Yohko Tsuboi

ASCA Detection of a Superhot 100 Million K X‐Ray Flare on the Weak‐lined T Tauri Star V773 Tauri

We present results of a ≈ 40 ks ASCA observation of the active weak-lined T Tauri star V773 Tau (HD 283447) and the surrounding Barnard 209 dark cloud, obtained in 1995 February. During this observation, V773 Tau exhibited a dramatic X-ray flare, with the X-ray count rate increasing rapidly by a factor of ~20, then decreasing exponentially with an e-folding timescale of ≈ 2.3 hr. The peak flare luminosity was at least ~1033 ergs s-1 (0.7-10 keV; distance = 150 pc), which is among the highest X-ray luminosities observed to date for T Tauri stars. The total energy release was ~1037 ergs. However, the most spectacular aspect of this flare was its temperature, which reached a maximum value of at least 100 million K. Spectral fits near flare maximum give a temperature of ~10 keV, which slowly declined to a value ~6 keV at the end of the observation. These temperature measurements are based on high signal-to-noise ratio spectra, and provide the first unambiguous evidence for superhot flaring plasma at temperatures of ~108 K in T Tauri stars. A simple cooling-loop model gives electron densities that are similar to those of solar flares, but requires loop sizes that are comparable to or larger than the star itself. The flare showed other interesting behavior, including a high (and possibly variable) absorption column density, NH = 4 × 1022 cm-2, and an apparent increase in the global metal abundance during the flare.

The Young Stellar Population in M17 Revealed by Chandra

We report here results from a Chandra ACIS observation of the stellar populations in and around the M17 H II region. The field reveals 886 sources with observed X-ray luminosities (uncorrected for absorption) between similar to 29.3 ergs s(-1) < log L(X) < 32.8 ergs s(-1), 771 of which have stellar counterparts in infrared images. In addition to comprehensive tables of X-ray source properties, several results are presented:

Discovery of X-rays from the protostellar outflow object HH2

Herbig–Haro (HH) objects have been known for 50 years to be luminous condensations of gas in star-forming regions, but their underlying physical nature is still being elucidated. Previously suggested models encompass newborn stars, stellar winds clashing with nebular material, dense pockets of interstellar gas excited by shocks from outflows, and interstellar ‘bullets’ (ref. 6). Recent progress has been made with the jet-induced shock model, in which material streams out of young stellar objects and collides with the surrounding interstellar medium. A clear prediction of this model is that the most energetic Herbig–Haro objects will emit X-rays, although they have not hitherto been detected. Here we report the discovery of X-ray emission from one of the brightest and closest Herbig–Haro objects, HH2, at a level consistent with the model predictions. We conclude that this Herbig–Haro object contains shock-heated material located at or near its leading edge with a temperature of about 106 K.

Chandra X-Ray Observatory Study of the Orion Nebula Cluster and BN/KL Region

About 1000 X-ray emitting young pre–main-sequence (PMS) stars distributed in mass from ~0.05 M☉ brown dwarfs to a ~50 M☉ O star are detected in an image of the Orion Nebula obtained with the Advanced CCD Imaging Spectrometer on board the Chandra X-Ray Observatory. This is the richest field of sources ever obtained in X-ray astronomy. Individual X-ray luminosities in the Orion Nebula cluster range from the sensitivity limit of 2 × 10^(28) ergs s^(-1) to ~10^(32) ergs s^(-1). ACIS sources include 85%–90% of V < 20 stars, plus a lower but substantial fraction of deeply embedded stars with extinctions as high as A_V ≃ 60. The relationships between X-ray and other PMS stellar properties suggest that X-ray luminosity of lower-mass PMS stars depends more on mass, and possibly stellar rotation, than on bolometric luminosity, as widely reported. In a subsample of 17 unabsorbed stars with mass ≃ 1 M☉, X-ray luminosities are constant at a high level around L_x ≃ 2 × 10^(30) ergs s^(-1) for the first ≃ 2 Myr while descending the convective Hayashi track, but diverge during the 2–10 Myr phase with X-ray emission plummeting in some stars but remaining high in others. This behavior is consistent with the distribution of X-ray luminosities on the zero-age main sequence and with current theories of their rotational history and magnetic dynamos. The sources in the Becklin-Neugebauer/Kleinman-Low region of massive star formation are discussed in detail. They include both unabsorbed and embedded low-mass members of the Orion Nebula cluster, the luminous infrared Source n, and a class of sources without optical or infrared counterparts that may be new magnetically active embedded PMS stars. Several X-ray sources are also variable radio emitters, an association often seen in magnetically active PMS stars. Faint X-ray emission is seen close to, but apparently not coincident with, the Becklin-Neugebauer object. Its nature is not clear.

Quasi-periodic X-Ray Flares from the Protostar YLW 15

With ASCA, we have detected three X-ray flares from the class I protostar YLW 15. The flares occurred every ~20 hr and showed an exponential decay with time constant 30-60 ks. The X-ray spectra are explained by a thin thermal plasma emission. The plasma temperature shows a fast rise and slow decay for each flare with kTpeak ~ 4-6 keV. The emission measure of the plasma shows this time profile only for the first flare, and remains almost constant during the second and third flares, at the level of the tail of the first flare. The peak flare luminosities, LX,peak, were ~5-20 × 1031 ergs s-1, which are among the brightest X-ray luminosities observed to date for class I protostars. The total energy released in each flare was 3-6 × 1036 ergs. The first flare is well reproduced by the quasi-static cooling model, which is based on solar flares, and it suggests that the plasma cools mainly radiatively, confined by a semicircular magnetic loop of length ~14 R☉, with diameter-to-length ratio ~0.07. The two subsequent flares were consistent with the reheating of the same magnetic structure as in the first flare. The large-scale magnetic structure and the periodicity of the flares imply that the reheating events of the same magnetic loop originate in an interaction between the star and the disk due to the differential rotation.

Suzaku Observations of the Local and Distant Hot ISM

Suzaku observed the molecular cloud MBM 12 and a blank field less than 3 ◦ away to separate the local and distant components of the diffuse soft X-ray background. Towards MBM 12, a local (D 275pc) O VII emission line was clearly detected with an intensity of 3.5 photons cm −2 s −1 sr −1 (or line units, LU), and the O VIII flux was < 0.34 LU. The origin of this O VII emission could be hot gas in the Local Hot Bubble (LHB), charge exchange within the heliosphere with oxygen ions from the solar wind (SWCX), or both. If entirely from the LHB, the emission could be explained by a region with emission measure of 0.0075cm −6 pc and a temperature of 1.2 ×10 6 K. However, this temperature and emission measure implies 1/4 keV emission in excess of observations. There is no evidence in the X-ray light curve or solar wind data for a significant contribution from geocoronal SWCX, although interplanetary SWCX is still possible. In any case, the observed O VII flux represents an upper limit to both the LHB emission and interplanetary SWCX in this direction. The blank field was observed immediately afterwards. The net off-cloud O VII and O VIII intensities were (respectively) 2.34 ±0.33 and 0.77 ±0.16 LU, after subtracting the on-cloud foreground emission. If this more distant O VII and O VIII emission is from a thermal plasma in collisional equilibrium beyond the Galactic disk, we infer it has a temperature of (2.1 ±0.1) ×10 6 K with an emission measure of (4 ±0.6) ×10 −3 cm −6 pc.

Discovery of X-Rays from Class 0 Protostar Candidates in OMC-3

We have observed the Orion Molecular Clouds 2 and 3 (OMC-2 and OMC-3) with the Chandra X-Ray Observatory (CXO). The northern part of OMC-3 is found to be particularly rich in new X-ray features; four hard X-ray sources are located in and along the filament of cloud cores. Two sources coincide positionally with the submillimeter-millimeter dust condensations of MMS 2 and 3 or an outflow radio source VLA 1, which are in a very early phase of star formation. The X-ray spectra of these sources show an absorption column of (1-3) × 1023 H cm-2. Assuming a moderate temperature plasma, the X-ray luminosity in the 0.5-10 keV band is estimated to be ~1030 ergs s-1 at a distance of 450 pc. From the large absorption, positional coincidence, and moderate luminosity, we infer that the hard X-rays are coming from very young stellar objects embedded in the molecular cloud cores.We found another hard X-ray source near the edge of the dust filament. The extremely high absorption of 3 × 1023 H cm-2 indicates that the source must be surrounded by dense gas, suggesting that it is either a young stellar object in an early accretion phase or a Type II AGN (e.g., a Seyfert 2), although no counterpart is found at any other wavelength. In contrast to the hard X-ray sources, soft X-ray sources are found spread around the dust filaments, most of which are identified with IR sources in the T Tauri phase.

Iron and Nickel Line Diagnostics for the Galactic Center Diffuse Emission

We have observed the diffuse X-ray emission from the Galactic center (GC) using the X-ray Imaging Spectrometer (XIS) on Suzaku. The high-energy resolution and the low-background orbit provide excellent spectra of the GC diffuse X-rays (GCDX). The XIS found many emission lines in the GCDX near the energy of K-shell transitions of iron and nickel. The most pronounced features are Fe I Kat 6.4 keV and K-shell absorption edge at 7.1 keV, which are from neutral and/or low ionization states of iron, and the K-shell lines at 6.7 keV and 6.9 keV from He-like (Fe XXV K�) and hydrogenic (Fe XXVI Ly�) ions of iron. In addition, Klines from neutral or low ionization nickel (Ni I K�) and He-like nickel (Ni XXVII K�), and Fe I K�, Fe XXV K�, Fe XXVI Ly�, Fe XXV K and Fe XXVI Ly are detected for the first time. The line center energies and widths of Fe XXV Kand Fe XXVI Lyfavor a collisional excitation (CE) plasma for the origin of the GCDX. The electron temperature determined from the line flux ratio of Fe XXV-K�/ Fe XXV-Kis similar to the ionization temperature determined from that of Fe XXV-K�/FeXXVI-Ly�. Thus it would appear that the GCDX plasma is close to ionization equilibrium. The 6.7 keV flux and temperature distribution to the galactic longitude is smooth and monotonic, in contrast to the integrated point source flux distribution. These facts support the hypothesis that the GCDX is truly diffuse emission rather than the integration of the outputs of a large number of unresolved point sources. In addition, our results demonstrate that the chemical composition of Fe in the interstellar gas near the GC is constrained to be about 3.5 times solar.

Iron Fluorescent Line Emission from Young Stellar Objects in the Orion Nebula

Wepresenttheresultofasystematic searchfortheironKfluorescentline at � 6.4keVamong1616X-ray sources detected by ultradeep Chandra observations of the Orion Nebula Cluster and the obscured Orion Molecular Cloud 1 population as part of the Chandra Orion Ultradeep Project (COUP). Seven sources are identified to have an excess emission at � 6.4 keV among 127 control sample sources with significant counts in the 6.0-9.0 keV band. These seven sources are young stellar objects (YSOs) characterized by intenseflarelikeflux variations, thermal spectra, and near-infrared (NIR) counterparts. The observed equivalent widths of the line cannot be attributed to the florescence by interstellar or circumstellar matter along the line of sight. The X-ray spectral fits and NIR colors of the 6.4 keV sources show that these sources have X-ray absorption of k1 ; 10 22 cm � 2 and NIR excess emission, which is not expected when thefluorescence occurs at the stellar photosphere. We therefore conclude that the iron fluorescent line of YSOs arises from reflection off of circumstellar disks, which are irradiated by the hard X-ray continuum emission of magnetic reconnection flares. Subject headingg ISM:individual(OMC-1) — openclustersandassociations:individual(OrionNebulaCluster) — scattering — stars: pre-main-sequence — X-rays: stars

X-Ray Properties of Young Stellar Objects in OMC-2 and OMC-3 from the Chandra X-Ray Observatory

We report X-ray results of the Chandra observation of OMC-2 and OMC-3. A deep exposure of ~100 ks detects ~400 X-ray sources in the field of view of the ACIS array, providing one of the largest X-ray catalogs in a star-forming region. Coherent studies of the source detection, time variability, and energy spectra are performed. We classify the X-ray sources into Class I, Class II, and Class III+MS based on the J-, H-, and K-band colors of their near-infrared counterparts and discuss the X-ray properties (temperature, absorption, and time variability) along these evolutionary phases.