arXiv subjects

J. A. D. L. Blommaert

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

At least 73 records · Page 4Linked to original sources

arXiv2010

HIFI Observations of Water in the Atmosphere of Comet C/2008 Q3 (Garradd)

High-resolution far-infrared and sub-millimetre spectroscopy of water lines is an important tool to understand the physical and chemical properties of cometary atmospheres. We present observations of several rotational ortho- and para-water transitions in comet C/2008 Q3 (Garradd) performed with HIFI on Herschel. These observations have provided the first detection of the 2_{12}-1_{01} (1669 GHz) ortho and 1_{11}-0_{00} (1113 GHz) para transitions of water in a cometary spectrum. In addition, the ground-state transition 1_{10}-1_{01} at 557 GHz is detected and mapped. By detecting several water lines quasi-simultaneously and mapping their emission we can constrain the excitation parameters in the coma. Synthetic line profiles are computed using excitation models which include excitation by collisions, solar infrared radiation, and radiation trapping. We obtain the gas kinetic temperature, constrain the electron density profile, and estimate the coma expansion velocity by analyzing the map and line shapes. We derive water production rates of 1.7-2.8 x 10^{28} s^{-1} over the range r_h = 1.83-1.85 AU.

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arXiv2010

PACS and SPIRE Spectroscopy of the Red Supergiant VY CMa

With a luminosity > 10^5 Lsun and a mass-loss rate of about 2.10-4 Msun/yr, the red supergiant VY CMa truly is a spectacular object. Because of its extreme evolutionary state, it could explode as supernova any time. Studying its circumstellar material, into which the supernova blast will run, provides interesting constraints on supernova explosions and on the rich chemistry taking place in such complex circumstellar envelopes. We have obtained spectroscopy of VYCMa over the full wavelength range offered by the PACS and SPIRE instruments of Herschel, i.e. 55 to 672 micron. The observations show the spectral fingerprints of more than 900 spectral lines, of which more than half belong to water. In total, we have identified 13 different molecules and some of their isotopologues. A first analysis shows that water is abundantly present, with an ortho-to-para ratio as low as 1.3:1, and that chemical non-equilibrium processes determine the abundance fractions in the inner envelope.

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arXiv2010

The detached dust shells of AQ And, U Ant, and TT Cyg

Detached circumstellar dust shells are detected around three carbon variables using Herschel-PACS. Two of them are already known on the basis of their thermal CO emission and two are visible as extensions in IRAS imaging data. By model fits to the new data sets, physical sizes, expansion timescales, dust temperatures, and more are deduced. A comparison with existing molecular CO material shows a high degree of correlation for TT Cyg and U Ant but a few distinct differences with other observables are also found.

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arXiv2010

Herschel PACS and SPIRE imaging of CW Leo

Herschel PACS and SPIRE images have been obtained over a 30'x30' area around the well-known carbon star CW Leo (IRC +10 216). An extended structure is found in an incomplete arc of ~22' diameter, which is cospatial with the termination shock due to interaction with the interstellar medium (ISM) as defined by Sahai & Chronopoulos from ultraviolet GALEX images. Fluxes are derived in the 70, 160, 250, 350, and 550 um bands in the region where the interaction with the ISM takes place, and this can be fitted with a modified black body with a temperature of 25+-3 K. Using the published proper motion and radial velocity for the star, we derive a heliocentric space motion of 25.1 km/s. Using the PACS and SPIRE data and the analytical formula of the bow shock structure, we infer a de-projected standoff distance of the bow shock of R0 = (8.0+-0.3)x10^17 cm. We also derive a relative velocity of the star with respect to the ISM of (106.6+-8.7)/sqrt(n_ISM) km/s, where n_ISM is the number density of the local ISM.

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arXiv2010

Herschel images of NGC 6720: H2 formation on dust grains

Herschel PACS and SPIRE images have been obtained of NGC 6720 (the Ring Nebula). This is an evolved planetary nebula with a central star that is currently on the cooling track, due to which the outer parts of the nebula are recombining. From the PACS and SPIRE images we conclude that there is a striking resemblance between the dust distribution and the H2 emission, which appears to be observational evidence that H2 forms on grain surfaces. We have developed a photoionization model of the nebula with the Cloudy code which we used to determine the physical conditions of the dust and investigate possible formation scenarios for the H2. We conclude that the most plausible scenario is that the H2 resides in high density knots which were formed after the recombination of the gas started when the central star entered the cooling track. Hydrodynamical instabilities due to the unusually low temperature of the recombining gas are proposed as a mechanism for forming the knots. H2 formation in the knots is expected to be substantial after the central star underwent a strong drop in luminosity about one to two thousand years ago, and may still be ongoing at this moment, depending on the density of the knots and the properties of the grains in the knots.

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arXiv2010

A study of the distant activity of comet C/2006 W3 (Christensen) using Herschel and ground-based radio telescopes

Comet C/2006 W3 (Christensen) was observed in November 2009 at 3.3 AU from the Sun with Herschel. The PACS instrument acquired images of the dust coma in 70- and 160-micrometers filters, and spectra covering several H2O rotational lines. Spectra in the range 450-1550 GHz were acquired with SPIRE. The comet emission continuum from 70 to 672 micrometers was measured, but no lines were detected. The spectral energy distribution indicates thermal emission from large particles and provides a measure of the size distribution index and dust production rate. The upper limit to the water production rate is compared to the production rates of other species (CO, CH3OH, HCN, H2S, OH) measured with the IRAM 30-m and Nancay telescopes. The coma is found to be strongly enriched in species more volatile than water, in comparison to comets observed closer to the Sun. The CO to H2O production rate ratio exceeds 220%. The dust to gas production rate ratio is on the order of 1.

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arXiv2010

When an old star smolders: On the detection of hydrocarbon emission from S-type AGB stars

Polycyclic aromatic hydrocarbons (PAHs) produce characteristic infrared emission bands that have been observed in a wide range of astrophysical environments, where carbonaceous material is subjected to ultraviolet (UV) radiation. Although PAHs are expected to form in carbon-rich AGB stars, they have up to now only been observed in binary systems where a hot companion provides a hard radiation field. In this letter, we present low-resolution infrared spectra of four S-type AGB stars, selected from a sample of 90 S-type AGB stars observed with the infrared spectrograph aboard the Spitzer satellite. The spectra of these four stars show the typical infrared features of PAH molecules. We confirm the correlation between the temperature of the central star and the centroid wavelength of the 7.9 μm feature, present in a wide variety of stars spanning a temperature range from 3 000 to 12 000 K. Three of four sources presented in this paper extend this relation towards lower temperatures. We argue that the mixture of hydrocarbons we see in these S-stars has a rich aliphatic component. The fourth star, BZ CMa, deviates from this correlation. Based on the similarity with the evolved binary TU Tau, we predict that BZ CMa has a hot companion as well.

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arXiv2010

Determining the forsterite abundance of the dust around Asymptotic Giant Branch stars

Aims. We present a diagnostic tool to determine the abundance of the crystalline silicate forsterite in AGB stars surrounded by a thick shell of silicate dust. Using six infrared spectra of high mass-loss oxygen rich AGB stars we obtain the forsterite abundance of their dust shells. Methods. We use a monte carlo radiative transfer code to calculate infrared spectra of dust enshrouded AGB stars. We vary the dust composition, mass-loss rate and outer radius. We focus on the strength of the 11.3 and the 33.6 μm forsterite bands, that probe the most recent (11.3 μm) and older (33.6 μm) mass-loss history of the star. Simple diagnostic diagrams are derived, allowing direct comparison to observed band strengths. Results. Our analysis shows that the 11.3 μm forsterite band is a robust indicator for 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. Applying our method to six high mass-loss rate AGB stars shows 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

870 um observations of evolved stars with LABOCA

During their evolution, Asymptotic Giant Branch (AGB) stars experience a high mass-loss which leads to the formation of a Circumstellar Envelope (CSE) of dust and gas. The mass-loss process is the most important phenomenon during this evolutionary stage. In order to understand it, it is important to study the physical parameters of the CSE. The emission of the CSE in the (sub)millimetre range is dominated by the dust continuum. This means that (sub)millimetre observations are a key tool in tracing the dust and improving our knowledge of the mass-loss process. In this paper we analyse new sub-millimetre observations of 9 evolved stars in order to constrain the CSE physical parameters. The data were taken by the new APEX bolometer LABoCa. The fluxes at 870 um are derived and the extended emission is investigated. We compute the spectral energy distribution (SEDs) using a 1D radiative transfer code, DUSTY which we compared to literature data. Grain properties are calculated using both spherical grains distribution and a Continuous Distribution of Ellipsoids (CDE) and a comparison between the two is drawn. Synthetic surface brightness maps have been derived from the modelling and were compared to the LABoCa brightness maps. We detected the presence of extended emission around four stars. Physical parameters of the circumstellar envelope are derived from SED modelling, such as the dust chemical composition, the dust condensation temperature and the total mass of the envelope. It proves difficult however to fit the SED and the intensity profile simultaneously.

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arXiv2009

Structure of the outer layers of cool standard stars

Context: Among late-type red giants, an interesting change occurs in the structure of the outer atmospheric layers as one moves to later spectral types in the Hertzsprung-Russell diagram: a chromosphere is always present, but the coronal emission diminishes and a cool massive wind steps in. Aims: Where most studies have focussed on short-wavelength observations, this article explores the influence of the chromosphere and the wind on long-wavelength photometric measurements. Methods: The observational spectral energy distributions are compared with the theoretical predictions of the MARCS atmosphere models for a sample of 9 K- and M-giants. The discrepancies found are explained using basic models for flux emission originating from a chromosphere or an ionized wind. Results: For 7 out of 9 sample stars, a clear flux excess is detected at (sub)millimeter and/or centimeter wavelengths. The precise start of the excess depends upon the star under consideration. The flux at wavelengths shorter than about 1 mm is most likely dominated by an optically thick chromosphere, where an optically thick ionized wind is the main flux contributor at longer wavelengths. Conclusions: Although the optical to mid-infrared spectrum of the studied K- and M-giants is well represented by a radiative equilibrium atmospheric model, the presence of a chromosphere and/or ionized stellar wind at higher altitudes dominates the spectrum in the (sub)millimeter and centimeter wavelength ranges. The presence of a flux excess also has implications on the role of these stars as fiducial spectrophotometric calibrators in the (sub)millimeter and centimeter wavelength range.

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arXiv2009

Metal-rich carbon stars in the Sagittarius Dwarf Spheroidal galaxy

We present spectroscopic observations from the {\it Spitzer Space Telescope} of six carbon-rich AGB stars in the Sagittarius Dwarf Spheroidal Galaxy (Sgr dSph) and two foreground Galactic carbon stars. The band strengths of the observed C$_2$H$_2$ and SiC features are very similar to those observed in Galactic AGB stars. The metallicities are estimated from an empirical relation between the acetylene optical depth and the strength of the SiC feature. The metallicities are higher than those of the LMC, and close to Galactic values. While the high metallicity could imply an age of around 1 Gyr, for the dusty AGB stars, the pulsation periods suggest ages in excess of 2 or 3 Gyr. We fit the spectra of the observed stars using the DUSTY radiative transfer model and determine their dust mass-loss rates to be in the range 1.0--3.3$\times 10^{-8} $M$_{\odot}$yr$^{-1}$. The two Galactic foreground carbon-rich AGB stars are located at the far side of the solar circle, beyond the Galactic Centre. One of these two stars show the strongest SiC feature in our present Local Group sample.

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arXiv2009

Mid-IR period-magnitude relations for AGB stars

Asymptotic Giant Branch variables are found to obey period-luminosity relations in the mid-IR similar to those seen at K_S (2.14 microns), even at 24 microns where emission from circumstellar dust is expected to be dominant. Their loci in the M, logP diagrams are essentially the same for the LMC and for NGC6522 in spite of different ages and metallicities. There is no systematic trend of slope with wavelength. The offsets of the apparent magnitude vs. logP relations imply a difference between the two fields of 3.8 in distance modulus. The colours of the variables confirm that a principal period with log P > 1.75 is a necessary condition for detectable mass-loss. At the longest observed wavelength, 24 microns, many semi-regular variables have dust shells comparable in luminosity to those around Miras. There is a clear bifurcation in LMC colour-magnitude diagrams involving 24 micron magnitudes.

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arXiv2007

Continuum emission around AGB stars at 1.2 mm

It is generally acknowledged that the mass loss of Asymptotic Giant Branch (AGB) stars undergoes variations on different time scales. We constructed models for the dust envelopes for a sample of AGB stars to assess whether mass-loss variations influence the spectral energy distribution (SED). To constrain the variability, extra observations at millimetre wavelengths (1.2 mm) were acquired. From the analysis of the dust models, two indications for the presence of mass-loss variations can be found, being (1) a dust temperature at the inner boundary of the dust envelope that is far below the dust condensation temperature and (2) an altered density distribution with respect to rho(r) \propto r^{-2} resulting from a constant mass-loss rate. For 5 out of the 18 studied sources a two-component model of the envelope is required, consisting of an inner region with a constant mass-loss rate and an outer region with a less steep density distribution. For one source an outer region with a steeper density distribution was found. Moreover, in a search for time variability in our data set at 1.2 mm, we found that WX Psc shows a large relative time variation of 34% which might partially be caused by variable molecular line emission.

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arXiv2006

Spitzer spectroscopy of carbon stars in the Small Magellanic Cloud

We present Spitzer Space telescope spectroscopic observations of 14 carbon-rich AGB stars in the Small Magellanic Cloud. SiC dust is seen in most of the carbon-rich stars but it is weak compared to LMC stars. The SiC feature is strong only for stars with significant dust excess, opposite to what is observed for Galactic stars. We argue that in the SMC, SiC forms at lower temperature than graphite dust, whereas the reverse situation occurs in the Galaxy where SiC condenses at higher temperatures and forms first. Dust input into the interstellar medium by AGB stars consists mostly of carbonaceous dust, with little SiC or silicate dust. Only the two coolest stars show a 30-micron band due to MgS dust. We suggest that this is due to the fact that, in the SMC, mass-losing AGB stars generally have low circumstellar (dust) optical depth and therefore effective heating of dust by the central star does not allow temperatures below the 650 K necessary for MgS to exist as a solid. Gas phase C$_2$H$_2$ bands are stronger in the SMC than in the LMC or Galaxy. This is attributed to an increasing C/O ratio at low metallicity. We present a colour-colour diagram based on Spitzer IRAC and MIPS colours to discriminate between O- and C-rich stars. We show that AGB stars in the SMC become carbon stars early in the thermal-pulsing AGB evolution, and remain optically visible for $\sim 6 \times 10^5$ yr. For the LMC, this lifetime is $\sim 3 \times 10^5$ yr. The superwind phase traced with Spitzer lasts for $\sim 10^4$ yr. Spitzer spectra of a K supergiant and a compact HII region are also given.

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arXiv2006

Spitzer observations of acetylene bands in carbon-rich AGB stars in the Large Magellanic Cloud

We investigate the molecular bands in carbon-rich AGB stars in the Large Magellanic Cloud (LMC), using the InfraRed Spectrograph (IRS) on board the Spitzer Space Telescope (SST) over the 5--38 micron range. All 26 low-resolution spectra show acetylene (C2H2) bands at 7 and 14 micron. The hydrogen cyanide (HCN) bands at these wavelengths are very weak or absent. This is consistent with low nitrogen abundances in the LMC. The observed 14 micron C2H2 band is reasonably reproduced by an excitation temperature of 500 K. There is no clear dilution of the 14 micron band by circumstellar dust emission. This 14 micron band originates from molecular gas in the circumstellar envelope in these high mass-loss rate stars, in agreement with previous findings for Galactic stars. The C2H2,column density, derived from the 13.7 micron band, shows a gas mass-loss rate in the range 3x10^-6 to 5x10^{-5} Msun yr-1. This is comparable with the total mass-loss rate of these stars estimated from the spectral energy distribution. Additionally, we compare the line strengths of the 13.7 micron C2H2 band of our LMC sample with those of a Galactic sample. Despite the low metallicity of the LMC, there is no clear difference in the C2H2 abundance among LMC and Galactic stars. This reflects the effect of the 3rd dredge-up bringing self-produced carbon to the surface, leading to high C/O ratios at low metallicity.

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arXiv2006

Near-IR spectroscopy of OH/IR stars in the Galactic Centre

Context. Galactic Centre (GC) OH/IR stars can be, based on the expansion velocities of their circumstellar shells, divided into two groups which are kinematically different and therefore are believed to have evolved from different stellar populations. Aims. To study the metallicity distribution of the OH/IR stars population in the GC on basis of a theoretical relation between EW(Na), EW(Ca) and EW(CO) and the metallicity. Methods. For 70 OH/IR stars in the GC, we obtained near-IR spectra. The equivalent line widths of NaI, CaI, 12CO(2,0) and the curvature of the spectrum around 1.6 micron due to water absorption are determined. Results. The near-IR spectrum of OH/IR stars is influenced by several physical processes. OH/IR stars are variable stars suffering high mass-loss rates. The dust that is formed around the stars strongly influences the near-IR spectra and reduces the equivalent line widths of NaI, CaI. A similar effect is caused by the water content in the outer atmosphere of the OH/IR star. Because of these effects, it is not possible with our low resolution near-infrared spectroscopy to determine the metallicities of these stars.

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arXiv2006

A Spitzer mid-infrared spectral survey of mass-losing carbon stars in the Large Magellanic Cloud

We present a Spitzer Space Telescope spectroscopic survey of mass-losing carbon stars (and one oxygen-rich star) in the Large Magellanic Cloud. The spectra cover the wavelength range 5--38 micron. They show varying combinations of dust continuum, dust emission features (SiC, MgS) and molecular absorption bands (C2H2, HCN). A set of four narrow bands, dubbed the Manchester system, is used to define the infrared continuum for dusty carbon stars. The relations between the continuum colours and the strength of the dust and molecular features are studied, and are compared to Galactic stars of similar colours. The circumstellar 7-micron C2H2 band is found to be stronger at lower metallicity, from a comparison of stars in the Galaxy, the LMC and the SMC. This is explained by dredge-up of carbon, causing higher C/O ratios at low metallicity (less O). A possible 10-micron absorption feature seen in our spectra may be due to C3. This band has also been identified with interstellar silicate or silicon-nitrite dust. The line-to-continuum ratio of theSiC and MgS bands shows some indication of being lower at low metallicity. The MgS band is only seen at dust temperatures below 600 K. We discuss the selection of carbon versus oxygen-rich AGB stars using the J-K vs. K-A colours, and show that these colours are relatively insensitive to chemical type. Metal-poor carbon stars form amorphous carbon dust from self-produced carbon. The formation efficiency of oxygen-rich dust depends more strongly on metallicity. We suggest that in lower-metallicity environments, the dust input into the Interstellar Medium by AGB stars is efficient but may be strongly biassed towards carbonaceous dust, as compare to the Galaxy.

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arXiv2005

ESO-VLT and Spitzer spectroscopy of IRAS05328-6827: a massive young stellar object in the Large Magellanic Cloud

We present the first thermal-infrared spectra of an extra-galactic Young Stellar Object (YSO), IRAS05328-6827 in the HII region LHA 120-N148 in the Large Magellanic Cloud. The observed and modelled spectral energy distribution reveals a massive YSO, M=20 Msun, which is heavily-embedded and probably still accreting. The reduced dust content as a consequence of the lower metallicity of the LMC allows a unique view into this object, and together with a high C/O ratio may be responsible for the observed low abundance of water ice and relatively high abundances of methanol and CO2 ices.

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