We report on follow-up observations of the gamma-ray burst GRB 060927 using the robotic ROTSE-IIIa telescope and a suite of larger aperture ground-based telescopes. An optical afterglow was detected 20 s after the burst, the earliest rest-frame detection of optical emission from any GRB. Spectroscopy performed with the VLT about 13 hours after the trigger shows a continuum break at λ ≈ 8070Å, produced by neutral hydrogen absorption at z ≈ 5.6. We also detect an absorption line at 8158Å which we interpret as Si II λ 1260 at z = 5.467. Hence, GRB 060927 is the second most distant GRB with a spectroscopically measured redshift. The shape of the red wing of the spectral break can be fitted by a damped Lyα profile with a column density with log(N HI /cm −2 ) = 22.50 ± 0.15. We discuss the implications of this work for the use of GRBs as probes of the end of the dark ages and draw three main conclusions: i) GRB afterglows originating from z 6 should be relatively easy to detect from the ground, but rapid near-infrared monitoring is necessary to ensure that they are found; ii) The presence of large H I column densities in some GRBs host galaxies at z > 5 makes the use of GRBs to probe the reionization epoch via spectroscopy of the red damping wing challenging; iii) GRBs appear crucial to locate typical star-forming galaxies at z > 5 and therefore the type of galaxies responsible for the reionization of the universe. Subject headings: gamma rays: bursts (GRB 060927) -cosmology 1 Partly based on observations carried out with the ESO telescopes under programmes 077.D-0661, 077.A-0667, 078.D-0416, and the large programme 177.A-f0591.

Using the Very Long Baseline Array (VLBA) we performed a high resolution OH maser survey in Galactic star-forming regions (SFRs). We observed all the ground state spectral lines: the main lines at 1665 and 1667 MHz and the satellite lines at 1612 and 1720 MHz. Due to the exceptionality of finding satellite lines in SFRs, we will focus our discussion on those lines. In our sample of 41 OH maser sources, five (12%) showed the 1612 MHz line and ten (24%) showed the 1720 MHz line, with only one source showing both lines. We find that 1720 MHz emission is correlated with the presence of HII regions, suggesting that this emission could be used to diagnose or trace high-mass star formation. We include an analysis of the possible mechanisms that could be causing this correlation as well as assessing the possible relationships between lines in our sample. In particular, the presence of magnetic fields seems to play an important role, as we found Zeeman splitting in four of our sources (W75 N, W3(OH), W51 and NGC 7538). Our results have implications for current understanding of the formation of high-mass stars as well as on the masing processes present in SFRs.

Estimation of bolometric fluxes (F BOL ) is an essential component of stellar effective temperature determination with optical and near-infrared interferometry. Reliable estimation of F BOL simply from broad-band K-band photometry data is a useful tool in those cases were contemporaneous and/or wide-range photometry is unavailable for a detailed spectral energy distribution (SED) fit, as was demonstrated in Dyck et al. (1974). Recalibrating the intrinsic F BOL versus observed F 2.2µm relationship of that study with modern SED fitting routines, which incorporate the significantly nonblackbody, empirical spectral templates of the INGS spectral library (an update of the library in Pickles 1998) and estimation of reddening, serves to greatly improve the accuracy and observational utility of this relationship. We find that F BOL values predicted are roughly 11% less than the corresponding values predicted in Dyck et al. (1974), indicating the effects of SED absorption features across bolometric flux curves.

Aims. We investigate the molecular and dusty environment of OH/IR stars in order to characterize the mass-loss process during the tip-AGB superwind phase. Methods. Employing the AMBER instrument at the VLT Interferometer we obtained near-infrared H-and K-band spectrointerferometric observations of the three OH/IR stars IRAS 13479-5436, IRAS 14086-6907 and IRAS 17020-5254 with a spectral resolution of about 35. We use a two-component geometrical model, consisting of a uniform disk and a Gaussian disk, to obtain characteristic angular sizes of the central stellar sources and their dust envelopes, as well as the flux ratios between these components. Results. Angular uniform disk diameters of the three central components of the objects above have values between 3.2 mas and 5.4 mas. For their dust envelopes, we find FWHM values between 17.1 mas and 25.2 mas. The three objects show significantly different flux contributions of the shells to the total near-IR flux of 61%, 38%, and 16% for IRAS 13479-5436, IRAS 14086-6907, and IRAS 17020-5254, respectively. According to distance estimates from the literature, the central stellar components have radii between 900 R and 1400 R , while their dust envelopes reach FWHM values between 9000 R and 13 000 R . The visibility functions of all three sources exhibit wavelength variations that resemble those of earlier VLTI/AMBER observations of semi-regular and Mira variable AGB stars. These are interpreted as characteristic of atmospheric molecular layers lying above the photosphere. Conclusions. The derived characteristic sizes of both, the central stellar atmospheres and dust envelopes are consistent with the canonical properties of OH/IR stars. The spectral visibility variations resemble those of other AGB stars and indicate the presence of molecular layers, confirming that these are a common phenomenon among AGB stars of very different luminosities and mass-loss rates, alike. We also find that the dust envelopes have a clearly larger optical depth than those known for Mira stars. We interpret this as an expected result of the "superwind" phase, the final 10 000 to 30 000 years of AGB-evolution, when the mass-loss rate increases by a factor of 10-100. By their different optical depths, the three dust shells studied here may represent different stages of the "superwind" and different initial masses.

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