arXiv subjects

C. Ngeow

Publications and source records attributed to C. Ngeow.

23 records · Page 2Linked to original sources

arXiv2005

Further empirical evidence for the non-linearity of the period-luminosity relations as seen in the Large Magellanic Cloud Cepheids

(abridged) Recent studies, using OGLE data for LMC Cepheids in the optical, strongly suggest that the period-luminosity (PL) relation for the Large Magellanic Cloud (LMC) Cepheids shows a break or non-linearity at a period of 10 days. In this paper we apply statistical tests, the chi-square test and the F-test, to the Cepheid data from the MACHO project to test for a non-linearity of the V- and R-band PL relations at 10 days, and extend these tests to the near infrared (JHK-band) PL relations with 2MASS data. We correct the extinction for these data by applying an extinction map towards the LMC. The statistical test we use, the F-test, is able to take account of small numbers of data points and the nature of that data on either side of the period cut at 10 days. With our data, the results we obtained imply that the VRJH-band PL relations are non-linear around a period of 10 days, while the K-band PL relation is (marginally) consistent with a single-line regression. The choice of a period of 10 days, around which this non-linearity occurs, is consistent with the results obtained when this "break" period is estimated from the data. Long period Cepheids are supplemented from the literature to increase our sample size. The photometry of these long period Cepheids is compared with our data and no trend with period is found. Our main results remain unchanged when we supplement our dataset with these long period Cepheids. By examining our data at maximum light, we also suggest arguments why errors in reddening are unlikely to be responsible for our results. The non-linearity of the mean V-band PL relation as seen in both of the OGLE and MACHO data, using different extinction maps, suggests that this non-linearity is real.

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arXiv2004

Cepheid Period-Luminosity Relations: Galactic vs. LMC and the Results from t-Test

The period-luminosity (PL) relation for Galactic Cepheids is derived with recent independent distance measurements taken from literature. Our PL relation confirms the work 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. The details of this work can be found in Ngeow & Kanbur (2004).

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arXiv2004

Period-colour and amplitude-colour relations in classical Cepheid variables II: the Galactic Cepheid model

In this paper, we construct full amplitude non-linear hydrodynamical models of fundamental mode Galactic Cepheids and analyze the resulting theoretical period-colour and amplitude-colour relations at maximum, mean and minimum light. These theoretical relations match the general form of the observed relations well. This agreement is, to some extent, independent of the mass-luminosity relations used, pulsation code, numerical techniques, details of the input physics and methods to convert theoretical quantities, such as bolometric luminosity and temperature, to observational quantities, such as V band magnitudes or $(V-I)$ colours. We show that the period-colour and amplitude-colour properties of fundamental mode Galactic Cepheids with periods such that $\log (P)>0.8$ can be explained by a simple application of the Stefan-Boltzmann law and the interaction of the photosphere with the hydrogen ionization front. We discuss the implications of our results for explaining the behavior of Galactic Cepheid period-colour, and period-luminosity relations at mean light.

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arXiv2004

Period-color and amplitude-color relations in classical Cepheid variables

In this paper we analyze the behavior of Galactic, LMC and SMC Cepheids in terms of period-color (PC) and amplitude-color (AC) diagrams at the phases of maximum, mean and minimum light. We find very different behavior between Galactic and Magellanic Cloud Cepheids. Motivated by the recent report of a break in LMC PC relations at 10 days (Tammann et al. 2002), we use the F-statistical test to examine the PC relations at mean light in these three galaxies. The results of the F-test support the existence of the a break in the LMC PC(mean) relation, but not in the Galactic or SMC PC(mean) relations. Furthermore, the LMC Cepheids also show a break at minimum light, which is not seen in the Galactic and SMC Cepheids. We further discuss the effect on the period-luminosity relations in the LMC due to the break in the PC(mean) relation.

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arXiv2003

The extra-galactic Cepheid distance scale from LMC and Galactic period-luminosity relations

In this paper, we recalibrate the Cepheid distance to some nearby galaxies observed by the HST Key Project (KP) and the Sandage-Tammann-Saha (STS) group. We use much of the KP methodology in our analysis but apply new techniques, based on Fourier methods to estimate the mean magnitudes to published Cepheid data. We also apply different calibrating PL relations to estimate Cepheid distances, and investigate the sensitivity of the distance moduli to the adopted calibrating PL relation. We re-determine the OGLE LMC PL relations using a more conservative approach and also study the effect of using Galactic PL relations on the distance scale. For the KP galaxies, we find good agreement with an average discrepancy of -0.002 and 0.075mag. when using the LMC and Galaxy, respectively, as a calibrating PL relation. However, the Cepheid distance to STS galaxies are shorter by -0.178mag. and -0.108mag. on average again when using the LMC and Galaxy PL relations, respectively. We also calculate the distance to these galaxies using PL relations at maximum light and find very good agreement with mean light PL distances. However, after correcting for metallicity effects, the difference between the distance moduli obtained using the two sets of calibrating PL relations becomes negligible. This suggests that Cepheids in the LMC and Galaxy do follow different PL relations, at least at the median periods of the target galaxies.

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