Mira: Star with a Comet-like Tail

by Paul Gilster on August 15, 2007

GALEX — the Galaxy Evolution Explorer — was an interesting mission to begin with, a space-based observatory conducting an all-sky survey of distant galaxies at ultraviolet wavelengths. Now it’s come up with a real newsmaker, a star moving at an unusually fast 130 kilometers a second and sporting a comet-like tail. The material blowing off the red giant Mira is, in fact, forming a wake some thirteen light years long. No such phenomenon has ever been seen around a star before.

Mira and its tail

Image: Mira appears as a small white dot in the bulb-shaped structure at right, and is moving from left to right in this view. The shed material can be seen in light blue. The dots in the picture are stars and distant galaxies. The large blue dot at left is a star that is closer to us than Mira. Credit: NASA/JPL-Caltech.

From what GALEX is telling us, the elements Mira is leaving behind, including carbon, oxygen and other building blocks for future star and planet formation, have been shed over a period of approximately 30,000 years. Although similar to our own Sun billions of years ago, the star has now swollen to variable red giant status, periodically growing bright enough to become visible to the naked eye. And as will happen to the Sun, its distant future involves its transformation into a white dwarf.

Nor does Mira travel alone. Mira B is itself a white dwarf [but see comments below] that orbits Mira A as the duo move through the constellation Cetus, some 350 light years from Earth. Interestingly, a bow shock has formed in which hot gases build up in front of the onrushing star, and astronomers have noted two streams of material that emerge from the star itself. Evidently the hot gas in the bow shock heats up the gas blowing off the star, causing it to fluoresce with ultraviolet light as it forms the wake.

Researchers admit to a sense of surprise. Here’s Mark Seibert (Carnegie Observatories, Pasadena), a co-author of the paper on this work:

“This is an utterly new phenomenon to us, and we are still in the process of understanding the physics involved. We hope to be able to read Mira’s tail like a ticker tape to learn about the star’s life.”

And from the discovery paper, this comment about putting the Mira findings to work:

The discovery of a two-degree-long wind wake emitting only in the far ultraviolet provides an unprecedented fossil record of post-main-sequence stellar evolution and mass loss, a laboratory for the study of astrophysical turbulence and the complex physics of a multiphase hydrodynamical flow, and suggests a new cooling process for hot gas that entrains a cool molecular phase. After 400 years of study, Mira continues to astound.

Indeed. A 30,000 year passage through the cosmos is now on display, discovered through a mission conceived to study much different things. If surprise is in the air, it’s understandable. A 13-light year long tail is not exactly standard issue, and who would have predicted that a star as well studied as Mira would turn out to have a wake that glowed only with ultraviolet light? How energizing it is to reflect that the pace of discovery is only accelerating, capable of blindsiding us at almost every turn.

The paper is Martin et al., “A turbulent wake as a tracer of 30,000 years of Mira’s mass loss history,” Nature Vol 448 (16 August 2007), pp. 780-783.

andy August 15, 2007 at 16:23

As far as I am aware, the traditional view that Mira B is a white dwarf is inconsistent with the accretion disc around it, it is more likely a main sequence K dwarf of about 0.7 solar masses.

See Born Again Protoplanetary Disk Around Mira B

Administrator August 15, 2007 at 19:35

Good point, Andy. From the Nature paper: “The secondary star, Mira B, is much less luminous and is usually classified as a white dwarf, but this is somewhat controversial.” With reference to the Ireland paper you cite.

Frank Taylor August 16, 2007 at 11:38

Truly an awesome discovery! This is the kind of unexpected discovery I imagined we would be making when I got a degree in astronomy (in the early 80s). It’s ironic that the technologies enabling these types of discoveries now (and enabling us to see the results) are some of the same technologies which diverted my career away from astronomy so long ago. I’m just glad I can now vicariously observe the discovery results and have some educational perspective to help me appreciate the wonder of the implications. But, the little boy who dreamed of the limitless unexplored wonders of the cosmos just looks at this story of Mira and says: “Wow!”.

Administrator August 16, 2007 at 13:59

You and me both, Frank…

Mark Seibert August 17, 2007 at 1:42

Frank…we said “Wow!” too. I find your comment inspiring.

Andy, regarding the nature of Mira B. Rumor has it that there is group with new strong evidence it is a white dwarf, but we will have to wait and see what that evidence is. Even if it is a WD, that does not rule out the wind accreted disk suggested by Ireland – just reduces the the chances of it being a proto-planetary disk.

Administrator August 17, 2007 at 10:14

Great to see you here on Centauri Dreams, Mark, and congratulations on this remarkable discovery!

george scaglione August 17, 2007 at 15:11

hello all,very strange, i just posted an idea under a practical positron rocket II that admits the possibility that there may be aliens advanced enough to move stars…then… this!! think maybe mira is a part of somebodies engineering project?! thank you george

Tim E Simmons September 2, 2007 at 16:58

There is an alternative explanation:-

http://www.preston.u-net.com/AGMatter/Index.htm

ljk May 7, 2008 at 6:48

A latitude-dependent wind model for Mira’s cometary head

Authors: A. C. Raga, J. Cantó, F. De Colle, A. Esquivel, P. Kajdic, A. Rodríguez-González, P. F. Velázquez

(Submitted on 5 May 2008)

Abstract: We present a 3D numerical simulation of the recently discovered cometary structure produced as Mira travels through the galactic ISM. In our simulation, we consider that Mira ejects a steady, latitude-dependent wind, which interacts with a homogeneous, streaming environment. The axisymmetry of the problem is broken by the lack of alignment between the direction of the relative motion of the environment and the polar axis of the latitude-dependent wind.

With this model, we are able to produce a cometary head with a “double bow shock” which agrees well with the structure of the head of Mira’s comet. We therefore conclude that a time-dependence in the ejected wind is not required for reproducing the observed double bow shock.

Comments: 4 pages, 4 figures, accepted for publication in ApJL

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0805.0549v1 [astro-ph]

Submission history

From: Fabio De Colle [view email]

[v1] Mon, 5 May 2008 14:54:57 GMT (360kb)

http://arxiv.org/abs/0805.0549

ljk May 14, 2008 at 11:29

Discovery of an HI Counterpart to the Extended Tail of Mira

Authors: L. D. Matthews (CfA), Y. Libert (Observatoire de Paris), E. Gerard (Observatoire de Paris), T. Le Bertre (Observatoire de Paris), M. J. Reid (CfA)

(Submitted on 12 May 2008)

Abstract: We report the detection of an HI counterpart to the extended, far-ultraviolet-emitting tail associated with the asymptotic giant branch star Mira (o Ceti). Using the Nancay Radio Telescope (NRT), we have detected emission as far as 88′ north of the star, confirming that the tail contains a significant atomic component (M_HI ~ 4x10e-3 M_sun). The NRT spectra reveal a deceleration of the tail gas caused by interaction with the local interstellar medium.

We estimate an age for the tail of ~1.2x10e5 years, suggesting that the mass-loss history of Mira has been more prolonged than previous observational estimates. Using the Very Large Array (VLA) we have also imaged the HI tail out to ~12′ (0.4 pc) from the star. The detected emission shows a “head-tail” morphology, but with complex substructure. Regions with detected HI emission correlate with far-ultraviolet-luminous regions on large scales, but the two tracers are not closely correlated on smaller scales (<1′). We propose that detectable tails of HI are likely to be a common feature of red giants undergoing mass-loss.

Comments: accepted to ApJ; version with full-resolution figures available at: this http URL

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0805.1730v1 [astro-ph]

Submission history

From: Lynn D. Matthews [view email]

[v1] Mon, 12 May 2008 20:03:10 GMT (606kb)

http://arxiv.org/abs/0805.1730

E. Moore May 22, 2008 at 22:35

I am very interested in viewing the star Mira. When would be the best time in 2008 to do this? (Austin, TX)

ljk May 23, 2008 at 13:45

The star Mira is located in the constellation Cetus the Whale,
which is generally an autumn/winter constellation for the
northern hemisphere in the evenings. Though you can see
Mira/Cetus at other times of the year if you don’t mind being
up in the wee hours of the morning.

Star maps are here:

http://www.astro.uiuc.edu/~kaler/sow/cm.html

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