HST images of globular clusters

Globular cluster photometry with the Hubble Space Telescope. VII. Color Gradients and Blue Stragglers in the Central Region of M30 (NGC 7099) from WFPC2 Observations

Author(s):Puragra Guhathakurta, Zodiac T. Webster, Brian Yanny, Donald P. Schneider, and John N. Bahcall

Journal:The Astronomical Journal, 116, 1757-1774 (October 1998); astro-ph/9805361.

Abstract: We present F555W (V), F439W (B), and F336W (U) photometry of 9507 stars in the central 2´ of the dense, post core collapse cluster M30 (NGC 7099) derived from Hubble Space Telescope Wide Field/Planetary Camera 2 images. These data are used to study the mix of stellar populations in the central region of the cluster. Forty eight blue straggler stars are identified; they are found to be strongly concentrated towards the cluster center. The specific frequency of blue stragglers, F_BSS = N(BSS)/N(V < V_HB + 2), is 0.25± 0.05 in the inner region of M30 (r < 20´´), significantly higher than the frequency found in other clusters: F_BSS = 0.05-0.15. The shape of M30's blue straggler luminosity function resembles the prediction of the collisional formation model and is inconsistent with the binary merger model, of Bailyn & Pinsonneault (1995, ApJ, 439, 705). An unusually blue star (B = 18.6, B-V = -0.97), possibly a cataclysmic variable based on its color, is found about 1.2´´ from the crowded cluster center. Bright red giant stars (B < 16.6) appear to be depleted by a factor of 2-3 in the inner r < 10´´ relative to fainter giants, subgiants, and main sequence turnoff stars (95% significance). We confirm that there is a radial gradient in the color of the overall cluster light, going from B-V ~ 0.82 at r ~ 1´ to B-V ~ 0.45 in the central 10´´. The central depletion of the bright red giants is responsible for about half of the observed color gradient; the rest of the gradient is caused by the relative underabundance of faint red main sequence stars near the cluster center (presumably a result of mass segregation). The luminosity function of M30's evolved stars does not match the luminosity function shape derived from standard stellar evolutionary models: the ratio of the number of bright giants to the number of turnoff stars in the cluster is 30% higher than predicted by the model (3.8 effect), roughly independent of red giant brightness over the range M_V = -2 to + 2.

Globular cluster photometry with the Hubble Space Telescope. VI. WF/PC-I observations of the stellar populations in the core of M13 (NGC 6205)

Author(s): Randi L. Cohen, Puragra Guhathakurta, Brian Yanny, Donald P. Schneider, and John N. Bahcall

Journal:The Astronomical Journal, 113, 669-681 (February 1997); astro-ph/9611151

Abstract: To probe the nature of stellar evolution in dense environments, we study the dense core of the globular cluster Messier 13 using pre-refurbishment Planetary Camera-I images obtained with the Hubble Space Telescope. We find 15 blue straggler star candidates and 10 other possible blue stragglers in this region of M13. Their specific frequency is in the range 0.04-0.07, comparable to what is observed near the centers of other dense clusters. A comparison between M13's observed V band stellar luminosity function and a theoretical model (Bergbusch & Vandenberg 1992) for the luminosity function of an old, metal-poor cluster shows that the model predicts too few of the brightest red giants (V~12.5-15) by a factor of two relative to subgiants/turnoff stars (> 6 sigma effect). The radial distributions of the red giants, blue stragglers, and subgiants are consistent with one another. Stars in the blue horizontal branch of M13, however, appear to be centrally depleted relative to other stellar types.
We combine data from three dense `King model clusters', M13, M3, and 47 Tuc, and two post core collapse clusters, M30 and M15, and compare the distributions of various stellar types as a function of (r/r_{half light}) and (r/r_core<). The horizontal branch stars in the combined sample appear to be centrally depleted relative to the giants (97% significance) - this depletion is only marginal (< about 1.5 sigma) in each of the clusters taken individually. The blue stragglers in the combined sample are centrally concentrated relative to the giants.