Marc Gagne

PI -STACKING INTERACTIONS : ALIVE AND WELL IN PROTEINS

A representative set of high resolution x-ray crystal structures of nonhomologous proteins have been examined to determine the preferred positions and orientations of noncovalent interactions between the aromatic side chains of the amino acids phenylalanine, tyrosine, histidine, and tryptophan. To study the primary interactions between aromatic amino acids, care has been taken to examine only isolated pairs (dimers) of amino acids because trimers and higher order clusters of aromatic amino acids behave differently than their dimer counterparts. We find that pairs (dimers) of aromatic side chain amino acids preferentially align their respective aromatic rings in an off-centered parallel orientation. Further, we find that this parallel-displaced structure is 0.5–0.75 kcal/mol more stable than a T-shaped structure for phenylalanine interactions and 1 kcal/mol more stable than a T-shaped structure for the full set of aromatic side chain amino acids. This experimentally determined structure and energy difference is consistent with ab initio and molecular mechanics calculations of benzene dimer, however, the results are not in agreement with previously published analyses of aromatic amino acids in proteins. The preferred orientation is referred to as parallel displaced π-stacking.

Chandra HETGS Multiphase Spectroscopy Of The Young Magnetic O Star Theta(1) Orionis C

We report on four Chandra grating observations of the oblique magnetic rotator � 1 Ori C (O5.5 V), covering a wide range of viewing angles with respect to the star’s 1060 G dipole magnetic field. We employ line-width and centroid analyses to study the dynamics of the X-ray–emitting plasma in the circumstellar environment, as well as line-ratio diagnostics to constrain the spatial location, and global spectral modeling to constrain the temperature distribution and abundances of the very hotplasma. We investigate these diagnostics as a function of viewing angle andanalyzetheminconjunctionwithnewMHDsimulationsofthemagneticallychanneledwindshockmechanism on � 1 Ori C. This model fits all the data surprisingly well, predicting the temperature, luminosity, and occultation of the X-ray–emitting plasma with rotation phase. Subject headingg stars: early-type — stars: individual (HD 37022) — stars: magnetic fields — stars: mass loss — stars: rotation — stars: winds, outflows — X-rays: stars Online material: color figure

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.

Periodic X-Ray Emission from the O7 V Star θ1 Orionis C

We report the discovery of large-amplitude, periodic X-ray emission from the O7 V star θ1 Orionis C, the central star of the Orion Nebula. Ten ROSAT HRI snapshots of the Trapezium cluster taken over the course of 21 days show that the count rate of θ1 Ori C varies from 0.26 to 0.41 counts s-1 with a clear 15 day period. The soft X-ray variations have the same phase and period as Hα and He II λ4686 variations reported by Stahl et al. and are in antiphase with the C IV and Si IV ultraviolet absorption features. We consider five mechanisms which might explain the amplitude, phase, and periodicity of the X-ray variations: (1) colliding-wind emission with an unseen binary companion, (2) coronal emission from an unseen late-type pre-main-sequence star, (3) periodic density fluctuations, (4) absorption of magnetospheric X-rays in a corotating wind, and (5) magnetosphere eclipses. The ROSAT data rule out the first three scenarios but cannot rule out either of the latter two which require the presence of an extended magnetosphere, consistent with the suggestion of Stahl et al. that θ1 Ori C is an oblique magnetic rotator. As such, θ1 Ori C may be the best example of a high-mass analog to the chemically peculiar, magnetic Bp stars.

Simultaneous Chandra and very large array observations of young stars and protostars in ρ Ophiuchus cloud core A

A 96 ks Chandra X-ray observation of ρ Ophiuchus cloud core A detected 87 sources, of which 60 were identified with counterparts at other wavelengths. The X-ray detections include 12 of 14 known classical T Tauri stars (CTTSs) in the field, 15 of 17 known weak-lined TTSs (WTTSs), and 4 of 15 brown dwarf candidates. The X-ray detections are characterized by hard, heavily absorbed emission. Most X-ray detections have visual extinctions in the range AV ≈ 10-20 mag, but several sources with visual absorptions as high as AV ≈ 40-56 mag were detected. The mean photon energy of a typical source is E ≈ 3 keV, and more than half of the detections are variable. Prominent X-ray flares were detected in the unusual close binary system Oph S1, the X-ray-bright WTTS DoAr 21, and the brown dwarf candidate GY 31 (M5.5). Time-resolved spectroscopic analysis of the DoAr 21 flare clearly reveals a sequence of secondary flares during the decay phase that may have reheated the plasma. We find that the X-ray luminosity distributions and spectral hardnesses of CTTSs and WTTSs are similar. We also conclude that the X-ray emission of detected brown dwarf candidates is less luminous than TTSs, but spectroscopically similar. Simultaneous multifrequency Very Large Array (VLA) observations detected 31 radio sources at 6 cm, of which 10 were also detected by Chandra. We report new radio detections of the optically invisible IR source WLY 2-11 and the faint Hα emission line star Elias 24 (class II). We confirm circular polarization in Oph S1 and report a new detection of circular polarization in DoAr 21. We find no evidence that X-ray and radio luminosities are correlated in the small sample of TTSs detected simultaneously with Chandra and the VLA. We describe a new nonparametric method for estimating X-ray spectral properties from unbinned photon event lists that is applicable to both faint and bright X-ray sources. The method is used to generate fX, log T, and LX light curves. In addition, we provide a publicly available electronic database containing multiwavelength data for 345 known X-ray, IR, and radio sources in the core A region.

Spectral and Temporal Characteristics of X-Ray-Bright Stars in the Pleiades

We follow up our deep ROSAT imaging survey of the Pleiades (Stauffer et al. 1994) with an analysis of the spectral and temporal characteristics of the X-ray-bright stars in the Pleiades. Raymond & Smith (1977) one and two-temperature models have been used to fit the position-sensitive proportional counter (PSPC) pulse-height spectra of the dozen or so brightest sources associated with late-type Pleiades members. The best-fit temperatures suggest hot coronal temperatures for K, M, and rapidly rotating G stars, and cooler temperatures for F and slowly rotating G stars. In order to probe the many less X-ray-luminous stars, we have generated composite spectra by combining net counts from all Pleiades members according to spectral type and rotational velocity. Model fits to the composite spectra confirm the trend seen in the individual spectral fits. Particularly interesting is the apparent dependence of coronal temperature on L(sub x)/L(sub bol). A hardness-ratio analysis also confirms some of these trends. The PSPC data have also revealed a dozen or so strong X-ray flares with peak X-ray luminosities in excess of approx. 10(exp 30) ergs/sec. We have modeled the brightest of these flares with a simple quasi-static cooling loop model. The peak temperature and emission measure and the inferred electron density and plasma volume suggest a very large scale flaring event. The PSPC data were collected over a period of approx. 18 months, allowing us to search for source variability on timescales ranging from less than a day (in the case of flares) to more than a year between individual exposures. On approximately year-long timescales, roughly 25% of the late-type stars are variable. Since the Pleiades was also intensively monitored by the imaging instruments on the Einstein Observatory, we have examined X-ray luminosity variations on the 10 yr timescale between Einstein and ROSAT and find that up to 40% of the late-type stars are X-ray variable. Since there is only marginal evidence for increased variability on decade-long timescales, the variability observed on long and short timescales may have a common physical origin.

NGC 1624-2: a slowly rotating, X-ray luminous Of?cp star with an extraordinarily strong magnetic field

This paper presents a first observational investigation of t he faint Of?p star NGC 1624-2, yielding important new constraints on its spectral and physical characteristics, rotation, magnetic field strength, X-ray emission and magnetospheric pro perties. Modeling the spectrum and spectral energy distribution, we conclude that NGC 1624-2 is a main sequence star of mass M≃ 30 M⊙, and infer an effective temperature of 35± 2 kK and log g = 4.0± 0.2. Based on an extensive time series of optical spectral observation s we report significant variability of a large number of spectral lines, and infer a unique period of 157.99± 0.94 d which we interpret as the rotational period of the star. We report the detec tion of a very strong - 5.35± 0.5 kG - longitudinal magnetic field h Bzi , coupled with probable Zeeman splitting of Stokes I profiles of metal lines confirming a surface field modulus h Bi of 14± 1 kG, consistent with a surface dipole of polar strength ∼ 20 kG. This is the largest magnetic field ever detected in an O-type star, and the first report of Zeeman splitting of Stoke s I profiles in such an object. We also report the detection of reversed Stokes V profiles associated with weak, high-excitation emission lines of Oiii, which we propose may form in the close magnetosphere of the star. We analyze archival Chandra ACIS-I X-ray data, inferring a very hard spectrum with an X-ray effi ciency log Lx/Lbol =−6.4, a factor of 4 larger than the canonical value for O-type sta rs and comparable to that of the young magnetic O-type starθ 1 Ori C and other Of?p stars. Finally, we examine the probable magnetospheric properties of the star, reporting in particular very strong magnetic confinement of the stellar wind, with η∗≃ 1.5× 10 4 , and a very large Alfven radius, RAlf = 11.4 R∗.

The Chandra Carina Complex Project: Deciphering the enigma of Carina's diffuse X-ray emission

We present a 1.42 deg2 mosaic of diffuse X-ray emission in the Great Nebula in Carina from the Chandra X-ray Observatory Advanced CCD Imaging Spectrometer camera. After removing >14,000 X-ray point sources from the field, we smooth the remaining unresolved emission, tessellate it into segments of similar apparent surface brightness, and perform X-ray spectral fitting on those tessellates to infer the intrinsic properties of the X-ray-emitting plasma. By modeling faint resolved point sources, we estimate the contribution to the extended X-ray emission from unresolved point sources and show that the vast majority of Carinas unresolved X-ray emission is truly diffuse. Line-like correlated residuals in the X-ray spectral fits suggest that substantial X-ray emission is generated by charge exchange at the interfaces between Carinas hot, rarefied plasma and its many cold neutral pillars, ridges, and clumps.

Chandra spectroscopy of the hot star β Crucis and the discovery of a pre-main-sequence companion

In order to test the O star wind-shock scenario for X-ray production in less luminous stars with weaker winds, we made a pointed 74-ks observation of the nearby early B giant, β Crucis (β Cru; B0.5 III), with the Chandra High Energy Transmission Grating Spectrometer. We find that the X-ray spectrum is quite soft, with a dominant thermal component near 3 million K, and that the emission lines are resolved but quite narrow, with half widths of 150 km s -1 . The forbidden-to-intercombination line ratios of Ne ix and Mg xi indicate that the hot plasma is distributed in the wind, rather than confined near the photosphere. It is difficult to understand the X-ray data in the context of the standard wind-shock paradigm for OB stars, primarily because of the narrow lines, but also because of the high X-ray production efficiency. A scenario in which the bulk of the outer wind is shock heated is broadly consistent with the data, but not very well motivated theoretically. It is possible that magnetic channelling could explain the X-ray properties, although no field has been detected on β Cru. We detected periodic variability in the hard (hv > 1 keV) X-rays, modulated on the known optical period of 4.58 h, which is the period of the primary β Cephei pulsation mode for this star. We also have detected, for the first time, an apparent companion to β Cru at a projected separation of 4 arcsec. This companion was likely never seen in optical images because of the presumed very high contrast between it and β Cru in the optical. However, the brightness contrast in the X-ray is only 3:1, which is consistent with the companion being an X-ray active low-mass pre-main-sequence star. The companions X-ray spectrum is relatively hard and variable, as would be expected from a post-T Tauri star. The age of the β Cru system (between 8 and 10 Myr) is consistent with this interpretation which, if correct, would add β Cru to the roster of Lindroos binaries - B stars with low-mass pre-main-sequence companions.

Radio, X‐Ray, and Extreme‐Ultraviolet Coronal Variability of the Short‐Period RS Canum Venaticorum Binary σ2 Coronae Borealis

We present the results of a coordinated observing campaign on the short-period (1.14 days) RS CVn binary ?2 Coronae Borealis with the VLA, ASCA, and RXTE. We also discuss earlier observations of the same system obtained by the Extreme Ultraviolet Explorer (EUVE). Dramatic coronal variability is present in all of these observations across the electromagnetic spectrum. ?2 CrB undergoes frequent large flares that occur close together in time. Radio flares are unambiguously correlated with X-ray flares; the X-ray flares peak as much as 1.4 hr before the corresponding radio maxima. Response to flares is more rapid in higher energy X-ray bandpasses, signaling an increase in temperature during the course of the flare. Flares are seen more frequently in the harder RXTE bandpass than in simultaneously taken ASCA observations. There is greater contrast between flaring and nonflaring conditions in the RXTE light curve. Complex flaring is seen in the radio at 3 and 6 cm, consistent with optically thin nonthermal gyrosynchrotron emission for most of the duration of the observation. Bursts of left-circularly polarized emission at 20 cm lasting ?15 minutes appear to be due to a coherent emission process. EUVE spectra reveal coronal material at ne ? 1012 cm-3, with no discernible density differences between flaring and quiescent time intervals. Quiescent ASCA spectra show lower than solar photospheric abundances with iron depleted by a factor of 4 from the solar photospheric value. The abundances increase during a large flare observed with ASCA, with iron enhanced to the solar photospheric value during the rise phase of the flare. Two-temperature fits to extracted spectra show a low-temperature quiescent value of 8 MK and a high-temperature component that varies from 22 MK during quiescence to 50 MK at the peak of the flare. Emission measure distributions measured from the ASCA observations are consistent with the temperatures derived from the discrete two-temperature fits and indicate the presence of very hot (>100 MK) plasma during the rise phase of the ASCA flare. Many of the flares observed with EUVE, ASCA, and RXTE show a double exponential decay phase, further highlighting the importance of this phenomenon in diagnosing flaring conditions. We also find that the observed second decay timescale cannot be explained by some of the currently used flare parameterizations.