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Post Info TOPIC: The Small Magellanic Cloud


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RE: The Small Magellanic Cloud
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Title: The Chandra Small Magellanic Cloud Wing Survey - the search for X-ray Binaries
Authors: K.E. McGowan (1), M.J. Coe (1), M.P.E. Schurch (1), V.A. McBride (1), J.L. Galache (2), W.R.T. Edge (1), R.H.D. Corbet (3), S. Lay**** (2), D.A.H. Buckley (4,5) ((1) University of Southampton, (2) CfA, (3) USRA/GSFC, (4) SAAO, (5) SALT)

We have detected 523 sources in a survey of the Small Magellanic Cloud (SMC) Wing with Chandra. By cross-correlating the X-ray data with optical and near-infrared catalogues we have found 300 matches. Using a technique that combines X-ray colours and X-ray to optical flux ratios we have been able to assign preliminary classifications to 265 of the objects. Our identifications include four pulsars, one high-mass X-ray binary (HMXB) candidate, 34 stars and 185 active galactic nuclei (AGNs). In addition, we have classified 32 sources as 'hard' AGNs which are likely absorbed by local gas and dust, and nine 'soft' AGNs whose nature is still unclear. Considering the abundance of HMXBs discovered so far in the Bar of the SMC the number that we have detected in the Wing is low.

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Magellanic Cloud Planetary Nebulae
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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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Posts: 128681
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NGC 346:KWBBe 200
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Title: Discovery of a New Dusty Be Star in the Small Magellanic Cloud
Authors: John P. Wisniewski (1), Karen S. Bjorkman (2), Jon E. Bjorkman (3), Mark Clampin (1) ((1) NASA GSFC, (2) University of Toledo)

We present new optical spectroscopic and archival Spitzer IRAC photometric observations of a B-type star in the SMC cluster NGC 346, NGC 346:KWBBe 200. We detect numerous Fe II, [O I], and [Fe II] lines, as well as strong P-Cygni profile H I emission lines in its optical spectrum. The star's near-IR colour and optical to IR SED clearly indicate the presence of an infrared excess, consistent with the presence of gas and warm, T ~800 K, circumstellar dust. Based on a crude estimate of the star's luminosity and the observed spectroscopic line profile morphologies, we find that the star is likely to be a B-type supergiant. We suggest that NGC 346:KWBBe 200 is a newly discovered B[e] supergiant star, and represents the fifth such object to be identified in the SMC.

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RE: The Small Magellanic Cloud
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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: Chemical evolution of the Small Magellanic Cloud based on planetary nebulae
Authors: T. P. Idiart, W. J. Maciel, R. D. D. Costa

We investigate the chemical evolution of the Small Magellanic Cloud (SMC) based on abundance data of planetary nebulae (PNe). The main goal is to investigate the time evolution of the oxygen abundance in this galaxy by deriving an age-metallicity relation. Such a relation is of fundamental importance as an observational constraint of chemical evolution models of the SMC. We have used high quality PNe data in order to derive the properties of the progenitor stars, so that the stellar ages could be estimated. We collected a large number of measured spectral fluxes for each nebula, and derived accurate physical parameters and nebular abundances. New spectral data for a sample of SMC PNe obtained between 1999 and 2002 are also presented. These data are used together with data available in the literature to improve the accuracy of the fluxes for each spectral line. We obtained accurate chemical abundances for PNe in the Small Magellanic Cloud, which can be useful as tools in the study of the chemical evolution of this galaxy and of Local Group galaxies. We present the resulting oxygen versus age diagram and a similar relation involving the [Fe/H] metallicity based on a correlation with stellar data. We discuss the implications of the derived age-metallicity relation for the SMC formation, in particular by suggesting a star formation burst in the last 2-3 Gyr.

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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)

ááá 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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N90
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A new image from the Hubble Space Telescope shows N90, one of the star-forming regions in the Small Magellanic Cloud. The rich populations of infant stars found here enable astronomers to examine star forming processes in an environment that is very different from that in our own Milky Way.
This new image taken with the Advanced Camera for Surveys onboard the NASA/ESA Hubble Space Telescope depicts bright blue newly formed stars that are blowing a cavity in the centre of a fascinating star-forming region known as N90.

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Posts: 128681
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RE: The Small Magellanic Cloud
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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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Title: The Spitzer Survey of the Small Magellanic Cloud: S3MC Imaging and Photometry in the Mid- and Far-Infrared Wavebands
Authors: Alberto D. Bolatto, Joshua D. Simon, Snezana Stanimirovic, Jacco Th. van Loon, Ronak Y. Shah, Kim Venn, Adam K. Leroy, Karin Sandstrom, James M. Jackson, Frank P. Israel, Aigen Li, Lister Staveley-Smith, Caroline Bot, Francois Boulanger, Monica Rubio

We present the initial results from the Spitzer Survey of the Small Magellanic Cloud (S3MC), which imaged the star-forming body of the Small Magellanic Cloud (SMC) in all seven MIPS and IRAC wavebands. We find that the F8/F24 ratio (an estimate of PAH abundance) has large spatial variations and takes a wide range of values that are unrelated to metallicity but anticorrelated with 24 um brightness and F24/F70 ratio. This suggests that photodestruction is primarily responsible for the low abundance of PAHs observed in star-forming low-metallicity galaxies. We use the S3MC images to compile a photometric catalogue of ~400,000 mid- and far-infrared point sources in the SMC. The sources detected at the longest wavelengths fall into four main categories: 1) bright 5.8 um sources with very faint optical counterparts and very red mid-infrared colours ({5.8}- {8.0}>1.2), which we identify as YSOs. 2) Bright mid-infrared sources with mildly red colours (0.16<{5.8}-{8.0}<0.6), identified as carbon stars. 3) Bright mid-infrared sources with neutral colours and bright optical counterparts, corresponding to oxygen-rich evolved stars. And, 4) unreddened early B stars (B3 to O9) with a large 24 Ám excess. This excess is reminiscent of debris disks, and is detected in only a small fraction of these stars (<5%). The majority of the brightest infrared point sources in the SMC fall into groups one to three. We use this photometric information to produce a catalogue of 282 bright YSOs in the SMC with a very low level of contamination (~7%).

s3mc1

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Posts: 128681
Date:
SNR 1E0102.2-7219
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Hubble Space Telescope has imaged a supernova remnant in the Small Magellanic Cloud.

The supernova remnant (SNR), known as "1E0102.2-7219", is the greenish-blue shell of debris just below the centre of the Hubble image. Its name is derived from its catalogued placement (or coordinates) in the celestial sphere. 1E0102.2-7219 is located almost 50 light-years away from the edge of the massive star-forming region, N 76, also known as Henize 1956 in the Small Magellanic Cloud. This structure, glowing a multitude of lavenders and peach hues, resides in the upper right of the image.

E0102
Expand (86kb, 800 x 641)
Position(2000): R.A. 01h 04m 1s.50 Dec. -72░ 01' 55".7
Credit NASA

Determined to be only about 2,000 years old, E0102 is relatively young on astronomical scales and is just beginning its interactions with the nearby interstellar medium. Young supernova remnants like E0102 allow astronomers to examine material from the cores of massive stars directly. This in turn gives insight on how stars form, their composition, and the chemical enrichment of the surrounding area. As well, young remnants are a great learning tool to better understand the physics of supernova explosions.
E0102 was observed in 2003 with the Hubble Advanced Camera for Surveys. Four filters that isolate light from blue, visible, and infrared wavelengths and hydrogen emission were combined with oxygen emission images of the SNR taken with the Wide Field Planetary Camera 2 in 1995.
The Small Magellanic Cloud is a nearby dwarf galaxy to our own Milky Way. It is visible in the Southern Hemisphere, in the direction of the constellation Tucana, and lies roughly 210,000 light-years distant.

-- Edited by Blobrana at 12:01, 2006-08-02

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