Search

Aims. Polycyclic aromatic hydrocarbons (PAHs) have been detected toward molecular clouds and some young stars with disks, but have not yet been associated with embedded young stars. We present a sensitive mid-infrared spectroscopic survey of PAH features toward a sample of low-mass embedded young stellar objects (YSOs). The aim is to put constraints on the PAH abundance in the embedded phase of star formation using radiative transfer modeling.

Context. The first magnetic fields in O- and B-type stars that do not belong to the Bp-star class, have been discovered. The cyclic UV wind-line variability, which has been observed in a significant fraction of early-type stars, is likely to be related to such magnetic fields.

Aims.Our aim is to determine the presence and location of the emission from polycyclic aromatic hydrocarbons (PAHs) towards low and intermediate mass young stars with disks using large aperture telescopes.

Context.Dust sedimentation is known to affect the infrared spectra and images of disks. In particular the far-infrared emission may be reduced by strong sedimentation. However, dust grains of different sizes sediment to different depths in the disk. Spectral features of one species may thus be enhanced, while those of other species may be suppressed.

Aims.We present spatially resolved mid-infrared images of the disk surrounding the young star IRS 48in the Ophiuchus cloud complex. The disk exhibits a ring-like structure at 18.7 μm, and is dominated by very strong emission from polycyclic aromatic hydrocarbons at shorter wavelengths. This allows a detailed study of the relative distributions of small and large dust grains.

Aims.The chemistry of, and infrared (IR) emission from, polycyclic aromatic hydrocarbons (PAHs) in disks around Herbig Ae/Be and T Tauri stars are investigated. PAHs can exist in different charge states and they can bear different numbers of hydrogen atoms. The equilibrium (steady-state) distribution over all possible charge/hydrogenation states depends on the size and shape of the PAHs and on the physical properties of the star and surrounding disk.

Aims.We search for Polycyclic Aromatic Hydrocarbon (PAH) features towards young low-mass (T Tauri) stars and compare them with surveys of intermediate mass (Herbig Ae/Be) stars. The presence and strength of the PAH features are interpreted with disk radiative transfer models exploring the PAH feature dependence on the incident UV radiation, PAH abundance and disk parameters.

V 2052 Ophis a βCep star with $v \sin i\sim 60$km s-1. The behavior of its stellar wind was studied in the ultraviolet wavelength region with the IUE satellite. It revealed periodic variations in the equivalent widths (EW) of the resonance lines of wind-sensitive ions with a period of 3.638833 d, which is identified as the rotational period. These variations are typical of magnetic stars. Therefore time-resolved circular spectropolarimetric observations were obtained with the Musicos échelle spectropolarimeter at the 2-m Télescope Bernard Lyot (TBL) to search for a magnetic field in the star. Stokes V patterns were observed, the inclination and magnetic angles were derived and a value was determined for the polar magnetic field ($250 \pm 190$G) using an oblique rotator dipole model. The spectroscopic information was used to search for periodicity in line-profile variations (lpv), radial velocity and minimum intensity curves. Multiperiodicity was found, corresponding to radial ($f_1=7.15$c d-1) and non-radial ($f_2=6.82$c d-1, $l=3$or 4) pulsation modes. The rotation period is also detected in rotationally modulated observables because of the magnetic poles passing through the observer's line of sight. We determined the stellar parameters of the star, which was found to be chemically peculiar, in particular He-enriched. This makes V 2052 Ophthe first magnetic He-strong early B star with known pulsational properties.

?Casis a B2 IV star with $v\sin i = 17$km s-1. Time-resolved circular spectropolarimetric observations of ?Casobtained in 2001 and 2002 with the Musicos échelle spectropolarimeter at the 2 m Télescope Bernard Lyot (TBL) show a sinusoidally varying longitudinal magnetic field with a strength between 10 G and -46 G for the averaged line-of-sight component, corresponding to $B_{\rm pol}=335^{+120}_{-65}$G. The period corresponds very accurately with the 5.37045 day period as derived from stellar wind variations observed in the ultraviolet. The epoch of the positive maximum field corresponds in phase with the maximum emission in the UV wind lines. This gives compelling evidence for a magnetic rotator model for this star, with an unambiguous rotation period of 5.37 days. We searched for periodicity in line-profile variations (lpv), radial velocity and minimum intensity curves in the ~400 optical spectra. We found a non-radial pulsation mode with $l=2 \pm 1$at the frequency $f=0.64$c d-1. From this periodicity and from stellar parameters derived from model fits, we propose to classify ?Casas a Slowly Pulsating B (SPB) star. This is the third detection of a magnetic field in an early B-type pulsating star and the first one in a SPB star.

We describe the design and performance of the Medium Resolution Spectrometer (MRS) for the JWST-MIRI instrument. The MRS incorporates four coaxial spectral channels in a compact opto-mechanical layout that generates spectral images over fields of view up to 7.7 × 7.7? in extent and at spectral resolving powers ranging from 1300 to 3700. Each channel includes an all-reflective integral field unit (IFU): an "image slicer" that reformats the input field for presentation to a grating spectrometer. Two 1024 × 1024 focal plane detector arrays record the output spectral images with an instantaneous spectral coverage of approximately one third of the full wavelength range of each channel. The full 5-28.5 ?m spectrum is then obtained by making three exposures using gratings and pass-band-determining filters that are selected using just two three-position mechanisms. The expected on-orbit optical performance is presented, based on testing of the MIRI Flight Model and including spectral and spatial coverage and resolution. The point spread function of the reconstructed images is shown to be diffraction limited and the optical transmission is shown to be consistent with the design expectations.

We report on our discovery of a magnetic field in the B2IV star ζCas. We determine the rotation period from the UV wind behaviour. We summarize the results from spectropolarimetric observations of ζCas obtained in 2001 and 2002 with the MuSiCoS échelle spectropolarimeter at the 2 m Télescope Bernard Lyot (TBL). We also find a non-radial pulsation mode and confirm that the star is N-enriched. This is the third detection of a magnetic field in an early B-type variable star and the first one in a SPB star.

We have searched for magnetic fields in several early B-type stars: βCephei, Slowly Pulsating B (SPB) and in particular Be stars. We recall the properties of these stars and highlight the problem of the Be phenomenon. We discuss two clues towards understanding this phenomenon in terms of pulsations and magnetic fields. We summarize results from spectropolarimetric observations obtained at the Pic du Midi (France) and highlight the results for the Be star ωOri and the βCep star V2052 Oph.