arXiv subjects

K. -H. Hofmann

Publications and source records attributed to K. -H. Hofmann.

At least 73 records · Page 4Linked to original sources

arXiv2005

High-resolution near-infrared speckle interferometry and radiative transfer modeling of the OH/IR star OH 104.9+2.4

We present near-infrared speckle interferometry of the OH/IR star OH 104.9+2.4 in the K' band obtained with the 6m telescope of the Special Astrophysical Observatory (SAO). At a wavelength of lambda = 2.12 micron the diffraction-limited resolution of 74 mas was attained. The reconstructed visibility reveals a spherically symmetric, circumstellar dust shell (CDS) surrounding the central star. The visibility function shows that the stellar contribution to the total flux at lambda = 2.12 micron is less than ~50%, indicating a rather large optical depth of the CDS. The azimuthally averaged 1-dimensional Gaussian visibility fit yields a diameter of 47 +/- 3mas (FHWM), which corresponds to 112 +/- 13 AU for an adopted distance of D = 2.38 +/- 0.24 kpc. To determine the structure and the properties of the CDS of OH 104.9+2.4, radiative transfer calculations using the code DUSTY were performed to simultaneously model its visibility and the spectral energy distribution (SED). We found that both the ISO spectrum and the visibility of OH 104.9+2.4 can be well reproduced by a radiative transfer model with an effective temperature T_eff = 2500 +/- 500 K of the central source, a dust temperature T_in = 1000 +/- 200 K at the inner shell boundary R_in = 9.1 R_star = 25.4 AU, an optical depth tau = 6.5 +/- 0.3 at 2.2 micron, and dust grain radii ranging from a_min = 0.005 +/- 0.003 micron to a_max = 0.2 +/- 0.02 micron with a power law with index -3.5. It was found that even minor changes in a_max have a major impact on both the slope and the curvature of the visibility function, while the SED shows only minor changes. Our detailed analysis demonstrates the potential of dust shell modeling constrained by both the SED and visibilities.

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arXiv2004

Bipolar outflow on the Asymptotic Giant Branch - the case of IRC+10011

Near-IR imaging of the AGB star IRC+10011 (= CIT3) reveals the presence of a bipolar structure within the central ~0.1 arcsec of a spherical dusty wind. We show that the image asymmetries originate from ~1E-4 Msun of swept-up wind material in an elongated cocoon whose expansion is driven by bipolar jets. We perform detailed 2D radiative transfer calculations with the cocoon modeled as two cones extending to ~1,100 AU within an opening angle of ~30deg, imbedded in a wind with the standard r^{-2} density profile. The cocoon expansion started <~200 years ago, while the total lifetime of the circumstellar shell is ~5,500 years. Similar bipolar expansion, at various stages of evolution, has been recently observed in a number of other AGB stars, culminating in jet breakout from the confining spherical wind. The bipolar outflow is triggered at a late stage in the evolution of AGB winds, and IRC+10011 provides its earliest example thus far. These new developments enable us to identify the first instance of symmetry breaking in the evolution from AGB to planetary nebula.

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arXiv2004

Interferometric observations of the Mira star o Ceti with the VLTI/VINCI instrument in the near-infrared

We present K-band commissioning observations of the Mira star prototype o Cet obtained at the ESO Very Large Telescope Interferometer (VLTI) with the VINCI instrument and two siderostats. The observations were carried out between 2001 October and December, in 2002 January and December, and in 2003 January. Rosseland angular radii are derived from the measured visibilities by fitting theoretical visibility functions obtained from center-to-limb intensity variations (CLVs) of Mira star models (Bessel et al. 1996, Hofmann et al. 1998, Tej et al. 2003). Using the derived Rosseland angular radii and the SEDs reconstructed from available photometric and spectrophotometric data, we find effective temperatures ranging from T_eff=3192 +/- 200 K at phase phi=0.13 to 2918 +/- 183 K at phi=0.26. Comparison of these Rosseland radii, effective temperatures, and the shape of the observed visibility functions with model predictions suggests that o Cet is a fundamental mode pulsator. Furthermore, we investigated the variation of visibility function and diameter with phase. The Rosseland angular diameter of o Cet increased from 28.9 +/- 0.3 mas (corresponding to a Rosseland radius of 332 +/- 38 R_sun for a distance of D=107 +/- 12 pc) at phi=0.13 to 34.9 +/- 0.4 mas (402 +/- 46 R_sun) at phi=0.4. The error of the Rosseland linear radius almost entirely results from the error of the parallax, since the error of the angular diameter is only approximately 1 %.

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arXiv2003

Evidence for bipolar jets in late stages of AGB winds

Bipolar expansion at various stages of evolution has been recently observed in a number of AGB stars. The expansion is driven by bipolar jets that emerge late in the evolution of AGB winds. The wind traps the jets, resulting in an expanding, elongated cocoon. Eventually the jets break-out from the confining spherical wind, as recently observed in W43A. This source displays the most advanced evolutionary stage of jets in AGB winds. The earliest example is IRC+10011, where the asymmetry is revealed in high-resolution near-IR imaging. In this source the jets turned on only ~200 years ago, while the spherical wind is ~4000 years old.

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arXiv2003

Speckle interferometry of the HAeBe star V376 Cas

We report H- and K'-band speckle interferometric observations of the HAeBe star V376 Cas. Our observations show that the object is partially resolved. The visibility curves suggest three separate components: a large scattering envelope visible only in the H band, a component approximately 100 mas in radius, and a component with a Gaussian HWHM of approximately 8+/-3 mas, corresponding to approximately 5 AU at 600 pc distance, which contributes most of the flux. We compare the smallest structure to the radius of dust sublimation in the radiation field of the star and find that the radius is approximately six times larger than that expected. This may indicate that the inner regions of the system are in fact obscured by a flaring circumstellar disk or torus seen close to edge-on.

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arXiv2002

Bispectrum speckle interferometry of the B[e] star MWC 349A

We present the results of bispectrum speckle interferometry of the B[e] star MWC 349A obtained with the SAO 6m telescope. Our diffraction-limited J-, H-, and K-band images (resolutions 43--74 mas) suggest the star is surrounded by a circumstellar disk seen almost edge-on. The observed visibility shape is consistent with a two-component elliptical disk model, probably corresponding to the gaseous and dusty components of the disk. We show that the classification of the object as a pre-main-sequence star or a young planetary nebula is problematic. An analysis of the uncertainties in the basic parameter determination lead us to the conclusion that MWC 349A is probably either a B[e] supergiant or a binary system, in which the B[e]-companion dominates the observed properties.

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arXiv2002

Spectral Observations of Stars in Late Stages of Stellar Evolution

Interferometric observations of stars in late stages of stellar evolution and the impact of VLTI observations are discussed. Special attention is paid to the spectral information that can be derived from these observations and on the corresponding astrophysical interpretation of the data by radiative transfer modelling. It is emphasized that for the robust and non-ambiguous construction of dust-shell models it is essential to take diverse and independent observational constraints into account. Apart from matching the spectral energy distribution, the use of spatially resolved information plays a crucial role for obtaining reliable models. The combination of long-baseline interferometry data with high-resolution single-dish data (short baselines), as obtained, for example, by bispectrum speckle interferometry, provides complementary information and will improve modelling and interpretation.

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arXiv2002

Observations of Mira stars with the IOTA/FLUOR interferometer and comparison with Mira star models

We present K'-band observations of five Mira stars with the IOTA interferometer. The interferograms were obtained with the FLUOR fiber optics beam combiner, which provides high-accuracy visibility measurements in spite of time-variable atmospheric conditions. For the M-type Miras X Oph, R Aql, RU Her, R Ser, and the C-type Mira V CrB we derived the uniform-disk diameters 11.7mas, 10.9mas, 8.4mas, 8.1mas, and 7.9mas (+/- 0.3mas), respectively. Simultaneous photometric observations yielded the bolometric fluxes. The derived angular Rosseland radii and the bolometric fluxes allowed the determination of effective temperatures. For instance, the effective temperature of R Aql was determined to be 2970 +/- 110 K. A linear Rosseland radius for R Aql of (250 +100/-60) Rsun was derived from the angular Rosseland radius of 5.5mas +/- 0.2mas and the HIPPARCOS parallax of 4.73mas +/- 1.19mas. The observations were compared with theoretical Mira star models of Bessel et al. (1996) and Hofmann et al. (1998). The effective temperatures of the M-type Miras and the linear radius of R Aql indicate fundamental mode pulsation.

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arXiv2002

Bispectrum speckle interferometry of IRC+10216: the dynamic evolution of the innermost circumstellar environment from 1995 to 2001

We present new near-infrared (JHK) bispectrum speckle-interferometry monitoring of the carbon star IRC+10216 obtained between 1999 and 2001 with the SAO 6m telescope. The J-, H-, and K-band resolutions are 50mas, 56mas, and 73mas, resp. The total sequence of K-band observations covers now 8 epochs from 1995 to 2001 and shows the dynamic evolution of the inner dust shell. The present observations show that the appearance of the dust shell has considerably changed compared to the epochs of 1995 to 1998. Four main components within a 0.2" radius can be identified in the K-band images. The apparent separation of the two initially brightest components A and B increased from ~191mas in 1995 to ~351mas in 2001. Simultaneously, component B has been fading and almost disappeared in 2000 whereas the initially faint components C and D became brighter (relative to peak intensity). These changes can be related to changes of the optical depth caused, e.g., by mass-loss variations or new dust condensation in the wind. Our 2D radiative transfer model suggests that the observed relative motion of A and B is not consistent with the known terminal wind velocity of 15 km/s. The apparent motion with a deprojected velocity of 19 km/s on average and of recently 27 km/s appears to be caused by adisplacement of the dust density peak due to dust evaporation in the optically thicker and hotter environment. Our monitoring, covering more than 3 pulsation periods, shows that the structural variations are not related to the stellar pulsation cycle in a simple way. This is consistent with the predictions of hydrodynamical models that enhanced dust formation takes place on a timescale of several pulsation periods. The timescale of the fading of component B can well be explained by the formation of new dust in the circumstellar envelope.

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arXiv2002

Diffraction-limited speckle interferometry and modeling of the circumstellar envelope of R CrB at maximum and minimum light

We present the first speckle interferometric observations of R CrB, the prototype of a class of peculiar stars which undergo irregular declines in their visible light curves. The observations were carried out with the 6m telescope at the Special Astrophysical Observatory near maximum light (V=7, 1996 Oct. 1) and at minimum light (V=10.61, 1999 Sep. 28). A spatial resolution of 75mas was achieved in the K-band. The dust shell around R CrB is partially resolved, and the visibility is approximately 0.8 at a spatial frequency of 10 cycles/arcsec. The two-dimensional power spectra obtained at both epochs do not show any significant deviation from circular symmetry. The visibility function and spectral energy distribution obtained near maximum light can be simultaneously fitted with a model consisting of the central star and an optically thin dust shell with density proportional to r^-2. The inner boundary of the shell is found to be 82Rstar (19mas) with a temperature of 920K. However, this simple model fails to simultaneously reproduce the visibility and spectral energy distribution obtained at minimum light. We show that this discrepancy can be attributed to thermal emission from a newly formed dust cloud.

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arXiv2001

Speckle interferometry and radiative transfer modelling of the Wolf-Rayet star WR 118

WR 118 is a highly evolved Wolf-Rayet star of the WC10 subtype surrounded by a permanent dust shell absorbing and re-emitting in the infrared a considerable fraction of the stellar luminosity. We present the first diffraction-limited 2.13micron speckle interferometric observations of WR 118 with 73 mas resolution. The speckle interferograms were obtained with the 6m telescope at the Special Astrophysical Observatory. The two-dimensional visibility function of the object does not show any significant deviation from circular symmetry. The visibility curve declines towards the diffraction cut-off frequency to 0.66 and can be approximated by a linear function. Radiative transfer calculations have been carried out to model the spectral energy distribution, given in the range of 0.5-25micron, and our 2.13micron visibility function, assuming spherical symmetry of the dust shell. Both can be fitted with a model containing double-sized grains (``small'' and ``large'') with the radii of a = 0.05micron and 0.38micron, and a mass fraction of the large grains greater than 65%. Alternatively, a good match can be obtained with the grain size distribution function n(a)~a^-3, with a ranging between 0.005micron and 0.6micron. At the inner boundary of the modelled dust shell (angular diameter (17 +/- 1)mas), the temperature of the smallest grains and the dust shell density are 1750K +/- 100K and (1 +/- 0.2)x10^-19 g/cm^3, respectively. The dust formation rate is found to be (1.3 +/- 0.5)x10^-7 Msol/yr assuming Vwind = 1200 km/s.

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arXiv2001

A multi-wavelength study of the oxygen-rich AGB star CIT 3: Bispectrum speckle interferometry and dust-shell modelling

(abridged) CIT 3 is an oxygen-rich long-period variable evolving along the AGB and one of the most extreme infrared AGB objects. We present the first bispectrum speckle-interferometry observations of CIT 3 in the J-, H-, and K'-band (resolution: 48mas, 56mas, and 73mas). The interferograms were obtained with the Russian SAO 6m telescope. While CIT 3 appears almost spherically symmetric in the H- and K'-band, it is clearly elongated in the J-band along a symmetry axis of position angle -28 degr. Two structures can be identified: a compact elliptical core (eccentricity ~0.8) and a fainter north-western fan-like structure (full opening angle ~40 degr). Extensive radiative transfer calculations have been carried out and confronted with the spectral energy distribution, our 1.24, 1.65 and 2.12 micron visibility functions, and 11micron ISI interferometry. The best model refers to a cool central star (Teff=2250K) surrounded by an optically thick dust shell (tau_V = 30). The central-star diameter is 10.9mas and the inner dust shell diameter 71.9mas. The inner dust-shell rim is located at r_1=6.6 Rstar and has a temperature of T_1=900K. A two-component model existing of an inner uniform-outflow shell region (rho~1/r^2; r < 20.5 r_1) and an outer region with rho ~ 1/r^1.5 proved to give the best overall match of the observations. Provided the outflow velocity kept constant, the more shallow density distribution in the outer shell indicates that mass-loss has decreased with time in the past of CIT 3. Adopting vexp=20km/s, the termination of that mass-loss decrease and the begin of the uniform-outflow phase took place 87yr ago. The present-day mass-loss rate can be determined to be Mdot = (1.3-2.1) x 10^-5 Msol/yr for d=500-800pc.

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arXiv2001

Bispectrum speckle interferometry observations and radiative transfer modelling of the red supergiant NML Cyg: Multiple dust-shell structures evidencing previous superwind phases

(abridged) NML Cyg is a highly evolved OH/IR supergiant and supposed to be among the most luminous supergiants in the galaxy. We present the first diffraction limited 2.13micron observations of NML Cyg with 73mas resolution. The speckle interferograms were obtained with the SAO 6m telescope, image reconstruction is based on the bispectrum speckle interferometry method. Radiative transfer calculations have been carried out to model the spectral energy distribution, our 2.13micron visibility function, and mid-infrared visibility functions. The observed dust shell properties do not appear to be in accordance with single-shell models but seem to require multiple components. Considering previous periods of enhanced mass-loss, various density enhancements in the dust shell were taken into account. An extensive grid of models was calculated for different locations and strenghts of such superwind regions in the dust shell. To match the observations from the optical to the sub-mm domain requires at least two superwind regions embedded in the shell. The best model includes a dust shell with a temperature of 1000K at its inner radius of 6.2Rstar, a close embedded superwind shell extending from 15.5Rstar to 21.7Rstar with amplitude 10 (factor of density enhancement), and a far-out density enhancement at 186Rstar with amplitude 5. The angular diameter of the inner dust-shell rim amounts to 105mas. Within the various parts of the dust shell, 1/r^2 density distributions could be maintained differing only in their amplitude A. The present-day mass-loss rate was determined to be 1.2 10^-4 Msol/yr. The inner embedded superwind shell corresponds to a phase of enhanced mass-loss which began ~59yr ago and lasted for ~18yr, and the outer superwind region to a high mass-loss period which terminated 529yr ago.

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arXiv2001

Computer simulations of interferometric imaging with the VLT interferometer and its AMBER instrument

We present computer simulations of interferometric imaging with the Very Large Telescope Interferometer (VLTI) of the European Southern Observatory (ESO) and the Astronomical MultiBEam Recombiner (AMBER) phase-closure instrument. These simulations include both the astrophysical modelling of a stellar object by radiative transfer calculations and the simulation of light propagation from the object to the detector (through atmosphere, telescopes, and the AMBER instrument), simulation of photon noise and detector read-out noise, and finally data processing of the interferograms. The results show the dependence of the visibility error bars on the following observational parameters: different seeing during the observation of object and reference star (Fried parameters r_0,object and r_0,ref ranging between 0.9m and 1.2m), different residual tip-tilt error (delta_tt,object and delta_tt,ref ranging between 0.1% and 20% of the Airy disk diameter), and object brightness (K_object=0.7mag to 10.2mag, K_ref=0.7mag). Exemplarily, we focus on stars in late stages of stellar evolution and study one of its key objects, the dusty supergiant IRC+10420 that is rapidly evolving on human timescales. We show computer simulations of VLT interferometry (visibility and phase closure measurements) of IRC+10420 with two and three Auxiliary Telescopes (ATs; AMBER wide-field mode, i.e. without fiber optics spatial filters) and discuss whether the visibility accuracy is sufficient to distinguish between different theoretical model predictions.

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arXiv2000

Diffraction-limited bispectrum speckle interferometry and speckle polarimetry of the young bipolar outflow source S140 IRS1

We present bispectrum speckle interferometry and speckle polarimetry of the deeply embedded infrared bipolar outflow source S140 IRS1, a massive protostellar object in the L1204 molecular cloud. Using the SAO 6 m telescope, we obtained 280 mas resolution polarization maps of S140 IRS1 as well as a K-band image with diffraction-limited resolution lambda/D of 76 mas, which is the highest angular resolution image of a young outflow source ever obtained in the infrared. Our data suggest that the central source is marginally resolved with a FWHM diameter of approximately 20 mas (~ 20 AU). The most remarkable feature in our image is a bright extended and very clumpy structure pointing away from the central source in exactly the same direction as the blue-shifted CO outflow lobe. A centro-symmetric pattern of high polarization in this feature suggests that we see scattered light from the central source. We interprete this feature as the clumpy inner surface of a partially evacuated cavity in the circumstellar envelope around IRS1, which has been excavated by the strong outflow from IRS1.

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arXiv2000

Observations of Mira stars with the IOTA/FLUOR interferometer and comparison with Mira star models

We present K-band observations of five Mira stars with the IOTA interferometer. The interferograms were obtained with the FLUOR fiber optics beam combiner which provides high-accuracy visibility measurements in spite of time-variable atmospheric conditions. For the Mira stars X Oph, R Aql, RU Her, R Ser, and V CrB we derived the uniform-disk diameters 11.7 mas, 10.9 mas, 8.4 mas, 8.1 mas, and 7.9 mas (+/-0.3 mas), respectively. Simultaneous photometric observations yielded the bolometric fluxes. The derived angular Rosseland radii and the bolometric fluxes allowed the determination of effective temperatures. For instance, the effective temperature of R Aql was determined to be 3072 K +/- 161 K. A Rosseland radius for R Aql of 250 Rsun +/- 63 Rsun was derived from the angular Rosseland radius of 5.5 mas +/- 0.2 mas and the HIPPARCOS parallax of 4.73 mas +/- 1.19 mas. The observations were compared with theoretical Mira star models of Bessel, Scholz and Wood (1996) and Hofmann, Scholz and Wood (1998).

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arXiv2000

Computer simulations of interferometric imaging with the VLT interferometer and the AMBER instrument

We present computer simulations of interferometric imaging with the VLT interferometer and the AMBER instrument. These simulations include both the astrophysical modelling of a stellar object by radiative transfer calculations and the simulation of light propagation from the object to the detector (through atmosphere, telescopes, and the AMBER instrument), simulation of photon noise and detector read-out noise, and finally data processing of the interferograms. The results show the dependence of the visibility error bars on the following observational parameters: different seeing during the observation of object and reference star (Fried parameters r_0,object=2.4m, r_0,ref.=2.5m, different residual tip-tilt error (delta_tt,object=2% of the Airy disk diameter, delta_tt,ref.=0.1%), and object brightness (K_object=3.5 mag and 11 mag, K_ref.=3.5 mag). Exemplarily, we focus on stars in late stages of stellar evolution and study one of its key objects, the dusty supergiant IRC+10420 that is rapidly evolving on human timescales. We show computer simulations of VLTI interferometry of IRC+10420 with two ATs (wide-field mode, i.e. without fiber optics spatial filters) and discuss whether the visibility accuracy is sufficient to distinguish between different theoretical model predictions.

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arXiv1999

The rapidly evolving hypergiant IRC+10420: High-resolution bispectrum speckle-interferometry and dust-shell modelling

The hypergiant IRC+10420 is a unique object for the study of stellar evolution since it is the only object that is believed to be witnessed in its rapid transition from the red supergiant stage to the Wolf-Rayet phase. Its effective temperature has increased by 1000-2000K within only 20yr. We present the first speckle observations of IRC+10420 with 73mas resolution. A diffraction-limited 2.11 micron image was reconstructed from 6m telescope speckle data using the bispectrum speckle-interferometry method. The visibility function shows that the dust shell contributes 40% to the total flux and the unresolved central object 60%. Radiative transfer calculations have been performed to model both the spectral energy distribution and visibility function. The grain sizes, a, were found to be in accordance with a standard distribution function, n(a)~a^(-3.5), with 0.005 micron < a < 0.45 micron. The observed dust shell properties cannot be fitted by single-shell models but seem to require multiple components. At a certain distance we considered an enhancement over the assumed 1/r^x density distribution. The best model for both SED and visibility was found for a dust shell with a dust temperature of 1000K at its inner radius of 69Rstar. At a distance of 308Rstar the density was enhanced by a factor of 40 and and its density exponent was changed from x=2 to x=1.7. The shell's intensity distribution was found to be ring-like.The ring diameter is equal to the inner diameter of the hot shell (69mas). The diameter of the central star is 1mas. The two-component model can be interpreted in terms of a termination of an enhanced mass-loss phase roughly 60 to 90 yr (for d=5kpc) ago.

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