Tiny globules around young and massive stars: what do they hide?

July 1, 2020

A unique combination of the IRAM 30-meter telescope and SOFIA images of the Orion Nebula reveals how the winds and UV radiation from young massive stars eat up their natal molecular cloud. The detection of small molecular globules inside the disrupted material can be explained as a result of the unavoidable cloud destruction process. These tiny globules, however, hide unexpected surprises.

Young massive stars, at least 8 times more massive than our Sun, have a profound impact on their surroundings. Their powerful winds and strong UV radiation disrupt the molecular clouds at the heart of which they are born. Wind and radiation sweep up large amounts of cloud material into enormous expanding bubbles. This process slowly destroys the natal cloud, and limits the rate at which new stars form.

Infrared images have previously revealed that a few hundred interstellar bubbles around massive stars populate the Milky Way. The images, however, provide a static picture and do not allow to study the bubbles’ motion, driving forces, and chemical composition.

Images of the Orion Nebula (M42). The left panel shows the emission from positively charged carbon atoms, observed with SOFIA, revealing a huge bubble blown by the winds from the most massive star in the Trapezium cluster. The 12CO and 13CO images taken with the IRAM 30-meter telescope show the molecular gas in the star-forming cloud behind the bubble. The numbers # show the position of some of the globules detected edge of the bubble. Credits: Goicoechea et al. (2020).

A team led by Javier Goicoechea (CSIC) and Cornelia Pabst (Leiden University) has now obtained wide-field images of the famous Orion Nebula, the closest region to us hosting on-going massive-star formation. Using the IRAM 30-meter telescope, as well as SOFIA, a stratospheric telescope, they detected the gas emission produced by carbon monoxide molecules (CO) as well as positively charged carbon atoms (C+). Once combined, these first velocity-resolved images show the properties of a 10 light-years-size bubble that is blown by the winds emanating from the most massive star in the Trapezium cluster, Orionis C, at the center of the Orion nebula. The bubble expands at incredible speed, nearly 50,000 km/h, and gives the peculiar appearance to this iconic region at the sword of Orion the hunter.

What the scientists didn’t expect was the existence of about a dozen of globules of dense molecular gas that had survived these harsh conditions. The tiny molecular clouds are not massive at all. Their typical size is about 200 times smaller than the Orion nebula itself, their typical mass about a third of the Sun’s mass. Most of these globules may be transient objects that eventually dilute or evaporate. However, once they looked at the globules more closely, another surprise was waiting for the scientist: one of them is evolving to a very young low-mass star.

Gallery of globules detected at the edge of Orion’s expanding bubble. The reddish colors represent the emission from carbon monoxide molecules detected with the IRAM 30-meter telescope. The bluish color is an infrared image taken by Spitzer space telescope. Globule #1 coincides with the position of a very young low-mass star. The white circle represents the angular resolution of the IRAM 30-meter telescope, roughly several times the size of the Solar System. Credits: Goicoechea et al. (2020).

The unique combination of carbon monoxide and ionized carbon velocity-resolved images is a powerful tool to study the impact of young massive stars on their cloud surroundings, with implications on how molecular clouds are destroyed and how star-formation is regulated in these clouds. It is also a very useful tool to detect and characterize the molecular globules that result from the cloud destruction process. It is still not clear whether these tiny objects can be a source of very low-mass stars, brown dwarfs, or free-floating planetary-mass objects. The present study caught first glimpses into the star formation processes going on inside these small globules.

Follow-up observations of the emission from other molecules, more sensitive to the presence of denser gas in their interior, will help to clarify the fate and destiny of these globules. Will they evaporate on the long term or, on the contrary, evolve to a nursery of newly forming stars? These are among the questions the researchers will attempt to answer.


Background Information:

This work is part of an international collaboration leading two complementary large observing programs. One using the IRAM 30-meter telescope at Pico Veleta, Spain (Dynamic and Radiative Feedback of Massive Stars, PI: J. R. Goicoechea) to map the 12CO, 13CO and C18O (J=2-1) emission at 11 arcsec resolution; the other one using NASA/DLR’s SOFIA airborne observatory (C+ Square-degree map of Orion, PI: Prof. A. G. G. M. Tielens) that has produced the largest velocity-resolved map of the [CII]158 μm line (typically the brightest line of the neutral interstellar medium) at 16 arcsec resolution. These C+ images of Orion are also relevant as a local template in the extragalactic context because IRAM-NOEMA can detect the high-redshift [CII]158 μm emission from very distant star-forming galaxies.

The consortium consists of the following institutions: CSIC, Leiden University, Cologne University, IRAP-CNRS, IRAM, Max-Planck-Institut für Radioastronomie, ESAC, NASA Ames, and University of Maryland.


Original papers:

J. R. Goicoechea, C. H. M. Pabst, S. Kabanovic, M. G. Santa-Maria, N. Marcelino, A. G. G. M. Tielens, A. Hacar, O. Berné, C. Buchbender, S. Cuadrado, R. Higgins, C. Kramer, J. Stutzki, S. Suri, D. Teyssier, and M. Wolfire. Molecular globules in Orion’s Veil bubble. IRAM 30 m 12CO, 13CO, and C18O (2-1) expanded maps of Orion A. Published in Astronomy & Astrophysics (2020).

C. H. M. Pabst, J.R. Goicoechea, D. Teyssier, O. Berné, R.D. Higgins, E. T. Chambers, S. Kabanovic, R. Güsten, J. Stutzki, and A.G.G.M. Tielens: Expanding bubbles in Orion A: [CII]158μm observations of M42, M43, and NGC 1977. Published in Astronomy & Astrophysics (2020).



Dr. Javier R. Goicoechea, IFF-CSIC, Madrid

Cornelia Pabst, Leiden Observatory and University

Dr. Carsten Kramer, IRAM, Grenoble