Red Clump Stars as Distance Indicator

See our press release for the 195th Meeting of theAmerican Astronomical Society (Poster #11.04).

Stellar cluster NGC 416 located in the nearby Small Magellanic Cloud galaxy. The cluster contains many red clump stars, allowing for accurate distance measurement to the host galaxy.This photograph was made with the 1.3-meter (51-inch) Warsaw University Telescope at Las Campanas Observatory, near La Sarena, Chile.


PHOTO CREDIT: Andrzej Udalski, Peter Garnavich and Krzysztof Stanek.

The accurate measurement of distances is one of the most basic goals of astronomers and one of the most difficult. The intrinsic brightness of pseudo-standard candles such as Type Ia supernovae is not known a priori, but must be calibrated from lower rungs on the distance ladder such as Cepheid variables which in turn must be calibrated by other means. Trigonometric parallax is the most direct method for estimating astronomical distances, but parallax is effective out to a very limited distance from the Sun. Even with the success of the Hipparcos satellite only a handful of Cepheids were calibrated with good enough precision and the base of the distance ladder remains unsteady.

The ideal distance indicator would be a standard candle abundant enough to provide many examples within reach of parallax measurements and sufficiently bright to be seen out to local group galaxies. As shown by Paczynski & Stanek (1998) and Stanek and Garnavich (1998), red clump stars (shown in the Hipparcos color-magnitude diagram above) precisely fit this description. These stars are the metal rich equivalent of the better known horizontal branch stars, and theoretical models predict that their absolute luminosity fairly weakly depends on their age and chemical composition. Indeed the absolute magnitude-color diagram of Hipparcos clearly shows how compact the red clump is. Stanek and Garnavich (1998) determined that the variance in the I-band is only about 0.15 mag (see the figure below).
In spite of their large number and sound theoretical understanding red clump stars have seldom been used as the distance indicators. However, recently Stanek (1995) and Stanek et al. (1994, 1997) used these stars to map the Galactic bar.

Paczynski & Stanek (1998) used the red clump stars observed by OGLE to obtain an estimate of the distance to the Galactic center. Stanek and Garnavich (1998) used the red clump stars observed by the HST in M31 to measure the distance to M31 in a single step (see the figure above). Udalski et al. (1998) have used the data collected by the OGLE collaboration to obtain distances to Large and Small Magellanic Clouds. Red clump giants can provide a very precise estimate of the distances to metal-rich Galactic globular clusters and nearby galaxies.

There have been many important papers discussing the red clump as a distance indicator. You can access them from the astro-ph preprint archive. You can also access here our AAS Meeting #193 poster (in PostScript or GIF format), which discusses red clump as distance indicator.

Some related links :
Bohdan Paczynski's page, which includes additional information about the red clump

- Distance determination to nearby galaxies

- Optical Gravitational Lensing Experiment

SINS - The Supernova INtensive Study

HST Key Project on the Extragalactic Distance Scale

Hipparcos Space Astrometry Mission

CASTLe - CfA-Arizona Space Telescope Lens Survey

This file is maintained by Krzysztof Z. Stanek (kstanek@cfa.harvard.edu). January 21st, 2000.