Atomic Physics with the Goddard High Resolution Spectrograph on the Hubble Space Telescope. V. Oscillator Strengths for Neutral Carbon Lines below 1200 Angstrom

We analyzed high resolution spectra of interstellar neutral carbon absorption toward $\lambda$ Ori, 1 Sco, and $\delta$ Sco that were obtained with the Goddard High Resolution Spectrograph on the Hubble Space Telescope. Several multiplets were detected within the wavelength interval 1150 to 1200 A, where most neutral carbon lines have ill-defined oscillator strength; multiplets at longer wavelengths with well-defined atomic parameters were also seen. We extracted accurate column densities and Doppler parameters from lines with precise laboratory-based f-values. These column densities and b-values were used to obtain a self-consistent set of f-values for all the observed neutral carbon lines. For many of the lines with wavelength below 1200 A, the derived f-values differ appreciably from the values quoted in the compilation by Morton (1991). The present set of f-values extends and in some cases supersedes those given in Zsargo et al. (1997), which were based on lower resolution data.Comment: 8 pages, 2 figures, 6 tables, accepted for publication in Ap

Atomic Physics with the Goddard High Resolution Spectrograph on the Hubble Space Telescope

High quality spectra of interstellar absorption from C I toward beta(sup 1) S(sub co), rho O(sub ph) A, and chi O(sub ph) were obtained with the Goddard High Resolution Spectrograph on HST. Many weak lines were detected within the observed wavelength intervals: 1150-1200 A for beta(sup 1) S(sub co) and 1250-1290 A for rho O(sub ph) A and chi O(sub ph). Curve-of-growth analyses were performed in order to extract accurate column densities and Doppler parameters from lines with precise laboratory-based f-values. These column densities and b-values were used to obtain a self-consistent set of f-values for all the observed C I lines. A particularly important constraint was the need to reproduce data for more than one line of sight. For about 50% of the lines, the derived f-values differ appreciably from the values quoted by Morton

Obscured clusters.IV. The most massive stars in [DBS2003]179

Aims. We report new results for the massive evolved and main sequence members of the young galactic cluster DBS2003 179. We determine the physical parameters and investigate the high-mass stellar content of the cluster, as well as of its close vicinity. Methods. Our analysis is based on ISAAC/VLT moderate-resolution (R\approx4000) infrared spectroscopy of the brightest cluster members. We derive stellar parameters for sixteen of the stellar members, using full non-LTE modeling of the obtained spectra. Results. The cluster contains three late WN or WR/LBV stars (Obj 4, Obj 15, and Obj 20:MDM32) and at least 5 OIf and 5 OV stars. According to the Hertzsprung-Russell diagram for DBS2003 179, the WR stars show masses above 85Msun, the OIf stars are between 40 and 80Msun, and the main sequence O stars are >20Msun. There are indications of binarity for Obj 4 and Obj 11, and Obj 3 shows a variable spectrum. The cluster is surrounded by a continuous protostar formation region most probably triggered by DBS2003 179. Conclusions. We confirm that DBS2003 179 is young massive cluster (2.5 10^4Msun) very close to the Galactic center at the distance of 7.9+-0.8 kpc.Comment: 14 pages, 16 figures, accepted in A&

Confirmation of the Luminous Blue Variable status of MWC 930

We present spectroscopic and photometric observations of the emission-line star MWC 930 (V446 Sct) during its long-term optical brightening in 2006--2013. Based on our earlier data we suggested that the object has features found in Luminous Blue Variables (LBV), such as a high luminosity (~3 10^5 Lsun, a low wind terminal velocity (~ 140 km/s), and a tendency to show strong brightness variations (~1 mag over 20 years). For the last ~7 years it has been exhibiting a continuous optical and near-IR brightening along with a change of the emission-line spectrum appearance and cooling of the star's photosphere. We present the object's $V$--band light curve, analyze the spectral variations, and compare the observed properties with those of other recognized Galactic LBVs, such as AG Car and HR Car. Overall we conclude the MWC 930 is a bona fide Galactic LBV that is currently in the middle of an S Dor cycle.Comment: 12 pages, 7 figure

The Lyman-alpha Forest and Heavy Element Systems of GB1759+7539

(abridged) We present observations of the high-redshift QSO GB1759+7539 (z=3.05) obtained with HIRES on the Keck 10m telescope. The spectrum has a resolution of FWHM = 7 km/s, and a typical signal-to-noise ratio per 2 km/s pixel of ~25 in the Ly-alpha forest region, and ~60 longward of the Ly-alpha emission. The observed Ly-alpha forest systems have a mean redshift of z=2.7. There is marginal evidence of clustering along the line of sight over the velocity range 100 < v < 250 km/s. The 1-point and 2-point joint probability distributions of the transmitted flux for the Ly-alpha forest were calculated, and shown to be very insensitive to the heavy element contamination. We could find no evidence of Voigt profile departures due to infalling gas, as observed in the simulated forest spectra. Twelve heavy-element absorption systems were identified, including damped Lyman-alpha systems at z=2.62 and 2.91. The C, N, O, Al, Si, P, S, Mg, Fe, and Ni absorption features of these systems were studied, and the elemental abundances calculated for the weak unsaturated lines. The systems have metallicities of Z(2.62) ~1/20 Z(solar) and Z(2.91) \~1/45 Z(solar). Both systems appear to have a low dust content. They show an over-abundance of alpha-elements relative to Fe-peak elements, and an under-abundance of odd atomic number elements relative to even. Nitrogen was observed, and found to be under-abundant relative to oxygen, in line with the time delay model of primary nitrogen production. C II* was also seen, allowing us to determine an upper limit to the cosmic microwave background temperature at z=2.62 of T(CMB)<12.9K.Comment: Added reference, and new S & N abundance determinations for z=2.91 syste

Further Evidence for Chemical Fractionation from Ultraviolet Observations of Carbon Monoxide

Ultraviolet absorption from interstellar 12CO and 13CO was detected toward rho Oph A and chi Oph. The measurements were obtained at medium resolution with the Goddard High Resolution Spectrograph on the Hubble Space Telescope. Column density ratios, N(12CO)/N(13CO), of 125 \pm 23 and 117 \pm 35 were derived for the sight lines toward rho Oph A and chi Oph, respectively. A value of 1100 \pm 600 for the ratio N(12C16O)/N(12C18O) toward rho Oph A was also obtained. Absorption from vibrationally excited H_2 (v" = 3) was clearly seen toward this star as well. The ratios are larger than the isotopic ratios for carbon and oxygen appropriate for ambient interstellar material. Since for both carbon and oxygen the more abundant isotopomer is enhanced, selective isotopic photodissociation plays the key role in the fractionation process for these directions. The enhancement arises because the more abundant isotopomer has lines that are more optically thick, resulting in more self shielding from dissociating radiation. A simple argument involving the amount of self shielding [from N(12CO)] and the strength of the ultraviolet radiation field premeating the gas (from the amount of vibrationally excited H_2) shows that selective isotopic photodissociation controls the fractionation seen in these two sight lines, as well as the sight line to zeta Oph.Comment: 40 pages, 8 figures, to appear in 10 July 2003 issue of Ap

Non-Thermal Chemistry in Diffuse Clouds with Low Molecular Abundances

High quality archival spectra of interstellar absorption from C I toward 9 stars, taken with the Goddard High Resolution Spectrograph on the Hubble Space Telescope, were analyzed. Our sample was supplemented by two sight lines, 23 Ori and B1 Sco, for which the C I measurements of Federman, Welty, & Cardelli were used. Directions with known CH+ absorption, but only upper limits on absorption from C2 and CN, were considered for our study. This restriction allows us to focus on regions where CH+ chemistry dominates the production of carbon-bearing molecules. Profile synthesis of several multiplets yielded column densities and Doppler parameters for the C I fine structure levels. Equilibrium excitation analyses, using the measured column densities as well as the temperature from H2 excitation, led to values for gas density. These densities, in conjunction with measurements of CH, CH+, C2, and CN column densities, provided estimates for the amount of CH associated with CH+ production, which in turn set up constraints on the present theories for CH+ formation in this environment. We found for our sample of interstellar clouds that on average, 30--40 % of the CH originates from CH+ chemistry, and in some cases it can be as high as 90 %. A simple chemical model for gas containing non-equilibrium production of CH+ was developed for the purpose of predicting column densities for CH, CO, HCO+, CH2+, and CH3+ generated from large abundances of CH+. Again, our results suggest that non-thermal chemistry is necessary to account for the observed abundance of CH and probably that of CO in these clouds.Comment: 45 pages, 3 figures, Accepted for publication in the Astrophysical Journa