Universe Today
Description:
"In NGC 6838 the giant stars are red and become bluer with decreasing brightness. In this respect, as well as in structure and richness of faint stars, NGC 6838 bears a striking resemblance to some of the globular clusters. The tendency of the color magnitude diagrams in the very rich galactic clusters to resemble the diagrams observed in the globular clusters emphasizes again the poorly defined boundaries between the two classes of objects."
"The X-ray light curve of the source coincident with this MSP shows marginal evidence for periodicity at the binary period of 4.2 hr. Its hard X-ray spectrum and luminosity resemble those of other eclipsing binary MSPs in 47 Tuc, suggesting a similar shock origin of the X-ray emission. A further 24 X-ray sources are found within the half-mass radius, reaching to a limiting luminosity of 1.5 × 10^30 ergs s/1 (0.3-8 keV). From a radial distribution analysis, we find that 18 ± 6 of these 29 sources are associated with M71, somewhat more than predicted, and that 11 ± 6 are background sources, both Galactic and extragalactic. M71 appears to have more X-ray sources in the range LX = 10^30–10^31 ergs s/1 than expected by extrapolating from other studied clusters using either mass or collision frequency. We explore the spectra and variability of these sources and describe the results of ground-based optical counterpart searches."
"This will allow us to use positional information about stars measured on multiple dithered exposures to improve the precision and depth of the photometry. The calibration to the standard Cousins VI system will be performed via comparisons with ground-based observations, most of which are already in hand. The ground-based data will also provide many more stars to populate the brighter portions of the clusters."
"From time-series CCD photometry, we find that QU Sge has an orbital period of 3.790818 days and a primary minimum depth of 1.333 mag. The eclipsing light curve solution shows that QU Sge has a semidetached binary configuration with the secondary component fully filling its Roche lobe. After subtracting the eclipses from the light curve, we discover an SX Phoenicis type pulsation feature. It is found to have a short period of about 0.03 days and a small amplitude of about 0.024 mag. This is the first eclipsing binary system in a globular cluster to exhibit a pulsating feature. This result supports the model in which the origin of some blue stragglers in globular clusters is mass transfer between two components in the primordial binary systems."
"The wind of an asymptotic giant branch star is sufficiently strong that if it is slightly asymmetric, it can propel the star outside the open cluster of its birth or significantly alter its trajectory through a globular cluster. Therefore, if these stellar winds are asymmetric, one would expect a deficit of white dwarfs of all ages in open clusters and for young white dwarfs to be less radially concentrated than either their progenitors or older white dwarfs in globular clusters. This latter effect has recently been observed. Hence detailed studies of the radial distribution of young white dwarfs in globular clusters could provide a unique probe of mass loss on the asymptotic giant branch and during the formation of planetary nebulae both as a function of metallicity and for a limited range of stellar mass."
History of Observation:
"71. 19h 43m 57s (295d 59' 06") +18d 13' 00" - Mechain: (296d 00' 04") +18d 14' 21" - Nebula discovered by M. Mechain on June 28, 1780, between the stars Gamma and Delta Sagittae. On October 4 following, M. Messier looked for it: its light is very faint and it contains no star; the least light makes it disappear. It is situated about 4 degrees below [south of] that which M. Messier discovered in Vulpecula. See No. 27. He reported it on the Chart of the Comet of 1779."
"A rich compressed Milky-Way cluster on the shaft of the arrow, and 10deg north-a quarter-east from Altair. It was discovered by Mechain in 1781, and described by Messier as a nebula unaccompanied by stars, and of a very feeble light. Piazzi seems to have observed it meridionally as a star of the 8th magnitude, by admitting the light of a lamp upon it, but his darkened field ought to have shown that it is flanked with four telescopic stars, besides other larger companions in view."