arXiv subjects

Michaela Kraus

Publications and source records attributed to Michaela Kraus.

67 records · Page 4Linked to original sources

arXiv2007

On the evolutionary stage of the unclassified B[e] star CD-42o11721

The star CD-42o11721 is a curious B[e] star sometimes pointed as an evolved B[e] supergiant and sometimes as a young HAeBe star, due to very uncertain or even unknown stellar parameters, especially the distance. In this paper, we present new data gained from high-resolution optical spectroscopy and a detailed description of IR data of this star. We present a qualitative study of the numerous emission lines in our optical spectra and the classification of their line profiles, which indicate a non-spherically symmetric circumstellar environment. The first real detection of numerous [Fe II] emission lines and of many other permitted and forbidden emission lines is reported. From our optical data, we derive an effective temperature of 14000 K, a radius of 17.3 +/- 0.6 Rsun, as well as a luminosity of 1.0 +/- 0.3 x 10^{4} Lsun. We advocate that CD-42o11721 might be a post-main sequence object, even though a pre-main sequence nature cannot be ruled out due to the uncertain distance. We further found that the SED in the optical and infrared can best be fitted with an outflowing disk-forming wind scenario rather than with a spherical symmetric envelope or with a flared disc, supporting our tentative classification as a B[e] supergiant.

astro-ph↗

arXiv2006

On the hydrogen neutral outflowing disks of B[e] supergiants

(abridged) B[e] supergiants are known to possess geometrically thick dusty disks. Disk-forming wind models have, however, been found to be insufficient in reproducing the observed dust emission. This problem arises due to the severe assumption that, as for classical Be stars, the near-infrared excess emission originates in the disk. Modeling of the free-free and free-bound emission therefore results in an upper limit for the disk mass loss rate, hampering dust condensation in the disk. We propose a revised scenario for the non-spherical winds of B[e] supergiants: a normal B-type line-driven polar wind and an outflowing disk-forming wind that is neutral in hydrogen at, or very close to the stellar surface. We concentrate on the pole-on seen LMC B[e] supergiant R126 and calculate the line luminosities of the optical [OI] emission lines with an outflowing disk scenario. In addition, we compute the free-free and free-bound emission from a line-driven polar wind and model the spectral energy distribution in the optical and near-infrared. Good fits to the [OI] line luminosities are achieved for an outflowing disk that is neutral in hydrogen right from the stellar surface. Neutral thereby means that hydrogen is ionized by less than 0.1%. Consequently, the free-free and free-bound emission cannot (dominantly) arise from the disk and cannot limit the disk mass loss rate. The hydrogen neutral outflowing disk scenario therefore provides an ideal environment for efficient dust formation. The spectral energy distribution in the optical and near-infrared range can be well fitted with the stellar continuum plus free-free and free-bound emission from the polar line-driven wind. Our modeling further delivers minimum values for \dot{M}(disk) > 2.5d-5 M_sun/yr and for the density contrast between equatorial and polar wind of ~10.

astro-ph↗

arXiv2006

Ionization structure in the winds of B[e] supergiants II. Influence of rotation on the formation of equatorial hydrogen neutral zones

Context: B[e] supergiants are known to have non-spherical winds, and the existence of disks that are neutral in hydrogen close to their stellar surface has been postulated. A suitable mechanism to produce non-spherical winds seems to be rapid rotation, and at least for three B[e] supergiants in the Magellanic Clouds rotation velocities at a substantial fraction of their critical velocity have been found. Aims: We want to find suitable recombination distances in the equatorial plane of rapidly rotating stars that explain the observed huge amounts of neutral material in the vicinity of B[e] supergiants. Methods: We perform ionization structure calculations in the equatorial plane around rapidly rotating luminous supergiants. The restriction to the equatorial plane allows us to treat the ionization balance equations 1-dimensionally, while the stellar radiation field is calculated 2-dimensionally, taking into account the latitudinal variation of the stellar surface parameters. The stellar parameters used correspond to those known for B[e] supergiants. The assumptions made in the computations all have in common that the total number of available ionizing photons at any location within the equatorial plane is overestimated, resulting in upper limits for the recombination distances. Results: We find that despite the drop in equatorial surface density of rapidly rotating stars (neglecting effects like bi-stability and/or wind compression), hydrogen and helium recombine at or close to the stellar surface, for mass loss rates Mdot > 5d-5 M_sun/yr and rotation speeds in excess of v(rot,eq)/v(crit)=0.8.

astro-ph↗

arXiv2005

The optical and IR properties of southern galactic unclB[e] stars: The case of CD-42 11721

(abridged) In this work we present the results of an optical and IR study for the curious B[e] star CD-42 11721. This object has a doubtful evolutionary stage, being either a pre-main sequence star (HAeB[e]) or an evolved star (sgB[e]). Our optical investigation could be splitted in two parts, a qualitative study based on the identification of the numerous emission lines present in the spectra and the classification of their line profiles, which indicate a non-spherically symmetric circumstellar environment, and a quantitative analysis of numerous forbidden lines, e.g. [OI], [OII], [NII] and [SII]. Assuming a typical circumstellar scenario for a sgB[e], i.e. a fast, low-density polar wind and a slow, high-density disc forming equatorial wind, we can reproduce very well the line luminosities of the forbidden lines. From this analysis, we can determine the mass loss rate of the star lying in the range from 4.4 x 10^(-6) to 2.2 x 10^(-5) M_sun/yr, depending on the considered redenning. Our IR study could also be splitted in two parts: the identification of several features in the SWS ISO spectrum, and the modeling of the SED of CD-42 11721. The first part shows the presence of a mixed chemistry, i.e. C- and O-rich dust in the same circumstellar medium. We have tried to model the SED, by using a numerical code written by us which considers a spherical circumstellar scenario. Although the answer concerning the evolutionary stage of CD-42 11721 is still not very clear, we believe that our analysis will improve the discussion about the nature of this curious star.

astro-ph↗

arXiv2005

The compact planetary nebula B[e] star Hen 2-90

We present a study of the optical spectrum of the fascinating B[e] star Hen 2-90 based on new high-resolution observations taken with FEROS at the ESO 1.52m telescope in La Silla (Chile). The recent HST image of Hen 2-90 (Sahai et al. 2002) reveals a bipolar, highly ionized region, a neutral disk-like structure seen almost perfectly edge-on, and an intermediate region of moderate ionization. The slits of our observations cover the same innermost region of Hen 2-90 as the HST image, which allows us to combine the observations. Our spectra contain a huge amount of permitted and forbidden emission lines of atoms in different stages of ionization. In addition, the line wings deliver velocity information of the emitting region. We find correlation between the different ionization states of the elements and the velocities derived from the line profiles: the highly ionized atoms have the highest outflow velocity, while the neutral lines have the lowest. When combining the velocity information with the HST image of Hen 2-90, it seems that a non-spherical stellar wind model is a good option to explain the ionization and spatial distribution of the circumstellar material. Our modeling of the forbidden emission lines results in strong evidence for Hen 2-90 being a compact planetary nebulae that has undergone a superwind phase of high, non-spherical mass loss, most probably triggered by a central star that was rotating with about 80% of the critical velocity. We find a total mass loss rate during this superwind phase on the order of 3 x 10^(-5) M_sun/yr.

astro-ph↗

arXiv2005

The neutral outflowing disks of some Magellanic Cloud B[e] supergiants

We report on the detection of [OI] emission lines in the high-resolution optical spectra of several Magellanic Cloud (MC) B[e] supergiants, which we took with FEROS at the ESO 1.52m telescope in La Silla (Chile). In addition, we model the [OI] line luminosities and show that the best location for the neutral oxygen material is the outflowing disk. In order to reproduce the observed line luminosities, we conclude that the disks must be neutral already very close to the stellar surface.

astro-ph↗

arXiv2005

Ionization structure in the disks and winds of B[e] stars

We investigate the ionization structure in the non-spherical winds and disks of B[e] stars. Especially the luminous B[e] supergiants seem to have outflowing disks which are neutral in hydrogen already close to the stellar surface. The existence of neutral material so close to the central star is surprising and needs to be investigated in detail. We perform our model calculations mainly in the equatorial plane, trying to find a plausible scenario that leads to recombination in the vicinity of the hot stars. Two different approaches are presented that both result in a hydrogen neutral equatorial region. We especially focus on the influence of stellar rotation which is known to play a significant role in shaping non-spherical winds. We show that a rotating star can have a neutral equatorial wind, even without the need of a density enhancement due to bi-stability or wind compression, but simply due to graviational darkening in combination with the known high mass-loss rates of the B[e] stars and especially the B[e] supergiants.

astro-ph↗

arXiv2005

The enigmatic B[e]-star Henize 2-90: The non-spherical mass loss history from an analysis of forbidden lines

(abridged) We study the optical spectrum of the exciting B[e] star Hen 2-90 based on new high-resolution observations that cover the innermost 2". Our investigation is splitted in two parts, a qualitative study of the presence of the numerous emission lines and the classification of their line profiles which indicate a circumstellar environment of high complexity, and a quantitative analysis of numerous forbidden lines, e.g. [OI], [OII], [OIII], [SII], [SIII], [ArIII], [ClII], [ClIII] and [NII]. We find a correlation between the different ionization states of the elements and the velocities derived from the line profiles: the highly ionized atoms have the highest outflow velocity while the neutral lines have the lowest outflow velocity. The recent HST image of Hen 2-90 reveals a bipolar, highly ionized region, a neutral disk-like structure and an intermediate region of moderate ionization. It seems that a non-spherical stellar wind model is a good option to explain the ionization and spatial distribution of the circumstellar material. We modelled the forbidden lines under the assumption of a non-spherically symmetric wind based on the HST image. We find that in order to fit the observed line luminosities, the mass flux, surface temperature, and terminal wind velocities need to be latitude dependent, which might be explained in terms of a rapidly rotating central star. A rotation speed of 75-80 % of the critical velocity has been derived. The total mass loss rate of the star was determined to be of order 3 10^{-5} M_sun/yr. Such a wind scenario and the fact that compared to solar abundances C, O, and N seem to be underabundant while S, Ar and Cl have solar abundances, might be explained in terms of a rapidly rotating post-AGB star.

astro-ph↗

arXiv2004

Parameters of PNe: constant density versus density distribution

We derive the stellar and circumstellar parameters of the Galactic compact planetary nebula Hen 2-90 with two models: the classical constant density nebula model where the emission (especially of the forbiden lines) is formed in a sphere or shell of constant density, and a model where the emission is formed in a shell with a $r^{-2}$ density distribution. Depending on the density $N_{\rm e,0}$ in the second model, the resulting range of valid values can deviate from the classical model results. This seems to be especially true for elemental abundances as is shown exemplary for S$^{+}$. We find that the abundance of S$^{+}$ is larger for the models with density distribution than for a constant density. A careful analysis of data with an appropriate input model is advisable when determining stellar and circumstellar parameters of planetary nebulae not only in our Galaxy.

astro-ph↗

arXiv2004

The outflowing disks of B[e] supergiants and unclassified B[e] stars

B[e] supergiants are known to possess outflowing cool disks but also some unclassified B[e] stars show clear indications for the presence of a neutral disk. We derive constraints on the disk mass loss rates, temperature distributions and disk opening angles for the Small Magellanic Cloud B[e] supergiant Hen S 18 and the unclassified galactic B[e] star Hen 2-90 by modeling the line luminosities of the [OI] lines arising in their optical spectrum. These lines are supposed to form in a hydrogen neutral disk. We find disk mass fluxes of order 3.4 10^-4 g s^-1 cm^-2 and 5.5 10^{-1} g s^{-1}cm^{-2} resulting in disk mass loss rates of 1.0 10^{-4} M_sun yr^{-1} and 1.5 10^{-5} M_sun yr^{-1} for Hen S 18 and Hen 2-90, respectively.

astro-ph↗

arXiv2004

The evolutionary phase of B[e] supergiants and unclassified B[e] stars

We present two classes of stars with yet unknown evolutionary phase: the B[e] supergiants and the so-called unclassified B[e] stars. While the B[e] supergiants are luminous post-main sequence stars with high mass progenitors, not much is known of the unclassified stars. We discuss how it might be possible to determine their evolutionary phases in the area of new large telescopes.

astro-ph↗

arXiv2004

The enigmatic B[e] star Hen 2-90 -- an interacting binary?

We present the enigmatic galactic unclassified B[e] star Hen 2-90. Its optical spectrum is discussed and analysed in terms of the forbidden emission lines coming from a non-spherical wind. The evolutionary phase of Hen 2-90 is discussed. We raise the question whether an interacting binary nature seems to be a possible solution to account for all observed characteristics.

astro-ph↗

arXiv2002

The Analysis of the Rotating Disk of some Southern Galactic B[e] Stars

The spectra of stars with the B[e] phenomenon are dominated by features that are related to physical conditions of circumstellar material around these objects and are not intrinsic to the stars. Previous studies have shown that emission lines present in the optical spectra of these objects are formed in an equatorial rotating disk. The analysis of high and low resolution spectra, obtained by us with 1.52 telescope in ESO for some Southern Galactic B[e] Stars, can give us information about the structure and velocity of the disk. We will describe the analysis of the unclassified B[e] star Hen 2-90.

astro-ph↗