MIRI (the Mid InfraRed Instrument) is one of the focal plane instruments of the James Webb Space Telescope. The instrument comprises a camera and a spectrometer module. The instrument plays the following key roles in the JWST science program.- Discovery of the "first light".- Assembly of galaxies: history of star formation, growth of black holes, production of heavy elements.- Formation of stars and planetary systems.- Evolution of planetary systems and conditions for life.The MIRI spectrometer covers the spectral range from 5 -28.3 mum with a spectral resolution better than 2000. The spectral module is an imaging spectrometer with a field of view greater than or equal to 3 arcsec. The spectrometer consists of 4 spectral channels that share 2 detectors of 1024 x 1024 pixels each. In a single measurement one third of the wavelength range of the channels is imaged onto the detectors. By changing gratings the whole spectral range is covered in 3 measurements.In this paper the optical design of the MIRI spectrometer is described.

Using a modified version of the CORS Baade-Wesselink method in the Walraven system, we derive the radii of a sample of Galactic Cepheids with period from few days up to 40 days. We test our results by performing a comparison with the radii obtained from interferometric measurements, finding an excellent agreement. The best fit Period-Radius relation log R=(0.75± 0.02)log P+(1.10±0.03) is obtained by fixing the projection factor to p=1.27 and is in good agreement with those from the literature.

We present a new derivation of the CORS Baade-Wesselink method in the Walraven photometric system. We solved the complete Baade-Wesselink equation by calibrating the surface brightness function with a recent grid of atmosphere models. The new approach was adopted to estimate the mean radii of a sample of Galactic Cepheids for which are available precise light curves in the Walraven bands. Current radii agree, within the errors, quite well with Cepheid radii based on recent optical and near-infrared interferometric measurements. We also tested the impact of the projection factor on the period-radius relation using two different values (p = 1.36, 1.27) that bracket the estimates available in the literature. We found that the agreement of our period-radius relation with similar empirical and theoretical period-radius relations in the recent literature improves by changing the projection factor from p = 1.36 to 1.27. Our period-radius relation is log R = (0.75 +/- 0.03)log P + (1.10 +/- 0.03), with an rms = 0.03 dex. Thanks to accurate estimates of the effective temperature of the selected Cepheids, we also derived the period-luminosity relation in the V band and we found M-V = (-2.78 +/- 0.11)log P + (-1.42 +/- 0.11) with rms = 0.13 mag, for p = 1.27. It agrees quite well with recent results in the literature, while the relation for p = 1.36 deviates by more than 2 Sigma. We conclude that, even taking into account the intrinsic dispersion of the obtained period-luminosity relations that is roughly of the same order of magnitude as the effect of the projection factor, the results of this paper seem to favour the value p = 1.27.

We provided accurate estimates of distances, radii and iron abundances for four metal-rich Cepheids, namely V340 Ara, UZ Sct, AV Sgr and VY Sgr. The main aim of this investigation is to constrain their pulsation properties and their location across the Galactic inner disk. We adopted new accurate NIR (J,H,K) light curves and new radial velocity measurements for the target Cepheids to determinate their distances and radii using the Baade-Wesselink technique. In particular, we adopted the most recent calibration of the IR surface brightness relation and of the projection factor. Moreover, we also provided accurate measurements of the iron abundance of the target Cepheids. Current distance estimates agree within one sigma with similar distances based either on empirical or on theoretical NIR Period-Luminosity relations. However, the uncertainties of the Baade-Wesselink distances are on average a factor of 3-4 smaller when compared with errors affecting other distance determinations. Mean Baade-Wesselink radii also agree at one sigma level with Cepheid radii based either on empirical or on theoretical Period-Radius relations. Iron abundances are, within one sigma, similar to the iron contents provided by Andrievsky and collaborators, thus confirming the super metal-rich nature of the target Cepheids. We also found that the luminosity amplitudes of classical Cepheids, at odds with RR Lyrae stars, do not show a clear correlation with the metal-content. This circumstantial evidence appears to be the consequence of the Hertzsprung progression together with the dependence of the topology of the instability strip on metallicity, evolutionary effects and binaries.

Aims. We investigate the quasar – radio galaxy unification scenario and detect dust tori within radio galaxies of various types. Methods. Using VISIR on the VLT, we acquired sub-arcsecond (∼0.40 �� ) resolution N-band images, at a wavelength of 11.85 μm, of the nuclei of a sample of 27 radio galaxies of four types in the redshift range z = 0.006−0.156. The sample consists of 8 edge-darkened, low-power Fanaroff-Riley class I (FR-I) radio galaxies, 6 edge-brightened, class II (FR-II) radio galaxies displaying low-excitation optical emission, 7 FR-IIs displaying high-excitation optical emission, and 6 FR-II broad emission line radio galaxies. Out of the sample of 27 objects, 10 nuclei are detected and several have constraining non-detections at sensitivities of 7 mJy, the limiting flux a point source has when detected with a signal-to-noise ratio of 10 in one hour of source integration. Results. On the basis of the core spectral energy distributions of this sample we find clear indications that many FR-I and several low-excitation FR-II radio galaxies do not contain warm dust tori. At least 57 ± 19 percent of the high-excitation FR-IIs and almost all of the broad line radio galaxies exhibit excess infrared emission, which must be attributed to warm dust reradiating accretion activity. The FR-I and low-excitation FR-II galaxies are all of low efficiency, which is calculated as the ratio of bolometric to Eddington luminosity Lbol/LEdd < 10 −3 . This suggests that thick tori are absent at low accretion rates and/or low efficiencies. The high-excitation FR-II galaxies are a mixed population with three types of nuclei: 1) low efficiency with dust torus; 2) low efficiency with weakly emitting dust torus; and 3) high efficiency with weak dust torus. We argue that the unification viewing angle range 0–45 degrees of quasars should be increased to ∼60 degrees, at least at lower luminosities.

Aims. The iron abundance gradient in the Galactic stellar disk provides fundamental constraints on the chemical evolution of this important Galaxy component, however the spread around the mean slope is, at fixed Galactocentric distance, more than the estimated uncertainties.Methods. To provide quantitative constraints on these trends, we adopted iron abundances for 265 classical Cepheids (more than 50% of the currently known sample) based either on high-resolution spectra or on photometric metallicity indices. Homogeneous distances were estimated using near-infrared period-luminosity relations. The sample covers the four disk quadrants, and their Galactocentric distances range from similar to 5 to similar to 17 kpc. We provided a new theoretical calibration of the metallicity-index-color (MIC) relation based on Walraven and NIR photometric passbands.Results. We estimated the photometric metallicity of 124 Cepheids. Among them 66 Cepheids also have spectroscopic iron abundances and we found that the mean difference is -0.03 +/- 0.15 dex. We also provide new iron abundances, based on high-resolution spectra, for four metal-rich Cepheids located in the inner disk. The remaining iron abundances are based on high-resolution spectra collected by our group (73) or available in the literature (130). A linear regression over the entire sample provides an iron gradient of -0.051 +/- 0.004 dex kpc(-1). The above slope agrees quite well, within the errors, with previous estimates based either on Cepheids or on open clusters covering similar Galactocentric distances. However, Cepheids located in the inner disk systematically appear more metal-rich than the mean metallicity gradient. Once we split the sample into inner (R(G) < 8 kpc) and outer disk Cepheids, the slope (-0.130 +/- 0.015 dex kpc(-1)) in the former region is approximate to 3 times steeper than the slope in the latter one (-0.042 +/- 0.004 dex kpc(-1)). In the outer disk the radial distribution of metal-poor (MP, [Fe/H] < -0.02 dex) and metal-rich (MR) Cepheids across the four disk quadrants does not show a clear trend when moving from the innermost to the external disk regions. The relative fractions of MP and MR Cepheids in the 1st and in the 3rd quadrants differ at the 8 sigma (MP) and 15 sigma (MR) levels. Finally, we found that iron abundances in two local overdensities of the 2nd and of the 4th quadrant cover individually a range in iron abundance of approximate to 0.5 dex.Conclusions. Current findings indicate that the recent chemical enrichment across the Galactic disk shows a clumpy distribution.

Context. Infrared (IR) spectroscopy and imaging provide a prime tool to study the characteristics of polycyclic aromatic hydrocarbon (PAH) molecules and the mineralogy in regions of star formation. Herbig Ae/Be stars are known to have varying amounts of natal cloud material present in their vicinity.Aims. Our aim is to study the characteristics of the mid-IR emission originating in Herbig Ae/Be stars, especially the extent of the emission and how this relates to the (proto-) stellar characteristics.Methods. Today's powerful ground-and space-based telescopes provide images and spectra at unprecedented spectral and spatial resolution. We analyse the images and spectra from four Herbig Ae/Be stars (IRAS 06084-0611, CD-42 11721, TY CrA, and HD176386), as obtained with TIMMI2 on the ESO 3.6 m telescope and VISIR on the VLT. These observations are supplemented with data from ISO-SWS and Spitzer-IRAC.Results. We find evidence for large-scale structure and extended emission in all four sources, except for HD176386, which only shows silicate emission, all sources show PAH emission in their spectra. In addition, a spatially resolved silicate and PAH spectrum could be extracted for TY CrA.Conclusions. The variety in emission scales distinguishes two classes. In the first, the morphology and spectral characteristics resemble those of reflection nebulae. In the second, the characteristics are in-line with Herbig A stars. This separation simply reflects a difference in stellar characteristics (e. g. luminosity). In Herbig B stars, dust emission from the surroundings dominates, where for Herbig A stars, the disk dominates the emission. In this scheme, IRAS 06084-0611 and CD-42 11721 resemble reflection nebulae and HD176386 a more typical Herbig Ae/Be star. TY CrA shows characteristics common to both genuine reflection nebulae and Herbig B stars. We propose a geometry for TY CrA, with most notably, a similar to 70 AU inner gap in the 340 AU circumtertiary disk cleared by a fourth stellar companion.

Context. Infrared (IR) spectroscopy and imaging provide a prime tool to study the characteristics of polycyclic aromatic hydrocarbon (PAH) molecules and the mineralogy in regions of star formation. Herbig Ae/Be stars are known to have varying amounts of natal cloud material present in their vicinity. Aims. Our aim is to study the characteristics of the mid-IR emission originating in Herbig Ae/Be stars, especially the extent of the emission and how this relates to the (proto-)stellar characteristics. Methods. Today’s powerful groundand space-based telescopes provide images and spectra at unprecedented spectral and spatial resolution. We analyse the images and spectra from four Herbig Ae/Be stars (IRAS 06084-0611, CD-42 11721, TY CrA, and HD 176386), as obtained with TIMMI2 on the ESO 3.6 m telescope and VISIR on the VLT. These observations are supplemented with data from ISO-SWS and Spitzer-IRAC. Results. We find evidence for large-scale structure and extended emission in all four sources, except for HD 176386, which only shows silicate emission, all sources show PAH emission in their spectra. In addition, a spatially resolved silicate and PAH spectrum could be extracted for TY CrA. Conclusions. The variety in emission scales distinguishes two classes. In the first, the morphology and spectral characteristics resemble those of reflection nebulae. In the second, the characteristics are in-line with Herbig A stars. This separation simply reflects a difference in stellar characteristics (e.g. luminosity). In Herbig B stars, dust emission from the surroundings dominates, where for Herbig A stars, the disk dominates the emission. In this scheme, IRAS 06084-0611 and CD-42 11721 resemble reflection nebulae and HD 176386 a more typical Herbig Ae/Be star. TY CrA shows characteristics common to both genuine reflection nebulae and Herbig B stars. We propose a geometry for TY CrA, with most notably, a ∼70 AU inner gap in the 340 AU circumtertiary disk cleared by a fourth stellar companion.

Context. Bipolar morphologies in planetary nebulae (PNe) are believed to be closely linked to binary central stars. Either by collimating a fast stellar wind or by driving a jet via accretion in the central system, dusty torii or stable disks may be crucial ingredients for the shaping of PNe. Aims. We study the dust distribution in the very young proto-planetary nebula (PPN) IRAS16342-3814, also known as the Water Fountain Nebula, which is known to show strong bipolar characteristics in the shape of two reflection lobes, and high-velocity collimated molecular outlfows. Methods. We use the new mid-infrared (MIR) instrument VISIR on the Very Large Telescope (VLT) both in imaging and spectroscopy mode at wavelengths from 8 to 13 mu m. Results. We present the first spatially resolved MIR observations of a dusty evolved star obtained with VISIR and find that the improved spatial resolution contradicts previous claims of an elliptical brightness distribution at the heart of IRAS16342: we find the waist region to be dark even in the MIR. We show that the filling angle of the obscuring dust lane, which is made mostly of amorphous silicates, is very large, possibly even close to a spherically symmetric superwind as seen in OH/IR stars. Conclusions. We conclude that, in contrast to the multitude of recent dusty-disk detections in post-AGB stars and PNe, IRAS16342 does not show this extreme equatorial density enhancement, at least not on the scale of the dusty environment which lends the object its IR appearance. Rather, it appears that the observed precessing jets are shaping the bipolar nature in the remains of a spherically symmetric AGB superwind.

Context. Bipolar morphologies in planetary nebulae (PNe) are believed to be closely linked to binary central stars. Either by collimating a fast stellar wind or by driving a jet via accretion in the central system, dusty torii or stable disks may be crucial ingredients for the shaping of PNe. Aims. We study the dust distribution in the very young proto-planetary nebula (PPN) IRAS16342-3814, also known as the Water Fountain Nebula, which is known to show strong bipolar characteristics in the shape of two reflection lobes, and high-velocity collimated molecular outlfows. Methods. We use the new mid-infrared (MIR) instrument VISIR on the Very Large Telescope (VLT) both in imaging and spectroscopy mode at wavelengths from 8 to 13 μm. Results. We present the first spatially resolved MIR observations of a dusty evolved star obtained with VISIR and find that the improved spatial resolution contradicts previous claims of an elliptical brightness distribution at the heart of IRAS16342: we find the waist region to be dark even in the MIR. We show that the filling angle of the obscuring dust lane, which is made mostly of amorphous silicates, is very large, possibly even close to a spherically symmetric superwind as seen in OH/IR stars. Conclusions. We conclude that, in contrast to the multitude of recent dusty-disk detections in post-AGB stars and PNe, IRAS16342 does not show this extreme equatorial density enhancement, at least not on the scale of the dusty environment which lends the object its IR appearance. Rather, it appears that the observed precessing jets are shaping the bipolar nature in the remains of a spherically symmetric AGB superwind.

We present new empirical and theoretical calibrations of two photometric metallicity indices based on Walraven photometry. The empirical calibration relies on a sample of 48 Cepheids for which iron abundances based on high resolution spectra are available in the literature. They cover a broad range in metal abundance (-0.5 < [Fe/H] < +0.5) and the intrinsic accuracy of the Metallicity Index Color (MIC) relations is better than 0.2 dex. The theoretical calibration relies on a homogeneous set of scaled-solar evolutionary tracks for intermediate-mass stars and on pulsation predictions concerning the topology of the instability strip. The metal content of the adopted evolutionary tracks ranges from Z=0.001 to Z=0.03 and the intrinsic accuracy of the MIC relations is better than 0.1 dex.

An overview is given of the Walraven V BLUW photometric system. We concentrate on the aspects of stability, internal and external precison of the standard system, flux calibration and theoretical model grids.

Aims. We present spatially resolved mid-infrared images of the disk surrounding the young star IRS 48 in the Ophiuchus cloud complex. The disk exhibits a ring-like structure at 18.7 mu m, 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.Methods. Images of IRS 48 in 5 mid-infrared bands from 8.6 to 18.7 mu m as well as a low resolution N-band spectrum are obtained with VLT-VISIR. Optical spectroscopy is used to determine the spectral type of the central star and to measure the strength of the H alpha line.Results. The 18.7 mu m ring peaks at a diameter of 110 AU, with a gap of similar to 60 AU. The shape of the ring is consistent with an inclination of i = 48 degrees +/- 8 degrees. In contrast, the 7.5 - 13 mu m PAH emission bands are centered on the source and appear to fill the gap within the ring. The measured PAH line strengths are 10 - 100 x stronger than those typically measured for young M0 stars and can only be explained with a high PAH abundance and/or strong excess optical/UV emission. The morphology of the images, combined with the absence of a silicate emission feature, imply that the inner disk has been cleared of micron-sized dust but with a significant population of PAHs remaining. We argue that the gap can be due to grain growth and settling or to clearing by an unseen planetary or low-mass companion. IRS 48 may represent a short-lived transitional phase from a classical to a weak-line T Tauri star.

MIRI ('Mid Infrared Instrument') is the combined imager and integral field spectrometer for the 5-29 micron wavelength range under development for the JWST. The Spectrometer Main Optics (SMO) system has been designed on the basis of a 'no adjustments' philosophy. This means that the optical alignment precision depends strongly on the design, tolerance analysis and detailed knowledge of the manufacturing process. Because in principle no corrections are possible after assembly, continuous tracking of the alignment performance during the design and manufacturing phases is important. This is done by controlling the "alignment budget" which allows a detailed comparison of the required and achieved alignment from component to system level. This paper will describe the development of the SMO alignment budget, and how it is used to bring the alignment performance under control. In addition, we will discuss the results of the actual alignment measurements on the SMO hardware and the feedback of these results into the alignment budget.

The MIRI Medium Resolution Spectrometer (MIRI-MRS) will increase the sensitivity of astronomical spectroscopy at thermal infrared wavelengths (from 5 to 28 microns), by a factor of 1000 over the best that can be achieved by existing ground-based instruments. This leap in performance is further enhanced by the first use at these wavelengths of all reflective Integral Field Units (image slicers) to provide the spectrometer with a rectangular field of view with a shortest dimension of 3.5 arcseconds. We describe the optical design of the MRS and present predictions for its delivered image quality.