Astronomers detect 22 new cataclysmic variables in globular cluster 47 Tucanae

May 30, 2017 by Tomasz Nowakowski, Phys.org report
This NASA/ESA Hubble Space Telescope image shows the globular cluster 47 Tucanae. Credit: NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration Acknowledgment: J. Mack (STScI) and G. Piotto (University of Padova, Italy)

(Phys.org)—An international team of astronomers led by Liliana Rivera Sandoval of the University of Amsterdam, Netherlands, has found 22 new cataclysmic variable stars in a globular cluster known as 47 Tucanae (47 Tuc for short). The discovery makes 47 Tuc the cluster with the highest number of cataclysmic variables identified so far. The findings were detailed in a paper published May 19 on arXiv.org.

Cataclysmic variables (CVs) are binary star systems consisting of a white dwarf and a normal star companion. They irregularly increase in brightness by a large factor, then drop back down to a quiescent state. These binaries have been found in different environments such as the center of the Milky Way galaxy, the solar neighborhood, and within open and globular clusters. Studying CVs in globular clusters is important for understanding how they affect the evolution of such groups of and to gain insights into how stellar interactions in a dense stellar environment affect the evolution of cataclysmic variables.

Located approximately 15,300 light years away from the Earth, 47 Tuc is a massive, non core-collapsed globular cluster in the constellation Tucana. This cluster shows low extinction, has a high interaction rate and has the most X-ray sources identified so far among any other . These properties make 47 Tuc an excellent target to study close binaries, what can potentially lead to detection of many cataclysmic variables.

That is why Sandoval and her team have analyzed the available near-ultraviolet and optical images from the Hubble Space Telescope, in combination with X-ray data from NASA's Chandra X-ray Observatory, which resulted in finding new CVs in 47 Tuc. The research increased the number of known cataclysmic variables in this cluster from 21 to 43.

"In this paper, we identify the largest number of CVs ever identified in a globular cluster through X-ray counterpart identification, more than doubling the number of known CVs in 47 Tuc," the astronomers wrote in the paper.

According to the study, CVs in 47 Tuc are more concentrated towards the cluster center than the main sequence turnoff stars. All the cataclysmic variables in this cluster have a blue color in the near-ultraviolet and in the optical color magnitude diagrams. This is due to emission from the accretor (white dwarf) and the accretion disk.

The researchers revealed that eighteen CVs in the cluster showcase hydrogen-alpha emission, which is likely coming from the or stream and contributions from the companion star. Moreover, four cataclysmic variables were found to have large amplitude variations (up to about 3 magnitudes) in time-scales of hours. However, the cause of these variations remains unexplained.

Given the fact that 47 Tuc is a non core-collapsed , the team tried to compared its CV population to those of NGC 6397 and NGC 6752 – two examples of core-collapsed clusters.

"47 Tuc has many more faint CVs than those core-collapsed clusters. This suggests that the CV population in 47 Tuc is a combination of primordial CVs and dynamically CVs formed a long time ago. (...) If we scale the masses of NGC 6397 and NGC 6752 to that of 47 Tuc, the core-collapsed clusters have many more bright CVs. This seems to indicate that the dynamical interactions play a major role in core-collapsed clusters to create CVs," the researchers concluded.

Explore further: A black hole in a low mass X-ray binary

More information: New Cataclysmic Variables and other Exotic Binaries in the Globular Cluster 47 Tucanae, arXiv:1705.07100 [astro-ph.SR] arxiv.org/abs/1705.07100

Abstract
We present 22 new (+3 confirmed) cataclysmic variables (CVs) in the non core-collapsed globular cluster 47 Tucanae (47 Tuc). The total number of CVs in the cluster is now 43, the largest sample in any globular cluster so far. For the identifications we used near-ultraviolet (NUV) and optical images from the Hubble Space Telescope, in combination with X-ray results from the Chandra X-ray Observatory. This allowed us to build the deepest NUV CV luminosity function of the cluster. We found that the CVs in 47 Tuc are more concentrated towards the cluster center than the main sequence turnoff stars. We compared our results to the CV populations of the core-collapsed globular clusters NGC 6397 and NGC 6752. We found that 47 Tuc has fewer bright CVs per unit mass than those two other clusters. That suggests that dynamical interactions in core-collapsed clusters play a major role creating new CVs. In 47 Tuc, the CV population is probably dominated by primordial and old dynamically formed systems. We estimated that the CVs in 47 Tuc have total masses of approx. 1.4 M_sun. We also found that the X-ray luminosity function of the CVs in the three clusters is bimodal. Additionally, we discuss a possible double degenerate system and an intriguing/unclassified object. Finally, we present four systems that could be millisecond pulsar companions given their X-ray and NUV/optical colors. For one of them we present very strong evidence for being an ablated companion. The other three could be CO- or He-WDs.

Related Stories

A black hole in a low mass X-ray binary

April 24, 2017

A globular cluster is a roughly spherical ensemble of stars (as many as several million) that are gravitationally bound together, and typically located in the outer regions of galaxies. Low mass X-ray binary stars (LMXBs) ...

Tidal tails detected around a distant globular cluster

May 22, 2017

(Phys.org)—Astronomers have found tidal tails around a distant globular cluster known as NGC 7492. The newly discovered features could provide important information about the nature of globular clusters. The findings were ...

Image: Hubble checks out a home for old stars

December 21, 2015

This image, taken with the Wide Field Planetary Camera 2 on board the NASA/ESA Hubble Space Telescope, shows the globular cluster Terzan 1. Lying around 20,000 light-years from us in the constellation of Scorpius (The Scorpion), ...

Image: A Hubble sky full of stars

August 8, 2016

Located approximately 22,000 light-years away in the constellation of Musca (The Fly), this tightly packed collection of stars—known as a globular cluster—goes by the name of NGC 4833. This NASA/ESA Hubble Space Telescope ...

Recommended for you

Long lost Galileo letter found at Royal Society library

September 26, 2018

Nature journalist Alison Abbott has published a News and Comment piece in the journal detailing the finding of a letter in a Royal Society library purported to have been written by famed early scientist Galileo Galilei. The ...

Hyper Suprime-Cam survey maps dark matter in the universe

September 26, 2018

Today, an international group of researchers, including Carnegie Mellon University's Rachel Mandelbaum, released the deepest wide field map of the three-dimensional distribution of matter in the universe ever made and increased ...

Software finds the best way to stick a Mars landing

September 26, 2018

Selecting a landing site for a rover headed to Mars is a lengthy process that normally involves large committees of scientists and engineers. These committees typically spend several years weighing a mission's science objectives ...

Tracking the interstellar object 'Oumuamua to its home

September 25, 2018

A team of astronomers led by Coryn Bailer-Jones of the Max Planck Institute for Astronomy has tracked the interstellar object 'Oumuamua to several possible home stars. The object was discovered in late 2017 – this was the ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.