arXiv subjects
Publications and source records attributed to R. Wesson.
In this paper we give a progress report on the Herschel imaging and spectroscopic observations of planetary nebulae that are carried out as part of the MESS guaranteed time key program. We present and discuss imaging and temperature maps of NGC 6720, NGC 650, and NGC 6853, as well as PACS and SPIRE spectroscopy of NGC 7027.
In 2007, R Coronae Borealis (R CrB) went into an historically deep and long decline. In this state, the dust acts like a natural coronagraph at visible wavelengths, allowing faint nebulosity around the star to be seen. Imaging has been obtained from 0.5 to 500 micron with Gemini/GMOS, HST/WFPC2, Spitzer/MIPS, and Herschel/SPIRE. Several of the structures around R CrB are cometary globules caused by wind from the star streaming past dense blobs. The estimated dust mass of the knots is consistent with their being responsible for the R CrB declines if they form along the line of sight to the star. In addition, there is a large diffuse shell extending up to 4 pc away from the star containing cool 25 K dust that is detected all the way out to 500 micron. The SED of R CrB can be well fit by a 150 AU disk surrounded by a very large diffuse envelope which corresponds to the size of the observed nebulosity. The total masses of the disk and envelope are 10^-4 and 2 M(Sun), respectively, assuming a gas-to-dust ratio of 100. The evidence pointing toward a white-dwarf merger or a final-helium-shell flash origin for R CrB is contradictory. The shell and the cometary knots are consistent with a fossil planetary nebula. Along with the fact that R CrB shows significant Lithium in its atmosphere, this supports the final-helium-shell flash. However, the relatively high inferred mass of R CrB and its high fluorine abundance support a white-dwarf merger.
We present an analysis of multi-epoch observations of the Type II-P supernova SN 2004et. New and archival optical spectra of SN 2004et are used to study the evolution of the Halpha and [O I] 6300A line profiles between days 259 and 646. Mid-infrared imaging was carried out between 2004 to 2010. We include Spitzer `warm' mission photometry at 3.6 and 4.5um obtained on days 1779, 1931 and 2151, along with ground-based and HST optical and near-infrared observations obtained between days 79 and 1803. Multi-wavelength light curves are presented, as well as optical-infrared spectral energy distributions (SEDs) for multiple epochs. Starting from about day 300, the optical light curves provide evidence for an increasing amount of circumstellar extinction attributable to newly formed dust, with the additional extinction reaching 0.8-1.5 magnitudes in the V-band by day 690. The overall SEDs were fitted with multiple blackbody components, in order to investigate the luminosity evolution of the supernova, and then with Monte Carlo radiative transfer models using smooth or clumpy dust distributions, in order to estimate how much new dust condensed in the ejecta. The luminosity evolution was consistent with the decay of 56Co in the ejecta up until about day 690, after which an additional emission source is required, in agreement with the findings of Kotak et al. (2009). Clumped dust density distributions consisting of 20% amorphous carbons and 80% silicates by mass were able to match the observed optical and infrared SEDs, with dust masses that increased from 8x10^{-5} Msun on day 300 to 1.5x10^{-3} Msun on day 690, still significantly lower than the values needed for core collapse supernovae to make a significant contribution to the dust enrichment of galaxies.
The extraction of chemical abundances of ionised nebulae from a limited spectral range is usually hampered by the lack of emission lines corresponding to certain ionic stages. So far, the missing emission lines have been accounted for by the ionisation correction factors (ICFs), constructed under simplistic assumptions like spherical geometry by using 1-D photoionisation modelling. In this contribution we discuss the results (Goncalves et al. 2011, in prep.) of our ongoing project to find a new set of ICFs to determine total abundances of N, O, Ne, Ar, and S, with optical spectra, in the case of non-spherical PNe. These results are based on a grid of 3-D photoionisation modelling of round, elliptical and bipolar shaped PNe, spanning the typical PN luminosities, effective temperatures and densities. We show that the additional corrections --to the widely used Kingsburgh and Barlow (1994) ICFs-- are always higher for bipolars than for ellipticals. Moreover, these additional corrections are, for bipolars, up to: 17% for oxygen, 33% for nitrogen, 40% for neon, 28% for argon and 50% for sulphur. Finally, on top of the fact that corrections change greatly with shape, they vary also greatly with the central star temperature, while the luminosity is a less important parameter.
We have obtained spectroscopic observations of the nebulae around seven Wolf-Rayet (WR) stars, four in the Milky Way (WR 8, 16, 18 and 40) and three in the LMC (BAT99-2, -11 and -38). They were observed using the ESO NTT with the EFOSC-2 instrument, except for one nebula, NGC 3199, which was observed using the UVES echelle spectrometer on the VLT. The aims of these observations were to (a) quantify the degree of chemical enrichment in WR nebulae which had previously been suggested to have abundances reflecting nucleosynthetic processing; and (b) to attempt to detect the far-red lines of neutral carbon (e.g. [C I] 9850A) in the nebulae around WC stars. Nebular densities, temperatures and elemental abundances were derived using standard emission line diagnostics. Our high spectral resolution UVES dataset for NGC 3199 allowed the determination of line broadening temperatures using lines from several different species. These showed consistent patterns and fair agreement with the nebular temperatures derived from diagnostic line ratios. Amongst the Galactic WR nebulae, the nebulae around WR 8, 16 and 40 were found to be strongly nitrogen-enriched while NGC 3199 displayed abundances which were similar to those of Galactic H II regions. Amongst the LMC WR nebulae, none displayed an N/O ratio that significantly exceeded LMC H II region values. The far-red [C I] lines were detected from NGC 3199, with the strength of the 8727A line indicating that at least part of it was produced by C+ recombination rather than collisional excitation. The line widths of the [C I] 9824 and 9850A lines were however the same as those of collisionally excited nebular lines, indicating that those lines originated largely from ionized regions within the nebula.
SN 2010jl was an extremely bright, Type IIn SNe which showed a significant IR excess no later than 90 days after explosion. We have obtained Spitzer 3.6 and 4.5 \mum and JHK observations of SN 2010jl \sim90 days post explosion. Little to no reddening in the host galaxy indicated that the circumstellar material lost from the progenitor must lie in a torus inclined out of the plane of the sky. The likely cause of the high mid-IR flux is the reprocessing of the initial flash of the SN by pre-existing circumstellar dust. Using a 3D Monte Carlo Radiative Transfer code, we have estimated that between 0.03-0.35 Msun of dust exists in a circumstellar torus around the SN located 6 \times 10 ^17 cm away from the SN and inclined between 60-80\cdot to the plane of the sky. On day 90, we are only seeing the illumination of approximately 5% of this torus, and expect to see an elevated IR flux from this material up until day \sim 450. It is likely this dust was created in an LBV-like mass loss event of more than 3 Msun, which is large but consistent with other LBV progenitors such as η Carinae.
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.
SN 2007it is a bright, Type IIP supernova which shows indications of both pre-existing and newly formed dust. The visible photometry shows a bright late-time luminosity, powered by the 0.09 M$_{\sun}$ of $^{56}$Ni present in the ejecta. There is also a sudden drop in optical brightness after day 339, and a corresponding brightening in the IR due to new dust forming in the ejecta. CO and SiO emission, generally thought to be precursors to dust formation, may have been detected in the mid-IR photometry of SN 2007it. The optical spectra show stronger than average [O I] emission lines and weaker than average [Ca II] lines, which may indicate a 16 - 27 M$_{\sun}$ progenitor, on the higher end of expected Type IIP masses. Multi-component [O I] lines are also seen in the optical spectra, most likely caused by an asymmetric blob or a torus of oxygen core material being ejected during the SN explosion. Interaction with circumstellar material prior to day 540 may have created a cool dense shell between the forward and reverse shocks where new dust is condensing. At late times there is also a flattening of the visible lightcurve as the ejecta luminosity fades and a surrounding light echo becomes visible. Radiative transfer models of SN 2007it SEDs indicate that up to 10$^{-4}$ M$_{\sun}$ of new dust has formed in the ejecta, which is consistent with the amount of dust formed in other core collapse supernovae.
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.
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).
We present time-resolved optical spectroscopy of V458 Vulpeculae (Nova Vul 2007 No. 1) spread over a period of 15 months starting 301 days after its discovery. Our data reveal radial velocity variations in the HeII λ5412 and HeII λ4686 emission lines. A period analysis of the radial velocity curves resulted in a period of 98.09647 \pm 0.00025 min (0.06812255 \pm 0.00000017 d) which we identify with the orbital period of the binary system. V458 Vul is therefore the planetary nebula central binary star with the shortest period known. We explore the possibility of the system being composed of a relatively massive white dwarf (M1 \gsim 1.0 Msun) accreting matter from a post-asymptotic giant branch star which produced the planetary nebula observed. In this scenario, the central binary system therefore underwent two common-envelope episodes. A combination of previous photoionisation modelling of the nebular spectra, post-asymptotic giant branch evolutionary tracks and the orbital period favour a mass of M2 \sim 0.6 Msun for the donor star. Therefore, the total mass of the system may exceed the Chandrasekhar mass, which makes V458 Vul a Type Ia supernova progenitor candidate.
SPIRE, the Spectral and Photometric Imaging Receiver, is the Herschel Space Observatory's submillimetre camera and spectrometer. It contains a three-band imaging photometer operating at 250, 350 and 500 μm, and an imaging Fourier transform spectrometer (FTS) covering 194-671 μm (447-1550 GHz). In this paper we describe the initial approach taken to the absolute calibration of the SPIRE instrument using a combination of the emission from the Herschel telescope itself and the modelled continuum emission from solar system objects and other astronomical targets. We present the photometric, spectroscopic and spatial accuracy that is obtainable in data processed through the "standard" pipelines. The overall photometric accuracy at this stage of the mission is estimated as 15% for the photometer and between 15 and 50% for the spectrometer. However, there remain issues with the photometric accuracy of the spectra of low flux sources in the longest wavelength part of the SPIRE spectrometer band. The spectrometer wavelength accuracy is determined to be better than 1/10th of the line FWHM. The astrometric accuracy in SPIRE maps is found to be 2 arcsec when the latest calibration data are used. The photometric calibration of the SPIRE instrument is currently determined by a combination of uncertainties in the model spectra of the astronomical standards and the data processing methods employed for map and spectrum calibration. Improvements in processing techniques and a better understanding of the instrument performance will lead to the final calibration accuracy of SPIRE being determined only by uncertainties in the models of astronomical standards.
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.
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.
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.
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.
We present far-infrared and submillimetre spectra of three carbon-rich evolved objects, AFGL 2688, AFGL 618 and NGC 7027. The spectra were obtained with the SPIRE Fourier transform spectrometer on board the Herschel Space Observatory, and cover wavelengths from 195-670 um, a region of the electromagnetic spectrum hitherto difficult to study in detail. The far infrared spectra of these objects are rich and complex, and we measure over 150 lines in each object. Lines due to 18 different species are detected. We determine physical conditions from observations of the rotational lines of several molecules, and present initial large velocity gradient models for AFGL 618. We detect water in AFGL 2688 for the first time, and confirm its presence in AFGL 618 in both ortho and para forms. In addition, we report the detection of the J=1-0 line of CH+ in NGC 7027.
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.