Accurate proper motions of the protostellar system VLA1623-2417

Imagine the night sky not as a static painting, but as a bustling cosmic dance floor where stars are born, spin, and sometimes crash into one another. To understand how these stellar families form and evolve, astronomers need to track their movements with incredible precision. This is the science of astrometry, the art of measuring exactly where things are and how fast they are moving across the sky. Think of it like trying to figure out if two cars on a highway are driving toward each other, away from each other, or just cruising in the same lane by taking snapshots of their positions years apart. For the youngest, most hidden baby stars—called protostars—this is especially tricky because they are wrapped in thick blankets of dust that block visible light. To see through the fog, astronomers use giant radio telescopes that act like super-powered eyes, listening to the radio waves these babies emit. By tracking these tiny movements over decades, scientists can figure out the invisible forces, like gravity, that are pulling these stars together or flinging them apart, helping us understand the chaotic nursery where our own Sun might have once lived.

In a new study, a team of astronomers has taken a long, hard look at a famous cosmic family called VLA 1623–2417. This isn't just a single baby star; it's a complex quadruple system, meaning it has four distinct components interacting in a tight, gravitational tango. The system is located about 137.3 parsecs away (roughly 448 light-years) in a cloud of gas and dust known as the Ophiuchi A core. The family consists of a tight binary pair (two stars orbiting each other closely, named Aa and Ab), a third star (B) orbiting them at a distance, and a fourth member (W) hanging out a bit further to the west.

The researchers, led by Ricardo Hernández Garnica, decided to play detective with a massive archive of data. They gathered 37 different observations taken over a span of about 34.5 years using some of the world's most powerful radio telescopes: the VLA (Very Large Array) and ALMA (Atacama Large Millimeter/submillimeter Array), along with older data from the SMA and BIMA arrays. By stitching these snapshots together, they created a high-definition movie of the system's motion, allowing them to measure the "proper motions"—the actual speed and direction each star is moving across the sky—with unprecedented accuracy.

Here is what they found, broken down by the family members:

The Tight Couple (Aa and Ab)
The innermost pair, Aa and Ab, are separated by only about 30 astronomical units (au)—a distance roughly the size of our entire solar system. The study revealed that they are definitely orbiting each other. However, because the team only had about 11 years of data on this specific pair, they only caught a tiny slice of their orbit (like watching a car drive past your house for 10 seconds and trying to guess its full lap time). While they can see the stars moving, they cannot yet calculate the exact mass of the stars or the full shape of their orbit with total certainty. The data suggests they are moving apart from each other, but the orbit is so long (estimated around 400 to 500 years) that 11 years is just a blink of an eye in cosmic time.

The Middle Sibling (B)
Star B is the middle child, orbiting the Aa/Ab pair at a distance of about 300 au. The team tracked B for a massive 34.5 years. Their measurements show that B is slowly drifting closer to the Aa/Ab pair. This inward motion confirms that B is gravitationally bound to the inner pair; they are a true family unit, not just passing strangers. The math suggests the whole A/B system has a total mass of about 2 times the mass of our Sun.

The Outlier (W)
The most exciting discovery concerns VLA 1623–2417 W, the star sitting about 10 arcseconds (roughly 1,400 au) away from the main group. For a long time, some scientists thought W might have been a "runaway" star—a member of the family that was violently kicked out by a gravitational slingshot during a chaotic early encounter.

The new data rules this out completely. By measuring W's motion over 32.5 years, the team found that W is actually moving toward the A/B system at a speed of 1 to 2 kilometers per second. If W had been ejected, it would be flying away, not coming back. So, what is W doing? The authors suggest two possibilities: either W is not part of the family at all and is just a neighbor moving in the same general direction by chance, or it is a family member on a very strange, highly tilted orbit that brings it closer to the others right now.

A One-Sided Jet?
The study also noticed something weird around star W. In some radio images, there is a faint "tail" of material sticking out to the west, perpendicular to W's disk. This looks like a jet of gas shooting out, but only in one direction. Usually, jets come out of both poles of a star like a double-sided firehose. The authors suggest W might be driving a "one-sided jet," perhaps because something is blocking the other side, though they admit they need more sensitive telescopes to be sure.

The Bottom Line
This paper doesn't just give us better numbers; it changes the story of how this family behaves. It proves that the "runaway" theory for star W is wrong. It confirms that the inner trio (Aa, Ab, and B) are a bound gravitational system. And while it can't yet solve the exact mass of the inner binary (Aa/Ab) because they need to watch them for a longer time, it provides the most accurate map of their movements to date.

The authors are careful to note that while they have solved the mystery of W's direction, the full orbital dance of the inner binary remains a puzzle for the future. They suggest that with even more data from next-generation telescopes like the SKA or ngVLA, we might finally be able to weigh these baby stars perfectly and understand exactly how they formed. For now, we know that VLA 1623–2417 is a stable, interacting family where the "outcast" is actually coming home, or at least walking in the same direction.