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Close Pass Through a Stellar Magnetosphere

Tau Boötis, a billion year old star some 50 light years from Earth, would be a fascinating place to see up close. The star is orbited by a gas giant some 4.4 times Jupiter’s mass, one of those ‘hot Jupiters’ that close to improbably tight distances with the primary. In this case, the planet/star separation is a mere 0.049 AU, making it about 5 percent of the distance between the Earth and the Sun.

‘Hot Jupiters’ are bound to be dramatic objects anyway, but this one has other attributes. A team of French astronomers has been able to measure the magnetic field of Tau Boötis itself, finding it just slightly larger than the Sun’s. That’s the first direct measurement of the magnetic field of a star hosting an exoplanet, and it sets up interesting studies of the interactions between the two bodies as the planet moves so breathtakingly close to its star.

Tau Bootis and known planet

Image: An artist’s conception of the giant exoplanet orbiting Tau Boötis through the star’s magnetic archs. Credit David Aguilar, CfA.

In particular, the revolution of the planet shows it to be synchronized with a rotating band of material on the star’s surface, suggestive of all kinds of interactions between magnetosphere and planet. A key factor in stellar magnetic fields is the fact that parts of the star rotate at different rates. In fact, matter at the equator of Tau Boötis rotates 18 percent faster than matter located at the poles.

We can suspect from all this that a lot is going on here, and it should trigger a memory. Remember Voyager’s early images of tortured Io? One thing we learned about the pizza-like Jovian moon was that it orbited well within Jupiter’s magnetic field, contributing to the charged particles in that field and concentrating them in what we came to call the ‘Io torus,’ a doughnut-shaped band surrounding Io’s orbit. Huge electrical currents are created between Jupiter and Io, and we can imagine similar exotic effects occurring around the distant star.

This first direct measurement should contribute to our understanding of how magnetic fields affect planetary development, perhaps giving us clues to why planets like this one make migrations into the extreme inner systems around their star. The paper is Catala et al., “The magnetic field of the planet-hosting star tau Boötis,” soon to appear in Monthly Notices of the Royal Astronomical Society and available in preprint form.

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  • ljk December 4, 2007, 14:23

    The On/Off Nature of Star-Planet Interactions

    Authors: Evgenya Shkolnik (University of Hawaii), David A. Bohlender (Herzberg Institute of Astrophysics, NRC), Gordon A.H. Walker (University of British Columbia), Andrew Collier Cameron (University of St. Andrews)

    (Submitted on 30 Nov 2007)

    Abstract: Evidence suggesting an observable magnetic interaction between a star and its hot Jupiter appears as a cyclic variation of stellar activity synchronized to the planet’s orbit. In this study, we monitored the chromospheric activity of 7 stars with hot Jupiters using new high-resolution echelle spectra collected with ESPaDOnS over a few nights in 2005 and 2006 from the CFHT. We searched for variability in several stellar activity indicators (Ca II H, K, the Ca II infrared triplet, Halpha, and He I). HD 179949 has been observed almost every year since 2001. Synchronicity of the Ca II H & K emission with the orbit is clearly seen in four out of six epochs, while rotational modulation with P_rot=7 days is apparent in the other two seasons. We observe a similar phenomenon on upsilon And, which displays rotational modulation (P_rot=12 days) in September 2005, in 2002 and 2003 variations appear to correlate with the planet’s orbital period. This on/off nature of star-planet interaction (SPI) in the two systems is likely a function of the changing stellar magnetic field structure throughout its activity cycle. Variability in the transiting system HD 189733 is likely associated with an active region rotating with the star, however, the flaring in excess of the rotational modulation may be associated with its hot Jupiter. As for HD 179949, the peak variability as measured by the mean absolute deviation for both HD 189733 and tau Boo leads the sub-planetary longitude by 70 degrees. The tentative correlation between this activity and the ratio of Mpsini to the planet’s rotation period, a quantity proportional to the hot Jupiter’s magnetic moment, first presented in Shkolnik et al. 2005 remains viable. This work furthers the characterization of SPI, improving its potential as a probe of extrasolar planetary magnetic fields.

    Comments: Accepted for publication in the Astrophysical Journal

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0712.0004v1 [astro-ph]

    Submission history

    From: Evgenya Shkolnik [view email]

    [v1] Fri, 30 Nov 2007 21:08:40 GMT (174kb)


  • ljk February 21, 2008, 12:00

    MOST detects variability on tau Bootis possibly induced by its planetary companion

    Authors: Gordon A.H. Walker, Bryce Croll, Jaymie M. Matthews, Rainer Kuschnig, Daniel Huber, Werner W. Weiss, Evgenya Shkolnik, Slavek M. Rucinski, David B. Guenther, Anthony F.J. Moffat, Dimitar Sasselov

    (Submitted on 20 Feb 2008)

    Abstract: (abridged) There is considerable interest in the possible interaction between parent stars and giant planetary companions in 51 Peg-type systems. We demonstrate from MOST satellite photometry and Ca II K line emission that there has been a persistent, variable region on the surface of tau Boo A which tracked its giant planetary companion for some 440 planetary revolutions and lies ~68deg (phi=0.8) in advance of the sub-planetary point. The light curves are folded on a range of periods centered on the planetary orbital period and phase dependent variability is quantified by Fourier methods and by the mean absolute deviation (MAD) of the folded data for both the photometry and the Ca II K line reversals. The region varies in brightness on the time scale of a rotation by ~1 mmag. In 2004 it resembled a dark spot of variable depth, while in 2005 it varied between bright and dark. Over the 123 planetary orbits spanned by the photometry the variable region detected in 2004 and in 2005 are synchronised to the planetary orbital period within 0.0015 d. The Ca II K line in 2001, 2002 and 2003 also shows enhanced K-line variability centered on phi=0.8, extending coverage to some 440 planetary revolutions. The apparently constant rotation period of the variable region and its rapid variation make an explanation in terms of conventional star spots unlikely. The lack of complementary variability at phi=0.3 and the detection of the variable region so far in advance of the sub-planetary point excludes tidal excitation, but the combined photometric and Ca II K line reversal results make a good case for an active region induced magnetically on the surface of tau Boo A by its planetary companion.

    Comments: 7 pages, 7 figures; accepted for publication in A&A

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0802.2732v1 [astro-ph]

    Submission history

    From: Gordon Walker [view email]

    [v1] Wed, 20 Feb 2008 01:20:51 GMT (580kb)


  • ljk February 28, 2008, 1:18

    Sun-like Star Flips Its Magnetic Field Like Our Sun:
    First Observation

    ScienceDaily (Feb. 26, 2008) — An international group of
    astronomers that includes the University of Hawaii’s Evgenya
    Shkolnik have discovered that the sun-like star tau Bootis
    flipped its magnetic field from north to south sometime
    during the last year.

    It has been known for many years that the Sun’s magnetic
    field changes its direction every 11 years, but this is the first
    time that such a change has been observed in another star.

    The team of astronomers, who made use of Canada-France-
    Hawaii Telescope atop Mauna Kea, are now closely monitoring
    tau Bootis to see how long it will be before the magnetic field
    reverses again.

    Full article here:


  • ljk December 19, 2008, 0:37

    arXiv: 0812.3641

    Date: Thu, 18 Dec 2008 20:12:18 GMT (69kb)

    Title: Planet influence on the shape of the hosting star – ellipsoidal variations of tau Bootis

    Authors: W. Dimitrov

    Categories: astro-ph

    Comments: 5 pages, 3 figures

    This paper presents estimations on the possibility of detection of ellipsoidal variations by means of measuring brightness of the star distorted by a close massive planet using Wilson-Devinney method. The problem was already discussed by Phafl et al. (2008) and earlier by Loeb and Gaudi (2003).

    The effect is well known in the case of binary stars where the effect can produce light curves with amplitudes of ellipsoidal variations of about 0.1 mag for distorted stars. For planets the effect is very small and usually less than 0.0001 mag.

    The detection of an exoplanet, by searching for small amplitude ellipsoidal variations, will be very difficult and affected by other photometric effects; however, it may be possible for some extreme cases.

    Observations of ellipsoidal variations can provide additional constraints on the model of the system. Light curves for few star/planet systems have been calculated using PHOEBE eclipsing binary software based on Wilson-Devinney method.

    As an example of ellipsoidal variations the synthetic light curve for tau Bootis is presented. The amplitude of ellipsoidal variation is 0.01 mmag. The companion is massive 7.3 Mjup and short-period hot Jupiter.

    http://arxiv.org/abs/0812.3641 , 69kb