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Register a new userOn the density profile of the globular cluster M92
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Alessandra Di Cecco
Publication date
2013
fields
Physics
and research's language is
English
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We present new number density and surface brightness profiles for the globular cluster M92 (NGC 6341). These profiles are calculated from optical images collected with the CCD mosaic camera MegaCam at the Canada-France-Hawaii-Telescope and with the Advanced Camera for Surveys on the Hubble Space Telescope. The ground-based data were supplemented with the Sloan Digital Sky Survey photometric catalog. Special care was taken to discriminate candidate cluster stars from field stars and to subtract the background contamination from both profiles. By examining the contour levels of the number density, we found that the stellar distribution becomes clumpy at radial distances larger than about 13 arcminutes, and there is no preferred orientation of contours in space. We performed detailed fits of King and Wilson models to the observed profiles. The best-fit models underestimate the number density inside the core radius. Wilson models better represent the observations, in particular in the outermost cluster regions: the good global agreement of these models with the observations suggests that there is no need to introduce an extra-tidal halo to explain the radial distribution of stars at large radial distances. The best-fit models for the number density and the surface brightness profiles are different, even though they are based on the same observations. Additional tests support the evidence that this fact reflects the difference in the radial distribution of the stellar tracers that determine the observed profiles (main sequence stars for the number density, bright evolved stars for the surface brightness).
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Although there have been recent claims that there is a large dispersion in the abundances of the heavy neutron capture elements in the old Galactic globular cluster M92, we show that the measured dispersion for the absolute abundances of four of the rare earth elements within a sample of 12 luminous red giants in M92 (less than or equal to 0.07 dex) does not exceed the relevant sources of uncertainty. As expected from previous studies, the heavy elements show the signature of the r-process. Their abundance ratios are essentially identical to those of M30, another nearby globular cluster of similar metallicity.
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Multiple populations in globular clusters are usually explained by the formation of stars out of material with a chemical composition that is polluted to different degrees by the ejecta of short-lived, massive stars of various type. Among other things, these polluters differ by the amount of helium they spread in the surrounding medium. In this study we investigate whether the present-day photometric method used to infer the helium content of multiple populations indeed gives the true value or underestimates it by missing very He-rich, but rare stars. We focus on the specific case of NGC6752. We compute atmosphere models and synthetic spectra along isochrones produced for this cluster for a very broad range of He abundances covering the predictions of different pollution scenarios, including the extreme case of the fast-rotating massive star (FRMS) scenario. We calculate synthetic photometry in HST filters best suited to study the helium content. We subsequently build synthetic clusters with various distributions of stars. We finally determine the maximum helium mass fraction of these synthetic clusters using a method similar to that applied to observational data. We build toy models of clusters with various distributions of multiple populations and ensure that we are able to recover the input maximum Y. We then build synthetic clusters with the populations predicted by the FRMS scenario and find that while we slightly underestimate the maximum Y value, we are still able to detect stars much more He-rich than the current observed maximum Y. It is easier to determine the maximum Y on main sequence stars than on red giant branch stars, but qualitatively the results are unaffected by the sample choice. We show that in NGC6752 it is unlikely that stars more He-rich than the current observational limit of about 0.3 are present.
In this paper we review the operational definition of the so-called main sequence knee (MS-knee), a feature in the color-magnitude diagram (CMD) occurring at the low-mass end of the MS. The magnitude of this feature is predicted to be independent of age at fixed chemical composition. For this reason, its difference in magnitude with respect to the MS turn-off (MS-TO) point has been suggested as a possible diagnostic to estimate absolute globular cluster (GC) ages. We first demonstrate that the operational definition of the MS-knee currently adopted in the literature refers to the inflection point of the MS (that we here more appropriately named MS-saddle), a feature that is well distinct from the knee and that cannot be used as its proxy. The MS-knee is only visible in near-infrared CMDs, while the MS-saddle can be also detected in optical-NIR CMDs. By using different sets of isochrones we then demonstrate that the absolute magni- tude of the MS-knee varies by a few tenths of a dex from one model to another, thus showing that at the moment stellar models may not capture the full systematic error in the method. We also demonstrate that while the absolute magnitude of the MS-saddle is almost coincident in different models, it has a systematic dependence on the adopted color combinations which is not predicted by stellar models. Hence, it cannot be used as a reliable reference for absolute age determination. Moreover, when statistical and systematic uncertainties are properly taken into ac- count, the difference in magnitude between the MS-TO and the MS-saddle does not provide absolute ages with better accuracy than other methods like the MS-fitting.
We have performed new wide-field photometry of the young open cluster NGC 6231 to study the shape of the initial mass function (IMF) and mass segregation. We also investigated the reddening law toward NGC 6231 from optical to mid-infrared color excess ratios, and found that the total-to-selective extinction ratio is Rv = 3.2, which is very close to the normal value. But many early-type stars in the cluster center show large color excess ratios. We derived the surface density profiles of four member groups, and found that they reach the surface density of field stars at about 10, regardless of stellar mass. The IMF of NGC 6231 is derived for the mass range 0.8 -- 45 Msun. The slope of the IMF of NGC 6231 (Gamma = -1.1 +/- 0.1) is slightly shallower than the canonical value, but the difference is marginal. In addition, the mass function varies systematically, and is a strong function of radius - it is is very shallow at the center, and very steep at the outer ring suggesting the cluster is mass segregated. We confirm the mass segregation for the massive stars (m >~ 8 Msun) by a minimum spanning tree analysis. Using a Monte Carlo method, we estimate the total mass of NGC 6231 to be about 2.6 (+/- 0.6) x 10^3 Msun. We constrain the age of NGC 6231 by comparison with evolutionary isochrones. The age of the low-mass stars ranges from 1 to 7 Myr with a slight peak at 3 Myr. However the age of the high mass stars depends on the adopted models and is 3.5 +/- 0.5 Myr from the non- or moderately-rotating models of Brott et al. as well as the non-rotating models of Ekstrom et al. But the age is 4.0 -- 7.0 Myr if the rotating models of Ekstrom et al. are adopted. This latter age is in excellent agreement with the time scale of ejection of the high mass runaway star HD 153919 from NGC 6231, albeit the younger age cannot be entirely excluded.
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