- ID:
- ivo://CDS.VizieR/J/A+A/616/A94
- Title:
- KIC red giants radial modes amplitude & lifetime
- Short Name:
- J/A+A/616/A94
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The space-borne missions CoRoT and Kepler have provided photometric observations of unprecedented quality. The study of solar-like oscillations observed in red giant stars by these satellites allows a better understanding of the different physical processes occurring in their interiors. In particular, the study of the mode excitation and damping is a promising way to improve our understanding of stellar physics that has, so far, been performed only on a limited number of targets. The recent asteroseismic characterization of the evolutionary status for a large number of red giants allows us to study the physical processes acting in the interior of red giants and how they are modified during stellar evolution. In this work, we aim to obtain information on the excitation and damping of pressure modes through the measurement of the stars' pressure mode widths and amplitudes and to analyze how they are modified with stellar evolution. The objective is to bring observational constraints on the modeling of the physical processes behind mode excitation and damping. We fit the frequency spectra of red giants with well-defined evolutionary status using Lorentzian functions to derive the pressure mode widths and amplitudes. To strengthen our conclusions, we used two different fitting techniques. Pressure mode widths and amplitudes were determined for more than 5000 red giants. With a stellar sample two orders of magnitude larger than previous results, we confirmed that the mode width depends on stellar evolution and varies with stellar effective temperature. In addition, we discovered that the mode width depends on stellar mass. We also confirmed observationally the influence of the stellar metallicity on the mode amplitudes, as predicted by models.
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- ID:
- ivo://CDS.VizieR/J/A+A/598/A62
- Title:
- KIC red giants showing depressed mixed modes
- Short Name:
- J/A+A/598/A62
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Seismic observations with the space-borne Kepler mission have shown that a number of evolved stars exhibit low-amplitude dipole modes, which is referred to as depressed modes. Recently, these low amplitudes have been attributed to the presence of a strong magnetic field in the stellar core of those stars. Subsequently, and based on this scenario, the prevalence of high magnetic fields in evolved stars has been inferred. It should be noted, however, that this conclusion remains indirect. We intend to study the properties of mode depression in evolved stars, which is a necessary condition before reaching conclusions about the physical nature of the mechanism responsible for the reduction of the dipole mode amplitudes. We perform a thorough characterization of the global seismic parameters of depressed dipole modes and show that these modes have a mixed character. The observation of stars showing dipole mixed modes that are depressed is especially useful for deriving model-independent conclusions on the dipole mode damping. We use a simple model to explain how mode visibilities are connected to the extra damping seen in depressed modes. Results. Observations prove that depressed dipole modes in red giants are not pure pressure modes but mixed modes. This result, observed in more than 90% of the bright stars (m_V_<=11), invalidates the hypothesis that depressed dipole modes result from the suppression of the oscillation in the radiative core of the stars. Observations also show that, except for visibility, seismic properties of the stars with depressed modes are equivalent to those of normal stars. The measurement of the extra damping that is responsible for the reduction of mode amplitudes, without any prior on its physical nature, potentially provides an efficient tool for elucidating the mechanism responsible for the mode depression. The mixed nature of the depressed modes in red giants and their unperturbed global seismic parameters carry strong constraints on the physical mechanism responsible for the damping of the oscillation in the core. This mechanism is able to damp the oscillation in the core but cannot fully suppress it. Moreover, it cannot modify the radiative cavity probed by the gravity component of the mixed modes. The recent mechanism involving high magnetic fields proposed for explaining depressed modes is not compliant with the observations and cannot be used to infer the strength and prevalence of high magnetic fields in red giants.
- ID:
- ivo://CDS.VizieR/J/ApJ/729/L10
- Title:
- KIC stars properties in NGC 6791 and NGC 6819
- Short Name:
- J/ApJ/729/L10
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present initial results on some of the properties of open clusters NGC 6791 and NGC 6819 derived from asteroseismic data obtained by NASA's Kepler mission. In addition to estimating the mass, radius, and log g of stars on the red giant branch (RGB) of these clusters, we estimate the distance to the clusters and their ages. Our model-independent estimate of the distance modulus of NGC 6791 is (m-M)_0_=13.11+/-0.06. We find (m-M)_0_=11.85+/-0.05 for NGC 6819. The average mass of stars on the RGB of NGC 6791 is 1.20+/-0.01M_{sun}_, while that of NGC 6819 is 1.68+/-0.03M_{sun}_. It should be noted that we do not have data that cover the entire RGB and the actual mass will be somewhat lower. We have determined model-dependent estimates of ages of these clusters. We find ages between 6.8 and 8.6Gyr for NGC 6791, however, most sets of models give ages around 7Gyr. We obtain ages between 2 and 2.4Gyr for NGC 6819.
- ID:
- ivo://CDS.VizieR/J/ApJ/768/172
- Title:
- Kinematic analysis of red giant in M31 dSphs
- Short Name:
- J/ApJ/768/172
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a homogeneous kinematic analysis of red giant branch stars within 18 of the 28 Andromeda dwarf spheroidal (dSph) galaxies, obtained using the Keck I/LRIS and Keck II/DEIMOS spectrographs. Based on their g-i colors (taken with the CFHT/MegaCam imager), physical positions on the sky, and radial velocities, we assign probabilities of dSph membership to each observed star. Using this information, the velocity dispersions, central masses, and central densities of the dark matter halos are calculated for these objects, and compared with the properties of the Milky Way dSph population. We also measure the average metallicity ([Fe/H]) from the co-added spectra of member stars for each M31 dSph and find that they are consistent with the trend of decreasing [Fe/H] with luminosity observed in the Milky Way population. We find that three of our studied M31 dSphs appear as significant outliers in terms of their central velocity dispersion, And XIX, XXI, and XXV, all of which have large half-light radii (>~700pc) and low velocity dispersions ({sigma}_v_<5km/s). In addition, And XXV has a mass-to-light ratio within its half-light radius of just [M/L]_half_=10.3_-6.7_^7.0^, making it consistent with a simple stellar system with no appreciable dark matter component within its 1{sigma} uncertainties. We suggest that the structure of the dark matter halos of these outliers have been significantly altered by tides.
- ID:
- ivo://CDS.VizieR/J/A+A/555/A91
- Title:
- Kinematics of bulge red clump stars
- Short Name:
- J/A+A/555/A91
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The Galactic bulge is X-shaped, caused by the two southern arms of the X both crossing the lines of sight, producing a double red clump (one bright and one faint) in the color magnitude diagram. In this paper the authors measure the radial velocities and proper motions for a sample of 454 individual bulge giant stars, roughly equally distributed between the two red clumps to determine how such a structure could be formed from bar instabilities. The radial velocity distribution of stars in the bright red clump, which traces the closer overdensity of bulge stars, shows an excess of stars moving towards the Sun. Similarly, an excess of stars receding from the Sun is seen in the more distant overdensity, which is traced by faint red clump stars. This can be explained by the presence of stars on elongated orbits, which are most likely streaming along the arms of the X-shaped bulge. Proper motions for these stars are consistent with qualitative predictions of dynamical models of peanut-shaped bulges. Surprisingly, stars on elongated orbits have preferentially metal-poor (subsolar) metallicities, while the metal rich ones, in both overdensities, are preferentially found in more axisymmetric orbits.
- ID:
- ivo://CDS.VizieR/J/ApJ/769/87
- Title:
- Kinematics of halo red giants
- Short Name:
- J/ApJ/769/87
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We explore the kinematics and orbital properties of a sample of red giants in the halo system of the Milky Way that are thought to have formed in globular clusters based on their anomalously strong UV/blue CN bands. The orbital parameters of the CN-strong halo stars are compared to those of the inner- and outer-halo populations as described by Carollo et al. (2007Natur.450.1020C, 2010ApJ...712..692C), and to the orbital parameters of globular clusters with well-studied Galactic orbits. The CN-strong field stars and the globular clusters both exhibit kinematics and orbital properties similar to the inner-halo population, indicating that stripped or destroyed globular clusters could be a significant source of inner-halo field stars, and suggesting that both the CN-strong stars and the majority of globular clusters are primarily associated with this population.
- ID:
- ivo://CDS.VizieR/J/AJ/115/168
- Title:
- Kinematics of red giant and RR Lyrae stars
- Short Name:
- J/AJ/115/168
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The kinematics of 122 red giant and 124 RR Lyrae stars in the solar neighborhood are studied using accurate measurements of their proper motions obtained by the Hipparcos astrometry satellite, combined with their published photometric distances, metal abundances, and radial velocities. A majority of these sample stars have metal abundances of [Fe/H]<=-1 and thus represent the old stellar populations in the Galaxy. The halo component, with [Fe/H]<=-1.6, is characterized by a lack of systemic rotation [(<U>, <V>, <W>)=(16+/-18, -217+/-21, -10+/-12)km/s] and a radially elongated velocity ellipsoid [(<U>, <V>, <W>)=(161+/-10, 115+/-7, 108+/-7)km/s]. About 16% of such metal-poor stars have low orbital eccentricities (e<0.4), and we see no evidence of a correlation between [Fe/H] and e. Based on the model for the e-distribution of orbits, we show that this fraction of low-e stars for [Fe/H]<=-1.6 is explained by the halo component alone, without introducing the extra disk component claimed by recent workers. This is also supported by the absence of a significant change in the e-distribution with height from the Galactic plane. In the intermediate-metallicity range (-1.6<[Fe/H]<=-1), we find that stars with disklike kinematics have only modest effects on the distributions of rotational velocities and e for the sample at |z|<1kpc. This disk component appears to constitute only 10% for -1.6<[Fe/H]<=-1 and 20% for -1.4<[Fe/H]<=-1. It is also verified that this metal-weak disk has a mean rotation of ~195km/s and a vertical extent of 1 kpc, which is consistent with the thick disk's dominating at [Fe/H]=-0.6 to -1. We find no metallicity gradient in the halo, whereas there is an indication of a metallicity gradient in the metal-weak tail of the thick disk. The implications of these results for the early evolution of the Galaxy are also presented.
- ID:
- ivo://CDS.VizieR/J/ApJ/836/202
- Title:
- Kinematic study of the Leo II dwarf galaxy
- Short Name:
- J/ApJ/836/202
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We conducted a large spectroscopic survey of 336 red giants in the direction of the Leo II dwarf galaxy using Hectochelle on the Multiple Mirror Telescope, and we conclude that 175 of them are members based on their radial velocities and surface gravities. Of this set, 40 stars have never before been observed spectroscopically. The systemic velocity of the dwarf is 78.3+/-0.6km/s with a velocity dispersion of 7.4+/-0.4km/s. We identify one star beyond the tidal radius of Leo II but find no signatures of uniform rotation, kinematic asymmetries, or streams. The stars show a strong metallicity gradient of -1.53+/-0.10dex/kpc and have a mean metallicity of -1.70+/-0.02dex.
- ID:
- ivo://CDS.VizieR/J/A+A/328/175
- Title:
- K & M giants equivalent widths
- Short Name:
- J/A+A/328/175
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The analyses of high resolution infrared spectra have been done for CN lines in oxygen-rich cool evolved stars including 2 K giants, 20 M giants and 1 S-type star. Since CN lines analyzed in the present work are weak and resolved well, they are appropriate for quantitative analyses. CN lines of {DELTA}v=-2 and -1 sequences (red system) which are in the K- and the H-window regions, respectively, give the consistent nitrogen abundance for each star. The analyses of NH lines in the L-window region have been done for 5 late M giants for which CN lines have been also analyzed. Although the triplet structure of NH lines cannot be fully resolved, they are preferable because determination of nitrogen abundance is almost independent of other elemental abundances while nitrogen abundance based on CN depends on carbon abundance. The nitrogen abundances derived from NH for late M giants agree well with those from CN for which we adopt 7.75eV as the dissociation energy in the analysis. The results show that the nitrogen abundances in late M giants are larger than those in early M giants while decrease of the carbon abundance was found in late M giants by our previous work (Tsuji, 1991A&A...245..203T). These variations of abundances can not be explained by the first dredge-up model but require additional processing by the CN cycle and mixing after the first dredge-up. However, there is no obvious evidence of other processes such as the 3{alpha}-process and subsequent hot bottom burning in our program stars. Such variation of the carbon and nitrogen abundances is not well understood by the present evolutionary models of low-mass and intermediate-mass stars.
- ID:
- ivo://CDS.VizieR/J/ApJ/777/82
- Title:
- Ks luminosity of AGB stars in LMC clusters
- Short Name:
- J/ApJ/777/82
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a study of K_s_-band luminosity evolution of the asymptotic giant branch (AGB) population in simple stellar systems using star clusters in the Large Magellanic Cloud (LMC). We determine physical parameters of LMC star clusters including center coordinates, radii, and foreground reddenings. Ages of 83 star clusters are derived from isochrone fitting with the Padova models, and those of 19 star clusters are taken from the literature. The AGB stars in 102 star clusters with log(age)=7.3-9.5 are selected using near-infrared color-magnitude diagrams based on Two Micron All Sky Survey photometry. Then we obtain the K_s_-band luminosity fraction of AGB stars in these star clusters as a function of ages. The K_s_-band luminosity fraction of AGB stars increases, on average, as age increases from log(age)~8.0, reaching a maximum at log(age)~8.5, and it decreases thereafter. There is a large scatter in the AGB luminosity fraction for given ages, which is mainly due to stochastic effects. We discuss this result in comparison with five simple stellar population models. The maximum K_s_-band AGB luminosity fraction for bright clusters is reproduced by the models that expect the value of 0.7-0.8 at log(age)=8.5-8.7. We discuss the implication of our results with regard to the study of size and mass evolution of galaxies.
