5 Discussion

   
5.1 Cluster membership

The membership probabilities for our blue-straggler candidates in NGC 7789 have been determined by several investigations:

Photometry, polarimetry and proper motion:

Pendl (1975), McNamara (1980), Twarog & Tyson (1985), Manteiga et al. (1991), Breger (1982).

Radial velocities:

Strom & Strom (1970), Stryker & Hrivnak (1984), Drilling & Schönberner (1987), Manteiga et al. (1989), Milone & Latham (1994).

Spectroscopy:

Schönberner & Napiwotzki (1994).

In discussing the cluster membership problem we exclusively rely only on the most stringent criteria, viz. proper motion, radial velocity and spectroscopy. Based on the proper motion study of McNamara (1980) and Pendl (1975) we selected those objects from McNamara's lists of blue straggler candidates with at least one measurement of the radial velocity. Instead of reporting the qualitative judgements of the respective authors, we retrieved the original data and listed them in Table 6.

We also revised the radial velocities for several blue stragglers previously analysed by Drilling & Schönberner (1987). We used the KPNO spectra and applied the very accurate method of line mirroring. The results are given in the corresponding column of Table 6. We checked the method by applying it to our reference star Vega. From 14 lines of its KPNO spectrum we derived the mean value of $V_{\rm r} = -12.7 \pm 0.6$ kms^-1, in good agreement with the recommended value of -13.9 kms^-1 (SIMBAD data base).

The problem of the determination of true, i.e. physical, cluster memberships is somewhat delicate and prone to personal opinions. The necessary condition for an object to be a (physical) cluster member is the agreement of its proper motion and radial velocity with those of the cluster.

Spectroscopy, as performed by Schönberner & Napiwotzki (1994), serves as a final criterion because it provides a distance information. If, however, spectroscopy indicates membership, it must be supplemented by the proper motion and radial velocity data in order to distinguish between the physical members and field interlopers (see the case of K677 in Table 6).

The membership assignments of Table 6 are based on the philosophy outlined above. Note that in the two cases (K409 and K1211) where spectroscopy indicates non-membership, this statement is supported by the radial velocity measurements. Altogether we consider 11 of the 26 blue-straggler candidates to be the members of NGC 7789.

   

Table 6: Radial velocities (in kms^-1) and memberships of blue stragglers in NGC 7789. The cluster velocity based on red giants is $V_{\rm r} = -55$ kms^-1 (Gim et al. 1998b)

Star number		Radial velocity							Spectroscopy	Membership
M	K	SS70	SH84	DS87(CA)	DS87(KPNO)	MPMR89	ML94	This work	SN94
257	2	-	-	-	-	-	N	-	-	N
317	68	-	-	-	-	-	N	-	-	N
325	88	-	-	-	-	-	Y	$-58\pm4(6)$	-	M*
377	168	-21	-	-	-	-	U	-	-	U
389	192	-	-	-	-	-	U	-	-	U
396	197	-31	-	-	-	-	N	-	-	N
419	234	-26	-	-	-	-	-	-	-	N
460	282	-	-47	-54	-	-43	M	-	M	U
482	316	-	-	-	-58	-	Y	$-59\pm3(15)$	-	M
502	342	-45	-51	-56	-	-51	U	-	M	M
500	349	-34	-	-	-	-	-	-	-	N
518	371	-56	-56	-55	-65	-58	Y	$-56\pm5(6)$	-	M
543	409	-20	-45	-27	-29	-	N	-28 (1)	N	N
574	453	-41	-52	-41	-	-71	U	-	M	M
747	677	-	-37	-27	-23	-18	N	$-17\pm4(14)$	M	N
752	696	-	-	-	-	-	U	-	-	U
789	746	-	-	-53	-52	-	Y	$-43\pm2(14)$	-	M
913	934	-	-	-	-	-	N	-	-	N
1011	1095	-	-	-	-	-	Y	-	-	M
1054	1168	-49	-	-	-	-	Y	-	-	M
1088	1211	-33	-41	-31	-	-	N	-	N	N
1133	1270	-	-	-	-61	-	Y	$-55\pm3(15)$	-	M
144	-	-	-				U	-	-	U
459	-	-	-	-44	-	-	U	-	M	M
808	-	-	-	-	-	-	U	-	-	U
1060	-	-	-	-	-	-	U	-	-	U

M - McNamara (1980) number.
K - Küstner (1923) number.
M, N, U - denotes the members, non-members and stars with an uncertain membership respectively.
SS70 - Strom & Strom (1970).
SH84 - Stryker & Hrivnak (1984).
DS87(CA) - Drilling & Schönberner (1987), Calar Alto spectra.
DS87(KPNO) - Drilling & Schönberner (1987), Kitt Peak spectra.
MPMR89 - Manteiga et al. (1989).
ML94 - Milone & Latham (1994).
SN94 - Schönberner & Napiwotzki (1994).
* - Based on the study of Milone & Latham (1994), Gim et al. (1998a) considered the membership of K88 to be uncertain.
However, by looking at the original data of Milone & Latham (Table 2 therein), it became evident that Gim et al.
used erroneously the data of K68 instead, which is a radial-velocity non-member.

  
5.2 Abundance peculiarities

The fact that the blue stragglers in NGC 7789 display abundance anomalies for some chemical elements, which are not seen in the convectively well-mixed cluster giants, indicates that these anomalies are purely surface phenomena. All investigated blue stragglers fall in the domain of late B - early/mid A-type stars. It is well known that stars of these spectral types demonstrate a great variety of chemical peculiarities. Our investigated blue stragglers possess projected rotational velocities which are lower than is expected for their spectral classes. It is therefore interesting to compare their abundance anomalies (for the confirmed cluster members only, see Sect. 5.1) with those of chemically peculiar stars.

Figure 3: Comparison of different abundance distributions. Open circles: mean values of "normal'' A-type stars from Caliskan & Adelman (1997) and Adelman (1999); open squares: mean values from Hill (1995); filled squares: individual values of the blue stragglers investigated in this paper; large open circles: our data for Vega. Note that the literature values are horizontally shifted by $\pm 0.2$, and that the typical error ($2\sigma$) is indicated

Normal A-type stars.

Numerous studies of sharp-lined late B - early A-type stars were performed by Adelman with co-authors (see, e.g. Caliskan & Adelman 1997 and references therein). They showed that some of these stars have solar-like elemental distributions. For example, Caliskan & Adelman (1997) and Adelman (1999) provide elemental abundances for 17 stars (without Vega and the $\lambda$ Boo type star 29 Cyg). From this sample one can derive a mean magnesium abundance (from the Mgii lines only) of $\mbox{\rm [Mg/H]} = -0.11 \pm 0.19$. This value is significantly larger than the magnesium abundance of our blue stragglers. In Fig. 3 we compare their abundance distribution (that of Vega from Table 3 included) with data from the literature on A stars possessing small projected rotational velocities. As one can see from Fig. 3 the blue straggler abundance distribution follows the general trend, with the exception of Mg and Sc which appear to be depleted.

Am stars.

According to the classical definition, Am stars have apparent surface underabundances of Ca and Sc (see Cayrel et al. 1991). This is also the case for the blue stragglers studied here. Am stars are, however, also characterized by a moderate overabundance of the iron-group and heavier elements, but this is not seen in NGC 7789 (we recall that we did not investigate elements heavier than iron). Also, the remarkable magnesium deficiency of the NGC 7789 blue stragglers is not reported for Am stars.

Ap stars.

Ap stars show even more pronounced abundance anomalies, especially for heavy species. On the other side, magnesium, calcium and scandium can often be substantially depleted. With the spectral material at our disposal we are not able to decide whether our blue-straggler sample contains Ap stars. For example, the Srii 4215 Å classification feature is not covered by the observed spectral region, although the rather strong lines of Zrii 4496.96 Å  and Yii 4398.02 Å  are seen in the spectra of K746 and K1270. It should be also mentioned that Stryker & Hrivnak (1984) noted that K1211 shows relatively strong Siii lines and an absence of the helium lines (the latter is in agreement with our result on helium abundance in this star). The above mentioned authors classified K1211 as Ap(Si) star.

Mg II 4481 Å weak stars.

The weak Mgii 4481 Å line is an inherent feature of some A stars, as described by Abt & Morrell (1995). These authors consider them as a mild version of the $\lambda$ Boo-type stars (see the next item). Using the data of Abt & Morrell (1995) we plot the equivalent widths of the magnesium line vs. spectral class for normal A-type stars, Mgii 4481 Å-weak stars, and NGC 7789 blue stragglers (see Table 7 for the spectral class assignments for program stars). As can be seen, several of the stragglers resemble the most extreme Mgii 4481 Å-weak stars. In spite of this, the stragglers have rather low projected rotational velocities (cf. Table 2) as compared with the Mgii 4481 Å-weak stars for which a mean of more than 100 kms^-1 appears to be appropriate (Abt & Morrell 1995). Interestingly, the two certain non-members, K409 and K677, show virtually solar magnesium abundances, thereby giving additional evidence for their non-membership.

$\lambda$ Boo and Vega-like stars.

Chemically peculiar A stars of the $\lambda$ Boo type occur at various rotational velocities with a typical $v\sin i$ value of $\approx$100 kms^-1. Several recent studies of the $\lambda$ Boo-type stars (see, Andrievsky et al. 1998; Paunzen et al. 1999) have shown that in their atmospheres many metals have significantly reduced abundances. In particular, magnesium, calcium and scandium may be depleted by a factor of about 10-100. Similar underabundances are also detected for titanium and iron. At present, there is accumulating evidence that there should be a link between the $\lambda$Boo phenomenon (extreme abundance anomalies) and Vega-like stars characterized by a milder metal deficiency. Dunkin et al. (1997) recently analysed several Vega-like stars and found in some cases relatively strong depletion of magnesium and calcium, while the abundances of iron-group elements are solar within 0.25 dex.

Summarizing, the abundance peculiarities found in the NGC 7789 blue straggler are difficult to attribute to any chemical peculiarity known among A-type stars. This peculiarity is also in contrast with the results reported in our Paper II. None of the straggler stars from three open clusters exhibits, for instance, such a profound magnesium deficiency as found in the present study. The only distinct feature which unites the blue stragglers from the present and previous studies is a low projected rotational velocity.

Figure 4: Equivalent width of the Mg II 4481 Å line vs. spectral class: small circles - class V, small filled squares - class IV, asterisks - class III, large filled squares - program blue stragglers, large open circle - Vega, dots - Mgii 4481 Å-weak stars

Concluding this investigation of the blue stragglers in the NGC 7789 field we state that:

a)

All the confirmed cluster members show a remarkably superficial magnesium deficiency, while the non-members possess a nearly normal abundance;

b)

The iron and titanium abundances in all blue stragglers agree with the mean cluster metallicity as derived from red giant stars;

c)

The (projected) rotational velocities of the confirmed blue stragglers are significanly lower than normal for their spectral types.

The present study was hampered by the fact that the most powerful telescopes available when the observations were made did not allow us to perform a high-resolution study of the total blue-straggler population of NGC 7789 relative to that cluster's turn-off stars. Such an ambitious project would certainly require the use of 8-10m class telescopes in order to get spectra of sufficient resolution, signal-to-noise ratio and wavelength coverage. Only then might we be able to answer the fundamental questions on the origin and evolution of blue straggler stars in open clusters.

Acknowledgements

Authors are thankful to the referee, Dr. S. J. Adelman, for his valuable 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.

 

 

Table 7: Mg II 4481 Å line in program stars

Star	$T_{\rm eff}$ (K)	Sp*	EW(4481)(mÅ)
K88	8900	A2-A3	320
K316	8950	A2	280
K371	8500	A4	490
K746	8600	A3-A4	353
K1270	8300	A4-A5	270
K409	9480	A0	640
K677	10680	B9	460
K1211	12630	B7-B8	280
Vega	9500	A0	311
* - spectral types are roughly estimated by us between
III-V luminosity classes using Lang (1992).