Nasty 1: a star unique setting

Thanks to the Hubble Space Telescope NASA a team of astronomers has discovered surprising new clues about a massive star rapidly aging. The star showed a behavior so strange, compared to others that we know in our galaxy, astronomers have nicknamed “Nasty 1” (literally “bad” or “sloppy”), a pun on the name of the catalog that NaSt1 comes from the first two letters of each of the two astronomers who discovered it in 1963: Jason Nassau and Charles Stephenson. Nasty 1 could represent a transitional phase of the evolution of very massive stars.
Nasty 1 has been identified as a Wolf-Rayet star, or a star in rapid evolution with much more massive mass than our sun. This type of stars loses their outer layers rich in hydrogen by exposing the core extremely bright and warm, composed essentially of helium.


Credit: NASA, ESA e G. Bacon (STScI)

Still Nasty 1 does not seem a typical star Wolf-Rayet. When astronomers have observed it, using the Hubble telescope, they were expected to see two symmetrical lobes of gas, similar to those seen around the massive star Eta Carinae, a Wolf-Rayet candidate. Instead Hubble has revealed a large disk of gas about 3,200 billion kilometers that may have formed from a companion star barely visible, positioned along the outer envelope of the newly Wolf-Rayet. According to current estimates, the nebula surrounding the star a few thousand years old, and is less than 3,000 light years from Earth.
“We’re very excited to see this disc, it could be demonstrated that a Wolf-Rayet star may form a binary system interacting,” said Jon Mauerhan the University of California, at the head of the study. “It’s very difficult to see this process in action, because this phase is short-lived, lasting perhaps only a hundred thousand years, while the time within which a disc produced by these processes remains visible may even get to 10,000 years or less. ”
According to the scenario proposed by the authors of the work, a massive star evolves very quickly and just beginning to be short of hydrogen expands. Its outer casing hydrogen is therefore more weakly bound and vulnerable to the gravitational pull or other types of cannibalism stellar by the mate. During this process, the more compact the star ends up increasing its mass and the star originally massive loses its hydrogen envelope exposing the helium nucleus and becoming a Wolf-Rayet star.
Another channel through which you think you can form the Wolf-Rayet is when a massive star ejects its outer envelope of hydrogen through a strong stellar wind flow. The binary interaction model, where there is a companion star, is gaining greater credibility because astronomers have realized that at least 70% of the massive stars are members of double star systems. The only mass loss, also cannot explain the fraction of Wolf-Rayet than other less evolved massive stars within the galaxy.
“We’re finding that it is difficult to form all of the Wolf-Rayet stars that we see through the mechanism of the stellar wind, because in this case the mass loss is not as effective as we thought,” said Nathan Smith of the University of Arizona in Tucson, co-author new work on NaSt1. ‘The exchange of material in binary systems seems to be vital to the formation of Wolf-Rayet stars and supernovae that give life. Capture images of binary stars that cross this short stage will help us to better understand this process. ”
But the process of mass transfer in binary systems is not always effective. A portion of the material torn from the massive star can be ejected during the push and pull of gravity between the stars, creating a disk around the binary system. “This is what we think is going to Nasty one,” said Mauerhan. “We think there is a Wolf-Rayet star buried in the nebula, and we think that the nebula was created by this process of mass transfer. This type of stellar cannibalism “sloppy” makes the nickname Nasty 1 quite suitable. ”
Observe in detail Nasty 1 was not easy. The system is surrounded by large amounts of gas and dust, which prevent even to Hubble to have a clear view of the stars in its interior. The team of Mauerhan has therefore not been able to accurately estimate the mass of each star, the distance that separates them or the amount of material that is transferred on the companion star.


Credit: NASA, ESA, e J. Mauerhan (University of California, Berkeley)

However, thanks to previous observations we have some information on the gas in the disk. The material travels at about 35,000 kilometers per hour in the outer nebula, a rate lower than similar stars. A relatively slow speed indicates that the star ejected its material through a very violent event, when compared to the explosions of Eta Carinae, the gas which has speeds of hundreds of thousands of kilometers per hour.
Nasty 1 could also spread the material sporadically. Earlier research in the infrared band showed a thickening powder compact hot, very close to the central star. Recent observations of Mauerhan and colleagues at the University of Arizona, made with the Magellan telescope at the Las Campanas Observatory in Chile, made it possible to reveal the presence of a thickening of dust at lower temperatures and larger dimensions that may be responsible for dispersion of light from the central stars. The observation of dust at intermediate temperatures implies that this has been formed in recent times, perhaps with jets, because of the collision of the material of the two chemically enriched stellar winds; it is then mixed, and then moved away on cooling. The structure with clumps and voids observed in the outer regions of the disk may also be explained by sporadic changes in wind or the rate at which the companion star attracts the outer layers of hydrogen the main star.
In order to measure the supersonic winds of each star, astronomers have requested observing time with Chandra X-ray Observatory of NASA. Observations have revealed the presence of plasma at very high temperatures, strong indicator that the winds of the stars that make up the system are actually colliding. These results are consistent with what astronomers have observed in other systems of Wolf-Rayet.
The transfer activities chaotic mass will cease at the time when the star Wolf-Rayet exhausted their own material. Subsequently, the gas in the disc will dissipate providing a clear view of the binary system.
“What evolution will take the star is not yet clear, but it certainly will not be boring,” said Mauerhan. “Nasty one could evolve into another type system of Eta Carinae. To get to accomplish this transformation, the companion star that gains mass could for example experience a major eruption, because of the instability due to the acquisition of material. Alternatively, the Wolf-Rayet may explode as a supernova. Another possible scenario is the merger of the two stars as a result of the evolution of the orbital system. The future may be full of exotic possibilities, depending on whether the system explodes or not, the length of the mass transfer and the survival time of the system at the end of the mass transfer. “

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