- ID:
- ivo://CDS.VizieR/J/A+A/640/A81
- Title:
- Abundances of 72 solar-type stars
- Short Name:
- J/A+A/640/A81
- Date:
- 29 Nov 2021 07:35:20
- Publisher:
- CDS
- Description:
- Previous high-precision studies of abundances of elements in solar twin stars are extended to a wider metallicity range to see how the trends of element ratios with stellar age depend on [Fe/H]. HARPS spectra with S/N>~600 at {lambda}~6000{AA} were analysed with MARCS model atmospheres to obtain 1D LTE abundances of C, O, Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni, Sr, and Y in 72 solar-type stars including the binary star zeta Reticuli and ASTEC stellar models were used to determine stellar ages from effective temperatures, luminosities obtained via Gaia DR2 parallaxes, and heavy element abundances. The age-metallicity distribution appears to consist of the following two distinct populations: a sequence of old stars with a steep rise of [Fe/H] to ~+0.3 dex at an age of ~7Gyr and a younger sequence with [Fe/H] increasing from about -0.3dex to ~+0.2dex over the last 6Gyr. Furthermore, the trends of several abundance ratios, [O/Fe], [Na/Fe], [Ca/Fe], and [Ni/Fe], as a function of stellar age, split into two corresponding sequences. The [Y/Mg]-age relation, on the other hand, shows no offset between the two age sequences and has no significant dependence on [Fe/H], but the components of a visual binary star, {zeta} Reticuli, have a large and puzzling deviation. The split of the age-metallicity distribution into two sequences may be interpreted as evidence of two episodes of accretion of gas onto the Galactic disk with a quenching of star formation in between. Some of the [X/Fe]-age relations support this scenario but other relations are not so easy to explain, which calls for a deeper study of systematic errors in the derived abundances as a function of [Fe/H], in particular 3D non-LTE effects.
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Search Results
- ID:
- ivo://CDS.VizieR/J/A+A/606/A58
- Title:
- Activity cycles in young solar-like stars
- Short Name:
- J/A+A/606/A58
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Magnetic cycles analogous to the solar cycle have been detected in tens of solar-like stars by analyzing long-term time series of different magnetic activity indexes. The relationship between the cycle properties and global stellar parameters is not fully understood yet. One reason for this is the lack of long-term time series for stars covering a wide range of stellar parameters. We searched for activity cycles in a sample of 90 young solar-like stars with ages between 4 and 95 Myr with the aim to investigate the properties of activity cycles in this age range. We measured the length P_cyc_ of a given cycle by analyzing the long-term time series of three different activity indexes: the period of rotational modulation, the amplitude of the rotational modulation and the median magnitude in the V band. For each star, we also computed the global magnetic activity index <IQR> that is proportional to the amplitude of the rotational modulation and can be regarded as a proxy of the mean level of the surface magnetic activity. We detected activity cycles in 67 stars. Secondary cycles were also detected in 32 stars of the sample. The lack of correlation between P_cyc_ and P_rot_ and the position of our targets in the P_cyc_/P_rot_-Ro^-1^ diagram suggest that these stars belong to the so-called transitional branch and that the dynamo acting in these stars is different from the solar dynamo and from that acting in the older Mt. Wilson stars. This statement is also supported by the analysis of the butterfly diagrams whose patterns are very different from those seen in the solar case. We computed the Spearman correlation coefficient r_S_ between P_cyc_, <IQR> and various stellar parameters. We found that P_cyc_ in our sample is uncorrelated with all the investigated parameters. The <IQR> index is positively correlated with the convective turnover timescale, the magnetic diffusivity timescale {tau}_diff_, and the dynamo number D_N_, whereas it is anti-correlated with the effective temperature Teff, the photometric shear {Delta}{Omega}_phot_ and the radius R_C_ at which the convective zone is located. We investigated how P_cyc_ and <IQR> evolve with the stellar age. We found that P_cyc_ is about constant and that <IQR> decreases with the stellar age in the range 4-95Myr. Finally we investigated the magnetic activity of the star AB Dor A by merging All Sky Automatic Survey (ASAS) time series with previous long-term photometric data. We estimated the length of the AB Dor A primary cycle as P_cyc_=16.78+/-2yr and we also found shorter secondary cycles with lengths of 400d, 190d, and 90d, respectively.
- ID:
- ivo://CDS.VizieR/J/AJ/138/312
- Title:
- Activity of bright solar analogs
- Short Name:
- J/AJ/138/312
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present 14 years of contemporaneous photometric and spectroscopic observations of 28 solar analog stars, taken with the Tennessee State University Automatic Photometric Telescopes at Fairborn Observatory and the Solar-Stellar Spectrograph at Lowell Observatory. These are the best observed and most nearly Sun-like of the targets in our magnitude-limited (V<=7.5) sample. The correlations between luminosity and activity reveal the expected inverse activity-brightness correlations for active stars. Strong direct correlations between activity and brightness are not prevalent for the less active solar age stars, but are precision limited.
- ID:
- ivo://CDS.VizieR/J/ApJ/687/1264
- Title:
- Age estimation for solar-type dwarfs
- Short Name:
- J/ApJ/687/1264
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- While the strong anticorrelation between chromospheric activity and age has led to the common use of the CaII H and K emission index (R'_HK_=L_HK_/L_bol_) as an empirical age estimator for solar-type dwarfs, existing activity-age relations produce implausible ages at both high and low activity levels. We have compiled R'_HK_, data from the literature for young stellar clusters, richly populating for the first time the young end of the activity-age relation. Combining the cluster activity data with modern cluster age estimates and analyzing the color dependence of the chromospheric activity age index, we derive an improved activity-age calibration for F7-K2 dwarfs (0.5<B-V<0.9mag). We also present a more fundamentally motivated activity-age calibration that relies on conversion of R'_HK_ values through the Rossby number to rotation periods and then makes use of improved gyrochronology relations.
- ID:
- ivo://CDS.VizieR/J/MNRAS/507/1847
- Title:
- A HARPS-N mass for the elusive Kepler-37d
- Short Name:
- J/MNRAS/507/1847
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- To date, only 18 exoplanets with radial velocity (RV) semi-amplitudes <2m/s have had their masses directly constrained. The biggest obstacle to RV detection of such exoplanets is variability intrinsic to stars themselves, e.g. nuisance signals arising from surface magnetic activity such as rotating spots and plages, which can drown out or even mimic planetary RV signals. We use Kepler-37 - known to host three transiting planets, one of which, Kepler-37d, should be on the cusp of RV detectability with modern spectrographs - as a case study in disentangling planetary and stellar activity signals. We show how two different statistical techniques - one seeking to identify activity signals in stellar spectra, and another to model activity signals in extracted RVs and activity indicators - can enable detection of the hitherto elusive Kepler-37d. Moreover, we show that these two approaches can be complementary, and in combination, facilitate a definitive detection and precise characterisation of Kepler-37d. Its RV semi-amplitude of 1.22+/-0.31m/s (mass 5.4+/-1.4M_{Earth}_) is formally consistent with TOI-178b's 1.05^+0.25^_-0.30_m/s, the latter being the smallest detected RV signal of any transiting planet to date, though dynamical simulations suggest Kepler-37d's mass may be on the lower end of our 1{sigma} credible interval. Its consequent density is consistent with either a water-world or that of a gaseous envelope (~0.4% by mass) surrounding a rocky core. Based on RV modelling and a re-analysis of Kepler-37 TTVs, we also argue that the putative (non-transiting) planet Kepler-37e should probably be stripped of its 'confirmed' status.
- ID:
- ivo://CDS.VizieR/J/A+A/615/A172
- Title:
- alpha Cen A and B chemical composition
- Short Name:
- J/A+A/615/A172
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The two solar-like stars alpha Cen A and B have long served as cornerstones for stellar physics in virtue of their immediate proximity, association in a visual binary, and masses that bracket the solar one. The recent detection of a terrestrial planet in the cool, suspected tertiary Proxima Cen now makes the system also of prime interest in the context of planetary studies. It is therefore of fundamental importance to tightly constrain the properties of the individual stellar components. We present a fully self-consistent, line-by-line differential abundance analysis of alpha Cen AB based on high-quality HARPS data. Various line lists are used and analysis strategies implemented to improve the reliability of the results. Abundances of 21 species with a typical precision of 0.02-0.03dex are reported. We find that the chemical composition of the two stars is not scaled solar (e.g., Na and Ni excess, depletion of neutron-capture elements), but that their patterns are strikingly similar, with a mean abundance difference (A-B) with respect to hydrogen of -0.01+/-0.04dex. Much of the scatter may be ascribed to physical effects that are not fully removed through a differential analysis because of the mismatch in parameters between the two components. We derive an age for the system from abundance indicators (e.g., [Y/Mg] and [Y/Al]) that is slightly larger than solar and in agreement with most asteroseismic results. Assuming coeval formation for the three components belonging to the system, this implies an age of about ~6Gyrs for the M dwarf hosting the terrestrial planet Proxima Cen b. After correction for Galactic chemical evolution effects, we find a trend between the abundance ratios and condensation temperature in alpha Cen A akin to that of the Sun. However, taking this finding as evidence for the sequestration of rocky material locked up in planets may be premature given that a clear link between the two phenomena remains to be established. The similarity between the abundance pattern of the binary components argues against the swallowing of a massive planet by one of the stars after the convective zones have shrunk to their present-day sizes.
- ID:
- ivo://CDS.VizieR/J/ApJ/745/24
- Title:
- A search for new companions with CHARA
- Short Name:
- J/ApJ/745/24
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the results of a comprehensive search for new companions to nearby solar-type stars using the separated fringe packet (SFP) technique at the Center for High Angular Resolution Astronomy (CHARA) Array. Our search included 636 observations of 186 stars, searching for companions with separations of approximately 8-80 mas and moderate brightness ratios ({Delta}K<~1.5). This survey was undertaken to support a comprehensive assessment of companions to solar-type stars within 25 pc. We detected separated fringe companions to two stars (HD 3196 and 79096) and found faint companion signatures to two more stars (HD 98231 and 137763). All of these companions are previously known by spectroscopic methods, and three of them have speckle interferometric observations as well. The faint companion seen to HD 98231 represents the first visual detection of this spectroscopic companion. Our null detection for new companions implies that the presumed gap between spectroscopic and visual techniques has largely been filled for nearby solar-type stars, thanks to systematic radial-velocity observations over multiple decades and a thorough coverage using visual techniques, especially speckle interferometric observations. We also generate simulated fringe packets to derive detection limits for SFP binaries using the CHARA Array.
- ID:
- ivo://CDS.VizieR/J/MNRAS/495/2363
- Title:
- Asteroseismology of 36 Kepler subgiants. I.
- Short Name:
- J/MNRAS/495/2363
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The presence of mixed modes makes subgiants excellent targets for asteroseismology, providing a probe for the internal structure of stars. Here we study 36 Kepler subgiants with solar-like oscillations and report their oscillation mode parameters. We performed a so-called peakbagging exercise, i.e. estimating oscillation mode frequencies, linewidths, and amplitudes with a power spectrum model, fitted in the Bayesian framework and sampled with a Markov Chain Monte Carlo algorithm. The uncertainties of the mode frequencies have a median value of 0.180uHz. We obtained seismic parameters from the peakbagging, analysed their correlation with stellar parameters, and examined against scaling relations. The behaviour of seismic parameters (e.g. {Delta}{nu}, {nu}_max_, {epsilon}_p_) is in general consistent with theoretical predictions. We presented the observational p-g diagrams: {gamma}_1_-{Delta}u for early subgiants and {Delta}{Pi}_1_-{Delta}u for late subgiants, and demonstrate their capability to estimate stellar mass. We also found a logg dependence on the linewidths and a mass dependence on the oscillation amplitudes and the widths of oscillation excess. This sample will be valuable constraints for modelling stars and studying mode physics such as excitation and damping.
- ID:
- ivo://CDS.VizieR/J/MNRAS/446/2959
- Title:
- Asterosismology for solar analogues 16 Cyg A/B
- Short Name:
- J/MNRAS/446/2959
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The solar analogues 16 Cyg A and B are excellent asteroseismic targets in the Kepler field of view and together with a red dwarf and a Jovian planet form an interesting system. For these more evolved Sun-like stars we cannot detect surface rotation with the current Kepler data but instead use the technique of asteroseimology to determine rotational properties of both 16 Cyg A and B. We find the rotation periods to be 23.8^1.5^_-1.8_ and 23.2^11.5^_-3.2_ d, and the angles of inclination to be 56^6^_-5_ {deg} and 36^17^_-7_ {deg}, for A and B, respectively. Together with these results we use the published mass and age to suggest that, under the assumption of a solar-like rotation profile, 16 Cyg A could be used when calibrating gyrochronology relations. In addition, we discuss the known 16 Cyg B star-planet eccentricity and measured low obliquity which is consistent with Kozai cycling and tidal theory.
- ID:
- ivo://CDS.VizieR/J/AJ/162/14
- Title:
- Astrometry & radial velocity, Alpha Centauri system
- Short Name:
- J/AJ/162/14
- Date:
- 14 Mar 2022 06:46:54
- Publisher:
- CDS
- Description:
- Alpha Centauri A is the closest solar-type star to the Sun and offers the best opportunity to find and ultimately to characterize an Earth-sized planet located in its habitable zone. Here, we describe initial results from an Atacama Large Millimeter/submillimeter Array (ALMA) program to search for planets in the {alpha} Cen AB system using differential astrometry at millimeter wavelengths. Our initial results include new absolute astrometric measurements of the proper motion, orbital motion and parallax of the {alpha} Cen system. These lead to an improved knowledge of the physical properties of both {alpha} Cen A and B. Our estimates of ALMA's relative astrometric precision suggest that we will ultimately be sensitive to planets of a few tens of Earth mass in orbits from 1 to 3au, where stable orbits are thought to exist.
