- ID:
- ivo://CDS.VizieR/J/MNRAS/411/2099
- Title:
- Fast-rotating M-dwarf stars in NGC 2547
- Short Name:
- J/MNRAS/411/2099
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- At fast rotation rates, the coronal activity of G- and K-type stars has been observed to "saturate" and then decline again at even faster rotation rates - a phenomenon dubbed "supersaturation". In this paper, we investigate coronal activity in fast-rotating M-dwarfs using deep XMM-Newton observations of 97 low-mass stars of known rotation period in the young open cluster NGC 2547 and combine these with published X-ray surveys of low-mass field and cluster stars of known rotation period.
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Search Results
- ID:
- ivo://CDS.VizieR/J/PASP/121/117
- Title:
- Fe and Ti abundances of 12 low-metallicity M stars
- Short Name:
- J/PASP/121/117
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have calculated Fe and Ti abundances in 12 low-metallicity main sequence M stars using high-resolution spectra. These subdwarf and extreme-subdwarf stars allow us to extend our calibration of a method to determine cool-dwarf-star metallicities using molecular band strength indices from low-resolution spectra. Our calibration can now be used to determine metallicity to within +/-0.3dex for stars with [Fe/H] between -1.5 and +0.05 and temperatures between 3500 and 4000K. We also report a method to estimate temperatures for M dwarfs using equivalent width measurements of the infrared CaII triplet and the KI line at 7699{AA}. Our metallicity measurements show that the recently proposed classification system for low-mass stars (dwarfs, subdwarfs, extreme subdwarfs, and ultrasubdwarfs) does represent a metallicity sequence, with the ultrasubdwarfs the most metal-poor stars.
- ID:
- ivo://CDS.VizieR/J/ApJS/161/444
- Title:
- [Fe/H] and Teff for luminosity class IV-V stars
- Short Name:
- J/ApJS/161/444
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- In this paper, an updated catalog containing averaged values of [Fe/H] is presented for FGK stars on and near the main sequence. The input data for the catalog are values of [Fe/H] derived from weak and moderately strong lines and published before 2005 July 1. Those data are corrected to a uniform temperature scale, and a statistical analysis is then applied to a subset of the data that did not contribute to a previous version of the catalog. In this way, it is found that an accurate zero point for the catalog can be established with an rms error of 0.005dex. After corrections are applied to a number of the newly added data, it is shown that those corrections help to produce satisfactory zero-point coherence among the catalog entries. Standard errors that are derived for the catalog data are shown to be accurate. It is also shown that those standard errors are based on pervasive scatter in the input data, as is expected if those data are affected by genuine random effects. Samples of the metallicity catalog and an accompanying temperature catalog are displayed and discussed. To make possible an effective extension of the catalog to stars without catalog entries, a database containing photometric metallicities derived by Nordstroem et al. (2004, Cat. <V/117>) is considered. Standard errors for those metallicities are derived, and zero-point corrections required to put those data on the catalog zero point are presented.
- ID:
- ivo://CDS.VizieR/J/A+A/398/731
- Title:
- [Fe/H] catalog of class IV-V stars
- Short Name:
- J/A+A/398/731
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- This catalog contains mean values of [Fe/H] (with rms errors) for 941 F-K stars which are on or near the main sequence. Literature references are given for the contributing values of [Fe/H]
- ID:
- ivo://CDS.VizieR/J/ApJ/833/225
- Title:
- -2.6<=[Fe/H]<=0.2 F and G dwarfs. II. Abundances
- Short Name:
- J/ApJ/833/225
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- For the first time, we present an extensive study of stars with individual non-LTE (NLTE) abundances for 17 chemical elements from Li to Eu in a sample of stars uniformly distributed over the -2.62<=[Fe/H]<=+0.24 metallicity range that is suitable for the Galactic chemical evolution research. The star sample has been kinematically selected to trace the Galactic thin and thick disks and halo. We find new results and improve earlier ones as follows: (i) the element-to-iron ratios for Mg, Si, Ca, and Ti form a metal-poor (MP) plateau at a similar height of 0.3 dex, and the knee occurs at common [Fe/H]~-0.8. The knee at the same metallicity is observed for [O/Fe], and the MP plateau is formed at [O/Fe]=0.61. (ii) The upward trend of [C/O] with decreasing metallicity exists at [Fe/H]<-1.2, supporting the earlier finding of Akerman et al. (iii) An underabundance of Na relative to Mg in the [Fe/H]<-1 stars is nearly constant, with the mean [Na/Mg]~-0.5. (iv) The K/Sc, Ca/Sc, and Ti/Sc ratios form well-defined trends, suggesting a common site of the K-Ti production. (v) Sr follows the Fe abundance down to [Fe/H]~-2.5, while Zr is enhanced in MP stars. (vi) The comparisons of our results with some widely used Galactic evolution models are given. The use of the NLTE element abundances gives increased credit to the interpretation of the data in the context of the chemical evolution of the Galaxy.
- ID:
- ivo://CDS.VizieR/J/AJ/160/217
- Title:
- 342 FGK-dwarfs ages using GALEX FUV magnitudes
- Short Name:
- J/AJ/160/217
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Stellar age cannot be directly measured, yet age determinations are fundamental to understanding the evolution of stars, planets, and galaxies. The work presented here builds upon the idea of a stellar-activity age. We utilized far-ultraviolet (FUV) photometry acquired by the Galaxy Evolution Explorer (GALEX) space telescope as an indicator of chromospheric activity to infer ages of late-F, G, and K type dwarf stars. We derived a purely empirical correlation between FUV magnitudes and stellar age in conjunction with (B-V) color. Our attention is restricted to Sun-like stars with color range 0.55<=(B-V)<=0.71 and absolute magnitude range 4.3<=MV<=5.3. The correlation is defined in terms of a FUV-excess parameter Q(FUV-B,B-V). We related stellar age, {tau}, to Q through the relation log_e_({tau})=log_e_(a)+bQ, where a and b are fit parameters and functions of (B-V). This correlation is functional up to 6Gyr for FGK dwarfs. With such a correlation, one only needs Johnson (B-V) and FUV measurements to estimate the stellar age for Population i dwarf stars of solar-like temperature and metallicity. Such a calibration has utility in population studies of FGK dwarfs for further understanding of the chemical evolution of the Milky Way. As an illustration of one such application, we have constructed activity and FUV-age distributions for a sample of thin and thick disk stars, as distinguished by their chemical abundances. Considerable overlap is found between the activity distribution and age range of the two populations. We discuss the possibility that some high-[{alpha}/Fe] thick disk stars were formed as a result of the accretion of dwarf galaxies as recently as 4Gyr ago.
- ID:
- ivo://CDS.VizieR/J/A+A/556/A86
- Title:
- FGK dwarf stars limb darkening coefficients
- Short Name:
- J/A+A/556/A86
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Limb darkening is a fundamental ingredient for interpreting observations of planetary transits, eclipsing binaries, optical/infrared interferometry and microlensing events. However, this modeling traditionally represents limb darkening by a simple law having one or two coefficients that have been derived from plane-parallel model stellar atmospheres, which has been done by many researchers. More recently, researchers have gone beyond plane-parallel models and considered other geometries. We previously studied the limb-darkening coefficients from spherically symmetric and plane-parallel model stellar atmospheres for cool giant and supergiant stars, and in this investigation we apply the same techniques to FGK dwarf stars. We present limb-darkening coefficients, gravity-darkening coefficients and interferometric angular diameter corrections from Atlas and SAtlas model stellar atmospheres. We find that sphericity is important even for dwarf model atmospheres, leading to significant differences in the predicted coefficients.
- ID:
- ivo://CDS.VizieR/J/ApJS/247/9
- Title:
- FGK stars magnetic activity in LAMOST-Kepler field
- Short Name:
- J/ApJS/247/9
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Monitoring chromospheric and photospheric indexes of magnetic activity can provide valuable information, especially the interaction between different parts of the atmosphere and their response to magnetic fields. We extract chromospheric indexes, S and R_HK_^+^, for 59816 stars from LAMOST spectra in the LAMOST-Kepler program, and photospheric index, Reff, for 5575 stars from Kepler light curves. The log Reff shows positive correlation with logR_HK_^+^. We estimate the power-law indexes between Reff and R_HK_^+^ for F-, G-, and K-type stars, respectively. We also confirm the dependence of both chromospheric and photospheric activity on stellar rotation. Ca ii H and K emissions and photospheric variations generally decrease with increasing rotation periods for stars with rotation periods exceeding a few days. The power-law indexes in exponential decay regimes show different characteristics in the two activity-rotation relations. The updated largest sample including the activity proxies and reported rotation periods provides more information to understand the magnetic activity for cool stars.
- ID:
- ivo://CDS.VizieR/J/A+A/645/A42
- Title:
- Flares in 5 open clusters
- Short Name:
- J/A+A/645/A42
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Magnetic fields are a key component in the main sequence evolution of low mass stars. Flares, energetic eruptions on the surfaces of stars, are an unmistakable manifestation of magnetically driven emission. The occurrence rates and energy distributions of flares 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 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 3.6Gyr (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 (690Myr) to the sample as a comparison cluster to Praesepe (750Myr), the 2.6Gyr old Ruprecht 147, and several hundred light curves from the late K2 Campaigns in the remaining clusters. We found that the flare energy distribution was similar in the entire parameter space, following a power law relation with an exponent between 1.84 and 2.39. We confirmed that flaring rates declined with age, and declined faster for higher mass stars. Our results are in good agreement with most previous statistical flare studies. 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 d, while higher mass stars had already transitioned to lower flaring rates, and lower mass stars still resided in the saturated activity regime. We conclude that some discrepancies between our results and flare studies that used rotation periods for their age estimates could be explained by sample selection bias toward more active stars, but others may hint at limitations of using rotation as an age indicator without additional constraints from stellar activity.
- ID:
- ivo://CDS.VizieR/J/ApJ/849/36
- Title:
- Flaring activity of M dwarfs in the Kepler field
- Short Name:
- J/ApJ/849/36
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Flare events are mainly due to magnetic reconnection and thus are indicative of stellar activity. The Kepler Space Observatory records numerous stellar activities with unprecedented high photometric precision in flux measurements. It is perfectly suitable for carrying out a statistical study of flares. Here we present 540 M dwarfs with flare events discovered using Kepler long-cadence data. The normalized flare energy, as defined by the ratio to bolometric stellar luminosity, L_flare_/L_bol_, is used to indicate the flare activity. We find that, similar to the X-ray luminosity relation, the L_flare_/L_bol_ versus P_rot_ relation can also be described with three phases, supersaturation, saturation, and exponential decay, corresponding to an ultra- short period, a short period, and a long period. The flare activity and the number fraction of flaring stars in M dwarfs rise steeply near M4, which is consistent with the prediction of a turbulent dynamo. The size of starspots are positively correlated with flare activity. The L_flare_/L_bol_ ratio has a power-law dependence on L_H{alpha}_/L_bol_, a parameter indicative of stellar chromosphere activity. According to this relation, a small enhancement in chromosphere activity may cause a huge rise in flare energy, which suggests that superflares or hyperflares may not need an extra excitation mechanism. Through a comparison study, we suggest that flare activity is a more suitable indicator for stellar activity, especially in the boundary region. However, contrary to what is expected, some M dwarfs with strong flares do not show any light variation caused by starspots. Follow-up observations are needed to investigate this problem.
