arXiv subjects

Martin A. T. Groenewegen

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

41 records · Page 3Linked to original sources

arXiv2008

Evolution of asymptotic giant branch stars II. Optical to far-infrared isochrones with improved TP-AGB models

We present a large set of theoretical isochrones, whose distinctive features mostly reside on the greatly improved treatment of the thermally pulsing asymptotic giant branch (TP-AGB) phase. Essentially, we have coupled the TP-AGB tracks described in Paper I, at their stages of pre-flash quiescent H-shell burning, with the evolutionary tracks for the previous evolutionary phases from Girardi et al. (2000). Theoretical isochrones for any intermediate value of age and metallicity are then derived by interpolation in the grids. We take care that the isochrones keep, to a good level of detail, the several peculiarities present in these TP-AGB tracks. Theoretical isochrones are then converted to about 20 different photometric systems -- including traditional ground-based systems, and those of recent major wide-field surveys such as SDSS, OGLE, DENIS, 2MASS, UKIDSS, etc., -- by means of synthetic photometry applied to an updated library of stellar spectra, suitably extended to include C-type stars. Finally, we correct the predicted photometry by the effect of circumstellar dust during the mass-losing stages of the AGB evolution, which allows us to improve the results for the optical-to-infrared systems, and to simulate mid- and far-IR systems such as those of Spitzer and AKARI. Access to the data is provided both via a web repository of static tables (http://stev.oapd.inaf.it/dustyAGB07 and CDS), and via an interactive web interface (http://stev.oapd.inaf.it/cmd) that provides tables for any intermediate value of age and metallicity, for several photometric systems, and for different choices of dust properties.

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arXiv2006

ISO Mid-Infrared Spectroscopy of Galactic Bulge AGB Stars

To study the nature of Bulge AGB stars and in particular their circumstellar dust, we have analysed mid-infrared spectra obtained with the ISOCAM CVF spectrometer in three Bulge fields. The ISOCAM 5-16.5 micron CVF spectra were obtained as part of the ISOGAL infrared survey of the inner Galaxy. A classification of the shape of the 10 micron dust feature was made for each case. The spectra of the individual sources were modelled using a radiative transfer model. Different combinations of amorphous silicates and aluminium-oxide dust were used in the modelling. Spectra were obtained for 29 sources of which 26 are likely to be Bulge AGB stars. Our modelling shows that the stars suffer mass loss rates in the range of 10^{-8} - 5 x 10^{-7} Msun / yr, which is at the low end of the mass-loss rates experienced on the Thermally Pulsing AGB. The luminosities range from 1,700 to 7,700 Lsun as expected for a population of AGB stars with Minit of 1.5 - 2Msun. In agreement with the condensation sequence scenario, we find that the dust content is dominated by Al_2O_3 grains in this sample of low mass-loss rate stars.

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arXiv2000

LMC and SMC Cepheids: combining OGLE with DENIS 2MASS infrared data

(Abridged) I cross-correlate the OGLE database of Cepheids in the Large and Small Magellanic Clouds (MCs) with the second incremental release of the 2MASS JHK survey, and the DENIS IJK point source catalog towards the MCs. Of the 3384 Cepheids in OGLE in both Clouds, 1745 have a counterpart in the 2MASS survey within an 1" radius and good-photometry in all three bands. Only 173 have a counterpart in the DENIS survey within an 1" radius and good-photometry in all three bands. The reason for this difference is that the limiting magnitudes of the 2MASS survey are considerably fainter than for DENIS. To describe to first order the geometrical configuration of the MCs a plane is fitted to the reddening-free Wesenheit (W) index. For the LMC an inclination angle of 18 $\pm$ 3\degr is derived, for the SMC a value of $i = 68 \pm 2$\degr is derived. These results are being used to take out the effect that some Cepheids are closer to the observer than others. Using single-epoch 2MASS data for the OGLE Cepheids, PL-relations are fitted in JHK for both Fundamental mode (FU) and First-overtone (FO) pulsators, taking out the effect of the inclined disk. The derived PL-relations are used to derive a relative distance modulus of 0.50 \pm 0.02 between SMC and LMC assuming no metallicity correction. Combining the zero points of the Galactic and MCs PL-relations distance moduli of 18.60 $\pm$ 0.11 (based on W) and 18.55 $\pm$ 0.17 (based on K) to the LMC, and of 19.11 $\pm$ 0.11 (W) and 19.04 $\pm$ 0.17 (K) to the SMC are derived (without taking into account possible metallicity corrections).

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arXiv1998

IRC +10 216 revisited II: the circumstellar CO shell

12CO and 13CO J = 6-5 observations of IRC +10 216 with the JCMT are presented. A spherically symmetric radiative transfer code is used to model these and other CO observations of the carbon star IRC~+10~216/CW Leo. Compared to previously published model calculations a much larger set of observational data is used as constraints. A grid of models is calculated with the following parameters: luminosity, mass loss rate, dust-to-gas ratio, dust opacity and CO abundance. A comparison with CO J = 1-0 data obtained from the literature points towards a preferred luminosity of 10000-15000 \lsol. Notwithstanding the overall good agreement, there remain discrepancies. The best fitting models (for each of the considered luminosities) cannot accommodate the more extended emission seen in the mapping data. To fit the data for radial offsets >50\arcsec the mass loss rate must be a factor of 5 higher in the outer envelope. A comparison is made between the mass loss rates and dust-to-gas ratios derived from the CO modelling and those derived from our previous dust modelling. The best agreement is obtained for the model with 15000 \lsol. We conclude that the likely luminosity of IRC +10 216 is between 10000 and 15000 \lsol and that its distance is between 110 and 135 pc. The present-day mass loss rate is (1.5 $\pm$ 0.3) $\times 10^{-5}$ \msolyr and the gas-to-dust ratio is a 700 $\pm$ 100. The dust opacity at 60 $μ$m is found to be of order of 250 cm$^2$gr$^{-1}$. The CO abundance is 1.1 $\times$ 10$^{-3}$ relative to H$_2$.

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arXiv1998

The Lutz-Kelker bias in trigonometric parallaxes

The theoretical prediction that trigonometric parallaxes suffer from a statistical effect, has become topical again now that the results of the Hipparcos satellite have become available. This statistical effect, the so-called Lutz-Kelker bias, causes measured parallaxes to be too large. This has the implication that inferred distances, and hence inferred luminosities are too small. Published analytic calculations of the Lutz-Kelker bias indicate that the inferred luminosity of an object is, on average, 30% too small when the error in the parallax is only 17.5%. Yet, this bias has never been determined empirically. In this paper we investigate whether there is such a bias by comparing the best Hipparcos parallaxes which ground-based measurements. We find that there is indeed a large bias affecting parallaxes, with an average and scatter comparable to predictions. We propose a simple method to correct for the LK bias, and apply it successfully to a sub-sample of our stars. We then analyze the sample of 26 `best' Cepheids used by Feast & Catchpole (1997) to derive the zero-point of the fundamental mode pulsators and leads to a distance modulus to the Large Magellanic Cloud - based on Cepheid parallaxes- of 18.56 +/- 0.08, consistent with previous estimates.

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