Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userA Search for Old Star Clusters in the Large Magellanic Cloud
170
0
0.0
(
0
)
Authors
Doug Geisler
Ask ChatGPT about the research
No Arabic abstract
We report the first results of a color-magnitude diagram survey of 25 candidate old LMC clusters. For almost all of the sample, it was possible to reach the turnoff region, and in many clusters we have several magnitudes of the main sequence. Age estimates based on the magnitude difference $delta T_1$ between the giant branch clump and the turnoff revealed that no new old clusters were found. The candidates turned out to be of intermediate age (1-3 Gyr) We show that the apparently old ages as inferred from integrated UBV colors can be explained by a combination of stochastic effects produced by bright stars and by photometric errors for faint clusters lying in crowded fields. The relatively metal poor candidates from the CaII triplet spectroscopy also turned out to be of intermediate age. This, combined with the fact that they lie far out in the disk, yields interesting constraints regarding the formation and evolution of the LMC disk. We also study the age distribution of intermediate age and old clusters This homogeneous set of accurate relative ages allows us to make an improved study of the history of cluster formation/destruction for ages $>1$Gyr. We confirm previous indications that there was apparently no cluster formation in the LMC during the period from 3-8 Gyr ago, and that there was a pronounced epoch of cluster formation beginning 3 Gyrs ago that peaked at about 1.5 Gyrs ago. Our results suggest that there are few, if any, genuine old clusters in the LMC left to be found.
rate research
Read More
We used resolved star counts from Hubble Space Telescope images to determine the center of gravity and the projected density profiles of 6 old globular clusters in the Large Magellanic Cloud (LMC), namely NGC 1466, NGC 1841, NGC 1898, NGC 2210, NGC 2257 and Hodge 11. For each system, the LMC field contribution was properly taken into account by making use, when needed, of parallel HST observations. The derived values of the center of gravity may differ by several arcseconds (corresponding to more than 1 pc at the distance of the LMC) from previous determinations. The cluster density profiles are all well fit by King models, with structural parameters that may differ from the literature ones by even factors of two. Similarly to what observed for Galactic globular clusters, the ratio between the effective and the core radii has been found to anti-correlate with the cluster dynamical age.
We present ESO/VLT FORS2 low resolution spectroscopy of red giant branch stars in three massive, intermediate age ($sim 1.7-2.3$ Gyr) star clusters in the Large Magellanic Cloud. We measure CH and CN index bands at 4300A, and 3883A, as well as [C/Fe] and [N/Fe] abundance ratios for 24, 21 and 12 member stars of NGC 1978, NGC 1651, NGC 1783, respectively. We find a significant intrinsic spread in CN in NGC 1978 and NGC 1651, a signal of multiple stellar populations (MPs) within the clusters. On the contrary, we report a null CN spread in NGC 1783 within our measurement precision. For NGC 1978, we separated the two populations in the CN distribution and we translated the CN spread into an internal N variation $Delta$[N/Fe]$=0.63pm0.49$ dex. For NGC 1651 and NGC 1783, we put upper limits on the N abundance variations of $Delta$[N/Fe]$leq 0.2, 0.4$ dex, respectively. The spectroscopic analysis confirms previous results from HST photometry, where NGC 1978 was found to host MPs in the form of N spreads, while slightly younger clusters (e.g. NGC 1783, $<$ 2 Gyr old) were not, within the limits of the uncertainties. It also confirms that intermediate age massive clusters show lower N abundance variations with respect to the ancient globular clusters, although this is in part due to the effect of the first dredge up at these stellar masses, as recently reported in the literature. We stress the importance of future studies to estimate the initial N abundance variations, free of stellar evolutionary mixing processes, by observing unevolved stars in young clusters.
The YMCA (Yes, Magellanic Clouds Again) and STEP ({The SMC in Time: Evolution of a Prototype interacting late-type dwarf galaxy) projects are deep g,i photometric surveys carried out with the VLT Survey Telescope (VST) and devoted to study the outskirts of the Magellanic System. A main goal of YMCA and STEP is to identify candidate stellar clusters and complete their census out to the outermost regions of the Magellanic Clouds. We adopted a specific over-density search technique coupled with a visual inspection of the color magnitude diagrams (CMDs) to select the best candidates and estimate their ages. To date, we analysed a region of 23 sq. deg. in the outskirts of the Large Magellanic Cloud, detecting 85 candidate cluster candidates, 16 of which have estimated ages falling in the so called age gap. We use these objects together with literature data to gain insight into the formation and interaction history of the Magellanic Clouds.
We present deep colour-magnitude diagrams for three rich intermediate-age star clusters in the Large Magellanic Cloud, constructed from archival ACS F435W and F814W imaging. All three clusters exhibit clear evidence for peculiar main-sequence turn-offs. NGC 1846 and 1806 each possess two distinct turn-off branches, while the turn-off for NGC 1783 shows a much larger spread in colour than can be explained by the photometric uncertainties. We demonstrate that although all three clusters contain significant populations of unresolved binary stars, these cannot be the underlying cause of the observed turn-off morphologies. The simplest explanation is that each cluster is composed of at least two different stellar populations with very similar metal abundances but ages separated by up to ~300 Myr. The origin of these unusual properties remains unidentified; however, the fact that at least three massive clusters containing multiple stellar populations are now known in the LMC suggests a potentially significant formation channel.
By using the multifiber spectrograph FLAMES mounted at the ESO-VLT, we have obtained high-resolution spectra for 18 giant stars, belonging to 3 old globular clusters of the Large Magellanic Cloud (namely NGC 1786, 2210 and 2257). While stars in each cluster showed quite homogeneous iron content, within a few cents of dex (the mean values being Fe/H]= -1.75+-0.01 dex, -1.65+-0.02 dex and -1.95+-0.02 dex for NGC 1786, 2210 and 2257, respectively), we have detected significant inhomogeneities for the [Na/Fe], [Al/Fe], [O/Fe] and [Mg/Fe] abundance ratios, with evidence of [O/Fe] vs [Na/Fe] and [Mg/Fe] vs [Al/Fe] anticorrelations. The trends detected nicely agree with those observed in Galactic Globular Clusters, suggesting that such abundance anomalies are ubiquitous features of old stellar systems and they do not depend on the parent galaxy environment. In NGC 1786 we also detected two extreme O-poor, Na-rich stars. This is the first time that a firm signature of extreme chemical abundance anomalies has been found in an extragalactic stellar cluster.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
