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Register a new userSmall scale behavior of the physical conditions and the abundance discrepancy in the Orion nebula
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Adal Mesa-Delgado Mesa-Delgado
Publication date
2007
fields
Physics
and research's language is
English
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We present results of long-slit spectroscopy in several positions of the Orion nebula. Our goal is to study the spatial distribution of a large number of nebular quantities, including line fluxes, physical conditions and ionic abundances at a spatial resolution of about 1. We find that protoplanetary disks (proplyds) show prominent spikes of T([N II]) probably produced by collisional deexcitation due to the high electron densities found in these objects. Herbig-Haro objects show also relatively high T([N II]) but probably produced by local heating due to shocks. We also find that the spatial distribution of pure recombination O II and [O III] lines is fairly similar, in contrast to that observed in planetary nebulae. The abundance discrepancy factor (ADF) of O^{++} remains rather constant along the slit positions, except in some particular small areas of the nebula where this quantity reaches somewhat higher values, in particular at the location of the most conspicuous Herbig-Haro objects: HH 202, HH 203, and HH 204. There is also an apparent slight increase of the ADF in the inner 40 around theta^1 Ori C. We find a negative radial gradient of T([O III]) and T([N II]) in the nebula based on the projected distance from theta^1 Ori C. We explore the behavior of the ADF of O^{++} with respect to other nebular quantities, finding that it seems to increase very slightly with the electron temperature. Finally, we estimate the value of the mean-square electron temperature fluctuation, the so-called t^2 parameter. Our results indicate that the hypothetical thermal inhomogeneities --if they exist-- should be smaller than our spatial resolution element.
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HST images, MUSE maps of emission-lines, and an atlas of high velocity resolution emission-line spectra have been used to establish for the firrst time correlations of the electron temperature, electron density, radial velocity, turbulence, and orientation within the main ionization front of the nebula. From the study of the combined properties of multiple features, it is established that variations in the radial velocity are primarily caused by the photo-evaporating ionization front being viewed at different angles. There is a progressive increase of the electron temperature and density with decreasing distance from the dominant ionizing star Theta1 Ori C. The product of these characteristics (NexTe) is the most relevant parameter in modeling a blister-type nebula like the Huygens Region, where this quantity should vary with the surface brightness in Halpha. Several lines of evidence indicate that small-scale structure and turbulence exists down to the level of our resolution of a few arcseconds. Although photo-evaporative ow must contribute at some level to the well-known non-thermal broadening of the emission lines, comparison of quantitative predictions with the observed optical line widths indicate that it is not the major additive broad- ening component. Derivation of Te values for H+ from radio+optical and optical-only ionized hydro- gen emission showed that this temperature is close to that derived from [Nii] and that the transition from the well-known at extinction curve that applies in the Huygens Region to a more normal steep extinction curve occurs immediately outside of the Bright Bar feature of the nebula.
We present results from integral field spectroscopy with PMAS. The observed field contains: five protoplanetary discs (also known as proplyds), the high-velocity jet HH 514 and a bowshock. Spatial distribution maps are obtained for different emission line fluxes, the c(H{beta}) coefficient, electron densities and temperatures, ionic abundances of different ions from collisionally excited lines (CELs), C2+ and O2+ abundances from recombination lines (RLs) and the abundance discrepancy factor of O2+, ADF(O2+). We find that collisional de-excitation has a major influence on the line fluxes in the proplyds. If this is not properly accounted for then physical conditions deduced from commonly used line ratios will be in error, leading to unreliable chemical abundances for these objects. We obtain the intrinsic emission of the proplyds 177-341, 170-337 and 170-334 by a direct subtraction of the background emission, though the last two present some background contamination due to their small sizes. A detailed analysis of 177-341 spectra reveals the presence of high-density gas (3.8times10^5 cm^-3) in contrast to the typical values observed in the background gas of the nebula (3800 cm^-3). We also explore how the background subtraction could be affected by the possible opacity of the proplyd. We construct a physical model for the proplyd 177-341 finding a good agreement between the predicted and observed line ratios. Finally, we find that the use of reliable physical conditions returns an ADF(O2+) about zero for the intrinsic spectra of 177-341, while the background emission presents the typical ADF(O2+) observed in the Orion Nebula. We conclude that the presence of high-density ionized gas is severely affecting the abundances determined from CELs and, therefore, those from RLs should be considered as a better approximation to the true abundances.
Intermediate mass protostars, the bridge between the very common solar-like protostars and the more massive, but rarer, O and B stars, can only be studied at high physical spatial resolutions in a handful of clouds. In this paper we present and analyze the continuum results from an observing campaign at the Submillimeter Array targeting two well-studied intermediate mass protostars in Orion, NGC 2071 and L1641 S3 MMS 1. The extended SMA (eSMA) probes structure at angular resolutions up to 0.2, revealing protostellar disks on scales of 200 AU. Continuum flux measurements on these scales indicate that a significant amount of mass, a few tens of M{odot}, are present. Envelope, stellar, and disk masses are derived using both compact, extended and eSMA configurations and compared against SED-fitting models. We hypothesize that fragmentation into three components occurred within NGC 2071 at an early time, when the envelopes were less than 10% of their current masses, e.g. < 0.5 M{odot}. No fragmentation occurred for L1641 S3 MMS 1. For NGC 2071 evidence is given that the bulk of the envelope material currently around each source was accreted after the initial fragmentation. In addition, about 30% of the total core mass is not yet associated to one of the three sources. A global accretion model is favored and a potential accretion history of NGC 2071 is presented. It is shown that the relatively low level of fragmentation in NGC 2071 was stifled compared to the expected fragmentation from a Jeans argument.
We investigate the physical conditions of the CO gas near the young star cluster, Trumpler 14 of the Carina Nebula. The observations presented in this work are taken with the Fourier Transform Spectrometer (FTS) of the Spectral and Photometric Imaging REceiver (SPIRE) onboard the Herschel Space Observatory. Our field of view covers the edge of a cavity carved by Trumpler 14 about $1,mathrm{Myr}$ ago and marks the transition from HII regions to photo-dissociation regions. With the state-of-the-art Meudon PDR code, we successfully derive the physical conditions, which include the thermal pressure ($P$) and the scaling factor of radiation fields ($G_{mathrm{UV}}$), from the observed CO spectral line energy distributions~(SLEDs) in the observed region. The derived $G_{mathrm{UV}}$ values generally show an excellent agreement with the UV radiation fields created by nearby OB-stars and thus confirm that the main excitation source of the observed CO emission are the UV-photons provided by the massive stars. The derived thermal pressure is between $0.5-3,times,10^{8},mathrm{K,cm^{-3}}$ with the highest values found along the ionization front in Car I-E region facing Trumpler 14, hinting that the cloud structure is similar to the recent observations of the Orion Bar. Comparing the derived thermal pressure with the radiation fields, we report the first observationally-derived and spatially-resolved $P sim 2times10^4,G_{mathrm{UV}}$ relationship. As direct comparisons of the modeling results to the observed $^{13}mathrm{CO}$, [OI] $63,mathrm{mu m}$, and [CII] $158,mathrm{mu m}$ intensities are not straightforward, we urge the readers to be cautious when constraining the physical conditions of PDRs with combinations of $^{12}mathrm{CO}$, $^{13}mathrm{CO}$, [CI], [OI] $63,mathrm{mu m}$, and [CII] $158,mathrm{mu m}$ observations.
The existence of multiple layers in the inner Orion Nebula has been revealed using data from an Atlas of spectra at 2 and 12 km/s resolution. These data were sometimes grouped over Samples of 10x10to produce high Signal to Noise spectra and sometimes grouped into sequences of pseudo-slit Spectra of 12.8--39 width for high spatial resolution studies. Multiple velocity systems were found: Vmif traces the Main Ionization Front (MIF), Vscat arises from back-scattering of Vmif emission by particles in the background Photon Dissociation Region (PDR), Vlow is an ionized layer in front of the MIF and if it is the source of the stellar absorption lines seen in the Trapezium stars, it must lie between the foreground Veil and those stars, Vnew may represent ionized gas evaporating from the Veil away from the observer. There are features such as the Bright Bar where variations of velocities are due to changing tilts of the MIF, but velocity changes above about 25arise from variations in velocity of the background PDR. In a region 25 ENE of the Orion-S Cloud one finds dramatic changes in the [OIII]components, including the signals from the Vlowoiii and Vmifoiii becoming equal, indicating shadowing of gas from stellar photons of >24.6 eV. This feature is also seen in areas to the west and south of the Orion-S Cloud.
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