|
|
| Herschel images of Fomalhaut - An extrasolar Kuiper belt at the height of its dynamical activity |
Context. Fomalhaut is a young (2 ± 1 × 108 years), nearby (7.7 pc), 2 MSun star that is suspected to harbor an infant planetary system, interspersed with one or more belts of dusty debris.
Aims. We present far-infrared images obtained with the Herschel Space Observatory with an angular resolution between 5.7" and 36.7" at wavelengths between 70 um and 500 um. The images show the main debris belt in great detail. Even at high spatial resolution, the belt appears smooth. The region in between the belt and the central star is not devoid of material; thermal emission is observed here as well. Also at the location of the star, excess emission is detected. We aim to construct a consistent image of the Fomalhaut system.
Methods. We use a dynamical model together with radiative-transfer tools to derive the parameters of the debris disk. We include detailed models of the interaction of the dust grains with radiation, for both the radiation pressure and the temperature determination. Comparing these models to the spatially resolved temperature information contained in the images allows us to place strong constraints on the presence of grains that will be blown out of the system by radiation pressure. We use this to derive the dynamical parameters of the system.
Results. The appearance of the belt points toward a remarkably active system in which dust grains are produced at a very high rate by a collisional cascade in a narrow region filled with dynamically excited planetesimals. Dust particles with sizes below the blow-out size are abundantly present.
- The remainder of the abstract is truncated - |
| Publication date: 11 Apr 2012 |
|
|
| The gas-to-dust mass ratio of Centaurus A as seen by Herschel |
We present photometry of the nearby galaxy NGC 5128 (Centaurus A) observed with the Photodetector Array Camera and Spectrometer (PACS) and Spectral and Photometric Imaging Receiver (SPIRE) instruments on board the Herschel Space Observatory, at 70, 160, 250, 350 and 500 micron, as well as new CO J=3-2 observations taken with the HARP-B instrument on the James Clerk Maxwell Telescope (JCMT). Using a single-component modified blackbody, we model the dust spectral energy distribution within the disc of the galaxy using all five Herschel wavebands and find dust temperatures of ~30 K towards the centre of the disc and a smoothly decreasing trend to ~20 K with increasing radius. We find a total dust mass of (1.59 ± 0.05) × 107 MSun and a total gas mass of (2.7 ± 0.2) × 109 MSun. The average gas-to-dust mass ratio is 103 ± 8, but we find an interesting increase in this ratio to approximately 275 towards the centre of Cen A. We discuss several possible physical processes that may be causing this effect, including dust sputtering, jet entrainment and systematic variables such as the XCO factor. Dust sputtering by X-rays originating in the active galactic nucleus or the removal of dust by the jets is our most favoured explanation.
Published online on 27 March 2012, to appear in forthcoming issue of MNRAS |
| Publication date: 27 Mar 2012 |
|
|
| Herschel observations of Cen A: stellar heating of two extragalactic dust clouds |
| We present the first results of a multiwavelength survey, incorporating Herschel-inline image, Spitzer, GALEX and Australian Telescope Compact Array (ATCA) observations, of a 1° × 1° field centred on Centaurus A. As well as detecting the inner lobes of the active galactic nucleus (AGN) jet and counterjet, we have found two clouds, bright at submillimetre wavelengths, ~15 kpc from the centre of Cen A that are co-aligned with the jets. Flux measurements at Herschel wavelengths have proved vital in constraining fits to the spectral energy distributions (SEDs). The clouds are well fitted by a single-temperature, modified blackbody spectrum (beta=2) indicating that we are looking at two cold dust clouds on the outskirts of Cen A. The temperature and masses of the clouds are T_north=12.6(+1.1, -1.2) K, T_south=15.1(+1.7, -1.6) K; log(M_north/M_Sun)=5.8(+0.2, -0.2); log(M_south/M_Sun)=5.6(+0.2, -0.2) and the gas-dust ratio for both clouds is ~100. The measured values for the northern dust cloud are consistent with previous measurements from ISO while the southern cloud is a new submillimetre detection. The two dust clouds are located at the termini of the partial HI ring that surrounds Cen A which is also where the gas column density peaks. The Herschel survey encompasses the partial HI ring yet we find no evidence of dust emission in any other part of the ring. Assuming that the gas-dust ratio is the same in the rest of the ring, dust mass upper limits in the HI ring are consistent with low column density dust being present but falling below the SPIRE detection limit. We have discussed the origin of these clouds and various possible heating mechanisms. The observations favour a scenario in which the gas and dust were once part of a late-type galaxy, which has since merged with Cen A. The dominant heating mechanism which adequately explains the observed temperatures in both clouds is heating from the evolved stellar population within Cen A. |
| Publication date: 15 Mar 2012 |
|
|
| The Herschel view of the on-going star formation in the Vela-C molecular cloud |
Aims: As part of the Herschel guaranteed time key programme "HOBYS", we present the PACS and SPIRE photometric survey of the star-forming region Vela-C, one of the nearest sites of low-to-high-mass star formation in the Galactic plane. Our main objectives are to take a census of the cold sources and to derive their mass distribution down to a few solar masses.
Methods: Vela-C was observed with PACS and SPIRE in parallel mode at five wavelengths between 70 micron and 500 micron over an area of about 3 square degrees. A photometric catalogue was extracted from the detections in each of the five bands, using a threshold of 5sigma over the local background. Out of this catalogue we selected a robust sub-sample of 268 sources, of which ~75 per cent are cloud clumps (diameter between 0.05 pc and 0.13 pc) and 25 per cent are cores (diameter between 0.025 pc and 0.05 pc). Their spectral energy distributions (SEDs) were fitted with a modified black body function. We classify 48 sources as protostellar, based on their detection at 70 um or at shorther wavelengths, and 218 as starless, because of non-detections at 70 micron. For two other sources, we do not provide a secure classification, but suggest they are Class 0 protostars.
Results: From the SED fitting we derived key physical parameters (i.e. mass, temperature, bolometric luminosity). Protostellar sources are in general warmer (< T > = 12.8 K) and more compact (< diameter > = 0.040 pc) than starless sources (< T > = 10.3 K, < diameter > = 0.067 pc). Both these findings can be ascribed to the presence of an internal source(s) of moderate heating, which also causes a temperature gradient and hence a more peaked intensity distribution. Moreover, the reduced dimensions of protostellar sources may indicate that they will not fragment further.
The remainder of the abstract is truncated. |
| Publication date: 12 Mar 2012 |
|
|
| First submillimetre detection of the TWA brown dwarf disc 2MASSW J1207334-393254 |
| We present Herschel/SPIRE observations for the 2MASS 1207334-393254 (2M1207) system. Based on radiative transfer modelling of near-infrared to submillimetre data, we estimate a disc mass of 3 ± 2 MJup and an outer disc radius of 50-100 AU for the 2M1207A disc. The relative disc mass for 2M1207A is similar to the T Tauri star TW Hya, which indicates that massive discs are not underabundant around substellar objects. In probing the various formation mechanisms for this system, we find that core accretion is highly uncertain mainly due to the large separation between the primary and the companion. Disc fragmentation could be a likely scenario based on analytical models, and if the disc initially was more massive than its current estimate. Considering that the TW Hydrae Association (TWA) is sparsely populated, this system could have formed via one of the known binary formation mechanisms (e.g. turbulent fragmentation of a core) and survived disruption at an early stage. |
| Publication date: 21 Feb 2012 |
|
|
| Detection of the Water Reservoir in a Forming Planetary System |
| Icy bodies may have delivered the oceans to the early Earth, yet little is known about water in the ice-dominated regions of extrasolar planet-forming disks. The Heterodyne Instrument for the Far-Infrared on board the Herschel Space Observatory has detected emission lines from both spin isomers of cold water vapor from the disk around the young star TW Hydrae. This water vapor likely originates from ice-coated solids near the disk surface, hinting at a water ice reservoir equivalent to several thousand Earth Oceans in mass. The water's ortho-to-para ratio falls well below that of solar system comets, suggesting that comets contain heterogeneous ice mixtures collected across the entire solar nebula during the early stages of planetary birth. |
| Publication date: 21 Oct 2011 |
|
|
| Ocean-like water in the Jupiter-family comet 103P/Hartley 2 |
Published online on 5 October 2011.
For decades, the source of Earth's volatiles, especially water with a deuterium-to-hydrogen ratio (D/H) of (1.558±0.001)×10-4, has been a subject of debate. The similarity of Earth's bulk composition to that of meteorites known as enstatite chondrites suggests a dry proto-Earth with subsequent delivery of volatiles via local accretion or impacts of asteroids or comets. Previous measurements in six comets from the Oort cloud yielded a mean D/H ratio of (2.96±0.25)×10-4. The D/H value in carbonaceous chondrites, (1.4±0.1)×10-4, together with dynamical simulations, led to models in which asteroids were the main source of Earth's water, with <10 per cent being delivered by comets. Here we report that the D/H ratio in the Jupiter-family comet 103P/Hartley 2, which originated in the Kuiper belt, is (1.61±0.24)×10-4. This result substantially expands the reservoir of Earth ocean-like water to include some comets, and is consistent with the emerging picture of a complex dynamical evolution of the early Solar System. |
| Publication date: 13 Oct 2011 |
|
|
| The lesser role of starbursts for star formation at z=2 |
Published online on 2 Sept 2011.
Two main modes of star formation are know to control the growth of galaxies: a relatively steady one in disk-like galaxies, defining a tight star formation rate (SFR)-stellar mass sequence, and a starburst mode in outliers to such a sequence which is generally interpreted as driven by merging. Such starburst galaxies are rare but have much higher SFRs, and it is of interest to establish the relative importance of these two modes. PACS/Herschel observations over the whole COSMOS and GOODS-South fields, in conjunction with previous optical/near-IR data, have allowed us to accurately quantify for the first time the relative contribution of the two modes to the global SFR density in the redshift interval 1.5 < z < 2.5, i.e., at the cosmic peak of the star formation activity. The logarithmic distributions of galaxy SFRs at fixed stellar mass are well described by Gaussians, with starburst galaxies representing only a relatively minor deviation that becomes apparent for SFRs more than 4 times higher than on the main sequence. Such starburst galaxies represent only 2% of mass-selected star forming galaxies and account for only 10% of the cosmic SFR density at z~2. Only when limited to SFR> 1000MSun/yr, off-sequence sources significantly contribute to the SFR density (46±20%). We conclude that merger-driven starbursts play a relatively minor role for the formation of stars in galaxies, whereas they may represent a critical phase towards the quenching of star formation and morphological transformation in galaxies. |
| Publication date: 01 Oct 2011 |
|
|
| Discovery of multiple dust shells beyond 1 arcmin in the circumstellar envelope of IRC+10216 using Herschel/PACS |
| We present new Herschel/PACS images at 70, 100, and 160 micron of the well-known, nearby, carbon-rich asymptotic giant branch star IRC+10216 revealing multiple dust shells in its circumstellar envelope. For the first time, dust shells (or arcs) are detected until 320". The almost spherical shells are non-concentric and have an angular extent between ~40° and ~200°. The shells have a typical width of 5"-8", and the shell separation varies in the range of ~10"-35", corresponding to ~500 -1700 yr. Local density variations within one arc are visible. The shell/intershell density contrast is typically ~4, and the arcs contain some 50% more dust mass than the smooth envelope. The observed (nested) arcs record the mass-loss history over the past 16 000 yr, but Rayleigh-Taylor and Kelvin-Helmholtz instabilities in the turbulent astropause and astrosheath will erase any signature of the mass-loss history for at least the first 200 000 yr of mass loss. Accounting for the bowshock structure, the envelope mass around IRC+10216 contains >2 solar masses of gas and dust mass. It is argued that the origin of the shells is related to non-isotropic mass-loss events and clumpy dust formation. |
| Publication date: 20 Sep 2011 |
|
|
| GOODS-Herschel: an infrared main sequence for star-forming galaxies |
We present the deepest 100 to 500 micron far-infrared observations obtained with the Herschel Space Observatory as part of the GOODS-Herschel key program, and examine the infrared (IR) 3-500 micron spectral energy distributions (SEDs) of galaxies at 0 < z < 2.5, supplemented by a local reference sample from IRAS, ISO, Spitzer and AKARI data. We determine the projected star formation densities of local galaxies from their radio and mid-IR continuum sizes.
We find that the ratio of total IR luminosity to rest-frame 8 micron luminosity, IR8 (=LtotIR/L8), follows a Gaussian distribution centered on IR8=4 (sigma=1.6) and defines an IR main sequence for star-forming galaxies independent of redshift and luminosity. Outliers from this main sequence produce a tail skewed toward higher values of IR8. This minority population (<20%) is shown to consist of starbursts with compact projected star formation densities. IR8 can be used to separate galaxies with normal and extended modes of star formation from compact starbursts with high-IR8, high projected IR surface brightness (SigmaIR>3×1010 LSunkpc-2) and a high specific star formation rate (i.e., starbursts). The rest-frame, UV-2700 Angstrom size of these distant starbursts is typically half that of main sequence galaxies, supporting the correlation between star formation density and starburst activity that is measured for the local sample.
- The remainder of the abstract is truncated - |
| Publication date: 13 Sep 2011 |
|
|
| Filaments and ridges in Vela C revealed by Herschel: from low-mass to high-mass star-forming sites |
| We present the first Herschel PACS and SPIRE results of the Vela C molecular complex in the far-infrared and submillimetre regimes at 70, 160, 250, 350, and 500 micron, spanning the peak of emission of cold prestellar or protostellar cores. Column density and multi-resolution analysis (MRA) differentiates the Vela C complex into five distinct sub-regions. Each sub-region displays differences in their column density and temperature probability distribution functions (PDFs), in particular, the PDFs of the "Centre-Ridge" and "South-Nest" sub-regions appear in stark contrast to each other. The Centre-Ridge displays a bimodal temperature PDF representative of hot gas surrounding the HII region RCW 36 and the cold neighbouring filaments, whilst the South-Nest is dominated by cold filamentary structure. The column density PDF of the Centre-Ridge is flatter than the South-Nest, with a high column density tail, consistent with formation through large-scale flows, and regulation by self-gravity. At small to intermediate scales MRA indicates the Centre-Ridge to be twice as concentrated as the South-Nest, whilst on larger scales, a greater portion of the gas in the South-Nest is dominated by turbulence than in the Centre-Ridge. In Vela C, high-mass stars appear to be preferentially forming in ridges, i.e., dominant high column density filaments. |
| Publication date: 07 Sep 2011 |
|
|
| Herschel detects a massive dust reservoir in supernova 1987A |
| Published online in Science Express, 7 July 2011.
We report far-infrared and submillimeter observations of Supernova 1987A, the star that exploded on 23 February 1987 in the Large Magellanic Cloud, a galaxy located 160 000 light years away. The observations reveal the presence of a population of cold dust grains radiating with a temperature of about 17 to 23 K at a rate of about 220 LSun. The intensity and spectral energy distribution of the emission suggests a dust mass of about 0.4 to 0.7 MSun. The radiation must originate from the supernova ejecta and requires the efficient precipitation of all refractory material into dust. Our observations imply that supernovae can produce the large dust masses detected in young galaxies at very high redshifts. |
| Publication date: 02 Sep 2011 |
|
|
| Herschel measurements of molecular oxygen in Orion |
| We report observations of three rotational transitions of molecular oxygen (O2) in emission from the H2 Peak 1 position of vibrationally excited molecular hydrogen in Orion. We observed the 487 GHz, 774 GHz, and 1121 GHz lines using HIFI on the Herschel Space Observatory, having velocities of 11 km s-1 to 12 km s-1 and widths of 3 km s-1. The beam-averaged column density is N(O2) = 6.5×1016 cm-2, and assuming that the source has an equal beam filling factor for all transitions (beam widths 44, 28, and 19'), the relative line intensities imply a kinetic temperature between 65 K and 120 K. The fractional abundance of O2 relative to H2 is 0.3-7.3×10-6. The unusual velocity suggests an association with a ~5' diameter source, denoted Peak A, the Western Clump, or MF4. The mass of this source is ~10 MSun and the dust temperature is >= 150 K. Our preferred explanation of the enhanced O2 abundance is that dust grains in this region are sufficiently warm (T >= 100 K) to desorb water ice and thus keep a significant fraction of elemental oxygen in the gas phase, with a significant fraction as O2. For this small source, the line ratios require a temperature >= 180 K. The inferred O2 column density ~5×1018 cm-2 can be produced in Peak A, having N(H2) ~4×1024 cm-2. An alternative mechanism is a low-velocity (10 to 15 km s-1) C-shock, which can produce N(O2) up to 1017 cm-2. |
| Publication date: 01 Aug 2011 |
|
|
| Direct detection of the Enceladus water torus with Herschel |
| Cryovolcanic activity near the south pole of Saturn's moon Enceladus produces plumes of H2O-dominated gases and ice particles, which escape and populate a torus-shaped cloud. Using submillimeter spectroscopy with Herschel, we report the direct detection of the Enceladus water vapor torus in four rotational lines of water at 557, 987, 1113, and 1670 GHz, and probe its physical conditions and structure. We determine line-of-sight H2O column densities of ~4 × 1013 cm-2 near the equatorial plane, with a ~50 000 km vertical scale height. The water torus appears to be rotationally cold (e.g. an excitation temperature of 16 K is measured for the 1113 GHz line) but dynamically excited, with non-Keplerian dispersion velocities of ~2 kms-1, and appears to be largely shaped by molecular collisions. From estimates of the influx rates of torus material into Saturn and Titan, we infer that Enceladus' activity is likely to be the ultimate source of water in the upper atmosphere of Saturn, but not in Titan's. |
| Publication date: 14 Jul 2011 |
|
|
| Massive molecular outflows and negative feedback in ULIRGs observed by Herschel-PACS |
| Mass outflows driven by stars and active galactic nuclei are a key element in many current models of galaxy evolution. They may produce the observed black hole-galaxy mass relation and regulate and quench both star formation in the host galaxy and black hole accretion. However, observational evidence of such feedback processes through outflows of the bulk of the star forming molecular gas
is still scarce. Here we report the detection of massive molecular outflows, traced by the hydroxyl molecule (OH), in far-infrared
spectra of ULIRGs obtained with Herschel-PACS as part of the SHINING key project. In some of these objects the (terminal) outflow velocities exceed 1000 km/s, and their outflow rates (up to ~1200 MSun/yr) are several times larger than their star formation rates. We compare the outflow signatures in different types of ULIRGs and in starburst galaxies to address the issue of the energy source (AGN or starburst) of these outflows. We report preliminary evidence that ULIRGs with a higher AGN luminosity (and higher AGN contribution to LIR) have higher terminal
velocities and shorter gas depletion time scales. The outflows in the observed ULIRGs are able to expel the cold gas reservoirs from the centres of these objects within ~106-108 years. |
| Publication date: 04 May 2011 |
|
|
| Characterizing interstellar filaments with Herschel in IC 5146 |
| We provide a first look at the results of the Herschel Gould Belt survey toward the IC 5146 molecular cloud and present a preliminary analysis of the filamentary structure in this region. The column density map, derived from our 70-500 micron Herschel data, reveals a complex network of filaments and confirms that these filaments are the main birth sites of prestellar cores. We analyze the column density profiles of 27 filaments and show that the underlying radial density profiles fall off as r-1.5 to r-2.5 at large radii. Our main result is that the filaments seem to be characterized by a narrow distribution of widths with a median value of 0.10 ± 0.03 pc, which is in stark contrast to a much broader distribution of central Jeans lengths. This characteristic width of ~0.1 pc corresponds to within a factor of ~2 to the sonic scale below which interstellar turbulence becomes subsonic in diffuse gas, which supports the argument that the filaments may form as a result of the dissipation of large-scale turbulence. |
| Publication date: 13 Apr 2011 |
|
|
| The Detection of a Population of Submillimeter-Bright, Strongly-Lensed Galaxies |
| Gravitational lensing is a powerful astrophysical and cosmological probe and is particularly valuable at submillimeter wavelengths for the study of the statistical and individual properties of dusty star-forming galaxies. However, the identification of gravitational lenses is often time-intensive, involving the sifting of large volumes of imaging or spectroscopic data to find few candidates. We used early data from the Herschel Astrophysical Terahertz Large Area Survey to demonstrate that wide-area submillimeter surveys can simply and easily detect strong gravitational lensing events, with close to 100% efficiency. |
| Publication date: 05 Nov 2010 |
|
|
| Warm water vapour in the sooty outflow from a luminous carbon star |
| The detection of circumstellar water vapour around the ageing carbon star IRC +10216 challenged the current understanding of chemistry in old stars, because water was predicted to be almost absent in carbon-rich stars. Several explanations for the water were postulated, including the vaporization of icy bodies (comets or dwarf planets) in orbit around the star, grain surface reactions, and photochemistry in the outer circumstellar envelope. With a single water line detected so far from this one carbon-rich evolved star, it is difficult to discriminate between the different mechanisms proposed. Here we report the detection of dozens of water vapour lines in the far-infrared and sub-millimetre spectrum of IRC +10216 using the Herschel satellite. This includes some high-excitation lines with energies corresponding to ~1000 K, which can be explained only if water is present in the warm inner sooty region of the envelope. A plausible explanation for the warm water appears to be the penetration of ultraviolet photons deep into a clumpy circumstellar envelope. This mechanism also triggers the formation of other molecules, such as ammonia, whose observed abundances are much higher than hitherto predicted. |
| Publication date: 01 Sep 2010 |
|
|
| From filamentary clouds to prestellar cores to the stellar IMF: Initial highlights from the Herschel Gould Belt Survey |
| We summarize the first results from the Gould Belt Survey, obtained toward the Aquila rift and Polaris Flare regions during the science demonstration phase of Herschel. Our 70-500 micron images taken in parallel mode with the SPIRE and PACS cameras reveal a wealth of filamentary structure, as well as numerous dense cores embedded in the filaments. Between ~350 and 500 prestellar cores and ~45-60 Class 0 protostars can be identified in the Aquila field, while ~300 unbound starless cores and no protostars are observed in the Polaris field. The prestellar core mass function (CMF) derived for the Aquila region bears a strong resemblance to the stellar initial mass function (IMF), already confirming the close connection between the CMF and the IMF with much better statistics than earlier studies. Comparing and contrasting our Herschel results in Aquila and Polaris, we propose an observationally-driven scenario for core formation according to which complex networks of long, thin filaments form first within molecular clouds, and then the densest filaments fragment into a number of prestellar cores via gravitational instability. |
| Publication date: 16 Jul 2010 |
|
|
| Herschel Space Observatory: An ESA facility for far-infrared and submillimetre astronomy |
Herschel was launched on 14 May 2009, and is now an operational ESA space observatory offering unprecedented observational capabilities in the far-infrared and submillimetre spectral range 55-671 mm. Herschel carries a 3.5 m diameter passively cooled Cassegrain telescope, which is the largest of its kind and utilises a 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, whose focal plane units are housed inside a superfluid helium cryostat. Herschel is an observatory facility operated in partnership among ESA, the instrument consortia, and NASA. The mission lifetime is determined by the cryostat hold time. Nominally approximately 20 000 h will be available for astronomy, 32% is guaranteed time and the remainder is open to the worldwide general astronomical community through a standard competitive proposal procedure. |
| Publication date: 16 Jul 2010 |
|
|
