Fissel Research Group

Fissel Research Group

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Studying Star and Planet Formation from The Stratosphere

In my research, I study how magnetic fields influence both star and planet formation. In particular, I design, build, and fly stratospheric balloon-borne polarimeters, which by operating above 99.5% of the atmosphere can map the magnetic fields of star-forming regions, and I then follow-up my observations with powerful ground-based telescopes.

How do stars and planetary systems like our own solar system form?

A far-infrared image of glowing dust tracing the structure of the Vela C Giant Molecular Cloud.
Star formation in the nearby Vela C molecular cloud as traced by light emitted by glowing dust grains in the far-infrared and sub-mm, using the Herschel Space Observatory. (Image credit: ESA)

Stars form out of vast clouds of molecular gas. However, this process is incredibly inefficient, in that typically at most a few percent of the gas in molecular clouds will end up in stars. Why is star formation so inefficient, and what physical processes set the properties of the stars and planetary systems that form in molecular clouds?  Answering these questions is crucial to understanding everything from the formation of our solar system to the chemical evolution of our universe.

Stars form out of vast clouds of molecular gas. However, this process is incredibly inefficient, in that typically at most a few percent of the gas in molecular clouds will end up in stars. Why is star formation so inefficient, and what physical processes set the properties of the stars and planetary systems that form in molecular clouds?  Answering these questions is crucial to understanding everything from the formation of our solar system to the chemical evolution of our universe.

To understand star formation we need to study the physics that regulates this process: the gravitational attraction between the gas particles, supersonic shocks and turbulent gas motions, feedback from young stars that can ionize and destroy their parent molecular clouds, and the magnetic fields that permeate molecular clouds. Of these magnetic fields are the hardest to observe, and therefore the least understood.

Do magnetic fields reduce the efficiency of star and planet formation?

A map of the magnetic field of the Vela C giant molecular cloud inferred from BLASTPol data.
A map of the magnetic field of the Vela C giant molecular cloud, made from the 500 mm dust polarization observations of my PhD thesis experiment BLASTPol.  This is to this date the most detailed magnetic field map ever made of a massive star-forming region.

Star-forming molecular clouds are threaded by magnetic field lines. If the cloud magnetic field is strong and well coupled to the gas then it is much easier for the gas to move parallel to the magnetic field lines rather than across field lines.  In this way magnetic fields can slow down, or perhaps even stop the gravitational contraction of gas clouds into stars, perhaps partially explaining why star formation is so inefficient.  

But are magnetic fields in molecular clouds actually strong enough to slow down star formation? We don’t really know because it is extremely difficult to directly observe the strength of magnetic fields in molecular gas. However, we can use the fact that dust grains in molecular clouds tend align perpendicular to their local magnetic field.  When these extremely cold dust grains glow they emit far-infrared or sub-mm light that is linearly polarized in the direction perpendicular to the magnetic field lines. We can therefore use this polarized radiation to map out the magnetic field of distant star-forming regions.

Unfortunately most far-infrared and sub-mm light is absorbed in the Earth’s atmosphere.  Even worse the atmosphere emits light at those same colours, making it very difficult to observe polarized light from the star forming regions in our galaxy.  To map magnetic fields we either need to look at even redder light (where the glow from dust grains is much fainter and harder to detect), or build a polarimeter that can somehow get above most of the atmosphere.

 

 

 

Stratospheric balloon-borne telescopes: an inexpensive ticket to near-space

A view of the Earth and sky from the Spider stratospheric balloon telescope, about 35 km above Antarctica.
A view looking out from the stratospheric balloon telescope Spider at roughly 35 km above Antarctica in late 2014. (Image Credit: the Spider Collaboration)

As part of an international collaboration of astrophysicists I build telescopes that operate while suspended from giant stratospheric helium balloons. These balloons can lift telescopes of > 2,500 kg up to altitudes as high as 46 km above sea level (about 4x as high as a commercial jet plane flies and above >99.5% of the Earth’s atmosphere).  Balloon astronomy offers us the chance to get nearly space-quality observations, for about 1% of the cost of an equivalent satellite telescope.

For my PhD thesis project I worked on building the Balloon-borne Large Aperture Sub-mm Telescope for Polarimetry (BLASTPol).  BLASTPol had a 1.8 meter aluminum primary mirror, and made maps of polarized dust emission in three different sub-mm colours (250, 350 and 500 microns).  We launched BLASTPol in December 2010 and 2012, both times from Antarctica in the summer when the sun never sets and the winds in the stratosphere push the balloon telescope around the continent.

We used BLASTPol to make an incredibly detailed magnetic field map of the massive molecular cloud Vela C.  By analyzing our map and comparing our observations with simulations of star formation, we have shown that in Vela C the magnetic field helps to shape the structure of the densest gas regions within the cloud, which are likely to be sites of star formation.  But to understand whether magnetic fields slow down star formation in general we need observations of many more clouds.

 

 

2012 Launch of the BLASTPol balloon-borne telescope from NASA's Long Duration Balloon facility near McMurdo Station, Antarctica (Image credit: Steven Benton)
2012 Launch of the BLASTPol balloon-borne telescope from NASA's Long Duration Balloon facility near McMurdo Station, Antarctica (Image credit: Steven Benton)

 

 

 

 

 

 

 

 

 

 

 

BLAST-TNG: studying magnetic fields in star-forming regions and in nearby galaxies 

The next generation BLAST polarimeter (BLAST-TNG) in Antarctica.
The next generation BLAST polarimeter (BLAST-TNG) in Antarctica.

Over the past few years I have been working with an international team of astrophysicists to develop the next generation BLAST polarimeter (BLAST-TNG). Like BLASTPol, BLAST-TNG will map the sky simultaneously in three colour bands centred at 250, 350, and 500 microns. However, BLAST-TNG uses new MKID detector arrays, which should result in at least a factor of 10 increase in mapping speed compared to BLASTPol, has a larger 2.5 meter primary mirror, which should give it 31 arcsecond best resolution (5x better than BLASTPol), and has a larger cryostat that will allow BLAST-TNG to operate for ~24 days instead of 10 days.

With BLAST-TNG we plan to map dozens of molecular clouds, 5 nearby galaxies, and several regions of diffuse high-galactic latitude dust (which is a foreground for measurements of the cosmic microwave background polarization). BLAST-TNG will therefore generate a crucial dataset for studies of star formation, the magnetic fields of galaxies, and for cosmology. We have also made 25% of our science time available for shared-risk observing programs from other astronomers, making BLAST-TNG the first ever balloon-borne observatory!

BLAST-TNG is currently in Antarctica, with a first launch scheduled for late December 2019. You can follow our Antarctic campaign on twitter, or learn more about the telescope on our webpage.

 

Looking at smaller scales and later stages of star and planet formation with ground-based telescopes

The Large Millimeter Telescope in Puebla, Mexico.
The Large Millimeter Telescope in Puebla, Mexico.

While telescopes on the Earth’s surface are not as sensitive as BLAST-TNG, we can build much larger telescopes on the ground.  With these telescopes we can ``zoom-in’’ to make detailed maps of magnetic fields on much smaller scales, such as dense filaments (the locations in molecular clouds where most stars form) and cores (the progenitors of individual stellar systems).

This is the goal of the Fields in Filaments survey that I am co-organizing using the TolTEC polarimeter on the 50 meter Large Millimeter Telescope in Mexico.  TolTEC is a new polarimeter that will start taking data in early 2020, which will have 5x better resolution than BLAST-TNG and more than 2x better resolution than any other single mm/sub-mm telescope.  With the Fields in Filaments survey we will use TolTEC to map dense filaments to determine whether they are supported against gravitational collapse by magnetic fields.

I am also studying magnetic fields on even smaller scales, using the Atacama Large Millimeter Array (ALMA) a telescope in the Chilean Atacama desert that combines mm and sub-mm light from more than 40 different dishes to make maps detailed maps of individual gas cores and protoplanetary disks.  By combining large scale maps from BLASTPol and soon BLAST-TNG, detailed maps of magnetic fields in filaments with polarimeters on large ground based telescopes like TolTEC on the LMT, and maps of protostars and protoplanetary disks with ALMA, my goal is to determine how magnetic fields affect all stages of star and planet formation.