Research ArticleASTROPHYSICS

Precise stellar surface gravities from the time scales of convectively driven brightness variations

See allHide authors and affiliations

Science Advances  01 Jan 2016:
Vol. 2, no. 1, e1500654
DOI: 10.1126/sciadv.1500654
  • Fig. 1 Examples of the Kepler light curves (top) of stars at different evolutionary stages (from left to right: upper main sequence, lower giant branch, and upper giant branch) and the corresponding power density spectra (bottom).

    (Top) Gray symbols represent relative intensity measurements as a function of time, and the red curves represent boxcar smoothings of the data. (Insets) Shorter segments of the light curves (as solid points) after high-pass filtering, where the black curves represent three-point running means and the spacings of the vertical red lines represent the dominant variability time scales measured from the ACF. (Bottom) The gray and black curves represent the power density spectra of the original and filtered data, respectively. The solid colored curves represent global model fits to the original spectra, and the dashed curves represent the components of these models (3). A long arrow points to the typical ACF frequency (νACF = τACF−1) in each panel. Each inset shows the squared ACF of the filtered time series (solid points) along with the fit (red curve) from which we estimate the typical ACF time scale τACF (vertical dashed line).

  • Fig. 2 Relation between asteroseismically determined surface gravity (log g) and ACF time scale (τACF) for a sample of Kepler light curves.

    Red dots represent long-cadence data (sampled every 29.4 min) for ~1200 red giants, whereas blue dots represent short-cadence data (59-s sampling) for 75 main sequence stars and subgiant stars. The solid black curve represents a second-order polynomial fit, deviating from linearity (dashed curve) because of a weak correlation between surface gravity and surface temperature and the fact that stars cool as they evolve up the giant branch. Error boxes represent typical uncertainties of g measured by photometric colors [ph (25)] ~100%; spectroscopy [sp (26)] ~50%; flicker method [fl (10)] ~25%; time scale technique (ts) ~4%; asteroseismology [as (8)] ~2%. (Top right inset) The same relation for stars observed with other instruments (Supplementary Materials). Cnc, Cancri; Cyg, Cygnus; Oph, Ophiuchi. (Bottom left inset) The correlation improves for main sequence stars when correcting for the temperature trend with log g. Empty circles represent original measurements of τACF, whereas filled circles represent τACF(Teff/TSun)½, where TSun = 5777 K. For more details on the insets, see Supplementary Materials.

  • Fig. 3 Example of a “noisy” star.

    Same as in the bottom panels of Fig. 1 but for a star with significantly more photon noise. The gray and black curves represent the original and heavily smoothed power density spectra of the ~1300-day-long Kepler photometry of the planet-hosting star Kepler 22. The red horizontal bar indicates the expected position of oscillation power excess. (Inset) The ACF computed from the time series (where the symbols are the same as in Fig. 1).

  • Fig. 4 Example of an active star.

    Same as in Fig. 1 but for a star with a strong rotation and/or magnetic activity signal. (Top) A 200-day-long subset of the Kepler short-cadence photometry showing a strong quasi-periodic signal with a time scale of about 4 days. (Bottom) Original spectrum (light gray) of the full (~1030 days) Kepler time series. The dark-gray and black curves show heavily smoothed versions of the original spectrum before and after filtering, respectively. The solid colored curves represent global model fits to the original spectrum. The 8-hour flicker amplitude roughly measures the signal between the vertical dashed lines, indicating the 8-hour filter cut (left) and the long-cadence Nyquist frequency (right). (Inset) The ACF computed from the time series (where the symbols are the same as in Figs. 1 and 3).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/1/e1500654/DC1

    Materials and Methods

    Fig. S1. Improving filter frequency.

    Fig. S2. Normalized squared ACFs for about 250 stars (gray dots) in the sample.

    Fig. S3. Robustness of the ACF time scale (τACF) with respect to filter frequency (νfilter).

    Fig. S4. Relation between asteroseismic surface gravity and ACF time scale, including stars with suppressed dipole modes (black circles).

    Fig. S5. Performance of the time scale method with different lengths of and noise levels in time series.

    Table S1. Reference (log gref) and time scale technique gravities (log gACF) for our independent calibration sample.

    References (2739)

  • Supplementary Materials

    This PDF file includes:

    • Materials and Methods
    • Fig. S1. Improving filter frequency.
    • Fig. S2. Normalized squared ACFs for about 250 stars (gray dots) in the sample.
    • Fig. S3. Robustness of the ACF time scale (τACF) with respect to filter frequency (νfilter).
    • Fig. S4. Relation between asteroseismic surface gravity and ACF time scale, including stars with suppressed dipole modes (black circles).
    • Fig. S5. Performance of the time scale method with different lengths of and noise levels in time series.
    • Table S1. Reference (log gref) and time scale technique gravities (log gACF) for our independent calibration sample.
    • References (2739)

    Download PDF

    Files in this Data Supplement: