"Our Milky Way Galaxy (I)"

Milchstraße (ii)

(iii)

(iv)

(v)

Zentrum (I)

(II)

(III)

(IV)

Distance GC

HVC — 21 cm surveys

Spiralstruktur

MWG-info

Literatur

Umgebung

Lokale Gruppe

SST: GLIMPSE

Collision

LBV

SDSS: Masse der Galaxis M_vir ~ 1.1 × 10¹²M
Zentrum der Milchstraße — :
Sgr A* multi-frequency (IV GC MWG)
Black hole in the centre of the Milky Way
IRS 13, Zweites Schwarzes Loch im Zentrum
The Central 200 Parsecs (Gemini / SST / H.E.S.S. / Swift)
Gal. Cen & The Arches Cluster
GC radio, Inner Galaxy , Molecular Ring
Sag A*: BH im Zentrum der MWG
Intrinsic Size of Sagittarius A
Das Herz der Milchstraße läuft heiß
1,001 nights at the telescopes Tomography with SDSS
Heaviest stars / star formation — :
LBV and the nature of Type IIn supernovae
Hellster Stern: Gigant LBV 1806-20 & Pistolenstern
Massengrenzen (Heaviest Stars in the Galaxy )
Eta Carinae in NGC 3372
LL Ori
Lynx Arc (z = 3.36) mega-cluster of stars
30 Dor (LMC) | Tarantula Nebel
Fusion der Elemente in Sternen — :
Life Products of Stars
Die Entstehung der Elemente im Kosmos
Explosive Nukleosynthese in Sternen
Thermonuclear Bursts in X-ray Binaries From progenitor to afterlife

Hyper-Velocity Stars — :
(I) (II)
The Dwarf Satellites — MWG origin — Mass:
Dark Matter Halo Mass
The Canis Major galaxy
The Future Fate
Accretion origin / Assembly History
Galactic globular clusters (GC) & — Open cluster (OC):
OC & GC
Super Sternhaufen
GC
Spin of galaxies - Oriantation
Origin of the Local Group of Galaxies
NGC6397: new theory on the formation of stars
NGC6397: Altersbestimmung mit Beryllium
Hochenergetische Gammastrahlung — :
H.E.S.S. (MIV)
Röntgenstrahlen-Archäologie
Galactic Archaeology
ESA's new view of the Milky Way - in gamma rays!
Gamma- und TeV-Licht
Dunkle Materie (e+e- annihilation)
OGLE: Opt. Grav. Lensing Experiment
Pictures — MWG:
Sky Maps
[1] The Milky Way in Stars and Dust

The Milky Way

Artikel zu "The Milky Way" SST: GLIMPSE (MIV) SSC: Super Star Cluster
Massengrenzen (Arches, Quintuplet) Sgr A* & Arches cluster Nebula NGC 6357 The Arches Cluster (X-ray) NGC 3603 Star Clusters and ImBHs in the Galactic Bulge K1 The Milky Way — Picture and Explanation K2 Spiral arm of Milky Way looms closer than thought K2.1 The Spiral Structure of the Milky Way K3.1 Arches K3.2 The proper motion of the Arches cluster K4 35 New Supernova Remnants	Galactic Center Radio Arc The Monoceros tidal stream Kugelsternhaufen Alter der Kugelsternhaufen Element Abundances — : Deuterium — Versteckspiel in der Milchstraße more D OB Associations K5 Two Young Star Disks in the Central Parsec of the Galaxy K6 Most Stars Are Single K7 Map of the Galaxy in the 6.7 keV emission line Literatur

K1 The Milky Way — Picture and Explanation

PDF

Young Massive Clusters in the Gal. Center: Arches, Quintuplet & Center (D. Figer)

Most massive star (D. Figer - Nature)

Arches (suppl) |

Mass limit (NV)

[1] The Milky Way in Stars and Dust

(2005 October 4) — Credit & Copyright: Serge Brunier
The disk of our Milky Way Galaxy is home to hot nebulae, cold dust, and billions of stars. This disk can be seen from a dark location on Earth as a band of diffuse light across the sky. This band crosses the sky in dramatic fashion in the above series of wide angle sky exposures from Chile. The deepness of the exposures also brings to light a vast network of complex dust filaments. Dust is so plentiful that it obscures our Galaxy's center in visible light, hiding its true direction until discovered by other means early last century.
The Galactic Center, though, is visible above as the thickest part of the disk. The diffuse glow comes from billions of older, fainter stars like our Sun, which are typically much older than the dust or any of the nebulae.
One particularly photogenic area of darkness is the Pipe Nebula visible above the Galactic Center.
Dark dust is not the dark matter than dominates our Galaxy -- that dark matter remains in a form yet unknown.

[2] Our Galaxy in Stars, Gas, and Dust

(2003 September 28) — Credit & Copyright: John P. Gleason, Steve Mandel
The disk of our Milky Way Galaxy is home to hot nebulae, cold dust, and billions of stars. The red nebulae visible in the above contrast-enhanced picture are primarily emission nebulae, glowing clouds of hydrogen gas heated by nearby, bright, young stars. The blue nebulae are primarily reflection nebulae, clouds of gas and fine dust reflecting the light of nearby bright stars. Perhaps the most striking, though, are the areas of darkness, including the Pipe Nebula visible on the image top left. These are lanes of thick dust, many times containing relatively cold molecular clouds of gas. Dust is so plentiful that it obscures the Galactic Center in visible light, hiding its true direction until discovered early last century. The diffuse glow comes from billions of older, fainter stars like our Sun, which are typically much older than any of the nebulae. Most of the mass of our Galaxy remains in a form currently unknown.

[3] The Pipe Dark Nebula

(June 21, 1997) — Credit and Copyright: Jerry Lodriguss
The dark nebula predominant at the lower left of the above photograph is known as the Pipe Nebula. The dark clouds, suggestively shaped like smoke rising from a pipe, are caused by absorption of background starlight by dust. These dust clouds can be traced all the way to the Rho Ophiuchi nebular clouds on the right. The brightest star in the field is Antares. Many types of nebula are highlighted here: the red are emission nebula, the blue are reflection nebula, and the dark are absorption nebula. This picture has been digitally enhanced.

K2 Spiral arm of Milky Way looms closer than thought

W3OH in the Perseus spiral arm — d = 1.95 ± 0.04
	Authors: Y. Xu, X. W. Zheng, Mark Reid, K.M. Menten
	Journal-ref: Science 311 (2006) 54 [astro-ph/0512223 ]
	Title: The Distance to the Perseus Spiral Arm in the Milky Way
Abstract: We have measured the distance to the massive star-forming region W3OH in the Perseus spiral arm of the Milky Way to be d = 1.95 ± 0.04 kilo-parsecs (5.86 × 10²¹ cm). This distance was determined by triangulation, with the Earth's orbit as one segment of a triangle, using the Very Long Baseline Array. This resolves a long-standing problem of a factor of two discrepancy between different techniques to determine distances. The reason for the discrepancy is that this portion of the Perseus arm has anomalous motions. The orientation of the anomalous motion agrees with spiral density-wave theory, but the magnitude is somewhat larger than most models predict.

One of the Milky Way's star-studded spiral arms lies twice as close to Earth as some previous estimates suggested. New research has produced the most accurate distance measurement ever made of the arm, which could help astronomers understand how our galaxy's spiral structure formed.
The Milky Way appears to be made up of four main arms that curve around its centre like a pinwheel. "However, our view from the interior makes it difficult to determine its spiral structure," writes a team led by Ye Xu.
Measuring the distance to the spiral arms can be particularly tricky. This is because astronomers can only measure the speed of an astronomical object in terms of how fast it is moving towards or away from the Earth. Comparing this speed to theoretical models, which assume the objects travel on circular paths around the centre of the galaxy, allows astronomers to deduce the object's distance from Earth.
Astronomers using this technique had previously estimated the distance to Perseus, the arm immediately beyond the Sun, at more than 13,000 light years. But other researchers arrived at half that distance using a method that compares the apparent brightness of massive, young stars with estimates of their intrinsic brightness.

VLBA / Xu et al.

W3OH inside the Perseus arm lies closer than thought

Now Xu's team has used a third technique - 100 times more accurate than the other two - to conclude the Perseus arm is indeed relatively close, at just 6400 light years from Earth.
They used a system of 10 radio dishes that boasts the sharpest vision of any telescope in existence. Called the Very Long Baseline Array (VLBA), the dishes - each spanning 25 metres - are scattered from Hawaii to the Caribbean Sea.
They focused on a star-forming region called W3OH inside the Perseus arm. Bright, young stars in the region heat methanol vapour in gas clouds around them, which in turn emits radio waves in what are called "masers".
The team tracked the masers at five intervals over the course of a year, determining their distance by "triangulating" their observed positions from different points along Earth's orbit.
"We used our changing vantage point to form one leg of a triangle," says team member Mark Reid. "Then, measuring the change in angle of the source as the Earth orbits the Sun, we can calculate the source's distance by simple trigonometry."
They found that W3OH is not moving in a perfectly circular orbit but instead follows an elliptical path, as if drawn along the Perseus spiral arm. "It seems to be indicating that the spiral arms may have a higher density than previously guessed," Reid told New Scientist.
The team will now use the VLBA to measure the distances to a dozen star-forming regions spread across several of the Milky Way's spiral arms. "We hope to use such data to better understand how spiral arms form," says Reid.

K2.1 The Spiral Structure of the Milky Way

—
	Authors: Levine, E. S.; Blitz, Leo; Heiles, Carl
	Journal-ref: Science 312 (2006) 1773-1777 [astro-ph/0605728 ]
	Title: The Spiral Structure of the Outer Milky Way in Hydrogen
Abstract: We produce a detailed map of the perturbed surface density of neutral hydrogen in the outer Milky Way disk, demonstrating that the Galaxy is a non-axisymmetric multiarmed spiral. Spiral structure in the southern half of the Galaxy can be traced out to at least 25 kiloparsec, implying a minimum radius for the gas disk. Overdensities in the surface density are coincident with regions of reduced gas thickness. The ratio of the surface density to the local median surface density is relatively constant along an arm. Logarithmic spirals can be fit to the arms with pitch angles of 20° to 25°.


Image credit: Science/Levine Abb. 1: Mit der modifizierten unscharfen Maskierung erzeugte Karte der Wasserstoffverteilung in der Milchstraße. Im Bild ist ein logarithmischer Fit an die vier größten Spiralarme eingetragen. Abb. 2: Im Bild zeigen die Linien das Modell einer vierarmigen, symmetrischen Spirale - das deutlich von den Beobachtungen abweicht.		Milchstraße neu kartiert — The Spiral Structure of the Outer Milky Way in Hydrogen Amerikanische Forscher präsentieren die bislang detailreichste Karte der Gasverteilung in der Milchstraße. Unsere Milchstraße ist eine Spiralgalaxie - aber ihre genaue Struktur, die Anzahl und die Länge ihrer Spiralarme sind nur schwer zu bestimmen. Denn im Gegensatz zu anderen Galaxien können die Astronomen die Milchstraße nicht von außen betrachten, sondern befinden sich selbst mitten in der zu untersuchenden Struktur. Amerikanische Forscher präsentieren nun in der Online-Ausgabe des Fachmagazins Science die bislang detailreichste Karte der Gasverteilung in der Milchstraße. „Wir konnten zeigen, dass die Milchstraße zwar eine mehrarmige, aber nicht achsensymmetrische Spiralgalaxie ist“, fassen die Autoren Evan Levine, Leo Blitz und Carl Heiles von der University of California in Berkeley ihr Ergebnis zusammen. Die Spiralarme sind also ungleichmäßig um das Zentrum der Milchstraße herum angeordnet. Im Gegensatz zu früheren Untersuchungen konnte das Team die Spiralarme bis zu einer Entfernung von 80.000 Lichtjahren vom galaktischen Zentrum hinaus verfolgen. Milchstraße hat fünf Arme A Magellanic Origin for the Warp of the Galaxy LMC + Galaxy coordinate system Galactic Plane Survey's Maps
Da die Beobachter selbst in der Scheibe der Galaxis sitzen, lässt sich die Struktur der Milchtrasse nicht mit optischen Methoden erfassen: Schon in wenigen tausend Lichtjahren wird die Absorption durch Staub in der Scheibenebene zu stark. Einen Ausweg bietet die Radioastronomie, denn die 21-Zentimeter-Linie des neutralen Wasserstoffs wird vom Staub nicht absorbiert. Die Intensität dieser Spektrallinie ist dabei in etwa proportional zur Dichte des Wasserstoffgases, liefert also einen guten Eindruck der Wasserstoffverteilung in der Milchstraße. Da die Milchstraße nicht starr rotiert liefert die Dopplerverschiebung der 21-Zentimeter-Linie zusammen mit einem Rotationsmodell die Position der jeweiligen Strahlungsquelle. So lässt sich aus dem genauen Profil der 21-Zentimete-Linie die Verteilung des Wasserstoffgases in der Milchstraße rekonstruieren. Die Spiralarme erscheinen dabei als Regionen, in denen die Dichte des Wasserstoffs im Mittel höher ist als in der Umgebung. Das Problem: Die Dichte des Wasserstoffs nimmt in der Milchstrasse sehr stark von innen nach außen ab. Gegen diese starke Abnahme ließen sich die schwächeren Kontraste zwischen den Spiralarmen und ihrer Umgebung bislang nur schwer sichtbar machen. Levine, Blitz und Heiles haben nun ein aus der optischen Astronomie entlehntes Verfahren in leicht modifizierter Weise auf die 21-Zentimeter-Daten angewendet: Die so genannte unscharfe Maskierung. Dabei wird eine absichtlich verschmierte Kopie des Bildes von dem Original abgezogen. Auf diese Weise wird der großräumige Trend beseitigt und kleinräumige Strukturen hervorgehoben. Die drei Astronomen haben dazu ein Gitter über die Galaxis gelegt und in jedem Gitterelement den Median der Dichte bestimmt. Während üblicherweise dann dieser Median von den ursprünglichen Daten subtrahiert wird, haben Levine und seine Kollegen die Originaldaten durch den lokalen Median geteilt. „Dadurch konnten wir die radiale Variation der Dichte um mehr als eine Größenordung kompensieren“, so die Forscher. Die resultierende dimensionslose Größe ist dann ein direktes Maß für die lokalen Abweichungen der Wasserstoffdichte von der Umgebung. In der sich so ergebenden Karte zeigen sich zahlreiche Spiralarme, wobei allerdings auf der einen Seite vom galaktischen Zentrum deutlich mehr Spiralarme zu erkennen sind. Im Gegensatz zu vielen großen Spiralgalaxien ist die Milchstraße also nicht achsensymmetrisch. Dank der unscharfen Maskierung können Levine und seine Kollegen die Spiralarme erstmals bis zu einer Entfernung von 80.000 Lichtjahren vom Zentrum der Milchstraße hinaus verfolgen. Die globale Form der Spiralarme, so zeigen die Wissenschaftler, ist logarithmisch, ähnelt also der Spiralstruktur eines tropischen Wirbelsturms.

K3 Arches

Hubble Weighs in on the Heaviest Stars in the Galaxy [ March 9, 2005 ]
References - PDF "An upper limit to the masses of stars" D. Figer - Nature 434 (2005) 192 - 194 Arches (Bild suppl) Stellar mass limited Pavel Kroupa - Nature 434 (2005) 148 - 149	Young Massive Clusters in the Gal. Center: Arches, Quintuplet & Center (D. Figer) Massive Stars in the Arches Cluster [7/8/2002] RSGC1 — A Behemoth Star Cluster

simulation — Arches
	Authors: Simon Portegies Zwart, Evghenii Gaburov, Hui-Chen Chen, M. Atakan Gürkan
	Journal-ref: MNRAS 378 (2007) L29 [astro-ph/0702693 ]
	Title: The present day mass function in the central region of the Arches cluster
Abstract: We study the evolution of the mass function in young and dense star clusters by means of direct N-body simulations. Our main aim is to explain the recent observations of the relatively flat mass function observed near the centre of the Arches star cluster. In this region, the power law index of the mass function for stars more massive than about 5-6 M, is larger than the Salpeter value by about unity; whereas further out, and for the lower mass stars, the mass function resembles the Salpeter distribution. We show that the peculiarities in the Arches mass function can be explained satisfactorily without primordial mass segregation. We draw two conclusions from our simulations: 1) The Arches initial mass function is consistent with a Salpeter slope down to ~1 M 2) The cluster is about half way towards core collapse. The cores of other star clusters with characteristics similar to those of the Arches are expected to show similar flattening in the mass functions for the high mass (>5 M) stars.

—
	Authors: F. Martins, D.J. Hillier, T. Paumard, F. Eisenhauer, T. Ott, R. Genzel
	Journal-ref: (2007) [0711.0657 ]
	Title: The most massive stars in the Arches cluster
Abstract: • Aims. We study a sample composed of 28 of the brightest stars in the Arches cluster. Our aim is to constrain their stellar and wind properties and to establish their nature and evolutionary status. • Methods. We analyze K-band spectra obtained with the integral field spectrograph SINFONI on the VLT. Atmosphere models computed with the code CMFGEN are used to derive the effective temperatures, luminosities, stellar abundances, mass loss rates and wind terminal velocities. • Results. We find that the stars in our sample are either H-rich WN7–9 stars (WN7–9h) or supergiants, two being classified as OIf+. All stars are 2–4 Myr old. There is marginal evidence for a younger age among the most massive stars. The WN7–9h stars reach luminosities as large as 2 × 10⁶L, consistent with initial masses of ~ 120 M. They are still quite H-rich, but show both N enhancement and C depletion. They are thus identified as core H-burning objects showing products of the CNO equilibrium at their surface. Their progenitors are most likely supergiants of spectral types earlier than O4–6 and initial masses > 60 M. Their winds follow a well defined modified wind momentum – luminosity relation (WLR): this is a strong indication that they are radiatively driven. Stellar abundances tend to favor a slightly super solar metallicity, at least for the lightest metals. We note however that the evolutionary models seem to under-predict the degree of N enrichment. 1. Introduction The center of our Galaxy is a unique environment to study massive stars. It harbors three massive clusters – the Arches, Quintuplet and central cluster – which together contain about 30% of the number of Wolf-Rayet stars known in the Galaxy (van der Hucht 2006). Interestingly, the three clusters have different ages, ranging from ~ 2 Myr for the Arches to ~ 6 Myr for the central cluster. Consequently, they host different populations of massive stars and sample the entire upper part of the HR diagram. Studying their stellar content gives us a unique opportunity to understand how massive stars evolve. In a previous study (Martins et al. 2007), we analyzed 18 massive stars in the central cluster of the Galaxy. This cluster is especially intriguing since it hosts the supermassive black hole SgrA. In spite of the drastic tidal forces, several tens of massive stars formed recently (Paumard et al. 2006). Some of them are approaching the black hole at distances of only a few light hours (Eisenhauer et al. 2005). The presence of young stars in the Galactic Center together with the apparent implausibility of forming stars so close to the central supermassive black hole is a puzzle usually referred to as “the paradox of youth”. Studying the dynamics of these young stars, Paumard et al. (2006) have shown that they orbit SgrA in two counter-rotating disks. Together with other evidences (total mass and structure of the disks), this points to a local, “in-situ” star formation event. The detailed analysis of the post-main sequence massive stars has revealed that, surprisingly, their evolution follows almost perfectly the predictions of evolutionary models (Martins et al. 2007). This implies that whatever the exact formation mechanism is, the subsequent evolution is not different from that predicted for normal stars. We found that all stars seem to have progenitors in the mass range 25–60 M and that they follow relatively well the evolutionary scenario proposed by Crowther et al. (1995) for this mass range. We have been able to refine this scenario, pinpointing the relation between different spectral types: O ®(Ofpe/WN9 <=> LBV) ® WN8 ® WN8/WC9 ® WC9. This was made possible by the detailed study of stellar abundances in various Wolf-Rayet stars and related objects. Abundance analysis is a powerful tool to constrain stellar evolution since it gives direct access to the evolutionary state of a star. References Eisenhauer, F., Genzel, R., Alexander, T., et al. 2005, ApJ, 628, 246 Martins, F., Genzel, R., Hillier, D. J., et al. 2007, A&A 468, 233 Paumard, T., Genzel, R., Martins, F., et al. 2006, ApJ, 643, 1011 van der Hucht, K. A. 2006, A&A 458, 453

K3.2 The proper motion of the Arches cluster

Zum Thema
Simulations of star formation in a gaseous disc around sgr A* - a failed AGN

—

Authors: Andrea Stolte, A.M. Ghez, Mark Morris, J.R. Lu, Wolfgang Brandner, Keith Matthews

Journal-ref: ApJ (2007) [0706.4133

]

Title: The proper motion of the Arches cluster with Keck Laser-Guide Star Adaptive Optics

Image credit: 2MASS/A.Stolte et al.

Fig. 5.— The Arches cluster proper motion (green arrow) with respect to the GC is displayed on a 2MASS JHK color image. The bright band of stars extending from the GC into the direction of the radius vector r_µ indicates the Galactic plane, with the Arches at a projected height of about 10 pc above the disk. The cluster is moving almost parallel to the plane, and away from the Sun. The vectors illustrate the coordinate system defined to estimate the 3d distance of the cluster to the GC under the assumption of a circular orbit.

Abstract: We present the first measurement of the proper motion of the young, compact Arches cluster near the Galactic center from near-infrared adaptive optics (AO) data taken with the recently commissioned laser-guide star (LGS) at the Keck 10-m telescope.
The excellent astrometric accuracy achieved with LGS-AO provides the basis for a detailed comparison with VLT/NAOS-CONICA data taken 4.3 years earlier. Over the 4.3 year baseline, a spatial displacement of the Arches cluster with respect to the field population is measured to be 24.0 ± 2.2 mas, corresponding to a proper motion of 5.6 ± 0.5 mas/yr or 212 ± 20 km s^-1 at a distance of 8 kpc.
In combination with the known radial velocity of the cluster, we derive a 3D space motion of 232 ± 22 km s^-1 of the Arches relative to the field.
The large proper motion of the Arches cannot be explained with any of the closed orbital families observed in gas clouds in the bar potential of the inner Galaxy, but would be consistent with the Arches being on a transitional trajectory from x1 to x2 orbits.
We investigate a cloud-cloud collision as the possible origin for the Arches cluster.
The integration of the cluster orbit in the potential of the inner Galaxy suggests that the cluster formed in the central 200 pc, and still resides inside 200 pc today.
A recent replenishment of the young stellar population in the inner few parsecs of the GC appears unlikely unless the present-day distance to the GC is smaller than ~30 pc, very close to the cluster's projected distance from the GC.

1. Introduction

The Arches cluster is among the very few massive starburst clusters observed in the inner Milky Way. At projected distances below 30 pc from the Galactic center (GC), only two other dense, young clusters are known: the Quintuplet and the central cluster.
Obtaining an unbiased sample of cluster members and thus an estimate of the stellar mass function is difficult for the latter two clusters; the central cluster contains a mixed variety of stars having different ages, and the 4 Myr old Quintuplet cluster appears already widely dispersed (see, e.g., Fig. 2 in Figer et al. 1999).
The compactness of the Arches cluster and its young, uniform age of only ~ 2 Myr (Najarro et al. 2004, Figer et al. 2002) characterize this cluster as a unique target to study the stellar mass function and dynamical properties of clusters forming in the immediate neighbourhood of the center of our Galaxy.
The inner ~ 100 pc of the nuclear region of the Galaxy is a hostile environment for star clusters. Clusters like the Arches and Quintuplet are expected to disrupt in the intense tidal eld on a time scale of ~ 10 Myr (Kim et al. 1999, Portegies Zwart et al. 2002). The strong influence of external tidal forces on the evolution of these clusters led to the suggestion by Gerhard (2001) that massive, young clusters migrate inwards from tens of parsecs to the inner few parsecs around the black hole. Gerhard suggested that the cores of these clusters might survive tidal disruption and supply the population of young, high-mass stars observed close to the black hole, where conditions are inhospitable to in-situ star formation (Morris 1993, Ghez et al. 2003, 2005).
However, dynamical simulations suggest that a cluster mass of M_cl > 10⁶M is required for a dense core to reach the central parsec within the lifetime of the highmass stars (Kim & Morris 2003), much more massive than the Arches today with M_cl ~ 10⁴M.

Arches cluster in near-infrared adaptive optics
Image credit: Keck/VLTA.Stolte et al.		Fig. 1.— Arches cluster — Keck/VLT comparison. The LGS-AO Keck/NIRC2 K0 image (left) with a spatial resolution of 53 mas (FWHM PSF) compared to the NGS-AO VLT/NACO Ks image with 84 mas (FWHM PSF). The Keck 10 m diffraction limited image displays the first order airy rings around each star. North is up and East to the left.

The amount of material that was already stripped from the cluster and thus the initial cluster mass depend on its orbit in the GC potential and the true distance of the cluster to the GC. At its projected distance of 26 pc, the Arches could have lost 50% of its initial mass, such that the cluster could have been as massive as 2 × 10⁴M initially. Thus far, both the orbit and the true distance to the GC, rGC, are observationally unconstrained.

K4 35 New Supernova Remnants

VLA — 35 New Supernova Remnants

Authors: C. L. Brogan, J. D. Gelfand, B. M. Gaensler, N. E. Kassim, T. J. Lazio

Journal-ref: ApJ 639 (2006) L25-L30 [astro-ph/0601451

]

Title: Discovery of 35 New Supernova Remnants in the Inner Galaxy

Abstract: We report the discovery of up to 35 new supernova remnants (SNRs) from a 42 arcsec resolution 90cm multi-configuration Very Large Array survey of the Galactic plane covering 4.5 deg< l <22.0 deg and |b| < 1.25 deg. Archival 20cm, 11cm, and 8 micron data have also been used to identify the SNRs and constrain their properties. The 90cm image is sensitive to SNRs with diameters 2.5 arcmin to 50 arcmin and down to a surface brightness limit of about 10^{-21} W m^{-2} Hz^{-1} sr^{-1}. This survey has nearly tripled the number of SNRs known in this part of the Galaxy, and represents an overall 15% increase in the total number of Galactic SNRs. These results suggest that further deep low frequency surveys of the inner Galaxy will solve the discrepancy between the expected number of Galactic SNRs and the significantly smaller number of currently known SNRs.

New Supernova Remnants
Three color images with blue=VLA 90cm, red=MSX 8 µm, and green=SGPS+VLA 20cm of the

(a) W30 region and

(b) W28 region.
New SNRs are indicated using Galactic coordinates; the previously known SNRs, H II regions, and a WBB are also labeled.

VLA / Brogan et al.

New Supernova Remnants

MAGPIS — Multi-Array Galactic Plane Imaging Survey

Authors: David J. Helfand, Robert H. Becker, Richard L. White, Adam Fallon, Sarah Tuttle

Journal-ref: AJ 131 (2005) 2525 [astro-ph/0510468

]

Title: MAGPIS: A Multi-Array Galactic Plane Imaging Survey

Abstract: We present the Multi-Array Galactic Plane Imaging Survey (MAGPIS), which maps portions of the first Galactic quadrant with an angular resolution, sensitivity and dynamic range that surpasses existing radio images of the Milky Way by more than an order of magnitude. The source detection threshold at 20 cm is in the range 1--2 mJy over the 85% of the survey region (5 deg < l < 32 deg, |b| < 0.8 deg) not covered by bright extended emission.

We catalog over 3000 discrete sources (diameters mostly <30 arcsec) and present an atlas of ~400 diffuse emission regions. New and archival data at 90 cm for the whole survey area are also presented. Comparison of our catalogs and images with the MSX mid-infrared data allow us to provide preliminary discrimination between thermal and non-thermal sources. We identify 49 high-probability supernova remnant candidates, increasing by a factor of seven the number of known remnants with diameters smaller than 5 arcmin in the survey region;
several are pulsar wind nebula candidates and/or very small diameter remnants (D<45 arcsec). We report the tentative identification of several hundred H II regions based on a comparison with the mid-IR data; they range in size from unresolved ultra-compact sources to large complexes of diffuse emission on scales of half a degree. In several of the latter regions, cospatial nonthermal emission illustrates the interplay between stellar death and birth. We comment briefly on plans for followup observations and our extension of the survey; when complemented by data from ongoing X-ray and mid-IR observations,
we expect MAGPIS to provide the most complete census yet obtained of the birth and death of massive stars in the Milky Way.

Catalogs and images are available on the MAGPIS web site

K5 Two Young Star Disks in the Central Parsec of the Galaxy

Central Parsec of the Galaxy — Two Young Star Disks
	Authors: T. Paumard, R. Genzel, F. Martins, S. Nayakshin, A. M. Beloborodov, Y. Levin, S. Trippe, F. Eisenhauer, T. Ott, S. Gillessen, R. Abuter, J. Cuadra, T. Alexander, A. Sternberg
	Journal-ref: ApJ 643 (2006) 1011 [astro-ph/0601268 ]
	Title: The Two Young Star Disks in the Central Parsec of the Galaxy: Properties, Dynamics and Formation
Abstract: We report the definite spectroscopic identification of 41 OB supergiants, giants and main sequence stars in the central parsec of the Galaxy. Detection of their absorption lines have become possible with the high spatial and spectral resolution and sensitivity of the adaptive optics integral field spectrometer SPIFFI/SINFONI on the ESO VLT. Several of these OB stars appear to be helium and nitrogen rich. Almost all of the ~80 massive stars now known in the central parsec (central arcsecond excluded) reside in one of two somewhat thick (<\|h\|/R> ~ 0.14) rotating disks. These stellar disks have fairly sharp inner edges (R~1") and surface density profiles that scale as R^-2. We do not detect any OB stars outside the central 0.5 pc. The majority of the stars in the clockwise system appear to be on almost circular orbits, whereas most of those in the `counter-clockwise' disk appear to be on eccentric orbits. Based on its stellar surface density distribution and dynamics we propose that IRS 13E is an extremely dense cluster (core density > 3x10⁸ M pc^-3), which has formed in the counter-clockwise disk. The stellar contents of both systems are remarkably similar, indicating a common age of ~ 6 ± 2 Myr. The K-band luminosity function of the massive stars suggests a top-heavy mass function and limits the total stellar mass contained in both disks to ~ 1.5 x 10⁴ M. Our data strongly favor in situ star formation from dense gas accretion disks for the two stellar disks. This conclusion is very clear for the clockwise disk and highly plausible for the counter-clockwise system.

Galactic Center — M_BH = (4.3 ± 0.5) x 10⁶ M for a distance to the Galactic Center R_o = 8 kpc
	Authors: A. M. Beloborodov, Y. Levin, F. Eisenhauer, R. Genzel, T. Paumard, S. Gillessen, T. Ott
	Journal-ref: ApJ 648 (2006) 405 [astro-ph/0601273 ]
	Title: Clockwise Stellar Disk and the Dark Mass in the Galactic Center
Abstract: Two disks of young stars have recently been discovered in the Galactic Center. The disks are rotating in the gravitational field of the central black hole at radii r = 0.1-0.3 pc and thus open a new opportunity to measure the central mass. We find that the observed motion of stars in the clockwise disk implies M = (4.3 ± 0.5) x 10⁶ M for the fiducial distance to the Galactic Center R_o = 8 kpc and derive the scaling of M with R_o. As a tool for our estimate we use orbital roulette, a recently developed method. The method reconstructs the three-dimensional orbits of the disk stars and checks the randomness of their orbital phases. The clockwise-disk stars are found to have modest orbital eccentricities.

—
	Authors: F. Martins, R. Genzel, D.J. Hillier, F. Eisenhauer, T. Paumard, S. Gillessen, T. Ott, S. Trippe
	Journal-ref: A&A 468 (2007) 233 [astro-ph/0703211 ]
	Title: Stellar and wind properties of massive stars in the central parsec of the Galaxy
Abstract: Context. How star formation proceeds in the Galactic Center is a debated question. Addressing this question will help us understand the origin of the cluster of massive stars near the supermassive black hole, and more generally starburst phenomena in galactic nuclei. In that context, it is crucial to know the properties of young massive stars in the central parsec of the Galaxy. Aims. The main goal of this study is to derive the stellar and wind properties of the massive stars orbiting the supermassive black hole SgrA* in two counter-rotating disks. Methods. We use non-LTE atmosphere models including winds and line-blanketing to reproduce H and K band spectra of these stars obtained with SINFONI on the ESO/VLT. Results. The GC massive stars appear to be relatively similar to other Galactic stars. The currently known population of massive stars emit a total N' = 6.0 × 10⁵⁰ s^-1 (resp. N' = 2.3 × 10⁴⁹ s^-1) H (resp. He I) ionising photons. This is sufficient to produce the observed nebular emission and implies that, in contrast to previous claims, no peculiar stellar evolution is required in the Galactic Center. We find that most of the Ofpe/WN9 stars are less chemically evolved than initially thought. The properties of several WN8 stars are given, as well as two WN/C stars confirmed quantitatively to be stars in transition between the WN and WC phase. We propose the sequence (Ofpe/WN9 <=> LBV) ® WN8 ® WN/C for most of the observed GC stars. Quantitative comparison with stellar evolutionary tracks including rotation favour high mass loss rates in theWolf-Rayet phase in these models. In the OB phase, these tracks nicely reproduce the average properties of bright supergiants in the Galactic Center. 1. Introduction The center of our Galaxy is a unique environment to study massive stars. It harbors three of the most massive clusters of the Galaxy – the Arches, Quintuplet and central clusters. Heavily extincted and only accessible at infrared (and longer) wavelengths or in X-rays, each of these clusters has a population of more than a hundred massive stars. Even more interesting is the difference in their ages: 2.5, 4 and 6 Myrs for the Arches, the Quintuplet and the central cluster respectively (Figer et al. 1999, 2002; Paumard et al. 2006). Such a spread implies the presence of different types of massive objects, naturally sampling stellar evolution in the upper HR diagram. The youth of these clusters, together with their total mass in excess of 10⁴M, partly explains the large number of massive stars in the Galactic Center. But another reason may be the top-heavy mass function: Stolte et al. (2002) for the Arches and Paumard et al. (2006) for the central cluster have shown that the slope G of the present-day mass function was shallower than the standard Salpeter value (-0.8 instead of -2.35). This may be due to mass segregation or to a true feature of the initial mass function. In that case the Galactic Center could be a peculiar environment for the formation of massive stars.

K6 Surprise! Most Stars Are Single

Red dwarfs (spectral type M, mass M < 0.5 M) are likely single and not binary
	Author: Charles J. Lada
	Journal-ref: ApJ 640 (2006) L63 [astro-ph/0601375 ]
	Title: Stellar Multiplicity and the IMF: Most Stars Are Single Born
Abstract: In this short communication I compare recent findings suggesting a low binary star fraction for late type stars with knowledge concerning the forms of the stellar initial and present day mass functions for masses down to the hydrogen burning limit. This comparison indicates that most stellar systems formed in the galaxy are likely single and not binary as has been often asserted. Indeed, in the current epoch two-thirds of all main sequence stellar systems in the Galactic disk are composed of single stars. Some implications of this realization for understanding the star and planet formation process are briefly mentioned.

The stellar IMF is one of the most fundamental distribution functions in astrophysics. A great deal of effort has been expended in determining its form since the first attempt to measure its shape by Salpeter (1954). He found that the IMF is a power-law which decreases with stellar mass for field stars with masses in the range between 1-10 M. More recent determinations of the IMF for field stars and young embedded clusters have expanded the mass range covered by Salpeter. These studies have found the IMF to break from a single power-law shape near 0.5 M and to have a broad peak between 0.1 - 0.5 M. On either side of this peak the IMF falls off rapidly.
[January 27, 2006]

Artwork courtesy NASA and G. Bacon (STScI)

An artist's conception of the outer giant planet orbiting the red-dwarf star Gliese 876. The inner planet is the tiny dot close to the star. Though redder and dimmer than the Sun, red dwarfs still have temperatures and surface brightnesses great enough to make them appear as dazzlingly white as a light-bulb filament to anyone seeing them up close — as the artist has correctly shown. Red dwarfs comprise over 80 percent of our galaxy's stars, and most of them do not have stellar companions.

Astronomers have known since the 1700s that a significant fraction of stars belong to binary or multiple systems. But what is that fraction? Given the observed fact that most solar-size and larger stars reside in binaries, many astronomers have concluded that more than half of our galaxy's stars belong to multiple-star systems.
But a new study by Charles Lada shows that conventional wisdom is almost certainly wrong. The problem, says Lada, is that astronomers have neglected to consider our galaxy's most common stellar denizens: red dwarfs (spectral type M). These low-mass, low-luminosity stars make up more than 70 percent of all the stars in the Milky Way.
Lada cites surveys from groups led by Geoff Marcy, Neil Reid, Xavier Delfosse, and others that find that the single-star fraction among red dwarfs is very high. And because red dwarfs are the dominant population of stars, single stars must account for upwards of two-thirds of all stellar systems in the galaxy Lada concludes in his paper.
"By assembling these pieces of the puzzle, the picture that emerged was the complete opposite of what most astronomers have believed," says Lada.

Courtesy Alan Boss.

This computer simulation shows a protoplanetary disk around one member of a binary-star system. Jupiter-mass clumps of gas (red and blue regions) form easily and later collapse to form gas-giant planets.

These results are consistent with the findings of the ongoing

RECONS study at Georgia State University, which surveys the Sun's local neighborhood. The RECONS study has found that only 43 of the 171 primary red dwarfs within 10 parsecs (32.6 light-years) of the Sun have either another red dwarf or brown dwarf as a companion. "This is a lower limit, of course, because we're still uncovering companions," says RECONS team leader Todd Henry. "But we've known that the binary fraction of red dwarfs is much lower than the fraction for more massive stars."
Studies like these have important ramifications for theories of star and planet formation, since planets probably have an easier time forming around single stars than they do in binary systems. In fact, a low-mass planet was announced earlier this week around a solitary red dwarf.
But the news isn't all bad for binary-star planets. At the American Astronomical Society meeting earlier this month, Henry and his Georgia State colleague Deepak Raghavan found that 29 of 131 stars known to host at least one exoplanet also have a stellar companion. Theoretical models by Alan Boss suggest that a distant stellar companion's gravity can actually induce planet birth around a star by triggering the coalescence of dense gas clumps within a protoplanetary disk. The clumps quickly collapse to form gas-giant planets. Jack Lissauer (NASA/Ames Research Center) presented his group's findings that Earth-size planets can form in wide orbits around two stars in a tight binary and in close orbits around one of the two stars in a widely separated binary.
"The take-home message is that half of all wide binary-star systems can harbor planets because the separation is great enough to permit both the formation and the subsequent stability of the resulting planetary orbits," says Marcy, coleader of the team that has discovered more than half of the 170-plus known exoplanets. "There are surely tens of billions of Sun-like stars in the galaxy that can easily harbor planets."

Milky Way brims with singleton stars

Image credit: ESO

Herzsprung-Russell diagram of the solar neighborhood.
Zwei Drittel der Sterne in der Milchstraße stehen allein, denn die meisten sind M-Zwerge, und bei denen ist der Anteil von Einzelsternen höher als bei den selteneren schwereren.

Most of the stars in the Milky Way are born alone and live out their lives without partners, a new analysis suggests. If true, the work overturns standard theories that stars are born in broods and also suggests planets – and potentially life – may be more common in the galaxy than thought.
Observations show that stars are born in nurseries of gas and dust that typically contain several hundred stars in a region 3 light years wide. According to most models, they are born there in clutches, with several stars condensing from each of many large, dense clouds of matter.
Now, astronomer Charles Lada of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, US, is challenging that notion and says most stars are born one by one in the nurseries. He says those models are based on early stellar surveys that focused on bright, relatively massive stars like the Sun. The surveys found that about 60% of these bright stars are found in pairs.
But in the last 15 years or so, astronomers have used more sensitive telescopes to survey smaller, dimmer stars called red dwarfs, which are between 10% and 50% the mass of the Sun. "The faint stars are harder to see, but they make up 85% of all stars in the galaxy, and three quarters of those are single," Lada told New Scientist. "I think the result strongly favours the idea that most stars form initially as single objects, not in multiple systems."
That goes against current models, which explain the existence of single stars by arguing they are born with siblings and are then separated after a gravitational interaction with another star. "You can certainly form a lot of stars that way, but with so many single red dwarfs, there's no simple way" to explain them all, says Lada.
Cloud turbulence Frank Shu, an astronomer and president of the National Tsinghua University in Taiwan, agrees. In order to explain the fact that red dwarfs are vastly more common than any other type of star, "the mechanism doesn't work unless one always breaks apart a pair of red dwarfs," he told New Scientist.
But he says that scenario is unlikely, as stellar home-wreckers are most likely to break up pairs of stars that orbit each other at relatively large distances. And observations show that such "wide binaries" are usually made up of stars of different masses.
Lada argues that whether a star forms alone or with a sibling depends on its mass, which in turn depends on the mass of its parent cloud. Observations show that the gas in large, star-forming clouds is more turbulent than the gas in small ones. Shu has done previous theoretical work suggesting this turbulence may cause large clouds to separate into groups of massive stars. "Turbulence becomes relatively unimportant" for clouds that form small stars, he adds.
"It means the majority of stars in the galaxy and the universe forms under considerably more quiescent conditions than have been promoted in some quarters," Shu says. "If Lada's results hold up, a lot of people who have made rash claims in the past will have to re-evaluate their positions."
Planets and life
If most red dwarfs form without a sibling, extrasolar planetary systems similar to our solar system may be common, says Lada. Kevin Luhman, an astronomer at Pennsylvania State University in University Park, US, agrees. He says planets would probably have a more difficult time forming around a star in a binary system because the other star's gravity would disturb them.
He adds that red dwarfs can live as long as a trillion years – 100 times longer than the Sun, which will heat and bloat up as a red giant in several billion years. "Those factors together make the stars very likely sites for the formation of planets and life," Luhman says.
The stars' longevity may make them ideal destinations when the Sun becomes a red giant and "makes life on Earth a living hell", says Shu. "In that case, travelling to a habitable planet (or making one by terraforming) around a red dwarf star could extend the lifetime of the human race by many, many billions of years."

K7 Map of the Galaxy in the 6.7 keV emission line

—

Authors: M. Revnivtsev, S. Molkov, S. Sazonov

Journal-ref: MNRAS 373 (2006) L11 [astro-ph/0605693

]

Title: Map of the Galaxy in the 6.7 keV emission line

Image credit:

Figure 4. Top: Time averaged map of the inner Galaxy in the energy band 3–20 keV obtained with RXTE/PCA. The domination of bright point sources is evident. The contours are iso-brightness contours of the NIR emission of the Galaxy (see middle panel) Middle: Near-infrared surface brightness map of the Galaxy (COBE/DIRBE 4.9µm data, corrected for reddening) Bottom: Map of the surface brightness of the inner Galaxy in the 6.7 keV emission line. The white contours are iso-brightness contours of NIR emission.

Abstract: We study the two dimensional surface brightness distribution of the Galactic X-ray background emission outside the central degree around Sgr A* in the 6.7 keV line as measured by the PCA spectrometer of the RXTE observatory. The use of the emission line instead of continuum (3-20 keV) radiation and application of time variability filtering to the long data set allows us to strongly suppress the contamination of the GRXE map by bright point sources. The surface brightness in the 6.7 keV line demonstrates very good correspondence with the near-infrared surface brightness over the whole Galaxy, supporting the notion that the GRXE consists mostly of integrated emission from weak Galactic X-ray sources. We find compatible linear correlations between near-infrared and 6.7 keV surface brightness for the bulge and disk of the Galaxy. This indicates that the populations of weak X-ray sources making up the GRXE in the disk and bulge are not significantly different.

INTRODUCTION

Galactic ridge X-ray emission (GRXE) — X-ray radiation concentrated to the Galactic plane and unresolvable into bright (> 0.1 - 1 mCrab) point sources — has a prominent spectral feature at energy ~ 6.7 keV that is typical of hot optically thin plasma emission. As the energy resolution of X-ray detectors increased more emission lines were found, additionally hinting at a thermal origin of the GRXE.
The hypothesis of a truly diffuse origin of the GRXE has met a number of practically unresolvable difficulties. The main problem is that the GRXE is apparently emission of optically thin plasma with temperature up to >5–10 keV. Such hot diffuse plasma cannot be bound to the gravitational potential or magnetic field of the Galaxy and should form a continous outflow with a very large energy loss rate ( L_X = 1 × 10⁴³ erg s^-1 ). To sustain stationary X-ray extended emission, this energy must somehow be supplied throughout the whole Galaxy.
The alternative explanation of the GRXE being cumulative emission of a large number of weak point X-ray sources emitting a strong 6.7 keV line has also faced difficulties due to the failure of X-ray telescopes (including the modern CHANDRA amd XMM-Newton) to resolve the GRXE.
A solution to all these problems has apparently been found recently through studies of the GRXE morphology. As the knowledge of the GRXE surface brightness distribution in the Galaxy progressively improved, it finally became possible to demonstrate that the GRXE closely follows the near-infrared emission of the Galaxy, which is in turn a good tracer of the stellar mass density.
It was consequently proposed that X-ray emissivity is proportional to stellar mass density. The determined unit-stellar-mass emissivity proved to be in good agreement with the cumulative emissivity of X-ray sources (cataclysmic variables and coronal stars) in the Solar neighborhood. These findings imply that the GRXE represents integrated emission of weak ( L_X < 10³⁴ erg s^-1 ) Galactic X-ray sources.

Literatur zu "The Milky Way " Galactic Cannibalism
Figer, D.	2005	Nature 434, 192 - 194	"An upper limit to the masses of stars"
Kroupa, P.	2005	Nature 434, 148 - 149	"Stellar mass limited"
Y. Xu, X. W. Zheng, Mark Reid, K.M. Menten	2006	Science 311, 54	"The Distance to the Perseus Spiral Arm in the Milky Way"
Levine, E. S.; Blitz, Leo; Heiles, Carl	2006	Science 312, 1773-77	"The Spiral Structure of the Outer Milky Way in Hydrogen"
C. L. Brogan, J. D. Gelfand, B. M. Gaensler, N. E. Kassim, T. J. Lazio	2006	ApJ 639, L25-L30	"Discovery of 35 New Supernova Remnants in the Inner Galaxy"
D.J. Helfand, R.H. Becker, R.L. White, A. Fallon, S. Tuttle	2005	AJ 131, 2525	"MAGPIS: A Multi-Array Galactic Plane Imaging Survey"
T. Paumard, R. Genzel, F. Martins, et al.	2006	ApJ 643, 1011	"The Two Young Star Disks in the Central Parsec of the Galaxy: Properties, Dynamics and Formation"
A.M. Beloborodov, Y. Levin, F. Eisenhauer, R. Genzel, T. Paumard, S. Gillessen, T. Ott	2006	ApJ 648, 405	"Clockwise Stellar Disk and the Dark Mass in the Galactic Center"
C.J. Lada	2006	ApJ 640, L63	"Stellar Multiplicity and the IMF: Most Stars Are Single Born"
M. Revnivtsev, S. Molkov, S. Sazonov	2006	MNRAS 373, L11	"Map of the Galaxy in the 6.7 keV emission line"
Martins, F., Genzel, R., Hillier, D. J., et al.	2007	A&A 468, 233	"Stellar and wind properties of massive stars in the central parsec of the Galaxy"
Portegies Zwart, S; Gaburov, E; et al.	2007	MNRAS 378, L29	"The present day mass function in the central region of the Arches cluster"

H. Heintzmann

( Eintrag vom 10.4.2008)

— Nr:

—