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TOPIC: Milky Way
Blobrana


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RE: Milky Way
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The Milky Way galaxy weighs about 1 trillion times as much as our sun, according to a new estimate.
Previous estimates had ranged from 750 billion solar masses to up to 2 trillion. Lately, researchers have been leaning toward the higher figure. But now astronomers have used a more refined method to conclude that our galaxy's mass is slightly less than 1 trillion solar masses.

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Blobrana


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Digel's Cloud 2
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Title: Star Formation in the Most Distant Molecular Cloud in the Extreme Outer Galaxy: A Laboratory of Star Formation in an Early Epoch of the Galaxy's Formation
Authors: Naoto Kobayashi (IoA, Univ. of Tokyo), Chikako Yasui (IoA, Univ. of Tokyo), Alan T. Tokunaga (IfA, Univ. of Hawaii), Masao Saito (ALMA, NAOJ)

We report the discovery of active star formation in Digel's Cloud 2, which is one of the most distant giant molecular clouds known in the extreme outer Galaxy (EOG). At the probable Galactic radius of ~20 kpc, Cloud 2 has a quite different environment from that in the solar neighbourhood, including lower metallicity, much lower gas density, and small or no perturbation from spiral arms. With new wide-field near-infrared (NIR) imaging that covers the entire Cloud 2, we discovered two young embedded star clusters located in the two dense cores of the cloud. Using our NIR and 12CO data as well as HI, radio continuum, and IRAS data in the archives, we discuss the detailed star formation processes in this unique environment. We show clear evidences of a sequential star formation triggered by the nearby huge supernova remnant, GSH 138-01-94. The two embedded clusters show a distinct morphology difference: the one in the northern molecular cloud core is a loose association with isolated-mode star formation, while the other in the southern molecular cloud core is a dense cluster with cluster-mode star formation. We propose that high compression by the combination of the SNR shell and an adjacent shell caused the dense cluster formation in the southern core. Along with the low metallicity range of the EOG, we suggest that EOG could be an excellent laboratory for the study of star formation processes, such as those triggered by supernovae, that occurred during an early epoch of the Galaxy's formation. In particular, the study of the EOG may shed light on the origin and role of the thick disk, whose metallicity range matches with that of the EOG well.

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Blobrana


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Posts: 131433
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RE: Milky Way
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Legions of tiny black holes created during the big bang may lurk at the centre of the galaxy, creating a prodigious antimatter factory, a new study suggests. The work could explain where the Milky Way's antimatter comes from one of the biggest mysteries in astrophysics.
Astronomers know the galactic centre is awash in antimatter because it generates a vast cloud of gamma rays when it collides with normal matter. But none of the proposed explanations for the antimatter's source which include supernovae, X-ray emitting binary stars and the decay of bizarre dark matter seem to precisely fit the bill.

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Blobrana


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antmatter
Integral mapped the glow of 511 keV gamma rays from electron-positron annihilation. The map shows the whole sky, with the galactic center in the middle. The emission extends to the right.
Credit: ESA/Integral/MPE/G. Weidenspointner

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Blobrana


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Posts: 131433
Date:
Extreme outer Galaxy
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Title: Extreme Outer Galaxy: A Laboratory of Star Formation in an Early Epoch of Galaxy Formation
Authors: Naoto Kobayashi, Chikako Yasui (IoA, Univ. of Tokyo), Alan T. Tokunaga (IfA, Univ. of Hawaii), Masao Saito (ALMA, NAOJ)

The extreme outer Galaxy (EOG) has a very different environment from that in the solar neighbourhood, with low metallicity (less than -0.5 dex), much lower gas density, and small or no perturbation from spiral arms. The EOG is an excellent laboratory for the study of the star formation processes that happened during the formation period of the Galaxy. In particular, the study of the EOG may shed light on the origin and role of the thick disk, whose metallicity range matches well with that of the EOG. We show an example of a molecular cloud in the EOG (Digel's Cloud 2), which is located at R_g ~ 20 kpc beyond the Outer arm. Based on our NIR and 12CO data as well as HI, radio continuum, and IRAS data in the archives, we examined the detailed star formation processes in this unique environment, especially the supernova triggered star formation, which should have been the major star formation mode during the halo and thick disk formation.

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Blobrana


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Posts: 131433
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RE: Milky Way
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Title: OGLE-II High Proper Motion Stars towards the Galactic centre
Authors: Nicholas J. Rattenbury, Shude Mao

The photometry data base of the second phase of the OGLE microlensing experiment, OGLE-II, is a rich source of information about the kinematics and structure of the Galaxy. In this work we use the OGLE-II proper motion catalogue to identify candidate stars which have high proper motions. 521 stars with proper motion mu > 50 mas/yr in the OGLE-II proper motion catalogue (Sumi 2004) were cross-identified with stars in the MACHO high proper motion catalogue, and the DENIS and 2MASS infra-red photometry catalogues. Photometric distances were computed for stars with colours consistent with G/K and M type stars. 6 stars were newly identified as possible nearby (< 50 pc) M dwarfs.

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Blobrana


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Title: Suzaku X-Ray Spectroscopy of a Peculiar Hot Star in the Galactic Centre Region
Authors: Yoshiaki Hyodo, Masahiro Tsujimoto, Katsuji Koyama, Shogo Nishiyama, Tetsuya Nagata, Itsuki Sakon, Hiroshi Murakami, Hironori Matsumoto

We present the results of a Suzaku study of a bright point-like source in the 6.7 keV intensity map of the Galactic centre region. We detected an intense FeXXV 6.7 keV line with an equivalent width of ~1 keV as well as emission lines of highly ionised Ar and Ca from a spectrum obtained by the X-ray Imaging Spectrometer. The overall spectrum is described very well by a heavily absorbed (~2x10^{23}cm^{-2}) thin thermal plasma model with a temperature of 3.8±0.6 keV and a luminosity of ~3x10^{34} erg s^{-1} (2.0--8.0 keV) at 8 kpc. The absorption, temperature, luminosity, and the 6.7 keV line intensity were confirmed with the archived XMM-Newton data. The source has a very red (J-Ks=8.2 mag) infrared spectral energy distribution (SED), which was fitted by a blackbody emission of ~1000 K attenuated by a visual extinction of ~31 mag. The high plasma temperature and the large X-ray luminosity are consistent with a wind-wind colliding Wolf-Rayet binary. The similarity of the SED to those of the eponymous Quintuplet cluster members suggests that the source is a WC-type source.

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Blobrana


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The Arches cluster
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A ragged company of stars is rushing through the chaotic core of our galaxy, travelling faster than can easily be explained. The new measurement of its path, made with the 10-metre Keck telescope in Hawaii, US, also deepens a mystery surrounding the Milky Way's central black hole.
The Arches cluster is a group of young stars only about 100 light years from the very centre of the galaxy, a dangerous region where strong gravitational fields can tear a star cluster apart.

"The Arches will only survive a few million years" - Andrea Stolte of the University of California in Los Angeles, US.

Stolte is a member of the team that has now tracked the cluster's trajectory. They took one image of the cluster in 2002 using the 8-metre Very Large Telescope (VLT) in northern Chile, and then another in 2006 using Keck.
They got a precise fix on the cluster using Keck's "laser guide star" system, in which a laser hits sodium atoms in the upper atmosphere to create an artificial star. By monitoring the guide star, the optics of the telescope can adapt in real time to disturbances in the atmosphere, which would otherwise blur the image.
The team found that after four years the cluster, which lies 25,000 light years away, had moved by seven millionths of a degree. That corresponds to a speed through space of more than 200 kilometres a second.
That is puzzlingly fast. The cluster is thought to have formed when two gas clouds in the inner galaxy collided but the most likely candidate clouds are travelling much more slowly than the Arches cluster.

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Position (J2000): RA 17h 46m 22s | Dec -28º 51' 36.4"

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Blobrana


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Posts: 131433
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RE: Milky Way
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Title: On the nature of the fast moving star S2 in the Galactic Centre
Authors: F. Martins (1), S. Gillessen (1), F. Eisenhauer (1), R. Genzel (1,2), T. Ott (1), S. Trippe (1) ((1) MPE Garching, (2) Berkeley)

We analyse the properties of the star S2 orbiting the supermassive black hole at the centre of the Galaxy. A high quality SINFONI H and K band spectrum obtained from coadding 23.5 hours of observation between 2004 and 2007 reveals that S2 is an early B dwarf (B0-2.5V). Using model atmospheres, we constrain its stellar and wind properties. We show that S2 is a genuine massive star, and not the core of a stripped giant star as sometimes speculated to resolve the problem of star formation so close to the supermassive black hole. We give an upper limit on its mass loss rate, and show that it is He enriched, possibly because of the presence of a magnetic field.

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Blobrana


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Posts: 131433
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Chemical evolution of the Galaxy
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Fossil magnetic fields protect helium 3 from dying stars

Les nébuleuses récalcitrantes - Quand les champs magnétiques fossiles protègent lhélium 3 des étoiles mourantes
Elles semblent préserver une substance que leurs consurs détruisent au cours de leur vie. Ce sont des nébuleuses planétaires rares, qui portent des noms comme NGC 3242 et J320. Grâce à leur champ magnétique fossile et contrairement à la majorité des étoiles évoluées de faible masse, elles ne détruisent pas lhélium 3 quelles ont produit dans leur jeunesse. Cette nouvelle hypothèse réconcilie lévolution stellaire avec lévolution chimique de la Galaxie dans les modèles quen donnent les spécialistes. Elle résulte dune recherche menée par une équipe dastrophysiciens de lUniversité de Genève (UNIGE), du Centre national de la recherche scientifique (CNRS) français et de lObservatoire de Paris.

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