Michael F. Corcoran

The periodicity of the η Carinae events

Extensive spectral observations of η Carinae over the last cycle, and particularly around the 2003.5 low-excitation event, have been obtained. The variability of both narrow and broad lines, when combined with data taken from two earlier cycles, reveal a common and well-defined period. We have combined the cycle lengths derived from the many lines in the optical spectrum with those from broad-band X-rays, optical and near-infrared observations, and obtained a period length of P pres = 2022.7 ± 1.3 d. Spectroscopic data collected during the last 60 yr yield an average period of P avg = 2020 ± 4 d, consistent with the present-day period. The period cannot have changed by more than AP/P = 0.0007 since 1948. This confirms the previous claims of a true, stable periodicity, and gives strong support to the binary scenario. We have used the disappearance of the narrow component of He I 6678 to define the epoch of the Cycle 11 minimum, To = JD 245 2819.8. The next event is predicted to occur on 2009 January 11 (±2 d). The dates for the start of the minimum in other spectral features and broad-bands are very close to this date, and have well-determined time-delays from the He I epoch.

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.

BULK VELOCITIES, CHEMICAL COMPOSITION, AND IONIZATION STRUCTURE OF THE X-RAY SHOCKS IN WR 140 NEAR PERIASTRON AS REVEALED BY THE CHANDRA GRATINGS

The Wolf-Rayet WC7+O4-5 binary WR 140 went through the periastron passage of its 8 yr eccentric binary orbit in early 2001 as the two stars made their closest approach. Both stars have powerful supersonic stellar winds that crash into each other between the stars to produce X-rays. Chandra grating observations were made when the X-rays were at their peak, making WR 140 the brightest hot-star X-ray source in the sky and giving the opportunity to study the velocity profiles of lines, all of which were resolved and blueshifted before periastron. In the general context of shock physics, the measurements constrain the flow of hot gas and where different ions were made. The brightness of lines relative to the strong continuum in conjunction with plasma models gives interim abundance estimates for eight different elements in WC-type material including an Ne/S ratio in good agreement with earlier long-wavelength measurements. The lower velocity widths of cool ions imply a plasma that was not in equilibrium, probably due to the collisionless nature of the shock transitions and the slow character of both the postshock energy exchange between ions and electrons and subsequent ionization. Electron heat conduction into fast-moving preshock gas was absent, probably suppressed by the magnetic field involved in WR 140s synchrotron emission. After periastron, the spectrum was weaker due mainly to absorption by cool Wolf-Rayet star material.

X-Ray Spectral Variation of η Carinae through the 2003 X-Ray Minimum

We report the results of an observing campaign on η Car around the 2003 X-ray minimum, mainly using the XMM-Newton observatory. These are the first spatially resolved X-ray monitoring observations of the stellar X-ray spectrum during the minimum. The hard X-ray emission, associated with the wind-wind collision (WWC) in the binary system, varied strongly in flux on timescales of days, but not significantly on timescales of hours. The X-ray flux in the 2-10 keV band seen by XMM-Newton was only 0.7% of the flux maximum seen by RXTE. The slope of the X-ray continuum above 5 keV did not vary in any observation, which suggests that the electron temperature of the hottest plasma did not vary significantly at any phase. Through the minimum, the absorption to the stellar source increased by a factor of 5-10 to NH ~ (3-4) × 1023 cm-2. These variations were qualitatively consistent with emission from the WWC plasma entering into the dense wind of the massive primary star. During the minimum, X-ray spectra also showed significant excesses in the thermal Fe XXV emission line on the red side, while they showed only a factor of 2 increase in equivalent width of the Fe fluorescence line at 6.4 keV. These features are not fully consistent with the eclipse of the X-ray plasma and may suggest an intrinsic fading of the X-ray emissivity. The drop in the WWC emission revealed the presence of an additional X-ray component that exhibited no variation on timescales of weeks to years. This component may be produced by the collision of high-speed outflows at v ~ 1000-2000 km s-1 from η Car with ambient gas within a few thousand AU from the star.

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.

DISCOVERY OF EXTREMELY EMBEDDED X-RAY SOURCES IN THE R CORONAE AUSTRALIS STAR-FORMING CORE

With the XMM-Newton and Chandra observatories, we detected two extremely embedded X-ray sources in the R Corona Australis (R CrA) star-forming core, near IRS 7. These sources, designated as XE and XW, have X-ray absorption columns of ~3 × 1023 cm-2 equivalent to AV ~ 180 mag. They are associated with the VLA centimeter radio sources 10E and 10W, respectively; XW is the counterpart of the near-infrared source IRS 7, whereas XE has no K-band counterpart above 19.4 mag. This indicates that XE is younger than typical Class I protostars, probably a Class 0 protostar, or in an intermediate phase between Class 0 and Class I. The X-ray luminosity of XE varied between 29 < log LX < 31.2 ergs s-1 on timescales of 3-30 months; XE also showed a monotonic increase in X-ray brightness by a factor of 2 in 30 ks during an XMM-Newton observation. The XMM-Newton spectra indicate emission from a hot plasma with kT ~ 3-4 keV and also show fluorescent emission from cold iron. Although the X-ray spectrum from XE is similar to flare spectra from Class I protostars in luminosity and temperature, the light curve does not resemble the light curves of magnetically generated X-ray flares, because the variability timescale of XE is too long and variations in X-ray count rate were not accompanied by variations in spectral hardness. The short-term variation of XE may be caused by the partial blocking of the X-ray plasma, while the month-long flux enhancement may be driven by mass accretion.

Constraints on decreases in η Carinae's mass-loss from 3D hydrodynamic simulations of its binary colliding winds

Recent work suggests that the mass-loss rate of the primary star Eta-A in the massive colliding wind binary Eta Carinae dropped by a factor of 2-3 between 1999 and 2010. We present result from large- (+/- 1545 au) and small- (+/- 155 au) domain, 3D smoothed particle hydrodynamics (SPH) simulations of Eta Cars colliding winds for three Eta-A mass-loss rates ( (dot-M(sub Eta-A) = 2.4, 4.8 and 8.5 10(exp 4) M(solar)/ yr), investigating the effects on the dynamics of the binary wind-wind collision (WWC). These simulations include orbital motion, optically thin radiative cooling and radiative forces. We find that dot-M Eta-A greatly affects the time-dependent hydrodynamics at all spatial scales investigated. The simulations also show that the post-shock wind of the companion star Eta-B switches from the adiabatic to the radiative-cooling regime during periastron passage (Phi approx.= 0.985-1.02). This switchover starts later and ends earlier the lower the value of dot-M Eta-A and is caused by the encroachment of the wind of Eta-A into the acceleration zone of Eta-Bs wind, plus radiative inhibition of Eta-Bs wind by Eta-A. The SPH simulations together with 1D radiative transfer models of Eta-As spectra reveal that a factor of 2 or more drop in dot-M EtaA should lead to substantial changes in numerous multiwavelength observables. Recent observations are not fully consistent with the model predictions, indicating that any drop in dot- M Eta-A was likely by a factor of approx. < 2 and occurred after 2004. We speculate that most of the recent observed changes in Eta Car are due to a small increase in the WWC opening angle that produces significant effects because our line of sight to the system lies close to the dense walls of the WWC zone. A modest decrease in dot-M Eta-A may be responsible, but changes in the wind/stellar parameter of Eta-B, while less likely, cannot yet be fully ruled out. We suggest observations during Eta-Cars next periastron in 2014 to further test for decreases in dot-M Eta-A. If dot-M Eta-A is declining and continues to do so, the 2014 X-ray minimum should be even shorter than that of 2009.

Chandra/Very Large Array Follow-Up of TeV J2032+4131, the Only Unidentified TeV Gamma-Ray Source

The High Energy Gamma Ray Astronomy (HEGRA) Cerenkov telescope array group recently reported a steady and extended unidentified TeV gamma-ray source lying at the outskirts of Cygnus OB2. This is the most massive stellar association known in the Galaxy, estimated to contain � 2600 OB-type members alone. It has been previously argued that the large-scale shocks and turbulence induced by the multiple interacting supersonic winds from the many young stars in such associations may play a role in accelerating Galactic cosmic rays. Indeed, Cyg OB2 also coincides with the nonvariable MeV–GeV range unidentified EGRET source, 3EG 2033+4118. We report on the near-simultaneous follow-up observations of the extended TeV source region with the Chandra X-Ray Observatory and the Very Large Array radio telescope, obtained in order to explore this possibility. Analysis of the CO, H i, and IRAS 100 lm emissions shows that the TeV source region coincides with an outlying subgroup of powerful OB stars that have evacuated or destroyed much of the ambient atomic, molecular, and dust material and that may be related to the very high energy emissions. An interesting supernova-remnant–like structure is also revealed near the TeV source region in the CO, H i, and radio emission maps. Applying a numerical simulation that accurately tracks the radio to gamma-ray emission from primary hadrons as well as primary and secondary e � , we find that the broadband spectrum of the TeV source region favors a predominantly nucleonic—rather than electronic—origin for the high-energy flux, although deeper X-ray and radio observations will help confirm this. A very reasonable, � 0.1%, conversion efficiency of Cyg OB2’s extreme stellar wind mechanical luminosity to nucleonic

ASCA solid state imaging spectrometer observations of O stars

We report ASCA Solid State Imaging Spectrometer (SIS) X-ray observations of the O stars delta Ori and lambda Ori. The energy resolution of the SIS allows us to resolve features in the O star X-ray spectra which are not apparent in spectra obtained by X-ray spectrometers with lower energy resolution. SIS spectra from both stars show evidence of line emission, suggesting the thermal nature of the X-ray source. However, the observed line strengths are different for the two stars. The observed stellar X-ray spectra are not well described by isothermal models although absorbed thermal emission models with two or more temperatures can provide an adequate fit to the data. For both stars we present evidence of absorbing columns significantly larger than the known Interstellar Medium (ISM) columns, indicative of absorption by a circumstellar medium, presumably the stellar winds. In addition, the lambda Ori spectrum shows the presence of emission at energies greater than 3 keV which is not seen in the delta Ori spectrum.

Modelling the RXTE light curve of η Carinae from a 3D SPH simulation of its binary wind collision

The very massive star system η Carinae exhibits regular 5.54-year (2024-day) period disruptive events in wavebands ranging from the radio to X-ray. There is a growing consensus that these events likely stem from periastron passage of an (as yet) unseen companion in a highly eccentric (ǫ � 0.9) orbit. This paper presents three-dimensional (3-D) Smoothed Particle Hydrodynamics (SPH) simulations of the orbital variation of the binary wind-wind collision, and applies these to modeling the X-ray light curve observed by the Rossi X-ray Timing Explorer (RXTE). By providing a global 3-D model of the phase variation of the density of the interacting winds, the simulations allow computation of the associated variation in X-ray absorption, presumed here to originate from near the apex of the wind-wind interaction cone. We find that the observed RXTE light curve can be readily fit if the observe r’s line of sight is within this cone along the general direction of apastron. Specifically, the data are well fit by an assumed inclination i = 45 ◦ for the orbit’s polar axis, which is thus consistent with orb ital angular momentum being along the inferred polar axis of the Homunculus nebula. The fits also constrain the position angle φ that an orbital-plane projection makes with the apastron si de of the semi-major axis, strongly excluding positions φ < 9 ◦ along or to the retrograde side of the axis, with the best fit position given by φ = 27 ◦ . Overall the results demonstrate the utility of a fully 3-D dynamical model for constraining the geometric and physical properties of this complex colliding-wind binary system.