Intermediate-age Star Clusters Research Articles

The Rotational Rates of Blue Stragglers Produced by Physical Stellar Collisions

Blue stragglers produced from physical stellar collisions are expected to be born rotating rapidly, while the blue stragglers observed in intermediate-age and old star clusters do not. We suggest that the collision and merger of two main-sequence stars results in an object that quickly swells into a pre-main-sequence-like star with a convective envelope, which can spin down via magnetic braking as it approaches its thermal equilibrium configuration. The result is a blue straggler with a rotation similar to that of a normal star of the same mass and age. Consequently, rapid rotation is not a signature of a collisionally merged blue straggler.

The star formation history of the Large Magellanic Cloud

view Abstract Citations (146) References (52) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Star Formation History of the Large Magellanic Cloud Bertelli, Gianpaolo ; Mateo, Mario ; Chiosi, Cesare ; Bressan, Alessandro Abstract We present deep photometric observations of stars in three fields of the Large Magellanic Cloud (LMC) and interpret these data using synthetic color-magnitude diagrams (CMDs) and luminosity functions (LFs) generated from the overshoot models of Bertelli et al., Bressan et al., and Aparicio et al. We can successfully model the field CMDs and LFs with a star formation rate that experienced a large increase (4 +/- 0.5) x 10^9^ yr ago. The precise age of this "burst" depends sensitively on the characteristics of the models. Classical (i.e., nonovershoot) models yield a burst age about 2 x 10^9^ yr younger than the value we obtain. An initial mass function with slope of 2.35 (the Salpeter value) and a mean field star metallicity of [Fe/H] ~ -0.7 are consistent with the photometric data and LFs. Furthermore, our primary conclusion, that is, the possibility of a single burst of star formation, depends on the distance modulus adopted for LMC. If (m - M)_0_ = 18.4, we find evidence for a single burst throughout the LMC. If (m - M)_0_ = 18.6, this is no longer possible. Given the differences between the ages derived using overshoot and classical models, the age distribution we derive for LMC field stars older than about 1 x 10^9^ yr is similar to that found by earlier studies using deep photographic observations of other fields throughout the LMC, and with the observed age distribution of intermediate-age and old LMC star clusters. This suggests that the star formation rate in the LMC was globally quite low during at least the first half of its lifetime, and that a major event triggered a substantial and relatively sudden increase in the star formation rate throughout the entire LMC which persisted for several 10^9^ yr and even up to the present epoch in some parts of that galaxy. Publication: The Astrophysical Journal Pub Date: April 1992 DOI: 10.1086/171163 Bibcode: 1992ApJ...388..400B Keywords: Astronomical Photometry; Color-Magnitude Diagram; Magellanic Clouds; Star Formation Rate; Stellar Evolution; Stellar Luminosity; Metallicity; Star Clusters; Stellar Models; Astrophysics; GALAXIES: MAGELLANIC CLOUDS; GALAXIES: STELLAR CONTENT; STARS: EVOLUTION; STARS: FORMATION full text sources ADS | data products SIMBAD (4) NED (1)

Similarities of the intermediate-age SMC star cluster NGC 152 with inner disk clusters in the Milky Way

A colour-magnitude diagram (CMD) of the region containing the intermediate-age SMC globular cluster NGC 152 was published recently (Melcher & Richtler 1989). A particularly interesting feature of this CMD is the “clump” of He-core burning stars, which are predominantly field stars. A selection of stars near the cluster centre leads to the CMD shown in Figure 1. The vertical extension of the clump (explainable by the evolution of stars younger than 1 Gyr) is replaced by a “tilted horizontal branch” (we use this expression for lack of a better one). The age of NGC 152 is about 1.3 Gyr and the reddening is small; the metallicity is unknown but less than −0.6 dex, which is the mean metallicity of the young SMC population. The tilted HB can be reproduced in CMD simulations using the method developed by Vallenari et al. (1990), and thus can be considered as a normal feature of star clusters like NGC 152. It is evident also in other intermediate-age MC clusters like Kron 3 (Rich et al. 1984).

The Nuclei of Nucleated Dwarf Elliptical Galaxies - Are they Globular Clusters?

It came as a great surprise that many dwarf elliptical galaxies of very low surface brightness in the Virgo Cluster have conspicuous bright star-like nuclei (Reaves 1983, Binggeli, Sandage and Tammann 1985). These nuclei are at least a factor of 10 more luminous than the brightest globular clusters in the Local Group and comparable only to the very brightest globulars surrounding M87. They contain a considerable fraction (1 to 20%) of the total light of the parent galaxy (Binggeli, priv. commun.). Their physical nature and origin are a matter of debate (Zinnecker et al. 1985, van den Bergh 1985, Norman 1986, Zinnecker 1986) but optical spectroscopy for 3 objects indicates a stellar composition with a range similar to globular clusters (Bothun et al. 1985). It has been suggested that a central nucleus is formed when off-center bound star clusters migrate to the center as a consequence of dynamical friction (Norman 1986). Support for such a scenario comes from CCD observations of IC 3475 which reveal numerous knots near the center of this dwarf irregular galaxy (Vigroux et al. 1986). These knots have the same color as the parent galaxy and are interpreted as intermediate age star clusters.

The evolution of red clump stars - Theoretical sequences

Two grids of horizontal-branch evolutionary sequences are presented which extend the grid of Sweigart and Gross (1976) to higher masses and higher metallicities. These two grids were constructed specifically for the study of red clump stars observed in intermediate-age star clusters in the Magellanic Clouds and in the Galaxy. An additional evolutionary track, with a thick hydrogen-burning shell which simulates the effect of a modest amount of mixing from the helium core into the shell at the time of the helium-core flash, is also presented. It is found that a small amount of mixing does not alter in a significant way the standard course of horizontal-branch evolution.

Intermediate-age core helium burning stars and the distance to the Magellanic Clouds

view Abstract Citations (29) References (26) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Intermediate-Age Core Helium-burning Stars and the Distance to the Magellanic Clouds Seidel, E. ; Da Costa, G. S. ; Demarque, Pierre Abstract The luminosities of the core helium burning stars in six Magellanic Cloud intermediate-age star clusters, whose ages have recently been determined from photometry reaching below the main-sequence turnoff, are compared with the predictions of a new grid of theoretical models. It is found that for the so-called 'short' distance moduli consistent results can be achieved with moderate amounts of mass loss from the main sequence to the core helium burning stage for the older clusters, and with rather more substantial mass loss for the younger objects. For the 'long' moduli, however, the results of the comparison give inconsistent results in that the required masses for the clump stars generally exceed the corresponding turnoff masses, in some cases by several tenths of a solar mass. The results support the adoption of smaller distances to these galaxies. Publication: The Astrophysical Journal Pub Date: February 1987 DOI: 10.1086/164961 Bibcode: 1987ApJ...313..192S Keywords: Distance; Gravitational Collapse; Helium; Magellanic Clouds; Star Clusters; Stellar Evolution; Horizontal Branch Stars; Nuclear Fusion; Stellar Cores; Stellar Mass Ejection; Astrophysics; CLUSTERS: GLOBULAR; GALAXIES: MAGELLANIC CLOUDS; STARS: EVOLUTION; STARS: HORIZONTAL-BRANCH full text sources ADS | data products NED (8) SIMBAD (6)

Ages of intermediate-age Magellanic Cloud star clusters

Ages of intermediate-age Large Magellanic Cloud star clusters have been estimated without locating the faint, unevolved portion of cluster main sequences. Six clusters with established color-magnitude diagrams were selected for study: SL 868, NGC 1783, NGC 1868, NGC 2121, NGC 2209, and NGC 2231. Since red giant photometry is more accurate than the necessarily fainter main-sequence photometry, the distributions of red giants on the cluster color-magnitude diagrams were compared to a grid of 33 stellar evolutionary tracks, evolved from the main sequence through core-helium exhaustion, spanning the expected mass and metallicity range for Magellanic Cloud cluster red giants. The time-dependent behavior of the luminosity of the model red giants was used to estimate cluster ages from the observed cluster red giant luminosities. Except for the possibility of SL 868 being an old globular cluster, all clusters studied were found to have ages less than 10 to the 9th yr. It is concluded that there is currently no substantial evidence for a major cluster population of large, populous clusters greater than 10 to the 9th yr old in the Large Magellanic Cloud.

Intermediate-Age Magellanic Cloud Star Clusters

Ages of six intermediate-age Large Magellanic Cloud star clusters have been estimated using the time dependent behavior of the luminosity of stellar interior models of red giants. All clusters studied, NGC 1783, NGC 1868, NGC 1978, NGC 2121, NGC 2209, and NGC 2231, were found to have ages < 109 yr. It is concluded that there is currently no substantial evidence for a major cluster population of large, populous clusters > 109 yr in the Large Magellanic Cloud.

Intermediate mass stellar models: Ages of Magellanic Cloud star clusters

Ages of six intermediate-age Large Magellanic Cloud star clusters have been estimated using the time dependent behavior of the luminosity of stellar interior models of red giants. All clusters studied, NGC 1783, NGC 1868, NGC 1978, NGC 2121, NGC 2209, and NGC 2231, were found to have ages < 109 yr. It is concluded that there is currently no substantial evidence for a major cluster population of large, populous clusters > 109 yr in the Large Magellanic Cloud.

Anomalous giants in Magellanic Cloud clusters

Many of the populous intermediate-age star clusters (theblue globulars) of the Magellanic Clouds have a few stars that occupy a “forbidden zone” in the colour-magnitude diagrams of the clusters, where stellar evolutionary theory does not predict that stars should lie. The properties of these stars in 13 clusters are gathered in Table 1, where it can be seen that the absolute magnitudes of the stars correlate with the cluster age, lying at a mean value of 2.7 mag above the tip of the main sequence. Several hypotheses regarding their nature are discussed, and it is concluded tentatively that they represent an unpredicted advanced stage of evolution from the giant branch, though no theoretical evidence supports such a conclusion. Considerably more observational data are needed before a more positive statement can be made.