Magnetic activity in HD 111456, a young F5–6 main-sequence star

Context. Eclipsing time variations are observed in many close binary systems. In particular, for several post-common-envelope binaries (PCEBs) that consist of a white dwarf and a main sequence star, the observed-minus-calculated (O–C) diagram suggests that real or apparent orbital period variations are driven by Jupiter-mass planets or as a result of magnetic activity, the so-called Applegate mechanism. The latter explains orbital period variations as a result of changes in the stellar quadrupole moment due to magnetic activity. Aims. In this work we explore the feasibility of driving eclipsing time variations via the Applegate mechanism for a sample of PCEB systems, including a range of different rotation rates. Methods. We used the MESA code to evolve 12 stars with different masses and rotation rates. We applied simple dynamo models to their radial profiles to investigate the scale at which the predicted activity cycle matches the observed modulation period, and quantifiy the uncertainty. We further calculated the required energies to drive the Applegate mechanism. Results. We show that the Applegate mechanism is energetically feasible in 5 PCEB systems. In RX J2130.6+4710, it may be feasible as well considering the uncertainties. We note that these are the systems with the highest rotation rate compared to the critical rotation rate of the main-sequence star. Conclusions. The results suggest that the ratio of physical to critical rotation rate in the main sequence star is an important indicator for the feasibility of Applegate’s mechanism, but exploring larger samples will be necessary to probe this hypothesis.

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A PULSATING FLARE STAR IN THE NEW PLANETARY SYSTEM: KOI-258

Revista Mexicana de Astronomía y Astrofísica ◽

10.22201/ia.01851101p.2021.57.01.02 ◽

2021 ◽

Vol 57 (1) ◽

pp. 39-55

Author(s):

E. Yoldaş ◽

H. A. Dal

Keyword(s):

Radial Velocity ◽

Small Amplitude ◽

Main Sequence ◽

Planetary System ◽

Magnetic Activity ◽

Main Sequence Star ◽

Velocity Variation ◽

Flare Activity ◽

The Third ◽

Radial Velocity Variation

We present findings about the nature of KOI-258. Its temperature was found to be 6500 ± 200 K from its spectrum, which also indicates that the target is a single main sequence star despite the existence of a radial velocity variation with a small amplitude. Ca II H, K lines indicate the existence of magnetic activity, though there is no remarkable excess or variation in the Hα line. We found 51 flares with a frequency of 0.00186 h−1, whose plateau value is 0.659 s. Apart from the flares, we found 420 frequencies due to solar-like oscillations at out-of-eclipses. Removing these 420 frequencies, we demonstrated three different transits caused by three exo-planets. Their radii were found to be 2.33 R_&oplus; for the first planet, 0.53 R_&oplus; for the second one, and 1.15 R_&oplus; for the third planet. Consequently, KOI-258 is an oscillating, single, main sequence star, with in a planetary system and remarkable flare activity.

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A large rotating structure around AB Doradus A at VLBI scale

Proceedings of the International Astronomical Union ◽

10.1017/s1743921319009785 ◽

2019 ◽

Vol 15 (S354) ◽

pp. 189-194

Author(s):

J. B. Climent ◽

J. C. Guirado ◽

R. Azulay ◽

J. M. Marcaide

Keyword(s):

Magnetic Reconnection ◽

Main Sequence ◽

Complex Structure ◽

Main Sequence Star ◽

Polar Cap ◽

Vlbi Observations ◽

Rotating Structure ◽

Source Structure ◽

Expected Position

AbstractWe report the results of three VLBI observations of the pre-main-sequence star AB Doradus A at 8.4 GHz. With almost three years between consecutive observations, we found a complex structure at the expected position of this star for all epochs. Maps at epochs 2007 and 2010 show a double core-halo morphology while the 2013 map reveals three emission peaks with separations between 5 and 18 stellar radii. Furthermore, all maps show a clear variation of the source structure within the observing time. We consider a number of hypothesis in order to explain such observations, mainly: magnetic reconnection in loops on the polar cap, a more general loop scenario and a close companion to AB Dor A.

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PHIBSS: MOLECULAR GAS CONTENT AND SCALING RELATIONS INz∼ 1-3 MASSIVE, MAIN-SEQUENCE STAR-FORMING GALAXIES

The Astrophysical Journal ◽

10.1088/0004-637x/768/1/74 ◽

2013 ◽

Vol 768 (1) ◽

pp. 74 ◽

Cited By ~ 589

Author(s):

L. J. Tacconi ◽

R. Neri ◽

R. Genzel ◽

F. Combes ◽

A. Bolatto ◽

...

Keyword(s):

Main Sequence ◽

Gas Content ◽

Main Sequence Star ◽

Scaling Relations ◽

Molecular Gas ◽

Star Forming

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The habitable zone of Kepler-16: impact of binarity and climate models

International Journal of Astrobiology ◽

10.1017/s1473550418000058 ◽

2018 ◽

Vol 18 (1) ◽

pp. 79-89 ◽

Cited By ~ 2

Author(s):

S. Y. Moorman ◽

B. L. Quarles ◽

Zh. Wang ◽

M. Cuntz

Keyword(s):

Binary System ◽

Main Sequence ◽

Climate Models ◽

Main Sequence Star ◽

Habitable Zone ◽

Lagrangian Points ◽

The Earth ◽

Stable Solutions

AbstractWe continue to investigate the binary system Kepler-16, consisting of a K-type main-sequence star, a red dwarf and a circumbinary Saturnian planet. As part of our study, we describe the system's habitable zone based on different climate models. We also report on stability investigations for possible Earth-mass Trojans while expanding a previous study by B. L. Quarles and collaborators given in 2012. For the climate models, we carefully consider the relevance of the system's parameters. Furthermore, we pursue new stability simulations for the Earth-mass objects starting along the orbit of Kepler-16b. The eccentricity distribution as obtained prefers values close to circular, whereas the inclination distribution remains flat. The stable solutions are distributed near the co-orbital Lagrangian points, thus enhancing the plausibility that Earth-mass Trojans might be able to exist in the Kepler-16(AB) system.

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Eclipsing spotted giant star with K2 and historical photometry

Astronomy and Astrophysics ◽

10.1051/0004-6361/201834106 ◽

2018 ◽

Vol 620 ◽

pp. A189 ◽

Cited By ~ 3

Author(s):

K. Oláh ◽

S. Rappaport ◽

T. Borkovits ◽

T. Jacobs ◽

D. Latham ◽

...

Keyword(s):

Binary System ◽

Main Sequence ◽

Rapid Evolution ◽

Magnetic Activity ◽

Primary Component ◽

Physical Parameters ◽

Giant Star ◽

Eclipsing Binary ◽

Giant Stars ◽

Active Stars

Context. Stars can maintain their observable magnetic activity from the pre-main sequence (PMS) to the tip of the red giant branch. However, the number of known active giants is much lower than active stars on the main sequence (MS) since the stars spend only about 10% of their MS lifetime on the giant branch. Due to their rapid evolution it is difficult to estimate the stellar parameters of giant stars. A possibility for obtaining more reliable stellar parameters for an active giant arises when it is a member of an eclipsing binary system. Aims. We have discovered EPIC 211759736, an active spotted giant star in an eclipsing binary system during the Kepler K2 Campaign 5. The eclipsing nature allows us to much better constrain the stellar parameters than in most cases of active giant stars. Methods. We have combined the K2 data with archival HATNet, ASAS, and DASCH photometry, new spectroscopic radial velocity measurements, and a set of follow-up ground-based BVRCIC photometric observations, to find the binary system parameters as well as robust spot models for the giant at two different epochs. Results. We determined the physical parameters of both stellar components and provide a description of the rotational and long-term activity of the primary component. The temperatures and luminosities of both components were examined in the context of the Hertzsprung–Russell diagram. We find that both the primary and the secondary components deviate from the evolutionary tracks corresponding to their masses in the sense that the stars appear in the diagram at lower masses than their true masses. Conclusions. We further evaluate the proposition that traditional methods generally result in higher masses for active giants than what is indicated by stellar evolution tracks in the HR diagram. A possible reason for this discrepancy could be a strong magnetic field, since we see greater differences in more active stars.

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