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Register a new userThe near-IR Surface Brightness Method applied to six Cepheids in the young LMC cluster NGC1866
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Jesper Storm
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We present new near-IR light curves for six Cepheids in the young blue LMC cluster NGC1866 as well as high precision radial velocity curves for ten Cepheids in NGC1866 and two in NGC2031. For the six Cepheids in NGC1866 with new J and K light curves we determine distances and absolute magnitudes by applying the near-IR surface brightness method. We find that the formal error estimates on the derived distances are underestimated by about a factor of two. We find excellent agreement between the absolute magnitudes for the low metallicity LMC Cepheids with the Period-Luminosity (P-L) relation determined by the near-IR surface brightness (ISB) method for Galactic Cepheids suggesting that the slope of the P-L relations for low metallicity and solar metallicity samples could be very similar in contrast to other recent findings. Still there appears to be significant disagreement between the observed slopes of the OGLE based apparent P-L relations in the LMC and the slopes derived from ISB analysis of Galactic Cepheids, and by inference for Magellanic Cloud Cepheids, indicating a possible intrinsic problem with the ISB method itself. Resolving this problem could reaffirm the P-L relation as the prime distance indicator applicable as well to metallicities significantly different from the LMC value.
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We derive individual distances to six Cepheids in the young populous star cluster NGC1866 in the Large Magellanic Cloud employing the near-IR surface brightness technique. With six stars available at the exact same distance we can directly measure the intrinsic uncertainty of the method. We find a standard deviation of 0.11 mag, two to three times larger than the error estimates and more in line with the estimates from Bayesian statistical analysis by Barnes et al. (2005). Using all six distance estimates we determine an unweighted mean cluster distance of 18.30+-0.05. The observations indicate that NGC1866 is close to be at the same distance as the main body of the LMC. If we use the stronger dependence of the p-factor on the period as suggested by Gieren et al. (2005) we find a distance of 18.50+-0.05 (internal error) and the PL relations for Galactic and MC Cepheids are in very good agreement.
Bayesian statistical calculations and linear-bisector calculations for obtaining Cepheid distances and radii by the infrared surface brightness method have been compared for a set of 38 Cepheids. The distances obtained by the two techniques agree to 1.5%+/-0.6% and the radii agree to 1.1%+/-0.7%. Thus the two methods yield the same distances and radii at the 2 sigma level. This implies that the short distance to the LMC found in recent linearbisector studies of Cepheids is not a result of simplifications in the mathematical approach. The computed uncertainties in distance and radius are larger in the Bayesian calculation typically by a factor of three.
I present light curves for two detached eclipsing binary stars in the region of the LMC cluster NGC 1850, which is possibly a young globular cluster still in formation. One, a likely spectral type O star, is a newly detected eclipsing binary in the region of the very young subcluster NGC 1850A. This binary is among a small number of highly massive O-type stars in binary systems found in LMC clusters. These two eclipsing binaries are the first discovered in the well studied NGC 1850, and the O-type star is the first eclisping binary found in NGC 1850A. Light curves for two NGC 1850 region Cepheid variables are also shown. Discovering two eclipsing binaries in the young globlular-like cluster NGC 1850 is discussed in terms of the importance of the binary fraction to globular cluster evolution.
This work continues our efforts to calibrate model surface brightness luminosities for the study of unresolved stellar populations, through the comparison with data of Magellanic Cloud star clusters. We present here the relation between absolute K_s-band fluctuation magnitude and (V - I) integrated colour, using data from the 2MASS and DENIS surveys, and from the literature. We compare the star cluster sample with the sample of early-type galaxies and spiral bulges studied by Liu et al. (2002). We find that intermediate-age to old clusters lie along a linear correlation with the same slope, within the errors, of that defined by the galaxies in the barM_{K_s} vs. (V - I) diagram. While the calibration by Liu et al. was determined in the colour range 1.05 < (V - I_c)_0 < 1.25, ours holds in the interval -5 >= barM_{K_s} >= -9, 0.3 <= (V - I) <= 1.25. This implies, according to Bruzual & Charlot (2003) and Mouhcine & Lancon (2003) models, that the star clusters and the latest star formation bursts in the galaxies and bulges constitute an age sequence. At the same time, there is a slight offset between the galaxies and the star clusters [the latter are ~ 0.7 mag fainter than the former at a given value of (V - I)], caused by the difference in metallicity of roughly a factor of two. The confrontation between models and galaxy data also suggests that galaxies with K_s fluctuation magnitudes that are brighter than predicted, given their (V - I) colour, might be explained in part by longer lifetimes of TP-AGB stars. (Abridged version.)
We reconsider the problem of the age of the stellar discs of late-type Low Surface Brightness (LSB) galaxies by making use of a new IMF recently derived from numerical fluid dynamical simulations (Padoan, Nordlund and Jones, 1997). While a Miller-Scalo IMF cannot adequately describe the photometric properties of LSBs, when we apply the new Padoan et al. (1997) IMF to a simple exponential disc model with parameters appropriate to LSBs, we get excellent fits of the colors and color gradients. We then conclude that: a) The star formation history of LSB disc galaxies can be described by an initial burst of a few times $10^7$ yr followed by a quiescent period with only sporadic star formation; b) LSBs discs are not young. The age of the LSB disc galaxies for which colors have been measured are all larger than about 10 Gyr$.
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