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Register a new userThe Age and Metallicity Dependence of the Near-Infrared Magnitudes of Red Clump Stars
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Hiroki Onozato
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Red clump (RC) stars are widely used as an excellent standard candle. To make them even better, it is important to know the dependence of their absolute magnitudes on age and metallicity. We observed star clusters in the Large Magellanic Cloud to fill age and metallicity parameter space, which previous work has not observationally studied. We obtained the empirical relations of the age and metallicity dependence of absolute magnitudes $M_{J}$, $M_{H}$, and $M_{K_{S}}$, and colours $J - H$, $J - K_{S}$, and $H - K_{S}$ of RC stars, although the coefficients have large errors. Mean near-infrared magnitudes of the RC stars in the clusters show relatively strong dependence on age for young RC stars. The $J - K_{S}$ and $H - K_{S}$ colours show the nearly constant values of $0.528 pm 0.015$ and $0.047 pm 0.011$, respectively, at least within the ages of 1.1--3.2 Gyr and [Fe/H] of $-0.90$ to $-0.40$ dex. We also confirmed that the population effects of observational data are in good agreement with the model prediction.
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The structural parameters, like the inclination, i and the position angle of the line of nodes (PA_lon) of the disk of the Large Magellanic Cloud (LMC) are estimated using the JH photometric data of red clump stars from the Infrared Survey Facility - Magellanic Cloud Point Source Catalog (IRSF-MCPSC). The observed LMC region is divided into several sub-regions and stars in each region are cross identified with the optically identified red clump stars to obtain the near infrared magnitudes. The peak values of H magnitude and (J-H) colour of the observed red clump distribution are obtained by fitting a profile to the distributions and also by taking the average value of magnitude and colour of the red clump stars in the bin with largest number. Then the dereddened peak H0 magnitude of the red clump stars in each sub-region is obtained. The RA, Dec and relative distance from the center of each sub-region are converted into x, y & z Cartesian coordinates. A weighted least square plane fitting method is applied to this x,y,z data to estimate the structural parameters of the LMC disk. A reddening map based on (J-H) colour of the RC stars is presented. When the peaks of the red clump distribution were identified by averaging, an inclination of 25.7 +/- 1.6 and PA_lon = 141.5 +/- 4.5 were obtained. We estimate a distance modulus of 18.47 +/- 0.1 mag to the LMC. Extra-planar features which are in front as well as behind the fitted plane are identified which match with the optically identified extra-planar features. The bar of the LMC is found to be part of the disk within 500 pc. The estimates of the structural parameters are found to be independent of the photometric bands used for the analysis. We find that the inner disk, within 3.0, is less inclined and has larger value of PA_lon when compared to the outer disk.
We present measurements of the V and I band magnitudes of red clump stars in 15 nearby galaxies obtained from recently published homogenous HST photometry. Supplementing these results with similar data for another 8 galaxies available in the literature the populational effects on the V and I band magnitudes of red clump stars were investigated. Comparing red clump magnitudes with the I-band magnitude of the TRGB in a total sample of 23 galaxies possessing very different environments we demonstrate that population effects strongly affect both the V and I band magnitude of red clump stars in a complex way. Our empirical results basically confirm the theoretical results of Girardi and Salaris, and show that optical (VI) photometry of red clump stars is not an accurate method for the determination of distances to nearby galaxies at the present moment, as long as the population effects are not better calibrated, both empirically and theoretically. Near infrared photometry is a much better way to measure galaxy distances with red clump stars given its smaller sensitivity to population effects.
Although core helium-burning red clump (RC) stars are faint at ultraviolet wavelengths, their ultraviolet-optical color is a unique and accessible probe of their physical properties. Using data from the GALEX All Sky Imaging Survey, Gaia Data Release 2 and the SDSS APOGEE DR14 survey, we find that spectroscopic metallicity is strongly correlated with the location of an RC star in the UV-optical color magnitude diagram. The RC has a wide spread in (NUV - G)$_0$ color, over 4 magnitudes, compared to a 0.7-magnitude range in (G$_{BP}$ - G$_{RP}$)$_0$. We propose a photometric, dust-corrected, ultraviolet-optical (NUV - G)$_0$ color-metallicity [Fe/H] relation using a sample of 5,175 RC stars from APOGEE. We show that this relation has a scatter of 0.28 dex and is easier to obtain for large, wide-field samples than spectroscopic metallicities. Importantly, the effect may be comparable to the spread in RC color attributed to extinction in other studies.
The zero point of the reddening toward the Large Magellanic Cloud (LMC) has been the subject of some dispute. Its uncertainty propagates as a systematic error for methods which measure the extragalactic distance scale through knowledge of the absolute extinction of LMC stars. In an effort to resolve this issue, we used three different methods to calibrate the most widely-used metric to predict LMC extinction, the intrinsic color of the red clump, $(V-I)_{RC,0}$, for the inner $sim$3 degrees of that galaxy. The first approach was to empirically calibrate the color zeropoints of the BaSTI isochrones over a wide metallicity range of ${Delta}rm{[Fe/H]} approx 1.10$ using measurements of red clump stars in 47 Tuc, the Solar Neighborhood, and NGC 6791. From these efforts we also measure these properties of the Solar Neighborhood red clump, ($V-I$, $G_{BP}-K_{s}$, $G-K_{s}$, $G_{RP}-K_{s}$, $J-K_{s}$, $H-K_{s}$, $M_{I}$, $M_{Ks}$)$_{RC,0} =$ (1.02, 2.75, 2.18, 1.52, 0.64, 0.15, $-$0.23, $-$1.63). The second and third methods were to compare the observed colors of the red clump to those of Cepheids and RR Lyrae in the LMC. With these three methods, we estimated the intrinsic color of the red clump of the LMC to be $(V-I)_{RC,0,rm{LMC}} = { approx 0.93,0.91 pm 0.02,0.89 pm 0.02}$ respectively, and similarly using the first and third method we estimated $ (V-I)_{RC,0,rm{SMC}} = {approx 0.85,0.84 pm 0.02 }$ respectively for the Small Magellanic Cloud. We estimate the luminosities to be $M_{I,RC,rm{LMC}}=-0.26$ and $M_{I,RC,rm{SMC}}=-0.37$. We show that this has important implications for recent calibrations of the tip of the red giant branch in the Magellanic Clouds used to measure $H_0$.
Red clump stars are fundamental distance indicators in astrophysics, although theoretical stellar models predict a dependence of absolute magnitudes with ages. This effect is particularly strong below 2 Gyr, but even above this limit a mild age dependence is still expected. We use seismically identified red clump stars in the Kepler field for which we have reliable distances, masses and ages from the SAGA survey to first explore this effect. By excluding red clump stars with masses larger than 1.6 Msun (corresponding to ages younger than 2 Gyr), we derive robust calibrations linking intrinsic colors to absolute magnitudes in the following photometric systems: Stromgren $by$, Johnson $BV$, Sloan $griz$, 2MASS $JHK_s$ and WISE $W1W2W3$. With the precision achieved we also detect a slope of absolute magnitudes 0.020(0.003) mag per Gyrin the infrared, implying that distance calibrations of clump stars can be off by up to 0.2 mag in the infrared (over the range from 2 Gyr to 12 Gyr) if their ages are unknown. Even larger uncertainties affect optical bands, because of the stronger interdependency of absolute magnitudes on colors and age. Our distance calibrations are ultimately based on asteroseismology, and we show how the distance scale can be used to test the accuracy of seismic scaling relations. Within the uncertainties our calibrations are in agreement with those built upon local red clump with Hipparcos} parallaxes, although we find a tension which if confirmed would imply that scaling relations overestimate radii of red clump stars by 2(+-20%. Data-releases post Gaia DR1 will provide an important testbed for our results.
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