arXiv subjects

Chow-Choong Ngeow

Publications and source records attributed to Chow-Choong Ngeow.

111 records · Page 7Linked to original sources

arXiv2005

The Linearity of the Wesenheit function for the Large Magellanic Cloud Cepheids

There is strong evidence that the period-luminosity (PL) relation for the Large Magellanic Cloud (LMC) Cepheids shows a break at a period around 10 days. Since the LMC PL relation is extensively used in distance scale studies, the non-linearity of the LMC PL relation may affect the results based on this LMC calibrated relation. In this paper we show that this problem can be remedied by using the Wesenheit function in obtaining Cepheid distances. This is because the Wesenheit function is linear although recent data suggests that the PL and the period-colour (PC) relations that make up the Wesenheit function are not. We test the linearity of the Wesenheit function and find strong evidence that the LMC Wesenheit function is indeed linear. This is because the non-linearity of the PL and PC relations cancel out when the Wesenheit function is constructed. We discuss this result in the context of distance scale applications. We also compare the distance moduli obtained from mu_0=mu_V-R(mu_V-mu_I) (equivalent to Wesenheit functions) constructed with the linear and the broken LMC PL relations, and find that the typical difference in distance moduli is ~ +-0.03mag. Hence, the broken LMC PL relation does not seriously affect current distance scale applications. We also discuss the random error calculated with equation mu_0=mu_V-R(mu_V-mu_I), and show that there is a correlation term that exists from the calculation of the random error. The calculated random error will be larger if this correlation term is ignored.

astro-ph↗

arXiv2004

Period-luminosity relations for Galactic Cepheid variables with independent distance measurements

In this paper, we derive the period-luminosity (PL) relation for Galactic Cepheids with recent independent distance measurements from open cluster, Barnes-Evans surface brightness, interferometry and HST astrometry techniques. Our PL relation confirms the results of Tammann et al. (2003), which showed that the Galactic Cepheids follow a different PL relation to their LMC counterparts. Our results also show that the slope of the Galactic PL relation is inconsistent with the LMC slope with more than 95% confidence level. We apply this Galactic PL relation to find the distance to NGC 4258. Our result of mu_o=29.49\pm0.06(random error)mag. agrees at the ~1.4sigma level with the geometrical distance of μ_{geo}=29.28\pm0.15mag. from water maser measurements..

astro-ph↗

arXiv2002

Reconstructing Cepheid Light Curve with Fourier Techniques I: The Fourier Expansion and Interrelations

Fourier decomposition is a well established technique used in stellar pulsation. However the quality of reconstructed light curves using this method is reduced when the observed data have uneven phase coverage. We use simulated annealing techniques together with Fourier decomposition to improve the quality of the Fourier decomposition for many OGLE LMC fundamental mode Cepheids. This method restricts the range that Fourier amplitudes can take. The ranges are specified by well sampled Cepheids in the Galaxy and Magellanic Clouds. We also apply this method to reconstructing Cepheid light curves observed by the HST. These typically consist of 12 V band and 4 I band points. We employ a direct Fourier fit to the 12 V band points using the simulated annealing method mentioned above and explicitly derive and use Fourier interrelations to reconstruct the I band light curve. We discuss advantages and drawbacks of this method when applied to HST Cepheid data over existing template methods. Application of this method to reconstruct the light curves of Cepheids observed in NGC 4258 shows that the derived Cepheid distance ($μ_0=29.38\pm0.06$, random error) is consistent with its geometrical distance ($μ_0=29.28\pm0.09$) derived from observations of water maser.

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