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We show that suprathermal particles, termed pick-up ions (PUIs), might reduce the postshock temperature of the fast wind and jets in some planetary nebulae (PNs) and in symbiotic systems. The goal is to explain the finding that the temperature of the "hot bubble" formed by the postshock gas in some PNs and symbiotic nebulae is lower, sometimes by more than an order of magnitude, than the value expected from simple hydrodynamical calculations. Although various explanations have been proposed, there is as yet no preferred solution for this "low temperature problem." PUIs have been invoked to explain the low temperature behind the termination shock of the solar wind. While in the case of the solar wind the neutral atoms that turn into PUIs penetrate the preshock solar wind region from the interstellar medium, in PNs the PUI source is more likely slowly moving clumps embedded in the fast wind or jets. These clumps are formed by instabilities or from backflowing cold gas. Our estimates indicate that in young PNs these PUIs will thermalize before leaving the system. Only in older PNs whose sizes exceed [?]5000 AU and for which the fast wind mass loss rate is $\dot{M}_w \lesssim 10^{-7}\; M_\odot { \;{\rm yr}}^{-1}$ do we expect the PUIs to be an efficient carrier of energy out of the postshock region (the hot bubble).

Close binary systems undergoing mass transfer or common envelope interactions can account for the morphological properties of some planetary nebulae. The search for close binary companions in planetary nebulae is hindered by the difficulty of detecting cool, late-type, main-sequence companions in binary systems with hot pre-white-dwarf primaries. However, models of binary planetary nebula progenitor systems predict that mass accretion or tidal interactions can induce rapid rotation in the companion, leading to X-ray-emitting coronae. To test such models, we have searched for, and detected, X-ray emission from three binary central stars within planetary nebulae: the post-common envelope close binaries in HFG 1 and DS 1 consisting of O-type subdwarfs with late-type, main-sequence companions and the binary system in LoTr 5 consisting of O-type subdwarf and rapidly rotating, late-type giant companion. The X-ray emission in each case is best characterized by spectral models consisting of two optically thin thermal plasma components with characteristic temperatures of [?]10 MK and 15-40 MK and total X-ray luminosities [?]1030 erg s-1. We consider the possible origin of the X-ray emission from these binary systems and conclude that the most likely origin is, in each case, a corona around the late-type companion, as predicted by models of interacting binaries.

We have imaged the disk surrounding the nearby (D [?] 73 pc), [?]12 Myr, classical T Tauri binary system V4046 Sgr with the Submillimeter Array (SMA) at an angular resolution of [?]2''. We detect a rotating disk in 12CO(2-1) and 13CO(2-1) emission and resolve the continuum emission at 1.3 mm. We infer disk gas and dust masses of [?]110 and [?]40 Earth masses, respectively. Fits to a power-law disk model indicate that the molecular disk extends to [?]370 AU and is viewed at an inclination of between [?]33deg and [?]39deg for dynamical stellar masses ranging from 1.8 M down to 1.5 M (the range of the total mass previously determined for the central, 2.4 day spectroscopic binary). This range of disk inclination is consistent with that assumed in deducing the central binary mass (i.e., 35deg), suggesting that the V4046 Sgr binary system and its circumbinary, molecular disk are coplanar. In light of the system's age and binarity, the presence of an extensive molecular disk orbiting V4046 Sgr provides constraints on the timescales of processes related to Jovian planet formation and demonstrates that circumbinary Jovian planets potentially could form around close binary systems.

We report the XMM-Newton/EPIC detection in 2008 March of a luminous (LX [?] 1032-33 erg s-1), variable X-ray source in the vicinity (within [?]6'') of the enigmatic star V838 Mon, which underwent a spectacular outburst in early 2002. Spectral modeling of the XMM-Newton X-ray source indicates the presence of two plasma components with characteristic temperatures of TX [?] 2 x 106 K and [?]1.5 x 107 K, attenuated by an absorbing column (NH [?] 4 x 1021 cm-2) that is consistent with the visual extinction measured toward V838 Mon (AV [?] 2). No such luminous source was detected in the immediate vicinity of V838 Mon in Chandra/ACIS-S observations obtained about one year after outburst or, most recently, in 2010 January. The two XMM -Newton source spectral components appear to be marginally spatially resolved, with the spatial centroid of the hard component lying closer to (within [?]2'' of) the position of V838 Mon than the broadband source or the soft source component; however, if there are two sources at or near V838 Mon, the Chandra nondetections would imply that both of them are variable. An alternative is that there is a single variable source, and that the apparent spatial separation may actually be due to photon-counting statistics or is perhaps instrumental in origin. We consider constraints placed by the X-ray detection and nondetections on a stellar merger model for the 2002 V838 Mon outburst, in which the spun-up merger product drives a powerful magnetic dynamo. Alternatively, the transient behavior of the X-ray source could indicate that the X-rays arose as a consequence of an interaction between V838 Mon's ejecta and its early-type (B3 V) companion.

X-rays from planetary nebulae (PNs) are believed to originate from a shock driven into the fast stellar wind (v [?] 1000 km s-1) as it collides with an earlier circumstellar slow wind (v[?] 10 km s-1). In theory, the shocked fast wind (hot bubble) and the ambient cold nebula can remain separated by magnetic fields along a surface referred to as the contact discontinuity (CD) that inhibits diffusion and heat conduction. The CD region is extremely difficult to probe directly owing to its small size and faint emission. This has largely left the study of CDs, stellar-shocks, and the associated microphysics in the realm of theory. This paper presents spectroscopic evidence of ions from the hot bubble (kT [?] 100 eV) crossing the CD and penetrating the cold nebular gas (kT [?] 1 eV). Specifically, a narrow radiative recombination continuum (RRC) emission feature is identified in the high resolution X-ray spectrum of the PN BD+30deg3639 indicating bare C VII ions recombine with cool electrons at kTe = 1.7 +- 1.3 eV. An upper limit to the flux of the narrow RRC of H-like C VI is obtained as well. The RRCs are interpreted as due to C ions from the hot bubble of BD+30deg3639 crossing the CD into the cold nebula, where they ultimately recombine with its cool electrons. The RRC flux ratio of C VII to C VI constrains the temperature jump across the CD to DkT > 80 eV, providing for the first time direct evidence of the stark temperature disparity between the two sides of an astrophysical CD and constraining the role of magnetic fields and heat conduction accordingly. Two colliding-wind binaries are noted to have similar RRCs suggesting a temperature jump, and CD crossing by ions may be a common feature of stellar-wind shocks.

We report the serendipitous detection by the XMM-Newton X-ray Observatory of an X-ray source at the position of the Type I (He- and N-rich) bipolar planetary nebula (PN) Hb 5. The Hb 5 X-ray source appears marginally resolved. While the small number of total counts ([?]170) and significant off-axis angle of the X-ray source ([?]7.'8) precludes a definitive spatial analysis, the morphology of the X-ray emission appears to trace the brightest features seen in optical images of Hb 5. The X-ray spectrum is indicative of a thermal plasma at a temperature between 2.4 and 3.7 MK and appears to display strong Neon emission. The inferred X-ray luminosity is LX = 1.5 x 1032ergs-1. These results suggest that the detected X-ray emission is dominated by shock-heated gas in the bipolar nebula, although we cannot rule out the presence of a pointlike component at the position of the central star. The implications for and correspondence with current models of shock-heated gas in PNe is discussed.

We present the results of the first X-ray gratings spectroscopy observations of a planetary nebula (PN)--the X-ray-bright, young BD +30deg3639. We observed BD +30deg3639 for a total of [?]300 ks with the Chandra X-ray Observatory's Low Energy Transmission Gratings in combination with its Advanced CCD Imaging Spectrometer (LETG/ACIS-S). The LETG/ACIS-S spectrum of BD +30deg3639 is dominated by H-like resonance lines of O VIII and C VI and the He-like triplet line complexes of Ne IX and O VII. Other H-like resonance lines, such as N VII, and lines of highly-ionized Fe are weak or absent. Continuum emission is evident over the range 6-18 A. Spectral modeling indicates the presence of a range of plasma temperatures from Tx [?] 1.7 x 106 K to 2.9 x 106 K and an intervening absorbing column NH [?] 2.4 x 1021 cm-2. The same modeling conclusively demonstrates that C and Ne are highly enhanced, with abundance ratios of C/O [?] 15-45 and Ne/O [?] 3.3-5.0 (90% confidence ranges, relative to the solar ratios), while N and Fe are depleted, with abundances N/O [?] 0.0-1.0 and Fe/O [?] 0.1-0.4, respectively. The intrinsic luminosity of the X-ray source determined from the modeling and the measured flux (FX = 4.1 x 10-13 ergs cm-2 s-1) is LX [?] 8.6 x 1032 erg s-1 (assuming D = 1.2 kpc). These gratings spectroscopy results are generally consistent with earlier results obtained from X-ray CCD imaging spectroscopy of BD +30deg3639, but are far more precise. Hence, the Chandra/LETG-S results for BD +30deg3639 place severe new constraints on models of PN wind-wind interactions in which X-ray emitting gas within PNs is generated via shocks and the plasma temperature is moderated by effects such as heat conduction or rapid evolution of the fast wind. The tight constraints placed on the (nonsolar) abundances directly implicate the present-day central star--hence, ultimately, the intershell region of the progenitor asymptotic giant branch star--as the origin of the shocked plasma now emitting in X-rays.

We have carried out a multiwavelength observational campaign demonstrating some of the remarkable properties of the infrared-bright variable star BP Psc. Surrounded by a compact dusty, gaseous disk, this little-studied late G (or early K) type star emits about 75% of its detected energy flux at infrared wavelengths. Evidence for accretion of gas in conjunction with narrow bipolar jets and Herbig-Haro objects is apparently consistent with classification of BP Psc as a pre-main-sequence star, as postulated in most previous studies. If young, then BP Psc would be one of the nearest and oldest known classical T Tauri stars. However, such an evolutionary classification encounters various problems that are absent or much less severe if BP Psc is instead a luminosity class III post-main-sequence star. In this case, it would be the first known example of a first-ascent giant surrounded by a massive molecular disk with accompanying rapid gas accretion and prominent jets and HH objects. In this model, the genesis of the massive dusty gaseous disk could be a consequence of the envelopment of a low-mass companion star. Properties in the disk may be conducive to the current formation of planets, a gigayear or more after the formation of BP Psc itself.

We present spatially resolved X-ray observations of the binary T Tauri star system V710 Tau. Using Chandra's Advanced CCD Imaging Spectrometer (ACIS), we imaged this 3.2 '' separation binary system, consisting of a classical T Tauri star, V710 Tau N, and a weak-lined T Tauri star, V710 Tau S. The Chandra ACIS-S3 images--obtained in two 9 ks exposures separated by about 3 months (2004 December and 2005 April)--cleanly resolve the V710 Tau binary, demonstrating that both stars emit X-rays, and thereby enabling the first spectral/temporal study of the individual components of this mixed (classical and weak-lined) T Tauri star binary system. The northern component, V710 Tau N, appears to have been in a flaring state during the first (2004 December) exposure. During this flare event, the X-ray flux of the classical T Tauri star hardened significantly. Single-component plasma models with plasma temperatures in the range kTX [?] 0.7-1.1 keV are adequate to fit the observed X-ray spectra of V710 Tau S in 2004 December and both stars in 2005 April. The 2004 December flare-state observation of V710 Tau N requires a higher temperature plasma component ( kTX [?] 2.5-3.0 keV) in addition to the soft component ( kTX [?] 0.5 keV), and is better fit by a model that includes a slightly enhanced Ne/Fe abundance ratio. These results are generally consistent with statistical contrasts between the X-ray emission properties of classical (rapidly accreting) versus weak-lined (weakly accreting or nonaccreting) T Tauri stars.

We report the discovery of quiescent emission from molecular hydrogen gas located in the circumstellar disks of six pre-main-sequence stars, including two weak-line T Tauri stars (TTSs) and one Herbig AeBe star, in the Chamaeleon I star-forming region. For two of these stars, we also place upper limits on the 2- 1 S(1) / 1- 0 S(1) line ratios of ~0.4 and 0.5. Of the 11 pre-main-sequence sources now known to be sources of quiescent near-infrared hydrogen emission, four possess transitional disks, which suggests that detectable levels of H2 emission and the presence of inner disk holes are correlated. These H2 detections demonstrate that these inner holes are not completely devoid of gas, in agreement with the presence of observable accretion signatures for all four of these stars and the recent detections of [Ne II] emission from three of them. The overlap in [Ne II] and H2 detections hints at a possible correlation between these two features and suggests a shared excitation mechanism of high-energy photons. Our models, combined with the kinematic information from the H2 lines, locate the bulk of the emitting gas at a few tens of AU from the stars. We also find a correlation between H2 detections and those targets which possess the largest Ha equivalent widths, suggesting a link between accretion activity and quiescent H2 emission. We conclude that quiescent H2 emission from relatively hot gas within the disks of TTSs is most likely related to ongoing accretion activity, the production of UV photons and/or X-rays, and the evolutionary status of the dust grain populations in the inner disks.

We report the serendipitous detection of the planetary nebula NGC 5315 by the Chandra X-Ray Observatory. The Chandra imaging spectroscopy results indicate that the X-rays from this PN, which harbors a Wolf-Rayet (W-R) central star, emanate from a TX [?] 2.5 x 106 K plasma generated via the same wind-wind collisions that have cleared a compact ([?]8000 AU radius) central cavity within the nebula. The inferred X-ray luminosity of NGC 5315 is ~ 2.5 x 1032 ergs s[?]1 (0.3-2.0 keV), placing this object among the most luminous such "hot bubble" X-ray sources yet detected within PNe. With the X-ray detection of NGC 5315, objects with W-R-type central stars now constitute a clear majority of known examples of diffuse X-ray sources among PNe; all such "hot bubble" PN X-ray sources display well-defined, quasi-continuous optical rims. We therefore assert that X-ray-luminous hot bubbles are characteristic of young PNe with large central star wind kinetic energies and closed bubble morphologies. However, the evidence at hand also suggests that processes such as wind and bubble temporal evolution, as well as heat conduction and/or mixing of hot bubble and nebular gas, ultimately govern the luminosity and temperature of superheated plasma within PNe.

We present high-resolution X-ray spectra of the nearby multiple T Tauri star (TTS) system Hen 3-600, obtained with the High Energy Transmission Grating Spectrograph (HETGS) aboard the Chandra X-Ray Observatory. Both principal binary components of Hen 3-600 (A and B, separation 1.4'') were detected in the zeroth-order Chandra HETGS X-ray image. Hen 3-600-A, the component with a large mid-infrared excess, is a factor of ~2-3 fainter in X-rays than Hen 3-600-B, due to a large flare at Hen 3-600-B during our observation. The dispersed X-ray spectra of the two primary components overlap significantly, so spectral analysis was performed primarily on the first-order spectrum of the combined A + B system, with analysis of the individual dispersed spectra limited to regions where the contributions of A and B can be disentangled via cross-dispersion profile fitting. This analysis results in two lines of evidence indicating that the X-ray emission from Hen 3-600 A + B is derived, in part, from accretion processes. (1) The line ratios of He-like O VII in the spectrum of Hen 3-600 A + B indicate that the characteristic density of its X-ray-emitting plasma is significantly larger than those of coronally active main-sequence and pre-main-sequence stars. (2) A significant component of low-temperature (2-3 MK) plasma is present in the Hen 3-600 A + B spectrum; this "soft excess" appears somewhat stronger in component A. These results for Hen 3-600 A + B are consistent with, although less pronounced than, results obtained from X-ray grating spectroscopy of more rapidly accreting TTS systems. Indeed, all of the emission signatures of Hen 3-600 A + B that are potential diagnostics of accretion activity--from its high-resolution X-ray spectrum, through its UV excess and Ha emission-line strengths, to its weak near-infrared excess--suggest that its components (and component A in particular) represent a transition phase between rapidly accreting, classical T Tauri stars and nonaccreting, weak-lined T Tauri stars.

Using XMM-Newton we detect faint unresolved X-ray emission from the compact steep-spectrum radio galaxy 3C 303.1. We detect a thermal component at kT [?] 0.8 keV, which seems likely to be produced in the interstellar medium (ISM) of the host galaxy. There is evidence for a second component in the spectrum whose nature is currently ambiguous. Plausible hypotheses for the second component include (1) hot gas shocked by the expansion of the radio source, and (2) synchrotron self-Compton emission from the southern radio lobe if the magnetic field is below the equipartition value by a factor of ~3.5.

We present the results of Chandra X-Ray Observatory observations of the planetary nebulae (PNs) NGC 40 and Hen 2-99. Both PNs feature late-type Wolf-Rayet central stars that are currently driving fast (~1000 km s-1), massive winds into denser, slow-moving (~10 km s-1) material ejected during recently terminated asymptotic giant branch (AGB) evolutionary phases. Hence, these observations provide key tests of models of wind-wind interactions in PNs. In NGC 40, we detect faint, diffuse X-ray emission distributed within a partial annulus that lies nested within a ~40'' diameter ring of nebulosity observed in optical and near-infrared images. Hen 2-99 is not detected. The inferred X-ray temperature (TX ~ 106 K) and luminosity (LX ~ 2 x 1030 ergs s-1) of NGC 40 are the lowest measured thus far for any PN displaying diffuse X-ray emission. These results, combined with the ringlike morphology of the X-ray emission from NGC 40, suggest that its X-ray emission arises from a "hot bubble" that is highly evolved and is generated by a shocked, quasi-spherical fast wind from the central star, as opposed to AGB or post-AGB jet activity. In contrast, the lack of detectable X-ray emission from Hen 2-99 suggests that this PN has yet to enter a phase of strong wind-wind shocks.

We present the results of XMM-Newton X-ray observations of the Mira AB binary system, which consists of a pulsating, asymptotic giant branch primary and nearby (~0.''6 separation) secondary of uncertain nature. The European Photon Imaging Camera (EPIC) CCD (MOS and pn) X-ray spectra of Mira AB are relatively soft, peaking at ~1 keV, with only very weak emission at energies >3 keV; lines of Ne IX, Ne X, and O VIII are apparent. Spectral modeling indicates a characteristic temperature TX ~ 107 K and intrinsic luminosity LX ~ 5 x 1029 ergs s-1 and suggests enhanced abundances of O and, possibly, Ne and Si in the X-ray-emitting plasma. Overall, the X-ray spectrum and luminosity of the Mira AB system more closely resemble those of late-type, pre-main-sequence stars or late-type, magnetically active main-sequence stars than those of accreting white dwarfs. We conclude that Mira B is most likely a late-type, magnetically active, main-sequence dwarf, and X-rays from the Mira AB system arise either from magnetospheric accretion of wind material from Mira A onto Mira B or from coronal activity associated with Mira B itself as a consequence of accretion-driven spin-up. In addition, one (or both) of these mechanisms could be responsible for the recently discovered, pointlike X-ray sources within planetary nebulae.

We further investigate subpixel event repositioning (SER) algorithms in application to Chandra X-Ray Observatory (Chandra) CCD imaging. SER algorithms have been applied to backside-illuminated (BI) Advanced CCD Imaging Spectrometer (ACIS) devices and demonstrate spatial resolution improvements in Chandra ACIS observations. Here a new SER algorithm that is charge split-dependent is added to the SER family. We describe the application of SER algorithms to frontside-illuminated (FI) ACIS devices. The results of SER for FI CCDs are compared with those obtained from SER techniques applied to BI CCD event data. Both simulated data and Chandra ACIS observations of the Orion Nebular Cluster were used to test and evaluate the achievement of the various SER techniques.

To probe the magnetic activity levels of asymptotic giant branch (AGB) stars, we used XMM-Newton to search for X-ray emission from two well-studied objects, TX Cam and T Cas. The former star displays polarized maser emission, indicating magnetic field strengths of B ~ 5 G, and the latter is one of the nearest known AGB stars. Neither star was detected by XMM-Newton. We use the upper limits on EPIC (CCD detector) count rates to constrain the X-ray luminosities of these stars and derive LX < 1031 ergs s-1 (<1030 ergs s-1) for an assumed X-ray emission temperature TX = 3 x 106 K (107 K). These limits represent [?]10% ([?]1%) of the X-ray luminosity expected under models in which AGB magnetic fields are global and potentially play an important role in collimating and/or launching AGB winds. We suggest, instead, that the B field strengths inferred from maser observations are representative of localized magnetic clouds.

We investigate the consequences of magnetic flares on the surface of asymptotic giant branch (AGB) and similar stars. In contrast to the solar wind, in the winds of AGB stars the gas cooling time is much shorter than the outflow time. As a result, we predict that energetic flaring will not inhibit, and may even enhance, dust formation around AGB stars. If magnetic flares do occur around such stars, we expect some AGB stars to exhibit X-ray emission; indeed, certain systems including AGB stars, such as Mira, have been detected in X-rays. However, in these cases, it is difficult to distinguish between potential AGB star X-ray emission and, e.g., X-ray emission from the vicinity of a binary companion. Analysis of an archival ROSAT X-ray spectrum of the Mira system suggests an intrinsic X-ray luminosity ~2 x 1029 ergs s-1 and temperature ~107 K. These modeling results suggest that magnetic activity, either on the AGB star (Mira A) or on its nearby companion (Mira B), is the source of the X-rays, but do not rule out the possibility that the X-rays are generated by an accretion disk around Mira B.

Subpixel event repositioning (SER) techniques have been demonstrated to significantly improve the already unprecedented spatial resolution of Chandra X-ray imaging with the Advanced CCD Imaging Spectrometer (ACIS). Chandra CCD SER techniques are based on the premise that the impact position of events can be refined, based on the distribution of charge among affected CCD pixels. ACIS SER models proposed thus far are restricted to corner split (3 and 4 pixel) events and assume that such events take place at the split-pixel corners. To improve the event-counting statistics, we modified the ACIS SER algorithms to include 2 pixel split events and single-pixel events, using refined estimates for photon impact locations. Furthermore, simulations that make use of a high-fidelity backside-illuminated (BI) CCD model demonstrate that mean photon impact positions for split events are energy dependent, leading to further modification of subpixel event locations according to event type and energy, for BI ACIS devices. Testing on Chandra CCD X-ray observations of the Orion Nebula Cluster indicates that these modified SER algorithms further improve the spatial resolution of Chandra/ACIS, to the extent that the spreading in the spatial distribution of photons is dominated by the high-resolution mirror assembly, rather than by ACIS pixelization.

We revisit Chandra observations of planetary nebulae NGC 7027 and BD +30deg3639 in order to address the question of abundance anomalies in the X-ray-emitting gas. Enhanced abundances relative to solar of magnesium (Mg) for NGC 7027 and neon (Ne) for BD +30deg3639 are required to fit their X-ray spectra, whereas observations at optical and infrared wavelengths show depleted Mg and Ne in these systems. We attribute the enhancement of Mg in NGC 7027 in the X-ray, relative to the optical, to the depletion of Mg onto dust grains within the optical nebula. For BD +30deg3639, we speculate that the highly enhanced Ne comes from a white dwarf companion, which accreted a fraction of the wind blown by the asymptotic giant branch progenitor and went through a novalike outburst that enriched the X-ray-emitting gas with Ne.

We report the detection of quiescent H2 emission in the v = 1 - 0 S(1) line at 2.12183 mm in the circumstellar environment of two classical T Tauri stars, GG Tau A and LkCa 15, in high-resolution (R [?] 60,000) spectra, bringing to four, including TW Hya and the weak-lined T Tauri star DoAr 21, the number of T Tauri stars showing such emission. The equivalent widths of the H2 emission line lie in the range 0.02-0.10 A, and in each case the central velocity of the emission line is centered at the star's systemic velocity. The line widths range from 9 to 14 km s-1, in agreement with those expected from gas in Keplerian orbits in circumstellar disks surrounding K-type stars at distances [?]10 AU from the sources. UV fluorescence and X-ray heating are likely candidate mechanisms responsible for producing the observed emission. We present mass estimates from the measured line fluxes and show that the estimated masses are consistent with those expected from the possible mechanisms responsible for stimulating the observed emission. The high temperatures and low densities required for significant emission in the v = 1 - 0 S(1) line suggests that we have detected reservoirs of hot H2 gas located in the low-density upper atmospheres of circumstellar disks of these stars.

We present archival ROSAT data for three recently identified, nearby (D < 70 pc), young (~10-40 Myr) stellar associations: the TW Hydrae association, the Tucana-Horologium association, and the b Pictoris moving group. The distributions of ROSAT X-ray hardness ratios (HR1, HR2) for these three groups, whose membership is dominated by low-mass, weak-lined T Tauri stars, are tightly clustered and very similar to one another. The value of HR1 for TW Hya itself--the only bona fide classical T Tauri star in any of the nearby groups--is clearly anomalous among these nearby young stars. We compare the hardness ratio distributions of stars in the three nearby groups with those of T Tauri stars, the Hyades, and main-sequence dwarfs in the field. This comparison demonstrates that the X-ray spectra of F through M stars soften with age and that F and G stars evolve more rapidly in X-ray spectral hardness than do K and M stars. It is as yet unclear whether this trend can be attributed to age-dependent changes in the intrinsic X-ray spectra of stars of type F and later, to a decrease in the column density of circumstellar gas (e.g., in residual protoplanetary disks), or to the diminishing contributions of star-disk interactions to X-ray emission. Regardless, these results demonstrate that analysis of archival ROSAT X-ray spectral data can help both to identify nearby, young associations and to ascertain the X-ray emission properties of members of known associations.

We examine mechanisms that may explain the luminosities and relatively low temperatures of extended X-ray emission in planetary nebulae. By building a simple flow structure for the wind from the central star during the proto- and early planetary nebula phase, we estimate the temperature of the X-ray-emitting gas and its total X-ray luminosity. We conclude that in order to account for the X-ray temperature and luminosity, both the evolution of the wind from the central star and the adiabatic cooling of the post-shocked wind's material must be considered. The X-ray-emitting gas results mainly from shocked wind segments that were expelled during the early planetary nebula phase when the wind speed was moderate, ~500 km s- 1. Alternatively, the X-ray-emitting gas may result from a collimated fast wind blown by a companion to the central star. Heat conduction and mixing between hot and cool regions are likely to occur in some cases and may determine the detailed X-ray morphology of a nebula but are not required to explain the basic properties of the X-ray-emitting gas.

Chandra X-Ray Observatory (CXO) images have revealed that the X-ray-emitting regions of the molecule-rich young planetary nebulae (PNe) BD +30deg3639 and NGC 7027 are much more asymmetric than their optical nebulosities. To evaluate the potential origins of these X-ray asymmetries, we analyze X-ray images of BD +30deg3639, NGC 7027, and another PN resolved by CXO, NGC 6543, within specific energy bands. Image resolution has been optimized by subpixel repositioning of individual X-ray events. The resulting subarcsecond resolution images reveal that the soft (E < 0.7 keV) X-ray emission from BD +30deg3639 is more uniform than the harder emission, which is largely confined to the eastern rim of the optical nebula. In contrast, soft X-rays from NGC 7027 are highly localized and this PN is more axially symmetric in harder emission. The broadband X-ray morphologies of BD +30deg3639 and NGC 7027 are highly anticorrelated with their distributions of visual extinction, as determined from high-resolution, space- and ground-based optical and infrared imaging. Hence, it is likely that the observed X-ray asymmetries of these nebulae are due in large part to the effects of nonuniform intranebular extinction. However, the energy-dependent X-ray structures in both nebulae and NGC 6543--which is by far the least dusty and molecule-rich of the three PNe and displays very uniform intranebular extinction--suggest that other mechanisms, such as the action of collimated outflows and heat conduction, are also important in determining the detailed X-ray morphologies of young PNe.

We study the conditions under which a main-sequence binary companion to the central ionizing star of a planetary nebula (PN) might become magnetically active and thereby display strong X-ray luminosity, LX [?] 5 x 1029 ergs s-1. Since most PNs are older than few billion years, any main-sequence companion will rotate too slowly to have magnetic activity and hence bright X-ray emission unless it is spun up. We demonstrate that if the orbital separation during the asymptotic giant branch (AGB) phase of the PN progenitor is a [?] 30-60 AU, main-sequence companions in the spectral type range F7-M4 (mass range 0.3 M [?] M2 [?] 1.3 M) will accrete enough angular momentum from the AGB wind to rotate rapidly, become magnetically active, and exhibit X-ray luminosities LX [?] 5 x 1029 ergs s-1. Lower mass M stars and brown dwarfs can also become magnetically active, but they should have small orbital separations and hence are less likely to survive the AGB phase of the progenitor. For orbital separation of a [?] 0.3 AU, i.e., for a binary systems that went through a common envelope phase, the fast wind from the central white dwarf (WD) star will interact with (and potentially disrupt) the companion's corona on the side facing the central star, while for a [?] 6 R, i.e., an orbital period of Porb [?] 30 hr, the WD's fast wind will compress a dense small region near the surface of the companion. This region may thermally emit X-rays with nonnegligible luminosity. We estimate that 20%-30% of elliptical PNs and 30%-50% of bipolar PNs are likely to have magnetically active companions that will reveal themselves in X-ray observations. Reanalysis of Chandra spectroscopy of the compact central source of NGC 7293 indicates that the emitting region of this object possesses abundance anomalies similar to those of coronally active main-sequence stars. High-resolution X-ray spectroscopy of this and other compact sources in PNs are necessary to confirm a coronal origin for the X-ray emission.

We present high-resolution X-ray spectra of the X-ray-bright classical T Tauri star, TW Hydrae, covering the wavelength range of 1.5-25 A. The differential emission measure derived from fluxes of temperature-sensitive emission lines shows a plasma with a sharply peaked temperature distribution, peaking at log T = 6.5. Abundance anomalies are apparent, with iron very deficient relative to oxygen, while neon is enhanced relative to oxygen. Density-sensitive line ratios of Ne IX and O VII indicate densities near log ne = 13. A flare with rapid (~1 ks) rise time was detected during our 48 ks observation; however, based on analysis of the emission-line spectrum during quiescent and flaring states, the derived plasma parameters do not seem strongly time-dependent. The inferred plasma temperature distribution and densities are consistent with a model in which the bulk of the X-ray emission from TW Hya is generated via mass accretion from its circumstellar disk. Assuming that accretion powers the X-ray emission, our results for log ne suggest an accretion rate of ~10-8 M yr-1.

Level populations and line intensity ratios are calculated in a 35 level model of Fe XV using most recently available atomic data and incorporating the effects of self-absorption through escape probability factors for slab and cylindrical sources. The computed optically thin spectrum is found to agree very well with solar active region observations, confirming the correctness of the atomic data. Under optically thick conditions, a different situation is found compared to the Fe XVII spectrum; all Fe XV subordinate lines and their intensity ratios are unaffected. At the same time the 284.16 A resonance line exhibits the counterintuitive effect of initial intensity increase with column density even more strongly than do Fe XVII resonance lines. A preliminary comparison is made with the available observations, which are found to be consistent with the present predictions.

We used the High-Energy Transmission Grating Spectrometer on board the Chandra X-Ray Observatoryto perform two observations, separated by 3 weeks, of the Orion Trapezium region. The zeroth-order images on the Advanced CCD Imaging Spectrometer provide spatial resolution of 05 and moderate energy resolution. Within a 160'' × 140'' region around the Orion Trapezium, we resolve 111 X-ray sources with luminosities between 7 × 1028and 2 × 1032ergs s-1. We do not detect any diffuse emission. All but six sources are identified. From spectral fits of the three brightest stars in the Trapezium, we determine the line-of-sight column density to be NH= (1.93 ± 0.29) × 1021cm-2. Many sources appear much more heavily absorbed, with NHin the range of 1022-1023cm-2. A large fraction of sources also show excursions in luminosity by more than a factor 5 on timescales greater than 50 ks; many are detected only in one of the observations. The main objective of this paper is to study the Orion Trapezium and its close vicinity. All five Trapezium stars are bright in X-rays, with ?1Ori C accounting for about 60% of the total luminosity of the Trapezium. The CCD spectra of the three very early-type members can be fitted with a two-temperature thermal spectrum with a soft component of kT~ 0.8 keV and a hard component of kT~ 2-3 keV. ?1Ori B is an order of magnitude fainter than ?1Ori E and shows only a hard spectrum of kT~ 3 keV. ?1Ori D is another order of magnitude fainter than ?1Ori B, with only a kT~ 0.7 keV component. We discuss these results in the context of stellar wind models. We detect eight additional, mostly variable X-ray sources in the close vicinity of the Trapezium. They are identified with thermal and nonthermal radio sources, as well as infrared and optical stars. Five of these X-ray sources are identified with proplyds, and we argue that the X-ray emission originates from class I, II, and III protostars at the cores of the proplyds.

We present high spectral resolution maps of near-infrared molecular hydrogen emission from the bipolar pre-planetary nebulae RAFGL 2688 and RAFGL 618, obtained with the NOAO Phoenix spectrometer. The measured velocity gradients along the polar axes of both nebulae indicate that the highest velocity gas lies closest to the central stars. These results support the suggestion that the polar H2 emission regions of both nebulae contain shocked gas formed as fast (~50-150 km s-1), collimated, post-asymptotic giant branch (AGB) winds collide with slower moving (~10-20 km s-1) material previously ejected while the central stars were still on the AGB. The kinematics of H2 emission perpendicular to the polar axis of RAFGL 2688 are consistent with a model combining expansion along the equator at 5-10 km s-1 with rotation about the polar axis at 5-10 km s-1. The rapid onset of the common envelope phase of a close binary system may explain both the bipolar structure of RAFGL 2688 and the presence and complex kinematics of its shocked H2 emission.

We have detected emission from the quiescent, molecular hydrogen gas in the disk around the X-ray bright, classical T Tauri star TW Hya in the 1-0 S(1) line at 2.1218 mm through high-resolution spectroscopy using CSHELL on NASA's Infrared Telescope Facility. The brightness of the observed emission line is consistent with that predicted from models for X-ray excitation of the 1-0 S(1) line of H2. This result demonstrates that X-ray ionization is a plausible mechanism for excitation of the H2 in the gaseous disks of T Tauri stars. In addition, from these high-resolution spectra we have measured the radial velocities of three T Tauri stars in the TW Hya association, CD -33deg7795, HD 98800, and TW Hya. We find that the radial velocities of these three stars are very similar to each other and to those of at least three other stars presumed to be members of the TW Hya association, Hen 600A, Hen 600B, and CD -29deg8887. This result, combined with the similar proper motions of all six of these stars, lends support to the hypothesis that all of these stars share a common origin in a now-dispersed molecular cloud.

We have used infrared polarimetric imaging with NICMOS to determine precisely the position of the star that illuminates (and presumably generated) the bipolar, preplanetary reflection nebula RAFGL 2688 (the Egg Nebula). The polarimetric data pinpoint the illuminating star, which is not detected directly at wavelengths [?]2 mm, at a position well within the dark lane that bisects the nebula, 0.''55 (~550 AU) southwest of the infrared peak that was previously detected at the southern tip of the northern polar lobe. The inferred position of the central star corresponds to the geometric center of the tips of the four principle lobes of near-infrared H2 emission; identifying the central star at this position also reveals the strong point symmetric structure of the nebula, as seen both in the intensity and polarization structure of the polar lobes. The polarimetric and imaging data indicate that the infrared peak directly detected in the NICMOS images is a self-luminous source and, therefore, is most likely a distant binary companion to the illuminating star. Although present theory predicts that bipolar structure in preplanetary and planetary nebulae is a consequence of binary star evolution, the separation between the components of the RAFGL 2688 binary system, as deduced from these observations, is much too large for the presence of the infrared companion to have influenced the structure of the RAFGL 2688 nebula.

We have obtained 1.64, 1.90, and 2.15 mm narrowband images of five T Tauri stars in the TW Hya Association (TWA) using the Near-Infrared Camera and Multiobject Spectrometer aboard the Hubble Space Telescope. Most of the T Tauri stars in our study show evidence of absorption by H2O vapor in their atmospheres; in addition, the low-mass brown dwarf candidate, TWA 5B, is brighter at 1.9 mm than predicted by cool star models that include the effects of H2O vapor but neglect dust. We conclude that the effect of atmospheric dust on the opacity is important at 1.9 mm for TWA 5B, the coolest object in our sample. The available evidence suggests that the TWA is 5-15 Myr old. Comparison of the colors of TWA 5B with theoretical magnitudes as a function of age and mass then confirms previous claims that TWA 5B is substellar with a mass in the range 0.02-0.03 M. The accurate single-epoch astrometry of the relative positions and separation of TWA 5A and TWA 5B reported here should permit the direct measurement of the orbital motion of TWA 5B within only a few years.

We have obtained images at 2.2, 450, and 850 mm of the pre-main-sequence binary T Tauri and of Hind's Nebula, which is located less than 1' west of T Tauri. The three maps reveal that the nebulosity around T Tauri is extended, with an FWHM size of several hundred AU in thermal emission and more than 1000 AU in reflected light. Dust clearly connects the circumstellar nebula around T Tauri to the southwestern tip of Hind's Nebula, where we have identified a secondary intensity peak in the submillimeter maps. Calculations of the long-wavelength emissivity demonstrate that the dust emissivity index b is ~2 for the large nebula but is ~1.2 for the secondary intensity peak. These results for b suggest that we have found a class I protostar 30'' southwest of T Tauri and that star formation is more active in the vicinity of T Tauri than previously had been thought. This protostar is detected directly at submillimeter wavelengths; however, polarization mapping at 2.2 mm demonstrates that the disk-shaped dust cloud around the protostar is illuminated externally at short wavelengths, making it similar to the Orion proplyds.

We present ASCA and ROSAT X-ray observations of the classical T Tauri star TW Hya, the namesake of a small association that, at a distance of ~50 pc, represents the nearest known region of recent star formation. Analysis of ASCA and ROSAT spectra indicates characteristic temperatures of ~1.7 and ~9.7 MK for the X-ray-emitting region(s) of TW Hya, with emission lines of highly ionized Fe dominating the spectrum at energies of ~1 keV. The X-ray data show variations in X-ray flux on timescales of [?]1 hr as well as indications of changes in the X-ray-absorbing column on timescales of several years, suggesting that flares and variable obscuration are responsible for the large-amplitude optical variability of TW Hya on short and long timescales, respectively. Comparison with model calculations suggests that TW Hya produces sufficient hard X-ray flux to produce significant ionization of molecular gas within its circumstellar disk; such X-ray ionization may regulate both protoplanetary accretion and protoplanetary chemistry.

This paper presents results obtained from high-resolution spectroscopy of the 2.1218 mm emission line of molecular hydrogen toward five bipolar pre-planetary nebulae. Our detections of IRAS 17441-2411 and AFGL 6815S bring to four, including AFGL 618 and AFGL 2688, the number of bipolar preplanetaries that have been detected as sources of H2 emission at 2.1218 mm. The spatially resolved H2 line profile of AFGL 6815S suggests that the bulk of the detected emission arises in an expanding molecular torus surrounding the central star. All of the H2-emitting pre-planetary nebulae are found at low galactic latitudes, consistent with previous results for planetary nebulae, and have central stars with intermediate or early spectral types. We were unable to detect H2 emission from the bipolar, post-main-sequence sources OH 231.8+4.2, IRAS 07131-0147, and IRAS 09371+1212, all of which have M-type central stars. These results suggest that the event that triggers the production of emission from shocked H2 occurs at an intermediate evolutionary epoch in the postasymptotic giant branch evolution of transition objects, after the generation of bipolar structure and before the nebular envelopes are ionized. From constraints imposed by the galactic distribution of H2-emitting planetaries, we estimate distances to the four pre-planetary nebulae that display H2 emission.

We describe a new method to identify young, late-type stars within [?]150 pc of the Earth that employs visual or near-infrared (NIR) data and the GALEX GR4/5 database. For spectral types later than K5, we demonstrate that the ratio of GALEX near-ultraviolet to visual and NIR emission is larger for stars with ages between 10 and 100 Myr than for older, main-sequence stars. A search in regions of the sky encompassing the TW Hya and Scorpius-Centaurus Associations has returned 54 high-quality candidates for follow up. Spectroscopic observations of 24 of these M1-M5 objects reveal Li 6708 A absorption in at least 17 systems. Because GALEX surveys have covered a significant fraction of the sky, this methodology should prove valuable for future young star studies.