Publication archive

Publication archive

A light echo around SN 1993J was observed 8.2 yr after explosion by a Hubble Space Telescope Wide Field Planetary Camera 2 observation, adding to the small family of supernovae with light echoes. The light echo was formed by supernova light scattered from a dust sheet that lies 220 pc away from the supernova, 50 pc thick along the line of sight, as inferred from the radius and width of the light echo. The dust inferred from the light echo surface brightness is 1000 times denser than the intercloud dust. The graphite-to-silicate fraction cannot be determined by our BVI photometric measurements. However, a pure graphite model can be excluded on the basis of comparison with the data. With future observations, it will be possible to measure the expansion rate of the light echo, from which an independent distance to M81 can be obtained.
Published: 15 January 2003
Brown dwarfs and giant gas planets are substellar objects whose spectral appearance is determined by the chemical composition of the gas and the solids/liquids in the atmosphere. Atmospheres of substellar objects possess two major scale regimes: large-scale convective motions + gravitational settling and small-scale turbulence + dust formation. Turbulence initiates dust formation spot-like on small scale, while the dust feeds back into the turbulent fluid field by its strong radiative cooling. Small, imploding dust containing areas result which eventually become isothermal. Multi-dimensional simulations show that these small-scale dust structures gather into large-scale structures, suggesting the formation of clouds made of dirty dust grains. The chemical composition of the grains, and thereby the chemical evolution of the gas phase, is a function of temperature and depends on the grains history.
Published: 02 May 2005
We present a relation between EUV flux and strength of the He I 10830 A line in active cool stars. The metastable lower level of the Helium 10830 line is believed to be populated by recombination following photoionization of neutral Helium present in the upper chromosphere. The EUV flux radiated in the corona (the ionization edge of He I is at 24.58 eV, or 504.27 A) is responsible for the photoionization. For the first time, we have measured the EUV flux in active stars directly using the Extreme Ultraviolet Explorer (EUVE) to verify a relation to the strength of the He I absorption, measured with the SOFIN echelle spectrograph (R~170000) on the Nordic Optical Telescope. The He I absorption increases with the EUV flux, displaying two different relations, one for subgiants and dwarfs, and another one for giants, which has stronger absorption. The presence of emission and chromospheric acceleration observed in the He 10830 line profiles will be also discussed.
Published: 02 September 2005
A free format discussion session was held following the presentations by Lee Hartmann, Karen Bjorkman, Georges Meynet and Keith MacGregor. Several subjects were discussed and debated during this session: a) The origin of the mass in the disks of Be stars that needs to be replenished on time scales of one to a few decades. b) The nature and cause of the hot plasmas and associated X-rays that arise from disks. c) The origin of magnetic fields that may play a role in providing angular momentum and mass to disks. d) The dissipation of primordial fields. e) The processes and time scales on which disks can be lost. The discussion then turns to specific properties of the disks around emission line Be stars: the fraction of B stars that may have emission line phases and the rotation rate relative to the maximal Keplerian rate of the Be stars.
Published: 02 May 2005
The massive star that underwent a collapse of its core to produce supernova (SN)1993J was subsequently identified as a non-variable red supergiant star in images of the galaxy M81 taken before explosion. It showed an excess in ultraviolet and B-band colours, suggesting either the presence of a hot, massive companion star or that it was embedded in an unresolved young stellar association. The spectra of SN1993J underwent a remarkable transformation from the signature of a hydrogen-rich type II supernova to one of a helium-rich (hydrogen-deficient) type Ib. The spectral and photometric peculiarities were best explained by models in which the 13-20 solar mass supergiant had lost almost its entire hydrogen envelope to a close binary companion, producing a 'type IIb' supernova, but the hypothetical massive companion stars for this class of supernovae have so far eluded discovery. Here we report photometric and spectroscopic observations of SN1993J ten years after the explosion. At the position of the fading supernova we detect the unambiguous signature of a massive star: the binary companion to the progenitor.
Published: 08 January 2004
We show that one can obtain a good fit to the measured main sequence mass function (MF) of a large sample of Galactic clusters (young and old) with a tapered Salpeter power law distribution function with an exponential truncation. The average value of the power law index is very close to Salpeter (~2.3), whereas the characteristic mass is in the range 0.1 - 0.5 Msolar and does not seem to vary in a systematic way with the present cluster parameters such as metal abundance and central concentration. However, a remarkable correlation with age is seen, in that the peak mass of young clusters increases with it. This trend does not extend to globular clusters, whose peak mass is firmly at ~0.35 Msolar. This correlation is due to the onset of mass segregation following early dynamical interactions in the loose cluster cores. Differences between globular and younger clusters may depend on the initial environment of star formation, which in turn affects their total mass.
Published: 02 May 2005
This article is based on a talk given by Professor Martin Huber to the Pro ISSI.
Published: 02 May 2005
I have developed an automated slitless-spectrum extraction software package that allows for the simultaneous obtention of hundreds of point-source spectra within a field. MULTISPEC is an spectroscopic analog to crowded-field PSF-fitting photometry packages such as DAOPHOT or HSTPHOT in the sense that it fits spatial profiles to the spectrum of each star simultaneously. The code has been written in IDL, and implemented and tested for the STIS NUV-MAMA objective-prism. It could be rather easily adapted to observations with other spectral elements with slitless capabilities (objective prisms, grisms, and gratings) in HST or in other telescopes. I discuss the techniques employed and I show the first results obtained with MULTISPEC.
Published: 02 May 2005
We report the results of a series of synchrotron characterizations of two epitaxial GaAs detectors of active areas 2.22 mm² and thicknesses 40 and 400 microns. In spite of an order of magnitude difference in depletion depths, the detectors were found to have comparable performances at ~ -40 °C, with energy resolutions of ~1 keV fwhm at 7 keV rising to ~2 keV fwhm at 200 keV and noise floors in the range 1-1.5 keV. At the lower energies, the energy resolution was dominated by leakage current and electromagnetic pick-up. At the highest energies, however, the measured resolutions appear to approach the expected Fano limit; e.g., ~950 eV at 200 keV. Both detectors were remarkably linear, with average rms non-linearities of 0.2% over the energy range 10-60 keV. By raster scanning the active areas with 20 x 20 micron² monoenergetic photon beams, it was found that the non-uniformity in the spatial response of both detectors was less than 1% and independent of energy. The material used to fabricate the detector is extremely pure. For example, low temperature photoluminescence measurements indicate that the density of the As anti-site defect (EL2) is of the order of 1012 cm-3, which is ~ 2-3 orders of magnitude lower than that generally reported. This indirect measurement of material purity is confirmed by Monte-Carlo simulations of the detector X-ray response, which show that in order to reproduce the observed energy-loss spectra, electron and hole trapping cross-section/density products must be <<1 cm-1.
Published: 16 October 1999
Using the Long Wavelength Spectrometer on board the Infrared Space Observatory, we have observed thermal water vapor emission from a roughly circular field of view approximately 750 in diameter centered on the Orion BN-KL region. The Fabry-Perot line strengths, line widths, and spectral line shifts observed in eight transitions between 71 and 125 mm show good agreement with models of thermal emission arising from a molecular cloud subjected to a magnetohydrodynamic C-type shock. Both the breadth and the relative strengths of the observed lines argue for emission from a shock rather than from warm quiescent gas in the Orion core. Although one of the eight transitions appears anomalously strong and may be subject to the effects of radiative pumping, the other seven indicate an H2O/H2 abundance ratio on the order of and a 5x10-4 corresponding gas-phase oxygen-to-hydrogen abundance ratio on the order of 4x10-4. Given current estimates of the interstellar, gas-phase, oxygen and carbon abundances in the solar vicinity, this value is consistent with theoretical shock models that predict the conversion into water of all the gas-phase oxygen that is not bound as CO. The overall cooling provided by rotational transitions of H2O in this region appears to be comparable to the cooling through rotational lines of CO but is an order of magnitude lower than cooling through H2 emission. However, the model that best fits our observations shows cooling by H2O and CO dominant in that portion of the postshock region where temperatures are below ~800 K and neither vibrational nor rotational radiative cooling by H is appreciable.
Published: 21 April 1998
We present the analysis of serendipitous sources in a deep, 500 ks, hard X-ray observation of the Coma Cluster region with the IBIS instrument on board INTEGRAL. In addition to the Coma Cluster, the final 20-50 keV image contains 12 serendipitous sources with statistical significance >4 sigma. We use these data (after correcting for expected number of false detections) to extend the extragalactic source counts in the 20-50 keV energy band down to a limiting flux of 1.0x10-11 ergs s-1 cm-2 (~=1 mcrab). This is a more than a factor of 10 improvement in sensitivity compared to the previous results in this energy band obtained with the HEAO 1 A-4 instrument. The derived source counts are consistent with the Euclidean relation N(>f)~f-3/2. A large fraction of identified serendipitous sources are low-redshift, z<0.02, active galactic nuclei (AGNs), mostly of Seyfert 1 type. The surface density of hard X-ray sources is (1.4+/-0.5)x10-2 deg-2 above a flux threshold of 10-11 ergs s-1 cm-2. These sources directly account for ~3% of the cosmic X-ray background in the 20-50 keV energy band. Given the low redshift depth of our sample, we expect that similar sources at higher redshifts account for a significant fraction of the hard X-ray background. Our field covers only 3% of the sky; a systematic analysis of other extragalactic INTEGRAL observations can produce much larger source samples and is, therefore, critically important.
Published: 21 May 2005
Photographs of the martian surface are undeniably beautiful, but uncovering the truth behind the sinuous river beds and suggestive gullies is not easy. Spacecraft and rover missions have turned up plenty of evidence that water once flowed on Mars, but today the planet's surface is bone dry. There is water ice at the poles and hints of a frozen sea near the equator - but any large reservoirs of water are likely to be kilometres down where conditions are warmer. So the hunt for liquid water, which might provide a habitat for martian life, has gone underground.
Published: 20 May 2005
On 10 December 2004, it was five years since ESA's XMM-Newton observatory was successfully put into orbit. It is therefore time to stand back and ask where we stand with the scientific results and what new perspectives the mission has brought us. The answers are to be found in more than 700 publications in the refereed literature and the manifold oversubscription for every observing Announcement of Opportunity for the mission.
Published: 16 May 2005
Through coupled thermal and orbital calculations including a full description of tidal dissipation, heat transfer and the H2ONH3 phase diagram, we propose a model for the internal structure and composition of Titan testable with Cassini Huygens measurements. The high value of Titan's orbital eccentricity provides a strong constraint on the amount of the tidal energy dissipation on its surface and within its interior since its formation. We show that only models with a few percent of ammonia (and not zero) in the primordial liquid water shell can limit the damping of the eccentricity over the age of the Solar System. The present models predict that a liquid ammonia-rich water layer should still be present within Titan under an ice I layer, a few tens of kilometers thick. Furthermore, we predict that any event linked to convective processes in the ice Ih layer (like the degassing of methane) could have occurred very late in Titan's history.
Published: 16 June 2005
We present a method for inferring the relative molar abundance of constituents of a liquid mixture, in this case methane, ethane, nitrogen and argon, from a measurement of a set of physical properties of the mixture. This problem is of interest in the context of the Huygens Surface Science Package, SSP, equipped to measure several physical properties of a liquid in case of a liquid landing on Saturn's moon Titan. While previous models emphasized the possibility of verifying a certain model proposed by atmospheric composition and equations of state, we use an inverse approach to the problem, i.e. we will infer the liquid composition strictly from our measurements of density, refractive index, permittivity, thermal conductivity and speed of sound. Other a priori information can later be used to improve (or reject) the model obtained from these measurements.
Published: 16 May 2005
We report on XMM-Newton observations of two X-ray transient millisecond pulsars (XRTMSPs). We detected XTE J0929-314 with an unabsorbed luminosity of ~ 7x 1031 erg s-1 (0.5-10 keV) at a fiducial distance of 10 kpc. The quiescent spectrum is consistent with a simple power law spectrum. The upper limit on the flux from a cooling neutron star atmosphere is about 20% of the total flux. XTE J1807-294 instead was not detected. We can put an upper limit on the source quiescent 0.5-10 keV unabsorbed luminosity ~< 4x1031 erg s-1 at 8 kpc. These observations strenghten the idea that XRTMSPs have quiescent luminosities significantly lower than classical neutron star transients.
Published: 16 May 2005
In an analysis of XMM-Newton archival observations of the bright Local Group spiral galaxy M 31 we study the population of X-ray sources (X-ray binaries, supernova remnants) down to a 0.2-4.5 keV luminosity of 4.4 x 1034 . EPIC hardness ratios and optical and radio information are used to distinguish between different source classes. The survey detects 856 sources in an area of 1.24 square degrees. We correlate our sources with earlier M 31 X-ray catalogues and use information from optical, infra-red and radio wavelengths. As sources within M 31 we detect 21 supernova remnants (SNR) and 23 SNR candidates, 18 supersoft source (SSS) candidates, 7 X-ray binaries (XRBs) and 9 XRB candidates, as well as 27 globular cluster sources (GlC) and 10 GlC candidates, which most likely are low mass XRBs within the GlC. Comparison to earlier X-ray surveys reveal transients not detected with XMM-Newton, which add to the number of M 31 XRBs. There are 567 sources classified as hard, which may either be XRBs or Crab-like SNRs in M 31 or background AGN. The number of 44 SNRs and candidates more than doubles the X-ray-detected SNRs. 22 sources are new SNR candidates in M 31 based on X-ray selection criteria. Another SNR candidate may be the first plerion detected outside the Galaxy and the Magellanic Clouds. On the other hand, six sources are foreground stars and 90 foreground star candidates, one is a BL Lac-type active galactic nucleus (AGN) and 36 are AGN candidates, one source coincides with the Local Group galaxy M 32, one with a background galaxy cluster (GCl) and another is a GCl candidate, all sources not connected to M 31.
Published: 16 May 2005
Faced with a $1 billion cost overrun, NASA managers last week began to search for cheaper designs for the $3.5 billion James Webb Space Telescope (JWST). But astronomers say the initial attempt to scale back the complexity of the spacecraft and its instruments is a nonstarter for the mission slated for a 2011 launch as a follow-on to the Hubble Space Telescope.
Published: 14 May 2005
The magnetic field signature obtained by Cassini during its first close encounter with Titan on 26 October 2004 is presented and explained in terms of an advanced model. Titan was inside the saturnian magnetosphere. A magnetic field minimum before closest approach marked Cassini's entry into the magnetic ionopause layer. Cassini then left the northern and entered the southern magnetic tail lobe. The magnetic field before and after the encounter was approximately constant for ~20 Titan radii, but the field orientation changed exactly at the location of Titan's orbit. No evidence of an internal magnetic field at Titan was detected.
Published: 14 May 2005
The Cassini Magnetospheric Imaging Instrument (MIMI) observed the interaction of Saturn's largest moon, Titan, with Saturn's magnetosphere during two close flybys of Titan on 26 October and 13 December 2004. The MIMI Ion and Neutral Camera (INCA) continuously imaged the energetic neutral atoms (ENAs) generated by charge exchange reactions between the energetic, singly ionized trapped magnetospheric ions and the outer atmosphere, or exosphere, of Titan. The images reveal a halo of variable ENA emission about Titan's nearly collisionless outer atmosphere that fades at larger distances as the exospheric density decays exponentially. The altitude of the emissions varies, and they are not symmetrical about the moon, reflecting the complexity of the interactions between Titan's upper atmosphere and Saturn's space environment.
Published: 14 May 2005
5-Jun-2020 13:59 UT

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