Don’t Cool Your Jets Quite Yet!

Title: The CALYPSO IRAM-PdBI Survey of Jets from Class 0 protostars
Are jets ubiquitous in young stars?

Authors: L. Podio, B. Tabone, C. Codella, et al.
First Author’s Institution: INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
Status: Accepted for publication by A&A [closed access]

Stars form within giant clouds of dust and gas, like this, this and this. The first stage of star formation is called the Class 0 phase, followed by the Class I, Class II, and Class III phases (see Figure 1 for more details). The Class 0 phase describes a currently forming star, which is accreting material around it, and is still surrounded by an ‘envelope’ of cold dust and gas. These Class 0 objects are vital in our understanding of how stars form, but have been very difficult to find, let alone learn much about. Nowadays, there is a sample of known Class 0 sources large enough to start surveys of these objects, with the goal of learning more about the Class 0 stage, as opposed to focused studies of individual objects. Many Class 0 objects have been observed to host jets that shoot out perpendicular toD the dusty disk around it. The process powering such jets, and the general properties of such jets are not well known, and could strongly affect future star formation or even planet formation around that star. Today’s paper seeks to learn more about the jets found around Class 0 objects.

Figure 1: A cartoon showing the different stages of star formation from molecular cloud (A) to solar system (F). The Class 0 object is in between B and C, where a protostar has started to form, but there is still leftover core material accreting onto the system. Class I would be the stage labeled C, and Class II the stage labeled D. [source]

Are Jets Universal?

This study contains 21 different Class 0 sources with a wide range of luminosities and envelope masses. Each source was observed with a radio interferometer telescope, which provides a very high resolution image, high enough to resolve the jets of each of the Class 0 sources. The first question the authors wanted to answer was, are jets found in all Class 0 protostars? To answer this question, they use observations of three molecules, CO, SO, and SiO each of which are commonly observed in star forming regions. The observations of CO are used to determine if there is any outflow from the Class 0 object. SiO is less abundant than CO, so a bright SiO detection would mean that there is a high density of gas in that region. An observation of a stream of SiO from the protostar would then suggest that there is a highly collimated jet. This means that the jet is dense and narrow. They also looked to see if SO (which is slightly less abundant than SiO) is detected in the jet. They find that all 21 Class 0 objects have CO emission indicative of outflow away from the central star, and 14 of those objects have an SiO jet. Eleven out of those fourteen also have a jet in SO. So right off the bat, we can say that outflow motion, or ejecta, is very common if not universal, and collimated jets are also common. Examples of the observations of SiO, CO, and SO are shown below!

Figure 2: Maps of CO (right), SO (middle), and SiO (left) towards two Class 0 objects from this survey. You can see that the CO is more widely extended than SiO and SO as well as farther out, which is typical of sources in this survey.  [Figure 1 from paper, altered]

However, the authors also find that they are more likely to find a jet around more luminous Class 0 objects. Luminosity corresponds to the ‘accretion rate’ of gas onto the star. Class 0 objects have continuing-to-form disks surrounding the star, and the rate of material moving from the disk onto the star is the accretion rate, which in turn drives the luminosity of the system. So, the fact that they don’t observe jets towards low luminosity sources is not entirely surprising. The observations may not be sensitive enough to pick up on those jets since they are expected to be very dim. 

What are the Jets Like?

The second goal of this study is to explore jet properties. How wide are typical jets? How fast is material outflowing? What are the abundances of certain molecules in jets? These observations of CO and SiO in the Class 0 objects with SiO jets show that CO emission traces a wider region and often much farther extent than SiO (see Figure 2). This suggests that CO traces not only the collimated jet, but also any other outflow motion caused by the jet, or interactions with the envelope. Half of the collimated SiO jets also show significant wiggles, so there may be a mechanism that keeps jets from being completely straight.

Figure 3: The widths of CO (black), SO (red), and SiO (blue) over distance from the central star in the observed Class 0 disks. The green dashed lines show constant opening angles for the jet at 3, 8, 12, and 35 degrees.  [Figure 4 in paper]

The authors are able to determine the speed of the jet using the SiO observations, and find a median velocity of 30 km/s (that’s over 67,000 mph!!). Believe it or not, this is two times slower than jets in more evolved protostars, the Class II sources. This is consistent with the fact that the protostar becomes more massive over time, thus increasing the jet outflow velocity. 

The authors found that eight of the 12 objects with SiO jets were SiO-‘rich’, meaning that there was a higher abundance of SiO than was previously expected. They presented two scenarios to explain the higher levels of SiO, one assuming a dust-rich jet and a dust-free jet. If the jet has dust in it, then shocks within the jet can produce SiO. If there is not any dust, then silicon (Si) can be released at the base of the jet where silicon-rich dust is being destroyed. They found that shocks probably aren’t the solution, as the time it takes to produce SiO from shocks is too long. In the dust-free scenario, if the jet starts with material taken directly from the disk, then there can be enough SiO and CO in the jet as seen in the observations.  

Finally, the authors also were able to calculate the rate of mass-loss from outflows using the CO observations. Interestingly, they found that the ratio of the mass-loss rate from the jet to the accretion rate onto the star for the Class 0 objects was very similar to the ratio for the Class II objects. This means that whatever ejection and accretion mechanisms are happening for protostellar objects, those mechanisms remain constant from the beginning of star formation all the way to the later stages. 

Today’s paper examines the results of a survey of 21 Class 0 objects specifically looking to understand and characterise jet properties. The authors were able to show how ubiquitous jets appear to be in young stars, and find some universal properties of the jets, including insight into the formation mechanism of SiO within Class 0 objects. And with that we are we know just a bit more about the complexities of star formation!

Astrobite edited by Jamie Sullivan

About Jenny Calahan

Hi! I am a third year graduate student at the University of Michigan. I study protoplanetary disks, which set the stage for planet formation. For the past few years I've been using high resolution ALMA data to pull out the 2D thermal structure of different types of disks using thermo-chemical modeling. Outside of astronomy, I love belting showtunes, eating Thai food, I enjoy crafting, and I love to travel and explore new places. Check out my website: https://sites.google.com/umich.edu/jcalahan

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