- ID:
- ivo://CDS.VizieR/J/A+A/639/A63
- Title:
- Active red giants asteroseismic & rotation param.
- Short Name:
- J/A+A/639/A63
- Date:
- 08 Feb 2022
- Publisher:
- CDS
- Description:
- Oscillating red-giant stars have provided a wealth of asteroseismic information regarding their interiors and evolutionary states, and access to their fundamental properties enable detailed studies of the Milky Way. The objective of this work is to determine what fraction of red-giant stars shows photometric rotational modulation, and understand its origin. One of the underlying questions is the role of close binarity in this population, standing upon the fact that red giants in short-period binary systems (less than 150 days or so) have been observed to display strong rotational modulation. We select a sample of about 4500 relatively bright red giants observed by Kepler, and show that about 370 of them (~8%) display rotational modulation. Almost all have oscillation amplitudes below the median of the sample, while 30 of them are not oscillating at all. Of the 85 of these red giants with rotational modulation chosen for follow-up radial-velocity observation and analysis, 34 show clear evidence of spectroscopic binarity. Surprisingly, 26 of the 30 non-oscillators are in this group of binaries. To the contrary, about 85% of the active red giants with detectable oscillations are not part of close binaries. With the help of stellar masses and evolutionary states computed from the oscillation properties, we shed light on the origin of their activity. It appears that low-mass red-giant branch stars tend to be magnetically inactive, while intermediate-mass ones tend to be highly active. The opposite trends are true for helium-core burning (red clump) stars, whereby the lower-mass clump stars are comparatively more active and the higher-mass ones less so. In other words, we find that low-mass red-giant branch stars gain angular momentum as they evolve to clump stars, while higher-mass ones lose angular momentum. The trend observed with low-mass stars leads to possible scenarios of planet engulfment or other merging events during the shell-burning phase. Regarding intermediate-mass stars, the rotation periods that we measure are long with respect to theoretical expectations reported in the literature, which reinforces the existence of an unidentified sink of angular momentum after the main sequence. This article establishes strong links between rotational modulation, tidal interactions, (surface) magnetic fields, and oscillation suppression. There is a wealth of physics to be studied in these targets not available in the Sun.
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- ID:
- ivo://CDS.VizieR/J/A+A/575/A4
- Title:
- Activity and accretion in {gamma} Vel and Cha I
- Short Name:
- J/A+A/575/A4
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We use the fundamental parameters (effective temperature, surface gravity, lithium abundance, and radial velocity) delivered by the GES consortium in the first internal data release to select the members of Gamma Vel and Cha I among the UVES and GIRAFFE spectroscopic observations. A total of 140 Gamma Vel members and 74 Cha I members were studied. The procedure adopted by the GES to derive stellar fundamental parameters provided also measures of the projected rotational velocity (vsini). We calculated stellar luminosities through spectral energy distributions, while stellar masses were derived by comparison with evolutionary tracks. The spectral subtraction of low-activity and slowly rotating templates, which are rotationally broadened to match the vsini of the targets, enabled us to measure the equivalent widths (EWs) and the fluxes in the H{alpha} and H{beta} lines. The H{alpha} line was also used for identifying accreting objects, on the basis of its equivalent width and the width at the 10% of the line peak (10%W), and for evaluating the mass accretion rate (M_acc_). The distribution of vsini for the members of Gamma Vel displays a peak at about 10km/s with a tail toward faster rotators. There is also some indication of a different vsini distribution for the members of its two kinematical populations. Most of these stars have H{alpha} fluxes corresponding to a saturated activity regime. We find a similar distribution, but with a narrower peak, for Cha I. Only a handful of stars in Gamma Vel display signatures of accretion, while many more accretors were detected in the younger Cha I, where the highest H{alpha} fluxes are mostly due to accretion, rather than to chromospheric activity. Accreting and active stars occupy two different regions in a T_eff-flux diagram and we propose a criterion for distinguishing them. We derive M_acc in the ranges 10^-11^-10^-9^M_{sun}_/yr and 10^-10^-10^-7^M_{sun}_/yr for Gamma Vel and Cha I accretors, respectively.
- ID:
- ivo://CDS.VizieR/J/AJ/133/2258
- Title:
- Activity and kinematics of ultracool dwarfs
- Short Name:
- J/AJ/133/2258
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the activity and kinematics of a representative volume-limited (20pc) sample of 152 late-M and L dwarfs (M7-L8) photometrically selected from the Two Micron All Sky Survey (2MASS). Using new proper-motion measurements and spectrophotometric distance estimates, we calculate tangential velocities. The sample has a mean tangential velocity of <V_tan_>=31.5km/s, a velocity dispersion of {sigma}_tan_=20.7km/s, and a maximum tangential velocity of V_tan_=138.8km/s. These kinematic results are in excellent agreement with previous studies of ultracool dwarfs in the local solar neighborhood. H{alpha} emission, an indicator of chromospheric activity, was detected in 63 of 81 late-M dwarfs and in 16 of 69 L dwarfs examined. We find a lack of correlation between activity strength, measured by log(FH{alpha}/Fbol), and V_tan_, although velocity distributions suggest that the active dwarfs in our sample are slightly younger than the inactive dwarfs. Consistent with previous studies of activity in ultracool dwarfs, we find that the fraction of H-emitting objects per spectral type peaks at spectral type M7 and declines through mid-L dwarfs. Activity strength is similarly correlated with spectral type for types later than M7. Eleven dwarfs out of 150 show evidence of variability, ranging from small fluctuations to large flare events. We estimate a flare cycle of 5% for late-M dwarfs and 2% for L dwarfs. Observations of strong, variable activity on the L1 dwarf 2MASS J10224821+5825453 and an amazing flare event on the M7 dwarf 2MASS J1028404-143843 are discussed.
- ID:
- ivo://CDS.VizieR/J/ApJ/795/161
- Title:
- Activity and rotation in Praesepe and the Hyades
- Short Name:
- J/ApJ/795/161
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Open clusters are collections of stars with a single, well-determined age, and can be used to investigate the connections between angular-momentum evolution and magnetic activity over a star's lifetime. We present the results of a comparative study of the relationship between stellar rotation and activity in two benchmark open clusters: Praesepe and the Hyades. As they have the same age and roughly solar metallicity, these clusters serve as an ideal laboratory for testing the agreement between theoretical and empirical rotation-activity relations at ~600 Myr. We have compiled a sample of 720 spectra - more than half of which are new observations - for 516 high-confidence members of Praesepe; we have also obtained 139 new spectra for 130 high-confidence Hyads. We have also collected rotation periods (P_rot_) for 135 Praesepe members and 87 Hyads. To compare H{alpha} emission, an indicator of chromospheric activity, as a function of color, mass, and Rossby number R_o_, we first calculate an expanded set of {chi} values, with which we can obtain the H{alpha} to bolometric luminosity ratio, L_H{alpha}_/L_bol_, even when spectra are not flux-calibrated and/or stars lack reliable distances. Our {chi} values cover a broader range of stellar masses and colors (roughly equivalent to spectral types from K0 to M9), and exhibit better agreement between independent calculations, than existing values. Unlike previous authors, we find no difference between the two clusters in their H{alpha} equivalent width or L_H{alpha}_/L_bol_ distributions, and therefore take the merged H{alpha} and P_rot_ data to be representative of 600 Myr old stars. Our analysis shows that H{alpha} activity in these stars is saturated for R_O_<=0.11\-0.03_^+0.02^. Above that value activity declines as a power-law with slope {beta}=0.73_-0.12_^+0.16^, before dropping off rapidly at R_o_{approx} 0.4. These data provide a useful anchor for calibrating the age-activity-rotation relation beyond 600 Myr.
- ID:
- ivo://CDS.VizieR/J/A+A/603/A52
- Title:
- Activity cycles in 3203 Kepler stars
- Short Name:
- J/A+A/603/A52
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- In recent years it has been claimed that the length of stellar activity cycles is determined by the stellar rotation rate. It has been observed that the cycle period increases with rotation period along two distinct sequences, known as the active and inactive sequences. In this picture the Sun occupies a solitary position between the two sequences. Whether the Sun might undergo a transitional evolutionary stage is currently under debate. Our goal is to measure cyclic variations of the stellar light curve amplitude and the rotation period using four years of Kepler data. Periodic changes in the light curve amplitude or the stellar rotation period are associated with an underlying activity cycle. Using a recent sample of active stars we compute the rotation period and the variability amplitude for each individual Kepler quarter and search for periodic variations of both time series. To test for periodicity in each stellar time series we consider Lomb-Scargle periodograms and use a selection based on a false alarm probability (FAP). We detect amplitude periodicities in 3203 stars between 0.5-6 years covering rotation periods between 1-40 days. Given our sample size of 23,601 stars and our selection criteria that the FAP is less than 5%, this number is almost three times higher than that expected from pure noise. We do not detect periodicities in the rotation period beyond those expected from noise. Our measurements reveal that the cycle period shows a weak dependence on rotation rate, slightly increasing for longer rotation periods. We further show that the shape of the variability deviates from a pure sine curve, consistent with observations of the solar cycle. The cycle shape does not show a statistically significant dependence on effective temperature. We detect activity cycles in more than 13% of our final sample with a FAP of 5% (calculated by randomly shuffling the measured 90-day variability measurements for each star). Our measurements do not support the existence of distinct sequences in the Prot-Pcyc plane, although there is some evidence for the inactive sequence for rotation periods between 5-25 days. Unfortunately,the total observing time is too short to draw sound conclusions on activity cycles with similar lengths to that of the solar cycle.
- 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/MNRAS/431/2240
- Title:
- Activity in A-type stars from Kepler
- Short Name:
- J/MNRAS/431/2240
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Two years of Kepler data are used to investigate low-frequency variations in A-type stars. In about 875 (40%) A-type stars, the periodogram shows a simple peak and its harmonic. If we assume that the photometric period is the period of rotation, we can derive the equatorial rotational velocity from a suitable radius estimate. It turns out that the distribution of equatorial velocities derived in this way is similar to the distribution of equatorial velocities of A-type main-sequence stars in the general field derived from spectroscopic line broadening, verifying our initial assumption. We suggest that the light variation is due to rotational modulation caused by starspots or some other corotating structure. In many stars the rotation peak in the periodogram has a characteristic shape which is not understood. The light amplitudes are highly variable. We deduce from the amplitude distribution that the sizes of starspots in A-type stars are similar to the largest sunspots. From the widths of the peaks in the periodogram we deduce that differential rotation in these stars is similar to that in the Sun. We find that the period-colour relationship used for gyrochronology in late-type stars extends to early F-type and probably late A-type stars as well. Flares in A-type stars have been recently detected. We add 13 additional A-type flare stars to this sample, which means that about 1.5% of A-type stars in the Kepler field show flares. We conclude that A-type stars are active and, like cooler stars, have starspots and flares.
- ID:
- ivo://CDS.VizieR/J/A+A/652/A28
- Title:
- Activity indicators across the M dwarf domain
- Short Name:
- J/A+A/652/A28
- Date:
- 22 Feb 2022
- Publisher:
- CDS
- Description:
- Stellar activity poses one of the main obstacles for the detection and characterisation of small exoplanets around cool stars, as it can induce radial velocity (RV) signals that can hide or mimic the presence of planetary companions. Several indicators of stellar activity are routinely used to identify activity-related signals in RVs, but not all indicators trace exactly the same activity effects, nor are any of them always effective in all stars. We evaluate the performance of a set of spectroscopic activity indicators for M dwarf stars with different masses and activity levels with the aim of finding a relation between the indicators and stellar properties. In a sample of 98 M dwarfs observed with CARMENES, we analyse the temporal behaviour of RVs and nine spectroscopic activity indicators: cross-correlation function (CCF) full-width-at-half-maximum (FWHM), CCF contrast, CCF bisector inverse slope (BIS), RV chromatic index (CRX), differential line width (dLW), and indices of the chromospheric lines H{alpha} and calcium infrared triplet. A total of 56 stars of the initial sample show periodic signals related to activity in at least one of these ten parameters. RV is the parameter for which most of the targets show an activity-related signal. CRX and BIS are effective activity tracers for the most active stars in the sample, especially stars with a relatively high mass, while for less active stars, chromospheric lines perform best. FWHM and dLW show a similar behaviour in all mass and activity regimes, with the highest number of activity detections in the low-mass, high-activity regime. Most of the targets for which we cannot identify any activity-related signals are stars at the low-mass end of the sample (i.e. with the latest spectral types). These low-mass stars also show the lowest RV scatter, which indicates that ultracool M dwarfs could be better candidates for planet searches than earlier types, which show larger RV jitter. Our results show that the spectroscopic activity indicators analysed behave differently, depending on the mass and activity level of the target star. This underlines the importance of considering different indicators of stellar activity when studying the variability of RV measurements. Therefore, when assessing the origin of an RV signal, it is critical to take into account a large set of indicators, or at least the most effective ones considering the characteristics of the star, as failing to do so may lead to false planet claims.
- ID:
- ivo://CDS.VizieR/J/A+A/531/A8
- Title:
- Activity indices and velocities for 890 stars
- Short Name:
- J/A+A/531/A8
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- In this work we present chromospheric activity indices, kinematics, radial-velocities, and rotational velocities for more than 850 FGK-type dwarfs and subgiant stars in the southern hemisphere and test how best to calibrate and measure S -indices from echelle spectra. We measured our parameters using the high-resolution and high-S/N FEROS echelle spectra acquired for this purpose. Results. We confirm the bimodal distribution of chromospheric activities for such stars and highlight the role that the more active K-dwarfs play in biasing the number of active stars. We show that the age-activity relationship does appear to continue to ages older than the Sun if we simply compare main sequence stars and subgiant stars with an offset of around 2.5Gyr between the peaks of both distributions. Also we show evidence of an increased spin-down timescale for cool K dwarfs compared with earlier F and G type stars. We highlight that activities drawn from low-resolution spectra (R<2.500') significantly increase the rms scatter when calibrating onto common systems of measurements like the Mt. Wilson system. Also we show that older and widely used catalogues of activities in the south appear to be offset compared to more recent works at the ~0.1dex level in logR'HK through calibrator drift. In addition, we show how kinematics can be used to preselect inactive stars for future planet search projects. We see the well known trend between projected rotational velocity and activity, however we also find a correlation between kinematic space velocity and chromospheric activity. It appears that after the Vaughan-Preston gap there is a quick step function in the kinematic space motion towards a significantly broader spread in velocities. We speculate on reasons for this correlation and provide some model scenarios to describe the bimodal activity distribution through magnetic saturation, residual low level gas accretion, or accretion by the star of planets or planetesimals. Finally, we provide a new empirical measurement for the disk-heating law, using the latest age-activity relationships to reconstruct the age-velocity distribution for local disk stars. We find a value of 0.337+/-0.045 for the exponent of this power law, in excellent agreement with those found using isochrone fitting methods and with theoretical disk-heating models.
- ID:
- ivo://CDS.VizieR/J/MNRAS/372/163
- Title:
- Activity indices for southern stars
- Short Name:
- J/MNRAS/372/163
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have acquired high-resolution echelle spectra of 225 F6-M5 type stars in the Southern hemisphere. The stars are targets or candidates to be targets for the Anglo-Australian Planet Search. CaII H&K line cores were used to derive activity indices for all of these objects. The indices were converted to the Mt. Wilson system of measurements and logR'_HK_ values determined. A number of these stars had no previously derived activity indices. In addition, we have also included the stars from Tinney et al. (2002MNRAS.332..759T) using our Mt. Wilson calibration. The radial-velocity instability (also known as jitter) level was determined for all 21 planet-host stars in our data set. We find the jitter to be at a level considerably below the radial-velocity signatures in all but one of these systems. 19 stars from our sample were found to be active (logR'_HK_>-4.5) and thus have high levels of jitter. Radial-velocity analysis for planetary companions to these stars should proceed with caution.
