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J. A. D. L. Blommaert

Publications and source records attributed to J. A. D. L. Blommaert.

At least 55 records · Page 3Linked to original sources
arXiv2011

A far-infrared survey of bow shocks and detached shells around AGB stars and red supergiants

Far-infrared Herschel/PACS images at 70 and 160 micron of a sample of 78 Galactic evolved stars are used to study the (dust) emission structures, originating from stellar wind-ISM interaction. In addition, two-fluid hydrodynamical simulations of the coupled gas and dust in wind-ISM interactions are used to compare with the observations. Four distinct classes of wind-ISM interaction (i.e. "fermata", "eyes", "irregular", and "rings") are identified and basic parameters affecting the morphology are discussed. We detect bow shocks for ~40% of the sample and detached rings for ~20%. De-projected stand-off distances (R_0) -- defined as the distance between the central star and the nearest point of the interaction region -- of the detected bow shocks ("fermata" and "eyes") are derived from the PACS images and compared to previous results, model predictions and the simulations. All observed bow shocks have stand-off distances smaller than 1 pc. Observed and theoretical stand-off distances are used together to independently derive the local ISM density. Both theoretical (analytical) models and hydrodynamical simulations give stand-off distances for adopted stellar properties that are in good agreement with the measured de-projected stand-off distance of wind-ISM bow shocks. The possible detection of a bow shock -- for the distance limited sample -- appears to be governed by its physical size as set roughly by the stand-off distance. In particular the star's peculiar space velocity and the density of the ISM appear decisive in detecting emission from bow shocks or detached rings. Tentatively, the "eyes" class objects are associated to (visual) binaries, while the "rings" generally appear not to occur for M-type stars, only for C or S-type objects that have experienced a thermal pulse.

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arXiv2011

X Her and TX Psc: Two cases of ISM interaction with stellar winds observed by Herschel

The asymptotic giant branch (AGB) stars X Her and TX Psc have been imaged at 70 and 160 microns with the PACS instrument onboard the Herschel satellite, as part of the large MESS (Mass loss of Evolved StarS) Guaranteed Time Key Program. The images reveal an axisymmetric extended structure with its axis oriented along the space motion of the stars. This extended structure is very likely to be shaped by the interaction of the wind ejected by the AGB star with the surrounding interstellar medium (ISM). As predicted by numerical simulations, the detailed structure of the wind-ISM interface depends upon the relative velocity between star+wind and the ISM, which is large for these two stars (108 and 55 km/s for X Her and TX Psc, respectively). In both cases, there is a compact blob upstream whose origin is not fully elucidated, but that could be the signature of some instability in the wind-ISM shock. Deconvolved images of X Her and TX Psc reveal several discrete structures along the outermost filaments, which could be Kelvin-Helmholtz vortices. Finally, TX Psc is surrounded by an almost circular ring (the signature of the termination shock?) that contrasts with the outer, more structured filaments. A similar inner circular structure seems to be present in X Her as well, albeit less clearly.

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arXiv2011

Herschel's view into Mira's head

Herschel's PACS instrument observed the environment of the binary system Mira Ceti in the 70 and 160 micron bands. These images reveal bright structures shaped as five broken arcs and fainter filaments in the ejected material of Mira's primary star. The overall shape of the IR emission around Mira deviates significantly from the expected alignment with Mira's exceptionally high space velocity. The observed broken arcs are neither connected to each other nor are they of a circular shape; they stretch over angular ranges of 80 to 100 degrees. By comparing Herschel and GALEX data, we found evidence for the disruption of the IR arcs by the fast outflow visible in both Halpha and the far UV. Radial intensity profiles are derived, which place the arcs at distances of 6-85" (550 - 8000 AU) from the binary. Mira's IR environment appears to be shaped by the complex interaction of Mira's wind with its companion, the bipolar jet, and the ISM.

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arXiv2011

Warm water vapour in the sooty outflow from a luminous carbon star

In 2001, the discovery of circumstellar water vapour around the ageing carbon star IRC+10216 was announced. This detection challenged the current understanding of chemistry in old stars, since water vapour was predicted to be absent in carbon-rich stars. Several explanations for the occurrence of water vapour 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. However, the only water line detected so far from one carbon-rich evolved star can not discriminate, by itself, between the different mechanisms proposed. Here we report on the detection by the Herschel satellite of dozens of water vapour lines in the far-infrared and sub-millimetre spectrum of IRC+10216, including some high-excitation lines with energies corresponding to ~1000 K. The emission of these high-excitation water lines can only be explained if water vapour is present in the warm inner region of the envelope. A plausible explanation for the formation of warm water vapour appears to be the penetration of ultraviolet (UV) photons deep into a clumpy circumstellar envelope. This mechanism triggers also the formation of other molecules such as ammonia, whose observed abundances are much higher than hitherto predicted.

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arXiv2010

MESS (Mass-loss of Evolved StarS), a Herschel Key Program

MESS (Mass-loss of Evolved StarS) is a Guaranteed Time Key Program that uses the PACS and SPIRE instruments on board the Herschel Space Observatory to observe a representative sample of evolved stars, that include asymptotic giant branch (AGB) and post-AGB stars, planetary nebulae and red supergiants, as well as luminous blue variables, Wolf-Rayet stars and supernova remnants. In total, of order 150 objects are observed in imaging and about 50 objects in spectroscopy. This paper describes the target selection and target list, and the observing strategy. Key science projects are described, and illustrated using results obtained during Herschel's science demonstration phase. Aperture photometry is given for the 70 AGB and post-AGB stars observed up to October 17, 2010, which constitutes the largest single uniform database of far-IR and sub-mm fluxes for late-type stars.

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arXiv2010

PACS spectroscopy of OH/IR stars

Observations of high-excitation molecular emission lines can greatly increase our understanding of AGB winds, as they trace the innermost regions of the circumstellar envelope. The PACS spectrometer on-board the Herschel Space Telescope, provides for the first time the spectral resolution and sensitivity necessary to trace these lines. We report on the first modelling efforts of a PACS spectral scan for the OH/IR star V669 Cas. Central to our methodology is the consistent treatment of both dust and gas by using a line radiative transfer and a continuum radiative transfer code conjointly. Water emission lines are found to be extremely sensitive to the dust-to-gas ratio, emphasizing the need of consistent modelling for dust and gas.

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arXiv2010

Determining the forsterite abundance of the dust around AGB stars

We present a diagnostic tool to determine the forsterite abundance of the dust ejected by AGB stars. Our method is based on a comparison between the observed strength of spectral bands of forsterite and model calculations. We show that the 11.3 μm forsterite band is a robust indicator of the forsterite abundance of the current mass-loss period for AGB stars with an optically thick dust shell. The 33.6 μm band of forsterite is sensitive to changes in the density and the geometry of the emitting dust shell, and so a less robust indicator. We apply this method to six high mass-loss rate AGB stars, showing that AGB stars can have forsterite abundances of 12% by mass and higher, which is more than the previously found maximum abundance of 5%.

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arXiv2010

Imaging Planetary Nebulae with Herschel-PACS and SPIRE

In this paper we will discuss the images of Planetary Nebulae that have recently been obtained with PACS and SPIRE on board the Herschel satellite. This comprises results for NGC 650 (the little Dumbbell nebula), NGC 6853 (the Dumbbell nebula), and NGC 7293 (the Helix nebula).

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arXiv2010

First results on Martian carbon monoxide from Herschel/HIFI observations

We report on the initial analysis of Herschel/HIFI carbon monoxide (CO) observations of the Martian atmosphere performed between 11 and 16 April 2010. We selected the (7-6) rotational transitions of the isotopes ^{13}CO at 771 GHz and C^{18}O at 768 GHz in order to retrieve the mean vertical profile of temperature and the mean volume mixing ratio of carbon monoxide. The derived temperature profile agrees within less than 5 K with general circulation model (GCM) predictions up to an altitude of 45 km, however, show about 12-15 K lower values at 60 km. The CO mixing ratio was determined as 980 \pm 150 ppm, in agreement with the 900 ppm derived from Herschel/SPIRE observations in November 2009.

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arXiv2010

Herschel/HIFI observations of Mars: first detection of O_2 at submillimetre wavelengths and upper limits on HCl and H_2O_2

We report on an initial analysis of Herschel/HIFI observations of hydrogen chloride (HCl), hydrogen peroxide (H_2O_2), and molecular oxygen (O_2) in the Martian atmosphere performed on 13 and 16 April 2010 (L_s ~ 77°). We derived a constant volume mixing ratio of 1400 +/- 120 ppm for O_2 and determined upper limits of 200 ppt for HCl and 2 ppb for H_2O_2. Radiative transfer model calculations indicate that the vertical profile of O_2 may not be constant. Photochemical models determine the lowest values of H_2O_2 to be around L_s ~ 75° but overestimate the volume mixing ratio compared to our measurements.

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arXiv2010

Water production in comet 81P/Wild 2 as determined by Herschel/HIFI

The high spectral resolution and sensitivity of Herschel/HIFI allows for the detection of multiple rotational water lines and accurate determinations of water production rates in comets. In this letter we present HIFI observations of the fundamental 110-101 (557 GHz) ortho and 111-000 (1113 GHz) para rotational transitions of water in comet 81P/Wild 2 acquired in February 2010. We mapped the extent of the water line emission with five point scans. Line profiles are computed using excitation models which include excitation by collisions with electrons and neutrals and solar infrared radiation. We derive a mean water production rate of $1.0 \times 10^{28}$ molecules s$^{-1}$ at a heliocentric distance of 1.61 AU about 20 days before perihelion, in agreement with production rates measured from the ground using observations of the 18-cm OH lines. Furthermore, we constrain the electron density profile and gas kinetic temperature, and estimate the coma expansion velocity by fitting the water line shapes.

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arXiv2010

First results of Herschel/PACS observations of Neptune

We report on the initial analysis of a Herschel/PACS full range spectrum of Neptune, covering the 51-220 micrometer range with a mean resolving power of ~ 3000, and complemented by a dedicated observation of CH4 at 120 micrometers. Numerous spectral features due to HD (R(0) and R(1)), H2O, CH4, and CO are present, but so far no new species have been found. Our results indicate that (i) Neptune's mean thermal profile is warmer by ~ 3 K than inferred from the Voyager radio-occultation; (ii) the D/H mixing ratio is (4.5+/-1) X 10**-5, confirming the enrichment of Neptune in deuterium over the protosolar value (~ 2.1 X 10**-5); (iii) the CH4 mixing ratio in the mid stratosphere is (1.5+/-0.2) X 10**-3, and CH4 appears to decrease in the lower stratosphere at a rate consistent with local saturation, in agreement with the scenario of CH4 stratospheric injection from Neptune's warm south polar region; (iv) the H2O stratospheric column is (2.1+/-0.5) X 10**14 cm-2 but its vertical distribution is still to be determined, so the H2O external flux remains uncertain by over an order of magnitude; and (v) the CO stratospheric abundance is about twice the tropospheric value, confirming the dual origin of CO suspected from ground-based millimeter/submillimeter observations.

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arXiv2010

Silicon in the dust formation zone of IRC +10216 as observed with PACS and SPIRE on board Herschel

The interstellar medium is enriched primarily by matter ejected from evolved low and intermediate mass stars. The outflows from these stars create a circumstellar envelope in which a rich gas-phase and dust-nucleation chemistry takes place. We observed the nearest carbon-rich evolved star, IRC+10216, using the PACS (55-210 μm) and SPIRE (194-672 μm) spectrometers on board Herschel. We find several tens of lines from SiS and SiO, including lines from the v=1 vibrational level. For SiS these transitions range up to J=124-123, corresponding to energies around 6700K, while the highest detectable transition is J=90-89 for SiO, which corresponds to an energy around 8400K. Both species trace the dust formation zone of IRC+10216, and the broad energy ranges involved in their detected transitions permit us to derive the physical properties of the gas and the particular zone in which each species has been formed. This allows us to check the accuracy of chemical thermodynamical equilibrium models and the suggested depletion of SiS and SiO due to accretion onto dust grains.

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arXiv2010

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 ~ 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.

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arXiv2010

Detection of anhydrous hydrochloric acid, HCl, in IRC+10216 with the Herschel SPIRE and PACS spectrometers

We report on the detection of anhydrous hydrochloric acid (hydrogen chlorine, HCl) in the carbon-rich star IRC+10216 using the spectroscopic facilities onboard the Herschel satellite. Lines from J=1-0 up to J=7-6 have been detected. From the observed intensities, we conclude that HCl is produced in the innermost layers of the circumstellar envelope with an abundance relative to H2 of 5x10^-8 and extends until the molecules reach its photodissociation zone. Upper limits to the column densities of AlH, MgH, CaH, CuH, KH, NaH, FeH, and other diatomic hydrides have also been obtained.

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arXiv2010

The β Pictoris disk imaged by Herschel PACS and SPIRE

We obtained Herschel PACS and SPIRE images of the thermal emission of the debris disk around the A5V star β Pic. The disk is well resolved in the PACS filters at 70, 100, and 160 μm. The surface brightness profiles between 70 and 160 μm show no significant asymmetries along the disk, and are compatible with 90% of the emission between 70 and 160 μm originating in a region closer than 200 AU to the star. Although only marginally resolving the debris disk, the maps obtained in the SPIRE 250 - 500 μm filters provide full-disk photometry, completing the SED over a few octaves in wavelength that had been previously inaccessible. The small far-infrared spectral index (β = 0.34) indicates that the grain size distribution in the inner disk (<200AU) is inconsistent with a local collisional equilibrium. The size distribution is either modified by non-equilibrium effects, or exhibits a wavy pattern, caused by an under-abundance of impactors which have been removed by radiation pressure.

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arXiv2010

The Vega Debris Disc: A view from Herschel

We present five band imaging of the Vega debris disc obtained using the Herschel Space Observatory. These data span a wavelength range of 70-500 um with full-width half-maximum angular resolutions of 5.6-36.9". The disc is well resolved in all bands, with the ring structure visible at 70 and 160 um. Radial profiles of the disc surface brightness are produced, and a disc radius of 11" (~ 85 AU) is determined. The disc is seen to have a smooth structure thoughout the entire wavelength range, suggesting that the disc is in a steady state, rather than being an ephemeral structure caused by the recent collision of two large planetesimals.

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