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Huge stellar dust cloud vanishes, leaving only questions

July 6, 2012
Courtesy of UCLA
and World Science staff

As­tro­no­mers are re­port­ing a baf­fling dis­cov­ery nev­er seen be­fore: A huge cloud of dust around a near­by star has dis­ap­peared.

“It’s like the clas­sic ma­gi­cian’s trick — now you see it, now you don’t,” said Carl Melis, a post­doc­tor­al schol­ar at Uni­vers­ity of Cal­i­for­nia San Die­go and lead au­thor of the re­search. “Only in this case, we’re talk­ing about enough dust to fill an in­ner so­lar sys­tem, and it really is gone!”

“It’s as if the rings around Sat­urn had dis­ap­peared,” said co-au­thor Ben­ja­min Zuck­er­man, a Uni­vers­ity of Cal­i­for­nia Los An­ge­les as­tron­o­mer. “This is even more shock­ing be­cause the dusty disc of rocky de­bris was big­ger and much more mas­sive than Sat­urn’s rings. The disc around this star, if it were in our so­lar sys­tem, would have ex­tend­ed from the sun half­way out to Earth, near the or­bit of Mer­cury.”

Re­search on the cos­mic van­ish­ing act ap­pears July 5 in the jour­nal Na­ture. The eerie event oc­curred around a star some 450 light years from Earth, in the di­rec­tion of the con­stella­t­ion Cen­tau­rus. A light year is the dis­tance light travels in a year.

“We don't have a satisfactory explanation,” said Melis. “There is no ev­i­dence to sug­gest that the star zapped the dust with some sort of mega-flare or any oth­er vi­o­lent event.”

Melis de­scribes the star, des­ig­nat­ed TYC 8241 2652, as a “young an­a­log of our sun” that only a few years ago dis­played all of the char­ac­ter­is­tics of “host­ing a so­lar sys­tem in the mak­ing,” be­fore trans­form­ing com­plete­ly. Now, very lit­tle of the warm, dusty ma­te­ri­al thought to orig­i­nate from col­li­sions of rocky plan­ets is ap­par­ent.

“Noth­ing like this has ev­er been seen in the many hun­dreds of stars that as­tron­o­mers have stud­ied for dust rings,” Zuck­er­man said. “This dis­ap­pear­ance is re­markably fast, even on a hu­man time scale, much less an as­tro­nom­i­cal scale. The dust dis­ap­pear­ance at TYC 8241 2652 was so bi­zarre and so quick, in­i­tially I fig­ured that our ob­serva­t­ions must simply be wrong in some strange way.”

Norm Mur­ray, di­rec­tor of the Ca­na­di­an In­sti­tute for The­o­ret­i­cal As­t­ro­phys­ics, who was not part of the re­search group, said, “The his­to­ry of as­tron­o­my has shown that events that are not pre­dicted and hard to ex­plain can be game-changers.”

The dust had en­cir­cled the star since at least 1983, and it con­tin­ued to glow brightly in in­fra­red light for 25 years. In 2009, it started to dim. By 2010, the dust emis­sion was gone; the as­tron­o­mers ob­served the star twice that year from the Gem­i­ni Ob­serv­a­to­ry in Chil­e, six months apart. An in­fra­red im­age ob­tained by the Gem­i­ni tel­e­scope May 1 of this year showed the warm dust had been gone for 2½ years.

Like Earth, warm dust ab­sorbs the en­er­gy of sun­light and re-radiates that heat en­er­gy as in­fra­red radia­t­ion. Be­cause so much dust had been or­biting around the star, plan­ets very likely are form­ing there, said Zuck­er­man, whose re­search is funded by NASA.

The lack of an ex­ist­ing mod­el for what is go­ing on around this star is forc­ing as­tron­o­mers to re­think what hap­pens with­in young so­lar sys­tems in the mak­ing. The dust likely re­sulted from a vi­o­lent col­li­sion — but that would not ex­plain where it went. Was it some­how swal­lowed by the star?

“Although we’ve iden­ti­fied a cou­ple of mech­a­nisms that are po­ten­tially vi­a­ble, none are really com­pelling,” Melis said. “In one case, gas pro­duced in the im­pact that re­leased the dust helps to quickly dra­g the dust par­t­i­cles in­to the star and thus to their doom. In anoth­er pos­si­bil­ity, col­li­sions of large rocks left over from an orig­i­nal ma­jor im­pact pro­vide a fresh in­fu­sion of dust par­t­i­cles in­to the disc, which then in­sti­gate a run­away pro­cess where small grains chip in­to ob­liv­i­on both them­selves and al­so larg­er grains.”

Ma­jor dusty re­gions are known to ex­ist in our own so­lar sys­tem and in­clude the as­ter­oid belt be­tween the or­bits of Mars and Ju­pi­ter and anoth­er be­yond Nep­tune’s orbit. Nearly 30 years ago, NASA’s In­fra­red As­tronomical Sat­el­lite disco­vered many si­m­i­lar re­gions or­biting oth­er stars — but no dis­ap­pear­ing act like the one at TYC 8241 2652 has ev­er been seen dur­ing these three dec­ades.

“We were lucky to catch this dis­ap­pear­ing ac­t,” Zuck­er­man said. “Such events could be rel­a­tively com­mon, with­out our know­ing it.”

* * *

Astronomers are reporting a baffling discovery never seen before: A huge cloud of dust around a nearby star has disappeared. “It’s like the classic magician’s trick — now you see it, now you don’t,” said Carl Melis, a postdoctoral scholar at University of California San Diego and lead author of the research. “Only in this case, we’re talking about enough dust to fill an inner solar system, and it really is gone!” “It’s as if the rings around Saturn had disappeared,” said co-author Benjamin Zuckerman, a University of California Los Angeles astronomer. “This is even more shocking because the dusty disc of rocky debris was bigger and much more massive than Saturn’s rings. The disc around this star, if it were in our solar system, would have extended from the sun halfway out to Earth, near the orbit of Mercury.” The research on the cosmic vanishing act, which occurred around a star some 450 light years from Earth, in the direction of the constellation Centaurus, appears July 5 in the journal Nature. “A perplexing thing about this discovery is that we don’t have a satisfactory,” said Melis. “The disappearing act appears to be independent of the star itself, as there is no evidence to suggest that the star zapped the dust with some sort of mega-flare or any other violent event.” Melis describes the star, designated TYC 8241 2652, as a “young analog of our sun” that only a few years ago displayed all of the characteristics of “hosting a solar system in the making,” before transforming completely. Now, very little of the warm, dusty material thought to originate from collisions of rocky planets is apparent. “Nothing like this has ever been seen in the many hundreds of stars that astronomers have studied for dust rings,” Zuckerman said. “This disappearance is remarkably fast, even on a human time scale, much less an astronomical scale. The dust disappearance at TYC 8241 2652 was so bizarre and so quick, initially I figured that our observations must simply be wrong in some strange way.” Norm Murray, director of the Canadian Institute for Theoretical Astrophysics, who was not part of the research group, said, “The history of astronomy has shown that events that are not predicted and hard to explain can be game-changers.” The dust had encircled the star since at least 1983, and it continued to glow brightly in infrared light for 25 years. In 2009, it started to dim. By 2010, the dust emission was gone; the astronomers observed the star twice that year from the Gemini Observatory in Chile, six months apart. An infrared image obtained by the Gemini telescope as recently as May 1 of this year confirmed that the warm dust has now been gone for two-and-a-half years. Like Earth, warm dust absorbs the energy of sunlight and re-radiates that heat energy as infrared radiation. Because so much dust had been orbiting around the star, planets very likely are forming there, said Zuckerman, whose research is funded by NASA. The lack of an existing model for what is going on around this star is forcing astronomers to rethink what happens within young solar systems in the making. The dust likely resulted from a violent collision — but that would not explain where it went. Was it somehow swallowed by the star? “Although we’ve identified a couple of mechanisms that are potentially viable, none are really compelling,” Melis said. “In one case, gas produced in the impact that released the dust helps to quickly drag the dust particles into the star and thus to their doom. In another possibility, collisions of large rocks left over from an original major impact provide a fresh infusion of dust particles into the disc, which then instigate a runaway process where small grains chip into oblivion both themselves and also larger grains.” Major dusty regions are known to exist in our own solar system and include the asteroid belt between the orbits of Mars and Jupiter and another located beyond the orbit of Neptune. Nearly 30 years ago, NASA’s Infrared Astronomical Satellite (IRAS) first discovered many similar regions orbiting other stars — but no disappearing act like the one at TYC 8241 2652 has ever been seen during these three decades. “We were lucky to catch this disappearing act,” Zuckerman said. “Such events could be relatively common, without our knowing it.”