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Surface Features on a Nearby Star

An image with 25 times the resolution the Hubble Space Telescope can produce has shown unprecedented levels of detail on the star Altair. Located 15 light years away in the constellation Aquila (The Eagle), Altair is a young, hot star about twice the size of the Sun, known to rotate at 300 kilometers per second at its equator. That’s about sixty times Sol’s rotation rate, fast enough to flatten Altair into an oval, its radius larger at the equator than the poles. In fact, it’s 22 percent wider than it is tall.

[kml_flashembed movie=”https://centauri-dreams.org/wp-content/uploads/2007/05/altair_small.swf” height=”300″ width=”450″ /]

Animation credit: Ming Zhao (University of Michigan)

How do you get a surface image of a star a million times farther away than our own? The technique is optical interferometry, combining the light from multiple telescopes to simulate a much larger instrument. In this case, the four telescopes used (at Georgia State University’s Center for High Angular Resolution Astronomy on Mt. Wilson, California) fed infrared light into a device called the Michigan Infrared Combiner. The result: A simulated instrument 265 meters by 195 meters in size.

And the imagery delivered a surprise. Rapidly rotating stars have been assumed to feature a dark band along their equators, the result of so-called ‘gravity darkening.’ The latter occurs because the star’s swollen equator is farther from its nuclear core, making that region cooler than the poles. The new work does confirm gravity darkening but shows a larger effect than earlier theories had predicted. The physics behind the darkening process may require an alternative model.

Hot, rapidly rotating stars are intriguing because of their effects on the stars around them. From the paper:

A significant fraction of hot stars are rapid rotators with surface rotational velocities of more than 100 km/s. These rapid rotators are expected to traverse very different evolutionary paths than their slowly rotating kin and rotation-induced mixing alters stellar abundances. While hot stars are relatively rare by number in the Milky Way Galaxy, they have a disproportional effect on galactic evolution due to their high luminosities, strong winds, and their final end as supernovae (for the most massive stars). Recently, rapid rotation in single stars has been invoked to explain at least one major type of gamma ray bursts and binary coalescence of massive stars/remnants for another.

Previous stellar imaging, apart from our own Sun, has focused on enormous red giant stars, but this is the first time a main-sequence star has been seen in such detail. The researchers hope to move on to other rapid rotators like Vega, but I like what Michigan graduate student Ming Zhao, who performed the stellar modeling on Altair, has to say about future work: “Imaging stars is just the start. We are going to next apply this technology to imaging extrasolar planets around nearby stars.”

The paper is Monnier et al., “Imaging the Surface of Altair,” published by Science online on May 31, 2007 (abstract available).

Comments on this entry are closed.

  • philw June 1, 2007, 11:52

    I always think of Altaira and her outfits in “Forbidden Planet”. Made quite an impression on me as a young male.

  • george scaglione June 1, 2007, 14:14

    philw, you bet!! are you kidding!!? me too!! met her in a bookstore many years later and was able to say hello! but when i was a boy forbidden planet was a very big deal to me.really a factor in cultivating my interest in space! i recall walking home from the movie theather on that fateful saturday.own my own copy on tape today and still watch it once in a while,sweet memories. all the very best your friend george

  • Adam June 1, 2007, 17:06

    I didn’t see “Forbidden Planet” until about 25 years after it came out, so I am a young pup by comparison. I always found the Krell machinery more impressive and the ID monster gave nightmares for a long time. The 1980s had more scantily clad women around, I guess.

  • Administrator June 2, 2007, 9:01

    Yes, the Krell machinery was something to see. The other day, I noticed an echo of that particular Forbidden Planet scene in a Babylon 5 first season episode, where an ancient, automated defense system is discovered inside the planet described as ‘Epsilon 3,’ which seems to be near the station. Some of the visuals could have been right out of the movie.

  • george scaglione June 2, 2007, 9:53

    paul,glad you enjoyed the movie it is a cherished childhood memory of mine.but one thing i notice,in the50’s when the movie was made bigger was better ergo the HUGE krell machinery! in reality things got smaller eg microchips but that would not have been as dramatic even if the films makers could have guessed. as to babylon 5 i must confess that i am barely aware of it don’t think i’ve ever seen a whole episode. but it is impossible for me to keep up with everything!! i am sure in the bargin i am missing some bets!! what you said sounded interesting though. thanks your friend george

  • Hans Bausewein June 2, 2007, 15:22

    Another nice interferometric study:

    http://www.eso.org/public/outreach/press-rel/pr-2007/pr-25-07.html

    … of a solar mass Mira-type star

  • george scaglione June 2, 2007, 15:51

    hans, thank you for passing that information along,we go farthest when we all work together. respectfully your friend george

  • ljk June 7, 2007, 17:13

    Imaging the Surface of Altair

    Authors: John D. Monnier (1), M. Zhao (1), E. Pedretti (2), N. Thureau (3), M. Ireland (4), P. Muirhead (5), J.-P. Berger (6), R. Millan-Gabet (7), G. Van Belle (7), T. ten Brummelaar (8), H. McAlister (8), S. Ridgway (9), N. Turner (8), L. Sturmann (8), J. Sturmann (8), D. Berger (1) ((1) U. Michigan, (2) St. Andrews, (3) Cambridge, (4) Caltech, (5) Cornell, (6) Grenoble, (7) Michelson Science Center, (8) Georgia State — CHARA, (9) NOAO)

    (Submitted on 6 Jun 2007 (v1), last revised 7 Jun 2007 (this version, v2))

    Abstract: Spatially resolving the surfaces of nearby stars promises to advance our knowledge of stellar physics. Using optical long-baseline interferometry, we present here a near-infrared image of the rapidly rotating hot star Altair with

  • ljk June 7, 2007, 17:16

    Not sure why posts keep getting cut off in mid-stream.

    Here is the Web site of the PI with all the details:

    http://www.astro.lsa.umich.edu/~monnier/Altair2007/altair2007.html

  • ljk June 25, 2007, 10:20

    Article on imaging Altair’s surface and exoplanet research here:

    http://tompkinsweekly.com/images/jpgs/Page_11.jpg

  • ljk March 24, 2008, 9:46

    The effect of rotation on the spectrum of Vega

    Authors: Jinmi Yoon (1), Deane M. Peterson (1), Robert J. Zagarello (2), J. Thomas Armstrong (3), Thomas Pauls (3), ((1) Stony Brook University, (2) PCPION, (3) Naval Research Laboratory)

    (Submitted on 21 Mar 2008)

    Abstract: The discovery that Vega is a rapidly rotating pole-on star has raised a number of questions about this fundamental standard, including such issues as its composition, and in turn its mass and age. We report here a reanalysis of Vega’s composition. A full spectral synthesis based on the Roche model derived earlier from NPOI interferometry is used. We find the line shapes in Vega’s spectrum to be more complex than just flat-bottomed, which have been previously reported; profiles range from slightly self-reversed to simple “V” shapes. A high SNR spectrum, obtained by stacking spectra from the ELODIE archive, shows excellent agreement with the calculations, provided we add about 10 km/s of macroturbulence to the predicted spectra.

    From the abundance analysis, we find that Vega shows the peculiar abundance pattern of a \W Bootis star as previously suggested. We investigate the effects of rotation on the deduced abundances and show that the dominant ionization states are only slightly affected compared to analyses using non-rotating models. We argue that the rapid rotation requires the star be fully mixed. The composition leads to masses and particularly ages that are quite different compared to what are usually assumed.

    Comments: 16 pages, 4 figures, accepted by ApJ

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0803.3145v1 [astro-ph]

    Submission history

    From: Deane Peterson [view email]

    [v1] Fri, 21 Mar 2008 15:32:15 GMT (55kb)

    http://arxiv.org/abs/0803.3145

  • ljk May 19, 2008, 11:30

    Abundances of lithium, sodium, and potassium in Vega

    Authors: Y. Takeda

    (Submitted on 16 May 2008)

    Abstract: Vega’s photospheric abundances of Li, Na, and K were determined by using considerably weak lines measured on the very high-S/N spectrum, while the non-LTE correction and the gravity-darkening correction were adequately taken into account. It was confirmed that these alkali elements are mildly underabundant ([Li/H] ~ -0.6, [Na/H] ~ -0.3, and [K/H] ~ -0.2) compared to the solar system values, as generally seen also in other metals. Since the tendency of Li being more deficient than Na and K is qualitatively similar to what is seen in typical interstellar cloud, the process of interstellar gas accretion may be related with the abundance anomaly of Vega, as suspected in the case of lambda Boo stars.

    Comments: Accepted for publication in MNRAS; 8 pages, 9 figures

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0805.2436v1 [astro-ph]

    Submission history

    From: Yoichi Takeda [view email]

    [v1] Fri, 16 May 2008 04:29:45 GMT (303kb)

    http://arxiv.org/abs/0805.2436

  • ljk March 8, 2009, 23:22

    Discovery of a magnetic field on Vega

    Authors: F. Lignieres, P. Petit, T. Bohm, M. Auriere

    (Submitted on 6 Mar 2009)

    Abstract: We report the detection of a magnetic field on Vega through spectropolarimetric observations. We acquired 257 Stokes V high signal-to-noise and high-resolution echelle spectra during four consecutive nights with NARVAL spectropolarimeter at the 2-m Telescope Bernard Lyot of Observatoire du Pic du Midi (France).

    A circularly polarized signal in line profiles is detected after gathering the contribution of about 1200 spectral lines for each spectrum and summing up the signal over the 257 spectra.

    Interpreting this polarization as a Zeeman signature leads to a value of $-0.6 \pm 0.3$ G for the disk-averaged line-of-sight component of the surface magnetic field. This is the first time a magnetic field is unambiguously detected in an A-type star which is not an Ap chemically peculiar star.

    Moreover, the Vega longitudinal magnetic field is smaller by about two orders of magnitude than the longitudinal magnetic field (taken at its maximum phase) of the most weakly magnetic Ap stars. Magnetic fields similar to the Vega magnetic field could be present but still undetected in many other A-type stars.

    Comments: 4 pages, submitted

    Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

    Cite as: arXiv:0903.1247v1 [astro-ph.SR]

    Submission history

    From: Lignieres Francois [view email]

    [v1] Fri, 6 Mar 2009 16:35:06 GMT (156kb,D)

    http://arxiv.org/abs/0903.1247

  • ljk September 9, 2009, 9:14

    On the Vega Debris Disc’s Dust Grains: Short-Lived or Long-Lived ?

    Authors: Ing-Guey Jiang (1), Li-Chin Yeh (2) ((1)Department of Physics and Institute of Astronomy, National Tsing-Hua University, Hsin-Chu, Taiwan, (2)Department of Applied Mathematics, National Hsinchu University of Education, Hsin-Chu, Taiwan)

    (Submitted on 8 Sep 2009)

    Abstract: Through Spitzer Space Telescope’s observations, Su et al. (2005) show that the Vega debris disc is dominated by grains which are small enough to be blown out by radiation pressure. This implies the lifetime of Vega debris disc’s grains is relatively short, about 1000 years, and a continuous dust production is necessary to maintain the observed debris disc.

    However, Krivov et al. (2006)’s theoretical calculations show that the Vega debris disc is dominated by 10 micro-meter grains, which would be in bound orbits and thus long-lived, provided that the disc is in a steady state.

    In order to solve the above contradiction, through dynamical simulations, we determine the grains’ orbital evolutions and density profiles and seek a model of size distribution which can reproduce the observed surface brightness.

    Our results show that a self-consistent dynamical model with a 1/R disc density profile can be constructed when the grains have a power-law size distribution. Moreover, both types of models, dominated by short-lived and long-lived grains, are consistent with the observational data.

    Comments: 3 pages, 1 figure, in Proceedings of the 10th Asian-Pacific Regional IAU Meeting, page 44-46, 2009

    Subjects: Solar and Stellar Astrophysics (astro-ph.SR); Earth and Planetary Astrophysics (astro-ph.EP)

    Cite as: arXiv:0909.1376v1 [astro-ph.SR]

    Submission history

    From: Ing-Guey Jiang [view email]

    [v1] Tue, 8 Sep 2009 03:08:33 GMT (35kb)

    http://arxiv.org/abs/0909.1376

  • ljk December 12, 2012, 10:08

    11 December 2012

    ** Contact information appears below. **

    Image:

    http://ns.umich.edu/Releases/2012/Dec12/vega.html

    AN OLDER VEGA: NEW INSIGHTS ABOUT THE STAR ALL OTHERS ARE MEASURED BY

    Vega, a star astronomers have used as a touchstone to measure other stars’ brightness for thousands of years, may be more than 200 million years older than previously thought. That’s according to new findings from the University of Michigan.

    The researchers estimated Vega’s age by precisely measuring its spin speed with a tool called the Michigan Infrared Combiner (MIRC), developed by John Monnier, associate professor of astronomy in U-M’s College of Literature, Science and the Arts.

    MIRC collects the light gathered by six telescopes to make it appear to be coming through one that’s 100 times larger than the Hubble Space Telescope. It’s installed at the Georgia State Center for High Angular Resolution Astronomy Array located on Mt. Wilson, California.

    The tool boosts resolution so astronomers can zoom in, relatively speaking, to observe the shape and surface characteristics of stars that would otherwise look like mere points even through the most powerful telescopes. By tracking stars’ surface characteristics, scientists can calculate how fast they rotate and deduce their inner workings.

    Vega is summer star in the Northern Hemisphere, just visible towards the west at sunset. It’s the brightest star in the constellation Lyra. At 25 light-years away, Vega is close on cosmic scales. A light-year is the distance light travels in one year.

    About six years ago astronomers discovered that Vega is rotating so fast it’s nearly flinging itself apart. They haven’t been able to agree on many of the related details, however. One of the debates centers on Vega’s exact rotation rate, which is essential to gauge both its mass and age. Other controversies deal with Vega’s tilt as viewed from Earth and the amount of turbulence in the system from roiling gases at the star’s surface.

    With MIRC’s unprecedented resolution, Monnier and his colleagues have taken steps to rectify competing estimates of Vega’s rotation rate and other properties. The new findings indicate that the star rotates once every 17 hours, rather than once every 12. The Sun’s equator, for comparison, rotates much slower — once every 27 days, or 648 hours. In addition to finding that Vega is older than previously thought, the Michigan group confirmed its mass to be just over two times the Sun’s.

    “Vega continues to challenge and surprise us,” Monnier said. “We found out not too long ago that it has a disk of dusty debris, or a leftover solar system, around it. Then we found out it was a rapid rotator. It’s a reference point for other stars, but it certainly isn’t boring or normal.”

    The work will help astronomers build more accurate computer models of stars, so they can simulate those too far away to observe and gain a better understanding of their life cycles.

    Contact:

    Nicole Casal Moore
    Public Relations Rep.
    +1 734-647-7087
    ncmoore@umich.edu

    A paper on the findings is published in the current edition of Astrophysical Journal Letters. It’s titled “Resolving Vega and the inclination controversy with CHARA/MIRC” by J. D. Monnier et al.:

    http://dx.doi.org/10.1088/2041-8205/761/1/L3

    The research is funded by the National Science Foundation and NASA.

    John Monnier:

    http://dept.astro.lsa.umich.edu/~monnier/