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Register a new userUVES-VLT Observations of the OIII Bowen Lines in RR Tel
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Authors
Pierluigi Selvelli
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The exceptional resolution of UVES has allowed the detection of weak spectral features and the separation of components in blended lines. The intensities of all of the OIII fluorescence lines produced by the O1, O3 and other channels, including the 5592 Angstrom charge-exchange line, have been measured and their ratios compared with models.
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We present here the first results of UVES observations of RR Tel. The exceptional performances of the instrument have allowed the detection of new spectral features and have led to an improvement in the identifications of several emission lines. A direct comparison with the IUE observations in the range 3045-3300 A has revealed the appearance of many weak and shallow lines, most of them lacking a convincing identification. The Balmer lines are visible up to H38 and are accompanied by the He II lines of the Pickering series. Also, all of the He II emissions of the Pfund series, from 5858 A to 6408 A have been detected. We made definite identifications of additional TiO bands at 4955 A (alpha R_2 1-0), 5167 A (alpha R_2 0-0), 5445 A (alpha R_2 0-1), 5598 A (beta R_1 0-0), 5847 A (gamma R_1 1-0), and 6148 A (gamma {^S} R_21 0-0). The Halpha line has very wide wings, extending to at least 5000 km/s, which are similar to those reported for the planetary nebula IC 4997 and attributed to Raman scattering by Ly beta photons. A selective pumping mechanism via the HeII 237 A emission is proposed to explain the intensity of the high-lying lines of O IV mult. 1 and 2.
A new measure of reddening (E$_{(B-V)}$$sim$0.00) has been obtained from the comparison between the observed and the theoretical intensity decrement for 20 emission lines of the $ion{He}{ii}$ Fowler (n$to$3) series. This value has been confirmed by the STIS and IUE continuum distribution, and by the value of n$_H$ from the damped profile of the IS H Ly-$alpha$ line. We have obtained very accurate measurements for about thirty Bowen lines of $ion{O}{iii}$ and a precise determination of the efficiency in the O1 and O3 excitation channels (18 % and 0.7 %, respectively). The relative $ion{O}{iii}$ intensities are in good agreement with the predictions by Froese Fischer (1994). A detailed study of the decays from all levels involved in the Bowen mechanism has lead to the detection of two new $ion{O}{iii}$ Bowen lines near $lambda$ 2190. High resolution IUE data have shown a nearly linear decline with time, from 1978 to 1995, in the efficiency of the O1 and O3 processes, with a steeper slope for the O3 channel. A detailed study of the $ion{N}{iii}$ $lambda$ 4640 lines and of their excitation mechanism has shown that, recombination and continuum fluorescence being ruled out, line fluorescence remains the only viable mechanism to pump the 3d $^2D_{5/2}$ and 3d $^2D_{3/2}$ levels of $ion{N}{iii}$. We point out the important role of multiple scattering in the resonance lines of $ion{O}{iii}$ and $ion{N}{iii}$ near $lambda$ 374 and show that the observed $ion{N}{iii}$ line ratios and intensities can be explained in terms of line fluorescence by the three resonance lines of $ion{O}{iii}$ at $lambda$$lambda$ 374.432, 374.162 and 374.073 under optically thick conditions.
From the available STIS data of RR Tel, that have provided a coverage with absolutely calibrated data in a wide wavelength range, we have obtained a new determination of its reddening (E(B-V)=0.00) from the comparison of the observed HeII Paschen lines decrement relative to HeII lambda 4686 (for 24 HeII Paschen lines down to the region of the head of the series near lambda 2060 A) with the theoretical one as given in Storey and Hummer (1995) for case B, T=10,000 K and log Ne=6. This new E(B-V)=0.0 value has been confirmed from a re-analysis of the IUE low resolution data. We recall that the so far generally adopted value in the literature has been E(B-V)=0.10 as obtained by Penston et al. (1983).
We discuss the abundances of interstellar CH, CH+, and CN in the Magellanic Clouds (MC), derived from spectra of 7 SMC and 13 LMC stars obtained (mostly) with the VLT/UVES. CH and/or CH+ are detected toward 3 SMC and 9 LMC stars; CN is detected toward 2 stars. In the MC, the CH/H2 ratio is comparable to that found for diffuse Galactic molecular clouds in some sight lines, but is lower by factors up to 10-15 in others. The abundance of CH in the MC thus appears to depend on local physical conditions -- and not just on metallicity. The observed relationships between the column density of CH and those of CN, CH+, Na I, and K I in the MC are generally consistent with the trends observed in our Galaxy. Using existing data for the rotational populations of H2, we estimate temperatures, radiation field strengths, and local hydrogen densities for the diffuse molecular gas. Densities estimated from N(CH), assuming that CH is produced via steady-state gas-phase reactions, are considerably higher; much better agreement is found by assuming that the CH is made via the (still undetermined) process(es) responsible for the observed CH+. The UVES spectra also reveal absorption from the diffuse interstellar bands at 5780, 5797, and 6284 A in the MC. On average, the three DIBs are weaker by factors of 7-9 (LMC) and about 20 (SMC), compared to those observed in Galactic sight lines with similar N(H I), and by factors of order 2-6, relative to E(B-V), N(Na I), and N(K I). The detection of several of the ``C2 DIBs, with strengths similar to those in comparable Galactic sight lines, however, indicates that no single, uniform scaling factor (e.g., one related to metallicity) applies to all DIBs (or all sight lines) in the MC. (abstract abridged)
We analyse the properties of MgII absorption systems detected along the sightlines toward GRBs using a sample of 10 GRB afterglow spectra obtained with VLT-UVES over the past six years. The S/N ratio is sufficiently high that we can extend previous studies to smaller equivalent widths (typically Wr>0.3A). Over a pathlength of Delta(z)~14 the number of weak absorbers detected is similar along GRB and QSO lines of sight, while the number of strong systems is larger along GRB lines of sight with a 2-sigma significance. Using intermediate and low resolution observations reported in the literature, we increase the absorption length for strong systems to Delta(z)=31.5 (about twice the path length of previous studies) and find that the number density of strong MgII systems is a factor of 2.1+/-0.6 higher (about 3-sigma significance) toward GRBs as compared to QSOs, about twice smaller however than previously reported. We divide the sample in three redshift bins and we find that the number density of strong MgII is larger in the low redshift bins. We investigate in detail the properties of strong MgII systems observed with UVES. Both the estimated dust extinction in strong GRB MgII systems and the equivalent width distribution are consistent with what is observed for standard QSO systems. We find also that the number density of (sub)-DLAs per unit redshift in the UVES sample is probably twice larger than what is expected from QSO sightlines which confirms the peculiarity of GRB lines of sight. These results indicate that neither a dust extinction bias nor different beam sizes of the sources are viable explanations for the excess. It is still possible that the current sample of GRB lines of sight is biased by a subtle gravitational lensing effect. More data and larger samples are needed to test this hypothesis. (abridged)
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