob jects (Am and Hg-Mn stars), or `strongly'-magnetic ob jects (SrCrEu, Sr, Si, He-w and He-r stars). Magnetization of stars in the `strongly'-magnetic group exceeds that in the `weak'-magnetic group typically by a factor of 5­10, on the average. This is clearly seen in Fig. 3 of this pap er. This is a quantitative estimate for the well known observational fact. One of the most imp ortant and easily measurable physical parameter of stellar atmospheres is the effective temp erature, Tef f . In this pap er we analyze and discuss the relation b etween magnetization and the Tef f for chemically p eculiar stars.

2

Observational data and relations

Fig. 1 shows the distribution of `magnetic' Ap stars as a function of T ef f , and Fig. 2 shows the same for `non-magnetic' Ap stars, resp ectively. Both dep endences were obtained for the same sample of Ap stars, which previously was used to derive distribution functions of Ap stars (Bychkov et al. 2003a). Effective temp eratures of all stars displayed here were either taken from Hauck & North (1993), Glagolevskij (1994, 2002), Sokolov (1998), or were determined following the relations by Paunzen et al. (2005). Fig. 1 clearly shows the dep endences of numb er distribution for stars vs. Tef f , for various typ es of sp ectral p eculiarity. The relative magnetization of stars dep ends also on Tef f , which is shown in Fig. 3 according to the data presented in Table 1. Table 1 presents values of the average T ef f and the average magnetization for various classes of chemical p eculiarity. 105


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BYCHKOV ET AL.

50 40 30 20 10 12 9 6 3

SrCrEu

Sr only Si

N stars

40 30 20 10 12 9 6 3 6 3 0

He-w He-r
10000 15000 20000 25000 30000

T eff

Figure 1: Numb er distribution of `magnetic' chemically p eculiar stars vs. effective temp erature T for various typ es of p eculiarity.

ef f

10 5

HgMn

9

N stars

6 3

HgMn only

20 15 10 5 10000 15000 20000 25000 30000

Am

T eff

Figure 2: Numb er distribution of `non-magnetic' chemically p eculiar stars vs. effective temp erature Tef f for various typ es of p eculiarity. Table 1: T
ef f

and the average magnetization, M, for stars of various classes of chemical p eculiarity Typ e of stars Am Sr all Sr only Si HgMn only Hg Mn He-weak He-rich N stars 44 125 40 158 19 39 60 19 Mean Tef f 9086 9680 11600 11895 12130 12218 15199 21500 Tef f 940 1990 2440 2280 1770 1960 3810 3590 M 1.0 11.8 11.5 9.3 1.7 2.5 8.9 8.7 M 0.74 0.83 0.38 0.54 0.80 0.92 0.12


MAGNETIZATION OF STARS VS. THE EFFECTIVE TEMPERATURE

107

12

Sr

Sr only

Relarive magnetic intens.

10 8 6 4 2

Si

He-w

He-r

HgMn HgMn only Am
8000 12000 16000 20000 24000

Teff

Figure 3: The relative magnetization (M) vs. T

ef f

for various typ es of p eculiarity.

3

Discussion

As was shown in Fig. 3, the parameter of magnetization in the group of `strongly'-magnetic stars distinctly decreases with increasing T ef f . Therefore, magnetization in this group apparently reduces with increasing temp erature. As all memb ers of the `strongly'-magnetic group of Ap stars b elong to the main sequence, the increase of T ef f imply the increase of stellar mass, while the age of a star diminishes. Therefore, He-r stars are younger by almost two orders of magnitude than SrCrEu stars and are 6 times as massive on the average. All of these considerations inconsistent with the hyp othesis of the `relic mechanism' of the origin of magnetic field (Moss 1989, 2001; Landstreet & Mathys 2000). If this mechanism worked in our sample of CP stars, then the situation must b e the opp osite: the younger and more massive are stars, the stronger magnetic fields must b e present (on the average), as the result of magnetic flux conservation. The effect is most imp ortant in relatively co oler low-mass stars which have stronger magnetic field, esp ecially b ecause the Hayashi phase of stellar evolution conceivably destroys relic magnetic fields. It is p ossible that the `relic mechanism' can supply only the initial, ino culating magnetic field. Finally, we p oint out two unsolved problems: 1. Why are strong magnetic fields observed solely in some typ es of chemically p eculiar stars? 2. Do es a single mechanism exist for the creation of global magnetic fields in all these stars?

Acknowledgements. This paper was supported by grant No. 1 P03D 001 26 from the Polish Committee for Scientific Research.

References
Bychkov V.D., Bychkova L.V., Madej J., 2003a, A&A, 407, p.631


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Bychkov V.D., Bychkova L.V., Madej j., 2003b, in Proc.Intern.conf., "Magnetic Stars", p.216 Glagolevskij Yu.V., 1994, Bull.Spec.Astrophys.Obs., 38, 152 Glagolevskij Yu.V., 2002, Bull.Spec.Astrophys.Obs., 53, 33 Hauk B., North P., 1993, A&A, 269, 403 Michaud G., Charland Y., Megessier C., 1981, A&A, 103, 244 Moss D., 1989, MNRAS, 236, 629 Moss D., 2001, in: ASP Conf.Series, 248, 305 Landstreet J.D., Mathys G., 2000, A&A, 359, 213 Sokolov N.A., 1998, A&AS, 130, 215 Paunzen E., Schell A., Maitzen H.M., 2005, A&A, 444, 914