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Register a new userObservational templates of star cluster disruption. The stellar group NGC 1901 in front of the Large Magellanic Cloud
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Added by
Giovanni Carraro dr
Publication date
2007
fields
Physics
and research's language is
English
Authors
Giovanni Carraro
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Observations indicate that present-day star formation in the Milky Way disk takes place in stellar ensembles or clusters rather than in isolation. Bound, long lived stellar groups are known as open clusters. They gradually lose stars and in their final evolutionary stages they are severely disrupted leaving an open cluster remnant made of a few stars. In this paper, we study in detail the stellar content and kinematics of the poorly populated star cluster NGC1901. This object appears projected against the Large Magellanic Cloud. The aim of the present work is to derive the current evolutionary status, binary fraction, age and mass of this stellar group. These are fundamental quantities to compare with those from N-body models in order to study the most general topic of star cluster evolution and dissolution.The analysis is performed using wide-field photometry in the UBVI pass-band, proper motions from the UCAC.2 catalog, and 3 epochs of high resolution spectroscopy, as well as results from extensive N-body calculations.The star group NGC1901 is found to be an ensemble of solar metallicity stars, 400+/-100 Myr old, with a core radius of 0.23 pc, a tidal radius of 1.0 pc, and located at 400+/-50 pc from the Sun. Out of 13 confirmed members, only 5 single stars have been found. Its estimated present-day binary fraction is at least 62%. The calculated heliocentric space motion of the cluster is not compatible with possible membership in the Hyades stream.Our results show that NGC1901 is a clear prototype of open cluster remnant characterized by a large value of the binary fraction and a significant depletion of low-mass stars. In the light of numerical simulations, this is compatible with NGC1901 being what remains of a larger system initially made of 500-750 stars.
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Whether or not the rich star cluster population in the Large Magellanic Cloud (LMC) is affected by significant disruption during the first few x 10^8 yr of its evolution is an open question and the subject of significant current debate. Here, we revisit the problem, adopting a homogeneous data set of broad-band imaging observations. We base our analysis mainly on two sets of self-consistently determined LMC cluster ages and masses, one using standard modelling and one which takes into account the effects of stochasticity in the clusters stellar mass functions. On their own, the results based on any of the three complementary analysis approaches applied here are merely indicative of the physical conditions governing the cluster population. However, the combination of our results from all three different diagnostics leaves little room for any conclusion other than that the optically selected LMC star cluster population exhibits no compelling evidence of significant disruption -- for clusters with masses, M_cl, of log(M_cl/M_sun) >= 3.0-3.5 -- between the age ranges of [3-10] Myr and [30-100] Myr, either infant mortality or otherwise. In fact, there is no evidence of any destruction beyond that expected from simple models just including stellar dynamics and stellar evolution for ages up to 1 Gyr. It seems, therefore, that the difference in environmental conditions in the Magellanic Clouds on the one hand and significantly more massive galaxies on the other may be the key to understanding the apparent variations in cluster disruption behaviour at early times.
Hubble Space Telescope V,I photometry of stars in the Large Magellanic Cloud Cluster NGC 1866 shows a well defined cluster main sequence down to V=25 mag, with little contamination from field or foreground stars. We use the main sequence fitting procedure to link the distance of NGC 1866 to the Hipparcos determination of the distance for the Hyades MS stars, making use of evolutionary prescriptions to allow for differences in the chemical composition. On this basis we find a true distance modulus for NGC 1866 of 18.35 +/- 0.05 mag. If the cluster is assumed to lie in the LMC plane then the LMC modulus is 0.02 mag less.
We use Monte-Carlo simulations, combined with homogeneously determined age and mass distributions based on multi-wavelength photometry, to constrain the cluster formation history and the rate of bound cluster disruption in the Large Magellanic Cloud (LMC) cluster system. We evolve synthetic star cluster systems formed with a power-law initial cluster mass function (ICMF) of spectral index $alpha =-2$ assuming different cluster disruption time-scales. For each of these disruption time-scales we derive the corresponding cluster formation rate (CFR) required to reproduce the observed cluster age distribution. We then compare, in a Poissonian $chi^2$ sense, model mass distributions and model two-dimensional distributions in log(mass) vs. log(age) space of the detected surviving clusters to the observations. Because of the bright detection limit ($M_V^{rm lim} simeq -4.7$ mag) above which the observed cluster sample is complete, one cannot constrain the characteristic disruption time-scale for a $10^4$ M$_odot$ cluster, $t_4^{rm dis}$ (where the disruption time-scale depends on cluster mass as $t_{rm dis} = t_4^{rm dis} (M_{rm cl} / 10^4 {rm M}_odot)^0.62$), to better than $t_4^{rm dis} ge 1$ Gyr. We conclude that the CFR has increased from 0.3 clusters Myr$^{-1}$ 5 Gyr ago, to a present rate of $(20-30)$ clusters Myr$^{-1}$. For older ages the derived CFR depends sensitively on our assumption of the underlying CMF shape. If we assume a universal Gaussian ICMF, then the CFR has increased steadily over a Hubble time from $sim 1$ cluster Gyr$^{-1}$ 15 Gyr ago to its present value. If the ICMF has always been a power law with a slope close to $alpha=-2$, the CFR exhibits a minimum some 5 Gyr ago, which we tentatively identify with the well-known age gap in the LMCs cluster age distribution.
The HST/ACS colour-magnitude diagrams (CMD) of the populous LMC star cluster NGC1751 present both a broad main sequence turn-off and a dual clump of red giants. We show that the latter feature is real and associate it to the first appearance of electron-degeneracy in the H-exhausted cores of the cluster stars. We then apply to the NGC1751 data the classical method of star formation history (SFH) recovery via CMD reconstruction, for different radii corresponding to the cluster centre, the cluster outskirts, and the underlying LMC field. The mean SFH derived from the LMC field is taken into account during the stage of SFH-recovery in the cluster regions, in a novel approach which is shown to significantly improve the quality of the SFH results. For the cluster centre, we find a best-fitting solution corresponding to prolonged star formation for a for a timespan of 460 Myr, instead of the two peaks separated by 200 Myr favoured by a previous work based on isochrone fitting. Remarkably, our global best-fitting solution provides an excellent fit to the data - with chi^2 and residuals close to the theoretical minimum - reproducing all the CMD features including the dual red clump. The results for a larger ring region around the centre indicate even longer star formation, but in this case the results are of lower quality, probably because of the differential extinction detected in the area. Therefore, the presence of age gradients in NGC1751 could not be probed. Together with our previous findings for the SMC cluster NGC419, the present results for the NGC1751 centre argue in favour of multiple star formation episodes (or continued star formation) being at the origin of the multiple main sequence turn-offs in Magellanic Cloud clusters with ages around 1.5 Gyr.
We present {it Hubble Space Telescope} {it V,I} photometry of the central region of the LMC cluster NGC 1866, reaching magnitudes as faint as V=27 mag. We find evidence that the cluster luminosity function shows a strong dependence on the distance from the cluster center, with a clear deficiency of low luminosity stars in the inner region. We discuss a {it global} cluster luminosity function as obtained from stars in all the investigated region, which appears in impressive agreement with the prediction from a Salpeter mass distribution. We also revisit the use of NGC 1866 as a probe for determining the efficiency of core overshooting, and conclude that a definitive answer to this question is not possible from this cluster.
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