"Wolf-Rayet stars"
Atomic & molecular spectra
(Super)giant stars
Hellste Sterne — LBV
Eta Carinae
I
II
III
Star Formation (SFR)
Population III stars
First Stars
Simulation
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Wolf-Rayet stars
K1.1 
The Stellar Population of IC 10
Image credit: Adam Block / NOAO / AURA / NSF
Fig. — This view of the dwarf galaxy IC 10 in Cassiopeia was obtained from Kitt Peak National Observatory
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Unlike most dwarf galaxies, IC 10 is busily forming new stars, and an inordinately high fraction of these are massive and luminous. In particular, IC 10 is chock-full of normally rare Wolf-Rayet stars. These blue giants are extremely hot and shed enormous amounts of material into space. In most galaxies, including our own, some W-R stars exhibit strong emissions from carbon (so-called WC stars) and others from nitrogen (WN stars). This is in line with widely accepted theories of star formation and evolution.
Oddly, though, in IC 10 astronomers have found almost no WN stars. Is there something wrong with stellar theory, or could IC 10 be hiding its WN stars?
This is where new technology comes in. At a distance of nearly 3 million light-years, it's exceedingly difficult to pick out individual stars in IC 10 using even the largest ground-based telescopes. To hunt for hidden WN stars, a team led by William Vacca (NASA/Ames Research Center) used the Hubble Space Telescope — whose images aren't blurred by Earth's atmosphere — and one of the twin Keck telescopes atop Mauna Kea, which use adaptive optics to "detwinkle" starlight to produce Hubble-quality images.
Vacca and his colleagues found many previously undiscovered stars in IC 10, and while they don't yet know if any of them are of the WN variety, additional observations can now tackle this question.
Now for the cautionary note. In the Wild West of Internet news, many media companies simply repost electronic press releases that they receive from sources. This latest work on IC 10 was published in the Astrophysical Journal on June 10th, but no one aside from professional astronomers noticed. So earlier this week, the Keck Observatory issued a press release to stir up broader interest. It contained the following quote from Keck director Taft Armandroff:
"We can now study individual stars of galaxies several million miles from Earth." I'm certain Armandroff meant to say "light-years" rather than "miles" — and a revised version of the press release on the Keck website reflects that — but already the Web is rife with news reports that repeat the original and obviously (to anyone who thinks about it) erroneous quote.
I see this happen all the time now.
I suppose it's inevitable when the news media and the public act as if it's more important to get a story first than to get it right
K1.2
‘One Of The Most Curious Objects In The Sky’ Delights Astronomers Again
Image credit: UCB/NASA/Keck Observatory
Fig. — The central starburst region of the IC 10 irregular dwarf galaxy
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Edwin Hubble once called IC 10 “one of the most curious objects in the sky,” and new observations of the extremely faint, lightweight dwarf galaxy are giving scientists new clues about how populations of stars are born.
Though the properties of stars is one of the most well-studied topics in astronomy, scientists still don’t fully understand all the mechanisms involved in star formation and evolution, particularly in galaxies with low levels of oxygen, nitrogen and other heavy elements. But scientists studying the IC 10 galaxy may soon understand how stars might have looked like in the distant past, when the universe was in a younger, more pristine form.
“A few years ago these types of studies would have been impossible from the ground,” said Taft Armandroff, director of the W. M. Keck Observatory, who’s own research includes the study of dwarf galaxies. “We can now study individual stars of galaxies several million light years from Earth to understand how star formation events may have affected the evolution of the Milky Way galaxy. This galaxy can teach us what the most common types of galaxies in the universe might be like.”
New images of IC 10 reveal a small region of space teeming with nearly a thousand stars. The image, obtained with NASA’s Hubble Space Telescope and the W. M. Keck Observatory in Hawaii, shows evidence of a vigorous star formation event that took place within the last 10 million years.
Dr. William Vacca at the NASA Ames Research Center led the study and says IC 10 may answer many unresolved questions about stellar evolution. “IC 10 is a remarkable galaxy,” he said. “It is the only one we’ve seen that falls outside an established pattern of having a certain number of massive nitrogen-type stars for each carbon-type star. This imbalance has caused us to wonder if our past conclusions about massive stars have been correct. Do we need to revise the models of stellar evolution?”
Astronomers have known that IC 10 has more giant, rare stars called “Wolf-Rayet stars” than all other nearby dwarf galaxies combined. Wolf-Rayet stars are extremely hot blue stars losing enormous amounts of mass to the interstellar medium. In addition, the proportion of Wolf-Rayet stars in IC 10 seems to be wildly out of balance. For the number of stars containing carbon, astronomers expected to see a certain number containing nitrogen. But so far, very few nitrogen stars have been found. Could IC 10 be hiding a population of stars?
Using a combination of Hubble and Keck telescope images, Vacca’s team found many previously undiscovered stars in the IC 10 galaxy. Each new star can now be measured to determine its chemical composition. If the newly found stars contain nitrogen, then part of the “missing nitrogen” puzzle might be solved.
“The combination of HST images in the optical and Keck Laser Guide Star images in the infrared has been a major breakthrough in our understanding of dense stellar regions,” said co-author Dr. James R. Graham, professor of astronomy at UC Berkeley. “IC 10 has so many stars in such a tiny region of space that ground-based studies have been confused. But the combination of HST and Keck has been revolutionary in our understanding of this object, and for any object with a dense region of stars.”
The new images of IC 10 are centered on a bright star first thought to possibly be the most luminous Wolf-Rayet star in IC 10. Follow up studies then found the star to be comprised of at least three or more components. Now, new data from Keck show the bright star ([MAC92] 24) is actually six or more stars, perhaps even a cluster of stars.
“This is the first time this sort of study has been done using adaptive optics,” said co-author Christopher Sheehy of the University of Chicago. “It gives us the ability to make these kinds of measurements accurately from the ground and there's no shortage of targets in need of a fresh look. The potential is exciting.”
The new data has also enabled scientists to measure the precise distance to IC 10, a figure that has eluded scientists since the object’s discovery more than 100 years ago. Dr. Vacca and his collaborators calculated the distance to IC 10 to be about 2.6 million light years from Earth, or 800 kiloparsecs. This is in good agreement with some previous estimates.
IC 10 was first discovered by Lewis Swift in 1889 at the Warner Observatory in Rochester, New York. The “Index Catalogue” (IC) is a catalogue of galaxies, nebulae and star clusters that supplements the more modern New General Catalogue (NGC). First published in 1895, the catalogue first described IC 10 as a “faint star involved in extremely faint and very large nebula.” It wasn’t until 1935 that IC 10 was first proposed as an extragalactic object and Edwin Hubble later proposed IC 10 might be a member of the Local Group. It took another 30 years before these suspicions could be confirmed using radial velocity and distance measurements.
Astronomers now know IC 10 is similar in many ways to the Large Magellenic Cloud of the Southern Hemisphere. But unlike the Large Magellenic Cloud, IC 10 orbits Andromeda, not the Milky Way. The study of IC 10 is giving astronomers a picture of what the Milky Way might have looked like billions of years ago before the galaxy’s interstellar medium was enriched with elements such as oxygen and nitrogen.
K1.3
Compact Object IC 10 X-1, a Black Hole
Nun haben Astronomen noch ein solches Objekt entdeckt, das aber zwischen 24 und 33 Mal so viel auf die Waage bringt wie die Sonne. "Wir hätten nicht gedacht, ein so massives stellares Schwarzes Loch zu finden", sagte Andrea Prestwich vom Harvard-Smithsonian Center for Astrophysics in Cambridge. "Wir wissen jetzt, dass Schwarze Löcher, die aus Sternen entstehen, viel größer sein können."
Stellare Schwarze Löcher sind Sternleichen und wesentlich kleiner als sogenannte supermassive Schwarze Löcher, die sich im Zentrum von Galaxien befinden. Sie entstehen, wenn ein Stern seinen Brennstoffvorrat verbraucht hat und unter dem eigenen Gewicht kollabiert. Dabei blitzt der Stern noch einmal kurz als Supernova auf. Übrig bleibt eine extrem dichte und massereiche Sternleiche, deren Anziehungskraft nicht einmal Licht zu entkommen vermag - ein Schwarzes Loch.
Der jetzt entdeckte Rekordhalter befindet sich nahe der Zwerggalaxie IC 10 im Sternbild Kassiopeia. Die Entfernung zur Erde beträgt 1,8 Millionen Lichtjahre. Wie auch bei der kürzlich entdeckten Sternleiche machte ein Begleiter die Massebestimmung erst möglich. Der Stern und das Schwarze Loch umkreisen einander. Der Stern verliert permanent Gas, das von dem Schwarzen Loch aufgesogen wird und dabei Röntgenstrahlung abgibt.
Bei Beobachtungen mit dem Teleskop "Chandra" war den Astronomen aufgefallen, dass die Intensität der Röntgenstrahlung des Sterns IC 10 X-1 stark schwankt - ein Hinweis auf ein großes Objekt, das die Quelle regelmäßig verdeckt. Der Satellit "Swift" konnte dies bestätigen und auch Details zum Orbit liefern, schreiben die Forscher im Fachblatt "Astrophysical Journal Letters".
Es gebe zwar noch einige Unsicherheiten, was die Berechnung der Masse betreffe, erklärte Prestwich. Diese sollten aber durch weitere Beobachtungen ausgeräumt werden. Nach Aussage der Astrophysikerin dürfte es, falls überhaupt, nur Korrekturen nach oben geben.
Die Forscher glauben nicht, dass der permanente Massezustrom vom benachbarten Stern das Objekt so schwer hat werden lassen. "Dieses Schwarze Loch kam dick auf die Welt, es ist nicht dick geworden", sagte Richard Mushotzky vom Goddard Space Flight Center der Nasa in Greenbelt, der selbst an der Studie nicht beteiligt war.
a minimum primary mass of 32.7 ± 2.6 MK1.4
IC 10 X-1: the immediate progenitor of a double black hole binary
K2
A Yellow Supergiant
K2.2
Yellow Supergiant Eclipsing Binary
"Mit so etwas rechnet man einfach nicht", sagt Stanek. Der Fund zeige, wie flexibel man als Astrophysiker sein müsse.
Das erste Sternenduo wurde mit dem Large Binocular Telescope (LBT) auf Mount Graham in Arizona entdeckt. Das System ist 4 Mpc von der Erde entfernt und liegt in der kleinen Galaxie Holmberg IX.
Die beiden Sterne bewegen sich umeinander: Je nach Phase versperrt einer der beiden Sterne den Blick auf seinen Nachbarn - oder beide sind nebeneinander zu sehen. Beide Sterne sind etwa gleich groß und 15 bis 20 Mal so schwer wie unsere Sonne, berichten die Astronomen im Fachblatt "Astrophysical Journal Letters".
José Prieto, der Hauptautor des Artikels, hatte das erdnussförmige System im Rahmen seiner Doktorarbeit untersucht. Als er ältere Beobachtungsdaten von Kollegen analysierte, fand er zum eigenen Erstaunen ein weiteres solches ungewöhnliches System - und zwar in 60 kpc Entfernung in der Kleinen Magellanschen Wolke. Das System war bereits in den achtziger Jahren entdeckt - seine Struktur aber falsch interpretiert worden. Prieto hatte in den alten Daten ein ähnliches Muster identifiziert wie beim Doppelstern in Holmberg IX.
"Wir brauchten das 8,4 Meter große LBT, um das erste Sternenpaar zu finden", sagt Stanek. Das zweite erdnussförmige System sei hingegen so hell, dass man es mit einem gewöhnlichen Fernglas vom eigenen Garten aus beobachten könne.
"Wenn wir das erste nicht gefunden hätten, wäre uns das zweite womöglich nie aufgefallen." Dies zeige, dass nach wie vor wertvolle Entdeckungen am Himmel versteckt seien. "Man muss nur die Augen offen halten", erklärt Stanek.
“You just have to keep your eyes open and connect the dots.”
Yellow supergiants as SN progenitors
The find may help solve another mystery. Of all the supernovae that have been studied over the years,
two have been linked to yellow supergiants -- and that’s two more than astronomers would expect.
Prieto explained why.
Over millions of years, a star will burn hotter or cooler as it consumes different chemical elements in its core. The most massive stars swing back and forth between being cool red supergiants or hot blue ones. They spend most of their lives at one end of the temperature scale or the other, but spend only a short time in-between, where they are classified as yellow. Most stars end their life in a supernova at the red end of the cycle; a few do at the blue end. But none do it during the short yellow transitional phase in between.
At least, that’s what astronomers thought.
Prieto, Stanek, and their colleagues suspect that yellow binary systems like the ones they found could be the progenitors of these odd supernovae.
“When two stars orbit each other very closely, they share material, and the evolution of one affects the other,” Prieto said. “It’s possible two supergiants in such a system would evolve more slowly, and spend more time in the yellow phase -- long enough that one of them could explode as a yellow supergiant.”
K3.1
Fate of supermassive stars
Zum Thema:
Die Entstehung der Elemente im Kosmos - Ursprungsmasse und Endprodukt | |
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K3.2
The very massive binary NGC 3603-A1
Zum Thema | |
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NGC 3603 — M1 = (116 ± 31) M
— M2 = (89 ± 16) M | |
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Authors: O. Schnurr, A.F.J. Moffat, N. St-Louis, J. Casoli, A.-N. Chené |
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Journal-ref: MNRAS (2008) [0806.2815  ] |
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Title: The very massive binary NGC3603-A1 |
| Abstract:
Using VLT/SINFONI, we have obtained repeated AO-assisted, NIR spectroscopy of the three central WN6ha stars
in the core of the very young (~1 Myr), massive and dense Galactic cluster NGC 3603.
One of these stars, NGC3603-A1, is a known 3.77-day, double-eclipsing binary, while another one, NGC3603-C, is one of the brightest X-ray sources among all known Galactic WR stars, which usually is a strong indication for binarity. Our study reveals that star C is indeed an 8.9-day binary, although only the WN6ha component is visible in our spectra; therefore we temporarily classify star C as an SB1 system. A1, on the other hand, is found to consist of two emission-line stars of similar, but not necessarily of identical spectral type, which can be followed over most the orbit. Using radial velocities for both components and the previously known inclination angle of the system, we are able to derive absolute masses for both stars in A1. We find M1 = (116 ± 31) M for the primary and
M2 = (89 ± 16) M for the secondary component
of A1.
While uncertainties are large, A1 is intrinsically half a magnitude brighter than WR20a, the current record holder with 83 and 82 M , respectively;
therefore, it is likely that the primary in A1 is indeed the most massive star weighed so far.
1. Introduction
While models maintain that in the early Universe, the first-generation of stars were very massive and reached
masses between 100 and 1000 M
(e.g. Schaerer 2002), it is generally accepted that under present-day
conditions, relatively fewer massive stars are formed, i.e. that the initial-mass function (IMF) is much steeper
and, more importantly, has a cut-off occurring around 150
M (Weidner & Kroupa 2004; Figer 2005).
So far, however, whenever Keplerian orbits of binary systems are used to weigh stars – the only way to obtain
reliable, least model-dependent masses –, measured masses fall short by almost a factor of two with respect to
the putative cut-off.
Currently, stars with the highest known masses are both WN6ha components of the Galactic WR binary WR20a, with 83 and 82 M , respectively (Rauw et al. 2004;
Bonanos et al. 2004), and the O3f/WN6 star in the Galactic binary WR21a with a
minimum mass of 87 M (Gamen et al. 2008). Significantly
more massive stars, however, have so far remained elusive.
References Figer D.F., 2005, Nature 434, 192 | |
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The Most Massive Star Yet?
So how heavy can a star actually be? For the universe’s present era, astronomers assume an upper limit of about 100 solar masses. This number comes from the need to preserve something called hydrostatic equilibrium. The term means that the two main forces working on a star have to balance out. One is gravity, which pulls a star together. The other is — no joke — the pressure of the star’s own heat and light. As energy streams out of the nuclear-fusing core, the photons push outward on the star’s layers. This pushing is called radiation pressure. For supermassive stars (which create a lot of heat and light), the radiation pressure on the star’s outermost layers overcomes the inward pull of gravity, blowing layers away as a stellar wind. This critical ratio between mass and radiation intensity is the Eddington limit, named after the astronomer who first figured it out in the 1920s. But astronomers have a hard time observing this upper mass limit in the universe. The problem is that high-mass stars are rare. They don’t form often, and when they do they burn themselves up much faster than smaller stars. The most massive stars may only live for a few million years (the Sun, on the other hand, will burn hydrogen for about 12 billion years). Even with today’s equipment astronomers still have yet to identify any stars weighing in at 100 solar masses. That’s not to say the theory’s wrong. Astronomers have found a handful of stars in the 85-solar-mass range. But the data have everyone wondering: are there any 100-solar-mass stars? A team at the University of Montreal now answers a tentative yes. Looking deep into NGC 3603, one of the youngest and most massive star clusters in the Milky Way, the scientists studied five extremely luminous stars. One is an only child, and the others are in two binary pairs. For one of the binaries, the team used shifts in the two stars’ spectral lines caused by the stars’ speeding around each other to calculate a rough mass for each companion. They came up with 85 to 145 solar masses for one, 75 to 105 solar masses for the other. |
K4
A bipolar X-ray jet from the T Tauri star DG Tauri
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Authors: Güdel, M.; Skinner, S. L.; Audard, M.; Briggs, K. R.; Cabrit, S. |
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Journal-ref: A&A 478 (2008) 797 [0712.1330 ] |
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Title: Discovery of a bipolar X-ray jet from the T Tauri star DG Tauri |
| Abstract:
• Aims. We have obtained and analyzed Chandra ACIS-S observations of the strongly accreting classical T Tauri star DG Tau. Our principal goals are to map the immediate environment of the star to characterize possible extended X-rays formed in the jet, and to re-visit the anomalous, doubly absorbed X-ray spectrum of DG Tau itself. • Methods. We combine our new ACIS-S data with a data set previously obtained. The data are superimposed to obtain flux and hardness images. Separate X-ray spectra are extracted for DG Tau and areas outside its point spread function. Results. We detect a prominent X-ray jet at a position angle of PA ~ 225° (tentatively suggested by Güdel et al. 2005), coincident with the optical jet axis. We also identify a counter jet at PA = 45°. The X-ray jets are detected out to a distance of ~ 5'' from the star, their sources being extended at the ACIS-S resolution. The jet spectra are soft, with a best-fit electron temperature of 3.4 MK. We find evidence for excess absorption of the counter jet. The spectrum of the DG Tau point source shows two components with largely different temperatures and absorption column densities. • Conclusions. The similar temperatures and small absorbing gas columns of the jet sources and the soft component of the “stellar” source suggest that these sources are related, produced either by shocks or by magnetic heating in the jets. Cooling estimates suggest that the pressure in the hot gas contributes to jet expansion. The hard “stellar” component, on the other hand, is associated with a stellar corona or magnetosphere. The excessive photoelectric absorption of this component suggests the presence of dust-depleted accretion streams above coronal magnetic fields. 1. Introduction
DG Tau is a most outstanding T Tauri star for X-ray studies. A Chandra high-resolutionX-ray image has shown
tentative evidence for the presence of faint X-rays along the optical jet. Both
XMM-Newton (Güdel et al., 2007b) and Chandra (Güdel et al., 2005) low-resolution CCD spectra of DG Tau are
anomalous, showing a “two-absorber X-ray” (TAX) spectrum in which two independent X-ray components are each
subject to different absorption column densities.
References Güdel, M., Skinner, S. L., Briggs, K. R., et al. 2005, ApJ, 626, L53 Güdel, M., Telleschi, A., Audard, M., et al. 2007b, A&A, 468, 515 | |
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DG Tau: Energetic Jets from a Budding Solar System
[April 9, 2008] Chandra’s image of DG Tau reveals the first double-sided X-ray jet ever detected from a young
star. The jet, which runs from the top left to the bottom right, extends about 70 billion miles away from the
star. Scientists think that a similar jet may have been launched from our young Sun and could have had a
significant impact on the early solar system.
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Image credit: NASA / Chandra X-ray Observatory
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The image on the left shows the first double-sided X-ray jet ever
detected from a young star. A similar jet may have been launched from the young Sun and could have
had a significant impact on the early solar system.
The young star, named DG Tau, is located in the Taurus star-forming region, about 450 light years from Earth. The bright source of X-rays in the middle of the image is DG Tau and the jet runs from the top left to the bottom right, extending to about 700 AU. A detailed analysis of this image, led by Manuel Güdel shows that the counter jet (top-left) has, on average, higher energy X-rays than the forward jet (bottom-right). The likely explanation is that some of the lower energy X-rays in the counter jet are absorbed by a disk around DG Tau, as shown in the accompanying illustration (right graphic), showing the star, disk and the inner regions of the jets. | |
| Highly energetic X-rays are also detected from the young star, partially absorbed by streams of material flowing from the disk onto the star. The disk itself is much too cool to be detected by Chandra. Note that the faint vertical feature below the star does not show evidence for an additional jet, but is a chance alignment of four photons. | ||
The effects of the jet on its surroundings may be significant. Other researchers have previously
suggested that X-rays from a typical young star can significantly affect the properties of its surrounding
disk, by heating it and creating charged particles by stripping electrons off atoms. These X-rays will
strike the disk at a low angle, mitigating their effects. In the case of the jets from DG Tau, the
combined X-ray power in the jet is similar to that of a young star with relatively modest X-ray
brightness, but X-rays from the jet have the advantage of striking the disk much more directly from
above and below.
Güdel and colleagues argue that powerful X-ray jets might develop at some stage during the evolution
of most young stars. They could, for example, have existed during the early stages of the solar system.
DG Tau has about the same mass as the Sun, but is much younger with an age of about one million years,
rather than about 4.5 billion years. Since it is surrounded by a disk where planets may be forming, this
new Chandra image suggests that the early Earth and its environment may have been bathed in X-rays from
a jet like DG Tau's. Although it is unknown if such X-rays would have had a significant impact on the
forming Earth, it is possible that they did more good than harm. By ionizing the disk the X-rays may
have generated turbulence, which could have had a substantial effect on the orbit of the young Earth,
possibly helping to prevent it from making a disastrous plunge into the Sun. Furthermore, X-ray irradiation
of disks may also be important in the production of complex molecules in the disk that will later end up
on the forming planets.
The new X-ray observations of X-ray jets add new features to the already complex story of star and
planet formation. The ionization and heating power of the X-rays rom jets will have to be included in
future model calculations that will help scientists understand the physical evolution and chemical
processing of environments that eventually lead to planets like those in our solar system.
| Literatur zu "" | |||
| W.D. Vacca, C.D. Sheehy, J.R. Graham | 2007 | ApJ 662, 272 |
"Imaging of the Stellar Population of IC10 with Laser Guide Star Adaptive Optics and the HST"
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| Prestwich, A. H., Kilgard, R. et al. | 2007 | ApJ 669, L21 |
"The Orbital Period of the Wolf-Rayet Binary IC 10 X-1"
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| Yungelson, L.R., van den Heuvel, E.P.J., et al. | 2008 | A&A 477, 223 |
"On the evolution and fate of super-massive stars"
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| Prieto, J.L.; Stanek, K.Z.; Kochanek, C.S.; et al. | 2008 | ApJ 673, L59 |
"LBT Discovery of a Yellow Supergiant Eclipsing Binary in the Dwarf Galaxy Holmberg IX"
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| Güdel, M. et al. | 2008 | A&A 478, 797 |
"Discovery of a bipolar X-ray jet from the T Tauri star DG Tauri"
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 | H. Heintzmann | ( Eintrag vom 20.11.2008) |
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