When old stars slow down

first_imgIn the Hollywood blockbuster “Speed,” a bomb on a bus is rigged to blow up if the bus slows down below 50 mph. The premise — slow down and you explode — makes for a great action movie plot, and also happens to have a cosmic equivalent.New research shows that some old stars might be held up by their rapid spins, and when they slow down, they explode as supernovae. Thousands of these “time bombs” could be scattered throughout our Galaxy.“We haven’t found one of these ‘time bomb’ stars yet in the Milky Way, but this research suggests that we’ve been looking for the wrong signs. Our work points to a new way of searching for supernova precursors,” said astrophysicist Rosanne Di Stefano of the Harvard-Smithsonian Center for Astrophysics (CfA).The specific type of stellar explosion Di Stefano and her colleagues studied is called a Type Ia supernova. It occurs when an old, compact star known as a white dwarf destabilizes.A white dwarf is a stellar remnant that has ceased nuclear fusion. It typically can weigh up to 1.4 times as much as the sun — a figure called the Chandrasekhar mass after the astronomer who first calculated it. Any heavier, and gravity overwhelms the forces supporting the white dwarf, compacting it and igniting runaway nuclear fusion that blows the star apart.There are two possible ways for a white dwarf to exceed the Chandrasekhar mass and explode as a Type Ia supernova. It can accrete gas from a donor star, or two white dwarfs can collide. Most astronomers favor the first scenario as the more likely explanation. But we would expect to see certain signs if the theory is correct, and we don’t for most Type Ia supernovae.For example, we should detect small amounts of hydrogen and helium gas near the explosion, but we don’t. That gas would come from matter that wasn’t accreted by the white dwarf, or from the disruption of the companion star in the explosion. Astronomers also have looked for the donor star after the supernova faded from sight, without success.Di Stefano and her colleagues suggest that white dwarf spin might solve this puzzle. A spin-up/spin-down process would introduce a long delay between the time of accretion and the explosion. As a white dwarf gains mass, it also gains angular momentum, which speeds up its spin. If the white dwarf rotates fast enough, its spin can help support it, allowing it to cross the 1.4-solar-mass barrier and become a super-Chandrasekhar-mass star.Once accretion stops, the white dwarf will gradually slow down. Eventually, the spin isn’t enough to counteract gravity, leading to a Type Ia supernova.“Our work is new because we show that spin-up and spin-down of the white dwarf have important consequences. Astronomers therefore must take angular momentum of accreting white dwarfs seriously, even though it’s very difficult science,” explained Di Stefano.The spin-down process could produce a time delay of up to a billion years between the end of accretion and the supernova explosion. This would allow the companion star to age and evolve into a second white dwarf, and any surrounding material to dissipate.In our Galaxy, scientists estimate that there are three Type Ia supernovae every thousand years. If a typical super-Chandrasekhar-mass white dwarf takes millions of years to spin down and explode, then calculations suggest that there should be dozens of pre-explosion systems within a few thousand light-years of Earth.Those supernova precursors will be difficult to detect. However, upcoming wide-field surveys conducted at facilities like Pan-STARRS and the Large Synoptic Survey Telescope should be able to spot them.“We don’t know of any super-Chandrasekhar-mass white dwarfs in the Milky Way yet, but we’re looking forward to hunting them out,” said co-author Rasmus Voss of Radboud University Nijmegen, the Netherlands.This research appears in a paper in the Sept. 1 issue of The Astrophysical Journal Letters and is available online. Authors are Di Stefano (CfA), Voss (Radboud University Nijmegen, the Netherlands), and J.S.W. Claeys (Universiteit Utrecht, the Netherlands).last_img read more

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Real estate prices are up 4 percent from the same month last year

first_imgAccording to research portal Njuškalo.hr   conducted on a sample of 156 thousand active real estate ads, their prices in Croatia are growing rapidly on an annual basis, so they are higher by 4 percent compared to the same period last year.If the current data is compared with the same month last year, apartments have risen in price by 6 and a half percent, while houses are as much as 4 percent more expensive. However, the news is that prices in July this year exceeded those of 2011, which leads to the conclusion that the crisis in the Croatian real estate sector is far behind us and that we may face a real price “boom” in the near future.“Our Real Estate category is the largest online real estate market in the Republic of Croatia and, with almost 3 million visits, the most visited real estate database for sale and rent. Interest in real estate in the Republic of Croatia is growing from year to year, so in July Njuškalo recorded an increase in visits in this category on an annual basis by almost 4 percent. ” stand out from Njuškalo.Looking at the counties in which real estate is most in demand, the City of Zagreb is in first place, while right behind it are the Split-Dalmatia, Primorje-Gorski Kotar and Zadar counties.Apartment prices in the largest Croatian cities have been growing rapidly in the last year. Thus, in Zagreb, the current average price per square meter of an apartment is 1.873 euros and is 9 percent higher compared to the same period last year. The largest annual increase in apartment prices in Croatia, as expected, was recorded in Dubrovnik. There, the average asking price per square meter of an apartment is 3.643 euros, and it is 18 percent higher than last year. The average price per square meter of a house in Dubrovnik has grown by 4 percent annually and currently stands at 4.684 euros.Apartment prices in Split rose by 11 percent and in Pula by almost 12 percentThe average asking price per square meter of an apartment in Split is 2.665 euros, while in Pula it is 1.570 euros. Osijek is still one of the cities where real estate prices are stagnating, so in July the average asking price there was 942 euros.A square meter of an apartment in Croatia is 38 percent more expensive than a square meter of a houseThus, in Zagreb, the square footage of a house recorded a growth of only 1 percent on an annual level, while in Osijek and Zadar, prices remained at the level of last year. The largest annual increase in the price of square meters of houses in Croatia from larger cities was recorded in Split and amounted to as much as 13 percent.The average asking price per square meter of an apartment in Zadar is 2.027 euros and is 11 percent higher than last year, while at the same time the average price per square meter of a house stagnates and amounts to 1.485 euros. As a rule, real estate on the coast and in Zagreb achieves a higher price, while in Slavonia the lowest real estate prices are recorded, so in Slavonski Brod, for example, the average price per square meter is 801 euros, while the average price per square meter is 643 euros. Even cheaper are Vinkovci, where an average of 710 euros will be needed for a square meter of an apartment, while the average price of a square meter of a house with a yard is around 200 euros more favorable than an apartment.Of the larger Croatian cities, Dubrovnik and Split are the only cities where square meters of houses are more expensive than square meters of apartments. This is mostly due to the high share of luxury houses in these cities, they conclude from Njuškalo.Source: Njuškalo.hrlast_img read more

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