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Register a new userThe discontinuous nature of chromospheric activity evolution
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G. Pace
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Chromospheric activity has been thought to decay smoothly with time and, hence, to be a viable age indicator. Measurements in solar type stars in open clusters seem to point to a different conclusion: chromospheric activity undergoes a fast transition from Hyades level to that of the Sun after about 1 Gyr of main--sequence lifetime and any decaying trend before or after this transition must be much less significant than the short term variations.
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We investigate the evolution and origin of small-scale chromospheric swirls by analyzing numerical simulations of the quiet solar atmosphere, using the radiative magnetohydrodynamic code CO$5$BOLD. We are interested in finding their relation with magnetic field perturbations and in the processes driving their evolution. For the analysis, the swirling strength criterion and its evolution equation are applied in order to identify vortical motions and to study their dynamics. We introduce a new criterion, the magnetic swirling strength, which allows us to recognize torsional perturbations in the magnetic field. We find a strong correlation between swirling strength and magnetic swirling strength, in particular in intense magnetic flux concentrations, which suggests a tight relation between vortical motions and torsional magnetic field perturbations. Furthermore, we find that swirls propagate upward with the local Alfven speed as unidirectional swirls, in the form of pulses, driven by magnetic tension forces alone. In the photosphere and low chromosphere, the rotation of the plasma co-occurs with a twist in the upwardly directed magnetic field that is in the opposite direction of the plasma flow. All together, these are characteristics of torsional Alfven waves. We also find indications of an imbalance between the hydrodynamic and magnetohydrodynamic baroclinic effects being at the origin of the swirls. At the base of the chromosphere, we find a net upwardly directed Poynting flux, which is mostly associated with large and complex swirling structures that we interpret as the superposition of various small-scale vortices. We conclude that the ubiquitous swirling events observed in simulations are tightly correlated with perturbations of the magnetic field. At photospheric and chromospheric levels, they form Alfven pulses that propagate upward and may contribute to chromospheric heating.
Vortex structures in the Suns chromosphere are believed to channel energy between different layers of the solar atmosphere. We investigate the nature and dynamics of two small-scale quiet-Sun rotating structures in the chromosphere. We analyse two chromospheric structures that show clear rotational patterns in spectropolarimetric observations taken with the Interferometric Bidimensional Spectrometer (IBIS) at the Ca II 8542 AA~ line. We present the detection of spectropolarimetric signals that manifest the magnetic nature of rotating structures in the chromosphere. Our observations show two long-lived structures of plasma that each rotate clockwise inside a 10 arcsec$^{2}$~ quiet-Sun region. Their circular polarization signals are 5-10 times above the noise level. Line-of-sight Doppler velocity and horizontal velocity maps from the observations reveal clear plasma flows at and around the two structures. An MHD simulation shows these two structures are plausibly magnetically connected. Wave analysis suggests that the observed rotational vortex pattern could be due to a combination of slow actual rotation and a faster azimuthal phase speed pattern of a magneto-acoustic mode. Our results imply that the vortex structures observed in the Suns chromosphere are magnetic in nature and that they can be connected locally through the chromosphere.
We present a catalogue of homogeneous determined chromospheric emission (CE), stellar atmospheric parameters and ages for 1,674 FGK main sequence (MS), subgiant, and giant stars. The analysis of CE level and variability is also performed. We measured CE in the CaII lines using more than 180,000 high-resolution spectra from the HARPS spectrograph, as compiled in the AMBRE project, obtained between 2003 and 2019. We converted the fluxes to bolometric and photospheric corrected chromospheric emission ratio, $R_text{HK}$. Stellar atmospheric parameters $T_text{eff}$, $log g$, and [Fe/H] were retrieved from the literature or determined using an homogeneous method. $M_star$, $R_star$, and ages were determined from isochrone fitting. We analysed the CE distribution for the different luminosity classes and spectral types and confirmed the existence of the very inactive stars (VIS) and very active stars (VAS) populations at $log R_text{HK}< -5.1$ and $> -4.2$ dex, respectively. We found indications that the VIS population is composed mainly of subgiant and giant stars and that $log R_text{HK}= -5.1$ dex marks a transition in stellar evolution. There appears to be at least three regimes of variability, for inactive, active and very active stars, with the inactive and active regimes separated by a diagonal Vaughan-Preston gap. We show that stars with low activity levels do not necessarily have low variability. In the case of K dwarfs which show high CE variability, inactive and active stars have similar levels of activity variability. This means that activity levels alone are not enough to infer about the activity variability of a star. We also explained the shape of the VP gap observed in the distribution of CE by using the CE variability-level diagram. In the CE variability-level diagram, the Sun is located in the high variability region of the inactive MS stars zone. (Abridged)
Using a model based on the rotational modulation of the visibility of active regions, we analyse the high-accuracy CoRoT lightcurve of the active young star CoRoT102899501. Spectroscopic follow-up observations are used to derive its fundamental parameters. We compare its chromospheric activity level with a model of chrosmospheric activity evolution established by combining relationships between the RHK index and the Rossby number with a recent model of stellar rotation evolution on the main sequence. We measure the spot coverage of the stellar surface as a function of time, and find evidence for a tentative increase from 5-14% at the beginning of the observing run to 13-29% 35 days later. A high level of magnetic activity on CoRoT102899501 is corroborated by a strong emission in the Balmer and Ca II HK lines (logRHK ~ -4). The starspots used as tracers of the star rotation constrain the rotation period to 1.625+/-0.002 days and do not show evidence for differential rotation. The effective temperature (Teff=5180+/-80 K), surface gravity (logg=4.35+/-0.1), and metallicity ([M/H]=0.05+/-0.07 dex) indicate that the object is located near the evolutionary track of a 1.09+/-0.12 M_Sun pre-main sequence star at an age of 23+/-10 Myrs. This value is consistent with the gyro-age of about 8-25 Myrs, inferred using a parameterization of the stellar rotation period as a function of colour index and time established for the I-sequence of stars in stellar clusters. We conclude that the high magnetic activity level and fast rotation of CoRoT102899501 are manifestations of its stellar youth consistent with its estimated evolutionary status and with the detection of a strong Li I 6707.8 A absorption line in its spectrum. We argue that a magnetic activity level comparable to that observed on CoRoT102899501 could have been present on the Sun at the time of planet formation.
Using recently established empirical calibrations for the impact of chromospheric activity on the radii, effective temperatures, and estimated masses of active low-mass stars and brown dwarfs, we reassess the shape of the initial mass function (IMF) across the stellar/substellar boundary in the Upper Sco star-forming region (age 5-10 Myr). We adjust the observed effective temperatures to warmer values using the observed strength of the chromospheric H$alpha$ emission, and redetermine the estimated masses of objects using pre--main-sequence evolutionary tracks in the H-R diagram. The effect of the activity-adjusted temperatures is to shift the objects to higher masses by 3-100%. While the slope of the resulting IMF at substellar masses is not strongly changed, the peak of the IMF does shift from ~0.06 to ~0.11 Msun. Moreover, for objects with masses <~0.2 Msun, the ratio of brown dwarfs to stars changes from ~80% to ~33%. These results suggest that activity corrections are essential for studies of the substellar mass function, if the masses are estimated from spectral types or from effective temperatures.
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