Guido De Marchi

The Resolved Stellar Population of the Poststarburst Galaxy NGC 1569

We present Hubble Space Telescope (HST) Wide Field Planetary Camera 2 (WFPC2) photometry of the resolved stellar population in the poststarburst galaxy NGC 1569. The color-magnitude diagram (CMD) derived in the F439W and F555W photometric bands contains ~2800 stars with a photometric error of ≤0.2 mag down to mF439, mF555 26 and is complete for mF555 23. Adopting the literature-distance modulus and reddening, our CMD samples stars more massive than ~4 M☉, allowing us to study the star formation (SF) history over the last ~0.15 Gyr. The data are interpreted using theoretical simulations based on stellar evolutionary models. The synthetic diagrams include photometric errors and incompleteness factors. Testing various sets of tracks, we find that the ability of the models to reproduce the observed features in the CMD is strictly related to the shape of the blue loops of the sequences with masses around 5 M☉. The field of NGC 1569 experienced a global SF burst of 0.1 Gyr duration, ending ~5-10 Myr ago. During the burst, the SF rate was approximately constant, and, if quiescent periods occurred, they lasted less than ~10 Myr. The level of the SF rate was very high; for a single-slope initial mass function (IMF) ranging from 0.1 to 120 M☉, we find values of 3, 1, and 0.5 M☉ yr-1 for α = 3, 2.6, and 2.35 (Salpeter), respectively. When scaled for the surveyed area, these rates are approximately 100 times larger than found in the most active dwarf irregulars in the Local Group. The data are consistent with a Salpeter IMF, though our best models indicate slightly steeper exponents. We discuss the implications of our results in the general context of the evolution of dwarf galaxies.

The Low End of the Initial Mass Function in Young Large Magellanic Cloud Clusters. I. The Case of R136

We report the result of a study in which we have used very deep broadband V and I Wide Field Planetary Camera 2 images of the R136 cluster in the Large Magellanic Cloud from the Hubble Space Telescope archive to sample the luminosity function below the detection limit of 2.8 M☉ previously reached. In these new deeper images, we detect stars down to a limiting magnitude of mF555W = 24.7 (1 mag deeper than previous works) and identify a population of red stars evenly distributed in the surrounding of the R136 cluster. A comparison of our color-magnitude diagram with recently computed evolutionary tracks indicates that these red objects are pre-main-sequence stars in the mass range 0.6-3 M☉. We construct the initial mass function (IMF) in the 1.35-6.5 M☉ range and find that, after correcting for incompleteness, the IMF shows a definite flattening below 2 M☉. We discuss the implications of this result for the R136 cluster and for our understanding of starburst galaxy formation and evolution in general.

The Structure of the Super-Star Clusters in NGC 1569 from Hubble Space Telescope WFPC2 Images

We present high-resolution observations of the super-star clusters in the nearby irregular galaxy NGC 1569, obtained with the WFPC2 on board the Hubble Space Telescope. Our analysis shows that the brightest of these clusters, NGC 1569A, results from the superposition of two clusters with an angular separation of ~02. Recent ground-based spectroscopy revealed that both red supergiant and Wolf-Rayet stars are associated with NGC 1569A. According to current evolutionary models, this finding requires the coexistence of two noncoeval stellar populations within the same cluster. Our discovery that NGC 1569A is double simplifies the interpretation, since red supergiants and Wolf-Rayet stars may belong to different clusters. We derive for the three clusters half-light radii of ~1.6-1.8 pc, smaller than indicated by previous HST observations. The size, mass, and luminosity of the brightest of these superclusters confirm that these objects could be young globulars, reinforcing previous conclusions.

VLT Observations of the Peculiar Globular Cluster NGC 6712. III. The Evolved Stellar Population

We present extensive UBVR photometry of the Galactic globular cluster (GGC) NGC 6712 obtained with the ESO Very Large Telescope (VLT) which reaches down to 2 mag below the main-sequence turn-off and allows us for the first time to determine the age of this cluster. By using the apparent luminosity of the zero age horizontal branch (ZAHB), VZAHB = 16.32 ± 0.05 and the stellar main-sequence (MS) turn-off (TO) magnitude VTO = 19.82 ± 0.10, we obtain ΔV = 3.5 ± 0.1 (a value fully compatible with that derived for other clusters), which suggests that, at an age of ~12 Gyr, NGC 6712 is coeval with other GGC of similar metallicity. We derive interstellar reddening by comparing the position and morphology of the red giant branch (RGB) with a wide variety of reference clusters and find E(B-V) = 0.33 ± 0.05, a value significantly lower than had been determined previously. Assuming this value for the reddening, we determine a true distance modulus of (m-M)0 = 14.55, corresponding to a distance of ~8 kpc. We find a population of 108 candidate blue straggler stars (BSSs), surprisingly large when compared with the typical BSS content of other low-concentration clusters. Moreover, we detect a very bright blue star in the core of NGC 6712 that might be a post-AGB star. These results, combined with those already presented in two companion papers, strongly support the hypothesis that NGC 6712 was, at some early epoch of its history, much more massive and concentrated. The continued interaction with the bulge and the disk of the Galaxy has driven it toward dissolution, and what we now observe is nothing but the remnant core of a cluster that once was probably one of the most massive in the Galaxy.

The Initial Mass Function of Low-Mass Stars in Globular Clusters*

We compare deep I-band luminosity functions of two globular clusters (NGC 6397 and NGC 6656) that have similar metal content but have experienced very different dynamical histories, both in terms of internal dynamics and as a result of different interactions with the Galactic tidal field. Their luminosity functions measured at the half-mass radius are indistinguishable from one another within the statistical errors over a range of more than 5 mag. They both show an increase with decreasing luminosity up to a peak at MI 8.5 and then drop all the way to the detection limit. We use the presently available theoretical mass-luminosity relations to convert these luminosity functions into mass functions and show that they all share one basic feature, namely, an exponential rise with decreasing mass down to about ~0.2 M☉ followed by a plateau. We conclude that an initial mass function that flattens out and possibly drops below ~0.2 M☉ is the only viable hypothesis for the low metallicity globular clusters studied so far.

On the Globular Cluster IMF Below 1 M

Accurate luminosity functions (LF) for a dozen globular clusters have now been measured at or just beyond their half-light radius using HST. They span almost the entire cluster main sequence (MS) below ti 0.75 Mo. Transformation of the LF into mass functions (MF) by means of the available mass luminosity (ML) relations that are consistent with all presently available data on the physical properties of low mass, low metallicity stars shows that all the LF observed so far can be obtained from MF having the shape of a log-normal distribution with characteristic mass m, = 0.33 + 0.03 Mo and standard deviation a = 0.34 + 0.04. After correction for the effects of mass segregation, the variation of the ratio of the number of higher to lower mass stars with cluster mass or any simple orbital parameter or the expected time to disruption shows no statistically significant trend over a range of this last parameter of more than a factor of ti 100. We conclude that the global MF of these clusters have not been measurably modified by evaporation and tidal interactions with the Galaxy and, thus, should reflect the initial distribution of stellar masses.