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太陽系物理学:疑問が投げ掛けられるダイナモ理論
http://www.asyura2.com/15/nature6/msg/391.html
投稿者 軽毛 日時 2016 年 7 月 29 日 11:26:54: pa/Xvdnb8K3Zc jHmW0Q
 

太陽系物理学:疑問が投げ掛けられるダイナモ理論
• Paul Charbonneau
• Full text恒星の磁場を駆動している機構を調べる手段であるX線放射観測が、今回4つの低温星について行われ、太陽と恒星における磁場生成に関して広く受け入れられているモデルに疑問を投げ掛ける結果が得られた。

http://www.nature.com/nature/journal/v535/n7613/standfirst/535500a_ja.html?lang=ja 
宇宙物理学:タコクラインのない全対流状態にある恒星内の太陽型ダイナモのふるまい
• Nicholas J. Wright & Jeremy J. Drake
Full text我々の太陽と同じように輻射層と対流層を持つ太陽型の恒星では、磁場は、紫外線波長からX線波長の彩層放射やコロナ放射だけでなく、恒星黒点やフレアなどの太陽現象にもエネルギーを供給している。磁場を生じさせているダイナモは、差動回転による内部磁場のせん断に依存している。このせん断は、タコクライン(tachocline)と呼ばれる輻射層と対流外層の境界面で生じていると長い間考えられていた。全対流星にはタコクラインがなく、ダイナモ機構は大きく異なると予想されているが、その正確な形状や物理的依存性は分かっていない。本論文では、太陽型の恒星と同じようにX線放射と自転が相関している4つの全対流星の観測について報告する。X線活動と自転の関係は、磁場ダイナモのふるまいを表すよく確立された代理指標であるため、これらの結果は全対流星でも太陽型ダイナモが働いていることを示唆している。従って、全対流星にタコクラインがないことは、タコクラインが太陽型ダイナモに不可欠な要素でないことを示唆しており、ダイナモの起源が対流層全体であるというモデルを裏付けている。
http://www.nature.com/nature/journal/v535/n7613/fp/nature18638_ja.html?lang=ja
 

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1. 2016年7月29日 11:27:40 : OO6Zlan35k : ScYwLWGZkzE[554]

Solar physics: Dynamo theory questioned
• Paul Charbonneau
Nature 535,500–501(28 July 2016)doi:10.1038/535500a27 July 2016
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Observations of X-ray emission — a diagnostic tool for the mechanisms driving stellar magnetic fields — from four cool stars call into question accepted models of magnetic-field generation in the Sun and stars. See Letter p.526
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Every star, including the Sun, hosts a magnetic field. One of the most notable products of stellar magnetic activity is X-ray emission (Fig. 1), most of which comes from active regions — areas on a star's surface where the magnetic field concentrates and whose best-known examples are sunspot groups. The commonly accepted theory of stellar magnetic-field generation is based on a dynamo process of electromagnetic induction: the mechanical energy of internal and surface plasma flows is converted into magnetic energy. According to this theory, less than one-tenth of 1% of the Sun's total luminosity is sufficient to drive the solar magnetic cycle, heat the corona, accelerate the solar wind and power all the eruptive phenomena that collectively make up solar activity.
Figure 1: Short-wavelength emission from the Sun.

In this composite image, extreme ultraviolet (red–yellow) was measured by the Solar Dynamics Observatory; low-energy X-rays (green) were measured by the Hinode spacecraft; and high-energy X-rays (blue) were measured by the Nuclear Spectroscopic Telescope Array. Wright and Drake1 present observations of X-ray emissions from four stars that cannot be explained by currently accepted models of the solar dynamo.
NASA/JPL-CALTECH/GSFC/JAXA
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But on page 526, Wright and Drake1 show that the details of the dynamo theory still escape us. They report on the X-ray emissions from four stars detected by the Chandra and ROSAT space observatories. These stars are substantially colder than the Sun and have a different internal structure. If the chain of physical mechanisms that lead to X-ray emission in the Sun conformed to the dynamo theory, we would expect a different pattern of X-ray emission from these stars. Yet, Wright and Drake show that their X-ray emissions have the same pattern as the Sun's.
The solar-surface magnetic field can be observed in great detail because of the Sun's proximity to Earth. Such observations have revealed several spatiotemporal patterns that are driven by the dynamo. Perhaps most notably, polarity reversals occur with a regular 11-year period, and magnetically active regions are found to emerge ever closer to the solar equator as activity cycles unfold2. Conversely, observations of the activity of other stars are usually restricted to surface-averaged, global measurements that lack spatial information.
But, where stars are concerned, what researchers lack in detail is made up for in numbers. Surveys of X-ray emissions from large samples of stars — which, among other things, determine the dependence of the emissions on stellar mass, luminosity and rotation rate — provide information on global stellar activity and, in turn, insight into the underlying magnetic processes.
In Sun-like stars, X-ray emission increases with rotation rate up to a value of a few times the rotation period, and then levels off3. Information about the details of the internal dynamo is thus lost for fast-rotating stars, which fall within this plateau region of emission behaviour. Wright and Drake present an updated version of the emission-behaviour profile (see Fig. 1 of the paper1), based on data for hundreds of stars of varying masses and luminosities.
Within the solar dynamo theory, a key element of the complex causal chain that links the internal dynamo to surface X-ray emission is the tachocline, a transition region between the radiative core of the Sun and its convective outer layer. Solar plasma in the convective zone rotates at different rates, depending on latitude, whereas the radiative core rotates more or less as a solid body. This difference in rotational rate between the two zones produces a strong 'shear' stress in the tachocline, which helps to concentrate the diffuse magnetic field into structures called flux ropes. These magnetic flux ropes emerge at the surface and generate active regions4.
Stars progressively less massive than the Sun have deeper convective envelopes, becoming fully convective at about 40% of the Sun's mass. Such stars no longer have a tachocline, so we should expect some qualitative change in their mode of dynamo action. This long-sought 'dynamo boundary' has not yet been detected. Fully convective stars have been found to emit the same level of X-rays as solar-type stars that harbour a tachocline. However, all previously reported stars of this type are fast rotators, and therefore fall on the plateau region of the diagram that plots the relationship between X-ray emission and rotation rate (see the red circles in Fig. 1 of the paper1), meaning that the details of the internal dynamo action might well be lost.
Enter Wright and Drake. The authors have uncovered the X-ray emissions from four slowly rotating, fully convective stars, and all four fall on the slope part of the X-ray emission–behaviour profile. X-ray emissions therefore scale identically with the Rossby number (the ratio of convective flow speed to rotation rate) in stars with and without a tachocline.The authors thus argue that the tachocline cannot be an essential ingredient for stellar dynamo action, as it is in the currently accepted theory.
There are several possible ways to explain this quandary. Perhaps low-mass stars are not fully convective all the way down to their centres. Or maybe the pattern of stellar X-ray emission is dominated by a re-organization of the magnetic field that occurs in the stellar upper atmosphere5, thus losing its 'memory' of the magnetic field's dynamo origin. And, of course, a tachocline might indeed be non-essential.
Numerical simulations6, 7 of global solar convection have demonstrated that solar-like large-scale magnetic fields undergoing regular polarity reversals can be produced wholly within a convection zone, without the need to extend the simulation down to the depth of the tachocline. Some of these simulations even generate rope-like structures of magnetic flux that rise to the top of the simulation domain in a solar-like manner8, 9. Wright and Drake's results, together with such simulations, provide an impetus to rethink what we know about the solar and stellar dynamo.
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1. Wright, N. J. & Drake, J. J. Nature 535, 526–528 (2016).
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2. Hathaway, D. H. Living Rev. Sol. Phys. 7, 1 (2010).
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3. Noyes, R. W., Hartmann, L. W., Baliunas, S. L., Duncan, D. K. & Vaughan, A. H. Astrophys. J. 279, 763–777 (1984).
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4. Fan, Y. Living Rev. Sol. Phys. 6, 4 (2009).
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5. Cheung, M. C. M., Rempel, M., Title, A. M. & Schüssler, M. Astrophys. J.720, 233–244 (2010).
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6. Charbonneau, P. Annu. Rev. Astron. Astrophys. 52, 251–290 (2014).
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7. Hotta, H., Rempel, M. & Tokoyama, T. Science 351, 1427–1430 (2016).
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8. Nelson, N. J., Brown, B. P., Sacha Brun, A., Miesch, M. S. & Toomre, J.Astrophys. J. 739, L38 (2011).
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9. Fan, Y. & Fang, F. Astrophys. J. 789, 35 (2014).
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1. Paul Charbonneau is in the Département de Physique, Université de Montréal, Québec H3C 3J7, Canada.
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• Solar physics: The planetary hypothesis revived
• Solar physics: Towards ever smaller length scales

http://www.nature.com/nature/journal/v535/n7613/full/535500a.html 

Solar-type dynamo behaviour in fully convective stars without a tachocline
• Nicholas J. Wright
• & Jeremy J. Drake
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Nature

535,

526–528

(28 July 2016)

doi:10.1038/nature18638
Received

25 April 2016
Accepted

02 June 2016
Published online

27 July 2016
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In solar-type stars (with radiative cores and convective envelopes like our Sun), the magnetic field powers star spots, flares and other solar phenomena, as well as chromospheric and coronal emission at ultraviolet to X-ray wavelengths. The dynamo responsible for generating the field depends on the shearing of internal magnetic fields by differential rotation1, 2. The shearing has long been thought to take place in a boundary layer known as the tachocline between the radiative core and the convective envelope3. Fully convective stars do not have a tachocline and their dynamo mechanism is expected to be very different4, although its exact form and physical dependencies are not known. Here we report observations of four fully convective stars whose X-ray emission correlates with their rotation periods in the same way as in solar-type stars. As the X-ray activity–rotation relationship is a well-established proxy for the behaviour of the magnetic dynamo, these results imply that fully convective stars also operate a solar-type dynamo. The lack of a tachocline in fully convective stars therefore suggests that this is not a critical ingredient in the solar dynamo and supports models in which the dynamo originates throughout the convection zone.
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• Solar physics
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Stars across the Hertzsprung–Russell diagram are known to emit X-rays, with only a few exceptions. In main-sequence solar-type and low-mass stars, the X-rays arise from a magnetically confined plasma known as a corona that reaches temperatures of several million kelvin5. Coronal X-ray emission is ultimately powered by the dissipation of the magnetic fields generated by an interior magnetic dynamo. A close relation between the surface magnetic flux and X-ray radiance based on both solar and stellar observations that span several decades for both quantities6 indicates that X-ray emission is a reliable proxy for magnetic activity. The dynamo is thought to be driven in part by differential rotation in the interior of the star1, 2, which itself is generated by the action of the Coriolis force on the rotating convective envelope, but the detailed mechanism remains to be properly understood7. The relationship between stellar rotation and tracers of magnetic activity is therefore an important probe of the stellar dynamo.
In solar-type stars, X-ray emission is observed to increase monotonically with increasing stellar rotational velocity for periods exceeding a few days8, 9. This relationship is often quantified in terms of the dependence of the ratio of stellar luminosity expended in X-rays to bolometric luminosity LX/Lbol, on the Rossby number, which is defined as Ro = Prot/τ, the ratio of the stellar rotation period and the mass-dependent convective turnover time10. A recent study9 using the largest available sample of 824 solar- and late-type stars fitted the rotation–activity relation as LX/Lbol = 5.3 × 10−6Ro−2.7.
This relationship has been observed in stars from late F-type through to early M-type, that is, those with radiative cores and convective envelopes. The interface layer between these two regions, named the tachocline, is believed to play an important role in the generation of the magnetic field3. Shear between the rigidly rotating core and the differential rotation of the convective envelope with latitude is thought to amplify and store the magnetic field11, generating what is known as an α–Ω dynamo, named for the interplay between cyclonic eddies (the α effect) and the shearing of the field (the Ω effect).
Others have argued that the latitudinal and radial gradients of the angular velocity in the convection zone may be sufficient for global dynamo action12, 13. In the Sun, the extremely strong levels of radial shear just beneath the surface are actually higher than in the tachocline, making it plausible that the solar dynamo is distributed across the convection zone rather than confined to the tachocline12.
Despite this hypothesis it has been widely accepted that magnetic structures in the convection zone would be disrupted by magnetic buoyancy or turbulent pumping, preventing large-scale magnetic fields from being established there14. However, some recent three-dimensional magneto-hydrodynamic simulations without a tachocline have produced persistent magnetic wreaths in the convection zone15 and shown that it is possible to produce large-scale magnetic fields in stellar convection layers16, 17.
For very fast rotators, the rotation–activity relationship has been found to break down, with X-ray luminosity reaching a saturation level of approximately LX/Lbol ≈ 10−3, independent of the spectral type18. This saturation level is reached at a Rossby number of approximately 0.13 ± 0.02 (ref. 9; the error represents one standard deviation), corresponding to a rotation period that increases towards later spectral types, from 2 days for a star similar to the Sun to up to about 20 days for low-mass M dwarfs, and is also seen in both chromospheric emission and magnetic field measurements. It is unclear whether this is caused by a saturation of either the dynamo mechanism or the transport of the magnetic flux to the corona, a change in the type of dynamo at work within the star9 or because coronal X-ray emission itself becomes insensitive to the strength of the magnetic field, the energy of which is then dissipated in other ways.
Main-sequence stars later than spectral type M3–3.5 (M < 0.4M⊙, where M is the mass of the star and M⊙ is the mass of the Sun) are predicted to be fully convective and therefore do not possess a tachocline. If the tachocline is critical to the operation of a solar-type dynamo, fully convective stars should not be able to sustain such a dynamo. Instead, it is generally thought that they generate magnetic fields entirely by helical turbulence4. Nevertheless, observations indicate that stars throughout the M-type spectral range exhibit high magnetic field strengths19 and high fractional X-ray luminosities9. In fact, no discernible difference in magnetic activity properties has been identified on either side of the fully convective boundary.
One problem with existing studies is that nearly all fully convective stars that have so far been studied have saturated levels of X-ray emission9. It is therefore unclear whether these stars all exhibit saturated levels of X-ray emission (possibly hinting at some facet of their dynamo mechanism), or whether slower rotators follow a more solar-like rotation–activity relationship. Studies of magnetic activity in slowly rotating fully convective stars have so far been lacking, adding to the uncertainty about their dynamo mechanism. Despite this, fully convective stars are common and, given their spin-down times of a few billion years20, at least half of all such stars are expected to be slow rotators.
Understanding the dynamo mechanism in these slowly rotating stars is important for various astrophysical problems, including the dynamo-driven angular momentum loss rate of low-mass stars, the particle and photon radiation environment of exoplanets and the notorious period gap in the cataclysmic variables. The lack of cataclysmic variables with periods in the 2–3 h range is often attributed to a change in rotational spin-down for fully convective stars with diminished magnetic dynamos and stellar winds21. Recent numerical simulations of stellar winds suggest that increasing the complexity of the surface magnetic morphology can also suppress angular momentum loss and spin-down without requiring any change in the total surface magnetic flux22.
Four slowly rotating, fully convective M-type stars were observed by either NASA’s Chandra X-ray Observatory or the ROSAT satellite. These stars are of spectral type M4–5.5, well beyond the fully convective boundary (M3–3.5) and are therefore genuine fully convective stars. Figure 1 illustrates the traditional rotation–activity diagram showing the fractional X-ray luminosity as a function of the Rossby number for all of the stars from the most recent large-scale study of the rotation–activity relationship in solar-type and low-mass stars9. The positions of the four slowly rotating fully convective stars studied in this work are also shown, and are in excellent agreement with the rotation–activity relationship of partly convective stars.
Figure 1: Rotation–activity relationship diagram for partly and fully convective stars.

Fractional X-ray luminosity, LX/Lbol, plotted against the Rossby number, Ro = Prot/τ, for 824 partly (grey circles) and fully (dark red circles) convective stars from the most recent large compilation of stars with measured rotation periods and X-ray luminosities7. The best-fitting saturated (horizontal) and unsaturated (diagonal) rotation–activity relationships from that study are shown as black dashed lines. The four slowly rotating fully convective M dwarfs studied here are shown in light red (error bars indicate one standard deviation). The uncertainties for the other data points are not quantified, but will be comparable to the M dwarfs for the Rossby number and approximately twice as large for LX/Lbol.
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The observations presented here provide clear evidence for unsaturated X-ray emission in fully convective stars and quantification of their rotation–activity relationship. The results show that fully convective stars, at least when they have spun down sufficiently, operate a dynamo that exhibits a rotation–activity relationship that is indistinguishable from that of solar-type stars. As the dynamo action in fully convective stars is expected to be different from that in solar-type stars owing to their lack of a tachocline4, where the magnetic field is thought to be amplified by radial shear, this is a surprising finding.
The most direct conclusion from these observations is that both partly and fully convective stars operate very similar rotation-dependent dynamos in which the tachocline is not a vital ingredient and differential rotation combined with the action of the Coriolis force is sufficient7. This implies that current models for the solar dynamo that rely on the tachocline layer to amplify the magnetic field are incorrect, and lends weight to recent three-dimensional magneto-hydrodynamic simulations without a tachocline that produce large-scale magnetic fields entirely within the convective layers23, 24. Recent studies with mean-field dynamo models have also suggested that differential rotation in the convection zone may play a greater role in the generation of the magnetic field than does the tachocline25, 26.
Another alternative possibility is that fully convective stars are able to generate a purely turbulent dynamo that exhibits a rotation–activity relationship that is similar to that in partly convective stars, but by a different mechanism that does not rely on a shear layer. Existing dynamo simulations for fully convective stars succeed in generating magnetic fields, but are unable to predict their behaviour as a function of the rotation rate17. However, it seems unlikely that both partly and fully convective stars would have the same rotation–activity relationship (requiring both their dynamo efficiency and rotational dependence to behave in the same way) without their dynamo mechanisms sharing a major feature.
A third possibility is that convection in the cores of fully convective stars could be magnetically suppressed27, leading to the existence of a solar-like tachocline, although some studies suggest that convection would not be completely halted, only made less efficient28. Furthermore, the field strengths that are necessary for such a transition are 107–108 G (refs 28, 29), orders of magnitude larger than the fields thought to exist in the solar interior and at levels that simulations suggest are impossible to maintain30.
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New observations
Two targets were chosen from the MEarth Transit survey20 of fully convective stars: G184-31 and GJ 3253, of spectral types M4.531 and M532 with rotation periods of 83.8 days (ref. 33) and 78.8 days (ref. 20), respectively.
The targets were observed with the Advanced CCD Imaging Spectrometer (ACIS)34 on the Chandra X-ray Observatory35 using the ACIS-S (spectroscopic) CCD array. Observations were performed in ‘very faint’ mode and were placed on the back-illuminated S3 (the 3rd CCD chip in the ACIS-S spectroscopic array) chip (owing to its higher sensitivity to soft X-rays. The observations were performed on 6 December 2012 and 25 September 2013 for G184-31 and GJ 3253 respectively, with exposure times of 8.0 ks and 21.0 ks.
Observations were processed using the CIAO 4.5 software tools36 and the CALDB 4.5.8 calibration files following standard procedures. The two sources were clearly identified and detected with significances of 5.0σ and 10.2σ at their expected positions. Point-source extraction was performed using CIAO 4.5. The total number of net counts was measured to be and for G184-31 and GJ 3253, respectively. Light curves were constructed for both sources to search for high levels of variability that might inflate the quiescent flux level measured, but no significant variability was detected.
Thermal plasma X-ray spectral models were fitted to the extracted spectra using XSPEC37 version 12.6.0 and compared to APEC38 single-temperature optically thin model spectra of an absorbed thermal plasma in collisional ionization equilibrium, allowing the plasma temperature (kBT, where kB is Boltzmann’s constant and T is temperature) and the hydrogen column density (NH) to vary freely. A grid of initial thermal plasma temperatures covering the range kBT = 0.1–3.0 keV was used to prevent fitting to local minima. The model with the lowest Cash statistic39 was selected as the best fit for each source (the Cash statistic is an application of the likelihood ratio test that is suitable for low-signal data). The best-fitting thermal plasma temperatures were found to be kBT = 0.78 ± 0.13 keV and 0.30 ± 0.05 keV for G184-31 and GJ 3253, respectively, consistent with the values found for other M-type dwarf stars. The fitted hydrogen column density is consistent with no absorption, as expected for the proximity of these sources.
Absorption-corrected broadband (0.5–8.0 keV) fluxes of FX = (2.08 ± 0.38) × 10−14 erg/s/cm2 and (4.35 ± 0.41) × 10−14 erg/s/cm2 were calculated from the model fits for G184-31 and GJ 3253 respectively. Combined with their known parallax distances31, 32 the fluxes were used to calculate X-ray luminosities in the ROSAT band (0.1–2.4 keV), for consistency and ease of comparison with previous studies7. Fractional X-ray luminosities were then calculated from the observed J-band magnitudes and the appropriate bolometric corrections40.
Literature data
We searched the literature for fully convective stars (spectral type M4 or later) with existing measured rotation periods and X-ray luminosities, excluding any with short rotation periods (Prot < 20 days), which would place the object in the saturated regime of Fig. 1. Two stars were found that met our criteria: GJ 699 (Barnard’s star, M4V; ref. 41) and GJ 551 (Proxima Centauri, M5.5V; ref. 42) with rotation periods43 and X-ray luminosity values44 existing in the literature. Rossby numbers and fractional X-ray luminosities for these stars were calculated as for the two newly observed stars.
We also uncovered a number of stars that are close to the convective boundary in the literature45, 46 (spectral types M3 or M3.5). The spectral types and colours9 of these stars imply that they are partly convective and so they are shown in Fig. 1as grey dots.
Sample size
No statistical methods were used to predetermine sample size.
Code availability
The CIAO code used to reduce the Chandra X-ray Observatory data are available at http://cxc.cfa.harvard.edu/ciao and the associated calibration database can be found at http://cxc.cfa.harvard.edu/caldb. The XSPEC code used to perform X-ray spectral fitting is available at https://heasarc.gsfc.nasa.gov/xanadu/xspec.
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Acknowledgements
• Main•
• Methods•
• References•
• Acknowledgements•
• Author information
N.J.W. acknowledges a Royal Astronomical Society Research Fellowship and an STFC Ernest Rutherford Fellowship. J.J.D. was supported by NASA contract NAS8-03060 to the Chandra X-ray Center. We thank J. Irwin, R. Jeffries and A. West for assistance and comments on an early draft of this paper. This research has made use of the Vizier (http://vizier.u-strasbg.fr/cgi-bin/VizieR) and SIMBAD (http://simbad.u-strasbg.fr/simbad/sim-fid) databases (operated at CDS, Strasbourg, France).
Author information
• Main•
• Methods•
• References•
• Acknowledgements•
• Author information
Affiliations
1. Astrophysics Group, Keele University, Keele ST5 5BG, UK
o Nicholas J. Wright
2. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA
o Jeremy J. Drake
Contributions
N.J.W. reduced the Chandra observations, measured the X-ray fluxes and made the necessary calculations to plot the stars in Fig. 1. N.J.W. and J.J.D. wrote the interpretation and discussion of the results.
Competing financial interests
The authors declare no competing financial interests.
Corresponding author
Correspondence to:
• Nicholas J. Wright
Reviewer Information Nature thanks D. Moss and the other anonymous reviewer(s) for their contribution to the peer review of this work.


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