scholarly journals Stellar rotation periods from K2 Campaigns 0–18

2020 ◽  
Vol 635 ◽  
pp. A43 ◽  
Author(s):  
Timo Reinhold ◽  
Saskia Hekker
Keyword(s):  
Gaussian Mixture Models ◽  
Rotation Period ◽  
Gaussian Mixture ◽  
Magnetic Activity ◽  
Stellar Rotation ◽  
Photometric Variability ◽  
Rotation Periods ◽  
Period Distribution ◽  
Brightness Variability ◽  
Stellar Magnetic Activity

Context. Rotation period measurements of stars observed with the Kepler mission have revealed a lack of stars at intermediate rotation periods, accompanied by a decrease of photometric variability. Whether this so-called dearth region is a peculiarity of stars in the Kepler field, or reflects a general manifestation of stellar magnetic activity, is still under debate. The K2 mission has the potential to unravel this mystery by measuring stellar rotation and photometric variability along different fields in the sky. Aims. Our goal is to measure stellar rotation periods and photometric variabilities for tens of thousands of K2 stars, located in different fields along the ecliptic plane, to shed light on the relation between stellar rotation and photometric variability. Methods. We use Lomb–Scargle periodograms, auto-correlation and wavelet functions to determine consistent rotation periods. Stellar brightness variability is assessed by computing the variability range, Rvar, from the light curve. We further apply Gaussian mixture models to search for bimodality in the rotation period distribution. Results. Combining measurements from all K2 campaigns, we detect rotation periods in 29 860 stars. The reliability of these periods was estimated from stars observed more than once. We find that 75–90% of the stars show period deviation smaller than 20% between different campaigns, depending on the peak height threshold in the periodograms. For effective temperatures below 6000 K, the variability range shows a local minimum at different periods, consistent with an isochrone age of ∼750 Myr. Additionally, the rotation period distribution shows evidence for bimodality, although the dearth region in the K2 data is less pronounced compared to the Kepler field. The period at the dip of the bimodal distribution shows good agreement with the period at the local variability minimum. Conclusions. We conclude that the rotation period bimodality is present in different fields of the sky, and is hence a general manifestation of stellar magnetic activity. The reduced variability in the dearth region is interpreted as a cancelation between dark spots and bright faculae. Our results strongly advocate that the role of faculae has been underestimated so far, suggesting a more complex dependence of the brightness variability on the rotation period.

scholarly journals Revisiting the connection between magnetic activity, rotation period, and convective turnover time for main-sequence stars

2018 ◽  
Vol 618 ◽  
pp. A48 ◽  
Author(s):  
M. Mittag ◽  
J. H. M. M. Schmitt ◽  
K.-P. Schröder
Keyword(s):  
Main Sequence ◽  
Rotation Period ◽  
Magnetic Activity ◽  
Turnover Time ◽  
Rossby Number ◽  
Stellar Rotation ◽  
Main Sequence Stars ◽  
Rotation Periods ◽  
Period Distribution ◽  
Cool Stars

The connection between stellar rotation, stellar activity, and convective turnover time is revisited with a focus on the sole contribution of magnetic activity to the Ca II H&K emission, the so-called excess flux, and its dimensionless indicator R+HK in relation to other stellar parameters and activity indicators. Our study is based on a sample of 169 main-sequence stars with directly measured Mount Wilson S-indices and rotation periods. The R+HK values are derived from the respective S-indices and related to the rotation periods in various B–V-colour intervals. First, we show that stars with vanishing magnetic activity, i.e. stars whose excess flux index R+HK approaches zero, have a well-defined, colour-dependent rotation period distribution; we also show that this rotation period distribution applies to large samples of cool stars for which rotation periods have recently become available. Second, we use empirical arguments to equate this rotation period distribution with the global convective turnover time, which is an approach that allows us to obtain clear relations between the magnetic activity related excess flux index R+HK, rotation periods, and Rossby numbers. Third, we show that the activity versus Rossby number relations are very similar in the different activity indicators. As a consequence of our study, we emphasize that our Rossby number based on the global convective turnover time approaches but does not exceed unity even for entirely inactive stars. Furthermore, the rotation-activity relations might be universal for different activity indicators once the proper scalings are used.

scholarly journals Evidence from stellar rotation for early disc dispersal owing to close companions

2019 ◽  
Vol 627 ◽  
pp. A97
Author(s):  
S. Messina
Keyword(s):  
Rotation Period ◽  
Equal Mass ◽  
Close Binaries ◽  
Stellar Rotation ◽  
Low Mass Stars ◽  
Components Separation ◽  
Rotation Periods ◽  
Period Distribution ◽  
Locking Mechanism ◽  
Average Rotation

Context. Young (≲600 Myr) low-mass stars (M ≲ 1 M⊙) of equal mass exhibit a distribution of rotation periods. At the very early phases of stellar evolution, this distribution is set by the star-disc locking mechanism, which forces stars to rotate at the same rate as the inner edge of the disc. The primordial disc lifetime and consequently the duration of the disc-locking mechanism, can be significantly shortened by the presence of a close companion, making the rotation period distribution of close binaries different from that of either single stars or wide binaries. Aims. We use new data to investigate and better constrain the range of ages, the components separation, and the mass ratio dependence at which the rotation period distribution has been significantly affected by the disc dispersal that is enhanced by close companions. Methods. We select a sample of close binaries in the Upper Scorpius association (age ∼8 Myr) whose components have measured the separation and the rotation periods and compare their period distribution with that of coeval stars that are single stars. Results. We find that components of close binaries have, on average, rotation periods that are shorter than those of single stars. More precisely, binaries with approximately equal-mass components (0.9 ≤ M2/M1 ≤ 1.0) have rotation periods that are shorter than those of single stars by ∼0.4 d on average; the primary and secondary components of binaries with smaller mass ratios (0.8 < M2/M1 < 0.9) have rotation periods that are shorter than those of single stars by ∼1.9 d and ∼1.0 d on average, respectively. A comparison with the older 25 Myr β Pictoris association shows that whereas in the latter, all close binaries with projected separation ρ ≤ 80 AU rotate faster than single stars, in the Upper Scorpius this is only the case for about 70% of stars. Conclusions. We interpret the enhanced rotation in close binaries with respect to single stars as the consequence of an early disc dispersal induced by the presence of close companions. The enhanced rotation suggests that disc dispersal timescales are longest for single stars and shorter for close binaries.

scholarly journals Stellar Magnetism in the Era of Space-Based Precision Photometry

2013 ◽  
Vol 9 (S302) ◽  
pp. 206-211
Author(s):  
Lucianne M. Walkowicz
Keyword(s):  
Differential Rotation ◽  
Magnetic Activity ◽  
Stellar Rotation ◽  
Photometric Variability ◽  
Fundamental Nature ◽  
Stellar Magnetic Activity

AbstractThe advent of precision space-based photometric missions such as MOST, CoRoT and Kepler has revealed stellar magnetic activity in unprecedented detail. These observations enable new investigations into the fundamental nature of stellar magnetism by furthering our understanding of the stellar rotation and differential rotation that generate the field, and the photometric variability caused by the surface manifestations of the field. In the case of stars with planetary candidates, these data also offer synergy between studies of stars and planets. Here, I review the possibilities and challenges for deepening our understanding of magnetism in solar-like stars in the era of space-based precision photometry.

scholarly journals The Sun Through Time

2020 ◽  
Vol 216 (8) ◽  
Author(s):  
Manuel Güdel
Keyword(s):  
Stellar Wind ◽  
Rotation Period ◽  
High Energy ◽  
Magnetic Activity ◽  
Energy Output ◽  
The Sun ◽  
Stellar Rotation ◽  
Solar Mass ◽  
Rotation Periods ◽  
Rotational Evolution

AbstractMagnetic activity of stars like the Sun evolves in time because of spin-down owing to angular momentum removal by a magnetized stellar wind. These magnetic fields are generated by an internal dynamo driven by convection and differential rotation. Spin-down therefore converges at an age of about 700 Myr for solar-mass stars to values uniquely determined by the stellar mass and age. Before that time, however, rotation periods and their evolution depend on the initial rotation period of a star after it has lost its protostellar/protoplanetary disk. This non-unique rotational evolution implies similar non-unique evolutions for stellar winds and for the stellar high-energy output. I present a summary of evolutionary trends for stellar rotation, stellar wind mass loss and stellar high-energy output based on observations and models.

scholarly journals Activity and rotation of the X-ray emitting Kepler stars

2019 ◽  
Vol 628 ◽  
pp. A41 ◽  
Author(s):  
D. Pizzocaro ◽  
B. Stelzer ◽  
E. Poretti ◽  
S. Raetz ◽  
G. Micela ◽  
...  
Keyword(s):  
White Light ◽  
Main Sequence ◽  
Magnetic Activity ◽  
Light Curves ◽  
Late Type ◽  
Main Sequence Stars ◽  
Photometric Variability ◽  
X Ray ◽  
Rotation Periods ◽  
Flare Frequency

The relation between magnetic activity and rotation in late-type stars provides fundamental information on stellar dynamos and angular momentum evolution. Rotation-activity studies found in the literature suffer from inhomogeneity in the measurement of activity indexes and rotation periods. We overcome this limitation with a study of the X-ray emitting, late-type main-sequence stars observed by XMM-Newton and Kepler. We measured rotation periods from photometric variability in Kepler light curves. As activity indicators, we adopted the X-ray luminosity, the number frequency of white-light flares, the amplitude of the rotational photometric modulation, and the standard deviation in the Kepler light curves. The search for X-ray flares in the light curves provided by the EXTraS (Exploring the X-ray Transient and variable Sky) FP-7 project allows us to identify simultaneous X-ray and white-light flares. A careful selection of the X-ray sources in the Kepler field yields 102 main-sequence stars with spectral types from A to M. We find rotation periods for 74 X-ray emitting main-sequence stars, 20 of which do not have period reported in the previous literature. In the X-ray activity-rotation relation, we see evidence for the traditional distinction of a saturated and a correlated part, the latter presenting a continuous decrease in activity towards slower rotators. For the optical activity indicators the transition is abrupt and located at a period of ~10 d but it can be probed only marginally with this sample, which is biased towards fast rotators due to the X-ray selection. We observe seven bona-fide X-ray flares with evidence for a white-light counterpart in simultaneous Kepler data. We derive an X-ray flare frequency of ~0.15 d−1, consistent with the optical flare frequency obtained from the much longer Kepler time-series.

scholarly journals Age, Activity and Rotation in Mid and Late-Type M Dwarfs from MEarth

2013 ◽  
Vol 9 (S302) ◽  
pp. 176-179
Author(s):  
Andrew A. West ◽  
Kolby L. Weisenburger ◽  
Jonathan Irwin ◽  
David Charbonneau ◽  
Jason Dittmann ◽  
...  
Keyword(s):  
Dimensional Space ◽  
Large Fraction ◽  
Rotation Period ◽  
Magnetic Activity ◽  
Rotating Stars ◽  
Late Type ◽  
M Dwarfs ◽  
Magnetic Field Generation ◽  
Rotation Periods ◽  
Wide Range

AbstractUsing spectroscopic observations and photometric light curves of 280 nearby M dwarfs from the MEarth exoplanet transit survey, we examine the relationships between magnetic activity (quantified by Hα emission), rotation period, and stellar age (derived from three-dimensional space velocities). Although we have known for decades that a large fraction of mid-late-type M dwarfs are magnetically active, it was not clear what role rotation played in the magnetic field generation (and subsequent chromospheric heating). Previous attempts to investigate the relationship between magnetic activity and rotation in mid-late-type M dwarfs were hampered by the limited number of M dwarfs with measured rotation periods (and the fact that vsini measurements only probe rapid rotation). However, the photometric data from the MEarth survey allows us to probe a wide range of rotation periods for hundreds of M dwarf stars (from less than one to over 100 days). Over all M spectral types we find that magnetic activity decreases with longer rotation periods, including late-type, fully convective M dwarfs. We find that the most magnetically active (and hence, most rapidly rotating) stars are consistent with a kinematically young population, while slow-rotators are less active or inactive and appear to belong to an older, dynamically heated stellar population.

scholarly journals Tidal effects on stellar activity

2016 ◽  
Vol 12 (S328) ◽  
pp. 308-314
Author(s):  
K. Poppenhaeger
Keyword(s):  
Orbital Period ◽  
Binary Systems ◽  
Semimajor Axis ◽  
Rotation Period ◽  
Magnetic Activity ◽  
Stellar Rotation ◽  
Stellar Activity ◽  
Tidal Interaction ◽  
Hot Jupiters ◽  
Orbital Periods

AbstractThe architecture of many exoplanetary systems is different from the solar system, with exoplanets being in close orbits around their host stars and having orbital periods of only a few days. We can expect interactions between the star and the exoplanet for such systems that are similar to the tidal interactions observed in close stellar binary systems. For the exoplanet, tidal interaction can lead to circularization of its orbit and the synchronization of its rotational and orbital period. For the host star, it has long been speculated if significant angular momentum transfer can take place between the planetary orbit and the stellar rotation. In the case of the Earth-Moon system, such tidal interaction has led to an increasing distance between Earth and Moon. For stars with Hot Jupiters, where the orbital period of the exoplanet is typically shorter than the stellar rotation period, one expects a decreasing semimajor axis for the planet and enhanced stellar rotation, leading to increased stellar activity. Also excess turbulence in the stellar convective zone due to rising and subsiding tidal bulges may change the magnetic activity we observe for the host star. I will review recent observational results on stellar activity and tidal interaction in the presence of close-in exoplanets, and discuss the effects of enhanced stellar activity on the exoplanets in such systems.

scholarly journals The Rotation of Low-Mass Pre-Main-Sequence Stars

2004 ◽  
Vol 215 ◽  
pp. 113-122 ◽  
Author(s):  
Robert D. Mathieu
Keyword(s):  
Angular Momentum ◽  
Main Sequence ◽  
Rotation Period ◽  
Rotating Stars ◽  
Stellar Rotation ◽  
Main Sequence Stars ◽  
Star Forming ◽  
Rotation Periods ◽  
Order Of Magnitude ◽  
Critical Velocities

Major photometric monitoring campaigns of star-forming regions in the past decade have provided rich rotation period distributions of pre-main-sequence stars. The rotation periods span more than an order of magnitude in period, with most falling between 1 and 10 days. Thus the broad rotation period distributions found in 100 Myr clusters are already established by an age of 1 Myr. The most rapidly rotating stars are within a factor of 2-3 of their critical velocities; if angular momentum is conserved as they evolve to the ZAMS, these stars may come to exceed their critical velocities. Extensive efforts have been made to find connections between stellar rotation and the presence of protostellar disks; at best only a weak correlation has been found in the largest samples. Magnetic disk-locking is a theoretically attractive mechanism for angular momentum evolution of young stars, but the links between theoretical predictions and observational evidence remain ambiguous. Detailed observational and theoretical studies of the magnetospheric environments will provide better insight into the processes of pre-main-sequence stellar angular momentum evolution.

scholarly journals Inflection point in the power spectrum of stellar brightness variations

2020 ◽  
Vol 633 ◽  
pp. A32 ◽  
Author(s):  
A. I. Shapiro ◽  
E. M. Amazo-Gómez ◽  
N. A. Krivova ◽  
S. K. Solanki
Keyword(s):  
Power Spectrum ◽  
Inflection Point ◽  
Rotation Period ◽  
Power Spectra ◽  
Light Curves ◽  
Stellar Rotation ◽  
Space Telescopes ◽  
Rotation Periods ◽  
Active Stars ◽  
Total Irradiance

Context. Considerable effort has gone into using light curves observed by such space telescopes as CoRoT, Kepler, and TESS for determining stellar rotation periods. While rotation periods of active stars can be reliably determined, the light curves of many older and less active stars, such as stars that are similar to the Sun, are quite irregular. This hampers the determination of their rotation periods. Aims. We aim to examine the factors causing these irregularities in stellar brightness variations and to develop a method for determining rotation periods for low-activity stars with irregular light curves. Methods. We extended the Spectral And Total Irradiance Reconstruction approach for modeling solar brightness variations to Sun-like stars. We calculated the power spectra of stellar brightness variations for various combinations of parameters that define the surface configuration and evolution of stellar magnetic features. Results. The short lifetime of spots in comparison to the stellar rotation period, as well as the interplay between spot and facular contributions to brightness variations of stars with near solar activity, cause irregularities in their light curves. The power spectra of such stars often lack a peak associated with the rotation period. Nevertheless, the rotation period can still be determined by measuring the period where the concavity of the power spectrum plotted in the log–log scale changes its sign, that is, by identifying the position of the inflection point. Conclusions. The inflection point of the (log–log) power spectrum is found to be a new diagnostic for stellar rotation periods which is shown to work even in cases where the power spectrum shows no peak at the rotation rate.

scholarly journals Quantization of Planetary Systems and its Dependency on Stellar Rotation

2011 ◽  
Vol 28 (3) ◽  
pp. 177-201 ◽  
Author(s):  
Jean-Paul A. Zoghbi
Keyword(s):  
Gravitational Instability ◽  
Rotation Period ◽  
Planetary Systems ◽  
Stellar Rotation ◽  
Tidal Dissipation ◽  
Half Integer ◽  
Pure Chance ◽  
Rotation Periods ◽  
Planetary Orbits ◽  
Orbital Periods

AbstractWith the discovery of now more than 500 exoplanets, we present a statistical analysis of the planetary orbital periods and their relationship to the rotation periods of their parent stars. We test whether the structural variables of planetary orbits, i.e. planetary angular momentum and orbital period, are ‘quantized’ in integer or half-integer multiples of the parent star's rotation period. The Solar System is first shown to exhibit quantized planetary orbits that correlate with the Sun's rotation period. The analysis is then expanded over 443 exoplanets to statistically validate this quantization and its association with stellar rotation. The results imply that the exoplanetary orbital periods are highly correlated with the parent star's rotation periods and follow a discrete half-integer relationship with orbital ranks n = 0.5, 1.0, 1.5, 2.0, 2.5, etc. The probability of obtaining these results by pure chance is p < 0.024. We discuss various mechanisms that could justify this planetary quantization, such as the hybrid gravitational instability models of planet formation, along with possible physical mechanisms such as the inner disc's magnetospheric truncation, tidal dissipation, and resonance trapping. In conclusion, we statistically demonstrate that a quantized orbital structure should emerge from the formation processes of planetary systems and that this orbital quantization is highly dependent on the parent star's rotation period.

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