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| Reliability Checks on the Indo-US Stellar Spectral Library Using Artificial Neural Networks and Principal Component Analysis
The Indo-US coudé feed stellar spectral library (CFLIB) madeavailable to the astronomical community recently by Valdes et al. (2004,ApJS, 152, 251) contains spectra of 1273 stars in the spectral region3460 to 9464Å at a high resolution of 1Å (FWHM) and a widerange of spectral types. Cross-checking the reliability of this databaseis an important and desirable exercise since a number of stars in thisdatabase have no known spectral types and a considerable fraction ofstars has not so complete coverage in the full wavelength region of3460-9464Å resulting in gaps ranging from a few Å to severaltens of Å. We use an automated classification scheme based onArtificial Neural Networks (ANN) to classify all 1273 stars in thedatabase. In addition, principal component analysis (PCA) is carried outto reduce the dimensionality of the data set before the spectra areclassified by the ANN. Most importantly, we have successfullydemonstrated employment of a variation of the PCA technique to restorethe missing data in a sample of 300 stars out of the CFLIB.
| Electron Density and Turbulence Gradients within the Extended Atmosphere of the M Supergiant Betelgeuse (α Orionis)
The extended atmosphere of the M supergiant Betelgeuse is complex withcool plasma dominating the structure by mass and small amounts ofembedded hotter chromospheric plasma. A major challenge is to understandthe interrelationship and juxtaposition of these different components,which in turn may provide clues to the nature of the process ofnonradiative heating and the mechanisms that drive mass loss. We examinethe chromospheric C II] λ2325 multiplet emission line electrondensity diagnostic using spatially scanned HST STIS echelle spectra.Escape probability models for the electron density-sensitive line ratiosreveal that the mean electron density decreases by 0.6 dex as the sightline goes from disk center to +/-75 mas. Radiative transfer simulationsusing spherical model atmospheres show that this trend can be explainedif the electron density declines with radius by nearly 2 dex acrossΔR~2R*. The emission profiles indicate that thechromospheric material corotates with the star and then becomesdecoupled by +/-75 mas from disk center. We find no evidence for radialoutflow in the chromospheric plasma. We find that the strongest C II]λ2325 emission lines are opacity broadened and that the gradientof atmospheric turbulence is surprisingly small. Using empiricalconstraints, we derive a relation between the relative C II columndensities in the cool and chromospheric atmospheric components and theexcitation temperature. These UV chromospheric results and previousradio analyses suggest that the chromosphere is pervasive but has asmall filling factor at ~3R*, suggestive of confinement andheating in magnetic structures.
| Infrared Spectra and Visibilities as Probes of the Outer Atmospheres of Red Supergiant Stars
In the light of the recent results of the stellar interferometry, weexamine the nature of the extra molecular layer outside the photosphereof red supergiant stars, so far studied mostly with the use of theinfrared spectra. Although the visibility data are more direct probes ofthe spatial structure of the outer atmosphere, it is essential that theyare analyzed in combination with the spectral data. In the case of theM2 supergiant μ Cephei, several sets of data, both spectra andvisibilities, strongly suggested the presence of an extra molecularlayer (which we referred to as ``MOLsphere'' for simplicity), and thebasic parameters of the MOLsphere are estimated to be excitationtemperature Tex~1600 K, column densities of CO andH2O molecules Ncol~3.0×1020cm-2, and located at about one stellar radius above thephotosphere or Rin~2.0R*. The result showsreasonable agreement with the one based on the infrared spectra alone,and the model inferred from the spectra is now fully supported with therecent visibility data. In the case of the M2 supergiant αOrionis, the infrared spectra and visibilities show a consistent picturein that its MOLsphere is closer to the photosphere(Rin~1.3R*) with higher gas temperature(Tex~2250 K) and lower gas column density(Ncol~1020 cm-2), compared with that ofμ Cep. Some controversy on the interpretation of the mid-infrareddata of α Orionis can be reconciled. Given that the presence ofthe extra molecular layer is reasonably well established, the majorunsolved problem is how to understand the origin of such a rather warmand dense layer in the outer atmosphere.
| Water Vapor on Supergiants: The 12 μm TEXES Spectra of μ Cephei
Several recent papers have argued for warm, semidetached, molecularlayers surrounding red giant and supergiant stars, a concept known as aMOLsphere. Spectroscopic and interferometric analyses have oftencorroborated this general picture. Here we present high-resolutionspectroscopic data of pure rotational lines of water vapor at 12 μmfor the supergiant μ Cep. This star has often been used to test theconcept of molecular layers around supergiants. Given the prediction ofan isothermal, optically thick water vapor layer in local thermodynamicequilibrium around the star (MOLsphere), we expected the 12 μm linesto be in emission or at least in absorption but filled in by emissionfrom the molecular layer around the star. Our data, however, show thecontrary; we find definite absorption. Thus, our data do not easily fitinto the suggested isothermal MOLsphere scenario. The 12 μm lines,therefore, put new, strong constraints on the MOLsphere concept and onthe nature of water seen in signatures across the spectra of early Msupergiants. We also find that the absorption is even stronger than thatcalculated from a standard, spherically symmetric model photospherewithout any surrounding layers. A cool model photosphere, representingcool outer layers, is, however, able to reproduce the lines, but thismodel does not account for water vapor emission at 6 μm. Thus, aunified model for water vapor on μ Cep appears to be lacking. It doesseem necessary to model the underlying photospheres of these supergiantsin their whole complexity. The strong water vapor lines clearly revealinadequacies of classical model atmospheres.
| Multigenerational Star Formation in L1551
The L1551 molecular cloud is undergoing a long and sustained period ofrelatively high efficiency star formation. It contains two smallclusters of Class 0 and I protostars, as well as a halo of more evolvedClass II and III YSOs, indicating a current and at least one past burstof star formation. We present here new, sensitive maps of 850 and 450μm dust emission covering most of the L1551 cloud; new CO J=2-1 dataof the molecular cloud; and a new, deep, optical image of [S II]emission (6730 Å). Compact submillimeter emitters are concentratedin two subclusters: L1551 IRS5 and L1551 NE, and the HL Tauri group.Both stellar groups show significant extended emission and outflow/jetactivity. A jet, terminating at HH 265 and with a very weak associatedmolecular outflow, may originate from LkHα 358 or from a binarycompanion to another member of the HL Tauri group. Several Herbig-Haroobjects associated with L1551 IRS5/L1551 NE were clearly detected in thesubmillimeter, as were faint ridges of emission tracing outflow cavitywalls. We confirm a large-scale molecular outflow originating from L1551NE, parallel to that from L1551 IRS5, and suggest that the ``hollowshell'' morphology is more likely due to two interacting outflows. Weconfirm the presence of a prestellar core (L1551 MC) of mass 2-3Msolar northwest of L1551 IRS5. The next-generation clustermay be forming in this core. The L1551 cloud appears cometary inmorphology and appears to be illuminated and eroded from the directionof Orion, perhaps explaining the multiple episodes of star formation.
| Changes in Apparent Size of Giant Stars with Wavelength due to Electron-Hydrogen Collisions
Interferometric measurements of stellar sizes in frequency bands rangingfrom the near-infrared to longer wavelengths give different results.Various explanations have been proposed to account for these variationsin apparent size with wavelength, but none have been entirelyconsistent. We propose that thermal ionization in the stellar atmosphereand resulting opacity, primarily due to free-free electron-hydrogencollisions, play a significant role. Such an opacity has a quadraticdependence on photon wavelength and produces variations in the opacityof the atmosphere with wavelength, consistent with pertinentmeasurements. This may be particularly important for Mira-type stars,and two examples, o Ceti and W Hya, are analyzed as examples. For starsthat are much smaller or with more concentrated mass, it is not likelyto be significant.
| Water Vapor on Betelgeuse as Revealed by TEXES High-Resolution 12 μm Spectra
The outer atmosphere of the M supergiant Betelgeuse is puzzling.Published observations of different kinds have shed light on differentaspects of the atmosphere, but no unified picture has emerged. They haveshown, for example, evidence of a water envelope (MOLsphere) that insome studies is found to be optically thick in the mid-infrared. In thispaper, we present high-resolution, mid-infrared spectra of Betelgeuserecorded with the TEXES spectrograph. The spectra clearly showabsorption features of water vapor and OH. We show that a spectrum basedon a spherical, hydrostatic model photosphere with Teff=3600K, an effective temperature often assumed for Betelgeuse, fails to modelthe observed lines. Furthermore, we show that published MOLspherescenarios are unable to explain our data. However, we are able to modelthe observed spectrum reasonably well by adopting a cooler outerphotospheric structure corresponding to Tmod=3250 K. Thesuccess of this model may indicate that the observed mid-infrared linesare formed in cool photospheric surface regions. Given the uncertaintiesof the temperature structure and the likely presence of inhomogeneities,we cannot rule out the possibility that our spectrum could be mostlyphotospheric, albeit nonclassical. Our data put new, strong constraintson atmospheric models of Betelgeuse, and we conclude that continuedinvestigation requires consideration of nonclassical model photospheres,as well as possible effects of a MOLsphere. We show that themid-infrared water vapor features have great diagnostic value for theenvironments of K and M (super)giant star atmospheres.
| A Survey of Merger Remnants. II. The Emerging Kinematic and Photometric Correlations
This paper is the second in a series exploring the properties of 51optically selected, single-nuclei merger remnants. Spectroscopic datahave been obtained for a subsample of 38 mergers and combined withpreviously obtained infrared photometry to test whether mergers exhibitthe same correlations as elliptical galaxies among parameters such asstellar luminosity and distribution, central stellar velocity dispersion(σ0), and metallicity. Paramount to the study is totest whether mergers lie on the fundamental plane. Measurements ofσ0 have been made using the Ca triplet absorption lineat 8500 Å for all 38 mergers in the subsample. Additionalmeasurements of σ0 were made for two of the mergers inthe subsample using the CO absorption line at 2.29 μm. The resultsindicate that mergers show a strong correlation among the parameters ofthe fundamental plane but fail to show a strong correlation betweenσ0 and metallicity (Mg2). In contrast toearlier studies, the σ0 values of the mergers areconsistent with objects that lie somewhere between intermediate-mass andluminous giant elliptical galaxies. However, the discrepancies withearlier studies appear to correlate with whether the Ca triplet or COabsorption lines are used to derive σ0, with the latteralmost always producing smaller values. Finally, the photometric andkinematic data are used to demonstrate for the first time that thecentral phase-space densities of mergers are equivalent to those inelliptical galaxies. This resolves a long-standing criticism of themerger hypothesis.Some of the data presented herein were obtained at the W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California, andthe National Aeronautics and Space Administration. The Observatory wasmade possible by the generous financial support of the W. M. KeckFoundation.
| Probing the mass-loss history of AGB and red supergiant stars from CO rotational line profiles. I. Theoretical model - Mass-loss history unravelled in VY CMa
Context: .Mass loss plays a dominant role in the evolution of low massstars while they are on the Asymptotic Giant Branch (AGB). The gas anddust ejected during this phase are a major source in the mass budget ofthe interstellar medium. Recent studies have pointed towards theimportance of variations in the mass-loss history of such objects.Aims: .By modelling the full line profile of low excitation CO linesemitted in the circumstellar envelope, we can study the mass-losshistory of AGB stars. Methods: .We have developed a non-LTEradiative transfer code, which calculates the velocity structure and gaskinetic temperature of the envelope in a self-consistent way. Theresulting structure of the envelope provides the input for the molecularline radiative calculations which are evaluated in the comoving frame.The code allows for the implementation of modulations in the mass-lossrate. This code has been benchmarked against other radiative transfercodes and is shown to perform well and efficiently. Results: .Weillustrate the effects of varying mass-loss rates in case of a superwindphase. The model is applied to the well-studied case of VYCMa. We show that both the observed integrated line strengthsas the spectral structure present in the observed line profiles,unambiguously demonstrate that this source underwent a phase of highmass loss (~3.2 × 10-4 Mȯyr-1) some 1000 yr ago. This phase took place for some 100yr, and was preceded by a low mass-loss phase (~1 ×10-6 Mȯ yr-1) taking some 800 yr.The current mass-loss rate is estimated to be in the order of 8 ×10-5 Mȯ yr-1. Conclusions:.In this paper, we demonstrate that both the relative strength of the COrotational line profiles and the (non)-occurrence of spectral structurein the profile offer strong diagnostics to pinpoint the mass-losshistory.
| A new analysis of the nucleus of NGC 1068 with MIDI observations
We present a new analysis of the first mid-infrared N-band long-baselineinterferometric observations of an extragalactic source: the nucleus ofthe Seyfert 2 galaxy NGC 1068, obtained with MIDI (Mid-InfrareDInterferometer), the mid-infrared beamcombiner at the European SouthernObservatory (ESO) Very Large Telescope Interferometer (VLTI). Theresolution of λ/B 10 mas allows us to study the compactcentral core of the galaxy between 8 and 13 μm. Both visibilitymeasurements and MIDI spectrum are well reproduced by a simple radiativetransfer model with two concentric spherical components. The derivedangular sizes and temperatures are ~35 and 83 mas, and ~361 K and 226 Kfor these two components respectively. Other evidence strongly supportssuch low temperatures. This modeling also provides the variation ofoptical depth as a function of wavelength for the extended componentacross the N-band suggesting the presence of amorphous silicate grains.This shows that MIDI has carried out the first direct observations ofthe distribution of dust around the central engine.
| Extended envelopes around Galactic Cepheids. I. ℓ Carinae from near and mid-infrared interferometry with the VLTI
We present the results of long-baseline interferometric observations ofthe bright southern Cepheid ℓ Carinae in the infrared N (8-13 μm)and K (2.0-2.4 μm) bands, using the MIDI and VINCI instruments of theVLT Interferometer. We resolve in the N band a large circumstellarenvelope (CSE) that we model with a Gaussian of 3 Rstar(≈500 Rȯ ≈ 2-3 AU) half width at half maximum. Thesignature of this envelope is also detected in our K band data as adeviation from a single limb darkened disk visibility function. Thesuperimposition of a Gaussian CSE on the limb darkened disk model of theCepheid star results in a significantly better fit of our VINCI data.The extracted CSE parameters in the K band are a half width at halfmaximum of 2 Rstar, comparable to the N band model, and atotal brightness of 4% of the stellar photosphere. A possibility is thatthis CSE is linked to the relatively large mass loss rate of ℓ Car.Though its physical nature cannot be determined from our data, wediscuss an analogy with the molecular envelopes of RV Tauri, redsupergiants and Miras.
| Amorphous alumina in the extended atmosphere of α Orionis
In this paper we study the extended atmosphere of the late-typesupergiant α Orionis. Infrared spectroscopy of red supergiantsreveals strong molecular bands, some of which do not originate in thephotosphere but in a cooler layer of molecular material above it.Lately, these layers have been spatially resolved by near and mid-IRinterferometry. In this paper, we try to reconcile the IRinterferometric and ISO-SWS spectroscopic results on α Orioniswith a thorough modelling of the photosphere, molecular layer(s) anddust shell. From the ISO and near-IR interferometric observations, wefind that α Orionis has only a very low density water layer closeabove the photosphere. However, mid-IR interferometric observations anda narrow-slit N-band spectrum suggest much larger extra-photosphericopacity close to the photosphere at those wavelengths, even when takinginto account the detached dust shell. We argue that this cannot be dueto the water layer, and that another source of mid-IR opacity must bepresent. We show that this opacity source is probably neither molecularnor chromospheric. Rather, we present amorphous alumina (Al2O3) as thebest candidate and discuss this hypothesis in the framework ofdust-condensation scenarios.
| An abundance analysis of the symbiotic star CH Cygni
The photospheric abundances for the cool component of the symbiotic starwere calculated for the first time using high-resolution near-infraredspectra and the method of of standard LTE analysis and atmosphericmodels. The iron abundance for CH Cyg was found to besolar, [Fe/H] = 0.0 ± 0.19. The atmospheric parameters (T_eff =3100 K, log g = 0.0 (cgs), ξt = 2.2 km s-1) andmetallicity for CH Cyg are found to be approximately equal to those fornearby field M7 giants. The calculated [C/H] = -0.15, [N/H] = +0.16,[O/H] = -0.07, and the isotopic ratios of 12C/13Cand 16O/17O are close to the mean values forsingle M giants that have experienced the first dredge-up. A reasonableexplanation for the absence of barium star-like chemical peculiaritiesseems to be the high metallicity of CH Cyg. The emission line techniquewas explored for estimating CNO ratios in the wind of the giant.
| Atlas and Catalog of Dark Clouds Based on Digitized Sky Survey I
We present a quantitative atlas and catalog of dark clouds derived byusing the optical database ``Digitized Sky Survey I''. Applying atraditional star-count technique to 1043 plates contained in thedatabase, we produced an AV map covering the entire region inthe galactic latitude range |b| ≤ 40°. The map was drawn at twodifferent angular resolutions of 6' and 18', and is shown in detail in aseries of figures in this paper. Based on the AV map, weidentified 2448 dark clouds and 2841 clumps located inside them. Somephysical parameters, such as the position, extent, and opticalextinction, were measured for each of the clouds and clumps. We alsosearched for counterparts among already known dark clouds in theliterature. The catalog of dark clouds presented in this paper lists thecloud parameters as well as the counterparts.
| A high-resolution stellar library for evolutionary population synthesis
We present a library of 1654 high-resolution stellar spectra, with asampling of 0.3 Åand covering the wavelength range from 3000 to7000 Å. The library was computed with the latest improvements instellar atmospheres, incorporating non-local thermodynamic equilibrium(non-LTE) line-blanketed models for hot, massive (Teff>=27500K) and line-blanketed models for cool (3000<=Teff<= 4500K) stars. The total coverage of the gridis 3000 K <=Teff<= 55000 K and -0.5 <= log g<=5.5, for four chemical abundance values: twice solar, solar, half solarand 1/10 solar. Evolutionary synthesis models using this library arepresented in a companion paper. We tested the general behaviour of thelibrary by calculating and comparing equivalent widths of numerous H andHeI lines, and some of the commonly used metallic indices. We alsocompared the library with the empirical libraries STELIB and Indo-US.The full set of the synthetic stellar spectra is available from ourwebsites (http://www.iaa.csic.es/~rosa andhttp://www.astro.iag.usp.br/~lucimara/library.htm).
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| Visual Star Colours from Instrumental Photometry
In order to display graphically the visual colours of stars and otherastronomical objects, photometric broadband R, V, B colours are used toproxy for the r, g, b colours of the three visual sensors of the eye.From photometric Johnson B-V and V-R colour indices, R, V, and Bmagnitudes (V = 0) are calculated, and from these the respectivebrightnesses (r, v = 1 = g, and b) are calculated. After suitablenormalization these are then placed in a ternary diagram having r, g,and b as the vertices. All B-V and V-R are adjusted so that the Sunfalls in the same place as a blackbody at 5800 K. The resulting ternaryplot shows all of its objects (stars, planets) in their visual coloursat their relative positions in the ternary diagram. The star coloursdisplayed on a computer monitor screen or as a print with a colourprinter are more vivid than the usual visual impressions of isolatedstars, undoubtedly because of properties of the dark-adapted eye, butdouble-star pairs with contrasting colours correspond nicely totelescopic visual impressions.
| A New Version of Reimers' Law of Mass Loss Based on a Physical Approach
We present a new semiempirical relation for the mass loss of coolstellar winds, which so far has frequently been described by ``Reimers'law.'' Originally, this relation was based solely on dimensional scalingarguments without any physical interpretation. In our approach, the windis assumed to result from the spillover of the extended chromosphere,possibly associated with the action of waves, especially Alfvénwaves, which are used as guidance in the derivation of the new formula.We obtain a relation akin to the original Reimers law, but whichincludes two new factors. They reflect how the chromospheric heightdepends on gravity and how the mechanical energy flux depends, mainly,on the effective temperature. The new relation is tested and sensitivelycalibrated by modeling the blue end of the horizontal branch of globularclusters. The most significant difference from mass-loss rates predictedby the Reimers relation is an increase by up to a factor of 3 forluminous late-type (super)giants, in good agreement with observations.
| The Effective Temperature Scale of Galactic Red Supergiants: Cool, but Not As Cool As We Thought
We use moderate-resolution optical spectrophotometry and the new MARCSstellar atmosphere models to determine the effective temperatures of 74Galactic red supergiants (RSGs). The stars are mostly members of OBassociations or clusters with known distances, allowing a criticalcomparison with modern stellar evolutionary tracks. We find we canachieve excellent matches between the observations and the reddenedmodel fluxes and molecular transitions, although the atomic lines Ca Iλ4226 and Ca II H and K are found to be unrealistically strong inthe models. Our new effective temperature scale is significantly warmerthan those in the literature, with the differences amounting to 400 Kfor the latest type M supergiants (i.e., M5 I). We show that the newlyderived temperatures and bolometric corrections give much betteragreement with stellar evolutionary tracks. This agreement provides acompletely independent verification of our new temperature scale. Thecombination of effective temperature and bolometric luminosities allowsus to calculate stellar radii; the coolest and most luminous stars (KWSgr, Case 75, KY Cyg, HD 206936=μ Cep) have radii of roughly 1500Rsolar (7 AU), in excellent accordance with the largeststellar radii predicted from current evolutionary theory, althoughsmaller than that found by others for the binary VV Cep and for thepeculiar star VY CMa. We find that similar results are obtained for theeffective temperatures and bolometric luminosities using only thedereddened V-K colors, providing a powerful demonstration of theself-consistency of the MARCS models.
| Near-Infrared Water Lines in V838 Monocerotis
V838 Monocerotis had an intriguing, nova-like outburst in 2002 Januarythat has subsequently led to several studies of the object. It is nowrecognized that the outburst of V838 Mon and its evolution are differentfrom those of a classical nova or other classes of well-known eruptivevariables. V838 Mon, along with two other objects that have analogousproperties, appears to make up a new class of eruptive variables. Thereare limited infrared studies of V838 Mon. Here we present near-infraredH-band (1.5-1.75 μm) spectra of V838 Mon from late 2002 to the end of2004. The principal new result from our work is the detection of severalrotation-vibration lines of water in the H-band spectra. The observedwater lines have been modeled to first establish that they are indeeddue to water. Subsequently the temperature and column densities of theabsorbing material, from where the water absorption features originate,are derived. From our analysis, we find that the water features arisefrom a cool, ~750-900 K, region around V838 Mon that appears to begradually cooling with time.
| Radio Photosphere and Mass-Loss Envelope of VY Canis Majoris
We have used the VLA to detect emission from the supergiant VY CMa atradio wavelengths and have constructed 3000-4500 K isothermal outeratmospheres constrained by the data. These models produce a radiophotosphere at 1.5-2 R*. An extrapolation of the model canaccount for the observed total mass-loss rate of the star. We alsopresent mid-infrared imaging of the supergiant which suggests that warmdust is extended in the same direction as the near-infrared reflectionnebula around VY CMa. The origin of the asymmetries in the outflowremains an unsolved problem.
| Spectroscopic Constants, Abundances, and Opacities of the TiH Molecule
Using previous measurements and quantum chemical calculations to derivethe molecular properties of the TiH molecule, we obtain new values forits rovibrational constants, thermochemical data, spectral line lists,line strengths, and absorption opacities. Furthermore, we calculate theabundance of TiH in M and L dwarf atmospheres and conclude that it ismuch higher than previously thought. We find that the TiH/TiO ratioincreases strongly with decreasing metallicity, and at high temperaturescan exceed unity. We suggest that, particularly for subdwarf L and Mdwarfs, spectral features of TiH near ~0.52 and 0.94 μm and in the Hband may be more easily measurable than heretofore thought. The recentpossible identification in the L subdwarf 2MASS J0532 of the 0.94 μmfeature of TiH is in keeping with this expectation. We speculate thatlooking for TiH in other dwarfs and subdwarfs will shed light on thedistinctive titanium chemistry of the atmospheres of substellar-massobjects and the dimmest stars.
| A Far-Ultraviolet Spectroscopic Survey of Luminous Cool Stars
The Far Ultraviolet Spectroscopic Explorer (FUSE) ultraviolet spectra ofeight giant and supergiant stars reveal that high-temperature(3×105 K) atmospheres are common in luminous cool starsand extend across the color-magnitude diagram from α Car (F0 II)to the cool giant α Tau (K5 III). Emission present in thesespectra includes chromospheric H Lyβ, Fe II, C I, and transitionregion lines of C III, O VI, Si III, and Si IV. Emission lines of FeXVIII and Fe XIX signaling temperatures of ~107 K and coronalmaterial are found in the most active stars, β Cet and 31 Com. Ashort-term flux variation, perhaps a flare, was detected in β Cetduring our observation. Stellar surface fluxes of the emission of C IIIand O VI are correlated and decrease rapidly toward the cooler stars,reminiscent of the decay of magnetically heated atmospheres. Profiles ofthe C III λ977 lines suggest that mass outflow is underway atT~80,000 K and the winds are warm. Indications of outflow at highertemperatures (3×105 K) are revealed by O VI asymmetriesand the line widths themselves. High-temperature species are absent inthe M supergiant α Ori. Narrow fluorescent lines of Fe II appearin the spectra of many giants and supergiants, apparently pumped by HLyα, and formed in extended atmospheres. Instrumentalcharacteristics that affect cool star spectra are discussed.
| Stellar dynamos - theoretical aspects
I review some dynamo models and constraints for stars other than theSun. The models include mean-field dynamo models for RS Cvn binarysystems and global magnetoconvection simulations for supergiants and forfully convective stars. In the latter case, we find generation ofmixed-parity large-scale magnetic fields once the dynamo reachessaturation. In addition to the cool, narrow downdrafts known from localsimulations, these models also show a hot convective plume stretchingfrom the centre towards the surface.
| The Orion Nebula in the Mid-Infrared
We present two wide-field (~5'×3&farcm;5),diffraction-limited (λ/D~=0.5" at 10 μm), broadband 10 and 20μm images of the Orion Nebula, plus six 7-13 μm narrowband(λ/Δλ~=1) images of the BN/KL complex taken at the3.8 m UKIRT telescope with the MPIA MAX camera. The wide-field images,centered on the Trapezium and BN/KL regions, are mosaics of35''×35'' frames obtained with standardchopping and nodding techniques and reconstructed using a newrestoration method developed for this project. They show the filamentarystructure of the dust emission from the walls of the H II region andreveal a new remarkable group of arclike structures ~1' to the south ofthe Trapezium. The morphology of the Ney-Allen Nebula, produced bywind-wind interaction in the vicinity of the Trapezium stars, suggests acomplex kinematical structure at the center of the cluster. We findindications that one of the most massive members of the cluster, theB0.5 V star θ1 Ori D, is surrounded by aphotoevaporated circumstellar disk. Among the four historic Trapezium OBstars, this is the only one without a binary companion, suggesting thatstellar multiplicity and the presence of massive circumstellar disks maybe mutually exclusive. In what concerns the BN/KL complex, we findevidence for extended optically thin silicate emission on top of thedeep 10 μm absorption feature. Assuming a simple two-component model,we map with ~=0.5" spatial resolution the foreground optical depth,color temperature, and mid-IR luminosity of the embedded sources. Weresolve a conspicuous point source at the location of the IRc2-A knot,approximately 0.5" north of the deeply embedded H II region ``I.'' Weanalyze the spectral profile of the 10 μm silicate absorption featureand find indication for grain crystallization in the harsh nebularenvironment. In the OMC-1 South region, we detect several point sourcesand discuss their association with the mass-loss phenomenology observedat optical and millimeter wavelengths. Finally, we list the position andphotometry of 177 point sources, the large majority of which aredetected for the first time in the mid-IR. Twenty-two of them lack acounterpart at shorter wavelengths and are therefore candidates fordeeply embedded protostars. The comparison of photometric data obtainedat two different epochs reveals that source variability at 10 μm ispresent up to a level of ~1 mag on a timescale of ~2 yr. With thepossible exception of a pair of OB stars, all point sources detected atshorter wavelengths display 10 μm emission well above thephotospheric level, which we attribute to disk circumstellar emission.The recent model of Robberto et al. provides the simplest explanationfor the observed mid-IR excess.
| High-Resolution, Long-Slit Spectroscopy of VY Canis Majoris: The Evidence for Localized High Mass Loss Events
High spatial and spectral resolution spectroscopy of the OH/IRsupergiant VY CMa and its circumstellar ejecta reveals evidence for highmass loss events from localized regions on the star occurring over thepast 1000 yr. The reflected absorption lines and the extremely strong KI emission lines show a complex pattern of velocities in the ejecta. Weshow that the large, dusty northwest arc, expanding at ~50 kms-1 with respect to the embedded star, is kinematicallydistinct from the surrounding nebulosity and was ejected about 400 yrago. Other large, more filamentary loops were probably expelled as muchas 800-1000 yr ago, whereas knots and small arcs close to the starresulted from more recent events 100-200 yr ago. The more diffuse,uniformly distributed gas and dust is surprisingly stationary, withlittle or no velocity relative to the star. This is not what we wouldexpect for the circumstellar material from an evolved red supergiantwith a long history of mass loss. We therefore suggest that the highmass loss rate for VY CMa is a measure of the mass carried out by thesespecific ejections accompanied by streams or flows of gas throughlow-density regions in the dust envelope. VY CMa may thus be our mostextreme example of stellar activity, but our results also bring intoquestion the evolutionary state of this famous star. In a separateappendix, we discuss the origin of the very strong K I and other rareemission lines in its spectrum.
| A library of high resolution synthetic stellar spectra from 300 nm to 1.8 μm with solar and α-enhanced composition
Libraries of stellar spectra are fundamental tools for the study ofstellar populations, and both empirical and synthetic libraries havebeen used for this purpose. In this paper, a new library of highresolution synthetic spectra is presented, ranging from thenear-ultraviolet (300 nm) to the near-infrared (1.8 μm). The libraryspans all the stellar types that are relevant to the integrated light ofold and intermediate-age stellar populations in the involved spectralregion (spectral types F through M and all luminosity classes). The gridwas computed for metallicities ranging from [Fe/H] = -2.5 to +0.5,including both solar and α-enhanced ([ α/Fe] = 0.4) chemicalcompositions. The synthetic spectra are a good match to observations ofstars throughout the stellar parameter space encompassed by the libraryand over the whole spectral region covered by the computations.
| WZ Cas - variability on multiple time-scales
We present results on our long-term velocity monitoring of the C-richsemi-regular variable WZ Cas with a Coravel instrument. Our findings arecompared with light changes of the star. We suggest that the two mainperiods, 366 and 186 d, are due to radial pulsation. Furthermore, wefind an interesting variation of the width and depth of thecross-correlation profile over a time-scale of at least 1000 d. Severalpossible explanations for this behaviour are discussed by comparison oftime-scales and expected variability amplitudes. The influence ofmovements of large convective cells on the line profiles seems to be themost likely explanation of some of the observed phenomena.
| An empirical formula for the mass-loss rates of dust-enshrouded red supergiants and oxygen-rich Asymptotic Giant Branch stars
We present an empirical determination of the mass-loss rate as afunction of stellar luminosity and effective temperature, foroxygen-rich dust-enshrouded Asymptotic Giant Branch stars and redsupergiants. To this aim we obtained optical spectra of a sample ofdust-enshrouded red giants in the Large Magellanic Cloud, which wecomplemented with spectroscopic and infrared photometric data from theliterature. Two of these turned out to be hot emission-line stars, ofwhich one is a definite B[e] star. The mass-loss rates were measuredthrough modelling of the spectral energy distributions. We thus obtainthe mass-loss rate formula log dot{M} = -5.65 + 1.05 log ( L / 10 000{L}_ȯ ) -6.3 log ( T_eff / 3500 K ), valid for dust-enshrouded redsupergiants and oxygen-rich AGB stars. Despite the low metallicity ofthe LMC, both AGB stars and red supergiants are found at late spectraltypes. A comparison with galactic AGB stars and red supergiants showsexcellent agreement between the mass-loss rate as predicted by ourformula and that derived from the 60 μm flux density fordust-enshrouded objects, but not for optically bright objects. Wediscuss the possible implications of this for the mass-loss mechanism.
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