arXiv subjects

O. De Marco

Publications and source records attributed to O. De Marco.

29 records · Page 2Linked to original sources

arXiv2011

Radial Velocity search for substellar companions to sdB stars

After the discovery of a substellar companion to the hot subdwarf HD 149382, we have started a radial velocity search for similar objects around other bright sdB stars using the Anglo-Australian Telescope. Our aim is to test the hypothesis that close substellar companions can significantly affect the post-main sequence evolution of solar-type stars. It has previously been proposed that binary interactions in this scenario could lead to the formation of hot subdwarfs. The detection of such objects will provide strong evidence that Jupiter-mass planets can survive the interaction with a solar-type star as it evolves up the Red Giant Branch. We present the first results of our search here.

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arXiv2008

A dense disk of dust around the born-again Sakurai's object

In 1996, Sakurai's object (V4334 Sgr) suddenly brightened in the centre of a faint Planetary Nebula (PN). This very rare event was interpreted as the reignition of a hot white dwarf that caused a rapid evolution back to the cool giant phase. From 1998 on, a copious amount of dust has formed continuously, screening out the star which has remained embedded in this expanding high optical depth envelope. The new observations, reported here, are used to study the morphology of the circumstellar dust in order to investigate the hypothesis that Sakurai's Object is surrounded by a thick spherical envelope of dust. We have obtained unprecedented, high-angular resolution spectro-interferometric observations, taken with the mid-IR interferometer MIDI/VLTI, which resolve the dust envelope of Sakurai's object. We report the discovery of a unexpectedly compact (30 x 40 milliarcsec, 105 x 140 AU assuming a distance of 3.5 kpc), highly inclined, dust disk. We used Monte Carlo radiative-transfer simulations of a stratified disk to constrain its geometric and physical parameters, although such a model is only a rough approximation of the rapidly evolving dust structure. Even though the fits are not fully satisfactory, some useful and robust constraints can be inferred. The disk inclination is estimated to be 75+/-3 degree with a large scale height of 47+/-7 AU. The dust mass of the disk is estimated to be 6 10^{-5} solar mass. The major axis of the disk (132+/-3 degree) is aligned with an asymmetry seen in the old PN that was re-investigated as part of this study. This implies that the mechanism responsible for shaping the dust envelope surrounding Sakurai's object was already at work when the old PN formed.

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arXiv2008

Towards a Spectral Technique for Determining Material Geometry Around Evolved Stars: Application to HD 179821

HD 179821 is an evolved star of unknown progenitor mass range (either post-Asymptotic Giant Branch or post-Red Supergiant) exhibiting a double peaked spectral energy distribution (SED) with a sharp rise from $\sim8-20$ $μ$m. Such features have been associated with ejected dust shells or inwardly truncated circumstellar discs. In order to compare SEDs from both systems, we employ a spherically symmetric radiative transfer code and compare it to a radiative, inwardly truncated disc code. As a case study, we model the broad-band SED of HD 179821 using both codes. Shortward of 40 $μ$m, we find that both models produce equivalent fits to the data. However, longward of 40 $μ$m, the radial density distribution and corresponding broad range of disc temperatures produce excess emission above our spherically symmetric solutions and the observations. For HD 179821, our best fit consists of a $T_{eff}=7000$ K central source characterized by $τ_V\sim1.95$ and surrounded by a radiatively driven, spherically symmetric dust shell. The extinction of the central source reddens the broad-band colours so that they resemble a $T_{eff}=5750$ K photosphere. We believe that HD 179821 contains a hotter central star than previously thought. Our results provide an initial step towards a technique to distinguish geometric differences from spectral modeling.

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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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arXiv2006

V605 Aql: The Older Twin of Sakurai's Object

New optical spectra have been obtained with VLT/FORS2 of the final helium shell flash (FF) star, V605 Aql, which peaked in brightness in 1919. New models suggest that this star is experiencing a very late thermal pulse. The evolution to a cool luminous giant and then back to a compact hot star takes place in only a few years. V605 Aql, the central star of the Planetary Nebula (PN), A58, has evolved from T$_{eff}\sim$5000 K in 1921 to $\sim$95,000 K today. There are indications that the new FF star, Sakurai's Object (V4334 Sgr), which appeared in 1996, is evolving along a similar path. The abundances of Sakurai's Object today and V605 Aql 80 years ago mimic the hydrogen deficient R Coronae Borealis (RCB) stars with 98% He and 1% C. The new spectra show that V605 Aql has stellar abundances similar to those seen in Wolf-Rayet [WC] central stars of PNe with ~55% He, and ~40% C. The stellar spectrum of V605 Aql can be seen even though the star is not directly detected. Therefore, we may be seeing the spectrum in light scattered around the edge of a thick torus of dust seen edge-on. In the present state of evolution of V605 Aql, we may be seeing the not too distant future of Sakurai's Object.

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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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arXiv2002

Revised Stellar Temperatures for Magellanic Cloud O Supergiants from FUSE and VLT-UVES Spectroscopy

We have undertaken quantitative analysis of four LMC and SMC O4--9.7 extreme supergiants using far-ultraviolet FUSE, ultraviolet IUE/HSTand optical VLT UVES spectroscopy. Extended, non-LTE model atmospheres that allow for the consistent treatment of line blanketing are used to analyse wind & photospheric spectral features simultaneously. Using Halpha to constrain dM/dt, HeI-II photospheric lines reveal T_eff's which are systematically (5-7.5kK) & substantially (15-20%) lower than previously derived from unblanketed, plane-parallel, non-LTE photospheric studies. We have confidence in these revisions, since derived T_eff's generally yield consistent fits across the entire 912-7000Ang observed spectral range. In particular, we are able to resolve previous UV-optical T_eff discrepancies. `Of' classification criteria are directly linked to (strong) nitrogen enrichment (via NIII 4097) & (weak) carbon depletion (via CIII 4647-51), providing evidence for mixing of unprocessed and CNO processed material at their stellar surfaces. Oxygen abundances are more difficult to constrain, except via OII lines in the O9.7 supergiant for which it is also found to be somewhat depleted. The effect of wind clumping is also investigated, for which PV 1118-28 potentially provides a useful diagnostic in O-star winds. Revised stellar properties affect existing calibrations of (i) Lyman continuum photons - a factor of two lower for the O4 supergiant; and (ii) kinetic energy released into the ISM by O supergiants. Our results also have importance for the calibration of the wind momentum-luminosity relationship for OB stars.

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arXiv2001

SwSt 1: an O-rich planetary nebula around a C-rich central star

The hydrogen-deficient carbon-rich [WCL] type central star HD167362 and its oxygen-rich planetary nebula (PN) SwSt~1 are investigated. The nebular chemistry might indicate a recent origin for the carbon-rich stellar spectrum. Its stellar and nebular properties might therefore provide further understanding of the origin of the [WCL] central star class. The UV-IR stellar spectra are modelled with state of the codes and show ~40kK central star with a wind and a C/O~3, indicative of efficient third dredge-up. The synthetic stellar flux distribution is used to model the high density, compact PN, which has a solar C/O ratio, is still enshrouded by 1200K and 230K dust shells and, reported here for the first time, in molecular hydrogen. Although it appears that the change in C/O ratio has been recent, the published spectroscopy since 1895 has been re-examined and no clear spectral change is seen. If an event occurred that has turned it into a hydrogen-deficient central star, it did not happen in the last 100 years.

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arXiv2000

Far-UV FUSE spectroscopy of the OVI resonance doublet in Sand2 (WO)

We present Far-Ultraviolet Spectroscopic Explorer (FUSE) spectroscopy of Sand 2, a LMC WO-type Wolf-Rayet star, revealing the OVI resonance P Cygni doublet at 1032-38A. These data are combined with HST/FOS ultraviolet and Mt Stromlo 2.3m optical spectroscopy, and analysed using a spherical, non-LTE, line-blanketed code. Our study reveals exceptional stellar parameters: T*=150,000K, v_inf=4100 km/s, log (L/Lo)=5.3, and Mdot=10^-5 Mo/yr if we adopt a volume filling factor of 10%. Elemental abundances of C/He=0.7+-0.2 and O/He=0.15(-0.05+0.10) by number qualitatively support previous recombination line studies. We confirm that Sand 2 is more chemically enriched in carbon than LMC WC stars, and is expected to undergo a supernova explosion within the next 50,000 yr.

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arXiv2000

FUSE Observations of the Stellar Winds of Two O7 Supergiants in the Magellanic Clouds

We compare the stellar wind features in far-UV spectra of Sk -67 111, an O7 Ib(f) star in the LMC, with Sk 80, an O7 Iaf+ star in the SMC. The most striking differences are that Sk 80 has a substantially lower terminal velocity, much weaker O VI absorption, and stronger S IV emission. We have used line-blanketed, hydrodynamic, non-LTE atmospheric models to explore the origin of these differences. The far-UV spectra require systematically lower stellar temperatures than previous determinations for O7 supergiants derived from plane-parallel, hydrostatic models of photospheric line profiles. At these temperatures, the O VI in Sk -67 111 must be due primarily to shocks in the wind.

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arXiv1999

Wolf-Rayet nebulae as tracers of stellar ionizing fluxes: I. M1-67

We use WR124 (WN8h) and its associated nebula M1-67, to test theoretical non-LTE models for Wolf-Rayet (WR) stars. Lyman continuum ionizing flux distributions derived from a stellar analysis of WR124, are compared with nebular properties via photo-ionization modelling. Our study demonstrates the significant role that line blanketing plays in affecting the Lyman ionizing energy distribution of WR stars, of particular relevance to the study of HII regions containing young stellar populations. We confirm previous results that non-line blanketed WR energy distributions fail to explain the observed nebular properties of M1-67, such that the predicted ionizing spectrum is too hard. A line blanketed analysis of WR124 is carried out using the method of Hillier & Miller (1998), with stellar properties in accord with previous results, except that the inclusion of clumping in the stellar wind reduces its wind performance factor to only approx2. The ionizing spectrum of the line blanketed model is much softer than for a comparable temperature unblanketed case, such that negligible flux is emitted with energy above the HeI 504 edge. Photo-ionization modelling, incorporating the observed radial density distribution for M1-67 reveals excellent agreement with the observed nebular electron temperature, ionization balance and line strengths. An alternative stellar model of WR124 is calculated, following the technique of de Koter et al. (1997), augmented to include line blanketing following Schmutz et al. (1991). Good consistency is reached regarding the stellar properties of WR124, but agreement with the nebular properties of M1-67 is somewhat poorer than for the Hillier & Miller code.

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