IoW_20191220_HyadesTails - Gaia

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The lost stars of the Hyades

The Hyades and its tidal tails. The dots represent those stars from Gaia DR2 that are in dense regions (at least 2.5 x 10^-3 stars per cubic parsec). In red is plotted the Hyades cluster proper. The green dots represent the leading tail while the blue dots represent the trailing tail. All other stars in the dense regions are plotted with pink dots. Indicated with yellow in the background is the location of the predicted Hyades tidal tails from the model by Kharchenko et al. (2009). Image credit: Siegfried Röser et al (2019).

The Hyades are the closest star cluster to Earth, being a mere 150 light years away, and can be easily spotted from Earth with the naked eye given some of its stars are so bright. Being so close to Earth makes the Hyades a perfect cluster to study in more detail. With the help of Gaia and its high-precision measurements of the positions, movements, and parallaxes (leading to derived distances), the three-dimensional structure of the cluster can be studied in great detail.

Stars in an open cluster are known to move together in space. While position and distance help to identify which stars are close to one another, the knowledge of the velocities of stars is essential to identify the ones that move along together. If nearby stars are moving in opposite directions, they most probably do not belong together. If nearby stars are moving in the same direction though, this is a good indicator of a genetic link.

A star cluster does not live forever. While star clusters can contain hundreds to thousands of stars, they continuously lose stars due to the gravitational pull of the Milky Way. This effect is felt most strongly by the star cluster members with the lowest masses and a slightly higher orbital velocity about the cluster centre. However, even stars which were lost from the cluster approximately still share the same velocity. If one therefore extends the search radius and traces back stars to their original place in space, former members of a cluster can be found.

Two independent research teams (from Heidelberg in Germany and from Vienna in Austria) searched through the Gaia DR2 data near the Hyades cluster, tracing back stars within a certain distance from the Hyades cluster with the help of their proper motions and radial velocities, and found traces of former members that the Hyades cluster has lost in the course of several hundred million years.

Based on the three-dimensional velocity data, evidence was found for the existence of extended tidal tails associated with the Hyades cluster. A leading tail extending up to 170 parsec from the centre of the Hyades and a trailing tail up to 70 parsec. More information can be found in the papers describing the results as referenced below. A video showing the cluster and its tidal tails can be found below.

The video first shows the direction to the constellation Taurus with the Hyades star cluster. The cluster is close on the sky to the bright star Aldebaran, marked in red. This star, however, is not a member of the Hyades, being located much closer to the Earth (at 66 light years from Earth). The directions of movements are then shown for many stars based on velocities measured by Gaia. The members and lost stars of the Hyades are flying in the same direction (both groups are marked in yellow). The video then shows the exaggerated movements of the Hyades (one trillion or 10¹² times faster than reality). The covered time interval amounts to 320,000 years. The time propagation is then reset to present time again and the camera moves backwards at superluminal velocity allowing to see the Hyades and their tidal tails. By orbiting around the star cluster, one gets an impression of its three-dimensional structure. The tidal arms are oriented parallel to the plane of the Milky Way and can be easily recognised.

This video was produced for ESA/Gaia/DPAC by Stefan Jordan and Toni Sagrista with Gaia Sky, a visualisation software developed at the university of Heidelberg.

References:

• Stefan Meingast and Joāo Alves, Extended stellar systems in the solar neighbourhood - the Hyades tidal tails and beyond, A&A, 621, L3
• Siegfried Röser, Elena Schilbach and Bertrand Goldman, Hyades tidal tails revealed by Gaia DR2, A&A, 621, L2

Credits: S. Jordan and T. Sagrista for the video; story based upon the work by S. Meingast and J. Alves and upon the work by S. Röser, E. Schilbach and B. Goldman.

[Published: 20/12/2019]