arXiv subjects

K. -H. Hofmann

Publications and source records attributed to K. -H. Hofmann.

At least 55 records · Page 3Linked to original sources

arXiv2009

A binary engine fuelling HD87643' s complex circumstellar environment, using AMBER/VLTI

Context. The star HD 87643, exhibiting the "B[e] phenomenon", has one of the most extreme infrared excesses for this object class. It harbours a large amount of both hot and cold dust, and is surrounded by an extended reflection nebula. Aims. One of our major goals was to investigate the presence of a companion in HD87643. In addition, the presence of close dusty material was tested through a combination of multi-wavelength high spatial 5Aresolution observations. Methods. We observed HD 87643 with high spatial resolution techniques, using the near-IR AMBER/VLTI interferometer with baselines ranging from 60 m to 130 m and the mid-IR MIDI/VLTI interferometer with baselines ranging from 25 m to 65 m. These observations are complemented by NACO/VLT adaptive-optics-corrected images in the K and L-bands, ESO-2.2m optical Wide-Field Imager large-scale images in the B, V and R-bands, Results. We report the direct detection of a companion to HD 87643 by means of image synthesis using the AMBER/VLTI instrument. The presence of the companion is confirmed by the MIDI and NACO data, although with a lower confidence. The companion is separated by ~ 34 mas with a roughly north-south orientation. The period must be large (several tens of years) and hence the orbital parameters are not determined yet. Binarity with high eccentricity might be the key to interpreting the extreme characteristics of this system, namely a dusty circumstellar envelope around the primary, a compact dust nebulosity around the binary system and a complex extended nebula witnessing past violent ejections.

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arXiv2009

Spatially resolving the inhomogeneous structure of the dynamical atmosphere of Betelgeuse with VLTI/AMBER

We present spatially resolved high-spectral resolution K-band observations of the red supergiant Betelgeuse (alpha Ori) using AMBER at the Very Large Telescope Interferometer (VLTI). Betelgeuse was observed between 2.28 and 2.31 micron using baselines of 16, 32, and 48m with spectral resolutions of 4800 -- 12000. Spectrally dispersed interferograms have been obtained in the 2nd, 3rd, and 5th lobes, which represents the highest spatial resolution (9 mas) achieved for Betelgeuse, corresponding to 5 resolution elements over its stellar disk. The AMBER data in the continuum can be reasonably fitted by a uniform disk with a diameter of 43.19+/-0.03 mas or a limb-darkening disk with 43.56+/-0.06 mas. The K-band interferometric data taken at various epochs suggest that Betelgeuse seen in the continuum shows much smaller deviations from the above uniform/limb-darkened disk than predicted by 3-D convection simulations. On the other hand, our AMBER data in the CO lines reveal that the blue and red wings of the CO lines originate in spatially distinct regions over the stellar disk, indicating an inhomogeneous velocity field. Our AMBER data in the CO lines can be roughly explained by a simple model, in which a patch of CO gas is moving outward or inward at velocities of 10--15 km s^-1, while the CO gas in the remaining region in the atmosphere is moving in the opposite direction at the same velocities. The AMBER data are also consistent with the presence of warm molecular layers at ~1.4--1.5 Rstar with a CO column density of ~1 x 10^20 cm^-2. Our AMBER observations of Betelgeuse are the first spatially resolved study of the so-called macroturbulence in a stellar atmosphere other than the Sun. The spatially resolved CO gas motion is likely to be related to convective motion or intermittent mass ejections in clumps or arcs.

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arXiv2009

A new spectroscopic and interferometric study of the young stellar object V645 Cyg

We present the results of high-resolution optical spectroscopy, low-resolution near-IR spectroscopy and near-infrared speckle interferometry of the massive young stellar object candidate V645 Cyg, acquired to refine its fundamental parameters and the properties of its circumstellar envelope. Speckle interferometry in the $H$- and $K$-bands and an optical spectrum in the range 5200--6680 Åwith a spectral resolving power of $R$ = 60 000 were obtained at the 6-m telescope of the Russian Academy of Sciences. Another optical spectrum in the range 4300--10500 Åwith $R$ = 79 000 was obtained at the 3.6-m CFHT. Low-resolution spectra in the ranges 0.46--1.4 $μ$m and 1.4--2.5 $μ$m with $R \sim$ 800 and $\sim$ 700, respectively, were obtained at the 3-m Shane telescope of the Lick Observatory. Using a novel kinematical method based on the non-linear modeling of the neutral hydrogen density profile in the direction toward the object, we propose a distance of $D = 4.2\pm$0.2 kpc. We also suggest a revised estimate of the star's effective temperature, T$_{\rm eff} \sim$25 000 K. We resolved the object in both $H$- and $K$-bands. We conclude that V645 Cyg is a young, massive, main-sequence star, which recently emerged from its cocoon and has already experienced its protostellar accretion stage. The presence of accretion is not necessary to account for the high observed luminosity of (2--6)$\times 10^4$ M$_{\odot}$ yr$^{-1}$. The receding part of a strong, mostly uniform outflow with a terminal velocity of $\sim$800 km s$^{-1}$ is only blocked from view far from the star, where forbidden lines form.

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arXiv2009

Tracing the young massive high-eccentricity binary system Theta 1 Orionis C through periastron passage

The nearby high-mass star binary system Theta 1 Orionis 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. Between January 2007 and March 2008, we observed T1OriC with VLTI/AMBER near-infrared (H- and K-band) long-baseline interferometry, as well as with bispectrum speckle interferometry with the ESO 3.6m and the BTA 6m 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 T1OriC system with a resolution of approx. 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- and K-band and is insensitive to calibration errors induced, for instance, by changing atmospheric conditions. 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 yrs) 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 (M_system=44+/-7 M_sun) and the dynamical distance (d=410+/-20 pc), which is in remarkably good agreement with recently published trigonometric parallax measurements obtained with radio interferometry.

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arXiv2008

The origin of hydrogen line emission for five Herbig Ae/Be stars spatially resolved by VLTI/AMBER spectro-interferometry

To trace the accretion and outflow processes around YSOs, diagnostic spectral lines such as the BrG 2.166 micron line are widely used, although due to a lack of spatial resolution, the origin of the line emission is still unclear. Employing the AU-scale spatial resolution which can be achieved with infrared long-baseline interferometry, we aim to distinguish between theoretical models which associate the BrG line emission with mass infall or mass outflow processes. Using the VLTI/AMBER instrument, we spatially and spectrally (R=1500) resolved the inner environment of five Herbig Ae/Be stars (HD163296, HD104237, HD98922, MWC297, V921Sco) in the BrG emission line as well as in the adjacent continuum. All objects (except MWC297) show an increase of visibility within the BrG emission line, indicating that the BrG-emitting region in these objects is more compact than the dust sublimation radius. For HD98922, our quantitative analysis reveals that the line-emitting region is compact enough to be consistent with the magnetospheric accretion scenario. For HD163296, HD104237, MWC297, and V921Sco we identify a stellar wind or a disk wind as the most likely line-emitting mechanism. We search for general trends and find that the size of the BrG-emitting region does not seem to depend on the basic stellar parameters, but correlates with the H-alpha line profile shape. We find evidence for at least two distinct BrG line-formation mechanisms. Stars with a P-Cygni H-alpha line profile and a high mass-accretion rate seem to show particularly compact BrG-emitting regions (R_BrG/R_cont<0.2), while stars with a double-peaked or single-peaked H-alpha-line profile show a significantly more extended BrG-emitting region (0.6<R_BrG/R_cont<1.4), possibly tracing a stellar wind or a disk wind.

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arXiv2008

VSI: the VLTI spectro-imager

The VLTI Spectro Imager (VSI) was proposed as a second-generation instrument of the Very Large Telescope Interferometer providing the ESO community with spectrally-resolved, near-infrared images at angular resolutions down to 1.1 milliarcsecond and spectral resolutions up to R=12000. Targets as faint as K=13 will be imaged without requiring a brighter nearby reference object. The unique combination of high-dynamic-range imaging at high angular resolution and high spectral resolution enables a scientific program which serves a broad user community and at the same time provides the opportunity for breakthroughs in many areas of astrophysic including: probing the initial conditions for planet formation in the AU-scale environments of young stars; imaging convective cells and other phenomena on the surfaces of stars; mapping the chemical and physical environments of evolved stars, stellar remnants, and stellar winds; and disentangling the central regions of active galactic nuclei and supermassive black holes. VSI will provide these new capabilities using technologies which have been extensively tested in the past and VSI requires little in terms of new infrastructure on the VLTI. At the same time, VSI will be able to make maximum use of new infrastructure as it becomes available; for example, by combining 4, 6 and eventually 8 telescopes, enabling rapid imaging through the measurement of up to 28 visibilities in every wavelength channel within a few minutes. The current studies are focused on a 4-telescope version with an upgrade to a 6-telescope one. The instrument contains its own fringe tracker and tip-tilt control in order to reduce the constraints on the VLTI infrastructure and maximize the scientific return.

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arXiv2007

GJ 900: A new hierarchical system with low-mass components

Speckle interferometric observations made with the 6 m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences in 2000 revealed the triple nature of the nearby ($π_{Hip}=51.80\pm1.74$ mas) low-mass young ($\approx200$ Myr) star GJ 900. The configuration of the triple system allowed it to be dynamically unstable. Differential photometry performed from 2000 through 2004 yielded $I$- and $K$-band absolute magnitudes and spectral types for the components to be $I_{A}$=6.66$\pm$0.08, $I_{B}$=9.15$\pm$0.11, $I_{C}$=10.08$\pm$0.26, $K_{A}$=4.84$\pm$0.08, $K_{B}$=6.76$\pm$0.20, $K_{C}$=7.39$\pm$0.31, $Sp_{A}$$\approx$K5--K7, $Sp_{B}$$\approx$M3--M4, $Sp_{C}$$\approx$M5--M6. The ``mass--luminosity'' relation is used to estimate the individual masses of the components: $\mathcal{M}_{A}$$\approx0.64\mathcal{M}_{\odot}$, $\mathcal{M}_{B}$$\approx0.21\mathcal{M}_{\odot}$, $\mathcal{M}_{C}$$\approx0.13\mathcal{M}_{\odot}$. From the observations of the components relative motion in the period 2000--2006, we conclude that GJ 900 is a hierarchical triple star with the possible orbital periods P$_{A-BC}$$\approx$80 yrs and P$_{BC}$$\approx$20 yrs. An analysis of the 2MASS images of the region around GJ 900 leads us to suggest that the system can include other very-low-mass components.

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arXiv2007

Visual/infrared interferometry of Orion Trapezium stars: Preliminary dynamical orbit and aperture synthesis imaging of the Theta 1 Orionis C system

Located in the Orion Trapezium cluster, Theta 1 Orionis C is one of the youngest and nearest high-mass stars (O5-O7) and also known to be a close binary system. 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 trace the orbital motion of the Theta 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 apply aperture synthesis techniques to our IOTA data to reconstruct a model-independent image of the Theta 1 Ori C binary system. The orbital solutions suggest a high eccentricity (e approx. 0.91) and short-period (P approx. 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. From these orbital solutions one can be favoured, implying a system mass of 48 M_sun and a distance to the Trapezium cluster of 434 pc. 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 Theta 1 Ori C1 (M=34.0 M_sun) and O9.5 for C2 (M=15.5 M_sun). 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 (approx. 1.5 AU, next passage around 2007.5), suggesting strong wind-wind interaction between the two O stars.

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arXiv2006

Direct constraint on the distance of y2 Velorum from AMBER/VLTI observations

In this work, we present the first AMBER observations, of the Wolf-Rayet and O (WR+O) star binary system y2 Velorum. The AMBER instrument was used with the telescopes UT2, UT3, and UT4 on baselines ranging from 46m to 85m. It delivered spectrally dispersed visibilities, as well as differential and closure phases, with a resolution R = 1500 in the spectral band 1.95-2.17 micron. We interpret these data in the context of a binary system with unresolved components, neglecting in a first approximation the wind-wind collision zone flux contribution. We show that the AMBER observables result primarily from the contribution of the individual components of the WR+O binary system. We discuss several interpretations of the residuals, and speculate on the detection of an additional continuum component, originating from the free-free emission associated with the wind-wind collision zone (WWCZ), and contributing at most to the observed K-band flux at the 5% level. The expected absolute separation and position angle at the time of observations were 5.1±0.9mas and 66±15° respectively. However, we infer a separation of 3.62+0.11-0.30 mas and a position angle of 73+9-11°. Our analysis thus implies that the binary system lies at a distance of 368+38-13 pc, in agreement with recent spectrophotometric estimates, but significantly larger than the Hipparcos value of 258+41-31 pc.

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arXiv2006

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

NGC 7538 IRS1 is a high-mass (approx. 30 M_sun) protostar with a CO outflow, an associated UCHII 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. We investigate the near-infrared morphology of the source to clarify the relations among the various axes. 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 the Spitzer Space Telescope Archive were used to relate the structures detected with the outflow at larger scales. High-dynamic range images show fan-shaped outflow structure in which we detect 18 stars and several blobs of diffuse emission. We interpret the misalignment of various outflow axes in the context of a disk precession model, including numerical hydrodynamic simulations of the molecular emission. The precession period is approx. 280 years and its half-opening angle is 40 degrees. A possible triggering mechanism is non-coplanar 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 between the outflow activities in IRS1 and IRS2. Indications of outflow precession have been discovered to date in a number of massive protostars, all with large precession angles 20--45 degrees. This might explain the difference between the outflow widths in low- and high-mass stars and add support to a common collimation mechanism.

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arXiv2005

Bispectrum speckle interferometry of the massive protostellar outflow source IRAS 23151+5912

We present bispectrum speckle interferometry of the massive protostellar object IRAS 23151+5912 in the near-infrared K' band. The reconstructed image shows the diffuse nebulosity north-east of two point-like sources in unprecedented detail. The comparison of our near-infrared image with mm continuum and CO molecular line maps shows that the brighter of the two point sources lies near the center of the mm peak, indicating that it is a high-mass protostar. The nebulosity coincides with the blue-shifted molecular outflow component. The most prominent feature in the nebulosity is a bow-shock-like arc. We assume that this feature is associated with a precessing jet which has created an inward-pointed cone in the swept-up material. We present numerical jet simulations that reproduce this and several other features observed in our speckle image of the nebulosity. Our data also reveal a linear structure connecting the central point source to the extended diffuse nebulosity. This feature may represent the innermost part of a jet that drives the strong molecular outflow (PA ~80 degr) from IRAS 23151+5912. With the aid of radiative transfer calculations, we demonstrate that, in general, the observed inner structures of the circumstellar material surrounding high-mass stars are strongly influenced by the orientation and symmetry of the bipolar cavity.

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arXiv2005

The vicinity of the galactic supergiant B[e] star CPD -57 2874 from near- and mid-IR long baseline spectro-interferometry with the VLTI (AMBER and MIDI)

We present the first spectro-interferometric observations of the circumstellar envelope (CSE) of a B[e] supergiant (CPD -57 2874), performed with the Very Large Telescope Interferometer (VLTI) using the beam-combiner instruments AMBER (near-IR interferometry with three 8.3 m Unit Telescopes or UTs) and MIDI (mid-IR interferometry with two UTs). Our observations of the CSE are well fitted by an elliptical Gaussian model with FWHM diameters varying linearly with wavelength. Typical diameters measured are $\simeq1.8\times3.4$ mas or $\simeq4.5\times8.5$ AU (adopting a distance of 2.5 kpc) at $2.2\micron$, and $\simeq12\times15$ mas or $\simeq30\times38$ AU at $12\micron$. We show that a spherical dust model reproduces the SED but it underestimates the MIDI visibilities, suggesting that a dense equatorial disk is required to account for the compact dust-emitting region observed. Moreover, the derived major-axis position angle in the mid-IR ($\simeq144\degr$) agrees well with previous polarimetric data, hinting that the hot-dust emission originates in a disk-like structure. Our results support the non-spherical CSE paradigm for B[e] supergiants.

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arXiv2005

VLTI/AMBER and VLTI/MIDI spectro-interferometric observations of the B[e] supergiant CPD-57 2874

We present the first high spatial and spectral observations of the circumstellar envelope (CSE) of a B[e] supergiant (CPD$-57 2874$), performed with the Very Large Telescope Interferometer (VLTI). Spectra, visibilities, and closure phase, were obtained using the beam-combiner instruments AMBER (near-IR interferometry with three 8.3 m Unit Telescopes or UTs) and MIDI (mid-IR interferometry with two UTs). The interferometric observations of the CSE are well fitted by an elliptical Gaussian model with FWHM diameters varying linearly with wavelength. Typical diameters measured are $\simeq1.8\times3.4$ mas or $\simeq4.5\times8.5$ AU (adopting a distance of 2.5 kpc) at $2.2\micron$, and $\simeq12\times15$ mas or $\simeq30\times38$ AU at $12\micron$. The size of the region emitting the Br$γ$ flux is $\simeq2.8\times5.2$ mas or $\simeq7.0\times13.0$ AU. The major-axis position angle of the elongated CSE in the mid-IR ($\simeq144\degr$) agrees well with previous polarimetric data, hinting that the hot-dust emission originates in a disk-like structure. In addition to the interferometric observations we also present new optical ($UBVR_{c}I_{c}$) and near-IR ($JHKL$) broadband photometric observations of CPD$-57 2874$. Our spectro-interferometric VLTI observations and data analysis support the non-spherical CSE paradigm for B[e] supergiants.

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arXiv2005

Disk and wind interaction in the young stellar object MWC 297 spatially resolved with VLTI/AMBER

The young stellar object MWC 297 has been observed with the VLT interferometer equipped with the AMBER instrument. MWC 297 has been spatially resolved in the continuum with a visibility of 0.50 as well as in the Br gamma emission line where the visibility decrease to a lower value of 0.33. This change in the visibility with the wavelength can be interpreted by the presence of an optically thick disk responsible for the visibility in the continuum and of a stellar wind traced by Br gamma and whose apparent size is 40% larger. We validate this interpretation by building a model of the stellar environment that combines a geometrically thin, optically thick accretion disk model consisting of gas and dust, and a latitude-dependent stellar wind outflowing above the disk surface. The continuum emission and visibilities obtained from this model are fully consistent with the interferometric AMBER data. They agree also with existing optical, near-infrared spectra and other broad-band near-infrared interferometric visibilities. A picture emerges in which MWC 297 is surrounded by an equatorial flat disk that is possibly still accreting and an outflowing wind which has a much higher velocity in the polar region than at the equator. [abridged]

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arXiv2005

High angular resolution N-band observation of the silicate carbon star IRAS08002-3803 with the VLTI/MIDI instrument

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 Rstar) between 8 and 10 micron, while it steeply increases longward of 10 micron to reach ~53 mas (106 Rstar) at 13 micron. 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.

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arXiv2005

Disentangling the wind and the disk in the close surrounding of the young stellar object MWC297 with AMBER/VLTI

The young stellar object MWC297 is a B1.5Ve star exhibiting strong hydrogen emission lines. This object has been observed by the AMBER/VLTI instrument in 2-telescope mode in a sub-region of the K spectral band centered around the Br gamma line at 2.1656 microns. The object has not only been resolved in the continuum with a visibility of 0.50+/-0.10, but also in the Br gamma line, where the flux is about twice larger, with a visibility about twice smaller (0.33+/-0.06). The continuum emission is consistent with the expectation of an optically thick thermal emission from dust in a circumstellar disk. The hydrogen emission can be understood by the emission of a halo above the disk surface. It can be modelled as a latitudinal-dependant wind model and it explains the width, the strength and the visibibility through the emission lines. The AMBER data associated with a high resolution ISAAC spectrum constrains the apparent size of the wind but also its kinematics.

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arXiv2005

A quasi-time-dependent radiative transfer model of OH104.9+2.4

We investigate the pulsation-phase dependent properties of the circumstellar dust shell (CDS) of the OH/IR star OH104.9+2.4 based on radiative transfer modeling (RTM) using the code DUSTY. Our previous study concerning simultaneous modeling of the spectral energy distribution (SED) and near-infrared (NIR) visibilities (Riechers et al. 2004) has now been extended by means of a more detailed analysis of the pulsation-phase dependence of the model parameters of OH104.9+2.4. In order to investigate the temporal variation in the spatial structure of the CDS, additional NIR speckle interferometric observations in the K' band were carried out with the 6 m telescope of the Special Astrophysical Observatory (SAO). At a wavelength of 2.12 micron the diffraction-limited resolution of 74 mas was attained. Several key parameters of our previous best-fitting model had to be adjusted in order to be consistent with the newly extended amount of observational data. It was found that a simple rescaling of the bolometric flux F_bol is not sufficient to take the variability of the source into account, as the change in optical depth over a full pulsation cycle is rather high. On the other hand, the impact of a change in effective temperature T_eff on SED and visibility is rather small. However, observations, as well as models for other AGB stars, show the necessity of including a variation of T_eff with pulsation phase in the radiative transfer models. Therefore, our new best-fitting model accounts for these changes.

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arXiv2005

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

We present diffraction-limited bispectrum speckle interferometry observations of four well-known Herbig Ae/Be (HAeBe) stars, LkHa 198, Elias 1, HK Ori and V380 Ori. For two of these, LkHa 198 and Elias 1, we present the first unambiguous detection of close companions. The plane of the orbit of the new LkHa 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.

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