Starspots, cycles, and magnetic fields

Saar, Steven H.

doi:10.1017/s1743921311015018

Cited by 12 publications

(18 citation statements)

References 29 publications

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“…ASAS V -band data show evidence for differential rotation, in the form of distinct P rot values in different epochs. The fractional DR rate is ∆P rot / P rot ∼ 0.16, similar to the solar value and broadly consistent with observed trends in single dwarfs (Saar 2011) Our analysis of four years of Swift data (2009-2013) strengthens the case for a stellar cycle by extending it to higher energies, with observed peak-to-peak variations of order 10% in the UVW1 band and roughly a factor of 1.5 in the 0.5-2 keV X-ray band, with Xray/UV variations anti-correlated with optical brightness. This anti-correlation is also seen (with less confidence) in rotational modulation, as would be expected if higher starspot coverage (which generates more Xray/UV emission) causes a net decrease in optical emission.…”

Section: Discussionsupporting

confidence: 88%

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Optical, UV, and X-ray evidence for a 7-yr stellar cycle in Proxima Centauri

Wargelin¹,

Saar²,

Pojmański³

et al. 2016

Mon. Not. R. Astron. Soc.

103

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Stars of stellar type later than about M3.5 are believed to be fully convective and therefore unable to support magnetic dynamos like the one that produces the 11-year solar cycle. Because of their intrinsic faintness, very few late M stars have undergone long-term monitoring to test this prediction, which is critical to our understanding of magnetic field generation in such stars. Magnetic activity is also of interest as the driver of UV and X ray radiation, as well as energetic particles and stellar winds, that affect the atmospheres of close-in planets that lie within habitable zones, such as the recently discovered Proxima b. We report here on several years of optical, UV, and X-ray observations of Proxima Centauri (GJ 551; dM5.5e): 15 years of ASAS photometry in the V band (1085 nights) and 3 years in the I band (196 nights), 4 years of Swift XRT and UVOT observations (more than 120 exposures), and 9 sets of X-ray observations from other X-ray missions (ASCA, XMM-Newton, and three Chandra instruments) spanning 22 years. We confirm previous reports of an 83-day rotational period and find strong evidence for a 7-year stellar cycle, along with indications of differential rotation at about the solar level. X-ray/UV intensity is anti-correlated with optical V -band brightness for both rotational and cyclical variations. From comparison with other stars observed to have X-ray cycles we deduce a simple empirical relationship between X-ray cyclic modulation and Rossby number, and we also present Swift UV grism spectra covering 2300-6000Å.

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Section: Discussionsupporting

confidence: 88%

“…The estimated ∆P rot implies ∆Ω/∆Ω ⊙ = 0.33, which puts Prox Cen at the edge of the observed ∆Ω− Ro −1 distribution for single dwarfs (a factor of ∼3 below than the overall trend; see Saar 2011, their Fig. 2 left).…”

Section: Optical Datamentioning

confidence: 85%

Optical, UV, and X-ray evidence for a 7-yr stellar cycle in Proxima Centauri

Wargelin¹,

Saar²,

Pojmański³

et al. 2016

Mon. Not. R. Astron. Soc.

103

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“…4.1) likewise reveal strong correlations between rotation and activity (Hempelmann et al 1996;Micela and Marino 2003;Güdel 2004). For recent surveys of activity in solar-like stars, see for example Wright et al (2004), Giampapa et al (2006), Saar (2011), Marsden et al (2014 or do and references therein.…”

Section: Main Sequence Solar-like Starsmentioning

confidence: 95%

“…Traditionally such dependency is written ΔΩ ∝ Ω n * , with n a positive exponent. Indeed, in Donahue et al (1996), , Saar (2011), n ∼ 0.6−0.7, in Hall (1991 and Henry et al (1995), n = 0.24 ± 0.06 and in Barnes et al (2005) and Collier Cameron (2007), n = 0.15 ± 0.1. Recent studies using asteroseismic data, have found a value in between n ∼ 0.3 (Reinhold and Gizon 2015).…”

Section: Main Sequence Solar-like Starsmentioning

confidence: 95%

“…A more sophisticated analysis reveals that at least two branches/scaling relations have been identified: an active one corresponding to relatively young stars and an inactive one corresponding to slow rotators (Saar and Brandenburg 1999;Saar 2002;Böhm-Vitense 2007;Hall et al 2009;Oláh et al 2009;Metcalfe et al 2010a;Saar 2011;See et al 2016). There are illustrated in Fig.…”

Section: Main Sequence Solar-like Starsmentioning

confidence: 99%

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Magnetism, dynamo action and the solar-stellar connection

Brun

Browning

2017

Living Rev Sol Phys

241

182

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The Sun and other stars are magnetic: magnetism pervades their interiors and affects their evolution in a variety of ways. In the Sun, both the fields themselves and their influence on other phenomena can be uncovered in exquisite detail, but these observations sample only a moment in a single star's life. By turning to observations of other stars, and to theory and simulation, we may infer other aspects of the magnetism-e.g., its dependence on stellar age, mass, or rotation rate-that would be invisible from close study of the Sun alone. Here, we review observations and theory of magnetism in the Sun and other stars, with a partial focus on the "Solar-stellar connection": i.e., ways in which studies of other stars have influenced our understanding of the Sun and vice versa. We briefly review techniques by which magnetic fields can be measured (or their presence otherwise inferred) in stars, and then highlight some key observational findings uncovered by such measurements, focusing (in many cases) on those that offer particularly direct constraints on theories of how the fields are built and maintained. We turn then to a discussion of how the fields arise in different objects: first, we summarize some essential elements of convection and dynamo theory, including a very brief discussion of mean-field theory and related concepts. Next we turn to simulations of convection and magnetism in stellar interiors, highlighting both some peculiarities of field generation in different types of stars and some unifying physical processes that likely influence dynamo action in general. We conclude with a brief

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The Solar-Stellar Connection

Brun¹,

García²,

Houdek³

et al. 2017

Space Sciences Series of ISSI

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We discuss how recent advances in observations, theory and numerical simulations have allowed the stellar community to progress in its understanding of stellar convection, rotation and magnetism and to assess the degree to which the Sun and other stars share similar dynamical properties. Ensemble asteroseismology has become a reality with the advent of large time domain studies, especially from space missions. This new capability has provided improved constraints on stellar rotation and activity, over and above that obtained via traditional techniques such as spectropolarimetry or CaII H&K observations. New data and surveys covering large mass and age ranges have provided a wide parameter space to confront theories of stellar magnetism. These new empirical databases are complemented by theoretical advances and improved multi-D simulations of stellar dynamos. We trace these pathways through which a lucid and more detailed picture of magnetohydrodynamics of solar-like stars is beginning to emerge and discuss future prospects.

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Starspots, cycles, and magnetic fields

Cited by 12 publications

References 29 publications

Optical, UV, and X-ray evidence for a 7-yr stellar cycle in Proxima Centauri

Optical, UV, and X-ray evidence for a 7-yr stellar cycle in Proxima Centauri

Magnetism, dynamo action and the solar-stellar connection

The Solar-Stellar Connection

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