- ID:
- ivo://CDS.VizieR/J/A+A/638/A9
- Title:
- A census of the near-by Psc-Eri stellar stream
- Short Name:
- J/A+A/638/A9
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Within a sphere of 400pc radius around the Sun, we search for members of the Pisces-Eridanus (Psc-Eri) stellar stream in the Gaia Data Release 2 DR2) data set. We compare basic astrophysical characteristics of the stream with those of the Pleiades. We used a modified convergent-point method to identify stars with 2D - velocities consistent with the space velocity of the Psc-Eri stream and the Pleiades, respectively. We found 1387 members of the Psc-Eri stream in a G-magnitude range from 5.1mag to 19.3mag at distances between 80 and 380pc from the Sun. The stream has a nearly cylindrical shape with a length of at least 700pc and a thickness of 100pc. The accumulated stellar mass of the 1387 members amounts to about 770M_{sun}_, and the stream is gravitationally unbound. For the stream we found an age of about 135Myr. In many astrophysical properties Psc-Eri is comparable to the open cluster M45 (the Pleiades): in its age, its luminosity function (LF), its present-day mass function (PDMF) as well as in its total mass. Nonetheless, the two stellar ensembles are completely unlike in their physical appearance. We cautiously give two possible explanations for this disagreement: (i) the star-formation efficiency in their parental molecular clouds was higher for the Pleiades than for Psc-Eri or/and (ii) the Pleiades had a higher primordial mass segregation immediately after the expulsion of the molecular gas of the parental cloud.
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- ID:
- ivo://CDS.VizieR/J/ApJ/871/193
- Title:
- A combined Chandra & LAMOST study of stellar activity
- Short Name:
- J/ApJ/871/193
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We probed stellar X-ray activity over a wide range of stellar parameters, using Chandra and Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) data. We measured the X-ray-to-bolometric luminosity ratio (R_X_=L_X_/L_bol_) for 484 main-sequence stars and found a bimodal distribution for G and K types. We interpret this bimodality as evidence of two subpopulations with different coronal temperatures, which are caused by different coronal heating rates. Using the metallicity and velocity information, we find that both of the subpopulations are mostly located in the thin disk. We find no trend of R_X_ with stellar age for stars older than ~4Gyr; there is a trough in the R_X_ versus age distribution, with the lowest range of R_X_ appearing at ages around 2Gyr. We then examined the correlation between R_X_ and R_H{alpha}_ (proxy of chromospheric activity): we find that the two quantities are well correlated, as found in many earlier studies. Finally, we selected a sample of 12 stars with X-ray flares and studied the light-curve morphology of the flares. The variety of flare profiles and timescales observed in our sample suggests the contribution of different processes of energy release.
- 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.
- 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/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/ApJ/812/40
- Title:
- Adiabatic mass loss in binary stars. II.
- Short Name:
- J/ApJ/812/40
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- In the limit of extremely rapid mass transfer, the response of a donor star in an interacting binary becomes asymptotically one of adiabatic expansion. We survey here adiabatic mass loss from Population I stars (Z=0.02) of mass 0.10M_{sun}_-100M_{sun}_ from the zero-age main sequence to the base of the giant branch, or to central hydrogen exhaustion for lower main sequence stars. The logarithmic derivatives of radius with respect to mass along adiabatic mass-loss sequences translate into critical mass ratios for runaway (dynamical timescale) mass transfer, evaluated here under the assumption of conservative mass transfer. For intermediate- and high-mass stars, dynamical mass transfer is preceded by an extended phase of thermal timescale mass transfer as the star is stripped of most of its envelope mass. The critical mass ratio q_ad_ (throughout this paper, we follow the convention of defining the binary mass ratio as q{equiv}M_donor_/M_accretor_) above which this delayed dynamical instability occurs increases with advancing evolutionary age of the donor star, by ever-increasing factors for more massive donors. Most intermediate- or high-mass binaries with nondegenerate accretors probably evolve into contact before manifesting this instability. As they approach the base of the giant branch, however, and begin developing a convective envelope, q_ad_ plummets dramatically among intermediate-mass stars, to values of order unity, and a prompt dynamical instability occurs. Among low-mass stars, the prompt instability prevails throughout main sequence evolution, with q_ad_ declining with decreasing mass, and asymptotically approaching q_ad_=2/3, appropriate to a classical isentropic n=3/2 polytrope. Our calculated q_ad_ values agree well with the behavior of time-dependent models by Chen & Han (2003MNRAS.341..662C) of intermediate-mass stars initiating mass transfer in the Hertzsprung gap. Application of our results to cataclysmic variables, as systems that must be stable against rapid mass transfer, nicely circumscribes the range in q_ad_ as a function of the orbital period in which they are found. These results are intended to advance the verisimilitude of population synthesis models of close binary evolution.
- ID:
- ivo://CDS.VizieR/J/ApJ/899/132
- Title:
- Adiabatic Mass Loss in Binary Stars. III.
- Short Name:
- J/ApJ/899/132
- Date:
- 14 Mar 2022 07:06:32
- Publisher:
- CDS
- Description:
- The distinguishing feature of the evolution of close binary stars is the role played by the mass exchange between the component stars. Whether or not the mass transfer is dynamically stable is one of the essential questions in binary evolution. In the limit of extremely rapid mass transfer, the response of a donor star in an interacting binary becomes asymptotically one of adiabatic expansion. We use the adiabatic mass-loss model to systematically survey the thresholds for dynamical timescale mass transfer over the entire span of possible donor star evolutionary states. We also simulate mass-loss process with isentropic envelopes, the specific entropy of which is fixed to be that at the base of the convective envelope, to artificially mimic the effect of such mass loss in superadiabatic surface convection regions, where the adiabatic approximation fails. We illustrate the general adiabatic response of 3.2M{odot} donor stars at different evolutionary stages. We extend our study to a grid of donor stars with different masses (from 0.1 to 100 M{sun} with Z=0.02) and at different evolutionary stages. We proceed to present our criteria for dynamically unstable mass transfer in both tabular and graphical forms. For red giant branch (RGB) and asymptotic giant branch (AGB) donors in systems with such mass ratios, they may have convective envelopes deep enough to evolve into common envelopes on a thermal timescale, if the donor star overfills its outer Lagrangian radius. Our results show that the RGB and AGB stars tend to be more stable than previously believed, and this may be helpful to explain the abundance of observed post-AGB binary stars with an orbital period of around 1000 days.
- ID:
- ivo://CDS.VizieR/J/A+A/569/A21
- Title:
- Age and mass of CoRoT exoplanet host HD 52265
- Short Name:
- J/A+A/569/A21
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- In the context of space missions CoRoT, Kepler, Gaia, TESS, and PLATO, precise and accurate stellar ages, masses and radii are of paramount importance. For instance, they are crucial to constrain scenarii of planetary formation and evolution.
- ID:
- ivo://CDS.VizieR/J/A+A/645/A85
- Title:
- Age dissection of the Milky Way discs
- Short Name:
- J/A+A/645/A85
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Ensemble studies of red-giant stars with exquisite asteroseismic (Kepler), spectroscopic (APOGEE), and astrometric (Gaia) constraints offer a novel opportunity to recast and address long-standing questions concerning the evolution of stars and of the Galaxy. Here, we infer masses and ages for nearly 5400 giants with available Kepler light curves and APOGEE spectra using the code PARAM, and discuss some of the systematics that may affect the accuracy of the inferred stellar properties. We then present patterns in mass, evolutionary state, age, chemical abundance, and orbital parameters that we deem robust against the systematic uncertainties explored. First, we look at age-chemical-abundances ([Fe/H] and [alpha/Fe]) relations. We find a dearth of young, metal-rich ([Fe/H]>0.2) stars, and the existence of a significant population of old (8-9Gyr), low-[alpha/Fe], super-solar metallicity stars, reminiscent of the age and metallicity of the well-studied open cluster NGC 6791. The age-chemo-kinematic properties of these stars indicate that efficient radial migration happens in the thin disc. We find that ages and masses of the nearly 400 alpha-element-rich red-giant-branch (RGB) stars in our sample are compatible with those of an old (~11Gyr), nearly coeval, chemical-thick disc population. Using a statistical model, we show that the width of the observed age distribution is dominated by the random uncertainties on age, and that the spread of the inferred intrinsic age distribution is such that 95% of the population was born within ~1.5Gyr. Moreover, we find a difference in the vertical velocity dispersion between low- and high-[alpha/Fe] populations. This discontinuity, together with the chemical one in the [alpha/Fe] versus [Fe/H] diagram, and with the inferred age distributions, not only confirms the different chemo-dynamical histories of the chemical-thick and thin discs, but it is also suggestive of a halt in the star formation (quenching) after the formation of the chemical-thick disc. We then exploit the almost coeval alpha-rich population to gain insight into processes that may have altered the mass of a star along its evolution, which are key to improving the mapping of the current, observed, stellar mass to the initial mass and thus to the age. Comparing the mass distribution of stars on the lower RGB (R<11R_{sun}_) with those in the red clump (RC), we find evidence for a mean integrated RGB mass loss DM=0.10+/-0.02M_{sun}_. Finally, we find that the occurrence of massive (M>~1.1M_{sun}_) alpha-rich stars is of the order of 5% on the RGB, and significantly higher in the RC, supporting the scenario in which most of these stars had undergone an interaction with a companion.
- ID:
- ivo://CDS.VizieR/J/A+A/377/911
- Title:
- Age-metallicity relation in solar neighbourhood
- Short Name:
- J/A+A/377/911
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We derive stellar ages, from evolutionary tracks, and metallicities, from Stroemgren photometry, for a sample of 5828 dwarf and sub-dwarf stars from the Hipparcos (Cat. <I/239>) Catalogue. This stellar disk sample is used to investigate the age-metallicity diagram in the solar neighbourhood. Such diagrams are often used to derive a so called age-metallicity relation. Because of the size of our sample, we are able to quantify the impact on such diagrams, and derived relations, due to different selection effects. Some of these effects are of a more subtle sort, giving rise to erroneous conclusions. In particular we show that [1] the age-metallicity diagram is well populated at all ages and especially that old, metal-rich stars do exist, [2] the scatter in metallicity at any given age is larger than the observational errors, [3] the exclusion of cooler dwarf stars from an age-metallicity sample preferentially excludes old, metal-rich stars, depleting the upper right-hand corner of the age-metallicity diagram, [4] the distance dependence found in the Edvardsson et al. (1993, Cat. <J/A+A/275/101>) sample by Garnett & Kobulnicky (2000ApJ...532.1192G) is an expected artifact due to the construction of the original sample. We conclude that, although some of it can be attributed to stellar migration in the galactic disk, a large part of the observed scatter is intrinsic to the formation processes of stars.
