Artikel zu "V838 Monocerotis (II)" MIV: V838 Mon (I) V838 Mon (III) MIV: Our Milky Way Galaxy
V838 Monocerotis (IV) Sterbende Sterne (Vorl.) Fressorgie erleuchtet die Milchstraße V838 Monocerotis — :    K1 Stern erzeugt spektakuläres Lichtecho K2 Fast Facts K3 Light continues to echo three years after stellar outburst K4 Evolution during and after the 2002 eruption K4.1 The first three years of light-echo evolution	— :    Literatur

V838 Monocerotis (II)

Rätselhafte Eruption Stern erzeugt spektakuläres Lichtecho
Einmal hellster Stern der Milchstraße sein: Einer zuvor unscheinbaren Sonne ist das vor einem Jahr gelungen. Das sehenswerte Nachleuchten des Kurzauftritts hat das Hubble-Teleskop eingefangen.

	NASA/ ESA/ H.E. Bond (STScI)
	Lichtecho um V838 Monocerotis: Nach außen wandernder Leuchtkranz

Der stellare Spuk war schnell vorbei, doch auch seine Nachwehen fielen spektakulär aus. Wie eine jetzt veröffentlichte Fotoserie des Weltraumteleskops Hubble zeigt, zog der Ausbruch ein Lichtecho nach sich, das Astronomen unerwartete Einblicke in die Umgebung des Sterns ermöglicht hat. Auf den vier Einzelbildern, die von Mai bis Dezember 2002 reichen, dehnt sich der leuchtende Ring um V838 Monocerotis immer weiter aus.

NASA/ ESA/ H.E. Bond (STScI)
Hubble-Fotoserie von V838 Monocerotis: "Keine befriedigende physikalische Erklärung"

Das Lichtecho entsteht durch Strahlung, die beim Ausbruch im Januar freigesetzt wurde und von Staubwolken im Umfeld des Sterns reflektiert wird. Weil das Licht dabei einen längeren Weg zurücklegt, erreicht es die Erde erst Monate nach dem Eruptionsblitz. Das Phänomen, das in dieser Deutlichkeit bislang noch nicht zu sehen war, stellen Forscher um Howard Bond in der aktuellen Ausgabe des Fachmagazins "Nature" vor.

"Hubbles Blick ist so scharf, dass wir zum ersten Mal den Raum um den Stern wie mit einer astronomischen Tomografie untersuchen können", erklärt Bond, der am Space Telescope Science Institute in Baltimore arbeitet. Bei den Aufnahmen kam die hochauflösende Advanced Camera for Surveys zum Einsatz, die vor einem Jahr bei der vorletzten Mission der Raumfähre "Columbia" am Weltraumteleskop montiert worden war.

Die Eruption selbst verlief ähnlich ungewöhnlich wie ihr Nachleuchten. Anders als explosivere Sterne, die als so genannte Novae oder Supernovae ihre Hüllen ins All sprengen, blähte sich V838 Monocerotis einfach zu einer kühlen Riesensonne auf, deren Oberfläche kaum heißer ist als eine Glühbirne. Auf dem Höhepunkt ihrer Leuchtkraft strahlte sie bläulich, danach wechselte sie zu einer rötlichen Farbe.

Diese Verwandlung lässt sich auch am expandierenden Lichtkreis ablesen, der außen blau und innen rot erscheint. Die Wolken um den etwa 20.000 Lichtjahre entfernten Stern, die normalerweise unsichtbar sind und nur durch das Lichtecho hervortreten, deuten auf eine turbulente Vorgeschichte von V838 Monocerotis hin: Sie wurden offenbar bei früheren Ausbrüchen abgestoßen.

"Wir kennen kein anderes Objekt, das sich vergleichbar verhält", gesteht Mark Wagner von der University of Arizona. "Der Ablauf des Ausbruchs ist genauso einzigartig wie sein spektakuläres Lichtecho." Die Extravaganz liegt womöglich im Aufbau des Sternes begründet. Wagner hat durch zusätzliche Beobachtungen vom Boden aus entdeckt, dass V838 Monocerotis ein Doppelsystem ist: Nur der kühlere der beiden Partner brach aus.

Dennoch haben die Wissenschaftler für das seltsame Schauspiel bislang "keine völlig befriedigende physikalische Erklärung", wie sie in "Nature" schreiben. Immerhin bleibt dem Team noch einige Zeit, um die Nachwirkungen der Sterneruption weiter zu untersuchen: Bond schätzt, dass der immer weiter nach außen wandernde Leuchtkranz noch bis zum Ende des Jahrzehnts zu beobachten ist.

Nachtrag: Space Phenomenon Imitates Art in Universe's Version of van Gogh Painting [8.2.2004] "Starry Night," Vincent van Gogh's famous painting, is renowned for its bold whorls of light sweeping across a raging night sky. Although this image of the heavens came only from the artist's restless imagination, a new picture from NASA's Hubble Space Telescope bears remarkable similarities to the van Gogh work, complete with never-before-seen spirals of dust swirling across trillions of miles of interstellar space. This image, obtained with the Advanced Camera for Surveys on February 8, 2004, is Hubble's latest view of an expanding halo of light around a distant star, named V838 Monocerotis (V838 Mon). The illumination of interstellar dust comes from the red supergiant star at the middle of the image, which gave off a flashbulb-like pulse of light two years ago. V838 Mon is located about 20,000 light-years away from Earth in the direction of the constellation Monoceros, placing the star at the outer edge of our Milky Way galaxy. Called a light echo, the expanding illumination of a dusty cloud around the star has been revealing remarkable structures ever since the star suddenly brightened for several weeks in early 2002. Though Hubble has followed the light echo in several snapshots, this new image shows swirls or eddies in the dusty cloud for the first time. These eddies are probably caused by turbulence in the dust and gas around the star as they slowly expand away. The dust and gas were likely ejected from the star in a previous explosion, similar to the 2002 event, which occurred some tens of thousands of years ago. The surrounding dust remained invisible and unsuspected until suddenly illuminated by the brilliant explosion of the central star two years ago. The Hubble telescope has imaged V838 Mon and its light echo several times since the star's outburst in January 2002, in order to follow the constantly changing appearance of the dust as the pulse of illumination continues to expand away from the star at the speed of light. During the outburst event, the normally faint star suddenly brightened, becoming 600,000 times more luminous than our Sun. It was thus one of the brightest stars in the entire Milky Way, until it faded away again in April 2002. The star has some similarities to a class of objects called "novae," which suddenly increase in brightness due to thermonuclear explosions at their surfaces; however, the detailed behavior of V838 Mon, in particular its extremely red color, has been completely different from any previously known nova. Nature's own piece of performance art, this structure will continue to change its appearance in coming years as the light from the stellar outburst continues to propagate outward and bounce off more distant black clouds of dust. Astronomers expect the echoes to remain visible for at least the rest of the current decade.

Orientation:

Object Name: V838 Monocerotis Object Description: Nova-like variable star and surrounding light echo Position (J2000): R.A. 07h 04m 04.8s Dec. –03° 50' 50" Constellation: Monoceros Distance: The star is ~20,000 light-years (~ 6 kiloparsecs) away. Dimensions: This image is 2.4 arcminutes (13.6 light-years or 4.2 parsecs) wide.

[Date: 03 Feb 2005] - Satellite: Hubble (ACS image) - Depicts: Erupting star V838 Monocerotis The Hubble Space Telescope's latest image of the star V838 Monocerotis (V838 Mon) reveals dramatic changes in the illumination of surrounding dusty cloud structures. The effect, called a light echo, has been unveiling never-before-seen dust patterns ever since the star suddenly brightened for several weeks in early 2002. Copyright: NASA, ESA, and The Hubble Heritage Team (AURA/STScI) Lars Lindberg Christensen

The illumination of interstellar dust comes from the red supergiant star at the middle of the image, which gave off a pulse of light three years ago, somewhat similar to setting off a flashbulb in a darkened room. The dust surrounding V838 Mon may have been ejected from the star during a previous explosion, similar to the 2002 event.

The echoing of light through space is similar to the echoing of sound through air. As light from the stellar explosion continues to propagate outwards, different parts of the surrounding dust are illuminated, just as a sound echo bounces off of objects near the source, and later, objects further from the source. Eventually, when light from the back side of the nebula begins to arrive, the light echo will give the illusion of contracting, and finally it will disappear.

V838 Mon is located about 20 000 light-years away from Earth in the direction of the constellation Monoceros, placing the star at the outer edge of our Milky Way galaxy. The Hubble telescope has imaged V838 Mon and its light echo several times since the star's outburst. Each time Hubble observes the event, different thin sections of the dust are seen as the pulse of illumination continues to expand away from the star at the speed of light, producing a constantly changing appearance. During the outburst event whose light reached Earth in 2002, the normally faint star suddenly brightened, becoming 600 000 times more luminous than our Sun.

The new image of V838 Mon, taken with Hubble's Advanced Camera for Surveys, was prepared from images obtained through filters that isolate blue, green, and infrared light. These images have been combined to produce a full-colour picture that approximates the true colours of the light echo and the very red star near the centre.

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	Author: R. Tylenda
	Journal-ref: A&A 436 (2005) 1009 [astro-ph/0502060 ]
	Title: Evolution of V838 Monocerotis during and after the 2002 eruption
Abstract: By fitting the available photometric data on V838 Mon with standard supergiant spectra we have derived principal stellar parameters, i.e. effective temperature, radius and luminosity, and followed the evolution of the object since its discovery in early January 2002. Our analysis shows that the 2002 outburst of V838 Mon consisted of two major phases: pre-eruption which was observed in January 2002 and a major outburst, called eruption, which started in the beginning of February 2002. During pre-eruption the object seemed to be relaxing after an initial event which had presumably taken place in last days of December 2001. The eruption phase, which lasted till mid-April 2002, resulted from a very strong energy burst, which presumably took place in last days of January at the base of the stellar envelope inflated in pre-eruption. The burst produced an energy wave, which was observed as a strong luminosity flash in the beginning of February, followed by a strong mass outflow in form of two shells, which was observed as an expanding photosphere in later epochs. In mid-April, when the outflow became optically transparent and most of its energy radiated away, the object entered the decline phase during which V838 Mon was evolving along the Hayashi track. This we interpret as an evidence that the main energy source during decline was due to gravitational contraction of the object envelope inflated in eruption. Late in 2002 a dust formation started in the expanding shells which gave rise to a strong infrared excess observed in 2003.

V838 Mon — d ~ 10 kpc
	Authors: U. Munari, A.Henden, A.Vallenari, H.E.Bond, R.L.M.Corradi, L.Crause, S.Desidera, E.Giro, P.M.Marrese, S.Ragaini, A.Siviero, R.Sordo, S.Starrfield, T.Tomov, S.Villanova, T.Zwitter, R.M.Wagner
	Journal-ref: A&A, 434 (2005) 1107 [astro-ph/0501604 ]
	Title: On the distance, reddening and progenitor of V838 Mon
Abstract: Extensive optical and infrared photometry as well as low and high resolution spectroscopy are used as inputs in deriving robust estimates of the reddening, distance and nature of the progenitor of V838 Mon. The reddening is found to obey the R_V=3.1 law and amounts to   • (i) E(B-V)=0.86 from the interstellar NaI and KI lines,   • (ii) E(B-V)=0.88 from the energy distribution of the B3V component and   • (iii) E(B-V)=0.87 from the progression of extinction along the line of sight. The adopted E(B-V)=0.87(+/-0.01) is also the amount required by fitting the progenitor with theoretical isochrones of appropriate metallicity. The distance is estimated from   • (a) the galactic kinematics of the three components of the interstellar lines,   • (b) the amount of extinction vs the HI column density and vs the dust emission through the whole Galaxy in that direction, from   • (c) spectrophotometric parallax to the B3V companion, from   • (d) comparison of the observed color-magnitude diagram of field stars with 3D stellar population models of the Galaxy, from (e) comparison of theoretical isochrones with the components of the binary system in quiescence and found to be around 10 kpc. Pre-outburst optical and IR energy distributions show that the component erupting in 2002 was brighter and hotter than the B3V companion. The best fit is obtained for a 50,000 K source, 0.5 mag brighter than the B3V companion. Comparison with theoretical isochrones suggests an age of 4 million year for the system and a mass around 65 M for the progenitor of the outbursting component, which at the time of the outburst was approaching the Carbon ignition stage in its core. The 2002 event is probably just a shell thermonuclear event in the outer envelope of the star.

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	Authors: U. Munari, A. Henden
	ref: Colloquium "Interacting Binaries: Accretion, Evolution and Outcome" (2005) [astro-ph/0501546 ]
	Title: The first three years of the outburst and light-echo evolution of V838 Mon and the nature of its progenitor
Abstract: V838 Mon has undergone one of the most mysterious stellar outbursts on record, with (a) a large amplitude (Delta B ~ 10 mag) and multi-maxima photometric pattern, (b) a cool spectral type at maximum becoming cooler and cooler with time during the descent, until it reached the never-seen-before realm of L-type supergiants, never passing through optically thin or nebular stages, (c) the development of a spectacular, monotonically expanding light-echo in the circumstellar material, and (d) the identification of a massive and young B3V companion, unaffected by the outburst. In this talk we review the photometric and spectroscopic evolution during the first three full years of outburst, the light-echo development and infer the nature of the progenitor, which was brighter and hotter in quiescence than the B3V companion and with an inferred ZAMS mass of about 65 M.

Literatur zu ""
U. Munari, A. Henden, A. Vallenari, et al.	2005	A&A 434, 1107	"On the distance, reddening and progenitor of V838 Mon"
Tylenda, R.	2005	A&A 436, 1009	"Evolution of V838 Monocerotis during and after the 2002 eruption"

H. Heintzmann

( Eintrag vom 12. 11. 2007)

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