arXiv subjects

M. A. T. Groenewegen

Publications and source records attributed to M. A. T. Groenewegen.

At least 145 records · Page 8Linked to original sources

arXiv2005

The mid- and far-infrared colours of AGB and post-AGB stars

With the advent of space missions, like SPITZER and ASTRO-F, with sensitive detectors in the near- and mid-infra red covering a reasonable field-of-view and having a good spatial resolution, it will be possible to detect individual AGB stars in Local Group galaxies. The filters used by these missions are non-standard and different from mission to mission. In this paper, the colours of mass-losing AGB and post-AGB stars are calculated in the broad-band filters of the SPITZER and ASTRO-F missions, as well as Bessell V,I and 2MASS J,H,K to connect these results to existing ground-based data. The models are calculated for carbon- and oxygen-rich chemistry and cover different effective temperatures and dust compositions.

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arXiv2005

The Chemical Composition of Cepheids in the Milky Way and the Magellanic Clouds

We have measured the elemental abundances of 68 Galactic and Magellanic Cepheids from FEROS and UVES high-resolution and high signal-to-noise spectra in order to establish the influence of the chemical composition on the properties of these stars (see Romaniello et al. 2005). Here we describe the robust analytical procedure we have developed to accurately determine them. The resulting iron abundances span a range between ~ -0.80 dex for stars in the Small Magellanic Cloud and ~ +0.20 dex for the most metal-rich ones in the Galaxy. While the average values for each galaxy are in good agreement with non-pulsating stars of similar age, Cepheids display a significant spread. Thus it is fundamental to measure the metallicity of individual stars.

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arXiv2005

ESO-VLT and Spitzer spectroscopy of IRAS05328-6827: a massive young stellar object in the Large Magellanic Cloud

We present the first thermal-infrared spectra of an extra-galactic Young Stellar Object (YSO), IRAS05328-6827 in the HII region LHA 120-N148 in the Large Magellanic Cloud. The observed and modelled spectral energy distribution reveals a massive YSO, M=20 Msun, which is heavily-embedded and probably still accreting. The reduced dust content as a consequence of the lower metallicity of the LMC allows a unique view into this object, and together with a high C/O ratio may be responsible for the observed low abundance of water ice and relatively high abundances of methanol and CO2 ices.

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arXiv2005

Populations of AGB stars and LPVs in the Galaxy and Local Group

In this review I will discuss recent developments on the topic of resolved Asymptotic Giant Branch (AGB) stars and Long Period Variables (LPVs) in our Galaxy, the Magellanic Clouds, and the Local Group in general. Since the main characteristics of AGB stars are (1) their infrared, (2) pulsational and (3) chemical properties, it is fitting that I will focus on recent results from the (a) 2MASS/DENIS and MSX near- and mid-IR surveys, the (b) micro-lensing surveys and (c) dedicated narrow-band imaging surveys.

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arXiv2005

Mira Variables in the OGLE Bulge fields

(abridged) The 222000 I-band light curves of variable stars detected by the OGLE-II survey in the direction of the Galactic Bulge have been fitted and have also been correlated with the DENIS and 2MASS databases. Results are presented for 2691 objects with I-band semi-amplitude larger than 0.45 magnitude, corresponding to classical Mira variables. The Mira period distribution of 6 fields at similar longitude but spanning latitudes from -1.2 to -5.8 degrees are statistically indistinguisable indicating similar populations with initial masses of 1.5-2 \msol (corresponding to ages of 1-3 Gyr). A field at similar longitude at b = -0.05 from Glass et al. (2001) does show a significantly different period distribution, indicating the presence of a younger population of 2.5-3 \msol and ages below 1 Gyr. The K-band period-luminosity relation is presented for the whole sample, and for sub-fields. The zero point depends on Galactic longitude. Simulations are carried out to show that the observed dependence of the zero point with $l$, and the number of stars per field are naturally explained using the model of disk and bulge stars of Binney et al. (1997), for a viewing angle of 43 +/- 17 degrees. A comparison is made with similar objects in the Magellanic Clouds, studied in a previous paper. The slope of the PL-relation in the Bulge and the MCs agree within the errorbars. A distance to the Galactic Centre between 8.6 and 9.0 kpc is derived (for an assumed DM of 18.50 to the LMC).

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arXiv2005

Eclipsing binaries in the Galactic Bulge: candidates for distance estimates

The 222~000 I-band light curves of variable stars detected by the OGLE-II survey in the direction of the Galactic Bulge have been searched for eclipsing binaries (EBs). A previously developed code to analyze lightcurve shapes and identify long period variables (LPVs) has been adapted to identify EBs. The parameters in the modified code have been optimised to recover a list of about 140 detached EBs in the Small Magellanic Cloud previously identified in the literature as particularly well suited for distance estimates (and wich have periods \more 0.85 days). The power of the code is demonstrated by identifying 16 and 178 previously uncatalogued EBs in the SMC and LMC, respectively. Among the 222~000 variable stars in the direction of the Galactic Bulge 3053 EBs have been identified. Periods and phased lightcurves are presented.

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arXiv2005

Star counts in the Galaxy. Simulating from very deep to very shallow photometric surveys with the TRILEGAL code

We describe TRILEGAL, a new populations synthesis code for simulating the stellar photometry of any Galaxy field. The code attempts to improve upon several technical aspects of star count models, by: dealing with very complete input libraries of evolutionary tracks; using a stellar spectral library to simulate the photometry in any broad-band system; being very versatile allowing easy changes in the input libraries and in the description of all of its ingredients -- like the SFR, AMR, IMF, and geometry of Galaxy components. In a previous paper (Groenewegen et al. 2002), the code was first applied to describe the very deep star counts of the CDFS stellar catalogue. Here, we briefly describe its initial calibration using EIS-deep and DMS star counts, which are adequate samples to probe both the halo and the disc components of largest scale heights (oldest ages). We then present the changes in the calibration that were necessary to cope with some improvements in the model input data, and the use of more extensive photometry datasets: the relatively shallower 2MASS catalogue, which probes mostly the disc at intermediate ages, and the immediate solar neighbourhood as sampled by Hipparcos, which contains a somewhat larger fraction of younger stars than deeper surveys. Remarkably, the same model calibration can reproduce well the star counts in all the above-mentioned data sets, that span from the very deep magnitudes of CDFS (16<R<23) to the very shallow ones of Hipparcos (V<8). Significant deviations are found just for fields close to the Galactic Center and Plane, and for a single set of South Galactic Pole data. The TRILEGAL code is ready to use for the variety of wide-angle surveys in the optical/infrared that will become available in the coming years.

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arXiv2004

The distance to the Pleiades: Main sequence fitting in the near infrared

Hipparcos parallax measurements of stars in the Pleiades notoriously result in a cluster distance of 118 pc; ~10% shorter than the `classical' results of MS-fitting. In an earlier paper we developed a purely empirical MS-fitting method to address this problem, which produced conflicting results between the (B-V) and (V-I) colour indices, indicating that the cluster's photometric metallicity is substantially lower than its (~solar) spectroscopic metallicity. We were able to reconcile the discrepancy by assuming [Fe/H]=-0.4 (as indicated from (B-V)/(V-I) colour-colour plots), and the distance moduli obtained from the 2 indices were in agreement with the Hipparcos result, within the 1sig errors. With the release of the 2MASS All Sky Catalogue, we now apply our MS-fitting method to the Pleiades using the infrared colours, in order to test the plausibility of our earlier result. Fitting in the V/(V-K) and K/(J-K) colour planes, we find that assuming a subsolar metallicity does not produce distances in agreement with the (B-V) and (V-I) results. However the infrared plus (V-I) distances are in mutual agreement when adopting the spectroscopic metallicity. By considering only stars with Mv<=6, the infrared and optical colour indices all yield consistent distances using the spectroscopic [Fe/H]. The concordant distances thus obtained from the (B-V), (V-I), (V-K) and (J-K) indices yield a mean of 133.8+/-3 pc, in excellent agreement with the pre-Hipparcos MS-fitting results, and the most recent determinations from other methods. We conclude that there are two distinct issues affecting the Pleiades: the Hipparcos parallax is in error by ~10%, as previously claimed; the (B-V) colours of the lower MS are anomalous, and we caution against their use for MS-fitting to the Pleiades and similarly young open clusters. (Abridged)

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arXiv2004

A purely geometric distance to the binary star Atlas, a member of the Pleiades

We present radial velocity and new interferometric measurements of the double star Atlas, which permit, with the addition of published interferometric data, to precisely derive the orbital parameters of the binary system and the masses of the components. The derived semi-major axis, compared with its measured angular size, allows to determine a distance to Atlas of 132+-4 pc in a purely geometrical way. Under the assumption that the location of Atlas is representative of the average distance of the cluster, we confirm the distance value generally obtained through main sequence fitting, in contradiction with the early Hipparcos result (118.3+-3.5 pc).

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arXiv2004

Evolution of planetary nebulae II. Population effects on the bright cut-off of the PNLF

We investigate the bright cut-off of the [OIII]l5007 planetary nebula luminosity function (PNLF), that has been suggested as a powerful extragalactic distance indicator. Theoretical PNLFs are constructed via Monte-Carlo simulations of populations of PNe, whose individual properties are described with the aid of recent PN synthetic models (Marigo et al. 2001), coupled to a detailed photoionisation code (CLOUDY). The basic dependences of the cut-off magnitude M* are then discussed. We find that: (i) In galaxies with recent or ongoing star formation, the modelled PNLF present M* values between -4 and -5, very close to the observationally-calibrated value for the LMC. (ii) In these galaxies, the PNLF cut-off is produced by PNe with progenitor masses of about 2.5 Msun, while less massive stars give origin to fainter PNe. As a consequence M* is expected to depend strongly on the age of the last burst of star formation, dimming by as much as 5 mag as we go from young to 10-Gyr old populations. (iii) Rather than on the initial metallicity of a stellar population, M* depends on the actual [O/H] of the observed PNe, a quantity that may differ significantly from the initial value (due to dredge-up episodes), especially in young and intermediate-age PN populations. (iv) Also the transition time from the end of AGB to the PN phase, and the nuclear-burning properties (i.e. H- or He-burning) of the central stars introduce non-negligible effects on M*. The strongest indication derived from the present calculations is a serious difficulty to explain the age-invariance of the cut-off brightness over an extended interval, say from 1 to 13 Gyr, that observations of PNLFs in galaxies of late-to-early type seem to suggest.

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arXiv2004

The metallicity dependence of the Cepheid PL-relation

A sample of 37 Galactic, 10 LMC and 6 SMC cepheids is compiled for which individual metallicity estimates exist and BVIK photometry in almost all cases. The Galactic cepheids all have an individual distance estimate available. For the MC objects different sources of photometry are combined to obtain improved periods and mean magnitudes. A multi-parameter Period-Luminosity relation is fitted to the data which also solves for the distance to the LMC and SMC. When all three galaxies are considered, without metallicity effect, a significant quadratic term in log P is found, as previously observed and also predicted in some theoretical calculations. For the present sample it is empirically determined that for log P < 1.65 linear PL-relations may be adopted, but this restricts the sample to only 4 LMC and 1 SMC cepheid. Considering the Galactic sample a metallicity effect is found in the zero point in the VIWK PL-relation (-0.6 \pm 0.4 or -0.8 \pm 0.3 mag/dex depending on the in- or exclusion of one object), in the sense that metal-rich cepheids are brighter. The small significance is mostly due to the fact that the Galactic sample spans a narrow metallicity range. The error is to a significant part due to the error in the metallicity determinations and not to the error in the fit. Including the 5 MC cepheids broadens the observed metallicity range and a metallity effect of about -0.27 \pm 0.08 mag/dex in the zero point is found in VIWK, in agreement with some previous empirical estimates, but now derived using direct metallicity determinations for the cepheids themselves.

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arXiv2003

`Thermal' SiO radio line emission towards M-type AGB stars: a probe of circumstellar dust formation and dynamics

An extensive radiative transfer analysis of circumstellar SiO `thermal' radio line emission from a large sample of M-type AGB stars has been performed. The sample contains 18 irregulars of type Lb (IRV), 7 and 34 semiregulars of type SRa and SRb (SRV), respectively, and 12 Miras. New observational material, which contains data in several SiO rotational lines in the ground vibrational state, is presented for the IRV/SRVs. The detection rate was about 60%. SiO fractional abundances have been determined through radiative transfer modelling. The abundance distribution of the IRV/SRV sample has a median value of 8E-6. The high mass loss rate Miras have a bimodal abundance distribution, a low abundance group (on average 4E-7) and a high abundance group (on average 7E-6). The derived SiO abundances are in all cases well below the abundance expected from stellar atmosphere chemistry, on average by a factor of ten. In addition, there is a trend of decreasing SiO abundance with increasing mass loss rate. This is interpreted in terms of depletion of SiO molecules by the formation of silicate grains in the circumstellar envelopes, with an efficiency which increases with the mass loss rate. The SiO and CO radio line profiles differ in shape. In general, the dominating part of the SiO profiles are narrower than the CO profiles, but they have low-intensity wings which cover the full velocity range of the CO profile. This is interpreted (as has been done also by others) as due to the influence of gas acceleration in the region which produces a significant fraction of the SiO line emission. Finally, a number of sources which have peculiar CO line profiles are discussed from the point of view of their SiO line properties.

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arXiv2003

The distance to the LMC cluster NGC 1866; clues from the cluster Cepheid population

(abridged) Recent investigations aimed at estimating the distance to the young LMC cluster NGC1866 have made use of Red Clump stars in the surrounding LMC field, together with empirical and theoretical Main-Sequence fitting methods, and have found significantly different distances for the field and the cluster, the latter being closer by in distance modulus by Δ(DM) \sim 0.20 mag. In this paper we (re-)consider the Cepheid star population of NGC1866, to try to shed some light on this discrepancy. By combining various extensive photometric datasets in BVI and single-epoch 2MASS JHK photometry, PL relationships for the cluster Cepheids are obtained. A comparison between the field LMC and cluster PL relationships for the reddening free Wesenheit index gives a firm determination of the distance between the cluster and the LMC main body (0.04 mag in distance modulus, the cluster being more distant) which, coupled to a model for the geometry of the LMC disk, provides Δ(DM) ranging between 0.0 and -0.11 mag. The simultaneous comparison of the PL relationships in BVI for the cluster and LMC field gives an estimate of the cluster reddening, which results to be E(B-V) = 0.12 \pm 0.02. This determination is higher than the canonical value of 0.06 mag used in all previous studies, but we show that it is not in contradiction with a re-analysis of independent estimates. The cluster Main Sequence fitting distance obtained with this new reddening is DM = 18.58 \pm 0.08, fully compatible with the Red Clump value of DM = 18.53 \pm 0.07(random)^{+0.02}_{-0.05} (systematic) and the Cepheid constraint on Δ(DM).

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arXiv2003

Long Period Variables detected by ISO in the SMC

This article presents the study of the light-curves extracted from the MACHO database of a sample of stars observed by the ISO in the SMC. These stars belong to the ISO-Mini-Survey catalogue of the Magellanic Clouds (ISO-MCMS, Loup et al. in preparation). Most of them are in the AGB and supergiant phases. The dominant period and amplitude of pulsation have been derived and the stars have been classified as Mira or Semi-Regular pulsators. Furthermore, the cross-identification with near-infrared DENIS and 2MASS magnitudes available within the ISO-MCMS allowed us: (i) to investigate the properties of these stars in the combined near- and mid-infrared colour-magnitude diagrams, (ii) to derive the bolometric magnitude by integrating the spectral energy distribution and (iii) to estimate the mass-loss rate. The stars have been divided into carbon- (C-) and oxygen-rich (O-rich) using the (J-Ks, Ks) colour-magnitude diagram and their period and amplitude distributions have been compared. C-rich AGB stars have a sharp peak in their period distribution at about 250 days and have on average a larger amplitude than O-rich AGB stars. This effect, not previously detected from the study of similar stars in the LMC and in the Baade's window, might be closely related to the metallicity of the environment in which the stars have formed.

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arXiv2002

Carbon stars in the Local Group

The current status of carbon stars in the Local Group and beyond, is discussed. Although many carbon stars and late M-stars have been identified in Local Group galaxies, a coherent understanding in terms of the chemical evolution- and star formation history of a galaxy is still largely lacking. Although a few major new surveys have been carried out over the last three years, the observational data is still incomplete in many respects: 1) for some of the larger galaxies only a small fraction in area has been surveyed so far, 2) surveys have been conducted using different techniques, and some of the older surveys are incomplete in bolometric magnitude, 3) only for some galaxies is there information about the late M-star population, or it is sometimes unpublished even when the data is available, 4) not all galaxies in the Local Group have been surveyed, 5) especially for some of the older work insufficient data is available to determine bolometric magnitudes. I have correlated carbon star positions with the 2MASS 2nd incremental data release to obtain $JHK$, and bolometric magnitudes, to remedy this situation in some cases. From the existing observations one can derive the following: the formation of carbon stars is both a function of metallicity and star-formation history. In galaxies with a similar star formation history, there will be relatively more carbon stars formed in the system with the lower metallicity. On the other hand, the scarcity of AGB type carbon stars in some galaxies with the lowest metallicity indicates that these systems have had a low, if any, star-formation over the last few Gyrs.

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arXiv2002

On the distance of Magellanic Clouds: first overtone Cepheids

We present a detailed comparison between predicted and empirical PL_{I,K} relations and Wesenheit function for Galactic and Magellanic Clouds (MCs) First Overtone (FO) Cepheids. We find that zero-points predicted by Galactic Cepheid models based on a noncanonical (mild overshooting) Mass-Luminosity (ML) relation are in very good agreement with empirical zero-points based on HIPPARCOS parallaxes, while those based on canonical (no overshooting) ML relation are about 0.2-0.3 mag brighter. We also find that predicted and empirical PL_K relation and Wesenheit function give, according to optical (V,I OGLE) and near-infrared (NIR, K, 2mass) data, mean distances to the MCs that agree at the 2% level. Individual distances to the Large and the Small Cloud are: 18.53+-0.08-19.04+-0.11 (theory) and 18.48+-0.13-19.01+-0.13 (empirical). Moreover, predicted and empirical FO relations do not present, within the errors, a metallicity dependence. Finaly, we find that the upper limit in the FO period distribution is a robust observable to constrain the accuracy of pulsation models. Current models agree within 0.1 in log P with the observed FO upper limits.

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arXiv2002

First spectroscopic evidence for carbon stars outside the Local Group: properties of a massive star cluster in NGC 7252

We present near-IR spectroscopy of the massive intermediate age star cluster W3 in the merger remnant galaxy NGC 7252, obtained with the NTT telescope. This cluster has an age when the integrated near-IR properties of a stellar population are dominated by the cool and luminous AGB stars. We compare the data with instantaneous curst model predictions from new evolutionary synthesis models that include: (1) the computation of the evolution through the TP-AGB for low- and intermediate-massive stars, with the initial mass and metallicity dependent formation of carbon stars; (2) spectroscopic data from a new stellar library in which differences betwenn static giants, vriable O-rich TPAGB stars and carbon stars are accounted for. The comparison of the data to the models clearly shows that carbon stars are present: for the first time, carbon star spectral features are thus detected directly outside the Local Group (abriged)

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arXiv2002

ESO Imaging Survey. The Stellar Catalogue in the Chandra Deep Field South

(abridged) Stellar catalogues in five passbands (UBVRI) over an area of approximately 0.3 deg^2, comprising about 1200 objects, and in seven passbands (UBVRIJK) over approximately 0.1 deg^2, comprising about 400 objects, in the direction of the Chandra Deep Field South are presented. The 90% completeness level of the number counts is reached at approximately U = 23.8, B = 24.0, V = 23.5, R = 23.0, I = 21.0, J = 20.5, K = 19.0. A scheme is presented to select point sources from these catalogues, by combining the SExtractor parameter CLASS_STAR from all available passbands. Probable QSOs and unresolved galaxies are identified by using the previously developed χ^2-technique (Hatziminaoglou et al 2002), that fits the overall spectral energy distributions to template spectra and determines the best fitting template. The observed number counts, colour-magnitude diagrams, colour-colour diagrams and colour distributions are presented and, to judge the quality of the data, compared to simulations based on the predictions of a Galactic Model convolved with the estimated completeness functions and the error model used to describe the photometric errors of the data. The resulting stellar catalogues and the objects identified as likely QSOs and unresolved galaxies with coordinates, observed magnitudes with errors and assigned spectral types by the $χ^2$-technique are presented and are publicly available.

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