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Post Info TOPIC: Pismis 24-1


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NGC6357
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Sweeping the Dust from a Cosmic Lobster New infrared VISTA image of NGC 6357

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A new image from ESO's VISTA telescope captures a celestial landscape of glowing clouds of gas and tendrils of dust surrounding hot young stars. This infrared view reveals the stellar nursery known as NGC 6357 in a surprising new light. It was taken as part of a VISTA survey that is currently scanning the Milky Way in a bid to map our galaxys structure and explain how it formed.
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 VLT Takes a Close Look at NGC 6357

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ESO's Very Large Telescope (VLT) has taken the most detailed image so far of a spectacular part of the stellar nursery called NGC 6357. The view shows many hot young stars, glowing clouds of gas and weird dust formations sculpted by ultraviolet radiation and stellar winds.
Deep in the Milky Way in the constellation of Scorpius (The Scorpion) lies NGC 6357, a region of space where new stars are being born in of chaotic clouds of gas and dust. The outer parts of this vast nebula have now been imaged by ESO's Very Large Telescope, producing the best picture of this region taken so far

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Title: Star formation and disk properties in Pismis 24
Authors: M. Fang, R. van Boekel, R. R. King, Th. Henning, J. Bouwman, Y. Doi, Y. K. Okamoto, V. Roccatagliata, A. Sicilia-Aguilar

(abridged) We investigate the properties of young stars and their disks in the NGC 6357 complex, concentrating on the most massive star cluster within the complex: Pismis 24. We discover two new young clusters in the NGC 6357 complex. We give a revised distance estimate for Pismis 24 of 1.7±0.2 kpc. We find that the massive star Pis 24-18 is a binary system, with the secondary being the main X-ray source of the pair. We derive the cluster mass function and find that up to the completeness limit at low masses it agrees well with the IMF of the Trapezium cluster. We derive a median age of 1 Myr for the Pismis 24 cluster members. We find five proplyds in HST archival imaging of the cluster, four of which are newly found. In all cases the proplyd tails are pointing directly away from the massive star system Pis 24-1. One proplyd shows a second tail, pointing away from Pis 24-2, suggesting this object is being photoevaporated from two directions simultaneously. We find that the global disk frequency (~30%) in Pismis 24 is much lower than some other clusters of similar age, such as the Orion Nebula Cluster. When comparing the disk frequencies in 19 clusters/star-forming regions of various ages and different (massive) star content, we find that the disks in clusters harbouring extremely massive stars (typically earlier than O5), like Pismis 24, are dissipated roughly twice as quickly as in clusters/star-forming regions without extremely massive stars. Within Pismis 24, we find that the disk frequency within a projected distance of 0.6 pc from Pis 24-1 is substantially lower than at larger radii (~19% vs. ~37%). We argue for a combination of photoevaporation and irradiation with ionising UV photons from nearby massive stars, causing increased MRI-induced turbulence and associated accretion activity, to play an important role in the dissipation of low-mass star disks in Pismis 24.

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Pismis 24-1.kmz
Google Sky File

-- Edited by Blobrana at 16:06, 2008-01-12

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Title: Pismis 24-1: The Stellar Upper Mass Limit Preserved
Authors: J. Maíz Apellániz, Nolan R. Walborn, N. I. Morrell, V. S. Niemela, E. P. Nelan
(revised v2)

Is there a stellar upper mass limit? Recent statistical work seems to indicate that there is and that it is in the vicinity of 150 solar masses. In this paper we use HST and ground-based data to investigate the brightest members of the cluster Pismis 24, one of which (Pismis 24-1) was previously inferred to have a mass greater than 200 solar masses, in apparent disagreement with that limit. We determine that Pismis 24-1 is composed of at least three objects, the resolved Pismis 24-1SW and the unresolved spectroscopic binary Pismis 24-1NE. The evolutionary zero-age masses of Pismis 24-1SW, the unresolved system Pismis 24-1NE, and the nearby star Pismis 24-17 are all approximately 100 solar masses, very large but under the stellar upper mass limit.

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The small open star cluster Pismis 24 lies in the core of the large emission nebula NGC 6357 in Sagittarius, about 8,000 light-years away from Earth. Some of the stars in this cluster are extremely massive and emit intense ultraviolet radiation. The brightest object in the picture is designated Pismis 24-1.

Credit: NASA/JPL/Space Science Institute

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How heavy can a star be? This conundrum has haunted astronomers for decades. Theory indicates that there should be an upper stellar mass limit somewhere between 120 and 300 solar masses. Even though heavy stars are very bright, measurements of their masses can be complicated. The majority of the heaviest stars – some researchers say as many as 90% or maybe even more – tend to be born in multiple systems that can easily disguise themselves as single very heavy stars. Moreover, these stellar heavyweights are so rare that only a few are close enough to the Sun to be examined in detail.
One of the top candidates for the title of “Milky Way stellar heavyweight champion” was, until now, Pismis 24-1, a bright young star that lies in the core of the small open star cluster Pismis 24 (the bright stars in the Hubble image) about 8,000 light-years away from Earth. Pismis 24-1 was thought to have an incredibly large mass of 200 to 300 solar masses. This would have made it by far the most massive known star in the Milky Way. New NASA/ESA Hubble measurements of the star, have, however, resolved Pismis 24-1 into two separate stars, and, in doing so, have “halved" its mass to around 100 solar masses.
These impressive observations, performed by a team of astronomers led by Jesús Maíz Apellániz of the Instituto de Astrofísica de Andalucía in Spain, were obtained with the highest-resolution optical-imaging instrument aboard Hubble - the High Resolution Channel (HRC) of the Advanced Camera for Surveys.

NGC6357
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The star cluster Pismis 24 lies in the core of the large emission nebula NGC 6357 that extends one degree on the sky in the direction of the Sagittarius constellation. Part of the nebula is ionised by the youngest (bluest) heavy stars in Pismis 24. The intense ultraviolet radiation from the blazing stars heats the gas surrounding the cluster and creates a bubble in NGC 6357.
The team also managed to determine the mass of another massive star in Pismis 24: Pismis 24-17. Its mass is around 100 times that of our Sun and brings the total number of heavy stars within this cluster to at least three, which is a very rare occurrence for a cluster this small: In our Milky Way, for every star with 65 solar masses or more that is born, another 18,000 solar-mass stars are produced. Furthermore, since a 65 solar-mass star lives for only 3 million years while a solar mass star can live for more than 3,000 times that long, there are actually millions of solar-mass stars for each very massive star.
Pismis 24-1’s “weight loss” may continue in the future: ground-based observations indicate that Pismis 24-1 could even be a triple star system. Although each of the three stars would then only average 70 solar masses, they would still make it to the top twenty-five for “Milky Way heavyweights”, but only for a few million years as they would be sure to end their lives as supernovae and then turn into black holes.
Jesús Maíz Apellániz’s team plans to study the Pismis 24 cluster further and to look at other systems in the hope of establishing just how massive a star can be.
Massive stars can be the precursors of either black holes or neutron stars, formed in the supernovae that are the final spectacular flaring of a collapsing massive star and the main sources of the heavy elements in the Universe. By studying massive stars astronomers gain a deeper insight into our current understanding of black holes, supernova explosions and the chemical composition of the Universe.
The results of this study were presented at the Massive Stars Workshop in Argentina in December 2006.

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