arXiv subjects

R. Wesson

Publications and source records attributed to R. Wesson.

85 records · Page 5Linked to original sources

arXiv2010

A Herschel PACS and SPIRE study of the dust content of the Cassiopeia A supernova remnant

Using the 3.5-m Herschel Space Observatory, imaging photometry of Cas A has been obtained in six bands between 70 um and 500 um with the PACS and SPIRE instruments, with angular resolutions ranging from 6 to 37". In the outer regions of the remnant the 70-um PACS image resembles the 24-um image Spitzer image, with the emission attributed to the same warm dust component, located in the reverse shock region. At longer wavelengths, the three SPIRE bands are increasingly dominated by emission from cold interstellar dust knots and filaments, particularly across the central, western and southern parts of the remnant. Nonthermal emission from the northern part of the remnant becomes prominent at 500 um. We have estimated and subtracted the contributions from the nonthermal, warm dust and cold interstellar dust components. We confirm and resolve for the first time a cool (~35 K) dust component, emitting at 70-160 um, that is located interior to the reverse shock region, with an estimated mass of 0.075 Msun.

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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

SN 2007od: A Type IIP SN with Circumstellar Interaction

SN 2007od exhibits characteristics that have rarely been seen in a Type IIP supernova (SN). Optical V band photometry reveals a very steep brightness decline between the plateau and nebular phases of ~4.5 mag, likely due to SN 2007od containing a low mass of 56Ni. The optical spectra show an evolution from normal Type IIP with broad Halpha emission, to a complex, four component Halpha emission profile exhibiting asymmetries caused by dust extinction after day 232. This is similar to the spectral evolution of the Type IIn SN 1998S, although no early-time narrow (~200 km s-1) Halpha component was present in SN 2007od. In both SNe, the intermediate-width Halpha emission components are thought to arise in the interaction between the ejecta and its circumstellar medium (CSM). SN 2007od also shows a mid-IR excess due to new dust. The evolution of the Halpha profile and the presence of the mid-IR excess provide strong evidence that SN 2007od formed new dust before day 232. Late-time observations reveal a flattening of the visible lightcurve. This flattening is a strong indication of the presence of a light echo, which likely accounts for much of the broad, underlying Halpha component seen at late-times. We believe the multi-peaked Halpha emission is consistent with the interaction of the ejecta with a circumstellar ring or torus (for the inner components at \pm1500 km s-1), and a single blob or cloud of circumstellar material out of the plane of the CSM ring (for the outer component at -5000 km s-1). The most probable location for the formation of new dust is in the cool dense shell created by the interaction between the expanding ejecta and its CSM. Monte Carlo radiative transfer modeling of the dust emission from SN 2007od implies that up to 4x 10-4Msun of new dust has formed. This is similar to the amounts of dust formed in other CCSNe such as SNe 1999em, 2004et, and 2006jc.

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arXiv2009

The destruction and survival of dust in the shell around SN 2008S

SN 2008S erupted in early 2008 in the grand design spiral galaxy NGC 6946. The progenitor was detected by Prieto et al. in Spitzer Space Telescope images taken over the four years prior to the explosion, but was not detected in deep optical images, from which they inferred a self-obscured object with a mass of about 10 Msun. We obtained Spitzer observations of SN 2008S five days after its discovery, as well as coordinated Gemini and Spitzer optical and infrared observations six months after its outburst. We have constructed radiative transfer dust models for the object before and after the outburst, using the same r^-2 density distribution of pre-existing amorphous carbon grains for all epochs and taking light-travel time effects into account for the early post-outburst epoch. We rule out silicate grains as a significant component of the dust around SN 2008S. The inner radius of the dust shell moved outwards from its pre-outburst value of 85 AU to a post-outburst value of 1250 AU, attributable to grain vaporisation by the light flash from SN 2008S. Although this caused the circumstellar extinction to decrease from Av = 15 before the outburst to 0.8 after the outburst, we estimate that less than 2% of the overall circumstellar dust mass was destroyed. The total mass-loss rate from the progenitor star is estimated to have been (0.5-1.0)x10^-4 Msun yr^-1. The derived dust mass-loss rate of 5x10^-7 Msun yr^-1 implies a total dust injection into the ISM of up to 0.01 Msun over the suggested duration of the self-obscured phase. We consider the potential contribution of objects like SN 2008S to the dust enrichment of galaxies.

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arXiv2009

Knots in the Helix Nebula found in H2

We present a deep and wide field-of-view (4'x 7') image of the planetary nebula (PN) NGC 7293 (the Helix Nebula) in the 2.12 micron H2 v=1-0 S(1) line. The excellent seeing (0.4'') at the Subaru Telescope, allows the details of cometary knots to be examined. The knots are found at distances of 2.2'-6.4' from the central star (CS). At the inner edge and in the inner ring (up to 4.5' fromthe CS), the knot often show a `tadpole' shape, an elliptical head with a bright crescent inside and a long tail opposite to the CS. In detail, there are variations in the tadpole shapes, such as narrowing tails, widening tails, meandering tails, or multi-peaks within a tail. In the outer ring (4.5'-6.4' from the CS), the shapes are more fractured, and the tails do not collimate into a single direction. The transition in knot morphology from the inner edge to the outer ring is clearly seen. The number density of knots governs the H2 surface brightness in the inner ring: H2 exists only within the knots. Possible mechanisms which contribute to the shaping of the knots are discussed, including photo-ionization and streaming motions. A plausible interpretation of our images is that inner knots are being overrun by a faster wind, but that this has not (yet) reached the outer knots. Based on H2 formation and destruction rates, H2 gas can survive in knots from formation during the late asymptotic giant branch (AGB) phase throughout the PN phase. These observations provide new constraints on the formation and evolution of knots, and on the physics of molecular gas embedded within ionized gas.

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arXiv2009

Swift X-ray and UV monitoring of the Classical Nova V458 Vul (Nova Vul 2007)

We describe the highly variable X-ray and UV emission of V458 Vul (Nova Vul 2007), observed by Swift between 1 and 422 days after outburst. Initially bright only in the UV, V458 Vul became a variable hard X-ray source due to optically thin thermal emission at kT=0.64 keV with an X-ray band unabsorbed luminosity of 2.3x10^{34} erg s^{-1} during days 71-140. The X-ray spectrum at this time requires a low Fe abundance (0.2^{+0.3}_{-0.1} solar), consistent with a Suzaku measurement around the same time. On day 315 we find a new X-ray spectral component which can be described by a blackbody with temperature of kT=23^{+9}_{-5} eV, while the previous hard X-ray component has declined by a factor of 3.8. The spectrum of this soft X-ray component resembles those typically seen in the class of supersoft sources (SSS) which suggests that the nova ejecta were starting to clear and/or that the WD photosphere is shrinking to the point at which its thermal emission reaches into the X-ray band. We find a high degree of variability in the soft component with a flare rising by an order of magnitude in count rate in 0.2 days. In the following observations on days 342.4-383.6, the soft component was not seen, only to emerge again on day 397. The hard component continued to evolve, and we found an anticorrelation between the hard X-ray emission and the UV emission, yielding a Spearman rank probability of 97%. After day 397, the hard component was still present, was variable, and continued to fade at an extremely slow rate but could not be analysed owing to pile up contamination from the bright SSS component.

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arXiv2008

A planetary nebula around nova V458 Vul undergoing flash ionization

Nova V458 Vul erupted on 2007 August 8th and reached a visual magnitude of 8.1 a few days later. H$α$ images obtained six weeks before the outburst as part of the IPHAS galactic plane survey reveal an 18th magnitude progenitor surrounded by an extended nebula. Subsequent images and spectroscopy of the nebula reveal an inner nebular knot increasing rapidly in brightness due to flash ionization by the nova event. We derive a distance of 13 kpc based on light travel time considerations, which is supported by two other distance estimation methods. The nebula has an ionized mass of 0.2 M$_{\odot}$ and a low expansion velocity: this rules it out as ejecta from a previous nova eruption, and is consistent with it being a ~14,000 year old planetary nebula, probably the product of a prior common envelope (CE) phase of evolution of the binary system. The large derived distance means that the mass of the erupting WD component of the binary is high. We identify two possible evolutionary scenarios, in at least one of which the system is massive enough to produce a Type Ia supernova on merging.

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arXiv2007

The hydrogen-deficient knot of the `born again' planetary nebula Abell 58 (V605 Aql)

We have analysed deep optical spectra of the `born-again' planetary nebula Abell 58 and its hydrogen-deficient knot, surrounding V605 Aql, which underwent a nova-like eruption in 1919. The electron temperature we derive for the central knot varies widely depending on the diagnostic used. The [O III] nebular-to-auroral transition ratio gives a temperature of 20800 K, while the ratio of the [N II] nebular and auroral lines give Te=15200 K. The helium line ratios 5876/4471 and 6678/4471 imply temperatures of 350 K and 550 K respectively. Weakly temperature-sensitive O II recombination line ratios imply similarly low electron temperatures. Abundances derived from recombination lines are vastly higher than those found from collisionally excited lines, with the abundance discrepancy factor (adf) for O2+ reaching 89 -- the second highest known value after that found for the hydrogen deficient knots in Abell 30. The observed temperature diagnostics and abundances support the idea that, like Abell 30, the knot of Abell 58 contains some very cold ionised material. Although the central star is carbon-rich (C/O>1), the knot is found to be oxygen-rich, a situation not predicted by the single-star `born again' theory of its formation. We argue that the abundances in the ejecta observed in A 30 and A 58 have more in common with neon novae than with Sakurai's Object, which is believed to have undergone a final helium flash. In particular, the C/O ratio of less than unity and presence of substantial quantities of neon in the ejecta of both Abell 30 and Abell 58 are not predicted by very late thermal pulse models.

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arXiv2004

Observations and 3D photoionisation modelling of the Wolf-Rayet planetary nebula NGC 1501

Deep optical spectra of the high excitation planetary nebula NGC 1501 and its W04 central star are presented. A recombination line abundance analysis of the central star's emission-line spectrum yields He:C:O mass fractions of 0.36:0.48:0.16, similar to those of PG1159 stars. A detailed empirical analysis of the nebular collisionally excited line (CEL) and optical recombination line (ORL) spectrum is presented, together with fully three-dimensional photoionisation modelling of the nebula. We found very large ORL-CEL abundance discrepancy factors (ADFs) for O^2+ (32) and Ne^2+ (33). The mean value of 5100K for the T_e derived from HeI recombination lines ratios is 6000K lower than the value of 11100K implied by the [OIII] line ratio. A three-dimensional photoionisation model of NGC 1501 was constructed using the photoionisation code MOCASSIN, based on our new spectroscopic data and using the three-dimensional electron density distribution determined from long-slit echellograms of the nebula by Ragazzoni et al. (2001). The initial models showed higher degrees of ionisation of heavy elements than indicated by observations. We investigated the importance of the missing low-temperature dielectronic recombination rates for third-row elements and have estimated upper limits to their rate coefficients. Our model heavily under-predicts the optical recombination line emission. We conclude that the presence of a hydrogen-deficient, metal-rich component is necessary to explain the observed ORL spectrum of this object. The existence of such knots could also provide a softening of the radiation field, via the removal of ionising photons by absorption in the knots, thereby helping to alleviate the over-ionisation of the heavy elements in our models.

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arXiv2004

Discovery of a WO star in the Scutum-Crux arm of the inner Galaxy

We report the discovery of only the fourth massive WO star to be found in the Milky Way, and only the seventh identified within the Local Group. This has resulted from the first observations made in a programme of follow-up spectroscopy of candidate emission line stars from the AAO/UK Schmidt Southern Galactic Plane H-alpha Survey. The optical spectrum of this star, to become WR 93b in the Catalogue of Galactic Wolf-Rayet stars, is presented and described. WR 93b is classified as WO3 and is shown to be highly reddened (E(B-V) = 2.1 pm 0.1). A recombination line analysis of the emission lines yields the abundance ratios C/He = 0.95 and O/He = 0.13 (by number). Comparisons at near infrared wavelengths of reddening corrected photometry between WR 93b and both of Sand 2 (WO3, D = 49 kpc) and Sand 5 (WO2, D = 1.75 kpc) yields a consistent distance to WR 93b of 3.4 kpc. Positioned at Galactic co-ordinates l = 353.27, b = -0.85, the star is most likely located in the Scutum-Crux Arm of the inner Milky Way. We note that none of the four Galactic WO stars lies significantly beyond the Solar Circle (with two well inside). Estimation of the wind terminal velocity in WR 93b at 5750 km/s makes this star the current wind speed record holder among all non-degenerate stars.

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arXiv2004

Electron temperatures and densities of planetary nebulae determined from the nebular hydrogen recombination spectrum and temperature and density variations

A method is presented to derive electron temperatures and densities of planetary nebulae (PNe) simultaneously, using the observed hydrogen recombination spectrum, which includes continuum and line emission. By matching theoretical spectra to observed spectra around the Balmer jump at about 3646A, we determine electron temperatures and densities for 48 Galactic PNe. The electron temperatures based on this method are found to be systematically lower than those derived from [O III] 4959/4363 and [O III] (88μ+52μ)/4959 ratios. And the electron densities based on this method are found to be systematically higher than those derived from forbidden line ratios. These results suggest that temperature and density fluctuations are generally present within nebulae. In addition, He/H abundances of PNe are found to be positively correlated with the difference between T_e([O III]) and T_e(Bal), suggesting that He/H abundances might have been overestimated generally because of the possible existence of H-deficient knots.

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arXiv2003

Physical conditions in the planetary nebula Abell 30

We have analysed optical spectra of two of the hydrogen-deficient knots (J1 & J3) in the born-again planetary nebula Abell 30, together with UV spectra of knots J3 & J4. We determine electron temperatures in the knots based on several diagnostics. The [O III] nebular-to-auroral transition ratio yields temperatures of the order of 17 000 K. The weak temperature dependence of the ratios of helium lines at 4471, 5876 and 6678 Angstroms is used to derive a temperature of 8850 K for knot J3 and 4600 K for knot J1. Ratios of O II recombination lines, which directly measure the temperature in the coldest regions of the knots, are used to derive temperatures of 2500 K for knot J3, and just 500 K for knot J1. We calculate abundances both from collisionally excited lines and from the well-observed recombination spectra of C, N, O and Ne ions. The forbidden line abundances agree well with previous determinations, but the recombination line abundances are several hundred times higher. These results confirm the scenario proposed by Harrington and Feibelman (1984) in which the knots contain a cold core highly enriched in heavy elements. Forbidden lines are almost entirely emitted by the hot outer part of the knot, while recombination lines are emitted predominantly from the cold core. The C/O ratios we derive imply that the knots are oxygen-rich, contrary to theoretical predictions for born-again nebulae (Iben et al 1983).

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