6 research outputs found
Heavy elements in Galactic and Magellanic Cloud HII regions: recombination-line versus forbidden-line abundances
We have obtained deep optical, long-slit spectrophotometry of the Galactic
HII regions M 17, NGC 3576 and of the Magellanic Cloud HII regions 30 Doradus,
LMC N11B and SMC N66, recording the optical recombination lines (ORLs) of CII,
NII and OII. Temperature-insensitive ORL C2+/O2+ and N2+/O2 ratios are obtained
for all nebulae except SMC N66. The ORL C2+/O2+ ratios show remarkable
agreement within each galactic system, while also being in agreement with the
corresponding CEL ratios. For all five nebulae, the O2+/H+ abundance derived
from multiple OII ORLs is found to be higher than the corresponding value
derived from the strong [OIII] 4959, 5007A CELs, by factors of 1.8--2.7 for
four of the nebulae. The LMC N11B nebula exhibits a more extreme discrepancy
factor for the O2+ ion, ~5. Thus these HII regions exhibit ORL/CEL abundance
discrepancy factors that are similar to those previously encountered amongst
planetary nebulae.
Our optical CEL O2+/H+ abundances agree to within 20-30 per cent with
published O2+/H+ abundances that were obtained from observations of infrared
fine-structure lines. Since the low excitation energies of the latter make them
insensitive to variations about typical nebular temperatures, fluctuations in
temperature are ruled out as the cause of the observed ORL/CEL O2+ abundance
discrepancies. We present evidence that the observed OII ORLs from these HII
regions originate from gas of very similar density (<3500 cm-3) to that
emitting the observed heavy-element optical and infrared CELs, ruling out
models that employ high-density ionized inclusions in order to explain the
abundance discrepancy. We consider a scenario whereby much of the heavy-element
ORL emission originates from cold (<=500 K) metal-rich ionized regions.Comment: 24 pages; 9 figures; accepted by Monthly Notices of the Royal
Astronomical Societ
A photoionization modeling study of 30 Doradus: the case for small-scale chemical inhomogeneity
Photoionization models of the giant HII region 30 Doradus are built and
confronted to available UV, optical, IR (ISO) and radio spectra, under
black-body or CoStar SEDs for the primary source and various density
distributions for the nebular gas. Chemically homogeneous models show very
small rms electron temperature fluctuations and fail to reproduce the heavy
element optical recombination line (ORL) spectrum of the nebula. Dual abundance
models incorporating small-scale chemical inhomogeneities in the form of
H-deficient inclusions which are in pressure balance with the normal
composition ambient gas, provide a better fit to the observed heavy element
ORLs and other nebular lines, while most spectral features are satisfactorily
accounted for. The inclusions, whose mass is ~2 per cent of the total gaseous
mass, are 2-3 times cooler and denser than the ambient nebula. Their O/H
abundance ratio is ~0.9 dex larger than in the normal composition gas and have
typical mass fractions of X = 0.687, Y = 0.273 and Z = 0.040. Helium is found
to be about as deficient as hydrogen in the inclusions, while elements heavier
than Ne, such as S and Ar, are quite possibly enhanced in proportions similar
to O. This suggests that the posited H-deficient inclusions may have arisen
from partial mixing of matter which was nucleosynthetically processed in a
supernova event with gas of normal LMC composition. Attention is drawn to a
bias in the determination of HII region helium abundances in the presence of
H-deficient inclusions. It is argued that these results provide evidence for
incomplete small-scale mixing of the ISM. The case for the existence of
abundance inhomogeneities in HII regions is examined in the light of current
theoretical considerations regarding the process of homogenization in the ISM.Comment: 19 pages; 8 figures; MNRAS in pres
First scientific observations with MEGARA at GTC
On June 25th 2017, the new intermediate-resolution optical IFU and MOS of the 10.4-m GTC had its first light. As part of the tests carried out to verify the performance of the instrument in its two modes (IFU and MOS) and 18 spectral setups (identical number of VPHs with resolutions R=6000-20000 from 0.36 to 1 micron) a number of astronomical objects were observed. These observations show that MEGARA@GTC is called to fill a niche of high-throughput, intermediateresolution IFU and MOS observations of extremely-faint narrow-lined objects. Lyman-α absorbers, star-forming dwarfs or even weak absorptions in stellar spectra in our Galaxy or in the Local Group can now be explored to a new level. Thus, the versatility of MEGARA in terms of observing modes and spectral resolution and coverage will allow GTC to go beyond current observational limits in either depth or precision for all these objects. The results to be presented in this talk clearly demonstrate the potential of MEGARA in this regard