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As­tro­chem­i­cal Dat­ing of a Stel­lar Nurs­ery


An in­ter­na­tional re­search team led by sci­en­tists from the Co­or­di­nated Re­search Cen­ter (CRC) 956 “Con­di­tions and Im­pact of Star For­ma­tion” at the Uni­ver­sity of Cologne has used ob­ser­va­tions made with the GREAT in­stru­ment on board the SOFIA air­craft ob­ser­va­tory and the APEX tele­scope to date the core of an in­ter­stel­lar cloud that is form­ing a group of Sun-like stars.

This work, to which sci­en­tists from the Uni­ver­sity of Helsinki as well as from the Max-Planck-In­sti­tutes for Radio As­tron­omy (MPIfR) and Ex­trater­res­trial Physics (MPE) con­tributed, is pub­lished in this week’s Na­ture jour­nal.

Our own solar sys­tem formed many bil­lion years ago when a dark in­ter­stel­lar cloud started to con­tract to form our pro­to­star – which later be­came the Sun. The du­ra­tion of this first step in stel­lar evo­lu­tion has now been de­ter­mined to last at least 1 mil­lion years in the sim­i­lar sys­tem IRAS 16293-2422, a col­lec­tion of pro­to­stars ~400 light years away in the con­stel­la­tion Ophi­uchus (back­ground image). This has been achieved by using mol­e­c­u­lar hy­dro­gen, H2, as a chem­i­cal clock. As hy­dro­gen is not di­rectly de­tectable, the chem­i­cally closely re­lated species H2D+ has been ob­served in­stead in the submm- and far-in­frared wave­length range, using the ground-based tele­scope APEX in the Chilean Andes and the air­borne ob­ser­va­tory SOFIA. (Fig­ure: Mar­tina Markus & Oskar As­vany, Cred­its: NASA/Carla Thomas, C. Durán/ESO/APEX (MPIfR/ESO/OSO), ESO/Dig­i­tized Sky Sur­vey 2/Da­vide De Mar­tin, ESO/ L. Calçada, Bill Sax­ton, NRAO/AUI/NSF)

The GREAT far-in­frared spec­trom­e­ter on board SOFIA, the air­borne ob­ser­va­tory. The in­stru­ment is mounted to the tele­scope flange, in­side the pres­sur­ized cabin. Dur­ing ob­ser­va­tions GREAT ro­tates ±20 de­grees from the ver­ti­cal. © R. Güsten

Stars like our Sun and their plan­e­tary sys­tems are born in­side clouds con­sist­ing of dust and mol­e­c­u­lar gas. Stel­lar evo­lu­tion be­gins with the con­trac­tion of dense ma­te­r­ial in these stel­lar nurs­eries until an em­bry­onic star, the pro­to­star, is formed. How this col­lapse hap­pens ex­actly, and on what timescales, is not very well un­der­stood. Is the gas “free-falling” to­wards the cen­ter due to grav­ity or is the col­lapse slowed down by other fac­tors?

“Since this process takes much longer than human his­tory, it can­not just be fol­lowed as a func­tion of time. In­stead, one needs to find an in­ter­nal clock that al­lows to read off the age of a par­tic­u­lar star form­ing cloud,” says the lead­ing au­thor San­dra Brünken from the Uni­ver­sity of Cologne.

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The hy­dro­gen mol­e­cule (H2), by far the most abun­dant mol­e­cule in space, could act as such an in­ter­nal, “chem­i­cal” clock. Mol­e­c­u­lar hy­dro­gen ex­ists in two dif­fer­ent forms, called ortho and para, which cor­re­spond to dif­fer­ent ori­en­ta­tions of the spins of the two hy­dro­gen nu­clei.

In the cold and dense mol­e­c­u­lar clouds out of which stars are formed, the rel­a­tive abun­dance of these two forms changes con­tin­u­ously with time by chem­i­cal ex­change re­ac­tions. There­fore the cur­rent abun­dance ratio ob­served is a mea­sure of the time elapsed since the for­ma­tion of H2, and thereby the mol­e­c­u­lar cloud it­self. Un­for­tu­nately, H2 can­not be di­rectly de­tected in the very cold in­ter­stel­lar “breed­ing grounds” of stars.

How­ever, H2D+, an ion­ized vari­ant in which a deuteron par­ti­cle is at­tached to the H2 mol­e­cule, can be ob­served. In­deed, the ortho and para forms of H2D+ emit and ab­sorb at char­ac­ter­is­tic wave­lengths, form­ing “spec­tral lines” that are ob­serv­able with dif­fer­ent tele­scopes. “We knew from our own lab­o­ra­tory ex­per­i­ments and from the­ory that H2D+ has a very close chem­i­cal re­la­tion to H2,” says Stephan Schlem­mer from the Uni­ver­sity of Cologne who pro­posed the ob­ser­va­tions.

“For the first time we could now ob­serve both vari­ants of H2D+, which al­lowed us to in­di­rectly de­ter­mine the ratio of ortho H2 to para H2. Read­ing this chem­i­cal clock we find an age of at least one mil­lion years for the parental cloud that is right now giv­ing birth to Sun-like stars.” This re­sult is ques­tion­ing the­o­ries of rapid star for­ma­tion.

The as­tro­nom­i­cal ob­ser­va­tions were very chal­leng­ing. The rel­e­vant spec­tral line of para- H2D+ lies in the far in­frared wave­length range (at 219 µm), where the Earth’s at­mos­phere ab­sorbs most of the ra­di­a­tion. “Its first un­am­bigu­ous de­tec­tion was only pos­si­ble due to the unique ca­pa­bil­i­ties of the GREAT (Ger­man RE­ceiver for As­tron­omy at Ter­a­hertz Fre­quen­cies) in­stru­ment on board the SOFIA (Stratos­pheric Ob­ser­va­tory for In­frared As­tron­omy) air­craft,” says Jürgen Stutzki, whose group at the Uni­ver­sity of Cologne is in­volved in the de­vel­op­ment of GREAT.

SOFIA is a joint pro­ject be­tween NASA and the DLR (Deutsches Zen­trum für Luft- und Raum­fahrt) car­ry­ing a 2.7 meter di­am­e­ter tele­scope as high as 13.7 km, above the at­mos­phere’s ab­sorb­ing lay­ers. The team also ob­served the cor­re­spond­ing ro­ta­tional line of or­tho- H2D+ at mm-wave­lengths with the ground-based APEX (At­a­cama Pathfinder Ex­per­i­ment) tele­scope lo­cated in the Chilean Andes at an al­ti­tude of 5100 m. APEX Prin­ci­pal In­ves­ti­ga­tor, the MPIfR’s Karl Menten points out that “It’s great to see the syn­ergy be­tween both tele­scopes!”

The age of this star-form­ing cloud, which is lo­cated in the Ophi­uchus con­stel­la­tion at a dis­tance of around 400 light years, was de­ter­mined by com­par­ing the data from the tele­scopes with ex­ten­sive com­puter sim­u­la­tions of the chem­istry that is chang­ing with time. “The sim­u­la­tions allow us a de­tailed look at the move­ment of our H2D+ clock,” ex­plains Jorma Harju of the Uni­ver­sity of Helsinki. “We find that our new chem­i­cal clock is more pre­cise than any of those used pre­vi­ously. Even more im­por­tantly, it keeps run­ning when other clocks have al­ready stopped work­ing.” The team is con­fi­dent that their new method will help to date other stel­lar birth­places.

Orig­i­nal Pub­li­ca­tion:

“H2D+ ob­ser­va­tions give an age of at least one mil­lion years for a cloud core form­ing Sun-like stars”
San­dra Brünken, Olli Sipilä, Ed­ward T. Cham­bers, Jorma Harju, Paola Caselli, Oskar As­vany, Cor­nelia E. Hon­ingh, Tomasz Kamin­ski, Karl M. Menten, Jürgen Stutzki, Stephan Schlem­mer


Prof. Stephan Schlem­mer
I. Physikalis­ches In­sti­tut
Uni­ver­sität zu Köln
Zülpicher Str. 77
50937 Köln
Phone: +49 (0)221 470 7880
email: schlem­mer@​ph1.​uni-koeln.​de
web: http://​www.​astro.​uni-koeln.​de/​labas­tro

Merle Hettesheimer
Presse und Kom­mu­nika­tion
Uni­ver­sität zu Köln
Phone: +49 (0)221 4701700
Email: m.​hettes­heimer(at)uni-koeln.​de

SOFIA, the Stratos­pheric Ob­ser­va­tory for In­frared As­tron­omy, is a joint pro­ject of the Na­tional Aero­nau­tics and Space Ad­min­is­tra­tion (NASA) and the Deutsches Zen­trum für Luft- und Raum­fahrt e.V. (DLR; Ger­man Aero­space Cen­tre, grant: 50OK0901). The Ger­man com­po­nent of the SOFIA pro­ject is being car­ried out under the aus­pices of DLR, with funds pro­vided by the Fed­eral Min­istry of Eco­nom­ics and Tech­nol­ogy (Bun­desmin­is­terium für Wirtschaft und Tech­nolo­gie; BMWi) under a res­o­lu­tion passed by the Ger­man Fed­eral Par­lia­ment, and with fund­ing from the State of Baden-Würt­tem­berg and the Uni­ver­sity of Stuttgart. Sci­en­tific op­er­a­tions are co­or­di­nated by the Ger­man SOFIA In­sti­tute (DSI) at the Uni­ver­sity of Stuttgart and the Uni­ver­si­ties Space Re­search As­so­ci­a­tion (USRA) head­quar­tered in Co­lum­bia, Mary­land, U.S.A.

GREAT, the Ger­man Re­ceiver for As­tron­omy at Ter­a­hertz Fre­quen­cies, is a re­ceiver for spec­tro­scopic ob­ser­va­tions in the far in­frared spec­tral regime at fre­quen­cies be­tween 1.25 and 5 ter­a­hertz (wave­lengths of 60 to 240 mi­crons), which are not ac­ces­si­ble from the ground due to ab­sorp­tion by water vapor in the at­mos­phere. GREAT is a first-gen­er­a­tion Ger­man SOFIA in­stru­ment, de­vel­oped and main­tained by the Max-Planck In­sti­tute for Radio As­tron­omy (MPIfR) and KOSMA at the Uni­ver­sity of Cologne, in col­lab­o­ra­tion with the Max Planck In­sti­tute for Solar Sys­tem Re­search and the DLR In­sti­tute of Plan­e­tary Re­search. Rolf Guesten (MPIfR) is the prin­ci­pal in­ves­ti­ga­tor for GREAT. The de­vel­op­ment of the in­stru­ment was fi­nanced by the par­tic­i­pat­ing in­sti­tutes, the Max Planck So­ci­ety and the Ger­man Re­search Foun­da­tion (Deutsche Forschungs­ge­mein­schaft; DFG).

APEX, the At­a­cama Pathfinder Ex­per­i­ment, is a col­lab­o­ra­tion be­tween the Max Planck In­sti­tute for Radio As­tron­omy (MPIfR), On­sala Space Ob­ser­va­tory (OSO), and the Eu­ro­pean South­ern Ob­ser­va­tory (ESO) to con­struct and op­er­ate a mod­i­fied pro­to­type an­tenna of ALMA (At­a­cama Large Mil­lime­tre Array) as a sin­gle dish on the Cha­j­nan­tor plateau at an al­ti­tude of 5,100 me­ters above sea level (At­a­cama Desert, Chile). The tele­scope was man­u­fac­tured by VER­TEX An­ten­nen­tech­nik in Duis­burg, Ger­many. The op­er­a­tion of the tele­scope is en­trusted to ESO.

Merle Hettesheimer | University of Cologne
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