arXiv subjects

Chow-Choong Ngeow

Publications and source records attributed to Chow-Choong Ngeow.

At least 109 records · Page 6Linked to original sources

arXiv2012

Period-Luminosity Relations for Cepheid Variables: From Mid-Infrared to Multi-Phase

This paper discusses two aspects of current research on the Cepheid period-luminosity (P-L) relation: the derivation of mid-infrared (MIR) P-L relations and the investigation of multi-phase P-L relations. The MIR P-L relations for Cepheids are important in the James Webb Space Telescope era for the distance scale issue, as the relations have potential to derive the Hubble constant within ~ 2% accuracy - a critical constraint in precision cosmology. Consequently, we have derived the MIR P-L relations for Cepheids in the Large and Small Magellanic Clouds, using archival data from Spitzer Space Telescope. We also compared currently empirical P-L relations for Cepheids in the Magellanic Clouds to the synthetic MIR P-L relations derived from pulsational models. For the study of multi-phase P-L relations, we present convincing evidence that the Cepheid P-L relations in the Magellanic Clouds are highly dynamic quantities that vary significantly when considered as a function of pulsational phase. We found that there is a difference in P-L relations as a function of phase between the Cepheids in each of the Clouds; the most likely cause for this is the metallicity difference between the two galaxies. We also investigated the dispersion of the multi-phase P-L relations, and found that the minimum dispersion do not differ significantly from the mean light P-L dispersion.

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arXiv2012

On the Application of Wesenheit Function in Deriving Distance to Galactic Cepheids

In this work, we explore the possibility of using the Wesenheit function to derive individual distances to Galactic Cepheids, as the dispersion of the reddening free Wesenheit function is smaller than the optical period-luminosity (P-L) relation. When compared to the distances from various methods, the averaged differences between our results and published distances range from -0.061 to 0.009, suggesting that the Wesenheit function can be used to derive individual Cepheid distances. We have also constructed Galactic P-L relations and selected Wesenheit functions based on the derived distances. A by-product from this work is the derivation of Large Magellanic Cloud distance modulus when calibrating the Wesenheit function. It is found to be 18.531 \pm 0.043 mag.

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arXiv2012

Constructing a Cepheid Period p-Factor Relation from Static Model Stellar Atmospheres

One of the largest uncertainties for using the Baade-Wesselink method to measure Cepheid distances is the value of the projection factor (p-factor). However, p-factors measured using the IRSB technique and from hydrodynamic models disagree. In this work, we compute spherically-symmetric static model stellar atmospheres and predict a period p-factor relation.

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arXiv2011

Period-Luminosity Relations for Small Magellanic Cloud Cepheid Based on AKARI Archival Data

In this work we matched the AKARI archival data to the Optical Gravitational Lensing Experiment-III (OGLE-III) catalog to derive the mid-infrared period luminosity (PL) relations for Small Magellanic Cloud (SMC) Cepheids. Mismatched AKARI sources were eliminated using random-phase colors obtained from the full I-band light curves from OGLE-III. It was possible to derive PL relations in the N3 and N4 bands only, although the S7, S11, L15, and L24 band data were also tested. Random-phase correction was included when deriving the PL relation in the N3 and N4 bands using the available time of observations from AKARI data. The final adopted PL relations were: N3 = -3.370 logP + 16.527 and N4 = -3.402 logP + 16.556. However, these PL relations may be biased due to the small number of Cepheids in the sample.

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arXiv2011

Recent Progress of Cepheid Research at National Central University: From Spitzer to Kepler

In this presentation I summarize recent work on Cepheid research carried out at the National Central University. The mid-infrared period-luminosity (P-L) relations for Cepheids are important in the James Webb Space Telescope era for distance scale work, as the relations have potential to derive the Hubble constant within ~2% accuracy - a critical constraint in the precision cosmology. Consequently, we have derived the mid-infrared P-L relations for Cepheids in the Large and Small Magellanic Clouds, using archival data from the Spitzer Space Telescope. Kepler Space Telescope is a NASA mission to search for Earth-size and larger planets around Sun-like stars, by observing continuously the stars in a dedicated patch of the sky. As a result, the almost un-interrupted observation is also used for stellar variability and asteroseismological study. However, Kepler observations are carried out with a single broad-band filter, hence ground-based follow-up observation needed to complement Kepler light curves to fully characterize the properties of the target stars. Here I present the ground-based optical follow-up observations for two Cepheid candidates located within the Kepler's field-of-view. Together with Kepler light curves, our ground-based data rule out V2279 Cyg being a Cepheid. Hence V1154 Cyg is the only Cepheid in the Kepler's field.

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arXiv2011

Period-Luminosity Relations For Magellanic Clouds Cepheids Based on OGLE-III Data: A Comparison

The period-luminosity (P-L) relation for Cepheid variables is important in modern astrophysics. In this work, we present the multi-band P-L relations derived from the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) Cepheids, based on the latest release of OGLE-III catalogs. In addition to the V I band mean magnitudes adopted from OGLE-III catalogs, we also cross-matched the LMC and SMC Cepheids to the 2MASS point source catalogs and publicly available Spitzer catalogs from SAGE program. Mean magnitudes for these Cepheids were corrected for extinction using available extinction maps. When comparing the P-L slopes, we found that the P-L slopes in these two galaxies are consistent with each others within ~2.5sigma level.

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arXiv2011

Statistical Tests for the Metallicity Dependency of the Synthetic Cepheid Period-Luminosity Relations in IRAC Bands

The mid-infrared (MIR) period-luminosity (P-L) relations for Cepheids will be important in the JWST era, as it holds the promise of deriving the Hubble constant within 2% accuracy. It is expected that the MIR P-L to be insensitive to metallicity. In this work, we test this assumption of metallicity independent of the IRAC band P-L relation by applying well-known statistical methods to the synthetic P-L slopes from a series of pulsating models with known metallicity. The statistical tests suggest that the P-L slopes in MIR are linearly depending on metallicity.

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arXiv2011

Theoretical Cepheid Period-Luminosity And Period-Color Relations In Spitzer IRAC Bands

In this paper the synthetic period-luminosity (P-L) relations in Spitzer's IRAC bands, based on a series of theoretical pulsation models with varying metal and helium abundance, were investigated. Selected sets of these synthetic P-L relations were compared to the empirical IRAC band P-L relations recently determined from Galactic and Magellanic Clouds Cepheids. For the Galactic case, synthetic P-L relations from model sets with (Y = 0.26, Z = 0.01), (Y = 0.26, Z = 0.02) and (Y = 0.28, Z = 0.02) agree with the empirical Galactic P-L relations derived from the Hubble Space Telescope parallaxes. For Magellanic Cloud Cepheids, the synthetic P-L relations from model sets with (Y = 0.25, Z = 0.008) agree with both of the empirical Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) P-L relations. Analysis of the synthetic P-L relations from all model sets suggested that the IRAC band P-L relations may not be independent of metallicity, as the P-L slopes and intercepts could be affected by the metallicity and/or helium abundance. We also derive the synthetic period color(P-C) relations in the IRAC bands. Non-vanishing synthetic P-C relations were found for certain combinations of IRAC band filters and metallicity. However, the synthetic P-C relations disagreed with the [3.6]-[8.0] P-C relation recently found for the Galactic Cepheids. The synthetic [3.6]-[4.5] P-C slope from (Y = 0.25, Z = 0.008) model set, on the other hand, is in excellent agreement to the empirical LMC P-C counterpart, if a period range of 1.0 < log(P) < 1.8 is adopted.

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arXiv2010

The Mid-Infrared Period-Luminosity Relations for the Small Magellanic Cloud Cepheids Derived from Spitzer Archival Data

In this paper we derive the Spitzer IRAC band period-luminosity (P-L) relations for the Small Magellanic Cloud (SMC) Cepheids, by matching the Spitzer archival SAGE-SMC data with the OGLE-III SMC Cepheids. We find that the 3.6micron and 4.5micron band P-L relations can be better described using two P-L relations with a break period at log(P)=0.4: this is consistent with similar results at optical wavelengths for SMC P-L relations. The 5.8micron and 8.0micron band P-L relations do not extend to sufficiently short periods to enable a similar detection of a slope change at log(P)=0.4. The slopes of the SMC P-L relations, for log(P)>0.4, are consistent with their LMC counterparts that were derived from a similar dataset. They are also in agreement with those obtained from a small sample of Galactic Cepheids with parallax measurements.

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arXiv2010

Cepheid Period-Luminosity Relation from the AKARI Observations

In this paper, we derive the period-luminosity (P-L) relation for Large Magellanic Cloud (LMC) Cepheids based on mid-infrared AKARI observations. AKARI's IRC sources were matched to the OGLE-III LMC Cepheid catalog. Together with the available I band light curves from the OGLE-III catalog, potential false matches were removed from the sample. This procedure excluded most of the sources in the S7 and S11 bands: hence only the P-L relation in the N3 band was derived in this paper. Random-phase corrections were included in deriving the P-L relation for the single epoch AKARI data, even though the derived P-L relation is consistent with the P-L relation without random-phase correction, though there is a \sim 7 per-cent improvement in the dispersion of the P-L relation. The final adopted N3 band P-L relation is N3 = -3.246 log(P) + 15.844, with a dispersion of 0.149.

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arXiv2010

Testing Mass Loss in Large Magellanic Cloud Cepheids using Infrared and Optical Observations II. Predictions and Tests of the OGLE-III Fundamental-Mode Cepheids

In this article, we test the hypothesis that Cepheids have infrared excesses due to mass loss. We fit a model using the mass-loss rate and the stellar radius as free parameters to optical observations from the OGLE-III survey and infrared observations from the 2MASS and SAGE data sets. The sample of Cepheids have predicted minimum mass-loss rates ranging from zero to $10^{-8}M_\odot$ $yr^{-1}$, where the rates depend on the chosen dust properties. We use the predicted radii to compute the Period-Radius relation for LMC Cepheids, and to estimate the uncertainty caused by the presence of infrared excess for determining angular diameters with the infrared surface brightness technique. Finally, we calculate the linear and non-linear Period-Luminosity (P-L) relations for the LMC Cepheids at VIJHK + IRAC wavelengths and we find that the P-L relations are consistent with being non-linear at infrared wavelengths, contrary to previous results.

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arXiv2009

The Connection Between Pulsation, Mass Loss and Circumstellar Shells in Classical Cepheids

Recent observations of Cepheids using infrared interferometry and Spitzer photometry have detected the presence of circumstellar envelopes (CSE) of dust and it has been hypothesized that the CSE's are due to dust forming in a Cepheid wind. Here we use a modified Castor, Abbott & Klein formalism to produce a Cepheid wind, and this is used to estimate the contribution of mass loss to the Cepheid mass discrepancy Furthermore, we test the OGLE-III Classical Cepheids using the IR fluxes from the SAGE survey to determine if Large Magellanic Cloud Cepheids have CSE's. It is found that IR excess is a common phenomenon for LMC Cepheids and that the resulting mass-loss rates can explain at least a fraction of the Cepheid mass discrepancy, depending on the assumed dust-to-gas ratio in the wind.

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arXiv2009

The Dark Energy Survey Data Management System: The Processing Framework

The Dark Energy Survey Data Management (DESDM) system will process and archive the data from the Dark Energy Survey (DES) over the five year period of operation. This paper focuses on a new adaptable processing framework developed to perform highly automated, high performance data parallel processing. The new processing framework has been used to process 45 nights of simulated DECam supernova imaging data, and was extensively used in the DES Data Challenge 4, where it was used to process thousands of square degrees of simulated DES data.

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arXiv2008

Testing Mass Loss in Large Magellanic Cloud Cepheids using Infrared and Optical Observations

It has been claimed that Period-Luminosity relations derived from infrared observations of Large Magellanic Cloud (LMC) Cepheids are less dependent on the metallicity of the Cepheids. In this work, infrared observations of LMC Cepheids from the SAGE survey are combined with OGLE II optical observation to model and predict mass-loss rates. The mass-loss rates are fit to the data and are predicted to range from about 10^{-12} to 10^{-7}M_\odot/yr; however, the rates depend on the assumed value of the dust-to-gas ratio. By comparing the relations derived from observations to the relations derived from predicted infrared stellar luminosities from the mass-loss model, it is shown mass loss affects the structure and scatter of the infrared Period-Luminosity relation. Mass loss produces shallower slopes of the infrared relations and a lower zero point. There is also evidence for non-linearity in the predicted Period-Luminosity relations, and it is argued that mass loss produces larger infrared excess at lower periods, which affects the slope and zero point, making the PL relations more linear in the wavelength range of 3.6 to 5.8 micron. Because the dust-to-gas ratio is metallicity dependent and mass loss may have a metallicity dependence, infrared Period-Luminosity relations have additional uncertainty due to metallicity.

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arXiv2008

Modeling Mass-Loss and Infrared Excess in Large Magellanic Cloud Cepheids

The purpose of this preliminary work is to determine if Large Magellanic Cloud (LMC) Cepheids have stellar winds. If a Cepheid undergoes mass loss then at some distance from the star, a fraction of the gas becomes dust, which causes an infrared excess. Mass loss is tested using OGLE II optical observations and SAGE infrared observations for a sample of 488 Cepheids. The resultant mass-loss rates range from $10^{-12}$ to $10^{-7}$ $M_\odot/yr$. Using the mass--loss model we compute infrared stellar luminosities for the sample of Cepheids and compare predicted infrared PL relations with observed relations. The predicted relations not only vary from the observed relations, implying mass loss plays a significant role, but also show evidence for non-linearity. It is determined that mass loss is important for LMC Cepheids.

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arXiv2008

The Dark Energy Survey Data Management System

The Dark Energy Survey collaboration will study cosmic acceleration with a 5000 deg2 griZY survey in the southern sky over 525 nights from 2011-2016. The DES data management (DESDM) system will be used to process and archive these data and the resulting science ready data products. The DESDM system consists of an integrated archive, a processing framework, an ensemble of astronomy codes and a data access framework. We are developing the DESDM system for operation in the high performance computing (HPC) environments at NCSA and Fermilab. Operating the DESDM system in an HPC environment offers both speed and flexibility. We will employ it for our regular nightly processing needs, and for more compute-intensive tasks such as large scale image coaddition campaigns, extraction of weak lensing shear from the full survey dataset, and massive seasonal reprocessing of the DES data. Data products will be available to the Collaboration and later to the public through a virtual-observatory compatible web portal. Our approach leverages investments in publicly available HPC systems, greatly reducing hardware and maintenance costs to the project, which must deploy and maintain only the storage, database platforms and orchestration and web portal nodes that are specific to DESDM. In Fall 2007, we tested the current DESDM system on both simulated and real survey data. We used Teragrid to process 10 simulated DES nights (3TB of raw data), ingesting and calibrating approximately 250 million objects into the DES Archive database. We also used DESDM to process and calibrate over 50 nights of survey data acquired with the Mosaic2 camera. Comparison to truth tables in the case of the simulated data and internal crosschecks in the case of the real data indicate that astrometric and photometric data quality is excellent.

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arXiv2006

Period-color and amplitude-color relations in classical Cepheid variables III: The Large Magellanic Cloud Cepheid models

Period-colour (PC) and amplitude-colour (AC) relations are studied for the Large Magellanic Cloud (LMC) Cepheids under the theoretical framework of the hydrogen ionization front (HIF) - photosphere interaction. LMC models are constructed with pulsation codes that include turbulent convection, and the properties of these models are studied at maximum, mean and minimum light. As with Galactic models, at maximum light the photosphere is located next to the HIF for the LMC models. However very different behavior is found at minimum light. The long period (P>10days) LMC models imply that the photosphere is disengaged from the HIF at minimum light, similar to the Galactic models, but there are some indications that the photosphere is located near the HIF for the short period (P<10 days) LMC models. We also use the updated LMC data to derive empirical PC and AC relations at these phases. Our numerical models are broadly consistent with our theory and the observed data, though we discuss some caveats in the paper. We apply the idea of the HIF-photosphere interaction to explain recent suggestions that the LMC period-luminosity (PL) and PC relations are non-linear with a break at a period close to 10 days. Our empirical PC and PL relations are also found to be non-linear with the F-test. Our explanation relies on the properties of the Saha ionization equation, the HIF-photosphere interaction and the way this interaction changes with the phase of pulsation and metallicity to produce the observed changes in the Cepheid PC and PL relations.

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arXiv2006

Period-colour and amplitude-colour relations in classical Cepheid variables IV: The multi-phase relations

The superb phase resolution and quality of the OGLE data on LMC and SMC Cepheids, together with existing data on Galactic Cepheids, are combined to study the period-colour (PC) and amplitude-colour (AC) relations as a function of pulsation phase. Our results confirm earlier work that the LMC PC relation (at mean light) is more consistent with two lines of differing slopes, separated at a period of 10 days. However, our multi-phase PC relations reveal much new structure which can potentially increase our understanding of Cepheid variables. These multi-phase PC relations provide insight into why the Galactic PC relation is linear but the LMC PC relation is non-linear. This is because the LMC PC relation is shallower for short (log P < 1) and steeper for long (log P > 1) period Cepheids than the corresponding Galactic PC relation. Both of the short and long period Cepheids in all three galaxies exhibit the steepest and shallowest slopes at phases around 0.75-0.85, respectively. A consequence is that the PC relation at phase ~0.8 is highly non-linear. Further, the Galactic and LMC Cepheids with log P > 1 display a flat slope in the PC plane at phases close to the maximum light. When the LMC period-luminosity (PL) relation is studied as a function of phase, we confirm that it changes with the PC relation. The LMC PL relation in V- and I-band near the phase of 0.8 provides compelling evidence that this relation is also consistent with two lines of differing slopes joined at a period close to 10 days.

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