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Physics of Magnetic Stars, 2007, pp. 32­60

Magnetic CP stars in our Galaxy
Iosif I. Romanyuk, Eugene A. Semenko
Special Astrophysical Observatory of the Russian AS, Nizhnij Arkhyz 369167, Russia

Abstract. A review of the mo dern state of the problem is presented. Spatial distribution of CP stars in the Galaxy is considered, it is shown that concentration of CP stars increases toward the Galaxy plane and toward the center of the Lo cal System. Kinematics was studied: it was demonstrated that magnetic stars rotate in synchronism with other nearby stars of the Lo cal System around the center of the Galaxy. Rotation velo cities, inclination angles b etween the axis of rotation and the line of sight, photometric indices a, ages and masses of magnetic stars dep ending on galactic longitude and latitude are analyzed, stars in the clusters of different age and binary systems are under investigation.

1

Intro duction

A considerable b o dy of unquestionable evidence of magnetic fields existing in b oth separate cosmic ob jects and in stellar systems have b een obtained by astronomers during the last half a century. In particular, the structure of the magnetic field of the Galaxy have b een studied well enough, magnetic fields of hundreds of stars and in the interstellar medium have b een found and studied. However, the main question -- how the magnetic field in our Galaxy (as a whole and in separate ob jects) is formed and supp orted, how it evolves -- has b een unclear in many asp ects up to now. In our opinion, the fundamental cause of this difficulty is the fact that data obtained from the observations very often lack information that can b e used for magnetic field analysis. Usually, such data can b e obtained from sp ectrop olarimetric observations, which are considerably rare, b ecause they need a sp ecial complicated instrumentation far from existing at all telescop es. Moreover, many physical effects p erfectly observed in magnetic field in the ground-based lab oratories, cannot b e measured in space b ecause of low sensitivity of facilities and b ecause it is imp ossible to make exp eriments with astronomical ob jects. For example, the question if there exists any connection b etween magnetic field parameters of separate ob jects and the structure of the magnetic field in different lo cal regions of the Galaxy, has b een not risen up to now, whereas such an investigation could give imp ortant information ab out the pro cesses of formation and evolution of magnetic fields in our stellar system. The deficit of information ab out cosmic magnetism presently analyzable is caused not only by difficulties and lab our intensity of magnetic field observations, but also by the fact that the interpretation of data is often ambiguous and dep ends on the mo del. The existing technique for the study of magnetic fields is far from b eing p erfect, it p ermits us to find only strong, regular and largescale fields of simple configuration. Studied ob jects must b e bright enough for sp ectrop olarimetric observations with high resolution and high photometric accuracy. Search for such ob jects is one of urgent directions in the problem of the stellar magnetism study. In this context attention is paid to the so-called chemically p eculiar (CP) stars of the Main Sequence. They are numerous enough, the fullest catalog (Renson et al., 1991a ) contains information ab out more than 6600 such ob jects of different typ es. 32


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2
2.1

CP stars in the lo cal part of the Galaxy
Some general parameters of CP stars

A detailed review of physical parameters and chemical comp osition of atmospheres of CP stars was published by Romanyuk (2007). In this pap er information ab out different-typ e chemically p eculiar stars is presented: frequency of o ccurrence, duplicity, rotation, magnetic fields, variability, luminosity, anomalies of chemical comp osition, etc. In the present review we restrict ourselves only to a discussion of some basic parameters and distinctive features of these Main Sequence ob jects. Unique feature of CP stars is their anomalous abundances (sometimes higher or lower by a few orders) of certain sp ecific chemical elements (for example, such as helium, chromium, strontium, silicon, rare-earth elements etc.). The frequency of o ccurrence of CP stars is ab out 15% ­ 20% of Main Sequence stars in the B2 ­ F0 sp ectral range. CP stars can b e divided into two large sub classes: magnetic and non-magnetic. The magnetic sub class contains approximately 3000 so-called Ap/Bp stars presented in the catalog by Renson et al. (1991b). Magnetic field was first discovered in 1947 by Bab co ck (1958) in the Ap star 78 Vir. Apparently all Ap/Bp stars p ossess regular large-scale magnetic fields, however practical sp ectrop olarimetric observations to search for such fields were made only for a small part (approximately 10%) of them. First of all it dep ends on the fact, that magnetic measurements need very large exp enditure of observing time at big telescop es. Besides one needs to use sp ecial equipment -- analyzers of circular p olarization and high resolution sp ectrographs. For this reason, magnetic field measurements are not widespread. For illustration, during the last 60 years passed from Bab co ck's discovery, direct Zeeman observations of 800 Ap/Bp stars were made; magnetic fields were found in 350 of them. Even using the world's largest telescop es it has b een p ossible to study only the brightest stars up to recent time. For example, in 1983, at the end of photographic era in astronomy Didelon (1983) published a catalog containing information on ab out 130 magnetic CP stars, only 4 of them were fainter than the 8th magnitude. Most of them were brighter than magnitude 6.5. Since, the typical absolute magnitude (M v ) for most of the ob jects indicated ab ove (excluding a small group of stars with anomalous helium lines), o ccupied the interval from -1 (Si anomalies) to +1 (SrCrEu anomalies), this means, that most of the observed magnetic stars are lo cated in the nearest neighb orho o d of the Sun, at the distances closer than 100­200 p c. Therefore the formulation of the problem of searching for any connections with the magnetic field of the Galaxy 20 years ago was to o untimely. However, the development of observational techniques, growing sizes of telescop es, improvement of sp ectrograph and detector parameters make it p ossible to study magnetic fields in fainter and more distant stars. Already in 2000, 212 magnetic CP stars were presented in the catalog by Romanyuk (2000), 30 of them are fainter than the 8th magnitude. All new found magnetic stars in the xxi century were observed in two observatories: 1) at the Sp ecial Astrophysical Observatory of the Russian Academy of Sciences (SAO RAS) using the 6 m telescop e BTA and 2) at the Europ ean Southern Observatory in Chile, using the 3.6 m and the 8 m telescop es. 75 new magnetic CP stars were found with the 6 m telescop e during 2000 ­ 2006, and 50 of them fainter than the 8th magnitude (Kudryavtsev et al. 2006). Bagnulo et al. (2006) found 41 new magnetic stars in op en clusters using observations in Chile, 28 of them are fainter than the 8th magnitude. At the same country, using the 8 m telescop e VLT, the magnetic field of the faintest star of 12.9 magnitude was measured. Thus, progress is evident: now approximately 350 magnetic CP stars are known and more than 100 of them are fainter than 8th magnitude. The region of the Galaxy with observed magnetic stars is extended: up to 200 ­ 300 p c for typical SrCrEu stars, up to 300 ­ 400 p c for silicon stars and


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Figure 1. Lo cal part of the Galaxy. up to 500 ­ 600 p c for helium stars. But is this information sufficient to search for any connections b etween top ology and orientation of magnetic fields of individual stars and the magnetic field of the Galaxy?

2.2

Lo cal part of the Galaxy

Consider first our lo cal region of the Galaxy. The Sun lo cates near the median plane of the Galaxy at a distance of approximately 8 kp c from center of the system and ab out 20 p c toward to the north of the galactic plane. Three regions of concentration of young ob jects exist in the solar neighb orho o d. In one of them, the so-called Orion arm, the Sun is lo cated. The second is observed in the direction opp osite to the center of the Galaxy at a distance of 1.5 kp c (Perseus arm), the third one lo cates in the direction towards the center of the Galaxy at a distance of ab out 1.2 kp c (Sagittarius arm). The Orion arm is b elieved to b e a small branch of large spiral arm, which is often observed in other galaxies ("Fizika Kosmosa" 1986) (see Fig. 1). The plane of concentration of nearby stars do es not coincide with the plane of the galactic equator. As far back as 1879, Gould (Agekian 1962) found a tilt of 17 degrees of the plane of largest concentration of stars brighter than 4th magnitude to the plane of the Galaxy. For fainter stars this inclination decreases, it reduces to 9 degree for stars brighter than 9th magnitude and is lower for fainter stars. The describ ed phenomenon may b e considered as evidence of existence of the Lo cal System, the main plane of which is oblate and do es not coincide with the plane of the Galaxy. The study of stellar density in the region around the Sun shows an essential increase of the density in the direction with the galactic longitude from l = 230 to l = 260 , up to distances of 300 ­ 400 p c. This scale is considered as approximate characteristics of the size of the Lo cal System (Agekian 1962). It is essential that increasing density takes place mainly due to B and partially A stars' contribution to general density. It is presently accepted that Lo cal System have sizes 200 â 500 p c, their center is lo cated at a distance of 100 ­ 150 p c from the Sun in the direction l = 275 , b = +12 .


MAGNETIC CP STARS IN OUR GALAXY

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Figure 2. Distribution of p olarization in our Galaxy. A radio-astronomical study shows that emission from the Galaxy is linearly p olarized. The generally accepted explanation is that p olarization arises from the light scattering on interstellar dust. It is oriented predominantly under the influence of magnetic field. The maps of galactic p olarization show clearly that p olarization in the Galaxy is directed along the spiral arms. Thus we have evidence that: 1) common magnetic field in the Galaxy exists; 2) it is directed along the spiral arms. For illustration, we demonstrate the distribution of p olarization versus galactic co ordinates l and b in Fig. 2 (Sp o elstra 1977). Comparison of Fig. 1 and Fig. 2 shows that there are "zero p oints" towards the galactic longitudes l = 40 and l = 220 where the sign of p olarization reverses.

2.3
2.3.1

Spatial distribution of CP stars
General pattern

Let us consider lo cation and kinematics of CP stars in the lo cal part of our Galaxy. For this purp ose, we use our data and information from different astronomical catalogs. The spatial distribution of 6600 CP stars from the catalog of Renson et al. (1991a) is demonstrated in Fig. 3. Extremely faint ob jects from the catalog are 12­13 magnitude stars, i.e. they are lo cated at distances of ab out 1 kp c. However, most of the catalog ob jects are brighter and nearer stars, b elonging to the Lo cal System. It is seen in Fig. 3 that the distribution of CP stars on galactic co ordinates is non-uniform: 1) evident concentration toward the plane of the Galaxy is observed along galactic latitude b; 2) highest concentration along galactic longitude l is seen towards the center of the Lo cal System, the lowest concentration -- towards the anti-center of it.


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Figure 3. Distribution of CP stars from the catalog of Renson et al. (1991a) versus galactic co ordinates l and b

The median of the distribution lays along galactic latitude b = -3.5 , which reflects the fact that the Sun is lo cated somewhat higher (to the north) ab ove the plane of the Galaxy. Thus, we can see that the spatial distribution of CP stars from the catalog of Renson et al. (1991a) is sensitive to morphology of the lo cal part of the Galaxy. That is why a search for any features in the distribution of magnetic stars dep ending on the morphology of the magnetic field in the Lo cal System acquire a sense. In order to b e able to connect parameters of individual stars with the structure of the Galaxy and the morphology of its magnetic field, it is needed to make numerous measurements of magnetic fields of stars in the whole region of the Lo cal System and even b eyond its b oundaries up to distances of 1 kp c. It is not realistic to study all 6600 stars with anomalous chemical comp osition (and brightness up to 12 magnitude) b ecause it demands to o much observing time at world's ma jor telescop es. For this reason sp ecial approaches were develop ed to search for new magnetic stars. An effective pro cedure has b een develop ed and employed at the 6-m telescop e for such investigations (see, for example, Kudryavtsev et al., 2006). Main criterion for selection of magnetic star candidates for the program of observation with BTA is the existence of strong anomalies in energy distribution in their continuous sp ectra. It is imp ortant that the development of new techniques numb er of stellar groups of different age, in which one can asso ciations, Bagnulo et al. (2006)). Comparative analysis and old ob jects may b e of imp ortance for understanding the magnetic fields. make it p ossible to largely increase the observe magnetic stars (70 clusters and of magnetic field parameters in young pro cess of origin and evolution of stellar


MAGNETIC CP STARS IN OUR GALAXY

37

2.3.2

Distribution of CP stars of different p eculiarity typ es

Consider, whether there are any differences in the distribution of chemically p eculiar stars in the Galaxy dep ending on the p eculiarity typ e? Since in further statements we will take interest in various relations with magnetic fields, we restrict ourselves only to comparison with the spatial distribution of unique p otentially magnetic sub classes of Ap/Bp stars: 1) stars with anomalous helium lines, 2) stars with anomalous silicon lines, 3) stars with strongly enhanced lines of strontium, chromium and europium. Data on the ob jects were taken from the catalog of Renson et al. (1991b). The results of this analysis are demonstrated in Fig. 4. Stars with anomalous helium lines (He-rich, He-weak) show conspicuous concentration towards the galactic plane. There are relatively not numerous young and hot B stars, an essential part of them are memb ers of op en stellar clusters and asso ciations. Any predominant concentration along galactic longitude is not seen (Fig.4, upp er panel). Stars with anomalous silicon lines (Si and Si+ typ es) can b e easily selected during sp ectral classification of stars, therefore they are found in a great numb er. These are late B and early A stars, memb ers of clusters and field stars are present among them. Si- and Si+-typ e stars show an essential concentration towards the plane of the Galaxy and non-uniform distribution in azimuth: the numb er of them towards galactic longitude l = 275 is many times higher than in opp osite direction (Fig.4, middle panel). The oldest and co olest (late A and F) stars of SrCrEu typ e show an essentially lower concentration toward the galactic plane in comparison with silicon stars (as it was exp ected). Higher concentration towards the center of the Lo cal System (galactic longitude l = 275 ) is preserved, but it is not seen as pronounced as for silicon stars (Fig.4, lower panel). Thus, we can see that the distribution of Ap/Bp stars of different typ es in our lo cal part of the Galaxy is different. In general, it corresp onds to the spatial distribution of normal stars with the same temp eratures and sp ectral classes in the Lo cal System.

3
3.1

CP stars with measured magnetic fields in the Galaxy
Intro duction

The overwhelming ma jority (more than 90%) of stars classified as chemically p eculiar are very p o orly studied. As a rule, only p eculiarity typ es, magnitudes and colors are known. However, it is p ossible to select a small part of them, at this moment ab out 350 magnetic Ap/Bp stars. Magnetic fields can to b e studied only by using high-quality observational data. Reduction of sp ectrop olarimetric data p ermits not only magnetic field parameters, but also radial velo cities, physical parameters and chemical comp osition of atmospheres to b e defined. Thus, magnetic Ap/Bp stars are usually well-studied. As a rule, these are bright (usually brighter than 10th magnitude) and relatively near ob jects. Let us consider their lo cation in the Lo cal System in more detail and make analysis: if there exist any dep endence of physical parameters and kinematic on their galactic co ordinates. The main sources of data on magnetic Ap/Bp stars are the catalog by Romanyuk (2000), and original pap ers (Kudryavtsev et al. 2006 and Bagnulo et al. 2006). Common parameters of the considered ob jects were adopted from the database SIMBAD. Of course, the authors of the present review understand that 350 ob jects is a to o small numb er for detailed investigation of their spatial characteristics. It is apparent that should b e continued intensive searches for new magnetic CP stars. But we cannot exp ect high (if only few times) increases in the numb er of magnetic Ap/Bp stars in the nearest 10 years. Thus, to assess the present day state of the problem is very imp ortant for selection of optimal strategy of investigation in this scientific direction. Our review serves for this purp ose.


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Figure 4. Distribution of CP stars from catalog of Renson et al. (1991b) versus galactic co ordinates l and b dep ending on p eculiarity typ e .


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3.2

Spatial distribution

Hereafter we will consider only Ap/Bp stars with measured magnetic fields. Since, only less than 50 magnetic stars with anomalous helium lines are known and most of them are memb ers of clusters, we will not demonstrate the spatial distribution of individual ob jects of this typ e in the Lo cal System. The distribution of magnetic silicon and SrCrEu stars versus galactic longitude l and latitude b is shown in Fig. 5. An analysis of Fig.5 shows that concentration of magnetic silicon stars towards the galactic plane is more pronounced than in co oler magnetic SrCrEu stars. The same is observed for all CP stars presented in Fig.4. Separate zones where magnetic stars are actually absent are observed in the plane of galactic co ordinates, for example the zone with l = 240 ­ 360 and b < -20 for SrCrEu stars (Fig. 5, lower panel). Note, that the location of the center of the Local System (l = 275 , b = +12 ) corresponds to the region with equatorial coordinates = 10 h 18m and = -42 . Such a feature is difficult to explain. Most probably, the effect of observational selection manifests itself. As we mentioned ab ove, magnetic field observations were p erformed earlier and made now only in a few observatories of the world, mainly lo cated in the Northern hemisphere of the Earth. That is why, mainly observations of southern stars, in particular, towards the direction of the center of the Lo cal System are deficient. Note, that a considerable part of magnetic observations using different telescop es lo cated in Chile were made by J. Landstreet and his colleagues with a Balmer-line magnetometer. They were concerned mainly with searching for magnetic fields in fast rotating stars with anomalous lines of helium and silicon (see, for instance, Borra et al., 1983; Bohlender et al., 1987; Bohlender et al., 1993). Magnetic fields were found in no less than 50 such ob jects. Now approximately the same numb er of magnetic silicon stars are known in b oth the northern and southern sky. Searches for magnetic fields in co ol SrCrEu stars were made by G. Mathys and his colleagues at ESO, using the study of Zeeman splitting of sp ectral lines. They found a smaller numb er of new magnetic stars than those discovered in observatories at the Northern hemisphere. Since G.Mathys made magnetic observations in a single observatory, the seasonal variations of weather conditions or particular character of telescop e time allo cation at ESO can, in principle, pro duce an effect of false non-uniform distribution of SrCrEu magnetic stars along galactic longitude l (Fig. 5, lower panel). We supp ose that effects of observational selection may affect significantly the numb er of ob jects that we observed in different directions, therefore it is to o early to make any conclusions concerning the existence of real features of the distribution of magnetic CP stars in the Lo cal System. HIPPARCOS parallaxes (ESA 1997) were measured for approximately 200 magnetic CP stars, which p ermits a three-dimensional picture of their distribution in the Galaxy to b e derived. For illustration, the lo cation of the sample of magnetic CP stars with measured parallaxes in our lo cal region in the Cartesian co ordinate system (X, Y , Z ) is demonstrated in Fig.6. It is well known that HIPPARCOS parallaxes p ermit us to measure reliable distance only for stars lo cated closer than 300 p c. For more distant ob jects the errors in distance determinations are to o large, which should b e b orne in mind when examining Fig.6. For instance, the long chain of ob jects toward l = 210 (or X = -500, Y = -300), presented by stars from the asso ciation Ori OB1, is unreal, it is due to the low accuracy of parallax determination. Thus, one can conclude that relatively nearby magnetic CP stars with measured parallaxes do not demonstrate on the whole any conspicuous features in their spatial distribution in the Lo cal System.

3.3

Radial velo cities

We will consider the kinematics of the ob jects studied using radial velo cities and prop er motions.


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Figure 5. Distribution of magnetic CP stars with anomalies of Si and SrCrEu versus galactic co ordinates.


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41

Figure 6. Distribution of magnetic CP stars in solar neighb orho o d.

Radial velo cities were measured for 193 magnetic CP stars. We use information taken from the database SIMBAD, and the new data from our pap er (Kudryavtsev et al., 2007). Radial velo cities of stars reduced to the system of galactic co ordinates l and b are presented in Fig.7. Radial velo cities are designated by circles of different diameters, the approaching and receding ob jects are represented by different tints. On can clearly see p eculiar movement of the Sun to the ap ex l = 210 , b = +22 with a velo city of V = +20 km/s relative to our sample. It is seen that there is not a single magnetic CP star whose radial velo city is strongly different from others. An analysis of prop er motions leads to the same conclusion. This means that the kinematics of magnetic stars in the observed lo cal part of the Galaxy is the same. They rotate in synchronism with other stars of the Lo cal System around the center of the Galaxy. Thus, the data on the spatial distribution and kinematics of magnetic CP stars give evidence that these are not alien ob jects, but they are an integral part of the Lo cal System. They formed and evolved in it.


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ROMANYUK, SEMENKO

90°

45°

b 0°

0 6 13 21

- 45°

- 90° 0° 60° 120° 180° 240°

0 < |Vr| < 10 10 < |Vr| < 20 20 < |Vr| < 30 30 < |Vr| < 40 40 < |Vr| < 50 |Vr| > 50 300° 360° 0 8 18 30

l
Figure 7. Distribution of magnetic CP stars with different radial velo cities in the system of galactic co ordinates l and b (filled circles are for receding stars, and op en circles -- for approaching).

3.4

Rotation velo cities

Let us analyze, whether the velo cities of axial rotation of magnetic stars lo cated in different regions of the Lo cal System are equal. Since the value of the equatorial rotation velo city v e has b een determined only for a relatively small numb er of stars with a known rotation p erio d, we will use and compare the data on the pro jection of the rotation velo city onto the line of sight v e sin i. Most of the data on ve sin i values for magnetic CP stars were compiled by Romanyuk (2004) in his Thesis, new data were obtained by Kudryavtsev et al. (2007). The distribution of 200 magnetic stars with different ve sin i values in the plane of galactic co ordinates (l, b) is present in Fig. 8. In this figure the diameter of the circle is prop ortional to value of a pro jection of rotation velo city of a star onto the line of sight ve sin i. In the lower part of Fig.8 two histograms are presented; they demonstrate the distribution function for stars with measured v e sin i northward (b > 0 ) and southward (b < 0 ) from the plane of the Galaxy. An analysis of this figure shows that in Northern part of the Lo cal System magnetic stars with ve sin i > 50 km/s are not observed, whereas in the Southern region they make up a considerable part of the sample. Consider how differs the average values of rotation velo cities v e sin i in the Northern and Southern parts of the Lo cal System (see Table 1). Assuming the distribution ve sin i values to b e random and applying Student's distribution cri-


MAGNETIC CP STARS IN OUR GALAXY

43

90°

45°

b 0°

- 45°

- 90° 0° 60° 120° 180° 240° 300°

10 48 86 124 162 200 360°

l
Frequency Frequency

- 90° < b < 0°
10 0

0° < b < 90°
0 15 0

0

50

100 150 200 250

50

100 150 200 250

Figure 8. Distribution of magnetic CP stars with different rotation velo cities in the Galaxy teria for comparison of average values, we find the t criterion to b e equal 2.64. With a high degree of probability it means that the differences b etween two samples are not o ccasional. The same picture (faster rotation of Southern ob jects) is observed for stars lo cated in high galactic latitudes (b > 45 ). Similar results, presented in Tables 1 and 2 make improbable an o ccasional cause of app earance of the differences indicated ab ove. Thus, from data available it follows that rotation velo cities of magnetic CP stars lo cated in the Southern hemisphere of the Lo cal System are 1.5 times higher than in the Northern. It seems to us that observational selection may has played an imp ortant role in the app earance of such an effect. It is rather difficult to determine it numerically, therefore we will make only a rough estimation. As we wrote ab ove, when analyzing the spatial distribution of different typ e magnetic stars, magnetic field measurements are relatively rare, esp ecially in the Southern hemisphere. Various techniques were used in different observatories and different typ e of stars were studied. In particular, a relatively large numb er of magnetic stars newly discovered using Chilean telescop es are fast rotators with anomalous lines of helium and silicon. Observations were made using a Balmer


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Table 1. Average values of ve sin i in the Lo cal System. Hemisphere b 0 b < 0 ve sin i km/s 33.2 ± 5.3 46.3 ± 6.4 Numb er of stars 106 94

Table 2. Average values of ve sin i of high latitude stars in the Lo cal System Hemisphere b +45 b -45 ve sin i km/s 30.9 ± 4.9 46.2 ± 6.4 Numb er of stars 25 13

magnetometer (Borra et al., 1983; Bohlender et al., 1987; Bohlender et al., 1993). This technique increases the relative numb er of stars known as fast rotators in the Southern hemisphere. Magnetic field observations mainly using lines of metals were made in the Northern hemisphere with large sp ectrographs and analyzers of circular p olarization. Preferences in such observation have b een made for stars with sharp and narrow lines in sp ectra. For example, such technique was used by Bab co ck (1958) and by our group using the 6 m telescop e (Kudryavtsev et al., 2006). Certainly, a numerical estimate of this effect is needed.

3.5
3.5.1 .

Inclination angles i b etween rotation axis and line of sight
Magnetic stars in the Lo cal System: general case

In the case of magnetic CP stars we are afforded a unique opp ortunity: it makes p ossible to determine a very imp ortant parameter, the spatial orientation of the rotation axis for a star in the Galaxy (to b e more correct -- inclination angle i b etween the rotation axes and the line of sight in the plane p erp endicular to picture plane). Note that now we cannot distinguish b etween different directions of the rotation axes in the picture plane. For magnetic CP stars it is p ossible to determine angle i b ecause one can find rotation parameters of a star by two indep endent ways: from the Doppler broadening of sp ectral lines it is p ossible to determine the ve sin i value, and given the p erio d of rotation of a star P and its radius R (which is p ossible to estimate from temp erature and/or sp ectral class), it is easily to find the velo city of equatorial rotation ve using the known formula (R in solar units, P in days): ve = 50.6 R , P (1)

and then the sin i value. There is not any reason for distribution of angles i to have predominant directions in the Galaxy as a whole. The random distribution was confirmed rep eatedly (for example, Stepien 1989 or Abt 2001). For instance, Abt (2001) found on the basis of v e sin i and rotation p erio d determinations for 102 Ap stars that rotation axes are oriented in a random manner within the measurement error. Abt found no relationship of the orientation of axes either with galactic longitude or galactic latitude. In the study rep orted here we were interested in the p oint if there exist any selected regions in our Galaxy where spatially close magnetic stars have the similar orientations of rotation axes.


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45

90°

45°

b

0°

- 45°

- 90° 0° 60° 120° 180° l
Figure 9. Distribution of inclination angles i for magnetic CP stars in the Galaxy (in the plane p erp endicular to the picture plane). The question what stars are close remains undecided. We will analyze data available from literature and will make a conclusion p ertaining the real distances b etween closest CP stars. Taking into account our previous study (Kudryavtsev and Romanyuk, 2001), we supp ose that they must not exceed a few tens of parsecs. We found in literature data on 160 CP stars (memb ers of clusters and field stars) with determined inclination angle i. The principal source of information is the pap er by Kopylov (1987). Data on inclination of axes for other stars, not considered in this pap er, were collected by Romanyuk (2004). Inclination angles of all found ob jects are displayed in Fig.9. There is a certain complication in demonstration of the distribution of angles i b ecause they are determined in the plane p erp endicular to the picture plane. Angle i is equal to 0 when rotational axis is parallel to the line of sight (parallel to the direction of galactic longitude l in Fig.9), and i = 90 when the rotation axis of a star is p erp endicular to it (parallel to the direction of galactic latitude b in Fig.9). Certainly, angles i = 0 and 180 are displayed identically. The p ositions of stars in the galactic co ordinate system l and b are marked by dots in Fig.9, the directions of rotation axes (emphasize: in the plane p erp endicular to the picture plane) are shown by arrows. A review of Fig.9 shows, at a first sight, that some predominant alignment of rotation axes at an angle of ab out 45 is observed. But this is due to large selection effects. The main difficulty is as follows. Knowing the equatorial velo city v e of a star (determined using

240°

300°

360°


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ROMANYUK, SEMENKO

formula (1)) and its pro jection onto the line of sight v e sin i (which is determined from the Doppler broadening of lines), one can not find directly angle i, but only the sin i value. Therefore, if one do es not take into account any additional arguments, one cannot distinguish to which quadrant the angle i b elongs. The ma jority of authors prefer the first quadrant(0 < i < 90 ), and b ecause of this visual illusion arises. However, over the last years pap ers have app eared (for example, by J. Landstreet and his team) in which magnetic mo dels of stars were derived. Parameters of these mo dels are angle i and angle b etween rotational and dip ole axes. The approach applied by these authors leads to deriving angle i > 90 for some stars. We will not discuss in this review if such an approach is realistic, we only note that the numb er of stars with angle i > 90 is extremely small, which is apparent from Fig. 9. Besides, the accuracy of determination of angle i is very low, esp ecially for slowly rotating CP stars. It is common, that 2­3 different estimates of angles i for the same stars but made by different authors differ by a few tens of degrees. Thus, we can consider the situation with determination of angles i to b e unsatisfactory. For this reason, we cannot make a definite conclusion if there exist obvious predominant orientation of rotation axes of magnetic CP stars in the Lo cal System and any relationship with its magnetic field structure. Do es there exist a certain predominant collective orientation of rotation axes of close magnetic stars lo cated in more compact regions of the Galaxy (for instance, in stellar clusters)? How are the rotational axes of binary stars oriented in space? These questions will b e considered b elow. 3.5.2 Magnetic CP stars in op en clusters

The p oint of the p ossible predominant collective spatial orientation of nearby stars is of certain interest: according to mo dern idea, group formation of stars takes place and their further evolution dep ending on their masses. It was proved that rotation velo cities of magnetic CP stars is threefold lower than those for normal Main Sequence stars (for example, Romanyuk 2004). Question of angular momentum loss is active discusses in literature (for example, Stepien 2000). Since 80s of the XX century a variety of investigations of magnetic stars in clusters of different age have b een p erformed. Note here the series of pap ers presented by I.M. Kopylov and his group (for instance, Klo chkova, Kopylov 1986, Glagolevskij et al. 1987). It was shown that p eculiarity typ es, rotation velo cities and magnetic fields do not change during the main sequence lifetime of stars. However, spatial orientation of rotational axes of magnetic stars ­ memb ers of clusters was not considered b efore. We will attempt to fill the gap. Formulation of the problem makes sense b ecause earlier we ( Romanyuk, Kudryavtsev 2001, Kudryavtsev, Romanyuk 2003) found evidence that in some cases relatively close stars (for instance, 53 Cam and 49 Cam, as well as 3 stars in Scorpius­Centaurus asso ciation) have similar physical characteristics, chemical comp osition and morphology of magnetic field. It is most reasonably to search for close magnetic stars among memb ers of clusters. Using the pap er by Kopylov (1987) and data from our catalog (Romanyuk 2004), we selected 4 stellar groups or clusters (Pleiades group, Ursa Ma joris Stream, Orion OB1 and Scorpius-Centaurus asso ciations) if only 5 CP stars with determined i values exist in each of them. Some general information making p ossible to estimate the spatial orientation of the rotational axis, distances b etween stars, magnetic field parameters and p eculiarity typ e is presented in 3. For each group in its columns are presented: the name of the star, angle i, galactic co ordinates l and b, parallax (in milliarcseconds, using HIPPARCOS data), extreme values of the longitudinal magnetic field and p eculiarity typ e. It is seen from Table 3 that Pleiades group and UMa stream o ccupy a large space in the plane of galactic co ordinates, while Orion and Scorpius-Centaurus asso ciations are essentially more spacesaving.


MAGNETIC CP STARS IN OUR GALAXY

47

Table 3. Orientation, lo cation and some parameters of CP stars, memb ers of clusters HD name 11503 25823 27309 74521 220825 15144 112185 118022 148112 152107 209515 36916 37017 37058 37479 37776 103192 122532 125823 130880 142301 142990 144334 147010 l


b



142.548 167.428 174.044 216.711 83.917 189.53 122.18 328.27 29.46 71.61 94.08 207.76 208.18 208.52 206.82 206.07 103192 122532 125823 130880 142301 142990 144334 147010

-41.201 -17.959 -19.828 +29.711 -55.083 -65.08 +61.16 +64.41 +38.63 +39.96 -8.58 -18.88 -18.96 -19.07 -17.32 -16.34 +27.41 +19.45 +20.02 +15.21 +21.51 +21.20 +20.85 +20.88

i , mas Pleiades group 47 15.96 30 6.60 37 10.32 42 8.00 11 20.12 UMa stream 24 15.24 63 40.30 25 17.79 19 13.87 15 18.62 21 6.16 Ori OB1 38 2.88 42 2.68 0 47 45 (90?) 1.96 Sco-Cen 21 8.93 0 5.91 90,5 7.79 32 7.90 35 7.16 30 6.68 45 6.70 61 6.98

B

extr

,G

p ec SiCr Si Si SiCr CrSr SrCr Cr SrCr SrCrEu SrCr SiMn He-wk He-r He-wk He-r He-r Si Si He-wk Si He-wk He-wk He-wk SiSr

-900/ + 410 -100/ + 1200 -1200/ - 200 -200/ + 1400 -400/ + 200 -1100/ - 530 -50/ + 150 -1800/ - 200 -250/ - 90 +500/ + 2000 -270/ + 560 -640/ - 615 -2300/ - 300 -800/ + 1000 -1650/ + 3500 -2000/ + 1000 -250/ - 100 -900/ + 900 -440/ + 370 -4400/ + 1920 -4100/ + 1600 -2500/ + 600 -1400/ + 500 -4500/ - 2500

We will search for close magnetic stars in these clusters (the distances b etween stars must not exceed tens of parsecs) and compare orientation of rotational axes and their other characteristics. Analyze each of the clusters separately. Pleiades group. This group o ccupies almost the whole hemisphere towards the anticenter of the Lo cal System. The cluster is nearby, stars lo cate at distances from 50 to 150 p c from the Sun. The directions of the rotational axes of 5 magnetic CP stars with known i values are shown in Fig. 10. It is seen from Fig.10 that predominant orientation of the rotational axes exists, the scattering is low. As follows from Table 3, the average angle i = 33 ± 6 degrees. We analyzed ab ove various disadvantages of the technique, which can give rise to a false opinion concerning the existence of the predominant alignment, therefore we will not dwell here on this problem. We calculated distances b etween all CP stars in Pleiades group from Table 3. We found that the closest to each other are two pairs of stars from this cluster. The distance b etween HD 25823 and HD 27309 in the first pair is 56 p c, and HD 11503 and HD 220825 in the second pair is 40 p c. It


48

ROMANYUK, SEMENKO

50°

Pleiades

80°

UMa stream

b

- 5°

b

5°

- 60° 80° 160° l 240°

- 70° 25° 192.5° l 360°

Figure 10. Orientation of rotation axes (angles i) in individual magnetic stars from Pleiades group and UMa stream (in the plane p erp endicular to the picture plane). follows from Table 3 that b oth stars from the first pair are silicon stars and they have approximately similar magnetic field strength. HD 11503 and HD 220825 lo cate closer to the Sun, they have chromium anomalies and approximately similar but weaker (than in the first pair) magnetic fields. Thus, we have a go o d ground to b elieve these stars to b e close enough and similar. Compare all found physical parameters for the two pair of stars: HD 25823 and HD 27309 as well as HD 11503 and HD 220825, which we have managed to determine by ourselves or to find in literature. Data on radial velo cities Vr (in km/s), prop er motions µ and µ (mas/year), rotation v sin i (km/s), rotation p erio ds P (days), duplicity and effective temp eratures T ef f were taken from the catalog by Romanyuk (2004), and rest of them -- distances d (in p c), absolute magnitude Mv , logarithm of luminosity log L, masses M , logarithm of age log t and fractional age -- were adopted from the pap er by Ko chukhov and Bagnulo (2006). The data are presented in Table 4. An analysis of Tables 3 and 4 shows that even close CP stars in Pleiades group are lo cated sufficiently far from each other, the distance b etween the closest of them is from 40 to 60 p c. A comparison of the kinematics and physical parameters of the pair of stars HD 25823 ­ HD 27309 shows following: b oth stars have approximately the same temp eratures, p eculiarity typ es, magnetic field strengths and orientation of rotational axes with the resp ect to the line of sight; however their rotation and radial velo cities, masses and luminosities strongly differs. According to Ko chukhov and Bagnulo (2006) ages t for them are ab out the same, but fractional ages are largely different: HD 25823 has already stayed more than 2/3 of its life on the Main Sequence, while HD 27309 finishes only the first 1/3. HD 25823 is a sp ectral binary with an orbital p erio d of 7.23 days, which coincides with the rotation p erio d. There is no information on the duplicity of HD 27309. For the stars HD 11503 and HD 220825 (the second pair) we can see approximately the same radial velo cities and prop er motions, temp eratures (b oth are co oler by 3000 degrees than the stars of the first pair), rotation p erio ds and ages log t. Rotation velo cities v sin i and fractional ages are different. HD 11503 is a well known binary star ADS 1507 A ( Ari A), while there is no information on the duplicity of HD 220825.


MAGNETIC CP STARS IN OUR GALAXY

49

Table 4. Kinematics and physical parameters of close CP stars in Pleiades group HD 25823 -2.0 22.13 -50.18 15 7.227d sp. bin 12900 151 ± 19 -0.63 ± 0.28 2.50 ± 0.12 3.80 ± 0.19 8.13 0.72 HD 27309 +12.4 29.77 -41.39 66 1.569d no data 12260 96 ± 7 0.40 ± 0.16 2.01 ± 0.07 3.04 ± 0.09 8.07 0.34 HD 11503 -0.6 79.43 -99.10 69 1.60920d ADS 1507 A 9850 62 ± 3 0.60 ± 0.12 1.78 ± 0.05 2.57 ± 0.06 8.47 0.55 HD 220825 -3.2 85.60 -94.43 34 1.412d no data 9100 49 ± 1 1.45 ± 0.09 1.35 ± 0.04 2.07 ± 0.04 8.57 0.37

Vr , km/s µ , mas/y µ , mas/y v sin i, km/s P duplicity Tef f , K d, p c Mv log L M log t

According to Klo chkova and Kopylov (1986) the age of Pleiades group is log t = 7.5, which is in a go o d agreement with the age of the two stars from the first pair, determined by Ko chukhov and Bagnulo (2006). The age of the stars from the second pair is half an order of magnitude higher. So, we can conclude: the difference b etween the stars inside each pair in Pleiades group is smaller than b etween the stars in different pairs. Ursa Ma joris Stream. It is seen from Table 3 that UMa stream contains stars distributed all over the sky. Actually, it means that our Solar system is lo cated inside this cluster (see Fig. 11 and right part of Fig. 10). This group contains co oler stars, than the group considered b efore. In Table 3 we found 3 closest stars in UMa stream: HD 148112, HD 152107 and HD 112185. The distance b etween HD 148112 and HD 152107 is 39 p c, b etween HD 152107 and HD 112185 it is 37 p c and b etween HD 148112 and HD 112185 of is 63 p c. By analogy with Table 4 construct Table 5. All symb ols are the same. According to Klo chkova and Kopylov (1986), the age of UMa stream is log t=8.6, which is in an excellent agreement with the data presented by Ko chukhov and Bagnulo (2006) for 3 stars from Table 4. Let us analyze the results presented in Tables 3 and 5. A comparison of the pair HD 148112 ­ HD 152107 shows that b oth stars have nearly the same temp eratures, p eculiarity typ es, inclination angles i (b etween the rotational axis and the line of sight) and rotation p erio ds. Kinematics, luminosities and masses differ insignificantly. Both stars are multiple: HD 148112 is a binary (ADS 10054 AB = CCDM J16254+1402AB), HD 152107 is a triple system (ADS 10227 ABC = CCDM J16492+4559ABC). In b oth cases p eculiar are the primary comp onents, which are substantially brighter than the secondary. According to Ko chukhov and Bagnulo (2006), ages t are approximately the same for b oth of them while fractional ages differ rather significantly. There are no essential differences in the pair HD 152107 ­ HD 112185. The main of them: HD 112185 (well known star UMa) is twice as close to the Sun (at a distance of 24 p c) as HD 152107, therefore kinematic parameters (prop er motions and radial velo cities) are different. Temp eratures, pro jections of rotation velo cities onto the line of sight, rotation p erio ds and ages t are very close. But luminosities, masses and fractional ages largely differ. HD 112185 is finishing


50

ROMANYUK, SEMENKO

Figure 11. Distribution of selected CP stars­memb ers of op en clusters in the plane of galactic co ordinates. its evolution on the Main Sequence, while HD 152107 has evolved only half of its Main Sequence lifetime. No data on the duplicity of HD 112185 are available. In general, we can conclude that parameters of close stars in UMa stream are essentially more alike than in the previous case of Pleiades group. Orion OB1 The op en cluster in Orion is reach in young hot stars. It o ccupies a rather compact area with a size of only a few square degrees in the galactic system of co ordinates. Five magnetic CP stars with measured angle i are presented in Table 3. All of them b elong to the sub class of chemically p eculiar stars with anomalous helium lines. Unfortunately, the cluster in Orion is far away and therefore the distances to the stars are determined with large errors: no parallax measurements for 2 stars out of 5. Information ab out physical parameters, kinematics and ages of these stars was collected by Romanyuk (2004). Inclination angles i for the Orion stars are presented in Fig. 12. Physical parameters of the 5 CP stars in Orion are presented in Table 6. The symb ols are the same as in Table 4, but R -- radius of a star (in solar radius units) -- is added. We can see that 4 ob jects out of 5 have a p erio d of ab out 1 day, while that of HD 37058 is ab out 15 days. Lines in the sp ectrum of this star are not broadened, most probably angle i is small. HD 37017 and HD 37479 are binary stars, information of multiplicity of HD 36916, HD 37058 and HD 37776 is absent. According to Klo chkova and Kopylov (1986) the CP stars in Orion have the following ages: in Ori OB1a log t = 7.3, in Ori OB1b -- log t = 6.6, in B Ori OB1c -- log t = 6.4 and in Ori OB1d -- log t < 6.0. We cannot determine reliable distances b etween the stars in Orion now, this is the sub ject of future studies. Note once more: this cluster is reach enough by p eculiar stars in anomalous helium lines. In the pap er by Klo chkova and Kopylov (1986) data on 35 of such stars in Orion were presented.


MAGNETIC CP STARS IN OUR GALAXY

51

Table 5. Kinematics and physical parameters of 3 close CP stars in UMA stream HD 148112 -5.9 39.39 -59.89 54 3.043d ADS 10054 AB 9250 72 ± 4 0.20 ± 0.15 1.87 ± 0.06 2.60 ± 0.08 8.63 0.81 HD 152107 -1.0 22.78 -51.37 24 3.858d ADS 10227 ABC 8800 53 ± 1 1.21 ± 0.07 1.43 ± 0.03 2.10 ± 0.04 8.74 0.58 HD 112185 -9.3 111.74 -8.99 35 5.0887d no data 8900 24 ± 0 -0.21 ± 0.04 2.01 ± 0.02 2.76 ± 0.03 8.61 0.94

Vr , km/s µ , mas/y µ ,mas/y v sin i, km/s P uplicity Tef f , K d, p c Mv log L M log t

Table 6. Physical parameters of five CP stars in Orion HD 36916 +10.7 100 150 1.565d 12950 298 -0.9 3.0 HD 37017 +29.0 150 80 0.901d 20450 692 -2.2 4.8 HD 37058 +22.8 0 14.6d 19600 HD 37479 +29.0 150 1.191d 22500 HD 37776 +27.0 80 1.539d 23050 603 -2.2 5.5

Vr , km/s v sin i, km/s P Tef f , K d, p c Mv R/R

4.7

5.7

A detailed investigation each of them and their comparative analysis are seen to b e imp ortant for clearing the details of the pro cess of group formation of the stars. Scorpius-Centaurus The young stellar cluster in Scorpius-Centaurus is reach also in CP stars (more than 30 such ob jects). V.G. Klo chkova and I.M. Kopylov for many years were concerned with the study of p eculiar stars of the northern part of this cluster amenable to observations with the 6 m telescop e. In particular, they estimated the ages of CP stars in the region of Upp er Scorpius: core -- log t = 6.6, inner zone -- log t = 6.7, eastern zone -- log t = 7.0, western zone --log t = 7.1 (Klo chkova, Kopylov 1986). Using data from Table 3 we can find that the stars closest to one another are three He-weak stars: HD 142301, HD 142990 and HD 144334. They o ccupy a zone of a few square degrees in the plane of galactic co ordinates, and the distances to them are approximately equal. Similarly to Table 3, the parameters of these 3 stars are listed in Table 7. We calculated the distances b etween these stars. The distance b etween HD142990 and HD142301 is 10.3 p c, b etween HD144334 and HD142301 -- 12.3 p c, and b etween HD144334 and HD142990 -- 5.5 p c. The stars o ccupy a volume of diameter less than 20 p c. HD 142990 and HD 144334 are particularly close.


52

ROMANYUK, SEMENKO

0°

Orion OB1

40°

Sco-Cen

b - 15°

b 25°

- 30° 200° 210° l 220°

10° 260° 310° l 360°

Figure 12. Orientation of inclination angles i of separate CP stars­memb ers of asso ciation Ori OB1 and cluster Sco­Cen (in the plane p erp endicular to the picture plane). From Tables 3 and 7 it follows that all the 3 stars are very similar: they have approximately the same sp ectral classes, temp eratures, rotation p erio ds, p eculiarity typ es, inclination angles of rotational axes to the line of sight, kinematics (b oth radial velo cities and prop er motions). Masses and luminosities are not much different. The three stars are single stars, no information on their duplicity is available. The ages log t differ by half an order of magnitude, fractional ages are less different: all the stars evolved less than 1/3 of their Main Sequence lifetime. Greatly different is the pro jection of rotation velo city onto the line of sight v sin i for HD 142990. However, it is not improbable that this effect may b e due to the large magnetic broadening of line profiles in strong magnetic field and, p ossibly, in magnetic fields of complex structure. This idea needs checking. Putting an end to the discussion of prop erties of magnetic CP stars in op en clusters it should b e noted, that these stars do not form groups in some separate regions of these systems. The distances b etween known CP stars exceed, as usual, a few tens of parsecs. Only in the Scorpius­Centaurus cluster we have found ob jects really close enough. At present we can observe only rare isolated cases where close stars have close physical parameters. For this reason it is needed to formulate an observational problem with the purp ose of reliable determination of physical parameters of magnetic stars (in the first place -- in Orion and Scorpius­Centaurus), including the magnetic field structure, orientation of the rotational axis to the line of sight i, and inclination angle b etween the dip ole and the rotational axes.

3.6

Binary magnetic CP stars

An evident new question arises -- how are rotational axes of comp onents of binary systems oriented, one of which (or b oth) is a magnetic CP star? The deficit of double systems among magnetic stars is well known, (see, for example, the review by Romanyuk (2007)). Nevertheless, actually all brightest and b est studied magnetic CP stars are either optical or sp ectral binaries: 2 CVn, CrB, 52 Her, Equ, 53 Cam and others. Carrier et al. (2002) presented information ab out 74 magnetic binary stars, 53 binaries are describ ed in our catalog of magnetic CP stars (Romanyuk, 2004). Peculiar are primary comp onents, all secondary comp onents b elong to the Main Sequence, no sp ecial cases (for instance, white dwarfs)


MAGNETIC CP STARS IN OUR GALAXY

53

Table 7. Kinematics and physical parameters of close CP stars in Scorpius­Centaurus cluster HD 142301 -8.7 -12.29 -25.45 47 1.459d no 17300 139 ± 23 -0.24 ± 0.37 2.53 ± 0.15 4.30 ± 0.21 7.21 0.11 HD 142990 -11.1 -11.41 -24.06 140 0.979d no 17800 149 ± 18 -0.74 ± 0.27 2.78 ± 0.11 4.87 ± 0.20 7.54 0.34 HD 144334 -6.6 -10.98 -29.18 25 1.495d no 15150 149 ± 19 -0.29 ± 0.27 2.50 + -0.12 4.08 ± 0.17 7.72 0.33

Vr , km/s µ , mas/y µ ,mas/y v sin i, km/s P duplicity Tef f , K d, p c Mv log L M log t

were found. Our search for data from literature shows that at the present time there is no system with a magnetic CP comp onent studied to a degree allowing the spatial orientation of b oth comp onents to b e found. This problem is to b e resolved in the future. Below, in Table 8 we prop ose a list of binary CP stars for first priority detailed investigation. It is obvious that one needs to consider bisp ectrum binaries, in which if only one of the comp onents is magnetic CP star while second would b e such that high quality data could b e obtained for a detailed study, including construction of mo dels and estimation of physical parameters. Most of the data were taken from the "Catalog of the observed p erio ds of Ap and Bp stars" (Catalano, Renson 1998) and from the pap er by Leone and Catanzaro (1999). Table 8. Bisp ectrum optical binary magnetic stars HD numb er BD +40 175 AB ADS numb er ADS 693 AB Comments Second comp onent -- magnetic CP star weaker by 0.5 magnitude at a distance of 3.7 (Elkin 1999). "HD 11502a, companion (HD 11503) at 8 often confused with the Ap star, the luminosity b eing ab out the same. HD 11502b (m less than 0.1"), (Catalano, Renson 1998). companion (Am star at 2" ) m = 0.9", companion A (mv = 4.33) -- Ap star, companion B (mv = 5.23) -- Am star at a distance of 3.6 (Catalano, Renson 1998).

HD 11503/2

ADS 1507 A

HD 12447

ADS 1615 A


54

ROMANYUK, SEMENKO

Table 8. Bisp ectrum optical binary magnetic stars (continued) HD numb er HD 15089 Aa ADS numb er ADS 1860 A Comments triple system, companions at 2 ( m = 2.9, orbital p erio d 840 years) and at 7 (m = 3.8), close HD 15089 Ab companion at 0.1 , revol. in 52.4 years (Catalano, Renson 1998). companion at 12 , m = 3.1 sp ectral binary, P = 2.9978d (Bab co ck 1958), sin i = 0.15 (Catalano, Renson 1998 and orbit from (Leone, Catanzaro 1999) Ori C. The system consists of 4 stars: the primary comp onent A = HR 1852 = HD 36486 is a sp ectral binary with V = 2.23, comp onent B lo cates at a distance of 33 from A and has a magnitude of 14.0m . Comp onent C (with enhanced helium lines) = HR 1851 = HD 36485 lo cates at a distance of 51.7 from A and have a magnitude of 6.85. HD 36485 is also binary (Bohlender et al. 1987). Orbital elements are available (Leone, Catanzaro 1999). Magnetic Ap star. Multiple system. A -- primary comp onent. Comp onent B -- Am star at a distance of 145 from A. Comp onent B -- sp ectral binary system (includes the comp onents CCDM J12289+2554BC) (Bab co ck 1958). Optical binary. Comp onent A -- famous magnetic CP star. Comp onent B -- on the distance 15 -- F star. The distance and kinematics of comp onents are as follows: comp. A -- = 29.60 mas, µ = -233.43 mas/y, µ = 54.98 mas/y, Vr = -3.3 km/s. Comp. B -- = 39.95 mas, µ = -203.89 mas/y, µ = 88.34 mas/y, Vr = -3.1 km/s (ESA 1997). Comp onent A -- 5.4 magnetic Ap star, visual binary with companion (A6p) of a 6.3 magnitude at a distance of 1.6 (Bab co ck 1958). Probably, comp onent B is also a magnetic star.

HD 15144 Aa

ADS 1849 A

HD 36485

ADS 4134 C

HD 108662 (17 Com A)

ADS 8568 A

HD 108651 (17 Com B)

ADS 8568 BC

HD 112413 (2 CVn)

ADS 8706A

HD 112412 (1 CVn)

ADS 8706 B

HD 130559 (µ Lib A)

ADS 9396 A


MAGNETIC CP STARS IN OUR GALAXY

55

Table 8. Bisp ectrum optical binary magnetic stars (continued)

HD numb er HD 145501 ( Sco BC)

ADS numb er ADS 9951 CD

HD 145502 ( Sco A) HD 165475 (HR 6758)

ADS 9951 AB ADS 11056 A

HD 165474

ADS 11056 B

Comments CCDM J16120-1928CD -- Binary or multiple star. Comp onent B = sp.B8p, comp onent C = sp. B9I I I. CD = visual binary (separation 2 ). Comp onent D is by 0.7m weaker than C (Borra et al. 1983). Comp onent A = B2 star at a distance of 40 . Sp ectral binary. Magnetic star ­ HD 145501. Comp onent A -- a typical A star with wide lines Peculiar star -- comp onent B, of approximately the same magnitude at a distance of 7 from A. (Bab co ck 1958)

Thus, we prop ose 11 binary systems with a magnetic CP comp onent for a detailed investigation. This problem needs to b e considered in more detail in future studies.

3.7

a photometry

The photometric index a was intro duced by Maitzen (1976), it is a characteristic of anomalies in energy distribution of magnetic CP stars, in particular, continuum depression at a wavelength 5200 ° The index a correlates in a certain degree with the magnetic field strength on the star's surface A. (Kudryavtsev et al. 2006), therefore it can b e used for indirect estimations of the field parameters. The distribution of stars with different a values versus the galactic co ordinates is shown in Fig.13, the source of data is the Thesis by Romanyuk (2004). It is seen from Fig.13 that most of the data were obtained for stars of the northern part of the Galaxy. It seems, that an essential contribution is intro duced by instrumentation selection, which we discussed rep eatedly ab ove. No systematic features in the distribution of a values versus galactic longitude l in Fig.13 are seen. Let us consider numerically if there are any differences in the distribution of stars with different a versus galactic latitude b. For an analysis we divided all the sample into 3 zones in the Galaxy: northern (b > 30 ), central(-30 < b < 30 ) and southern (b < -30 ). The average indices a in the zones are presented in Table 9. Table 9. Distribution of CP stars with different a indices with galactic latitude b a 0.034 ± 0.003 0.037 ± 0.003 0.029 ± 0.003 Numb er of stars 29 70 18

b > +30 -30 < b < +30 b < -30



It follows from Table 9 that most CP stars with known a indices are accumulated in the plane of the Galaxy. No significant difference across the galactic latitude is observed.


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ROMANYUK, SEMENKO

90°

45°

b

0° b

- 45°

-0.001 0.022 0.034 0.04775 0.146

- 90° 0° 60° 120° 180° l 240° 300° 360°

Figure 13. Distribution of a indices of magnetic stars along galactic co ordinates.

3.8

Age

Consider the age distribution of CP stars in the Lo cal System. It will b e recalled once again that these ob jects b elong to the Main Sequence. The age designated as t is the chronological age, it shows how many years a star lives on the Main Sequence. is fractional age -- the fraction of the Main Sequence lifetime completed. Ages (log t) and fractional ages ( ) for a large sample of stars are presented in the pap er by Ko chukhov and Bagnulo (2006). We have already used the results of this pap er when making an analysis of magnetic CP stars in stellar clusters. Consider now, if there are any systematic regularities in the distribution of stars versus age in the Lo cal System. We will make the same analysis as for indices a. The distribution of stars of different age log t versus galactic co ordinates is presented in Fig.14. There is no pronounced p eculiarities in the distribution of log t versus galactic longitude. By analogy with the study of a distribution, divide the region of the Lo cal System into 3 zones across the galactic latitude b and find an average log t for stars in them. Results are presented in Table 10. Table 10. Distribution of CP stars with different ages log t versus galactic latitude log 8.61 ± 8.11 ± 8.23 ± t 0.05 0.08 0.14 Numb er of stars 33 66 17

b > +30 -30 < b < +30 b < -30





The differences are clearly seen. To the north of the plane of the Galaxy, considerably older stars are observed. We consider this as a result of different distribution of Si and SrCrEu stars across galactic latitude: Si stars concentrate towards the galactic equator much more largely, and therefore


MAGNETIC CP STARS IN OUR GALAXY

57

90°

45°

b

0° b

- 45°

6.39 7.985 8.42 8.6375 9.16

- 90° 0° 60° 120° 180° l 240° 300° 360°

Figure 14. The distribution of stars of different age (log t) versus galactic co ordinates. we can observe mainly older SrCrEu stars in the high galactic latitudes. A very small numb er of data was obtained in the southern part of the Galaxy (b < -30 ) and the contribution of observational selection is large. In particular, as we mentioned ab ove, J. Landstreet and his colleagues in Chilean observatories studied mainly hot helium and silicon CP stars using the Balmer-line magnetometer. Table 11 and Fig. 15 are constructed by analogy with Table 10 and Fig. 14, using fractional age instead of age log t. Table 11. Distribution of CP stars with different fractional ages versus galactic latitude 0.61 ± 0.04 0.50 ± 0.03 0.49 ± 0.06 Numb er of stars 33 66 17

b > +30 -30 < b < +30 b < -30





The same picture is seen from Table 11: high latitude stars (b > 30 ) have sp ent more time of its evolution on the Main Sequence in comparison with stars of the equatorial zone of the Galaxy.

3.9

Masses of magnetic CP stars

We made the same analysis for the mass distribution of magnetic CP stars, using the data from the pap er by Ko chukhov and Bagnulo (2006). Results are presented in Fig.15 and in Table 12. It is seen that masses of north high latitude stars (b > +30 ) and stars from the plane of the Galaxy are the same within errors of estimation. Southern stars show unexp ectedly low masses. Probably, this is a result of low statistics and observational selection. This p oint should b e studied in more details.


58
0 43.0 55.0 57.0 1 b

ROMANYUK, SEMENKO

90°

45°

b

0°

- 45°

- 90° 0° 60° 120° 180° l 240° 300° 360°

Figure 15. Distribution of CP stars of different fractional ages ( ) across galactic co ordinates. Table 12. Distribution of CP stars of different masses across galactic latitude b M 2.88 3.07 2.38 /M ± 0.24 ± 0.13 ± 0.08 Numb er of stars 33 66 17

b > +30 -30 < b < +30 b < -30





4

CONCLUSIONS

We have made a review of magnetic CP stars in our Galaxy. The spatial distribution of CP stars in the Lo cal System has b een considered. It is shown that the ma jority of them are lo cated at the distances less than 1 kp c. The distribution is non-uniform: the accumulation of CP stars grows towards the galactic plane and towards the center of the Lo cal System. Stars with anomalous helium and silicon lines show a higher concentration towards the galactic plane than SrCrEu stars. A study of radial velo cities and prop er motions shows that magnetic CP stars rotate in synchronism with other nearby stars of Lo cal System around the galactic center. These data give evidence that magnetic CP stars were formed in the Lo cal System and evolve in it. Rotation velo cities of stars lo cated in the southern hemisphere of the Galaxy are 1.5 times higher then those in its northern hemisphere. An imp ortant role in the app earance of such a false effect may b e plaid by observational selection. The inclination angles b etween the rotational axes and the line of sight are distributed randomly. However, for some spatially close cluster stars evidence of collective predominant orientation of rotational axes exists. We prop ose a list of binary systems in which if only one comp onent is a magnetic star for a detailed study.


MAGNETIC CP STARS IN OUR GALAXY

59

90°

45°

b

0° b

- 45°

1.55 2.2 2.62 3.215 6

- 90° 0° 60° 120° 180° l 240° 300° 360°

Figure 16. Distribution of CP stars with different masses versus galactic co ordinates. We demonstrated, that photometric indices a and masses of magnetic stars do not dep end on galactic latitudes, young helium and silicon stars show high concentration to the plane of the Galaxy, while older stars (b oth, chronological and evolution ages) spread up to the high galactic latitudes. Neither lo cation of magnetic star in the Lo cal System nor their kinematics show any anomalies or features to distinguish them from normal stars of the Main Sequence. It means that they did not come from anywhere, but they are an intrinsic part of the Lo cal System. Magnetic stars in op en clusters do not show any concentration within them, they spread over a cluster. Certainly, the existing numb er of magnetic CP stars is to o small for construction of reliable statistical distributions. To search for a connection b etween magnetic field of single stars and the structure of magnetic field of the Galaxy requires to find if only 500 new magnetic CP stars, mainly brighter than 10­12 magnitude, preferably in different clusters. It is needed to construct magnetic mo dels of these ob jects and compare them with configuration of magnetic field in different lo cal parts of the Galaxy. This is a very imp ortant observational pro ject, requiring co op erative efforts of astronomers from different countries.
Acknowledgements. The authors are grateful to D.O. Kudryavtsev for discussion of the problem, to the Russian Foundation for Basic Research for financial support (RFBR grant no. 06-02-16110a). We used information from the databases SIMBAD and VALD.

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