Stragglers

Abstract. We present preliminary results of lithium abundances in turnoff stars in the open cluster NGC 2506. Some fifty turnoff stars and a few blue stragglers have been observed using the FLAMES facility on the VLT, during half a night from the French Guaranteed Time on this instrument. 

The lithium abundance, using the Li I 6707 doublet, will be used as a signature of internal mixing to confirm whether or not internal mixing is one of the mechanisms responsible for the existence of blue straggler stars. 

The formation mechanisms that can explain the existence of blue stragglers are still very unclear. The different explanations involve mass-transfer in binary systems, or internal mixing. 

Moreover the proportion of turnoff stars supposed to be future blue stragglers (i.e. the subturnoff-mass blue stragglers) would give an independent estimate of the ratio found by [8] the number of blue stragglers to the number of giant stars. 

Using FLAMES we observed, 45 turnoff stars, 7 giant stars with GIRAFFE (LR2 from 396.4 to 456.7 nm, R 6400 and H15 from 660.6 to 696.5 nm, R 19300). At the same time, 3 stars known as blue stragglers ([1]) were observed by UVES-link R = 47000, and a 200 nm wide spectral range centered around 580 nm. 

Fig. 1. HR diagram of NGC 2506 crosses are the data from [4]. The targets observed with FLAMES-GIRAFFE are indicated by circles (open circles are the outliers in the radial velocities distribution), and the blue stragglers observed with FLAMES-UVES are indicated by double circles.

The organizing laboratory performs statistical tests on the results from participating laboratories, and how outliers are treated depends on the nature of the trial. Grubbs s tests for single and paired outliers are recommended (see chapter 2). In interlaboratory studies outliers are usually identified at the 1% level (rejecting H0 at a = 0.01), and values between 0.01 < a < 0.05 are flagged as stragglers. As with the use of any statistics, all data from interlaboratory studies should be scrutinized before an outlier is declared. 

Since a few large size stragglers were always present, the upper bound was taken ... 

All the tests previously mentioned are sensitive to so-called outliers. Outliers ( stragglers ) are defined as single wrong values within a series of values which seem to be right . As already mentioned outliers may influence decisions. There are, therefore, numerous tests for identifying and eliminating possible outliers [RECHENBERG, 1982],... 

In the late 1980s, French astrophysicist Hubert Reeves speculated on the origin of mysterious stars called blue stragglers, first identified in 1952. These... 

Hubert Reeves, Atoms of Silence An Exploration of Cosmic Evolution (Cambridge MIT Press, 1984) M. Beech, Blue stragglers as indicators of extraterrestrial civilizations Earth, Moon, and Planets 49 177-186 (1990) Guillermo A. Lemarchand, Detectability of extraterrestrial technological activities. ... 

Rex Saffer and Dave Zurek, Blue stragglers in globular cluster 47 Tucanae, http // oposite.stsci.edu/pubinfo/PR/97/35/a.html... 

Typically, two types of extreme values can exist in our experimentally measured results, namely stragglers and outliers. The difference between the two is the confidence level required to distinguish between them. Statistically, stragglers are detected between the 95% and 99% confidence levels whereas outliers are detected at >99% confidence limit. It is always important to note that no matter how extreme a data point may be in our results, the data point could in fact be correct, and we need to remember that, when using the 95% confidence limit, one in every 20 samples we examine will be classified incorrectly. 

Grubbs statistic is compared with the critical values in Table 6.5. If the Grubbs statistic is significant at a probability of 5 percent, the outlying values are considered stragglers. Values are considered outliers if significant at the 1 percent level. 

The (1-statistic is compared with the critical values in Table 6.6. The lowest or largest value in the dataset is identified as a straggler or as an outlier if the (1-statistic is larger than the critical value with a statistical certainty of 95 or 99 percent, respectively. 

The test statistics at all concentrations exceed the critical values at 5%, but not at 1%. By ISO 5725-2 definition, this classifies them as stragglers, but not as outliers. Application of Grubb s outlier test [Eqs. (9.26) and (9.27)] to the data submitted by laboratory 5 (Table 9.14, where G represents Grubb s outlier statistic) suggests that there are no statistically significant outliers. 

ISO 5725-2 Section 133.2 defines data significant at 5% but not at the 1% critical value as stragglers only, ISO 5725-2 Section 7.3.3.6 states that if several stragglers are reported consistently for a specific laboratory, the whole of... 

Room in it for you and the family," Hanly said when the stragglers had moved away. 

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