Evolution Of Nebulae Research Articles

Unresolved questions regarding the origins of Solar System solids

Restricted accessMoreSectionsView PDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareShare onFacebookTwitterLinked InRedditEmail Cite this article Cassen Patrick 2001Unresolved questions regarding the origins of Solar System solidsPhil. Trans. R. Soc. A.3591935–1947http://doi.org/10.1098/rsta.2001.0888SectionRestricted accessUnresolved questions regarding the origins of Solar System solids Patrick Cassen Patrick Cassen SETI Institute, 2035 Landings Drive, Mountain View, CA 94043, USA and NASA Ames Research Center, 245–3, Moffett Field, CA 94035, USA Google Scholar Find this author on PubMed Search for more papers by this author Patrick Cassen Patrick Cassen SETI Institute, 2035 Landings Drive, Mountain View, CA 94043, USA and NASA Ames Research Center, 245–3, Moffett Field, CA 94035, USA Google Scholar Find this author on PubMed Search for more papers by this author Published:15 October 2001https://doi.org/10.1098/rsta.2001.0888AbstractIt is remarkable that we possess samples of Solar System solids which have retained a record of galactic and stellar events that occurred well before the birth of the Sun, as well as samples which contain evidence of Solar System processes that occurred during the earliest stages of planet building. The signatures of such processes are most commonly recognized in isotopic anomalies, identifiable against a pervasive background of isotopic homogeneity, and systematic trends in the elemental abundance patterns of primitive meteoritic materials. Although the significance of these anomalies and patterns is unmistakable, their interpretation in terms of specific processes is problematic. Central to such interpretations are questions of spatial scale: do cosmogonically significant isotopic and elemental compositions reflect processes that occurred on the grand scale of the proto–solar cloud, or do they result from many localized events within a nebular or planetary environment? The question is fundamental to our understanding of the formation of solid objects in the Solar System. This question will be examined here, with specific reference to theoretical models of nebular evolution and planet building, and evidence regarding the survival of presolar signatures, the origin of short–lived radionuclides and oxygen isotopic systematics. Previous ArticleNext Article VIEW FULL TEXT DOWNLOAD PDF FiguresRelatedReferencesDetailsCited by Scott E and Krot A (2014) Chondrites and Their Components Treatise on Geochemistry, 10.1016/B978-0-08-095975-7.00104-2, (65-137), . Scott E and Krot A (2007) Chondrites and Their Components Treatise on Geochemistry, 10.1016/B0-08-043751-6/01145-2, (1-72), . Marcus R (2004) Mass-independent isotope effect in the earliest processed solids in the solar system: A possible chemical mechanism, The Journal of Chemical Physics, 10.1063/1.1803507, 121:17, (8201), . Hutchison R, Pillinger C, Turner G, Russell S and Allègre C (2001) Condensed matter astrophysics: constraints and questions on the early development of the Solar System, Philosophical Transactions of the Royal Society of London. Series A: Mathematical, Physical and Engineering Sciences, 359:1787, (2137-2155), Online publication date: 15-Oct-2001. Vollstaedt H, Mezger K and Alibert Y (2020) Carbonaceous Chondrites and the Condensation of Elements from the Solar Nebula, The Astrophysical Journal, 10.3847/1538-4357/ab97b4, 897:1, (82) This Issue15 October 2001Volume 359Issue 1787Discussion Meeting Issue ‘Origin and early evolution of solid matter in the Solar System’ organized by Robert Hutchison, Colin Pillinger, Grenville Turner and Sara Russell Article InformationDOI:https://doi.org/10.1098/rsta.2001.0888Published by:Royal SocietyPrint ISSN:1364-503XOnline ISSN:1471-2962History: Published online15/10/2001Published in print15/10/2001 License: Citations and impact Keywordsplanetssolar nebulameteoriteselemental compositionisotopic composition

Multiwavelength analyses of the extraordinary nova LMC 1991

LMC 91 was a very fast, classical nova and the brightest nova ever observed in the Large Magellanic Cloud. It was extensively observed during both its early optically thick and its nebular evolution in the optical and UV wavelength regions. We successfully fit all the optically thick spectra using a grid of spherically symmetric, non-LTE, line-blanketed, expanding synthetic spectra created with the model atmosphere code PHOENIX. The emission lines of the nebular spectra have been fitted using an optimization technique for the emission-line luminosities predicted by the photoionization code CLOUDY. Our analyses show the following: the bolometric luminosity was super-Eddington before visual maximum and reached the ejected mass was and nuclear burning on the white dwarf ceased after ∼100 days. The elemental abundances (by number) with respect to solar of the ejecta are and all other elements ∼0.1. These abundances were determined from both the optically thick and nebular analyses. The extreme luminosity, high ejected mass, rapid turn-off time, and low metallicity (except for enhanced CNO) represent the extreme values of observational nova parameters and thus present interesting challenges for understanding the nature of the outburst of LMC 91.

New detected CO(1-0) emission from planetary nebulae

We report the results of CO(1-0) (J=l-0) observations for 20 selected planetary nebulae (PNe), using the 13.7 m radio telescope of Purple Mountain Observatory at the Qinghai Station. Due to weak CO(1-0) emission, long integrated time observations have been carried out for most of sampled PNe. Among these PNe, nine are first detections in CO(1-0) named NGC 6445, Ml-59, M4-9, M2-51, M4-18, He2-459, Sh2-71, K3-31, M2-52, and one is the first possible detection named V-Vl-8, a probably misclassified PN. NGC 6445, Ml-59, M4-9 and M2-51 have been detected in CO(2-1)(J=2-1). Although having been observed in previous CO surveys, Sh2-71 and M4-18 were not detected in CO; M2-52, K3-31, He2-459 are the first detected PNe in CO emission and V-Vl-8 is given with a first completed spectrum detected in CO(1-0). Comparing these data with other previous observational ones, we discuss the relationships between CO(1-0) emission and evolution of PNe: While the nebular radii increase, the CO(l-0) line integrated intensities multiplied by distances have a decreasing trend. Also the masses of molecular envelopes (from 0.001 to 1 M⊙) for PNe are decreased with increasing nebular radii. All the results mean the CO(1-0) emission decreases dramatically with the nebular evolution. The detectability of CO restricted by different PNe which have different dust properties and different evolutionary stages is also analyzed.

Distribution and Evolution of Water Ice in the Solar Nebula: Implications for Solar System Body Formation

Water is important in the solar nebula both because it is extremely abundant and because it condenses out at 5 AU, allowing all three phases of H2O to play a role in the composition and evolution of the Solar System. In this paper, we undertake a thorough examination of and model the inward radial drift of ice particles from 5 AU. We then link the drift results to the outward diffusion of vapor, in one overall model based on the two-dimensional diffusion equation, and numerically evolve the global model over the lifetime of the nebula. We find that while the inner nebula is generally depleted in water vapor, there is a zone in which the vapor is enhanced by 20–100%, depending on the choice of ice grain growth mechanisms and rates. This enhancement peaks in the region from 0.1 to 2 AU and gradually drops off out to 5 AU. Since this result is somewhat sensitive to the choice of nebular temperature profile, we examine representative hot (early) and cool (later) conditions during the quiescent phase of nebular evolution. Variations in the pattern of vapor depletion and enhancement due to the differing temperature profiles vary only slightly from that given above. Such a pattern of vapor enhancement and depletion in the nebula is consistent with the observed radial dependence of water of hydration bands in asteroid spectra and the general trend of asteroid surface darkening. This pattern of water vapor abundance will also cause variations in the C:O ratio, shifting the ratio more in favor of C in zones of relative depletion, affecting local and perhaps even global nebular chemistry, the latter through quenching and radial mixing processes.

Toroidal Magnetic Fields and the Evolution of Wind-driven Nebulae

Recent analytical work on magnetic shaping of wind-driven nebulae suggests that diverging stellar winds with frozen-in toroidal magnetic fields (occurring naturally when the wind originates from a rotating source) may evolve into collimated outflows. We report numerical simulations which show that the MHD collimation of stellar winds is indeed possible provided that the field carried by the wind is sufficiently strong. Our collimation mechanism operates in the shocked wind region of the nebula, leading to the formation of an astrophysical plasma gun recently invoked to accelerate jets associated with accretion disks.

Processing of chondritic and planetary material in spiral density waves in the nebula

Abstract— A widely held view of nebular evolution is that during the ∼0.5 Ma while interstellar material was collapsing onto the disk, the latter grew in mass to the point of gravitational instability. It responded to this by losing axial symmetry, growing spiral arms that had the capacity to tidally redistribute disk mass (inward) and angular momentum (outward) and prevent further increase in the disk/protosun mass ratio. The spiral arms (density waves) rotated differently than the substance of the nebula, and in some parts of the disk, nebular material may have encountered the arms at supersonic velocities. The disk gas, and solid particles entrained in it, would have been heated to some degree when they passed through shock fronts at the leading edges of the spiral arms. The present paper proposes this was the energetic nebular setting or environment that has long been sought, in which the material now in the planets and chondritic meteorites was thermally processed.

Determination of oxygen self-diffusion in åkermanite, anorthite, diopside, and spinel: Implications for oxygen isotopic anomalies and the thermal histories of Ca-Al-rich inclusions

Oxygen self-diffusion coefficients have been measured for three natural diopsidic clinopyroxenes, a natural anorthite, a synthetic magnesium aluminate spinel, and a synthetic åkermanite for oxygen fugacities ranging from the NNO to IW buffers. The experiments employed a gas-solid isotopic exchange technique utilizing 99% 18O-enriched CO-CO2 gas mixtures to control both the oxygen fugacity and the isotopic composition of the exchange reservoir. Diffusion profiles of the 18O tracer were obtained by in-depth analysis with an ion microprobe. The experimental results, fit to the Arrhenius relation D = D0e(−QRT), yield the following: Do (m2 s−1)Q (kJ mol−1)diopside4.3− 3.8+32.6× 10 − 4457 ± 26åkermanite4.7− 4.4+83.5× 10 − 7457 ± 26278 ± 33spinel2.2− 1.8+8.7× 10 − 7404 ± 21anorthite8.4− 8.0+174× 10 − 13162 ± 36At a given temperature, oxygen diffuses about 100 times more slowly in diopside than indicated by previous bulk-exchange experiments (Connolly and Muehlenbachs, 1988). Our data for anorthite, spinel, and åkermanite agree well with prior results obtained by gas-solid exchange and depth profiling methods (Elphick et al., 1988; Reddy and Cooper, 1981; Yurimoto et al., 1989, respectively). Since these other experiments were conducted at different oxygen fugacities, this agreement indicates that diffusion of oxygen in these nominally Fe-free minerals is not greatly affected by fO2 in the range between pure oxygen and the iron-wüstite buffer. However, our diffusion coefficients for anorthite, melilite, and spinel are also uniformly lower than those obtained by bulk analysis of crushed powders at similar temperatures (Muehlenbachs and Kushiro, 1974; Hayashi and Muehlenbachs, 1986; Ando and Oishi, 1974).The oxygen diffusion data are used to evaluate the effects of three different types of thermal histories upon the oxygen isotopic compositions of minerals found in Type B Ca-Al-rich inclusions (CAIBs) in carbonaceous chondrites: 1.(1) gas-solid exchange during isothermal heating,2.(2) gas-solid exchange as a function of cooling rate subsequent to instantaneous heating, and3.(3) isotopic exchange with a gaseous reservoir during partial melting and recrystallization. With the assumptions that the mineral compositions within a CAIB were uniformly enriched in 16O prior to any thermal processing, that effective diffusion dimensions may be estimated from observed grain sizes, and that diffusion in diopside is similar to that in fassaitic clinopyroxene, none of the above scenarios can reproduce the relative oxygen isotopic anomalies observed in CAIBs without improbably long or unrealistically intense thermal histories relative to current theoretical models of nebular evolution. The failure of these simple models, coupled with recent observations of “disturbed” magnesium isotopic abundances and correlated petrographic features in anorthite and melilite indicative of alteration and recrystallization, suggests that the oxygen isotopic compositions of these phases may have also been modified by alteration and recrystallization possibly interspersed with multiple melting events. Because the modal abundance of spinel remains relatively constant for plausible melting scenarios, and its relatively sluggish diffusion kinetics prevent substantial equilibration, Mg-Al spinel is the most reliable indicator of the oxygen isotopic composition of precursor material which formed Type B CAIs.

The radiation gas dynamics of planetary nebulae. 4. From the Owl to the Eskimo

We present the results of two-dimensional radiation-gasdynamic simulations of aspherical Planetary Nebulae (PNe) evolution. These simulations were constructed using the Generalized Interacting Stellar Winds (GISW) scenario of Balick (1987) where a fast, tenuous wind from the central star expands into a toroidal, slow, dense wind. We demonstrate that the GISW model can produce a wide range of aspherical flow patterns. We have constructed self-consistent synthetic observations of the models from forbidden line emissivities used in the energy loss term. We present integrated intensity and long-slit spectrum, (Position-Velocity) maps of the models projected at different angles on the sky. These synthetic observations are compared with real intensity and Position-Velocity maps of PNe. We find that there is a very good match between the synthetic and real observations in terms of morphologies, kinematics, and physical conditions. From the results of these simulations we conclude that the GISW scenario can account for most, if not all, PNe morphologies, thus confirming Balick's (1987) conjecture.

ON THE EXPANSION VELOCITIES OF PLANETARY NEBULAE AND THE CIRCUMSTELLAR ENVELOPES OF AGB STARS

ABSTRACT The distribution of expansion velocities of the gas of planetary nebulae are compared to the distribution of the circumstellar envelopes of AGB stars. We have examined the relationship between ionized and neutral expansion velocities for the nebulae with detected circumnebular neutral shells. It is found that compact planetary nebulae with ionized radii less than about 0.05 pc show a distribution of expansion velocities that is similar to AGB envelopes, while large planetary nebulae with ionized radii greater than about 0.30 pc show a significantly different expansion velocity distribution than AGB envelopes. Among the large planetary nebula subset, only those nebulae of Greig's class C have an expansion velocity distribution significantly different than AGB stars, while large Grieg's class B nebulae have the same expansion velocity distribution as AGB stars. The average velocity difference between AGB stars and large Grieg's class C nebulae is well predicted by the interacting stellar winds model of planetary nebula evolution. However, the acceleration of ionized gas predicted by the interaction of stellar winds is not evident in the observed expansion velocity distribution of large Grieg's class B nebulae.

Evolution of the Solar Nebula. II. Thermal Structure during Nebula Formation

view Abstract Citations (63) References (52) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Evolution of the Solar Nebula. II. Thermal Structure during Nebula Formation Boss, Alan P. Abstract Models of the thermal structure of protoplanetary disks are required for understanding the physics and chemistry of the earliest phases of planet formation. Numerical hydrodynamical models of the protostellar collapse phase have not been evolved far enough in time to be relevant to planet formation, i.e., to a relatively low-mass disk surrounding a protostar. One simplification is to assume a pre-existing solar-mass protostar, and calculate the structure of just the disk as it forms from the highest angular momentum vestiges of the placental cloud core. A spatially second-order accurate, axisymmetric (two-dimensional), radiative hydrodynamics code has been used to construct three sets of protoplanetary disk models under this assumption. Because compressional heating has been included, but not viscous or other heating sources, the model temperatures obtained should be considered lower bounds. The first set started from a spherically symmetric configuration appropriate for freely falling gas: ρ ∝ r-3/2, υr ∝ r-1/2, but with rotation (Ω ∝ r-1, where r is the spherical coordinate radius). These first models turned out to be unsatisfactory because in order to achieve an acceptable mass accretion rate onto the protostar (Mṡ ≤ 10-5 Msun yr-1 for low-mass star formation), the disk mass became much too small (∼ 0.0002 Msun). The second set improved on the first set by ensuring that the late-arriving, high angular momentum gas did not accrete directly onto the protosun. By starting from a disklike cloud flattened about the equatorial plane and flowing vertically toward the midplane, these models led to Mṡ → 0, as desired. However, because the initial cloud was not chosen to be close to equilibrium, the disk rapidly contracted vertically, producing an effective disk mass accretion rate Mṡd ∼ 10-2 Msun yr-1, again too high. Hence, the third (and most realistic) set started from an approximate equilibrium state for an adiabatic, self-gravitating "fat" Keplerian disk, with surface density σ ∝ r-1/2, surrounded by a much lower density "halo" infalling onto the disk. This initial condition produced Mṡs → 0 and Mṡd ∼ 10-6 to 10-5 Msun yr-1, as desired. The resulting nebula temperature distributions show that midplane temperatures of at least 1000 K inside 2.5 AU, falling to around 100 K outside 5 AU, are to be expected during the formation phase of a minimum mass nebula containing ∼0.02 Msun within 10 AU. This steady state temperature distribution appears to be consistent with cosmochemical evidence which has been interpreted as implying a phase of relatively high temperatures in the inner nebula. The temperature distribution also implies that the nebula would be cool enough outside 5 AU to allow ices to accumulate into planetesimals even at this relatively early phase of nebula evolution. Publication: The Astrophysical Journal Pub Date: November 1993 DOI: 10.1086/173318 Bibcode: 1993ApJ...417..351B Keywords: ACCRETION; ACCRETION DISKS; HYDRODYNAMICS; SOLAR SYSTEM: FORMATION full text sources ADS | Related Materials (8) Part 1: 1989ApJ...345..554B Part 3: 1996ApJ...469..906B Part 4: 1998ApJ...503..923B Part 5: 2002ApJ...576..462B Part 6: 2004ApJ...616.1265B Part 7: 2005ApJ...629..535B Part 8: 2007ApJ...660.1707B Part 9: 2011ApJ...739...61B

Numerical Calculations of the Linear Response of a Gaseous Disk to a Protoplanet

In this paper we present calculations of the linear response of a flat gaseous disk to the presence of an embedded protoplanet. The calculations are numerical and take into account terms neglected by analytic theory and also examine the effects of gradients in surface density and sound speed in the disk. We have also developed a means of rapid calculation of Laplace coefficients for a softened potential (or for a disk of nonzero height). In general, the classical analytic theory is confirmed. However, the magnitude of calculated torque due to the corotation resonance disagrees with previously derived analytical results. The calculated corotation torque is smaller than the analytic value, is much less sensitive to the value of the perturbing potential at corotation, and falls off more rapidly as a function of the azimuthal wavenumber m. It is possible that the rapid variation of the perturbing potential near corotation is responsible for this behavior. We find that the dominant effect of a pressure gradient is to increase the net torque, due to the inward displacement of the Lindblad resonances. The effect countervails the opposing effect of a gradient in disk surface density, in general. We have calculated the inferred orbital evolution timescales for the Earth and outer planets for a variety of model nebulae. In general the orbital evolution timescale for the Earth is on the order of 10 6 years. Timescales for the outer planets are much shorter: 10 3-10 4 years in general. Although the effects of the reaction of the solar nebula to the torques have been neglected in our calculations, the results indicate the importance of the protoplanets for the later stages of nebular evolution.

Young bipolar nebulae

Young bipolar nebulae are those rather rare exceptions among deeply embedded pre-main sequence objects driving bipolar outflows, which are amenable to detailed optical studies — by that means they provide unique information about this whole class of objects. The aim of this review is to work out, what their observations have told us about their structure and about the winds of young stars and their interaction with circumstellar matter. While the emphasis lies on the discussion of the optical observations, the large body of infrared and radio data is also considered. First, four of the best studied examples, S 106, Cep A, R Mon, and L 1551 IRS 5, are analyzed in depth. Then, recent observational results about disks and winds of T Tauri stars are considered, which shed new light also on the essential structural elements of young bipolar nebulae. After the description of some peculiar cases, the common properties characterizing the whole class of young bipolar nebulae are worked out. We are led to a unified picture of the bipolar winds causing both, the broad optically bright reflection lobes and molecular outflows, as well as the highly collimated optical jets observed in some cases on their polar axes; structure and dynamics of the bipolar winds are determined by the close interaction between the central stars and their massive circumstellar disks. Finally, current theoretical concepts about the mechanisms driving the winds are discussed, and present ideas about the evolution of bipolar nebulae are described. The Appendix includes a catalog of young bipolar nebulae.

Energy Distribution of Planetary Nebulae (UV to Radio)

The past decade has seen significant progress in our understanding of spectral energy distribution of planetary nebulae over the entire wavelength range from UV to radio. In this review we show the detailed breakdown of the energy budget for a planetary nebula as a system of the three components, i.e., the central star, the gaseous nebula and the dust shell. This picture of the energy distribution is further discussed in the context of planetary nebula evolution.

Planetary Nebula Evolution Traced by Distance-Independent Parameters

Making use of the results from recent infrared and radio surveys of planetary nebulae, we have selected 431 nebulae to form a sample where a number of distance-independent parameters (e.g., Tb, Td, I60μm and IRE) can be constructed. In addition, we also made use of other distance-independent parameters ne and T∗ where recent measurements are available. We have investigated the relationships among these parameters in the context of a coupled evolution model of the nebula and the central star. We find that most of the observed data in fact lie within the area covered by the model tracks, therefore lending strong support to the correctness of the model. Most interestingly, we find that the evolutionary tracks for nebulae with central stars of different core masses can be separated in a Tb-T∗ plane. This implies that the core masses and ages of the central stars can be determined completely independent of distance assumptions. The core masses and ages have been obtained for 302 central stars with previously determined central-star temperatures. We find that the mass distribution of the central stars strongly peaks at 0.6 M⊙, with 66% of the sample having masses <0.64 MM⊙. The luminosities of the central stars are then derived from their positions in the HR diagram according to their core masses and central star temperatures. If this method of mass (and luminosity) determination turns out to be accurate, we can bypass the extremely unreliable estimates for distances, and will be able to derive other physical properties of planetary nebulae.

Planetary Nebulae: Origin and Evolution

AbstractThere has been a great deal of progress in our understanding of planetary nebulae and their central stars during the past decade and a half. Most of this has come about through progress in observational techniques covering almost the entire electromagnetic spectrum. Theories of planetary nebula evolution have been put to better and better tests as more and more discriminating data have become available.This review describes some of the progress made in observations and their interpretation, particularly in the context of the evolution of the nebulae and the central stars. It includes a discussion on the improved determinations of magnitudes and temperatures of the central stars, and of progress in the measurement of distances, and a reassessment of the observed mass-distribution of the central stars. The last topic has been at the centre of a lively debate for almost a decade now and has been responsible for a large number of studies of central star evolution, some this review briefly touchs upon.

The evolutionary sequence of planetary nebulae

A first-order solution to the problem of nebular evolution is presented and used to construct photoionization models with the correct ionized gas geometry with respect to the central star. The models are used to investigate the evolution of the central star in terms of a transformed H-R diagram in which H-beta luminosity replaces stellar luminosity and the nebular excitation class takes the place of stellar effective temperature. It is shown that the optically thick PN in the Magellanic Clouds behave as ram-pressure confined H II regions trapped between the shocked remnants of AGB wind and the high-velocity stellar wind of the PN nuclei. 72 refs.

On the formation of ansae in planetary nebulae

Formation mechanisms for the two optical bright knots in planetary nebulae, 'ansae', are investigated. Adiabatic two-dimensional numerical simulations of the interacting winds flow at early stages of the planetary nebulae evolution are performed. It is found that no real focusing of the shocked fast wind material toward the symmetry axis occurs, and thus the interaction of winds by itself will not form the ansae. A model is proposed, in which jets from the central star form the ansae. These jets are presumably being formed during the few hundred years in the period between the end of the slow wind and the beginning of the fast wind. It is shown that a companion to the progenitor red giant can lead to the degree of asymmetry observed in elliptical planetary nebulae.

Evolution of the solar nebula. I - Nonaxisymmetric structure during nebula formation

Numerical solutions of the equations of hydrodynamics, gravitation, and radiative transfer in three spatial dimensions are used to model the formation and time evolution of the early solar nebula in order to learn whether or not gravitational torques between nonaxisymmetric structures in the solar nebula can transport angular momentum rapidly enough to produce nebula clearing on astronomically indicated (10 to the 5 to 10 to the 7 yr) time scales. The models involve solutions for the collapse of spherical clouds with assumed initial density and rotation profiles onto protosuns of variable mass. Most of the models assume uniform initial density and rotation, and have variations in the initial parameters of cloud mass, cloud rotation rate, and protosun mass which are chosen to simulate a range of possible phases of early solar nebula evolution. The models show little tendency for directly forming small numbers of giant gaseous protoplanets through gaseous gravitational instability.

Near Infrared Photometry of Compact Planetary Nebulae

The study of young planetary nebulae can provide information about the ejection process of the nebular material as well as on nebular evolution and the enrichment of the interstellar medium. The youngest PN are expected to be compact and very dense, they usually show signs of variability and IR excess due to the presence of warm dust which, in some cases, seems to be mixed with the ionized gas.

IUE observations of stars in the neighborhood of the Lambda Orionis nebula

Results are presented from an IUE archive study of five stars along the line of sight towards the Lambda Ori H II region. H I, Fe II, Si II, and Mn II column densities have been found using a profile-fitting technique for H I and the curve-of-growth method for the ions. HD 36861 and HD 36822 lie at or near the center of the H II region, while HD 35468 and HD 37490 are imbedded in the dense neutral shell surrounding the H II region. HD 38899 is a foreground star. The depletion factors in the gas of this nebula are indicative of some shock processing of the material along these lines of sight. This processing probably occurred in the early stages of the nebula's evolution.