arXiv subjects

F. Kerber

Publications and source records attributed to F. Kerber.

35 records · Page 2Linked to original sources

arXiv2007

Dust observations of Comet 9P/Tempel 1 at the time of the Deep Impact

On 4 July 2005 at 05:52 UT, the impactor of NASA's Deep Impact (DI) mission crashed into comet 9P/Tempel 1 with a velocity of about 10 km/s. The material ejected by the impact expanded into the normal coma, produced by ordinary cometary activity. The characteristics of the non-impact coma and cloud produced by the impact were studied by observations in the visible wavelengths and in the near-IR. The scattering characteristics of the "normal" coma of solid particles were studied by comparing images in various spectral regions, from the UV to the near-IR. For the non-impact coma, a proxy of the dust production has been measured in various spectral regions. The presence of sublimating grains has been detected. Their lifetime was found to be about 11 hours. Regarding the cloud produced by the impact, the total geometric cross section multiplied by the albedo was measured as a function of the color and time. The projected velocity appeared to obey a Gaussian distribution with the average velocity of the order of 115 m/s. By comparing the observations taken about 3 hours after the impact, we have found a strong decrease in the cross section in J filter, while that in Ks remained almost constant. This is interpreted as the result of sublimation of grains dominated by particles of sizes of the order of some microns.

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arXiv2007

The enigma of the oldest `nova': the central star and nebula of CK Vul

CK Vul is classified as, amongst others, the slowest known nova, a hibernating nova, or a very late thermal pulse object. Following its eruption in AD 1670, the star remained visible for 2 years. A 15-arcsec nebula was discovered in the 1980's, but the star itself has not been detected since the eruption. We here present radio images which reveal an 0.1-arcsec radio source with a flux of 1.5 mJy at 5 GHz. Deep Halpha images show a bipolar nebula with a longest extension of 70 arcsec, with the previously known compact nebula at its waist. The emission-line ratios show that the gas is shock-ionized, at velocities >100 km/s. Dust emission yields an envelope mass of ~0.05 Msun. Echelle spectra indicate outflow velocities up to 360 km/s. From a comparison of images obtained in 1991 and 2004 we find evidence for expansion of the nebula, consistent with an origin in the 1670 explosion; the measured expansion is centred on the radio source. No optical or infrared counterpart is found at the position of the radio source. The radio emission is interpreted as thermal free-free emission from gas with Tel ~ 10000 K. The radio source may be due to a remnant circumbinary disk, similar to those seen in some binary post-AGB stars. We discuss possible classifications of this unique outburst, including that of a sub-Chandrasekhar mass supernova, a nova eruption on a cool, low-mass white dwarf, or a thermal pulse induced by accretion from a circumbinary disk.

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arXiv2007

The onset of photoionization in Sakurai's Object (V4334 Sgr)

We investigate the reheating of the very late thermal pulse (VLTP) object V4334 Sgr (Sakurai's Object) using radio observations from the Very Large Array, and optical spectra obtained with the Very Large Telescope. We find a sudden rise of the radio flux at 5 and 8 GHz - from <= 90 micro-Jy and 80 +/- 30 micro-Jy in February 2005 to 320 micro-Jy and 280 micro-Jy in June 2006. Optical line emission is also evolving, but the emission lines are fading. The optical line emission and early radio flux are attributed to a fast shock (and not photoionization as was reported earlier) which occurred around 1998. The fading is due to post-shock cooling and recombination. The recent rapid increase in radio flux is evidence for the onset of photoionization of carbon starting around 2005. The current results indicate an increase in the stellar temperature to 12 kK in 2006. The mass ejected in the VLTP eruption is M_ej >= 1e-4 Msol, but could be as high as 1e-2 Msol, depending mainly on the distance and the clumping factor of the outflow. We derive a distance between 1.8 and 5 kpc. A high mass loss could expose the helium layer and yield abundances compatible with those of [WC] and PG1159 stars.

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arXiv2007

High-resolution FUSE and HST ultraviolet spectroscopy of the white dwarf central star of Sh 2-216

LS V +4621 is the DAO-type central star of the planetary nebula Sh 2-216. We perform a comprehensive spectral analysis of high-resolution, high-S/N ultraviolet observations obtained with FUSE and STIS aboard the HST as well as the optical spectrum of LS V +4621 by means of state-of-the-art NLTE model-atmosphere techniques in order to compare its photospheric properties to theoretical predictions from stellar evolution theory as well as from diffusion calculations. From the N IV - NV, O IV - O VI, Si IV - Si V, and Fe V - Fe VII ionization equilibria, we determined an effective temperature of 95 +/- 2 kK with high precision. The surface gravity is log g = 6.9 +/- 0.2. An unexplained discrepancy appears between the spectroscopic distance d = 224 +46/-58 pc and the parallax distance d = 129 +6/-5 pc of LS V +4621. For the first time, we have identified Mg IV and Ar VI absorption lines in the spectrum of a hydrogen-rich central star and determined the Mg and Ar abundances as well as the individual abundances of iron-group elements (Cr, Mn, Fe, Co, and Ni). With the realistic treatment of metal opacities up to the iron group in the model-atmosphere calculations, the so-called Balmer-line problem (found in models that neglect metal-line blanketing) vanishes. Spectral analysis by means of NLTE model atmospheres has presently arrived at a high level of sophistication, which is now hampered largely by the lack of reliable atomic data and accurate line-broadening tables. Strong efforts should be made to improve upon this situation.

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arXiv2007

Sulphur and zinc abundances in Galactic halo stars revisited

High resolution UVES spectra of 40 main-sequence stars with -3.3 < [Fe/H] < -1.0 are used to derive S, Fe and Zn abundances from lines in the 400 - 950 nm region. For one star we also present novel observations of the SI triplet at 1.046 micron carried out with the ESO VLT CRIRES spectrograph. Comparison of sulphur abundances from the weak and strong SI lines provides important constraints on non-LTE effects. The high sulphur abundances reported by others for some metal-poor stars are not confirmed; instead, when taking non-LTE effects into account, the Galactic halo stars distribute around a plateau at [S/Fe] = +0.2 dex with a scatter of 0.07 dex only. This indicates that sulphur in Galactic halo stars has been made by alpha-capture processes in massive SNe. The observed scatter in S/Fe is, however, much smaller than predicted from current stochastic models of the chemical evolution of the early Galaxy, suggesting that either the models or the calculated yields of massive SNe should be revised. [Zn/Fe] is close to zero for metallicities in the range -2.0 < [Fe/H] < -1.0 but increases to a level of [Zn/Fe] = +0.15 dex in the range -2.7 < [Fe/H] < -2.0. At still lower metallicities [Zn/Fe] rises steeply to a value around [Zn/Fe] = +0.5 dex at [Fe/H] = -3.2. We also examine the behaviour of S/Zn and find that departures from the solar ratio are significantly reduced at all metallicities if non-LTE corrections to the abundances of these two elements are adopted. This effect, if confirmed, would reduce the usefulness of the S/Zn ratio as a diagnostic of past star-formation activity, but would bring closer together the values measured in damped Lyman-alpha systems and in Galactic stars.

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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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arXiv2006

The Real-Time Evolution of Sakurai's Star (V4334 Sgr) and other (V)LTP Objects

We report on the progress of our on-going campaign to monitor the evolution of the VLTP objects V4334 Sgr and V605 Aql, as well as the suspected (V)LTP object CK Vul. V4334 Sgr does not show signs of increased ionization compared to our previous observations in 2004. We obtained the first radio detection of V605 Aql, indicating a strong increase in radio flux since 1987. We also present the first radio detection of CK Vul and discuss the expansion of the material ejected during the 1670 event.

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arXiv2005

Report by the ESA-ESO Working Group on Extra-Solar Planets

Various techniques are being used to search for extra-solar planetary signatures, including accurate measurement of radial velocity and positional (astrometric) displacements, gravitational microlensing, and photometric transits. Planned space experiments promise a considerable increase in the detections and statistical knowledge arising especially from transit and astrometric measurements over the years 2005-15, with some hundreds of terrestrial-type planets expected from transit measurements, and many thousands of Jupiter-mass planets expected from astrometric measurements. Beyond 2015, very ambitious space (Darwin/TPF) and ground (OWL) experiments are targeting direct detection of nearby Earth-mass planets in the habitable zone and the measurement of their spectral characteristics. Beyond these, `Life Finder' (aiming to produce confirmatory evidence of the presence of life) and `Earth Imager' (some massive interferometric array providing resolved images of a distant Earth) appear as distant visions. This report, to ESA and ESO, summarises the direction of exo-planet research that can be expected over the next 10 years or so, identifies the roles of the major facilities of the two organisations in the field, and concludes with some recommendations which may assist development of the field. The report has been compiled by the Working Group members and experts over the period June-December 2004.

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arXiv2004

Dynamics and star formation activity of CG J1720-67.8 unveiled through integral field spectroscopy and radio observations

CG J1720-67.8 is an ultra compact group of several galaxies with a low velocity dispersion, and displaying the hallmarks of mutual interaction and possible tidal dwarf galaxy formation. In hierarchical models, the system is a possible precursor to a massive elliptical galaxy. In this paper, we use new optical integral field spectroscopic and radio observations to investigate the evolutionary status of the group in more detail: global star-formation rates are estimated using H$α$ and 1.4 GHz radio continuum measurements; H {\sc i} observations provide an upper limit to the global neutral gas content; optical broadband colours and spectra provide ages and stellar mass estimates for the tidal dwarf candidates; the bidimensional H$α$ velocity field is used to trace the kinematics of the group and its members, which are compared with numerical simulations of galaxy encounters. The observations suggest a model in which multiple interactions have occurred, with the latest strong encounter involving at least two major components within the last 200 Myr. Debris from the encounter fuels ongoing star formation at the global level of $\sim20$ M$_{\odot}$ yr$^{-1}$, with self-gravity within the tidal tail possibly providing a mechanism to enhance the star formation rate of the tidal dwarf candidates, with bursts of star-formation in clumps of mass $\sim2\times 10^7$ M$_{\odot}$ appearing within the last 10 Myr. The amount of time required for final merging of all group components remains uncertain.

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arXiv2003

NLTE Spectral Analysis of Central Stars of PNe Interacting with the ISM

The analysis of Planetary Nebulae (PNe) provides a tool to investigate the properties of their exciting central stars (CSPN) at the moment of the PN ejection as well as on the properties of the ambient interstellar medium (ISM). The spectral analysis of the CSPN is a prerequisite to calculate the ionizing flux which is a crucial input for reliable PN modeling. In the framework of a systematic study of PNe interacting with the ISM, we present preliminary results of ongoing NLTE spectral analyses of ten of their CS based on new optical medium-resolution spectra.

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arXiv2003

3D kinematics of white dwarfs from the SPY project

We present kinematics of a sample of 107 DA white dwarfs from the SPY project and discuss kinematic criteria for a distinction of thin disk, thick disk, and halo populations. This is the first homogeneous sample of white dwarfs for which 3D space motions have been determined. Studies of white dwarf kinematics can help determine the fraction of the total mass of our Galaxy contained in the form of thick disk and halo white dwarfs. Radial velocities and spectroscopic distances obtained by the SPY project are combined with our measurements of proper motions to derive 3D space motions. Galactic orbits and further kinematic parameters are computed. Candidates for thick disk and halo members are selected in a first step from the classical U-V-velocity diagram. Our final assignment of population membership is based on orbits and position in the angular-momentum-eccentricity diagram. We find four halo and twelve thick disk white dwarfs.

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arXiv2003

Candidate Tidal Dwarf Galaxies in the Compact Group CG J1720-67.8

This is the second part of a detailed study of the ultracompact group CG J1720-67.8: in the first part we have focused the attention on the three main galaxies of the group and we have identified a number of candidate tidal dwarf galaxies (TDGs). Here we concentrate on these candidate TDGs. Absolute photometry of these objects in BVRJHKs bands confirms their relatively blue colors, as we already expected from the inspection of optical and near-infrared color maps and from the presence of emission-lines in the optical spectra. The physical conditions in such candidate TDGs are investigated through the application of photoionization models, while the optical colors are compared with grids of spectrophotometric evolutionary synthesis models from the literature. Although from our data self-gravitation cannot be proved for these objects, their general properties are consistent with those of other TDG candidates. Additionally we present the photometry of a few ``knots'' detected in the immediate surroundings of CG J1720-67.8 and consider the possibility that these objects might belong to a dwarf population associated with the compact group.

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arXiv2002

Freshly ionized matter around the final Helium shell flash object V4334 Sgr (Sakurai's object)

We report on the discovery of recently ionized hydrogen-deficient gas in the immediate circumstellar environment of the final helium shell flash star V4334 Sgr (Sakurai's object). On spectra obtained with FORS2 multi-object spectroscopy we have found spatially extended (about 2") emission from [N II], [O I], [O II] and very faint Halpha and [S II]. In the [N II] (ll6548,83) lines we have identified two components located at velocities -350 +/-50 and +200 +/-50 km/s, relative to V4334 Sgr itself. The full width of the [N II] l6583 feature at zero intensity corresponds to a velocity spread of about 1500 km/s. Based on the available data it is not possible to conclusively determine the mechanism of ionization. Both photo-ionization, from a rapidly evolving central star, and shock excitation, as the result of the collision of the fast ouflows with slower circumstellar matter, could account for the observed lines. The central star is still hidden behind strong dust absorption, since only a faint highly reddened continuum is apparent in the spectra. Theory states that it will become hotter and will retrace its post-asymptotic giant branch evolution towards the planetary nebula domain. Our detection of the ionized ejecta from the very late helium shell flash marks the beginning of a new phase in this star's amazingly rapid evolution.

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arXiv2002

CG J1720-67.8: A Detailed Analysis of Optical and Infrared Properties of a New Ultracompact Group of Galaxies

We present here optical spectroscopy and BVRJHK(s) photometry of the recently discovered ultra-compact group of galaxies CG J1720-67.8. This work represents a considerable extension of the preliminary results we presented in a previous paper. Despite the complicated morphology of the group, a quantitative morphological classification of the three brightest members of the group is attempted based on photometric analysis. We find that one galaxy is consistent with a morphological type S0, while the other two are most probably late-type spirals that are already losing their identity due tothe interaction process. Information on the star formation activity and dust content derived from both spectroscopic data and optical and near-infrared colors are complemented with a reconstruction of far-infrared (FIR) maps from IRAS raw data. Enhanced star formation activity is revealed in all the group's members, including the early-type galaxy and the extended tidal tail, along which several tidal dwarf galaxy candidates are identified. The metallicity of the gaseous component is investigated and photoionization models are applied to the three main galaxies of the group, while a detailed study of the tidal dwarf candidates will appear in a companion paper. Subsolar metal abundances are found for all the three galaxies, the highest values being shown by the early-type galaxy (Z ~ 0.5 Zsolar).

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arXiv1999

On the Verge of Coalescence: a Dusty Group of Galaxies

We found a new, very compact group of galaxies characterized by a median projected separation between galaxies of 5.2/h kpc and a very low velocity dispersion. All the observed member galaxies show emission line spectra and a disturbed morphology. From these properties it emerges that this compact group is in an extremely advanced stage of evolution.

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arXiv1999

A New Ultra-dense Group of Obscured Emission-Line Galaxies

We present the discovery of an isolated compact group of galaxies that is extremely dense (median projected galaxy separation: 6.9 kpc), has a very low velocity dispersion ($σ_{\rm 2D}$ = 67 km s$^{-1}$), and where all observed members show emission lines and are morphologically disturbed. These properties, together with the lack of spirals and the presence of a prominent tidal tail make this group one of the most evolved compact groups.

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arXiv1999

The Ever Changing Circumstellar Nebula Around UW Centauri

We present new images of the reflection nebula surrounding the R Coronae Borealis Star, UW Cen. This nebula, first detected in 1990, has changed its appearance significantly. At the estimated distance of UW Cen, this nebula is approximately 0.6 ly in radius so the nebula cannot have physically altered in only 8 years. Instead, the morphology of the nebula appears to change as different parts are illuminated by light from the central star modulated by shifting thick dust clouds near its surface. These dust clouds form and dissipate at irregular intervals causing the well-known declines in the R Coronae Borealis (RCB) stars. In this way, the central star acts like a lighthouse shining through holes in the dust clouds and lighting up different portions of the nebula. The existence of this nebula provides clues to the evolutionary history of RCB stars possibly linking them to the Planetary Nebulae and the final helium shell flash stars.

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