Symposium Proceedings

Stars and WD

Cataloging the Young Stars Around the Sigma Orionis Region

Nancy Adams, Scott Wolk (SAO), Fredrick Walter (SUNY at Stony Brook), William Sherry (NSO)

The Sigma Orionis region is a relatively nearby young star forming region. We have X-ray observations from Chandra and XMM, allowing us to study the X-ray luminosity of a well understood sample of pre-main sequence stars and detecting X-ray emission from known young brown dwarfs(BDs). The Sigma Ori cluster is uniquely suited to this study; it spans different stellar interior conditions, almost 3 orders of magnitude in mass, it is relatively close, almost dust free (5% of the ONC), the proper angle to have a reasonable mix of stars with and without disks. Here we present the combined observations from the Chandra X-ray Observatory, XMM, and ground based photometry and spectra to study the probability of this area being a star formation cluster.

Strip-mining the Coronal Graveyard

Thomas Ayres, Alexander Brown, Graham Harper (CASA)

In a series of Chandra programs, we have conducted High Resolution Camera (HRC-I) pointings on optically bright, but coronally dead, late-type giants and supergiants. So far, we have observed the red giants $\alpha$ Bootis (Arcturus: K1 III) and $\alpha$ Tauri (Aldebaran: K5 III), collecting 4 counts at the two detect circles combined (constituting a positive detection of the former, but not the latter); and (with more significant detections) the yellow supergiants $\alpha$ Aquarii (G2 Ib) and $\beta$ Aquarii (G0 Ib), members of the so-called hybrid chromosphere class. Previous ROSAT observations had been inconclusive: in the cases of $\alpha$ Boo and $\alpha$ Tau owing to lack of sensitivity; in the case of $\alpha$ Aqr due to a 38$^{\prime}$ mispointing; and for $\beta$ Aqr, because of a small positional discrepancy of the apparent source. The Chandra HRC-I, with its superior spatial resolution and sensitivity (and freedom from the CCD red leak), recorded positive detections of Arcturus (albeit only 3 counts) and $\alpha$ Aqr; and recovered faint emission at the location of $\beta$ Aqr, now well separated from the stronger source to the SE that dominated the earlier ROSAT image. The coronal $L_{\rm X}/L_{\rm C~IV}$ luminosity ratios (or upper limit in the case of Aldebaran) of all four stars are extremely depressed relative to solar-type dwarfs, continuing the puzzling X-ray deficiency syndrome originally identified in late-F/early-G luminosity class III coronal giants of the Hertzsprung gap. One additional target--$\gamma$ Draconis (K5 III)--remains to be observed in the Cycle 6 part of the program; and new pointings on $\alpha$ Trianguli Australis (K2 I) and $\beta$ Indi (K1 II) are planned for Cycle 7.

[PDF of the poster]

Close to the Dredge

Bill Ball, Jeremy Drake (SAO)

We present Chandra LETG measurements of the C/N abundance ratio in the corona of the giant in the active RS CVn binary $\lambda$ And (G8 III-IV +). We found a ratio by number $N(C)/N(N) \sim$ 3.1, which is similar to its inferred original surface value and indicates that the products of main-sequence nuclear burning have not been significantly dredged-up. Comparison with evolutionary tracks indicate instead that the star, whose mass lies in the range $\sim 1.7$-$2.0 M_{\odot}$, is in a post dredge-up stage of evolution, near the base red giant branch. $\lambda$ And therefore poses a problem for standard stellar evolution predictions. We discuss this result in the context of C/N abundance ratios of other active binaries and possible implications for dredge-up theory, and speculate on mechanisms by which the $\lambda$ And giant might have avoided mixing its envelope with deeper processed layers as it evolved toward the cooler giant phase.

[PDF of the poster]

Flare Analysis for Multiple Stellar Cluster Data from ANCHORS Database

Natalia Bizunok (Harvard-Smithsonian Center for Astrophysics), Megan Bruck (Williams College), Scott Wolk, Brad Spitzbart, Nancy Evans (Harvard-Smithsonian Center for Astrophysics)

We conduct a study of flares for multiple young stellar clusters using ANCHORS (An Archive of Chandra Observations of Regions of Star Formation) data and flare criteria derived previously from COUP (Chandra Orion Ultradeep Project). Stellar flares are strong disturbances in magnetic field structure in stellar atmospheres that result in violent outbursts of plasma and radiation. Understanding flares and identifying stars and young stellar objects that flare aids in understanding stellar magnetic field structures as well as evolution of protoplanetary disks and stellar environments. As a part of the ANCHORS database creation, we subject our sources' light curves to Bayesian Blocks analysis, which allows us to search for flaring behavior. While COUP presents a large catalog of stars with extensive variability sampling from the Orion Cloud region, ANCHORS data offers a complimentary sample from a variety of environments. We present the results of our study in a form of select light curves, detailed description of flaring detection algorithm, and a summary of flares found and some of their physical properties. The ANCHORS project is supported by Chandra archival grant AR5-6002A and NASA contract NAS8-39073.

[PDF of the poster]

Simultaneous Chandra HETG and Radio Observations of the Decay of a Large Flare on the RS CVn Binary Sigma Gem

Alexander Brown, Thomas R. Ayres, Edmund Hodges-Kluck, Fonda Day (CASA, Univ. of Colorado), James E. Neff (College of Charleston), Rachel A. Osten (NRAO - Charlottesville)

During the period 2005 May 16-18 we obtained Chandra HETG spectra of the coronal emission from the 19.6 day orbital period RS CVn binary $\sigma$ Gem. Simultaneous with these X-ray observations we made a comprehensive set of radio observations using the VLA, VLBA, and MERLIN arrays and the Arecibo 305m telescope. The Chandra spectra were obtained in two 65 ksec segments and show the steady decay of a large X-ray flare. The most prominent spectral evolution is the decrease of a strong high temperature continuum, while the bulk of the dominant 10$^{7}$ K plasma component maintains a nearly constant emission measure. We present a detailed description of the evolution of the coronal plasma during the two days of our Chandra observation. The radio observations show strong radio continuum emission at 3 and 20 cm varying independently from the X-ray emission with a strong radio flare, whose rapid onset occurs at 2005 May 17 16 hrs UT. Almost continuous radio monitoring was obtained using the VLA and MERLIN arrays. High time resolution 20 cm Arecibo observations were made during two 2.5 hour intervals, while the VLBA provided high spatial resolution imaging on both days of the Chandra observations.

X-ray Flares in Orion Low Mass Stars

Marilena Caramazza (Unversita` di Palermo), Ettore Flaccomio, Giusi Micela (INAF - Oss. Astronomico di Palermo), Fabio Reale (Unversita` di Palermo), Scott Wolk (SAO), Eric Feigelson (Penn State Univ.)

We present a study of X-ray variability of 250 COUP (Chandra Orion Ultradeep project) X-sources, associated with low mass stars (0.1-0.3 Msun). Using a method similar to that used by Wolk et al. (2005), we detect in these stars 270 flares and derive their frequency-intensity relation. Comparing this analysis with the results for solar type stars (0.9-1.2 Msun), we establish that, at $\sim$1Myr, low mass and solar type stars have very similar flares frequencies. The intensity distribution of flares for low mass stars is a power-law with index $\sim$2.2. We propose the following model/scenario: the light curves are entirely built by overlapping flares with a power-law intensity distribution, the intense ones are detected, the weak ones merge and form a pseudo-quiescent level, which we indicate as the characteristic level. Through several simulation set, we constrain the model parameters for every single light curve. The study of simulated light curves with the same analysis method used for real sources shows a good agreement between data and model.

Eta Carinae: X-ray Line Variations during the 2003 X-ray Minimum

Michael Corcoran, Kenji Hamaguchi (USRA & XAL-NASA/GSFC), Kazunori Ishibashi (MIT), Ted Gull (NASA/GSFC), Augusto Daminel (IAGUSP), Julian Pittard (Leeds), Kris Davidson (UMN), Rob Petre (XAL-NASA/GSFC), Yousaf Butt (Harvard-Smithsonian Center for Astrophysics)

The nearby, superluminous star $\eta$ Carinae has been a mystery since its giant eruption in the 1840s. Based on periodic changes in the emission line spectrum, $\eta$ Car is now widely (but not universally) believed to be two stars bound in a long period, eccentric orbit. The presence of a companion star provides an in-situ probe of $\eta$ Car and may affect the atmosphere and perhaps even the evolution of the massive star. In mid-2003 $\eta$ Car underwent a minimum in its emission line spectrum, and in its X-ray emission. This event was observed by an unprecedentedly wide range of ground-based and space-based observatories, including 5 100-ksec HETGS observations, which arguably provide the most sensitive probe of this system. We describe the results of this campaign, focussing on the X-ray emission line changes, and show how these observations impact our understanding of the star and its evolutionary state.

[PDF of the poster]

A Comparison of Fe XVIII and Fe XIX, EUV to X-Ray Line Ratios of Active Stars using Chandra Grating Observations

Priya Desai, Nancy Brickhouse, Andrea Dupree, Ronnie Hoogerwerf (Harvard-Smithsonian Center for Astrophysics)

Recent Fe XVIII and Fe XIX line observations of Capella (Desai et al. 2005) have shown significant discrepancies between modern theory and observations for the X-ray and EUV resonance lines. We investigate these lines further by analyzing a sampleof six active stars (Algol, AB Dor, Eps Eri, Capella, HR 1099, and Lambda And) also observed with the Low Energy Transmission Grating Spectrometer. The sample covers a range of coronal temperatures and will help shed light on these puzzling disagreements.

X-ray Spectroscopic Signatures of the Extended Corona of FK Comae

Jeremy Drake, Sun Mi Chung, Vinay Kashyap (SAO), Heidi Korhonen (Astrophysikalisches Institut Potsdam), Adriaan Van Ballegooijen (SAO), Detlef Elstner (Astrophysikalisches Institut Potsdam)

High resolution Chandra X-ray spectra and surface Doppler images obtained from optical spectra of the rapidly rotating giant FK Com have been analysed in order to investigate links between coronal and surface magnetic structures. Net redshifts were detected at the $3\sigma$ level in the light of Ne X $\lambda 12.13$ amounting to $\sim
140$ km s$^{-1}$. smaller shifts of $\sim 60$ km s$^{-1}$ at the $\sim
2\sigma$ level are seen in the X-ray spectrum as a whole, while the observed position of O VIII $\lambda 18.97$--the second strongest line in the spectrum after Ne X--is consistent with its rest wavelength. There is no statistical difference between redshifts during the first and second halves of the observation. Spectral line widths are most consistent with thermal broadening combined with rotational broadening at a scale height of $\sim 1R_\star$, though are also statistically consistent with surface rotational broadening. We interpret the results as indicative of pervasive emission at temperatures $<\sim 3\times
10^6$ K, but with plasma at temperatures $>\sim 3\times 10^6$ K residing predominantly in extended structures centred at phase $\phi=0.75$ with a size similar to that of the star itself. The contemporaneous Doppler images of the surface of FK Com reveal active longitudes at phases $\phi
\sim 0.6$ and 0.9. We speculate that extended coronal structures correspond to magnetic fields joining the two active longitudes which theoretical models predict are of opposite magnetic polarity. Such structures are supported by coronal potential field extrapolations of typical theoretical model surface magnetic field distributions.

X-ray Evidence for a Pole-Dominated Corona on AB Dor

Jeremy Drake, Sun Mi Chung, Vinay Kashyap, David Garcia-Alvarez (SAO)

A fine analysis of spectral line widths and Doppler shifts employing Fourier transform and cross-correlation techniques has been applied to the Chandra HETG spectra of the rapidly rotating young star AB Doradus in order to investigate its coronal topology. We find no significant Doppler shifts that could be attributed to rotation of dominant coronal structures. Individual spectral line widths are statistically consistent with thermal broadening and formally require no rotational broadening, while the $1\sigma$ limit to rotational broadening corresponds to a compact corona restricted to latitudes $>30\deg$. Fourier analysis suggests a small amount of additional rotational broadening is present consistent with a corona restricted to the poles, and excludes models with surface rotational broadening or greater. These results present direct spectroscopic evidence that the dominant coronal activity on rapidly-rotating active stars is associated with the dark polar spots commonly seen in photospheric Doppler images, and support models in which these spots are of mixed magnetic polarity.

Chandra Observations of Open Cluster h Per

Nancy Remage Evans, Scott J. Wolk, Natalya Bizunok, Brad Spitzbart, Fred Seward, Scott Kenyon (SAO), Tom Barnes (Univ. of Texas), Jay Pasachoff (Williams)

We are analysing the 200 sources found in a 40 ksec observation of the open cluster h Per. The data are being processed with the ANCHORS pipeline which provides fluxes and low resolution X-ray spectra. For the stronger sources temperatures are derived from spectral fitting; for weaker sources interpretation is through quantiles. The luminosity distribution is discussed, including the effects of shallow sampling on the source population. The distribution of these properties on optical and infrared color magnitude diagrams is investigated for the cool pre-main sequence stars in this 10 Myr cluster.

[PDF of the poster]

Chemical Composition and Geometry Diagnostics in High Resolution X-ray Spectra of T Tauri Stars

David Garcia-Alvarez, Jeremy Drake (Harvard-Smithsonian Center for Astrophysics), Paola Testa (MIT), Vinay Kashyap, Liwei Lin, Bill Ball (Harvard-Smithsonian Center for Astrophysics)

Chandra archival spectra of the weak-lined T Tauri Stars HDE 283572 and DoAr 21, and the classical T Tauri SU Aur have been analysed in order to investigate diagnostics of chemical composition and hot plasma geometry during flaring and quiescent states. Temperature-insensitive line ratios have been used to estimate the coronal abundance ratios Ne/O, Mg/Si, Si/S and Ca/Ar. Limits have been placed on plasma densities using transitions of He-like Mg. We also place limits on the strength of cold Fe fluorescence induced by X-ray irradiation of circumstellar disks. We discuss the results in the context of chemical fractionation, coronal structure and disk geometry in pre-main sequence stars.

[PDF of the poster]

High Resolution Spectroscopy of the Super-saturated Contact Binary, VW Cep

David Huenemoerder, Paola Testa (MIT), Derek Buzasi (USAFA)

Short-period binaries represent extreme cases in the generation of stellar coronae via a rotational dynamo. Such stars are important for probing the origin and nature of coronae in the regimes of rapid rotation and activity saturation. VW Cep (P=0.28d) is relatively bright, partially eclipsing, and very active object. Light curves made from Chandra HETGS data show flaring and rotational modulation, but no obvious eclipses. Velocity modulation of emission lines indicates that the larger, more massive component dominates the X-ray emission. The emission measure is highly structured, having three peaks. Helium-like triplet lines give electron densities of about 3.0e+10 to 3.0e+11 /cm$^3$. The modulation, emission measure, and densities together suggest that the emitting structures are compact.

[PDF of the poster]

New Spectral Models for Sirius B and HZ 43 and their Implication for the LETGS of Chandra

Jelle Kaastra (SRON), Thierry Lanz (Univ. of Maryland), Ivan Hubeny (Univ. of Arizona)

We have calculated new spectral models for the white dwarfs Sirius B and HZ 43. Both stars have been used widely as effective area calibration sources for soft X-ray missions, including the LETGS of Chandra. The new models take Compton scattering into account, a process that is unimportant for the global UV/optical spectrum but which diminishes the flux at the hardest X-rays significantly. Using these models together with the observed LETGS spectra of both stars allows us to refine the basic parameters of these stars as well as to re-calibrate the effective area of the LETGS. In particular at the longer wavelengths we find significant (>20 percent) differences with the previously used effective area. In Sirius B, we find no evidence for significant amounts of helium in the atmosphere.

High-resolution Imaging of the Mira AB Accreting System

Margarita Karovska, Eric Schlegel, John Raymond (Harvard-Smithsonian Center for Astrophysics) Warren Hack (STScI) Brian Wood (JILA)

Chandra's spatial and spectral resolution offers unprecedented opportunities for detailed studies of many astrophysical sources. Our recent observations of Mira AB demonstrate the power of Chandra's sub-arcsecond angular resolution by separating the components of this 0.6" interacting binary for the first time at X-ray wavelengths. Mira AB is the nearest symbiotic system composed of an evolved mass lossing AGB star (Mira A) and a wind accreting white dwarf.

In December 2003, Chandra ACIS observations detected a powerful soft X-ray outburst originating from the cool AGB star. This outburst is possibly associated with a jet-like activity, as evidenced by the extended structures in the X-ray and UV images of Mira A. Chandra also detected a bridge between the components showing that Mira B is accreting not only from the wind of the AGB star, but also via direct mass exchange. This was unexpected, because the components are separated by  70 AU and it has been assumed that the interaction between the components is only via wind accretion. We discuss the implications that these recent results have on our understanding of accretion processes in detached binaries and other wind accreting systems.

X-ray Monitoring of the Rapid Pre-MS Accretion Episode of V1647 Ori

Joel Kastner (Rochester Inst. Technology), Nicolas Grosso (Lab Astrophys. Grenoble), Michael Richmond (Rochester Inst. Technology), David Weintraub (Vanderbilt Univ.), Ted Simon (Inst. for Astronomy, HI), Kenji Kamaguchi (NASA/GSFC), Hideki Ozawa (Lab Astrophys. Grenoble), Adam Frank (Univ. of Rochester)

although it is widely accepted that low-mass, pre-main sequence (pre-MS) stars emit X-rays as a consequence of solar-like coronal activity (e.g., Preibisch et al. 2005, astro-ph/0506526), evidence is accumulating that X-ray emission also may be a direct result of mass accretion onto pre-MS stars (e.g., Kastner et al. 2002, ApJ, 567, 434; Schmitt et al. 2005, 432, L35). Perhaps the most striking example of such accretion-driven pre-MS X-ray emission is that of V1647 Ori. Chandra and XMM observations obtained before and shortly after this young stellar object underwent a spectacular, accretion-generated optical/IR outburst revealed a large, contemporaneous increase in X-ray flux (Kastner et al. 2004, Nature, 430, 429; Grosso et al. 2005, A&A, 438, 159). I describe the latest results from ongoing Chandra and XMM monitoring of V1647 Ori. These observations offer a unique opportunity to establish the timescale, variability, and spectral characteristics of accretion-enhanced X-ray emission from erupting YSOs, thereby probing star-disk interactions on size and temporal scales that are inaccessible to observations at longer wavelengths.

The Darkest Bright Star: Chandra X-ray Observations of Vega

Deron Pease, Jeremy Drake, Vinay Kashyap (SAO)

We present X-ray observations of Vega obtained with the Chandra High Resolution Camera and Advanced CCD Imaging Spectrometer. After a total of 29 ks of observation with Chandra, X-rays from Vega remain undetected. We derive upper limits to the X-ray luminosity of Vega as a function of temperature over the range of $10^5 - 10^7$ K and find a 99.7% upper limit as low as sim 2 times $10^{25}$ ergs$^{-1}$ at logT=6.2K. We also compare these new deeper observations with the limit derived from a re-analysis of ROSAT PSPC data. Our X-ray luminosity limit for Vega is still greater than predictions of post-Herbig Ae phase X-rays from the shear dynamo model proposed by Tout & Pringle for a Vega age of 350My. If the age of Vega is closer to 100Myr, as suggested by some indicators, our X-ray limit is then similar to Tout-Pringle model predictions. Current X-ray observations of Vega are therefore unable to discriminate between different scenarios explaining the X-ray activity of the convectively stable Herbig Ae/Be stars. Further progress is more likely achieved through X-ray observations of younger main sequence early-type A stars whose conjectured residual post-Herbig Ae phase X-ray activity would be significantly higher.

DOP, JJD, and VLK were supported by NASA contract NAS8-39073 to the Chandra X-ray Center during the course of this research.

[PDF of the poster]

A New Paradigm for the X-ray Emission from the Winds of Hot Stars

Andy Pollock (European Space Agency XMM-SOC)

XMM-Newton observations of the O supergiant $\zeta$ Orionis have suggested a new framework for the interpretation of the X-ray spectra of hot stars. They probably originate far out in the wind in collisionless shocks behind which the exchange of energy between ions and electrons is so slow that electron heating does not take place. The observed plasma is not in equilibrium and an electron bremsstrahlung continuum is essentially absent. The excitation and ionization is more likely to be produced by protons whose thermalized post-shock velocities ensure appropriately high cross-sections. In general, the form of X-ray spectra in both single and binary stars is likely principally to be determined by the amount of post-shock electron heating: magnetically confined X-ray plasma in binary systems can evolve to high electron temperatures while in single stars this does not take place. The accumulated Chandra high-resolution data of hot stars are assessed in this new context.

[PDF of the poster]

Invisible Giant: Chandra's Limits on X-rays from Betelgeuse

Jennifer Posson-Brown, Vinay Kashyap, Deron Pease, Jeremy Drake (SAO)

We have analyzed calibration observations of Betelgeuse ($\alpha$ Ori, M1 Iab, $m_{V} = 0.58$, 130 pc) performed by Chandra instruments. Betelgeuse is undetected in 8 ks of HRC-I, 8 ks of HRC-S, and 5 ks of ACIS-I data. We derive upper limits to its X-ray count rates for these observations by computing the counts necessary to obtain a 3$\sigma$ detection in the presence of the observed background. We compute corresponding X-ray luminosity upper limits assuming isothermal coronal plasma over a range of temperatures, T = 0.3 - 10 MK. We place a limit as low as $L_{x} \sim 4\cdot10^{27}$ ergs $s^{-1} (L_{x}/L_{bol}
\sim 4\cdot10^{-11})$ on the coronal luminosity of Betelgeuse, a limit roughly an order of magnitude lower than that obtained from ROSAT All-Sky Survey data.

This work was supported by NASA contract NAS8-39073 to the CXC.

Chandra/HETGS Detection of Fluorescent X-ray Emission in the Flaring Single Giant HR 9024

Paola Testa, David Huenemoerder (MIT), David Garcia-Alvarez (SAO), Fabio Reale (Universita' di Palermo)

We present evidence of Fe 6.4 keV fluorescent emission in the Chandra/HETGS spectrum of the single G-type giant HR 9024 during a large flare. The unprecedented spectral resolution of Chandra/HETGS allows us to detect the weak fluorescent emission, and to distinguish it from the strong thermal Fe emission line complex near 6.7 keV. Constraints on geometry derived from the analysis of the fluorescent emission will be examined. The high flux provides us with line diagnostics at both high temporal and spectral resolution. Single line spectral diagnostics and lightcurves determine plasma temperature, emission measure, abundances, and density evolution during the flare. From this information, we can construct time-dependent hydrodynamic loop models to constrain loop morphology and heating.

Chandra Observations of the Massive Stellar Cluster Trumpler 14 in Carina

Leisa Townsley, Patrick Broos, Eric Feigelson (Penn State Univ.)

We present the first high-spatial-resolution X-ray images and spectra of the massive cluster Trumpler 14, obtained in a single 57-ksec observation with Chandra/ACIS in March 2002. Tr 14 is an extremely rich, young ($\sim$1 My), compact OB cluster near the center of the Carina complex, containing at least 30 O and early B stars. This rich high-mass star-forming region reveals a complex mix of point source and diffuse X-ray emission. The OB association is resolved at the arcsecond level into more than 900 sources. HD 93129AB, a very early-type (O2I-O3.5V) binary, is resolved and the components are shown to possess very different X-ray spectra. Soft diffuse X-ray emission pervades the H II region and is resolved from the point source population. The extended emission that pervades Trumpler 14 is most likely from the fast O-star winds that thermalize and shock the surrounding media. We also see softer, bright diffuse emission in the off-axis CCDs of the ACIS array, far from any of the Carina massive clusters. A spectral fit to this diffuse emission requires high abundances of O, Ne, Si, and Fe; this is evidence that the emission may be from an old cavity supernova remnant that exploded inside the Carina superbubble.

[PDF of the poster]

A Chandra HETGS Survey of O-Stars

Wayne Waldron (L-3 Communications GSI), Joy Nichols (SAO), Joseph Cassinelli (Univ. of Wisconsin), Nathan Miller (Univ. of Wisc. - Eau Claire), Nancy Evans (SAO), Nolan Walborn (STScI), Alex Fullerton (JHU), Pete Mendygral (SAO)

The high energy resolution capabilities of the Chandra HETGS are particularly useful for studies of X-ray emission from O-stars. The X-rays are believed to arise from within and/or below a moderately ionized, high density, rapidly expanding plasma flow. Model calculations predicted that the X-ray line profiles would be very broad, asymmetric, and highly blue-shifted. although the line broadness is observed in several O-stars (typical HWHM $\sim$ 1000 km/s), the line profiles are found to be symmetric and to have minimal blue-shifts in essentially every O-star that has been observed, with one exception, the early O-star, Zeta Puppis. To understand these discrepancies between model predictions and observations, we were granted a Chandra Very Large Project to obtain HETGS data for several other early O-stars. Our O-star survey targets along with the HETGS archival data will provide complete coverage of the O-star domain in the H-R diagram. The collection will allow us to search for morphological trends in the emission lines as a function of stellar and wind parameters. These trends will guide the development of improved models to explain the behavior of the X-ray emission lines of O-type stars. In this poster, we present our preliminary results for the HETGS observations of three early O-stars (9 Sgr, HD93250, and HD93129AB), and we discuss the trends that are emerging from the data. We also address the question, is Zeta Puppis still the only O-star to display highly asymmetric and blue-shifted X-ray line profiles?

Chandra Observation of Massive Star-Forming Complex NGC6357: the HII Region G353.2+0.9 and the Massive Open Cluster Pismis 24

Junfeng Wang, Leisa Townsley, Eric Feigelson, Gordon Garmire (Penn State Univ.)

We present the first high-spatial-resolution X-ray image of the massive star forming complex NGC 6357 at a distance of 2.5 kpc, obtained in a 40-ksec observation using the Advanced CCD Imaging Spectrometer (ACIS) on board Chandra. The OB association illuminating this HII region, Pismis 24, is one of the richest star clusters in the galaxy with eight O stars including two of the O3 stars. The cluster is resolved at the arcsecond level, $\sim$800 point sources are detected, and more than half of them have optical/IR counterparts. X-ray emission was detected from $\sim$20 O-type and early B-type stars, including two O3 stars and a Wolf-Rayet/O6 binary HD 157504, and from hundreds of pre-main-sequence stars. Given the exposure time and typical spectra (e.g., $\theta^{1}$ Ori C), Chandra can detect 10$^{33}$ ergs/s O stars through heavy absorption (Av 50 mag). The nondetection of embedded O stars supports models where the HII region is ionized by the unobscured O stars in Pismis 24. One of the X-ray sources arises from a very young low-mass star, an evaporating gaseous globule (EGG) lying at the tip of an evaporating elephant trunk molecular structure.

Support for this work was provided to Gordon Garmire, the ACIS Principal Investigator, by the National Aeronautics and Space Administration (NASA) through NASA Contract NAS8-38252 and Chandra Contract SV4-74018 issued by the Chandra X-ray Observatory Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of NASA under contract NAS8-03060.

[PDF of the poster]

Chandra Observations of the RCW 108 Star Forming Region

Scott Wolk, Bradley Spitzbart, Tyler Bourke (SAO)

We present our recent Chandra observations of the embedded star forming region RCW 108. RCW 108 contains a deeply embedded young cluster lying in a dark cloud to the west of the young open cluster NGC 6193 (excited by two early O stars). Our motivation for this obsrvation was to followup on our discovery of diffuse X-ray emission in the older less embedded RCW 38. If the diffuse emission seen in RCW 38 is the result of plasma being trapped within the star forming region by overlaying molecular material, then RCW 108 should contain even more trapped plasma.

At 8-20 $\mu$m microns, the Midcourse Space Experiment Galactic Plane Survey data shows a ridge of warm dust passing through the eastern edge of the emission peak parallel to but west of an optical ridge. Our SEST mm continuum observations show a cold component to this dust ridge peaking strongly at the cluster. The far infrared luminosity suggests that there is more than one significant heating source, i.e., OB stars and/or intermediate mass protostars. The extended infrared nebulosity to the east of the main cluster is due to emission and not reflection, suggesting a break-out of radiation in this direction.

We detected 339 sources divided into 3 groups: A) $\sim$ 3 Myr Stars associated with the older star forming region, B) Stars still forming and C) background sources. Over 80 sources exceed 100 counts. 53 Sources are variable. ABout 175 have IR counterparts bright than K of 15.

The goals of the observations are to: 1) Study triggered star formation. 2) Investigate the origin and nature of diffuse X-ray emission within regions of massive star formation. 3) Identify deeply embedded PMS stars via their X-ray emission and derive an IMF and XLF. 4) Investigate the X-ray properties of the embedded sources.

Star Formation in the Era of the Three Great Observatories: A White Paper

Scott J. Wolk (SAO), SOC and Attendees of the Workshop on Star Formation

At the behest of the Chandra Director's Office, a workshop was held 13-15 July 2005 entitled Star Formation in the Era of Three Great Observatories ( The goal of the workshop, which was co-sponsored by the Spitzer Science Center, was to develop a white paper which could serve as a roadmap for the field. We sought to review topics in star-formation which are inherently multiwavelength, and define both the current state of our knowledge and the points of current controversy where new observations are most needed. We focused on topics for which the Great Observatories (HST, Chandra and Spitzer) have the most to contribute during this unique period of simultaneous operation. We also considered observations from other facilities including radio and ground based optical in addition to theoretical work. We covered star formation in both galactic and local-group star forming regions. One of the goals we define for star formation is to understand how stars and their associated accretion disks are assembled from molecular material. We identified the following key areas of physics which highlight the complimentary aspects of the great observatories: 1)Stellar populations. 2)The formation and evolution of disk systems. 3)Rotation and dynamos.

Chandra Spectroscopy of the 70 Oph and 36 Oph Binaries

Brian Wood, Jeffrey Linsky (JILA, Univ. of Colorado)

Last year Chandra resolved the nearby 70 Oph (K0 V+K5 V) and 36 Oph (K1 V+K1 V) binary systems for the first time in X-rays. The LETG/HRC-S spectra of all four of these stars are presented and compared with a higher S/N archival LETG spectrum of another K dwarf, Epsilon Eri. One particularly surprising finding concerns the relative coronal abundances of the two 70 Oph stars. The coronae and winds of the Sun and many solar-like stars have been found to have a curious abundance pattern where the abundances of elements with low first ionization potential (FIP) are enhanced relative to high FIP elements. Surprisingly, 70 Oph A shows a significantly stronger FIP bias than does 70 Oph B, despite these stars being nearly identical in almost all respects. They have the same age, the same rotation period (20 days), nearly identical coronal X-ray fluxes, and very similar spectral types. Any theoretical explanation for the FIP effect will have to explain how two stars so similar in all other respects can have coronae with different degrees of FIP bias.

[PDF of the poster]