* Astronomy

Title: Spitzer Infrared Spectrograph Observations of Magellanic Cloud Planetary Nebulae: the nature of dust in low metallicity circumstellar ejecta
Authors: L. Stanghellini, P. Garcia-Lario, D. A. Garcia-Hernandez, J. V. Perea-Calderon, J. E. Davies, A. Manchado, E. Villaver, R. A. Shaw

We present 5 - 40 micron spectroscopy of 41 planetary nebulae (PNe) in the Magellanic Clouds, observed with the Infrared Spectrograph on board the Spitzer Space Telescope. The spectra show the presence of a combination of nebular emission lines and solid-state features from dust, superimposed on the thermal IR continuum.
By analysing the 25 LMC and 16 SMC PNe in our sample we found that the IR spectra of 14 LMC and 4 SMC PNe are dominated by nebular emission lines, while the other spectra show solid-state features. We observed that the solid-state features are compatible with carbon-rich dust grains (SiC, polycyclic aromatic hydrocarbons (PAHs), etc.) in most cases, except in three PNe showing oxygen-rich dust features. The frequency of carbonaceous dust features is generally higher in LMC than in SMC PNe.
The spectral analysis allowed the correlations of the dust characteristics with the gas composition and morphology, and the properties of the central stars. We found that: 1) all PNe with carbonaceous dust features have C/O>1, none of these being bipolar or otherwise highly asymmetric; 2) all PNe with oxygen-rich dust features have C/O<1, with probable high mass progenitors if derived from single-star evolution (these PNe are either bipolar or highly asymmetric); 3) the dust temperature tracks the nebular and stellar evolution; and 4) the dust production efficiency depends on metallicity, with low metallicity environments not favouring dust production.

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Title: Evolution of the Small Magellanic Cloud
Authors: Kenji Bekki

Based on the results of N-body simulations on the last 2.5 Gyr evolution of the Large and Small Magellanic Clouds (LMC and SMC, respectively) interacting with the Galaxy, we firstly show when and where the leading arms (LAs) of the Magellanic stream (MS) can pass through the Galactic plane after the MS formation. We secondly show collisions between the outer Galactic HI disk and the LAs of the MS can create giant HI holes and chimney-like structures in the disk about 0.2 Gyr ago. We thirdly show that a large amount of metal-poor gas is stripped from the SMC and transferred to the LMC during the tidal interaction between the Clouds and the Galaxy about 0.2 and 1.3 Gyr ago. We thus propose that this metal-poor gas can closely be associated with the origin of LMC's young and intermediate-age stars and star clusters with distinctively low-metallicities with [Fe/H] < -0.6.

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Title: Three clusters of the SMC from ACS/WFC HST archive data: NGC 265, K~29 and NGC 290 and their fields
Authors: E. Chiosi, A. Vallenari (INAF, Osservatorio di Padova, Padova, Italy)

    We determine the age, metallicity and initial mass function of three clusters, namely NGC 265, K~29, NGC 290, located in the main body of the Small Magellanic Cloud. In addition, we derive the history of star formation in the companion fields. We make use of ACS/WFC HST archive data. For the clusters, the age and metallicity are derived fitting the integrated luminosity function with single synthetic stellar population by means of the \chi^2 minimisation. For the companion fields, the history of star formation is derived using the \chi^2 minimisation together with the downhill-simplex method. For the clusters we find the following ages and metallicities: NGC 265 has log(Age)=8.5±0.3 yr and metallicity 0.004±0.003(or [Fe/H]=-0.62); \object{K~29} has log(Age)=8.2±0.2 yr and metallicity Z=0.003±0.002 (or [Fe/H]=-0.75); NGC 290 has log(Age)=7.8±0.5 yr and metallicity 0.003±0.002(or [Fe/H]=-0.75). The superior quality of the data allows the study of the initial mass function down to M \sim 0.7 M_\odot. The initial mass function turns out to be in agreement with the standard Kroupa model. The comparison of the NGC 265 luminosity function with the theoretical ones from stellar models both taking overshoot from the convective core into account and neglecting it, seems to suggest that a certain amount of convective overshoot is required. The star formation rate of the field population presents periods of enhancements at 300-400 Myr, 3-4 Gyr and finally 6 Gyr. However it is relatively quiescent at ages older than 6 Gyr. This result suggests that at older ages, the tidal interaction between the Magellanic Clouds and the Milky Way was not able to trigger significant star formation events.

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Title: The Spitzer Survey of the Small Magellanic Cloud: FIR Emission and Cold Gas in the SMC
Authors: Adam Leroy, Alberto Bolatto, Snezana Stanimirovic, Norikazu Mizuno, Frank Israel, Caroline Bot

We present new far infrared maps of the Small Magellanic Cloud (SMC) at 24, 70, and 160 microns obtained as part of the Spitzer Survey of the Small Magellanic Cloud (S3MC,Bolatto et al. 2006). These maps cover most of the active star formation in the SMC Bar and the more quiescent Wing. We combine our maps with literature data to derive the dust surface density across the SMC. We find a total dust mass of Mdust = 3 10^5 Msun, implying a dust-to-hydrogen ratio over the region studied of log D/H = -2.86, or 1-to-700, which includes H_2. Assuming the dust to trace the total gas column, we derive H_2 surface densities across the SMC. We find a total H_2 mass M_H2 = 3.2 10^7 Msun in a distribution similar to that of the CO, but more extended. We compare profiles of CO and H_2 around six molecular peaks and find that on average H_2 is more extended than CO by a factor of ~ 1.3. The implied CO-to-H_2 conversion factor over the whole SMC is XCO = 13 ± 1 10^21 cm^-2 (K km/s)^-1. Over the volume occupied by CO we find a lower conversion factor, XCO = 6 ± 1 10^21 cm^-2 (K km/s)^-1, which is still a few times larger than that found using virial mass methods. The molecular peaks have H_2 surface densities \Sigma_H2 \approx 180 ± 30 Msun pc^-2, similar to those in Milky Way GMCs, and correspondingly low extinctions, A_V ~ 1 - 2 mag. To reconcile these measurements with predictions by the theory of photoionisation-regulated star formation, which requires A_V ~ 6, the GMCs must be ~ 3 times smaller than our 46 pc resolution element. We find that for a given hydrostatic gas pressure, the SMC has a 2 - 3 times lower ratio of molecular to atomic gas than spiral galaxies. Combined with the lower mean densities in the SMC this may explain why this galaxy has only 10% of its gas in the molecular phase.

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