scholarly journals Rotation & differential rotation of the active Kepler stars

2013 ◽  
Vol 9 (S302) ◽  
pp. 216-219
Author(s):  
Timo Reinhold ◽  
Ansgar Reiners ◽  
Gibor Basri
Keyword(s):  
Temperature Dependence ◽  
Differential Rotation ◽  
Large Scatter ◽  
Stellar Rotation ◽  
Rotation Periods ◽  
Active Stars ◽  
Magnetic Braking ◽  
The Absolute ◽  
First Time ◽  
Second Period

AbstractStellar rotation is a well-known quantity for tens of thousands of stars. In contrast, differential rotation (DR) is only known for a handful of stars because DR cannot be measured directly. We present rotation periods for more than 24,000 active stars in the Kepler field. Thereof, more than 18,000 stars show a second period, which we attribute to surface differential rotation. Our rotation periods are consistent with previous measurements and the theory of magnetic braking. Our results on DR paint a rather different picture: The temperature dependence of the absolute shear δΩ is split into two groups separated around 6000 K. For the cooler stars δΩ only slightly increases with temperature, whereas stars hotter than 6000 K show large scatter. This is the first time that DR has been measured for such a large number of stars.

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 Activity and differential rotation of the early M dwarf Kepler-45 from transit mapping

2020 ◽  
Vol 492 (4) ◽  
pp. 5141-5151
Author(s):  
S M Zaleski ◽  
A Valio ◽  
B D Carter ◽  
S C Marsden
Keyword(s):  
Differential Rotation ◽  
Rotation Period ◽  
Activity Cycle ◽  
Theoretical Modelling ◽  
Stellar Rotation ◽  
M Dwarfs ◽  
Convective Envelope ◽  
Rotation Periods ◽  
Solar Type ◽  
Rotational Shear

ABSTRACT Little is known of the activity and differential rotation of low luminosity, early M dwarfs from direct observation. We present the first stellar activity analysis of star-spots and faculae for the hot Jupiter hosting M1V dwarf Kepler-45 from $\it Kepler$ transit light curves. We find star-spot and facula temperatures contrasting a few hundred degrees with the quiet photosphere, hence similar to other early M dwarfs having a convective envelope surrounding a radiative core. Star-spots are prominent close to the centre of the stellar disc, with faculae prominent towards the limbs, similar to what is observed for the Sun. Star-spot and facula mean sizes are about 40 and 45 × 103 km, respectively, and thus faculae occupy a 10 per cent larger surface area than the star-spots. A short-term activity cycle of about 295 d is observed that is reminiscent of those seen for other cool dwarfs. Adopting a solar-type differential rotation profile (faster equatorial rotation than polar rotation), our star-spot and facula temporal mapping indicates a rotation period of 15.520 ± 0.025 d at the transit latitude of −33.2°. From the mean stellar rotation of 15.762 d, we estimate a rotational shear of 0.031 ± 0.004 rad d−1, or a relative differential rotation of 7.8 ± 0.9 per cent. Kepler-45’s surface rotational shear is thus consistent with observations and theoretical modelling of other early M dwarfs that indicate a shear of less than 0.045 rad d−1 and no less than 0.03 rad d−1 for stars with similar stellar rotation periods.

scholarly journals Effect of metallicity on the detectability of rotational periods in solar-like stars

2020 ◽  
Vol 634 ◽  
pp. L9 ◽  
Author(s):  
V. Witzke ◽  
T. Reinhold ◽  
A. I. Shapiro ◽  
N. A. Krivova ◽  
S. K. Solanki
Keyword(s):  
Accurate Determination ◽  
Theoretical Estimate ◽  
Light Curves ◽  
Stellar Rotation ◽  
Photometric Variability ◽  
Rotation Periods ◽  
Irregular Pattern ◽  
Stellar Ages ◽  
Solar Metallicity ◽  
First Time

The accurate determination of stellar rotation periods is important for estimating stellar ages and for understanding stellar activity and evolution. While rotation periods can be determined for about thirty thousand stars in the Kepler field, there are over one hundred thousand stars, especially with low photometric variability and irregular pattern of variations, for which rotational periods are unknown. Here we investigate the effect of metallicity on the detectability of rotation periods. This is done by synthesising light curves of hypothetical stars that are identical to our Sun with the exception of the metallicity. These light curves are then used as an input to the period determination algorithms. We find that the success rate for recovering the rotation signal has a minimum close to the solar metallicity value. This can be explained by the compensation effect of facular and spot contributions. In addition, selecting solar-like stars with near-solar effective temperature and photometric variability, and with metallicity between M/H = −0.35 and M/H = 0.35 from the Kepler sample, we analyse the fraction of stars for which rotational periods have been detected as a function of metallicity. In agreement with our theoretical estimate we find a local minimum for the detection fraction close to the solar metallicity. We further report rotation periods of 87 solar-like Kepler stars for the first time.

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

2020 ◽  
Vol 642 ◽  
pp. A225
Author(s):  
E. M. Amazo-Gómez ◽  
A. I. Shapiro ◽  
S. K. Solanki ◽  
G. Kopp ◽  
M. Oshagh ◽  
...  
Keyword(s):  
Power Spectrum ◽  
Inflection Point ◽  
Active Regions ◽  
Light Curves ◽  
The Sun ◽  
Stellar Rotation ◽  
Rotation Periods ◽  
Active Stars ◽  
Effective Temperatures

Context. Stellar rotation periods can be determined by observing brightness variations caused by active magnetic regions transiting visible stellar disk as the star rotates. Successful stellar photometric surveys stemming from the Kepler and TESS observations have led to the determination of rotation periods in tens of thousands of young and active stars. However, there is still a lack of information on the rotation periods of older and less active stars like the Sun. The irregular temporal profiles of light curves caused by the decay times of active regions, which are comparable to, or even shorter than, stellar rotation periods, in combination with the random emergence of active regions make period determination for such stars very difficult. Aims. We tested the performance of a new method for the determination of stellar rotation periods against stars with previously determined rotation periods. The method is based on calculating the gradient of the power spectrum (GPS) and identifying the position of the inflection point (i.e. point with the highest gradient). The GPS method is specifically aimed at determining rotation periods of low-activity stars like the Sun. Methods. We applied the GPS method to 1047 Sun-like stars observed by the Kepler telescope. We considered two stellar samples individually: one with near-solar rotation periods (24–27.4 d) and a broad range of effective temperatures (5000–6000 K) and the other with near-solar effective temperatures (5700–5900 K) and a broad range of rotation periods (15–40 d). Results. We show that the GPS method returns precise values for stellar rotation periods. Furthermore, it allows us to constrain the ratio between facular and spot areas of active regions at the moment of their emergence. We also show that the relative facular area decreases with the stellar rotation rate. Conclusions. Our results suggest that the GPS method can be successfully applied to retrieve the periods of stars with both regular and non-regular light curves.

Coleridge's “ideal Realism”: an alternative to the “doctors of the Absolute”?

1995 ◽  
Vol 16 (01) ◽  
pp. 1-16
Author(s):  
Mary Anne Perkins
Keyword(s):  
Political Philosophy ◽  
Common Ground ◽  
Samuel Taylor Coleridge ◽  
The Political ◽  
Close Reading ◽  
Magnum Opus ◽  
The Absolute ◽  
First Time

A few months ago I read Peter Nicholson's The Political Philosophy of the British Idealists for the first time. In the index I found more than a hundred references to Hegel and only one to Samuel Taylor Coleridge. However, as many of the latter's writings, published for the first time in recent years, become generally accessible there is an increasing sense that he has been unfairly deprived of his due status as a philosopher. This is partly, no doubt, the syndrome of the prophet in his own country and partly the inevitable consequence of much of his later work remaining unpublished until recent years. Coleridge himself, with what some would take to be confirmation of an over-sensitivity to criticism, felt the neglect of his work went deeper and betrayed an anti-philosophical trait in British character. Despite his close reading of the work of many of his German contemporaries it seems that he did not read more than sixtyone pages of Hegel's Wissenschaft der Logik. His margin notes to this work are, on the whole, negative in their criticism. However, despite significant disagreements, there is much common ground in theme, argument and conclusion between his many drafts of the ‘Logosophia’, his intended magnum opus, and Hegel's system.

The Tartaria Tablets: a Chronological Issue

Antiquity ◽  
1968 ◽  
Vol 42 (165) ◽  
pp. 32-35 ◽  
Author(s):  
Evžen Neustupný
Keyword(s):  
Neolithic Period ◽  
Central European ◽  
The Absolute ◽  
First Time

In the June number of ANTIQUITY Mr Sinclair Hood introduced to the readers of this journal the sensational find of the so-called Neolithic inscribed tablets from Transylvania. When this find was announced for the first time [I], it made a great impression upon everybody who appreciated its significance. It was a kind of deus ex machina which seemed to solve once and for all one of the crucial issues of Central European archaeology: the absolute chronology of the Neolithic Period.

scholarly journals Inverse tides in pulsating binary stars

2020 ◽  
Vol 501 (1) ◽  
pp. 483-490
Author(s):  
Jim Fuller
Keyword(s):  
Binary Stars ◽  
Close Binary ◽  
Close Binaries ◽  
Spin Orbit ◽  
Misalignment Angle ◽  
Stellar Rotation ◽  
Transfer Energy ◽  
Pulsating Stars ◽  
Mode Amplitude ◽  
Rotation Periods

ABSTRACT In close binary stars, the tidal excitation of pulsations typically dissipates energy, causing the system to evolve towards a circular orbit with aligned and synchronized stellar spins. However, for stars with self-excited pulsations, we demonstrate that tidal interaction with unstable pulsation modes can transfer energy in the opposite direction, forcing the spins of the stars away from synchronicity, and potentially pumping the eccentricity and spin–orbit misalignment angle. This ‘inverse’ tidal process only occurs when the tidally forced mode amplitude is comparable to the mode’s saturation amplitude, and it is thus most likely to occur in main-sequence gravity mode pulsators with orbital periods of a few days. We examine the long-term evolution of inverse tidal action, finding the stellar rotation rate can potentially be driven to a very large or very small value, while maintaining a large spin–orbit misalignment angle. Several recent asteroseismic analyses of pulsating stars in close binaries have revealed extremely slow core rotation periods, which we attribute to the action of inverse tides.

The Apogee of India - Nepal relations under NDA First Modi Government

2021 ◽  
pp. 166-171
Author(s):  
Afroz Ahmad ◽  
Usha Roopnarain
Keyword(s):  
Federal Government ◽  
Prime Minister ◽  
Parliamentary Election ◽  
Bharatiya Janata Party ◽  
The Absolute ◽  
The Government ◽  
Democratic Norms ◽  
First Time

The last Indian parliamentary election held in 2014, proved to be the finest example of India’s age-old commitment towards the pinnacle of democratic norms. India had set a niche by conducting the largest democratic franchise in history. First time ever since the 1984, Bharatiya Janata Party (BJP) achieved the majority in the Lok Sabha without clubbing with coalition partners. It also got the absolute mandate to rule India’s federal government by ending the Congress monopoly. Interestingly, the Prime Ministerial candidate Narendra Modi in his campaigns criticized Congress-led United Progressive Alliance II (UPA II) for its impotency towards establishing friendly and cooperative relations with India’s neighbors. He also gave assurance that if his party (BJP) got the mandate, his leadership would adopt appropriate measures to resuscitate convivial ties with neighbors. Since forming the government, Prime Minister Modi has been persistently trying to pursue those promises by proceeding towards friendly ties with India’s neighbors. In the light of above discussion, this paper seeks to critically analyze the progress in Indo-Nepal relations under BJP government led by Prime Minister Narendra Modi.

TRANSPORT CHARACTERISTICS IN c-AXIS La2-xSrxCuO4 (LSCO) SINGLE CRYSTALS

2005 ◽  
Vol 19 (01n03) ◽  
pp. 447-450
Author(s):  
SANG-JAE KIM ◽  
TAKESHI HATANO
Keyword(s):  
Temperature Dependence ◽  
Single Crystals ◽  
Critical Current ◽  
Focused Ion Beam ◽  
Ion Beam ◽  
Etching Method ◽  
First Time ◽  
Small Voltage

c-axis micro-bridges of La 2-x Sr x CuO 4 ( LSCO ) single crystals were fabricated by the focused-ion-beam (FIB) etching method. Small rectangular LSCO pieces were fabricated by cutting and grinding single crystals of underdoped LSCO of x=0.09. The size of LSCO single crystals between electrodes was cut to 20×40μm2 in ab-plane by using the FIB etching method. Superconductor-insulator-superconductor (SIS) like-branch structures on I-V curves of the LSCO stacks were observed for the first time. The branch structures exhibited voltage jumps of several tens mV in the range of from 1.7 K to 5 K with temperature dependence. When the temperature is changed from 5 K to 1.7 K , the critical current and the next branch split into a few of small voltage jumps with the intervals of several mV in the range of from 0.1 mV and 2.0 mV .

Dithiolium and diselenolium compounds of the transition metals

1969 ◽  
Vol 22 (1) ◽  
pp. 83 ◽  
Author(s):  
RL Martin ◽  
IM Stewart
Keyword(s):  
Transition Metals ◽  
Temperature Dependence ◽  
Mössbauer Spectra ◽  
Mossbauer Spectra ◽  
First Time

The preparations are reported of an extended series of compounds of the type ML2X4 where M is Mn, Fe, Co, or Ni; X is Cl or Br; and L is analytically identical with dithioacetylacetone. ��� Electronic, infrared, and Mossbauer spectra together with the temperature dependence of the magnetism (80-300�K) of these compounds establish-contrary to previous suggestions in the literature-that they should be properly formulated as tetrahalometallate salts of the 3,5- dimethyl-1,2-dithiolium cation. Simple salts of this cation (such as the iodide) have also been isolated. ��� During the course of this work salts (I- and [CoCl4]2-) of the hitherto unknown 3,5-dimethyl-1,2-diselenolium ion were synthesized and characterized for the first time.

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