High Resolution X-ray Spectra of the Brightest OB Stars in the Cygnus OB2 Association

The Cygnus OB2 Association contains some of the most luminous OB stars in our Galaxy, the brightest of which are also among the most luminous in X-rays. We have obtained a Chandra High Energy Transmission Grating Spectrometer (HETGS) observation centered on Cyg OB2 No. 8a, the most luminous X-ray source in the Association. Although our analysis will focus on the X-ray properties of Cyg OB2 No. 8a, we also present limited analyses of three other OB stars (Cyg OB2 Nos. 5, 9, and 12). Applying standard diagnostic techniques as used in previous studies of early-type stars, we find that the X-ray properties of Cyg OB2 No. 8a are very similar to those of other OB stars that have been observed using high-resolution X-ray spectroscopy. From analyses of the He-like ion "fir" emission lines, we derive radial distances of the He-like line emission sources and find these fir-inferred radii are consistent with their corresponding X-ray continuum optical depth unity radii. Contrary to other O-star results, the emission lines of Cyg OB2 No. 8a show a large range in line centroid shifts (roughly -800 to +250 km/s). We discuss the implications of our results in light of the fact that Cyg OB2 No. 8a is a member of a rather tight stellar cluster, and shocks could arise at interfaces with the winds of these other stars.Comment: 36 pages (including 4 tables and 12 figures). LaTeX. Submitted to Ap

An Extensive Collection of Stellar Wind X-ray Source Region Emission Line Parameters,Temperatures, Velocities, and Their Radial Distributions as Obtained from Chandra Observations of 17 OB Stars

Chandra high energy resolution observations have now been obtained from numerous non-peculiar O and early B stars. The observed X-ray emission line properties differ from pre-launch predictions, and the interpretations are still problematic. We present a straightforward analysis of a broad collection of OB stellar line profile data to search for morphological trends. X-ray line emission parameters and the spatial distributions of derived quantities are examined with respect to luminosity class. The X-ray source locations and their corresponding temperatures are extracted by using the He-like f/i line ratios and the H-like to He-like line ratios respectively. Our luminosity class study reveals line widths increasing with luminosity. Although the majority of the OB emission lines are found to be symmetric, with little central line displacement, there is evidence for small, but finite, blue-ward line-shifts that also increase with luminosity. The spatial X-ray temperature distributions indicate that the highest temperatures occur near the star and steadily decrease outward. This trend is most pronounced in the OB supergiants. For the lower density wind stars, both high and low X-ray source temperatures exist near the star. However, we find no evidence of any high temperature X-ray emission in the outer wind regions for any OB star. Since the temperature distributions are counter to basic shock model predictions, we call this the "near-star high-ion problem" for OB stars. By invoking the traditional OB stellar mass loss rates, we find a good correlation between the fir-inferred radii and their associated X-ray continuum optical depth unity radii. We conclude by presenting some possible explanations to the X-ray source problems that have been revealed by this study.Comment: Published in 2007, ApJ, 668, 456. An Erratum scheduled for publication in 2008, ApJ, 680, is included as an Appendix. The Erratum corrects some tabulated data in 5 tables and 2 figure

X-ray emitting young stars in the Orion Nebula

The Orion Nebula Cluster and the molecular cloud in its vicinity have been observed with the ACIS-I detector on board the Chandra X-ray Observatory with 23 hours exposure. We detect 1075 X-ray sources: 91% are spatially associated with known stellar members of the cluster, and 7% are newly identified deeply embedded cloud members. This provides the largest X-ray study of a pre-main sequence stellar population. We examine here the X-ray properties of Orion young stars as a function of mass. Results include: (a) the discovery of rapid variability in the O9.5 31 M_o star \theta^2A Ori, and several early B stars, inconsistent with the standard model of X-ray production in small wind shocks; (b) support for the hypothesis that intermediate-mass mid-B through A type stars do not themselves produce significant X-ray emission; (c) confirmation that low-mass G- through M-type T Tauri stars exhibit powerful flaring but typically at luminosities considerably below the `saturation' level; (d) confirmation that the presence or absence of a circumstellar disk has no discernable effect on X-ray emission; (e) evidence that T Tauri plasma temperatures are often very high with T >= 100 MK, even when luminosities are modest and flaring is not evident; and (f) detection of the largest sample of pre-main sequence very low mass objects showing high flaring levels and a decline in magnetic activity as they evolve into L- and T-type brown dwarfs.Comment: 82 pages, 16 figures, 6 tables. To appear in the Astrophysical Journal. For a version with high quality images and electronic tables, see ftp://ftp.astro.psu.edu/pub/edf/orion1

X-ray Point Sources in The Central Region of M31 as seen by Chandra

We report on \chandra observations of the central region of M31. By combining eight \chandra ACIS-I observations taken between 1999 and 2001, we have identified 204 X-ray sources within the central $\sim 17'\times17'$ region of M31, with a detection limit of $\sim 2\times10^{35}$\lum. Of these 204 sources, 22 are identified with globular clusters, 2 with supernova remnants, 9 with planetary nebula, and 9 as supersoft sources. By comparing individual images, about 50% of the sources are variable on time scales of months. We also found 13 transients, with light curves showing a variety of shapes. We also extracted the energy spectra of the 20 brightest sources; they can be well fit by a single power-law with a mean photon index of 1.8. The spectral shapes of 12 sources are shown to be variable, suggesting that they went through state changes. The luminosity function of all the point sources is consistent with previous observations (a broken power-law with a luminosity break at $1.7\times10^{37}$\lum). However, when the X-ray sources in different regions are considered separately, different luminosity functions are obtained. This indicates that the star-formation history might be different in different regions.Comment: 42 pages, 9 figures, ApJ, accepted, Higher-resolution figures available on reques

O VI Observations of the Onset of Convection Zones in Main-Sequence A Stars

If magnetic activity in outer stellar atmospheres is due to an interplay between rotation and subsurface convection, as is generally presumed, then one would not expect to observe indicators of activity in stars with T_eff > 8300 K. Any X-ray or ultraviolet line emission from hotter stars must be due either to a different mechanism or to an unresolved, active, binary companion. Due to their poor spatial resolution, X-ray instruments have been especially susceptible to source confusion. At wavelengths longward of 1216 Angstroms, the near ultraviolet spectra of stars hotter than this putative dividing line are dominated by photospheric continuum. We have used FUSE to obtain spectra of the subcoronal O VI emission lines, which lie at a wavelength where the photospheric continuum of the mid- and early-A stars is relatively weak. We observed 14 stars spanning a range in T_eff from 7720 to 10,000 K. Eleven of the 14 stars showed O VI emission lines, including 6 of the 8 targets with T_eff > 8300 K. At face value, this suggests that activity does not fall off with increasing temperature. However, the emission lines are narrower than expected from the projected rotational velocities of these rapidly-rotating stars, suggesting that the emission could come from unresolved late-type companions. Furthermore, the strength of the O VI emission is consistent with that expected from an unseen active K or M dwarf binary companinon, and the high x-ray to far uv luminosity ratios observed indicate that this must be the case. Our results are therefore consistent with earlier studies that have shown a rapid drop-off in activity at the radiative/convective boundary expected at T_eff about 8300 K, in agreement with conventional stellar structure models

Inverse Compton Scattering as the Source of Diffuse EUV Emission in the Coma Cluster of Galaxies

We have examined the hypothesis that the majority of the diffuse EUV flux in the Coma cluster is due to inverse Compton scattering of low energy cosmic ray electrons (0.16 < epsilon < 0.31 GeV) against the 3K black-body background. We present data on the two-dimensional spatial distribution of the EUV flux and show that these data provide strong support for a non-thermal origin for the EUV flux. However, we show that this emission cannot be produced by an extrapolation to lower energies of the observed synchrotron radio emitting electrons and an additional component of low energy cosmic ray electrons is required.Comment: 11 pages, 5 figure

X-Ray Line Profiles from Parameterized Emission Within an Accelerating Stellar Wind

Motivated by recent detections by the XMM and Chandra satellites of X-ray line emission from hot, luminous stars, we present synthetic line profiles for X-rays emitted within parameterized models of a hot-star wind. The X-ray line emission is taken to occur at a sharply defined co-moving-frame resonance wavelength, which is Doppler-shifted by a stellar wind outflow parameterized by a `beta' velocity law, $v(r)=v_{\infty} (1-\R_{\ast}/r)^\beta$. Above some initial onset radius $R_o$ for X-ray emission, the radial variation of the emission filling factor is assumed to decline as a power-law in radius, $f(r) \sim r^{-q}$. The computed emission profiles also account for continuum absorption within the wind, with the overall strength characterized by a cumulative optical depth $\tau_\ast$. In terms of a wavelength shift from line-center scaled in units of the wind terminal speed $v_{\infty}$, we present normalized X-ray line profiles for various combinations of the parameters $\beta$, $\tau_\ast$, $q$ and $R_o$, and including also the effect of instrumental broadening as characterized by a Gaussian with a parameterized width $\sigma$. We discuss the implications for interpreting observed hot-star X-ray spectra, with emphasis on signatures for discriminating between ``coronal'' and ``wind-shock'' scenarios. In particular, we note that in profiles observed so far the substantial amount of emission longward of line center will be difficult to reconcile with the expected attenuation by the wind and stellar core in either a wind-shock or coronal model.Comment: Submitted to Ap.J. 17 pages; includes 5 figures. Preprint also available at http://www.bartol.udel.edu/~owocki/preprint

High-Resolution Chandra X-Ray Imaging And Spectroscopy Of The Sigma Orionis Cluster

We present results of a 90 ks Chandra X-ray observation of the young sigma Orionis cluster ( age similar to 3 Myr) obtained with the HETGS. We use the high-resolution grating spectrum and moderate-resolution CCD spectrum of the massive central star sigma Ori AB (O9.5 V + B0.5 V) to test wind shock theories of X-ray emission and also analyze the high spatial resolution zero-order ACIS-S image of the central cluster region. Chandra detected 42 X-ray sources on the primary CCD (ACIS-S3). All but five have near-IR or optical counterparts and about one-fourth are variable. Notable high-mass stellar detections are sigma Ori AB, the magnetic B star sigma Ori E, and the B5 V binary HD 37525. Most of the other detections have properties consistent with lower mass K- or M-type stars. We present the first X-ray spectrum of the unusual infrared source IRS 1, located approximate to 3 \u27\u27 north of sigma Ori AB. Its X-ray properties and elongated mid-IR morphology suggest that it is an embedded low-mass T Tauri star whose disk/envelope is being photoevaporated by sigma Ori AB. We focus on the radiative wind shock interpretation of the soft luminous X-ray emission from sigma Ori AB, but also consider possible alternatives including magnetically confined wind shocks and colliding wind shocks. Its emission lines show no significant asymmetries or centroid shifts and are moderately broadened to HWHM approximate to 264 km s(-1), or one-fourth the terminal wind speed. Forbidden lines in He-like ions are formally undetected, implying strong UV suppression. The Mg XI triplet forms in the wind acceleration zone within one stellar radius above the surface. These X-ray properties are consistent in several respects with the predictions of radiative wind shock theory for an optically thin wind, but explaining the narrow line widths presents a challenge to the theory

CBR model for the intelligent management of customer support centers

[EN] In this paper, a new CBR system for Technology Management Centers is presented. The system helps the staff of the centers to solve customer problems by finding solutions successfully applied to similar problems experienced in the past. This improves the satisfaction of customers and ensures a good reputation for the company who manages the center and thus, it may increase its profits. The CBR system is portable, flexible and multi-domain. It is implemented as a module of a help-desk application to make the CBR system as independent as possible of any change in the help-desk. Each phase of the reasoning cycle is implemented as a series of configurable plugins, making the CBR module easy to update and maintain. This system has been introduced and tested in a real Technology Management center ran by the Spanish company TISSAT S.A.Financial support from Spanish government under grant PROFIT FIT-340001-2004-11 is gratefully acknowledgeHeras Barberá, SM.; Garcia Pardo Gimenez De Los Galanes, JA.; Ramos-Garijo Font De Mora, R.; Palomares Chust, A.; Julian Inglada, VJ.; Rebollo Pedruelo, M.; Botti, V. (2006). CBR model for the intelligent management of customer support centers. En Lecture Notes in Computer Science. Springer Verlag (Germany). 663-670. https://doi.org/10.1007/11875581_80S663670Acorn, T., Walden, S.: SMART: SupportManagement Automated Reasoning Technology for Compaq Customer Service. In: Scott, A., Klahr, P. (eds.) Proceedings of the 2 International Conference on Intelligent Tutoring Systems, ITS-92 Berlin, vol. 4, pp. 3–18. AAAI Press, Menlo Park (1992)Simoudis, E.: Using Case-Based Retrieval for Customer Technical Support. IEEE Intelligent Systems 7, 10–12 (1992)Kriegsman, M., Barletta, R.: Building a Case-Based Help Desk Application. IEEE Expert: Intelligent Systems and Their Applications 8, 18–26 (1993)Shimazu, H., Shibata, A., Nihei, K.: Case-Based Retrieval Interface Adapted to Customer-Initiated Dialogues in Help Desk Operations. In: Mylopoulos, J., Reiter, R. (eds.) Proceedings of the 12th National Conference on Artificial Intelligence, vol. 1, pp. 513–518. AAAI Press, Menlo Park (1994)Raman, R., Chang, K.H., Carlisle, W.H., Cross, J.H.: A self-improving helpdesk service system using case-based reasoning techniques. Computers in Industry 2, 113–125 (1996)Kang, B.H., Yoshida, K., Motoda, H., Compton, P.: Help Desk System with Intelligent Interface. Applied Artificial Intelligence 11, 611–631 (1997)Roth-Berghofer, T., Iglezakis, I.: Developing an Integrated Multilevel Help-Desk Support System. In: Proceedings of the 8th German Workshop on Case-Based Reasoning, pp. 145–155 (2000)Goker, M., Roth-Berghofer, T.: The development and utilization of the case-based help-desk support system HOMER. Engineering Applications of Artificial Intelligence 12, 665–680 (1999)Roth-Berghofer, T.R.: Learning from HOMER, a case-based help-desk support system. In: Melnik, G., Holz, H. (eds.) Advances in Learning Software Organizations, pp. 88–97. Springer, Heidelberg (2004)Bergmann, R., Althoff, K.D., Breen, S., Göker, M., Manago, M., Traphöner, R., Wess, S.: Developing Industrial Case-Based Reasoning Applications. In: The INRECA Methodology, 2nd edn. LNCS (LNAI), vol. 1612. Springer, Heidelberg (2003)eGain (2006), http://www.egain.comKaidara Software Corporation (2006), http://www.kaidara.com/Empolis Knowledge Management GmbH - Arvato AG (2006), http://www.empolis.com/Althoff, K.D., Auriol, E., Barletta, R., Manago, M.: A Review of Industrial Case-Based Reasoning Tools. AI Perspectives Report. Goodall, A., Oxford (1995)Watson, I.: Applying Case-Based Reasoning. Techniques for Enterprise Systems. Morgan Kaufmann Publishers, Inc. California (1997)empolis: empolis Orenge Technology Whitepaper. Technical report, empolis GmbH (2002)Tissat, S.A. (2006), http://www.tissat.esGiraud-Carrier, C., Martinez, T.R.: An integrated framework for learning and reasoning. Journal of Artificial Intelligence Research 3, 147–185 (1995)Corchado, J.M., Borrajo, M.L., Pellicer, M.A., Yanez, J.C.: Neuro-symbolic system for Business Internal Control. In: Perner, P. (ed.) ICDM 2004. LNCS (LNAI), vol. 3275, pp. 1–10. Springer, Heidelberg (2004)Aamodt, A., Plaza, E.: Case-based reasoning: foundational issues, methodological variations and system approaches. AI Communications 7(1), 39–59 (1994)Tversky, A.: Features of similarity. Psychological Review 84(4), 327–352 (1997