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Inflow and outflow at the tip of a lost pillar of creation

posted Oct 3, 2014, 12:45 PM by Jeremy Drake
Stars and planets are born in the murky depths of gas clouds that are sufficiently dense for gravitational collapse to overcome their own pressure.  The proto-stellar cores that form are cocooned within residual gas and dust, and infant stars like the Sun can remain embedded for a million years or more - provided there are no massive stars nearby. When massive stars form - stars from several times to a hundred times the mass of the Sun - their own cocoons are expelled relatively quickly, over tens to hundreds of millennia, by copious radiation and outflow from the collapsing protostars within.  In massive star forming regions, the high mass stars also tend to blow away the gas in their environments.  In the process, "pillars of creation" may be created - residual columns of denser gas that have resisted erosion for a while. The tips of the famous pillars seen in the Eagle Nebula host "EGGs" - evaporating gaseous globules of denser gas and sites where stars and planets are forming. 

SAO postdoc Nick Wright, now at the University of Hertfordshire, discovered similar globule-like structures that are likely the last vestiges of larger pillar-like structures in the Cygnus OB2 Association - a 30,000 solar mass maelstrom of star formation 1.4 kpc away.  Each of them has a core in which a star is forming.  A team of us lead by astronomer Mario Guarcello, ex-SAO postdoc and now at the Osservatorio Astronomico di Palermo, obtained spectra of two of these cores using the 10.4 meter "Gran Telescopio CANARIAS".  Doppler shifts in spectral lines allow us to determine if the line emission is produced by outflowing or inflowing gas.  Inflow is expected because the stellar cores are is still in the process of accreting gas from their surroundings, and this was indeed seen at rates of about a millionth of a solar mass per year.  But one of the objects also showed strong outflow, and at a much higher rate.  The likely culprit responsible for driving off this gas is photoevporation due to the intense UV radiation from all the massive stars in Cygnus OB2.  Essentially, the combined light from these extremely luminous stars heats the outer layer of gas around the stellar core enough for the gas to gain the energy required to escape the protostellar gravitational field.  There is, then, a competition between evaporation and accretion.  If evaporation is fast enough, it can curtail the growth of the star and dissipate the gas where planets might otherwise form.  More observations of this type of object can reveal whether or not massive star forming regions such as Cygnus OB2 are really inhospitable for planet formation.  This work was published in the 2014 September 20 edition of the Astrophysical Journal.