CN variations in NGC 7006

Abstract

Abstract. Rotationally induced mixing with subsequent dredge-up of nucleosynthesized material is discussed as a second parameter of the horizontal branch morphology in globular clusters. CNO abundances have been proposed as tracers of the dredge up of processed material. NGC 7006 is a prominent example of a second parameter GC: Its HB morphology is too red for its metallicity. We present spectroscopic measurements of CN molecular band strengths S(3839) and CH band CH(4300) strengths for 12 giants in NGC 7006 to test rotationally-driven mixing as a second parameter in this cluster. Our observations reveal (i) a scatter in star-to-star CN absorption strengths with the same amplitude as seen in other GCs of the same metallicity, but different HB morphologies; (ii) a possible continuous distribution of CN absorption strength with a preference for CN-enriched stars, and (iii) a possible weak radial gradient in the number ratio of CN-strong and CN-weak stars. We argue against the hypothesis that CN-variations are directly correlated with the second parameter effect of the HB morphology. However, the small sample of stars measured in NGC 7006 prevents us from drawing firm conclusions. Finally, we identify one star of our sample as a foreground dwarf carbon star.

Comparing CN and CH line strengths in a homogeneous spectroscopic sample of 8 Galactic globular clusters

“…In particular, the inclusion of our results with those listed in Smith (2002) and Harbeck et al (2003b) further confirms the lack of a correlation between cluster ellipticity and the number ratio of CN-strong stars. We observe a large scatter in Fig.…”

“…A similar analysis has been done before by, e.g., Norris (1987), Smith & Mateo (1990), Smith (2002), and Harbeck et al (2003b). However their studies were based on compilations of upper RGB star measurements in various clusters from different sources and therefore different techniques.…”

“…For a more statistically complete investigation, we combined our results with those by Smith (2002) and Harbeck et al (2003b). In Sect.…”

“…Similarly, based on the CN and CH measurements of stars in the second parameter globular cluster NGC 7006, Harbeck et al (2003b) argued against the hypothesis that CN-variations are directly correlated with the second parameter effect. They found the scatter in CN to be similar to those in other GCs of the same metallicity but different horizontal branch ratios.…”

A Survey of Cn and Ch Variations in Galactic Globular Clusters From Sloan Digital Sky Survey Spectroscopy

“…A common approach for investigation of the star-to-star lightelement abundance variations within GCs is measurement of the 3883 Å CN molecular absorption band (Norris & Freeman 1979;Norris et al 1981;Smith et al 1996;Harbeck et al 2003aHarbeck et al , 2003bPancino et al 2010). This measurement does not require high-resolution spectroscopy, making it ideal for low-resolution spectroscopic surveys such as SDSS.…”