Antonella Nota

Spitzer survey of the large magellanic cloud: Surveying the agents of a Galaxy's evolution (SAGE). I. Overview and initial results

We are performing a uniform and unbiased imaging survey of the Large Magellanic Cloud (LMC; ~7° × 7°) using the IRAC (3.6, 4.5, 5.8, and 8 μm) and MIPS (24, 70, and 160 μm) instruments on board the Spitzer Space Telescope in the Surveying the Agents of a Galaxys Evolution (SAGE) survey, these agents being the interstellar medium (ISM) and stars in the LMC. This paper provides an overview of the SAGE Legacy project, including observing strategy, data processing, and initial results. Three key science goals determined the coverage and depth of the survey. The detection of diffuse ISM with column densities >1.2 × 10^(21) H cm^(-2) permits detailed studies of dust processes in the ISM. SAGEs point-source sensitivity enables a complete census of newly formed stars with masses >3 M_☉ that will determine the current star formation rate in the LMC. SAGEs detection of evolved stars with mass-loss rates >1 × 10^(-8) M_☉ yr^(-1) will quantify the rate at which evolved stars inject mass into the ISM of the LMC. The observing strategy includes two epochs in 2005, separated by 3 months, that both mitigate instrumental artifacts and constrain source variability. The SAGE data are nonproprietary. The data processing includes IRAC and MIPS pipelines and a database for mining the point-source catalogs, which will be released to the community in support of Spitzer proposal cycles 4 and 5. We present initial results on the epoch 1 data for a region near N79 and N83. The MIPS 70 and 160 μm images of the diffuse dust emission of the N79/N83 region reveal a similar distribution to the gas emissions, especially the H I 21 cm emission. The measured point-source sensitivity for the epoch 1 data is consistent with expectations for the survey. The point-source counts are highest for the IRAC 3.6 μm band and decrease dramatically toward longer wavelengths, consistent with the fact that stars dominate the point-source catalogs and the dusty objects detected at the longer wavelengths are rare in comparison. The SAGE epoch 1 point-source catalog has ~4 × 10^6 sources, and more are anticipated when the epoch 1 and 2 data are combined. Using Milky Way (MW) templates as a guide, we adopt a simplified point-source classification to identify three candidate groups—stars without dust, dusty evolved stars, and young stellar objects—that offer a starting point for this work. We outline a strategy for identifying foreground MW stars, which may comprise as much as 18% of the source list, and background galaxies, which may comprise ~12% of the source list.

Age Determination of Six Intermediate-Age Small Magellanic Cloud Star Clusters with Hst/acs

We present a photometric analysis of the star clusters Lindsay 1, Kron 3, NGC 339, NGC 416, Lindsay 38, and NGC 419 in the Small Magellanic Cloud (SMC), observed with the Hubble Space Telescope Advanced Camera for Surveys (ACS) in the F555W and F814W filters. Our color-magnitude diagrams (CMDs) extend ~3.5 mag deeper than the main-sequence turnoff points, deeper than any previous data. Cluster ages were derived using three different isochrone models: Padova, Teramo, and Dartmouth, which are all available in the ACS photometric system. Fitting observed ridgelines for each cluster, we provide a homogeneous and unique set of low-metallicity, single-age fiducial isochrones. The cluster CMDs are best approximated by the Dartmouth isochrones for all clusters, except for NGC 419 where the Padova isochrones provided the best fit. Using Dartmouth isochrones we derive ages of 7.5 ± 0.5 Gyr (Lindsay 1), 6.5 ± 0.5 Gyr (Kron 3), 6 ± 0.5 Gyr (NGC 339), 6 ± 0.5 Gyr (NGC 416), and 6.5 ± 0.5 Gyr (Lindsay 38). The CMD of NGC 419 shows several main-sequence turnoffs, which belong to the cluster and to the SMC field. We thus derive an age range of 1.2-1.6 Gyr for NGC 419. We confirm that the SMC contains several intermediate-age populous star clusters with ages unlike those of the Large Magellanic Cloud and the Milky Way. Interestingly, our intermediate-age star clusters have a metallicity spread of ~0.6 dex, which demonstrates that the SMC does not have a smooth, monotonic age-metallicity relation. We find an indication for centrally-concentrated blue straggler star candidates in NGC 416, while these are not present for the other clusters. Using the red clump magnitudes, we find that the closest cluster, NGC 419 (~50 kpc), and the farthest cluster, Lindsay 38 (~67 kpc), have a relative distance of ~17 kpc, which confirms the large depth of the SMC. The three oldest SMC clusters (NGC 121, Lindsay 1, and Kron 3) lie in the northwestern part of the SMC, while the youngest (NGC 419) is located near the SMC main body.

Spitzer survey of the Large Magellanic Cloud, Surveying the Agents of a Galaxy's Evolution (SAGE) IV: dust properties in the interstellar medium

The goal of this paper is to present the results of a preliminary analysis of the extended infrared (IR) emission by dust in the interstellar medium (ISM) of the Large Magellanic Cloud (LMC). We combine Spitzer Surveying the Agents of Galaxy Evolution (SAGE) and Infrared Astronomical Satellite (IRAS) data and correlate the infrared emission with gas tracers of H I, CO, and Hα. We present a global analysis of the infrared emission as well as detailed modeling of the spectral energy distribution (SED) of a few selected regions. Extended emission by dust associated with the neutral, molecular, and diffuse ionized phases of the ISM is detected at all IR bands from 3.6 μm to 160 μm. The relative abundance of the various dust species appears quite similar to that in the Milky Way (MW) in all the regions we have modeled. We construct maps of the temperature of large dust grains. The temperature map shows variations in the range 12.1-34.7 K, with a systematic gradient from the inner to outer regions, tracing the general distribution of massive stars and individual H II regions as well as showing warmer dust in the stellar bar. This map is used to derive the far-infrared (FIR) optical depth of large dust grains. We find two main departures in the LMC with respect to expectations based on the MW: (1) excess mid-infrared (MIR) emission near 70 μm, referred to as the 70 μm excess, and (2) departures from linear correlation between the FIR optical depth and the gas column density, which we refer to as FIR excess emission. The 70 μm excess increases gradually from the MW to the LMC to the Small Magellanic Cloud (SMC), suggesting evolution with decreasing metallicity. The excess is associated with the neutral and diffuse ionized gas, with the strongest excess region located in a loop structure next to 30 Dor. We show that the 70 μm excess can be explained by a modification of the size distribution of very small grains with respect to that in the MW, and a corresponding mass increase of ≃13% of the total dust mass in selected regions. The most likely explanation is that the 70 μm excess is due to the production of large very small grains (VSG) through erosion of larger grains in the diffuse medium. This FIR excess could be due to intrinsic variations of the dust/gas ratio, which would then vary from 4.6 to 2.3 times lower than the MW values across the LMC, but X_(CO) values derived from the IR emission would then be about three times lower than those derived from the Virial analysis of the CO data. We also investigate the possibility that the FIR excess is associated with an additional gas component undetected in the available gas tracers. Assuming a constant dust abundance in all ISM phases, the additional gas component would have twice the known H I mass. We show that it is plausible that the FIR excess is due to cold atomic gas that is optically thick in the 21 cm line, while the contribution by a pure H_2 phase with no CO emission remains a possible explanation.

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.

Chemical Composition and Origin of Nebulae around Luminous Blue Variables

We use the analysis of the heavy element abundances (C, N, O, S) in circumstellar nebulae around luminous blue variables to infer the evolutionary phase in which the material has been ejected. We concentrate on four aspects. (1) We discuss the diUerent eUects that may have changed the gas composition of the nebula since it was ejected: mixing with the swept up gas from the wind-blown bubble, mixing with the gas from the faster wind of the central star, and depletion by CO and dust. (2) We calculate the expected abundance changes at the stellar surface due to envelope convection in the red supergiant phase. We show that this depends strongly on the total amount of mass that was lost prior to the onset of the envelope convection. If the observed LBV nebulae are ejected during the red supergiant phase, the abundances of the LBV nebulae require a signi—cantly smaller amount of mass to be lost than assumed in the evolutionary calculations of Meynet et al. (3) We calculate the changes in the surface composition during the main-sequence phase by rotation-induced mixing. If the nebulae are ejected at the end of the main-sequence phase, the abundances in LBV nebulae are compatible with mixing times between 5 ] 106 and 1 ] 107 yr. These values are reasonable, considering the high rotational velocities of mainsequence O-stars. The existence of ON stars supports this scenario. (4) The predicted He/H ratio in the nebulae, derived from the observed N/O ratios, are signi—cantly smaller than the current observed photospheric values of their central stars. This indicates that either (1) the nebula was ejected from a star that had an abundance gradient in its envelope or (2) that fast mixing on a timescale of 104 yr must have occurred in the stars immediately after the nebula was ejected. Combining various arguments, we show that the LBV nebulae are ejected during the blue supergiants phase and that the stars have not gone through a red supergiant phase. The chemical enhancements are due to rotation-induced mixing, and the ejection is possibly triggered by near-critical rotation. During the ejection, the out—ow was optically thick, which resulted in a large eUective radius and a low eUective temperature. This explains why the observed properties of the dust around LBVs closely resemble the properties of dust formed around red supergiants.

Past and present star formation in the SMC: NGC 346 and its neighborhood

In the quest to understand how star formation occurs and propagates in the low-metallicity environment of the Small Magellanic Cloud (SMC), we acquired deep F555W (~V) and F814W (~I) Hubble Space Telescope ACS images of the young and massive star-forming region NGC 346. These images and their photometric analysis provide us with a snapshot of the star formation history of the region. We find evidence for star formation extending from ?10 Gyr in the past until ?150?Myr in the field of the SMC. The youngest stellar population (~3 ? 1?Myr) is associated with the NGC 346 cluster. It includes a rich component of low-mass pre-main-sequence stars mainly concentrated in a number of subclusters spatially colocated with CO clumps previously detected by Rubio and coworkers. Within our analysis uncertainties, these subclusters appear coeval with each other. The most massive stars appear concentrated in the central subclusters, indicating possible mass segregation. A number of embedded clusters are also observed. This finding, combined with the overall wealth of dust and gas, could imply that star formation is still active. An intermediate-age star cluster, BS 90, formed ~4.3 ? 0.1?Gyr ago, is also present in the region. Thus, this region of the SMC has supported star formation with varying levels of intensity over much of the cosmic time.

The stellar mass distribution in the giant star forming region NGC 346

Deep F555W and F814W Hubble Space Telescope (HST ) Advanced Camera for Survey (ACS) images are the basis for a study of the present-day mass function (PDMF) of NGC 346, the largest active star-forming region in the Small Magellanic Cloud (SMC). We find a PDMF slope of Γ = −1.43 ± 0.18 in the mass range 0.8-60 M☉, in excellent agreement with the Salpeter initial mass function (IMF) in the solar neighborhood. Caveats on the conversion of the PDMF to the IMF are discussed. The PDMF slope changes, as a function of the radial distance from the center of the NGC 346 star cluster, indicating a segregation of the most massive stars. This segregation is likely primordial considering the young age (~ 3 Myr) of NGC 346, and its clumpy structure which suggests that the cluster has likely not had sufficient time to relax. Comparing our results for NGC 346 with those derived for other star clusters in the SMC and the Milky Way (MW), we conclude that, while the star formation process might depend on the local cloud conditions, the IMF does not seem to be affected by general environmental effects such as galaxy type, metallicity, and dust content.

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.

O Stars in Transition. II. Fundamental Properties and Evolutionary Status of Ofpe/WN9 Stars from HST Ultraviolet Observations*

We present new HST/FOS ultraviolet spectroscopic observations of seven LMC Ofpe/WN9 stars. We find that Ofpe/WN9 stars have slow winds with terminal velocities of about 400 km s-1 and high mass-loss rates of the order of 2-5 × 10-5 M☉ yr-1. Ofpe/WN9 stellar temperatures and radii are in the range 30,000-39,000 K, and 19-39 R☉, respectively. Stellar luminosities are between log (L/L☉) = 5.6 and 6.3. We study the Ofpe/WN9 stars winds and examine their evolutionary status. We find that Ofpe/WN9 stars are intermediate between O and W-R stars in terms of the wind momentum flux. We also find that the stellar properties and wind momentum of the Ofpe/WN9 sample place them in the evolutionary sequence: O → Of → H-rich WNL → Ofpe/WN9, for initial stellar masses less than ~100 M☉. In view of persisting discrepancies of standard massive star models with observations, we compute massive main-sequence models according to three different evolutionary scenarios. We find that both higher mass-loss rate and enhanced mixing between core and envelope are required in order to yield models compatible with the derived stellar and wind properties of Ofpe/WN9 stars. The emerging picture may be consistent with earlier evidence of Ofpe/WN9 stars being quiescent luminous blue variables (LBVs). This idea is further strengthened by the highly reduced surface H mass fractions of the Ofpe/WN9 stars. We derive Xs = 0.5 to 0.3, which still excludes Ofpe/WN9 stars from being core He-burning objects, but is almost identical to the Xs values recently measured in LBVs.