Jim Morrison
High-mass stars, defined as 8 times the mass of the sun (solar masses) or greater, are influential components of our Universe. They produce powerful radiation, stellar winds and supernovae, shaping the surrounding interstellar medium, while fusion in their cores are the source of the keystone molecules that are the basis of our existence. Despite their impact, their formation is still relatively unknown. High-mass stars are rare and their massive luminosities cause them to live short lifetimes. Furthermore, they are often formed completely surrounded by gas and dust, and practically invisible to many observable wavelengths.
In my project, I use the available data from the ALMA (Atacama Large Millimeter Array) archive to study a sample of high-mass stars; ALMA is able to, with high resolution, detect radio waves which can pierce the surrounding gas and dust. I aim to see if they form in the same manner as low-mass stars (2 solar masses or less), i.e. through rotating disks surrounding the star. These disks are called Keplerian as they follow Kepler’s 3rd law of motion. I determine a wide range of properties, including temperatures, densities, masses, stability, morphology and kinematics. My results show that of nine objects studies, there are possibly up to six variations of disk structure, including multiple showing signatures of Keplerian rotation! While this is promising, much more research is needed to understand this variation in disk structure. Are there other methods of massive star formation? Or is it simply different stages of the same process?
This work is based on many excellent studies, but the discoveries in the field are only just beginning; the majority of findings have been published only within the last 5-10 years. The future looks bright with ALMA and other interferometers continuing to improve data quality. Much more is yet to come!
Title: Disk Structure and the Formation of Young, Massive Stars
Author: Jim Morrison
MSc Thesis Project at the Kapteyn Institute, University of Groningen
Advisor: Professor Floris van der Tak
Slide 2:
Title: Low-Mass vs High-Mass Star Formation
The formation of low-mass stars, 2x the mass of the sun (solar masses) or less, is well-known. There are many low-mass stars visible in our local field of view and formation timescales are long!
Figure 1 depicts the evolutionary sequence of low-mass star formation, taken from Greene, 2001. A dark cloud containing dense cores will collapse into a central dense region. This region will further collapse into a protostar, which forms a surrounding disk, while simultaneously producing outflows perpendicular to the disk. The system is surrounded by the cloud, now called the envelope. Mass is transferred from the envelope to the disk to the star until the envelope dissipates. The disk stops accreting matter onto the star and the outflows cease as well. The remaining disk is made of debris which begins to form planets. Finally, a central star surrounded by rotating planets is left.
High-mass star (8 solar masses or greater) formation is trickier to determine. Massive stars are much less populous. They form very quickly and radiate so much energy that they die quickly as well. They form so fast that they are still completely surrounded by gas and dust (called the envelope) when they are born. Thus, they are practically invisible in most observable wavelengths. Radio observations would work, but waves are so big they produce unresolved (blurry) images of small objects…
Slide 3:
ALMA to the rescue!
Radio interferometers such as the Atacama Large (sub)Millimeter Array (ALMA) use many antennae and can produced resolved images of small, distant radio sources. ALMA began observing in 2011 and many objects were now visible that could not be resolved before.
My Project:
1. Use ALMA archive data to look at a sample of disks surrounding young massive stars.
2. Determine many properties: temperatures, densities, velocities, disk shapes, stability, and sizes.
3. Determine if massive stars form with similar disks as their low-mass counterparts.
Figure 2 contains an image of the ALMA array, a set of about a dozen radio antenna in the Atacama Desert in Chile. Image taken from eso.org.
Slide 4:
My Project Results:
1. Nine objects produce six different disk types
a. Two disks were deemed to be the stable, Keplerian disks (i.e. following Kepler’s 3rd law of motion) that are seen around low-mass stars!
b. Several showed aspects of the expected shape and rotation, but it is clear that there are many variations.
c. Are the variations a result of different formation routes? Or maybe are the disks in different stages of evolution? Much more research is needed!
A “Bright” Future:
This field of study has really picked up since the development of interferometers such as ALMA. Many findings have occurred only within the last 5-10 years! Much more is yet to come, and hopefully this mystery of the Universe will be solved in the upcoming years.
FIGURE 3: Emission images of four different high-mass stars named AFGL 4176, G23.01 -0.41, G34.43 -0.24 mm1, and G35.03 +0.35, produced from ALMA archive data. Only the central point represents the star, while the surrounding regions are the disks. The images have a colour scale, where red represent the high emission intensity and black/purple the lowest. The structures range from single disks, to clumps of multiple disks in a star-forming region.