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Magnetic stars, 2004, 198-204

Spectrophotometry of the He-weak star HD 217833
Glagolevskij Yu. V., Leushin V. V., Kevlishvili A. G., Filonova S. V. , Chountonov G. A.
Special Astrophysical Observatory of the Russian AS, Nizhnij Arkhyz 369167, Russia

Abstract. Using sp ectra obtained at the 6 m telescop e, the chemical comp osition was measured, an estimate of the strength of the surface magnetic field, Bs = 4500 G, was made from magnetic intensification of sp ectral lines, and the turbulent velo city value, Vt = 0.75 ± 1.00 km/s, was derived. The small value of Vt may indicate that the atmosphere of the star is stabilized by the magnetic field, which explains the presence of diffusion pro cesses causing chemical anomalies. Fe has slight excess (0.6 dex), He is strongly deficient, N(He)/N(H) = 0.001. Seemingly, this star can b e classified as an extreme He-deficient one. Another characteristic distinction of HD 217833 is the presence of strong and numerous chromium lines in its sp ectrum, which are not observed in the sp ectrum of normal ° ° stars. At the same time the doublet SiI I 4128 Aand 4131 A have an approximately the same intensity as a normal star. The mo del atmosphere metho d was used to obtain the temp erature Te = 16000K and the acceleration of gravity lg g = 3.88, which corresp ond to the p osition of the star on the main sequence b etween the ZAMS and V luminosity class stars. Key words: stars: chemically p eculiar ­ stars: magnetic fields ­ techniques: sp ectrophotometry ­ stars: individual: HD 217833

1

Intro duction

He-weak stars are remarkable for that they possess a number of characteristic features similar to the rest of the CP stars, but show very strong anomaly of helium, the second in abundance element. The neutral helium lines in the visual region of the spectrum are actually invisible in the spectra of some He-weak stars, although in normal stars having the same temperatures and gravity accelerations the HeI lines are the most intensive after hydrogen. It is apparent that since these stars are comparatively young, helium must account for 30 % of their mass. All the more so its nearly complete absence in the atmospheres of some of these stars is striking. Physical conditions that favor removal of nearly 1/3 of the mass consisting of helium from the atmosphere must be very specific. For this reason, a thorough analysis of the atmospheres of such stars is very important. Here we present results of investigation of physical conditions in the atmosphere of one of He-weak stars HD 217833.

2

Parameters of the atmosphere of the He-weak star HD 217833

The He-weak star 217833 is the A component of the visual binary system ADS 16474. The star shows brightness variability with period of 5.393 days (Veto 1993), v sini = 30 km/s (Abt et al. 2002). The distance of the system derived from the satellite HIPPARCOS data is 221.7 pc ( = 4.51mas), the apparent stellar magnitude V = 6m 15, consequently, MV = -0m 213. From the data of different authors B­V lies within the . . limits 0.008 Â -0.15, U­B equals -0.55 Â -0.56. From measurements made by Glagolevskij, Chunakova (1985) the star has a strong magnetic field varying from -2000 to -5000 G. Glagolevskij (2002) and Glagolevskij,
c Sp ecial Astrophysical Observatory of the Russian AS, 2004


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Table 1: Parameters of the He-weak star HD 217833 V , mas M M
V

6.516 4.51 -0.213 -1.513 2.49 16000 2.04 4.6 4.38

6.516 2.08 -1.90 -3.200 3.17 16000 4.00 4.6 3.88

6.516 4.51 -0.213 -1.513 2.49 14000 3.20 4.6 4.03

bol

log L/L T, K R/R M /M log g

Kopylova (1990) estimated the temperature of the star to be 15500 K and log g = 3.86. The temperature estimates from the values of B­V computed by Kuruz (1998) yield 16000 K. It should be noted that the effective temperatures of CP stars are determined not quite reliably from calibrations of colors for normal stars because of chemical anomalies of the atmospheres of CP stars. At a correct temperature all the observed values must be consistent with one another, which is not always the case. Table 1 presents sets of coordinated parameters for the He-weak star HD 217833. It follows from the data of the table that the star has a parallax twice as small as the one given by the HIPPARCOS catalog, or a temperature considerably lower than 16000 K. If, for instance, we take the parameters of the first column, then the star has log g = 4.38, which means that with its spectral class and mass it must be on the ZAMS. To make the values of Tef f and log g more precise, we have calculated a synthetic spectrum around the line H by using the program SyntVa developed by Tsymbal (2003) and allowing for the whole list of the lines VALD. The theoretical spectrum was computed for different values of Tef f and log g varying the abundances of some elements from the models interpolated from the grid of models (Kuruz 1999). Fig. 1 gives a comparison of the observed (see Section 3) spectrum of HD 217833 with the theoretical one for the model atmosphere with Tef f = 16000 K and log g = 3.88. The abundances of helium, iron and chromium proved to be as follows: [He] = 0.001, [Fe] = +0.6, [Cr] = +2.0 (see further). Herein there also presented a theoretical spectrum for the model with the normal atmosphere, with the same parameters Tef f = 16000 K and log g = 3.88. The conclusion is obvious: the parameters derived when Tef f = 16000 K and log g = 3.88 represent best the atmosphere of the He-weak star HD 217833. (By the way, the difference with the temperature from (Glagolevskij 2002) is only 3%, which is insignificant). The parameters obtained are collected in the second column of the table. It is remarkable that the value of differs from the HIPPARCOS value by more than a factor of 2. The last column lists the parameters of the star derived when using the temperature 14000 K. This temperature does not correspond to the spectrum of the star.

3

Sp ectral lines in the sp ectrum of the He-weak star HD 217833

Observational data were obtained with the Main Stellar Spectrograph of the 6 m telescope with a CCD of 1Kâ1K and with a laminated slicer (Chountonov, Perepelitsin 2000). The spectral resolution corresponded to 0.14 ° A/pixel, the S/N ratio was equal to 500, the spectral region was 4000­4265 A. We have made careful identification and measurement of the spectrum of the He-weak star HD 217833 in the spectral region under investigation. It is noteworthy that the lines HeI 4009.27 ° HeI 4023.97 ° A, A, HeI 4026.19 ° HeI 4120.82 ° HeI 4143.76 ° are practically absent in the spectral region studied. The A, A, A


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Figure 1: Comparison of the observed H profile (dots) with the theoretical one. The best fit is reached for the model with the parameters: Tef f =16000 K and log g=3.88, N(He)/N(H)=0.001, [Fe]=+0.6, [Cr]=+2.0. Here also is presented the synthetic spectrum for the normal atmosphere model with T ef f =14000 K and log g=3.88. ° equivalent widths of these lines in spectra of normal stars reach 300 mA, whereas in the spectrum of HD 217833 the lines FeII and FeIII with an equivalent width of about 20 m° are identified in these regions of spectrum. A The weakest lines measured from our spectra have 1 Â 3 m° The mean helium abundance that we have derived A. is N(He)/N(H) = 0.001. In the paper by Glagolevskij, Kopylova (1990) N(He)/N(H) = 0.014. The difference in the estimates by more than an order of magnitude is indicative of strong inhomogeneity of chemical composition over the surface. A characteristic distinction of the line spectrum of the star being discussed is the presence of comparatively strong lines of FeIII, which is evidence that the effective temperature of the atmosphere is obviously higher than 14000 K. Another distinction of the spectrum of HD 217833 is the presence of strong and numerous lines of chromium (see Fig. 2), which are not observed in the spectrum ° ° of normal stars (HD119857 in Fig. 2). At the same time the silicon doublet Si 4128 A and 4131 A has approximately the same intensity as in a normal star spectrum. The abundance of chromium turns out to be [Cr] = 2.0.

4

Determination of microturbulent velo city and magnetic field in the atmosphere of HD 217833 from iron lines

In the spectral region being investigated, 34 lines of FeII and FeIII were measured. These lines were divided into two groups according to the value of the Lande factor (z). The first group included the lines with z < 1.20, the second group was comprised of the lines with z > 1.20. Thus, the sensitivity of the lines of the = former to the influence of magnetic intensification was lower than that of the latter. The lines of the first group were used to determine microturbulent velocity. Computations were conducted for the model chosen above with Tef f = 16000 K and log g = 3.88 and the helium abundance N(He)/N(H) modified for calculations by the program KONTUR (Leushin, Topil'skaya 1986; Leushin 1995). The parameters of the lines were selected


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201

Figure 2: Comparison of spectra of the He-weak star HD 217833 and the normal star HD 11857 (B6V), having close parameters. from the list VALD (Kupka et al. 1999). Comparison of the values of log N (F e), derived for each line, with the equivalent width, when varying the microturbulent velocity, allows a correct value of this parameter to be found. For this purpose, the linear regression in the form log N (F e) = log N (F e)0 + k w is used. The results of calculations for the first group of 16 lines are presented in Table 2. The relationships between log N (F e) and equivalent width presented by the linear regression yield the coefficient k which indicates the direction of the necessary correction of the microturbulent velocity value. Thus, the data of Table 2 show that the microturbulent velocity in the atmosphere of the He-weak star HD 217833 is Vt = 0.75 ± 1.00 km/s. The second group consisting of 18 lines was used for determination of the surface magnetic field. Here too the linear regression log N (F e) vrs line equivalent width for different magnetic field values were constructed. The results of calculation of log N (F e) from these lines for Vt = 0.75 km/s and different field strengths are collected in Table 3. The same considerations as stated above show that the magnetic intensification of lines in the atmosphere of the He-weak star HD 217833 corresponds to the mean surface field H s = 4500 ± 1000 G. It should be noted that this value may prove to be the lower limit of the surface field since the relationship between spectral line intensity and surface field undergoes saturation at a field strength above 5000 G. Several measurements of the mean effective field showed the value Be = -5500 Â 2000 G (Glagolevskij, Chunakova 1985; El'kin 1994).

5

Iron abundance in the atmosphere of HD 217833

The data of Table 2 and 3 show that the iron abundance in the atmosphere of HD 217833 calculated from the two groups of lines, when varying the microturbulent velocity and magnetic field values, may change


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Table 2: Parameters of the linear regression for the relationship between iron abundance and equivalent width for different values of microturbulent velocity Vt , km/s 1.50 1.00 0.80 0.75 0.50 log N (F e) 8.266 8.266 8.265 8.263 8.104
o

k -0.00315 -0.00091 -0.00021 -0.00007 +0.00756

log N (F e) 8.237 8.258 8.265 8.239 8.205

±0.072 ±0.072 ±0.072 ±0.070 ±0.104

Table 3: Parameters of the linear regression for the `iron abundance ­ equivalent width' relation for different magnetic field values H, gs 3000 3500 4000 4500 5000 7000 log N (F e) 8.308 8.305 8.304 8.300 8.297 8.205 k +0.00289 +0.00187 +0.00093 -0.00002 -0.00054 -0.00293 log N (F e) 8.358 8.336 8.319 8.301 8.288 8.243 ±0.099 ±0.099 ±0.099 ±0.100 ±0.100 ±0.102

o


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Figure 3: Two estimates of helium abundance obtained at different times (asterisks). The circles stand for the mean relationship of helium abundance in He-weak stars and age. within 0.16 dex at a formal determination error of ±0.1 dex. At the same time, our data point definitely to overabundance of iron in the atmosphere of HD 217833 by ±0.6 dex (log N (F e) = 7.5) in comparison with normal. It can be noted that the same overabundance of the iron peak elements is also found in CP stars by other authors (Ryabchikova 1992). However, we have to take into account the fact that the value 7.50 is calculated with respect to the abundance of hydrogen log N (F e) = 12.00. In our case, helium deficit in the atmosphere (helium mass accounts for 30 % of the total for normal stars) causes hydrogen abundance to increase to log N (F e) = 12.15. Thus, the iron abundance that we have found above should be smaller by 0.15 dex. Table 4 gives the abundance of iron computed from all the lines that we have measured for the model atmosphere with the parameters Tef f = 16000 K, log g = 3.88, helium abundance N(He)/N(H) = 0.001, microturbulent velocity Vt = 0.75 km/s, and magnetic field Hs = 4500 G. The mean value of log N (F e) = 8.089 ± 0.070.

6

Conclusions

The spectrophotometric analysis made for the atmosphere of HD 217833 shows that this is a He-weak star, it has the microturbulent velocity close to zero and a rather strong mean surface magnetic field. The field is likely to stabilize the atmosphere of the star, favoring the diffusion processes that cause separation of elements in the atmosphere. The separation is mainly displayed by movement of helium from the atmosphere deep into the star and causing in it iron to be in excess of ±0.6 dex. Based on the data obtained earlier (Glagolevskij, Kopylova 1990), we have derived a smoothed relationship between helium underabundance (lg N (H e) - lg N (H e) ) in He-weak stars and relative radius R/Rz , which is displayed in Fig. 3. R is the radius of the star at the present time, while Rz is its radius on the ZAMS. The parameter R/Rz lg g is also proportional to the age of the star on the main sequence. The solid line exhibits the mean relationship which, as expected, vanishes at the ZAMS, the minimum of the relationship falls on


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the main sequence band corresponding to stars of luminosity class V. This relation is not in contrast with our preliminary conclusions that stars became magnetic chemically peculiar ones near the ZAMS (Leushin et al. 2002), prior to that they had no anomalies (Glagolevskij et al. 1987). Between the point R/R z = 1 that corresponds to ZAMS and the first points on the relationship there is a gap due to observational selection. The asterisks mark the values of the relative abundance of helium for HD 217833, which have been obtained herein and in (Glagolevskij, Kopylova 1990). We draw the conclusion that the distribution of helium over the surface is extremely nonuniform.

7

Sp ectral lines in the sp ectrum of the He-weak star HD 217833

It can be readily seen from Fig. 3 that HD 217833 is located between the ZAMS and stars of luminosity class V, i.e. this is a relatively young star. The error of location of the star on this diagram on the horizontal axis is, on average, ±0.1 (Glagolevskij 2002). The underabundance of helium in HD 217833 is much higher than the average for stars of the same age. This is why it can be ranked among extreme He-weak stars.

References
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