arXiv subjects

A. de Koter

Publications and source records attributed to A. de Koter.

215 records · Page 12Linked to original sources

arXiv2003

Water ice growth around evolved stars

We present a model of the growth of water ice on silicate grains in the circumstellar envelopes of Asymptotic Giant Branch (AGB) stars and Red Super Giants. We consider the growth of ice by gas grain collisions, the thermal evaporation of ice from a grain, and sputtering. Our model contains several improvements compared to earlier models, including a detailed treatment of the effects of sputtering, a detailed calculation of the radiation pressure on the grain, and the treatment of subsonic drift velocities. In terms of drift velocity between the grains and gas in the envelope, we find that the ice formation process can be divided into three regimes: (i) a sputtering dominated regime where ice growth is heavily suppressed, (ii) an intermediate regime with moderately efficient condensation and (iii) a thermally dominated regime where ice formation is highly efficient. Sputtering is the critical factor which determines if ice formation can occur at all. We find that in Red Supergiants, ice formation is suppressed, while the winds of OH/IR stars allow for efficient condensation and can convert significant fractions of the available water vapor (tens of percent) into ice mantles on grains. Population II AGB stars hardly form ice due to their low dust to gas ratios. We also modify an analytical equation describing condensation and depletion (Jura and Morris, 1985) in order to give reasonable results for high and low drift velocities. Initially, ice will condense in crystalline form, but continuing condensation at low temperatures, and damage caused by interstellar UV photons favor the production of amorphous ice as well. We predict that a significant fraction of the ice formed will be amorphous.

astro-ph↗

arXiv2003

The origin of crystalline silicates in the Herbig Be star HD100546 and in comet Hale-Bopp

We have investigated the spatial distribution, and the properties and chemical composition of the dust orbiting HD 100546. This system is remarkably different from other isolated Herbig Ae/Be stars in both the strength of the mid-IR excess and the composition of the circumstellar dust. We speculate that the formation and spatial distribution of the crystalline dust observed in the HD 100546 system may be linked to the formation of a proto-Jupiter in the disk around HD 100546. Such a proto-Jupiter could gravitationally stir the disk leading to a collisional cascade of asteroidal sized objects producing small crystalline grains, or it could cause shocks by tidal interaction with the disk which might produce crystalline dust grains through flash heating. As shown by Malfait et al. (1998), the infrared spectrum of HD 100546 is very similar to that of C/1995 O1 Hale-Bopp (Crovisier et al. 1997). Using an identical methodology, we have therefore also studied this solar system comet. Both objects have an almost identical grain composition, but with the important difference that the individual dust species in Hale-Bopp are in thermal contact with each other, while this is not the case in HD 100546. This suggests that if similar processes leading to the dust composition as seen in HD 100546 also occurred in our own solar system, that Hale-Bopp formed after the formation of one or more proto-gas giants.

astro-ph↗

arXiv2002

The absence of the 10 um silicate feature in the isolated Herbig Ae star HD 100453

We analyse the optical and IR spectra, as well as the spectral energy distribution (UV to mm) of the candidate Herbig Ae star HD100453. This star is particular, as it shows an energy distribution similar to that of other isolated Herbig Ae/Be stars (HAEBEs), but unlike most of them, it does not have a silicate emission feature at 10 um, as is shown in Meeus (2001). We confirm the HAEBE nature of HD100453 through an analysis of its optical spectrum and derived location in the H-R diagram. The IR spectrum of HD100453 is modelled by an optically thin radiative transfer code, from which we derive constraints on the composition, grain-size and temperature distribution of the circumstellar dust. We show that it is both possible to explain the lack of the silicate feature as (1) a grain-size effect - lack of small silicate grains, and (2) a temperature effect - lack of small, hot silicates, as proposed by Dullemond (2001), and discuss both possibilities.

astro-ph↗

arXiv2002

Hydrogen infrared recombination lines as diagnostic tool for hot star wind geometry

We have analysed the infrared hydrogen recombination lines of a sample of well studied hot massive stars observed with the Infrared Space Observatory. Our sample contains stars from several classes of objects, whose circumstellar environment is believed to be dominated by an ionized stellar wind (the Luminous Blue Variables) or by a dense disk-like geometry (Be stars and B[e] stars). We show that hydrogen infrared recombination lines can be used as a diagnostic tool to constrain the geometry of the ionized circumstellar material. The line strengths are sensitive to the density of the emitting gas. High densities result in optically thick lines for which line strengths are only dependent on the emitting surface. Low density gas produces optically thin lines which may be characterized by Menzel case B recombination. The ISO observations show that stellar winds are dominated by optically thin \hi\ recombination lines, while disks are dominated by optically thick lines. Disks and winds are well separated in a diagnostic diagram using the \hu/\bra and the \hu/\pfg line flux ratios. This diagnostic tool is useful to constrain the nature of hot star environments in case they are highly obscured, for instance while they are still embedded in their natal molecular cloud.

astro-ph↗

arXiv2002

An atlas of 2.4 to 4.1 microns ISO/SWS spectra of early-type stars

We present an atlas of spectra of O- and B-type stars, obtained with the Short Wavelength Spectrometer (SWS) during the Post-Helium program of the Infrared Space Observatory (ISO). This program is aimed at extending the Morgan & Keenan classification scheme into the near-infrared. Later type stars will be discussed in a seperate publication. The observations consist of 57 SWS Post-Helium spectra from 2.4 to 4.1 microns, supplemented with 10 spectra acquired during the nominal mission with a similar observational setting. For B-type stars, this sample provides ample spectral converage in terms of subtype and luminosity class. For O-type stars,the ISO sample is coarse and therefore is complemented with 8 UKIRT L'-band observations. In terms of the presence of diagnostic lines, the L'-band is likely the most promising of the near-infrared atmospheric windows for the study of the physical properties of B stars. Specifically, this wavelength interval contains the Brackett alpha, Pfund gamma, and other Pfund lines which are probes of spectral type, luminosity class and mass loss. Here, we present simple empirical methods based on the lines present in the 2.4 to 4.1 microns interval that allow the determination of: the spectral type of B dwarfs and giants to within two subtypes; the luminosity class of B stars to within two classes; the mass-loss rate of O stars and B supergiants to within 0.25 dex.

astro-ph↗

arXiv2002

Dust and the spectral energy distribution of the OH/IR star OH 127.8+0.0: Evidence for circumstellar metallic iron

We present a fit to the spectral energy distribution of OH 127.8+0.0, a typical asymptotic giant branch star with an optically thick circumstellar dust shell. The fit to the dust spectrum is achieved using non-spherical grains consisting of metallic iron, amorphous and crystalline silicates and water ice. Previous similar attempts have not resulted in a satisfactory fit to the observed spectral energy distributions, mainly because of an apparent lack of opacity in the 3--8 micron region of the spectrum. Non-spherical metallic iron grains provide an identification for the missing source of opacity in the near-infrared. Using the derived dust composition, we have calculated spectra for a range of mass-loss rates in order to perform a consistency check by comparison with other evolved stars. The L-[12 micron] colours of these models correctly predict the mass-loss rate of a sample of AGB stars, strengthening our conclusion that the metallic iron grains dominate the near-infrared flux. We discuss a formation mechanism for non-spherical metallic iron grains.

astro-ph↗

arXiv2001

Discovery of a double ring in the core of eta Carinae

We report the discovery of a double ring structure in the waist of the nebula surrounding eta Carinae. The rings are detected in the mid-IR dust continuum at wavelengths of 7.9, 11.9, 12.9 and 20 micronmeter. The dust in the rings has a temperature of about 300 K. The orientation of the rings is inclined with respect to the axis of the homunculus by either 37 or 58 degrees. The central star is not in the projected centre of the structure defined by the two rings. This geometry is reminiscent of that seen in SN1987A and some planetary nebulae. We discuss several possible origins for this remarkable geometry and its orientation.

astro-ph↗

arXiv2001

Crystallinity versus mass-loss rate in Asymptotic Giant Branch stars

Infrared Space Observatory (ISO) observations have shown that O-rich Asymptotic Giant Branch (AGB) stars exhibit crystalline silicate features in their spectra only if their mass-loss rate is higher than a certain threshold value. Usually, this is interpreted as evidence that crystalline silicates are not present in the dust shells of low mass-loss rate objects. In this study, radiative transfer calculations have been performed to search for an alternative explanation to the lack of crystalline silicate features in the spectrum of low mass-loss rate AGB stars. It is shown that due to a temperature difference between amorphous and crystalline silicates it is possible to include up to 40% of crystalline silicate material in the circumstellar dust shell, without the spectra showing the characteristic spectral features. Since this implies that low mass-loss rate AGB stars might also form crystalline silicates and deposit them into the Interstellar Medium (ISM), the described observational selection effect may put the process of dust formation around AGB stars and the composition of the predominantly amorphous dust in the Interstellar Medium in a different light. Our model calculations result in a diagnostic tool to determine the crystallinity of an AGB star with a known mass-loss rate.

astro-ph↗

arXiv2000

The gamma Velorum binary system. II. WR stellar parameters and the photon loss mechanism

In this paper we derive stellar parameters for the Wolf-Rayet star in the gamma Velorum binary system (WR11), from a detailed non-LTE model of its optical and infrared spectra. Compared to the study of Schaerer et al., the parameters of the WC8 star are revised to a hotter effective temperature (Teff~57kK),a higher luminosity (log(L/Lsun = 5.00), and a lower mass loss rate (log(Mdot / Msun/yr) = -5.0 - using a 10% clumping filling factor). These changes lead to a significant decrease in wind efficiency number, from 144 to 7, so that the driving mechanism of the wind of this WR star may be simply radiation pressure on lines. The derived spectroscopic luminosity is found to be 40% lower than that derived by De Marco & Schmutz through the mass-luminosity relationship for WR stars (log(L/Lsun = 5.2). The paper furthermore presents a comparison of the independently-developed modelling programs, CMFGEN and ISA-Wind . Overall, there seems to be very reasonable agreement between the derived parameters for WR11, except for the carbon content, which is 2 times higher for CMFGEN (C/He=0.15 vs. 0.06, by number).The comparison also confirms a disparity in the predicted flux at lambda<$400A, found by Crowther et al., which will have effects on several nebular line strengths. The paper also presents the first independent check of the photon loss mechanism proposed by Schmutz. We conclude that, not only is it important to include very many lines to realistically model line blanketing, but in particular those ones that critically interact with strong resonance lines (e.g. HeII lambda303.78). The inclusion of these latter lines may significantly alter the wind ionization structure.

astro-ph↗

arXiv1999

The infrared spectrum of the Be star gamma Cassiopeiae

We present the 2.4-45 micrometer ISO-SWS spectrum of the Be star gamma Cas (B0.5 IVe). The spectrum is characterised by a thermal continuum which can be well fit by a power-law S_nu ~ nu^0.99 over the entire SWS wavelength range. For an isothermal disc of ionized gas with constant opening angle, this correponds to a density gradient rho(r) ~ r^(-2.8). We report the detection of the Humphreys bound-free jump in emission at 3.4 micrometer. The size of the jump is sensitive to the electron temperature of the gas in the disc, and we find T~9000 K, i.e. much lower than the stellar effective temperature (25000-30000 K). The spectrum is dominated by numerous emission lines, mostly from HI, but also some HeI lines are detected. Several spectral features cannot be identified. The line strengths of the HI{\sc i} emission lines do not follow case B recombination line theory. The line strengths and widths suggest that many lines are optically thick and come from an inner, high density region with radius 3-5 R_star and temperature above that of the bulk of the disc material. Only the alpha, beta and gamma transitions of the series lines contain a contribution from the outer regions. The level populations deviate significantly from LTE and are highly influenced by the optically thick, local (disc) continuum radiation field. The inner disk may be rotating more rapidly than the stellar photosphere.

astro-ph↗

arXiv1999

Mass-loss rates and luminosity functions of dust-enshrouded AGB stars and red supergiants in the LMC

A radiative transfer code is used to model the spectral energy distributions of 57 mass-losing Asymptotic Giant Branch (AGB) stars and red supergiants (RSGs) in the Large Magellanic Cloud (LMC) for which ISO spectroscopic and photometric data are available. As a result we derive mass-loss rates and bolometric luminosities. A gap in the luminosity distribution around M_bol = -7.5 mag separates AGB stars from RSGs. The luminosity distributions of optically bright carbon stars, dust-enshrouded carbon stars and dust-enshrouded M-type stars have only little overlap, suggesting that the dust-enshrouded AGB stars are at the very tip of the AGB and will not evolve significantly in luminosity before mass loss ends their AGB evolution. Derived mass-loss rates span a range from Mdot about 10^-7 to 10^-3 M_sun/yr. More luminous and cooler stars are found to reach higher mass-loss rates. The highest mass-loss rates exceed the classical limit set by the momentum of the stellar radiation field, L/c, by a factor of a few due to multiple scattering of photons in the circumstellar dust envelope. Mass-loss rates are lower than the mass consumption rate by nuclear burning, Mdot_nuc, for most of the RSGs. Two RSGs have Mdot >> Mdot_nuc, however, suggesting that RSGs shed most of their stellar mantles in short phases of intense mass loss. Stars on the thermal pulsing AGB may also experience episodes of intensified mass loss, but their quiescent mass-loss rates are usually already higher than Mdot_nuc.

astro-ph↗

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.

astro-ph↗

arXiv1999

The composition and nature of the dust shell surrounding the binary AFGL 4106

We present infrared spectroscopy and imaging of AFGL~4106. The 2.4-5 micron ISO-SWS spectrum reveals the presence of a cool, luminous star (T_eff ~ 3750 K) in addition to an almost equally luminous F star (T_eff ~ 7250 K). The 5-195 micron SWS and LWS spectra are dominated by strong emission from circumstellar dust. We find that the dust consists of amorphous silicates, with a minor but significant contribution from crystalline silicates. The amorphous silicates consist of Fe-rich olivines. The presence of amorphous pyroxenes cannot be excluded but if present they contain much less Fe than the amorphous olivines. Comparison with laboratory data shows that the pure Mg-end members of the crystalline olivine and pyroxene solid solution series are present. In addition, we find strong evidence for simple oxides (FeO and Al2O3) as well as crystalline H2O ice. Several narrow emission features remain unidentified. Modelling of the dust emission using a dust radiation transfer code shows that large grains (~1 micron) must be present and that the abundance of the crystalline silicates is between 7 and 15% of the total dust mass, depending on the assumed enstatite to forsterite ratio, which is estimated to be between 1 and 3. The amorphous and crystalline dust components in the shell do not have the same temperature, implying that the different dust species are not thermally coupled. We find a dust mass of ~3.9 x 10^-2 M_sol expelled over a period of 4 x 10^3 years for a distance of 3.3 kpc. The F-star in the AFGL~4106 binary is likely a post-red-supergiant in transition to a blue supergiant or WR phase.

astro-ph↗

arXiv1998

Crystalline silicates in Planetary Nebulae with [WC] central stars

We present ISO-SWS spectroscopy of the cool dusty envelopes surrounding two Planetary Nebulae with [WC] central stars, BD+30~3639 and He~2-113. The lambda < 15micron region is dominated by a rising continuum with prominent emission from C-rich dust (PAHs), while the long wavelength part shows narrow solid state features from crystalline silicates. This demonstrates that the chemical composition of both stars changed very recently (less than 1000 years ago). The most likely explanation is a thermal pulse at the very end of the AGB or shortly after the AGB. The H-rich nature of the C-rich dust suggests that the change to C-rich chemistry did not remove all H. The present-day H-poor [WC] nature of the central star may be due to extensive mass loss and mixing following the late thermal pulse.

astro-ph↗

arXiv1995

Combined stellar structure and atmosphere models for massive stars I. Interior evolution and wind properties on the main sequence

We present the first "combined stellar structure and atmosphere models" (CoStar) for massive stars, which consistently treat the entire mass loosing star from the center out to the asymptotic wind velocity. The models use up-to-date input physics and state-of-the-art techniques to model both the stellar interior and the spherically expanding non--LTE atmosphere including line blanketing. Our models thus yield consistent predictions regarding not only the basic stellar parameters, including abundances, but also theoretical spectra along evolutionary tracks. On the same ground they allow us to study the influence of stellar winds on evolutionary models. In this first paper, we present our method and investigate the wind properties and the interior evolution on the main sequence (MS) at solar metallicity. The wind momentum and energy deposition associated with the MS evolution is given and the adopted wind properties are discussed. From our atmosphere calculations, we also derive theoretical estimates of mass loss driven by radiation pressure. These values are compared with the predictions from recent wind models of the Munich group. We find an overall agreement with most of their results. In addition, our models are better in reproducing the strong wind momentum rates observed in supergiants than those of Puls et al. (1995). A comparison between boundary conditions given by the conventional plane parallel and the new spherically expanding atmosphere approach is made. For the MS evolution the evolutionary tracks and the interior evolution are found to be basically unchanged by the new treatment of the outer layers. Given the small spherical extension of the continuum forming layers in the considered

astro-ph↗

arXiv1995

Combined stellar structure and atmosphere models for massive stars II. Spectral evolution on the main sequence

In Schaerer et al. (1995, Paper I) we have presented the first "combined stellar structure and atmosphere models" (CoStar) for massive stars, which consistently treat the entire mass loosing star from the center out to the outer region of the stellar wind. The models use up-to-date input physics and state-of-the-art techniques to model both the stellar interior and the spherically expanding non--LTE atmosphere including line blanketing. Paper II covers the spectral evolution corresponding to the MS interior evolution discussed in Paper I. The CoStar results presented comprise: a) flux distributions, from the EUV to the far IR, and the ionizing fluxes in the H and He continua, b) absolute UBVRIJHKL MN photometric data and UV colors, c) detailed line blanketed UV spectra, and d) non-LTE H and He line spectra in the optical and IR, including theoretical K band spectra. These results may, e.g., be used for population synthesis models intended to study the massive star content in young starforming regions. We compare our results with other predictions from LTE and non-LTE plane parallel models and point out the improvements and the importance of using adequate atmosphere models including stellar winds for massive stars. We compare the UV spectral evolution with observations, including continuum indices and several metal line signatures of P-Cygni lines and broad absorption features. Good agreement is found for most UV features. We are able to reproduce the strong observed FeIII 1920 A feature in late O and early B giants and supergiants. This feature is found to depend sensitively on temperature and may be used to derive effective temperatures for these stars. We also derive a simple formula to determine mass loss rates from the equivalent width of hydrogen recombination lines for OB stars showing net

astro-ph↗

arXiv1994

Combined Stellar Structure and Atmosphere Models: Exploratory Results for Wolf-Rayet Stars

In this paper we present Complete Stellar models (CoStar) for massive stars, which treat the stellar interior and atmosphere, including its wind. Particular emphasis is given to Wolf-Rayet stars. We address the question of the effective temperatures of WNE and WC stars. Our first results show a satisfactory agreement between the CoStar models and the simple temperature correction method applied by Schaller et al. (1992). An analyses of the subphotospheric structure of the WR star models shows the importance of metal opacity. This may be essential for understanding the driving mechanism of Wolf-Rayet winds.

astro-ph↗