From AAS NOVA: “Building a Pair of Blue Stragglers”

AASNOVA

From AAS NOVA

31 May 2019
Susanna Kohler

1
This star cluster, NGC 6397, is one of many clusters that play host to a number of bright blue stragglers. A new study explores one way in which blue stragglers may form. [ESA/Hubble]

Unusually blue and bright stars may not have only themselves to thank for their uniqueness. A new study looks at one way these unconventional objects might form in clusters … with a little help from a friend.

Cluster Stand-Outs

A stellar cluster is a group of stars that were all born together and should evolve in a consistent way. According to stellar evolution theory, for a given cluster, the stars of the cluster should fall onto a well-defined track on a Herzsprung–Russell (H–R) diagram — a plot of stellar brightness vs. color — with the stars’ location on the track dependent only on their initial mass.

But a few stars defy this logic. These so-called “blue stragglers” seem to have been left behind as their fellow cluster inhabitants evolved without them; on the H–R diagram, blue stragglers lie alone above the main-sequence turnoff point, shining brighter and bluer than they should be.

Just a Little Boost?

What causes these unorthodox stars? The simplest explanation is that they are main-sequence stars that belatedly received a bump in their mass. Theorists favor two possible formation channels:

Mass transfer from an evolved donor onto a main-sequence star in a binary, which increases the main-sequence star’s mass and consequently causes it to become brighter and hotter.
Collision and merger of two main-sequence stars, which forms a new, more massive main-sequence star that is brighter and hotter than usual.

But these two channels can only explain some observed blue stragglers; other systems — like WOCS ID 7782, a binary consisting of two blue stragglers in a 10-day orbit — are unlikely to have formed in either of these ways.

With WOCS ID 7782 in mind, scientists Simon Portegies Zwart (Leiden University) and Nathan Leigh (American Museum of Natural History; Stony Brook University; and University of Concepción, Chile) have now proposed an alternative formation channel.

A Third Star in the Mix

Portegies Zwart and Leigh’s model relies on one important element: a third star. In their proposed scenario, two main-sequence stars in a close binary are orbited by a giant, evolved companion star. As this evolved star ages and overflows its Roche lobe, gas flows from it onto the main-sequence binary, increasing the masses of the two inner stars.

2
Snapshot from one of the authors’ simulated triple systems. The binary system at left is being fed by gas from the outer tertiary companion on the right. [Portegies Zwart & Leigh 2019]

The authors use simulations to show that the final result of this process can be a close binary with two similar-mass blue stragglers, just as seen in WOCS ID 7782. In this scenario, the outer companion eventually evolves into a hard-to-spot white dwarf on a wide orbit with a period of more than ~5.8 years.

In addition to potentially explaining WOCS ID 7782, Portegies Zwart and Leigh’s model can produce a number of other masses, geometries, and configurations for blue-straggler systems, depending on the initial masses and separations of the binary and the outer companion. This formation scenario — which relies on just a little help from a friend — may therefore go a long way toward explaining the formation of the blue-straggler systems that have stumped us before now.

Citation

“A Triple Origin for Twin Blue Stragglers in Close Binaries,” Simon Portegies Zwart and Nathan W. C. Leigh 2019 ApJL 876 L33.
https://iopscience.iop.org/article/10.3847/2041-8213/ab1b75/meta

See the full article here .


five-ways-keep-your-child-safe-school-shootings

Please help promote STEM in your local schools.

Stem Education Coalition

1

AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

Adopted June 7, 2009