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Posts: 131433
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Messier 8
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Lagoon Nebula: Stormy seas in Sagittarius

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Nebulae are often named based on their key characteristics - particularly beautiful examples include the Ring Nebula, the Horsehead Nebula and the Butterfly Nebula. This new NASA/ESA Hubble Space Telescope image shows the centre of the Lagoon Nebula, otherwise known as Messier 8, in the constellation of Sagittarius (The Archer).
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Posts: 131433
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Herschel 36
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Title: The Hourglass as seen with HST/WFPC2
Author: J. Maíz Apellániz, L. Ubeda, R. H. Barbá, J. W. MacKenty, J. I. Arias, A. I. Gómez de Castro

We present a multi-filter HST/WFPC2 UV-optical study of the Hourglass region in M8. We have extracted the stellar photometry of the sources in the area and obtained the separations and position angles of the Herschel 36 multiple system: for Herschel 36 D we detect a possible orbital motion between 1995 and 2009. We have combined our data with archival IUE spectroscopy and measured the Herschel 36 extinction law, obtaining a different result from that of Cardelli et al. (1989) due to the improvement in the quality of the optical-NIR data, in agreement with the results of Ma\'iz Apell\'aniz et al. (2014). A large fraction of the UV flux around Herschel 36 arises from the Hourglass and not directly from the star itself. In the UV the Hourglass appears to act as a reflection nebula located behind Herschel 36 along the line of sight. Finally, we also detect three new Herbig-Haro objects and the possible anisotropic expansion of the Hourglass Nebula.

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Posts: 131433
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RE: Lagoon Nebula
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Title: Resolving the stellar components of the massive multiple system Herschel 36 with AMBER/VLTI
Author: J. Sanchez-Bermudez, A. Alberdi, R. Schödel, C. A. Hummel, J. I. Arias, R.H. Barbá, J. Maíz Apellániz, J.-U. Pott
(Version v2)

Context: Massive stars are extremely important for the evolution of the galaxies; there are large gaps in our understanding of their properties and formation, however, mainly because they evolve rapidly, are rare, and distant. It may well be that almost all massive stars are born as triples or higher multiples, but their large distances require very high angular resolution to directly detect the companions at milliarcsecond scales.
Aims: Herschel36 is a young massive system located at 1.3 kpc. It has a combined smallest predicted mass of 45 M_sun. Multi-epoch spectroscopic data suggest the existence of at least three gravitationally bound components. Two of them, system Ab, are tightly bound in a spectroscopic binary, and the third one, component Aa, orbits in a wider orbit. Our aim was to image and obtain astrometric and photometric measurements of components Aa and Ab using, for the first time, long-baseline optical interferometry to further constrain its nature.
Methods: We observed Herschel 36 with the near-infrared instrument AMBER attached to the ESO VLT Interferometer, which provides an angular resolution of approx. 2 mas. We used the code BSMEM to perform the interferometric image reconstruction. We fitted the interferometric observables using proprietary IDL routines and the code LitPro.
Results: We imaged the Aa+Ab components of Herschel 36 in H and K filters. Component Ab is located at a projected distance of 1.81 mas, at a position angle of approx. 222 deg. east of north, the flux ratio between components Aa and Ab is close to one. The small measured angular separation indicates that system Ab and Ab may be approaching the periastron of their orbits. These results, only achievable with long-baseline near-infrared interferometry, constitute the first step toward a thorough understanding of this massive triple system.

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Posts: 131433
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Herschel 36
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Title: Anomalous Diffuse Interstellar Bands in the Spectrum of Herschel 36. II. Analysis of Radiatively Excited CH+, CH, and DIBs
Authors: Takeshi Oka (1), Daniel E. Welty (1), Sean Johnson (1), Donald G. York (1), Julie Dahlstrom (2), L. M. Hobbs (1) ((1) University of Chicago, (2) Carthage College)

Absorption spectra toward Herschel 36 for the A¹Pi <-- X¹Sigma transitions of CH+ in the J=1 excited rotational level and the A²Delta <-- X²Pi transition of CH in the J=3/2 excited fine structure level have been analysed. These excited levels are above their ground levels by 40.1 K and ~25.7 K and indicate high radiative temperatures of the environment, 14.6 K and 6.7 K, respectively. The effect of the high radiative temperature is more spectacular in some diffuse interstellar bands (DIBs) observed toward Her 36; remarkable extended tails toward red (ETR) were observed. We interpret these ETRs as due to a small decrease of rotational constants upon excitation of excited electronic states. This, together with radiative pumping of high J rotational levels cause the ETRs. In order to study this effect quantitatively, we have developed a model calculation in which the effects of collision and radiation are treated simultaneously. The simplest case of linear molecules is considered. It has been found that the ETR is reproduced if the fraction of the variation of the rotational constant, beta = (B'-B)/B, is sufficiently high (3-5%) and the radiative temperature is high (T_r > 50 K). Although modelling for general molecules is beyond the scope of this paper, the results indicate that the prototypical DIBs at 5780.5, 5797.1, and 6613.6 A which show the pronounced ETRs are due to polar molecules sensitive to the radiative excitation. The requirement of high beta favours relatively small molecules with 3-6 heavy atoms. DIBs at 5849.8, 6196.0, and 6379.3 A which do not show the pronounced ETRs are likely due to non-polar molecules or large polar molecules with small beta.

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Posts: 131433
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NGC 6530
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Title: Spectral classification and HR diagram of pre-main sequence stars in NGC6530
Authors: L. Prisinzano (1), G. Micela (1), S. Sciortino (1), L. Affer (1), F. Damiani (1) ((1) INAF - Osservatorio Astronomico di Palermo, Palermo)

Mechanisms involved in the star formation process and in particular the duration of the different phases of the cloud contraction are not yet fully understood. Photometric data alone suggest that objects coexist in the young cluster NGC6530 with ages from ~1 Myr up to 10 Myrs. We want to derive accurate stellar parameters and, in particular, stellar ages to be able to constrain a possible age spread in the star-forming region NGC6530. We used low-resolution spectra taken with VIMOS@VLT and literature spectra of standard stars to derive spectral types of a subsample of 94 candidate members of this cluster. We assign spectral types to 86 of the 88 confirmed cluster members and derive individual reddenings. Our data are better fitted by the anomalous reddening law with R_{V}=5. We confirm the presence of strong differential reddening in this region. We derive fundamental stellar parameters, such as effective temperatures, photospheric colors, luminosities, masses, and ages for 78 members, while for the remaining 8 YSOs we cannot determine the interstellar absorption, since they are likely accretors, and their V-I colours are bluer than their intrinsic colours. The cluster members studied in this work have masses between 0.4 and 4 solar masses and ages between 1-2 Myrs and 6-7 Myrs. We find that the SE region is the most recent site of star formation, while the older YSOs are loosely clustered in the N and W regions. The presence of two distinct generations of YSOs with different spatial distribution allows us to conclude that in this region there is an age spread of ~6-7 Myrs. This is consistent with the scenario of sequential star formation suggested in literature.

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Posts: 131433
Date:
Messier 8
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Gemini Images a Psychedelic Stellar Nursery

An all-time favourite of skywatchers on both hemispheres, the Lagoon Nebula (Messier 8, or M8) is among the most striking examples of a stellar nursery in our neighbourhood of the Milky Way galaxy. Visible in small telescopes and binoculars its fuzzy glow reveals the type of chaotic environment where new stars are born.
Argentinean astronomers Julia Arias (Universidad de La Serena) and Rodolfo Barbá (Universidad de La Serena and ICATE-CONICET) have used the Gemini South telescope in Chile, to obtain a dramatic new image of the nursery that could be described as psychedelic. Actually, since M8 is located some 5,000 lightyears away, the multi-hued scene is truly a psychedelic "flashback" as its photons had to travel through space for that same number of years before they reached the gigantic Gemini 8-meter mirror. Astronomers sometimes call the region imaged the "Southern Cliff" because it resembles a sharp drop-off. Beyond the "cliff," light from a spattering of young background stars in the upper left of the image shines through the cloudscape.
Arias and Barbá obtained the imaging data to explore the evolutionary relationship between the newborn stars and what are known as Herbig-Haro (HH) objects. HH objects form when young stars eject large amounts of fast-moving gas as they grow. This gas ploughs into the surrounding nebula, producing bright shock fronts that glow as the gas is heated by friction and surrounding gas is excited by the high-energy radiation of nearby hot stars. The researchers found a dozen of these HH objects in the image, spanning sizes that range from a few thousand astronomical units (about a trillion kilometres) to 1.4 parsecs (4.6 light-years), i.e. a little greater than the distance from the Sun to its nearest neighbour Proxima Centauri.

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Posts: 131433
Date:
RE: Lagoon Nebula
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VISTA Stares Deeply into the Blue Lagoon

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This new infrared image of the Lagoon Nebula was captured as part of a five-year study of the Milky Way using ESO's VISTA telescope at the Paranal Observatory in Chile. This is a small piece of a much larger image of the region surrounding the nebula, which is, in turn, only one part of a huge survey.

Astronomers are currently using ESO's Visible and Infrared Survey Telescope for Astronomy (VISTA) to scour the Milky Way's central regions for variable objects and map its structure in greater detail than ever before. This huge survey is called VISTA Variables in the Via Lactea (VVV). The new infrared image presented here was taken as part of this survey. It shows the stellar nursery called the Lagoon Nebula (also known as Messier 8), which lies about 4000-5000 light-years away in the constellation of Sagittarius (the Archer).



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Posts: 131433
Date:
Messier 8
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A spectacular new NASA/ESA Hubble Space Telescope image reveals the heart of the Lagoon Nebula. Seen as a massive cloud of glowing dust and gas, bombarded by the energetic radiation of new stars, this placid name hides a dramatic reality.
The Advanced Camera for Surveys (ACS) on the NASA/ESA Hubble Space Telescope has captured a dramatic view of gas and dust sculpted by intense radiation from hot young stars deep in the heart of the Lagoon Nebula (Messier 8). This spectacular object is named after the wide, lagoon-shaped dust lane that crosses the glowing gas of the nebula.
This structure is prominent in wide-field images, but cannot be seen in this close-up. However the strange billowing shapes and sandy texture visible in this image make the Lagoon Nebula's watery name eerily appropriate from this viewpoint too.

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RE: Lagoon Nebula
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Posts: 131433
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twisters in space
 This Hubble Telescope snapshot unveils a pair of one-half, light-year-long interstellar "twisters" -- eerie funnels and twisted-rope structures (upper left) -- in the heart of the Lagoon Nebula (M8) which lies 5,000 light-years from Earth in the direction of the constellation Sagittarius.

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