ASTRONOMY AND ASTROPHYSICS. Blue stragglers in open clusters. Part II. S.M. Andrievsky 1,2,D.Schönberner 1, and J.S. Drilling 3

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1 Astron. Astrophys. 356, (2000) Blue stragglers in open clusters Part II S.M. Andrievsky 1,2,D.Schönberner 1, and J.S. Drilling 3 ASTRONOMY AND ASTROPHYSICS 1 Astrophysikalisches Institut Potsdam, An der Sternwarte 16, Potsdam, Germany (DeSchoenberner@aip.de) 2 Odessa State University, Department of Astronomy, Shevchenko Park, Odessa, Ukraine (scan@deneb.odessa.ua) 3 Louisiana State University, Department of Physics and Astronomy, Baton Rouge, Louisiana 70803, USA (Drilling@baton.phys.lsu.edu) Received 11 June 1999 / Accepted 8 December 1999 Abstract. We present the results of a spectroscopic study of blue straggler and main sequence B and A type stars in the open clusters NGC 3496, NGC 6475, NGC 6633 and IC A detailed analysis based on Kurucz s atmosphere models showed that the main sequence stars rotate rather rapidly and differ from a normal (solar) metallicity for only a few light elements. The blue stragglers have significantly smaller projected rotational velocities. As a group, they show the same chemical peculiarities as ordinary cluster and galactic field stars of the same spectral type. Two blue stragglers and one main sequence star possess a rather low helium content. All investigated stars for which the carbon abundance could be measured showed a moderate-tostrong deficiency of this element. The phenomenon of the blue stragglers is briefly discussed on the basis of our study. Key words: stars: blue stragglers stars: abundances 1. Introduction The enigmatic phenomenon of the blue straggler stars has been widely discussed in the literature for almost half a century. Nowadays there is no doubt that they do not really follow the standard evolutionary scenario of the other cluster members. Despite the numerous efforts to explain this phenomenon, no general mechanism resulting in the remarkable apparent delay in their evolution has been proposed (see e.g. the detailed review by Stryker 1993 or the paper by Leonard & Linnell 1992). This discouraging result may be due to the absence of any systematic observational studies of these stars (i.e. spectroscopic analyses). Available fragmentary results do not allow a general picture to be drawn of the variations among the spectra of the blue stragglers themselves, or between the blue stragglers and normal stars of the same spectral type. In this paper, which is to be regarded as part of a series, we continue a systematic observational study of blue stragglers in open clusters that was started by Andrievsky (1998), hereafter Paper I. We also follow up the detailed investigation of Send offprint requests to: S. Andrievsky Based on observations collected at European Southern Observatory. blue stragglers in several open clusters by Schönberner & Napiwotzki (1994). Those authors have shown that the most attractive scenario for the blue straggler phenomenon based on the evolution of single stars is probably not valid. With that conclusion, they clearly stress the urgent necessity of searching for other ways to account for the blue straggler phenomenon. It is quite clear that without any systematic observational work this can hardly be done. We investigated objects in the open clusters NGC 3496, NGC 6475, NGC 6633 and IC 2602 (ages 0.23, 0.22, 0.66 and Gyr respectively, according to Ahumada & Lapasset 1995). NGC 3496 is a very poorly investigated open cluster. Ahumada & Lapasset (1995) mention four stars as its blue stragglers. They are, according to the Sher (1965) nomenclature, S 43, S 59, S 115 and S 117. No reliable information about their cluster membership is available. NGC 6475 contains two blue stragglers. In the compilation of Ahumada & Lapasset (1995), HD is mentioned as a He-weak star with variable radial velocity (within 30 km s 1 ). Borra, Landstreet & Thompson (1983) tried to measure the surface magnetic field, but their result was not convincing: the detected field was of the same order of magnitude as the errors ( G). The second blue straggler, HD , is known to be a visual binary. For HD , Gieseking (1985) gives a membership probability of 92%. Also, Snowden (1976) qualitatively confirms the membership of both blue stragglers. Several objects have been investigated in the open cluster NGC Among them two stars are blue stragglers and three main sequence stars. The very young open cluster IC 2602 contains only one very hot blue straggler, θ Car, but this star was not the subject of present investigation. We observed several main sequence stars in this cluster. 2. Observations 2.1. Spectroscopic material The stars selected for observation are described in Table 1. Classification as a blue straggler is given in accordance with the cat-

2 518 S.M. Andrievsky et al.: Blue stragglers in open clusters. II alogue of Ahumada & Lapasset (1995). Because some of the program stars belong to spectral class A, we decided also to observe the A-type star α CMa (Sirius) as a control on our data reduction method and abundance analysis. The observations were performed with the ESO 3.6-m telescope equipped with the CASPEC echelle spectrograph during May Resolving power and S/N ratio were and about 100 at the continuum level, respectively (for the faintest main sequence stars the S/N is slightly smaller) Reduction of the spectra All of the echelle orders were extracted from the images, flatfielded and wavelength calibrated using the MIDAS software. Preliminary visual inspection showed that the blue stragglers usually possess sharp, well-defined spectral lines, while the observed main sequence stars appeared to be rather fast rotators. For sharp-lined spectra the continuum placement is rather reliable, but for the rotationally broadened spectra, continuum within some orders is not well defined. We excluded those orders from the further analysis. Where it was possible (i.e. only for the sharp unblended lines), equivalent widths were measured directly under the assumption that the shapes of the lines can be approximated by a gaussian profile. After visual inspection of the rotationally broadened spectra of the main sequence stars, it became clear that the Am star α CMa could hardly be considered an appropriate reference for our abundance analysis of those stars. Nevertheless, we kept α CMa as a check on the abundance determinations. We compared our equivalent widths from the αcma spectrogram with those given by Sadakane & Ueta (1989). It appeared that on the average our values are systematically about 10% greater than those of Sadakane & Ueta (1989). This small difference is likely due to the higher quality of the spectroscopic material used by these authors (the reciprocal dispersion was 1 Åmm 1, while we estimate our value as about 7Åmm 1 ). 3. Method of the analysis To derive elemental abundances we employed both the spectral synthesis technique and equivalent width analysis (the latter only for stars with sharp lines). The spectral synthesis was performed using the SYNSPEC code developed and described by Hubeny, Lanz & Jeffery (1994), while the equivalent width analysis was based on the well-known Kurucz WIDTH9 code. While performing the spectral synthesis for the hot stars of our sample, special attention was paid to the accurate calculation of the He i line profiles using updated Stark broadening parameters (for details see Hubeny, Lanz & Jeffery 1994). Absolute abundance determinations depend dramatically upon the oscillator strengths selected for the analysis. For stars with intermediate temperatures, one can employ oscillator strengths based on solar spectroscopic data and the solar chemical composition (Grevesse, Noels & Sauval 1996), because the great majority of the prominent lines in their spectra are also Table 1. Program stars Star V Sp Region Date Type NGC 3496 S May 13 BS S May 13 BS S May 13 BS NGC 6475 HD B6V May 13 BS HD B8V May 13 BS NGC 6633 HD A0III May 13 BS HD A May 13 MS HD B6IV May 13 BS May 15 HD A May 15 MS HD A May 15 MS IC 2602 HD B8V May 13 MS HD A0V May 15 MS HD A0V May 13 MS HD B5V May 15 MS HD B6V May 13 MS α CMa A1V May 15 BS - blue straggler; MS - main sequence star present in the solar spectrum. We obtained so-called solar oscillator strengths by fitting the synthetic and observed solar spectrum (Kurucz et al., 1984), using the synthetic spectrum with the solar atmosphere model from the Kurucz (1992) grid and a microturbulence velocity V t =1kms 1 (see Paper I for details). For the hottest stars of our sample, a great number of the lines belong to ionized carbon, magnesium, silicon, and sulphur. These lines are not visible in the solar spectrum and we have to rely on other sources for the oscillator strengths. In the present study we used an updated compilation provided by the Vienna Atomic Line Database (VALD). For the investigation of the main sequence stars, a larger number of lines has been used. However, we note that most of the Ti ii, Cr ii and Fe ii lines which are visible in the spectra of our hot stars also dominate the spectra of the program main sequence stars. The list of used oscillator strengths is available from the authors. 4. Stellar parameters 4.1. T eff estimates based on photometry Strömgren uvbyβ and Geneva photometry (if available) has been used to derive effective temperatures and surface gravities for our program stars. The calibrations provided by Castelli (1991), Napiwotzki (1992) and Kunzli et al. (1997) were applied. The photometric data for NGC 6475, NGC 6633 and IC 2602 were taken from the SIMBAD data base. For NGC 3496 the situation with the photometric data appeared to be more complicated. Fortunately, Balona & Laney (1995) have carried out

3 Table 2. Results of the T eff determination for program stars. S.M. Andrievsky et al.: Blue stragglers in open clusters. II 519 T eff Star Type C N KET NGC 3496 S 43 BS S BS NGC 6475 HD BS HD BS NGC 6633 HD BS HD MS 8700 HD BS HD MS HD MS 8500 IC2602 HD MS HD MS HD MS 9500 HD MS HD MS C Castelli (1991), N Napiwotzki (1992), KET Kunzli et al. (1997). 1 β index is not available; 2 Geneva photometric data have been provided by Kienzle (1999) Fig. 1. Comparison of the observed (thick line) and calculated (thin line) UV spectra for HD CCD Strömgren photometry of NGC From their finding chart we have identified our program stars S 43 and S 115 as SAAO 175 and SAAO 30, respectively. It should be noted that Balona & Laney (1995) found that E(b y) varies within the cluster field from 0.25 to Therefore we did not use the mean reddening value for NGC 3496 (E(b y) = 0.39), but determined individual reddenings for each program star. For S 43 we have obtained E(b y) = 0.48, while for S 115, E(b y) = Unfortunately, S 117, which is situated in the direct vicinity of S115 (see finding chart from Sher 1965), appeared to be out of the region observed by Balona & Laney. Thus, for S 117 we have adopted the same reddening as for S 115 and used it to make a rough estimate of the effective temperature of S 117 from the UBV T eff calibration of Castelli (1999), and we obtained T eff (B V )=13000 K and T eff (U B)=11000 K. All T eff determinations are listed in Table UV spectra As a check on the temperature for some stars (HD , HD and HD 93194), we used the large-aperture IUE spectra provided by SIMBAD. The original spectra in the short and long wavelength regions for HD (images SWP21337 and LWR02120, exposed on 23 October 1983), for HD (images SWP14048 and LWR10696 exposed on 24 May 1981 and SWP14085 and LWR10722 on 26 May 1981), and for HD (images SWP and LWP89701, exposed on 30 August 1986) were combined and then dereddened using the Seaton (1979) law for selective interstellar extinction Fig. 2. Same as Fig. 1, but for HD (observations on 24 May 1981). in the UV. The dereddened UV spectrum for each star has then been converted into emergent fluxes using the stellar angular diameter. Assuming that it is not dependent upon wavelength, the latter was found for the effective wavelength of the V band using the absolute flux calibration given by Heber et al. (1984) and the corresponding theoretical flux calculated by means of the SYNSPEC code. All of these calculations were performed using the photometric values V = 8.20 and E(b y) = (HD ), V = 5.90 and E(b y) = (HD ), V = 4.83 and E(b y)=0.02 (HD 93194). The synthetic spectra in the region 1000 Å Å are smoothed to a resolution comparable to the observed one (about 6 Å), and compared with the observations for blue straggler stars in Figs As one can see, in each case the temperature estimated by uvbyβ photometry gives good agreement Surface gravity determination Gravity values were estimated from the photometric data (using the same calibrations as for the T eff determination) and also independently by interpolating in a two-dimensional T eff log g grid of theoretical Hδ and Hγ line profiles based on Kurucz s model atmospheres. The results are given in Ta-

4 520 S.M. Andrievsky et al.: Blue stragglers in open clusters. II Table 3. Results of the log g determination log g Star Type C N KET Hδ Hγ NGC 3496 S 43 BS S 115 BS S 117 BS NGC 6475 HD BS HD BS NGC 6633 HD BS HD MS HD BS HD MS HD MS IC2602 HD MS HD MS HD MS HD MS HD MS α CMa C Castelli (1991), N Napiwotzki (1992), KET Kunzli et al. (1997). ble 3, and individual fits for some of the stars are presented in Figs Note that in these cases the typical error of the gravity determination is about dex. Our finally adopted atmospheric parameters for the program stars, those which were used for atmosphere model interpolation in the Kurucz s (1992) grid, are collected in Table 4. The adopted gravities are weighted averages (weight 3 has been given for gravities based on the hydrogen profiles, and 1 for photometric estimates) Microturbulence and projected rotational velocities Note that for all stars (except α CMa) we adopted a microturbulent velocity of V t =3kms 1, a value that is most appropriate for late B - early A stars. For α CMa, the number of lines in the spectrum is large enough to determine this value using the iron lines. We have obtained 2.5 km s 1 by requiring that there should be no dependence of the iron abundance on equivalent width. Our value is 0.5 km s 1 higher than that determined by Sadakane & Ueta (1989). Projected rotational velocities for our program stars were derived by matching observed and calculated profiles for specified spectral lines (for fast rotators it was the Mg ii 4481 Å line only). These results are also given in Table 4. Note that our v sin i determination for HD and HD are in the complete accordance with Abt s (1975) measurements (he gives for both stars v sin i approximately 40 km s 1 or slightly less). Our rotational velocity of HD agrees fairly well Table 4. Adopted parameters for our program stars Star Type T eff log g v sin i NGC 3496 S 43 BS S 115 BS S 117 BS NGC 6475 HD BS HD BS NGC 6633 HD BS HD MS HD BS HD MS HD MS IC2602 HD MS HD MS HD MS HD MS HD MS α CMa *-T eff and v sin i were adopted following to Sadakane & Ueta (1989) with that obtained by Andersen & Nordström (1983), v sin i = 40 km s 1, but their value is marked as being uncertain. Levato (1975) determined the rotational velocities for some stars from IC In particular, his results on HD (220 km s 1 ), HD (310 km s 1 ) and HD (305 km s 1 ) generally agree with our values. The only exception is HD For this star Levato gives v sin i = 215 km s 1, while we found 135 km s 1. The origin of such strong disaccord is unknown, but in Fig. 7 we show for this star the fit between observed and synthetic spectra which supports our estimate. Here we have to note that high rotation may affect the spectral classification (Gray & Garrison, 1987). The photometric indices ((b y) and β, in particular) of a rapidly rotating star can mimic those of a slowly rotating one with a lower temperature. Fortunately, as it was shown by Gray & Garrison (1987), this effect is small. Up to v sin i km s 1, the (b y) correction is not greater than 0.02, and it was therefore not taken into account. 5. Elemental abundances With T eff, log g and V t values determined, one can then derive elemental abundances. Note that no detailed abundance analysis has previously been performed for any star of our sample. As a first step, we used WIDTH9 to derive the elemental abundances for the sharp-lined stars. These abundances were then used as input for a detailed spectral synthesis. It should be noted that use of the WIDTH9 code was restricted by the necessity of taking into account only unblended or slightly blended lines, the number of which is small even in the spectra of the blue

5 S.M. Andrievsky et al.: Blue stragglers in open clusters. II 521 Fig. 3. Observed (solid line) and fitted (dashed line) profiles of Hδ line for some blue stragglers and α CMa Fig. 6. Same as Fig. 5, but for the main sequence stars Fig. 7. The synthetic spectrum for HD convolved with v sin i = 135 km s 1 (solid line), and compared with observed one (circles). Fig. 4. Same as Fig. 3, but for some main sequence stars ted together with the observed ones. For rapidly rotating stars, we also show the deconvolved synthetic spectra with the aim of demonstrating to what extent the original spectrum is altered due to rapid rotation. Note that for blends only approximate wavelengths are given. 6. Results and error estimates Fig. 5. Same as Fig. 3, but for Hγ line in blue stragglers and α CMa stragglers. Abundances for the main sequence stars with rotationally broadened spectral lines can be found only by means of the SYNSPEC code, which allows one to find an optimal fit of the synthetic spectrum to the observed one. This procedure was performed separately for each order. Generally, the fit was quite satisfactory for all lines of a given element for a single value of the abundance. This indicates that the oscillator strengths of the calculated lines are rather accurate and that there are no significant errors in the continuum placement. Therefore, we will not discuss line-to-line abundance scattering here, but will restrict ourselves to a discussion of systematical errors caused by a poor choice of the atmospheric parameters and NLTE effects (see next section). In Figs we give some examples of the synthetic spectra convolved with the instrumental and rotational profiles, plot- The final abundances are presented in Table 5 for the blue stragglers and in Table 6 for the main sequence stars. For α CMa in Table 7 we also give for comparison the results obtained by Sadakane & Ueta 1989 and the more recent results of spectral synthesis by Hill & Landstreet (1993). In this study we did not aim to update the list of measured elemental abundances for α CMa, which has been the subject of several investigations. Nevertheless, note that we are in very good agreement with other investigators. It is quite likely that some of the abundances are influenced by NLTE effects. Some errors may also come 1) from uncertainties in the continuum level within the echelle orders (especially for rapidly rotating stars), 2) from oscillator strengths (in particular from those that can not be checked using the solar spectrum), 3) from Stark broadening constants, which strongly affect abundances derived from the lines of ionized species (prevalent in the blue straggler spectra), but which are poorly known for the bulk of the lines. To this list we should also add the unavoidable errors introduced by temperature, gravity, microturbulence parameter and projected rotational velocity determination which are discussed below. As an example, in Table 8 we give error estimates for the He and Mg abundances in the blue straggler HD based

6 522 S.M. Andrievsky et al.: Blue stragglers in open clusters. II Table 5. Final relative elemental abundances [El/H] for blue straggler stars NGC 3496 NGC 6475 NGC 6633 El S 43 S 115 S 117 HD HD HD HD He / 0.10 C / 0.40 Mg /+0.30 Al 0.20/ 0.20 Si / 0.60 S +0.00/+0.00 Ti Cr / 0.20 Fe /+0.00 Sr * - for HD we give results from two spectra. Table 6. Final relative elemental abundances [El/H] for main sequence stars NGC 6633 IC 2602 El HD HD HD HD HD HD HD HD He C Mg Si Ca Sc Ti Cr Fe Table 7. Relative elemental abundances [El/H] for α CMa El our H&L S&U Mg Ca Sc Ti Cr Mn Fe Ni Sr H&L - Hill & Landstreet (1993); S&U - Sadakane & Ueta (1989) on the analysis of two selected lines: He i 4471 Å and Mg ii 4481 Å. In each individual case presented in Table 8, the abundance correction should be regarded as the largest necessary to compensate for the visible differences between the observed and calculated profiles. Inspecting Table 8 and taking into account that the temperature and gravity values for our blue stragglers are probably more accurate than the extreme cases supposed for Table 8, one can conclude that the elemental abundances for the stars with narrow-lined spectra are reliable with an accuracy better than ± dex. This is not the case for the main sequence stars with broadened and blended lines, where errors connected with the continuum placement are more pronounced than for the blue stragglers. We would be optimists if we could say that in this case abundances are known with an accuracy better than ± 0.2 dex. 7. Discussion We start this section with an examination of the individual cluster membership Cluster membership Although our CASPEC spectrograms cannot be used for precise radial velocity measurements, we have tried to investigate the membership problem for our program stars. To avoid any instrumental errors, we have measured only the mean shifts between calculated and observed spectra and compared them with other stars in the same cluster. Since all of the stars were observed in just a few days, these relative shifts should be equal to the real radial velocity differences between the stars. We did not investigate possible measurement errors, but we do not expect the accuracy to be less than ± 5kms 1. Otherwise, wavelength variations in the shifts between the calculated and observed spectra become clearly visible.

7 Table 8. Parameter variation and abundance corrections ( [El/H]) S.M. Andrievsky et al.: Blue stragglers in open clusters. II 523 T eff V t log g Line 500 K +500 K 1 kms +1 kms 0.3 dex +0.3 dex He I < 0.05 < Mg II Fig. 8a c. Fragments of observed (circles) and synthetic (solid line) spectra for HD (T eff = K, log g = 4.05, v sin i =30 km s 1 ) NGC 3496 We found that the shift is practically the same for S 115 and S 117 (mean value of about +0.1 Å), while for S 43 the shift appeared to be much greater ( 0.6 Å), corresponding to a velocity difference about 50 km s 1. Such a difference could imply either that S 43 is a variable star, or that there is no connection between this star and S 115 and S 117, while the latter stars are physically connected. This explanation is supported by estimates of the distance moduli of these stars. For S 43 and S 115 we have the necessary photometric data, and using Crawford s relation for B stars (Crawford 1978), we get (V 0 M V ) and (V 0 M V ) The lack of uvbyβ photometric data for S 117 prevents such an estimate for this star. In this case we attempted to find the distance modulus by supposing that it is B8- B9 main sequence object (see values of T eff and log g are from Table 4). A star of this spectral type should have R 2.8R. With this value we get M v 0.2, or (V 0 M V ) Fig. 9a c. Fragments of observed and synthetic spectra for HD (T eff = K, log g = 4.30, v sin i =20kms 1 ). For NGC 3496 Balona & Laney (1995) have estimated the distance modulus as approximately If we accept this value, then the distance moduli of S 115 and S 117 are consistent with cluster membership whereas that of S 43 is not, in agreement with the differences in radial velocity. However, we should be careful here, since the above mentioned authors have stated that, we strongly suspect that NGC 3496 is not a true cluster but contains at least two different group of stars of different age, very likely at different distances NGC 6475 For both blue stragglers from this cluster we obtained the same shift between the observed and calculated spectra, with an uncertainty of only a few km s 1. This is in agreement with the results of Mermilliod (1982) and Gieseking (1985), who found that the radial velocities of these two stars are close to each other and also close to the mean radial velocity for NGC 6475 cluster.

8 524 S.M. Andrievsky et al.: Blue stragglers in open clusters. II Fig. 10a and b. Fragments of observed and synthetic (solid line - convolved, dashed line v sin i = 0 km s 1 ) spectra for HD (T eff = 8700 K, log g = 3.70, v sin i = 130 km s 1 ) NGC 6633 According to the compilation of Ahumada & Lapasset (1995), the membership probabilities for the two blue stragglers HD and HD are 90% and 70%, respectively. For the main sequence stars, HD and HD , only a qualitative photometric membership probability was reported (Schmidt 1976, Hiltner, Iriart & Johnson 1958). According to Levato & Abt (1977), HD is also a member of NGC 6633 (but the authors do not give any details). In the present study we determined the differences in radial velocities V r = V r (star) V r (HD ), i.e. between each star and HD , whose membership probability is highest. As a result we have got the following V r : +19 km s 1 (HD ), +1 km s 1 and +6 km s 1 (HD , two spectra), +1 km s 1 (HD ), +6 km s 1 (HD ). As one can see, our estimates based on the CASPEC spectra seem to support the membership of the program stars. Only one main sequence star of the sample possesses an anomalous radial velocity, but for the final conclusion about its membership in NGC 6633, additional studies will be needed IC 2602 All of the stars observed by us in the field of this cluster are physical members (Hill and Perry 1993). The relative radial velocities (with respect to the velocity of HD 93540) estimated from our spectra are the following: 9 kms 1 (HD 92385), +2 km s 1 (HD 92837), -4 km s 1 (HD 93098), +5 km s 1 Fig. 11a and b. Fragments of observed and synthetic spectra for HD (T eff = 9200 K, log g = 3.90, v sin i = 190 km s 1 ). (HD 93194). These results also support the membership of the investigated stars in IC 2602 (note that HD with a highest deviation is known as variable star of the α 2 CVn type) Rotational velocities From Table 4 one can immediately conclude that all the investigated blue stragglers have small projected rotational velocities, while the main sequence stars are fast rotators. For the latter objects (from NGC 6633 and IC 2602) v sin i values are close to those which are expected for their spectral types. Similar small v sin i values were reported by Mathys (1991) for ten blue stragglers from M 67 and by Andrievsky for 40 Cnc (Paper I). Another well-known blue straggler of the Hyades cluster, 68 Tau, is also very likely to be a slow rotator (projected rotational velocity is v sin i=15kms 1, Abt & Morrell 1995). A detailed study of blue stragglers from four open clusters undertaken by Schönberner & Napiwotzki (1994) also revealed that all blue stragglers investigated have small projected rotational velocities (seven stars from NGC 7789, one from M 67, one from NGC 752 and one from NGC Cnc) Chemical peculiarities Helium and carbon In our present study, we discovered two new He-weak stars: the blue straggler HD and the main sequence star HD In Fig. 12 we show the synthetic spectrum for HD in the vicinity of the He i 4471 Å line calculated with a tenfold

9 S.M. Andrievsky et al.: Blue stragglers in open clusters. II 525 Fig. 12. Fragment of observed and synthetic spectra for HD in the vicinity of the He i 4471 Å line: a solar helium abundance was adopted for the synthetic spectrum. Fig. 13a and b. Fragment of observed and synthetic spectra for HD in the vicinity of the He i 4471 Å a and C ii 4267 Å b lines. Solar helium and carbon abundances were adopted for the synthetic spectrum. increase in the He abundance (i.e. solar). One can see that for this case the synthesized line appears to be much too strong. The blue straggler HD is also an extremely helium deficient star. In fact, this has been known for a long time (see e.g. Norris 1971), but an accurate quantitative estimate of the helium deficiency in the atmosphere of this star has not been performed before. We show that this star possesses an atmospheric helium abundance which is about 25 times lower than solar. In Fig. 13a we demonstrate how the helium 4471 Å lines should look in the spectrum of this star for a solar abundances. The two remaining program blue stragglers do not show any significant helium anomalies. All of the investigated stars (both blue stragglers and main sequence stars) with measured carbon abundances show a moderate-to-strong underabundance of this element. For example, the C ii 4267 Å line of the blue straggler HD is shown in Fig. 13b and, for comparison, the profile synthesized with a solar carbon abundance is also shown. To be sure that the reduced helium content and NLTE effects in hot B star atmospheres do not affect our conclusions about the helium and carbon deficiency detected for some stars, 1) Dr. F. Kupka calculated at our request two atmosphere models with the same parameters as those of our He-weak blue stragglers and with a reduced helium content, 2) Dr. S.A. Korotin applied the NLTE MULTI code (see for reference Korotin, Andrievsky & Kostynchuk 1999) to investigate NLTE corrections to the carbon abundances of some blue stragglers. It was found that: 1) the atmosphere models with a reduced helium content gave practically the same result as the Kurucz models with a normal helium abundance. 2) non-lte corrections for the stars having effective temperatures in the range K K appeared to be rather small (about dex or less with respect to LTE carbon abundance). For example, the relative LTE carbon abundance in HD is 1.30, while the NLTE calculation gives This indicates that the strong carbon deficiency in the atmospheres of some program stars is real. He and CNO anomalies are known for B-A stars from clusters and the galactic field. There exists a group of so-called He-weak stars (Norris 1971) of spectral class B. The stars of that group show a relative helium abundance (He /H ) Almost all of the investigated field B stars and those from OB associations show a moderate carbon deficiency (see, e.g. the recent results concerning the NLTE carbon abundance in B stars published by Daflon, Cunha & Becker 1999 and Andrievsky et al. 1999). Similar carbon underabundances are found also for chemically peculiar stars in the temperature region from K to K. According to Roby & Lambert (1990) [C/H] varies from approximately solar to 0.7. These authors also reported a reduction of the carbon deficit towards higher effective temperatures. For an explanation of the He-weak star phenomenon, the mechanism of gravitational settling of neutral helium in a stable stellar atmosphere is usually invoked (Michaud 1970). Michaud et al. (1979) also state that if the envelope is stable enough, then diffusion always leads to helium underabundance in the atmosphere. Gonzalez, Artu & Michaud (1995) considered radiative acceleration for CNO atoms in A-F star envelopes and found that, in particular, carbon should be as underabundant as [C/H] 1 for the hottest model of their sample. Our results marginally agree with the prediction of diffusion theory. Nevertheless there is a problem that spoils this picture based on the diffusion theory predictions for He and CNO anomalies in B-F stars. The high projected rotational velocities for HD and even higher values for S 43 and HD are hardly consistent with the supposition about the stability of their atmospheres. Nevertheless, these stars show a remarkable helium (and carbon) deficiency. For S 43, one of the faintest stars of our sample, we show in Fig. 14 a fragment of the observed spectrum and the synthetic spectrum calculated with a solar carbon abundance. Note that this star has v sin i =90 km s 1. No doubt, carbon is really deficient. The blue straggler HD has v sin i =70kms 1 which implies that its equatorial velocity could be as high as 100 km s 1, which is

10 526 S.M. Andrievsky et al.: Blue stragglers in open clusters. II Fig. 14. Fragment of observed and synthetic spectra for S 43 in the vicinity of the C ii 4267 Å line: adopted for synthetic spectrum carbon abundance is solar. Fig. 15. Corellation between the helium and silicon abundances. regarded to be limiting value for the operation of gravitational diffusion. Note that most of the helium-weak stars investigated by Norris (1971) have v sin i parameters smaller than 70 km s Other elements Among the α-elements, silicon shows an interesting behaviour. When He is normal in the atmosphere, silicon shows an apparent deficiency. In stars with decreased helium content, silicon is enhanced. The correlation is presented in Fig. 15, which is based on data from the present work and from Paper I in the case of HD The abundances of the remaining elements (iron-group, in particular) in blue stragglers show no anomalies when compared to those in main sequence stars Blue straggler phenomenon The present-day situation Standard theory is not able to explain the existence of rather massive main sequence stars (having masses greater than those of turn-off stars) in stellar clusters. These stars seemed to be delayed in their evolution. Indeed, instead of evolving off the main sequence towards the red giant region, they prefer to spend on the main sequence a time interval which is longer than the main sequence lifetime for the corresponding stellar mass. Are they single stars, binaries or merged remnants? Several hypotheses have been proposed to explain their peculiar origin. None of them was able to elucidate all the observed properties of blue stragglers from the different stellar systems: open and globular clusters, stellar associations. The single star evolution hypothesis (Saio & Wheeler, 1980) faces some problems with localizing the source for the strong internal mixing or the non-thermal pressure which prevents stellar core contraction and thus prolongs the lifetime of the main sequence phase. Moreover, this hypothesis was criticized from the observational side by Schönberner & Napiwotzki (1994). An examination of the blue straggler problem which supposes that they are stars formed via stellar collisions and tidal captures in open clusters was put forward by Leonard & Linnell (1992). These authors were able to account for only 10% of the observed blue stragglers in this manner. The mass transfer hypothesis (McCrea 1964) and stellar coalescence scenario (Zinn & Searle 1976 ; Mateo et al. 1990) also face observational difficulties (slow rotation and a lack of the radial velocity variations for the great majority of blue stragglers). It seems that nowadays only one definite answer concerning the origin of blue straggler star phenomenon can be given: no known cause! To change this situation significantly, new observational efforts should be started. Up to now we still have only a few spectroscopic investigations of blue stragglers based on high resolution spectroscopic material, from which we can learn something about the properties of only about dozen stars, but the properties of the great majority blue stragglers in open clusters remain unknown. Even the primary characteristic of these stars, their membership probability, is not known for the majority with the necessary precision What is new we can learn from the present study? Let us consider briefly how our data fit the most popular hypotheses. Mass transfer, tidal captures and stellar mergers as an origin of open cluster blue stragglers should produce the rather high rotation of the newly formed stars, which is the result of the angular momentum conservation. Otherwise, one needs to assume some special condition for these processes. A high rotational velocity should be also an important input parameter for the single star evolution hypothesis. In fact, the high projected velocities are not observed for blue stragglers. Mass transfer and stellar mergers must also produce some specific chemical peculiarities in blue stragglers. One can expect to detect an increased helium abundance on the surfaces of blue stragglers, provided they are formed via these mechanisms, due to the appearance in the upper atmospheric layers of the CNO-processed material from the stellar interior. But the blue stragglers investigated show either normal helium abundances, or an underabundance of this element. Global mixing of the merger s remnant caused by a coalescence of two stars should result in some decreasing of the surface carbon abundance and respective nitrogen enhancement (again due to the

11 S.M. Andrievsky et al.: Blue stragglers in open clusters. II 527 mixing of the atmospheric gas with CNO-processed material). The carbon abundances derived for the blue stragglers seem to support this hypothesis, but similar underabundances have been also derived for the main sequence stars. Our spectroscopic analysis of a restricted sample of blue stragglers showed that as a group, these stars demonstrate the same chemical peculiarities as main sequence stars. The only obvious distinction between the blue stragglers and main sequence stars of the same spectral type are the projected rotational velocities. We think that any proposed hypothesis for the blue straggler stars must first be able to account for two features: 1) the low projected rotational velocities, 2) an absent of specific chemical peculiarities inherent to the class of blue straggler stars. 8. Conclusion We have investigated several blue stragglers and main sequence stars from the following open clusters: NGC 3496, NGC 6475, NGC 6633 and IC For all of these stars a detailed spectroscopic study, based on the LTE assumption, has been performed for the first time. The main results are as follows: 1). Our present knowledge about NGC 3496 does not allow a confident classification of the three stars S 43, S 115 and S 117 as blue stragglers. 2). All investigated blue stragglers showed low projected rotational velocities, while investigated B-A main sequence stars appeared to be the fast rotators in accordance with their spectral types (the case of S 115 and S 117 is disputable). 3). We have discovered two new He-weak stars (one is a blue straggler) and obtained a quantitative estimate of the helium content for the already known He-weak blue straggler HD For the quantitative analysis of the He-weak stars, atmosphere models were calculated with decreased helium content. 4).All the investigated middle-to-late B stars showed a subsolar carbon abundance. These abundances have been checked by NLTE calculations. 5). The severe helium deficiency found for some stars and the moderate-to-strong carbon underabundances could be explained as a result of atomic diffusion in the stellar atmospheres. However, three stars with apparent helium/carbon deficiencies possess rather high projected rotational velocities (v sin i 70 km s km s 1 ). 6). The silicon abundance in our program stars shows a clear dependence upon the helium content. In the helium deficient stars this element is increased, while its abundance is solar (or slightly subsolar) for the He-normal stars. 7). The iron-group elements do not show any significant deviations from the solar abundances in either blue stragglers or main sequence stars. 8). Generally, based on their chemical compositions, the blue stragglers stars are not different from main sequence stars. This conclusion is made for a very restricted sample of blue stragglers and requires further investigation. To summarize, one can note that any theory which attempts an exhaustive explanation of the blue straggler phenomenon should be able to account for the two important features of these enigmatic stars: a) their small projected rotational velocities; b) the similarities of their chemical compositions with those of ordinary cluster and galactic field stars. Acknowledgements. Authors are thankful to Drs. G.-L. Hildebrandt, M. Steffen, I. Hubeny and F. Herwig for kind assistance with the necessary software installation. Special thanks to Drs. F. Kupka and S.A. Korotin for the NLTE calculations and computation of the atmosphere models for helium deficient stars, to Dr. F. Kienzle for providing unpublished photometric data for one of the program stars, and to anonymous referee for several useful comments. SMA is also grateful to the Astrophysikalisches Institut of Potsdam (Germany) for the financial support and the opportunity to perform this work using its institutional facilities. The necessary information has been obtained through the SIMBAD and VALD data bases. 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