Scientific Interest:

Cepheids, Extragalactic Distance Scale and Cosmology, Stellar Pulsations & Astro-Statistics

Past Projects (include Thesis rojects):

I: Cepheid Light Curve Reconstruction with Fourier Techniques.
--- Using the Fourier techniques, the Fourier expansion and interrelations, to reconstruct the light curves for sparsely sampled HST Cepheid data ---
Fourier expansion is a well established technique used in stellar pulsation. To reconstruct the Cepheid light curves, we use simulated annealing techniques to fit the data with Fourier expansion, and restricting the ranges of Fourier amplitudes can take. These methods can improve the quality of the light curves. For HST observation of extra-galactic Cepheids, which 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 method mentioned above and explicitly derive and use Fourier interrelations (linear relations connecting the Fourier parameters in V and I bands) to reconstruct the I band light curve. Error analysis of these light curve reconstruction methods shows the errors of the Fourier amplitudes are around ~0.03mag. Application of these methods to reconstruct the light curves of Cepheids observed in NGC 4258 shows that the derived Cepheid distance (mu_0=29.38+-0.06mag., random error) is consistent with its geometrical distance (mu_0=29.28+-0.09mag.) derived from observations of water maser.

II: Cepheid Distance to NGC 1637.
--- Determine the Cepheid distance to NGC 1637 and compared to EPM (Expanding Photosphere Method) distance measured from a Type II supernova, SN 1999em ---
NGC1637 is a nearby, barred spiral galaxy. Recently, the explosion of supernova 1999em (Li, 1999) in NGC1637 allowed the measurement of its distance indenpendent of the distance ladder. Since this supernova is shown to be a Type II-P event, the distance to the supernova is measured via the Expanding Photosphere Method (EPM), and yielded D = 8.2+-0.6 Mpc (Leonard et al, 2002). This distance is consistent with the distance measured by using the red supergiant stars, as D = 7.8 +1.0-0.9 Mpc (Sohn \& Davidge, 1998). Because NGC1637 is a spiral galaxy, it should host some Cepheids in the disk. However, there is no Cepheids been reported for this galaxy, and hence no Cephied distance is available. Hence, it is desiable to derive the distance to this galaxy and compared to the distance from other estimators. In addtion, based on the distance measurements from Cepheid and EPM to 5 galaxies that host Type II supernovae, Leonard et al (2002) found that DCeph/DEPM=0.96+-0.09. Therefore, the measurement of Cepheid distance to NGC1637 could enhance the statstics of the distance ratio between Cepheid distance and EPM (or Type II supernova) distance. The recent HST observations of NGC1637 has completed, and the data set contains 12 V band epochs and 6 I band epochs. This data set is currently being processed by Leonard. Once it finishes, the data set will be used to search and identify the Cepheids. Finally, the Cepheid distance can be derived using the observed Cepheids in this galaxy.

III: Extragalactic Distance with LMC and Galactic PL relations.
--- Using the LMC and Galactic period-luminosity (PL) relations to obtain the Cepheid distance to about two dozen galaxies ---
In this paper, we recalibrate the Cepheid distance to some nearby galaxies observed by the HST Key Project and the Sandage-Tammann-Saha group. We use much of the Key Project methodology in our analysis but apply new techniques, based on Fourier methods to estimate the mean of a sparsely sampled Cepheid light curve, to published extra-galactic 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 Key Project galaxies after accounting for charge transfer effects, 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. For NGC 4258 which has a geometric distance of 29.28mag., we find a distance modulus of 29.44+-0.06(random)mag., after correcting for metallicity. In addition we have calculated the Cepheid distance to 8 galaxies observed by the Sandage-Tammann-Saha group and find shorter distance moduli by -0.178mag. (mainly due to the use of different LMC PL relations) and -0.108mag. on average again when using the LMC and Galaxy, respectively, as a calibrating PL relation. However care must be taken to extrapolate these changed distances to changes in the resulting values of the Hubble constant because STS also use distances to NGC 3368 and 4414 and because STS calibration of SN Ia is often decoupled from the distance to the host galaxy through their use of differential extinction arguments. We also calculate the distance to all 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 and constrains the sign for the coefficient of the metallicity correction, gamma, to be negative, at least at the median period log(P)~1.4, of the target galaxies.

My PhD Thesis project:

Title of the thesis: Investigating the Break of Cepheid Period-Luminosity Relations and its Implications.

--- Purpose: The aim of this thesis is to investigate the observed breaks in the Cepheid period-luminosity (PL) relations seen in the OGLE LMC Cepheids and its implications for stellar structure, evolution, and cosmology. The work involved in this project includes data collecting and analysis to further characterize the breaks, and pulsation modeling to understand the physical reasons behind such breaks in Cepheid PL relations.

Sub-project:

A. Period-Luminosity Relations for Galactic Cepheids with independent distance measurements.

--- Description: We compile a list of Galactic Cepheids with independent distace measurements from literature and obtain the Galactic PL relations.

B. Period-Color and Amplitude-Color Relations in Cepheids.

--- Description: Statistical (F test) study of the period-color and amplitude-color relations for short and long period Cepheids, at maximum, mean and minimum light for the Cepheids in Galaxy, LMC and SMC.

C. Modelling Period-Color and Amplitude-Color Relations for Galactic Cepheids.

--- Description: Using the Florida pulsation code (include convection) to model the interaction of hydrogen ionization front with photosphere, and compare the resulted PC and AC relations with observations for the Galactic Cepheids.

E. Modelling Period-Color and Amplitude-Color Relations for LMC Cepheids.

--- Description: Using the Florida pulsation code (include convection) to model the interaction of hydrogen ionization front with photosphere, and compare the resulted PC and AC relations with observations for the LMC Cepheids.

Publications (until mid-2005)

1. Reconstructing Cepheid Light Curves with Fourier Techniques I: Fourier Expansion and Interrelations, by Ngeow, C. C., Kanbur, S. M., Nikolaev, S., Tanvir, N. & Hendry, M., 2003, ApJ 586:959. astro-ph/0212106, Click here for astro-ph preprint.
2. Reconstructing Cepheid Light Curve with Fourier Expansion and Interrelations, by Ngeow, C. C., Kanbur, S. M., Nikolaev, S., Tanvir, N. & Hendry, M., 2003, in "Focus on Astrophysics Research", Ed. Ross, Nova Science Publisher.
3. The Cepheid Distance to NGC 1637: A Direct Comparison with the EPM Distance to SN 1999em, by Leonard, D., Kanbur, S., Ngeow, C. C & Tanvir, N., 2003, ApJ 594:247. astro-ph/0305259, Click here for astro-ph preprint.
4. Extra-Galactic Cepheid Distance from Large Magellanic Cloud and Galactic Period-Luminosity Relations, by Kanbur, S. M., Ngeow, C. C, Nikolaev, S., Tanvir, N. & Hendry, M., 2003, A&A 411:361. astro-ph/0309235 Click here for astro-ph preprint. VizieR On-Line Catalogue: J/A+A/411/361
5. Period-Luminosity Relations for Galactic Cepheid Variables with Independent Distance Measurements, by Ngeow, C. C. & Kanbur, S., 2004, MNRAS 349:1130. astro-ph/0401038 Click here for astro-ph preprint. (Erratum: 2004, MNRAS, 351, 768)
6. Period-Color and Amplitude-Color Relations for Classical Cepheid Variables, by Kanbur, S. & Ngeow, C. C. , 2004, MNRAS 350:962. astro-ph/0402145 Click here for astro-ph preprint.
7. Period-Color and Amplitude-Color Relations for Classical Cepheid Variables II: the Galactic Cepheid models, by Kanbur, S., Ngeow, C. C. & Buchler, J. R., 2004, MNRAS 354:212. astro-ph/0404578 Click here for astro-ph preprint.
8. Cepheid Period-Luminosity Relations: Galactic vs. LMC and the Results from t-Test, by Ngeow, C. C. & Kanbur, S., to be published in "GAIA: The Three Dimensional Universe with GAIA", ESA. Click here for astro-ph preprint.
9. The Physics Behind the Non-Universality of the Cepheid Period-Luminosity Relation, by Kanbur, S. M. & Ngeow, C. C., to be published in "GAIA: The Three Dimensional Universe with GAIA", ESA.
10. The Linearity of the Wesenheit Function for the Large Magellanic Cloud Cepheids, by Ngeow, C. C. & Kanbur, S. M., 2005, MNRAS submitted.

Conference Presentations:

1. Extragalactic Cepheid Distance with Period-Luminosity (Max) Relation, by Kanbur, S. M., Ngeow, C. C. & Nikolaev, S., Conference for "New Era in Cosmology", Durham UK, September 2001 (Poster presentation). [Note: absent at the conference]
2. Reconstructing Cepheid Light Curves with Fourier Techniques, by Ngeow, C. C., Kanbur, S. M. & Nikolaev, S., American Astronomical Society Meeting 200, #07.05, Alberquerque NM, June 2002 (Poster presantation). Click here for NASA ADS abstract.
3. The Distance to NGC 1637: Comparison of Cepheid Distance with the EPM Distance to SN 1999em, by Leonard, D., Kanbur, S., Tanvir, N. & Ngeow, C. C, Workshop for "Standard Candles for the Extra-galactic Distance Scale", Concepcion Chile, December 2002 (Oral presentation).
4. Is There a Difference Between Short and Long Period Cepheids?, by Kanbur, S. & Ngeow, C. C, Workshop for "Standard Candles for the Extra-galactic Distance Scale", Concepcion Chile, December 2002 (Poster presentation).
5. The Cepheid Distance to NGC 1637: A Direct Comparison with the EPM Distance to SN 1999em, by Leonard, D., Kanbur, S., Ngeow, C. C & Tanvir, N., American Astronomical Society Meeting 201, #23.03, Seattle WA, January 2003 (Poster presantation). Click here for NASA ADS abstract. [Note: absent at the conference]
6. The Physics Behind the Non-Universality of the Cepheid Period-Luminosity Relation, by Kanbur, S. M. & Ngeow, C. C., Symposium for "The Three Dimensional Universe with GAIA", Meudon, France, October 2004 (Poster presentation).
7. Cepheid Period-Luminosity Relations: Galactic vs. LMC and the Results from t-Test, by Ngeow, C. C. & Kanbur, S., Symposium for "The Three Dimensional Universe with GAIA", Meudon, France, October 2004 (Poster presentation).
8. Investigating the Break of Cepheid Period-Luminosity Relations and its Implications, by Ngeow, C. C. & Kanbur, S., American Astronomical Society Meeting 205, #34.02D, San Diego CA, January 2005 (Oral presantation). Click here for NASA ADS abstract.

1. Recent Developments in Cepheid Period-Luminosity Relations, Departmental lunch talk, U. of Massachusetts, March 2004. Click here for the viewgraphs.
2. The break of the Cepheid Period-Luminosity Relation and the Possible Interpretation, Departmental lunch talk, U. of Illinois (Urbana-Champaign), December 2004. Click here for the viewgraphs.

Computer Codes:

Some c++ programs I wrote in my research (not complete):

FITLINE_SIMPLE.cxx - A simple program to a fit straight line to data and find the corresponding slope and zero-point.
INTENSITY_MEAN.cxx - Calculate the intensity mean and phase-weighted intensity mean for the Cepheid observations.
FTEST.cxx - F-test for two regressions vs one regression.

Useful Links:

A. Papers / Pre-prints:

1. Astro-ph.

2. NASA ADS.

1. DDO Galactic data.

2. McMaster database.

3. OGLE database.

4. EROS database.

5. DIRECT database.

6. HST Ho Key Project database.

7. Fourier Coefficient.

8. Faint Cepheid Programe.

9. Variables in Globular Clusters.

10. Harvard Variables.

11. Araucaria Project.

12. MACHO Data.

13. WeCAPP.

C. Others:

1. Simbad Astronomical Database.

2. Astronomy resources.

3. Astronomy job rumor mill.

4. Bayesian Reprints.

5. LMC extintion tool.

6. All-Sky Automated Survey.

7. Padova Evolutionary Tracks.

8. BaSeL Atmosphere.

9. Kurucz Models.

10. F calculator.

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