RV Tau Variables in Globular Clusters - clues on their nature?

Stephen C. Russell, PASA, 15 (2), 189
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The Results

The Globular Cluster stars

The mean of results from Russell (1997) for the stars V11 in M2 and SH11 in M13; from Gonzalez & Lambert (1997) for stars V11 in M2, V84 in M5, V17 in M28, and V2 in M10; and from Gonzalez & Wallerstein (1994) for V1 in tex2html_wrap_inline149Cen, are presented in column 4 of Table 1. For V11 in M2, the weighted mean of the results from the two references was used. The star V6 in M56 has been discarded since the abundances are discrepant. The solar abundances adopted in Russell (1997) were used in each case, except for V1 in tex2html_wrap_inline149Cen, where a differential solar analysis was used.

The results are also shown in Figure 1, where the filled circles with error bars had more than one measure for the abundance, and the open circles had just one measure. The error bars are the standard deviations in the mean of the abundances derived from all the stars.

The first result is that the [Fe/H] abundances for the RV Tau variables, compared with the [Fe/H] abundances of the red giants in the same clusters, are -0.07tex2html_wrap_inline1530.04. This value was calculated without including V2 in M10, as this star was felt to be discrepant by the authors. This shows that there are negligible differences between the RV Tau variable iron abundances and those of the local environs. Certainly there is no suggestion of an over abundance of iron. This means it is unlikely that these variables suffer from a hydrogen deficiency.

There is also no sign of an underabundance in s-process elements relative to iron. Figure 1 shows clearly that there is actually an overabundance of s-process elements, relative to the sun. This is likely to be true also of the variable stars compared to their local environs, as shown by two of the stars studied by Russell (1997). Based on these results, after correcting for the detailed differences between the companion red giants, and the sun, all s-process elements, including Y, would lie above the zero line.

Although the results are provocative, it is hard to say how significant they are at this stage. Barium is the most significantly overabundant s-process element, and it depends on two or three saturated resonance lines. Further work on refining the measurements in the future, may hold the key to determining the exact processes involved in production of the s-process elements.

The second major result from this work is the significant overabundance of the light elements Na, Mg, Al, & Si. These data are on a firmer footing than the s-process elements, since they have been observed in more stars, and Si particularly, depends on many more lines.

The Na excess has already been discussed in Gonzalez & Lambert (1997), with regards to V84 in M5, and the possibility that it is a product of ON-cycling (Langer et al., 1993). The present work demonstrates that the Na excess is common to the RV Tau variables in globular clusters. What is more, the Al excess suspected by Gonzalez & Lambert (1997), was demonstrated by Russell (1997) to be a true excess in two RV Tau type variables, compared with their companion red-giants. This more firmly supports the ON-cycling explanation for light element excesses.

Mg and Si are both tex2html_wrap_inline155-elements, and the work of Russell (1997) illustrates that the over-abundances in these elements, relative to the sun, are not significantly different from the abundances observed in the companion red giants. This is in agreement with the tex2html_wrap_inline149Cen RV Tau variable V1 (Gonzalez & Wallerstein 1994), though in apparent disagreement with the star V84 in M5 (Gonzalez & Lambert 1997). The statistics are too poor to make firm conclusions here, but present indications are that the tex2html_wrap_inline155-elements in RV Tau variables in globular clusters, are usually in normal abundance ratio to Fe, compared with neighboring red giants.

 

Element

A Solar Globular sigma Field 1 sigma Field 2
[M/H] [M/Fe] [M/Fe] [M/Fe]

Na

11 6.33 0.46 0.13 0.99 0.26 0.55
Mg 12 7.58 0.32 0.10 0.14 0.10 0.49
Al 13 6.47 0.54 0.35 0.98 ... 0.53
Si 14 7.55 0.47 0.06 0.71 0.09 0.28
S 16 7.38 0.88 ... 1.29 0.23 ...
Ca 20 6.36 0.35 0.08 -0.21 0.14 0.00
Sc 21 3.10 0.06 0.07 -0.99 0.12 -0.49
Ti 22 4.99 0.26 0.01 -0.58 0.19 0.09
V 23 4.00 0.35 0.39 ... ... 0.01
Cr 24 5.67 0.08 0.14 0.08 0.05 -0.10
Mn 25 5.39 -0.36 0.11 -0.19 0.20 -0.28
Co 27 4.92 0.08 0.60 0.18 0.06 0.07
Ni 28 6.25 0.10 0.05 0.04 0.08 0.08
Cu 29 4.21 -0.29 ... ... ...-0.29
Zn 30 4.60 0.13 0.12 1.09 0.27 -0.04
Y 39 2.24 -0.06 0.06 -0.51 0.24 -0.72
Zr 40 2.60 0.51 0.17 ... ...-0.83
Ru 44 1.84 0.14 ... ... ... ...
Ba 56 2.13 0.41 0.10 -0.20 ...-0.38
La 57 1.22 0.09 0.31 ... ...-0.55
Ce 58 1.55 0.10 0.15 0.69 ...-0.53
Pr 59 0.71 0.13 ... ... ...-1.77
Nd 60 1.50 0.11 0.17 ... ...-0.66
Sm 62 1.00 0.38 0.02 0.52 ...-0.61
Eu 63 0.51 0.47 0.11 0.04 ...-0.05
Dy 66 1.10 0.46 ... ... ... ...

Table 1: Adopted Abundances
 

  figure36
Figure 1: Average [M/Fe] ratios for RV Tau variables in globular clusters

The Field Stars

The full results for the average abundances for field RV Tau variable stars are shown in Table 1. Columns 6 & 7 (`Field 1' & `sigma', respectively) give the average abundances and standard deviations from the work of Gonzalez, Lambert, & Giridhar, 1997a; Gonzalez, Lambert, & Giridhar, 1997b; and Giridhar, Rao, & Lambert (1994). Column 8 ('Field 2') gives the results from Luck & Bond (1989), and references therein, for comparison purposes.

It is becoming increasingly apparent that the detailed chemical abundance distribution of field RV Tau variables is best explained if we plot the abundances against the condensation temperatures for those elements. In Figure 2 the [M/Fe] abundance ratios for the `Field 1' field stars (column 6 of Table 1) are plotted as open circles, against the condensation temperatures. For comparison, the mean abundances of the globular cluster stars (column 4 of Table 1) are plotted as filled circles, with the appropriate error bars. This clearly demonstrates the difference between the two sets of stars. The field stars track the abundance distribution of the interstellar medium (see, for instance, Gonzalez, Lambert, & Giridhar 1997b), while the globular cluster stars show no such trend.

The only problem is that the work of Luck & Bond seems to disagree with these results. Figure 3 compares the results of Luck & Bond (1989) with the more recent results given in column 6 of Table 1. Clearly, it is the critical elements S, Zn, Na, Sc, and Ti that will decide the issue. These are the very same elements that will decide the differences and similarities between field and globular cluster stars. On the grounds of condensation temperature alone, therefore, there is still a case to be answered. There is no overlap in objects between the field stars studied in `Field 1' & those in `Field 2'. Work has to be done to repeat observations and analyses of the stars studied by Luck & Bond, and the references quoted in that paper.

Further clues can be obtained by asking how the abundances of the s-process elements match between the Gonzalez and Luck & Bond studies. The Luck & Bond study shows uniformly negative relative abundances to iron, for all s-process elements. Unfortunately, the Gonzalez study did not concentrate on s-process elements, and only returned significant results for Ba, and these were consistent with the Luck & Bond study. Again, it would be profitable to carry out a new study of the Luck & Bond stars, and as well, re-observe the Gonzalez study stars to determine the s-process elements.

The difference between field stars and globular cluster stars

No matter what assumptions we make about the field RV Tau variables, it still seems that the globular cluster RV Tau variables are different. If the results of `Field 2' are to be believed, we must explain why cluster variables don't show s-process element under abundances. No explanations for this difference have been published in the literature. Presumably one of the options suggested by Luck & Bond (1989) operates in field variables, but not in cluster variables. Since the `over-ionization' option should reasonably be expected to operate in both field and globular variables, this seems the least likely explanation. At this stage, however, it seems inappropriate to pursue the explanation in detail, until the results have been confirmed.

If the results of `Field 1' are to be believed, then three options are offered by Gonzalez & Lambert (1997), to explain the differences. The explanations depend on the assumption that field variables undergo dust/gas separation, and cluster variables do not. Though the photospheric metallicities of field stars range through the metallicities observed in globular clusters, they are supposed to have achieved these low metallicities through selective loss of dust compared with gas. Therefore, either the higher intrinsic metallicity of the stars, or a higher evolutionary mass, might be responsible for the more intense, or longer duration stellar wind.

A third possibility is that the environment in a globular cluster is responsible for the difference, by precluding binary formation. There is a possibility that binary stars are responsible for the field RV Tau abundance patterns, as they are for the peculiar abundance patterns of Ba-stars. Evidence that binaries may be rare in globular clusters comes from Mayor et al. (1996), who report that in tex2html_wrap_inline149Cen there are only about 20% as many binary systems as are found in the field.

Again, there is still a great deal of work to be done to establish the facts of the case, before too much effort is directed towards explaining the apparent systematic effects.

  figure49
Figure 2: Average [M/Fe] for RV Tau variables in the field and in globular clusters, against condensation temperature

  figure54
Figure 3: Luck & Bond (1989) compared with Gonzalez, Lambert and co-workers


Next Section: Conclusions
Title/Abstract Page: RV Tau Variables in
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Contents Page: Volume 15, Number 2

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