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Posts: 131433
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NGC346-013
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Title: The VLT-FLAMES survey of massive stars: NGC346-013 as a test case for massive close binary evolution
Authors: B.W. Ritchie, V.E. Stroud, C.J. Evans, J.S. Clark, I. Hunter, D.J. Lennon, N. Langer, S.J. Smartt

NGC346-013 is a peculiar double-lined eclipsing binary in the Small Magellanic Cloud discovered by the VLT-FLAMES survey of massive stars. Spectra obtained with VLT-FLAMES are used to construct a radial velocity curve and photometry obtained with the Faulkes Telescope South is then used to derive orbital parameters, while spectra of the secondary are compared with synthetic spectra from TLUSTY model atmospheres. The orbital period is found to be 4.20381(12) days, with masses of 19.11.0 and 11.90.6 Msun. The primary is a rapidly rotating late-O dwarf while the secondary, an early-B giant, displays near-synchronous rotation and has filled its Roche lobe, implying that it was originally the more massive component with recent mass transfer 'spinning up' the primary to near-critical rotation. Comparison with synthetic spectra finds temperatures of 34.5kK and 24.5kK for the primary and secondary respectively, with the nitrogen abundance of the secondary enhanced compared to baseline values for the SMC, consistent with the predictions of models of interacting binaries. NGC346-013 likely evolved via non-conservative mass transfer in a system with initial masses ~22+15Msun, with the well-constrained orbital solution and atmospheric parameters making it an excellent candidate for tailored modelling with binary evolution codes. This system will form a cornerstone in constraining the physics of thermal timescale mass transfer, and the associated mass transfer efficiency, in massive close binary systems.

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Posts: 131433
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NGC 346
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Today ESO has released a dramatic new image of NGC 346, the brightest star-forming region in our neighbouring galaxy, the Small Magellanic Cloud, 210 000 light-years away towards the constellation of Tucana (the Toucan). The light, wind and heat given off by massive stars have dispersed the glowing gas within and around this star cluster, forming a surrounding wispy nebular structure that looks like a cobweb. NGC 346, like other beautiful astronomical scenes, is a work in progress, and changes as the aeons pass. As yet more stars form from loose matter in the area, they will ignite, scattering leftover dust and gas, carving out great ripples and altering the face of this lustrous object.
NGC 346 spans approximately 200 light-years, a region of space about fifty times the distance between the Sun and its nearest stellar neighbours. Astronomers classify NGC 346 as an open cluster of stars, indicating that this stellar brood all originated from the same collapsed cloud of matter. The associated nebula containing this clutch of bright stars is known as an emission nebula, meaning that gas within it has been heated up by stars until the gas emits its own light, just like the neon gas used in electric store signs.

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Title: NGC 346 in The Small Magellanic Cloud. IV. Triggered Star Formation in the HII Region N66
Authors: Dimitrios A. Gouliermis, You-Hua Chu, Thomas Henning, Wolfgang Brandner, Robert A. Gruendl, Eva Hennekemper, Felix Hormuth
(Version v4)

Stellar feedback, expanding HII regions, wind-blown bubbles, and supernovae are thought to be important triggering mechanisms of star formation. Stellar associations, being hosts of significant numbers of early-type stars, are the loci where these mechanisms act. In this part of our photometric study of the star-forming region NGC346/N66 in the Small Magellanic Cloud, we present evidence based on previous and recent detailed studies, that it hosts at least two different events of triggered star formation and we reveal the complexity of its recent star formation history. In our earlier studies of this region (Papers I, III) we find that besides the central part of N66, where the bright OB stellar content of the association NGC346 is concentrated, an arc-like nebular feature, north of the association, hosts recent star formation. This feature is characterized by a high concentration of emission-line stars and Young Stellar Objects, as well as embedded sources seen as IR-emission peaks that coincide with young compact clusters of low-mass pre-main sequence stars. All these objects indicate that the northern arc of N66 encompasses the most current star formation event in the region. We present evidence that this star formation is the product of a different mechanism than that in the general area of the association, and that it is triggered by a wind-driven expanding HII region (or bubble) blown by a massive supernova progenitor, and possibly other bright stars, a few Myr ago. We propose a scenario according to which this mechanism triggered star formation away from the bar of N66, while in the bar of N66 star formation is introduced by the photo-ionising OB stars of the association itself.

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RE: NGC346
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By combining multi-wavelength data of NGC 346, Dimitrios Gouliermis of the Max Planck Institute for Astronomy, says he and his team were able to pinpoint the trigger as a very massive star that blasted apart in a supernova explosion about 50,000 years ago. According to the astronomers, this very massive star spurred the isolated young stars into existence before it died, but through a different type of triggered star formation than that which occurred near the centre of the region. Fierce winds from the massive star, and not radiation, pushed dust and gas together, compressing it into new stars.

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Stellar work of art
HI-RES JPEG (Size:1121 kb)
spacer.gifCredits: NASA/JPL-Caltech/ESA/ XMM-Newton/NTT/MPIAspacer.gif

This portrait of a star-forming cloud, called NGC 346, is a combination of multiwavelength data from ESAs XMM-Newton space-borne X-ray observatory, NASA's Spitzer Space Telescope and the European Southern Observatory's New Technology Telescope.

NGC 346 is the brightest star-forming region in the Small Magellanic Cloud, an irregular dwarf galaxy that orbits our Milky Way galaxy,
210 000 light-years away.

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-- Edited by Blobrana at 21:10, 2008-10-08

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Born from the Wind - Unique Multi-wavelength Portrait of Star Birth
Telescopes on the ground and in space have teamed up to compose a colourful image that offers a fresh look at the history of the star-studded region NGC 346. This new, ethereal portrait, in which different wavelengths of light swirl together like watercolours, reveals new information about how stars form.

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Posts: 131433
Date:
LMC-N66
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Title: A new outburst in LMC-N66
Authors: Miriam Pena, M. Teresa Ruiz, Patricio Rojo, Silvia Torres-Peimbert, Wolf-Rainer Hamann

This is the first report on the new outburst presented by the central star of the LMC-N66 nebula. This object was classified as a planetary nebula, however, its true nature is under debate. In the period 1955-1990 the central star was almost undetectable and only nebular emission lines were observed. In 1990, the beginning of an outburst was detected and in few months it became much brighter and developed wide He and N lines, typical of a Wolf Rayet star of the N-sequence. The maximum occurred in 1994 and afterwards the star slowly faded. Analysis of its evolution showed that it has a variable mass-loss rate which occasionally increases enormously, creating a false photosphere at a much larger radius, making it appear a few magnitudes brighter. The present outburst has occurred 13 years after the episode from 1994 to 2000. So far this new event has similar characteristics although there are some significant differences in the spectral features. We present optical and FUSE spectra showing the main properties of this latter event.

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RE: NGC346
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Title: Triggered Star Formation in the Region NGC 346/N 66 in the Small Magellanic Cloud
Authors: Dimitrios A. Gouliermis, Thomas Henning, Eva Hennekemper, Wolfgang Brandner, Felix Hormuth

Stellar feedback and supernovas are the most important triggering mechanisms of star formation, and stellar associations, being hosts of significant numbers of early-type stars, are the loci where both mechanisms may act. We present evidence that this is the case of the association NGC 346, related to the nebula N~66, the brightest HII region in the Small Magellanic Cloud. We find that except of the central part of N~66, where the bright stellar OB content of the association is concentrated, an arc-like nebular feature, north of the association, includes the most recent star formation. This feature is characterized by a high concentration of emission stars and young stellar objects, as well as embedded sources seen as IR-emission peaks coinciding with young compact clusters of low-mass pre-main sequence stars, and therefore it encompasses the most current star formation in the region. We argue that star formation in the northern arc of N 66 was triggered by the rear side of the post-explosion shock of a core-collapse supernova about 2 Myr ago, and we propose a scenario according to which the supernova remnant in addition to the photo-ionising OB stars of the association shapes the current star formation history of NGC 346/N 66.

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Title: NGC 346 in the Small Magellanic Cloud. III. Recent Star Formation and Stellar Clustering Properties in the Bright HII Region N 66
Authors: Eva Hennekemper, Dimitrios A. Gouliermis, Thomas Henning, Wolfgang Brandner, Andrew E. Dolphin

In the third part of our photometric study of the star-forming region NGC 346/N~66 and its surrounding field in the Small Magellanic Cloud (SMC), we focus on the large number of low-mass pre-main sequence (PMS) stars revealed by the Hubble Space Telescope Observations with the Advanced Camera for Surveys. We investigate the origin of the observed broadening of the pre-main sequence population in the V-I, V CMD. The most likely explanations are either the presence of differential reddening or an age spread among the young stars. Assuming the latter, simulations indicate that we cannot exclude the possibility that stars in NGC 346 might have formed in two distinct events occurring about 10 and 5 Myr ago, respectively. We find that the PMS stars are not homogeneously distributed across NGC 346, but instead are grouped in at least five different clusters. On spatial scales from 0.8'' to 8'' (0.24 to 2.4 pc at the distance of the SMC) the clustering of the PMS stars as computed by a two-point angular correlation function is self-similar with a power law slope \gamma ~ -0.3. The clustering properties are quite similar to Milky Way star forming regions like Orion OB or
ho Oph. Thus molecular cloud fragmentation in the SMC seems to proceed on the same spatial scales as in the Milky Way. This is remarkable given the differences in metallicity and hence dust content between SMC and Milky Way star forming regions.

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Title: The Spitzer Survey of the Small Magellanic Cloud: Discovery of Embedded Protostars in the HII Region NGC 346
Authors: Joshua D. Simon (Caltech), Alberto D. Bolatto (UC Berkeley), Barbara A. Whitney (Space Science Institute), Thomas P. Robitaille (SUPA, St. Andrews), Ronak Y. Shah (Boston University), David Makovoz (Spitzer Science Center), Snezana Stanimirovic (Wisconsin), Rodolfo H. Barba (Universidad de La Serena), Monica Rubio (Universidad de Chile)

We use Spitzer Space Telescope observations from the Spitzer Survey of the Small Magellanic Cloud (S3MC) to study the young stellar content of N66, the largest and brightest HII region in the SMC. In addition to large numbers of normal stars, we detect a significant population of bright, red infrared sources that we identify as likely to be young stellar objects (YSOs). We use spectral energy distribution (SED) fits to classify objects as ordinary (main sequence or red giant) stars, asymptotic giant branch stars, background galaxies, and YSOs. This represents the first large-scale attempt at blind source classification based on Spitzer SEDs in another galaxy. We firmly identify at least 61 YSOs, with another 50 probable YSOs; only one embedded protostar in the SMC was reported in the literature prior to the S3MC. We present color selection criteria that can be used to identify a relatively clean sample of YSOs with IRAC photometry. Our fitted SEDs indicate that the infrared-bright YSOs in N66 have stellar masses ranging from 2 Msun to 17 Msun, and that approximately half of the objects are Stage II protostars, with the remaining YSOs roughly evenly divided between Stage I and Stage III sources. We find evidence for primordial mass segregation in the HII region, with the most massive YSOs being preferentially closer to the center than lower-mass objects. Despite the low metallicity and dust content of the SMC, the observable properties of the YSOs appear consistent with those in the Milky Way. Although the YSOs are heavily concentrated within the optically bright central region of N66, there is ongoing star formation throughout the complex and we place a lower limit on the star formation rate of 3.2 x 10^-3 Msun/yr over the last ~1 Myr.

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