Mónica Rodríguez

A reappraisal of the chemical composition of the Orion nebula based on Very Large Telescope echelle spectrophotometry

We present Very Large Telescope (VLT) UVES echelle spectrophotometry of the Orion nebula in the 3100-10 400 A range. We have measured the intensity of 555 emission lines, many of them corresponding to permitted lines of different heavy-element ions. This is the largest set of spectral emission lines ever obtained for a Galactic or extragalactic H II region. We have derived He + , C 2+ , O + , O 2+ and Ne 2+ abundances from pure recombination lines. This is the first time that O + and Ne 2+ abundances have been obtained from these kinds of lines in the nebula. We have also derived abundances from collisionally excited lines for a large number of ions of different elements. In all cases, ionic abundances obtained from recombination lines are larger than those derived from collisionally excited lines. We have obtained remarkably consistent independent estimations of the temperature fluctuation parameter, t 2 , from different methods, which are also similar to other estimates from the literature. This result strongly suggests that moderate temperature fluctuations (t 2 between 0.02 and 0.03) are present in the Orion nebula. We have compared the chemical composition of the nebula with those of the Sun and other representative objects. The heavy-element abundances in the Orion nebula are only slightly higher than the solar ones, a difference that can be explained by the chemical evolution of the solar neighbourhood.

Carbon and Oxygen Galactic Gradients: Observational Values from H II Region Recombination Lines*

We present results of deep echelle spectrophotometry of eight Galactic H ii regions located at Galactocentric distances between 6.3 and 10.4 kpc. The data have been taken with the Very Large Telescope Ultraviolet Echelle Spectrograph in the 3100–10360 range. We have derived C and O abundances from recombination lines for u A all the objects as well as O abundances from this kind of line for three of the nebulae. The intensity of recombination lines is almost independent of the assumed electron temperature as well as of the possible presence of spatial temperature variations or fluctuations inside the nebulae. These data allow the determination of the gas-phase C and O abundance gradients of the Galactic disk, of paramount importance for chemical evolution models. This is the first time the C gradient is derived from such a large number of H ii regions distributed in such a wide range of Galactocentric distances. Abundance gradients are found of the form dex kpc 1 , D log (O/H) p 0.044 0.010 dex kpc 1 , and dex kpc 1 . D log (C/H) p 0.103 0.018 D log (C/O) p 0.058 0.018 Subject headings: Galaxy: abundances — H ii regions — ISM: abundances

The Localized Chemical Pollution in NGC 5253 Revisited: Results from Deep Echelle Spectrophotometry*

WepresentechellespectrophotometryofthebluecompactdwarfgalaxyNGC 5253obtainedwiththeVLTUVES. We have measured the intensities of a large number of permitted and forbidden emission lines in four zones of the centralpartof thegalaxy.Wedetect faint C iiand O iirecombinationlines, thefirsttimethat these areunambiguously detected in a dwarf starburst galaxy. The physical conditions of the ionized gas have been derived using a large number of different line intensity ratios. Chemical abundances of He, N, O, Ne, S, Cl, Ar, and Fe have been determined following standardmethods.C ++ andO ++ abundanceshavebeenderivedfrompurerecombinationlinesand arelargerthanthoseobtainedfromcollisionallyexcitedlines(from0.30to0.40dexforC ++ andfrom0.19to0.28dex for O ++ ). This result is consistent with atemperaturefluctuation parameter (t 2 ) between 0.050 and 0.072. Weconfirm previous results that indicate the presence of a localized N enrichment in certain zones of NGC 5253 and detect a possible slight He overabundance in the same zones. The enrichment pattern agrees with that expected for the pollution by the ejecta of Wolf-Rayet (W-R) stars. The amount of enriched material needed to produce the observed overabundance is consistent with the mass lost by the number of W-R stars estimated in the starbursts. We discuss the possible origin of the difference between abundances derived from recombination and collisionally excited lines (the so-called abundance discrepancy problem) in H ii regions, finding that a recent hypothesis based on the delayed enrichment by SN ejecta inclusions seems not to explain the observed features. Subject headingg galaxies: abundances — galaxies: clusters: individual (NGC 5253) — galaxies: kinematics and dynamics — galaxies: starburst Online material: color figures, machine-readable table

Optical Recombination Lines of Heavy Elements in Giant Extragalactic H II Regions

We present high-resolution observations of the giant extragalactic H II regions NGC 604, NGC 2363, NGC 5461, and NGC 5471, based on observations taken with the ISIS spectrograph on the William Herschel Telescope. We have detected, for the first time, C II and O II recombination lines in these objects. We find that recombination lines give larger C++ and O++ abundances than collisionally excited lines, suggesting that temperature variations may be present in the objects. We detect [Fe IV] lines in NGC 2363 and NGC 5471, the most confident detection of optical lines of this kind in H II regions. Considering the temperature structure, we derive their H, He, C, N, O, Ne, S, Ar, and Fe abundances. From the recombination lines of NGC 5461 and NGC 5471, we determine the presence of C/H and O/H gradients in M101. We calculate the ΔY/ΔO and ΔY/ΔZ values considering the presence of temperature variations and under the assumption of constant temperature. We obtain a better agreement with models of galactic chemical evolution by considering the presence of temperature variations than by assuming that the temperature is constant in these nebulae.

Properties of the ionized gas in HH 202 – II. Results from echelle spectrophotometry with Ultraviolet Visual Echelle Spectrograph

We present results of deep echelle spectrophotometry of the brightest knot of the Herbig– Haro object HH 202 in the Orion Nebula – HH 202-S – using the Ultraviolet Visual Echelle Spectrograph in the spectral range from 3100 to 10 400 A. The high spectral resolution of the observations has permitted to separate the component associated with the ambient gas from that associated with the gas flow. We derive electron densities and temperatures from different diagnostics for both components, as well as the chemical abundances of several ions and elements from collisionally excited lines, including the first determinations of Ca + and Cr + abundances in the Orion Nebula. We also calculate the He + ,C 2+ ,O + and O 2+ abundances from recombination lines. The difference between the O 2+ abundances determined from collisionally excited and recombination lines – the so-called abundance discrepancy factor – is 0.35 and 0.11 dex for the shock and nebular components, respectively. Assuming that the abundance discrepancy is produced by spatial variations in the electron temperature, we derive values of the temperature fluctuation parameter, t 2 , of 0.050 and 0.016 for the shock and nebular components, respectively. Interestingly, we obtain almost coincident t 2 values for both components from the analysis of the intensity ratios of He I lines. We find significant departures from case B predictions in the Balmer and Paschen flux ratios of lines of high principal quantum number n. We analyse the ionization structure of HH 202-S, finding enough evidence to conclude that the flow of HH 202-S has compressed the ambient gas inside the nebula trapping the ionization front. We measure a strong increase of the total abundances of nickel and iron in the shock component, the abundance pattern and the results of photoionization models for both components are consistent with the partial destruction of dust after the passage of the shock wave in HH 202-S.

Faint emission lines in the Galactic H ii regions M16, M20 and NGC 3603★

We present deep echelle spectrophotometry of the Galactic H II regions M16, M20 and NGC 3603. The data have been taken with the Very Large Telescope Ultraviolet-Visual Echelle Spectrograph in the 3100‐10 400 A range. We have detected more than 200 emission lines in each region. Physical conditions have been derived using different continuum and line intensity ratios. We have derived He + ,C ++ and O ++ abundances from pure recombination lines as well as collisionally excited lines (CELs) for a large number of ions of different elements. We have obtained consistent estimations of the temperature fluctuation parameter, t 2 , using different methods. We also report the detection of deuterium Balmer lines up to Dδ (M16) and to Dγ (M20) in the blue wings of the hydrogen lines, which excitation mechanism seems to be continuum fluorescence. The temperature fluctuation paradigm agrees with the results obtained from optical CELs, and the more uncertain ones from far-infrared fine-structure CELs in NGC 3603, although, more observations covering the same volume of the nebula are necessary to obtain solid conclusions.

Multiepoch VLBA Observations of T Tauri South

In this Letter, we present a series of seven observations of the compact, nonthermal radio source associated with T Tauri South made with the Very Long Baseline Array (VLBA) over the course of 1 year. The emission is found to be composed of a compact structure most certainly originating from the magnetosphere of an underlying pre-main-sequence star and a low brightness extension that may result from reconnection flares at the star-disk interface. The accuracy of the absolute astrometry offered by the VLBA allows us to make very precise determinations of the trigonometric parallax and proper motion of T Tau South. The proper motion derived from our VLBA observations agrees with that measured with the Very Large Array over a similar period to better than 2 mas yr-1, and it is fully consistent with the infrared proper motion of T Tau Sb, the pre-main-sequence M star with which the radio source has traditionally been associated. The parallax, π = 7.07 ± 0.14 mas, corresponds to a distance of 141.5 pc.

The [Fe IV] discrepancy : Constraining the iron abundances in nebulae

We study the current discrepancy between the model-predicted and measured concentrations of Fe++ and Fe+3 in ionized nebulae. We calculate a set of photoionization models, updated with the atomic data relevant to the problem, and compare their results with those derived for the available nebulae where both [Fe III] and [Fe IV] lines have been measured. Our new model results are closer to the measured values than the results of previous calculations, but a discrepancy remains. This discrepancy translates into an uncertainty in the derived Fe abundances of a factor of up to ~4. We explore the possible causes of this discrepancy and find that errors in the Fe atomic data may be the most likely explanation. The discrepancy can be fully accounted for by any of the following changes: (1) an increase by a factor of ~10 in the recombination rate (radiative plus dielectronic, or charge transfer) for Fe+3, (2) an increase by a factor of 2-3 in the effective collision strengths for Fe++, or (3) a decrease by a factor of 2-3 in the effective collision strengths for Fe+3. We derive the Fe abundances implied by these three explanations and use the results to constrain the degree of depletion of Fe in our sample nebulae. The Galactic H II regions and planetary nebulae are found to have high depletion factors, with less than 5% of their Fe atoms in the gas phase. The extragalactic H II regions (LMC 30 Doradus, SMC N88A, and SBS 0335-052) have somewhat lower depletions. The metal-deficient blue compact galaxy SBS 0335-052 could have from 13% to 40% of Fe in the gas phase. The depletions derived for the different objects define a trend of increasing depletion at higher metallicities.

Deep echelle spectrophotometry of S 311, a Galactic H ii region located outside the solar circle

We present echelle spectrophotometry of the Galactic H II region S 311. The data have been taken with the Very Large Telescope Ultraviolet-Visual Echelle Spectrograph in the 3100‐ 10 400 A range. We have measured the intensities of 263 emission lines; 178 are permitted lines of H 0 ,D 0 (deuterium), He 0 ,C 0 ,C + ,N 0 ,N + ,O 0 ,O + ,S + ,S i 0 ,S i + ,A r 0 and Fe 0 ; some of them are produced by recombination and others mainly by fluorescence. Physical conditions have been derived using different continuum- and line-intensity ratios. We have derived He + , C ++ and O ++ ionic abundances from pure recombination lines as well as abundances from collisionally excited lines for a large number of ions of different elements. We have obtained consistent estimations of t 2 applying different methods. We have found that the temperature fluctuations paradigm is consistent with the T e(He I )v ersus T e(H I) relation for H II regions, in contrast with what has been found for planetary nebulae. We report the detection of deuterium Balmer lines up to Dδ in the blue wings of the hydrogen lines, whose excitation mechanism seems to be continuum fluorescence. Ke yw ords: ISM: abundances ‐ H II regions ‐ ISM: individual: S 311.

Enrichment of the interstellar medium by metal-rich droplets and the abundance bias in H ii regions

We critically examine a scenario for the enrichment of the interstellar medium (ISM) in which supernova ejecta follow a long (10 8 yr) journey before falling back onto the galactic disk in the form of metal-rich “droplets”, These droplets do not become fully mixed with the interstellar medium until they become photoionized in H ii regions. We investigate the hypothesis that the photoionization of these highly metallic droplets can explain the observed “abundance discrepancy factors” (ADFs), which are found when comparing abundances derived from recombination lines and from collisionally excited lines, both in Galactic and extragalactic H ii regions. We derive bounds of 10 13 –10 15 cm on the droplet sizes inside H ii regions in order that (1) they should not have already been detected by direct imaging of nearby nebulae, and (2) they should not be too swiftly destroyed by diffusion in the ionized gas. From photoionization modelling we find that, if this inhomogeneous enrichment scenario holds, then the recombination lines strongly overestimate the metallicities of the fully mixed H ii regions. The abundances derived from collisionally excited lines also suffer some bias, although to a much lesser extent. In the absence of any recipe for correcting these biases, we recommend the discarding of all objects showing large ADFs from studies of galactic chemical evolution. These biases must also be kept in mind when comparing the galactic abundance gradients for elements derived from recombination lines with those derived from collisionally excited lines. Finally, we propose a set of observations that could be undertaken to test our scenario and improve our understanding of element mixing in the ISM.