15090215
The Araucaria Project: The Local Group Galaxy WLM—Distance and Metallicity from Quantitative Spectroscopy of Blue Supergiants
The quantitative analysis of low-resolution spectra of A and B supergiants is used to determine a distance modulus of 24.99 ± 0.10 mag (995 ± 46 kpc) to the Local Group galaxy WLM. The analysis yields stellar effective temperatures and gravities, which provide a distance through the flux-weighted gravity-luminosity relationship (FGLR). Our distance is 0.07 mag larger than the most recent results based on Cepheids and the tip of the red giant branch. This difference is within the 1 σ overlap of the typical uncertainties quoted in these photometric investigations. In addition, non-LTE spectral synthesis of the rich metal-line spectra (mostly iron, chromium, and titanium) of the A supergiants is carried out, which allows the determination of stellar metallicities. An average metallicity of –0.87 ± 0.06 dex with respect to solar metallicity is found.
56042891
Evidence for a universal slope of the period-luminosity relation from direct distances to Cepheids in the LMC.
We have applied the infrared surface brightness (ISB) technique to derive distances to 13 Cepheid variables in the LMC which have periods from 3-42 days. The corresponding absolute magnitudes define PL relations in VIWJK bands which agree exceedingly well with the corresponding Milky Way relations obtained from the same technique, and are in significant disagreement with the observed LMC Cepheid PL relations, by OGLE-II and Persson et al., in these bands. Our data uncover a systematic error in the p-factor law which transforms Cepheid radial velocities into pulsational velocities. We correct the p-factor law by requiring that all LMC Cepheids share the same distance. Re-calculating all Milky Way and LMC Cepheid distances with the revised p-factor law, we find that the PL relations from the ISB technique both in LMC and in the Milky Way agree with the OGLE-II and Persson et al. LMC PL relations, supporting the conclusion of no metallicity effect on the slope of the Cepheid PL relation in optical/near infrared bands.
118621475
THE ARAUCARIA PROJECT. INFRARED TIP OF THE RED GIANT BRANCH DISTANCES TO FIVE DWARF GALAXIES IN THE LOCAL GROUP
We have obtained accurate near-infrared photometry of the tip of the red giant branch (TRGB) in the Local Group galaxies Sculptor, NGC 6822, NGC 3109, IC 1613, and WLM. We have used the derived TRGB magnitudes together with the absolute magnitude calibration of the near-infrared TRGB magnitude of Valenti et al. to determine the distances of these five galaxies. The statistical errors in the distance moduli are typically 4%. The systematic uncertainties are dominated by the knowledge of the mean metallicities of the red giant branches and are in the range of 5%-8%. We observe a slight (2%) systematic difference between the distances derived from the J and K bands, respectively, which is within the 1{sigma} errors of the distances. We compare the new distances derived in this paper with other recent distance determinations for our target galaxies and find excellent agreement. In particular, the near-infrared TRGB distances to the four dwarf irregular galaxies in the sample agree to better than 5% in each case with their Cepheid distances obtained from infrared photometry, indicating that there is no appreciable systematic offset between these two fundamental techniques using old and young stellar populations, respectively.
18443556
The ARAUCARIA Project: Deep Near-Infrared Survey of Nearby Galaxies. I. The Distance to the Large Magellanic Cloud from K-Band Photometry of Red Clump Stars*
We have obtained deep imaging in the near-infrared J and K bands for two nearby fields in the bar of the LMC with the ESO New Technology Telescope, under exquisite seeing conditions. The K, J-K color-magnitude diagrams constructed from these data are of outstanding photometric quality and reveal the presence of several hundred red clump stars. Using the calibration of Alves for the K-band absolute magnitude of Hipparcos-observed red clump stars in the solar neighborhood, we derive a distance modulus to our observed LMC fields of 18.487 mag. Applying a correction for the tilt of the LMC bar with respect to the line of sight according to the geometrical model of van der Marel et al., the corresponding LMC barycenter distance is 18.501 mag. The random error on this result is ±0.008 mag, whereas the systematic uncertainty on this distance result due to the photometric zero-point uncertainty in our LMC K-band photometry, to the uncertainty of the Hipparcos-based absolute magnitude calibration of red clump stars in the solar neighborhood, and to reddening uncertainties mounts up to ±0.048 mag. If we adopt a K-band population correction of -0.03 mag, as done by Alves et al., to account for the difference in age and metallicity between the solar neighborhood and LMC red clump star populations, we obtain an LMC barycenter distance modulus of 18.471 mag from our data. This is in excellent agreement with the results of Alves et al. and those of another very recent study by Sarajedini et al. obtained from K-band photometry. However, we emphasize that current model predictions about the uncertainties of population corrections seem to indicate that errors up to about 0.12 mag may be possible, probably in any photometric band. Therefore, work must continue to tighten the constraints on these corrections. We also determine the mean red clump star magnitude in our LMC fields in the J band, which could be a useful alternative to the K band should future work reveal that population effect corrections for red clump stars in the J band are smaller or more reliably determined than those for the K band.
1966849
The araucaria project : VLT spectra of blue supergiants in WLM-classification and first abundances
As part of the Araucaria Project, we present the first spectral catalog of supergiant stars in the Local Group dwarf irregular galaxy WLM. In assigning a spectral classification to these stars we accounted for the low metal content of WLM relative to the galactic standards used in the MK process, by using classification criteria developed for B and A supergiants contained in the Small Magellanic Cloud. Our spectral catalog shows that our higher signal-to-noise ratio (S/N) spectroscopic sample of 19 objects contains at least six early B (B0-B5) supergiants and six late B and early A (B8-A2) stars of luminosity class between Ia and II, as well as an O7 V star and an O9.7 Ia star. The spectra of several of these stars are of sufficiently high quality for a determination of the stellar parameters and abundances. We have also acquired a second set of lower S/N spectra for mostly BA stars; however, their quality does not allow a further analysis. We have carried out a quantitative analysis for three early B supergiants. The mean oxygen abundance we derive is 12 + log(O/H) = 7.83 ± 0.12. This value agrees very well with the measurement that is obtained from H II regions. We therefore find no additional evidence for the discrepancy between stellar and nebular oxygen abundances measured for a single A-type supergiant by Venn et al. The analysis of B- and A-type supergiants yields compatible results for nitrogen, silicon, and magnesium. We show that the photometric variability of the blue supergiants included in our spectroscopic sample is negligible for the use of these stars as distance indicators.
54761402
The Araucaria Project
Results from a long-term observational project called the Araucaria Project are presented. Based on Wide Field optical monitoring of 8 nearby galaxies, covering a large range of metallicities, more than 500 Cepheids and a few hundred Blue Supergiant candi- dates were identified. From the analysis of Cepheid P-L relations of outstanding quality de- rived from our data we conclude that the slope of these relations in the I band and Wesenheit index are not dependent on metallicity. Comparing the I-band magnitudes of Cepheids of a period of ten days, as computed from our P-L relations, to the I-band magnitudes of the tip of the RGB, which is widely believed to be independent of population e ects, we cannot see any obvious dependence of the zero point of the I-band P-L relation on metallicity. A preliminary analysis of IR follow-up observations of sub-samples of the identified Cepheids in various galaxies of the project show that the distances obtained from these data are sys- tematically shorter by about of 0.1 mag than those derived from the optical photometry. It is likely that this e ect can be attributed to the internal reddening in the program galaxies. The selected Blue Supergiant candidates were observed spectroscopically with 8m-class telescopes to determine their element abundances, and their luminosities from the Flux- weighted Gravity-Luminosity Relationship. Results on this aspect of the Araucaria Project are presented in the review of Kudritzki presented during this conference.
15295782
Probing quintessence: reconstruction and parameter estimation from supernovae
We explore the prospects for using future supernova observations to probe the dark energy. We focus on quintessence, an evolving scalar field that has been suggested as a candidate for the dark energy. After simulating the observations that would be expected from the proposed SuperNova/Acceleration Probe satellite (SNAP), we investigate two methods for extracting information concerning quintessence from such data. First, by expanding the quintessence equation of state as w Q (z) = w Q (0) - a In(1 + z), to fit the data, it is possible to reconstruct the quintessence potential for a wide range of smoothly varying potentials. Secondly, it will be possible to test the basic properties of the dark energy by constraining the parameters Ω Q , w Q and a. We show that it may be possible, for example, to distinguish between quintessence and the cosmological constant in this way. Furthermore, when supernova data are combined with other planned cosmological observations, the precision of reconstructions and parameter constraints is significantly improved, allowing a wider range of dark energy models to be distinguished.