162 research outputs found
Rotating models of young solar-type stars : Exploring braking laws and angular momentum transport processes
We study the predicted rotational evolution of solar-type stars from the
pre-main sequence to the solar age with 1D rotating evolutionary models
including physical ingredients. We computed rotating evolution models of
solar-type stars including an external stellar wind torque and internal
transport of angular momentum following the method of Maeder and Zahn with the
code STAREVOL. We explored different formalisms and prescriptions available
from the literature. We tested the predictions of the models against recent
rotational period data from extensive photometric surveys, lithium abundances
of solar-mass stars in young clusters, and the helioseismic rotation profile of
the Sun. We find a best-matching combination of prescriptions for both internal
transport and surface extraction of angular momentum. This combination provides
a very good fit to the observed evolution of rotational periods for solar-type
stars from early evolution to the age of the Sun. Additionally, we show that
fast rotators experience a stronger coupling between their radiative region and
the convective envelope. Regardless of the set of prescriptions, however, we
cannot simultaneously reproduce surface angular velocity and the internal
profile of the Sun or the evolution of lithium abundance. We confirm the idea
that additional transport mechanisms must occur in solar-type stars until they
reach the age of the Sun. Whether these processes are the same as those needed
to explain recent asteroseismic data in more advanced evolutionary phases is
still an open question.Comment: 16 pages, 16 figures, accepted for publication in A&
Impact of internal gravity waves on the rotation profile inside pre-main sequence low-mass stars
We study the impact of internal gravity waves (IGW), meridional circulation,
shear turbulence, and stellar contraction on the internal rotation profile and
surface velocity evolution of solar metallicity low-mass pre-main sequence
stars. We compute a grid of rotating stellar evolution models with masses
between 0.6 and 2.0Msun taking these processes into account for the transport
of angular momentum, as soon as the radiative core appears and assuming no more
disk-locking from that moment on.IGW generation along the PMS is computed
taking Reynolds-stress and buoyancy into account in the bulk of the stellar
convective envelope and convective core (when present). Redistribution of
angular momentum within the radiative layers accounts for damping of prograde
and retrograde IGW by thermal diffusivity and viscosity in corotation
resonance. Over the whole mass range considered, IGW are found to be
efficiently generated by the convective envelope and to slow down the stellar
core early on the PMS. In stars more massive than ~ 1.6Msun, IGW produced by
the convective core also contribute to angular momentum redistribution close to
the ZAMS. Overall, IGW are found to significantly change the internal rotation
profile of PMS low-mass stars.Comment: Accepted for publication in A&A (15 pages
Tidal dissipation in rotating low-mass stars and implications for the orbital evolution of close-in planets I. From the PMS to the RGB at solar metallicity
Star-planet interactions must be taken into account in stellar models to
understand the dynamical evolution of close-in planets. The dependence of the
tidal interactions on the structural and rotational evolution of the star is of
peculiar importance and should be correctly treated. We quantify how tidal
dissipation in the convective envelope of rotating low-mass stars evolves from
the pre-main sequence up to the red-giant branch depending on the initial
stellar mass. We investigate the consequences of this evolution on planetary
orbital evolution. We couple the tidal dissipation formalism described in
Mathis (2015) to the stellar evolution code STAREVOL and apply it to rotating
stars with masses between 0.3 and 1.4 M. In addition, we generalize the
work of Bolmont & Mathis (2016) by following the orbital evolution of close-in
planets using the new tidal dissipation predictions for advanced phases of
stellar evolution. On the PMS the evolution of tidal dissipation is controlled
by the evolution of the internal structure of the contracting star. On the MS
it is strongly driven by the variation of surface rotation that is impacted by
magnetized stellar winds braking. The main effect of taking into account the
rotational evolution of the stars is to lower the tidal dissipation strength by
about four orders of magnitude on the main-sequence, compared to a normalized
dissipation rate that only takes into account structural changes. The evolution
of the dissipation strongly depends on the evolution of the internal structure
and rotation of the star. From the pre-main sequence up to the tip of the
red-giant branch, it varies by several orders of magnitude, with strong
consequences for the orbital evolution of close-in massive planets. These
effects are the strongest during the pre-main sequence, implying that the
planets are mainly sensitive to the star's early history.Comment: 13 pages, 7 figures, accepted for publication in A&
Further evidence of the link between activity and metallicity using the flaring properties of stars in the Kepler field
The magnetic activity level of low-mass stars is known to vary as a function
of the physical properties of the star. Many studies have shown that the
stellar mass and rotation are both important parameters that determine magnetic
activity levels. In contrast, the impact of a star's chemical composition on
magnetic activity has received comparatively little attention. Data sets for
traditional activity proxies, e.g. X-ray emission or calcium emission, are not
large enough to search for metallicity trends in a statistically meaningful
way. Recently, studies have used the photometric variability amplitude as a
proxy for magnetic activity to investigate the role of metallicity because it
can be relatively easily measured for large samples of stars. These studies
find that magnetic activity and metallicity are positively correlated. In this
work, we investigate the link between activity and metallicity further by
studying the flaring properties of stars in the Kepler field. Similar to the
photometric variability, we find that flaring activity is stronger in more
metal-rich stars for a fixed mass and rotation period. This result adds to a
growing body of evidence that magnetic field generation is correlated with
metallicity.Comment: 6 pages, 5 figures, accepted for publication in MNRA
When public values and user-centricity in e-government collide – A systematic review
User-centricity in e-government is a double-edged sword. While it helps governments design digital services tailored to the needs of citizens, it may also increase the burden on users and deepen the digital divide. From an institutional perspective, these fundamental conflicts are inevitable. To better understand the role and effect of user-centricity in e-government, this paper analyses academic literature on user-centricity and public values. The analysis leads to three main insights: First, there is a conflict in citizen representation that may result from the normative dominance of decision-makers. Second, we identify an accountability conflict that can prevent user-centric innovation from thriving in a highly institutionalized environment. Third, we identify a pluralism conflict that emerges from a clash between the reality of a diverse society and the assumed homogeneity of actors. The need to address these conflicts increases with rapid technological innovation, such as distributed ledger technologies, artificial intelligence, and trust infrastructures. These technologies put the user at the center stage and permeate aspects of social life beyond government. In response to these insights, we outline suggestions for further research and practice
User-centricity and Public Values in eGovernment: Friend or Foe?
In their delivery of services, public administrations seek to develop a ‘citizen-centric’ approach. Concomitantly, user-centricity is emerging as a widely accepted construct for Web 3.0 applications supporting the digital interaction between service providers and recipients. The digitalization of public services can positively impact important public values, such as efficiency and transparency. However, the digital divide highlights that information and communication technologies can simultaneously neglect public needs. This begs the question of whether user-centricity reflects or conflicts with public values. To answer this question, we present a systematic review of existing literature on user-centricity and public values. The contribution of this paper is an extended taxonomy of public values for user- centricity, as well as recommendations for public policy to address conflicts between public values and user-centricity
User-centricity and Public Values in eGovernment: Friend or Foe?
In their delivery of services, public administrations seek to develop a ‘citizen-centric’ approach. Concomitantly, user-centricity is emerging as a widely accepted construct for Web 3.0 applications supporting the digital interaction between service providers and recipients. The digitalization of public services can positively impact important public values, such as efficiency and transparency. However, the digital divide highlights that information and communication technologies can simultaneously neglect public needs. This begs the question of whether user-centricity reflects or conflicts with public values. To answer this question, we present a systematic review of existing literature on user-centricity and public values. The contribution of this paper is an extended taxonomy of public values for user-centricity, as well as recommendations for public policy to address conflicts between public values and user-centricity
The impact of metallicity on the evolution of the rotation and magnetic activity of Sun-like stars
This is the author accepted articleThe rotation rates and magnetic activity of Sun-like and low-mass (.1.4M) main-sequence stars are knownto decline with time, and there now exist several models for the evolution of rotation and activity. However,the role that chemical composition plays during stellar spin-down has not yet been explored. In this work,we use a structural evolution code to compute the rotational evolution of stars with three different masses (0.7, 1.0, and 1.3M) and six different metallicities, ranging from [Fe/H]=−1.0 to [Fe/H]= +0.5. We also implement three different wind-braking formulations from the literature (two modern and one classical) and compare their predictions for rotational evolution. The effect that metallicity has on stellar structural properties,and in particular the convective turnover timescale, leads the two modern wind-braking formulations to predict a strong dependence of the torque on metallicity. Consequently, they predict that metal rich stars spin-down more effectively at late ages (>1 Gyr) than metal poor stars, and the effect is large enough to be detectable with current observing facilities. For example, the formulations predict that a Sun-like (solar-mass and solar-aged) star with [Fe/H]=−0.3 will have a rotation period of less than 20 days. Even though old, metal poor stars are predicted to rotate more rapidly at a given age, they have larger Rossby numbers and are thus expected to have lower magnetic activity levels. Finally, the different wind-braking formulations predict quantitative differences in the metallicity-dependence of stellar rotation, which may be used to test themER
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