An asteroseismic membership study of the red giants in three open clusters observed by Kepler: NGC6791, NGC6819, and NGC6811

The Astrophysical Journal (Impact Factor: 5.99). 07/2011; 739(1). DOI: 10.1088/0004-637X/739/1/13
Source: arXiv


Studying star clusters offers significant advances in stellar astrophysics
due to the combined power of having many stars with essentially the same
distance, age, and initial composition. This makes clusters excellent test
benches for verification of stellar evolution theory. To fully exploit this
potential, it is vital that the star sample is uncontaminated by stars that are
not members of the cluster. Techniques for determining cluster membership
therefore play a key role in the investigation of clusters. We present results
on three clusters in the Kepler field of view based on a newly established
technique that uses asteroseismology to identify fore- or background stars in
the field, which demonstrates advantages over classical methods such as
kinematic and photometry measurements. Four previously identified seismic
non-members in NGC6819 are confirmed in this study, and three additional
non-members are found -- two in NGC6819 and one in NGC6791. We further
highlight which stars are, or might be, affected by blending, which needs to be
taken into account when analysing these Kepler data.

Download full-text


Available from: S. Hekker, Oct 08, 2015
29 Reads
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The frequency of maximum oscillation power measured in dwarfs and giants exhibiting solar-like pulsations provides a precise, and potentially accurate, inference of the stellar surface gravity. An extensive comparison for about 40 well-studied pulsating stars with gravities derived using classical methods (ionisation balance, pressure-sensitive spectral features or location with respect to evolutionary tracks) supports the validity of this technique and reveals an overall remarkable agreement with mean differences not exceeding 0.05 dex (although with a dispersion of up to ~0.2 dex). It is argued that interpolation in theoretical isochrones may be the most precise way of estimating the gravity by traditional means in nearby dwarfs. Attention is drawn to the usefulness of seismic targets as benchmarks in the context of large-scale surveys.
    Monthly Notices of the Royal Astronomical Society 10/2011; 419(1). DOI:10.1111/j.1745-3933.2011.01172.x · 5.11 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Mass-loss of red giant branch (RGB) stars is still poorly determined, despite its crucial role in the chemical enrichment of galaxies. Thanks to the recent detection of solar-like oscillations in G–K giants in open clusters with Kepler, we can now directly determine stellar masses for a statistically significant sample of stars in the old open clusters NGC 6791 and 6819. The aim of this work is to constrain the integrated RGB mass-loss by comparing the average mass of stars in the red clump (RC) with that of stars in the low-luminosity portion of the RGB [i.e. stars with L≲L(RC)]. Stellar masses were determined by combining the available seismic parameters νmax and Δν with additional photometric constraints and with independent distance estimates. We measured the masses of 40 stars on the RGB and 19 in the RC of the old metal-rich cluster NGC 6791. We find that the difference between the average mass of RGB and RC stars is small, but significant [ (random) ±0.04 (systematic) M⊙]. Interestingly, such a small does not support scenarios of an extreme mass-loss for this metal-rich cluster. If we describe the mass-loss rate with Reimers prescription, a first comparison with isochrones suggests that the observed is compatible with a mass-loss efficiency parameter in the range 0.1 ≲η≲ 0.3. Less stringent constraints on the RGB mass-loss rate are set by the analysis of the ∼2 Gyr old NGC 6819, largely due to the lower mass-loss expected for this cluster, and to the lack of an independent and accurate distance determination. In the near future, additional constraints from frequencies of individual pulsation modes and spectroscopic effective temperatures will allow further stringent tests of the Δν and νmax scaling relations, which provide a novel, and potentially very accurate, means of determining stellar radii and masses.
    Monthly Notices of the Royal Astronomical Society 01/2012; 419(3):2077 - 2088. DOI:10.1111/j.1365-2966.2011.19859.x · 5.11 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Kepler ultra-high precision photometry of long and continuous observations provide a unique dataset in which surface rotation and variability can be studied for thousands of stars. Because many of these old field stars also have independently measured asteroseismic ages, measurements of rotation and activity are particularly interesting in the context of age-rotation-activity relations. These relations generally lack good calibrators at old ages, a problem that this Kepler sample of stars is uniquely suited to address. We study the surface rotation and the photometric magnetic activity of a subset of 540 solar-like stars on the main-sequence and the subgiant branch for which stellar pulsations have been measured. The rotation period is determined by comparing the results from two different sets of calibrated data and from two complementary analyses. Global photometric levels of magnetic activity in this sample of stars are also extracted by using a photometric activity index, which takes into account the rotation period of the stars. Out of the 540 solar-like pulsating stars in our sample, we successfully measured the rotation period of 310 stars (excluding known binaries and candidate planet host stars). The rotation periods lay between 1 and 100 days. The remaining stars are classified into two categories: those not showing any surface rotation (6 stars), and those in which the four analyses did not converge to a single and robust rotation period (213). The photometric magnetic activity levels were computed and for 61.5% of the dwarfs, its value is comparable to the solar one. We then extract an age-rotation relation only for the dwarfs with very precise asteroseismic age estimations, highlighting the necessity of excluding the hot stars and the subgiants when inferring such relations. We also studied age-activity-rotation relations with a hint of correlation for the subgiants.