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Study said to crack mystery of “monster” stars

Aug. 11, 2012
Courtesy of the Royal Astronomical Society
and World Science staff

Four “mon­ster” stars whose colossal size has de­fied ex­plana­t­ion turn out to have been formed thanks to stel­lar merg­ers, sci­en­tists re­port.

In 2010 sci­en­tists dis­cov­ered the stars, the big­gest of which weighs the equiv­a­lent of more than 300 Suns, in a near­by gal­axy known as the Large Mag­el­lanic Cloud. De­spite their in­credible bright­ness, si­m­i­lar ob­jects have turned up no­where else. 

An art­ist's il­lus­tra­tion of the Wolf-Ray­et star R136a1, the most mas­sive star known. (Credit: Wiki­pedia)


Now a group of as­tro­no­mers at the Uni­vers­ity of Bonn in Ger­ma­ny pre­s­ent their find­ings on how the ob­jects arose, in a pa­per in the jour­nal Monthly No­tices of the Roy­al As­tro­nom­i­cal So­ci­e­ty. Their con­clu­sion: the “ul­tra­mas­sive” stars were cre­at­ed from the merg­er of stars in tight bi­na­ry sys­tems, which con­sist of a pair of stars or­bit­ing each oth­er.

The Large Mag­el­lanic Cloud is the third near­est sat­el­lite of the Milky Way gal­axy we live in and it con­tains around 10 bil­lion stars. It has many ac­tively star-forming re­gions, with by far the most ac­tive be­ing one that’s 1,000 light-years wide (a light-year is the dis­tance light trav­els in a year).

This ar­ea, called the Ta­ran­tu­la Neb­u­la, is where the four su­per­mas­sive stars lie. It’s a cloud of gas and dust that serves as a highly fer­tile breed­ing ground of stars, al­so known as the “30 Do­ra­dus” (30 Dor) com­plex. Near the cen­ter of 30 Dor is a gi­ant star clus­ter called R136, by far the bright­est stel­lar nurse­ry not just in that ga­la­xy but in our en­tire “Lo­cal Group” of more than 50 ga­lax­ies.

Un­til the mon­ster stars ap­peared, it was thought that stars heav­i­er than about 150 Suns were im­pos­si­bil­i­ties. “The star birth pro­cess seems to be uni­ver­sal,” and simply does­n’t al­low for that, ex­plained Pa­vel Krou­pa of the Uni­vers­ity of Bonn, a co-author of the find­ings. But the mon­ster stars pre­s­ented a stark chal­lenge to that as­sump­tion. What gives?

The Bonn group, al­so in­clud­ing lead in­ves­ti­ga­tor Sam­baran Baner­jee, mod­elled the in­ter­ac­tions be­tween stars in a R136-like clus­ter. Their com­put­er sim­ula­t­ion as­sem­bled the mod­el clus­ter star by star, so as to re­sem­ble the real clus­ter as closely as pos­si­ble, cre­at­ing a group of more than 170,000 stars packed closely to­geth­er.

To com­pute how this sys­tem changes over time, the mod­el had to solve 510,000 equa­t­ions many times over. The sim­ula­t­ion is com­pli­cat­ed by the ef­fect of the nu­clear re­ac­tions and hence en­er­gy re­leased by each star and what hap­pens when two stars hap­pen to col­lide, a fre­quent event in such a crowd­ed en­vi­ron­ment.

These highly in­ten­sive, star by star cal­cula­t­ions are known as “di­rect N-body sim­ula­t­ions” and are con­sid­ered the most re­li­a­ble and ac­cu­rate way to mod­el clus­ters of stars. The Bonn re­search­ers used a com­put­er code called “N­BODY6,” de­vel­oped pri­marily by Sverre Aarseth of the In­sti­tute of As­tron­o­my in Cam­bridge, and took ad­van­tage of the un­prec­e­dent­ed hard­ware ac­celera­t­ion of video-gaming cards in­stalled in oth­erwise or­di­nary worksta­t­ions to fast for­ward their cal­cula­t­ions.

With all this, the sim­ula­t­ion in­volved the most dif­fi­cult, in­ten­sive N-body cal­cula­t­ions ev­er made, said the astron­omers. “Once these cal­cula­t­ions were done, it quickly be­came clear that the ul­tra­mas­sive stars are no mys­tery,” added Ban­er­jee. “They start ap­pear­ing very early in the life of the clus­ter. With so many mas­sive stars in tight bi­na­ry pairs, them­selves packed closely to­geth­er, there are fre­quent ran­dom en­coun­ters, some of which re­sult in col­li­sions where two stars co­a­lesce in­to heav­i­er ob­jects. The re­sulting stars can then quite easily end up be­ing as ul­tra­mas­sive as those seen in R136.

“Imag­ine two bulky stars closely cir­cling each oth­er but where the du­o gets pulled apart by the gravita­t­ional at­trac­tion from their neigh­bour­ing stars. If their in­i­tially cir­cu­lar or­bit is stretched enough, then the stars crash in­to each oth­er as they pass and make a sin­gle ul­tra­mas­sive star,” Baner­jee ex­plained.

“This helps us re­lax,” added Kroupa. “The uni­ver­sal­ity of star forma­t­ion pre­vails af­ter al­l.”


* * *

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Homepage image: R136a1 and its stellar neighborhood (courtesy European Southern Observatory)







 

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Four “monster” stars whose extreme immensity has defied explanation turn out to have been formed thanks to stellar mergers, scientists report. In 2010 scientists discovered the stars, the biggest of which weighs the equivalent of more than 300 Suns, in a giant star cluster called R136 in a nearby galaxy known as the Large Magellanic Cloud. Despite their incredibly brightness, similar objects have turned up nowhere else. Now a group of astronomers at the University of Bonn in Germany present their findings on how the objects arose, in a paper in the journal Monthly Notices of the Royal Astronomical Society. Their conclusion: the “ultramassive” stars were created from the merger of stars in tight binary systems, which consist of a pair of stars orbiting each other. The Large Magellanic Cloud is the third nearest satellite of the Milky Way galaxy we live in and it contains around 10 billion stars. It has many actively star-forming regions, with by far the most active being one that’s 1,000 light-years wide (a light-year is the distance light travels in a year). This area, called the Tarantula Nebula, is where the four supermassive stars lie. It’s a cloud of gas and dust that serves as a highly fertile breeding ground of stars, also known as the “30 Doradus” (30 Dor) complex. Near the centre of 30 Dor is R136, by far the brightest stellar nursery not just in the LMC but in our entire “Local Group” of more than 50 galaxies. Until the monster stars appeared, it was thought that stars heavier than about 150 Suns were impossibilities. “The star birth process seems to be universal,” and simply doesn’t allow for that, explained Pavel Kroupa of the University of Bonn, a co-author of the findings. But the monster stars presented a stark challenge to that assumption. What gives? The Bonn group, also including lead investigator Sambaran Banerjee, modelled the interactions between stars in a R136-like cluster. Their computer simulation assembled the model cluster star by star, so as to resemble the real cluster as closely as possible, creating a cluster of more than 170,000 stars packed closely together. To compute how even this relatively basic system changes over time, the model had to solve 510,000 equations many times over. The simulation is complicated by the effect of the nuclear reactions and hence energy released by each star and what happens when two stars happen to collide, a frequent event in such a crowded environment. These highly intensive, star by star calculations are known as ‘direct N-body simulations’ and are considered the most reliable and accurate way to model clusters of stars. The Bonn researchers used a computer code called “NBODY6,” developed primarily by Sverre Aarseth of the Institute of Astronomy in Cambridge, and took advantage of the unprecedented hardware acceleration of video-gaming cards installed in otherwise ordinary workstations to fast forward their calculations. With all this, the simulation involved the most difficult, intensive N-body calculations ever made, said Pavel and Seungkyung. “Once these calculations were done, it quickly became clear that the ultramassive stars are no mystery,” added Banerjee. “They start appearing very early in the life of the cluster. With so many massive stars in tight binary pairs, themselves packed closely together, there are frequent random encounters, some of which result in collisions where two stars coalesce into heavier objects. The resulting stars can then quite easily end up being as ultramassive as those seen in R136. “Imagine two bulky stars closely circling each other but where the duo gets pulled apart by the gravitational attraction from their neighbouring stars. If their initially circular orbit is stretched enough, then the stars crash into each other as they pass and make a single ultramassive star”, Banerjee explained. “This helps us relax,” added Kroupa. “The universality of star formation prevails after all.”