Thierry Montmerle

10 MK Gas in M17 and the Rosette Nebula: X-Ray Flows in Galactic H II Regions

We present the first high spatial resolution X-ray images of two high-mass star forming regions, the Omega Nebula (M17) and the Rosette Nebula (NGC 2237-2246), obtained with the Chandra X-Ray Observatory Advanced CCD Imaging Spectrometer instrument. The massive clusters powering these H II regions are resolved at the arcsecond level into more than 900 (M17) and 300 (Rosette) stellar sources similar to those seen in closer young stellar clusters. However, we also detect soft diffuse X-ray emission on parsec scales that is spatially and spectrally distinct from the point-source population. The diffuse emission has luminosity LX 3.4 × 1033 ergs s-1 in M17 with plasma energy components at kT 0.13 and 0.6 keV (1.5 and 7 MK), while in Rosette it has LX 6 × 1032 ergs s-1 with plasma energy components at kT 0.06 and 0.8 keV (0.7 and 9 MK). This extended emission most likely arises from the fast O star winds thermalized either by wind-wind collisions or by a termination shock against the surrounding media. We establish that only a small portion of the wind energy and mass appears in the observed diffuse X-ray plasma; in these blister H II regions, we suspect that most of it flows without cooling into the low-density interstellar medium. These data provide compelling observational evidence that strong wind shocks are present in H II regions.

On the Periodic X-Ray Emission from θ1 Orionis C

The O7 V star u 1 Orionis C was very recently found to present a strong periodic variation of the X-ray emission (Gagneet al. 1997) that is in agreement with the period determined from the variability of spectral lines (Stahl et al. 1996). The authors suggested that the variations are caused by an oblique magnetic rotator with a large-scale magnetic field modulating the wind. We present here the hypothesis of a dipolar magnetic field embedded in the radiation-driven wind of u 1 Orionis C and analyze the X-ray data in the framework of the magnetically confined wind shock (MCWS) model (Babel & Montmerle 1997). We find that both the ROSAT HRI count rate and the periodic variability of u 1 Orionis C are quantitatively reproduced by the model, provided that the field has an intensity at the stellar surface of B * 3 300 G. The variability is caused in large part by the circumstellar cooling disk predicted by the MCWS model. These results give strong support to a magnetic origin for the variability of u 1 Orionis C and render this star the best candidate for a high-mass analog to magnetic Bp stars. Subject headings: stars: early-type — stars: individual (u 1 Orionis C) — stars: magnetic fields —

ROSAT X-ray study of the Chamaeleon I dark cloud. I: The stellar population

Two soft X-ray images of the Chamaeleon I star-forming cloud obtained with the ROSAT Position Sensitive Proportional Counter are presented. Seventy reliable, and 19 possible additional, X-ray sources are found. Eighty percent of these sources are certainly or probably identified with T Tauri stars formed in the cloud. Nineteen to 39 are proposed new weak T Tauri (WTT) stars which, when confirmed by optical spectroscopy, will significantly enlarge the known cloud population. Individual T Tauri X-ray luminosities range from ∼6×10 28 to ∼2×10 31 ergs s −1 (0.4-2.5 keV), or ∼10 2 -10 4 times solar levels. The ROSAT images are a order of magnitude more sensitive, with 3-4 times more stellar identifications, than earlier Einstein Observatory images of the cloud

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.

Evolution of very small particles in the southern part of Orion B observed by ISOCAM

We present ISOCAM observations (5 18 m) of the southern part of Orion B, including the reflection nebula NGC 2023 and the Horsehead nebula illuminated by the B star HD 37903 and the O star Orionis, respectively. Due to the limited radiation eld, the emission in these wavelengths is due to very small particles which are heated each time they absorb a UV photon. A lamentary structure is detected at small angular scales (down to the angular resolution of 6 00 ) on top of a smoother background. The particular case of the Horsehead nebula suggests that the laments in general result from the illuminated surfaces of dense structures, while the smoother background comes from lower density matters probably ionised. Striking spatial variations of the infrared colour (5 8.5 m/12{18 m) are also detected. Spectroscopic observations show that they are due to variations of the intensity of the aromatic features (especially at 7:7 m) relative to a continuum emission increasing in intensity towards longer wavelengths. The contribution of the continuum relative to the aromatic features appears signicantly higher at the illuminated surfaces of dense structures than in lower density matter. This eect could be the signature of the evolution of the very small particles from shielded molecular material to photo-dissociated and photo-ionised matter. We also show that size segregation due to grain dynamics in uni-directional radiation elds may play a major role.

Young radio-emitting magnetic B star in the Rho Ophiuchi cloud

A VLA study of continuum radio sources associated with the Rho Oph molecular cloud has resulted in the detection of circular polarization from the luminous infrared source S1. The presence of a weak compact H II region indicates that the source is an early-type very young star. The results are consistent with a model of optically thin gyrosychrotron radiation from energetic electrons in the outer parts of a magnetosphere similar to that of chemically peculiar magnetic stars. 75 references.

ISOCAM and molecular observations of the edge of the Horsehead nebula

We present ISOCAM observations (5-18 µm) of the Horsehead nebula, together with observations of the (J = 1−0) and (J = 2−1) transitions of 12 CO, 13 CO and C 18 O taken at the IRAM 30-m telescope. The Horsehead nebula presents a typical photodissociation region illuminated by the O9.5 V system σ Ori. The ISOCAM emission is due to very small particles transiently heated to high temperature each time they absorb a UV photon. A very sharp filament (width: ∼10 �� or ∼0.02 pc) is detected by ISOCAM at the illuminated edge of the nebula. This filament is due to a combined effect of steep increase of the column density and extinction of incident radiation, on typical sizes below ∼0.01 pc. Both the three-dimensional shape and the local density of the illuminated interface are strongly constrained. The dense material forming the edge of the Horsehead nebula appears illuminated edge-on by σ Ori, and the particles located beyond the border should not be affected by the incident radiation field. This structure may be due to dense filaments in the parental cloud which have shielded the material located in their shadow from the photo-dissociating radiations. The measurement of the penetration depth of the incident radiation from the infrared data (∼0.01 pc) gives a density of a few 10 4 cm −3 just behind the bright filament. This value is comparable to the estimate of the density beyond the edge and deduced from our molecular observations, and also to the density behind the ionization front calculated in the stationary case. The material behind the illuminated edge could also be non-homogeneous, with clump sizes significantly smaller than the observed penetration depth of ∼0.01 pc. In that case no upper limit on the average density just behind the illuminated edge can be given.

X-ray emitting T Tauri stars in the L1551 cloud

Low mass pre-main sequence stars in the nearby Lynds 1551 star forming cloud are studied with the ROSAT and ASCA X-ray satellites. An 8 ksec ROSAT image reveals 38 sources including 7 well-known T Tauri stars, 2 likely new weak-lined T Tauri stars, 5 potential new weak-lined T Tauri stars, one is a young B9 star, and the remaining sources are unrelated to the cloud or poorly identified. A 40 ksec ASCA image of the cloud detects seven of the ROSAT sources. Spectral fitting of the brighter X-ray emitting stars suggests the emission is produced in either a multi-temperature plasma, with temperatures near 0.2 and 1 keV, or a single-temperature plasma with low metal abundances. XZ Tau, a young classical T Tauri star, is much stronger in ASCA than ROSAT observations showing a harder (1.5-2.0 kev) component. Timing analysis reveals all but one of the T Tauri stars are variable on timescales ranging from one hour to a year. A powerful flare, emitting 3 x 10(exp 34) ergs within a 40 minute rise and fall, was observed by ASCA on the weak-lined T Tauri star V826 Tau. The event was preceded and followed by constant quiescent X-ray emission. The extreme classical T Tauri star XZ Tau was also caught during both high and low states, varying by a factor of 15 between the ASCA and ROSAT observations. Neither of the luminous infrared embedded protostars L1551-IRS 5 or L1551NE were detected by ROSAT or ASCA.

Spatial Study with the Very Large Telescope of a New Resolved Edge-on Circumstellar Dust Disk Discovered at the Periphery of the ρ Ophiuchi Dark Cloud*

We report the discovery in near-infrared (NIR) with SofI at the New Technology Telescope (NTT) of a resolved circumstellar dust disk around a 2MASS source at the periphery of the ? Ophiuchi dark cloud. We present follow-up observations in J, H, and Ks bands, obtained with ISAAC at the Very Large Telescope (VLT) under 04 seeing conditions, which unveil a dark dust lane oriented east-west between two characteristic northern and southern reflection nebulae. This new circumstellar dust disk has a radius of 215 (300 AU at 140 pc) and a width of 12 (170 AU at 140 pc). Thanks to its location at the periphery of the dense cores, it suffers small foreground visual extinction (AV = 2.1 ? 2.6 mag). Although this disk is seen close to edge-on, the two reflection nebulae display very different colors. We introduce a new NIR data visualization called pixel NIR color mapping (PICMap for short), which allows us to visualize directly the NIR colors of the nebula pixels. Thanks to this method, we identify a ridge, 03 (40 AU at 140 pc) to the north of the dark lane and parallel to it, that displays an NIR color excess. This ridge corresponds to an unusual increase of brightness from J to Ks, which is also visible in the NTT observations obtained 130 days before the VLT ones. We also find that the northern nebula shows ~3 mag more extinction than the southern nebula. We compute axisymmetric disk models to reproduce the VLT scattered-light images and the spectral energy distribution from optical to NIR. Our best model, with a disk inclination i = 86? ? 1?, correctly reproduces the extension of the southern reflection nebula, but it is not able to reproduce either the observed NIR color excess in the northern nebula or the extinction difference between the two reflection nebulae. We discuss the possible origin of the peculiar, asymmetrical NIR color properties of this object.

A VLA survey of radio-emitting young stars in the Rho Ophiuchi dark cloud

An extensive radio-continuum survey of the Rho Ophiuchi molecular cloud was conducted with the Very Large Array to look for counterparts of several dozen pre-main-sequence stars found in X-ray, optical, and infrared surveys. The authors detect 93 radio sources, significantly more than in previous observations. Thirteen radio sources are found to be probably stellar. They include five variable X-ray stars, seven near- or far-infrared sources, five stars visible on the Palomar Sky Survey, and ten sources with flat or ascending radio spectra. No optical T Tauri star is detected. The remaining sources are extragalactic, in agreement with radio-source counts. The spatial distribution of the stellar radio sources is weakly clustered to the northeast of the core region of the cloud, like the moderately embedded infrared sources. 65 references.