Whizzing and Whirling Stars in the Tarantula’s Web

Title: The VLT-FLAMES Tarantula Survey: Observational evidence for two distinct populations of massive runaway stars in 30 Doradus

Authors: H. Sana, O. H. Ramírez-Agudelo, V. Hénault-Brunet, et al.

First Author’s Institution: Institute of Astronomy, KU Leuven, Celestijnlaan 200D, 3001 Leuven, Belgium

Status: Accepted to A&A Letters [open access]

If you’ve been keeping up with JWST news, you might remember seeing its breathtaking image of the Tarantula Nebula a few months ago, an HII region in the Large Magellanic Cloud (LMC), a satellite galaxy of the Milky Way. This spectacular display of gaseous wisps and filaments is made possible by the thousands of young stars that were born in this “stellar nursery”. These stars are quite hot and bright, and they emit very energetic photons that excite atoms in the gas around them, lighting it up and making it visible to our telescopes. 

While most stars that were born within the dense environment of the Tarantula Nebula (also called 30 Doradus or 30 Dor) won’t wander far from the home they help keep lit, a few of them make a run for it and travel many parsecs from their original birthplace. These kinds of stars – moving anywhere from tens to hundreds of kilometers per second through the surrounding interstellar medium – are what astronomers call “runaway stars”. But they don’t just get up and move, but rather can be flung out of their natal homes by two main processes: dynamical interaction or binary ejection. In dynamical interaction, a star is tossed about by the gravitational pull of its siblings until it is eventually kicked out of the system. In binary ejection, one of the partners in a binary star system goes supernova, and the energy generated from the explosion propels its partner out into space. 

Though both mechanisms are possible, we aren’t quite sure how efficiently they produce runaways or how they contribute to different runaway stars’ fates. The authors of today’s paper hunt for these runaway stars in the Tarantula Nebula in order to constrain their properties and uncover what mechanism exiled these stars from their birth clusters.

In Search of the Runaways

Instead of looking at images, the authors used data from the VLT-Flames Tarantula Survey which took optical spectra of 800 OB-type stars (the hottest and most massive stars) in 30 Dor, centered on a central stellar concentration called R136. Spectrographs break up light from a star by its wavelength, much like a prism breaks up white light into its visible rainbow constituents. However, we usually don’t see a continuous spectrum and instead see peaks and troughs that result from emission and absorption of light by elements in the star’s atmosphere, respectively. While arguably not as pretty as a JWST photograph, the squiggles of a spectrum tell us crucial information about a star’s properties, chemical composition, and motion. Just as you can tell when an ambulance is moving towards or away from you by the Doppler shift of its siren’s pitch, astronomers are able to measure how much each star’s spectral lines have shifted from their rest-frame wavelengths to calculate their radial velocities (RV), or how fast these stars are moving through space. 

From these measurements, the authors picked out the stars whose velocities differed significantly (3-sigma, or 25.8 km/s) from the overall average velocity of the general population of massive O-stars in the system. The difference between these two velocities is what the authors call the peculiar line-of-sight (LOS) velocity – basically the velocity of objects relative to some rest-frame. They found that 23 single stars and 1 binary meet their threshold RV cutoff (about 7% of O-type stars in 30 Dor). Five of these stars have peculiar LOS velocities of up to 98 km/s! 

Figure 1 below shows an image of 30 Dor and its stellar populations. The dots represent O stars – blue are non-runaway stars, red are single runaway stars, and the green dot is the binary runaway pair. The dashed yellow circles named NGC 2070 and NGC 2060 are areas we call OB associations –  groups of 10-100 OB-type stars in a very close cluster where the stars aren’t gravitationally bound to each other but are still moving through space together. Looking at the positions of all these stars, we see that the runaway candidates are mostly located outside the two OB associations shown in Fig 1, which suggests that field stars around NGC 2070 and 2060 were ejected from these associations.

A black and white image of the Tarantula Nebula, which is white and looks very wispy and gaseous, like smoke. The axes read "Dec (J200) and RA(J2000) in y and x, respectively. In the middle and lower right of the image are yellow dashed circles. Around the image, mostly concentrated within the circles, are blue dots. Scattered around are also a few red dots, and one green square.
Figure 1: A mosaic of the Tarantula Nebula from the Wide Field Imager camera at La Silla Observatory. Blue dots are non-runaway stars, red dots are single runaway stars, and the green dot is the binary runaway pair. The dashed yellow circles represent the extent of two OB associations. Most of the runaway stars are located outside the OB associations, likely because they were ejected from there. Figure 1 in the paper.

You Spin Me Right ‘Round

While some stars race about their neighborhood, others dance a cosmic ballet and pirouette around at dizzyingly fast rates. To see what hobbies the stars in 30 Dor have, the authors looked at data showing their projected rotational velocities. They find that the distributions of rotational velocities in runaway and non-runaway stars differ significantly, and they plot each runaway star’s rotational velocity against its peculiar LOS velocity in Figure 2 below. We see an interesting pattern – there’s a population of rapidly spinning but slower moving runaway stars (red) and a population of slow spinning but fast moving runaway stars (blue). But no runaway stars are both spinning and moving very rapidly at the same time, creating a “runaway desert” (green).

A plot of the projected rotational velocity (y axis) vs. the peculiar LOS velocity (x-axis) of stars in 30 Dor. The plot is broken up into 5 sections: a thin, yellow rectangle at the left where non-runaway stars are located, a larger red box to the right where rapidly spinning runaway stars are, a large green box to the right of the red one that is the avoidance region, a blue box below that for fast runaway stars, and a black/grey box to the left of that for slow runaway stars.
Figure 2: The projected rotational velocity vs. the peculiar LOS velocity of runaway stars. Most runaway stars are rapid rotators but slower movers (compared with other runaways). There is a “Zone of Avoidance” (in green), telling us that there are no fast rotating and fast moving runaway stars in 30 Dor. Figure 4 in the paper.

What can this tell us about the mechanisms that create runaway stars in the first place? Many papers have found that rapidly rotating stars can be a sign of post-binary interaction, where a star can be spun up by its companion either by merging with it or transferring mass from it. However, we can’t use this mechanism to confidently account for the population of slow rotating, fast moving runaways, since how efficiently binaries can produce fast runaways depends on the model of binary evolution you use. Instead, these could be chalked up to recent dynamical interactions, especially within the dense environment of the young R136 cluster in the center of the nebula, which astronomers estimate is not old enough to have had any supernova explosions. In general, as a star cluster ages, stars are more likely to be ejected by binaries and not through dynamical interactions.

Though the authors do find two distinct populations of runaway stars in 30 Dor, there is a statistically significant overabundance of runaway fast rotators compared to non-runaway fast rotators. Therefore, the authors conclude that the majority of runaway stars in 30 Dor were ejected due to binary interaction instead of dynamical interaction, and that binary interaction will be the main channel for creating runaway stars in the future.

This paper provides new insight into the dynamics of massive star formation and evolution in crowded star-forming regions like the Tarantula Nebula. The authors hope that with new data coming out from Gaia, they’ll be able to pinpoint the location from where these runaway stars started their journey and compare the Tarantula’s speedy siblings with other chaotic stellar nurseries around the Milky Way.

Astrobite edited by Sahil Hegde

Featured image credit: NASA, ESA, CSA, STScI, Webb ERO Production Team

About Katya Gozman

Hi! I’m a third year PhD candidate at the University of Michigan. I’m originally from the Northwest suburbs of Chicago and did my undergrad at the University of Chicago. There, my research primarily focused on gravitational lensing and galaxies while also dabbling in machine learning and neural networks. Nowadays I’m working on galaxy mergers and stellar halos, currently studying the spiral galaxy M94. I love doing astronomy outreach and frequently volunteer with a STEAM education non-profit in Wisconsin called Geneva Lake Astrophysics and STEAM, as well as work at our on-campus observatory and planetarium.

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