The Metallicity of the Red Giant Branch in the Disk of NGC 6822

Deep J, H, and K' images obtained with the Canada-France-Hawaii Telescope adaptive optics system are used to investigate the metallicity of red giant branch (RGB) stars in three fields in the disk of the Local Group dwarf irregular galaxy NGC 6822. The slope of the RGB on the (K, J-K) color-magnitude diagrams indicates that = -1.0 +/- 0.3. The locus of the RGB is bluer than that of globular clusters with the same RGB slope, by an amount that is consistent with the majority of RGB stars in these fields having an age near 3 Gyr. It is demonstrated that if RGB stars in NGC 6822 are this young then the metallicity computed from the RGB slope may be ~ 0.05 dex too low.Comment: 19 pages of text; 10 figures; to appear in the PAS

Nano positioning system reveals the course of upstream and nontemplate DNA within the RNA polymerase II elongation complex

Crystallographic studies of the RNA polymerase II (Pol II) elongation complex (EC) revealed the locations of downstream DNA and the DNA-RNA hybrid, but not the course of the nontemplate DNA strand in the transcription bubble and the upstream DNA duplex. Here we used single-molecule Fluorescence Resonance Energy Transfer (smFRET) experiments to locate nontemplate and upstream DNA with our recently developed Nano Positioning System (NPS). In the resulting complete model of the Pol II EC, separation of the nontemplate from the template strand at position +2 involves interaction with fork loop 2. The nontemplate strand passes loop β10-β11 on the Pol II lobe, and then turns to the other side of the cleft above the rudder. The upstream DNA duplex exits at an approximately right angle from the incoming downstream DNA, and emanates from the cleft between the protrusion and clamp. Comparison with published data suggests that the architecture of the complete EC is conserved from bacteria to eukaryotes and that upstream DNA is relocated during the initiation–elongation transition

The Recent Star Formation in NGC 6822: an Ultraviolet Study

We characterize the star formation in the low-metallicity galaxy NGC 6822 over the past few hundred million years, using GALEX far-UV (FUV, 1344-1786 A) and near-UV (NUV, 1771-2831 A) imaging, and ground-based Ha imaging. From GALEX FUV image, we define 77 star-forming (SF) regions with area >860 pc^2, and surface brightness <=26.8 mag(AB)arcsec^-2, within 0.2deg (1.7kpc) of the center of the galaxy. We estimate the extinction by interstellar dust in each SF region from resolved photometry of the hot stars it contains: E(B-V) ranges from the minimum foreground value of 0.22mag up to 0.66+-0.21mag. The integrated FUV and NUV photometry, compared with stellar population models, yields ages of the SF complexes up to a few hundred Myr, and masses from 2x10^2 Msun to 1.5x10^6 Msun. The derived ages and masses strongly depend on the assumed type of interstellar selective extinction, which we find to vary across the galaxy. The total mass of the FUV-defined SF regions translates into an average star formation rate (SFR) of 1.4x10^-2 Msun/yr over the past 100 Myr, and SFR=1.0x10^-2 Msun/yr in the most recent 10 Myr. The latter is in agreement with the value that we derive from the Ha luminosity, SFR=0.008 Msun/yr. The SFR in the most recent epoch becomes higher if we add the SFR=0.02 Msun/yr inferred from far-IR measurements, which trace star formation still embedded in dust (age <= a few Myr).Comment: Accepted for publication in ApJ, 21 pages, 6 figures, 3 table

The ARAUCARIA project: Grid-Based Quantitative Spectroscopic Study of Massive Blue Stars in NGC55

The quantitative study of the physical properties and chemical abundances of large samples of massive blue stars at different metallicities is a powerful tool to understand the nature and evolution of these objects. Their analysis beyond the Milky Way is challenging, nonetheless it is doable and the best way to investigate their behavior in different environments. Fulfilling this task in an objective way requires the implementation of automatic analysis techniques that can perform the analyses systematically, minimizing at the same time any possible bias. As part of the ARAUCARIA project we carry out the first quantitative spectroscopic analysis of a sample of 12 B-type supergiants in the galaxy NGC55 at 1.94 Mpc away. By applying the methodology developed in this work, we derive their stellar parameters, chemical abundances and provide a characterization of the present-day metallicity of their host galaxy. Based on the characteristics of the stellar atmosphere/line formation code FASTWIND, we designed and created a grid of models for the analysis of massive blue supergiant stars. Along with this new grid, we implemented a spectral analysis algorithm. Both tools were specially developed to perform fully consistent quantitative spectroscopic analyses of low spectral resolution of B-type supergiants in a fast and objective way. We present the main characteristics of our FASTWIND model grid and perform a number of tests to investigate the reliability of our methodology. The automatic tool is applied afterward to a sample of 12 B-type supergiant stars in NGC55, deriving the stellar parameters and abundances. The results indicate that our stars are part of a young population evolving towards a red supergiant phase. The derived chemical composition hints to an average metallicity similar to the one of the Large Magellanic Cloud, with no indication of a spatial trend across the galaxy.Comment: 19 pages, 12 figures and 9 tables. Accpeted for publication in A&

Stellar spectroscopy far beyond the Local Group

Multi-object spectroscopic observations of blue supergiants in NGC 3621, a spiral galaxy at a distance of 6.7 Mpc, carried out with the ESO VLT and FORS are presented. We demonstrate the feasibility of quantitative stellar spectroscopy at distances approaching a ten-fold increase over previous investigations by determining chemical composition, stellar parameters, reddening, extinction and wind properties of one of our targets, a supergiant of spectral type A1 Ia located in the outskirts of NGC 3621. The metallicity (determined from iron group elements) is reduced by a factor of two relative to the sun in qualitative agreement with results from previous abundance studies based on H II region oxygen emission lines. Reddening and extinction are E(B-V) = 0.12 and Av = 0.37, respectively, mostly caused by the galactic foreground. Comparing stellar wind momentum and absolute V magnitude with galactic and M31 counterparts we confirm the potential of the wind momentum-luminosity relationship as an alternative tool to estimate extragalactic distances.Comment: 9 pages, 4 figures, 1 table, accepted for publication in The Astrophysical Journal Letter

Spectral Types of Red Supergiants in NGC 6822 and the Wolf-Lundmark-Melotte Galaxy

We present moderate-resolution spectroscopic observations of red supergiants (RSGs) in the low-metallicity Local Group galaxies NGC 6822 (Z = 0.4Zsun) and Wolf-Lundmark-Melotte (WLM; Z = 0.1Zsun). By combining these observations with reduction techniques for multislit data reduction and flux calibration, we are able to analyze spectroscopic data of 16 RSGs in NGC 6822 and spectrophotometric data of 11 RSGs in WLM. Using these observations we determine spectral types for these massive stars, comparing them to Milky Way and Magellanic Clouds RSGs and thus extending observational evidence of the abundance-dependent shift of RSG spectral types to lower metallicities. In addition, we have uncovered two RSGs with unusually late spectral types (J000158.14-152332.2 in WLM, with a spectral type of M3 I, and J194453.46-144552.6 in NGC 6822, with a spectral type of M4.5 I) and a third RSG (J194449.96-144333.5 in NGC 6822) whose spectral type has varied from a M2.5 in 1997 to a K5 in 2008. All three of these stars could potentially be members of a recently-discovered class of extreme RSG variables.Comment: 14 pages, 7 figures; accepted for publication in A

A Survey of Local Group Galaxies Currently Forming Stars: II. UBVRI Photometry of Stars in Seven Dwarfs and a Comparison of the Entire Sample

We have obtained UBVRI images with the Kitt Peak and Cerro Tololo 4-m telescopes and Mosaic cameras of seven dwarfs in (or near) the Local Group, all of which have known evidence of recent star formation: IC10, NGC 6822, WLM, Sextans B, Sextans A, Pegasus,and Phoenix. We construct color-magnitude diagrams (CMDs) of these systems, as well as neighboring regions that can be used to evaluate the degree of foreground contamination by stars in the Milky Way. Inter-comparison of these CMDs with those of M31, M33, the LMC, and the SMC permits us to determine improved reddening values for a typical OB star found within these galaxies. All of the CMDs reveal a strong or modest number of blue supergiants. All but Pegasus and Phoenix also show the clear presence of red supergiants in the CMD, although IC10 appears to be deficient in these objects given its large WR population. The bright stars of intermediate color in the CMD are badly contaminated by foreground stars (30-100%), and considerable spectroscopy is needed before statistics on the yellow supergiants in these systems will be known. This study is intended to serve both as the impetus and "finding charts" for further space-based imaging, and for many spectroscopic programs at large aperture.Comment: Added Erratum. Revised versions of Tables 12 and 13 can be found at http://www.lowell.edu/users/massey/Table12Revised.txt and http://www.lowell.edu/users/massey/Table13Revised.tx