Th. Preibisch

X-RAY EMISSION FROM EARLY-TYPE STARS IN THE ORION NEBULA CLUSTER

The X-ray properties of twenty ~1 Myr old O, B, and A stars of the Orion Trapezium are examined with data from the Chandra Orion Ultradeep Project (COUP). On the basis of simple theories for X-ray emission, we define two classes separated at spectral type B4: hotter stars have strong winds that may give rise to X-ray emission in small- or large-scale wind shocks, and cooler stars that should be X-ray dark due to their weaker winds and absence of outer convection zones where dynamos can generate magnetic fields. Emission by late-type magnetically active companions may be present in either class. Sixteen of the 20 stars are detected with a wide range of X-ray luminosities, log LX ~ 29-33, and X-ray efficiencies, log(LX/Lbol) ~ -4 to -8. Only two stars, ?1 Ori D (B0.5) and NU Ori (B1), show exclusively the constant soft-spectrum emission at log(LX/Lbol) ~ -7 expected from the standard model involving many small shocks in an unmagnetized radiatively accelerated wind. Most of the other massive O7-B3 stars exhibit some combination of soft-spectrum wind emission, hard-spectrum flaring, and/or rotational modulation indicating large-scale inhomogeneity. Magnetic confinement of winds with large-scale shocks can be invoked to explain these phenomena. This is supported in some cases by nonthermal radio emission and/or chemical peculiarities, or direct detection of the magnetic field (?1 Ori C). Most of the stars in the weak-wind class exhibit X-ray flares and log LX < 31 ergs s-1, consistent with magnetic activity from known or unseen low-mass companions. In most cases, the X-ray spectra can be interpreted in terms of a two-temperature plasma model with a soft component of 3-10 MK and a hard component up to 40 MK. All nondetections belong to the weak-wind class. A group of stars exhibit hybrid properties?flarelike behavior superimposed on a constant component with log LX ~ 32 ergs s-1?which suggest both magnetic activity and wind emission.

Visual/infrared interferometry of orion trapezium stars : preliminary dynamical orbit and aperture synthesis imaging of the θ1 orionis C system

Context. Located in the Orion Trapezium cluster, θ^1 Ori C is one of the youngest and nearest high-mass stars (O5-O7) known. Besides its unique properties as a magnetic rotator, the system is also known to be a close binary. Aims. By tracing its orbital motion, we aim to determine the orbit and dynamical mass of the system, yielding a characterization of the individual components and, ultimately, also new constraints for stellar evolution models in the high-mass regime. Furthermore, a dynamical parallax can be derived from the orbit, providing an independent estimate for the distance of the Trapezium cluster. Methods. Using new multi-epoch visual and near-infrared bispectrum speckle interferometric observations obtained at the BTA 6 m telescope, and IOTA near-infrared long-baseline interferometry, we traced the orbital motion of the θ^1 Ori C components over the interval 1997.8 to 2005.9, covering a significant arc of the orbit. Besides fitting the relative position and the flux ratio, we applied aperture synthesis techniques to our IOTA data to reconstruct a model-independent image of the θ^1 Ori C binary system. Results. The orbital solutions suggest a highly eccentricity (e ≈ 0.91) and short-period (P ≈ 10.9 yrs) orbit. As the current astrometric data only allows rather weak constraints on the total dynamical mass, we present the two best-fit orbits. Of these two, the one implying a system mass of 48 M_☉ and a distance of 434 pc to the Trapezium cluster can be favored. When also taking the measured flux ratio and the derived location in the HR-diagram into account, we find good agreement for all observables, assuming a spectral type of O5.5 for θ^1 Ori C1 (M = 34.0 M_☉, T_(eff) = 39 900 K) and O9.5 for C2 (M = 15.5 M_☉, T_(eff) = 31 900 K). Using IOTA, we also obtained first interferometric observations on θ^1 Ori D, finding some evidence for a resolved structure, maybe by a faint, close companion. Conclusions. We find indications that the companion C2 is massive itself, which makes it likely that its contribution to the intense UV radiation field of the Trapezium cluster is non-negligible. Furthermore, the high eccentricity of the preliminary orbit solution predicts a very small physical separation during periastron passage (~1.5 AU, next passage around 2007.5), suggesting strong wind-wind interaction between the two O stars.

Tracing the young massive high-eccentricity binary system

Context. The nearby high-mass star binary system θ 1 Ori C is the brightest and most massive of the Trapezium OB stars at the core of the Orion Nebula Cluster, and it represents a perfect laboratory to determine the fundamental parameters of young hot stars and to constrain the distance of the Orion Trapezium Cluster. Aims. By tracing the orbital motion of the θ 1 Ori C components, we aim to refine the dynamical orbit of this important binary system. Methods. Between January 2007 and March 2008, we observed θ 1 Ori C with VLTI/AMBER near-infrared (H -a ndK-band) longbaseline interferometry, as well as with bispectrum speckle interferometry with the ESO 3.6 m and the BTA 6 m telescopes (B � and V � -band). Combining AMBER data taken with three different 3-telescope array configurations, we reconstructed the first VLTI/AMBER closure-phase aperture synthesis image, showing the θ 1 Ori C system with a resolution of ∼2 mas. To extract the astrometric data from our spectrally dispersed AMBER data, we employed a new algorithm, which fits the wavelength-differential visibility and closure phase modulations along the H -a ndK-band and is insensitive to calibration errors induced, for instance, by changing atmospheric conditions. Results. Our new astrometric measurements show that the companion has nearly completed one orbital revolution since its discovery in 1997. The derived orbital elements imply a short-period (P ≈ 11.3 yr) and high-eccentricity orbit (e ≈ 0.6) with periastron passage around 2002.6. The new orbit is consistent with recently published radial velocity measurements, from which we can also derive the first direct constraints on the mass ratio of the binary components. We employ various methods to derive the system mass (Msystem = 44 ± 7 M� ) and the dynamical distance (d = 410 ± 20 pc), which is in remarkably good agreement with recently published trigonometric parallax measurements obtained with radio interferometry.

\theta^{\mathsf 1}

We present the results of N-band spectro-interferometric observations of the silicate carbon star IRAS08002-3803 with the MID-infrared Interferometric instrument (MIDI) at the Very Large Telescope Interferometer (VLTI) of the European Southern Observatory (ESO). The observations were carried out using two unit telescopes (UT2 and UT3) with projected baseline lengths ranging from 39 to 47 m. Our observations of IRAS08002-3803 have spatially resolved the dusty environment of a silicate carbon star for the first time and revealed an unexpected wavelength dependence of the angular size in the N band: the uniform-disk diameter is found to be constant and ∼36 mas (72 R� ) between 8 and 10 µm, while it steeply increases longward of 10 µm to reach ∼53 mas (106 R� )a t 13µm. Model calculations with our Monte Carlo radiative transfer code show that neither spherical shell models nor axisymmetric disk models consisting of silicate grains alone can simultaneously explain the observed wavelength dependence of the visibility and the spectral energy distribution (SED). We propose that the circumstellar environment of IRAS08002-3803 may consist of two grain species coexisting in the disk: silicate and a second grain species, for which we consider amorphous carbon, large silicate grains, and metallic iron grains. Comparison of the observed visibilities and SED with our models shows that such disk models can fairly – though not entirely satisfactorily – reproduce the observed SED and N-band visibilities. Our MIDI observations and the radiative transfer calculations lend support to the picture where oxygen-rich material around IRAS08002-3803 is stored in a circumbinary disk surrounding the carbon-rich primary star and its putative low-luminosity companion.

Orionis C through periastron passage

We present the results of the first mid-infrared interferometric observations of the Mira variable RR Sco with the MID-infrared Interferometer (MIDI) coupled to the European Southern Observatorys (ESO) Very Large Telescope Interferometer (VLTI), together with K-band observations using VLTI VINCI. The observations were carried out in June 2003, when the variability phase of the object was 0.6, using two unit telescopes (UT1 and UT3), as part of the Science Demonstration Time (SDT) program of the instrument. Projected baseline lengths ranged from 73 to 102 m, and a spectral resolution of 30 was employed in the observations, which enabled us to obtain the wavelength dependence of the visibility in the region be- tween 8 and 13 µm. The uniform-disk diameter was found to be 18 mas between 8 and 10 µm, while it gradually increases at wavelengths longer than 10 µm to reach 24 mas at 13 µm. The uniform-disk diameter between 8 and 13 µm is significantly larger than the K-band uniform-disk diameter of 10.2 ± 0.5 mas measured using VLTI VINCI with projected baseline lengths of 15-16 m, three weeks after the MIDI observations. Our model calculations show that optically thick emission from a warm molecular envelope consisting of H2O and SiO can cause the apparent mid-infrared diameter to be much larger than the con- tinuum diameter. We find that the warm molecular envelope model extending to ∼2.3 Rwith a temperature of ∼1400 K and column densities of H2 Oa nd SiO of 3× 10 21 cm −2 and 1 × 10 20 cm −2 , respectively, can reproduce the observed uniform-disk diameters between 8 and 10 µm. The observed increase of the uniform-disk diameter longward of 10 µm can be explained by an optically thin dust shell consisting of silicate and corundum grains. The inner radius of the optically thin dust shell is derived to be 7-8 Rwith a temperature of ∼700 K, and the optical depth at 10 µm is found to be ∼0.025.

High angular resolution n-band observation of the silicate carbon star IRAS08002-3803 with the VLTI/MIDI instrument : Dusty environment spatially resolved

Context. NGC 7538 IRS1 is a high-mass (30 M⊙) protostar with a CO outflow, an associated ultracompact H II region, and a linear methanol maser structure, which might trace a Keplerian-rotating circumstellar disk. The directions of the various associated axes are misaligned with each other. Aims. We investigate the near-infrared morphology of the source to clarify the relations among the various axes. Methods. K’ -band bispectrum speckle interferometry was performed at two 6-meter-class telescopes—the BTA 6m telescope and the 6.5m MMT. Complementary IRAC images from theSpitzer Space Telescope Archive were used to relate the structures detected with the outflow at larger scales. Results. High-dynamic range images show fan-shaped outflow structur e in which we detect 18 stars and several blobs of diffuse emission. We interpret the misalignment of various outflow axes in the con text of a disk precession model, including numerical hydrodynamic simulations of the molecular emission. The precession period is∼ 280 years and its half-opening angle is∼ 40 ◦ . A possible triggering mechanism is noncoplanar tidal interaction of an (undiscovered) close companion with the circumbinary protostellar disk. Our observations resolve the nearby massive protostar NGC 7538 IRS2 as a close binary with separation of 195 mas. We find indications for shock interaction bet ween the outflow activities in IRS1 and IRS2. Finally, we find prominent sites of star formation at the interface between two bubble-like structures in NGC 7538, suggestive of a triggered star formation scenario. Conclusions. Indications of outflow precession have been discovered to da te in a number of massive protostars, all with large precessi on angles (∼ 20‐45 ◦ ). This might explain the difference between the outflow widt hs in low- and high-mass stars and add support to a common collimation mechanism.

Mid-infrared interferometry of the Mira variable RR Sco with the VLTI MIDI instrument

We present diffraction-limited bispectrum speckle interferometry observations of four well-known Herbig Ae/Be (HAeBe) stars, LkHα 198, Elias 1, HK Ori and V380 Ori. For two of these, LkHα 198 and Elias 1, we present the first unambiguous detection of close companions. The plane of the orbit of the new LkHα 198 companion appears to be significantly inclined to the plane of the circumprimary disk, as inferred from the orientation of the outflow. We show that the Elias 1 companion may be a convective star, and suggest that it could therefore be the true origin of the X-ray emission from this object. In the cases of HK Ori and V380 Ori, we present new measurements of the relative positions of already-known companions, indicating orbital motion. For HK Ori, photometric measurements of the brightness of the individual components in four bands allowed us to decompose the system spectral energy distribution (SED) into the two separate component SEDs. The primary exhibits a strong infrared excess which suggests the presence of circumstellar material, whereas the companion can be modelled as a naked photosphere. The infrared excess of HK Ori A was found to contribute around two thirds of the total emission from this component, suggesting that accretion power contributes significantly to the flux. Submillimetre constraints mean that the circumstellar disk cannot be particularly massive, whilst the near-infrared data indicates a high accretion rate. Either the disk lifetime is very short, or the disk must be seen in an outburst phase.

Outflows from the high-mass protostars NGC 7538 IRS1/2 observed with bispectrum speckle interferometry Signatures of flow precession

Context. The Scorpius-Centaurus association is the most-nearby group of massive and young stars. As nuclear-fusion products are ejected by massive stars and supernovae into the surrounding interstellar medium, the search for characteristic γ-rays from radioactivity is one way to probe the history of activity of such nearby massive stars on a My time scale through their nucleosynthesis. 26 Al decays with a radioactivity lifetime τ ∼1 My, 1809 keV γ-rays from its decay can be measured with current γ-ray telescopes. Aims. We aim to identify nucleosynthesis ejecta from the youngest subgroup of Sco-Cen stars, and interpret their location and bulk motion from 26 Al observations with INTEGRAL’s γ-ray spectrometer SPI. Methods. Following earlier 26 Al γ-ray mapping with NASA’s Compton observatory, we test spatial emission skymaps of 26 Al for a component which could be attributed to ejecta from massive stars in the Scorpius-Centaurus group of stars. Such a model fit of spatial distributions for large-scale and local components is able to discriminate 26 Al emission associated with Scorpius-Centaurus, in spite of the strong underlying nucleosynthesis signal from the Galaxy at large. Results. We find an 26 Al γ-ray signal above 5σ significance, which we associate with the locations of stars of the Sco-Cen group. ]

Close binary companions of the HAeBe stars LkHα 198, Elias 1, HK Ori and V380 Ori

We present a bispectrum speckle interferometry study of the embedded young stellar objects in Mon R2 IRS 3 in the near-infrared H and K bands. Our images with a resolution of 75 mas (b 62 AU) show a close triple system surrounded by strong diuse nebulosity and three additional infrared sources (K magnitudes11.8-13.7) within 3 00 of the brightest object IRS 3 A (K 7:9). We use HST/NICMOS archive images to derive near-infrared photometry for the sources and estimate the stellar masses of the three brightest objects IRS 3 A, B, and C to be in the range5-15 M. IRS 3 A is surrounded by a bipolar nebula (position angle30), suggesting it to be embedded in a thick circumstellar disk or a torus with polar cavities. IRS 3 B shows a remarkable jet-like emission feature pointing towards the north-east (position angle 50). This feature consists of at least three individual knots with projected separations of 130, 230, and 290 mas (110, 190, and 240 AU) from IRS 3 B, which are much brighter in the K band than in the H band. This strongly indicates outflow activity from IRS 3 B, which is therefore probably the source of the compact high velocity molecular outflow reported from Mon R2 IRS 3. We also analyze Chandra X-ray archive data for the Mon R2 region and find IRS 3 A and IRS 3 C to be sources of hard (3 10 keV) and variable X-ray emission, suggesting plasma temperatures of at least 50 10 6 K. The similarity of the X-ray properties to those of Class I protostars suggests magnetic interaction between the protostars and their circumstellar disks to be the origin of the X-ray emission; this provides indirect evidence for the presence of circumstellar disks in the intermediate- to high-mass young stellar objects IRS 3 A and IRS 3 C.

Radioactive 26Al from the Scorpius-Centaurus association

Context. IC 348 is a nearby (≈310 pc), young (∼2–3 Myr) open cluster with >300 members identified from optical and infrared observations. It comprises young stellar objects in various evolutionary phases from protostars over disk-bearing to diskless premain sequence stars. This gives us the opportunity to study evolutionary effects in the high-energy emissions in a homogeneous environment. Aims. We study the properties of the coronae of the young low-mass stars in IC 348 combining X-ray and optical/infrared data. In particular, we intend to shed light on the dependence of X-ray luminosity and spectral hardness on evolutionary stage and on stellar parameters such as mass, effective temperature, and bolometric luminosity. Methods. The four existing Chandra observations of IC 348 were merged, thus providing a deeper and spatially more complete X-ray view than previous X-ray studies of the cluster. We have compiled a comprehensive catalog of IC 348 members taking into account recent updates to the cluster census. Our data collection comprises fundamental stellar parameters, infrared excess indicating the presence of disks, Hα emission as a tracer of chromospheric emission or accretion, and mass accretion rates. Results. We have detected 290 X-ray sources in four merged Chandra exposures, of which 187 are associated with known cluster members, which corresponds to a detection rate of ∼60% for the cluster members of IC 348 that are identified in optical/infrared studies. According to the most recent spectral classification of IC 348 members, only four of the X-ray sources are brown dwarfs (spectral type M6 and later). The detection rate is highest for diskless Class III stars and increases with stellar mass. This may be explained with higher X-ray luminosities for higher mass and later evolutionary stage that is evident in the X-ray luminosity functions. In particular, we find that for the lowest examined masses (0.1−0.25 M� )t here is ad ifference between the X-ray luminosity functions of accreting and non-accreting stars (classified on the basis of their Hα emission strength) as well as those of disk-bearing and diskless stars (classified on the basis of the slope of the spectral energy distribution). These differences disappear for higher masses. This is related to our finding that the Lx/Lbol ratio is non-constant across the mass/luminosity sequence of IC 348 with a decrease toward lower luminosity stars. Our analysis of an analogous stellar sample in the Orion Nebula Cluster suggests that the decline of Lx/Lbol for young stars at the low-mass end of the stellar sequence is likely universal.