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Starts With A Bang

Perhaps ‘Stellar Jets’ Come From Protoplanetary Disks, Not Stars Alone

An almost 40-year-old theory finally has ‘smoking gun’ evidence for it.

During most of their lives, stars burn stably, changing imperceptibly.

The rotten egg nebula, at lower right (and shown in detail in the inset box, as imaged by Hubble) is a preplanetary nebula that’s part of a larger star cluster that also contains a full-blown planetary nebula, at the upper left. Most of the stars, however, are burning stably through their fuel, and are unchanging to humanity’s instruments. (ADAM BLOCK/MOUNT LEMMON SKYCENTER/UNIVERSITY OF ARIZONA (MAIN); ESA/HUBBLE & NASA ACKNOWLEDGEMENT: JUDY SCHMIDT (INSET))

But it isn’t merely death that induces spectacular, cataclysmic phenomena.

A vast star-forming region has rich, gaseous structures illuminated by starlight. The giant red nebula consists of many massive stars, with the smaller, blue nebula created by a single massive star. This was chosen to be Hubble’s official 30th anniversary image. With more than 1.4 million observations of ~47,000 objects, Hubble has been the most scientifically prolific observatory in history. (NASA, ESA AND STSCI)

Stars exhibit extreme violence in their youth: when planetary systems are still forming.

The Bubble Nebula, also known as NGC 7635, is an emission nebula 8,000 light-years away. The individual stellar features inside of it can be clearly seen in this Hubble image, including the central star responsible for ‘blowing’ this bubble by emitting intense radiation into the denser interstellar medium. (NASA, ESA, HUBBLE HERITAGE TEAM)

Stars come into existence when nuclear fusion begins in a dense, collapsed molecular cloud.

The very young protostar M17-SO1, as imaged way back in 2005 with the ground-based Subaru telescope. This image shows features of a protoplanetary disk around a newly-forming star, but without the ability to fully see through the surrounding matter or to resolve any features like gaps or bands present in the disk. (SUBARU / NAOJ)

The ensuing energetic radiation and stellar winds ionize and blow off surrounding material.

The gas burning off in the Carina Nebula may be clumping into planet-like and planet-sized objects, but the luminosity and the ultraviolet radiation from the massive star driving the evaporation will certainly boil it all away before any clumps can grow into a star. The “clumps” that do remain will likely form failed stars and failed solar systems: a bevy of rogue planets. (NASA, THE HUBBLE HERITAGE TEAM AND NOLAN R. WALBORN (STSCI), RODOLFO H. BARBA’ (LA PLATA OBSERVATORY, ARGENTINA), AND ADELINE CAULET (FRANCE))

Volatile ices are boiled away, while gravitational imperfections grow into protoplanetary seeds.

A schematic of a protoplanetary disk, showing the Soot and Frost Lines. For a star like the Sun, estimates put the Frost Line at somewhere around three times the initial Earth-Sun distance, while the Soot Line is significantly further in. The exact locations of these lines in our Solar System’s past is hard to pin down. (NASA / JPL-CALTECH, ANNONATIONS BY INVADER XAN)

But all the while, magnetic fields are present, interacting between the young star and the surrounding disk.

Ultra-hot, young stars can sometimes form jets, like this Herbig-Haro object in the Orion Nebula, just 1,500 light years away from our position in the galaxy. The radiation and winds from young, massive stars can impart enormous kicks to the surrounding matter, where we find organic molecules as well. (ESA / HUBBLE & NASA, D. PADGETT (GSFC), T. MEGEATH (UNIVERSITY OF TOLEDO), AND B. REIPURTH (UNIVERSITY OF HAWAII))

For many decades, young stellar jets have been observed in Herbig-Haro objects.

This Hubble image shows two Herbig-Haro objects HH46 and HH47. The color image was made from separate exposures taken in the visible and infrared regions of the spectrum with Hubble’s Wide Field Camera 3 (WFC3). It is based on data obtained through six filters. The color results from assigning different hues to each monochromatic image associated with an individual filter. (NASA / ESA / HUBBLE / B. NISINI)

Material gets ejected outward from the star: bidirectionally, but along a single axis.

The Herbig-Haro object, HH34, is a protostar still in the early stages of star formation. Two jets in opposite directions can clearly be seen, and it’s long been suspected that these jets arise from a magnetic interaction between the central proto-star and the surrounding, disk-like material. This picture, however, has not been confirmed for this system just yet. (ESO / FORS2 INSTRUMENT)

Despite appearances, the star alone cannot account for these jet-like features.

Protostar HH46/47 ejects a bipolar jet, with a volatile-rich composition. There is no way that material such as water ice, carbon dioxide ice, plus methyl alcohol, methane gas, and silicate minerals should arise from the protostar itself, but these materials should be copious in the protoplanetary disk, creating indirect evidence for a star-disk interaction. (NASA/JPL/CALTECH)

Instead, a rotating protoplanetary disk interacts with the star’s magnetic field.

The protoplanetary disk around the star HL Tauri in a young star cluster may well be the best analogue of a Sun-like star forming, with planets around it, that we’ve ever seen. This was ALMA’s first protoplanetary disk to display the rings and gaps, and over the past 4 years our knowledge of protoplanetary evolution has taken us ever closer to a complete understanding of these systems. (ALMA (ESO/NAOJ/NRAO)/NASA/ESA)

The theory, proposed in 1982 by Blandford and Payne, appears validated by new multiwavelength observations.

A composite radio/visible image of the protoplanetary disk and jet around HD 163296. The protoplanetary disk and features are revealed by ALMA in the radio, while the blue optical features are revealed by the MUSE instrument aboard the ESO’s Very Large Telescope. They are at perpendicular, 90 degree angles to one another. (VISIBLE: VLT/MUSE (ESO); RADIO: ALMA (ESO/NAOJ/NRAO))

On solar system scales, magnetic fields funnel material into or perpendicularly away from the protoplanetary disk.

A new star, very massive and in the earliest stages of its life cycle, displays a protoplanetary disk and bipolar jets that only ALMA has the capability of revealing. The combination of radio observations, for revealing protoplanetary disks, and visible light observations, which could reveal jets, can finally shed light on the mystery of the jet-like origins in Herbig-Haro systems. (ALMA (ESO/NAOJ/NRAO))

Those fields then collimate the ejected material, launching the observed jets.

Herbig-Haro object HH34 and its outflows, which are highly collimated. What looks to be a single star with a small jet can actually be seen continuing many light-years farther away, as the ejected material slams into material farther out, producing the ionized shock front seen at the image’s lower left. (ESA/HUBBLE & NASA)

Observationally, the jets and disk are indeed perpendicular.

The outflows from the young star HD 163296, as observed by MUSE. When the three-dimensional orientation of both the jet and the disk are determined, they are found to be perpendicular: exactly as the theory of magnetic fields in the rotating disk predicts. (C. XIE ET AL. (2021), A&A ACCEPTED, ARXIV:2016.01661)

Long-term, multiwavelength studies should reveal evolutionary explanations for systems like HD 163296.

A combination of edge-on and face-on protoplanetary disks, as imaged with DSHARP. The Disk Substructures at High Angular Resolution Project (DSHARP) collaboration, is one of multiple cutting-edge instruments optimized for revealing the features found in newly-forming planetary systems. The gaps in the disk are likely the locations of newly-forming planets, with the largest gaps likely corresponding to the most massive proto-planets. (S. M. ANDREWS ET AL. AND THE DSHARP COLLABORATION, ARXIV:1812.04040)

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.

Starts With A Bang is written by Ethan Siegel, Ph.D., author of Beyond The Galaxy, and Treknology: The Science of Star Trek from Tricorders to Warp Drive.


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