Precise Records of Baby-Stars’ Fast Growth Caught at Millimeter Wavelengths

(ASIAA Science Highlight, August 22, 2018)

Babies grow up fast in the blink of an eye and thus their parents wish to record their growth without missing any moment. This is true not only for human babies but also for baby-stars, called protostars, although the recorders are not parents but astronomers in this case. Protostars’ age, or evolutionary stages, has been determined from observations at near and mid-infrared wavelengths. The youngest stage, called Class 0, is defined by non-detection at near and mid-infrared wavelengths, corresponding to <300,000 years old. This definition cannot differentiate younger and older Class 0 protostars. On the other hand, it is expected from studies on even older protostars that they grow up faster at earlier stages than at later stages, as human babies do, implying that many precious moments of their growth are missed.

As we all know, human “babies” (fetuses) in mothers’ wombs also grow at a fast rate - just as the star babies do. Using ultrasound scanning techniques, parents can hear the baby's heart beating during the regular prenatal examinations; not only so, they could even detect how much the thigh bone grows, how much the head circumference is, or perhaps, getting some hints about girl or boy! All of these are important indicators informing us about how much progress our babies are making -- in terms of their growth.

Similarly, in order to record the important evolutionary stages of baby stars, rather than ultrasound scanners, astronomers would use millimeter/ sub-millimeter telescopes. To probe the fast growth of Class 0 protostars, an international team led by Dr. Yusuke Aso of Academia Sinica Institute of Astronomy and Astrophysics (ASIAA, Taiwan) has observed three Class 0 protostars using the Atacama Large Millimeter/submillimeter Array (ALMA) and has differentiated evolutionary stages of those protostars in multiple aspects. Thanks to ALMA’s strong capabilities, the team revealed four evolutionary indicators in details: (1) dusty disk growth on 100 astronomical-unit scales, (2) widening of outflow opening angles, (3) carbon-monoxide (CO) desorption from icy grains due to temperature rising, and (4) weakening of accretion shock, all of which are consistent with theoretical predictions for young protostars.

Their work demonstrates the importance of millimeter wavelength on probing young protostars’evolution. The work was also accomplished by ALMA’s high spatial resolution differentiating morphology on the small scale and its high sensitivity detecting the faint molecular line from the cold regions.

Dr. Aso the lead author says: “From now on, the precious moments of young baby-stars’ fast growth will be recorded more precisely on millimeter wavelength.”

The background image shows the Serpens Main star forming cluster in near infrared. The inset image shows the location of the two Class 0 protostars SMM4A and SMM4B in the entire cluster, in 1.3 mm wavelength.

Credit: ESO/ALMA(ESO/NAOJ/NRAO)/Aso et al.

Schematic view of evolution in the Class 0 phase, showing the disk grows, the outflow is widened, and CO gas increases. Credit: Aso/ASIAA

(animation by: Lauren Huang/ASIAA)

Paper and Information

“The Distinct Evolutionary Nature of Two Class 0 Protostars in Serpens Main SMM4” by Aso Yusuke et al., was published on 2018 Aug. 7 in the Astrophysical Journal [http://iopscience.iop.org/article/10.3847/1538-4357/aacf9b/meta].