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Magnetic stars, 2004, 121-127

Monitoring magnetic fields of sharp-lined Ap stars with the 6 m telescope
Wade G. A.1 , Smolkin S.1 , Romanyuk I. I.2 , Kudryavtsev D.
1 2

2

Physics Department, Royal Military College of Canada, Kingston, Canada Special Astrophysical Observatory, Nizhny Arkhyz, Russia

Abstract. We rep ort results of an ongoing programme aimed at measuring the longitudinal magnetic fields of a sample of approximately 20 Ap stars with sharp, magnetically-split sp ectral lines, using the 6 m telescop e at the Sp ecial Astrophysical Observatory. Key words: stars: magnetic fields ­ stars: p eculiar ­ metho ds: observational

1

Intro duction

This talk represents a progress report of a programme of long-term monitoring of longitudinal magnetic fields of sharp-lined magnetic stars. These stars have been identified by the resolved splitting of Zeeman components ° in their spectra, primarily in the Zeeman doublet of Fe ii at 6149.3 A, and were reported primarily by Mathys et al. (1997). The sharp lines of these stars (with intrinsic widths < 3 km s-1 after magnetic broadening is taken into account) imply small pro jected rotational velocities ve sin i. For those sharp-lined stars with relatively short rotational periods, it is the combination of moderate equatorial rotational velocity v e and significant inclination sin i that lead to their sharp lines. However, for those stars with rotational periods longer than about 50 days, their sharp lines are entirely attributable to slow rotation, notwithstanding the inclination of the stellar rotational axis. These ultra-slow rotators represent about one-half of the known sample of Ap stars with resolved, magnetically-split lines, and their measured and estimated rotational periods extend to years, and in some cases (HD 201601= Equ) decades. The rotational angular momentum of an Ap star with a rotational period of 50 d (v e 3 km s-1 ) is -1 less than 2% of that of a typical normal A star rotating with ve sin i = 120 km s ; that of an Ap star with a rotational period of 50 years is less than 5 в 10-3 %. It is of enormous physical interest to decipher the mechanism by which these stars have shed the vast ma jority of their angular momentum (e.g. Stepien & Landstreet 2002). Unfortunately, their extremely long rotational periods make them poorly suited to study. With the de-commissioning of the CASPEC spectropolarimeter on the ESO La Silla 3.6 m telescope, the Special Astrophysical Observatory provides essentially the only facility worldwide adapted technologically and philosophically to continued study of the magnetic fields of ultra slowly-rotating Ap stars. Since 1996, we have been observing a sample of about 20 of these stars in circular spectro-polarisation, using the main stellar spectrograph on the 6 m telescope. In the remainder of this paper, we describe the properties of the sample, and present some of the first results of our study.

2

Bulk prop erties of the sharp-lined Ap stars
Ap stars studied within the context of period" stars, is reported in Table 1. Of for 5, and the results are reported in the unpublished magnetic phase curves have this programme, roughly divided into the 23 stars listed in Table 1, we have refereed literature. For an additional 4 been obtained. Finally, for another 12

The sample of sharp-lined "shorter period" and "long completed detailed studies stars, complete but as yet

c Sp ecial Astrophysical Observatory of the Russian AS, 2004


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Table 1: List of targets. Individual columns give the ob ject designation, rotational period (if known), apparent visual magnitude, the average value of the mean magnetic field modulus as measured by Mathys et al. (1997), the number of observations obtained in this programme, and comments regarding published models and binarity, and completeness. Ob ject H AV # obs. Comments (kG) Shorter period (Prot < 50 d) 34.9 d 7.8 8.0 10 Wade et al. (2000a), SB1 26.9 d 8.3 7.5 30 Wade et al. (2000a) 34.0 d 7.2 8.4 6 Wade et al. (2000b), SB1 1 month 10.0 3.1 0 incomplete 4.17 d? 7.5 3.1 10 incomplete 3.37 d 8.0 5.0 12 SB1, complete 2.54 d? 7.5 6.5 11 complete 6.42 d 7.4 4.7 12 Wade et al. (1996a) 49 d? 9.2 9.7 12 complete Long period (Prot > 50 d) >> 2 y 8.6 4.4 6 incomplete 21: y 6.4 4.4 6 SB1, incomplete ? 8.3 3.7 14 complete long 9.2 17.5 10 incomplete >> 4 y 9.0 4.7 3 SB1, incomplete 1360 d 8.0 3.1 5 SB2, incomplete long 9.8 7.2 0 SB1 4900: d 6.4 4.1 11 incomplete >> 3 y 7.0 6.0 9 SB1, incomplete > 75 y ? 6.7 4.7 4 52.01 d 7.7 8.7 11 Wade et al. (1997) 223.9 d 5.6 3.7 3 incomplete > 70 y 4.7 3.8 7 incomplete >> 3 y 7.6 5.6 1 SB1
V

Rotational period

m

HD HD HD HD HD HD HD HD HD HD HD HD HD HD HD HD HD HD HD HD HD HD HD

12288 14437 81009 119027 134214 142070 165474 192678 335238 965 9996 18078 47103 50169 59435 61468 110066 116114 137949 200311 188041 201601 216018

stars some magnetic data has been obtained (ranging from a single observation in the case of HD 216018, to 11 observations in the case of 110066). In total, as of the date of this meeting, approximately 300 measurements of the longitudinal magnetic fields of the programme stars have been obtained. The histogram of rotational periods of these stars is shown in Fig. 1. Stars to the left in this figure are characterised, as described above, by shorter periods and larger inclinations. For the ma jority of these stars, full rotational phase coverage has been obtained and their periods are well-determined. On the other hand, stars to the right in this diagram represent true slow rotators. The ma jority of these stars have poorly determined rotational periods, and in particular the values reported in the figure tend to be lower limits (as indicated by the red arrow). Therefore, the marginal separation of these two populations evident in Fig. 1 is very probably much more significant, and it would not be unreasonable to assume that they are representative of two independent distributions. The programme stars have been located on the HR diagram, with effective temperatures determined using Geneva and Stromgren photometry. Heliocentric distances and luminosities were inferred using Hipparcos trigonometric parallaxes (no Lutz-Kelker correction has been applied). Finally, masses and ages were inferred by locating each star on the (log T , log L) Hertzsprung-Russell diagram and comparing with the model predictions of Schaller et al. (1992). The resultant HR diagrams for all programme stars and for those stars with well-determined Hipparcos parallaxes ( / < 0.2) are shown in Fig. 2. Hubrig et al. (2000) reported that the sharp-lined Ap stars appear preferentially away from the ZAMS. Our analysis confirms this general result. However, our analysis also suggests that this may not be true for lower mass stars (e.g. HD 134214, HD 216018). This may suggest that there is a minimum age of stars


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7 Large v, small sin i 6 Small v 5

# OF STARS

4

3

2

1

0

0

0.5

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2 2.5 3 LOG (PERIOD) (DAYS)

3.5

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Figure 1: Period histogram. The dash line separates true slow rotators (to the right) from stars with general ly moderate rotational speeds and low inclinations (to the left).
3.5 HR Diagram for Select Ap Stars

3

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log L/L

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HR Diagram for Select Ap Stars with good Hipparcos parallax data

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eff

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Figure 2: Derived HR diagrams for the observational sample. Upper frame ­ al l stars. Lower frame ­ stars with wel l-determined Hipparcos paral laxes. Note the general tendency for these stars to reside away from the ZAMS.


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included in this sample. We also find that no clear distinction exists on the HR diagram between long-period and shorter-period sharp-lined stars. There may exist a marginal tendency for long-period stars to have lower masses. However, this is clearly not a firm rule (exceptions include HD 9996, HD 110066).

3
3.1

Results for individual stars
HD 142070

In this section we highlight results for 3 stars in our sample: HD 142070, HD 335238 and HD 59435.

HD 142070 is an intermediate-mass Ap star with an effective temperature of about 8400 K. Mathys et al. (1997) find a mean field modulus of 5.0 kG averaged over the rotational cycle of 3.3748 days. Landstreet & Mathys (2000) show phased field modulus and longitudinal field measurements of this star. Our 12 new measurements of the longitudinal magnetic field are well phased according to a period of 3.3714 days, somewhat shorter than the period adopted by Mathys et al. (1997). The results are illustrated in Fig. 3.

3.2

HD 335238

HD 335238 is an intermediate-mass Ap star with an effective temperature of about 9100 K. Mathys et al. (1997) find a mean field modulus of 9.7 kG averaged over a supposed rotational cycle of 44 days. To our knowledge, apart from a single measurement by Mathys & Hubrig (1997), no other longitudinal field data have been obtained. Our 12 new measurements of the longitudinal magnetic field are well phased according to a period of 49 days, somewhat longer than the period proposed by Mathys et al. (1997). The results are illustrated in Fig. 4.

3.3

HD 59435

HD 59435 is an intermediate-mass Ap star with an effective temperature of about 8600 K, and a member of a double-lined spectroscopic binary. Wade et al. (1996) studied this system in detail, deriving the mean field modulus variation and a rotational period of 1360 days. To our knowledge, no measurements of the longitudinal magnetic field have ever been published. Our 5 new measurements agree well with the period proposed by Wade et al. (1996b), although continued observation is clearly necessary. The results are illustrated in Fig. 5.

4

Summary

New longitudinal field observations have been obtained for 21 sharp-lined Ap stars using the 6 m telescope. Multipolar magnetic field models have been published for 5 of these stars, and firm new period constraints have been obtained for an additional 4 shorter-period stars. Preliminary data have been obtained for an additional 12 stars, 11 of which have rotational periods estimated to be longer than 50 days. In order to obtain significant constraints on the magnetic fields of these extremely slowly-rotating Ap stars, observations continuing on timescales of months, to years and possibly to decades will be required.

References
Hubrig S., Mathys G., & North P., 2000, Astrophys. J., 539, 352 Landstreet J. D., Mathys G., 2000, Astron. Astrophys., 359, 213 Mathys G., Hubrig S., Landstreet J. D., Lanz T., Manfroid J., 1997, Astron. Astrophys. Suppl. Ser., 123, 353 Mathys G. & Hubrig S., 1997, Astron. Astrophys. Suppl. Ser., 124, 475 Schaller G. et al., 1992, Astron. Astrophys. Suppl. Ser., 96, 269 Stepien K. & Landstreet J.D., 2002, Astron. Astrophys., 383, 218 Wade G.A., Elkin V.G., Landstreet J.D., Leroy J.-L., Mathys G. and Romanyuk I.I., 1996a, Astron. Astrophys., 313, 209 Wade G.A., Mathys G., North P., Hubrig S., 1996b, Astron. Astrophys., 314, 491 Wade G.A., Elkin V.G., Landstreet J.D., Romanyuk I.I., 1997, Mon. Not. R. Astron. Soc., 292, 748 Wade G.A., Debernardi Y., Bohlender D.A., Hill G.M., Landstreet J.D., Mathys G., Elkin V.G., 2000a, Astron. Astrophys., 361, 991 Wade G.A., Kudryavtsev D., Romanyuk, I.I., Landstreet J.D., Mathys G., 2000b, Astron. Astrophys., 355, 1080


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4

HD 142070 on the HR Diagram

5300

Mean Magnetic Field Modulus vs. Julian Date for HD 142070

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Variation in the Hipparcos Photometric Magnitude for HD 142070

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Mean Longitudinal Magnetic Field vs. Julian Date for HD 142070

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Figure 3: HD 142070. Upper frame ­ HR diagram position, and time variations of Hipparcos magnitude, mean field modulus (Mathys et al. 1997) and longitudinal field (this work). Lower frame ­ phased field modulus and longitudinal magnetic field measurements.


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4

HD 335238 on the HR Diagram

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Mean Magnetic Field Modulus vs. Julian Date for HD 335238

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Variation in the Hipparcos Photometric Magnitude for HD 335238

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Mean Longitudinal Magnetic Field vs. Julian Date for HD 335238

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Hipparcos Photometric Magnitude

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Phase (P=49.04 days)

Figure 4: HD 335238. Upper frame ­ HR diagram position, and time variations of Hipparcos magnitude, mean field modulus (Mathys et al. 1997) and longitudinal field (this work). Lower frame ­ phased field modulus and longitudinal magnetic field measurements.


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4

HD 59465 on the HR Diagram

Mean Magnetic Field Modulus vs. Julian Date for HD 59435 4500

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Variation in the Hipparcos Photometric Magnitude for HD 59435

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Figure 5: HD 59435. Upper frame ­ HR diagram position, and time variations of Hipparcos magnitude, mean field modulus (Mathys et al. 1997) and longitudinal field (this work). Lower frame ­ phased field modulus and longitudinal magnetic field measurements.