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Register a new userStellar Activity in Coeval Open Clusters: Praesepe and The Hyades
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David Barrado y Navascues
Publication date
1998
fields
Physics
and research's language is
English
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Randich and Schmitt [1995, A&A 298, 115] found that the coronal activity of solar-type and low mass stars in Praesepe is significantly lower than that of stars in the Hyades cluster. We have carried out several tests in order to find a possible explanation for this result. We have measured radial velocities of two groups of Praesepe stars (a dF-dK sample and a dM sample) and have measured H$alpha$ as a chromospheric activity index for the dM sample. We conclude that the Praesepe catalog used in the X-ray analysis does not contain a significant number of non-members. The comparison of the H$alpha$ equivalent widths for the M dwarfs in Praesepe with those in the Hyades indicates that, at least for stars in this mass range, the Praesepe stars are as active or more active than their Hyades counterparts. We have also analyzed a few ROSAT PSPC pointings of Praesepe in order to obtain a new and independent estimate of the X-ray luminosities and upper limits for a small sample of Praesepe members concluding that the small differences between the old and new upper limits are not large enough to explain the dichotomy in the X-ray properties of Praesepe and the Hyades. Therefore, our examination of the available data does not provide a clear reason to explain why the X-ray luminosity functions of the two clusters are different. Part of the explanation could be found in the binaries. Speculatively, these clusters could have different orbital period distributions, with more short period binaries among the Hyades, which would show larger coronal activity.
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Flares, energetic eruptions on the surfaces of stars, are an unmistakable manifestation of magnetically driven emission. Their occurrence rates and energy distributions trace stellar characteristics such as mass and age. But before flares can be used to constrain stellar properties, the flaring-age-mass relation requires proper calibration. This work sets out to quantify flaring activity of independently age-dated main sequence stars for a broad range of spectral types using optical light curves obtained by the Kepler satellite. Drawing from the complete K2 archive, we searched 3435 $sim 80$ day long light curves of 2111 open cluster members for flares using the open-source software packages K2SC to remove instrumental and astrophysical variability from K2 light curves, and AltaiPony to search and characterize the flare candidates. We confirmed a total of 3844 flares on high probability open cluster members with ages from zero age main sequence (Pleiades) to 3.6 Gyr (M67). We extended the mass range probed in the first study of this series to span from Sun-like stars to mid-M dwarfs. We added the Hyades (690 Myr) to the sample as a comparison cluster to Praesepe (750 Myr), the 2.6 Gyr old Ruprecht 147, and several hundred light curves from the late K2 Campaigns in the remaining clusters. The flare energy distribution was similar in the entire parameter space, following a power law relation with exponent $alphaapprox 1.84-2.39$. The flaring rates declined with age, and declined faster for higher mass stars. We found evidence that a rapid decline in flaring activity occurred in M1-M2 dwarfs around Hyades/Praesepe age, when these stars spun down to rotation periods of about 10 days, while higher mass stars had already transitioned to lower flaring rates, and lower mass stars still resided in the saturated activity regime. (abridged)
Context. Open clusters are ideal test particles to study the chemical evolution of the Galactic disc. However the existing high-resolution abundance determinations, not only of [Fe/H], but also of other key elements, is largely insufficient at the moment. Aims. To increase the number of Galactic open clusters with high quality abundance determinations, and to gather all the literature determinations published so far. Methods. Using high-resolution (R~30000), high-quality (S/N$>60 per pixel), we obtained spectra for twelve stars in four open clusters with the fiber spectrograph FOCES, at the 2.2 Calar Alto Telescope in Spain. We use the classical equivalent widths analysis to obtain accurate abundances of sixteen elements: Al, Ba, Ca, Co, Cr, Fe, La, Mg, Na, Nd, Ni, Sc, Si, Ti, V, Y. Oxygen abundances have been derived through spectral synthesis of the 6300 A forbidden line. Results. We provide the first determination of abundance ratios other than Fe for NGC 752 giants, and ratios in agreement with the literature for the Hyades, Praesepe and Be 32. We use a compilation of literature data to study Galactic trends of [Fe/H] and [alpha/Fe] with Galactocentric radius, age, and height above the Galactic plane. We find no significant trends, but some indication for a flattening of [Fe/H] at large Rgc, and for younger ages in the inner disc. We also found a possible decrease of [Fe/H] with |z| in the outer disc, and a weak increase of [alpha/Fe] with Rgc.
We present the results of photometric surveys for stellar rotation in the Hyades and in Praesepe, using data obtained as part of the SuperWASP exoplanetary transit-search programme.We determined accurate rotation periods for more than 120 sources whose cluster membership was confirmed by common proper motion and colour-magnitude fits to the clusters isochrones. This allowed us to determine the effect of magnetic braking on a wide range of spectral types for expected ages of ~600Myr for the Hyades and Praesepe. Both clusters show a tight and nearly linear relation between J - Ks colour and rotation period in the F,G and K spectral range. This confirms that loss of angular momentum was significant enough that stars with strongly different initial rotation rates have converged to the same rotation period for a given mass, by the age of Hyades and Praesepe. In the case of the Hyades our colour-period sequence extends well into the M dwarf regime and shows a steep increase in the scatter of the colour- period relation, with identification of numerous rapid rotators from 0.5SM down to the lowest masses probed by our survey (0.25SM). This provides crucial constraints on the rotational braking timescales and further clears the way to use gyrochronology as an accurate age measurement tool for main-sequence stars.
We identify member stars of more than 90 open clusters in the LAMOST survey. With the method of Fang et al.(2018), the chromospheric activity (CA) indices logRCaK for 1091 member stars in 82 open clusters and logRH{alpha} for 1118 member stars in 83 open clusters are calculated. The relations between the average logRCaK, logRH{alpha} in each open cluster and its age are investigated in different Teff and [Fe/H] ranges. We find that CA starts to decrease slowly from logt = 6.70 to logt = 8.50, and then decreases rapidly until logt = 9.53. The trend becomes clearer for cooler stars. The quadratic functions between logR and logt with 4000K < Teff < 5500K are constructed, which can be used to roughly estimate ages of field stars with accuracy about 40% for logRCaK and 60% for logRH{alpha}.
Mixing mechanisms bring the Li from the base of the convective zone to deeper and warmer layers where it is destroyed. These mechanisms are investigated by comparing observations of Li abundances in stellar atmospheres to models of stellar evolution. Observations in open cluster are especially suitable for this comparison, since their age and metallicity are homogeneous among their members and better determined than in field stars. In this work, we compare the evolution of Li abundances in three different clusters: the Hyades, NGC752, and M67. Our models are calculated with microscopic diffusion and transport of chemicals by meridional circulation, and calibrated on the Sun. These comparisons allow us to follow the evolution of Li abundance as a function of stellar mass in each cluster and as a function of the age by comparing this evolution in each cluster. We evaluate the efficiency of the mixing mechanisms used in the models, and we try to identify the lacking mechanisms to reproduce the observed evolution of Li abundance.
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