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Y. K. Ng

Publications and source records attributed to Y. K. Ng.

15 recordsLinked to original sources
arXiv2002

Automatic Observation Rendering (AMORE) I. On a synthetic stellar population's colour-magnitude diagram

A new method, AMORE - based on a genetic algorithm optimizer, is presented for the automated study of colour-magnitude diagrams. The method combines several stellar population synthesis tools developed in the last decade by or in collaboration with the Padova group. Our method is able to recover, within the uncertainties, the parameters -- distance, extinction, age, metallicity, index of a power-law initial mass function and the index of an exponential star formation rate -- from a reference synthetic stellar population. No a priori information is inserted to recover the parameters, which is done simultaneously and not one at a time. Examples are given to demonstrate and to better understand biases in the results, if one of the input parameters is deliberately set fixed to a non-optimum value.

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arXiv1998

Are there carbon stars in the Bulge ?

The bulge carbon stars have been a mystery since their discovery, because they are about 2.5mag too faint to be regarded as genuine AGB stars, if located inside the metal-rich bulge (m-M=14.5mag). Part of the mystery can be solved if these carbon stars are related to the Sagittarius dwarf galaxy (SDG; m-M=17.0mag). They are in that case not old and metal-rich, but young, ~0.1 Gyr, with SMC-like metallicity. The sigma_RV=113+/-14 km/s radial velocity dispersion of the stars appears to be consistent with bulge membership. On the other hand, a similar velocity dispersion could be the result from an induced star formation event when the SDG crosses the galactic midplane. It is suggested that the carbon stars are tracers of such an event and that they therefore are located at distances related to the SDG. However, the majority of the carbon stars are not member of the SDG, nor are they similar to the C-stars which are member of the SDG. The radial velocities can be used to determine a possible membership to the SDG. However, they do not give information about the distance of the stars. In particular, if the stars are located at a distance comparable to the SDG. This implies that only the period-luminosity relation can be used to distinguish unambiguously if the carbon stars are located at bulge-like or SDG-like distances. Thus far only carbon stars with reliable periods have been identified at a SDG related distance.

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arXiv1998

Field #3 of the Palomar-Groningen Survey II. Near-infrared photometry of semiregular variables

Near-infrared photometry (JHKL'M) was obtained for 78 semiregular variables (SRVs) in field #3 of the Palomar-Groningen survey (PG3, l=0, b=-10). Together with a sample of Miras in this field a comparison is made with a sample of field SRVs and Miras. The PG3 SRVs form a sequence (period-luminosity & period-colour) with the PG3 Miras, in which the SRVs are the short period extension to the Miras. The field and PG3 Miras follow the same P/(J--K)o relation, while this is not the case for the field and PG3 SRVs. Both the PG3 SRVs and Miras follow the SgrI period-luminosity relation adopted from Glass et al. (1995, MNRAS 273, 383). They are likely pulsating in the fundamental mode and have metallicities spanning the range from intermediate to approximately solar.

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arXiv1998

Are the Bulge C-stars related to the Sagittarius dwarf galaxy? II. Metallicity -- a link with the Galactic disc

The photometric estimate of the metallicity and the age of the Azzopardi et al. (1991, A&AS 88, 265) carbon stars (Ng 1997, A&A 328, 211) is revised to respectively Z~0.004 and ~0.1 Gyr. Under the hypothesis that the carbon stars are located at a distance related to the Sagittarius dwarf galaxy, the broad velocity dispersion of the stars can only be explained if they were formed out of Galactic material during a recent crossing of the Sagittarius dwarf galaxy through the Galactic plane.

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arXiv1998

Stellar population synthesis diagnostics

A quantitative method is presented to compare observed and synthetic colour-magnitude diagrams (CMDs). The method is based on a chi^2 merit function for a point (c_i,m_i) in the observed CMD, which has a corresponding point in the simulated CMD within n*sigma(c_i,m_i) of the error ellipse. The chi^2 merit function is then combined with the Poisson merit function of the points for which no corresponding point was found within the n*sigma(c_i,m_i) error ellipse boundary. Monte-Carlo simulations are presented to demonstrate the diagnostics obtained from the combined (chi^2, Poisson) merit function through variation of different parameters in the stellar population synthesis tool. The simulations indicate that the merit function can potentially be used to reveal information about the initial mass function. Information about the star formation history of single stellar aggregates, such as open or globular clusters and possibly dwarf galaxies with a dominating stellar population, might not be reliable if one is dealing with a relatively small age range.

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arXiv1998

On the galactic disc age-metallicity relation

A comparison is made between the age-metallicity relation obtained from four different types of studies: F and G stars in the Solar Neighbourhood, analysis of open clusters, galactic structure studies with the stellar population synthesis technique, and chemical evolution models. Metallicities of open clusters are corrected for the effects of the radial gradient, which we find to be -0.09 dex/kpc and to be most likely constant in time. We do not correct for the vertical gradient, since its existence and value are not firmly established. Stars and clusters trace a similar age-metallicity relation, showing an excess of rather metal-rich objects in the age range 5-9 Gyr. Galactic structure studies tend to give a more metal-poor relation than chemical evolution models. Both relations do not explain the presence of old, relatively metal-rich stars and clusters. This might be due to uncertainties in the ages of the local stars, or to pre-enrichment of the disc with material from the bulge, possibly as a result of a merger event in the early phases of the formation of our Galaxy.

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arXiv1997

Revised ages for the stars in the solar neighbourhood

New ages are computed for the stars from the Edvardsson et al. (1993) data set. The revised values are systematically larger toward older ages (t>4 Gyr), while they are slightly lower for t<4 Gyr. A similar, but considerably smaller trend is present when the ages are computed with the distances based on Hipparcos parallaxes. The resulting age-metallicity relation has a small, but distinct slope of about 0.07 dex/Gyr

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arXiv1997

Are the galactic bulga and bar the same ?

An overview is given of the results obtained about Galactic Structure studies towards the Galactic Centre from star counts. The results indicate the presence of an old, metal-rich population (Z=0.005-0.08; t=13-15 Gyr), presumably the old Galactic Bulge. In addition, a younger, less metal-rich population was found (Z=0.005-0.03}; t=8-9 Gyr), likely related to the tri-axial bar found in other studies. In Baade's Window a ratio bulge/bar stars of approximately 1/2 was obtained from star counts.

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arXiv1997

The galactic disc age-metallicity relation

New ages are computed for stars in the Solar Neighbourhood from the Edvardsson et al. (1993) data set. Distances derived from the Hipparcos parallaxes were adopted to obtain reliable ages (uncertainty less than 12%) for a subset of stars. There is no apparent age-metallicity relation for stars with an age less than 10 Gyr. Only if we consider older stars a slope of ~0.07 dex/Gyr appears. This relation is compared with those obtained from other methods, i.e. galactic open clusters, stellar population synthesis (star counts), and chemical evolution models.

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arXiv1997

Are the Bulge C-stars in the Sagittarius dwarf Galaxy ?

Part of the mystery around the Bulge carbon stars from Azzopardi et al. (1991) is solved, if they are related to the Sagittarius dwarf galaxy. The carbon stars are in that case not metal-rich as previously thought, but they have a metallicity comparable to the LMC, with an age between 0.1 to 1 Gyr. A significant fraction of the carbon stars still have luminosities fainter than the lower LMC limit of Mbol~-3.5. A similar trend is present among some of the carbon stars found in other dwarf spheroidals, but they do not reach a limit as faint as Mbol~-1.4 found for the SMC. At present, the TP-AGB evolution models cannot explain the origin of carbon stars with Mbol>-3.5 through a single-star evolution scenario, even if they form immediately after entering the TP-AGB phase. Mass transfer through binary evolution is suggested as a possible scenario to explain the origin of these low luminosity carbon stars.

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arXiv1997

Probing the Galaxy I. The galactic structure towards the galactic pole

Observations of (B-V) colour distributions towards the galactic poles are compared with those obtained from synthetic colour-magnitude diagrams to determine the major constituents in the disc and spheroid. The disc is described with four stellar sub-populations: the young, intermediate, old, and thick disc populations, which have respectively scale heights of 100 pc, 250 pc, 0.5 kpc, and 1.0 kpc. The spheroid is described with stellar contributions from the bulge and halo. The bulge is not well constrained with the data analyzed in this study. A non-flattened power-law describes the observed distributions at fainter magnitudes better than a deprojected R^{1/4}-law. Details about the age, metallicity, and normalizations are listed in Table 1. The star counts and the colour distributions from the stars in the intermediate fields towards the galactic anti-centre are well described with the stellar populations mentioned above. Arguments are given that the actual solar offset is about 15 pc north from the galactic plane.

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arXiv1996

Field #3 of the Palomar-Groningen Survey I. Variable stars at the edge of the Sagittarius dwarf galaxy

A catalogue is presented with variable (RR Lyrae, semiregular and Mira) stars located inside field #3 of the Palomar-Groningen Survey, at the outer edge of the Sagittarius dwarf galaxy. One of the semiregular variables is a carbon star, comparable with those found by Azzopardi et al. (1991). Serendipity provides the suggestion, that their carbon stars might not be located inside, but behind the bulge in the Sagittarius dwarf galaxy.

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arXiv1996

Stellar population synthesis

Monte-Carlo simulations of optical and near-infrared colour-magnitude diagrams in Baade's Window (l=1.0,b=-3.9) are compared with each other. The morphological structure of the red horizontal branch in those CMDs is likely due to a combination of extinction and age-metallicity of the stars from the bulge/bar. In contrast with the optical (V,I) passbands, the simulations indicate that it is feasible to separate metallicity from extinction with near-infrared data.

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arXiv1996

On the extinction towards Baade's Window

An analysis is made on the morphology of the sequences, formed by stars distributed along the line of sight in (V,V--I) colour-magnitude diagrams from Baade's Window (l=1.0,b=-3.9). The extinction is due to an absorbing layer with an effective thickness of about 100 pc. A linearly, with distance, increasing extinction up to a maximum value inside the absorbing layer is in first order a good approximation for the actual extinction present along the line of sight. We show that the morphology of the disc sequence gives a good indication of the nearby extinction along the line of sight. The morphology of the bulge/bar red horizontal branch is likely due to a combination of differential reddening and a significant metallicity spread among the horizontal branch stars. This is irrespective of the type of extinction law adopted. There is in Baade's Window globally no big difference between a Poissonian or a patchy type of extinction, except for substructures in the morphology of the sequences for the latter.

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