Astronomy: Young Stellar Moving Groups

Open cluster M25. Credit:  J.-C. Cuillandre (CFHT), G. Anselmi (Coelum Astronomia), Hawaiian Starlight

Stars are born in groups, as clusters of stars. Some groups stay well-knit and the members remain together many hundreds of millions of years later. Others, however, are loosely bound to each other, and, after traveling a bit through the Galaxy, get dispersed. However, the initial bulk motion of the stars in these groups is preserved. So, if you search carefully, you can find groups of widely separated stars throughout the sky all moving in approximately the same direction and with the same properties like age and composition. These are stellar moving groups, and here I'm going to tell you why astronomers love them.

The ages of stars are difficult to determine. There are plenty of uncertainties involved in finding the age of a star and although various methods are used, they don't always match up. As an example, you can use the rotation of a star to infer something about its age given that stars slow down as they age. However, projection effects mean you could be observing the star pole-on and you would not be able to accurately measure the true rotation rate. You can consider features in the spectra, like certain emission or absorption lines of particular elements. However, the models can disagree on when exactly a particular feature will appear or disappear for a particular star's mass. The mass of an individual star can also be very difficult to determine. Some methods only work for high-mass stars, others only work for low-mass stars.

Moving groups provide an interesting way to approach the problem of determining ages. Since all the stars have approximately the same age, you can apply the different methods throughout the stars and get a better idea for what's going on. If you have a star that is just too tricky to determine, you have the advantage that by being part of the group it has the same age as other stars that may be much easier to study. In addition, you can trace back the motions of these stars through the Galaxy to find out when they all were together. This gives you a trace-back age for the whole group and you can add it to your list of age determinations. For example, the beta Pictoris moving group has a trace-back age of about 12 million years: quite young!

Tracing back the locations of beta Pictoris moving group members. Credit: I. Song et al. (2003)

Young stars tend to be far away. The closest star forming regions are hundreds of light-years away. That makes studying young stars a challenge. However, for the past few decades astronomers have identified a number of moving groups near the Earth that contain young stars. These groups have ages of roughly 10 to 100 million years and are much closer than distant star forming regions. The age may seem like a lot, but it's actually quite young for stars. Most nearby stars tend to be many hundreds of millions of years old or more. In fact, our own Sun is 4.5 billion (4,500 million) years old. The young age of these nearby moving groups presents a very interesting opportunity to study the late stages of planet and disk evolution.

Four planets orbiting HR8799 star system, a proposed member of the 30 million year old Columba Association. 

Planets and disks are easier to search for and study for nearby stars. It should not be too surprising that the closer something is, the easier it is to study. A planet that is half an AU away, like Mars can be readily observed and we can even travel to it. The farther something is, the fainter and smaller it appears and so a tiny object like a planet will be very difficult, but not impossible, to observe.

The beta Pictoris star system with disk and planet. Credit: ESO/A.-M. Lagrange et al.

The above shows the beta Pictoris star system with it's Saturn-mass planet. Beta Pic is only 19 parsecs (63 light-years) away so it's relatively close. It's also very young (~12 million years old), which is important for big planets like this one. Planets glow in the infrared from the leftover heat of their formation, but this is quickly dissipated as they age. Despite this, their infrared glow is many times brighter than what they look like in the optical, where they only reflect a tiny portion of their star's light. Given that this infrared glow dissipates over time, the best targets to attempt to directly image planets are those that are young and nearby: like those members of the nearby moving groups I've been mentioning!

Stars can also possess circumstellar gas and/or dust disks. These are either the sites of planet formation or the remnants left behind from the process. The star beta Pic itself contains a dusty disk. V4046 Sagitarii (Sgr), another member of the beta Pic moving group, also contains a disk. The disk around V4046 Sgr is rich in gas and was imaged at submillimeter wavelengths with the SMA telescope in Mauna Kea, Hawaii. Here's an animation I produced of our data:

The colors represent velocities of the gas moving towards (blue) or away (red) from us. This animation shows different velocities per time step and thus we are probing different regions, which is why the shape changes. For comparison, the orbit of Neptune is shown at the corner. The white ellipse on the left is a measure of the telescope resolution.

Disks like those around V4046 Sgr are actually rare for such old stars. You expect gas-rich disks to disperse in just a few million years, not last for so long. In fact, this is why gas giant planets are expected to form within 10 million years: because most disks are gone by then. And yet this is one of 4 or 5 such disks known around the nearby stars. Studies of these disks can help us understand how they tie together to the late stages of planet formation. Furthermore, the proximity of such systems means we can study them in great detail.

Members of 4 nearby moving groups (ages ~10-100 Myr) in Galactic coordinates. Members of these moving groups are spread over large areas of the sky.

There are plenty of young stars out there and a lot of effort is being done to identify even more, particularly at the low-mass end. With the advent of observatories like ALMA and the next generation of adaptive optics instruments, we'll have a powerful set of tools to study how planets form in these nearby groups and understand better how we came to be.