Göran Pilbratt

Kuiper belts around nearby stars

Context. The existence of dusty debris disks around a large fraction of solar type main-sequence stars, inferred from excess far-IR and submillimetre emission compared to that expected from stellar photospheres, suggests that leftover planetesimal belts analogous to the asteroid- and comet reservoirs of the solar system are common. Aims. Sensitive submillimetre observations are essential to detect and characterise cold extended dust originating from collisions of small bodies in disks, belts, or rings at Kuiper-belt distances (30–50 AU or beyond). Measurements of the flux densities at these wavelengths will extend existing IR photometry and permit more detailed modelling of the Rayleigh-Jeans tail of the disks spectral energy distribution (SED), effectively constraining dust properties and disk extensions. By observing stars spanning from a few up to several hundred Myr, the evolution of debris disks during crucial phases of planet formation can be studied. Methods. We observed 22 exo-Kuiper-belt candidates at 870 μm, as part of a large programme with the LABOCA bolometer at the APEX telescope. Dust masses (or upper limits) were calculated from integrated 870 μm fluxes, and fits to the SED of detected sources revealed the fractional dust luminosities fdust, dust temperatures Tdust, and power-law exponents β of the opacity law. Results. A total of 10 detections with at least 3σ significance were made, out of which five (HD 95086, HD 131835, HD 161868, HD 170773, and HD 207129) have previously never been detected at submillimetre wavelengths. Three additional sources are marginally detected with >2.5σ significance. The best-fit β parameters all lie between 0.1 and 0.8, in agreement with previous results indicating the presence of significantly larger grains than those in the ISM. From our relatively small sample we estimate fdust ∝t −α , with α ∼ 0.8−2.0, and identify an evolution of the characteristic radial dust distance Rdust that is consistent with the t 1/3 increase

Herschel mission overview and key programmes

Herschel is the next astronomy mission in the European Space Agency (ESA) science programme. It is currently in the final stages of assembly and verification in ESAs ESTEC facility in Noordwijk, and is scheduled to be flown to the launch site at Europes Spaceport in Kourou later this year. Herschel will carry a 3.5 metre diameter passively cooled Cassegrain telescope which is the largest of its kind and utilises novel silicon carbide technology. The science payload comprises three instruments: two direct detection cameras/medium resolution spectrometers, PACS and SPIRE, and a very high resolution heterodyne spectrometer, HIFI. The focal plane units are housed inside a superfluid helium cryostat based on ISO legacy. Herschel will be launched by an Ariane 5 ECA together with the Planck satellite into a transfer trajectory towards the operational orbit around L2. When operational Herschel will provide unprecedented observational opportunities in the 55-672 μm spectral range, much of which has never before been accessible from a space observatory. It is an observatory facility available to the worldwide astronomical community, nominally almost 20,000 hours will be available for astronomy, 32% is guaranteed time and the remainder is open to the general astronomical community through a standard competitive proposal procedure. The initial Key Programme Announcement of Opportunity (AO) was issued in Feb 2007. Both the guaranteed and open time Key Programmes have been selected and are introduced, and future observing opportunities are outlined.

Herschel mission: status and observing opportunities

Herschel is the fourth cornerstone mission in the European Space Agency (ESA) science programme. It will perform imaging photometry and spectroscopy in the far infrared and submillimetre part of the spectrum, covering approximately the 57-670 μm range. The key science objectives emphasize current questions connected to the formation of galaxies and stars, however, having unique capabilities in several ways, Herschel will be a facility available to the entire astronomical community. Herschel will be equipped with a 3.5 metre diameter passively cooled telescope. The science payload complement - two cameras/medium resolution spectrometers (PACS and SPIRE) and a very high resolution heterodyne spectrometer (HIFI) - will be housed in a superfluid helium cryostat. The ground segment will be jointly developed by the ESA, the three instrument teams, and NASA/IPAC. Herschel is scheduled to be launched into a transfer trajectory towards its operational orbit around the Earth-Sun L2 point by an Ariane 5 (shared with the ESA cosmic background mapping mission Planck) in 2007. Once operational Herschel will offer a minimum of 3 years of routine observations; roughly 2/3 of the available observing time is open to the general astronomical community through a standard competitive proposal procedure.

The Herschel and Planck Space Telescopes

The European Space Agency launches in 2009 two flagship missions in the domain of submillimeter space astronomy. The Herschel Space Observatory is a common user facility featuring a 3.5 m aperture Cassegrain telescope passively cooled to 80 K. The Planck survey mission includes a 1.5 m unobscured aperture off-axis aplanatic telescope passively cooled to 40 K. Herschel will make pointed target observations of astrophysical objects and phenomena in the frequency range 448 GHz to 5.3 THz (or 672 to 55 mum wavelength). Planck, on the other hand, will map the entire sky by strip scanning at a spin rate of one revolution per minute covering a frequency bandwidth of 30-857 GHz (or wavelength from 10 to 0.35 mm). Its spin axis is pointed antisunward and can be oriented within a 10deg cone around that direction. The telescope line of sight is fixed at an angle of 85deg to the spacecraft spin-axis. This paper describes briefly the specific telescopes developed for each mission; their design characteristics, the development process for each, their achieved performances (from on ground testing) and their expected performances in flight.

Herschel view of the large-scale structure in the Chamaeleon dark clouds

The Chamaeleon molecular cloud complex is one of the nearest star-forming sites encompassing three molecular clouds with a different star-formation history, from quiescent (Cha III) to actively forming stars (Cha II), and reaching the end of star-formation (Cha I). To charactize its large-scale structure, we derived column density and temperature maps using PACS and SPIRE observations from the Herschel Gould Belt Survey, and applied several tools, such as filament tracing, power-spectra, \Delta-variance, and probability distribution functions of column density (PDFs), to derive physical properties. The column density maps reveal a different morphological appearance for the three clouds, with a ridge-like structure for Cha I, a clump-dominated regime for Cha II, and an intricate filamentary network for Cha III. The filament width is measured to be around 0.12\pm0.04 pc in the three clouds, and the filaments found to be gravitationally unstable in Cha I and II, but mostly subcritical in Cha III. Faint filaments (striations) are prominent in Cha I showing a preferred alignment with the large-scale magnetic field. The PDFs of all regions show a lognormal distribution at low column densities. For higher densities, the PDF of Cha I shows a turnover indicative of an extended higher density component, culminating with a power-law tail. Cha II shows a power-law tail with a slope characteristic of gravity. The PDF of Cha III can be best fit by a single lognormal. The turbulence properties of the three regions are found to be similar, pointing towards a scenario where the clouds are impacted by large-scale processes. The magnetic field could possibly play an important role for the star-formation efficiency in the Chamaeleon clouds if proven that it can effectively channel material on Cha I, and possibly Cha II, but probably less efficiently on the quiescent Cha III cloud.

A submillimetre search for cold extended debris disks in the β Pictoris moving group

Context: Previous observations with the Infrared Astronomical Satellite and the Infrared Space Observatory, and ongoing observations with Spitzer and AKARI, have led to the discovery of over 200 debris disks, based on detected mid- and far infrared excess emission, indicating warm circumstellar dust. To constrain the properties of these systems, e.g., to more accurately determine the dust mass, temperature and radial extent, follow-up observations in the submillimetre wavelength region are needed. Aims: The β Pictoris moving group is a nearby stellar association of young ( 12 Myr) co-moving stars including the classical debris disk star β Pictoris. Due to their proximity and youth, they are excellent targets when searching for submillimetre emission from cold, extended, dust components produced by collisions in Kuiper-Belt-like disks. They also allow an age independent study of debris disk properties as a function of other stellar parameters. Methods: We observed 7 infrared-excess stars in the β Pictoris moving group with the LABOCA bolometer array, operating at a central wavelength of 870 μm at the 12-m submillimetre telescope APEX. The main emission at these wavelengths comes from large, cold dust grains, which constitute the main part of the total dust mass, and hence, for an optically thin case, make better estimates on the total dust mass than earlier infrared observations. Fitting the spectral energy distribution with combined optical and infrared photometry gives information on the temperature and radial extent of the disk. Results: From our sample, β Pic, HD 181327, and HD 172555 were detected with at least 3σ certainty, while all others are below 2σ and considered non-detections. The image of β Pic shows an offset flux density peak located near the south-west extension of the disk, similar to the one previously found by SCUBA at the JCMT. We present SED fits for detected sources and give an upper limit on the dust mass for undetected ones. Conclusions: We find a mean fractional dust luminosity bar{f}_dust = 1.1 × 10-3 at t ≈ 12 Myr, which together with recent data at 100 Myr suggests an f_dust ∝ t-α decline of the emitting dust, with α > 0.8. Based on observations with APEX, Llano Chajnantor, Chile (ESO programmes 079.F-9307(A) and 079.F-9308(A)).

The pointing system of the Herschel space observatory

We present the activities carried out to calibrate and characterise the performance of the elements of attitude control and measurement on board the Herschel spacecraft. The main calibration parameters and the evolution of the indicators of the pointing performance are described, from the initial values derived from the observations carried out in the performance verification phase to those attained in the last year and half of mission, an absolute pointing error around or even below 1 arcsec, a spatial relative pointing error of some 1 arcsec and a pointing stability below 0.2 arsec. The actions carried out at the ground segment to improve the spacecraft pointing measurements are outlined. On-going and future developments towards a final refinement of the Herschel astrometry are also summarised. A brief description of the different components of the attitude control and measurement system (both in the space and in the ground segments) is also given for reference. We stress the importance of the cooperation between the different actors (scientists, flight dynamics and systems engineers, attitude control and measurement hardware designers, star-tracker manufacturers, etc.) to attain the final level of performance.

Herschel far-IR observations of the Chamaeleon molecular cloud complex . Chamaeleon I: A first view of young stellar objects in the cloud

Context. The Herschel Gould Belt survey of nearby star forming regions is providing great insights into the early stages of the formation and the evolution of stars and their circumstellar disks. The Chamaeleon I dark cloud is an elongated region of dense dust and gas where star formation is ongoing in two centres, a northern region centred on Ced 112 and a southern cluster subdivided into the two regions Ced 110 and 111. Aims. In this initial study we present Herschel data of previously identified young stellar objects (YSOs) in the cluster, focusing on the spatial distribution of the YSOs and the determination of the relative colours of the protostars and the disk-bearing stars in Chamaeleon I. Methods. Chamaeleon I has been observed as part of the Herschel Gould Belt Survey, using the PACS and SPIRE parallel mode imaging at 70, 160, 250, 350, and 500 μm. Source extraction was performed using the getsources software. Results. We have detected 397 sources over the five available PACS and SPIRE bands, and through comparison with previously identified objects in the cluster we have identified 49 YSOs, 4 bright nebular emission features, five CO clumps, and twenty-eight candidate prestellar or starless cores in the Herschel sample. The remaining sample consists of candidate prestellar cores, condensations within the cloud, or background galaxies. The Herschel detected YSOs are highly clustered and mainly associated with the three known Cederblad groups. The observations clearly show that the young stars are forming coincident with the denser regions of cold dust and gas, visible at longer Herschel wavelengths. Those YSOs detected with Herschel were found to have an IRAC m3.6 μm < 10. No difference was found in the Herschel colours between the class I and class II young stars, however the class I sources were brighter than the class II at most Herschel wavelengths. One class III star and three transition disks are detected.

q1 Eridani : a solar-type star with a planet and a dust belt

Context. Far-infrared excess emission from main-sequence stars is due to dust produced by orbiting minor bodies. In these disks, larger bodies, such as planets, may also be present and the understa ...

The Herschel Gould Belt Survey in Chamaeleon II - Properties of cold dust in disks around young stellar objects

Context. We report on the Herschel Gould Belt survey (HGBS) of the Chamaeleon II (Cha II) star-forming region, focusing on the detection of Class I to III young stellar objects (YSOs). Aims. We aim at characterizing the circumstellar material around these YSOs and at understanding which disk parameters are most likely constrained by the new HGBS data, which are expected to be crucial for studying the transition from optically thick disks to evolved debris-type disks. Methods. We recovered 29 of the 63 known YSOs in Cha II with a detection in at least one of the PACS/SPIRE pass-bands: 3 Class I YSOs (i.e.,100%), 1 flat source (i.e., 50%), 21 Class II objects (i.e., 55%), 3 Class III objects (i.e, 16%), and the unclassified far-infrared source IRAS 12522-7640. We explored PACS/SPIRE colors of this sample and modeled their spectral energy distributions (SEDs) from the optical to Herschel’s wavelengths with the RADMC-2D radiative transfer code. Results. We find that YSO colors are typically confined to the following ranges: −0.7 ≲ log (F_(70)/F_(160)) ≲ 0.5, −0.5 ≲ log (F_(160)/F_(250)) ≲ 0.6, 0.05 ≲ log (F_(250)/F_(350)) ≲ 0.25 and −0.1 ≲ log (F_(350)/F_(500)) ≲ 0.5. These color ranges are expected to be only marginally contaminated by extragalactic sources and field stars and, hence, provide a useful YSO selection tool when applied together. We were able to model the SED of 26 of the 29 detected YSOs. We discuss the degeneracy/limitations of our SED fitting results and adopted the Bayesian method to estimate the probability of different values for the derived disk parameters. The Cha II YSOs present typical disk inner radii ≲0.1 AU, as previously estimated in the literature on the basis of Spitzer data. Our probability analysis shows that, thanks to the new Herschel data, the lower limits to the disk mass (M_(disk)) and characteristic radius (R_C) are well constrained, while the flaring angle (1 + φ) is only marginally constrained. The lower limit to R_C is typically around 50 AU. The lower limits to M_(disk) are proportional to the stellar masses with a typical 0.3% ratio, i.e., in the range estimated in the literature for young Class II stars and brown dwarfs across a broad range of stellar masses. The estimated flaring angles, although very uncertain, point toward very flat disks (1 + φ ≲ 1.2), as found for low-mass M-type YSO samples in other star-forming regions. Thus, our results support the idea that disk properties show a dependence on stellar properties.