Wayne L. Waldron

AN INTRODUCTION TO THE CHANDRA CARINA COMPLEX PROJECT

The Great Nebula in Carina provides an exceptional view into the violent massive star formation and feedback that typifies giant H II regions and starburst galaxies. We have mapped the Carina star-forming complex in X-rays, using archival Chandra data and a mosaic of 20 new 60 ks pointings using the Chandra X-ray Observatorys Advanced CCD Imaging Spectrometer, as a testbed for understanding recent and ongoing star formation and to probe Carinas regions of bright diffuse X-ray emission. This study has yielded a catalog of properties of > 14,000 X-ray point sources;> 9800 of them have multiwavelength counterparts. Using Chandras unsurpassed X-ray spatial resolution, we have separated these point sources from the extensive, spatially-complex diffuse emission that pervades the region; X-ray properties of this diffuse emission suggest that it traces feedback from Carinas massive stars. In this introductory paper, we motivate the survey design, describe the Chandra observations, and present some simple results, providing a foundation for the 15 papers that follow in this special issue and that present detailed catalogs, methods, and science results.

NEW CHALLENGES FOR WIND SHOCK MODELS: THE CHANDRA SPECTRUM OF THE HOT STARORIONIS

The Chandra spectrum ofOri A shows emission lines from hydrogen- and helium-like states of Si, Mg, Ne, and O, along with Nvii Lyand lines from ions in the range Fe xvii-Fexxi. In contrast to the broad lines seen inPup andOri (850 � 40 and 1000 � 240 km s � 1 half-width at half-maximum (HWHM), respec- tively), these lines are broadened to only 430 � 60 km s � 1 HWHM. This is much lower than the measured wind terminal velocity of 2000 km s � 1 . The forbidden, intercombination, and resonance ( fir) lines from He-like ions indicate that the majority of the X-ray line emission does not originate at the base of the wind, in agreement with the standard wind shock models for these objects. However, in that model the X-ray emission is distributed throughout an expanding, X-ray-absorbing wind, and it is therefore surprising that the emis- sion lines appear relatively narrow, unshifted, and symmetric. We compare the observed line profiles to recent detailed models for X-ray line profile generation in hot stars, but none of them offers a fully satisfac- tory explanation for the observed line profiles.

X-Ray and Radio Observations of the Cygnus OB2 Association

Several OB stars in the Cygnus OB2 association are among the strongest stellar X-ray and radio sources in the Galaxy. The radio emission is particularly unusual, displaying a high level of variability and nonthermal behavior. We obtained two ROSAT PSPC observations, a ROSAT HRI observation, and three VLA observations of the association during a 2 yr time span. Our study will focus on four stars, Cyg OB2 No. 5, No. 8A, No. 9, and No. 12. Three of the four (Cyg No. 5, No. 9, and No. 12) were relatively constant in their X-ray emission over the 2 yr time frame. The fourth, Cyg OB2 No. 8A, increased in intensity by ~34%. No short-term (hourly) variability was detected. The observed X-ray characteristics (e.g., luminosity, temperature) are found to be consistent with the X-ray properties of other OB stars. The exception is Cyg OB2 No. 12, whose X-ray characteristics are found to be inconsistent with its spectral classification. Detailed spectral analyses of the PSPC data are presented for two absorption models: (1) ISM (cold absorber) and (2) Wind + ISM (warm absorber). The spectral fits suggest that the X-ray sources are located within the stellar wind, and estimates of the X-ray locations are presented. Adopting the radio-derived mass-loss rates, these X-ray locations are found to be consistent with the shock scenario proposed for OB stars. As expected, the radio emission has continued to be highly variable. Nonthermal characteristics are observed in Cyg OB2 No. 8A and No. 12. One of the most unusual nonthermal radio sources, Cyg OB2 No. 9, was found to be thermal in one of our observations. An observation of Cyg OB2 No. 5 also displayed a thermal radio spectrum. A comparison of the observed and intrinsic X-ray fluxes with the observed radio fluxes suggests that these quantities are anticorrelated; the strongest X-ray source is the weakest radio source. This is contrary to normal expectations for a wind-generated model of X-ray and radio emission. We investigate the long-term temporal behavior of both the X-ray and radio emission by comparing our newly acquired data with the previous X-ray (IPC) and radio data over the past 15 yr. Except for three events observed in Cyg OB2 No. 5, which displayed significant increases in its X-ray emission, the X-ray emission has remained relatively constant with a variability level less than 20% over this time span, whereas the radio emission has stayed highly variable with various levels of nonthermal behavior. We present a model to investigate the case in which the X-ray and radio emission are controlled by stellar wind properties and find that the predicted variability should be comparable in both emission processes. This is not observed. It is very intriguing that every time we observe the radio emission, it is different, whereas the X-ray emission always appears to be constant. If the stellar wind is as variable as suggested by the radio data, we believe it is highly implausible that we just happened to miss all periods of X-ray variability. We also investigate the implications of the observed nonthermal radio spectrum of Cyg OB2 No. 8A. Using the synchrotron emission model of White, we find that this nonthermal radio spectrum predicts a mass-loss rate almost 2 orders of magnitude less than that expected for a thermal radio spectrum. This lower mass-loss rate is consistent with an X-ray source located at the base of the stellar wind, contrary to the basic shock scenario. Since these stars show evidence of changing from thermal to nonthermal radio characteristics, it is difficult to understand how such a large change in mass loss did not produce a significant change in the observed X-rays.

A multi-wavelength investigation of the non-thermal radio emitting O-star 9 Sgr

We report the results of a multi-wavelength investigation of the O4 V star 9 Sgr (= HD 164794). Our data include observations in the X-ray domain withXMM-Newton, in the radio domain with the VLA as well as optical spectroscopy. 9 Sgr is one of a few presumably single OB stars that display non-thermal radio emission. This phenomenon is attributed to synchrotron emission by relativistic electrons accelerated in strong hydrodynamic shocks in the stellar wind. Given the enormous supply of photospheric UV photons in the wind of 9 Sgr, inverse Compton scattering by these relativistic electrons is a priori expected to generate a non-thermal power law tail in the X-ray spectrum. Our EPIC and RGS spectra of 9 Sgr reveal a more complex situation than expected from this simple theoretical picture. While the bulk of the thermal X-ray emission from 9 Sgr arises most probably in a plasma at temperature3 10 6 K distributed throughout the wind, the nature of the hard emission in the X-ray spectrum is less clear. Assuming a non-thermal origin, our best fitting model yields a photon index of2: 9f or the power law component which would imply a low compression ratio of1:79 for the shocks responsible for the electron acceleration. However, the hard emission can also be explained by a thermal plasma at a temperature2 10 7 K. Our VLA data indicate that the radio emission of 9 Sgr was clearly non-thermal at the time of the XMM-Newton observation. Again, we derive a low compression ratio (1.7) for the shocks that accelerate the electrons responsible for the synchrotron radio emission. Finally, our optical spectra reveal long-term radial velocity variations suggesting that 9 Sgr could be a long-period spectroscopic binary.

A COORDINATED X-RAY AND OPTICAL CAMPAIGN OF THE NEAREST MASSIVE ECLIPSING BINARY, δ ORIONIS Aa. IV. A MULTIWAVELENGTH, NON-LTE SPECTROSCOPIC ANALYSIS

T.S. is grateful for financial support from the Leibniz Graduate School for Quantitative Spectroscopy in Astrophysics, a joint project of the Leibniz Institute for Astrophysics Potsdam (AIP) and the institute of Physics and Astronomy of the University of Potsdam. L.M.O. acknowledges support from DLR grant 50 OR 1302. M.F.C., J.S.N., and W.L.W. are grateful for support via Chandra grants GO3-14015A and GO3-14015E. A.F.J.M. acknowledges financial aid from NSERC (Canada) and FRQNT (Quebec). J.M.A. acknowledges support from (a) the Spanish Government Ministerio de Economia y Competitividad (MINECO) through grants AYA2010-15 081 and AYA2010-17 631 and (b) the Consejeria de Educacion of the Junta de Andalucia through grant P08-TIC-4075. Caballero N.D.R. gratefully acknowledges his Centre du Recherche en Astrophysique du Quebec (CRAQ) fellowship. Y.N. acknowledges support from the Fonds National de la Recherche Scientifique (Belgium), the Communaute Francaise de Belgique, the PRODEX XMM and Integral contracts, and the “Action de Recherche Concertee” (CFWB-Academie Wallonie Europe). J.L.H. acknowledges support from NASA award NNX13AF40G and NSF award AST-0807477. I.N. is supported by the Spanish Mineco under grant AYA2012-39364-C02-01/02, and the European Union.

On the Weak-wind Problem in Massive Stars: X-Ray Spectra Reveal a Massive Hot Wind in μ Columbae

United States. National Aeronautics and Space Administration (Chandra X-ray Observatory (U.S.) Award GO1-12017B)

Effects of coronal and shock-produced X-rays on the ionization distribution in hot star winds

We investigate the effects of X-ray radiation on the ionization distribution in the wind of ζ Pup (O4 If). In particular, we have studied how the distribution of the superionization species O VI is affected by X-ray source characteristics. In our calculations, detailed statistical equilibrium and atomic physics models are used to ensure that critical processes such as photoionization out of excited states and Auger ionization are considered. For shock-produced X-ray sources, we show how the distribution of X-ray emission affects the O VI P Cygni profile and the resultant X-ray spectrum. For coronal sources, we examine the sensitivity of the X-ray spectrum and O VI profile to the mass-loss rate and attenuation by the overlying wind

ASCA X-Ray Spectroscopy of the Unusual B0 V Star τ Scorpii

We have obtained a high-quality ASCA spectrum of the MK standard B0 V star τ Sco in order to test the standard wind-shock picture of OB star X-ray production. The fluxes in three line complexes from ions indicative of hot plasma—Mg+10, Si+12, and S+14—are measured, and we also present a global spectral fit using a fairly standard multitemperature, optically thin, collisional equilibrium model. We were able to achieve a statistically good fit, but only by using the MeKaL plasma emission code (Mewe, Kaastra, & Liedahl) and fixing the elemental abundances at the photospheric values as determined by optical spectroscopy. The parameters of the model are T1 = 7 MK, EM1 = 3.5 × 1054 cm-3, T2 = 12 MK, EM2 = 8.1 × 1053 cm-3, T3 > 27 MK, EM3 > 3.0 × 1053 cm-3. The quantity of material with temperature in excess of 107 K on τ Sco is comparable to that with temperature in excess of 106 K on most other early B stars. The data cannot be explained by the standard line-force instability wind-shock mechanism. However, more unusual shock mechanisms involving magnetically confined wind shocks or interactions between infalling matter and the ambient stellar wind cannot be ruled out. Alternately, a dynamo driven by differential rotation could be powering coronal plasma. If magnetic fields are involved in any way, then the stars extreme youth could play a role.

[ITAL]Chandra[/ITAL] Discovers a Very High Density X-Ray Plasma on the O Star ζ Orionis

We report on a Chandra line spectrum observation of the O supergiant ζ Orionis (O9.7 Ib). A 73.4 ks High-Energy Transmission Grating Spectrometer observation shows a wide range of ionization stages and line strengths over the wavelength range of 5-26 A. The observed emission lines indicate a range in temperature of 2-10 MK, which is consistent with earlier X-ray observations of ζ Ori. Many lines are spectrally resolved showing Doppler broadening of 900 ± 200 km s-1. The observed He-like ions (O VII, Ne IX, Mg XI, and Si XIII) provide information about the spatial distribution of the X-ray emission. Although the observations support a wind distribution of X-ray sources, we find three conflicting results. First, line diagnostics for Si XIII indicate that this line emission forms very close to the stellar surface, where the density is of order 1012 cm-3, but the velocity there is too small to produce the shock jump required for the observed ionization level. Second, the strong X-ray line profiles are symmetric and do not show any evidence of Doppler-blueshifted line centroids, which are expected to accompany an outwardly moving source in a high-density wind. Third, the observed velocity dispersions do not appear to correlate with the associated X-ray source radii velocities, contrary to expectations of wind-distributed source models. A composite source model involving wind shocks and some magnetic confinement of turbulent hot plasma in a highly nonsymmetric wind appears to be needed to explain the line diagnostic anomalies.

THE CORRELATION BETWEEN X-RAY LINE IONIZATION AND OPTICAL SPECTRAL TYPES OF THE OB STARS

Marked correlations are reported between the ionization of the X-ray line spectra of normal OB stars, as observed by the Chandra X-Ray Observatory, and their optical spectral types. These correlations include the progressive weakening of the higher ionization relative to the lower ionization X-ray lines with advancing spectral type, and the similarly decreasing intensity ratios of the H-like to He-like lines of the α ions. These relationships were not predicted by models, nor have they been clearly evident in astrophysical studies of a few objects; rather, they have emerged from morphological analysis of an adequate (albeit still small) sample, from which known peculiar objects such as magnetic stars and very rapid rotators have been isolated to reveal the normal trends. This process is analogous to that which first demonstrated the strong relationships between the UV wind profiles and the optical spectral types of normal OB stars, which likely bear a physical as well as a historical connection to the present X-ray results. Since the optical spectral types are calibrated in terms of fundamental stellar parameters, it follows that the winds and X-ray spectra are determined by the latter. These observations provide strong guidance for further astrophysical modeling of these phenomena.