Solar-like stars (
M
≲ 1.3
M
⊙
) lose angular momentum through their magnetized winds. The resulting evolution of the surface rotation period, which can be directly measured photometrically, has ...the potential to be an accurate indicator of stellar age, and is constrained by observations of rotation periods of coeval stars, such as members of Galactic open clusters. A prominent observational feature of the mass–rotation period diagrams of open clusters is a sequence of relatively slower rotators. The formation and persistence of this slow-rotator sequence across several billion years imply an approximately coherent spin-down of the stars that belong to it. In particular, the sequence is observed to evolve coherently toward longer periods in progressively older clusters. Recent observations of the ≈700 Myr Praesepe and the 1 Gyr NGC 6811 clusters, however, are not fully consistent with this general pattern. While the stars of 1
M
⊙
on the slow-rotator sequence of the older NGC 6811 have longer periods than their counterparts in the younger Praesepe, as expected, the two sequences essentially merge at lower masses (≲0.8
M
⊙
). In other words, it seems that low-mass stars have not been spinning down in the intervening 300 Myr. Here we show that this behavior is a manifestation of the variable rotational coupling in solar-like stars. The resurfacing of angular momentum from the interior can temporarily compensate for that lost at the surface due to wind braking. In our model the internal redistribution of angular momentum has a steep mass dependence; as a result, the re-coupling occurs at different ages for stars of different masses. The semi-empirical mass dependence of the rotational coupling timescale included in our model produces an evolution of the slow-rotator sequence in very good agreement with the observations. Our model, in particular, explains the stalled surface spin-down of low-mass stars between Praesepe and NGC 6811, and predicts that the same behavior should be observable at other ages in other mass ranges.
Context. The observed relationship between mass, age and rotation in open clusters shows the progressive development of a slow-rotator sequence among stars possessing a radiative interior and a ...convective envelope during their pre-main sequence and main-sequence evolution. After 0.6 Gyr, most cluster members of this type have settled on this sequence. Aims. The observed clustering on this sequence suggests that it corresponds to some equilibrium or asymptotic condition that still lacks a complete theoretical interpretation, and which is crucial to our understanding of the stellar angular momentum evolution. Methods. We couple a rotational evolution model, which takes internal differential rotation into account, with classical and new proposals for the wind braking law, and fit models to the data using a Monte Carlo Markov chain (MCMC) method tailored to the problem at hand. We explore to what extent these models are able to reproduce the mass and time dependence of the stellar rotational evolution on the slow-rotator sequence. Results. The description of the evolution of the slow-rotator sequence requires taking the transfer of angular momentum from the radiative core to the convective envelope into account. We find that, in the mass range 0.85–1.10 M⊙, the core-envelope coupling timescale for stars in the slow-rotator sequence scales as M-7.28. Quasi-solid body rotation is achieved only after 1–2 Gyr, depending on stellar mass, which implies that observing small deviations from the Skumanich law (\hbox{$P \propto \sqrt{t}$}P∝t) would require period data of older open clusters than is available to date. The observed evolution in the 0.1–2.5 Gyr age range and in the 0.85–1.10 M⊙ mass range is best reproduced by assuming an empirical mass dependence of the wind angular momentum loss proportional to the convective turnover timescale and to the stellar moment of inertia. Period isochrones based on our MCMC fit provide a tool for inferring stellar ages of solar-like main-sequence stars from their mass and rotation period that is largely independent of the wind braking model adopted. These effectively represent gyro-chronology relationships that take the physics of the two-zone model for the stellar angular momentum evolution into account.
The Yale-Potsdam Stellar Isochrones Spada, F.; Demarque, P.; Kim, Y.-C. ...
The Astrophysical journal,
04/2017, Letnik:
838, Številka:
2
Journal Article
Recenzirano
Odprti dostop
We introduce the Yale-Potsdam Stellar Isochrones (YaPSI), a new grid of stellar evolution tracks and isochrones of solar-scaled composition. In an effort to improve the Yonsei-Yale database, special ...emphasis is placed on the construction of accurate low-mass models ( ), and in particular on their mass-luminosity and mass-radius relations, both crucial for characterizing exoplanet-host stars, and, in turn, their planetary systems. The YaPSI models cover the mass range 0.15- densely enough to permit detailed interpolation in mass, and the metallicity and helium abundance ranges Fe/H = −1.5 to +0.3 and Y0 = 0.25-0.37 are specified independently of each other (i.e., no fixed relation is assumed). The evolutionary tracks are calculated from the pre-main sequence up to the tip of the red giant branch. The isochrones, with ages between 1 Myr and 20 Gyr, provide UBVRI colors in the Johnson-Cousins system, and JHK colors in the homogenized Bessell & Brett system, derived from two different semi-empirical -color calibrations from the literature. We also provide utility codes, such as an isochrone interpolator, in age, metallicity, and helium content, and an interface of the tracks with an open-source Monte Carlo Markov-Chain tool for the analysis of individual stars. Finally, we present comparisons of the YaPSI models with the best empirical mass-luminosity and mass-radius relations available to date, as well as isochrone fitting of well-studied stellar clusters.
Context.
Asteroseismic measurements of the internal rotation of subgiants and red giants all show the need for invoking a more efficient transport of angular momentum than theoretically predicted. ...Constraints on the core rotation rate are available starting from the base of the red giant branch (RGB) and we are still lacking information on the internal rotation of less evolved subgiants.
Aims.
We identify two young
Kepler
subgiants, KIC 8524425 and KIC 5955122, whose mixed modes are clearly split by rotation. We aim to probe their internal rotation profile and assess the efficiency of the angular momentum transport during this phase of the evolution.
Methods.
Using the full
Kepler
data set, we extracted the mode frequencies and rotational splittings for the two stars using a Bayesian approach. We then performed a detailed seismic modeling of both targets and used the rotational kernels to invert their internal rotation profiles using the MOLA inversion method. We thus obtained estimates of the average rotation rates in the
g
-mode cavity (⟨Ω⟩
g
) and in the
p
-mode cavity (⟨Ω⟩
p
).
Results.
We found that both stars are rotating nearly as solid bodies, with core-envelope contrasts of ⟨Ω⟩
g
/⟨Ω⟩
p
= 0.68 ± 0.47 for KIC 8524425 and ⟨Ω⟩
g
/⟨Ω⟩
p
= 0.72 ± 0.37 for KIC 5955122. This result shows that the internal transport of angular momentum has to occur faster than the timescale at which differential rotation is forced in these stars (between 300 Myr and 600 Myr). By modeling the additional transport of angular momentum as a diffusive process with a constant viscosity
ν
add
, we found that values of
ν
add
> 5 × 10
4
cm
2
s
−1
are required to account for the internal rotation of KIC 8524425, and
ν
add
> 1.5 × 10
5
cm
2
s
−1
for KIC 5955122. These values are lower than or comparable to the efficiency of the core-envelope coupling during the main sequence, as given by the surface rotation of stars in open clusters. On the other hand, they are higher than the viscosity needed to reproduce the rotation of subgiants near the base of the RGB.
Conclusions.
Our results yield further evidence that the efficiency of the internal redistribution of angular momentum decreases during the subgiant phase. We thus bring new constraints that will need to be accounted for by mechanisms that are proposed as candidates for angular momentum transport in subgiants and red giants.
Context. Using asteroseismic techniques, it has recently become possible to probe the internal rotation profile of low-mass (≈1.1−1.5 M⊙) subgiant and red giant stars. Under the assumption of local ...angular momentum conservation, the core contraction and envelope expansion occurring at the end of the main sequence would result in a much larger internal differential rotation than observed. This suggests that angular momentum redistribution must be taking place in the interior of these stars. Aims. We investigate the physical nature of the angular momentum redistribution mechanisms operating in stellar interiors by constraining the efficiency of post-main sequence rotational coupling. Methods. We model the rotational evolution of a 1.25M⊙ star using the Yale Rotational stellar Evolution Code. Our models take into account the magnetic wind braking occurring at the surface of the star and the angular momentum transport in the interior, with an efficiency dependent on the degree of internal differential rotation. Results. We find that models including a dependence of the angular momentum transport efficiency on the radial rotational shear reproduce very well the observations. The best fit of the data is obtained with an angular momentum transport coefficient scaling with the ratio of the rotation rate of the radiative interior over that of the convective envelope of the star as a power law of exponent ≈3. This scaling is consistent with the predictions of recent numerical simulations of the Azimuthal Magneto-Rotational Instability. Conclusions. We show that an angular momentum transport process whose efficiency varies during the stellar evolution through a dependence on the level of internal differential rotation is required to explain the observed post-main sequence rotational evolution of low-mass stars.
Aims. Digital images of observations printed in the books Rosa Ursina sive solis and Prodromus pro sole mobili by Christoph Scheiner, as well as the drawings from Scheiner’s letters to Marcus Welser, ...are analysed to obtain information on the positions and sizes of sunspots that appeared before the Maunder minimum. Methods. In most cases, the given orientation of the ecliptic is used to set up the heliographic coordinate system for the drawings. Positions and sizes are measured manually on screen. Very early drawings have no indication of their orientation. A rotational matching using common spots of adjacent days is used in some cases, while in other cases, the assumption that images were aligned with a zenith-horizon coordinate system appeared to be the most probable. Results. In total, 8167 sunspots were measured. A distribution of sunspot latitudes versus time (butterfly diagram) is obtained for Scheiner’s observations. The observations of 1611 are very inaccurate, the drawings of 1612 have at least an indication of their orientation, while the remaining part of the spot positions from 1618−1631 have good to very good accuracy. We also computed 697 tilt angles of apparently bipolar sunspot groups observed in the period 1618−1631. We find that the average tilt angle of nearly 4 degrees is not significantly different from 20th-century values.
A long-standing issue in the theory of low-mass stars is the discrepancy between predicted and observed radii and effective temperatures. In spite of the increasing availability of very precise ...radius determinations from eclipsing binaries and interferometric measurements of radii of single stars, there is no unanimous consensus on the extent (or even the existence) of the discrepancy and on its connection with other stellar properties (e.g., metallicity, magnetic activity). We investigate the radius discrepancy phenomenon using the best data currently available (accuracy < ~5%). We have constructed a grid of stellar models covering the entire range of low-mass stars (0.1-1.25 M sub(middot in circle)) and various choices of the metallicity and mixing length parameter, alpha . We used an improved version of the Yale Rotational stellar Evolution Code, implementing surface boundary conditions based on the most up-to-date PHOENIX atmosphere models. Our models are in good agreement with others in the literature and improve and extend the low mass end of the Yale-Yonsei isochrones. Our calculations include rotation-related quantities, such as moments of inertia and convective turnover timescales, useful in studies of magnetic activity and rotational evolution of solar-like stars. Consistent with previous works, we find that both binaries and single stars have radii inflated by about 3% with respect to the theoretical models; among binaries, the components of short orbital period systems are found to be the most deviant. We conclude that both binaries and single stars are comparably affected by the radius discrepancy phenomenon.
We demonstrate with a nonlinear magnetohydrodynamic (MHD) code that angular momentum can be transported because of the magnetic instability of toroidal fields under the influence of differential ...rotation, and that the resulting effective viscosity may be high enough to explain the almost rigid-body rotation observed in radiative stellar cores. We only consider stationary, current-free fields, and only those combinations of rotation rates and magnetic field amplitudes which provide maximal numerical values of the viscosity. We find that the dimensionless ratio of the effective over molecular viscosity, νT/ν, linearly grows with the Reynolds number of the rotating fluid multiplied by the square-root of the magnetic Prandtl number, which is approximately unity for the considered red subgiant star KIC 7341231. For the interval of magnetic Reynolds numbers considered – which is restricted by numerical constraints of the nonlinear MHD code – the magnetic Prandtl number has a remarkable influence on the relative importance of the contributions of the Reynolds stress and the Maxwell stress to the total viscosity, which is magnetically dominated only for Pm ≳ 0.5. We also find that the magnetized plasma behaves as a non-Newtonian fluid, i.e., the resulting effective viscosity depends on the shear in the rotation law. The decay time of the differential rotation thus depends on its shear and becomes longer and longer during the spin-down of a stellar core.
While males have dominated the physician lines over the last decades the recent female doctors' number increasing might progressively reduce this gender gap. This might be not fully true in the ...academic/research area. We aimed to analyze the gender distribution of first/senior Italian authors on neuroendocrine neoplasm papers published on peer reviewed journals.
Publications from January 2019 to September 2023 were reviewed; only papers with first and/or senior Italian authors were included. First/senior author gender, type of article, co-authorship with foreign authors were the variable analyzed.
742 papers with Italian first and/or senior authors were retrieved, 449 (60.5%) multicentric, 285 (38.4%) original articles. A female author was first and senior author in 386/742 (52%) and in 228/742 (31%) papers, respectively. 150 (20.2%) papers included foreign coauthors, being an Italian female researcher first author in 50 papers (33%), senior author in 28 (18.6%). The number of Italian female first/senior authors has been increasing over the years (22 in 2019, 113 in 2022; 16 in 2019, 62 in 2022, respectively). The first/senior female authors were mainly Oncologists/Endocrinologists/Pathologists rather than Gastroenterologists/Nuclear Medicine doctors/Surgeons/Radiologists.
There has been an increase in the prevalence of female authorship of published research in the neuroendocrine setting over the last 5 years, which partially reflects the current distributions in this field, taking into account that several specialties with different gender distribution are involved. However, senior authorship continues to be primarily men. Efforts should be made to improve proportionate gender representation in both clinical and academic/research setting.
Main-sequence, solar-like stars (M 1.5 M ) have outer convective envelopes that are sufficiently thick to affect significantly their overall structure. The radii of these stars, in particular, are ...sensitive to the details of inefficient, superadiabatic convection occurring in their outermost layers. The standard treatment of convection in stellar evolution models, based on the mixing-length theory (MLT), provides only a very approximate description of convection in the superadiabatic regime. Moreover, it contains a free parameter, MLT, whose standard calibration is based on the Sun and is routinely applied to other stars, ignoring the differences in their global parameters (e.g., effective temperature, gravity, chemical composition) and previous evolutionary history. In this paper, we present a calibration of MLT based on 3D radiation hydrodynamics (RHD) simulations of convection. The value of MLT is adjusted to match the specific entropy in the deep, adiabatic layers of the convective envelope to the corresponding value obtained from the 3D RHD simulations, as a function of the position of the star in the plane and its chemical composition. We have constructed a model of the present-day Sun using such entropy-based calibration. We find that its past luminosity evolution is not affected by the entropy calibration. The predicted solar radius, however, exceeds that of the standard model during the past several billion years, resulting in a lower surface temperature. This illustrative calculation also demonstrates the viability of the entropy approach for calibrating the radii of other late-type stars.
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